Multiple‐Trapping Model for the Charge Recombination Dynamics in Mesoporous‐Structured Perovskite Solar Cells

Abstract

The photovoltaic performance of organic-inorganic hybrid perovskite solar cells has reached a bottleneck after rapid development in last few years. Further breakthrough in this field requires deeper understanding of the underlying mechanism of the photoelectric conversion process in the device, especially the dynamics of charge-carrier recombination. Originating from dye-sensitized solar cells (DSSCs), mesoporous-structured perovskite solar cells (MPSCs) have shown many similarities to DSSCs with respect to their photoelectric dynamics. Herein, by applying the multiple-trapping model of the charge-recombination dynamic process for DSSCs in MPSCs, with rational modification, a novel physical model is proposed to describe the dynamics of charge recombination in MPSCs that exhibits good agreement with experimental data. Accordingly, the perovskite- and TiO -dominating charge-recombination processes are assigned and their relationships with the trap-state distribution are also discussed. An optimal balance between these two dynamic processes is required to improve the performance of mesoporous-structured perovskite devices.

Extraction technique of trap states based on transient photo-voltage measurement

“…Because of the trap states in the perovskite absorber layer, the behavior of trapping and de-trapping of carrier by the trap states determines the recombination rate, which can be described by the multiple-trapping model 46,[50][51][52] . According to the multiple-trapping model, the relation of carrier lifetime τ n and the free carrier lifetime τ f satisfies 46,[50][51][52] Here n = n t + n c denotes the sum of electron density in trap states n t and electron density in conduct band n c . Therefore, we have which can be rewritten as The density of electron in trap states satisfies 46,52 Here ρ t (E) denotes the DOS T distribution.…”

“…According to the multiple-trapping model, the relation of carrier lifetime τ n and the free carrier lifetime τ f satisfies 46,[50][51][52] Here n = n t + n c denotes the sum of electron density in trap states n t and electron density in conduct band n c . Therefore, we have which can be rewritten as The density of electron in trap states satisfies 46,52 Here ρ t (E) denotes the DOS T distribution.…”

“…The carrier density in conductor band satisfies 46,52,53 Here N c is the density of effective states in conduction band. E c is the conduction band energy level position 53 We define a derivation factor…”

“…Wang et al 46 proposed a transient photo-voltage (TPV) method for DOS T distribution extraction. Based on the hypothesis of exponential type DOS T distribution 46 – 49 , multiple-trapping model 46 , 50 – 52 , and the zero-temperature approximation 46 , 52 , they find that when the DOS T distribution is exponential type, the logarithm of carrier lifetime and the photo-voltage satisfy linear relation. They use this relation to extract the DOS T distribution based on the TPV result 46 – 48 .…”

“…In this article, we put forward a new technique for extraction of DOS T distribution based on the TPV measurement result. We give up the hypothesis of exponential type DOS T distribution 46 – 49 and zero-temperature approximation 46 , 52 . The method given in our work is based on the single hypothesis of multiple-trapping model 46 , 50 – 52 .…”