Perovskite Solar Cells Processed by Solution Nanotechnology

CHAPTE R 5

Perovskite Solar Cells Processed by Solution Nanotechnology Jiangsheng Xie, Feng Liu, Keyou Yan School of Environment and Energy, State Key Laboratory of Luminescent Materials and Devices, National Engineering Laboratory for VOCs Pollution Control Technology and Equipment, South China University of Technology, Guangzhou, People’s Republic of China; Department of Electronic Engineering, The Chinese University of Hong Kong, Sha Tin, Hong Kong

The word perovskite originated from the name of the Russian mineralogist Lev Perovski, who is related to the discovery of calcium titanium oxide (CaTiO3) by the Russian mineralogist Gustav Rose in 1839 (see Wikipedia). It lends its name to the class of compounds that have the same type of crystal structure as CaTiO3, known as the perovskite structure (Wells, 1893). Oxide perovskites have a cubic structure with the general formula of ABO3, consisting of an A-site ion on the corners of the lattice, B-site ions on the center of the lattice, and oxygen at the facet centers. Halide peroviskite is similar in structure to oxide perovskite in the ABX3 formula. Although the crystal structure of organic-inoganic perovskites were first reported by Weber (1978a,b) and later developed for field-effect transistors in the 1990s (Kagan, 1999; Mitzi, 1994, Mitzi, 1995), the excellent photovoltaic (PV) properties of halide perovskites were first exploited in 2009 (Kojima et al., 2009). The halide perovskite was first applied in the liquid dye-sensitized solar cell (DSSC) structure as a visible-light sensitizer by Miyasaka’s group, which delivered power conversion efficiency (PCE) values as low as 3.8% (Kojima et al., 2009). In 2011, Park’s group demonstrated a solar-to-electrical PCE of 6.54% by increasing the concentration of perovskite-coating solution and modifying the mesoporous TiO2 nanocrystal surface (Im et al., 2011). However, perovskite quantum dot-sensitized solar cells (QDSSCs) exhibited poor performance stability under continuous irradiation, owing to the instability of perovskite in the polar redox electrolyte. Perovskite-sensitized liquid solar cells were not stable during the measurement, and it was difficult to repeat the performance. A surge of progress was achieved after the breakthrough development of solid-state perovskite solar cells (PSCs) in the group of Park and Snaith in 2012, independently (Kim et al., 2012; Lee et al., 2012).

Advanced Nanomaterials for Solar Cells and Light-Emitting Diodes. https://doi.org/10.1016/B978-0-12-813647-8.00005-9 © 2019 Elsevier Inc. All rights reserved.

119

120  Chapter 5

5.1  Properties of Halide Perovskite 5.1.1  Chemical Structure The organic-inorginc halide perovskites are prepared by the solution process, and the ABX3 structure is shown in Fig. 5.1, in which X is a monovalent halide/pseudohalide anion (X = Cl, Br, I, SCN…), and A and B are monovalent and divalent cations of different sizes (A being larger than B; A = CH3NH3+, CH(NH2)2+, Cs+…; B = Pb2+, Sn2+…). Their crystallographic stability and probable structure can be deduced by considering a tolerance factor t and an octahedral factor μ. The values of t and μ can be expressed as (Li et al., 2008) t=

RA + RX

2 ( RB + RX )

;

µ=

RB RX

(5.1)

where rA, rB, and rX are the ionic radius of A-, B-, and X-site elements, respectively. It is generally required that 0.81 < t < 1.11 and 0.44 < μ < 0.90 for a stable perovskite structure (Li et al., 2008). If t lies in the narrower range of 0.89–1.0, the cubic structure is energetically stable, while lowering the t-values slightly leads to less symmetric tetragonal or orthorhombic structures. For example, CH3NH3PbBr3 adopts the cubic structure, with t = 0.844 and u = 0.607. CH3NH3PbI3, the most widely studied perovskite, has a little smaller t = 0.834 and u = 0.541, respectively; and thus, the tetragonal structure is preferable at room temperature (McKinnon et al., 2011). (rA = 0.18 nm, rB = 0.119 nm, rI = 0.22 nm, rBr = 0.196 nm). Replacing the MA+ ion of MAPbI3 with the FA+ ion of FAPbI3 perovskite gives rise to a t-value of 0.88, and thus it changes easily to nonperovskite yellow phase (Stoumpos et al., 2013). Estimated t- and μ-factors are calculated for a series of perovskites (Fig. 5.2). This type of ABX3 is termed three-dimensional perovskite (3DP). Besides the intrisinc ionic radius, the temperature has a large influence on whether a perovskite will form. Taking CH3NH3PbI3 as an example, the orthogonal phase forms at low temperatures (T < 165 K) (D’Innocenzo et al., 2014; Onoda-Yamamuro et al., 1992; Wang et al., 2015b), whereas it transforms into the tetragonal and cubic phases at 165 K

Fig. 5.1 Cubic crystal structure of halide perovskite.

Perovskite Solar Cells Processed by Solution Nanotechnology  121 GUA

1.1 FA

Tolerance factor

1.0 Stable Perovskite

MA Cs

0.9

k

0.8 0.7

Rb

Na Li

0.6 0.05

0.10

0.15 0.20 Ionic radius (nm)

APbl3 APbBr3 APbCl3 ASnl3

u

0.54 0.61 0.66 0.50 ASnBr3 0.56 ASnCl3 0.61

0.25

0.30

Fig. 5.2 Calculated t- and μ-factors for a series of halide perovskites.

and 327 K, respectively (Baikie et al., 2013; Poglitsch and Weber, 1987; Stoumpos et al., 2013). For FAPbI3, the black perovskite phase is thermodynamically stable only above 160°C, forming a yellow phase below this phase transition temperature (Jeon et al., 2015). For the all-inorganic lead (Pb)-halide perovskite CsPbI3, it can form a black cubic perovskite phase with an optical band gap of 1.73 eV at 320°C (Eperon et al., 2014a); however, the orthorhombic d phase (Eg = 2.82 eV) is thermodynamically preferred bellow 320°C (Møller, 1958; Sharma et al., 1992; Stoumpos et al., 2013). In addition, the black cubic phase is unstable in ambient conditions at room temperature, returning to the yellow nonperovskite phase in a matter of minutes (Eperon et al., 2015). Hence, slightly adjusting the ionic radius of the A, B, and X sites can improve the intrinsic phase stability of halide perovskite. The A cation plays the key role in determining its structure and dimensionality. For example, FA/MA, FA/Cs, and FA/MA/Cs mixed perovskites have stablized the black-perovskite phase at room temperature (Aharon et al., 2015; Lee et al., 2015; Saliba et al., 2016a,b; Yi et al., 2016). If the small A+ is replaced by a much larger organic primary ammonium cation, the 3DP would change to a two-dimensional perovskite (2DP) in the layered structure as a result of steric effects. The general formula of 2DP is (L)2(S)n−1BX3n+1, where L and S are long- and short-chain ammonium cations, respectively; B is a divalent metal ion; and X is a halide anion. Karunadasa et al. demonstrated the first 2DP (PEA)2(MA)2[Pb3I10] solar cell by incorporating C6H5(CH2)2NH3+(PEA+) into a 3DP MAPbI3 (Fig. 5.3A) with improved stability (Smith et al., 2014). Recently, 2DPs have attracted a good deal of attention due to their moisture stability (Tsai et al., 2016) (Fig. 5.3B). B sites also enable some tuning of interesting properties. For example, Pb/Sn perovskites not only improve the stability of tin (Sn) perovskite, but also narrow the band gap to harvest more sunlight (Hao et al., 2014a). Furthermore, X-site tuning is widely employed to address the stability issue as well. For example, all-inorganic perovskites, such as CsPbI3, CsPbI2Br, and

122  Chapter 5 b

(BA)2(MA)3Pb3l10 Pb

c a

I

C H N (BA)2(MA)3Pb4l13

(A)

(PEA)2(MA)2(Pb3l10)

(B)

Fig. 5.3 (A) Crystal structures of 2DP (PEA)2(MA)2[Pb3I10]; Smith et al. (2014). (B) Crystal structures of 2DP (BA)2(MA)3Pb3I10 (Tsai et al., 2016).

CsPbBr3 recently have been investigated intensively because bromide (Br−) incorporation results in the improved phase stability (Liang et al., 2016; Nam et al., 2017; Swarnkar et al., 2016).

5.1.2  Tunable Band Gap Fig. 5.4 shows the calculated Shockley-Queisser (S-Q) efficiency limit versus the solar cell band gap, together with the highest experimental results (Green and Bremner, 2017). The PV efficiency is ultimately dependent on the semiconductor band gap. Based on the S-Q limit calculation, the two highest efficiency peaks are 1.1 and 1.4 eV, respectively, with peak values of about 34% for single-junction cells. As PV semiconductors, one of the more interesting advantages of perovskite is the tunability of the band gap through compositional engineering of the constituent elements (Fig. 5.5) (Green et al., 2014). The most effective method is halide composition engineering. Theoretical study has revealed that the electronic structure of MAPbX3 is related to the p-orbital of X and the 6p-orbital of Pb, and thus the band gap generally can be tuned by the p-orbital of X (Brivio et al., 2014). Taking MAPbI3 as an example, the valence band maximum (VBM) is mainly constructed from the 5p states of I, partially mixed with the 6s states of Pb, while the conduction band minimum (CBM) originates primarily from the 6p states of Pb, hybridized with a small amount of the 5p states of I (Fig. 5.4B) (Brivio et al., 2014). For MAPbX3 perovskite, the valence orbital of halide changes from 3p to 4p to 5p when X changes from chlorine (Cl) to bromine (Br) to iodine (I). The absorbance of the MAPbX3 (X = I, Br, Cl) single crystals has confirmed that the band-gap increases, and it is found to be 2.97, 2.24, and 1.53 eV for the Cl-, Br-, and I-based perovskite, respectively (Fig. 5.5) (Dong et al., 2015a,b; Liu et al., 2015). The values measured for polycrystalline films are slightly higher—3.1 and 2.3 eV for the Cl and Br perovskites, respectively, and 1.6 for the I-perovskite (Jacobsson et al., 2016; Sadhanala et al., 2015). Seok et al. first

Perovskite Solar Cells Processed by Solution Nanotechnology  123 45 40 30

c-Si

25

CIGS

20 10

c-GalnP Organic

nc-Si CZTS CGS

c-Ge

5

(A)

c-InP Perovskite

mc-Si CdTe CIS CZTSSe Dye

15

0

2 c-GaAs

AM1.5G

Energy (eV)

Efficiency (%)

35

4

AM1.5D 46,200x concentration

SQ limits

0 –2 –4 MAPbl3 Pbl3–

–6

a-Si

MAPbl3

Pbl3–

–8 0.4

0.8

1.2

1.6

Bandgap (eV)

2.0

2.4

(B)

Γ

R

X M

Γ X

l 5p Pb 6p Pb 6s N 2p C 2p H 1s

PDOS

Fig. 5.4 (A) S-Q PCE limits (solid line) under global sunlight (AM 1.5G) versus the band gap, the highest experimental values for various semiconducting materials (data points) and limits for direct AM 1.5G sunlight conversion (Green and Bremner, 2017). (B) The band structure of MAPbI3 and projected density of state (PDOS) distribution (Brivio et al., 2014).

Fig. 5.5 The versatility of hybrid perovskite materials and their tunable band gap via A, B, and X composition engineering.

