New Progress in the Device Physics of Polymer-fullerene Solar Cells

The efficiencies of polymer-fullerene solar cells (PSCs) have now exceeded 10% and are approaching 15% which is the criterion for commercial production. With the increase of the efficiency of organic solar cell, the field has drawn lots of interest. In this review, we present a detailed description on the operating principles of PSCs and an overview of recently developed techniques and new designs that improve the commercial merit of the final product. Firstly, the principles of key steps, including photo-absorption, creation of charge-transfer (CT) state, photocurrent generation and collection, which strongly influence the device performance such as the short circuit current, open circuit voltage, fill factor and overall efficiency are identified, as well as the methods to improve these parameters and the relationship among the work steps of PSC. Theoretical analysis to the up-to-date experiments leads to the novel descriptions of the photocurrent generation, such as the hot exciton dissociation and non-integer-order recombination. The former one highlights that excess energy of exciton dominates the dissociation yield, and the later one is related to the disorder property of organic solar cells. Secondly, we review advanced techniques for boosting the device performance, which include the employment of inverted structures, plasmonic effects of metal nano-particles, tandem cells and ternary blends. Recent experiments demonstrate proper combinations of these strategies are required to achieve higher efficiencies of PSCs. Comprehensions of device physics in semiconducting polymers is necessary to optimize the performance of polymeric solar cells, which would require researches focusing on the synthesis, characterization, design and simulation of organic semiconductors. Toward a better understanding of the microscopic working mechanism of high-efficiency PSCs, multi-perspective investigations combined with first-principle calculation such as the density functional theory (DFT) are necessary to provide the detailed information at the donor/acceptor interface.