Solar cells as a sustainable and clean energy have attracted a great amount of attentions in both academia and industry. In various types of solar cells, polymer solar cells have unique attractions and opportunities. Such cells can be manufactured using roll-to-roll technologies similar to how newspapers are printed, thus greatly reducing costs. Polymer solar cells usually contain amixture of polymer chains which can donate electrons and “bucky-ball” molecules which accept electrons. The working principle of polymer solar cells is substantiallydifferent from traditional silicon solar cells. Under solar irradiation, electron excitations generate mobile electron-hole pairs named excitons. The excitons then diffuse through the active layer of the cell, separating at donor-acceptor interfaces into free charge carriers: electrons and holes, which are then collected as electrical current when they reach electrodes.
In spite of the unique attractions of polymer solar cells, the challenge has been improving the power conversion efficiency of solar cells, which is defined as the percentage of the power generated by the cell versus the power of the incident sunlight. The power produced by a solar cell is the product of three cell performance parameters, the open circuit voltage, the short circuit current, and the fill factor. While there are reliable approaches to increase the cell open circuit voltage and short circuit current, the realization of high fill factors has proven elusive, with fill factors of most polymer solar cells typically substantially below 70%.
In polymer solar cells, achieving high fill factors is mainly limited by the recombination (self-annihilation) of the photogenerated electrons and holes. In typical polymer solar cells, the randomly distributed donor and acceptor components lead to formation of disordered domains and segregated islands, as well as incorrect contacts with the electrodes. Such inherent disordered structures leads to many recombinations,thus results in low fill factors. Therefore achieving high fill factors inpolymer solar cells represents a significant progress.
Figure1. Polymer solar cells with a high fill factor. The cell shows close packed donor-acceptor microstructure with horizontal separation and vertical gradation of the electron donor and acceptor regions. Such morphology results in low electron-hole recombination, efficient charge sweep-out, and thus high fill factor.
On August 11, Nature Photonics published the study by associate professor Xugang Guo of Materials Science and Engineering at SUSTC: http://www.nature.com/nphoton/journal/vaop/ncurrent/abs/nphoton.2013.207.html. The paper, entitled “Polymer Solar Cells with Enhanced Fill Factors”,reports the realizations of unprecedented fill factors of 76%-80% through materials design and device engineering. The research shows that the exceptional fill factors arise from high levels of order in the mixture of polymer donor chains and bucky ball acceptor components, how these two components are distributed within the cell active layer, and a “face-on”orientation of the polymer chains on the electrode. The film morphology greatly suppressed the recombinations of charge carriers, leads to efficient charge collections in the respective electrodes. The 80% fill factor of polymersolar cells comes close to that of silicon cells, and heralds a brighter future of polymer solar cells. Many media report this research, including Science Daily and Phys.org,
Figure 2. High-performance organic semiconductors designed based on sulfur and oxygen non-bonding interaction, The polymer semiconductor achieves high charge carrier mobility and enhanced device air stability.
Journal of the American Chemical Society reports the research progress in organic thin-film transistors by Xugang Guo (J. Am. Chem.Soc., 2013, 135, 1986-1996). In the design of organic semiconductors, the semiconductors should have highly coplanar structure in order to maximize π-orbital overlap to enhance carrier mobility. At meantime, the solublizing alkyl chains are introduced to achieve the desired solubilities for low-cost printed electronics. The introduction of the alkyl chain usually imposes the steric hindrance, which leads to backbone torsion. By introducing alkoxy chain, the non-bonding interactions between sulfur and oxygen lock the backbone conformations. Therefore the resulting organic semiconductors can achieve high degree of backbone copolanrity and substantial charge carrier mobility, and meantime the device shows improved device airstability. Such study offers a new strategy for designing high-performance organic semiconductors. Xuguang Guo was also invited by SPIE for a keynote paper,which is published in the Proceedings of SPIE (Proc. SPIE, 2013, 8622, 86220K).
Associate Professor Xugang Guo is the first author of all the papers, both the South University of Science and Technology of China (SUSTC) and Northwestern University are the affiliations of Xugang Guo.