124  Chapter 5 demonstrated a colorful band gap (1.6–2.2 eV) in the mixed-halide perovskite MAPbI3−xBrx through adjusting the Br/I ratios, as illustrated in Fig. 5.5 (Noh et al., 2013). This result illustrates the tremendous potential of combining diverse band-gap Pb halide perovskites for tandem solar cells. However, several theoretical reports showed that the mixed-(I, Cl) perovskite is difficult to form with high Cl content, whereas mixed-(Br, Cl) and mixed-(I, Br) alloys can be formed at room temperature (Nedelcu et al., 2015). Another tuning method is the change of A+ cation. The formamidinium lead iodide perovskite (FAPbI3) has a narrower band gap 1.48 eV compared to MAPbI3 (about 1.57 eV) due to its strong steric size and the symmetric effect of the amino group (Eperon et al., 2014a,b). The band-gap value is close to the first peak (1.4 eV) of the S-Q limit curve. In contrast, fully Cs-based perovskite films exhibit wider band gaps than the MA-based perovskite films. A third method is B-site tuning. Sn-based perovskite has a promising lower band gap (about 1.3 eV); however, lead-free MASnI3 PSCs have shown much lower PV performance than MAPbI3 due to the oxidation state of tin (Sn4+) (Hao et al., 2014a,b). It is interesting that tin‑lead (SnPb)–alloyed perovskites induce even lower band gaps than pure Sn or Pb perovskite (Fig. 5.6) (Hao et al., 2014a,b). The smallest band gap obtained (1.16 eV) is close to the second peak (1.1 eV) of the S-Q limit of 34% (Hao et al., 2014a,b). The low-band-gap cells using PbSn alloyed perovskites are well suited for use as the rear cell in the tandem cell. The band-gap tunability offers many ideal perovskite candidates for the front cell in tandem solar cell in combination with silicon (Si) cells (Eperon et al., 2017). Tuning the band gap to 1.7 eV, which matches well with Si cells, can be realized by different compositions using

Fig. 5.6 The tunability of the band gap through compositional engineering and abnormal band-gap narrowing of Pb-Sn alloyed perovskite.

Perovskite Solar Cells Processed by Solution Nanotechnology  125 A-site and X-site mixed perovskites, as previously mentioned. In addition, connecting two perovskites in a tandem solar cell with ideal band gaps would break the fundamental S-Q efficiency limit up to 46% for PSC under the standard solar spectrum (Eperon et al., 2017).

5.1.3 Absorption Strong optical absorption is the key parameter for the outstanding performance of the solar cell since the S-Q limit is calculated for a step function in absorption. A large absorption coefficient of these perovskite materials could both reduce the required thickness and address the challenges in collecting photogenerated carriers. This means that a thin photoactive layer within 1 μm achieves low cost and high performance. Fig. 5.7 shows the absorption onset of CH3NH3PbI3 perovskite compared to other solar cell materials (Sun et al., 2014; Wehrenfennig et al., 2014). Absorption measurements demonstrate that perovskites are among the most efficient PV absorbers. The absorption coefficient of MAPbI3 is similar to that of gallium arsenide (GaAs), and more than one order of magnitude larger than that of Si. Thus, the thickness of the present high-efficiency PSCs is in the range of 0.5–1 μm, whereas that of crystalline Si solar cells is usually about 300 μm due to the indirect band gap. Absorption measurements are consistent with calculations showing direct-band-gap properties for perovskites of interest. Two strong, spin-orbit split, excitonic absorption thresholds are apparent as the direct-band-gap III-V semiconductors (Green et al., 2014; Sell and Lawaetz, 1971). The absorption coefficient below the band gap shows a so-called Urbach tail, which comes from various sources of disorder that generate exponentially decaying densities of states below the conduction and above the valence band. Those could be impurities, ionic positional disorders, vibrational

Fig. 5.7 Absorption coefficient of CH3NH3PbI3 and CH3NH3PBI3−xClx, compared to other solar cell materials (Sun et al., 2014; Wehrenfennig et al., 2014).

126  Chapter 5 fluctuations of atoms, and other issues (Huang et al., 2017). Their common feature is that they locally perturb the periodic crystal potential, giving rise to fluctuations of the potential that electrons close to the band edges see. Besides efficient light harvesting, a large absorption coefficient also enables a large opencircuit voltage (VOC). Because a very thin active layer is able to extinct the solar light fully, it can reduce the charge recombination-induced, saturated dark current. This is described by the S-Q model of the relationship between the film thickness and VOC (Jensen et al., 2002): VOC =

kT  J SC  kT  J SC N Dτ eff  ln  ln   = q  J 0  q  qni 2 d 

where k is the Boltzmann constant, T is the temperature, q is elemental charge, JSC is shortcircuit current density, J0 is initial current density, ND is the doping concentration, τeff is the effective carrier recombination lifetime, ni is the intrinsic carrier concentration, and d is the thickness of the light absorber.

5.1.4  Carrier Behaviors 5.1.4.1  Charge generation and transport The exciton binding energy (EB) is the energy required to dissociate excitons into free charge carriers before they are recombined or collected. At the low-coupling extreme, the electron and hole will be independent, and relative concentrations will be determined by the spectral composition of illumination. For PV applications, EB, which determines the fraction of carriers present under typical optoelectronic device conditions, must be small enough to minimize the energy loss. Many inorganic semiconductors, such as Si and GaAs, exhibit easy charge separation owing to their small binding energies [Si: about 15 meV (Shaklee and Nahory, 1970); GaAs: about 4 mV (Fehrenbach et al., 1985)], while a donor-acceptor (DA) heterojunction organic PVs (OPV) need a large energy offset (>0.3 eV) to provide the internal electrochemical driving force for exciton dissociation (Giebink et al., 2011). For PSCs, the first photophysical question is whether the primary photoexcited carriers are bound excitons or free carriers. Thus, the exciton EB in the tetragonal phase MAPbI3 has been intensely studied and determined to the values varying from just 2 to 75 meV (Fig. 5.8) (D’Innocenzo et al., 2014; Hirasawa et al., 1994; Lin et al., 2015; Saba et al., 2014). Considering the effective dielectric constant much larger than 6.5, the exciton binding energies are possibly as low as about 1–10 meV. Nevertheless, even for the upper bound of about 75 meV, these results suggest that free charges are generated spontaneously following light absorption, as opposed to a large population of bound excitons. Although different exciton binding energies have been obtained using different techniques, a consensus has been reached that the direct generation of free charges dominates the conversion process from photons to photocurrent in PSCs (Huang et al., 2017).

Perovskite Solar Cells Processed by Solution Nanotechnology  127

EB =

m

1

m0 e

2

2(4πe 0)

EB = 35 meV EB = 55 meV

0.8

m0q4

101 100

1.0

2

¬ (nFC /n)

Static ε′

ε′

102

e′ e″

Ionic conductivity 1/f

103

0.6 0.4

EB = 75 meV PV regime

0.2

Cut-off frequency 1/2π

0.0

(A)

100 101 102 103 104

105

4 × 1014

Frequency (Hz)

6 × 1014

(B)

1013 1014 1015 1016 1017 1018 1019 1020 1021 n (cm–3)

Fig. 5.8 Exciton or free carriers under illumination in perovskite. (A) Real (ε′) and imaginary (ε″) parts of the dielectric constants of MAPbI3. (B) Simulation of the free charge fraction over the total excitation density (x = nFC/n) at thermal equilibrium, where nFC and n are the density of free charges and total excitation, respectively (D’Innocenzo et al., 2014).

For devices with a continuous perovskite layer, transport across the perovskite is important for device operation. The key expectation for high efficiency is that the electron and hole diffusion lengths (LD) are much longer than the film thickness required to obtain complete solar light absorption. The free electrons and holes both have high mobilities in the range about 10–30 cm2/V/s with a decay time of tens of microseconds, and the diffusion length has been observed to be over 1 μm in polycrystalline films and 100 μm in single crystals (Dong et al., 2015a,b; Shi et al., 2015). The dimensionality of the crystal structure also influences exciton binding energies. For instance, 2DPs have much greater exciton binding energies owing to spatial quantum confinement. Exciton binding energies of 380 and 270 meV have been reported in (BA)2PbI4 and (BA)2(MA) Pb2I7 exfoliated single crystals, respectively. (BA)2(MA)n–1PbnI3n+1 with the exciton binding quickly approaches to that of the 3DP, with increasing n (Blancon et al., 2017; Yaffe et al., 2015). Although 2DPs have achieved high efficiency (PCE>12%) with good stability, more in-depth investigations of the charge-carrier dynamics in 2DPs, especially for low values of n, are urgently needed to enable further optoelectronic applications (Blancon et al., 2017). 5.1.4.2  Charge Recombination Recombination of electrons and holes is a process wherein the electrons fall into the empty state that is associated with the hole. In real solar cells, charge transfer (CT) and charge transport rates compete with recombination rates, demanding low recombination rates that allow for efficient charge-carrier extraction. When the cell is working, bulk, surface, and interface defects would introduce recombination centers that lead to fast nonradiative losses. The charge recombination in a semiconductor material can be described by the following equation (Johnston and Herz, 2015):

128  Chapter 5 dn = G − k1n − k2 n 2 − k3 n3 dt where G is the charge generation rate, k1 is the monomolecular charge recombination rate constant, k2 is the bimolecular charge recombination rate constant, and k3 is the Auger charge recombination rate. Auger recombination is a process in which an electron and a hole recombine in a band-to-band transition; however, the resulting energy is given off to another electron or hole. This recombination is generally weak compared with other recombination channels at normal solar cell operating conditions. The very weak bimolecular charge recombination makes monomolecular charge recombination the dominant recombination process in most polycrystalline perovskite films under low excitation intensity. An additional loss mechanism is surface recombination, where charges are lost at the interface, such as between the perovskite/transport layer or the transport layer/electrode. The role of grain boundaries in recombination in the PSC is still under debate. In addition, it was found that impurities in the precursor material could be a further source of defects. 5.1.4.3  Hot-carrier behavior The Beard group showed that the sharp optical absorption onset is due to an exciton transition that is inhomogeneously broadened with a binding energy of 9 meV (Yang et al., 2016b). They fully characterized the transient absorption spectrum by free-carrier-induced bleaching of the exciton transition, quasi-Fermi energy, carrier temperature, and bandgap renormalization constant. The photoinduced carrier temperature is extracted from the transient absorption spectra and monitored as a function of delay time for various excitation wavelengths and photon fluences. They find an efficient hot-phonon bottleneck that slows down the cooling of hot carriers by three to four orders of magnitude in time (Yang et al., 2016a,b). By comparing three single-crystal lead Br− perovskites: MAPbBr3, FAPbBr3, and CsPbBr3, the Zhu group observed hot fluorescence emissions from energetic carriers with about 102-ps lifetimes in MAPbBr3 or FAPbBr3 (Zhu et al., 2016b). Hot fluorescence is correlated with liquidlike molecular reorientational motions, suggesting that dynamic screening protects energetic carriers via solvation on time scales competitive with that of ultrafast cooling. Similar protections likely exist for band-edge carriers. Huang et al. reported direct visualization of hot-carrier migration in MAPbI3 thin films by ultrafast transient absorption microscopy, demonstrating three distinct transport regimes. Quasi-ballistic transport was observed to correlate with excess kinetic energy, resulting in up to 230 nm of transport distance that could overcome grain boundaries (Guo et al., 2017). The nonequilibrium transport persisted over tens of picoseconds and about 600 nm before reaching the diffusive transport limit. The long-lived energetic carriers and long-range hot carriers may enable hot-carrier solar cells with efficiencies exceeding the S-Q limit.

