Perovskite solar cells

CHAPTER FIVE

Perovskite solar cells Narges Yaghoobi Nia1, * Danila Saranin2, Alessandro Lorenzo Palma1, 3, Aldo Di Carlo1, 2 1

Centre for Hybrid and Organic Solar Energy, University of Rome Tor Vergata, Rome, Italy Laboratory for Advanced Solar Energy, National University of Science and Technology ‘‘MISiS’’, Moscow, Russia 3 Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), Energy Efficiency Unit Department, Rome, Italy *Corresponding author. 2

5.1 Introduction Hybrid organiceinorganic halide perovskite solar cells (PSCs) have risen to stardom owing to the peculiar characteristics of the halide perovskite absorber such as high charge carrier mobility, broad and strong optical absorption, long free carrier diffusion length, low exciton binding energy, as well as their cost-effective and easy solution process manufacture [1,2]. PSCs have achieved remarkable progress with power conversion efficiency (PCE), developing from 3.8% in 2009 to 23.7% in 2018 [3,4]. PSCs have become an important technology in the photovoltaic (PV) field, rivaling the well-consolidated silicon-based solar cells, copper indium gallium selenide (CIGS) and cadmium telluride (CdTe) solar cells [5]. This excellent improvement in PCEs is largely due to the optimization of the device’s architecture, the use of interface engineering, improvement of hole and electron transport materials, and the development of fabrication processes with high-quality perovskite films [9e14]. These progresses have promptly motivated the PV communities’ efforts to commercialize PSCs. Many efforts have been focused to scale up PSCs, and great progress has been achieved in perovskite module performance [5e7]. Many excellent review papers have been published to summarize the advances of upscaling of PSC [15e19]. However, considering that large-scale manufacturing of perovskite films is mainly based on solution processes, an in-depth understanding of nucleation and growth processes of perovskite crystals is essential to obtain a high-quality perovskite film, which is directly linked with the performance of PSCs.

Solar Cells and Light Management ISBN: 978-0-08-102762-2 https://doi.org/10.1016/B978-0-08-102762-2.00005-7

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In this chapter, we first summarize the main properties of halide perovskite, and then we review the mechanism of nucleation and growth of perovskite thin films. PSC architectures and scaling up procedures are then discussed in the last two sections of the chapter.

5.2 Unique properties of metaldhalide perovskites for photovoltaics 5.2.1 Molecular composition and basic materials PSCs are based on an absorber with ABX3 perovskite crystal structure, where “A” and “B” are cation elements and “X” is an anion element. Originally, the terminology of “perovskite” materials was accepted in the first half of the 19th century, when the group of Lev Perovskii and Gustav Rose discovered mineral-CaTiO3 with ABX3 structure. Currently, the family of perovskite materials has a lot of fields for applications like superconductors [20,21], optical waveguides [22], transparent electrodes [23,24], etc. Mainly based on oxides, they achieved progress and lasting successes in PV in the form of hybrid organiceinorganic halide perovskites. The organometallic composition for ABX3 crystal (schematically presented in Fig. 5.1) has showed the most advantageous semiconductor properties for use in thin film solar cells [25]. “A”dsite represents small organic cations such as methyl ammonium (MAþ, CH3 NH3 þ ); formamidinium (FAþ, CH4 N2 þ ); guanidinium (GUAþ, C3 N3 H6 þ ), as well as a single element from the first group of periodic table (PT) such as Csþ, Kþ, Rbþ. “B”dsite cation represents an element from the 14th group of the PT such as Pb, Sn, and Ge. “X”dsite anions represent halogens of the 17th group such as I; Br, and Cl. Appropriate formulation of A, B, and X components should satisfy the Goldschmidt tolerance factor [26], a specific ratio between the ionic radius of cations and anions, described in Eq. (5.1): rAeff þ rX t ¼ pffiffiffi 2ðrBþ rX Þ

(5.1)

where rA,B,X is the effective ion radius of A, B, and X. t should be in a range between 0.80 and 1.15 [27] to form an organometallic semiconducting perovskite crystal. The range of values between 0.8 and 1 characterizes perfect fitting between A-cation and BX6 octahedrons, while the structures with smaller t (around 0.8) has increased tilting of the BX6 that tends to form lower symmetry crystals. If the ionic radius of A is too big (t > 1), a more

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Figure 5.1 The ideal cubic perovskite unit cell. (A) A cations(blue) occupy the lattice corners, B cations (green) occupy the interstitial site, and X anions (red) occupy lattice faces. (B) An alternative view depicting B cations assembled around X anions to form BX6 octahedra, as B X bonds are responsible for determining electrical properties. (C) Tilting of BX6 octahedra occurring from nonideal size effects and other factors, inducing strain on the B X bonds. Reprinted from Q. Chen, N. De Marco, Y. Yang, T. Bin Song, C.C. Chen, H. Zhao, Z. Hong, H. Zhou, Y. Yang, Under the spotlight: the organicinorganic hybrid halide perovskite for optoelectronic applications, Nano Today. 10 (2015) 355e396. https://doi.org/10.1016/j.nantod.2015.04.009

complex structure is formed with intermolecular distortions. Typically, if t < 0.8, A-B-X ions do not form perovskite crystals. In opposite to inorganic oxide compositions, metaleorganic halide perovskites do not have spherical symmetry of the organic cation. In this case, the correct estimation of organic cation size becomes more complicated, as well as a precise calculation of tolerance factor with accounting of cationeanion interactions. Thus, Goldschmidt tolerance factor is not a comprehensive parameter that completely describes stability [28] of the crystal structures (Fig. 5.1) that can be formed between A cation and the corner-sharing BX6 octahedra in dependence to their size. For this reason, a special geometric octahedral factor, m [29], was also assessed for determination of tilting between B and X sites. An objective approach for the prediction of ABX3 stability requires combinational account of t-m parameters. The ionic size of cation and anions in A-B-X sites influences the ordering/disordering of the molecular symmetry and can shift the orientation alignment, off-centering, and bonding [30]. Thus, phase transformation of metalorganic halide perovskites strongly depends on the stoichiometry and composition. Moreover, organic parts (representing A site cation) add functionality for interaction with inorganic BX6 parts. Hybrid halide perovskite crystals have several phase transitions in 100e400 K temperature range [31]. For example, for MAPbI3 crystals [32], reorientation of A site

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Figure 5.2 Octahedra tilting pattern for the tetragonal (left) to the cubic (right) structure of a MAPbX3 perovskite (X ¼ halide); the order parameter of the transition. Reprinted from C. Quarti, E. Mosconi, J.M. Ball, V. D’Innocenzo, C. Tao, S. Pathak, H.J. Snaith, A. Petrozza, F. De Angelis, Structural and optical properties of methylammonium lead iodide across the tetragonal to cubic phase transition: implications for perovskite solar cells, Energy Environ. Sci. (2016). https://doi.org/10.1039/c5ee02925b.

CH3NHþ 3 first cause transition from Pnma to tetragonal I4/mcm symmetry at w170 K, then to photoactive, cubic Pm3m phase at w330 K (see Fig. 5.2), and finally to P4mm at a temperature >400 K with large cation disordering. The use of mono FAþ or Csþ cations can lift up the transition temperature to “black” photoactive phase at temperature >420 K [33] and >570 K [34], respectively. The most studied perovskite composition for prototypes of PSCs is MAPbI3, due to a low temperature crystallization (100 C) and high solar cell performance (>20%). However, due to a weak alignment between MA and PbI6 sites (tfactor ¼ 0.83), low temperature transitions into cubic photoactive phase (w320 K) occur and cause several instabilities [35], which will be discussed in the next subchapters. Actual trends for perovskite molecule stabilization include double/triple cation compositions to balance the molecular packing density.

5.2.2 Band gap structure In general, organometallic perovskites have direct band gap structure (Fig. 5.3A), which minimizes the thermalization losses which occur in nondirect band gap Si [36]. Many simulations and theoretical calculations, mainly based on density functional theory, have been performed to estimate the electronic band structure [36,39,40], and comparisons with experimental data are quite good. Optoelectronic properties of halide perovskites can be easily tuned by replacing cations and anions in the composition. A change in

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Figure 5.3 (A) Calculated (SOC-GW) band structure (VB and CB highlighted as thick solid lines) for MAPbI3 (blue lines). (B) Absorption coefficient of (MA)Pb(BrxI1-x)3 measured by diffuse spectral reflection and transmission measurements on thin films and photocurrent spectroscopy of solar cells. SOC, spineorbit coupling; VB, valence band; CB, conduction band. (A) Reprinted from P. Umari, E. Mosconi, F. De Angelis, Relativistic GW calculations on CH3NH3PbI3 and CH3NH3SnI3 perovskites for solar cell applications, Sci. Rep. 4 (2015) 4467. https://doi.org/10.1038/srep04467. (B) Reprinted from E.T. Hoke, D.J. Slotcavage, E.R. Dohner, A.R. Bowring, H.I. Karunadasa, M.D. McGehee, Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics, Chem. Sci. (2015). https://doi.org/10.1039/c4sc03141e.

the anionic composition from Cl to Br (X-sites) tunes the Eg from 3.16 to 2.30 eV [40], and down to 1.55 eV with I anion [41], as shown for MAPbX3 perovskite in Fig. 5.3B. The optimal band gap of perovskite for terrestrial single-junction solar cell approximately lies in the range between 1.2 and 1.62 eV, as it was shown for best-performing devices [42]. The classical calculations of ShockleyeQueisser limit [46,47] show that the maximum solar cell efficiency of 33.4% can be achieved with a 1.34-eV band gap semiconductor, and organometallic halide perovskite allows to fit this value with appropriate chemical composition. Eg can be also tuned by varying the A and B site cations. To narrow the band gap and expand the absorption spectrum into the infrared region, the cation B (typically Pb) can be gradually replaced by tin (up to 1.2 eV) [45]. The tuning of A site cation also can shift the Eg width [46] in comparison to standard MAPbI3 crystal. Replacing MA with FA cation decreases Eg to 1.4 eV [47], while CsPbI3 has an Egw1.70 eV [48]. So, all compositional varieties of the cations and anions can effectively change the optoelectronic properties for different applications of the solar cells as single-junction devices, tandems, and detectors for shortwave optical spectrum of light.

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Figure 5.4 Effective absorption coefficient of a CH3NH3PbI3 perovskite thin film compared with other typical photovoltaic materials, including amorphous silicon (aSi), GaAs, CIGS, CdTe, and crystalline silicon (c-Si), all measured at room temperature. €per, B. Niesen, M. Ledinsky, F.J. Reprinted from S. De Wolf, J. Holovsky, S.J. Moon, P. Lo Haug, J.H. Yum, C. Ballif, Organometallic halide perovskites: sharp optical absorption edge and its relation to photovoltaic performance, J. Phys. Chem. Lett. (2014). https://doi.org/10. 1021/jz500279b.

