Overcoming Short-Circuit in Lead-Free CH3NH3SnI3 Perovskite Solar

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Overcoming Short-Circuit in Lead-Free CHNHSnI Perovskite Solar Cells via Kinetically Controlled Gas-Solid Reaction Film Fabrication Process Takamichi Yokoyama, Duyen H. Cao, Constantinos C. Stoumpos, TzeBin Song, Yoshiharu Sato, Shinji Aramaki, and Mercouri G. Kanatzidis J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00118 • Publication Date (Web): 15 Feb 2016 Downloaded from http://pubs.acs.org on February 15, 2016

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The Journal of Physical Chemistry Letters

Overcoming Short-circuit in Lead-free CH3NH3SnI3 Perovskite Solar Cells via Kinetically Controlled Gas-solid Reaction Film Fabrication Process Takamichi Yokoyama1, 2, Duyen H. Cao1, Constantinos C. Stoumpos1, Tze-Bin Song1, Yoshiharu Sato2, Shinji Aramaki2 and Mercouri G. Kanatzidis1* 1

Department of Chemistry, Northwestern University, 2145, Sheridan Road, Evanston, Illinois

60208, United States 2

Mitsubishi Chemical Group Science & Technology Research Center, Inc., 1000 Kamoshida-

cho, Aoba-ku, Yokohama 227-8502, Japan Corresponding Author *Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT The development of Sn-based perovskite solar cells has been challenging since devices often show short-circuit behavior due to poor morphologies and undesired electrical properties of the thin-films. A low-temperature vapor assisted solution process (LT-VASP) has been employed as a novel kinetically controlled gas-solid reaction film fabrication method to prepare lead-free CH3NH3SnI3 thin-films. We show that the solid SnI2 substrate temperature is the key parameter in achieving high surface-coverage and excellent-uniformity perovskite films. The resulting high quality CH3NH3SnI3 films allow the successful fabrication of solar cells with drastically improved reproducibility, reaching an efficiency of 1.86%. Furthermore, our Kelvin probe studies show the VASP-films to have lower doping level than films prepared from the conventional one-step method, effectively lowering the film conductivity. Above all, with (LT)VASP, the short-circuit behavior often obtained from the conventional one-step-fabricated Snbased perovskite devices has been overcome. This study facilitates the path to more successful Sn-perovskite photovoltaic research. TOC GRAPHICS


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The efficiency of organic-inorganic lead halide perovskite-based solar cells has been improved rapidly in the past few years and a certified power conversion efficiency (PCE) of 21.0% has already been obtained in a small size device.1-7 Tremendous effort on thin film fabrication such as thermal co-evaporation in high vacuum,8 sequential deposition,9 vaporassisted solution process (VASP),10 chemical vapor deposition,11 solvent engineering,4 intermolecular exchange,7 has led to this drastic improvement. Despite the rapid advancement of perovskite solar cells, the toxicity of lead remains a major concern for the commercialization of this technology. Therefore, it is highly desirable for lead to be replaced with other non-toxic elements. Among the lead-free perovskite materials for solar cell application reported so far,12-16 tin-based perovskites which have the chemical formula of ASnX3 where A can be Cs, methylammonium (MA), or formamidinium (FA) and X can be I, Br, Cl or F, are the most promising alternatives since Sn and Pb both belong to group 14 of the periodic table and thus are expected to possess comparable physical and chemical properties.17,18 The reported PCE of ~6% of lead-free device using MASnI3-xBrx as a light absorber has proven its feasibility.19,20 Despite the many similarities between the APbI3 and ASnI3 systems, there are significant differences between the two which stem from the different ionic radii of the Sn2+ and Pb2+ ions. As a result, the nature of the chemical bonding becomes more covalent in the case of ASnI3 systems because of the relatively larger degree of orbital overlap in the shorter Sn-I bond compared to the Pb-I bond. The consequences of this subtle difference in chemical bonding has a strong impact on the semiconducting properties of the materials. Another marked difference comes from the relativistic effects which are stronger in Pb than in Sn and lead to a larger stabilization of the 6s2 electrons of Pb relative to the 5s2 electrons of Sn. The valence band of Sn, consisting of the 5s2 electrons, thus lies significantly higher in energy with respect to the 6s2


