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Solution-processable electron transport layer for efficient hybrid perovskite solar cells beyond fullerenes
Solar Energy Materials and Solar Cells 169 (2017) 78–85
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Solution-processable electron transport layer for efficient hybrid perovskite solar cells beyond fullerenes
MARK
Priyadharsini Karuppuswamya,b,c,1, Chintam hanmandluc,d,1, Karunakara Moorthy Boopathic, ⁎ Packiyaraj Perumale, Chi-ching Liuc, Yang-Fang Chene, Yun-Chorng Changc, Pen-Cheng Wanga, , ⁎⁎ Chao-Sung Laid,f,g, , Chih-Wei Chuc,h a
Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan Nano Science and Technology Program, Taiwan International Graduate Program, Academia Sinica and National Tsing Hua University, Taiwan c Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan d Department of Electronic Engineering, Chang Gung University, Taoyuan 33302, Taiwan e Department of Physics, National Taiwan University, Taipei 10617, Taiwan f Department of Nephrology, Chang Gung Memorial Hospital, Taiwan g Department of Materials Engineering, Ming Chi University of Technology, Taiwan h College of Engineering, Chang Gung University, Taoyuan 33302, Taiwan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Hybrid perovskite solar cells Perylene diimide Electron transport layer Space charge limited current Carrier recombination
A solution-processable planar perylene diimide, N,N´-dipentyl-3,4,9,10-perylenedicarboximide (PDI), has been demonstrated as a suitable acceptor material to replace the conventional fullerene derivatives (e.g., PCBM) found in inverted perovskite solar cells (PSCs). The energy offset between the perovskite and PDI layers was optimized by varying the ratio of halides (iodide to bromide) to improve the exciton dissociation and carrier transport. The PDI acceptor material had higher electron mobility and smoother morphology on perovskite relative to those of PCBM, leading to enhancements in the short circuit current density, fill factor, and power conversion efficiency. The incorporation of PDI in the perovskite devices decreased recombination losses and improved the device stability. The performance of the best PSC containing PDI as the electron transport layer (11%) was higher than that of the best device featuring PCBM (10%).
1. Introduction Perovskite solar cells (PSCs) have gained considerable attention recently because their high efficiencies (> 20%) [1] are close to those of commercial Si-based and thin film solar cells [2]. Perovskites are organometal halides exhibiting high degrees of light absorption, high fluorescence yields, long carrier lifetimes, high diffusion lengths and highly tunable bandgaps [3–7]. Because of these properties as an ideal light absorber, the ready and simple solution-processability of perovskites [8,9] caters to the needs of low-cost photovoltaics. Continuous improvements in device architectures [10,11], carrier extraction layers [8,12], interface layers [13,14] and perovskite composition [15–17] are leading to more efficient and stable PSCs. Generally, perovskite device structure falls into two categories: mesoscopic and planar-heterojunction; the former structure requires high-temperature processing, while the latter is mostly solution-processable and, hence, more attractive. In
⁎
PSCs, the carrier extraction layers play several important roles: effectively extracting electrons and holes from the perovskite absorber material and helping to prevent leakage currents [18]. Electron transport materials have not been the focus of research attention as much as hole transport layers, with fullerenes being the most common type of electron transport layer (ETL) materials [19–21]. PCBM, a fullerene derivative, is commonly used as the ETL in planar heterojunction PSCs displaying high performance [9,22]. Unfortunately, PCBM comes with high production costs and photochemical instability [23], hindering its commercial viability. Apart from fullerenes, only a few polymers and perylene diimide small molecules have been studied as alternative materials for ETLs. Polymers have found their use in different types of solar cells as electrolytes and carrier transport layers. The coexistence of dye-sensitised solar cells (DSSC) and PSCs could be possible with the use of solid polymer electrolytes or perovskites to replace liquid electrolytes in DSSCs [24]. The first n-type
Corresponding author. Corresponding author at: Department of Electronic Engineering, Chang Gung University, Taoyuan 33302, Taiwan. E-mail addresses: [email protected] (P.-C. Wang), [email protected] (C.-W. Chu). 1 These authors contributed equally. ⁎⁎
http://dx.doi.org/10.1016/j.solmat.2017.04.043 Received 30 March 2017; Received in revised form 25 April 2017; Accepted 28 April 2017 0927-0248/ © 2017 Published by Elsevier B.V.
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Fig. 1. (a) Device architecture of p–i–n mixed-halide hybrid PSCs. (b) Schematic representation of energy band levels of PSCs featuring various perovskite compositions and ETLs. (c) Molecular structure of PDI. (d) UV absorption spectra of perovskites; arrow indicates the increase in Br– content.
In this paper, we demonstrate the successful preparation of planar heterojunction PSCs incorporating a PDI (N,N´-dipentyl-3,4,9,10-perylenedicarboximide) with terminal alkyl groups as a replacement for fullerene derivatives. To facilitate charge transport and exciton dissociation, we added methyl ammonium bromide (MABr) to better align the energy levels of the organometal halide and the PDI. Solutionprocessed deposition of PDI resulted in smooth, continuous films on the perovskite surface. Electrical and photophysical studies of the perovskite–ETL interfaces revealed that trap-assisted recombination occurred in both the PCBM and PDI ETLs, but bimolecular recombination was absent in the PDI-containing device. PDI provided higher mobility, a lower degree of recombination, and better performance than did PCBM, making it a potential ETL material for replacing PCBM in PSCs.
polymer ETL from the naphthalene diimide polymer family was reported to have an efficiency of 7% [25] in PSCs, later improved to 10% by Shao et al. [18]. An exhaustive review of the polymer electron transport materials used in PSCs has been reported recently [26]. Apart from ETL materials, a type of fluoro polymer coating has been shown to improve the stability of PSCs, which is another important parameter to be studied for commercial viability of PSCs [27]. Perylene diimide derivatives, employed commercially as cheap dyes and pigments, have high photostability and tunable energy levels because of the wide range of choices for derivatizing their structures. Their use in PSCs was introduced by Das et al. [28] who demonstrated that a thiophene-modified perylene diimide could be used as the hole transport layer. The use of perylene diimides as ETLs in PSCs was reported recently for a device incorporating its dimer doped with an organic molecule, displaying 10% efficiency [29]. The planar nature of perylene diimides makes them easy to synthesize and leads to high degrees of charge transport as a result of their π-stacking [30]. Zhang et al. reported that TiO2 could be replaced by an amino-substituted perylene diimide (N-PDI) in an n-i-p structure [31]. They proposed that a perylene diimide with terminal alkyl groups (C-PDI) could not be used for solar cell fabrication because perovskites cannot form complete film on non-wetting surface of C-PDI. This disadvantage of non-wetting surfaces might be overcome in an inverted architecture, where the ETL is fabricated on top of the perovskite layer; such a structure might also enhance the stability of the PSCs against moisture.
2. Experimental section 2.1. Materials Hydroiodic acid (HI), hydrobromic acid (HBr), and methylamine (CH3NH2) were purchased from Alfa Aesar and used without purification. PDI was obtained from Lumtec Corporation, Taiwan.