Halide metalorganic perovskites (MAPbI3, for example) are characterized by a sharp onset and abrupt edge in the absorption coefficient above the band gap (Fig. 5.4) [49], typical of direct band gap materials. Lead halide perovskite crystal, due to the heavy atoms present in the structure, has an interesting characteristic caused by strong spineorbit coupling (SOC) of lead and iodine that provides the splitting of electron states. In detail, splitting occurs for electrons with spins directed up and down along the axis of the wave vectors, and this process is called Rashba effect [50]. Such state in the electronic structure appears in the organometallic perovskites that have no inversion and center of symmetry. SOC creates an effective magnetic field that influences the electron states, removes the degeneration of spins, and splits the edges of valence and conduction bands, that finally result in a slight split of the band gap from direct to nondirect with DEw50 meV [51] (as shown in Fig. 5.5). As it was showed in several works [54e58], Rashba splitting in halide perovskite significantly reduces the rates of radiative recombination (for more than 35%), ensures a high density of states, enhances the optical absorption, and increases the charge carrier lifetimes.

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αC =

εC≠ 2ΔkC

ε

ε+C

ΔkC

ε+

εC– k

ΔkV

V

εC≠

V

ε–

ε≠ V

αV =

εV≠ 2ΔkV

Figure 5.5 Schematic representation of Rashba splitting for parabolic bands, along with relevant interaction parameters, see text for definitions. The valence and conduction bands (VB and CB) in the presence (absence) of Rashba splitting are the solid red and blue (dashed) lines, respectively. Reprinted from E. Mosconi, T. Etienne, F. De Angelis, Rashba band splitting in organohalide lead perovskites: bulk and surface effects, J. Phys. Chem. Lett. (2017). https://doi.org/10.1021/acs.jpclett.7b00328.

5.2.3 Crystal instability While PSCs already have an advantage in efficiency competition with CIGS and CdTe solar cells, stability remains a main problem that delays PSC commercialization. In the list of degradation factors of PSCs, crystal instability is a major obstacle for the long-term device operation. Intrinsic instability is mainly caused by photo and thermal factors that induce chemical and structural degradation. Both degradation processes cause decomposition of ABX3 crystal to organic cations, gaseous forms of anions, BX2 salts, and restored pure element B. Several investigations for the MAPbI3 films showed significant phase and composition changes at temperatures higher than 50 C. As it is shown in the work of Juarez-Perez et al. [57], photo stress of sun illumination and thermal stress in 40e80 C induce three degradation mechanisms: (1) organic cation decomposition: CH3NH2 þ HI (reversible path); (2) formation of gaseous products: NH3 þ CH3I (the irreversible or detrimental path); (3) a reversible cationeanion decomposition: Pb(0) þ I2(g). Reversibility of processes and self-healing of MAPbI3 films after thermal and photo stressing can be obtained in dark conditions and low temperatures [58], but nevertheless decomposition and recrystallization processes left light-activated metastable trap states which accumulate from cycle to cycle, and finally significantly decrease PV performance with nonradiative

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recombination effects. Sequential transition from tetragonal MAPbI3 to trigonal PbI2 under illumination was described in the work of Yang et al. [59], where authors indicated the increase of signal from PbI2 component in the electron diffraction patterns during analysis of thermos-kinetics processes with time. Several approaches have been proposed for the stabilization of MAPbI3 device operation [60], like solar cell encapsulation (to prevent leakage of volatile decomposition products) and selection of chemically stable transport layers, that must be chemically inert with acidic molecules (HI), methylation reagents (CH3I), oxidizing agents (I2), and weak bases (CH3NH2, NH3). Nevertheless, MAPbI3 absorbers do not show longterm stability, due to cycling transition processes which spoil the morphology and increase the level of bulk recombination on defectdtrap centers. For this reason, cation engineering should be applied to increase the tolerance factor by using larger cation size and to increase the temperature of transition to cubic phase, to prevent unfavorable thermal decomposition processes at the typical temperatures of the solar cell operation (w350 K). FAþ cation with ionic radius of 2.79 Å (vs. 2.70 Å for MAþ) provides higher temperature of black phase formation w420 K, but, at the same time, single-cation FAPbI3 is not completely stable at room temperature, due to the formation of both hexagonal d-phase (“yellow phase”) and photoactive perovskite a-phase [61]. Inorganic CsPbI3 also forms yellow orthorhombic phase at room temperature and becomes stable only near w300 C [62]. So, it is possible to conclude that monocation halide perovskites cannot perform stable operation because of low-temperature phase transitions. Thus, precision mixing of the cations and halides should result in perovskite thermal and structural stabilization. A big step forward for the reduction of perovskite instability was done with cesium-containing triple cation engineering [63]. Authors showed that double MA/FA compositions still have intrinsic thermal instability that tends to form the less crystalline films. Incorporation of inorganic cesium with small ionic radius (1.81 Å) to A-site suppresses transition processes to yellow phase and improves morphology. A composition of Cs5(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 was found to be optimal for the performance and stability, with achievement of PCE up to 21.1% and outputs of 18%, even after 250 h of power point tracking (Fig. 5.6). Another approach for the crystal stabilization was realized by Grancini and coworkers [64] with 2De3D modification of perovskite submicron grain structure using large AVAI (HOOC(CH2)4NH3I) cation (Fig. 5.7) that results in bottom-up phase-segregated structures with combination of

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Figure 5.6 Improvement in the long-term device stability with Cs cation incorporation. PCE, power conversion efficiency. Reprinted from M. Saliba, T. Matsui, J.-Y. Seo, K. Domanski, J.-P. Correa-Baena, M.K. Nazeeruddin, S.M. Zakeeruddin, W. Tress, A. Abate, A. €tzel, Cesium-containing triple cation perovskite solar cells: improved staHagfeldt, M. Gra bility, reproducibility and high efficiency, Energy Environ. Sci. 9 (2016) 1989e1997. https:// doi.org/10.1039/C5EE03874J.

Figure 5.7 (A) Local density of state of the 3D/2D interface. (B) Interface structure with the 2D phase contacting the electron transport layer. Reprinted from I. Zimmermann, E. Mosconi, X. Lee, D. Martineau, S. Narbey, F. Oswald, G. Grancini, C. Rolda, One-Year stable perovskite solar cells by 2D/3D interface engineering, Nat. Commun. (2017) 1e8. https:// doi.org/10.1038/ncomms15684.

the 2D layer acting as a protective buffer against moisture, preserving the 3D perovskite of phase transition caused by thermal stress. The use of advanced mesoscopic structure with ZrO2 scaffold and stable carbon cathodes resulted in the most stable configuration of the perovskite device to date, with >10,000 h of stable efficiency under light soaking.

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5.2.4 Charge transport Charge transport is one of the most critical topics for metal halide organometallic perovskites. The core of solar cells functionality lies in the electronic structure and charge carrier conditions. As it was noted before, halide perovskites are direct semiconductors; moreover, tetragonal and “black” cubic phases both have similar positions for valence band maximum and conductive band minimum in the Brillouin zone, so the optical transitions of lightexcited charge carriers will occur without phonon interactions [65]. For MAPbI3 perovskite, the generated electronehole (eeh) couples has binding energy (Eb) around 0.055 eV [66], thus charge splitting does not require high-electric fields across the absorber. As it was found in the literature, basic charge transport characteristics of MAPbI3 microcrystals [65] have a diffusion length (LD) up to 1 mm, a carrier lifetime up to 1 ms [67], and a charge carrier mobility (m) up to 101 cm2/(V*s) [67]. It is important to point out that, differently to classic semiconductors, the listed charge transport parameters approximately apply to both electrons and holes. The origin of such behavior is mainly related to the electronic structures of conduction and valence band. The electronic structure of the metaleorganic halide perovskite presents similar curvature for the bottom of the conduction band and the top of the valence band, consequently the electron and hole effective masses are equal, m*e ¼ m*hw(0.1e0.15)m0 [68, 71], where m0 is a free electron mass. Such values for MAPbI3 polycrystalline films are comparable with conventional inorganic semiconductors for optoelectronic application such as GaAs, InP, etc.

5.2.5 Bulk recombination There are two main types of recombination occurring in solar cells: radiative and nonradiative. In case of PSCs, it was shown [69] that radiative recombination of free electrons and holes in MAPbI3 cannot give a serious negative impact for solar cells operation because of low bimolecular charge recombination constants, high mobilities (>10 cm2/(V*s), and large diffusion lengths (up to 1 mm). On the other hand, nonradiative, trap-assisted recombination (NRR) is a major mechanism for losses in PSCs [70]. Trap centers in perovskite thin films with submicron crystallinity are mainly concentrated at grain boundaries in the bulk due to favorable energy states and at the interface between perovskite and transport materials (Fig. 5.8A and B). In general, intrinsic defects of the halide perovskite are represented by vacancies of anions, organic and metal cations; interstitials, antisite

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(A)

(B) FTO

Ec G

Re

TiO2

Steady-State Model

Er Rh

Ef

Electron Hole Trap

Case 2 Case 1 Case 3

CH3NH3Pbl3 spiro-OMeTAD

Ev

Au

X

Figure 5.8 (A) Schematic energy level diagram of a single-effective recombination center model, representing the carrier generation by illumination and the processes of electron and hole capture in the recombination centers. (B) Device structure of perovskite solar with illustrated recombination processes: Case 1: bulk recombination at grain boundaries, denoted as “Bulk”; Case 2: electron transport layer/perovskite interfacial recombination at its interface, denoted as “Top”; Case 3: perovskite/hole transport layer interfacial recombination at its interface, denoted as “Bottom.” (A) Reprinted from I. Levine, S. Gupta, T.M. Brenner, D. Azulay, O. Millo, G. Hodes, D. Cahen, I. Balberg, Mobilityelifetime products in MAPbI 3 films, J. Phys. Chem. Lett. 7 (2016) 5219e5226. https://doi.org/10.1021/acs.jpclett.6b02287. (B) Reprinted from L.X. Shi, Z.S. Wang, Z. Huang, W.E.I. Sha, H. Wang, Z. Zhou, The effects of interfacial recombination and injection barrier on the electrical characteristics of perovskite solar cells, AIP Adv. (2018). https://doi. org/10.1063/1.5021293.

occupations, that give a main contribution to NRR, for example, vacancies of the anion sites in MAPbI3, have relatively low energy of activation in the range of 0.08e0.90 eV and can be electrically mobile [75,76]. Measurements of temperature-dependent capacitance frequency for MAPbI3 films demonstrated that shallow traps are present with an energy level of around 0.20 eV from the conduction band, and deep traps of w0.50e1.1 eV can be obtained with concentrations of 1015ee1016cm3 [78e80]. Actual efforts for grain boundaries passivation are based on advanced methods of crystallization as well as incorporation of special additives for morphology modification. Niu and coauthors [76] showed that addition of DMSO to perovskite solution improves the coordination of Pb2þ during nucleation and passivate antisite PbeI states via more heterogeneous growth of perovskite frameworks. As a result, PCE was improved from 16.5% to 18.5%. Ye et al. [77] proposed the incorporation of Cu(thiourea) I to the MAPbI3 absorber for coordination of metal cations and halide anions with a passivation agent at grain surface. Such intergrain passivation improved transient PL spectra with delayed quenching, provided increased Voc for the devices from 1.00 to 1.11 V, and boosted the efficiency from 8.5% to 18.6%.

