Direct charge-transfer mechanism (Type-II) in coordination complexes for sensitization in solar cells: A comprehensive review

Photovoltaics employ absorbers to harness light energy for power generation. One huge class of absorbers is molecular coordination complexes, which are naturally available and can be obtained by synthetic procedures as well. To be used in solar energy harvesting, chemists consider anchoring groups as a crucial component. The anchoring moiety determines the extent of interaction between these complexes and inorganic substrates onto which they are attached in solar cell devices. This introduces several synthetic difficulties and energy loss mechanisms. To solve these issues, a new class of materials known as Type-II absorbers have arisen among chemists. Herein, the absorbers of the useful solar region (visible and infrared spectra) are rather generated after in-situ interactions between high-bandgap organic molecules and the inorganic substrates. This process eliminates non-radiative energy losses and shifts synthetic focus from thermodynamic alignments to interface binding strengths. Dye-sensitized solar cells (DSSCs), which employ these absorbers, can be classified into two types based on the electron transfer mechanism at the photoanode. The conventional one, Type-I, involves a two-step process consisting of light absorption by the dye and the subsequent injection of photoexcited electrons into the conduction band of TiO2. In contrast, Type-II employs direct single-step injection of excited electrons from the sensitizer. In Type-II, simple molecular sensitizers, rather than complex light-harvesting dyes, directly bind with TiO2 to form a charge-transfer complex, exhibiting broad-range absorption over 300–800 nm. This range is consistent with a highly effective and direct injection of excited electrons into the semiconducting TiO2, hence the designation “coordination complex sensitized solar cells.” However, the overall PCE is generally low, primarily due to the fast back-electron transfer (BET) process from the TiO2 to the oxidized sensitizer. Significant efforts have been devoted to suppressing the BET to enhance the performance of Type-II DSSCs both computationally and experimentally. However, these efforts were limited to using 1,2-benzenediol (catechol) as the backbone for sensitizing molecules, resulting in a PCE still as high as approximately 1 %. Recently, the employment of alternative sensitizers without the catechol backbone has been suggested to reduce the BET, which is often facilitated by the pi-electron system of catechol. In this paper, we examine the trends and various strategies employed to minimize the BET and propose a novel molecular design approach that could ultimately lead to better-performing coordination complex sensitized solar cells in the future.