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atmosphere with stirring and then the solvent was evaporated (rotary evaporator) until a precipitate was formed [32]. The white precipitate was filtered off, washed thrice with diethyl ether, dried under vacuum at 60 °C overnight, and stored in a glove box until required for further use. 2.3. MABr Equal volume proportions of HBr (47 wt% in water) and CH3NH2 (33 wt% in absolute EtOH) were mixed with stirring at 0 °C in a threeneck round-bottom flask under a N2 atmosphere and then the solvent was evaporated (rotary evaporator) [33]. The precipitate was purified using the same procedure described above for MAI. 2.4. Device fabrication ITO substrates (< 10 Ω cm–1, RiT display) were patterned by etching through HCl followed by rinsing (20 min each) once with detergent and twice with DI water with ultrasonication. The substrates were blown dry under N2 and stored in an oven until required for use. The cleaned substrates were subjected to UV/ozone treatment for 15 min to remove organic contaminants and to make the surface more hydrophilic. PEDOT:PSS was spin-coated (4000 rpm, 40 s) on top of the ITO layer, followed by annealing at 130 °C for 30 min. The substrates were transferred to a glove box and left under a N2 atmosphere until required for further processing. The perovskite precursor PbI2 (99.998%, Alfa Aesar) along with a salt additive (7.5 mg of KCl) [34] was dissolved in DMSO (40 wt%); MAI:MABr was dissolved in 2propanol at various concentrations; these solutions were left overnight at 70 °C. The different MAI:MABr concentrations (weight percent ratios) used are as follows: 1.5:1.5 (450 mg each), 2.0:1.0 (600:300 mg) and 2.5:0.5 (750:150 mg) dissolved in 3 mL of 2-propanol. The perovskite layer was deposited (6000 rpm, 40 s) using a twostep method—first the PbI2 precursor layer and then the MAI: MABr layer—and then the system was subjected to post-annealing for 1.5 h at 100 °C. A solution of PDI (PenPTC, Lumtech Corporation) in chlorobenzene (1 wt%) was spin-coated onto the perovskite layer at various spin speeds (3000–6000 rpm) for 40 s and then the samples were annealed at 100 °C for 30 min. Sequential thermal evaporation of C60 (30 nm), Bathocuprine (BCP, 10 nm), and the Al electrode (100 nm) completed the device structure. 2.5. Material and device characterization The valence band level of the perovskite layers and ETLs were investigated at room temperature using photoelectron spectroscopy in air (PESA, Model AC-2, Riken Keiki). The electrochemical measurements were performed by using a potentiostat/galvanostat MacLab model ML160 controlled by NOVA software (1.8 version for Windows) using a conventional single-compartment three-electrode cell with a Pt working electrode, Ag wire as reference electrode, and Pt wire as the counter electrode. All measurements in deaerated dichloromethane were performed with freshly distilled solvent with a solute concentration of 1.0 mm in the presence of NBu4PF6 as the supporting electrolyte and a scan rate of 100 mV/s. Ferrocene (Fc) was added at the end of the experiment as internal standard to calibrate the redox potentials. The Fc/Fc+ redox couple with a half-wave potential E1/2 was set as 0.690 V vs. NHE in CH2Cl2. XRD patterns were recorded at room temperature using a Bruker D8 diffractometer, with a diffracted beam monochromator set for Cu Kα radiation (λ=1.54056 Å), in the 2θ range 10–80° with a step size of 0.03939° and a step time of 99.45 s. UV–Vis absorbance spectra were recorded using a Jacobs V-670 UV–Vis spectrophotometer. SEM images of the perovskites on PEDOT:PSScoated glass/ITO substrates were recorded using an FEI Noval 200 microscope (15 kV). PL emission spectra were recorded using an optically excited Q-switched Nd:YAG laser (266 nm, 3–5 ns pulse,
Fig. 2. (a) Tauc plots (corresponding to the UV absorbances), (b) XRD patterns, and (c) XRD spectra (plotted in the range 27°≤ 2θ≤ 31°) of the various perovskites.
2.2. Methyl ammonium iodide preparation HI (57% in water, 15 mL) was added to CH3NH2 (40 wt% in water, 13.5 mL) at 0 °C in a three-neck round-bottom flask under a N2 80
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Fig. 3. (a) J–V Characteristics of the best PSCs incorporating PCBM and PDI as ETLs, measured under illumination from an AM 1.5 G solar simulator (reverse scan). (b) EQE spectra of CH3NH3PbI3−xBrx perovskite (MAI:MABr =2.5:0.5) devices incorporating PDI and PCBM as ETLs; the integrated Jsc values were 18.91 for perovskite/PDI devices (black) and 17.22 for perovskite/PDI devices (red). Table 1 Performance parameters of mixed-halide CH3NH3PbI3–xBrx (MAI: MABr =2.5:0.5) PSCs featuring PCBM and PDI as ETLs, measured under AM 1.5 G illumination. The numbers represent the average of 10 devices prepared in two batches; the numbers in the brackets represent the parameters of the champion device. Device
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
Rs (Ω/cm2)
Rsh (kΩ/cm2)
Perovskite/PCBM
1.00 ± 0.02 (0.99)
16.91 ± 0.62 (17.26)
58.47 ± 2.90 (58.52)
9.87 ± 0.15 (10.00)
11.49
0.57
Perovskite/PDI
0.93 ± 0.01 (0.93)
19.30 ± 0.39 (19.31)
58.45 ± 2.75 (61.48)
10.49 ± 0.43 (11.04)
3.80
10.00
study. The perovskite layer was deposited in a two-step method, forming continuous and pinhole-free CH3NH3PbI3–xBrx films through annealing-driven interdiffusion of I– and Br– ions. The device fabricated with methyl ammonium iodide (MAI) alone displayed no significant performance, due to energy level mismatch (Table S1). The energy level diagram was drawn by the valence band values obtained from photoelectron spectroscopy in air (PESA) and band gap values obtained from Tauc plot (Fig. 1b, Fig. 2a); it reveals that this misalignment would lead to carrier recombination at the CH3NH3PbI3–PDI junction interface, whereas the mixed halide perovskites align more favorably. The ratio of MAI and MABr was optimized to achieve better alignment of the conduction band of the perovskite with the LUMO of PDI. Fig. 1c presents the molecular structure of PDI. A uniform layer of 100 nm thick PDI was spin coated on top the perovskite. Fig. S1a shows the cross-section SEM image of the perovskite device indicating the thicknesses of the different layers. Further, the presence of PDI was confirmed by the XPS depth profile analysis. Fig. S1b shows the X-ray photoelectron spectroscopy (XPS) depth profile analysis of Pb in the perovskite device clearly representing the increase in Pb content with continuous removal of the PDI top layer by sputtering. The HOMO and LUMO of PDI were calculated from cyclic voltammetry (powder form) and photoelectrospectroscopy (thin film) measurements, explained in detail in the experimental section. Cyclic voltammetry results of PDI are shown in Fig. S2. We used three different weight percentage ratios of MAI to MABr (1.5:1.5, 2.0:1.0 and 2.5:0.5) to evaluate the effect of incorporating Br– ions. UV–Vis absorbance spectra revealed that increasing the MABr concentration led to a blue shift of the exciton absorption edge (Fig. 1d), due to widening of the perovskite band gap, because the energy of Br(4p) is lower than that of Pb(6s) [37]. A systematized shift of the bandgaps to the wide bandgap region is observed with the addition of MABr in precursor solutions. The MAI:MABr ratios of 1.5:1.5, 2.0:1.0, and 2.5:0.5 yielded band gaps of
