ORGANIC PHOTOVOLTICS

Solid-state PV cells based on carbonaceous (organic) matter were first discovered 20 years ago(Tang 1986). Early work on organic photovoltaics using molecular-based systems demonstratedthe concept; however energy conversion efficiencies were low. Considerable excitement in thisarea was generated by a report published in the mid-1990s of 2.9%-efficient cells based onconducting organic polymers mixed with derivatives of C60 (fullerene) (Yu et al. 1995). Duringthe past decade, refinements in the chemical components of the cells, improvements in cellphysics, and device engineering have led to individual demonstration cells that operate at greaterthan 5% solar-to-electrical-power conversion efficiency. The opportunities and potential payoffhere are significant: low-cost, lightweight, large-area, flexible, high-efficiency solar cells. Thematerials are basically like those used in video display technology, and they offer the possibilityof very significant cost reduction, as well as flexibility in installation, form factor, etc. The basicresearch goal is to develop sufficient understanding of such materials and structures to improvetheir conversion efficiency by a factor of 5–10, and thus obtain robust, scalable efficiencies of15–25% in cheap, plastic-type solar cells .
The basic principle of operation in an organicPV cell is that illumination of a mixture ofelectron-rich (donor) and electron-poor(acceptor) species leads to separation of (+) and(-) charge carriers. Collection of these carriers atelectrodes produces an electrical current that cando work on a load in an external circuit.Although the basic principles behind theoperation of organic PV cells and conventionalp-n junction cells are similar, the details of keyevents, such as charge carrier generation andcharge transport, are very different in the organiccells. Consequently, the basic understanding ofthe physics and chemistry of organic PV systemslags behind that of conventional p-n junction devices.
The molecules and materials from which these devices are made vary considerably, withpolymers, organic molecules, inorganic molecules, quantum dots, and other nanostructures allcontributing equally to the field. The large array of options provides ample scope for researchersto develop novel solutions for improving device performance.