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Severe Morphological Deformation of Spiro-OMeTAD in (CHNH)PbI Solar Cells at High Temperature 3
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Ajay Kumar Jena, Masashi Ikegami, and Tsutomu Miyasaka ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00582 • Publication Date (Web): 14 Jul 2017 Downloaded from http://pubs.acs.org on July 16, 2017
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ACS Energy Letters
Severe Morphological Deformation of SpiroOMeTAD in (CH3NH3)PbI3 Solar Cells at High Temperature Ajay Kumar Jena*, Masashi Ikegami, Tsutomu Miyasaka* Toin University of Yokohama, 1614 Kurogane-cho, Aoba, Yokohama, Kanagawa AUTHOR INFORMATION Corresponding Author *Ajay Kumar Jena
[email protected] *Tsutomu Miyasaka E-mail:
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ABSTRACT
The hole transport material, spiro-OMeTAD, in MAPbI3 perovskite solar cells undergoes a severe morphological deformation at high temperature, showing big voids in the layer when the devices are heated at 80 oC and above. It is puzzling that the voids emerge only in the area where the spiro-OMeTAD is capped with Au film and only in the case where the HTM contains both LiTFSI and TBP as additives.
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MAIN TEXT The organic-inorganic hybrid perovskite solar cells1 have shown an incredibly fast progress in the last few years. However, the two issues that still stand firm against its commercialization are long term stability and toxicity of lead. Decomposition of perovskite into PbI2 by atmospheric humidity 2, and thermal
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and photoinduced peroformance degradation
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are equally important
concerns. In addition, the hole transport material (HTM) and the additives in it also hold responsible for peroformance degradation. Indeeed, perovskite is quite stable in dry conditions and at temperature up to 85 oC 5. In such cases, where perovskite has not changed much, degradation has been found to be either due to crystallization5 or photo-oxidation of spiroOMeTAD6, or diffusion of Au into Spiro-OMeTAD7. In our present investigation on performance degrdation in methyammonium lead iodide (MAPbI3) perovskite solar cells, we witnessed a surprisingly interesting physical change in the spiro-OMeTAD layer at high temperatures (80, 100 and 120 oC). The spiro-OMeTAD layer, when heated, was found to form large voids, which depended on the site (Au-deposited or nondeposited area) and additives, such as lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) and 4-tertiary butyl pyridine (TBP), present in it. Photovoltaic characteristics of MAPbI3 solar cells (fabrication and characterization details are given in SI) before and after heat-treatment (60, 80, 100 and 120 oC for one hour), as shown in figure S1 in SI, revealed a continuous drop in cell efficiency with increasing temperature and the drop was essentially due to remarkable reduction in FF while Jsc and Voc decreased slightly. However, surprisingly, there was no apparent change in color of perovskite and X-ray diffraction (XRD) patterns of the devices (Figure S2) showed that perovskite remains almost unaffected up
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to 80 oC and degrades slightly to form PbI2 at higher temperatures (100 and 120 oC). Although PbI2 can be involved, it was unconvincing that such an insignificant amount of PbI2 could be the only factor causing such a remarkable decrease in cell performance because it has been reported and we have also seen that slight excess of PbI2 is rather beneficial for the cells8. The cross-sectional scanning electron microscope (SEM) images (Figure 1) of the devices revealed a daunting fact of existence of macro-defects (voids) in the spiro-OMeTAD layer of the devices heated at higher temperature (80, 100 and 120 oC) while there was no change in the perovskite. The size of these voids in spiro-OMeTAD increased with temperature (from 80 oC to 120 oC). More interestingly, these voids existed only in the Au-deposited area, where Au film capped the HTM (Figure 2 a). No voids were seen in areas outside the Au coating (non-deposited area) (Figure 2 b).