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Lee and coworkers presented a concept of 2D/3D crystal structure formation with the addition of phenethylammonium cation (PEAþ) [78]. A small amount of additive (<10% by volume) provided passivation of grain boundaries that increased carrier lifetimes and opened circuit voltages of the devices. Such approach, with incorporation of the large cation into perovskite molecules to form the quasi 2D perovskite films among the grains of 3D perovskites, was realized in many reports and also resulted in significant structural stabilization of organometallic perovskites [84e86].

5.2.6 Ferroelectric properties The ferroelectric properties of inorganic perovskites are known from early works [87,88]. The distortions in the BX6 octahedral positions result in appearance of the electric dipole with A-site cation and increase molecular response for the polarization. Concerning halide perovskites with organic cations, large values of dielectric constants were reported of the order of 101 [84] to 103 (under illumination) [85], attesting the possibility of cellstructure orientation under an electric field. The strict explanation of polarization effects in organometallic perovskites is still debated, and some reports on antiferroelectricity [86], ferroelectricity [87], and ferroelasticity [88] have been presented. Interesting effects were showed in theoretical papers of Liu et al. [89] and Pecchia [90], where authors demonstrated that in presence of polarization effect, ferroelectric domains in MAPbI3 films (Fig. 5.9) tend to reduce the recombination between domain boundaries (relatively to not polarized grains). Accordingly to this proposal, Sherkar et al. [91] studied the influence of ferroelectricity in MAPbI3 perovskite film on the device performance by using a 3D diffusion model. Authors proved that the depolarizing field induced by bound charges can effectively separate holes and electrons in the bulk of the semiconductor, with accumulation of charge carriers at domain boundaries. An improvement in the device can be achieved only if ordered domains are perpendicular to the electrodes and have polarization in the plane of the device. This case effect resulted in higher JSC and FF, as electrons and holes cannot be screened on domain boundaries during their transport. In the opposite case, if polarization goes along the thickness of the device, electrons and holes will be accumulated at the domain boundaries and finally recombine. Further investigation performed by Rossi and coworkers [92] included combination of experimental piezo-force response measurements (PFM) with drift diffusion simulations.

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Figure 5.9 Typical piezo-force response measurement (PFM) view of ferroelectric do€hm, T. Leonhard, M.J. Hoffmann, A. Colsmann, mains in MAPbI3. Reprinted from H. Ro Ferroelectric domains in methylammonium lead iodide perovskite thin-films, Energy Environ. Sci. (2017). https://doi.org/10.1039/c7ee00420f.

Based on PFM measurements, calculations were performed accounting for the orientation of polarization vectors in the domains. A good fitting of experiment JV characteristics has been shown based on the presence of ferroelectricity. Finally, it was concluded that ordered ferroelectric domains enhance the device performance due to decreased rate of SRH recombination losses via the formation of highly conductive channels improving carrier separation and transport.

5.3 Perovskite crystallization According to in situ X-ray scattering studies, the nucleation and crystallization activation energy of a perovskite (56.6e97.3 kJ mol1) is much lower than that of amorphous silicon (280e470 kJ mol1) [99,100]. This low-crystallization energy barrier allows the perovskite films to be easily prepared by a variety of low-temperature large-scale fabrication processes including blade coating, ink-jet printing, and roll-to-roll print, which set up a bridge between industrial applications and academic research [19,101,102]. The perovskite crystallization involves a classical nucleation/growth mechanism which includes three stages: the solution reaches supersaturation, nucleation, and subsequent growth toward a large crystal, which, depending

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Figure 5.10 (A) Schematic showing nucleation and crystal growth processes. (B) Relations of the nucleation and growth rates with solute concentration. Reprinted from F. Huang, M. Li, P. Siffalovic, G. Cao, J. Tian, From scalable solution fabrication of perovskite films towards commercialization of solar cells, Energy Environ. Sci. 12 (2019) 518e549. https://doi.org/10.1039/C8EE03025A.

on the deposition method, could also pass through stable or metastable intermediate phases [6,103e105]. The LaMer model in Fig. 5.10A schematically illustrates the nucleation and subsequent growth processes of the crystals. A supersaturated solution is a prerequisite for nucleation, when the precursor solution is dropped on the substrate and rapid surface evaporation of the solvent is happening, the concentration of solute increases, and the solution quickly reaches saturation (Cs). Because of a critical energy barrier, the nucleation process cannot happen at the saturation concentration (Cs).Onlywhen the solvent continues to evaporate, a supersaturated solunu exists, with a Gibbs free energy higher than the surface energy of tion Cmin the newly formed nuclei. At this point, the second stage, i.e., the nucleation process, starts and the ions, atoms, or molecules in the solution form a new phase as nuclei or embryos. The nucleation rate increases with increased supersaturation, so a higher supersaturation leads to a higher nucleation rate and density (more nuclei) with a larger number of smaller crystals. Once the nuclei are formed, subsequent crystal growth proceeds immediately. With continuous consumption of the solute for formation of nuclei, the nucleation process terminates when the solution concentration is lower nu . The crystal will continue to grow until the concentration of than Cmin growth species drops below Cs. Once the nuclei are formed (Fig. 5.10B), the subsequent growth occurs rapidly and simultaneously. Subsequent

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growth contains two sequential and parallel steps: (1) the growth species diffuse to the growth surface and (2) the surface growth, i.e., the growth species are irreversibly incorporated to the kink, ledge-kink, and ledge sites of the crystal’s growth surface. For a given solution concentration, high nucleation density leads to more nuclei. The final grain size depends on the nucleation density and subsequent solute supplement. In classical nucleation theory, the nucleation rate (N) is affected by the nucleation factor (P) and the probability of atomic diffusion (G), so it follows from this equation [98]:     C0 KT DG * N ¼ PG ¼ exp kT 3pl3 h where C0 is the initial solution concentration, l is the nucleus diameter, h is the solution viscosity, and DG* is the critical energy barrier. The equation illustrates that a high initial concentration or supersaturation helps to form more nuclei, while low viscosity or a critical energy barrier facilitates the diffusion of atoms/ions to the liquidesolid interface and then incorporation to the solid surface. In order to improve the coverage of high-quality perovskite films, many efforts have been done to control the nucleation parameters (e.g., volatility and solution viscosity (h)) [6,106e108]. So far, many processes have been exploited to deposit a perovskite thin film, which could be simply divided into two kinds, one-step and two-step methods.

5.3.1 One-step perovskite formation The formation process of perovskite crystals in a one-step deposition method involves the crystallization of the perovskite phase during a single deposition. In this method, a fast nucleation occurs from the precursor solution, without formation of any stable intermediate phase or with a fast passing through a metastable intermediate. For the fabrication of perovskite films, dimethylformamide (DMF), gbutyrolactone (GBL), N-methyl pyrrolidone (NMP), dimethyl sulfoxide (DMSO), and dimethylacetamide (DMAc) are the most commonly used solvents [103e106]. However, all these solvents have a rather low vapor pressure at room temperature and high boiling point. In one-step deposition, slow evaporation of the solvent limits the nucleation rate which consequently leads to low nucleation density and fast crystal growth. When the Pb/DMF precursor solution (MAI:PbI2 ¼ 1:1, molar ratio) is spin-coated

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(A)

(B)

Figure 5.11 SEM images of perovskite films prepared by one-step deposition methods. (A) Conventional spin coating. (B) FDC (antisolvent method). FDC, fast crystallization deposition. Reprinted from M. Xiao, F. Huang, W. Huang, Y. Dkhissi, Y. Zhu, J. Etheridge, A. Gray-Weale, U. Bach, Y.-B. Cheng, L. Spiccia, A fast deposition-crystallization procedure for highly efficient lead iodide perovskite thin-film solar cells, Angew. Chem. Int. Ed. 53 (2014) 9898e9903. https://doi.org/10.1002/anie.201405334.

onto the substrate at room temperature, a unique dendritic structure of MAPbI3 film will be immediately formed due to low nucleation density and fast growth rate [107]. As shown in Fig. 5.11A, the branchlike crystal images display a low surface coverage. To solve these problems, some essential tricks and approaches have been developed which led to rapid improvement of PV performance. 5.3.1.1 Antisolvent-induced and solvent engineering In 2014, Xiao et al. reported a one-step, antisolvent-induced, fast crystallization deposition method [107]. This method involved spin coating of the precursor solution followed by immediate exposure to chlorobenzene as antisolvent. The antisolvent extracted the initial solvent quickly, resulting in a high degree of supersaturation, leading to the generation of more crystal nuclei. Dense and uniform perovskite films could be readily prepared by this method, as presented in Fig. 5.11(b). In addition, Jeon et al. used a mixed solvent of GBL and DMSO followed by toluene drop-casting, nominated as “solvent engineering,” showing extremely uniform and dense perovskite layers via a CH3NH3IePbI2eDMSO metastable intermediate phase, and enabling the fabrication of solar cells with a certified power-conversion efficiency of 16.2% without hysteresis [105]. Although these processes facilitate crystallization and generate a better film morphology, the quality of perovskite films and cells performance still varies appreciably with the additional time and amount of antisolvent, which makes this method difficult to control and hard to scale up.

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Figure 5.12 Schematic illustration of the hot-casting process. Reprinted from W. Nie, H. Tsai, R. Asadpour, J.-C. Blancon, A.J. Neukirch, G. Gupta, J.J. Crochet, M. Chhowalla, S. Tretiak, M.A. Alam, H.-L. Wang, A.D. Mohite, Solar cells. High-efficiency solution-processed perovskite solar cells with millimeter-scale grains., Science. 347 (2015) 522e525. https://doi. org/10.1126/science.aaa0472.

