Utilizing Temperature-Dependent Photocurrent Spectroscopy to Extract the Exciton Binding Energy of CH3NH3PbI3 Perovskite Thin-Films
Jay Patel a, Qianqian Lin b, Olga Zadvorna a, Christopher Davies a, Laura Herz a, Michael Johnston a
a Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU, United Kingdom
b Wuhan University, Wuhan, 430072, China
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Poster, Jay Patel, 073
Publication date: 11th February 2019

Solar cells consisting of metal-halide perovskite thin films show great energy harvesting capabilities in terrestrial photovoltaic installations.[1][2] We utilize Fourier-transform photocurrent spectroscopy (FTPS) to understand the temperature-dependence of photocurrent spectra of CH3NH3PbI3 devices. Interestingly we find that there are clear features in the low-temperature photocurrent spectra that indicate photocurrent contribution from PbI2. The evidence of PbI2 in the photocurrent spectra at low temperature shows that PbI2 plays a dynamic role (other than passivation) in CH3NH3PbI3 solar cells. Moreover, at 15 K exciton formation leads to a blue-shift in the photocurrent-onset with respect to the absorption onset. At such low temperatures, there is very little photocurrent contribution from the excitonic states.[3] Hence our low temperature photocurrent and absorption measurements of CH3NH3PbI3, allow us to extract a lower limit on the exciton binding energy of 9.1 meV for CH3NH3PbI3.[4] Additionally, we assess the device characteristics of the prototypical perovskite solar cells based on CH3NH3PbI3 over a temperature range from 15 K to 350 K. We observe a peak in the short circuit current and open circuit voltage at 200 K with reasonable operation maintained up to 350 K. However, the device current-voltage characteristics are significantly poor below 200 K with the short circuit current dropping by over 5 orders of magnitude at 15 K. 

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