Orthogonal Thin Film Photovoltaics on Vertical Nanostructures

Ahnood et al. Nanoscale Research Letters (2015) 10:486 DOI 10.1186/s11671-015-1187-6 NANO EXPRESS Open Access Orthogonal Thin Film Photovoltaics on Vertical Nanostructures Arman Ahnood1*, H. Zhou2, Y. Suzuki3, R. Sliz4, T. Fabritius4, Arokia Nathan5 and G. A. J. Amaratunga5 Abstract Decoupling paths of carrier collection and illumination within photovoltaic devices is one promising approach for improving their efficiency by simultaneously increasing light absorption and carrier collection efficiency. Orthogonal photovoltaic devices are core-shell type structures consisting of thin film photovoltaic stack on vertical nanopillar scaffolds. These types of devices allow charge collection to take place in the radial direction, perpendicular to the path of light in the vertical direction. This approach addresses the inherently high recombination rate of disordered thin films, by allowing semiconductor films with minimal thicknesses to be used in photovoltaic devices, without performance degradation associated with incomplete light absorption. This work considers effects which influence the performance of orthogonal photovoltaic devices. Illumination non-uniformity as light travels across the depth of the pillars, electric field enhancement due to the nanoscale size and shape of the pillars, and series resistance due to the additional surface structure created through the use of pillars are considered. All of these effects influence the operation of orthogonal solar cells and should be considered in the design of vertically nanostructured orthogonal photovoltaics. Keywords: Thin film solar cells, Orthogonal solar cells, Illumination uniformity, Series resistance, Electric field confinement Background Thin film photovoltaic devices, also known as the second generation solar cells, have provided a complimentary platform to the first generation solar cells based on bulk materials, by catering for the low cost, and low efficiency applications [1]. Orthogonal solar cells, a subgroup of the third generation solar cells, are an extension of the thin film solar cells and operate based on the principle of perpendicular path of illumination with respect to photocarrier collection path [2, 3]. In conventional thin film photovoltaic devices, light travels in the same direction as the photogenerated carriers within the absorber layer as illustrated in Fig. 1a. Here, photogenerated carrier lifetime imposes a design limit on the upper value of the absorber layer thickness. This typically leads to incomplete light absorption, as maximizing the light absorption requires increasing the thickness of the absorber layer. Conventional thin film solar cells’ photoabosorber layer thickness is optimized to minimize the recombination losses while * Correspondence: 1 School of Physics, University of Melbourne, Melbourne, Australia Full list of author information is available at the end of the article maximizing the light absorption [4]. In addition to this, optical enhancements such as textured electrodes, back reflectors, and anti-reflective coatings serve to further improve the light absorption without increasing absorber layer thickness and subsequently prevent the increase in recombination of the photogenerated carrier [5]. Solar cells with an orthogonal structure offer an alternative solution to address this challenge. The structure of such device is shown in the Fig. 1b. It consists of thin film photovoltaic devices grown on an array of vertically aligned nanopillars [6, 7] and other vertical nanostructures such as spikes [8, 9]. Here, decoupling of photogenerated carriers and optical light pathways allows the use of a thin absorber layer to maximize the collection of the photogenerated carriers, while providing sufficient depth for complete light absorption [10, 11]. Despite the simplicity of the concept of orthogonal solar cells, there are a number of underlying physical mechanisms which need to be accounted when considering the form factor of orthogonal solar cells. These require development of a design framework which is tailored for the orthogonal devices based on the physical © 2015 Ahnood et al. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Ahnood et al. Nanoscale Research Letters (2015) 10:486 Page 2 of 10 Results and Discussions Illumination Uniformity Fig. 1 Cross-sectional diagram of a planar solar cell and b orthogonal solar cell. As illustrated, in the case of planar solar cells, photocarrier collection path is in the same direction as the light path. However, orthogonal solar cells allow the photocarrier’s collection path to be decoupled, in this case perpendicularly, from the optical path. This makes it possible to use a thin photoabsorber layer, for enhance the collection efficiency, while maintaining the necessary length of optical path to prevent losses associated with incomplete light absorption effects uniquely present in this class of devices. Earlier works have demonstrated the clear influence of pillar height and diameter on the efficiency of thin film orthogonal solar cells [3, 12]. This paper builds on the earlier works by considering (i) non-uniformity of the illumination across the depth of the device, (ii) electric field enhancement effects at the nanoscales, and (iii) increased series resistance due to the higher device surface area. Methods The test structures were fabricated in this study consisted of silicon thin film PV cells deposited on vertical nanostructures and on a flat ITO-coated glass substrate as reference samples. Where vertical nanostructures were used, they consisted of either an array of MWCNTs or ZnO nanowires with their growth deposition methods reported in earlier works [6, 13]. The PV cells consisted of p-i-n type structure deposited using plasma-enhanced chemical vapor deposition with their deposition methods reported in earlier works [14]. The thicknesses of active layers used here are p-type amorphous silicon carbide (20 nm), intrinsic a-Si:H (300 nm), n-type nanocrystalline silicon (40 nm). PV cell measurements were performed using Keithley source meter 2400, under dark and various illuminated conditions. Simulations were performed using SPICE based module on a double diode circuit module with series and parallel parasitic resistances (AimSpice software). Conventional planar solar cells are two terminal electrical devices which can be considered as an array of parallel-connected smaller planar segments, as shown in Fig. 2a. In conventional solar cells, the segments are illuminated uniformity across the planar device, leading to uniform electrical characteristics for all segm (...truncated)