Figure 1 Cross-sectional SEM micrographs of the planar MAPbI3 perovskite solar cells heated at (a) 80, (b) 100 and (c) 120 oC for 1 hour. SEM micrographs of the devices heated at 0 and 60 o
C are given in figure S3.
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ACS Energy Letters
Figure 2 Cross-sectional SEM micrographs of the planar perovskite solar cell heated at 100 oC for 1 hour at the site where Au (a) capped (Au-deposited area) and (b) did not cap (non-deposited area) the spiro-OMeTAD. We suspected any role of perovskite in generating defects after chemical reactions, for example, iodine diffusion to spiro-OMeTAD9 under electric field and/or at high temperature. However, samples free of perovskite (i.e. FTO/spiro-OMeTAD/Au) also produced the same voids (Figure S4), again in the Au-deposited area only, confirming no role of perovskite in creating such defects in the HTM. Additionally, it is unlikely that electric field had any influence on the defect development because FTO/Spiro-OMeTAD/Au samples that were not subjected to electric field (only heat-treated) also showed same voids (Figure S5). This implied that formation of the voids is intrinsically related to spiro-OMeTAD, releasing some component at high temperature. The component that we suspected to get out easily is tertiary butyl pyridine
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which is volatile. However, to our surprise, we found that spiro-OMeTAD layers including only TBP (no LiTFSI) and only LiTFSI (no TBP) did not have defects even after heating at 100 and 120 oC (Figure S6) whereas the HTM containing both of them showed the voids (Figure 2 a). LiTFSI and TBP together, but not in isolation, modifies the HTM in such a way that some
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unknown component gets eliminated when the HTM is heated at temperatures like 100 and 120 o
C. Emergence of voids only in the Au-deposited area was suspected to be related to Au diffusion
into spiro-OMeTAD, which has been recently reported to play role in performance degradation7, 11
. However, in an inverse perovskite-free structure, where Au was underneath spiro-OMeTAD
instead of on the top (i.e. FTO/Au/spiro-OMeTAD), no such voids were seen even after heating at 80, 100 and 120 oC (Figure S7 in SI). This suggests that the voids are rather results of some gaseous component produced and physically trapped in the space between Au and FTO while it is released out from sites where the HTM is not covered by Au. Further investigation is needed to know if any chemical reaction (catalyzed by gold) is involved in production of gaseous component, which is formed only in the co-existence of LiTFSI and TBP. More work in the line of finding the effect of different metal contacts, dopants, solvents and even atmospheric humidity on defect generation in spiro-OMeTAD is being undertaken. Nevertheless, the evidences were compelling to believe that the performance reduction was essentially caused by changes in spiroOMeTAD or at the perovsktie/spiro-OMeTAD interface instead of perovskite itself. The conductivity of spiro-OMeTAD, as measured on the FTO/spiro-OMeTAD/Au structures, was found to increase with heating (Figure S8 in SI). This was also consistent with the fact that performance of heated devices dropped because of low shunt resistance (enhanced recombination), not increased series resistance. Further investigation is in progress to know the direct effects of these large defects in the spiro-OMeTAD on carrier recombination. ASSOCIATED CONTENT Supporting Information
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Experimental details, photovoltaic characteristics, additional SEM micrographs of the cells are given in supplementary information. AUTHOR INFORMATION 1. Ajay Kumar Jena Toin University of Yokohama, Japan Email-
[email protected] 2. Masashi Ikegami Toin University of Yokohama, Japan
[email protected] 3. Tsutomu Miyasaka Toin University of Yokohama, Japan Email-
[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The present research has been supported by Japan Science and Technology Agency -Advanced Low Carbon Technology R&D program (ALCA). The authors acknowledge Professor H. Segawa for giving access to research facilities at Research Center for Advanced Science and Technology (RCAST), University of Tokyo. T. M. thanks the supports of New Energy and Industrial Technology Development Organization (NEDO) as well as Japanese Society for Promotion of Science (JSPS) Grant-in-Aid for Scientific Research B.
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