Two quasi-interfacial p-n junctions observed by a dual-irradiation system in perovskite solar cells

In general, perovskite solar cells (PSC) with a sensitized or thin-film architecture absorb light from a single-side illumination, and carrier separation and transport only take place inside the active layer of the perovskite film. Herein, we demonstrated a dual-irradiation PSC system in which light passes through both the fluorinated tin oxide (FTO) side and the Au electrode side, resulting in much faster interfacial charge carrier extraction and transportation than that in a single-irradiation system, in which light passes through from either the FTO or semitransparent Au electrode side. This dual-irradiation PSC system with a configuration of FTO/Cl-TiO₂/Mp-TiO₂/mixed perovskite/spiro-OMeTAD/Au/ITO can form two quasi-interfacial p-n junctions, which occur separately at the interfaces of TiO₂/perovskite and perovskite/spiro-OMeTAD. When the PSC device was illuminated simultaneously from both the FTO and Au/ITO sides, the PSC achieved a total power conversion efficiency (PCE) as high as 20.1% under high light intensity (1.4 sun), which is higher than PCE (18.4%) of a single-irradiation system. The time of flight (TOF) photoconductivity, small perturbation transient photovoltaic (TPV), finite-difference time-domain (FDTD) optical simulations, and dual illumination-side-dependent impedance spectroscopy (ISD-IS) were used to authenticate the presence of two quasi-interfacial p-n junctions in the PSC, creating more charge carriers than only one quasi p-n junction, and thus leading to a fast recombination process.