Letter pubs.acs.org/JPCL
Charge Transport and Recombination in Perovskite (CH3NH3)PbI3 Sensitized TiO2 Solar Cells Yixin Zhao and Kai Zhu* Chemical and Materials Science Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States S Supporting Information *
ABSTRACT: We report on the effect of TiO2 film thickness on the charge transport, recombination, and device characteristics of perovskite (CH3NH3)PbI3 sensitized solar cells using iodide-based electrolytes. (CH3NH3)PbI3 is relatively stable in a nonpolar solvent (e.g., ethyl acetate) with a low iodide concentration (e.g., 80 mM). Frequency-resolved modulated photocurrent/photovoltage spectroscopies show that increasing TiO2 film thickness from 1.8 to 8.3 μm has little effect on transport but increases recombination by more than 10-fold, reducing the electron diffusion length from 16.9 to 5.5 μm, which can be explained by the higher degree of iodide depletion within the TiO2 pores for thicker films. The changes of the charge-collection and light-absorption properties of (CH3NH3)PbI3 sensitized cells with varying TiO2 film thickness strongly affect the photocurrent density, photovoltage, fill factor, and solar conversion efficiency. Developing alternative, compatible redox electrolytes is important for (CH3NH3)PbI3 or similar perovskites to be used for potential photoelectrochemical applications. SECTION: Energy Conversion and Storage; Energy and Charge Transport
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Herein, we present results of our investigation of the effect of TiO2 film thickness on charge transport, recombination, and device characteristics of perovskite (CH3NH3)PbI3 sensitized solar cells using iodide-based liquid electrolytes. The liquid electrolyte can easily permeate the entire mesoporous TiO2 electrode. In contrast, the use of solid-state hole conductors has been reported to lead to variations in the pore-filling fraction and overlayer thickness of the hole conductor (depending on the electrode morphology),15 which would complicate the interpretation of the transport and recombination studies. In this work, we study charge transport and recombination in (CH3NH3)PbI3 sensitized cells by intensity-modulated photocurrent/photovoltage spectroscopies (IMPS/IMVS). We find that increasing film thickness affects strongly the electron diffusion length and recombination lifetime, which in turn, significantly influences the device characteristics of (CH3NH3)PbI3 sensitized cells. Figure 1 compares the X-ray diffraction patterns of the (CH3NH3)PbI3-covered and as-grown mesoporous TiO2 film on the fluorine-doped tin oxide glass (FTO) substrate. Strong diffraction peaks are observed at approximately 14.02, 19.92, 28.32, 31.76, 40.46, and 43.02°, respectively corresponding to the reflections from (110), (112), (220), (310), (224), and (314) crystal planes of the tetragonal perovskite structure.12,13 It is worth noting that several perovskite peaks expected for the bulk (CH3NH3)PbI3 sample3 are absent in Figure 1. Similar observations were reported previously for both (CH3NH3)PbI3
rganometallic halide perovskites (e.g., (CH3NH3)PbI3) represent a novel class of light absorbing materials that have recently attracted increasing attention for solar conversion applications.1−14 The attractiveness of the organometallic halide perovskites can be primarily ascribed to their suitable, direct bandgap with large absorption coefficients and the lowcost, solution-based fabrication process. The bandgap of these absorbers can also be tuned by adjusting their inorganic and organic components. The perovskite absorber was initially used as the sensitizer to replace dye molecules in the conventional dye-sensitized TiO2 nanocrystalline solar cell (DSSC) by using the liquid, iodide-based electrolyte with efficiency levels of approximately 3−6%.2,3 The cell performance was found to decrease with increasing electrode film thickness (3.6−8.6 μm),3 which is in contrast to the dependence normally observed for DSSCs. Recent efforts have centered on improving the performance of perovskite solar cells using solid-state hole conductors (e.g., spiro-MeOTAD) with efficiencies reaching 10−15%.5,6,10,15−17 Although the perovskite layer thickness for solid-state cells has been reduced to less than 2 μm, the solar conversion efficiency was still found to be greater for thinner samples (i.e., those that are a few hundred nanometers in thickness). However, the thinner samples exhibited insufficient light absorption, especially in the long wavelength range of the solar spectrum. Despite the rapid progress in improving cell performance, the dependence of the device characteristics on thickness is still poorly understood for both liquid-type and solid-state perovskite solar cells. This is largely due to the lack of studies on charge transport and recombination properties in perovskite solar cells. © XXXX American Chemical Society
Received: July 18, 2013 Accepted: August 9, 2013
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than 4 μm, owing to the overall strong absorption of (CH3NH3)PbI3. Figures 3 compares the J−V characteristics of the (CH3NH3)PbI3 sensitized cells with Electrolytes A and B
Figure 1. X-ray diffraction patterns of the (a) (CH3NH3)PbI3-covered and (b) as-grown mesoporous TiO2 film on the FTO substrate. Peaks associated with the perovskite structure are labeled.
