Strategies for increasing the efficiency of heterojunction organic solar cells: material selection and device architecture

Paul Heremans, David Cheyns, Barry P Rand
Accounts of Chemical Research 2009 November 17, 42 (11): 1740-7
Thin-film blends or bilayers of donor- and acceptor-type organic semiconductors form the core of heterojunction organic photovoltaic cells. Researchers measure the quality of photovoltaic cells based on their power conversion efficiency, the ratio of the electrical power that can be generated versus the power of incident solar radiation. The efficiency of organic solar cells has increased steadily in the last decade, currently reaching up to 6%. Understanding and combating the various loss mechanisms that occur in processes from optical excitation to charge collection should lead to efficiencies on the order of 10% in the near future. In organic heterojunction solar cells, the generation of photocurrent is a cascade of four steps: generation of excitons (electrically neutral bound electron-hole pairs) by photon absorption, diffusion of excitons to the heterojunction, dissociation of the excitons into free charge carriers, and transport of these carriers to the contacts. In this Account, we review our recent contributions to the understanding of the mechanisms that govern these steps. Starting from archetype donor-acceptor systems of planar small-molecule heterojunctions and solution-processed bulk heterojunctions, we outline our search for alternative materials and device architectures. We show that non-planar phthalocynanines have appealing absorption characteristics but also have reduced charge carrier transport. As a result, the donor layer needs to be ultrathin, and all layers of the device have to be tuned to account for optical interference effects. Using these optimization techniques, we illustrate cells with 3.1% efficiency for the non-planar chloroboron subphthalocyanine donor. Molecules offering a better compromise between absorption and carrier mobility should allow for further improvements. We also propose a method for increasing the exciton diffusion length by converting singlet excitons into long-lived triplets. By doping a polymer with a phosphorescent molecule, we demonstrate an increase in the exciton diffusion length of a polymer from 4 to 9 nm. If researchers can identify suitable phosphorescent dopants, this method could be employed with other materials. The carrier transport from the junction to the contacts is markedly different for a bulk heterojunction cell than for planar junction cells. Unlike for bulk heterojunction cells, the open-circuit voltage of planar-junction cells is independent of the contact work functions, as a consequence of the balance of drift and diffusion currents in these systems. This understanding helps to guide the development of new materials (particularly donor materials) that can further boost the efficiency of single-junction cells to 10%. With multijunction architectures, we expect that efficiencies of 12-16% could be attained, at which point organic photovoltaic cells could become an important renewable energy source.

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