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electrons of Pb leading to i) the narrowing of the band gap and ii) the broadening of the band widths.21,22 The latter effect suggests that the hole and electron mobilities could be actually superior in the case of ASnX3 and therefore these can be considered as more efficient semiconductors in terms of charge transport characteristics. On the downside, however, the destabilization effect is so intense that leads to facile oxidation of Sn2+ when exposed to the atmosphere. Quick oxidation of the Sn-based perovskites hinders proper studies on the system. More specifically, the easy oxidation of Sn2+ to Sn4+ results in highly conductive hole doped perovskite films that lead to poor photovoltaic performance due to short-circuit behavior.20,23-26 Successful device performance in CsSnI3-xBrx, FASnI3 and MASnI3 with SnF2 as an additive highlights the importance of understanding and controlling the electrical properties of Sn-based perovskite materials.24,27-31 Besides their chemical instability, film growth of Sn-based perovskites is also problematic as the Sn-perovskites tend to quickly crystalize at room temperature (or during spincoating), which impedes a uniform film growth. ASnI3 films obtained by the conventional onestep method often have randomly oriented growth, resulting in poor surface coverage.19,20,24 Recently, our group reported film growth with improved surface coverage utilizing dimethyl sulfoxide (DMSO)-SnI2 intermediate phase.29 To the best of our knowledge, all reports so far on Sn-based perovskite solar cells have utilized only the traditional one-step film deposition approach, where the organic halide and metal halide salts are dissolved in polar solvents, followed by spin-coating the solution on substrates. Considering the improvement of Pb-based perovskite solar cell efficiencies upon inventing different film fabrication techniques,32 the development of a more advanced process for film growth for the Sn perovskites would be necessary for both device engineering and better scientific understanding of this system. Due to


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the good solubility of Sn-based perovskites in isopropyl alcohol, the sequential deposition method cannot be applied in the Sn perovskite fabrication process. We then set out to study our Sn-based perovskite devices using a vapor assisted solution processed (VASP) film fabrication method. In this work, we demonstrate that the film growth of MASnI3 can be successfully controlled by modifying the conventional VASP method. Devices fabricated by our new VASP method show drastically improved reproducibility with the best power conversion efficiency (PCE) reaching 1.86%. In addition, the obtained films show better air-stability, which allows us to better characterize the Fermi levels and the MASnI3 film degradation dynamics in air with the Kelvin probe technique and offer some new insights about this lead-free perovskite material. We report on a systematic study of the thin film growth, optical and electronic properties of MASnI3 perovskites and their corresponding photovoltaic performances. Our main focus stays on developing a film fabrication method which yields films with uniform and high surface coverage compared to those obtained from the conventional one-step fabrication method. We prepared our MASnI3 films on mesoporous TiO2 substrates using a VASP method (more experimental details can be found in the Supporting Information).10 As shown in Figure 1, the SnI2 film formed on mesoporous TiO2 substrate and MAI powder placed on the petri dish react at elevated temperature to form a MASnI3 perovskite film through MAI (gas) - SnI2 (solid) reaction. For comparison, we also made MASnI3 films with the conventional one-step method. We observed that in VASP, the transformation of SnI2 to MASnI3 upon its reaction with MAIgas is much faster than that for Pb-perovskites. The black perovskite phase is formed within 1 min (Figure S1) while it was reported that VASP of 2 hours was necessary to complete the transformation of PbI2 to MAPbI3,10 suggesting that the diffusion of MAI gas into SnI2 network


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is faster and/or the Gibbs free energy to form MASnI3 perovskite is lower than that of its Pbbased analogue. The phenomenon can be attributable to the higher Madelung energy because of the more covalent character and the shorter length of the Sn-I bond.33 From the viewpoint of industrial processing, the faster perovskite conversion could make VASP a highly practical approach for the Sn-based perovskite solar cell manufacturing.

Figure 1. (left) Schematic of VASP and LT-VASP and (right) crystal structure of MASnI3.

