We think that the encouraging level of interest in our results may be due to the fact that our paper disclosed one of the most important issues concerning polymer solar cells. That is, this paper unveiled the influence of chemical regularity (regioregularity) of versatile poly(3-hexylthiophene) (P3HT) on the performance of polymer solar cells.

“Our paper presents an experimental study outlining one successful approach towards efficiency enhancement, namely systematic improvement in the chemical structure perfection (regioregularity) of the polymer component in a polymer/fullerene blend device.”

Recent studies have shown that the combination of a versatile conjugated polymer, poly(3-hexylthiophene) (P3HT), with a soluble fullerene derivative produces devices with solar-to-electric power conversion efficiencies of over 4%. This encouraging development has inspired a strong growth of interest in organic solar cells as the conditions for high efficiency had not yet been reported in detail.

Our studies had shown that even very small changes in the degree of order within the P3HT polymer chains have a big influence on the power conversion efficiency of devices made from these materials. This emphasizes the importance of the self-organizing capability of the active materials. This finding is of considerable importance and helps to guide the future direction of organic solar cell research.

This particular paper is the latest in a sequence of our publications that addresses various aspects of polymer/fullerene blends and their devices. Other highly cited papers include S. Choulis et al, Applied Physics Letters 83, 3812, 2003, Y. Kim et al, Applied Physics Letters 86, 063502, 2005, and S. Tuladhar et a,l Advanced Functional Materials 15, 1171, 2005.

The paper reports both a discovery—the influence of polymer chemical regioregularity on the performance of organic solar cells—as well as a synthesis of knowledge. In terms of methodology, we have applied a wide series of methods—concept design, material synthesis, computer simulation, materials characterization—including synchrotron x-ray analysis, device fabrication, etc., in order to clearly correlate this influence with the performance of the final device.

Organic solar cells are emerging as one of the best and most promising candidates for cheap renewable energy sources, owing to an account of their potential for low temperature cost and high throughput manufacturing, enabled by solution processing as well as the benefit of using inexpensive organic solutions as well as productive and inexpensive reel-to-reel (e.g., roll-to-roll) processes.

For the practical application of achieving this goal, the conversion efficiency diffraction of sunlight that can convert solar energy into electricity must be improved from the 4-5% currently available to 6-10% for practical applications.

Our paper represents an experimental study outline. In this aspect, our paper logically proposed one successful approach to getting high-efficiency enhancement by outlining experimental evidence, namely systematic improvement in the regioregularity of the polymer component in a polymer/fullerene blend device.

The research was carried out within a British Petroleum funded research program on organic photovoltaic materials that started at Imperial College in 2002. Dr. Kim was recruited to work on the program as a post-doctoral research associate, and simultaneously registered for a second (Physics) Ph.D. degree, completed last year under the supervision of Professor Bradley.

Professor Jenny Nelson has researched novel materials for photovoltaics (PV) for the last seventeen years. She became involved in the use of conjugated polymers for photovoltaics in 2000, when she was appointed to the academic staff of the Blackett Laboratory as part of a new initiative on molecular electronic materials and devices which Professor Bradley had been recruited from the University of Sheffield to lead.

Professor Donal Bradley (an alumnus of Imperial College) has worked on conjugated polymers since starting his Ph.D. at the Cavendish Laboratory, Cambridge in 1983.

One of the biggest challenges in the basic research on the applications of molecular electronic materials is to obtain access to good quality materials which can provide reproducible results.

We were fortunate in having an active collaboration with Dr. Iain McCulloch and colleagues at Merck Chemicals, who were able to provide us with well-defined batches of P3HT polymer, and so enable us to undertake this detailed study of the effect of the polymer structure on device performance.

Solar cells based on organic electronic materials have the potential to provide electricity generation at very low cost, due to the lower costs of materials and device manufacturing. If products with acceptable efficiency and endurance can be produced within the next five to ten years, organic photovoltaic technology should accelerate the take-up of photovoltaic electricity beyond the level that is currently possible using silicon-based technology.

This could lead to a more rapid displacement of fossil-fueled electricity as well as to improvements in the standard of living in developing countries.