Role of carrier delocalization in enhancing the photostability and defect passivation of 2D/3D hybrid metal halide perovskites

Photodegradation and defect passivation of hybrid metal halide perovskites are still challenges toward related highly stable optoelectronic devices such as solar cells. Herein, we show that photodegradation and defect density can be drastically reduced in 2D/3D perovskites by engineering the conditions for carrier delocalization. For this purpose, conjugated, partially conjugated and non-conjugated spacer molecules of similar sizes as well as n values have been systematically changed in 2D/3D perovskites based on archetypical MAPbI₃. The decrease of calculated HOMO-LUMO gaps of complexes with spacer molecules (E_HL-complex) correlated well with the decrease of the Urbach energy, the decrease of the trap-state density, and the strong reduction of photodegradation of 2D/3D perovskite films, whereas the photodegradation was practically independent of the n value for conjugated spacer molecules. In addition, the short circuit current density (J_sc) and open circuit voltage (V_oc) of the prepared solar cells increased with decreasing E_HL-complex; hence, maximum initial values of J_sc and V_oc larger than those of identically prepared reference solar cells based on 3D MAPbI₃ were reached. We also demonstrate that the photodegradation of solar cells decreased monotonously with increasing initial power conversion efficiency. Thus, delocalization by optimum coupling of electron wavefunctions between conjugated spacer molecules and binding moieties at the perovskite enhances defect passivation and strongly reduces photodegradation in 2D/3D perovskites.