Perovskite Solar Cells Processed by Solution Nanotechnology  129 5.1.4.4  The Rashba effect on direct-indirect band gaps Although substantial progress has been made in modeling the dynamics of photoexcited charge carriers in perovskite, we are yet to fully understand what drives the second-order electron-hole recombination in this material. Currently, the conventional idea is that MAPbI3 behaves as a direct-band-gap semiconductor, where the absorption and emission of photons occur via allowed transitions. This is fundamentally different from indirect bandgap semiconductors such as Si, in which both absorption and recombination involve not only photons, but also phonons. This results in lower absorption coefficients than direct semiconductors, but at the same time, recombination is much slower. The presence of a Rashba band-splitting mechanism mediated by spin-orbit coupling and breaking of inversion symmetry has been suggested as a possible cause for the reduced recombination rates observed in organohalide perovskites. Recent theoretical calculations of the band structure of MAPbI3 suggest that its CBM is slightly shifted in k-space with respect to the VBM, making the fundamental band-gap indirect (Motta et al., 2015). The direct-indirect behavior can interpret the varying absorption band edges and photoluminescence (PL) between the thin film and single crystal. Namely, the indirect band gap locates at the tail of the direct band-gap edge. Due to the much lower absorption coefficient for indirect semiconductors, indirect behavior is difficult to observe in the ultraviolet-visible spectroscopy (UV-Vis) absorption in ultrathin films. 5.1.4.5 Electroluminescence In a real-life device, defects always introduce recombination centers that lead to fast, nonradiative losses. An efficient solar cell device should also be a good light emitter according to the S-Q formulation. The EQEEL has already been introduced as an important insight on the VOC of the device according to the following formula (Miller et al., 2012; Tress, 2017): VOC =

 J kT k BT  −1 = VOC ,rad − ∆VOC ,non −rad ln  EQEEL SC + 1  ≈ VOC ,rad − B ln EQEEL   e e e Φ , 0 em  

where kB is the Boltzmann constant, T is temperature, q is the elementary charge, EQEEL is the external quantum efficiency (EQE) of electroluminescence (EL) from the cell. The ideal value of VOC, is achieved only if the EQE for light emission by the solar cell measured at VOC is 100%. This S-Q limit is 1.33 V for MAPbI3 with a band gap of 1.6 eV (Correa-Baena et al., 2017a). To date, the highest VOC of 1.24 V at a band gap of 1.63 eV has been achieved, leading to loss-in-potential of 0.39 V, versus 0.4 V for commercial Si cells (Fig. 5.9) (Saliba et al., 2016b).

130  Chapter 5 1.6 1.4

V=E g

1.2

oret

The

Voc (V)

1.0

0.4 0.2

MAPbI3

GaAs InP

Si

0.8 0.6

RbCsMAFAPbI3 V oc ical

CdTe

CulnGaSe

CulnS

CulnSe CuZnSnSSe CuZnSnSe

ZnP2

0.0 1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

Eg (eV)

Fig. 5.9 The maximum VOC of a solar cell is set by the S-Q limit and the highest experimental values for various materials.

5.1.5  Defect Tolerance In commercialized semiconductors, such as Si, the understanding and control of defects have been cornerstones of their successful application. To date, it is generally thought that perovskites have a relatively high tolerance for defect-related losses because the PCEs of these solar cells are increased rapidly (Green et al., 2014). However, the understanding of the charge traps in PSCs is still at an early stage. The highest PCEs are still far below the thermodynamic limit of about 30%–33% for band gaps in the range 1.2–1.6 eV. Crystal defects in conventional semiconductors have been studied extensively, which include point defects, such as atomic vacancies, interstitials, antisite substitutions, and higher-dimensional defects, such as dislocations and grain boundaries (Fig. 5.10). 5.1.5.1  Point defects The most widely studied defects in perovskite-halides are the native point defects in CH3NH3PbI3 due to its single phase. Researchers have reported that CH3NH3PbI3 has native point defects: the vacancies VMA, VPb, and VI; the interstitials MAi, Pbi, and Ii; and the antisite occupations MAPb, MAI, PbMA, PbI, IMA, and IPb (Buin et al., 2014). The transition energies for all possible intrinsic point defects in CH3NH3PbI3 have been calculated, as shown in Fig. 5.11 (Buin et al., 2014). The results show that all the vacancy defects and most interstitial defects exhibit very shallow transition energy levels. The formation energy of the point defects as a function of the Fermi-level position is shown in Fig. 5.12 (Buin et al., 2014). It is seen that the defects that have low formation energy, such

Perovskite Solar Cells Processed by Solution Nanotechnology  131

Fig. 5.10 Types of defects in perovskite crystal.

as VPb, Ii, VI, and Pbi, have transition energy levels <0.05 eV above (below) the VBM (CBM) of CH3NH3PbI3. On the other hand, all the defects that create deep levels, such as IPb and PbI, have a high formation energy. This might suggest that point defects should not greatly reduce the VOC of these solar cells due to the relatively low density of nonradiative recombination centers, whose energies usually lie deep within the band gap. Some studies directly exhibited the defect-tolerant electronic properties in the perovskite. For example, the mobility-lifetime products of a superior MAPbBr3 single crystal were only slightly improved over that of a MAPbBr3 single crystal with relatively high defect density (Wei et al., 2016). The energy levels of MAPbI3 were observed to be unchanged even after the loss of about 20% of the iodine from the films (Motta et al., 2015). It is surprising to note that although most perovskite film and single crystals were fabricated under low-temperature conditions, the trap density is much lower than that of other polycrystalline inorganic materials. The bulk trap density in polycrystalline perovskite thin-film-based devices is in the range of 1015–1017 cm−3, and the perovskite single crystals even have an extremely low

132  Chapter 5

Fig. 5.11 Energy levels associated with the defect states corresponding to vacancies (VPb, VI, and VMA), interstitials (Pbi, Ii, and MAi), and neutral and antisites (PbI and IPb) (Buin et al., 2014).

Fig. 5.12 Formation energies and volume densities of key defects in tetragonal MAPbI3. Defect formation energies for (A) iodine-poor and (B) iodine-rich growth conditions (Buin et al., 2014).

trap density of about 1010 cm−3, which is comparable to the best value reported in intrinsic crystalline Si (Dong et al., 2015a,b). The shallow point defects could cause unintentional doping at room temperature. The acceptor defects with shallow-level energies are VPb, VMA, MAPb, and Ii, and the donors are MAi, VI, and MAI. Both shallow donors and acceptors can form with low formation energies, allowing CH3NH3PbI3 to be intrinsically doped from p-type to n-type when carefully controlling the growth conditions.

Perovskite Solar Cells Processed by Solution Nanotechnology  133 5.1.5.2  Grain boundary charge traps Although there is a common consensus that the grain boundary in polycrystalline Si has a negative effect on efficiency, the question of whether grain boundaries in perovskite are benign remains under debate. In the early stages of PSC development, a relative low VOC loss of 0.4–0.5 V could be achieved using small grains as an active layer, which indicates that grain boundaries might not be that defective. However, it was also found that VOC improved after passivating the grain boundaries or increasing the grain size. Another example shows that the grain boundaries were dimmer and exhibited faster nonradiative decay, whereas chemical treatment with pyridine could activate previously dark grains. Theoretical studies have shown that “ideal” grain boundaries—which are formed by connecting two clean surfaces—are electronically benign with a negligible number of deep charge traps. However, it is worth noting that grain boundaries in a real PSC are highly likely to be nonideal. Similar to surfaces, excess intrinsic and extrinsic impurities are likely to segregate at grain boundaries, affecting the electronic properties of the thin films. Bulk defects in devices can be effectively reduced through morphological control, defect management, and even forming single crystals as an absorbed layer. However, the surface charge traps at the perovskite surface seem to form inevitably and are much more complicated because the periodic array of the lattice is broken at the crystal terminal. With the development of film preparation technology, the detect state might no longer be limited by the quality of the bulk perovskite, but rather by its surface. Charge recombination at the surface of perovskite materials has proved to be more complicated than initially thought. Even the surfaces of OIHP single crystals can be rich in defects, as demonstrated by narrowband photodetectors made of single crystals (Fig. 5.13) (Huang et al., 2017). In another example, each facet of the perovskite surfaces is different in terms of defect density and atomic force microscopy (AFM) measurement has revealed a VOC variation of 0.6 V between different facets on the same grain. Tailoring the chemical composition of perovskite films can be an effective strategy to passivate surface defects, which has been successfully demonstrated via small molecules, such as pyridine, thiophene, tri-n-octylphosphine oxide, fullerene, and even gas (O2) (Vorpahl et al., 2015).

5.1.6  Ion Motion Behavior Unlike other PV materials, hybrid perovskite materials show very strong ionic migration, which has been known for >30 years (Dong et al., 2015a,b). To date, this property has received broad attention due to J-V hysteresis under both forward-to-reverse and reverseto-forward scanning directions. Both theoretical calculations and experimental results show that the ions in perovskite materials have low activation barriers and modest ionic diffusion coefficients to move within perovskite devices, especially when subjected to external bias or under light illumination. This characteristic of hybrid perovskites has triggered many exciting optoelectronic/electronic applications, such as switchable PV and perovskite memristors.

134  Chapter 5 Recombination

Au + + + +

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80

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Fig. 5.13 Schematic illustration showing that the charge recombination and collection under above-band-gap excitation is severely affected by surface traps, whereas under below-band-gap excitation, it is less susceptible to surface-defect-induced recombination, resulting in significantly different PL lifetimes (Huang et al., 2017).

Simulation work demonstrated that Schottky defect formation energy is low (0.14 eV per defect), as shown in the following reaction in the Kroger-Vink notation for the full Schottky defect (Walsh et al., 2015): / nil → VMA + VPb/ / + 3VI• + MAPbI 3

where nil represents the perfect CH3NH3PbI3 lattice, V indicates a vacancy, the subscripts show the ionic species, and superscripts show the effective defect charge. A significant equilibrium concentration of I−, Pb2+, and CH3NH3+ vacancies can be formed at room temperature, which could support vacancy-mediated diffusion. The anomalous current density-voltage (J-V) hysteresis was initially noticed in the PSCs and subsequently demonstrated by other phenomena, such as solar cell performance modulation by precondition biasing and light soaking, slow photoconductivity response,

Perovskite Solar Cells Processed by Solution Nanotechnology  135 and halide redistribution and segregation. Several recent studies indicated that ionic movement in perovskites will lead to fast (seconds to minutes) and slow (minutes to hours) performance degradation, showing hysteresis and reversible losses, respectively (Bag et al., 2015; Domanski et al., 2017; Tress et al., 2015). A common observation about devices after illumination is that they can partially or completely recover their initial efficiency after dark storage for a few hours (Bag et al., 2015). The self-healing effect is also most closely related to the role of ion diffusion. A few studies also reported the beneficial role of ion migration on device efficiency and stability. For example, ion migration and associated ion accumulation could cause a chemical doping effect, leading to an ohmic contact for high-efficiency solar cells with a p–i–n or n–i–p structure (Xiao et al., 2015). The device performance shows an enhancement due to the accumulation of ions under operation, which could provide an additional electric field to improve charge extraction. Some studies also proposed that ion migration could decrease the defect pairs in perovskites under light illumination and thus reduce the nonradiative recombination in PSCs. Regardless, it is important to establish how organic and inorganic contacts affect the accumulation of ions at interfaces in the long term because mild reversible losses might be an obstacle for long-term stability. To date, there are several methods to reduce the anion migration. For example, it is known that the phenyl-C₆₁-butyric acid methyl ester (PCBM) is able to suppress the hysteresis of PSCs because the mobile ions in the perovskite could interact with PCBM to form a PCBM halide radical and then reduce electric field–induced anion migration (Xu et al., 2015). Almost simultaneously, several research groups demonstrated that ion migration could be completely suppressed through the incorporation of extrinsic alkali cations (Rb+, K+, Na+, and Li+) in perovskite materials (Fig. 5.14). However, how K+ suppresses the ion migration

Fig. 5.14 Schematic of the passivated mechanism for ion migration. (A) The surplus halide is immobilized through complexing with K into benign compounds at the grain boundaries and surfaces. (B) The K ion is able to prevent I− migration because K+ energetically prefers the interstitial site. (C) The K ion was incorporated into the lattice of perovskite through substituting MA+ or FA+.

136  Chapter 5 remains a question. The focus of the controversy is whether potassium (K) was incorporated into the lattice of perovskite (Abdi-Jalebi et al., 2018; Son et al., 2018; Tang et al., 2017).