5.3.1.2 Hot-casting In 2015, Nie et al. applied a hot-casting method to rapidly evaporate the high-boiling point solvents (NMP, B.P. 202 C, or DMF, B.P. 150 C), where pinhole free and uniform perovskite films with crystalline grain sizes in the millimeter-scale were obtained. Fig. 5.12 shows a schematic illustration of the hot-casting method [101]. The grain size could be controlled by tuning the substrate temperature. The grain size is markedly increased by increasing the substrate temperature. By increasing the grain size from w1 to w180 mm, the overall PCE is enhanced from 1% to 18% due to a reduced number of bulk defects. The authors attributed the formation of large grains to a prolonged growth of the perovskite crystals. An excess solution present on the substrate kept supplying enough growth species for continuous crystal growth in an appropriate temperature region. 5.3.1.3 Vacuum pumping A vacuum flasheassisted solution process (VASP) was also developed to enable rapid removal of solvent to generate a burst of supersaturation for fast crystallization of the FA0.81MA0.15PbI2.51Br0.45 perovskite [108]. As illustrated in Fig. 5.13, the substrate was first spin-coated with perovskite precursor solution and then transferred to a vacuum chamber to allow rapid evaporation of the solvent. Finally, the films were annealed at 100  C owing to complete crystal growth. As confirmed by the UV-Vis spectra and SEM images, with the presence of DMSO, as Lewis acidebase adduct, this method allows the deposition of large grainesized perovskite films with uniform coverage on the substrate compared with the conventional process. A maximum PCE of 20.5% and a certified PCE of 19.6% were achieved for solar cells with a large square aperture area of 41 cm2.

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Figure 5.13 Schematic illustration of the deposition of a perovskite film via VASP. CP, conventional process; FF, fill factor; PCE, power conversion efficiency; PSC, perovskite solar cell; VASP, vacuum flasheassisted solution process. Reprinted from X. Li, D. Bi, C. €tzel, A vacuum flash-assisted Yi, J.-D. Décoppet, J. Luo, S.M. Zakeeruddin, A. Hagfeldt, M. Gra solution process for high-efficiency large-area perovskite solar cells., Science. 353 (2016) 58e62. https://doi.org/10.1126/science.aaf8060.

5.3.1.4 Gas flow Recently, Ding et al. developed a gas floweinduced gas pump approach to regulate nucleation and grain growth of perovskite crystals, and obtained uniform, dense, and full-coverage large area perovskite films [109]. Using this approach, normal planar PSCs were fabricated at pressures of 100, 500, and 1500 Pa, achieving average efficiencies of 19.25  0.50%, 19.17  0.46%, and 18.98  0.51%, respectively, for 0.1 cm2 devices with high reproducibility. Although large grainesized perovskite films were obtained by effective control of the supersaturated solution, the formation mechanism of perovskite crystals from the precursor solution seems more complex, as demonstrated by Pascoe’s work [97]. They reported a nitrogen gas floweassisted method with postannealing for formation of a perovskite layer. Through precise control of the gas flow rate, the grain size of the produced perovskite films reached up to about 100 nm. However, their large grains were constituted of many smaller grains instead of being single monocrystals. Still, there are many factors that can affect the film deposition process, e.g., the nitrogen gas flow might have generated prolonged supersaturation on the solution surface, resulting in secondary or continued nucleation, and quick agglomeration of primary crystals may stop further growth with the depletion of growth species.

5.3.2 Two-step perovskite formation For the two-step method (see Fig. 5.14), PbX2 layer is deposited first, followed by the conversion to perovskite in organic cation/halide solution

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Figure 5.14 A sketch illustrating two-step perovskite deposition.

[6,116,117]. Since the deposition processes and the control strategy of PbX2 layer are versatile and flexible, uniform and full coverage could be easily obtained, which tends to improve the quality of the perovskite film. The first successful work on the two-step method for PSCs was reported by Burschka et al. in 2013, who obtained a record PCE of 15% at that time [110]. Soon after, a tremendous effort has been conducted on this method, due to its high operability and good performance reproducibility. Most importantly, the great progress on the reaction/growth mechanism resulted in modified two-step processes, which well improved perovskite quality and enhanced device performance. Accordingly, two formation mechanisms of the MAPbI3 perovskite have been proposed: one is a direct solideliquid interfacial reaction at low methyl ammonium iodide (MAI) concentration and the other is a dissolutioncrystallization at high MAI concentration [116,118]. Fu et al. further studied these two formation mechanisms in two-step deposition method [113]. As shown in Fig. 5.15, when the MAI concentration is less than 8 mgmL1, an in situ transformation (interfacial reaction) proceeds within 2 min. Whereas, if the MA concentration is higher than 10 mgmL1, MAPbI3 perovskite crystals form immediately through a solideliquid interfacial reaction. A further reaction of MAI with underlying PbI2 is suppressed, leading to an incomplete reaction.

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Figure 5.15 Schematic illustration of the reaction process from the precursor to monolithic perovskite grains. Reprinted from C. Fei, B. Li, R. Zhang, H. Fu, J. Tian, G. Cao, Highly efficient and stable perovskite solar cells based on monolithically grained CH 3 NH 3 PbI 3 film, Adv. Energy Mater. 7 (2017) 1602017. https://doi.org/10.1002/aenm.201602017.

5.3.2.1 Crystal Engineering approach Recently, Yaghoobi Nia et al. introduced the crystal engineering (CE) method for deposition of pure cubic-phase thin film of MAPbI3 at the surface of a mesoporous TiO2 layer under ambient atmospheric condition [13]. Using CE approach in the general two-step spin coating deposition, they realized high-efficiency PSC and perovskite solar modules (PSMs) using several hole transport layers (HTLs). This CE approach was divided in three main steps (see Fig. 5.16): (1) Preactivation of mp-TiO2 surface to increase the active nucleation sites. (2) Slow evaporation, a well-known technique for single-crystal growth, was performed first by spin coating of a supersaturated PbI2 solution in DMF and then using a well-defined annealing program. (3) Solvent-assisted SN1 perovskite formation was achieved by dipping the PbI2.DMF intermediate layer in the diluted solution of MAI in 2-propanol, to form a pure cubic structure of MAPbI3 perovskite. Considering the SN1 mechanism of the perovskite formation, breaking of the PbeO bonding between Pb and DMF is in the rate determination step (RDS) of the reaction kinetic. In fact, due to the low kinetic constant for dissociation of a ligand in the octahedral symmetry and to high-energy 5coordinate intermediate state, this reaction state is the lowest kinetical step in

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Figure 5.16 (A) Solvent-assisted SN1 ligand exchange reaction mechanism. The labeled terms in the dipping time can introduce as (1) dissociation of dimethylformamide (DMF) ligand from octahedral and formation of a square pyramid structure (rate determinate step of the reaction), (2) rearrangement of the square pyramid to a triangular bipyramid structure, and (3) association of methyl ammonium iodide (MAI) with Ie in axial position of Pb coordination and MAþ in octahedral surrounding sites of perovskite structure using hydrogen bonding. (B) A schematic which represents the procedure of CE approach for fabrication of pure cubic MAPbI3 perovskite layer in sequential spin , F. Matcoating deposition. Reprinted from N. Yaghoobi Nia, M. Zendehdel, L. Cina teocci, A. Di Carlo, A crystal engineering approach for scalable perovskite solar cells and module fabrication: a full out of glove box procedure, J. Mater. Chem. A. 6 (2018) 659e671. https://doi.org/10.1039/C7TA08038G.

the SN1 mechanism and could be classified as the RDS. The weak bonding of PbeO could be easily broken and increases the rate of perovskite nucleation. Furthermore, DMF molecules, as a polar solvent, could facilitate the reaction’s rate by remaining in the coordination sphere and stabilizing the triangular bipyramid structure of SN1 intermediate structure. The PV results of this work show that the CE approach remarkably improved the device performance, achieving a PCE of 17%, 16.8%, and 7% for SpiroOMeTAD, poly (3-hexylthiophene) (P3HT) and HTL free (direct contact of the perovskite to the gold), respectively, with ultrahigh reproducibility. Furthermore, PSMs (active area of 10.1 cm2) could reach an overall efficiency of 13% and 12.1% by using Spiro-OMeTAD and P3HT as HTLs, respectively. Sealed modules showed promising results in terms of stability, maintaining 70% of the initial efficiency after 350 h of light soaking at maximum power point [13].

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5.3.2.2 Vapor-assisted process Perovskite films can also be prepared by the thermal evaporation method. Co-evaporation of MAI and PbCl2 leads to the formation of a MAPbI3 phase [114]. Since vacuum deposition uses two chemical sources, it can be regarded as a two-step coating process. Thus, MAI can be also deposited onto the PbI2 film by evaporation. In particular, a vapor-assisted solution process was proposed to grow perovskite films via the in situ reaction of an as-deposited film of PbI2 with MAI [115]. The perovskite film prepared by this method presented a well-defined grain structure, with grain size up to the microscale, small surface roughness, and full surface coverage. A sequential two-step vacuum deposition method (see Fig. 5.17) was applied to prepare planar structure PSCs, where MAI was evaporated after PbCl2 evaporation onto an indium-doped tin oxide/poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate) (ITO/PEDOT:PSS) substrate. In this process, the substrate was heated while depositing MAI vapor, and it was found that the substrate temperature was critical for the PV performance. Indeed, the PV performance was better when the substrate was heated to 75 C compared to a substrate temperature of 65 C or 85 C.

Perovskite Substrate heating

ITO/PEDOT:PSS

PbCI2deposition

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Ca/Ag C60/Bphen Perovskite

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Figure 5.17 Sequential two-step vacuum deposition using PbCl2 and methyl ammonium iodide (MAI) to fabricate a planar-structured perovskite solar cell. MAI was evaporated onto a pre-evaporated PbCl2 film while the substrate was heated. Reprinted from Q. Chen, H. Zhou, Z. Hong, S. Luo, H.-S. Duan, H.-H. Wang, Y. Liu, G. Li, Y. Yang, Planar heterojunction perovskite solar cells via vapor-assisted solution process, J. Am. Chem. Soc. 136 (2014) 622e625. https://doi.org/10.1021/ja411509g.

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5.3.3 Summary of crystallization properties Owing to the low activation energy of perovskites, nucleation and subsequent crystal growth are readily occurring at room temperature during solution processes. Such a low activation energy also tends to a rapid nucleation and crystal growth process, which allows significant influences from subtle changes of other processing parameters, including pressure, temperature, additives, solvents, impurities, and types of precursors, which at least partially explain the fact that PCE varies widely from one research group to another, even following the same synthesis procedure and parameters. Extensive research with lab-scale PSCs has revealed the fact that PCE, hysteresis, and stability are widely dependent on the quality of perovskite crystals, including the crystallinity or the degree of perfection, morphology, and interfaces. Such ample knowledge and experience have laid a solid foundation for further exploration and development of large-scale solution manufacturing and commercialization. It is imperative to develop scalable fabrication technologies capable of retaining the ability of nucleation, subsequently crystal growth processes, and control of the environmental parameters. Solution fabrication processes directly inherit many advantages and similarities from lab-scale fabrication processes. Some generic considerations and requirements could be mentioned, as: (1) Heterogeneous nucleation at the interface, preferably at the solid solution, with subsequent oriented growth across the perovskite films, i.e., preventing homogeneous and secondary nucleation, to obtain a ‘‘single crystal state’’ across the layer. (2) Forcing to low nucleation density so that each crystallite grows to several or tens of micrometers in lateral dimension, resulting in a small number of grain boundaries. (3) Keeping the crystal growth to proceed in a near thermodynamic equilibrium condition, so to reach very high crystal perfection with minimal defects or impurities. (4) Developing an effective annealing process to convert a polycrystalline film to a single crystal layer or to improve crystallinity and remove defects from the perovskite film. (5) Developing reliable equipment and precursor solutions to realize the scalable fabrication. (6) Developing a fabrication process which is less sensitive to the environmental situations such as temperature, vapor pressure, and solvent, impurities/additives.