We observed a significant difference in the surface morphology between the one-stepand the VASP-film as shown in their corresponding scanning electron microscope (SEM) images (Figures 2a and 2b). While a large part of the mesoporous TiO2 surface remains uncovered in the one-step MASnI3 film, the VASP-film surface is smoother and better covered, which is beneficial for device performance. There is, however, still room for VASP-film uniformity optimization. For example, big crystals are formed on the surface of the conventional VASPfilm, causing incomplete coverage. This prompted us to further investigate the film growth mechanism by means of SEM. While as-spincoated SnI2 film has rather high surface coverage (Figure 2A), we find the coverage becomes poorer after the SnI2 film was subjected to annealing at 100°C for 10 min (Figure 2B). We attribute this behavior to the fast crystallization of SnI2 in the conventional VASP method, such as during the vaporization process of MAI at high


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temperature, the SnI2 film is simultaneously being crystallized (referred to as Process 1). The final MASnI3 perovskite film morphology depends on the rates of Process 1 and the gas-solid reaction of MAI and SnI2 (referred to as Process 2). Specifically, if Process 2 is faster than Process 1, the resulting film will possess a continuous and smooth perovskite surface. Conversely, if Process 1 is faster than Process 2, the film instead will be composed of separated MASnI3 islands. Aiming for a high quality film, we thus lowered the SnI2 substrate temperature during VASP to suppress the detrimental SnI2 crystallization process by inserting a glass spacer between the bottom of the petri dish and the sample substrate (Figure 1). By changing the height (or the number) of the glass spacers, the substrate temperature during VASP can be well controlled. We call this process low-temperature (LT-) VASP. SEM images of LT-VASP films are shown in Figure 2c and Figure S2. As expected, a drastic improvement in film surface coverage was obtained in our LT-VASP-film.

Figure 2. SEM images of films on FTO/TiO2 substrate. (a) one-step MASnI3, (b) VASP-MASnI3, (c) LT-VASP-MASnI3, (A) as-deposited SnI2, (B) annealed SnI2,. The scale bars indicate 2 µm. (a) shows one-step films have exposed mesoporous TiO2 and (b) shows VASP-films have a good


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surface coverage. (c) LT-VASP-films have both good film surface coverage and uniformity. (A) and (B) show the surface coverage of SnI2 films become poor after annealing due to fast crystallization of SnI2.

Furthermore, we found that the film growth during LT-VASP was quite different from that in the conventional VASP such that the SnI2 film initially started turning black from the edge and then the black part spread until the entire film fully changed to black in LT-VASP (Figure 1), whereas the whole SnI2 film homogeneously turned black in VASP. The film growth edge-to-edge pathway was observed when the SnI2 substrate temperature was between about 60°C and 80°C. As we mentioned earlier, when the substrate temperature was higher than 80°C, SnI2 crystalized before reacting with MAI. On the other hand, at lower temperature, the SnI2 film did not turn black upon reaction with MAI presumably because of insufficient activation energy and/or slow diffusion dynamics that inhibit the perovskite formation. We reasoned that the 6080oC window is the optimal temperature range to control the perovskite growth because only the outer SnI2 layer gets “activated” to react with the evaporated MAI leading to a kinetically controlled process. Therefore, the substrate temperature during VASP is a key parameter for controlling the film morphology of MASnI3. In the case of Pb-perovskites, it is not critical to strictly control the PbI2 substrate temperature; films with high surface coverage have been reported with conventional VASP,10 presumably because PbI2 films do not change their morphology at this temperature range. It has been reported that MAPbI3 films can be formed through PbCl2 solid-MAI gas reaction at the substrate temperature between 145°C to 170°C which is the same temperature range where VASP is normally performed.11 We validated the perovskite formation and studied the film crystallinity using X-ray diffraction (XRD). Figure 3a shows the XRD patterns of different MASnI3 films prepared from


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the one-step and VASP methods. The MASnI3 pseudocubic phase is formed in all films regardless of the film fabrication method with the two prominent Bragg diffraction peaks at 14.3° and 28.0° corresponding to the (100) and (200) planes, respectively.34 Notably, we found that the LT-VASP films have stronger preferred orientation along the [001] direction (along the C-N bond of the MA cation) which is shown by the intense (100) reflection. The UV-vis spectra of these films are shown in Figure 3b. The band gaps of one step-, VASP- and LT-VASP films are estimated to be 1.35 eV, 1.27 eV and 1.26 eV, respectively (from Tauc plot of direct band gap, Figure S3).35 The experimental band gap values are very close to those of the previous MASnI3 reports.19,20 The subtle optical absorption loss in the one-step-films could be a result of the perovskite phase degradation upon exposing to air.