10 Hz) focused on a beam diameter of approximately 0.5 mm; all perovskite films for PL measurement were prepared on glass substrates. The EQE spectra of the encapsulated devices were measured under short-circuit conditions using a 75-W Xe lamp (Enlitech, QE-R3011) as the light source; the light output from the monochromator was focused on the photovoltaic cell being tested (DC mode). The devices were illuminated inside a glove box under simulated 1.5 AM illumination (100 mW cm–2) using a solar simulator (Thermal Oriel) featuring a Xe lamp. The light intensity was calibrated prior to device measurement, using a standard silicon photodiode (Hamamatsu). 2.6. Energy level diagram The HOMO of the PDI films on glass was obtained from PESA measurements. Band gap values calculated from Tauc plot of PDI films on glass substrates along with HOMO level values were used to calculate the LUMO of PDI thin films. LUMO of the PDI in powder form was also calculated by cyclic voltammetry measurements. The UVabsorbance data of PDI powder dissolved in chloroform was used to calculate bandgap and hence the HOMO of PDI in powder form. 3. Results and discussion An important aspect affecting the optical properties of any solaractive material is band gap tunability. The band gaps of hybrid perovskites can be tuned by varying the type and concentration of halide anions surrounding the metal (lead) cation [35]. In addition to iodide, bromide and chloride are also common halide ions in perovskites. Here, we use Br– to tune the bandgaps as Br– is advantageous compared to Cl– ions in terms of formation energy and grain orientation, as suggested from previous studies [36]. Fig. 1a provides a schematic representation of the device structure used in this present 81
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corresponded to a tetragonal structure in the absence of Br–, with the crystal structure changing to cubic after its addition [35]. Magnification of these XRD spectra in the range 27°≤ 2θ≤ 31° (Fig. 2c) revealed (220) and (004) peaks corresponding to the tetragonal structure of CH3NH3PbI3 perovskite and (002) peaks corresponding to the cubic structure of the CH3NH3PbI3–xBrx perovskite. A shift of the (002) peaks to higher angle occurred upon increasing the Br– concentration, resulting in a respective decrease of the lattice constant (Table S2). The observed changes in crystal structure and respective lattice constants are consistent with the ionic radius of a Br– ion being smaller than that of an I– ion. Fig. 3a displays the J–V characteristics of the as-fabricated devices, featuring PCBM and PDI as ETLs, under illumination; Fig. 3b presents their external quantum efficiency (EQE) spectra; Table 1 lists the device performance parameters obtained from the J–V curves. The device incorporating PDI as the ETL exhibited an open-circuit voltage (Voc) of 0.93 V, a short-circuit current density (Jsc) of 19.31 mA cm–2, a fill factor (FF) of 61.49%, and a power conversion efficiency (PCE) of 11.04%; for the device based on PCBM, these values were 0.99 V, 17.26 mA cm–2, 58.52%, and 10.0% respectively. The histograms representing the distribution of the J-V parameters of PCBM and PDI PSCs are given in Figs. S4 and S5, respectively. The J-V parameters of forward and reverse scans for perovskite/PDI device are given in Table S3 and the respective J-V graph is shown in Fig. S6. The integrated Jsc values from EQE spectra (Fig. 3b), 17.22 mA/cm2 for perovskite/PCBM and 18.92 mA/cm2 for perovskite/PDI, agree within 3% to the Jsc measured in white light (Table 1). The series resistances (Rs) of the PDIand PCBM-based PSCs were 3.8 and 11.49 Ω cm–2, respectively; the higher value for the PCBM-perovskite device indicates a higher degree of charge recombination, as evidenced by its lower values of Jsc and FF. The higher value of Jsc for the perovskite/PDI device may also have arisen from the relatively greater absorbance of PDI over a wider range of wavelengths of visible light (Fig. S7a). Assuming that the ETL–Al interface featured efficient charge injection, due to the favorable energy level alignment, and because the anode used for both devices was the same, the difference in the values of Rs must have originated only from the charge transport properties of the ETL layers and their energy level alignments with the perovskite. The shunt resistances (Rsh) of the PSCs incorporating PDI and PCBM were 10 and 0.57 kΩ cm–2, respectively. The greater value for the former device indicates that the PDI film effectively blocked the leakage current, due to its smoother film surface and favorable energy alignment. For the perovskite/PCBM device, the rougher and discontinuous film morphology led to ineffective blockage of the leakage current and, therefore, a lower value of Rsh. We attribute the lower value of Jsc in the perovskite/PCBM device to the low mobility in the PCBM film, a result of its high surface roughness when formed on top of our mixed lead halide perovskite layer. Atomic force microscopy (AFM) images revealed (Fig. 4) that the initial roughness of the perovskite layer decreased only slightly, from 13.05 to 9.41 nm, after the deposition of PCBM, whereas the deposition of PDI decreased the roughness to 3.59 nm—that is, it formed a smoother surface suitable for improving charge transport. The value of Voc of the perovskite/PDI device was lower than that of the perovskite/PCBM device, despite the LUMO energy level of PDI being higher than that of PCBM, presumably because of grain boundary recombination occurring in the perovskite/PDI device. Grain boundary recombination is a common phenomenon in PSCs because their small crystallite sizes; it mainly affects the open-circuit voltage [38]. A double fullerene layer can effectively passivate grain boundaries and, thereby, minimize grain boundary recombination [34,39]. We also investigated the behavior of the PSCs under constant illumination. The steady-state PCE of the devices with an initial value of 10.15% exhibited a steady value of 10% over a period of 15 min (Fig. S8). After 15 min, the PCE was recorded to be 9.97%, which does not noticeably deviate from the initial PCE. This conforms to the J-V parameters observed from the champion device measured under white light.
Fig. 4. Surface topographic AFM images (10 µm×10 µm) of (a) the CH3NH3PbI3–xBrx (MAI:MABr =2.5:0.5) perovskite film on a PEDOT:PSS layer and (b, c) the surfaces obtained after deposition of (b) PCBM and (c) PDI films on the PEDOT:PSS/perovskite layer.
1.72, 1.67, and 1.62 eV, respectively, as calculated from Tauc plots (Fig. 2a). A plot of the band gap with respect to the Br content was linear, following the equation, Eg =0.1x+1.57, where x is Br content in weight percentage (Fig. S3). The equation is consistent with the reported values for the band gap of MAPbI3 (i.e., for x=0). The steady state photoluminescence (PL) of different MAI: MABr ratios showed a blue shift of the peaks with increasing MABr concentrations, indicating the change in bandgaps (Fig. S7a). X-ray diffraction (XRD) spectra (Fig. 2b) revealed a change in the crystal structure after incorporating Br– ions. The diffraction peaks 82
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Fig. 5. (a) Steady state PL spectra of CH3NH3PbI3–xBrx (MAI:MABr =2.5:0.5) perovskite without (black line) and with PCBM (red line) and PDI (blue line) as ETLs. (b) TRPL spectra of perovskites without ETL and with PCBM/PDI as ETLs.