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(7) Improving interfacial and surface properties. It is clear that PSC performance is strongly affected by the interface and surface properties of the perovskite layer in contact with electron transport layer (ETL) and HTL, which required some interfacial modifications to achieve a desired surface/interface chemistry, coherency, and passivation.

5.4 Device architectures 5.4.1 Evolution of the PSC architectures The first device architecture using metaleorganic halide perovskite MAPbX3 was developed using the dye-sensitized solar cell (DSSC) configuration, as reported in the famous pioneer work of Kojima et al. [116] in 2009. In this paper, authors presented the novel concept of a perovskite sensitizer for the mesoporous TiO2. Despite the fact that device lifetime was just few minutes (the electrolyte dissolved perovskite quantum dots), the authors achieved a PCE of 3.8%. Although such efficiencies were not competitive even for DSSC applications, a new era for PV was open. To solve the problem of perovskite dissolution, liquid electrolyte was replaced with solid state organic HTL. Kim and coworkers [117] introduced organic small molecules like Spiro, OMETAD (2,20 ,7,70 -Tetrakis[N,Ndi(4-methoxyphenyl)amino]-9,90 -spirobifluorene), HTL, and increased PCE of mesoscopic device with optimized thickness of the TiO2 layer up to 9.7% in 2012, but still perovskite material was used only as a sensitizer. The next important milestone was achieved with the development of meso-superstructured solar cells (MSSCs) (see Fig. 5.18). In 2012, Snaith and coworkers [118] presented a novel device architecture replacing the TiO2 ETL with insulating Al2O3. They demonstrated that in such device structure, MAPbI3 perovskite can transport both electron and holes without efficiency losses (10.9% was achieved with Al2O3 scaffold) with respect to reference solar cell with TiO2. In the MSSC, the perovskite film was used as a semiconductor absorber, a significant difference with DSSC. Then, the device structure was optimized by using a sandwich-type mesoscopic configuration where the perovskite absorber was surrounded by an ETL and an HTL (Fig. 5.18A). Heo et al. [119] demonstrated threedimensional nanocomposite of mesoporous (mp)-TiO2 with MAPbI3 with submicron (600 nm) thickness and novel high-performing HTLPTAA. Burschka et al. demonstrated optimized crystallization procedure with sequential precursor deposition in mesoscopic device configuration and improved device efficiency up to 15.0%. Currently, the mesoscopic

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Figure 5.18 Schematic diagrams of perovskite solar cells in the (A) neiep mesoscopic, (B) neiep planar, (C) peien planar, and (D) peien mesoscopic structures. Reprinted from Z. Song, S.C. Watthage, A.B. Phillips, M.J. Heben, Pathways toward high-performance perovskite solar cells: review of recent advances in organo-metal halide perovskites for photovoltaic applications, J. Photonics Energy 6 (2016) 022001. https://doi.org/10.1117/1. JPE.6.022001.

device architecture became classical for the PSC, allowing to achieve PCE records for the device performance above 20% with this structure. In 2013, Snaith et al. [114] published a paper with the description of planar PSC with thermal evaporation method and demonstrated that perovskite absorbers can operate as high-performing heterostructure (PCE>15%) without the need for complex mesostructures (Fig. 5.18B). In order to follow the concept of organic PV devices with HTL on transparent electrode, Guo et al. [120] demonstrated the first peien oriented planar solar cells (Fig. 5.18C) with organic transport layers PEDOT:PSS (poly(3,4ethylenedioxythiophene) polystyrene sulfonate) and PCBM (Phenyl-C61butyric acid methyl ester). The unprecedented progress in the efficiency growth of PSCs has been achieved with a scientific supply of experience from the development or DSSC and OPV. Even so, halide perovskite rapidly became a separate class of semiconductor technology for PV, with their own specifics of device physics. The actual classification of PSC architectures can be divided in two main groups accordingly to the orientation of transport layers: “direct” structure with n-i-p orientation and “inverted” structure with p-i-n sequence. These groups can be further divided into mesoscopic and planar configurations (see Fig. 5.18). Efficiencies of described device architectures can exceed 20% PCE by using optimized perovskite crystallization methods, performing transport layers and passivation techniques [130e132]. So far, all the PSC records

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[124] have been achieved by using the direct mesoscopic structure, while planar configurations are slightly below: neiep planar structure recently demonstrated 21.6% [125] and inverted peien structure showed 20.91% [126]. PSC structures can be also realized in pen heterostructure configuration (FTO/TiO2/perovskite/metal) as presented by Etgar [127]; however, the performance of such architectures is not competitive with neiep or pe ien structures. The variety of transport layers including numerous quantities of inorganic and organic materials [137e140] is presented in Fig. 5.19 (see energy levels for typical transport layers). In principle, a charge transport layer should provide perfect level alignment with perovskite’s conduction and valence band for the collection of hþ and e, respectively (see schematic band diagram on Fig. 5.19). The absence of ohmic losses at heterojunctions will provide high values of Voc, since its value is related to the difference between quasi-Fermi levels (QFLs). For ideal conditions, in PSC the QFLs are constant across the perovskite film and the difference between them is the open circuit voltage (Fig. 5.20)

0

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Figure 5.19 Energy level diagrams with respect to vacuum (0 eV) for typical hole transport layer (HTL) and electron transport layer (ETL) with mono-cation perovskites. Reprinted from V. Zardetto, B.L. Williams, A. Perrotta, F. Di Giacomo, M.A. Verheijen, R. Andriessen, W.M.M. Kessels, M. Creatore, Atomic layer deposition for perovskite solar cells: research status, opportunities and challenges, Sustain. Energy Fuels. (2017). https://doi.org/ 10.1039/c6se00076b.

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Figure 5.20 Band diagram of an ideal solar cell under open circuit conditions, with the absorber.

[132]. As noted before, diffusion lengths in perovskite for both types of charges can exceed the thickness of the perovskite film. In this case, the open circuit voltage can be affected by nonradiative recombination in the bulk and at the interfaces with transport layers.

5.4.2 Tandem solar cells with perovskites The development of tandem solar cells attracts the interest of the scientific community as an effective approach to overcome the ShockeyeQueisser limit, formulated for ideal single cell with 1.34 eV band gap and maximum efficiency of 33.7% [133]. Multijunction solar cells with aligned spectral splitting of the subcells already reached high PCE 32.8% for two-junction and 37.9% for three-junction devices, as reported for AIIIBV semiconductors. Yet, unfortunately, manufacturing of large-scale AIIIBV devices is still an expensive and complex technology for mass production. Progress in the increase of the output performance of silicon and CIGS reached the saturation at 26% and 23% PCE without concentration of light [124]. The rapid growth of PSC efficiency demonstrated that this novel class of semiconductors has a big potential for tandem integration with commercially available Si and CIGS solar cells, due to a comparable level of performance and easy manufacture. Moreover, cost estimates and economic analyses performed for PSCs showed that the expected price for a 1-m2 module will be w30 US$, dominated by the cost of glass substrates and back cover (10 US$). The cost of absorber, transport materials, contacts, and TCOs does not exceed 5 US$ per m2, other part of the prices going for packing, lamination, etc. [144e146]. In this case, integration of PSCs with silicon solar cells that present cost of 70e80 US$/m2 will not significantly increase the cost of fabrication [136].

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Figure 5.21 Schematic illustration of spectral splitting in tandem solar cells with decrease of photons energy absorbed in bottom subcells. Reprinted from S. Kurtz, J. Geisz, Multijunction solar cells for conversion of concentrated sunlight to electricity, Optic Express 18 (2010) A73. https://doi.org/10.1364/OE.18.000A73.

Opposite to single-junction solar cells, tandem configuration allows to minimize thermalization losses with light absorption via spectral splitting in subcells with different Eg. In general, tandem architecture has a stack of subcells with large band gap on the top and narrow band gap on the bottom. The top cell is transparent to low-energy photons that go through the device and are absorbed in the bottom cell (see Fig. 5.21). Tandem devices can be realized in two-terminal (2T) and fourterminal (4T) configuration (Fig. 5.22). The 2T configuration requires a series connection between the cells and consequently a current matching between them. On the opposite, 4T tandem does not require current matching, but introduces an increased level of parasitic absorption and reflection at the electrodes between the subcells and an increased PV system cost due to the request of independent maximum power point tracker. Simulations of the theoretical tandem efficiency considering (1) a 100% EQE above the band gap, (2) a reverse saturation current calculated assuming 100% radiative emission, and (3) 1.1 eV band gap for the bottom subcell (representing Si and CIGS SC) [136] showed that the highest PCE can be obtained with a top perovskite subcell with Eg ¼ 1.60e1.75 eV, resulting in a predicted performance of 44%.

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(A)

(B) transparent electrode

Perovskite top cell high Eg recombination or tunnel junction

Silicon bottom cell low Eg back reflector

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Perovskite top cell high Eg transparent electrode

transparent electrode

Silicon bottom cell low Eg back reflector

Figure 5.22 Device schematics of two-terminal (A) and four-terminal tandems (B) on the example of perovskiteeSi structures. Reprinted from J. Werner, B. Niesen, C. Ballif, Perovskite/Silicon Tandem Solar Cells: Marriage of Convenience or True Love Story?-An Overview, (2017). https://doi.org/10.1002/admi.201700731.

Present record for perovskite/Si 2T tandem shows a PCE ¼ 28.0%, achieved by Oxford Photovoltaics [133,149], while perovskite/CIGS tandem developed by IMEC (Solliance) gave PCE ¼ 24.60% [138], but the details about spectral matching and the compositions of the perovskite subcell were not disclosed. The most efficient perovskite/Si tandems were designed with multication perovskite compositions with 1.63e1.74 eV band gaps. Bush et al. [139] developed tandem Si heterojunction solar cell with double cation CsFAPbI3xBrx perovskite absorber with 1.63 eV band gap; the device structure was monolithically integrated and balanced at 18.5 mA/cm2 current density, and finally a 23.6% PCE was achieved with 1000-hour damp heat test at 85  C and 85% relative humidity (Fig. 5.23). Peng et al. [140] used triple cation Cs0.05(MA0.17FA0.83)0.95 Pb(I0.83Br0.17)3 with Eg ¼ 1.63 eV in 4T mechanically stacked tandem with two ITO electrodes in PSC structure with IBC silicon cell and obtained a 24.5% PCE. Duong et al. [141] developed four-cation RbCsMAFAPbI3xBrx composition of perovskite absorber with Eg ¼ 1.74 eV in same device stack and achieved 26.4% PCE via reduced optical losses in transport layers (with use of wide band gap TiOx-doped with In doping), improved transmittance in visible region and decreased contact resistance. In all cited papers, the efficiency of subcell was exceeded by 1%e3% when used in tandem structure with perovskite.

