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The Controlling Mechanism for Potential Loss in CH3NH3PbBr3 Hybrid Solar Cells Xiaojia Zheng, Bo Chen, Mengjin Yang, Congcong Wu, Bruce Orler, Robert B. Moore, Kai Zhu, and Shashank Priya ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00215 • Publication Date (Web): 25 Jul 2016 Downloaded from http://pubs.acs.org on July 25, 2016

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ACS Energy Letters

The Controlling Mechanism for Potential Loss in CH3NH3PbBr3 Hybrid Solar Cells Xiaojia Zheng,* a Bo Chen,a Mengjin Yang,b Congcong Wu,a Bruce Orler,c Robert B. Moore,c Kai Zhu,* b and Shashank Priya* a a

Center for Energy Harvesting Materials and System, Virginia Tech, Blacksburg, Virginia 24061, United States

b

Chemistry and Nanoscience Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States c

Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, United States AUTHOR INFORMATION Corresponding Author * Email: [email protected], [email protected], [email protected]

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ABSTRACT We investigated moisture and thermal stability of MAPbBr3 perovskite material. Cubic MAPbBr3 was found to be moisture-insensitive and can avoid the thermal stability issues introduced by low-temperature phase transition in MAPbI3. MAPbBr3 and MAPbI3 hybrid solar cells with efficiencies of ~7.1% and ~15.5%, respectively, were fabricated and we identified the correlation between the working temperature/light intensity and the photovoltaic performance. No charge-carrier transport barriers were found in the MAPbBr3 and MAPbI3 solar cells. The MAPbBr3 solar cell displays a better stability under high working temperature due to its closed packed crystal structure. Temperature-dependent photocurrent-voltage characteristics indicate that, unlike the MAPbI3 solar cell with an activation energy (EA) nearly equal to its bandgap (Eg), the EA for the MAPbBr3 solar cell is much lower than its Eg. This indicates that a high interface recombination process limits the photovoltage and consequently the device performance of the MAPbBr3 solar cell.

TOC GRAPHICS


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Methylammonium lead trihalide perovskites (CH3NH3PbX3 or MAPbX3, X =Br, Cl, or I) have recently emerged as a strong candidate for photovoltaic and general optoelectronic applications due to their facile fabrication process, suitable bandgap (Eg), high absorption coefficient, long hole and electron diffusion length, and the capability for ambipolar carrier transport.1-3 Since the demonstration of perovskite solar cell (PSC) in 20094 based upon MAPbX3 (X=Br or I) as the sensitizer in a liquid quantum dot solar cell with power conversion efficiency (PCE) of ~4%, the interest in the academic community and the performance of the PSC have both increased rapidly. Recent advances in efficiency with certified PCEs as high as ~22%, together with a PCE >15% for active areas on the square-centimeter scale, have opened the opportunity for the future application of this class of solar cells.5-7 The challenge remains in resolving the low stability of MAPbI3 toward heat and moisture mainly due to the facile phase transition at ~330 K and formation of hydrated phases. This challenge has hindered the practical scale-up of the perovskite technology using MAPbI3 perovskite.8-9 Unlike MAPbI3, MAPbBr3 shows much better stability under both heat and moisture, and its larger Eg shows a great potential as the top cell in tandem solar cells. To match the Eg of crystalline Si (1.12 eV)—although the optimum Eg required is in the 1.7–1.8 eV range—an Eg between 1.6 and 2.3 eV can provide a PCE as high as 40% for a dual-junction tandem structure based on the detailed balance principle. A multijunction structure based on three cells with Eg1 = 2.3 eV, Eg2 = 1.4 eV, and Eg3 = 0.8 eV can provide an efficiency of 49%.10 MAPbBr3 with an Eg of 2.3 eV can be easily tuned to larger or smaller values by composition engineering with great potential in colored, partially transparent, or tandem solar cells.11-12 However, compared to the optimized iodide-based solar cell with an Eg/q–Voc loss of ~0.4 V (where Voc is the open-circuit voltage), the potential loss in MAPbBr3 is usually higher (0.7 V or more), which hinders the Voc and device performance of Br-based solar cells far below the predicted value.13-15 Similar problems related to high potential losses also exist in the chalcopyrite solar cells, such as Cu(In,Ga)(Se,S)2 (CIGS) and Cu2ZnSn(Se,S)4 (CZTS).16-19 Especially for CZTS, the Eg/q–Voc value is always larger than 0.6 V, which significantly affects its efficiency. Much work in the literature has focused on identifying the origin of the large potential loss in the chalcopyrite solar cells, and the results suggest that the primary reason is the conduction-band offset (CBO) between the window layer (buffer) and the light-absorber layer.16, 20-21 For a given


