Differentiation Between Bulk and Interfacial Properties: Analysis of Time-Dependent Carrier Injection in Perovskite Solar Cells

Perovskite solar cells (PSCs) have developed rapidly in the last decade, primarily because of improvements in their photovoltaic (PV) performance. However, methods of characterization and analyses for PSCs remain immature, mostly because of the complex multilayered PSC structure. The sandwiched structure of i-type perovskite photoabsorbers having both p-type hole transport materials (HTMs) and n-type electron transport materials (ETMs) facilitates charge separation in the perovskite photoabsorber and thus plays a crucial role in developing efficient PSCs. However, this structure forms multiple heterointerfaces, which more easily undergo structural changes than the bulk and are difficult to characterize independently. Herein, we propose a method for differentiating the bulk and heterointerface properties in PSCs in terms of their carrier dynamics by using excitation power dependence on photoluminescence lifetime (EPD-PLL) measurements. This method exploits the intrinsically different carrier mobility behavior of the PSC components; specifically, the carrier mobilities within perovskite photoabsorbers are significantly higher (by over three orders of magnitude) than those in carrier-transport materials (CTMs). Strong excitation causes carrier accumulation in the CTM bulk, resulting in time-dependent carrier injection, which could be indicative of the bulk CTM properties. In contrast, weak excitation leads to time-independent carrier injection from the perovskite to CTMs, correlating with heterointerface properties. Hence, EPD-PLL measurements differentiate carrier dynamics between the CTM bulk and heterointerfaces. The method was tested by investigating the thermal degradation mechanism of typical Spiro-OMeTAD-based PSCs, revealing that the contribution of heterointerface deterioration by thermal stress can be neglected. The degraded PSCs had unusual PV properties, unlike those of fresh PSCs, and the EPD-PLL results confirmed these unusual properties, facilitating effective PSC characterization. The proposed method can be applied to the characterization of other PSCs, contributing to the further development of PSCs by addressing a missing point: the characterization and analysis of PSCs.