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Figure 5.23 Device schematic of monolithically integrated perovskite-Si tandem (A), balanced spectral splitting between Si and perovskite subcells. Reprinted from K.A. Bush, A.F. Palmstrom, Z.J. Yu, M. Boccard, R. Cheacharoen, J.P. Mailoa, D.P. McMeekin, R.L.Z. Hoye, C.D. Bailie, T. Leijtens, I.M. Peters, M.C. Minichetti, N. Rolston, R. Prasanna, S. Sofia, D. Harwood, W. Ma, F. Moghadam, H.J. Snaith, T. Buonassisi, Z.C. Holman, S.F. Bent, M.D. McGehee, 23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stability, Nat. Energy. 2 (2017) 17009. https://doi.org/10.1038/nenergy.2017.9.

For high-efficiency CIGS perovskite tandems, Han and coauthors [142] used Cs0.09FA0.77MA0.14Pb(I0.86Br0.14)3 (Eg e 1.60 eV) with monolithical integration, and boosted the original PCE of CIGS subcell from 18.73% to 22.43%, with current density balance at 17.76 mA/cm2 (Fig. 5.24). Shen and coworkers [143] developed a mechanically stacked perovskite/CIGS tandem solar cell, where 1.62 eV band gap perovskite was used with four-cation Cs0.05Rb0.05FA0.765MA0.135PbI2.55Br0.45 composition, with final PCE ¼ 23.9% and increased PCE with respect to the two subcells (PCE ¼ 16.5% for CIGS and 18.0% for PSC) (Fig. 5.24). Concerning the manufacture of tandem solar cells, we can point out that the fabrication processes of the top PSC in 2T tandem should be compatible with the bottom cell. Silicon heterostructure solar cell consists of Si crystals that support temperature up to 500 C and a-Si passivation layers that degrade for T > 200 C. This temperature limitation excludes the high temperature sintering processes needed for transport layers used in PSCs, such as

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Figure 5.24 Device schematic of mechanically stacked perovskite-CIGS tandem with cross-sectional image (A), balanced spectral splitting between CIGS and perovskite subcells (B). CIGS, copper indium gallium diselenide. Reprinted from H. Shen, T. Duong, J. Peng, D. Jacobs, N. Wu, J. Gong, Y. Wu, S.K. Karuturi, X. Fu, K. Weber, X. Xiao, T.P. White, K. Catchpole, Mechanically-stacked perovskite/CIGS tandem solar cells with efficiency of 23.9% and reduced oxygen sensitivity, Energy Environ. Sci. (2018). https://doi.org/10.1039/ c7ee02627g.

TiO2 or NiO. Thus, low-temperature PSC processes are required, which can be obtained by solution processes or vacuum deposition. PSC fabrication requires the deposition of a top transparent electrode to ensure the transmission of infrared light to the bottom subcell. To date, conductive oxides, deposited via magnetron sputtering (MS) processes, are still the best solution for transparent electrodes, in comparison to alternative nanostructured materials such as nanoparticles, carbon nanotubes, metal nanowires, etc. We should point out that high-performing organic transport materials such as Spiro-OMeTAD or PTAA (for HTL) or PCBM and ITIC (for ETL) can be damaged by the high-energy ion flux while depositing TCO with radiofrequency or DC MS methods [156e158]. For this reason, organic transport materials should be completely replaced with an inorganic one, or a special protection barrier should be deposited to organic TL prior electrode deposition. The use of molybdenum oxide (MoOx) as a buffer layer is a standard solution to protect the underlying organic layer [159e161] but requires accurate control of environment conditions such as humidity.

5.4.3 Special structures The introduction of a so versatile perovskite material inspired also alternative architectures which have not been considered in other PV technologies. In the following, we will discuss some of them.

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5.4.3.1 Devices with carbon electrode One of the most crucial interface instabilities in PSCs is the diffusion of metal into the bulk of transport and absorber films that cause shorts, an increased level of nonradiative recombination and formation of metal complexes. In alternative to commonly used back metal electrodes, such as Au, Ag, Al, Cu, conductive carbon was used, that provided, together with an engineered 2D/3D perovskite, a very stable PSC (>10,000 h of stable device operation under light soaking) [64]. In general, device structures with carbon electrodes consider the following stack: FTO/compact TiO2/mesoTiO2/ZrO2 or Al2O3 scaffold/perovskite/carbon [162,163] (see Fig. 5.25). In this configuration, all the layers are printed and then sintered, while the perovskite is infiltrated afterward. Typical thickness for carbon electrodes is in the range of several micrometers (Fig. 5.25B). Solution processing of carbon is not compatible with use of HTL on the top of perovskite, and the absence of electron blocking at anode increases recombination rates and decreases device efficiency, with reduced Voc and high series resistance [152]. Also, the deposition of carbon layers forms poor contact with perovskite film, affecting the whole collection [153]. 5.4.3.2 Resonant NP for light harvesting management Inducing of plasmonic effects is one of the promising efforts for absorption enhancement and photoconversion in solar cells. Liyang et al. [154] (A)

(B)

Figure 5.25 Device schematics of mesoscopic perovskite solar cell (PSC) with ZrO2 scaffold and carbon electrode; (B) Cross section of the ms-PSC with carbon electrode. Reprinted from A. Mei, X. Li, L. Liu, Z. Ku, T. Liu, Y. Rong, M. Xu, M. Hu, J. Chen, Y. Yang, €tzel, H. Han, A hole-conductor-free, fully printable mesoscopic perovskite solar cell M. Gra with high stability, Science (80-.). (2014). https://doi.org/10.1126/science.1254763. (B) Reprinted from K. Lee, J. Kim, H. Yu, J.W. Lee, C.M. Yoon, S.K. Kim, J. Jang, A highly stable and efficient carbon electrode-based perovskite solar cell achieved: via interfacial growth of 2D PEA2PbI4 perovskite, J. Mater. Chem. (2018). https://doi.org/10.1039/c8ta09433k.

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considered a full-wave simulation approach to investigate the contributions of light absorptions across the device illuminated with solar light. Authors revealed that the absorption of the perovskite layer is dominant in UVVis spectral part, while the electrode layers contribute in the red and IR regions. These results demonstrate that it is possible to use silver plasmonic nanoparticles for 58.2% enhancement of the light absorption in the infrared region, by placing a 140-nm-diameter particle arrays (2 nm spacing) via highly localized plasmonic near-fields. Hajjiah and coauthors [155] performed a more detailed modeling for a semitransparent device by placing gold and silver nanoparticles with a diameter of w40 nm at rear electrode and showed a Jsc/EQE enhancement. Furasova and coworkers experimentally realized a novel approach for advanced light harvesting [156]. Si nanoparticles of w50 nm were inserted at the heterojunction interface between perovskite absorber and meso-TiO2 for light trapping in NIR region induced by Mie resonance. The improved device showed 18.8% PCE with >1% of efficiency increase in comparison to the reference cell (Fig. 5.26). Aeineh et al. introduced core/shell Au@SiO2 structures with 10-nm size between compact and meso TiO2 layers, and improved photoconversion. ((A))

( ) (B)

Figure 5.26 Device schematics of mesoscopic perovskite solar cell (PSC) with ZrO2 scaffold and carbon electrode. (B) Cross section of the ms-PSC with carbon electrode. Reprinted from A. Mei, X. Li, L. Liu, Z. Ku, T. Liu, Y. Rong, M. Xu, M. Hu, J. Chen, Y. Yang, €tzel, H. Han, A hole-conductor-free, fully printable mesoscopic perovskite solar cell M. Gra with high stability, Science (80-.). (2014). https://doi.org/10.1126/science.1254763. (B) Reprinted from K. Lee, J. Kim, H. Yu, J.W. Lee, C.M. Yoon, S.K. Kim, J. Jang, A highly stable and efficient carbon electrode-based perovskite solar cell achieved: via interfacial growth of 2D PEA2PbI4 perovskite, J. Mater. Chem. (2018). https://doi.org/10.1039/c8ta09433k.

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Devices with modified ETL with resonant nanoparticles exhibited higher external quantum efficiency in all the visible wavelength range with improved Jsc.

5.5 Stability of PSCs 5.5.1 Light-induced degradation Stability is one of the key challenges for industrial scale commercialization of PSCs. Degradation mechanisms depend on materials, bias conditions, light soaking, and temperature [157]. One of the first studies on metal halide PSCs illustrated that encapsulated MAPbI3 films can be exposed to 1000 h of full sun spectrum, simulated with AM1.5 irradiation, without showing any sign of decomposition as shown by perovskite absorption spectra. This result has been supported by many reports of cells maintaining their full current generation over hundreds of hours of light exposure [151,170,171]. In many studies, however, it is shown that significant changes occur in perovskite films during illumination, such as, ion migration, halide segregation, and compositional degradation [172,173]. While these phenomena present challenges to the perovskite community, photoinduced changes may also be beneficial. For example, in CIGS solar cells, light soaking causes metastable defect reactions with generated charge carriers which lead to increase conductivity, fill factor, and open circuit voltage [174,175]. In addition to the absorber layer, the charge transport layers used in PSCs show variable photostability, resulting in limiting the stability of some types of PSCs. 5.5.1.1 Photostability of charge transport layers Organic semiconductors can form free radicals in the excited state, which can be paired with breaking of carbonenitrogen, carbonecarbon, and carboneoxygen bonds, typically on side chains of the conjugated backbone [175,176]. Under inert conditions, this can result in cross-linking of various free radical species and increases the disorder in the semiconductor films. In fullerene-based materials, which are used as ETLs in PSCs, the photodimerization under photoexcitation creates disorder and inhibits charge transport [164]. Indeed, solar cells using both small molecule hole transport materials and fullerene as electron acceptor materials have been demonstrated to perform stably after hundreds of hours of continuous illumination under 1 sun [151,177,178]. It has been shown that PSCs are robust against minor

