Planar FAPbBr3 Solar Cells with Power Conversion Efficiency above

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Planar FAPbBr Solar Cells With the Power Conversion Efficiency Above 10% Yongfei Zhang, Yongqi Liang, Yajuan Wang, Fengwan Guo, Licheng Sun, and Dongsheng Xu ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 2, 2018

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ACS Energy Letters

Planar FAPbBr3 Solar Cells with the Power Conversion Efficiency above 10% Yongfei Zhang1, Yongqi Liang,1* Yajuan Wang,1Fengwan Guo2, Licheng Sun1,3 Dongsheng Xu2* 1

State Key Laboratory of Fine Chemicals, Institute of Artificial Photosynthesis, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT), Dalian 116024, China 2 Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China 3 Department of Chemistry, School of Chemical Science and Engineering, KTH Royal Institute of Technology, 10044 Stockholm, Sweden [email protected] [email protected] [email protected]

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ABSTRACT.

Bromide-based hybrid perovskites are of particular interest not only due to the fact that they offer a way to go beyond the Shockley-Queisser limit via the tandem cell scheme, single-junction devices of them can also achieve reasonably high efficiency with high stability for solar energy conversion. However, the highest power conversion efficiency achieved up to now for FAPbBr3 single-junction solar cells is only 8.2%, which is far below the efficiency of ~17% predicted from detailed balance analysis. Here, a twostep method (the inter-molecule exchange pathway) was systematically optimized for the fabrication of high quality FAPbBr3 films. The molecule of urea, structurally similar to formamidinium, is introduced as the additive to tune the intermolecular ion exchange procedure. SnO2 is introduced as the electron selective contact to the planar structured FAPbBr3 solar cells. As a result, a power conversion efficiency of 10.61% and a Voc of 1.56V are achieved with planar structured solar cells, both of which represent the highest value ever reported for FAPbBr3 solar cells.


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Table of Contents graphic


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Impressive progresses have been made for inorganic-organic hybrid perovskite (ABX3, A=MA+, FA+, Cs+ and etc.; B=Pb2+, Sn2+ etc., X=I-, Br-, Cl-, SCN- and etc.) solar cells during the past few years, minimization the losses of photons in the solar spectrum can drive further the perovskite device performance to surpass the Shockley-Queisser limit.1 With a large bandgap of 2.2 eV~2.4 eV, bromidebased lead perovskites (FAPbBr3, MAPbBr3 and CsPbBr3) are of particular interest to serve as the top layer in a dual-junction tandem cell 2. In addition, bromide based perovskites have been demonstrated to be stable against the humidity and O2 in the ambient air

3,4

. Thus, it is now of crucial importance to

increase the device efficiency for bromide based perovskites to release their full potential for solar cell applications. On the other hand, the research on device fabrication of bromide based perovskites has byfar been lagging behind that of their iodide counterpart

3-9

. Among the limited number of reports on

FAPbBr3 solar cells, a recent demonstration of 8.2% by Arora et al.3 stands out as the highest power conversion efficiency through improving the interface between mesoporous TiO2 and FAPbBr3. However, the fabrication process had to be carried out inside a controlled atmosphere and mesoporous TiO2, which is not compatible to the tandem cell fabrication, was adopted. Yet, the power conversion efficiency for the FAPbBr3 solar cells is still low. Here, we adopt a two-step method for the fabrication of FAPbBr3 films under ambient air condition. After systematic optimization of the inter-molecule exchange pathway, the new fabrication process has resulted in high quality FAPbBr3 films. A power conversion efficiency of 10.61% under simulated AM1.5 sunlight is achieved for FAPbBr3 solar cells with a planar n-i-p architecture. A Voc of 1.56V is also highlighted as the highest Voc ever reported for FAPbBr3 solar cells.