5.2  Development of Photovoltaic Device The solid-state PSC was realized by using mesoporous metal oxide (TiO2 or Al2O3) and spiroMeOTAD as an electron-transporting layer (ETL) and hole-transporting layer (HTL), respectively, and it exhibited highly improved stability by testifying in ambient conditions without encapsulation >500 h (Kim et al., 2012; Lee et al., 2012). Then PSC became a worldwide research of interest in a few years. Owing to the outstanding intrinsic photoelectric properties of metal halide perovskites, the initial PCE (around 10% in 2012) was swiftly increased to a certified 22.1% by Seok’s group in 2017. The high efficiency was achieved by optimizing the thin-film fabrication method and composition engineering (Yang et al., 2017b). It is still challenging to build reliable and reproducible methods to produce highly efficient and stable devices that could compete with conventional Si cells for the huge PV market. Very recently, an improved PCE of 23.25% was newly announced by the Gratzel group.

5.2.1  Architecture Development The conventional configuration of PSCs was to form DSSCs and fabricated with mesoporous architecture where the perovskite nanocrystals were utilized as dye sensitizers. The working mechanism was similar to that of the dyes in DSSCs and solid-state HTL was used to replace the liquid electrolyte (Im et al., 2011). Later, it was reported that the metal halide perovskite could form a dense photoactive layer and still work without the mesoporous layer, although it delivered much lower PCE (Lee et al., 2012). Researchers also revealed that the CH3NH3PbI3 thin film exhibited balanced and long-range charge-carrier diffusion lengths of 100 nm (Xing et al., 2013) and even reached 1 μm in CH3NH3PbI3−xClx thin film (Stranks et al., 2013). This finding ensured effective charge transporting in metal halide perovskite thin film and led to the subsequent development of new device configurations for PSCs (Etgar et al., 2012). Nowadays, two device architectures have dominated in PSCs: the mesostructured cell and the planar heterojunction cell (including n-i-p and p-i-n configuration) (Fig. 5.15). For a mesoscopic device architecture, a transparent conducting glass [fluorine-doped tin oxide (FTO)] was used as the substrate where a very thin and compact titanium dioxide (TiO2) layer (ca. 20–50 nm) was deposited and worked as a hole-blocking layer, followed by a metal oxide mesoporous layer infiltrated with perovskite material (ca. 100–300 nm) and finally capped by an organic or inorganic HTL and a gold or silver metal electrode (Fig. 5.16). The mesoporous metal oxide scaffold was utilized to assist in the formation of a homogeneous film on a large area and collect the generated electron charges in the bulk layer. The n-type TiO2 or nonelectron accepting, insulating metal oxide and aluminum oxide (Al2O3) were commonly used as a mesostructured scaffold. For the state-of-the-art, most efficient PSCs,

Perovskite Solar Cells Processed by Solution Nanotechnology  137

Fig. 5.15 Schematic diagram of PSCs based on mesoporous layer (A), planar regular n-i-p (B), and planar inverted p-i-n (C) configurations.

Au HTM TiO2/CH3NH3Pbl3

FTO 200 nm Glass

Current density (mA cm-2)

(A) 20 15 10 5

96.4 mW cm-2 20.0 mA cm-2

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Fig. 5.16 Cross-section SEM image (A) and corresponding J-V curves (B) of the mesostructured PSCs (Burschka et al., 2013).

138  Chapter 5 the mesostructured scaffold was always topped by an additional perovskite layer of around 100–300 nm as a capping layer and the compact perovskite layer was optimized to 500 nm to act as a light absorber (Correa-Baena et al., 2016; Jeon et al., 2014). However, the mesostructured scaffold typically needed a high-temperature processing step that limited its application on the flexible plastic substrate with low melting points. Generally, the planar heterojunction structure is classified into two categories—that is, regular (n-i-p) and inverted (p-i-n) structures—that are determined by which charge selective layer is utilized on the bottom. For regular planar PSCs, the intrinsic perovskite thin film sandwiched between the compact n-type ETL and the solution-processed p-type HTL, which is similar to that of the mesostructured device, except that the metal oxide scaffold is totally eliminated. Lee et al., 2012 made the first attempt to fabricate the planar heterojunction solar cell on compact TiO2 by using MAPbI3−xClx, and a very low PCE of 1.8% was achieved due to the poor surface coverage. Later, Liu et al. (2013) developed a vapor-deposited method to improve the quality of the MAPbI3−xClx perovskite film by coevaporation of PbCl2 and MAI, and the champion solar cell yielded a PCE of 15.4% (Fig. 5.17). Compared with traditional TiO2, SnO2 has a deeper conduction band and higher electron mobility, which can enhance charge-carrier transport and reduce charge accumulation at the interface (Bob et al., 2013; Snaith and Ducati, 2010). By introducing low-temperature, solution-processed SnO2 nanoparticles as an ETL, Jiang et al. fabricated n-i-p planar heterojunction solar cells with a PbI2 passivation phase in the perovskite layer and exhibited a high certified efficiency of 19.9% ± 0.6% (Jiang et al., 2016).

Fig. 5.17 (A) Cross-section SEM image and (B) top-view SEM image of vapor-deposited planar perovskite thin film. (C) J-V curves of the planar heterojunction PSCs (Liu et al., 2013).

Perovskite Solar Cells Processed by Solution Nanotechnology  139

Fig. 5.18 Device configuration (A) and energy-level diagram (B) of the planar p-i-n heterojunction PSCs (Jeng et al., 2013).

Another planar architecture of PSCs is the inverted heterojunction configuration, in which the perovskite film is deposited on an HTL, such as poly(3,4-ethylenedioxythiophene):poly(styr ene sulfonate) (PEDOT:PSS), nickel oxide (NiO), molybdenum trioxide (MoO3), and copper thiocyanate (CuSCN). Holes are collected through the bottom FTO or ITO electrode. Jeng et al. (2013) first successfully demonstrated the planar p-i-n PSCs with configuration of ITO/ PEDOT:PSS/MAPbI3/C60/bathocuproine (BCP)/Al, where MAPbI3/C60 was considered as a planar heterojunction (Jeng et al., 2013). The resulted devices displayed a PCE of 1.6% and improved to 3.9% when PC61BM was used to replace C60 (Fig. 5.18). Recently, the efficiency of the planar structure was pushed over 19% through interface engineering (Zhou et al., 2014).

5.2.2  Perovskite Material Structure diversity is one of the most intriguing properties of metal halide perovskite (ABX3), which can incorporate miscellaneous anions or cations, such as monovalent cations A (MA+, FA+, and Cs+), divalent cations M (Sn2+), and halide ions X [bromide (Br–) and chloride (Cl−)], to construct cubic perovskite structures with a tolerance factor (t) between 0.8 and 1.0. The chemical and physical properties of metal halide perovskite can be finely tuned by composition engineering, using elements such as crystal structure parameters, band gaps, carrier behaviors, defect properties, phase stabilities, and moisture and/or thermal resistances. 5.2.2.1  Pure perovskite solar cells In the first few years of developing PSCs, MAPbI3 was exclusively applied as a light absorber, where MA served as a monovalent A cation alone. MAPbI3 thin films exhibited a high absorption coefficient in the UV-Vis-NIR region with an absorption edge at 800 nm,

140  Chapter 5 corresponding with a band gap of 1.55 eV. MAPbI3-based solar cells show a high PCE, approaching 20%. However, the intrinsic unstable properties of MAPbI3-based solar cells impede its commercial application. Most important, MAPbI3 shows poor stability in ambient environments, including moisture stability, thermal stability, and electric field stability, which may be ascribed to significant ion migration (Koh et al., 2016; Leijtens et al., 2015; Niu et al., 2015). It is noteworthy that thermal annealing can induce MAPbI3 degradation at temperatures above 85°C, which has been confirmed by accelerating stress tests according to ISOS protocols (Conings et al., 2015; Roesch et al., 2015). FAPbI3, where the formamidinium ion (FA+) with an ionic size of ≈2.2 Å was used as a monovalent cation, shows a smaller absorption band gap of 1.48 eV than that of MAPbI3 (Zhou et al., 2016). The broader light-absorption region of FAPbI3 can enhance the nearinfrared (NIR) light harvesting in solar cells. It is to be expected that FAPbI3 exhibits better thermal stability than does MAPbI3. However, FAPbI3-based solar cells shows inferior device performance because the black perovskite phase (α-FAPbI3) is stable only at high temperatures (>150°C), while it can easily transform to a yellow nonperovskite phase (δ-FAPbI3) at device operating conditions (Lee et al., 2014). Similar to FAPbI3, the monovalent cation CsPbI3 exhibits a stable perovskite phase only at temperatures higher than 300°C, which recovers to a photoinactive orthorhombic δ-phase at room temperature and is not suitable for solar cell fabrication (Møller, 1958). Swarnkar et al. synthesized CsPbI3 quantum dots by using methyl acetate as an antisolvent and found that it can be stabilized at low temperatures for months, which may be ascribed to the irremovable surface species with reduced surface energy. The solar cells based on the CsPbI3 quantum dot films delivered a PCE of up to 10.77%, with a large VOC of 1.23 eV (Swarnkar et al., 2016). 5.2.2.2  X-site mixed perovskite solar cells The X site, which is usually halide anion, such as Cl−, Br−, and I−, can finely applied to tune the optoelectronic properties of perovskite materials for better PV performance. Lee et al. first applied the Cl− mixed perovskite of MAPbI3−xClx into a mesostructured device, where inert Al2O3 was used as a mesoporous scaffold and delivered a PCE of 10.9%, with a high VOC of 1.13 V (Lee et al., 2012). In this device, MAPbI3−xClx acted as a light absorber and electron transporter. Interestingly, the Cl− mixed perovskite of MAPbI3−xClx shows negligible band-gap tunability to the pristine MAPbI3 (Conings et al., 2015; Edri et al., 2014; Zhao and Zhu, 2014). X-ray photoelectron spectroscopy (XPS) analysis indicated that the ratio of Cl/(Cl + I) was determined to be 2.2% (You et al., 2014). However, the small amount of Cl− have a great effect on the optoelectronic properties of perovskite. The introduction of Cl− in the precursor induces grain orientation and promotes the rapid formation of a MAPbCl3 template acted as a seed and resulted in larger crystal size (Deepa et al., 2016; Dong et al., 2015a,b; Grancini et al., 2014; Unger et al., 2014). Moreover, MAPbI3−xClx

Perovskite Solar Cells Processed by Solution Nanotechnology  141 100

x=0 x=0.06 x=0.13 x=0.20 x=0.29 x=0.58 x=1.0

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Fig. 5.19 (A) EQE response spectrum of MAPbI3−xBrx perovskite thin films. (B) Device stability of the MAPbI3−xBrx-based PSCs (Noh et al., 2013).

exhibited an almost one order of magnitude higher charge-carrier diffusion length than that of MAPbI3 (Stranks et al., 2013). Unlike Cl− mixed MAPbI3−xClx, the introduction of Br− can easily tune the absorption edge of the resulting thin film and usually is applied to construct large band-gap PSCs (Herz, 2016; Rehman et al., 2015). The absorption edge of MAPbI3−xBrx perovskite thin films exhibit a hypochromatic shift by increasing Br content and the band gap can be tuned from 1.58 eV (786 nm) to 2.28 eV (544 nm) (McMeekin et al., 2016). In addition, the incorporation of Br− in the precursor can enhance thin-film quality and device stability in humid environments (Fig. 5.19) (Noh et al., 2013).

142  Chapter 5 5.2.2.3  A-site mixed perovskite solar cells To solve the instability of unitary A-site cation perovskite materials and improve optoelectronic properties, mixed perovskite materials have been demonstrated. Pellet et al., 2014 first demonstrated that the black perovskite phase of FAPbI3 (α-FAPbI3) can be efficiently stabilized by the addition of 20% MA cation into FA solution, where the two cations are both located in the same crystal lattice (Fig. 5.20). They prepared the mixed-cation MAxFA1−xPbI3 thin film by a two-step deposition method and found that MA0.6FA0.4PbI3 exhibited a broad light-harvesting region, and the same is true of FAPbI3, and superior chargecarrier collection efficiency. Devices based on the optimized composition MA0.4FA0.6PbI3 exhibited a PCE of 14.9% under AM 1.5G simulated solar illumination. Bein et al. (Binek et al., 2015) also demonstrated that the desired α-FAPbI3 phase can be stabilized by FAI MAI:FAI 1:4 MAI:FAI 2:3 MAI:FAI 3:2 MAI:FAI 4:1 MAI

Light harvesting (%)

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Fig. 5.20 XRD patterns (A) and light absorption spectrum (B) of perovskite thin films with different MA/FA ratios. (C) J-V curves of PSCs based on MAxFA1−xPbI3 (x = 0, 0.6, and 1) (Pellet et al., 2014).

