(Invited) Sn Based Perovskite Solar Cells on Glass Substrate and Plastic Substrate
Authors: Hayase Shuzi 
Published: December 2020

Lead-based perovskite solar cells (PSCs) with small cell area have achieved a conversion efficiencies of higher than 25 %. Because of the Pb ion concern, various Pb-free PSCs also have been explored such as Bi, Ge, Ag, and Sn-PSCs. Among then, Sn-PSCs are is the most promising candidate. In this presentation, to begin with, the effect of the photo-voltaic performance caused by Ge ion doping, A site cation replacement, and surface passivation are reviewed briefly from the view point of the carrier concentration, carrier life time, crystal lattice strain, and band alignment. Recently, We reported the correlation between lattice strain relaxation and the power conversion efficiency of Sn-PSCs by substituting the A site with cations of various ionic radii(1). From the study, we knew the unique behaviors of ethylammonium iodide (EAI) replacement at A site cation, giving the efficiency over 13 %. After the brief review, especially, the effect of EAI addition on the enhanced efficiency is discussed. There is some works previously on EAI replacement at A site(2).
The formulation with Ge ion doped Sn-perovskite with an inverted structure is used as the discussion. The lattice disorder induced by the A site cation size has detrimental effects on the charge carrier dynamics in perovskite semiconductors. Among them, EAI promoted crystal growth towards preferred crystal orientation and caused the crystal lattice expansion, suggesting that bigger EA+ cation is incorporated in the perovskite crystal structure. The addition of EAI decreased the lattice strain, shifted the band edge absorption towards lower wavelength, and caused blueshift of the PL, showing that the introduction of EAI makes band gap a little wider. PL lifetime became longer from 12.9 ns to 20.1 ns after the addition of EAI. Efficiency of solar cells consisting of FTO/PEDOT:PSS/Sn-PVK/C60/BCP/Ag/Au was enhanced from 9.20 % ( JSC of 23.38 mA cm-2, VOC of 0.56 V and FF of 0.70) to 9.86 % ( JSC of 23.41 mA cm-2, VOC of 0.59 V and FF of 0.71) after EAI addition. After optimization, the efficiency reached a high value of 10.80 % with a JSC of 23.05 mA cm-2, a VOC of 0.62 V and a FF of 0.75. The increased in the device performance with EAI addition was attributed to the significant increase in the VOC. We have already published a report regarding this phenomenon in which we attribute this to the lattice strain relaxation of the perovskite crystal(1). The ideality factor is decreased from 1.47 to 1.26. By using open-circuit voltage decay (OCVD) method, we proved that the addition of EAI minimizes the charge carrier recombination process and as a result showed longer carrier lifetime. Further optimization of the best device is performed by surface passivation using Lewis base following our previous report(3). This helped to increase the efficiency even further to an unprecedented efficiency of 13.24 % with a JSC of 20.32 mA cm-2, FF 0.78 of and VOC of 0.84 V. The cause is explained by using the results of Hall effect measurement and Mott-Schottkey plot from the view point of carrier concentration, mobility, and built in potential. The carrier diffusion length increased from 88.7 nm, to 348 nm after addition of EAI. This justifies the higher VOC. This agrees with the trend obtained from Hall effect measurement. XPS study proved that by the introduction of EA cation, the oxygen content has been reduced compared to the pristine perovskite film. Tuning the A site cation not only has a significant effect on the crystal stability but also the lattice strain of the perovskites. This work is expected to establish a trend on fabricating tin halide PSCs that is able to compete with lead-based PSCs.
References
1. K. Nishimura, S. Hayase, et al., ACS Appl. Mater. Interfaces, 2019, 11, 31105–31110.
2. S. Shao, at. Al., Nano Energy, 2019, 60, 810–816.
3. M. A. Kamarudin, S. Hayase, at al., J. Phys. Chem. Lett., 2019, 10, 5277–5283