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10

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ACS Energy Letters

* (112)

AGC-8 substrate

8 Voc=1.552V

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2

Jsc= 8.94mA/cm

Au Spiro-OMeTAD

FF= 0.76 η = 10.61%

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FAPbBr3 SnO2

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Figure 1: Structural characterization and the photovoltaic performance of the FAPbBr3 solar cells. (A) Morphological SEM image of the FAPbBr3 film and (B) Cross-sectional SEM image of a complete planar FAPbBr3 solar cell. (C) XRD spectrum of the complete solar cell (with Au layer and spiroOMeTAD). The diffraction peak at 38.18º from Au layer (marked as #) was used as the internal reference. The peaks at 26.46 º, 33.68 º, and 37.72 º (marked as *) are due to the SnO2 from theAGC-8 substrate, the diffraction pattern of which is also shown. (D) J-V curve for the best-performing FAPbBr3 solar cell measured under the simulated AM1.5 sunlight. The structure of the solar cell is schemed as inset. Systematic optimization throughout the FAPbBr3 film fabrication process and the solar cell fabrication process, which will be discussed in detail in the following parts of this paper, has been carried out for the FAPbBr3 solar cells and Figure 1 shows both the structural and the photovoltaic characterization of the optimized solar cells. Based on statistical analysis of the area covered by each grain in SEM (Figure 1A), the average grain size is estimated to be 440 nm with a wide distribution of 180 nm for the FAPbBr3 film. A layer thickness of 370 nm is estimated from the cross-section SEM image of the FAPbBr3 film (Figure 1B). The XRD pattern of the FAPbBr3 films on FTO substrates (Figure 1C) ACS Paragon Plus Environment

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ACS Energy Letters

matches well with the perovskite structure10. The strong peaks at 14.70 º, 20.92 º, 29.70 º, 33.36 º and 36.68 º can be indexed to the crystal planes of FAPbBr3 perovskite. The diffraction peak at 12.18º most probably comes from δ-FAPbBr3

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and the diffraction peak at 18.56º comes from the PbBr2 residue in

the film. We note the peak-splitting (shoulder peaks appear at 29.82 º and 33.46º, Figure S1) in the XRD spectrum indicates a structure of lower symmetry, most likely orthorhombic, for FAPbBr3 films12,13 rather than the cubic structure reported in the literature.3

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Figure 2: (A) The IPCE spectrum of the FAPbBr3 solar cell (champion cell in efficiency). The dashed line indicates the integrated photocurrent along the standard AM1.5 solar spectrum (http://rredc.nrel.gov/solar/spectra/am1.5/), which is scaled to the right-hand axis. (B) Light intensity dependence of Jsc for the FAPbBr3 device on SnO2 coated FTO substrate (black solid squares). Red color solid line: linear fit. The device was illuminated at 450 nm via a diode laser. (C) The evolution of Voc over time under simulated AM1.5 sunlight. The light is shuttered on/off for two cycles during the measuring period. (D) Light intensity dependence of Voc for a FAPbBr3 device on SnO2 coated FTO substrate (black solid squares). Red color solid line: linear fit. The device was illuminated at 450 nm via a diode laser.