Perovskite Solar Cells Processed by Solution Nanotechnology  143 inserting a small amount of MA (<20%) content without any significant lattice shrinkage and shows phase stability in the temperature range of 25°C–250°C. The stability of the αFAPbI3 phase may be ascribed to the stronger interactions between the MA cation and the inorganic [PbI6] cage. Meanwhile, the incorporation of a MA cation into FAPbI3 exhibited a positive effect on the lifetime of the photoexcited charge carrier and resulted in higher PV performance. Choi et al., 2014 incorporated cesium cation (Cs+) into MAPbI3 and demonstrated that a stable mixed CsxMA1−xPbI3 phase can be formed at room temperature. However, CsxMA1−xPbI3-based solar cells exhibited a low PCE of 7.68% and poor stability in ambient conditions with an inverted planar device configuration, with PEDOT:PSS used as a HTL and Al used as an electrode. Then, Lee et al. (2015) fabricated high-efficient planar heterojunction solar cells by employing FA0.9Cs0.1PbI3 as a light absorber and exhibited an average PCE of 16.5%. The incorporation of a cesium cation into a FAPbI3 structure can efficiently reduce charge-carrier recombination, which resulted in the increase of open-circuit voltage (VOC) and fill factor (FF). More interestingly, FA0.9Cs0.1PbI3 exhibited enhanced photostability and moisture stability, which is ascribed to the enhanced interaction between FA cations and iodide anions in the contracted cubo-octahedral structure (Fig. 5.21).

Fig. 5.21 (A) XRD profiles of the FA1−xCsxPbI3 thin films. (B) Light-absorption spectrum of FAPbI3 and FA1−xCsxPbI3 thin films. (C) The device parameters of FAPbI3 (red)- and FA0.9Cs0.1PbI3 (blue)-based PSCs under continuous white light (sulfur lamp, ≈100 mW/cm2) illumination (Lee et al., 2015).

144  Chapter 5 Except for the double-cation mixed perovskite materials, Saliba et al. first applied triplecation (Cs/FA/MA) (Saliba et al., 2016a,b) and quadruple-cation (Rb/Cs/FA/MA) (Saliba et al., 2016a,b) perovskite materials as a light-harvesting layer and further improved the PV performance and device stability. The incorporation of cesium cations into MA/FA mixtures can effectively reduce the production of the photoinactive yellow phase and induce high-quality perovskite thin film. The resulting perovskite thin film exhibited good thermal stability and nonsensitivity to device fabrication conditions, which enabled reproducibility to increase. The triple-cation PSCs exhibited a high PCE of 21.1% and about 18% under operating conditions after 250 h (Fig. 5.22). Then, they further introduced rubidium cation (Rb+) into the Cs/FA/MA mixtures by considering the slightly smaller ionic size of Rb+ than that of Cs+. They also studied the role of Rb+ in the quadruple-cation perovskite materials. The introduction of Rb+ into the mixed-cation PVSK effectively eliminates the appearance of PbI2 and the yellow-phase impurities as well. Normalized absorbance, PL (a.u.)

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Fig. 5.22 (A) XRD pattern of perovskite with different amount of Cs. (B) The corresponding absorption and PL spectra. (C) J-V curves of perovskite devices based on CsMAFA-based film; the inset shows the power output under MPP tracking for 60 s. (D) Device stability test of CsMAFA-based PSCs under continuous illumination and MPP tracking in a nitrogen atmosphere (Saliba et al., 2016a,b).

Perovskite Solar Cells Processed by Solution Nanotechnology  145 The addition of inorganic cations facilitated the formation of the perovskite phase during the crystallization of thin films, which was confirmed by the study of nonannealed films. The Rb/Cs/FA/MA-based PSCs displayed a stabilized PCE of up to 21.6% for a small-area device with a high VOC of 1.24 V at a band of 1.63 eV, which delivered a low-energy loss of 0.39 eV, versus 0.4 eV for Si solar cells. Moreover, by replacing spiroOMeTAD with a thin layer of polytriarylamine (PTAA), the Rb/Cs/FA/MA-based PSCs could maintain 95% of their initial PCE at 85°C for 500 h under continuous illumination and maximum power point (MPP) tracking in an inert atmosphere. 5.2.2.4  B-site mixed perovskite solar cells Owing to the ionic size of Sn2+ is similar to that of Pb2+ and low toxicity, Sn is an excellent candidate to substitute or partially replace Pb-based perovskites. The introduction of Sn2+ into MAPbI3 can efficiently tune the band gap of the resulting B-mixed perovskite (Koh et al., 2015; Lee et al., 2016b; Liao et al., 2016). Hao et al. (2014a,b) synthesized MASnxPn1−xI3 with different Sn/Pb ratios, and anomalous band-gap behaviors were observed in the solid solution of MAPbI3 and MASnI3. The band gap of MASnxPn1−xI3 did not follow a linear trend (Vegard’s law) between these two extremes of 1.55 and 1.35 eV, respectively, and the MASn0.5Pn0.5I3 (x = 0.5) perovskite exhibited the lowest band gap (1.17 eV) and the light absorption extended to the near-infrared (about 1050 nm). The band-gap difference between theory and experiment may be ascribed to the real crystal-structure variation of the mixed perovskites because MAPbI3 has a tetragonal crystal structure at room temperature, while MASnI3 has a cubic structure. The resulting MASn0.5Pn0.5I3 perovskites exhibited the broadest light response (up to 1050 nm) and highest Jsc of ≈20 mA/cm2 (Fig. 5.23A–C). However, Sn-Pb-based PSCs always displayed a low VOC (Hayase, 2017; Kapil et al., 2017; Ogomi et al., 2014; Shi et al., 2017), which were mainly contributed by vacancies created due to the oxidation of Sn2+ and consequently affected its interaction with HTLs and ETLs. Inspired by highly efficient Cu(In,Ga)Se2 solar cells, Kapil et al. (2018) introduced an n-type interlayer (PCBM) with a spike structure between the FA0.5MA0.5Sn0.5Pn0.5I3 (x = 0.5) perovskites absorber and ETL (C60). The introduction of a spike structure can effectively reduce interfacial traps and charge-carrier recombination, thus facilitating the charge flow at the interface. The resulting devices exhibited a high VOC of 0.75 V, with energy loss of <0.5 eV and a high PCE of 17.6% (Fig. 5.23D–E). Except for Sn2+, many other kinds of divalent metal elements (Frolova et al., 2016; Klug et al., 2017), such as cobalt (Co), copper (Cu), iron (Fe), magnesium (Mg), nickel (Ni), strontium (Sr), and zinc (Zn), also can partially or completely replace Pb2+ in the perovskite lattice, and some of it would not influence the PV performance at low levels of substitution.

5.2.3  Process Development Different kinds of solution-processed methodologies have emerged for fabricating thin films of metal halide perovskites; however, each processing protocol induces a different thin-film

146  Chapter 5

Fig. 5.23 (A) The crystal structure of MASn0.5Pn0.5I3 (x = 0.5) perovskite. J-V curves (B) and corresponding IPCE spectra (C) of the MASnxPn1−xI3-based PSCs with different Sn/Pb ratios (Hao et al., 2014a,b). Inverted device configuration (D) and J-V curves (E) of FA0.5MA0.5Sn0.5Pn0.5I3 (x = 0.5)–based PSCs (Kapil et al., 2018).

Perovskite Solar Cells Processed by Solution Nanotechnology  147 morphology, such as surface coverage, film roughness, grain size, and crystal quality. The different thin-film qualities, in turn, have a great effect on the optoelectronic properties of perovskite thin film, including defect states, charge-carrier mobilities, and diffusion lengths. Generally, metal halide perovskite thin films were fabricated by combining an organic salt, such as MAI or FAI, with a Pb salt, such as PbI2 or PbBr2. 5.2.3.1  One-step fabrication method For the one-step fabrication method, both organic and inorganic halides are mixed in an organic solvent such as dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or mixed solvents, the organic solution is spin-coated on a substrate, and then the thin film is annealed at around 100°C to form the perovskite phase. The first high-performance PSCs were obtained by one-step spin-coating the mixed precursor on a mesoporous structure, which stands on a thin n-type compact TiO2 layer (Kim et al., 2012; Lee et al., 2012). However, solution-processed planar heterojunction solar cells, where the precursor solution (1,1 M PbI2:MAI) directly spin-coated on the compact TiO2 layer, resulted in discontinuous, needlelike morphology and exhibited low PCE (Ball et al., 2013; Burlakov et al., 2014; Eperon et al., 2014a,b; Saliba et al., 2014). A precursor solution plays an important role in the conversion of perovskite thin film (Gao et al., 2014; Gratzel, 2014; Pellet et al., 2014; Snaith, 2013; Stamplecoskie et al., 2015). Yan et al. (2015) revealed that the metal halide perovskite precursor solutions are generally colloidal dispersion rather than real solutions, and they proposed a general principle for the coordination engineering (Fig. 5.24). The presented precursor solution with a different compound ratio exhibited a colloid diameter from 10 nm to the micrometer scale, and a typical Tyndall effect was observed with a green laser light. The layered PbI2 first separated into (001)-aligned pieces due to the weak van der Waals interaction between (001) interlayers and coordinated with DMF when dissolved into a DMF solution. Owing to the dangling bond of PbI2, CH3NH3I selectively coordinated with PbI2 (001) planar edges and acted as a surfactant to form colloidal PbI2 nanorods. As organic salts increased, MAI coordinated with the back-precipitated trigonal PbI2 colloid toward tetragonal transformation with maximum coordination. Further increasing the amount of MAI decreased the iodoplumbate colloidal size. By utilizing coordination engineering, particularly through employing additional methylammonium halide over the stoichiometric ratio for tuning the coordination degree and mode in the initial colloidal solution, along with a thermal leaching for the selective release of excess methylammonium halides, it achieved full and even coverage, the preferential orientation, and high purity of planar perovskite thin films and delivered a high PCE of 17%. In addition, several solvent additives have been discovered that result in smoother, continuous films when added in small amounts to PbI2/MAI solutions (Chueh et al., 2015; Jin et al., 2017; Lee et al., 2017a; Li et al., 2017; Liang et al., 2014). Also, HI and HCl have shown dramatic improvements in film coverage when used as solution-processing additives (Eperon et al., 2014a,b; Heo et al., 2015a,b; Li et al., 2015a).

148  Chapter 5

Fig. 5.24 (A) Tyndall effect photograph of perovskite precursors with different MAI/PbI2 ratios at room temperature. (B) Schematic illustration of the colloidal intermediate phase in the solution of perovskite from starting material (PbI2 + CH3NH3I → soft coordination colloid → CH3NH3PbI3, which is actually a corner-shared octahedral soft framework (denoted as [−PbIx−… −PbIx−]n), and CH3NH3 is not sterically arranged in a lattice at this stage due to its higher solubility). (C) Possible organicinorganic coordination products at different levels, and with different content of organic compounds in perovskite precursors. (D) Supporting SEM micrographs of calcined thin films with CH3NH3I coordination engineering. (E) Supporting SEM micrographs of the final perovskites with optimized film morphology by CH3NH3Cl coordination engineering (Yan et al., 2015).