Figure 3. Effect of electrolyte composition (Electrolytes A and B) on the J−V curves of (CH3NH3)PbI3 sensitized cells under simulated AM 1.5G sunlight. The TiO2 film thickness is 1.8 μm. The inset shows the IPCE of a cell using Electrolyte A. See text for details of the electrolyte compositions.
and (CH3NH3)PbBr3 grown on mesoporous metal oxides (e.g., TiO2 and Al2O3).3,7,18 Presumably, this is a result of the preferred growth orientation when perovskites were deposited on the nanoporous substrates.7 No impurity peaks other than the ones attributable to (CH3NH3)PbI3 and TiO2/FTO were observed in the XRD patterns, suggesting that (CH3NH3)PbI3 grown on the mesoporous TiO2 substrate is phase pure. Figure 2a shows a typical ultraviolet−visible (UV−vis) absorption spectrum of (CH3NH3)PbI3 deposited on a 2-μm thick mesoporous TiO2 film. The optical bandgap (Eg) of (CH3NH3)PbI3 can be determined from the Tauc plot of the absorption spectrum as shown in the inset of Figure 2a. The Tauc plot follows the expression (αhν)m ∝ (hν − Eg), where α is the absorption coefficient, hν is the photon energy, and m is 2 for a direct bandgap semiconductor.19 By extrapolating the linear region of the Tauc plot, the bandgap is found at about 1.53 eV, which is consistent with other reports for (CH3NH3)PbI3.5,13 The (CH3NH3)PbI3 deposited on a 2-μm thick mesoporous TiO2 film appears sufficient to absorb most light below 600 nm. The absorbance decreases slightly with increasing wavelength approximately from 600 to 750 nm, followed by a sharp decrease when the wavelength approaches the bandgap near 800 nm. Consistent with this absorption characteristic, we found as shown in Figure 2b that when the TiO2 film thickness increases from 1.8 to 8.3 μm, the absorption is enhanced primarily in the 600−800 nm spectral range, where (CH3NH3)PbI3 absorbs relatively weakly. The increase is less significant when the film thickness is greater
using 1.8-μm thick mesoporous TiO2 films under simulated AM 1.5G sunlight. Electrolyte A comprises 0.08 M LiI, 0.04 M I2, 0.05 M tBP (4-tert-butylpyridine), and 0.005 M urea in ethyl acetate. Electrolyte B comprises 0.5 M LiI, 0.25 M I2, 0.3 M tBP, and 0.03 M urea in ethyl acetate. Electrolytes A and B differ only in the concentration of the electrolyte components. The cell with Electrolyte A demonstrates a short-circuit photocurrent density (Jsc) of 14.52 mA/cm2, an open-circuit voltage (Voc) of 0.649 V, and a fill factor (FF) of 0.486 to give a solar conversion efficiency (η) of 4.58%. When Electrolyte B is used, the cell efficiency improves to 6.77% with a Jsc of 18.57 mA/cm2, a Voc of 0.663 V, and a FF of 0.549. The efficiency enhancement of the cell using Electrolyte B results primarily from the improved Jsc and FF relative to the cell using Electrolyte A, which is attributable to the different iodide concentrations of the electrolytes.20 The inset of Figure 3 shows the incident photon-to-current efficiency (IPCE) spectrum of the cell using Electrolyte A. The IPCE spectrum approximately follows the absorption spectrum (Figure 2b) with an onset near 800 nm, which agrees with the measured bandgap (ca. 1.53 eV; Figure 2a) for (CH3NH3)PbI3.
