2015 Fiscal Year Research-status Report
High stability perovskite solar cells
| Project/Area Number |
15K17925
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| Research Institution | Okinawa Institute of Science and Technology Graduate University |
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Principal Investigator |
Qi Yabing 沖縄科学技術大学院大学, Energy Materials and Surface Sciences Unit, 准教授 (10625015)
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| Project Period (FY) |
2015-04-01 – 2017-03-31
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| Keywords | Hybrid perovskite / Solar cell / Lifetime / Surface science |
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| Outline of Annual Research Achievements |
In my proposed research plan, strategies for building high efficiency organic-inorganic hybrid perovskite-based solar cells with long lifetimes were described. In FY2015, we have made important progress, e.g. 4 peer-refereed papers have been published acknowledging Kakenhi Grant Number 15K17925 (see Journal Articles section). I was invited to write a review paper [1]. The main findings from our research are summarized below:
(1) Alternative high throughput method of hybrid chemical vapor deposition (HCVD) was used for the growth of formamidinium (NH2CH=NH2+, FA+) based perovskite solar cells. Solar cells based on FAPbI3 generated solar-to-energy power conversion efficiencies as high as 14.2%, which is attractive for industry. Large advantages regarding stability has been demonstrated for FAPbI3 materials as compared to the conventional methylammonium (CH3NH3+, MA+) based, i.e. MAPbI3.
(2) Co-evaporation and sequential evaporation of SnBr2 and MABr for fabricating Pb-free perovskites. MASnBr3 showed photovoltaic properties generating 1.12% efficiency. We have demonstrated that the fast formation of Sn oxide by air exposure can be avoided employing vacuum vapor-deposition methods, which is unavoidable in solution processing techniques [4].
(3) We were the first to succeed in revealing the real-space atomic structure of single crystal methylammonium lead bromide (CH3NH3PbBr3) using low-temperature scanning tunneling microscopy (LT-STM). This study revealed the orientation of CH3NH3+ within the Pb-Br cage providing fundamental insights for further design of perovskite materials.
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| Current Status of Research Progress |
Current Status of Research Progress
1: Research has progressed more than it was originally planned.
Reason
In addition to the descriptions provided in the “Summary of Research Achievements” section, below are further outcomes from the present research:
(1) One publication from my group have received large attention and was featured as cover page in the Journal of Materials Chemistry A [3].
(2) Recently, our group found that the top-most layer in perovskite solar cells, spin-coated spiro-MeOTAD films using chlorobenzene as the solvent showed a high density of pinholes (small sized pinholes ~4 pinholes/um2 with an average diameter of ~135 nm and large sized pinholes ~289 pinholes/mm2 with diameters in the range of 1－20 m). Additional cross-sectional view scanning electron microscopy (SEM) measurements revealed that these pinholes form channels wiggling across the film thickness (~240 nm). These pinholes were observed to facilitate the inward diffusion of gas molecules present in ambient air (e.g. H2O and O2) leading to detrimental effects of the underneath perovskite layer. In addition, these pinholes can also facilitate the outward diffusion of chemical elements/compounds with high vapor pressure such as iodine-containing volatile species (MAI and/or HI) as a result of the degradation and/or decomposition of the CH3NH3PbI3 perovskite film. Moreover, the pinholes in spiro-MeOTAD HTL were demonstrated to be one of the causes for the reduced device performance as a function of cell’s operation time.
(3) The above research was featured in several local and overseas media. Please see Remarks/Webpage section.
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| Strategy for Future Research Activity |
Organic-inorganic perovskite materials have the general ABX3 structure where organic compounds such as MA and FA occupy site ‘A’. Metals (Sn and Pb) and halogen elements (Cl-, I-. Br-) occupy sites 'B' and 'X', respectively. If the tolerance factor for perovskite formation does not deviate substantially from 1, it is possible to synthesize alternative perovskite materials to be tested as candidates for solar cell applications. From my current research, I have gained substantial knowledge working with MAI, MABr, FAI, PbI2, PbCl2, PbBr2, and SnBr2 compounds reaching >10% efficiency solar cells [2,4]. As shown in the timeline of my original proposed research (Table 1, Year 2), I plan to study systematically the possible new chemical compound candidates: FABr, ethylammonium (CH3CH2NH3+), PEA [C6H5(CH2)2NH3+] and several other suitable compounds to combine with PbI2, PbCl2, SnI2, and SnBr2 as well as mixed cations and halides perovskite films based not only on solution processing, but also from our originally developed vapor deposition techniques, which includes MS-MLD, HCVD, and HB (see Figure 1 of original proposal).
In addition to the optimization of solar cell devices, guidance and feedback/verification from surface science analytical tools are also essential for efficiently achieving our goals. Particular efforts will be given to the LT-STM technique on new perovskite materials as described in the proposal. Atomic-level understanding of perovskite structure is crucial for rational design of new photovoltaic materials.
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| Causes of Carryover |
NONE.
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| Expenditure Plan for Carryover Budget |
NONE.
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Research Products
(7 results)
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[Journal Article] The presence of CH3NH2 neutral species in organometal halide perovskite films2016
Author(s)
M.-C. Jung, Y. M. Lee, H.-K. Lee, J. Park, S. R. Raga, L. K. Ono, S. Wang, M. R. Leyden, B. D. Yu, S. Hong and Y. B. Qi
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Journal Title
Appl. Phys. Lett. 108, 073901 (2016)
Volume: 108
Pages: 073901
DOI
Peer Reviewed / Open Access / Int'l Joint Research / Acknowledgement Compliant
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