Controlling the Morphology and Efficiency of Hybrid ZnO:Polythiophene Solar Cells Via Side Chain Functionalization

• Published in: Advanced energy materials Vol. 1; no. 1; pp. 90 - 96
• Main Authors: Oosterhout, Stefan D., Koster, L. Jan Anton, van Bavel, Svetlana S., Loos, Joachim, Stenzel, Ole, Thiedmann, Ralf, Schmidt, Volker, Campo, Bert, Cleij, Thomas J., Lutzen, Laurence, Vanderzande, Dirk, Wienk, Martijn M., Janssen, René A. J.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 01-01-2011; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: Chains (polymeric); Conjugated Polymers; Efficiency; Hybrid Materials; Morphology; Phase separation; Photovoltaic cells; Semiconductors; Solar Cells; Solar energy; Thin films; Three dimensional; Zinc Oxide
• ISSN: 1614-6832; 1614-6840
• DOI: 10.1002/aenm.201000022

The efficiency of polymer – metal oxide hybrid solar cells depends critically on the intimacy of mixing of the two semiconductors. The effect of side chain functionalization on the morphology and performance of conjugated polymer:ZnO solar cells is investigated. Using an ester‐functionalized side chain poly(3‐hexylthiophene‐2,5‐diyl) derivative (P3HT‐E), the nanoscale morphology of ZnO:polymer solar cells is significantly more intimately mixed compared to ZnO:poly(3‐hexylthiophene‐2,5‐diyl) (ZnO:P3HT), as evidenced experimentally from a 3D reconstruction of the phase separation using electron tomography. Photoinduced absorption reveals nearly quantitative charge generation for the ZnO:P3HT‐E blend but not for ZnO:P3HT, consistent with the results obtained from solving the 3D diffusion equation for excitons formed in the polymer within the two experimental ZnO morphologies. For thin ZnO:P3HT‐E active layers (∼50 nm) this yields a significant improvement of the solar cell performance. For thicker cells, however, the reduced hole mobility and a reduced percolation of ZnO pathways hinders charge carrier collection, limiting the power conversion efficiency.
The efficiency of ZnO:polythiophene hybrid solar cells depends critically on the intimacy of mixing of the ZnO and the polymer. An ester functionalized polythiophene copolymer is able to greatly enhance mixing of the materials, leading to superb exciton generation efficiency, but the fine phase separation hinders charge carrier transport to the electrodes, limiting device performance.