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The bandgap of FAPbBr3 was reported to be between 2.2 eV and 2.3 eV,14 and the photon absorption predicts a maximum Jsc obtainable is ~ 10 mA/cm2 under AM1.5 sunlight. For the FAPbBr3 solar cell with the highest power conversion efficiency, a Jsc of 8.94 mA/cm2 is derived from the J-V curve measured under simulated AM1.5 sunlight. The photocurrent integrated from IPCE spectrum over the standard AM1.5 sunlight spectrum is 7.69 mA/cm2 (Figure 2A), which is 14% less than the value of Jsc extracted from the J-V curve. Since the IPCE values might vary as the light intensity increases from ~0.1 mW/cm2 (under which the IPCE spectrum was collected, Figure S6) to 100 mW/cm2 (under which J-V curve was measured), the photoresponse of FAPbBr3 devices at different light intensity was initially suspected by us to account for the difference between the integrated Jsc and the measured Jsc. To test such a hypothesis, the photocurrent response of FAPbBr3 solar cells under different light intensity was checked and the data is shown in Figure 2B. A linear plot develops in the logi-logI plot when the FAPbBr3 devices are illuminated at 450 nm. The slope across the light intensity range is found to be 0.97, which is very close to 1. This indicates the IPCE value (at 450 nm) will be the same or even lower at high light intensity than the IPCE value at low light intensity. Since the wavelength of 450 nm is positioned inside the plateau for the IPCE spectrum (Figure 2A), similar light-intensity dependent photoresponse is expected for FAPbBr3 solar cells at other wavelengths. As a result, the expected Jsc under AM1.5 sunlight should be the same or even lower than the integrated Jsc. The apparent discrepancy between the measured Jsc and expected Jsc needs other explanation rather than the lightintensity dependent photoresponse. The spectra mismatch between the simulated AM1.5 sunlight and the standard AM1.5 sunlight was then checked and is found not be able to fully account for the discrepancy (Figure S7 and Table S5). Future research to push FAPbBr3 closer to theoretical limits might help to elucidate the exact reasons behind this discrepancy. Upon exposing the device to simulated AM1.5 sunlight, the open-circuit voltage (Voc) increases to a maximum of 1.56V and slowly decreases to 1.51V during a period of 200 seconds (Figure 2C). The drop in Voc might be caused by the generation of defects inside FAPbBr3 films or at the interfaces. However, we notice the device gets recovered in the following dark period and the Voc restore to 1.55V when the device is exposed to light ACS Paragon Plus Environment

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for the 2nd cycle. We notice similar finding is observed in a recent paper.15 In Figure 2D, the dependence of Voc on light intensity is shown. The slope is 0.122V/decade, corresponding to an ideality factor of 2.06 for the FAPbBr3 device. This indicates that trap assisted recombination, instead of bimolecular recombination, is dominant inside FAPbBr3 solar cells.16 During our research, the fabrication process of FAPbBr3 films was systematically optimized in the following aspects: the solvent for dissolving FABr (Table S1) and the annealing temperature (Table S2). As a result of these efforts, methanol was chosen as the solvent for FABr and annealing at a relatively high temperature of 140°C was selected to get highly efficient FAPbBr3 solar cells. In between the first step and the second step of inter-molecular cation-exchange, 60ºC annealing of the PbBr2-(DMSO)x precursor films for 30 seconds is found to be necessary for getting uniform FAPbBr3 films. Otherwise, the PbBr2-(DMSO)x films without the 60ºC annealing step will get partially dissolved when FABr solution in methanol is spin-casted on top. Changes in two aspects, either more ordered (better crystallized) PbBr2 framework or the loss of DMSO molecules from the precursor films, might be expected from this mild annealing process. In our experiment, no obvious difference in the XRD spectra is found for the PbBr2-(DMSO)x precursor films after the 60ºC heating process. Crystalline PbBr2 films form only after a long period of time (10 minutes) heating at 80ºC change (Figure S2A). Though this proves that 60ºC annealing is not harsh enough to crystallize the PbBr2 framework by completely evaporating DMSO molecules, a structurally more ordered, though still amorphous, PbBr2 framework precursor films is still possible due to the 60ºC annealing process. As a result, the mild annealing process finely tunes the incorporation rate of FABr and uniform FAPbBr3 films form during the transformation step.

Though the combination of TiO2 and spiro-OMeTAD as the electron selective layer and hole selective layer has served as a cornerstone during the development of solid-state perovskite solar cells,17 concerns have been raised over the existence of a Schottky barrier between the compact TiO2 layer and FTO.18 It is also known that perovskite solar cells with the compact TiO2 as electron selective layer show ACS Paragon Plus Environment