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photoinduced degradation within the organic charge transport layers. TiO2 ETLs have been reported to be extremely unstable to UV light not only in the presence of oxygen but even in inert conditions. Photoinduced desorption of oxygen from oxygen vacancies on TiO2 surfaces results in rapid trap-induced recombination across the TiO2 interface [167]. Bella et al. demonstrated that by applying a downconverting fluoropolymer able to transmit the incoming UV light onto the perovskite film as visible light, it is possible to ensure that the TiO2 layer cannot be photoexcited and no photocurrent is lost from the device [168]. 5.5.1.2 Effects on ion distribution in metal halide perovskites DeQuilettes et al. showed that the photoluminescence (PL) intensity of MAPbI3 layers increases over time under 1-sun equivalent irradiation intensity [169]. This increasing of PL intensity is correlated with migration of I species away from the illuminated area. Some of the first direct physical evidence migration of iodide in metal halide perovskite films were provided by this group. This effect could be due to the slow diffusion of ionic species throughout the perovskite layer, and also it is probably related to the presence of point defects, particularly halide vacancies. This group has also found that combining light exposure with oxygen and humidity exposure resulted in even more drastic improvements to PL intensity, causing internal PL quantum yields approaching 90%. Open circuit voltages have been increased in corresponding PSCs [170]. An additional passivating effect of superoxide molecules is suggested, where molecular oxygen can adsorb to surface-trapped electrons and form a stable and passivating superoxidedefect complex by moving the defect energy level outside the band gap. During exposure to humidity and light, an amorphous shell of passivating PbI2 is likely to form, bringing long-lived passivation. 5.5.1.3 Light-induced halide segregation There are many efforts to tune the band gap of PSKs by bromide substitution at halide sites and to increase the thermal stability through Cs and FA substitution on the small cation site. In doing this, however, new lightinduced phenomena have been perceived [66,183]. Under light exposure, Pb(BryI1y)3 perovskites undergo reversible phase segregation into Br- and I-rich perovskite phases; indeed, this observation well aligns with the observation that migration of halide species in perovskite films is induced by light [173]. By removing the light, the materials regain their normal compositional distribution. This phase segregation means that the smaller band gap

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iodine-rich phase inclusions are effectively carrier trapping domains, as confirmed by the fact that PL comes from a low-energy state in such phase-segregated materials. This phenomenon named as the “Hoke effect” provided some definitive evidence that the ions in the perovskite structure can be mobile and that phase segregation can happen in materials having both Br and I on the X site. 5.5.1.4 Light-induced cation segregation There are many studies on cation segregation of the perovskite layer against irradiation [185,186]. Christians et al. revealed that, while the halide and lead distributions remained unchanged all over the depth of the triple catione based PSC, there was a significant shift in Csþ content from the bulk of the material to the hole transporter after 25 h of illumination [174]. The formamidinium and methyl ammonium cations were also redistributed throughout the bulk of the device, which shows further evidence for light-induced compositional changes in PSCs. Large changes in compositional distribution were observed after several hours of operation under full illumination by Domanski et al. [175], with a reversible relative drop in efficiency, up to 15%. The combination of both light and electrical bias was found to be essential to observe the phenomenon. For improvement of the structural stability of the perovskite material, A-site engineering has been applied, but it is critical to specify whether the improved structural stability in the dark translates to structural stability in the light or whether miscibility gaps appear in the compositional spaces for materials in their excited states [176]. 5.5.1.5 Photochemical reactions Metal halides are known to photodecompose. Photodecomposition of silver halide is the most obvious example, which decomposes to halogen and silver upon photoexcitation and formed the basis for early photography [177]. A similar effect has been proposed in PbI2, where carriers trapped at iodide vacancies oxidize iodide to iodine and reduce Pb2þ to Pb0 [178]. Recently, a similar reaction mechanism has been proposed for lead halide perovskites (Fig. 5.27). Kim et al. showed that the halide vacancy concentration dramatically increases by illumination, causing an increased ionic conductivity [161]. This observation is consistent with other studies reporting a rising in ionic conductivity under illumination [179]. They suggest that iodide ions in the lattice are oxidized by photogenerated holes, resulting in coupled formation of neutral iodine interstitials and iodide vacancies.

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Figure 5.27 (A) Photodecomposition mechanism. (B) The movement of ions and the formation of molecular iodine and metallic lead upon illumination. (A) Reprinted from M.G. Albrecht, M. Green, The kinetics of the photolysis of thin films of lead iodide, J. Phys. Chem. Solids 38 (1977) 297e306. https://doi.org/10.1016/0022-3697(77)90106-8. (B) €m, V. Lanzilotto, F.O.L. Johansson, K. Aitola, B. Reprinted from U.B. Cappel, S. Svanstro €hlisch, S. Svensson, N. MårPhilippe, E. Giangrisostomi, R. Ovsyannikov, T. Leitner, A. Fo tensson, G. Boschloo, A. Lindblad, H. Rensmo, Partially reversible photoinduced chemical changes in a mixed-ion perovskite material for solar cells, ACS Appl. Mater. Interfaces. 9 (2017) 34970e34978. https://doi.org/10.1021/acsami.7b10643.

5.5.2 Reactions with electrodes Au, as electrode in PSKs devices, does not form a redox couple with perovskite while some metals can form redox couples with the perovskite itself, even reacting with PbI2 [180]. Some of them erode in the presence of reactive polyiodide melts, formed from perovskite decomposition in visible light [179,192]. Thus, while some metals may be stable with respect to the perovskite structure, almost all metals react with decomposition products of the perovskite, such as HI, MAI, CH3I, and I2, that could be produced even in an encapsulated PSC. For metal contacteinduced degradation in PSCs, there are three major mechanisms, all of which cause the device performance to drop significantly: (1) halide anions diffuse to the metal electrode, corroding the metal and resulting in a halide deficiency in the perovskite absorber layer (Fig. 5.28A) [182]; (2) metal diffuses under activation of light and/or heat into the perovskite film, potentially forming insulating metal halide species or defect states in the bulk or at the perovskite interface (Fig. 5.28, panels b and c) [183]; (3) metal contacts form a redox couple with Pb2þ in perovskite layer, accelerating the loss of halide species and forming Pb0 [180].

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Figure 5.28 (A) Schematic detailing reaction of a metal electrode with halide/halogen species created via decomposition of the perovskite by moisture. (B) TOF-SIMS depth profile showing diffusion of Au into the perovskite film after heating at 70  C under illumination, with (C) a corresponding 3D elemental map. (A) The figure has elected from Y. Kato, L.K. Ono, M. V. Lee, S. Wang, S.R. Raga, Y. Qi, Silver iodide formation in methyl ammonium lead iodide perovskite solar cells with silver top electrodes, Adv. Mater. Interfaces. 2 (2015) 1500195. https://doi.org/10.1002/admi.201500195. (B) Reprinted from K. Domanski, J.-P. Correa-Baena, N. Mine, M.K. Nazeeruddin, A. Abate, M. Saliba, W. Tress, A. €tzel, Not all that glitters is gold: metal-migration-induced degradation in Hagfeldt, M. Gra perovskite solar cells, ACS Nano. 10 (2016) 6306e6314. https://doi.org/10.1021/acsnano. 6b02613.

5.6 Upscaling of perovskite solar devices The devices of the second [192e198] and third [199e222] generations of PV are based on transparent conducting oxides (TCOs) [223], employed as photoelectrodes. These materials are characterized by a good degree of transparency and relatively low sheet resistances, typically in the order of 7e15 U/, [224e226]. In the case of large-area devices, the latter values could be deleterious for the electrical performance, generating a high series resistance that opposes to current flux. Therefore, to realize large-area PSMs, it is mandatory to divide full-sized TCO substrates into multiple cells. Two main architectures could be employed, one that uses series-connected cells and the other that uses parallel-connected cells.

5.6.1 Series-connected solar modules The architecture that is principally employed to fabricate PSMs is the seriesconnected one [227e236]. In this case, the voltage of the cells will be ideally

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Figure 5.29 Cross section of a typical thin-film series-connected solar module. The P1, P2, and P3, discussed below, constitute a zone called dead area ([, in the figure), dedicated to the interconnection between subsequent cells. The photoactive area is here indicated with L. RS, top and RS, bottom represent the series resistance offered by the two electrodes, Rint represents the contact resistance between the electrodes of subsequent cells in the series architecture. Reprinted from L. Lucera, F. Machui, P. Kubis, H.J. Egelhaaf, C.J. Brabec, Highly efficient, large area, roll coated flexible and rigid solar modules: design rules and realization, in: Conference Record of the IEEE Photovoltaic Specialists Conference, 2016, pp. 234e237, https://doi.org/10.1109/PVSC.2016.7749585. Copyright (2016) IEEE.).

summed to obtain the voltage of the module, while the current of the latter will be dominated by one of the less performing cells in the series. In Fig. 5.29, different resistances are shown, two generated by the electrodes and one that characterizes the contact between the TCO of a cell and the counter-electrode of the subsequent one.

5.6.2 Parallel-connected solar modules In order to raise the current output of the module, while, ideally, maintaining the same voltage of a single cell, the parallel-connection architecture can be employed. The latter is normally realized maintaining the same TCO layer for all of the cells of the module. Due to the TCO resistivity, above introduced, this leads to a strong reduction of performance [237e239], which has brought to a scarcer scientific investigation of the parallel-connection architecture with respect to the series-connection one [237,240,241]. Different stratagems have been adopted to improve the performance of parallel-connection architecture, e.g., the employment of a metal grid printed over the TCO, to increase the charge collection on the photoelectrode [240,241], as shown in Figs. 5.30 and 5.31.

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(A)

(B)

Figure 5.30 Device structure (A) and layout (B) by Hambsch et al. [240]. The aluminum grid was deposited directly onto the ITO layer by thermal evaporation and, then, exposed to an UV-ozone plasma to realize an aluminum oxide layer; the latter improves the hydrophilic property of the film, reducing the possibility of device short circuit between the grids and top electrode and can prevent corrosion of aluminum grids from the perovskite film. Adapted from M. Hambsch, Q. Lin, A. Armin, P.L. Burn, P. Meredith, Efficient, monolithic large area organohalide perovskite solar cells, J. Mater. Chem., 4 (2016) 13830-13836, https://doi.org/10.1039/C6TA04973G. Copyright (2016) The Royal Society of Chemistry.

Figure 5.31 Device layout and certified IeV curve by Kim et al. [241] The authors did not specify which metal has been employed for the grid, but only its resistivity ¼ 9.3$106 U cm. Adapted from J. Kim, J.S. Yun, Y. Cho, D.S. Lee, B. Wilkinson, A.M. Soufiani, X. Deng, J. Zheng, A. Shi, S. Lim, S. Chen, Z. Hameiri, M. Zhang, C.F.J. Lau, S. Huang, M.A. Green, A.W.Y. Ho-Baillie, Overcoming the challenges of large-area highefficiency perovskite solar cells, ACS Energy Letters, 2 (2017) 1978e1984, https://doi.org/ 10.1021/acsenergylett.7b00573. Copyright (2017) American Chemical Society.

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The optimization of such a layout has brought to a maximum certified PCE ¼ 12.1% on a device with AA ¼ 16 cm2 [241]. This result sets a record for PSMs based on parallel-connected architecture. Nevertheless, the series-connected architecture evidently makes the scalability of devices easier, and a wider research activity has been focused on series-connected architecture, which holds all of the current records for each dimension-category of PSMs [228,235,242]. To realize the interconnections of such an architecture, the P1eP2eP3 process is employed.

