Correlation between Energy and Spatial Distribution of Intragap Trap States in the TiO₂ Photoanode of Dye‐Sensitized Solar Cells

Abstract

The energy and spatial distribution of intragap trap states of the TiO2 photoanode of dye-sensitized solar cells and their impact on charge recombination were investigated by means of time-resolved charge extraction (TRCE) and transient photovoltage (TPV). The photoanodes were built from TiO2 nanospheroids with different aspect ratios, and the TRCE results allowed differentiation of two different types of trap states, that is, deep and shallow ones at the surface and in the bulk of the TiO2 particles, respectively. These trap states exhibit distinctly different characteristic energy with only a slight variation in the particle size, as derived from the results of the density of states. Analyses of the size-dependent TPV kinetics revealed that in a moderate photovoltage regime of about 375-625 mV, the dynamics of electron recombination are dominated by shallow trap states in the bulk, which can be well accounted for by the mechanism of multiple-trap-limited charge transport.

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

“…As ar esult, charger ecombination directly occurs among the free electrons and acceptors, exhibiting an almost constant recombination rate. [35] The reasonisascribed to the alternative recombination pathway at low voltage (i.e., the isoenergetic electrons transfer from surfacestate of the TiO 2 to the electrolyte acceptors), which cannot be described by multiple-trapping mechanism. Figure 1s hows the ideal multiple-trapping assistedc harge recombination;h owever,m odificationsa re often required when describing real experiments.…”

“…The simulation parameters are as follows: N c = 10 20 cm À3 , N T = 10 19 cm À3 ,[33][34][35] k B T = 25.8 meV, E c = E redox + 900 meV,and k c = 100 s À1 . The simulation parameters are as follows: N c = 10 20 cm À3 , N T = 10 19 cm À3 ,[33][34][35] k B T = 25.8 meV, E c = E redox + 900 meV,and k c = 100 s À1 .…”

The Influence of Morphology and PbI2 on the Intrinsic Trap State Distribution in Perovskite Films Determined by Using Temperature‐Dependent Fluorescence Spectroscopy

“…Therefore, the electron and hole transfers from perovskite to PbI 2 are forbidden; in other words, PbI 2 could not act as a trap state to capture charge carriers. In addition, the trap states are expected to localize at the surface or interface of perovskite particles, so films assembled from small particles may possess higher DOS due to the larger surface area . However, with the decrease in particle size, the contact between particles becomes more intimate and the neck area becomes larger (see Figure ).…”

“…[54,55] In recent years, we have reported that time-resolved charge extraction (TRCE) could act as another effective candidate to study quantitatively the DOS distribution in dye-sensitized solar cells, quantum-dot-sensitized solar cells, and perovskite solar cells. [56][57][58][59][60] However,asignificant limitation of these methods is that the tested system should be connected to as hort or open circuit,s ot hey are unable to investigate the trap states in intrinsic perovskite films without selected electrodes. In contrast, transient infrareda bsorption spectroscopy can be used to measure the DOS distribution of thin films, but it is not easy to identify the characteristic optical signals and the requirement of instrumentsa nd technology is relatively high.…”

Enhanced Efficiency and Stability of Perovskite Solar Cells Through Nd‐Doping of Mesostructured TiO2

“…[14][15][16][17] Nevertheless, the relatively high density of electronic trap states below the CB are a signifi cant drawback for the application of TiO 2 electrodes in solar cells. [19][20][21] One method to reduce the trap states in TiO 2 is doping. [19][20][21] One method to reduce the trap states in TiO 2 is doping.…”