Figure 2. (a) A typical UV−vis absorption spectrum of (CH3NH3)PbI3 sensitized mesoporous TiO2 film. (b) Normalized absorption spectra as a function of the TiO2 film thickness. 2881
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9.18 mA/cm2, despite the enhanced absorption with thicker TiO2 films (Figure 2b). Voc and FF decrease respectively from 0.649 to 0.558 V and from 0.486 to 0.374 with increasing TiO2 film thickness from 1.8 to 8.3 μm. As a result of these changes, the cell efficiency decreases from 4.58 to 1.92% with TiO2 film thickness varied from 1.8 to 8.3 μm. To understand the effect of TiO2 film thickness on the J−V characteristics, we performed IMPS and IMVS measurements to investigate the charge transport and recombination properties in (CH3NH3)PbI3 sensitized cells with varying TiO2 film thickness. The typical IMPS and IMVS response curves for the (CH3NH3)PbI3 sensitized cells are shown in Figure S1 (Supporting Information). Both IMPS and IMVS spectra display a semicircle in the complex plane, which is similar to that observed for the conventional DSSCs. The characteristic transport and recombination frequencies, and consequently, the diffusion coefficient and recombination lifetime can be determined from these plots as detailed in our previous reports.21,22 Figure 4a shows the effect of TiO2 film thickness on the electron diffusion coefficient (D) for (CH3NH3)PbI3 sensitized cells (using Electrolyte A) as a function of photoelectron density (n) in cells. For each TiO2 film thickness, D displays a power-law dependence on n. For DSSCs, this nonlinear dependence has typically been attributed to electrons undergoing multiple trapping and detrapping during their transit through the TiO2 network.22−29 The trapping time is usually described by a power-law distribution of release times in the forms of t−1−α, where the parameter α is related to the disorder of the TiO2 film and has values between 0 and 1.30−32 Within the framework of a random walk model, D is related to n by the − 1/α 1/α−1 n , where Ntot is the total trap equation D = C1N1/3 tot density and C1 is a constant.30 Best fits to the data yield α = 0.47 ± 0.05 for all samples. Thus, no obvious film thickness dependence of the disorder parameter α is found for (CH3NH3)PbI3 sensitized cells. Moreover, for a given photoelectron density (e.g., 1 × 1017 cm−3), the D values for all samples differ only within a factor of 2. These results suggest that the density and distribution of transport-limiting traps in the (CH3NH3)PbI3 sensitized TiO2 films depend little on the TiO2 film thickness. Consequently, no significant thickness dependence is found for the electron diffusion coefficient in these cells. Figure 4b shows the effect of TiO2 film thickness on the electron recombination lifetime (τ) in (CH 3 NH 3 )PbI 3
Despite their higher performance, the cells based on Electrolyte B are only functional for several minutes, allowing only enough time for the J−V measurement. In contrast, the cells using Electrolyte A are stable for 1−2 h, which is sufficient for conducting the J−V and other photoelectrochemical measurements reported in this study. Table S1 (Supporting Information) shows the effect of aging time on the device characteristics of (CH3NH3)PbI3 sensitized cells using Electrolyte A. The degradation of the cell performance is normally accompanied by a color bleaching of perovskites even when the cells are stored in the dark, suggesting that the chemical instability of perovskites in the iodide electrolyte contributes to the degradation of the cell performance over time. Using electrolytes with even higher concentrations (e.g., 1 M iodide) than Electrolyte B (0.5 M iodide) takes seconds to completely bleach the (CH3NH3)PbI3 deposited on the TiO2 film. Although the exact degradation or bleaching mechanism is unknown, it is important to develop new