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ACS Energy Letters

pronounced hysteresis in their J-V curve under illumination: the current density at the same voltage bias is higher when the bias is scanned from forward bias (reverse scan) than when the bias is scanned from the reverse bias (forward scan).19 Mesoporous-TiO2 on top of the compact TiO2 layer was found to obviously suppress the hysteresis in the J-V curve for perovskite devices.20 However, the role of the mesoporous TiO2 layer is still under debate.21 To elucidate the working mechanism of a perovskite solar cell and the possible effects from the interface between perovskite and the electron selective layer, different materials rather than TiO2 should be studied and compared. Due to the different position of the conduction band edge, the high electron mobility and the lack of photocatalytic activity, SnO2 is proposed to have advantages over TiO2 toward device efficiency and the stability for the solar cells.22,23 Recent studies on iodide based perovskite solar cells have indeed demonstrated SnO2 can effectively serve as the electron selective layer, especially with the suppression of the J-V hysteresis.24 Here, compact SnO2 films were adopted as the electron selective contact in FAPbBr3 solar cells in the planar architecture. Table 1 summarizes the photovoltaic parameters of them with side-by-side comparison to devices with compact TiO2 films as the electron selective layer. Voc(V)

Jsc (mA/cm2)

FF

%

SnO2-FTO

1.393±0.073

8.87±0.26

0.68±0.02

8.50±0.71

TiO2 –FTO

1.174±0.087

8.79±0.29

0.70±0.03

7.22±0.62

Table 1: Comparison of the photovoltaic performance (reverse scan) between solar cells on TiO2 and on SnO2 coated FTO glass substrates (AGC-8). The statistical analysis was done on 21 devices for each series.

Various positions of the conduction band edge have been reported for both SnO2 and TiO2, and it is generally believed that the conduction band edge for SnO2 is lower than that for TiO2.22-24 As a result, the conduction band offset between FAPbBr3 and SnO2 will be larger than that between FAPbBr3 and TiO2. An energy loss across the interface between FAPbBr3 and TiO2/SnO2 is expected from the classical view of photovoltaic devices4,25 and the energy loss for FAPbBr3 on SnO2 should be larger ACS Paragon Plus Environment

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compared to that for FAPbBr3 on TiO2. However, higher photovoltaic performance, especially larger Voc, for FAPbBr3 devices with SnO2 as the electron selective contact than that for FAPbBr3 devices with TiO2 as the electron selective contact is observed here (Table 1). Our finding suggests that the carrier separation process is not determined by the energy band alignment at the interface between FAPbBr3 and electron selective contact. This also agrees with a recent observation that Voc is governed by the separation of the quasi-Fermi level of carriers inside perovskite films.26 As a result, the position of the energy level for the electron selective contact (and the hole selective contact) will only affect the Voc through carrier recombination process at the interface and the separation of energy levels between electron selective contact and hole selective contact will not directly determine the Voc for perovskite solar cells. During optimizing the inter-molecule exchange procedure for the growth of FAPbBr3 films, a small molecule additive of urea is explored for tuning the fabrication process. Similar to DMSO molecules, urea is able to coordinate to Pb2+ (Figure S4 and S5). The structure similarity between urea and FA molecules further suggests that urea might be involved during the growth of FAPbBr3 films. Thus, urea molecules are expected to affect the nucleation and growth process for FAPbBr3 films.


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Figure 3. SEM image of the surface morphology for FAPbBr3 films. Scale bars: 2 um. Effects of urea as the additive on grain size distribution. For the FAPbBr3 films appearing in A, B, C, and D, different amounts of urea (molar ratio to FABr): 0%, 5%, 10%, and 15% respectively, were added to the FABr solution in methanol.

Figure 3 shows the surface morphology of the FAPbBr3 films fabricated when urea is added to the FAI solution in methanol. As the amounts of urea increases, the average grain size of the FAPbBr3 films increases from 446±195 nm (0% urea) to 441±144 nm(5% urea), 682±248 nm(10% urea), and 818±300 nm(15% urea). It is clear that the additive of urea during the FAPbBr3 film growth increases the size of FAPbBr3 grains. We note that cracks between grains exist in FAPbBr3 films after electron beam exposure during SEM characterization (Figure 3D), and this phenomenon becomes more obvious as more urea is added to the precursor solution.