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buffer layer, the Voc has a limiting value independent of the absorber bandgap due to interface recombination. Reducing carrier recombination at the buffer/absorber interface by minimizing the CBO is a prerequisite toward obtaining high Voc and, consequently, high-efficiency thin-film solar cells.16 We hypothesize that the issues observed in the chalcopyrite cells could apply to MAPbBr3-based PSCs. However, limited systematic studies have been conducted in the literature on understanding the origin of the high potential losses in MAPbBr3 solar cells, and thus, the reasons remain unclear. The temperature dependence of the photovoltaic parameters of PSCs are very useful in understanding the origin of loss behavior, and the temperature dependence is also one of the critical concerns in using PSCs because cell performance decreases with increasing temperature due to the increased internal carrier recombination rates caused by increased carrier excitation.22 Here, we first investigate the heat and moisture stability of MAPbBr3 perovskite materials, and then evaluate the impact of working temperature and light intensity on the photovoltaic properties of MAPbBr3 solar cells. We also investigated the MAPbI3 solar cell as a reference. Unlike MAPbI3, MAPbBr3 perovskite material is less sensitive to moisture, and it can maintain its structure even after a 5-month aging period in ambient atmosphere with a high relative humidity (RH) due to its closely packed cubic crystal structure.23 The cubic structure can also avoid the thermal stability issues introduced by phase transition. External quantum efficiency (EQE) and photoresponse under different light intensities both show efficient charge generation/separation/collection in MAPbBr3 solar cell, similar to that of MAPbI3 solar cells. However, unlike the MAPbI3 solar cell, which has an activation energy (EA) close to its Eg, the EA of the MAPbBr3 solar cell is much lower than its Eg, which indicates a high interface recombination process and this could be the reason for its higher potential loss.


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ACS Energy Letters

Figure 1. (a) Photograph, (b) X-ray diffraction patterns and (c) UV-vis absorption spectra and the corresponding Tauc plot of the as-synthesised MAPbBr3 thin film and the sample left in ambient atmosphere for five months. (d) DSC heating (solid trace) and cooling (dashed trace) curves for the perovskite single crystal heated at 10 °C min−1.

It is well known that MAPbI3 is very sensitive to moist air and will degrade in several hours under high humidity.24 However, MAPbBr3 shows excellent stability under ambient environment. Figure 1a shows photographs of the MAPbBr3 film fabricated on glass slide substrates. A yellow thin film with smooth surface can be obtained by the anti-solvent assisted crystallization (ASAC) approach, which is consistent with its Eg (~2.3 eV). More importantly, there was no change in the color of the thin film after an aged period in the ambient atmosphere with a RH as high as ~70% for 5 months, which indicates that MAPbBr3 can maintain its form for a prolonged period of time. To confirm our hypothesis, we also performed X-ray diffraction (XRD) and Ultraviolet-visible (UV-vis) absorption spectra for all the samples. From the XRD results shown in Figure 1b, the sharp reflection peaks at 15.2° and 30.4° correspond to the (100)


