Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
In polymer-based bulk heterojunction solar cells, the absorber blend has no preferential transport direction for photogenerated charge carriers due to the statistical intermixing of both the donor and acceptor phase. Therefore, charge-selective interfacial layers, which are semipermeable membranes for either electrons or holes in an ideal case, are widely applicated to achieve efficient charge extraction at the respective contacts and determine the device polarity.[1] Amongst the materials for electron collection and transport, ZnO nanoparticles attracted a lot of attention in polymer photovoltaics, as they combine the favorable properties of crystalline ZnO (e.g., high electron mobility, wide band gap, n-type conductivity) with the low temperature and solution-based processing methods of colloidal chemistry.[2,3]
However, the most critical issue related to thin films made of ZnO nanoparticles is their large internal surface area and one prominent surface-related phenomenon is the adsorption of ambient oxygen. Once being attached to the ZnO surface, oxygen species such as O2 are able to trap electrons from the conduction band, which gives rise to depletion of free carriers next to the surface and, consequently, impedes charge transport within the nanoparticle film.[4]
In this study, we synthesized ZnO nanoparticles with varying surface-area-to-volume ratio and implemented them into inverted polymer-fullerene solar cells with gas-permeable top-electrodes. This allowed us to examine the impact of oxygen adsorption on working devices under different atmospheric scenarios. We find that the availability of ambient oxygen strongly affects the current-voltage characteristics (see Fig. 1) and, thus, deteriorates the efficiency of the devices. The magnitude of the oxygen adsorption effect is observed to be strongly dependent on the amount of internal surface area in the ZnO nanoparticle film. Different origins for the inferior device performance under oxygen exposure are discussed, e.g., Ohmic losses in the current injection regime, leading to an increased series resistance (larger particles), and the occurrence of space-charge-limited currents (SCLC), leading to a non-Ohmic current-voltage behavior (smaller particles).[5]
Fig. 1: Dark current density-voltage characteristics of inverted P3HT:PCBM solar cells with interfacial layers made from rod-like (a) and spherical (b) shaped ZnO nanoparticles and comprising a gas-permeable PEDOT:PSS top electrode depending on the exposure time to ambient air. Insets: TEM images of the different ZnO nanoparticles used in this study.
[1] Ratcliff, E. L.; Zacher, B.; Armstrong, N. R. Selective Interlayers and Contacts in Organic Photovoltaic Cells. J. Phys. Chem. Lett. 2011, 2, 1337-1350. [2] Wilken, S.; Scheunemann, D.; Wilkens, V.; Parisi, J.; Borchert, H. Improvement of ITO-free inverted polymer-based solar cells by using colloidal zinc oxide nanocrystals as electron-selective buffer layer. Org. Electron. 2012, 13, 2386-2394. [3] Pacholski, C.; Kornowski, A.; Weller, H. Self-Assembly of ZnO: From Nanodots to Nanorods. Angew. Chem. Int. Ed. 2002, 41, 1188-1191. [4] Li, M.; Xing, G.; Qune, L. F. N. A.; Xing, G.; Wu, T.; Huan, C. H. A.; Zhang, X.; Sum, T. C. Tailoring the charge carrier dynamics in ZnO nanowires: the role of surface hole/electron traps. Phys. Chem. Chem. Phys. 2012, 14, 3075-3082. [5] Wilken, S.; Parisi, J.; Borchert, H. Manuscript in preparation.
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