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Figure 3. (a) XRD patterns of each film. * indicates peaks from the FTO/TiO2 substrates. (b) Absorption spectra of one-step-, VASP- and LT-VASP-films. The measurements were performed again after films were exposed to air for 15 min and 25 min to obtain time evolution of the spectra. The perovskite film thickness is the sum of the 350 nm mesoporous TiO2 and the thickness of the MASnI3 overlayer which depends on the film fabrication process.

We investigated the absorption behaviors of our perovskite films as a function of air exposure time with a UV-vis spectrometer (Figure 3b). Remarkably, our VASP-based films had better stability in air, as judged by the consistency of their spectra over 25 min. The films lost their black color in approximately 24 hours. Contrarily, the one-step-based films lost their


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absorption within a few minutes of exposure in air as it has been previously reported.20 The results suggest the stability of the films strongly depends on the fabrication process. The temperatures of the one-step and LT-VASP methods are comparable, so the presence of any remnant solvent that favors the Sn2+ to Sn4+ redox reaction, should not be the cause of the difference in film air-stability. We hypothesize that the air-stability difference between the two methods arises from the differences in doping levels and/or the film morphology. An in-depth investigation in the possible causes lies outside the scope of our report. Nevertheless, the high quality and the improved air-stability of our perovskite films proves the (LT-)VASP method feasible and desirable for the Sn-based perovskite film fabrication. Noel et al. used 2,2’,7,7’-tetrakis(N,N’-di-p-methoxyphenylamine)-9,9’-spirobiuorene (spiro-OMeTAD) with hydrogen bis(trifluoromethanesulfonyl)imide (H-TFSI) and tertbutylpyridine (tBP) additives as hole transporting material (HTM) for their MASnI3 devices.20 However, the same HTM solution mixture broke down our MASnI3 films because of the reaction between tBP and MASnI3 that can form colorless coordination complexes or hydroxides. Our group has introduced the use of 2,6-lutidine (pKa = 6.7) instead of tBP (pKa = 6.0) which although it acts as a stronger base, it also inhibits direct coordination to the Sn metal due to the steric hindrance provided by the methyl groups.19 Our film color remains intact when 2,6lutidine is used. However, as lutidine is still a base itself, it may potentially damage the film when used in a large scale or over longer periods of time. In this work, we employed a 4isopropyl-4'-methyldiphenyliodonium tetrakis(pentafluorophenyl)borate as a dopant in HTM, which has been used in organic light emitting diode technologies.36,37 Our devices comprise six typical layers including FTO/compact TiO2/mesoporous TiO2/perovskite/hole transport layer (HLT)/Au. A picture and a cross-sectional SEM image of a


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functional LT-VASP device is shown in Figure 4a. J-V curves and device performance summary of all devices are plotted in Figure 4b and shown in Table 1, respectively. We observed hysteresis behavior in our devices (Figure S4). The J-V characteristics were measured with a reverse bias scan (that is, positive bias to negative bias scan). A promising PCE of 1.86% was obtained from LT-VASP device with a JSC of 17.8 mA/cm2, VOC of 0.273 V and FF of 39%. With the same device fabrication recipe except the higher SnI2 substrate temperature used in the conventional VASP method, solar devices produce comparable VOC and FF but lower JSC which could be attributed to the inferior uniformity and surface coverage of the perovskite film. The one-step devices showed consistently poor performance due to significant leakage current as the dark current indicates. The low FF of ~25% (or sometimes 0% from lack of photocurrent) of the one-step devices suggest an Ohmic behavior. Note that although the performance of the VASPand LT-VASP-devices might vary from cell to cell, the VASP-devices rarely showed shortcircuited behavior as summarized in Figure 4c and Table 1. We further studied the photo response of our solar cells by collecting the incident photon to current conversion efficiency (IPCE) spectrum of LT-VASP device (Figure 4d). The photocurrent onset begins at ~1000 nm which is in good agreement with the optical absorption spectra. The IPCE value reaches 60% at maximum and it gradually decreases for wavelengths longer than 510nm. This indicates that although the band gap of MASnI3 is smaller than of MAPbI3, its absorbance above the bandgap is weaker and therefore thicker films would be necessary to take advantage of its smaller band gap for photo-electrical conversion. The estimated JSC obtained by integrating the EQE spectrum was 14.5 mA/cm2 which is somewhat smaller than the measured JSC (17.36 mA/cm2) by AM1.5G solar simulator. We did ensure that the measured JSC is not overestimated due to possible underestimation of device active area by