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and (200) reflections of MAPbBr3, respectively, which shows preferred orientation in the (100) direction of the thin films. After five months, no new peaks were found, which confirmed the purity of the MAPbBr3 component. The sharp optical band edge for both fresh and aged samples of the MAPbBr3 films given in Figure 1c also indicates excellent stability of the sample. The ionic radius of I- and Br- are 0.220 nm and 0.196 nm, respectively. The smaller Br- can provide a higher bond strength, thereby resulting in a more-compact crystal lattice for higher stability. In addition to the moisture-related stability, thermal stability is another important factor that affects the solar cell application. In normal practical conditions, direct exposure of solar cells to sunlight will increase the temperature of the solar panel. The accumulation of heat can increase the temperature of the solar cell as high as ~358 K assuming that the environmental temperature is ~313 K.25 Although the encapsulation can overcome moisture stability, it cannot solve the thermal instability without additional cooling support. Differential Scanning Calorimetry (DSC) has been used to monitor polymorphic crystal transitions of perovskite samples. To understand the effect of the halide ion on the polymorphic transitions, only MAPbX3 (X=Br, I) single crystals were used in this measurement (Figure S1). Figure 1d compares the thermal behavior of the Br- and I-based perovskite samples. To better understand the thermal reversibility of these materials, both the heating and cooling scans were recorded. For the MAPbI3-based perovskite, the heating scan shows a 0.3 J/g endothermic peak at 330.2 K. Upon cooling, a similar exothermic peak was observed in the same temperature range, implying phase reversibility. These results are in good agreement with the reported tetragonal to cubic phase transition of MAPbI3.26-27 The phase transition around 330 K will cause a large volume change of the light absorber due to an obvious refined lattice parameter change of the crystallographic c axis,27 which may degrade solar cell performance during thermal cycling. In contrast, the MAPbBr3 sample did not display any phase transitions between 273 K and 373 K. The absence of phase transitions for the MAPbBr3 suggests that this material will be less sensitive to thermal cycling, thus giving this material a higher tolerance to environmental constraints.


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Figure 2. (a) Microstructures of the MAPbBr3 solar cell. (b) Diagram of energy levels (relative to the vacuum level) of each functional layer in the perovskite photovoltaic device. (c) J-V curves and (d) EQE of the perovskite solar cells.

Table 1. Photovoltaic parameters of the typical MAPbI3 and MAPbBr3 perovskite solar cells at room temperature. Samples

Voc (V)

Jsc (mA cm-2)

FF (%)

η (%)

Eg (eV)

Eg/q-Voc (V)

MAPbI3

1.08

20.43

70.29

15.51

1.53

0.45

MAPbBr3

1.38

7.15

71.65

7.07

2.31

0.93

To investigate the influence of working temperature on the photovoltaic properties of MAPbBr3 solar cells, several samples were fabricated using the ASAC approach. To overcome the low PCE of planar MAPbBr3 caused by low short-circuit current,11, 28 a 300-nm mesoporous TiO2 (electron transport material or ETM) layer was used to collect and transport the electrons. Figure 2a shows the microstructure of the MAPbBr3 device. A continuous, flat, and dense