Perovskite Solar Cells Processed by Solution Nanotechnology  149 In all these recipes, the excess MAI in precursors can improve the film morphology, but it cannot lead to high-performance PSCs. The excess MACl can improve both the film morphology and performance due to facile removal of MACl. This suggests that the pure film quality is the most important for high-performance PSC. Due to the passivation effect of PbI2, perovskite with a little excess PbI2 has even better high photovoltage and PCE. 5.2.3.2  Two-step fabrication methods The morphology of perovskite thin film also can be controlled by sequentially depositing the two precursors of MAI and PbI2 (Burschka et al., 2013). The simplest way to form the perovskite phase consists of depositing a thin layer of Pb halide (commonly PbI2), and then immersing this thin film into a solution containing an organic salt, where the solvent must be carefully chosen so as not to dissolve the already-deposited Pb salt film or the developing perovskite film. The perovskite phase conversion occurred at room temperature within seconds. Burschka et al. (2013) first utilized this method to fabricate solution-processed PSCs by using a mesoporous TiO2 scaffold and achieve a high PCE of 15.3%. It is noteworthy that the size of the perovskite crystal is around 20 nm, limited by the mesoporous scaffold. Without the mesoporous TiO2 scaffold, the spin-coated solid films of PbI2 also can convert to a perovskite phase, allowing the unconstrained growth of perovskite crystal to a micrometer scale. Liu and Kelly (2013) then reported the planar metal halide PSCs by utilizing this fabrication protocol, achieving a PCE of 15.7%. Wu et al. (2014) found that the crystallization of the PbI2 layer can be retarded by using strong coordinative DMSO instead of DMF as the solvent, resulting in amorphous PbI2 film. The perovskite thin film exhibited a more narrowly distributed particle size, flat surface morphology, and improved reproducibility (Fig. 5.25A–D). Xiao et al. (2014b) demonstrated that the conversion of the perovskite phase can be completed by the “interfusion” of the bilayer stacks, which were sequentially deposited by spin-coating from the two precursor salts. The planar structure device based on the resulting pin-hole-free thin film exhibited a PCE of 15.4%, with a thickness of PbI2/MAI of 140/190 nm, and annealed at 100°C for 2 h (Fig. 5.25E–H). Instead of the perovskite phase conversion by dipping in solution, Chen et al. (2014b) developed a novel, low-temperature vapor-assisted solution process (VASP) to form perovskite film by exposing the PbI2 thin film into MAI vapor (Fig. 5.26). The resulting thin film exhibited a welldefined grain structure, crystal size up to the micrometer scale, and full surface coverage with small roughness. Devices based on the as-prepared thin film delivered a PCE of 12.1%. Recently, Long et al. (2016) investigated the degradation process of MAPbI3 perovskite and revealed that the CH3NH2 gas was first missed, and then HI leaves behind PbI2 solid through an intermediate of HPbI3. The degradation processes are depicted as follows: CH 3 NH 3 PbI 3 + H 2 O → CH 3 NH 3 PbI 3 ⋅ H 2 O → HPbI 3 + CH 3 NH 2 ⋅ H 2 O ↑ moisture

moisture

→ HPbI 3 + CH 3 NH 2 ↑ + H 2 O → PbI 2 + HI ↑ + H 2 O ↑

moisture

moisture

150  Chapter 5

Fig. 5.25 SEM images of perovskite obtained from DMF (A) and DMSO (B)-based PbI2 films. (C) and (D) The analysis of the particle size distributions (Wu et al., 2014). (E) Schematics of spin-coating of PbI2 and MAI using orthogonal solvents and the conversion of the stacking layer into a perovskite layer upon annealing. The SEM image of the PbI2 film (F), the annealed perovskite layer formed by an interdiffusion process (G), and the annealed perovskite film spun from the premixed PbI2 and MAI solution (H) (Xiao et al., 2014a,b,c).

Perovskite Solar Cells Processed by Solution Nanotechnology  151 (A)

(B)

CH3NH3I Vapor

(C) Perovskite c-TiO2 FTO

PbI2

Perovskite

500 nm

500 nm

Fig. 5.26 (A) Schematic illustration of perovskite thin-film conversion by VASP. Cross-section SEM image (B) and top-view SEM image (C) of the resulting perovskite film (Chen et al., 2014a,b).

Interestingly, based on this discovery, they developed an alternative two-step nonstoichiometric acid-base reaction (NABR) for the fabrication of perovskite thin film; that is, perovskite can be reconverted by the reaction of HPbI3 with columnar face-sharing PbI6 octahedra and excess CH3NH2 gas (Fig. 5.27). The excess and volatile MA gas promotes PbI2 stoichiometric conversion to MAPbI3 and effectively reduces lattice vacancy by eliminating the penetration of undesirable H2O molecules into vacant sites and avoiding the formation of monohydrate degradation product associated with H-bonding between H2O and MA+. Therefore, the as-prepared MAPbI3 thin film exhibited high stability in 65% humidity for up to 2 months without appreciable PbI2-impurity. 5.2.3.3  Solution engineering The control of the crystallization, and thus the kinetics of film formation during deposition and annealing, are the keys to the optimization of device performance. Solvent engineering is a key breakthrough in the development of high-performance perovskite thin film and control of the nucleation and growth of the perovskite (Ahn et al., 2015; Jeon et al., 2014; Xiao et al., 2014a). Solvent engineering is a class of fabrication techniques that combine both precursors in solution, but that also purposely introduce a solvent that coordinates with PbI2 to form PbI2-solvent complexes, and thus prevent further chemistry from happening in solution (Lee et al., 2014, 2016a,b). A film of this precursor-solvent complex is then cast and subsequently converted to the Pb halide perovskite. Jeon et al., 2014 developed a solvent-engineering technology to fabricate the PSCs and utilize MAPb(I1−xBrx)3 as the light-harvesting layer (Fig. 5.28). The precursor salts were dissolved into a mixture solvent of gamma-butyrolactone (GBL) and DMSO (7:3 v/v), and toluene was dripped on the film during spin-coating in order to stabilize the formation of a crystalline, transparent MAI-PbI2-DMSO complex that occurred via an intercalation process, retarding the crystallization kinetics of perovskite formation. Then the intermediate phase was converted to a smooth, continuous perovskite film via thermal annealing, which removes the coordinated DMSO molecules.

152  Chapter 5

Fig. 5.27 (A) Crystallographic illustration between NABR conversion from PbI2, to HPbI3 (H+ ions are included to indicate stoichiometry, but are actually mobile around a [PbI3]− column), then to intermediate CH3NH3PbI3‧DMF, and finally to CH3NH3PbI3; and conventional conversion from MAI + PbI2 using DMF as the solvent, to CH3NH3PbI3‧DMF, then to CH3NH3PbI3. (B) Quantitative estimation of degradation through the XRD external standard method (65% moisture with ambient light soaking) (Long et al., 2016).

Perovskite Solar Cells Processed by Solution Nanotechnology  153

100 °C

100 °C

(A)

Perovskite solution spreading MAI

Spinning

Toluene dripping

Intermediate phase film

DMSO

+MAI and +DMSO

–DMSO Annealing at ∼100 °C

Toluene dripping

(B)

Dense and uniform perovskite film

(ii) MAI–Pbl2–DMSO as intermediate phase

(i) Pbl2 in medium

(iii) MAPbl3 perovskite

100 Height (nm)

(C)

3

3 2

x(

µm 1 )

1 0

y

2 ) m (µ

–100 100 Height (nm)

(D)

–100

3

3 2

x(

µm

2 )

1

1 0

y

)

m

(µ

(E)

Fig. 5.28 (A) Scheme illustration of solvent-engineering procedure for preparing the uniform and dense perovskite film. (B) Scheme illustration the formation of the perovskite material via the MAI– PbI2–DMSO intermediate phase. AFM topography (left) and 3D views (right) of the surface of the intermediate phase (C) and resulting perovskite film (D). The size of the AFM images is 3 × 3 μm2. (E) Top-view SEM image of perovskite thin film (Jeon et al., 2014).

This procedure allowed a uniform grain growth of perovskite thin film and resulted in the formation of compact and dense capping layer with large grain size (100–500 nm) and complete surface coverage. Devices based on mesoporous configuration demonstrated with certified efficiencies of 16.2%. Many research groups have further improved PCEs to >20% by optimizing the precursor composition and preparation recipe of this advanced processing technique. Almost the same time, Xiao et al. (2014a,b,c) constructed planar heterojunction solar cells by developing a fast deposition-crystallization method to deposit high-quality

154  Chapter 5 perovskite thin film, which is almost the same with Jeon’s method except chlorobenzene was utilized as an antisolvent, and delivered a maximum PCE of 16.2%. Other solvent methodologies besides antisolvent engineering have been developed for the fabrication of high-performance PSCs. Xiao et al. (2014a,b,c) applied a solvent-annealing technique, which is commonly used in organic solar cells, for the two-step perovskite thin-film deposition (Fig. 5.29). During the thermal annealing process, the DMF vapor can dissolve some of the precursor salt (PbI2 and MAI) and provide a wet environment so that the metal halide and organic halide could diffuse more efficiently than that of all-solid-state thermal annealing. The solvent-annealed method facilitates the crystal growth and the resulting thin film exhibited a much larger grain size than the thermally growth films. The devices based on the as-prepared perovskite thin film exhibited a PCE of 15.6%, which may be ascribe to the much longer charge-carrier diffusion length (>1 μm). Several other solvents, such as pyridine (Vorpahl et al., 2015) and MAI (Yang et al., 2016a,b), have been used in solvent annealing to fabricate high-quality perovskite thin film, which showed increased PL and longer charge-carrier lifetimes. 5.2.3.4  Thermal vapor deposition Thermal evaporation technology (Era et al., 1997; Mitzi et al., 1999) is intensively applied in the semiconductor manufacturing industries. Compared with solution-processed perovskite

Fig. 5.29 (A) Scheme illustration of the interfusion approach and solvent-annealing-induced grain size increase. Top view SEM images of the thermally annealed (TA) (B) and solvent-annealed (SA) (C) perovskite film with thicknesses of 1015 nm and corresponding grain-size distributions of the SEM images to the left (Xiao et al., 2014a,b,c).

Perovskite Solar Cells Processed by Solution Nanotechnology  155 thin film, thermal vapor deposition thin films exhibited more uniform, pinhole-free and complete coverage surface (Sessolo et al., 2015). Liu et al. (2013) first applied this deposition method to fabricate the planar heterojunction solar cells by dual coevaporation of PbCl2 and MAI. The resulting perovskite thin film exhibited a dense and uniform surface, and a high PCE of 15.4% was achieved. After that, a series of alternative thermal vapor deposition methods have been developed to further optimize perovskite thin film, such as chemical vapor deposition (Tavakoli et al., 2015), flash evaporation (Longo et al., 2015), and layer-bylayer vacuum evaporation (Chen et al., 2014a). In addition, vapor deposition method is also a practical technology to fabricate large-area perovskite thin films (Ono et al., 2016). Leyden et al. (2016) applied the chemical vapor deposition (CVD) method to fabricate MAPbI3- and FAPbI3-based solar modules with PCE of 8.5% (6-cell module, active area 15.4 cm2) and 9.0% (6-cell module, active area 12 cm2), respectively. However, the PCE of vapor-deposited PSCs is lagging solution-processed counterparts, which may be ascribed to the low thermal stability of both the precursor and the resulted perovskite thin film, which are easily affected during thermal deposition, and resulted in nonstoichiometric compounds (Chen et al., 2014a,b; Liu et al., 2013; Zhao et al., 2016).