redox electrolytes that are more compatible with (CH3NH3)PbI3 or similar perovskite absorbers in order for these promising materials to find use for photoelectrochemical applications in the future. It is also notable that the (CH3NH3)PbI3 film is not stable (dissolved or bleached) in polar solvents such as acetonitrile, which is commonly used in DSSCs. In contrast, no change is found for (CH3NH3)PbI3 in a nonpolar solvent such as ethyl acetate for a few weeks, which is consistent with a previous report.3 Table 1 compares the J−V characteristics of (CH3NH3)PbI3 sensitized cells using Electrolyte A with different TiO2 film Table 1. Effect of TiO2 Film Thickness on Short-Circuit Photocurrent Density (Jsc), Open-Circuit Voltage (Voc), Fill Factor (FF), Solar Conversion Efficiency (η), and Electron Diffusion Length (Ld) of (CH3NH3)PbI3 Sensitized Solar Cells Using Electrolyte A Thickness (μm)
Jsc (mA/cm2)
Voc (V)
FF
η (%)
Ld (μm)
1.8 3.9 5.8 8.3
14.52 15.14 10.38 9.18
0.649 0.623 0.573 0.558
0.486 0.470 0.431 0.374
4.58 4.43 2.57 1.92
16.9 11.7 8.2 5.5
thicknesses. The Jsc first increases from 14.52 to 15.14 mA/cm2 when the TiO2 film thickness is changed from 1.8 to 3.9 μm. This increase of Jsc is consistent with the enhanced absorption shown in Figure 2b. However, further increasing film thickness from 3.9 to 5.8 to 8.3 μm reduces Jsc from 15.14 to 10.38 to
Figure 4. Effect of TiO2 film thickness on the (a) electron diffusion coefficient and (b) recombination lifetime for (CH3NH3)PbI3 sensitized cells (using Electrolyte A) as a function of photoelectron density. 2882
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TiO2 film thickness, faster recombination for cells with thicker TiO2 films significantly limits the solar conversion process for (CH3NH3)PbI3 sensitized cells using the iodide electrolyte in this study. Alternative redox electrolytes (new redox couples or different composition/concentration of the iodide electrolyte), light trapping schemes, or both are thus needed to improve the performance of (CH3NH3)PbI3 sensitized photoelectrochemical cells. In summary, we investigated the effect of TiO2 film thickness (1.8−8.3 μm) on charge transport, recombination, and device characteristics of perovskite (CH3NH3)PbI3 sensitized photoelectrochemical cells using iodide-based electrolyte. (CH3NH3)PbI3 is found to be relatively stable in the nonpolar solvent (e.g., ethyl acetate) with a low iodide concentration (e.g., 80 mM). In contrast, using polar solvent (e.g., acetonitrile) or high iodide concentration (e.g., 0.5−1 M) leads to rapid degradation or bleaching of (CH3NH3)PbI3. The absorption of (CH3NH3)PbI3 increases with film thickness, especially in the long wavelength range of the visible solar spectrum. While the electron diffusion coefficient depends little on the TiO2 film thickness, recombination becomes much faster with thicker films, which is attributable to the higher degree of iodide depletion within the TiO2 pores for thicker films resulting from the low iodide concentration in the bulk of the electrolyte. The electron diffusion length decreases from 16.9 to 5.5 μm as TiO2 film thickness increases from 1.8 to 8.3 μm. Consistent with the transport and recombination results, the open-circuit voltage, fill factor, and solar conversion efficiency of (CH3NH3)PbI3 sensitized cells decrease significantly with increasing TiO2 film thickness. As a net effect of the higher absorption and faster recombination with thicker TiO2 film, the short-circuit photocurrent density first increases slightly with film thickness from 1.8 to 3.9 μm and then decreases by about 40% with film thickness from 3.9 to 8.3 μm. Developing alternative redox electrolytes that are more compatible with (CH3NH3)PbI3 or similar perovskite absorbers is important for these materials to find potential use for photoelectrochemical applications.