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MAPbBr3 film fully covered the top surface of the mesoporous TiO2 layer, along with the filling of the perovskite in the mesoporous TiO2 layer. Spiro-OMeTAD was chosen as the holetransport material (HTM). A MAPbI3 solar cell was also fabricated as a reference, and the device architecture of the MAPbI3 solar cell is shown in Figure S2. The energy levels of TiO2, MAPbBr3, MAPbI3, and spiro-OMeTAD are shown in Figure 2b. The conduction band (CB) of the TiO2 layer lies at about -4.0 eV whereas the CB of MAPbBr3 is around -3.38eV (band position analysis of MAPbBr3 can be found in Figure S3).13-14, 29 The ~0.6 eV difference between the conduction bands provides an efficient charge-separation driving force for the electron collection. The valence band (VB) of the MAPbBr3 lies around -5.69 eV, and the highest occupied molecular orbital (HOMO) of spiro-OMeTAD is reported to be located at -5.2 eV, which indicates that it can smoothly extract the holes from MAPbBr3. For the MAPbI3, with a CB of -3.93 eV and a VB of -5.43 eV,13 the energy level is well aligned with TiO2 and spiroOMeTAD for the charge-carrier separation and collection. The current density–voltage (J-V) curves and the EQE of the TiO2/perovskite/spiro-OMeTAD solar cell devices are shown in Figure 2c–d. The corresponding photovoltaic parameters are listed in Table 1. For the MAPbBr3 solar cell, a Voc of 1.38 V, a short-circuit current density (Jsc) of 7.15 mA cm2, and a fill factor (FF) of 71.65% were obtained with an overall PCE of 7.07%. The device shows a small amount of J-V hysteresis due to the room-temperature processing that minimizes the formation of surface electron traps and corresponding slow transient current (Figure S4).11 The MAPbI3 solar cell exhibits a Voc of 1.08 V, Jsc of 20.43 mA/cm2, and FF of 70.29%, leading to a PCE of 15.51%. Compared to the MAPbI3 solar cell, the MAPbBr3 solar cell displays higher Voc (consistent with larger Eg) and lower Jsc (consistent with the lower cut-off wavelength of photoresponse). However, the Eg/q–Voc value for the MAPbBr3 solar cell is 0.93 V, which is much higher than that of the MAPbI3 solar cell with a value of 0.45 V. This limits the photovoltaic performance of the Br-based solar cell. The EQE spectra shown in Figure 2d demonstrate that both materials exhibit

good

photoresponse

in

the

absorption

region,

implying

that

charge

generation/separation/collection are efficient in both kinds of solar cells, benefiting from the sufficient driving force for the charge separation as discussed in Figure 2b.


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Figure 3. (a) Schematic diagram of the setup for performance measurement at different temperatures. Comparative analysis of the performance parameters: (b) Voc, (c) Jsc, (d) FF, and (e) PCE of MAPbBr3 solar cell at different temperatures. (Black dots represent the PV parameters obtained at heated temperature and the red stars represent the PV parameters obtained at room temperature after the solar cell is cooled down from the corresponding “high temperature.”) Figure 3 shows the thermal behaviors of the MAPbBr3 solar cell, which was measured under simulated one-sun illumination with 100 mW/cm2. As shown in Figure 3a, we used a hotplate to heat the solar cell and a thermometer to monitor the working temperature of the device. Figure 3b–e compare the performance parameters of the same solar cell under different working temperatures. There are two sets of data shown in this figure, where the black dots represent the PV parameters of the solar cell as a function of temperature and the red stars


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represent the PV parameters of the same solar cell after the working temperature is cooled down from the corresponding “high temperature” to the room temperature (The simplified working procedure for data collecting can be found in supporting information in Figure S5). Through the data shown by red stars, we can identify the safe temperature range for the device and thereby exclude the impact caused by the irreversible damage to the solar cell at high temperatures, which may come from the degradation of perovskite layer, HTM, or both. Similar data for MAPbI3 solar cell are shown in Figure S6. The results show that the performance of MAPbBr3 at temperatures higher than 364 K exhibits irreversible damage. In contrast, for the MAPbI3 solar cell, a temperature higher than 341 K begins to degrade the device performance (Figure S6). From the parameters collected at high temperatures (the black circles), as shown in Figure 3b–e, we can see that the Voc, FF, and PCE decrease with temperature, while Jsc is almost independent of the temperature. For a non-ideal solar cell, the Voc can be written as: ࢂ࢕ࢉ =

࢔࢑ࢀ ࢗ

‫(ܖܔ‬૚ −

ࡳ࢙ࢎ ࢂ࢕ࢉ ࡶ૙

ࡶ

+ ࡶࡸ )

(1)

૙

where J0, Gsh, n, k, and JL are the diode saturation current density, parallel conductance, diode ideality factor, Boltzmann constant, and photocurrent density, respectively. The diode saturation current density can be further written as: ࡱ