5.2.4  Interface Materials Using Thin Film and Nanotechnology 5.2.4.1  Organic interface material For organic interface materials, one of the most important application is as an ETL or a HTL to effectively collect and transport the photogenerated charge carriers at the perovskite/ electrode interface (Bai et al., 2018; Calió et al., 2016). In 2012, Kim and Lee et al. independently applied spiroOMeTAD as hole-transporting material (HTM) and fabricated the first solid-state PSCs and delivered a high PCE around of 10% (Kim et al., 2012; Lee et al., 2012). When spiroOMeTAD is applied as a HTL, polar additives (dopants), such as tert-butylpyridine (t-BP) and bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI), were widely used to hold high conductivity. However, the dopants exhibited deliquescence, which easily caused device degradation under an ambient environment and halogenated solvents were commonly used to process the HTM, which is severely toxic to the human body (Kim et al., 2016; Lee et al., 2017a,b). Therefore, different kinds of green solvent-processable and dopant-free organic HTM have been developed for the fabrication of high efficient PSCs (Bai et al., 2018; Calió et al., 2016; Zhou et al., 2015). Meanwhile, the interface materials play an important role in tuning the perovskite thin-film morphology. For example, due to the hydrophobic surface, PTAA, which is one of the most presentative HTMs for PSCs, is facilitated to the growth of perovskite crystal with large grain size (Bi et al., 2015; Dong et al., 2015a,b). Due to the fine alignment of energy level with perovskite and efficient charge extraction from the perovskite surface, fullerene, and its derivatives are the mostly used as ETMs in PSCs (Jeng et al., 2013). In addition, many other

156  Chapter 5 kinds of n-type organic materials with a proper energy level and band gap, high electron mobility, and chemical stability were applied in PSCs as ETMs (Lin et al., 2017; Meng et al., 2017; Niu et al., 2018). Moreover, organic interface materials can act as a passivator to reduce the imperfection at the gain boundaries of perovskite thin film, which were usually regarded as nonradiative recombination center and caused hysteresis phenomenon (Dong et al., 2015a,b; Nie et al., 2015; Shao et al., 2014; Zhao et al., 2015). Yang et al. (2015a,b,c) found that the spiroOMeTAD HTM can penetrate the grain boundaries to form vertically aligned bulk heterojunction in the perovskite thin film. The vertical aligned bulk heterojunction could efficiently suppress nonradiative recombination and worked as charge-carrier collection channels. Several groups (Chiang and Wu, 2016; Shao et al., 2014; Xiao et al., 2016) also confirmed that the introduction of PCBM molecule into the grain boundaries of perovskite thin films have a significant effect on electronic properties and can effectively reduce the interface charge recombination loss and minimize the hysteresis effect in planar PSCs. To reduce the contact barrier between the high work function metal electrode (such as Al, Ag) and the ETM (such as PCBM), organic interfacial layered also has been utilized to improve charge extraction and injection. Wang et al. (2014) fabricated planar PSCs with a device structure of ITO/PEDOT:PSS/perovskite/PCBM/C60/BCP/Al and found that the double fullerene layers could form a Schottky junction with the anode and greatly reduce the dark current leakage and resulted in large FF. Zhang et al. (2014) introduced an ultrathin polyelectrolyte layer (PEIE or P3TMAHT) between PCBM and Ag electrodes and thought that the formation of surface dipole reduced the work function of Ag electrodes and facilitated electron injection to PCBM. In addition to the organic semiconductor, ultrathin organic insulated layers have been applied in order to fabricate high-performance PSCs. Wang et al. (2016) introduced PS, Teflon, PVDF-TrFE, and a fluorosilane ultrathin layer between C60 and perovskite and found that the photogenerated electron could effectively inject into the C60 layer through the tunneling effect and block the holes back into the perovskite layer. Interestingly, the resulting PSC exhibited good stability due to the insulating layer serving as an encapsulating layer to prevent the perovskite film from damage caused by water or moisture. 5.2.4.2  Inorganic interface material For the inorganic interface materials, solution-processed nanotechnology and vacuum-based nanotechnology are widely employed (Fig. 5.30). Several methods have mainly been used for the fabrication of nanoscale thin films. The first is the sol-gel method to prepare the organometal precursor film, followed by pyrolysis oxidation. For example, a TiO2 precursor was prepared by mixing 0.6 mL of titanium isopropoxide and 0.15 mL of 37 wt% HCl solution dissolved in 15 mL of ethanol, with 5 mol% Nb-doping by niobium ethoxide. In this method,

Perovskite Solar Cells Processed by Solution Nanotechnology  157

NiOx

Al ZnO

ITO

–4.2 eV

CH3NH3Pbl3 ZnO/AI

–4.7 eV CH3NH3Pbl3

–3.9 eV

–5.05 eV –5.4 eV

NiOx Glass/ITO

(A)

(B) ZnO Perovskite NiOx ITO

1µm

(C) Fig. 5.30 Solution-processed nanocrystals in PSC. (A) Diagram of the cell configuration using a nanocrystal, consisting of glass/ITO/NiOx/MAPbI3/ZnO/Al. (B) Band alignments of the solar cell. (C) Crosssectional SEM image of a complete solar cell (You et al., 2016).

organometal precursor film is deposited on FTO by spin-coating or ultrasonic spray, followed by pyrolysis oxidization and surface oxidation in an oxidizing atmosphere in the oven according to previous reports (e.g., Chen et al., 2015). Tetragonal phase SnO2 thin films were synthesized by the sol-gel method using SnCl2·2H2O and polyethylene glycol (PEG). NOx thin films are prepared using 1:1 Ni(II) acetylacetonate/diethanolamine in ethanol (0.5 M) after stirring for 24 h at 70°C. Growth parameters such as composition, doping concentration, temperature, and reaction time were systematically adjusted to place a highly conductive selective layer on the substrates. Another option is the solution growth method, which involves synthesizing nanoparticles and then modifying the surface of the nanoparticles that is dispersive in a compatible solution as nano-ink. During a layer-by-layer process, it is expected that the following deposition will not damage the as-formed perovskite thin film. The solution choice thus becomes very important. The hydrothermal method was widely used for synthesizing nanomaterial and control over the morphology, electronic properties, and surface of the nanomaterials to meet our needs (Qiu et al., 2011; Xie et al., 2013). A series of n-type nanoparticles were synthesized, such

158  Chapter 5

Fig. 5.31 The all-inorganic interfacial charge-transporting layers for PSC with improved stability (Zhang et al., 2017).

as TiO2, Nb2O5, SnO2, and ZnO nanoparticles, and p-type nanoparticles, such as NiOx, NiO-MgO, and CuMO2 (M = Al, Ga, Cr, etc.) are accompanied by doping to enhance charge extraction (Fig. 5.31) (Chen et al., 2015; Zhang et al., 2017). Other vacuum-deposition methods are employed to fabricate charge-transport films, such as thermal evaporation and magnetic sputtering. Variation of evaporation/sputtering conditions was performed to achieve a dense packing morphology and well crystallization for the films. Using an alloyed sputtering source, such as Zn-Mg-O, Ni-Mg-O, or Cu(Al/Cr)O2, or dual-source co-sputtering, such as SnO2-In2O3 or SnO2-Ga2O3, the multicomponent alloyed and graded doping films could be realized, with continuous tuning of band gaps and alignment. Besides, carbon-based nanomaterial was modified to be suitable for charge extraction. Interface engineering is critical to enhance the contact and reduce the recombination in p-i-n junction solar cells. Energy-band engineering has been incorporated, which allowed the devices to reduce the voltage loss. In this step, the coordination chemistry was transferred and molecular linker in a monolayer was fabricated to enhance the interface, like in situ fabrication using bifunctional material (Yan et al., 2013). Synthesis conditions such as concentration, temperature, ligands, and reaction time were systematically varied to understand the bottom-up growth mechanism and control the nanostructures, and interface engineering was performed to enhance the device performance.

5.2.5  Stability Issues 5.2.5.1  Humidity stability Water molecules will catalyze perovskite degradation irreversibly, as shown in Fig. 5.32, and so humidity is an important degradation factor (Frost et al., 2014). The water will extract MAI

Perovskite Solar Cells Processed by Solution Nanotechnology  159 6

Absorbance

5 4 3 2 1 0 400

500

600

700

800

Wavelength (nm)

Fig. 5.32 H2O-induced perovskite degradation (Frost et al., 2014).

in the form of HI and MA from perovskite, leaving behind PbI2 solid, and thus the humidity will degrade the performance of PSCs. Yang et al. employed an in situ absorption spectrum and GIXRD to observe the decomposition process. The absorption coefficient reduces by half after 4 h under 98% humidity, but there is no change even after 500 h under 20% humidity, indicating the humidity effect (Yang et al., 2015a,b,c). Scientists developed several strategies to enhance the humidity stability. The first was the change of coordination compound to lead (II) centers. Noh et al. employed the bromide for a partial substation of iodide (CH3NH3Pb(I1−xBrx)3) and reduced the crystal lattice parameters for the cubic structure, leading to stability enhancement (Noh et al., 2013). Second, Cao et al. employed a 2D perovskite (CH3(CH2)3NH3)2(CH3NH3)n−1PbnI3n+1 for stability enhancement. Due to the hydrophobic effect of butylamine and high alignment of closely packed 2D layers, the perovskites are stable for 2 months under 40% humidity (Cao et al., 2015). Third, an inert shielding layer and crystal cross-linkage have been used to retard the water penetration and enhanced the stability (Li et al., 2015a,b). 5.2.5.2  Thermal stability Compared to traditional inorganic PV semiconductors, the hybrid perovskite has small formation energy and chemical binding energy, leading to instability under heating stress. Phillippe et al. investigated the heating effect on MAPbI3 and MAPbI3−xClx under high vacuum and found that the perovskite degraded to PbI2 at 100°C for 20 min, releasing HI and MA vapor in the reaction (Philippe et al., 2015). According to the Standard Testing for Photovoltaic Panels (IEC61646, 2008; JISC8938, 1995), 85°C is a threshold that should be passed in PV performance testing. Conings found that MAPbI3 changed significantly

160  Chapter 5

Au P3HT perovskite TiO2 ITO

100 nm

100 nm

N2

pristine

100 nm

100 nm

O2

air

Fig. 5.33 The cross-section high-angle annular dark field of perovskite degradation uder 85°C with O2 and air for 24 h (Conings et al., 2015).

after 24 h of aging at 85°C, regardless of the O2 and N2 atmosphere (Fig. 5.33). Theoretical calculations indicated that the formation energy was about 0.11–0.14 eV per unit cell and the thermal energy at 85°C was 0.093 eV. Their equivalent values created the instability problem (Conings et al., 2015). Many independent groups found that MAPbI3 degraded to PbI2 due to the decomposition of MA+ into volatile MA vapor. Scientists thus have used large organic FA+ or nonvolatile inorganic Cs+ to replace MA+ for thermal stability enhancement afterward (Wang et al., 2015b). Indeed, FAPbI3 had much higher decomposition temperatures and was able to endure 160°C calcination for 80 min with no decomposition (Wang et al., 2015a,b). The Park group and Snaith group incorporated the small Cs+ into FAPbI3 and reduced the tolerance

Perovskite Solar Cells Processed by Solution Nanotechnology  161 factor, leading to even more stable CsFAPbI3 perovskite (Lee et al., 2014, 2015). Saliba et al. incorporated 5% Ru+ for RbCsMAFA-based perovskite and found that the PV device retained 95% of its initial efficiency even after 500 h under 85°C and 1 sun photo illumination (Saliba et al., 2016a,b). All-inorganic perovskites, such as CsPbI3 and CsSnI3, have a suitable band gap for solar cells (1.7 and 1.3 eV, respectively), optoelectronic performance, and excellent thermal stability. (Chung et al., 2012a,b). CsPbI2Br that used Br to partially replace I demonstrated that there was no thermal decomposition at 350°C (Sutton et al., 2016). There, the inorganic incorporation enhanced the thermal stability. 5.2.5.3  Phase and structure stability Changing A/B/X species and tuning the Goldschmidt tolerance factor to satisfy the highsymmetric requirements are crucial to the stability of 3D perovskite. For example, the blackphase FAPbI3 was easy to transform to the yellow phase under ambient conditions, and the Snaith group in Oxford University used FA0.83Cs0.17Pb(I0.6Br0.4)3, which incorporated small Cs+ to enhance the phase stability. Therefore, a multicomponent strategy is an effective method to achieve high stability of perovskite (McMeekin et al., 2016). All-inorganic perovskite with no organic component had much more thermal stability (Swarnkar et al., 2016). For example, CsPbI3 and CsSnI3, meet the basic requirements for PSCs, with a suitable band gap (Eg = 1.7 and 1.3 eV, respectively), excellent optoelectronic properties, and high-temperature stability above 360°C (Chung et al., 2012a,b; Yakunin et al., 2015; Yuan et al., 2016). However, the photoactive black phase is not thermodynamically stable at room temperature, converting to the undesired yellow phase (Eg = 2.82 and 2.55 eV, respectively) (Chung et al., 2012a,b). Black phase stabilization by alloying with Br has been explored because bulk CsPbIBr2 also shows a much-reduced phase-transition temperature of 100°C. The composition change of halide leads to an undesired increase in the band gap. Fortunately, the alloyed perovskite between Sn and Pb can cause abnormal band-gap narrowing in the organic-inorganic perovskite (Hao et al., 2014a,b). Therefore, the optimization of A/B/X species is promising for obtaining phase-stable and band-suitable all-inorganic perovskites. In addition, through nanoscale confinement and nanostructure design, phase stabilization can be achieved at room temperature. At the National Renewable Energy Laboratory (NREL), Luther et al. exhibited improved room-temperature black cubic-phase stability and attractive optical properties in CsPbI3 quantum dots (QDs). Under ambient conditions for 30 days, the CsPbI3 QDs had no structure change (Fig. 5.34) (Liu et al., 2017). 5.2.5.4  Poling voltage stability The Huang Group at the University of Nebraska-Lincoln employed in situ microscopy to directly observe the ion motion of MAPI3 under 1.2 V/μm and the material interface change. The interface change induced the built-in field and photocurrent to reverse, leading

162  Chapter 5

Intensity (a.u.)