value of Jsc. The value of Voc of a PSC depends on the difference between the valence band of the perovskite layer and the LUMO of the ETL; in our case, we observed a clear increase in the value of Voc upon the increase of MABr content, which resulted from the increase in bandgap of the perovskites. Fig. S9 displays scanning electron microscopy (SEM) images of the perovskites prepared using the various MAI:MABr ratios. The film quality decreased dramatically upon increasing the Br– content. We attribute the poorer performance of the devices prepared with MAI:MABr ratios of 1.5:1.5 and 2.0:1.0 to their poorer-quality perovskite films formed on PEDOT:PSS, and the poorer performance of the CH3NH3PbI3-based device to the energy level mismatch between the perovskite and the ETL. We recorded photoluminescence (PL) spectra to study the charge extraction phenomena of the ETL layers used in our PSCs. The steady state PL spectra of the perovskite films revealed significant quenching after the deposition of PCBM and PDI (Fig. 5a). The quenching after deposition of PDI was superior to that after deposition of PCBM, due to more favorable band alignment with the perovskite film. Time-resolved photoluminescence (TRPL) spectroscopy verified the charge transport properties of these perovskite films (Fig. 5b). The lifetime was obtained from the PL decay curves by fitting them into an exponential decay function and they mainly follow a single exponential decay with the expression for intensity, I˭I0 exp(−t/τ), where I0 is the initial emission intensity and τ is the PL life time. The TRPL of the perovskite film in the absence of an ETL layer had a time constant of 26.2 ns; the presence of ETLs resulted in much lower time constants, with that for PDI (6.3 ns) being shorter than that for PCBM (9.8 ns). These substantially shorter lifetimes suggest that the ETLs induced faster carrier extraction from the perovskite layer. Thus, both PCBM and PDI are capable of electron extraction from the perovskite, with PDI being slightly more efficient, exhibiting a much shorter lifetime. The efficiency of photocurrent generation in solar cells depends on the rate of charge generation, recombination, and charge transportation. An important characteristic of an ETL in a PSC is its charge carrier mobilities, which determine the efficiency of charge transport to the contacts. The difference in the series resistance of the PSCs incorporating PCBM and PDI ETLs may have originated from the differences in their charge carrier mobilities. We investigated the electron transport in PCBM and PDI films by measuring the dark J–V characteristics of electron-only devices having the device architecture indium tin oxide (ITO)/ZnO/(PCBM or PDI)/BCP/Al (see Supporting Information for details). The electron mobilities of PCBM and PDI were calculated by fitting the experimental data with the space-charge limited current equation (Fig. S10)
Fig. 6. (a) Short-circuit current densities of PSCs plotted with respect to the light intensity on a double-logarithmic scale. (b) Open-circuit voltages of PSCs, plotted on a semilogarithmic scale.
Table S1 presents the variation in the J–V parameters of the PSCs upon varying the Br– content. Widening of the band gap occurred upon increasing in Br– content in the PSC, as evident from the blue-shift of the signal in the UV–Vis absorption spectra, thereby suppressing the
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J=
(Vapp − Vbias )2 9 ε0 εr μ 8 d3
Taiwan (104-2221-E-001-014-MY3 and 104-2221-E-009-096-MY3) and the Career Development Award of Academia Sinica, Taiwan (103-CDAM01), for financial support.
(1)
where ε0 , εr , µ, and d represent the vacuum dielectric constant, the relative dielectric constant of the molecule, the electron mobility, and the thickness of the ETL, respectively. We calculated the electron mobilities of PCBM and PDI to be 1×10–4 and 1.2×10−4 cm2 V–1 s–1, respectively. The electron mobility in PDI was slightly higher than that in PCBM, presumably because the former's planar π-stacking is beneficial for efficient electron transport. Lower mobility creates spacecharge buildup and eventually leads to severe recombination losses. To further investigate the charge carrier recombination mechanisms in our PSCs, we studied the dependence of the values of Voc and Jsc on the light intensity. A power law relationship between the value of Jsc and the illuminating light intensity, on a double-logarithmic scale, is observed in solar cells [40],
Jsc ∝ I α
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(2)
A space-charge limited solar cell exhibits a value of α close to 0.75, whereas a device with no space-charge effects will have a value of α close to 1 [41]. Variation of current densities with the illumination light intensities, on a double-logarithmic scale, along with the corresponding linear fit is given in Fig. 6a. The near-unity value of α for our perovskite/PDI device (α=0.98) implies balanced charge carrier mobility, no substantial space-charge buildup [42], a predominance of monomolecular recombination (due to defects/traps), an absence of bimolecular recombination, [19] and efficient charge extraction; in contrast, the value of α of 0.91 for the perovskite/PCBM device suggests a certain degree of bimolecular recombination. The relationship between the value of Voc and the intensity of the illuminating light is given by
∂Voc nkT = ∂lnI0 q
(3)
where n is the ideality factor, k is the Boltzmann constant, T is the temperature (in Kelvin), and q is the electric charge. Fig. 6b presents a semi-logarithmic plot of the value of Voc with respect to the light intensity at room temperature. The slopes of these plots for the perovskite/PCBM and perovskite/PDI devices are 3.81 and 1.81 kt/q, respectively, suggesting the presence of trap-assisted recombination in both devices. We attribute the higher levels of recombination in perovskite/PCBM device, indicated by the higher value of n (3.81), to the poor morphology and poor interface (low value of Rsh) leading to considerable charge accumulation and, therefore, poor electron extraction. The value of n of 1.8 for the perovskite/PDI device suggests a considerable level of trap-assisted recombination, but comparatively lower than that in the PCBM-based device. 4. Conclusions Commercially available PDI can be used as low-cost alternative to PC60BM in PSCs. We have investigated the structural, elemental, morphological, photophysical, and charge transport properties of mixed lead halide perovskites in the presence of two different ETLs. The energy levels of the perovskite were modified through incorporation of Br– ions to align well with the LUMO of PDI. The PSCs incorporating PDI as the ETL exhibited improved morphologies with lower surface roughness. Trap-assisted recombination in the perovskite/PDI device was lower than that in the PCBM device. We achieved a champion efficiency of 11.04% when using PDI as the ETL in PSCs, confirming that planar PDI derivatives are potential alternatives for PCBM in hybrid PSCs. Acknowledgements Dr. Chu thanks the Ministry of Science and Technology (MOST) of 84
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P. Karuppuswamy et al. devices, Electrochim. Acta 175 (2015) 151–161. 25 W. Wang, J. Yuan, G. Shi, X. Zhu, S. Shi, Z. Liu, L. Han, H.-Q. Wang, W. Ma, Inverted planar heterojunction perovskite solar cells employing polymer as the electron conductor, ACS Appl. Mater. Interfaces 7 (2015) 3994–3999. 26 A. Isakova, P.D. Topham, Polymer strategies in perovskite solar cells, J. Polym. Sci. Part B Polym. Phys. 55 (2017) 549–568. 27 F. Bella, G. Griffini, J.-P. Correa-Baena, G. Saracco, M. Gratzel, A. Hagfeldt, S. Turri, C. Gerbaldi, Improving efficiency and stability of perovskite solar cells with photocurable fluoropolymers, Science 354 (2016) 203–206. 28 J. Das, R.B.K. Siram, D. Cahen, B. Rybtchinski, G. Hodes, Thiophene-modified perylenediimide as hole transporting material in hybrid lead bromide perovskite solar cells, J. Mater. Chem. A. 3 (2015) 20305–20312. 29 S.S. Kim, S. Bae, W.H. Jo, A perylene diimide-based non-fullerene acceptor as an electron transporting material for inverted perovskite solar cells, RSC Adv. 6 (2016) 19923–19927. 30 H.E. Katz, A.J. Lovinger, J. Johnson, C. Kloc, T. Siegrist, W. Li, Y.Y. Lin, A. Dodabalapur, A soluble and air-stable organic semiconductor with high electron mobility, Nature 404 (2000) (478–81). 31 H. Zhang, L. Xue, J. Han, Y.Q. Fu, Y. Shen, Z. Zhang, Y. Li, M. Wang, New generation perovskite solar cells with solution-processed amino-substituted perylene diimide derivative as electron-transport layer, J. Mater. Chem. A 4 (2016) 8724–8733. 32 Q. Wang, Y. Shao, Q. Dong, Z. Xiao, Y. Yuan, J. Huang, Large fill-factor bilayer iodine perovskite solar cells fabricated by a low-temperature solution-process, Energy Environ. Sci. 7 (2014) 2359–2365. 33 N.K. Kumawat, A. Dey, K. Narasimhan, D. Kabra, Near infrared to visible electro-
