RESEARCH ARTICLE Design, fabrication, and analysis of transparent silicon solar cells for multi-junction assemblies Christopher Kerestes * , Yi Wang, Kevin Shreve, James Mutitu, Tim Creazzo, Paola Murcia and Allen Barnett University of Delaware, Electrical and Computer Engineering, Newark, DE USA ABSTRACT Transparent silicon solar cells can lead to an increased efficiency of silicon-based multi-junction assemblies by transmitting near and below band gap energy light for conversion in a low band gap solar cell. This analysis shows that the maximum efficiency gain for a low band gap solar cell beneath silicon at a concentration of 50 suns is 5.8%, based on ideal absorption and conversion of the photons. This work analyzes the trade-offs between increased near band edge absorption in the silicon and silicon solar cell transparency. Application of these results to real cases including a germanium bottom solar cell is analyzed, leading to a range of cases with increased system efficiency. Non-ideal surfaces and real silicon and germanium solar cell device performance are presented. The range of practical system gains may be as low as 2.2 – 1% absolute when compared with the efficiency of a light-trapped silicon solar cell for 1-sun operation, based on this work. Copyright © 2012 John Wiley & Sons, Ltd. KEYWORDS silicon solar cell; multi-junction; high efficiency; concentrator cells *Correspondence Christopher Kerestes, University of Delaware, Electrical and Computer Engineering, Newark, DE, USA. E-mail: kerestec@udel.edu Received 17 July 2011; Revised 27 September 2011; Accepted 4 October 2011 1. INTRODUCTION Transparent silicon solar cells can be integrated into multi- junction solar cells for high efficiency conversion of sun- light to electrical power. Collection of photons and the conversion to electrons within a material and the voltage at which the cell operates dictate the power conversion effi- ciency. Allowing light that is not absorbed in one material to transmit to a material of a lesser band gap will result in an increased range of the absorbed solar spectrum and increased conversion albeit at a lower voltage. By transmit- ting photons not absorbed in 300 mm of silicon to a low band gap (0.73 eV) solar cell can add 2.9% absolute effi- ciency. Utilizing the power of concentration, the perfor- mance of the low band gap cell doubles to 5.8% under a concentration factor of 50 suns. The work presented herein analyzes the optical properties of silicon to determine the optimal structure of transparent silicon in a multi-junction assembly. The effects of bulk absorption and surface textur- ing are the focus of examination to determine the optimum management of photons between silicon and a low band gap material. 2. REVIEW OF TRANSPARENT SOLAR CELLS A transparent solar cell refers to a device that transmits a spe- cific part of the solar spectrum for conversion by another ma- terial while it too converts sunlight to electrical power. The idea of utilizing the transparency of photovoltaic materials to more efficiently convert a broader range of the solar spectrum was proposed in 1955 [1]. Current records of the highest independently verified solar cell efficiencies comply with the fact that multi-junction solar cells are more efficient than single junction. A three-junction solar cell achieves 42.3% at 406suns [2] and for single junction 29.1% at 117 suns [3]. The three-junction solar cell has a monolithi- cally stacked structure that connects all the junctions in elec- trical and optical series. Electrical series connection of the junctions limits the current of the entire structure to that of the lowest of the individual junctions. One method to circum- vent the need to current match is independent contacting, which allows each junction to independently operate at max- imum power. Such a structure referred to as mechanical- stacked solar cells have been used to integrate materials that PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS Prog. Photovolt: Res. Appl. (2012) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/pip.1232 Copyright © 2012 John Wiley & Sons, Ltd.