SILICON NANOWIRE HYBRID PHOTOVOLTAICS Erik C. Garnett, Craig Peters, Mark Brongersma, Yi Cui and Mike McGehee Stanford Univeristy, Department of Materials Science, Stanford, CA, USA ABSTRACT Silicon nanowire Schottky junction solar cells have been fabricated using n-type silicon nanowire arrays and a spin-coated conductive polymer (PEDOT). The polymer Schottky junction cells show superior surface passivation and open-circuit voltages compared to standard diffused junction cells with native oxide surfaces. External quantum efficiencies up to 88% were measured for these silicon nanowire/PEDOT solar cells further demonstrating excellent surface passivation. This process avoids high temperature processes which allows for low- cost substrates to be used. INTRODUCTION It is now widely accepted that solar energy is a leading candidate for large-scale renewable power generation. Despite a remarkable increase in production capacity and decrease in cost over the last decade, photovoltaics are still 2-5 times more expensive than traditional power sources. One strategy for further cost reduction is to use a lower quality silicon for the starting wafer, which is a major contributing component to the overall cell cost. This approach does not work well in planar silicon solar cells because the minority carrier diffusion length becomes smaller than the thickness of silicon needed to absorb most of the above-band gap photons. To avoid this problem, there has been significant interest in recent years in using a radial p-n junction which allows for short charge separation lengths even in thick samples [1-3]. A second approach to reduce the cost of silicon solar cells is to use a much thinner high purity silicon wafer, reducing the quantity of silicon needed and thus the cost of the cell. Due to silicon’s poor absorption coefficient in the red and infrared parts of the solar spectrum, planar silicon solar cells that are only a few microns thick are less than half as efficient as thick silicon cells even with traditional light trapping schemes, primarily due to a low photocurrent. However, recent work has demonstrated that periodic silicon nanowire arrays fabricated from sub-10 micron absorbers have extraordinary light trapping capabilities with optical path length enhancement factors up to 73 over the integrated AM1.5 solar spectrum [1]. Unfortunately, the high- temperature diffusion and surface passivation steps involved in standard silicon solar cell fabrication are not compatible with low-cost substrates such as glass, plastic and aluminum foil. To ameliorate this problem we have developed silicon nanowire Schottky junction solar cells that use a transparent conductive polymer (polyethylenedioxythiophene: PEDOT) instead of a metal to form the junction. As demonstrated nearly twenty years ago on planar substrates, conductive polymers with high work functions deposited on n-type silicon show open- circuit voltages (Voc) approximately equal to the theoretical maximum for a given minority carrier diffusion length [4]. This is quite different from metal-semiconductor junction solar cells that almost always show much lower Voc than expected from theoretical calculations due to Fermi level pinning. The lack of Fermi level pinning in the n- Si/conductive polymer system suggests surface dangling bonds are effectively passivated in this device. Since nanowire solar cells have a much larger surface area than planar cells, surface passivation is even more important. RESULTS AND DISCUSSION Since the previous work on planar n- Si/conductive polymer solar cells did not use PEDOT, which is the most common conductive polymer in organic solar cells and light emitting diodes, first we compared a n- Si/PEDOT solar cell to a standard diffused p-n junction cell. We started with a highly doped (~1*10 20 cm -3 ) n-type wafer that has a moderately doped (~1*10 17 ) n-type 978-1-4244-5892-9/10/$26.00 ©2010 IEEE 000934