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Graphene Cathode-based ZnO Nanowire Hybrid Solar Cells

• Authors: S. Chang, H. Park, J. Jean, J. J. Cheng, P. T. Araujo, M. Wang, M. G. Bawendi, M. S. Dresselhaus, V. Bulović, J. Kong, S. Gradečak
• Sponsorship: Eni S.p.A.

Semiconducting nanowire-based solar cells have gained interest because of their potential to achieve one-dimensional charge transport pathways and large interfacial areas of well-ordered bulk heterojunction geometry^[1]. Simultaneously, due to the outstanding electrical, mechanical, and optical properties, graphene is being explored as a potential replacement for indium tin oxide (ITO) as a transparent conducting electrode. The growth of semiconducting nanowires on graphene would open up the opportunities to develop flexible solar cell devices, but challenges remain in preserving the structural and electrical properties of graphene during the process. Here, we demonstrate the graphene cathode-based hybrid solar cells composed of ZnO nanowire as an electron transporting layer and two different photoactive materials: conjugated polymers and PbS quantum dots (Figure 1).

The growth of highly uniform, well-aligned ZnO nanowire arrays on graphene is enabled by the interfacial modification with conductive polymer that preserves the structural and electrical properties of grapheme. We developed efficient graphene cathode-based solar cells with the power conversion efficiencies of 4.2% and 0.5% for PbS quantum dot and P3HT polymer structures, respectively, efficiencies that are comparable to the similar ITO-based devices (Figure 2)^[2],^[3]. This behavior can be attributed to the interfacial modification of the graphene electrode that facilitates conformal, smooth wetting of the ZnO seed layer and subsequent ordered growth of the ZnO nanowires. The advances demonstrated by this work suggest that graphene can serve as a viable replacement for ITO in various photovoltaic device configurations.



Figure 1: Hybrid graphene/ZnO nanowire solar cells. (a) Schematic diagram of the graphene cathode hybrid solar cells: graphene deposited on quartz is covered by a polymer (PEDOT:PEG(PC) or RG-1200), followed by the ZnO seed layer and ZnO nanowires. The nanowires are then infiltrated and covered with PbS QDs or P3HT, and finally with MoO3/Au; (b) Flat-band energy level diagram of the solar cells.



Figure 2: (a) J–V characteristics of the graphene-based PbS QD devices with different polymer interlayers, demonstrating performance comparable to that of an ITO reference cell. (b) J–V characteristics of representative graphene-based P3HT devices with different polymer interlayers, compared with an ITO reference device. Insets in (a) and (b) show SEM cross-section images of the complete devices.




1. S. Ren, N. Zhao, S. C. Crawford, M. Tambe, V. Bulovic, and S. Gradecak, “Heterojunction Photovoltaics Using GaAs Nanowires and Conjugated Polymers,” Nano Lett., vol. 11, pp. 408–413, Feb. 2011. [↩]
2. J. Jean, S. Chang, P. R. Brown, J. W. J. Cheng, M. G. Bawendi, S. Gradečak, and V. Bulović, “ZnO Nanowire Arrays for Enhanced Photocurrent in PbS Quantum Dot Solar Cells,” Adv. Mater. Published online, Feb. 2013, DOI: 10.1002/adma.201204192. [↩]
3. H. Park, S. Chang, J. Jean, J. W. J. Cheng, P. T. Araujo, M. Wang, M. G. Bawendi, M. S. Dresselhaus, V. Bulović, J. Kong, and S. Gradečak, “Graphene Cathode-based ZnO Nanowire Hybrid Solar Cells,” Nano Lett., vol. 13, pp. 233–239, Jan. 2013. [↩]