sensitized cells. Recombination in conventional DSSCs refers to the process by which electrons are transferred to either the oxidized species of the redox couple or the oxidized dyes. For a standard DSSC, the recombination of electrons to the oxidized dyes can generally be ignored because the high concentration (e.g., 0.5−1 M) of I− in the standard electrolyte can effectively regenerate oxidized dyes following electron injection.33,34 For conventional DSSCs, recombination is usually found to be independent of the electrode film thickness.21,35 In contrast, Figure 4b shows that τ depends strongly on the TiO2 film thickness for the (CH3NH3)PbI3 sensitized cells. Analyses of Figure 4b show that at a given photoelectron density (e.g., 3 × 1017 cm−3), τ decreases by more than 1 order of magnitude with increasing TiO2 film thickness from 1.8 to 8.3 μm. The decrease of τ is likely caused by the unusually low concentration (0.08 M) of I− in Electrolyte A compared to the standard iodide electrolyte (0.5−1 M) used for DSSCs. During the normal operation of a DSSC, I− diffuses into the pores of TiO2 to regenerate the oxidized dye whereas I3− diffuses out of the pores of TiO2 toward the counter electrode to be reduced.34 In general, neither of these diffusion processes limits the photocurrent extraction in DSSCs.36 However, a recent study showed that the rate of regenerating the oxidized dyes could drop by more than 1 order of magnitude when the iodide concentration was lowered from 0.5 M to 70 mM, leading to a significant increase of the electron recombination with the oxidized dyes and a reduction of the Jsc value by about 30%.20 Thus, because of the low iodide concentration in the bulk of the electrolyte used for our (CH3NH3)PbI3 sensitized cells, the depletion of I− within the TiO2 pores is presumably enhanced to a higher degree when the iodide diffusion pathways through the TiO2 pores are elongated with increasing TiO2 film thickness. The increased level of I− deficiency with thicker TiO2 films explains the faster recombination (or reduced lifetime) with increasing film thickness (Figure 4b). Consistent with this interpretation, the Jsc value of (CH3NH3)PbI3 sensitized cells increases with higher I− concentration and reaches a saturation level when the I− concentration is greater than 0.3 M (Figure S2 in the Supporting Information). From the experimentally determined D and τ values (Figure 4), we can calculate the electron diffusion length Ld using the expression Ld = (Dτ)1/2. A longer Ld corresponds to a higher charge-collection efficiency. For a standard DSSC, Ld is usually several (>3) times greater than the electrode film thickness in order to achieve the optimum cell performance.34 Analyses of the data in Figure 4 at a constant photoelectron density (e.g., 3 × 1017 cm−3) shows that Ld decreases monotonically from 16.9 to 11.7 to 8.2 to 5.5 μm when the TiO2 film thickness increases sequentially from 1.8 to 3.9 to 5.8 to 8.3 μm (Table 1). It is evident that only the first two thinner samples (1.8 and 3.9 μm) have Ld values that are much longer than the TiO2 film thickness. Therefore, the enhancement of Jsc with the TiO2 film thickness from 1.8 to 3.9 μm is primarily ascribed to the improved light absorption (Figure 2b). Further increase of TiO2 film thickness reduces Ld significantly to be either close to or less than the TiO2 film thickness, and thus should limit charge collection34 and reduce Jsc (Table 1), despite the enhanced light absorption (Figure 2b). The substantially lower Voc and FF for cells based on thicker TiO2 films (Table 1) are also consistent with the more rapid recombination for thicker TiO2 films (Figure 4b).37 Therefore, we conclude that although the light absorption increases (especially in the long wavelength range of the visible solar spectrum; Figure 2b) with increasing
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ASSOCIATED CONTENT
S Supporting Information *
Experimental method, typical IMPS and IMVS responses in the complex plane, and Jsc dependence on the iodide concentration. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy, under contract No. DE-AC36-08GO28308 with the National Renewable Energy Laboratory.
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