Differentiation between bulk and interfacial properties: analysis of time-dependent carrier injection in perovskite solar cells

Abstract

Perovskite solar cells (PSCs) have developed rapidly over the past decade, primarily because of improvements in their photovoltaic (PV) performance. However, methods of characterization and analysis for PSCs remain immature, mostly because of the complex multilayered PSC structures. The sandwiched structure of i-type perovskite photoabsorbers, having both p-type hole transport materials (HTMs) and n-type electron transport materials, is crucial for efficient charge separation. However, this structure forms multiple heterointerfaces, which undergo structural changes more easily 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 approach exploits the emissive nature of perovskite photoabsorbers and the contrast in carrier mobilities between perovskite photoabsorbers and carrier transport materials (CTMs), which exhibit an approximately three-orders-of-magnitude difference. Strong excitation causes carrier accumulation in the CTM bulk, resulting in time-dependent carrier injection, which presumably indicates 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. This method is validated by investigating the thermal degradation mechanism of PSCs comprising a commonly used spiro-OMeTAD HTM and strongly suggests that CTM/perovskite heterointerfaces was not thermally degraded. The proposed method can be applied to other PSC characterizations, contributing to the further development of PSCs by addressing a missing gap: the characterization and analysis of PSCs.