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measuring the device with shadow masks of different apertures. We also measured our device again after the IPCE measurement and observed almost the same JSC value. We surmise that the JSC could be dependent of incident light intensity due to trap states present in the perovskite film and/or TiO2.38 Since the IPCE measurements were performed without light bias, the carrier density in the MASnI3 film is lower than that under 1 sun illumination for the J-V measurements. When the excited carrier density is higher, these traps are filled, which would make carrier transport easier.

Figure 4. (a) A photo of an encapsulated LT-VASP device and a cross-sectional SEM image of a functional LT-VASP device. About a 100-nm-thick overlayer is observed. (b) J-V characteristics of devices fabricated from one-step-, VASP- and LT-VASP MASnI3 films. (c) Device parameters of 12 devices for each film fabrication process. (d) IPCE spectrum of the best LT-VASP-device.

Table 1 Device parameters of the best cells. Values in parentheses show average and standard deviation of 12 cells for each film fabrication process. FF values of one-step devices showing short-circuit behavior are excluded.


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To rationalize the obtained device results, we further looked at the Fermi levels of our perovskite films. Several different values of the valence band (VB) energy of MASnI3 have been reported by different research groups, ranging from 4.73eV to 5.47eV.19,23 This divergence is probably due to the easy oxidization of Sn2+ during sample handling and a non-sharp threshold in the ultraviolet photoemission spectroscopy (UPS) spectra.19 Incidentally, the 5.47eV value corresponds well with the value obtained for Cs2SnI6,39 and therefore it may be assumed that it belongs to the fully oxidized analogue, namely MA2SnI6. In this study, we determined the Fermi levels and their degradation dynamics in air using a Kelvin probe. Figure 5a shows the VASPfilms having higher Fermi level energy (vs vacuum, i.e. smaller absolute value means higher levels) than films made with the one-step method despite the fact that they are nominally the same compound. The initial Fermi levels are estimated to be at 4.70 eV and 4.65 eV for the onestep- and LT-VASP-films, respectively. Both values should be already affected by some oxidization, so it is reasonable to presume that the initial Fermi levels of pristine MASnI3 are slightly higher. In particular, we could expect even lower initial value for LT-VASP film from Figure 5a if we extrapolate to zero exposure time. The previously reported VB of 4.73eV is thus reasonable since the MASnI3 should be a p-type semiconductor due to oxidization in air.23 Longer-range degradation dynamics of the Fermi-level as shown in Figure 5a further supports the value. With increased air-exposure time the Fermi level became lower in energy for both the one-step and LT-VASP derived films, and then saturated only for LT-VASP films. The so-called