Voc(V)

Jsc (mA/cm2)

FF

η (%)

FW-RE

1.419±0.048

8.07±0.18

0.69±0.03

7.89±0.44

RE-FW

1.412±0.032

8.12±0.13

0.53±0.04

6.08±0.42

FW-RE

1.439±0.051

8.34±0.12

0.70±0.02

8.49±0.47

RE-FW

1.413±0.049

8.29±0.07

0.58±0.03

6.86±0.57

10% urea FW-RE

1.479±0.055

8.39±0.14

0.73±0.01

9.08±0.49

RE-FW

1.475±0.049

8.18±0.33

0.66±0.01

7.99±0.56

0% urea

5% urea

Table 2: Comparison between the photovoltaic performance of FAPbBr3 planar solar cells on SnO2 fabricated with different amounts of urea adding to the FABr solution. The molar ratio is between urea and FABr. The bias applied on the solar cells was scanned in both directions, from forward bias to reverse bias (FW-RE) and from reverse bias to forward bias (RE-FW).

The additive of urea increases the power conversion efficiency by improving all the three parameters of Voc, Jsc, and Fill Factor for FAPbBr3 solar cells (Table 2). Though the grain size was proposed not to be the main factor to determine the perovskite solar cell performance,

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improvements of the

photovoltaic performance are clearly observed for FAPbBr3 films with larger grain size. This can be explained by the decrease of trap density as the grain size increases. As also evidenced from the dataset ACS Paragon Plus Environment

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shown in Table 2, hysteresis in the J-V curves still exists for all the FAPbBr3 solar cells. The additive of urea to the precursor solution shows certain extent of suppression of the hysteresis.

Shockley and Queisser have pointed out that a single-junction solar cell will have the maximized power conversion efficiency of 33% when the bandgap of a semiconductor is close to 1.3 eV.28 Iodide based perovskites MAPbI3 (FAPbI3) with a small bandgap (1.4 -1.6 eV) are adopted particularly for this reason, and research along this direction has witnessed the stunning rise up of power conversion efficiency to 22.7% for perovskite solar cells.29 Although the power conversion efficiency for bromide based hybrid perovskites (Eg, from 2.2 eV to 2.4eV) based single junction devices will not exceed 18%, research on bromide based hybrid perovskites promises stable single-junction devices of high-Voc. We note, on the other hand, research based on alloying bromide with iodide for hybrid perovskite might eventually lead to highly efficient dual-junction tandem cells. Various pathways29,30 might exist for the two-step deposition method31 adopted here for the fabrication of FAPbBr3 films. Through annealing the PbBr2-(DMSO)x precursor films at different temperatures, we verified that high quality FAPbBr3 films here form through an intermolecular cationexchange pathway (Figure S3). The difference of this pathway from the conventional two-step pathway lies in the fact that DMSO molecules are intercalated inside PbBr2 precursor films and the significant change in volume (film thickness) are avoided during the second step of FABr incorporation

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. As a

contrast, the FAPbBr3 films of the cubic structure reported recently would rather form through the conventional two-step pathway.3 Bromide based perovskites promise high open-circuit photovoltage for solar cells under AM1.5 sunlight, of which a single junction device might be able to drive uphill reactions such as electrochemical reduction of CO2 and water splitting.32 With the FAPbBr3 solar cells fabricated with the presence of 5% urea in the precursor solution, an open-circuit voltage of 1.56V is achieved (Voc of 1.567 V, Jsc of 7.67 mA/cm2, a Fill Factor of 0.70 and an overall power conversion efficiency of 8.49%). This represents the highest value ever reported for FAPbBr3 solar cells. On the other hand, we have to note ACS Paragon Plus Environment

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that the majority loss for FAPbBr3 solar cells is still on Voc since the theoretical prediction is ~ 2.0V.25,33 The Voc deficit of ~0.44 V might be located either at the interface

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or inside bulk FAPbBr3 films34.