ࡱ

࡭ ࡭ ) ≈ ࡭ࢀ૜ ࢋ࢞࢖[− ࢔࢑ࢀ ] ࡶ૙ = ࡶ૙૙ ‫(ܘܠ܍‬− ࢔࢑ࢀ

(2)

where EA (activation energy for recombination) and J00 (prefactor) mainly depend on the dominating recombination mechanisms.30 If JL >> GshVoc and JL >> J0, then according to Equations (1) and (2), the temperature dependence of the open-circuit voltage can be simplified to: ࢂ࢕ࢉ =

ࡱ࡭ ࢗ

−

࢔ࡷࢀ ࢗ

× ‫ܖܔ‬

ࡶ૙૙

(3)

ࡶࡸ

In general, the ideality factor n and J00 have minor temperature dependence. According to Equation (3), we know that Voc depends strongly on the temperature, decreasing linearly with the increase of temperature. By data fitting (using data in the safe temperature range where there is no device degradation), we find that Voc increases with decreasing temperature by 1.8 mV/K and


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2 mV/K for MAPbI3 and MAPbBr3 solar cells, respectively. Similar values have been reported for the crystalline Si solar cells (~2 mV/K).31 As shown in Figure 3d, FF also decreases monotonically with increasing temperature. It is difficult to derive a generic expression for the temperature dependence of FF because it can be affected by a variety of cell parameters including cell ideality factors, parasitic losses, and shunt resistances. However, it is generally observed for good solar cells that FF usually decreases with temperature.32 Because both Voc and FF decrease with temperature and the short-circuit current is relatively unaffected by temperature, the cell performance decreases with increased temperature.

Figure 4. (a) Temperature dependence of Voc with data extrapolated to 0 K. (b) A “cliff-type” band alignment for perovskite solar cells with large conduction-band offset (CBO; ∆Ec) at the electron transfer layer /absorber interface.


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Assuming n, J00, and JL to be temperature independent (which is reasonable for reasons discussed above), the plot of Voc vs T should exhibit a straight line and its linear extrapolation to T = 0 K will yield the EA / q. The plots of Voc vs T data for MAPbI3 and MAPbBr3 solar cells are shown in Figure 4a. It clearly shows that unlike the MAPbI3 solar cell with an EA nearly equal to its Eg, the EA for the MAPbBr3 solar cell is much lower than its Eg. As shown in Figure 2b, the CB difference between both perovskite layers and TiO2 can provide an efficient chargeseparation driving force facilitating the electron injection. However, the larger CBO between MAPbBr3/TiO2 can lead to a “cliff-type” band alignment (Figure 4b). As suggested in previous reports, although there is no barrier for the electron-transfer process, Voc will decrease monotonically with the increasing CBO value due to the cliff that can decrease the EA.33-35 In PSCs, the CBO of the ETM/absorber layers but also the valence band offset (VBO) of the absorber and HTM layers are important. There are experimental reports on the variation of device behaviors with different HTMs that could be partly ascribed to the effect of the band offset.14, 36


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Figure 5. J-V curves of (a) MAPbBr3 and (b) MAPbI3 solar cells under different light intensities from 8 to 100 mW cm−2. Plots of (c) Jsc and (d) Voc as a function of light intensity from 8 to 100 mW cm−2.

To further understand the recombination mechanism in these devices, we measured both the dark J-V curves for the MAPbBr3- and MAPbI3-based solar cells under different working temperature (Figure S7) and the J-V curves under light intensities ranging from 8 to 100 mW cm–2 (Figure 5). From Equation (1), it is clear that the values of both J0 and n are important indicators of the recombination in a device and consequently affect the Voc values of a solar cell. The diode equation provides an expression for the current through a diode as a function of voltage, which can be described by the following equation: ࢗࢂ

ࡶ = ࡶ૙ (ࢋ࢔࢑ࢀ − ૚)

(4)