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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
Solution-processable electron transport layer for efficient hybrid perovskite solar cells beyond fullerenes
MARK
Priyadharsini Karuppuswamya,b,c,1, Chintam hanmandluc,d,1, Karunakara Moorthy Boopathic, ⁎ Packiyaraj Perumale, Chi-ching Liuc, Yang-Fang Chene, Yun-Chorng Changc, Pen-Cheng Wanga, , ⁎⁎ Chao-Sung Laid,f,g, , Chih-Wei Chuc,h a
Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan Nano Science and Technology Program, Taiwan International Graduate Program, Academia Sinica and National Tsing Hua University, Taiwan c Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan d Department of Electronic Engineering, Chang Gung University, Taoyuan 33302, Taiwan e Department of Physics, National Taiwan University, Taipei 10617, Taiwan f Department of Nephrology, Chang Gung Memorial Hospital, Taiwan g Department of Materials Engineering, Ming Chi University of Technology, Taiwan h College of Engineering, Chang Gung University, Taoyuan 33302, Taiwan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Hybrid perovskite solar cells Perylene diimide Electron transport layer Space charge limited current Carrier recombination
A solution-processable planar perylene diimide, N,N´-dipentyl-3,4,9,10-perylenedicarboximide (PDI), has been demonstrated as a suitable acceptor material to replace the conventional fullerene derivatives (e.g., PCBM) found in inverted perovskite solar cells (PSCs). The energy offset between the perovskite and PDI layers was optimized by varying the ratio of halides (iodide to bromide) to improve the exciton dissociation and carrier transport. The PDI acceptor material had higher electron mobility and smoother morphology on perovskite relative to those of PCBM, leading to enhancements in the short circuit current density, fill factor, and power conversion efficiency. The incorporation of PDI in the perovskite devices decreased recombination losses and improved the device stability. The performance of the best PSC containing PDI as the electron transport layer (11%) was higher than that of the best device featuring PCBM (10%).
1. Introduction Perovskite solar cells (PSCs) have gained considerable attention recently because their high efficiencies (> 20%) [1] are close to those of commercial Si-based and thin film solar cells [2]. Perovskites are organometal halides exhibiting high degrees of light absorption, high fluorescence yields, long carrier lifetimes, high diffusion lengths and highly tunable bandgaps [3–7]. Because of these properties as an ideal light absorber, the ready and simple solution-processability of perovskites [8,9] caters to the needs of low-cost photovoltaics. Continuous improvements in device architectures [10,11], carrier extraction layers [8,12], interface layers [13,14] and perovskite composition [15–17] are leading to more efficient and stable PSCs. Generally, perovskite device structure falls into two categories: mesoscopic and planar-heterojunction; the former structure requires high-temperature processing, while the latter is mostly solution-processable and, hence, more attractive. In
⁎
PSCs, the carrier extraction layers play several important roles: effectively extracting electrons and holes from the perovskite absorber material and helping to prevent leakage currents [18]. Electron transport materials have not been the focus of research attention as much as hole transport layers, with fullerenes being the most common type of electron transport layer (ETL) materials [19–21]. PCBM, a fullerene derivative, is commonly used as the ETL in planar heterojunction PSCs displaying high performance [9,22]. Unfortunately, PCBM comes with high production costs and photochemical instability [23], hindering its commercial viability. Apart from fullerenes, only a few polymers and perylene diimide small molecules have been studied as alternative materials for ETLs. Polymers have found their use in different types of solar cells as electrolytes and carrier transport layers. The coexistence of dye-sensitised solar cells (DSSC) and PSCs could be possible with the use of solid polymer electrolytes or perovskites to replace liquid electrolytes in DSSCs [24]. The first n-type
Corresponding author. Corresponding author at: Department of Electronic Engineering, Chang Gung University, Taoyuan 33302, Taiwan. E-mail addresses: [email protected] (P.-C. Wang), [email protected] (C.-W. Chu). 1 These authors contributed equally. ⁎⁎
http://dx.doi.org/10.1016/j.solmat.2017.04.043 Received 30 March 2017; Received in revised form 25 April 2017; Accepted 28 April 2017 0927-0248/ © 2017 Published by Elsevier B.V.
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Fig. 1. (a) Device architecture of p–i–n mixed-halide hybrid PSCs. (b) Schematic representation of energy band levels of PSCs featuring various perovskite compositions and ETLs. (c) Molecular structure of PDI. (d) UV absorption spectra of perovskites; arrow indicates the increase in Br– content.
In this paper, we demonstrate the successful preparation of planar heterojunction PSCs incorporating a PDI (N,N´-dipentyl-3,4,9,10-perylenedicarboximide) with terminal alkyl groups as a replacement for fullerene derivatives. To facilitate charge transport and exciton dissociation, we added methyl ammonium bromide (MABr) to better align the energy levels of the organometal halide and the PDI. Solutionprocessed deposition of PDI resulted in smooth, continuous films on the perovskite surface. Electrical and photophysical studies of the perovskite–ETL interfaces revealed that trap-assisted recombination occurred in both the PCBM and PDI ETLs, but bimolecular recombination was absent in the PDI-containing device. PDI provided higher mobility, a lower degree of recombination, and better performance than did PCBM, making it a potential ETL material for replacing PCBM in PSCs.
polymer ETL from the naphthalene diimide polymer family was reported to have an efficiency of 7% [25] in PSCs, later improved to 10% by Shao et al. [18]. An exhaustive review of the polymer electron transport materials used in PSCs has been reported recently [26]. Apart from ETL materials, a type of fluoro polymer coating has been shown to improve the stability of PSCs, which is another important parameter to be studied for commercial viability of PSCs [27]. Perylene diimide derivatives, employed commercially as cheap dyes and pigments, have high photostability and tunable energy levels because of the wide range of choices for derivatizing their structures. Their use in PSCs was introduced by Das et al. [28] who demonstrated that a thiophene-modified perylene diimide could be used as the hole transport layer. The use of perylene diimides as ETLs in PSCs was reported recently for a device incorporating its dimer doped with an organic molecule, displaying 10% efficiency [29]. The planar nature of perylene diimides makes them easy to synthesize and leads to high degrees of charge transport as a result of their π-stacking [30]. Zhang et al. reported that TiO2 could be replaced by an amino-substituted perylene diimide (N-PDI) in an n-i-p structure [31]. They proposed that a perylene diimide with terminal alkyl groups (C-PDI) could not be used for solar cell fabrication because perovskites cannot form complete film on non-wetting surface of C-PDI. This disadvantage of non-wetting surfaces might be overcome in an inverted architecture, where the ETL is fabricated on top of the perovskite layer; such a structure might also enhance the stability of the PSCs against moisture.
