Bulk Heterojunction Photovoltaics Utilizing Semiconducting Nanowires and Conjugated Polymers

Organic photovoltaics represent an attractive approach for the production of cost competitive solar cells, as they incorporate solution-based processes that are compatible with large-area, high-throughput manufacturing infrastructure. Enhanced device performance can be achieved incorporating organic/inorganic blends to form hybrid BHJ solar cells with improved electron mobility, dielectric constant, photon absorption, as well as enhanced physical and chemical stability. In this study, a solution-based method has been investigated to fabricate a new type of hybrid BHJ solar cell. By achieving self-alignment of semiconducting NW structures within the active layer, performance improvements were realized through enhanced carrier extraction pathways.

Reviewed by Jeff Morse, PhD., National Nanomanufacturing Network

• Ren S, Zhao N, Crawford SC, Tambe M, Bulovic V, Gradecak S. 2010. Heterojunction Photovoltaics Using GaAs Nanowires and Conjugated Polymers. Nano Letters Article ASAP 20 December 2010. doi:10.1021/nl1030166.

Organic photovoltaics represent an attractive approach for the production of cost competitive solar cells, as they incorporate solution-based processes that are compatible with large-area, high-throughput manufacturing infrastructure. One of the drawbacks associated with these photovoltaics, which are also referred to as bulk heterojunction (BHJ) solar cells, is that the polymer blends—mixtures of conjugated polymers and C60 derivatives— form random networks of electron acceptor/donor regions within the active area of the cell, resulting in somewhat inefficient exciton dissociation and charge collection pathways. In order to improve the efficiencies of these processes, an ordered, interpenetrating network of electron acceptor and donor regions having domain sizes on the order of 10-30 nm matching the exciton diffusion length are ideal. Further enhancements in device performance can be achieved incorporating organic/inorganic blends to form hybrid BHJ solar cells with improved electron mobility, dielectric constant, photon absorption, as well as enhanced physical and chemical stability. The inclusion of inorganic nanoparticles such as TiO2 or ZnO in a hybrid BHJ cell is still limited in performance by the charge hopping transport through the discontinuous percolation pathways.

An alternate approach to realize hybrid BJH cells would utilize semiconductor nanowires (NW) that provide a continuous conduction pathway for electron transport, as well as exploit the high electron mobility of single crystal, one-dimensional NW structures. This can be done by infiltrating conjugated polymers between arrays of as-grown NWs, however, the process remains limited by the NW packing density and the challenge of effectively infiltrating the polymer between the high aspect ratio NWs, as well as the poor crystalline quality of the polymer. Recently, Ren et. al. have investigated organic/inorganic BHJ solar cell devices blending GaAs NWs with poly(3-hexylthiophene) (P3HT) in a single solvent to form a NW dispersion in a polymer matrix through solution processing. By controlling the NW concentration, P3HT ordering, and NW alignment, the authors presented a pathway to improving the performance of the hybrid BHJ cell.

In their approach, the authors prepared a NW powder from as-grown GaAs NWs having uniform size and shape. The NW powder was then mixed with ethanol/1,2-dichloro-benzene (1,2-DCB), then blended with P3HT in 1,2-DCB solution at different weight ratios of GaAs NWs ranging up to 50% GaAs wt%. Cell fabrication consisted of first spin-casting a film of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) onto prepatterned indium-tin-oxide (ITO) electrodes, followed by casting a 160 nm active layer of the P3HT/GaAs NW blend which was annealed overnight in 1,2-DCB  solvent. After a 175°C heat treatment for 10 minutes, a 10 nm thick bathocuproine (BCP) hole blocking layer and top Mg/Ag layers were evaporated.

Devices having GaAs NW weight percentages of 0, 20, 30, 40, and 50% in the active layer blend were fabricated and characterized. Under white light exposure, the authors observed a threshold limit of 30 wt% GaAs NW loading where the short circuit current begins to increase. This loading correlates with the formation of GaAs NW percolation networks, thereby enhancing the extraction of charge from the device. A similar trend was observed for cell open circuit voltage and fill factor. Additionally, the dark leakage current decreases with NW loading. The authors attribute these performance enhancements to an increase in the molecular ordering in the P3HT phase with increased NW loading. Characterization of the active layer film by techniques including TEM, AFM, and grazing incidence X-ray spectroscopy indicated some level of vertical alignment of the NWs, with average spacing of ~35 nm for the 50 wt% NW loaded films. To further enhance cell performance, the authors engineered the interface between the P3HT and NW to better facilitate charge separation by preparing GaAs-TiOx core shell NWs by coating the GaAs NWs using a solgel chemistry. The power conversion efficiency for cells incorporating the core-shell NWs was measured to be 2.36% at 50 wt% NW loading, an increase of 20% over the bare GaAs NWs at similar loadings.

In summary, a solution-based method has been investigated to fabricate a new type of hybrid BHJ solar cell. By achieving self-alignment of semiconducting NW structures within the active layer, performance improvements were realized through enhanced carrier extraction pathways. The molecular ordering of the conjugated polymer facilitated by the one-dimensional NWs at loadings exceeding the percolation threshold can potentially provide new approaches that can be extended to other systems having one-dimensional elements. Additional investigations of the hybrid BHJ solar cell devices might include better matching of the semiconducting NW bandgap with that of the conjugated polymers.

Images reproduced with permission from  Ren S, et al. Nano Letters Article ASAP 20 December 2010. doi:10.1021/nl1030166.Copyright2010 American Chemical Society.