Surface polarization and recombination in organic-inorganic hybrid perovskite solar cells based on photo- and electrically induced negative capacitance studies

Recent studies have shown that organic-inorganic hybrid perovskites (OIHPs) exhibit unprecedented optoelectronic properties for photovoltaic applications. Complete realizations of photo- and electrically induced surface polarization and recombination are critically important for extensively upgrading solar cell performance. We have explored such effects in a planar solar cell comprising ITO(glass)/PEDOT:PSS/CH3NH3PbI3-xClx/PC60BM/PEI/Ag with a maximum power conversion efficiency (PCE) of 15.4%. A remarkable frequency-dependence of surface polarization was observed for the operating device under illumination (i.e. short-circuit current J(SC) condition) and at steady state (i.e. open-circuit voltage V-OC condition). A sign reversal of the capacitance (i.e. negative capacitance) associated with the surface polarization and recombination was detected at some specific steady states. Based on capacitance-voltage (C-V) spectroscopic measurements and theoretical fittings, they can be understood that, (i) a naturally formed surface depletion region due to energy mismatch can be fully suppressed at a very low fluence and moderate forward bias voltage; (ii) a surface capacitance (C-s) appears at lower ac-modulation frequencies only; (iii) remarkable trap-assist and band to band surface recombination may play the dominant roles for generating the negative capacitive effect in the CH3NH3PbI3-xClx based photovoltaic system.