2. Experimental section 2.1. Materials Hydroiodic acid (HI), hydrobromic acid (HBr), and methylamine (CH3NH2) were purchased from Alfa Aesar and used without purification. PDI was obtained from Lumtec Corporation, Taiwan.
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atmosphere with stirring and then the solvent was evaporated (rotary evaporator) until a precipitate was formed [32]. The white precipitate was filtered off, washed thrice with diethyl ether, dried under vacuum at 60 °C overnight, and stored in a glove box until required for further use. 2.3. MABr Equal volume proportions of HBr (47 wt% in water) and CH3NH2 (33 wt% in absolute EtOH) were mixed with stirring at 0 °C in a threeneck round-bottom flask under a N2 atmosphere and then the solvent was evaporated (rotary evaporator) [33]. The precipitate was purified using the same procedure described above for MAI. 2.4. Device fabrication ITO substrates (< 10 Ω cm–1, RiT display) were patterned by etching through HCl followed by rinsing (20 min each) once with detergent and twice with DI water with ultrasonication. The substrates were blown dry under N2 and stored in an oven until required for use. The cleaned substrates were subjected to UV/ozone treatment for 15 min to remove organic contaminants and to make the surface more hydrophilic. PEDOT:PSS was spin-coated (4000 rpm, 40 s) on top of the ITO layer, followed by annealing at 130 °C for 30 min. The substrates were transferred to a glove box and left under a N2 atmosphere until required for further processing. The perovskite precursor PbI2 (99.998%, Alfa Aesar) along with a salt additive (7.5 mg of KCl) [34] was dissolved in DMSO (40 wt%); MAI:MABr was dissolved in 2propanol at various concentrations; these solutions were left overnight at 70 °C. The different MAI:MABr concentrations (weight percent ratios) used are as follows: 1.5:1.5 (450 mg each), 2.0:1.0 (600:300 mg) and 2.5:0.5 (750:150 mg) dissolved in 3 mL of 2-propanol. The perovskite layer was deposited (6000 rpm, 40 s) using a twostep method—first the PbI2 precursor layer and then the MAI: MABr layer—and then the system was subjected to post-annealing for 1.5 h at 100 °C. A solution of PDI (PenPTC, Lumtech Corporation) in chlorobenzene (1 wt%) was spin-coated onto the perovskite layer at various spin speeds (3000–6000 rpm) for 40 s and then the samples were annealed at 100 °C for 30 min. Sequential thermal evaporation of C60 (30 nm), Bathocuprine (BCP, 10 nm), and the Al electrode (100 nm) completed the device structure. 2.5. Material and device characterization The valence band level of the perovskite layers and ETLs were investigated at room temperature using photoelectron spectroscopy in air (PESA, Model AC-2, Riken Keiki). The electrochemical measurements were performed by using a potentiostat/galvanostat MacLab model ML160 controlled by NOVA software (1.8 version for Windows) using a conventional single-compartment three-electrode cell with a Pt working electrode, Ag wire as reference electrode, and Pt wire as the counter electrode. All measurements in deaerated dichloromethane were performed with freshly distilled solvent with a solute concentration of 1.0 mm in the presence of NBu4PF6 as the supporting electrolyte and a scan rate of 100 mV/s. Ferrocene (Fc) was added at the end of the experiment as internal standard to calibrate the redox potentials. The Fc/Fc+ redox couple with a half-wave potential E1/2 was set as 0.690 V vs. NHE in CH2Cl2. XRD patterns were recorded at room temperature using a Bruker D8 diffractometer, with a diffracted beam monochromator set for Cu Kα radiation (λ=1.54056 Å), in the 2θ range 10–80° with a step size of 0.03939° and a step time of 99.45 s. UV–Vis absorbance spectra were recorded using a Jacobs V-670 UV–Vis spectrophotometer. SEM images of the perovskites on PEDOT:PSScoated glass/ITO substrates were recorded using an FEI Noval 200 microscope (15 kV). PL emission spectra were recorded using an optically excited Q-switched Nd:YAG laser (266 nm, 3–5 ns pulse,
Fig. 2. (a) Tauc plots (corresponding to the UV absorbances), (b) XRD patterns, and (c) XRD spectra (plotted in the range 27°≤ 2θ≤ 31°) of the various perovskites.
2.2. Methyl ammonium iodide preparation HI (57% in water, 15 mL) was added to CH3NH2 (40 wt% in water, 13.5 mL) at 0 °C in a three-neck round-bottom flask under a N2 80
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Fig. 3. (a) J–V Characteristics of the best PSCs incorporating PCBM and PDI as ETLs, measured under illumination from an AM 1.5 G solar simulator (reverse scan). (b) EQE spectra of CH3NH3PbI3−xBrx perovskite (MAI:MABr =2.5:0.5) devices incorporating PDI and PCBM as ETLs; the integrated Jsc values were 18.91 for perovskite/PDI devices (black) and 17.22 for perovskite/PDI devices (red). Table 1 Performance parameters of mixed-halide CH3NH3PbI3–xBrx (MAI: MABr =2.5:0.5) PSCs featuring PCBM and PDI as ETLs, measured under AM 1.5 G illumination. The numbers represent the average of 10 devices prepared in two batches; the numbers in the brackets represent the parameters of the champion device. Device
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
Rs (Ω/cm2)
Rsh (kΩ/cm2)
Perovskite/PCBM
1.00 ± 0.02 (0.99)
16.91 ± 0.62 (17.26)
58.47 ± 2.90 (58.52)
9.87 ± 0.15 (10.00)
11.49
0.57
Perovskite/PDI
0.93 ± 0.01 (0.93)
19.30 ± 0.39 (19.31)
58.45 ± 2.75 (61.48)
10.49 ± 0.43 (11.04)
3.80
10.00
study. The perovskite layer was deposited in a two-step method, forming continuous and pinhole-free CH3NH3PbI3–xBrx films through annealing-driven interdiffusion of I– and Br– ions. The device fabricated with methyl ammonium iodide (MAI) alone displayed no significant performance, due to energy level mismatch (Table S1). The energy level diagram was drawn by the valence band values obtained from photoelectron spectroscopy in air (PESA) and band gap values obtained from Tauc plot (Fig. 1b, Fig. 2a); it reveals that this misalignment would lead to carrier recombination at the CH3NH3PbI3–PDI junction interface, whereas the mixed halide perovskites align more favorably. The ratio of MAI and MABr was optimized to achieve better alignment of the conduction band of the perovskite with the LUMO of PDI. Fig. 1c presents the molecular structure of PDI. A uniform layer of 100 nm thick PDI was spin coated on top the perovskite. Fig. S1a shows the cross-section SEM image of the perovskite device indicating the thicknesses of the different layers. Further, the presence of PDI was confirmed by the XPS depth profile analysis. Fig. S1b shows the X-ray photoelectron spectroscopy (XPS) depth profile analysis of Pb in the perovskite device clearly representing the increase in Pb content with continuous removal of the PDI top layer by sputtering. The HOMO and LUMO of PDI were calculated from cyclic voltammetry (powder form) and photoelectrospectroscopy (thin film) measurements, explained in detail in the experimental section. Cyclic voltammetry results of PDI are shown in Fig. S2. We used three different weight percentage ratios of MAI to MABr (1.5:1.5, 2.0:1.0 and 2.5:0.5) to evaluate the effect of incorporating Br– ions. UV–Vis absorbance spectra revealed that increasing the MABr concentration led to a blue shift of the exciton absorption edge (Fig. 1d), due to widening of the perovskite band gap, because the energy of Br(4p) is lower than that of Pb(6s) [37]. A systematized shift of the bandgaps to the wide bandgap region is observed with the addition of MABr in precursor solutions. The MAI:MABr ratios of 1.5:1.5, 2.0:1.0, and 2.5:0.5 yielded band gaps of
