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 (E_g) 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 (V_OC) (0.2–0.5 V) for tin-based devices.  In other words, there is a difference in photon energy loss E_loss (= E_g − qV_OC) between the two devices. 

Herein, we studied the origin of E_loss in both lead-based and tin-based perovskite solar cells.  By measuring the temperature dependence of V_OC, we discussed the difference in E_loss between them in terms of charge recombination mechanism.  As a result, we found that an activation energy of the diode (E_a), evaluated from a linear extrapolation at 0 K on the basis of qV_OC = E_a – k_BTln(J₀₀/J_SC), is almost the same as the E_g of perovskites for lead-based perovskite solar cells.  In contrast, it is substantially smaller than the E_g of perovskites for tin-based devices.^[1,2]  This suggests that the charge recombination mechanism underlying E_loss 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 V_OC in both devices.