Deciphering the capacitance frequency technique for performance-limiting defect-state parameters in energy-harvesting perovskites.

With emerging thin-film PIN-based optoelectronics devices, a significant research thrust is focused on the passivation of trap states for performance enhancement. Among various methods, the capacitance frequency technique (CFT) is widely employed to quantify the trap-state parameters; however, the trapped charge-induced electrostatic effect on the same is not yet established for such devices. Herein, we present a theoretical methodology to incorporate such effects in the CF characteristics of well-established, but not limited to, carrier-selective perovskite-based PIN devices. We show that the electrostatic effect of trapped charges leads to non-linear energy bands in the perovskite layer, which results in the underestimation of the trap density from existing CFT models. Consequently, a parabolic band approximation with effective length (PBAEL) model is developed to accurately predict the trap density for shallow or deep states from CFT analysis. In addition, we demonstrate that the attempt-to-escape frequency, which dictates the trapping dynamics, can be well extracted by eliminating the electrostatic effect at a reduced perovskite thickness. We believe that our work provides a unified theoretical platform for CFT to extract trap-state parameters for a broad class of organic, inorganic, and hybrid semiconductor-based thin-film devices for energy-conversion applications such as solar cells, LEDs, artificial photosynthesis, etc.

On the Uniqueness of Ideality Factor and Voltage Exponent of Perovskite-Based Solar Cells.

Perovskite-based solar cells have attracted much recent research interest with efficiency approaching 20%. While various combinations of material parameters and processing conditions are attempted for improved performance, there is still a lack of understanding in terms of the basic device physics and functional parameters that control the efficiency. Here we show that perovskite-based solar cells have two universal features: an ideality factor close to two and a space-charge-limited current regime. Through detailed numerical modeling, we identify the mechanisms that lead to these universal features. Our model predictions are supported by experimental results on solar cells fabricated at five different laboratories using different materials and processing conditions. Indeed, this work unravels the fundamental operation principle of perovskite-based solar cells, suggests ways to improve the eventual performance, and serves as a benchmark to which experimental results from various laboratories can be compared.