Open-Circuit Voltage Loss in Organic–Inorganic Halide Perovskite Solar Cells
Hyung Do Kim a, Atsushi Wakamiya b, Hideo Ohkita a
a Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Japan
b Institute for Chemical Research, Kyoto University, Japan
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of International Conference on Perovskite and Organic Photovoltaics and Optoelectronics (IPEROP19)
Kyōto-shi, Japan, 2019 January 27th - 29th
Organizers: Hideo Ohkita, Atsushi Wakamiya and Mohammad Nazeeruddin
Oral, Hyung Do Kim, presentation 043
Publication date: 23rd October 2018

Solution-processed organic–inorganic perovskites based on metal halides have recently gained considerable interest as one of the most promising materials for next-generation photovoltaic devices.  This is because of their excellent optoelectronic properties such as direct bandgap with high absorption coefficient, high charge carrier mobility, long diffusion length of charge carriers, Wannier–Mott exciton, and low trap densities.  Since the pioneering work of Miyasaka coworkers in 2009, the power conversion efficiency (PCE) of lead-based perovskite solar cells has steeply increased from 3.8 to more than 23%.  A further improvement in PCE is still possible by considering the Shockley–Queisser (SQ) limit (≈30%) for photovoltaic efficiency with an optical bandgap (Eg) of absorber (≈1.6 eV).  On the other hand, environmentally friendly perovskite solar cells without lead(ΙΙ) element have been in the spotlight from a commercial point of view.  More recently, several research groups have successfully fabricated lead-free perovskite solar cells by replacing lead(ΙΙ) element with tin(ΙΙ) element.  In most cases, however, the PCE was as low as less than 10%, which still lag far behind that of lead-based counterparts.  This is mainly ascribed to a small open-circuit voltage (VOC) (0.2–0.5 V) for tin-based devices.  In other words, there is a difference in photon energy loss Eloss (= Eg − qVOC) between the two devices. 

Herein, we studied the origin of Eloss in both lead-based and tin-based perovskite solar cells.  By measuring the temperature dependence of VOC, we discussed the difference in Eloss between them in terms of charge recombination mechanism.  As a result, we found that an activation energy of the diode (Ea), evaluated from a linear extrapolation at 0 K on the basis of qVOC = Ea – kBTln(J00/JSC), is almost the same as the Eg of perovskites for lead-based perovskite solar cells.  In contrast, it is substantially smaller than the Eg of perovskites for tin-based devices.[1,2]  This suggests that the charge recombination mechanism underlying Eloss is different.  In other words, the main loss mechanism is the bulk recombination in a perovskite layer for lead-based perovskite solar cells while that is the surface recombination at the interface for tin-based perovskite solar cells.  We further discuss the potential strategies for further improvement of VOC in both devices.

This study was partly based on the results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO) and was partly supported by JST ALCA Grant Number JPMJAL1603, Japan.

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