Day 0 Day 30

10

20

30

40

50

2-theta (deg) Fig. 5.34 The XRD data of freshly synthesized CsPbI3 QDs after being stored for 30 days under ambient conditions (Liu et al., 2017).

to field-switchable PV effects (Xiao et al., 2015). Venkataraman et al. studied the Warburg impedance and demonstrated that the ion diffusion coefficient could be sorted as follows: MAPbI3 > FAxMA1−xPbI3 ≫ FAPbI3. Structural studies using powder X-ray diffraction (PXRD) showed that for MAPbI3, a structural change and lattice expansion occurred at device operating temperatures. On the basis of EIS and PXRD studies, they postulated that in MAPbI3, the predominant mechanism of accelerated device degradation under sunlight involved thermally activated fast ion transport coupled with a lattice-expanding phase transition, both of which were facilitated by absorption of the infrared component of the solar spectrum. Using these findings, they correlated ion motion to the performance devices (Bag et al., 2015). In addition to bulk defects, the surface and grain boundaries of the crystal grain are an extremely important ion migration channel. Due to the relatively open structure at grain boundaries and surfaces, the activation energy (EA) for ion migration at grain boundaries and surfaces is roughly half of that in the bulk because only half of the chemical bonds are reserved. The ion migration cross-section area along grain boundaries is much smaller than that for bulk migration. Much direct and indirect evidence has overwhelmingly confirmed that ion migration existed in perovskite polycrystalline materials. However, more research is needed to address many open questions and to fully understand the impact of ion migration on solar cell performance. 5.2.5.5  Stability related to interfacial material The interfacial layer of electron transport/hole transport materials (ETMs/HTMs) plays a role in charge extraction to the contact electrode. The optimization of charge-transporting layer was also extremely important to performance and stability.

Perovskite Solar Cells Processed by Solution Nanotechnology  163 (i) Stability of interfacial layers of HTMs/ETMs Spiro-OMeTAD is a standard HTM for regular n-i-p-type PSC, in which the Li-salt, Co-salt, and tributyl phosphate (TBP) additive are included for optimized electronic properties. These instable compounds cause long-term stability problems. The air-slaked metal-salt, intractable oxidization of spiroOMeTAD, and perovskite-corrosive TBP accounted for the degradation of PSC. Recently, researchers employed additive-free HTMs or inorganic compounds for hole extraction, such as carbon electrode, which showed much greater stability after 30 days (Mei et al., 2014). In the inverted p-i-n structure, people employed PEDOT:PSS as the bottom-hole selective layer. However, PEDOT:PSS is water-sensitive and has strong acidity that corrodes the ITO substrate, which influence the long-term stability. Zhu et al. employed inorganic NiOx/SnO2 for hole/electron extraction and demonstrated better long-term stability (Zhu et al., 2016a,b). (ii) Interfacial layer-induced perovskite degradation TiO2 is the most common ETM in PSCs, but its photocatalyst capability causes the degradation of perovskite under UV light, resulting in performance decay of PSCs when exposed to sunlight. Researchers found that Sb2S3-modified TiO2/MAPbI3 retarded this photodegradation and increased the stability of PSC (Ito et al., 2014). Bella et al. employed photocurable fluoropolymers and converted UV light to visible light for stability enhancement. Therefore, the device can be exposed to outdoor conditions and retain 95% of its initial efficiency (Bella et al., 2016). The instable ZnO and mismatched interfacial layer also can degrade the perovskite indirectly (Yang et al., 2015c). Recently, Seok developed perovskite oxide and enhanced the performance and stability (Shin et al., 2017). (iii) Shielding effects by interfacial layer The interfacial layer can protect the perovskite if it is stable enough. For example, PCBM and PCBM/TiNbOx, PCBM/AZO/SnOx, carbon electrode, ZrO2, and Cr/Cr2O3 demonstrated shielding effects from the moisture in previous studies (Brinkmann et al., 2017; Kaltenbrunner et al., 2015). In summary, the employment of compact and chemically stable inorganic represented an important direction for the stability enhancement of PSC.

5.3 Perspective 5.3.1 Efficiency For certified efficiency over 22%, in terms of performance metrics, photocurrents have been maximized toward the theoretical and practical limits owing to the optical absorption of transparent conductive glass. In these devices, however, there is still room to improve VOC

164  Chapter 5 and FF. This target requires identifying and suppressing recombination pathways, along with optimizing the interface for efficient charge separation, to boost these crucial PV parameters. In the device, the work function difference between n/p interface layers could produce a built-in field across the intrinsic perovskite in equilibrium or similar to traditional Si solar cells, with perovskite to be n- or p-doped and sandwiching the intrinsic perovskite, forming a p-i-n junction in the device. However, the electric field distribution is influenced further by intrinsic defects induced by ion motion and stoichiometric deviation. Low mobilities may be responsible for the frequently observed hysteresis in the J-V curve. The defects could induce trap-mediate recombination and accelerate thin film degradation. Therefore, intrinsic trap reduction, surface passivation, and interface CT engineering may be helpful for the increase of the photovoltage and FF, along with photocurrent. Thus, hysteresis can be minimized by either immobilizing the ionic charge or increasing charge-carrier transport and extraction to make the latter less sensitive to the electric field. Namely, reaction optimization and the interface engineering, in combination with surface passivation, will the next steps for improving efficiency (Correa-Baena et al., 2017a,b). Furthermore, the efficiency enhancement can be promoted by tandem devices. Tandem solar cells have a higher efficiency limit than that of single-junction solar cells. Right now, Snaith et al. announced that PSC-Si tandem solar cells have achieved above 27%. Tandem solar cells could also improve the device stability, and they can combine the advantages of different types of solar cells through integration.

5.3.2 Scalability PSCs have improved efficiency using laboratory-scale spin-coating methods owing to advances in solution engineering and process optimization. However, large-area devices with high performance are still lagging state-of-the-art spin-coated devices because perovskite crystallization is not controlled on a large scale from a colloidal precursor state. At present, using stoichiometric ratio ink, the precursor films will crystallize in dendritic morphology due to the formation of solvate MAPbI3‧DMF. In the antisolvent method, the solvent is removed immediately and its formation is avoided, leading to direct transfer to perovskite. For scalable fabrication, one should address the solvate issue. Recently, the vacuum-assisted solution process is employed to balance the partial pressure of the solvent and retard its formation. As a scalable fabrication, this method may be employed to posttreat the precursor film because it is uniform enough to control in large areas (Li et al., 2016). In addition, the Zhu group at NREL demonstrated a chlorine-containing methylammonium Pb iodide precursor formulation, along with solvent tuning, to enable a wide precursor-processing window (up to 8 min) and a rapid grain-growth rate (taking as little as 1 min). Coupled with antisolvent extraction, this precursor ink delivers high-quality perovskite films with large-scale

Perovskite Solar Cells Processed by Solution Nanotechnology  165 uniformity. The ink can be used by both spin-coating and blade-coating methods, with indistinguishable film morphology and device performance. (Yang et al., 2017a,b). Also, complex fluid dynamics and the uncontrollable solvent drying balance have limited the uniform deposition of pinhole-free perovskite thin films over large areas. The Huang group developed a surfactant-assisted one-step method for solar modules with a large area. Small amounts of surfactants dramatically alter the fluid-drying dynamics and increase the adhesion of the perovskite ink to the underlying nonwetting charge- transport layer. The polymer additives enable the blading of smooth perovskite films and passivate charge traps, resulting in improved efficiencies by over 20% for small-area solar cells and stabilized module efficiencies of 15% in mini-module, reported recently (Deng et al., 2018).

5.3.3 Cost The costs of PSC modules were found to be lower than those of other PV technologies. The levelized cost of electricity (LCOE) of PSCs was calculated to be 3.5–4.9 cents/kWh, below the cost of traditional energy sources, with an efficiency and lifetime of >12% and 15 years, respectively. Sensitivity analysis indicated that an increase of module efficiency could reduce module cost significantly. The fabrication of high-efficiency modules through high-precision processes was the most promising approach for further reducing the cost. We estimated the cost for 100-MW scale solar modules and found that the major cost is not the deposition of perovskite thin film (see Fig. 5.35). Because it is a solution process with low-cost material, it carries only 5%–6% of the total cost. The deposition of transparent

Fig. 5.35 The estimated cost distrubition for 100-MW PSC modules.

166  Chapter 5 conductive substrate, interface material, and electrodes account for 70% of the total cost. PSCs can ultimately reduce the cost, with only half of that of Si solar cells.

5.3.4  Operation Stability PCE and long-term stability are two factors that determine the commercial promise of a new solar technology. The PCE degradation over time pays back the investment, and thus it is important to assess the longtime operation of new PV technology accurately. The market reference standard is crystalline Si solar cells, which has a 25-year operating lifetime and an average degradation rate of 0.5% PCE per year. To compete within the PV market, PSCs must achieve similar levels of stability in the range of 15–25 years. First, a robust measurement protocol for long-term stability under operation may perform at the MPP efficiency. Anyway, long-term stability remains one of the key issues that retard rapid commercialization in the coming years. Both intrinsic and extrinsic degradation in the PSC solar modules must be considered, and in particular, the photoaging stability should be addressed by either encapsulation/shielding or improving the intrinsic stability.

5.3.5 Toxicity Researchers have taken the first step in comparing the environmental impact of Pb- and Snbased PSCs. The structural failure of a solar panel has been taken into consideration. There are three basic types of pollution for solar panels at various stages: (1) the starting raw materials during fabrication could bring a large amount of chemical waste, such as volatile organic compounds, water pollution, and solid waste; (2) the full degradation of its perovskite absorber material when being exposed to ambient can cause solid waste; (3) the dissolution of the decomposition products in water on a rainy day later would bring a series of wastes in vapor, liquid and solid states. Thus, as a first step, the main degradation products of Pb and Sn-based perovskites need to be established before one proceeds to their toxicological assessment in an aqueous environment (Babayigit et al., 2016). Basically, Pb toxicity in the solar panels should be addressed. However, even using Pb-free perovskite, the toxicity cannot be underestimated. For example, tin halide (SnX2) is not toxic like the raw materials (silicon tetrachloride, SiCl4) for Si solar cells. Besides, Sn has even Sn4+ oxidation states, which is even more stable than Sn2+. The exposure of Sn2+ compounds is ever more toxic than the Pb2+ solid, and SnX2 is less stable than Pb halide under ambient conditions. Therefore, in the long run, toxicity may be the most challenging aspect to handle if we want to commercialize such products.

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