Promising excitonic absorption for efficient perovskite solar cells

Tuning the band gap of perovskites toward the ideal band gap enables the enhancement of the power conversion efficiency (PCE) of perovskite solar cells (PSCs). Here, we demonstrate that the optical band-gap narrowing can be achieved by employing the excitonic absorption in perovskites through tuning their exciton binding energy (E_b), which directly leads to a photocurrent gain and hence improves the PCE of PSCs. With combined theoretical and experimental studies, it is revealed that the E_b is deeply correlated with the density of vacancy defects in perovskites due to their potential screening effect. Using the precursor engineering, we enhance the E_b by decreasing the density of vacancy defects in perovskites films. As a result, the improved excitonic absorption in formamidinium lead iodide (FAPbI₃) obviously broadens the spectral response and thus boosts the efficiency of the champion PSC up to 26.31% (certified 26.09%), mainly due to an enhanced photocurrent.