10 Hz) focused on a beam diameter of approximately 0.5 mm; all perovskite films for PL measurement were prepared on glass substrates. The EQE spectra of the encapsulated devices were measured under short-circuit conditions using a 75-W Xe lamp (Enlitech, QE-R3011) as the light source; the light output from the monochromator was focused on the photovoltaic cell being tested (DC mode). The devices were illuminated inside a glove box under simulated 1.5 AM illumination (100 mW cm–2) using a solar simulator (Thermal Oriel) featuring a Xe lamp. The light intensity was calibrated prior to device measurement, using a standard silicon photodiode (Hamamatsu). 2.6. Energy level diagram The HOMO of the PDI films on glass was obtained from PESA measurements. Band gap values calculated from Tauc plot of PDI films on glass substrates along with HOMO level values were used to calculate the LUMO of PDI thin films. LUMO of the PDI in powder form was also calculated by cyclic voltammetry measurements. The UVabsorbance data of PDI powder dissolved in chloroform was used to calculate bandgap and hence the HOMO of PDI in powder form. 3. Results and discussion An important aspect affecting the optical properties of any solaractive material is band gap tunability. The band gaps of hybrid perovskites can be tuned by varying the type and concentration of halide anions surrounding the metal (lead) cation [35]. In addition to iodide, bromide and chloride are also common halide ions in perovskites. Here, we use Br– to tune the bandgaps as Br– is advantageous compared to Cl– ions in terms of formation energy and grain orientation, as suggested from previous studies [36]. Fig. 1a provides a schematic representation of the device structure used in this present 81
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corresponded to a tetragonal structure in the absence of Br–, with the crystal structure changing to cubic after its addition [35]. Magnification of these XRD spectra in the range 27°≤ 2θ≤ 31° (Fig. 2c) revealed (220) and (004) peaks corresponding to the tetragonal structure of CH3NH3PbI3 perovskite and (002) peaks corresponding to the cubic structure of the CH3NH3PbI3–xBrx perovskite. A shift of the (002) peaks to higher angle occurred upon increasing the Br– concentration, resulting in a respective decrease of the lattice constant (Table S2). The observed changes in crystal structure and respective lattice constants are consistent with the ionic radius of a Br– ion being smaller than that of an I– ion. Fig. 3a displays the J–V characteristics of the as-fabricated devices, featuring PCBM and PDI as ETLs, under illumination; Fig. 3b presents their external quantum efficiency (EQE) spectra; Table 1 lists the device performance parameters obtained from the J–V curves. The device incorporating PDI as the ETL exhibited an open-circuit voltage (Voc) of 0.93 V, a short-circuit current density (Jsc) of 19.31 mA cm–2, a fill factor (FF) of 61.49%, and a power conversion efficiency (PCE) of 11.04%; for the device based on PCBM, these values were 0.99 V, 17.26 mA cm–2, 58.52%, and 10.0% respectively. The histograms representing the distribution of the J-V parameters of PCBM and PDI PSCs are given in Figs. S4 and S5, respectively. The J-V parameters of forward and reverse scans for perovskite/PDI device are given in Table S3 and the respective J-V graph is shown in Fig. S6. The integrated Jsc values from EQE spectra (Fig. 3b), 17.22 mA/cm2 for perovskite/PCBM and 18.92 mA/cm2 for perovskite/PDI, agree within 3% to the Jsc measured in white light (Table 1). The series resistances (Rs) of the PDIand PCBM-based PSCs were 3.8 and 11.49 Ω cm–2, respectively; the higher value for the PCBM-perovskite device indicates a higher degree of charge recombination, as evidenced by its lower values of Jsc and FF. The higher value of Jsc for the perovskite/PDI device may also have arisen from the relatively greater absorbance of PDI over a wider range of wavelengths of visible light (Fig. S7a). Assuming that the ETL–Al interface featured efficient charge injection, due to the favorable energy level alignment, and because the anode used for both devices was the same, the difference in the values of Rs must have originated only from the charge transport properties of the ETL layers and their energy level alignments with the perovskite. The shunt resistances (Rsh) of the PSCs incorporating PDI and PCBM were 10 and 0.57 kΩ cm–2, respectively. The greater value for the former device indicates that the PDI film effectively blocked the leakage current, due to its smoother film surface and favorable energy alignment. For the perovskite/PCBM device, the rougher and discontinuous film morphology led to ineffective blockage of the leakage current and, therefore, a lower value of Rsh. We attribute the lower value of Jsc in the perovskite/PCBM device to the low mobility in the PCBM film, a result of its high surface roughness when formed on top of our mixed lead halide perovskite layer. Atomic force microscopy (AFM) images revealed (Fig. 4) that the initial roughness of the perovskite layer decreased only slightly, from 13.05 to 9.41 nm, after the deposition of PCBM, whereas the deposition of PDI decreased the roughness to 3.59 nm—that is, it formed a smoother surface suitable for improving charge transport. The value of Voc of the perovskite/PDI device was lower than that of the perovskite/PCBM device, despite the LUMO energy level of PDI being higher than that of PCBM, presumably because of grain boundary recombination occurring in the perovskite/PDI device. Grain boundary recombination is a common phenomenon in PSCs because their small crystallite sizes; it mainly affects the open-circuit voltage [38]. A double fullerene layer can effectively passivate grain boundaries and, thereby, minimize grain boundary recombination [34,39]. We also investigated the behavior of the PSCs under constant illumination. The steady-state PCE of the devices with an initial value of 10.15% exhibited a steady value of 10% over a period of 15 min (Fig. S8). After 15 min, the PCE was recorded to be 9.97%, which does not noticeably deviate from the initial PCE. This conforms to the J-V parameters observed from the champion device measured under white light.
Fig. 4. Surface topographic AFM images (10 µm×10 µm) of (a) the CH3NH3PbI3–xBrx (MAI:MABr =2.5:0.5) perovskite film on a PEDOT:PSS layer and (b, c) the surfaces obtained after deposition of (b) PCBM and (c) PDI films on the PEDOT:PSS/perovskite layer.