Recently, there has been increasing concern that a majority Voc deficit could be from the non-ideal radiative process inside preovskites.35,36 This suggests us to increase the Voc through increasing the fluorescence quantum yield, so to say, improving the intrinsic quality of FAPbBr3 films. 1.0

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Figure 4: (A) Photoluminescence spectrum of the FAPbBr3 film at room temperature. The film was excited at 450 nm. (inset) UV-Vis absorbance spectrum of the FAPbBr3 film. (B) Time-resolved photoluminescence decay of the FAPbBr3 film on SnO2 coated FTO substrate. The FAPbBr3 film was fabricated through urea assisted two-step deposition. The fluorescence signal was probed at 543 nm. A nanoLED from Horiba at 450 nm was used to excite the film.

The optical properties of FAPbBr3 films have been studied via steady-state photoluminescence and time–resolved fluorescence decay to assess their quality. A representative photoluminescence spectrum shows a single peak at 543 nm, which is consistent with the band edge emission from FAPbBr3, with a full-width at half-maximum (FWHM) of 20 nm (Figure 4A). The relatively narrow FWHM and absence of emission till 900 nm from deep levels associated with impurities and defects demonstrate the excellent optical properties of FAPbBr3 films. Time-resolved photoluminescence decay of FAPbBr3 films (fabricated through urea assisted two-step deposition) on SnO2 is shown in Figure 4B. The fitting of the photoluminescence decay signal via a two-exponential component model gives a fast decay lifetime τ1= 48 ns (relative contribution 10%) and a slow decay lifetime τ2= 285 ns (relative contribution, 90%). The fluorescence lifetime for the FAPbBr3 films here is very close to the ~680 ns ACS Paragon Plus Environment

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reported for FAPbBr3 single crystals (excitation carrier density ~4*1014 cm-3).10 For comparison, the fluorescence lifetime of the FAPbBr3 film fabricated by Arora et al. is ~40 ns (excitation carrier density ~4*1015 cm-3) 3. The long carrier lifetime, together with the luminescence yield determined below, indicates that the defects accounting for the non-radiative loss are much less inside FAPbBr3 films fabricated here37. The absolute fluorescence quantum yield determined for FAPbBr3 film on SnO2 coated FTO substrate is ~0.5%. According to the theoretical analysis,38 such a photoluminescence quantum yield of FAPbBr3 films will account for a Voc loss of at least 0.14 V inside solar cells. The remaining Voc loss of 0.30 V for the FAPbBr3 solar cells is most probably caused by the non-radiative recombination at the interfaces.

To summarize, high-quality FAPbBr3 films have been successfully fabricated through the two-step method after systematic optimization of the intermolecular ion exchange procedure. Together with the adoption of SnO2 film as the electron selective contact, this allows us to achieve a power conversion efficiency of 10.61% with planar structured FAPbBr3 solar cells with a normal n-i-p architecture. A record high Voc of 1.56V is also obtained for FAPbBr3 solar cells. Pushing forward the research on FAPbBr3 solar cells will not only offer us a paradigm to elucidate detailed working mechanisms for perovskite solar cells, it also will eventually lead to stable dual-junction tandem cells with power conversion efficiencies beyond Shockley-Queisser limit.

Supporting information Experimental details for the fabrication and characterization FAPbBr3 solar cells, the XRD spectra of the FAPbBr3 films and the precursor PbBr2-(DMSO)x films, the differentiation of the pathways for the two-step method, the precursor PbBr2-(urea)x films, the NMR spectra showing the absence of urea in the annealed FAPbBr3 films, comparison between different solvents and annealing temperature for the


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fabrication of FAPbBr3 films, the lists of the high performance FAPbBr3 solar cells, the discussion of the spectra mismatch between sunlight from the solar simulator and the standard AM1.5 sunlight, and the current density-time response of a FAPbBr3 solar cell at the maximum power point.

Acknowledgements The authors thank the National Natural Science Foundation of China (91233201), the Swedish Energy Agency, and the Ministry of Science and Technology of China (973 project, grant No. 2013CB932601) for financial support. Liang thanks prof. Qi Li (PKU) and prof Gagik Gurzadyan (DLUT) for active discussions. We thank prof. Mingxing Chen (PKU) for FL QY measurement.

References

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Planar FAPbBr3 Solar Cells with Power Conversion Efficiency above

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