5.6.3 The P1eP2eP3 process In a series-connected PSM, due to the considerable sheet resistance of TCO, a subdivision of the substrate into subcells is necessary, as previously mentioned. In this way a monolithic interconnection scheme [228] is realized, as shown in Fig. 5.32 [236]. The latter is constituted by some active zones that are dedicated to the photogeneration of electric charges and, thus, to the PV energy conversion, and dead zones, dedicated to the interconnection between adjacent cells. It is possible to define the active area (AA) as that part of the substrate dedicated to the PV energy conversion and the dead area (DA) as the part where the interconnection of cells takes

Figure 5.32 Schematic representation of a typical mesostructured perovskite solar module. In the zoomed circle, an active area width (WA) and a dead area width (WD) are highlighted. The latter is formed by the P1, P2, and P3 processes lines and by the two safety areas (SA) between these. Reprinted from A.L. Palma, F. Matteocci, A. , L. Vesce, S. Christiansen, M. Schmidt, A.D. Carlo, LaserAgresti, S. Pescetelli, E. Calabro patterning engineering for perovskite solar modules with 95% aperture ratio, IEEE Journal of Photovoltaics, 7 (2017) 1674e1680, https://doi.org/10.1109/JPHOTOV.2017. 2732223. Copyright (2017) IEEE.

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place, and which does not contribute to the energy conversion. The DA is formed by the P1, P2, and P3 zones and by the two safety areas (SA) that are interposed between these. The respective functions will be described in the following paragraphs. In Fig. 5.32 [236], the relative widths (WA, WD ¼ P1þSA þ P2þSA þ P3) are highlighted. It is now possible to define the aperture ratio (AR) as AR ¼

WA WD þ WA

(5.2)

where the aperture area width is defined as the sum of WD and WA. It has been shown that the interconnections in series-connected PSMs can be the realized employing several patterning approaches [228,231,233,234]. Nevertheless, laser processing has been demonstrated to be an optimal choice for industrial applications, already for secondgeneration technologies [221,243e246], since it provides for the best trade-off between manufacturability and minimization of DAs [234]. The P1 process insulates the photoanodes of neighboring cells, for series connection. The P2 patterning step selectively removes the entire stack of active materials between adjacent cells to permit their series connection through the subsequent deposition of a conductive layer. Finally, the latter is homogeneously deposited on the entire module and the P3 step is applied to separate the just realized adjacent counter-electrodes. The P1eP2eP3 process on PSMs has to be performed taking into account the geometrical losses due to the presence of DAs, the resistive losses presented by the discrete sheet resistance of the TCO, and the ohmic contact losses due to the limited width of the interconnection areas and to the characteristic transfer length (LT) [228,247e250] of the realized contact between the electrodes. These can be resumed in a parameter, named total loss (TL) [251,252]. The TL can be expressed as a function of AA width (WA), DA width (WD), LT, and, in particular, P2 process width. By extending the procedure in Ref. [252] and according to well-known mathematical models [251,253,254], the TL has recently been defined, in terms of the modules parameters [236], as: Total Loss ¼

TCO R, WD J WA3 þ MPP WA þ WD VMPP 3 WA þ WD  ! WA2 P2 P2 þ LT R, coth LT WA þ WD

(5.3)

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where JMPP and VMPP are the current density and voltage at maximum TCO is the sheet resistance of the power point of a small area single cell, R, P2 active area TCO photoelectrode, R, is the sheet resistance of the TCO in the interconnection area after the realization of the P2 process, which could possibly modify the TCO surface. The realization of a P1eP2eP3 procedure is represented in Fig. 5.33. 5.6.3.1 P1 process The P1 is the first process in the fabrication of PSMs. It is realized to create the photoelectrodes of the cells, starting from a substrate homogeneously covered by a TCO. In PSMs, the P1 is typically realized with the application of IR laser machines that achieve the ablation through thermal vaporization of the FTO [228e230,235,236,255] or ITO [234,242] on glass substrates and ITO on plastics substrates [256,257].

Figure 5.33 Production process flow of a typical p-i-n structured perovskite solar module based on compact and mesoscopic TiO2 ETM.

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The width of P1 ablation lines should be the thinnest that still realizes an isolation between adjacent TCO zones. In the fabrication of PSMs, P1 widths in the range of 40e50 mm have been reported [236,258]. 5.6.3.2 P2 process The most delicate step in the realization of PSMs interconnections is the P2. The latter is actuated after the deposition of all of the active materials on the TCO and just before the deposition of the counter-electrode. The P2 step is needed to remove the active stack from the zones of TCO dedicated to the interconnection with subsequent cells in the series and that have to be preserved intact. Apart from very few mechanical microblade scribing techniques [233,234], which are complicated and not scalable, laser processing has been the most employed technique for the realization of the P2 step. In fact, laser techniques are wavelength selective and permit to ablate the active materials, which are highly absorptive in the visible range of the light spectrum, from the TCOs, which, instead, are lowly absorptive in the same wavelengths range. Therefore, the mainly employed laser wavelengths for the P2 step have been in the range of 515e532 nm [244,259] and 355 nm [236,242]. Different kinds of sources have been used, typically in the nanosecond (ns)-range [246], and picosecond (ps)-range [236,242], like fiber lasers [260], and diode-pumped solid-state Q-switched lasers [244]. Due to the discrete resistivity of the TCOs, the P2 scribe width cannot be indiscriminately reduced, like in the P1 case, to avoid high contact resistances between the electrodes. For PSMs, P2 dimensions in the range of 45e200 mm have been reported [231,234,236,242]. 5.6.3.3 P3 process After the P2 step, a homogeneous counter-electrode material layer is deposited and, then, the P3 is realized to isolate, one to each other, the counterelectrodes of the single cells. Typically, the same laser systems and parameters employed for the P2 step can also be used in the P3 step realization [234,236]. As for the P1, the P3 scribing can be performed realizing the thinnest width that still isolates the electrodes. In PSMs fabrication, P3 process widths as low as 25 mm have been reported [236]. 5.6.3.4 Safety areas While realizing a P1eP2eP3 procedure, in the steps between the laser processes, when the substrate is removed from the laser stage to deposit the

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materials, some possible misalignments are introduced, with respect to the position assumed during the P1 step. Therefore, even considering top-inclass subsequent alignment steps, the realization of SAs between P1eP2 and P2eP3 is necessary to avoid detrimental overlapping of these steps (see Fig. 5.32). SAs as thin as 50 mm have been reported [45].

5.6.4 Deposition techniques The morphology of the perovskite layer plays a crucial role in order to obtain high-efficiency PSCs. Several deposition techniques have been used for the perovskite deposition such as spin coating, blade coating, slot-die coating. The main constraints are related to the optimization of the solutions and deposition parameters in order to obtain highly uniform perovskite layer in terms of growth and substrate coverage. All the layers of a perovskite-based device can be deposited starting from solutions. Two main deposition strategies can be adopted, the sheet-to-sheet production and the roll-to-roll production. The first one involves a single glass or plastic substrate, on which the materials are deposited. The most used deposition techniques are the spin coating, the blade coating, or the slot-die coating. The spin coating is mostly used for small-area solar cells and modules rapid prototyping since it shows high thickness and film quality control, obtained by setting a series of parameters like the acceleration time, speed, and time of rotation. Nevertheless, while increasing the dimensions of the substrates, this technique has been shown not to be optimal for an industrial-oriented production, mostly for the impressive rate of material that is wasted during such a deposition process. An alternative way of depositing active layers of a large-area PSM is the blade coating (or doctor blade). This technique can be adopted both on rigid and on flexible substrates. In the sheet-to-sheet processes, typically, the substrate is fixed and a liquid solution is deposited on an external part of the latter; then, a blade moves over it, spreading the liquid precursor [229,261]. The precursor solution, the speed of the blade, and the height of the latter from the substrate are parameters that can be set. The slot die coating is one of the most promising deposition techniques. In this case, a liquid solution passes through the slot-die head that, moving over the substrate, distributes the precursor on it. Respect to the blade coating, the distribution and the deposition of the solution are part of the same process.

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Figure 5.34 Schematics of a roll-to-roll production system that includes a slot-die coating process. Reprinted from B. Dou, J.B. Whitaker, K. Bruening, D.T. Moore, L.M. Wheeler, J. Ryter, N.J. Breslin, J.J. Berry, S.M. Garner, F.S. Barnes, S.E. Shaheen, C.J. Tassone, K. Zhu, M.F.A.M. van Hest, Roll-to-Roll printing of perovskite solar cells, ACS Energy Letters, 3 (2018) 2558e2565, https://doi.org/10.1021/acsenergylett.8b01556. Copyright © 2019 American Chemical Society.)

The roll-to-roll production takes inspiration from the journals printing processes. As depicted in Fig. 5.34 [262], this production procedure involves rolls that move a flexible substrate along a path. Distributed on the latter, different production devices are placed, such as the deposition part, like a blade coating or a slot-die machine, the zone dedicated to the annealing, such as a double-sided furnace; a zone can be dedicated to the laser patterning. The roll-to-toll production procedure, clearly, permits to reach a higher throughput respect to the sheet-to-sheet one and has been investigated to reach an industrially oriented production rate for perovskite-based largearea modules [263e265].

5.7 Conclusions and perspectives This chapter provided a brief introduction to working principles and main directions for the development of PSC technology. The unprecedented rapid progress in the research of halide perovskites for PV

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applications during the last decade demonstrates real industrial potential for a cost-effective, low capital expenditure (CAPEX) printed thin-film technology that can be the competitive to traditional semiconductors such as Si, CIGS, CdTe, etc. At the same time, the unique semiconductor characteristics, related to band structure properties, bipolar charge transport, ionic motion, and traps still need to be clarified and related to the exceptional performance of these materials. The main strategies for future development of PSC are oriented to the optimization of photo and thermal stability of absorber and transport layers. In addition, interface engineering for improvement of charge extraction, passivation of perovskite grain-boundaries/surfaces, and to prevent metal electrode diffusion needs to be further developed to achieve an international standard assessment of PSC stability. In turn, this will open industrial exploitation of perovskite PV technology. In this context, the possibility to tune on demand the optoelectronic properties of the absorber, by varying the halide perovskite composition, opens promising perspectives for spectral management in tandem integration with silicon and CIGS PVs. The PCE of 28% achieved so far for perovskite/silicon tandem allows us to forecast a bright future of the emerging technology of perovskite multijunction solar cells.

Acknowledgments D.S. and A.D.C. gratefully acknowledge the financial support of the Ministry of Science and Higher Education of the Russian Federation in the framework of Increase Competitiveness Program of NUST «MISiS» (N L2-2019-013), implemented by a governmental decree dated 16th of March 2013, N 211.

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