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Controlled growth of a molecular bulk heterojunction photovoltaic cell

Nature Materials volume 4, pages 37–41 (2005)Cite this article

Abstract

The power conversion efficiency of organic photovoltaic cells has increased with the introduction of the donor–acceptor heterojunction that serves to dissociate strongly bound photogenerated excitons1. Further efficiency increases have been achieved in both polymer2,3 and small-molecular-mass4 organic photovoltaic cells through the use of the bulk heterojunction (BHJ), where the distance an exciton must diffuse from its generation to its dissociation site is reduced in an interpenetrating network of the donor and acceptor materials. However, the random distribution of donor and acceptor materials in such structures can lead to charge trapping at bottlenecks and cul-de-sacs in the conducting pathways to the electrodes. Here, we present a method for growing crystalline organic films into a controlled bulk heterojunction; that is, the positions and orientations of donor and acceptor materials are determined during growth by organic vapour-phase deposition (OVPD5), eliminating contorted and resistive conducting pathways while maximizing the interface area. This results in a substantial increase in power conversion efficiency compared with the best values obtained by 'random' small-molecular-weight BHJ solar cells formed by high-temperature annealing, or planar double heterojunction photovoltaic cells using the same archetypal materials systems.

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Figure 1: Schematic diagram of the types of organic donor–acceptor heterojunctions and the structure of the controlled bulk heterojunction (BHJ) device.
Figure 2: SEM and atomic force microscope (AFM) images of CuPc films.
Figure 3: Bragg–Brentano X-ray diffractograms of CuPc films grown on ITO using the Cu-Kα line.
Figure 4: Morphology of films grown on top of a highly folded underlying CuPc layer by VTE and OVPD.
Figure 5: Performance of an OVPD-grown, controlled CuPc/ PTCBI BHJ photovoltaic cell.

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Acknowledgements

We thank J. Xue, S. Uchida, R. Holmes, B. Rand and P. Peumans for discussions. We also thank the US Air Force Office of Scientific Research, the National Renewable Energy Laboratory and Global Photonic Energy Corporation for financial support.

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Authors and Affiliations

  1. Department of Electrical Engineering, Princeton Institute for the Science and Technology of Materials (PRISM), Princeton University, Princeton, 08544, New Jersey, USA

    Fan Yang & Stephen R. Forrest

  2. Department of Chemical Engineering, Princeton Institute for the Science and Technology of Materials (PRISM), Princeton University, Princeton, 08544, New Jersey, USA

    Max Shtein

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Correspondence to Stephen R. Forrest.

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Yang, F., Shtein, M. & Forrest, S. Controlled growth of a molecular bulk heterojunction photovoltaic cell. Nature Mater 4, 37–41 (2005). https://doi.org/10.1038/nmat1285

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