Inorganic/organic hybrid solar cells: optimal carrier transport in vertically aligned silicon nanowire arrays† Keisuke Sato, * ab Mrinal Dutta a and Naoki Fukata * a Inorganic/organic hybrid radial heterojunction solar cells that combine vertically-aligned n-type silicon nanowires (SiNWs) with poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) have great potential for replacing commercial Si solar cells. The chief advantage of such solar cells is that they exhibit higher absorbance for a given thickness than commercial Si solar cells, due to incident light- trapping within the NW arrays, thus enabling lower-cost solar cell production. We report herein on the effects of NW length, annealing and surface electrode on the device performance of SiNW/PEDOT:PSS hybrid radial heterojunction solar cells. The power conversion efficiency (PCE) of the obtained SiNW/ PEDOT:PSS hybrid solar cells can be optimized by tuning the thickness of the surface electrode, and the etching conditions during NW formation and post-annealing. The PCE of 9.3% is obtained by forming efficient transport pathways for photogenerated charge carriers to electrodes. Our approach is a significant contribution to design of high-performance and low-cost inorganic/organic hybrid heterojunction solar cells. Introduction Most recently, solar cells using one-dimensional architecture, such as silicon (Si) wires 1,2 and Si nanowires (SiNWs), 3–7 have raised hope for the realization of highly efficient solar cell modules. The main advantage of SiNWs is that they show greater light absorption (minimal reectivity) due to incident light-trapping within the NW arrays. In short, nanostructured Si solar cells lead to higher absorbance per unit thickness than achieved by commercial Si solar cells, 8–10 signicantly reducing the quantity of Si materials needed for cell fabrication. 11,12 Other important advantages are that they permit the enlargement of radial p–n junction areas to improve carrier separation 12 and to provide efficient transport pathways for photogenerated charge carriers to electrodes. 13,14 In the design of such SiNW solar cells, the formation of vertically aligned SiNWs is of crucial impor- tance. The vertically aligned SiNWs can assist with light-trap- ping and suppress photon reections on the surface, thus simultaneously improving both light absorption and carrier generation. 15,16 The formation of metal-free SiNWs is also necessary for the production of highly efficient solar cells. We fabricated SiNWs by catalytic chemical vapour deposition (CVD) with vapour–liquid–solid (VLS) growth. 17 Using this particular growth technique, the electrically active level derived from a metal (e.g., gold) used as catalyst has been found to be trans- ferred onto the NW sidewall surfaces and into the NW volume due to metal diffusion from the NW tip during the growth process. 18 When metal-contaminated SiNWs were used as the active layer in solar cells, this metal contamination proved detrimental to cell performance, causing a dramatic drop in power conversion efficiency (PCE). This is due to the trapping of most of the photogenerated charge carriers in the metal-related electronic levels, leading to carrier transport losses. This problem can be solved by employing a top-down approach, such as a solution-based etching process. 19–21 Metal-assisted chem- ical etching, 19,21 which is a simple and low-cost technique, has several advantages. For instance, it can enable not only easy and complete removal of the metal by post-treatment immersion in a solution, but also control of the morphology, including the diameter, length, and orientation of SiNWs relative to the substrate, allowing the fabrication of metal-free and vertically aligned SiNWs. A matter of key importance in the development of solar cells is how the p–n junction is formed. In SiNW solar cells, the p–n junction is normally formed by a thermal diffu- sion process using an n-type dopant, employing a furnace at 845–850  C for p-type NWs 22,23 or by deposition of a p-type polycrystalline Si sheath using low-pressure CVD at 450  C and subsequent rapid thermal annealing at 1000  C for n-type NWs. 4 As a step toward the practical use of SiNW solar cells, there has been signicant interest in forming p–n junctions using low- a World Premier International Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. E-mail: satok@mail.dendai.ac.jp; FUKATA.Naoki@nims.go.jp b Department of Electrical and Electronic Engineering, Tokyo Denki University, 5 Senju- Asahi-cho, Adachi-ku, Tokyo 120-8551, Japan † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4nr00733f Cite this: Nanoscale, 2014, 6, 6092 Received 10th February 2014 Accepted 25th March 2014 DOI: 10.1039/c4nr00733f www.rsc.org/nanoscale 6092 | Nanoscale, 2014, 6, 6092–6101 This journal is © The Royal Society of Chemistry 2014 Nanoscale PAPER Published on 01 April 2014. Downloaded by NIMS Namiki Library on 22/05/2014 13:15:49. View Article Online View Journal | View Issue