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one-step film is quite unstable in air as shown in Figure 3b, so the linearly declining trend in their Fermi level should be ascribed to the decomposition of MASnI3 (presumably to MA2SnI6) rather than to the oxidative doping effect. On the other hand, it is reasonable to consider that the LT-VASP films are gradually p-doped by oxidation and the Fermi level moves closer to the valence band maximum. The saturated value was 4.75eV which is very close to the value that Ogomi et al. reported as VB.23 We then consider this value as VB in the following discussion. It is worth mentioning that the electrical properties of the LT-VASP-films in air do change with time, even though their absorption spectra remain almost identical as displayed in Figure 3b. Thus, it would be still challenging to realize air-stable MASnI3-based devices. Figure 5b shows the energy level diagram of LT-VASP MASnI3 together with previously reported values for MAPbI3, TiO2 and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA).40,41 The Kelvin probe study shows that one-step films have Fermi levels closer to the MASnI3’s VB, hinting higher doping levels than LT-VASP-films (i.e. higher hole carrier density). This higher carrier density could be a major cause for the poor performance (often short-circuited) of the one-step devices. LT-VASP films show a good rectifying property with very small leakage current as the dark J-V characteristics indicate in Figure 4b. One might think that there is an unfavorable energy barrier between the MASnI3’s VB and the PTAA’s highest occupied molecular orbital (HOMO) level (5.2eV). However, the surface of the perovskite film should be doped when exposed to air before the Au evaporation step, which essentially forms an Ohmic contact with the p-type HTL, although such uncontrolled doping may lead to the degradation of the FF. Assuming the VB of MASnI3 to be at ~4.75 eV, the conduction band (CB) is calculated to be at 3.49 eV by subtracting the VB from the band gap of 1.26 eV. One possible explanation then for the low VOC in the MASnI3 system, despite possessing the band gap of 1.26


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eV, could be stemming from the energy loss in the electron injection process from the CB of MASnI3 to the CB of TiO2. This loss in VOC is much larger than that in MAPbI3 perovskite devices; more precisely, the difference between the CBs of TiO2 and perovskite is 0.77eV for MASnI3 while 0.31eV for MAPbI3 (Figure 5b).3 Based on this, an alternative electron transporting layer (ELT) with higher Fermi level could be used to enhance the currently low VOC of MASnI3 devices. High quality MASnI3 films have been fabricated by the (LT)-VASP method. We found that MASnI3 formation by VASP takes place on a minute timescale which is much faster than that in the Pb-analogue, and this makes VASP a more practical thin film fabrication process for MASnI3. We established here that the substrate temperature during VASP is a key parameter in achieving high quality MASnI3 films since the high substrate temperature promotes quick change in the morphology of the SnI2 films before their reaction with MAI vapor. Our modified LTVASP method produces MASnI3 films with preferentially oriented growth and drastically improved uniformity. Solar devices reproducibly fabricated from VASP-based-films did not show short-circuit behavior and the best performance of 1.86% PCE has been achieved. Our devices were exposed to air for a few minutes before and after Au evaporation because of the nature of our experimental set up, and this should cause performance degradation. We thus consider our LT-VASP devices to potentially have even higher efficiency. Our Kelvin probe study strongly suggests VASP-films have lower doping level than films deposited with the onestep method. Based on the energy levels of MASnI3, we propose that alternative ETLs with higher CB than TiO2’s will produce higher VOC. Most importantly, this study demonstrates a two-step-based fabrication process for Sn-based perovskite for the first time with successful results. With LT-VASP, we have overcome the short-circuit phenomenon normally obtained


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from the one-step-based devices with MASnI3 based perovskites. This study opens a new avenue for developing high efficiency Sn-perovskite devices.

Figure 5. (a) Fermi levels of one-step- and LT-VASP-films of MASnI3 measured by a Kelvin probe in air. The saturating value in the LT-VASP film was confirmed by measuring two different points on the same film. (b) Energy level diagram of a LT-VASP MASnI3 film estimated from the Kelvin probe study. Reported values for MAPbI3, TiO2 and PTAA are also depicted for comparison.40, 41

ASSOCIATED CONTENT


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Supporting Information: Experimental details, additional UV-vis spectra, a cross-sectional SEM image, Tau plots of absorption spectra and hysteresis behavior in J-V characteristics used in this work are provided in Supporting Information.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] NOTES The authors declare no competing financial interests.

ACKNOWLEDGEMENT D.H.C. acknowledges support from the Link Foundation through the Link Foundation Energy Fellowship Program. This work was supported in part by the ANSER Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Award Number DE-SC0001059. This work made use of the EPIC facility (NUANCE Center-Northwestern University), which has received support from the MRSEC program (NSF DMR-1121262) at the Materials Research Center, and the Nanoscale Science and Engineering Center (EEC-0118025/003), both programs of the National Science Foundation; the State of Illinois; and Northwestern University.

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Overcoming Short-Circuit in Lead-Free CH3NH3SnI3 Perovskite Solar

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