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The values of J0 and n are estimated from the linear fit of the semi-logarithmic plot in a range near Voc for both cells at different cell working temperatures, and the results are compared in Figure S7. For both solar cells, the J0 value increases with increasing temperature, which can be attributed to the increase of the intrinsic charge-carrier concentration at higher temperature. For the MAPbI3 solar cell, the fitting results show that n is insensitive to temperature and exhibits a lower value compared to that for the MAPbBr3 solar cell. It is well known that in the case where there is no recombination in the space-charge region, n should have a value of 1. On the other hand, if the current is dominated by recombination in the space-charge region, n should be 2 or even larger.19, 37 From Figure S7, it is notable that the value of n is between 1 and 2 for MAPbI3, whereas for the MAPbBr3 solar cell, a larger n value (>2) is observed across the entire test temperature range, indicating a much higher recombination. The J-V curves of the PSCs under different light intensities are shown in Figure 5a and 5b. With an increase in light intensity, both the Voc and Jsc increased monotonously due to a higher photocarrier density. The power law dependence of the Jsc with light intensity (J ∝ Iα) is shown in Figure 5c. A solar cell with no space-charge effect will have an α value close to 1 whereas a solar cell with space-charge-limited current due to carrier imbalance will have a power law relationship with α = 0.75.38-39 Both the MAPbBr3 and MAPbI3 solar cells show a linear relation of photocurrent with light intensity (with a slope of 1 on a double-logarithmic scale), indicating that charge-collection efficiency is independent of light intensity. This implies sufficient electron and hole mobility and no chargetransport barrier existing in the solar cells, which is in agreement with the good photoresponse in EQE. Figure 5d plots Voc as a function of the logarithm of the light intensity. We can see that Voc increases monotonically with light intensity for both solar cells, while the slope for MAPbBr3 solar cell is higher than that for the MAPbI3 solar cell, which is consistent with the larger n value. The efficient charge generation/separation/collection and an n value between 1 and 2 indicate that Shockley-Read-Hall (SRH) recombination dominates in the absorber layer of the MAPbI3 solar cell. In contrast, EA2) indicates that both SRH and the interface recombination mechanisms are important to the MAPbBr3 solar cells. This can be associated primarily with the larger CBO, leading to a larger potential loss in the MAPbBr3 solar cell. In summary, a higher bond strength and a more compact crystal lattice is associated with the smaller Br- in comparison to I-. Consequently, the cubic MAPbBr3 shows a moisture-


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ACS Energy Letters

insensitive response and can enhance the thermal stability that is problematic to MAPbI3, which is partially caused by its low-temperature phase transition. Both EQE and photoresponse under different light intensity show no charge-carrier transport barrier in the MAPbBr3 solar cells. However, the larger CBO between MAPbBr3 and TiO2 leads to a “cliff-type” band alignment, which lowers the activation energy and limits the Voc at room temperature. Unlike the MAPbI3 solar cell, which is dominated by a SRH recombination in the absorber layer, both the SRH and the interface recombination mechanisms are important to the MAPbBr3 solar cells. The Eg/q–Voc value for the MAPbBr3 solar cell is 0.93 V, which is much higher than that of the MAPbI3 solar cell with a value of 0.45 V, and thus limits the photovoltaic performance of the Br-based solar cell. To overcome this problem, investigation on alternative ETM and/or HTM or surface modification is needed in the future.

ASSOCIATED CONTENT Supporting Information. Experimental details, photographs of perovskite single crystals, cross-sectional SEM image, photoelectron spectroscopy results, J–V curves, illustration of the data collection sequence shown in Figure 3, temperature dependent PV parameters for MAPbI3 solar cells, dark J–V curves, and temperature dependent Voc for MAPbBr3 solar cells.

AUTHOR INFORMATION Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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The authors acknowledge the financial support from the Institute of Critical Technology and Applied Science (ICTAS). Authors S.P. and X.Z. would also like to acknowledge the financial support from Office of Naval Research through the MURI program. The work at the National Renewable Energy Laboratory is supported by the U.S. Department of Energy under Contract No. DE-AC36-08-GO28308. K.Z. and M.Y. acknowledge the support by the hybrid perovskite solar cell program of the National Center for Photovoltaics funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Solar Energy Technologies Office.

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