1.72, 1.67, and 1.62 eV, respectively, as calculated from Tauc plots (Fig. 2a). A plot of the band gap with respect to the Br content was linear, following the equation, Eg =0.1x+1.57, where x is Br content in weight percentage (Fig. S3). The equation is consistent with the reported values for the band gap of MAPbI3 (i.e., for x=0). The steady state photoluminescence (PL) of different MAI: MABr ratios showed a blue shift of the peaks with increasing MABr concentrations, indicating the change in bandgaps (Fig. S7a). X-ray diffraction (XRD) spectra (Fig. 2b) revealed a change in the crystal structure after incorporating Br– ions. The diffraction peaks 82
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Fig. 5. (a) Steady state PL spectra of CH3NH3PbI3–xBrx (MAI:MABr =2.5:0.5) perovskite without (black line) and with PCBM (red line) and PDI (blue line) as ETLs. (b) TRPL spectra of perovskites without ETL and with PCBM/PDI as ETLs.
value of Jsc. The value of Voc of a PSC depends on the difference between the valence band of the perovskite layer and the LUMO of the ETL; in our case, we observed a clear increase in the value of Voc upon the increase of MABr content, which resulted from the increase in bandgap of the perovskites. Fig. S9 displays scanning electron microscopy (SEM) images of the perovskites prepared using the various MAI:MABr ratios. The film quality decreased dramatically upon increasing the Br– content. We attribute the poorer performance of the devices prepared with MAI:MABr ratios of 1.5:1.5 and 2.0:1.0 to their poorer-quality perovskite films formed on PEDOT:PSS, and the poorer performance of the CH3NH3PbI3-based device to the energy level mismatch between the perovskite and the ETL. We recorded photoluminescence (PL) spectra to study the charge extraction phenomena of the ETL layers used in our PSCs. The steady state PL spectra of the perovskite films revealed significant quenching after the deposition of PCBM and PDI (Fig. 5a). The quenching after deposition of PDI was superior to that after deposition of PCBM, due to more favorable band alignment with the perovskite film. Time-resolved photoluminescence (TRPL) spectroscopy verified the charge transport properties of these perovskite films (Fig. 5b). The lifetime was obtained from the PL decay curves by fitting them into an exponential decay function and they mainly follow a single exponential decay with the expression for intensity, I˭I0 exp(−t/τ), where I0 is the initial emission intensity and τ is the PL life time. The TRPL of the perovskite film in the absence of an ETL layer had a time constant of 26.2 ns; the presence of ETLs resulted in much lower time constants, with that for PDI (6.3 ns) being shorter than that for PCBM (9.8 ns). These substantially shorter lifetimes suggest that the ETLs induced faster carrier extraction from the perovskite layer. Thus, both PCBM and PDI are capable of electron extraction from the perovskite, with PDI being slightly more efficient, exhibiting a much shorter lifetime. The efficiency of photocurrent generation in solar cells depends on the rate of charge generation, recombination, and charge transportation. An important characteristic of an ETL in a PSC is its charge carrier mobilities, which determine the efficiency of charge transport to the contacts. The difference in the series resistance of the PSCs incorporating PCBM and PDI ETLs may have originated from the differences in their charge carrier mobilities. We investigated the electron transport in PCBM and PDI films by measuring the dark J–V characteristics of electron-only devices having the device architecture indium tin oxide (ITO)/ZnO/(PCBM or PDI)/BCP/Al (see Supporting Information for details). The electron mobilities of PCBM and PDI were calculated by fitting the experimental data with the space-charge limited current equation (Fig. S10)
Fig. 6. (a) Short-circuit current densities of PSCs plotted with respect to the light intensity on a double-logarithmic scale. (b) Open-circuit voltages of PSCs, plotted on a semilogarithmic scale.
Table S1 presents the variation in the J–V parameters of the PSCs upon varying the Br– content. Widening of the band gap occurred upon increasing in Br– content in the PSC, as evident from the blue-shift of the signal in the UV–Vis absorption spectra, thereby suppressing the
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J=
(Vapp − Vbias )2 9 ε0 εr μ 8 d3
Taiwan (104-2221-E-001-014-MY3 and 104-2221-E-009-096-MY3) and the Career Development Award of Academia Sinica, Taiwan (103-CDAM01), for financial support.
(1)
where ε0 , εr , µ, and d represent the vacuum dielectric constant, the relative dielectric constant of the molecule, the electron mobility, and the thickness of the ETL, respectively. We calculated the electron mobilities of PCBM and PDI to be 1×10–4 and 1.2×10−4 cm2 V–1 s–1, respectively. The electron mobility in PDI was slightly higher than that in PCBM, presumably because the former's planar π-stacking is beneficial for efficient electron transport. Lower mobility creates spacecharge buildup and eventually leads to severe recombination losses. To further investigate the charge carrier recombination mechanisms in our PSCs, we studied the dependence of the values of Voc and Jsc on the light intensity. A power law relationship between the value of Jsc and the illuminating light intensity, on a double-logarithmic scale, is observed in solar cells [40],
Jsc ∝ I α
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2017.04.043. References 1 D. Bi, W. Tress, M.I. Dar, P. Gao, J. Luo, C. Renevier, K. Schenk, A. Abate, F. Giordano, J.-P.C. Baena, Efficient luminescent solar cells based on tailored mixed-cation perovskites, Sci. Adv. 2 (2016) e1501170. 2 M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Solar cell efficiency tables (version 47), Prog. Photo.: Res. Appl. 24 (2016) 3–11. 3 M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites, Science 338 (2012) 643–647. 4 Y.H. Kim, H. Cho, J.H. Heo, T.S. Kim, N. Myoung, C.L. Lee, S.H. Im, T.W. Lee, Multicolored organic/inorganic hybrid perovskite light emitting diodes, Adv. Mater. 27 (2015) 1248–1254. 5 G. Xing, N. Mathews, S.S. Lim, N. Yantara, X. Liu, D. Sabba, M. Grätzel, S. 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(2)
A space-charge limited solar cell exhibits a value of α close to 0.75, whereas a device with no space-charge effects will have a value of α close to 1 [41]. Variation of current densities with the illumination light intensities, on a double-logarithmic scale, along with the corresponding linear fit is given in Fig. 6a. The near-unity value of α for our perovskite/PDI device (α=0.98) implies balanced charge carrier mobility, no substantial space-charge buildup [42], a predominance of monomolecular recombination (due to defects/traps), an absence of bimolecular recombination, [19] and efficient charge extraction; in contrast, the value of α of 0.91 for the perovskite/PCBM device suggests a certain degree of bimolecular recombination. The relationship between the value of Voc and the intensity of the illuminating light is given by
∂Voc nkT = ∂lnI0 q
(3)
where n is the ideality factor, k is the Boltzmann constant, T is the temperature (in Kelvin), and q is the electric charge. Fig. 6b presents a semi-logarithmic plot of the value of Voc with respect to the light intensity at room temperature. The slopes of these plots for the perovskite/PCBM and perovskite/PDI devices are 3.81 and 1.81 kt/q, respectively, suggesting the presence of trap-assisted recombination in both devices. We attribute the higher levels of recombination in perovskite/PCBM device, indicated by the higher value of n (3.81), to the poor morphology and poor interface (low value of Rsh) leading to considerable charge accumulation and, therefore, poor electron extraction. The value of n of 1.8 for the perovskite/PDI device suggests a considerable level of trap-assisted recombination, but comparatively lower than that in the PCBM-based device. 4. Conclusions Commercially available PDI can be used as low-cost alternative to PC60BM in PSCs. We have investigated the structural, elemental, morphological, photophysical, and charge transport properties of mixed lead halide perovskites in the presence of two different ETLs. The energy levels of the perovskite were modified through incorporation of Br– ions to align well with the LUMO of PDI. The PSCs incorporating PDI as the ETL exhibited improved morphologies with lower surface roughness. Trap-assisted recombination in the perovskite/PDI device was lower than that in the PCBM device. We achieved a champion efficiency of 11.04% when using PDI as the ETL in PSCs, confirming that planar PDI derivatives are potential alternatives for PCBM in hybrid PSCs. Acknowledgements Dr. Chu thanks the Ministry of Science and Technology (MOST) of 84
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