Results: Group III-V Cell Materials

1. (3) Mechanically Stacked Solar Cells for Concentrator Photovoltaics Ian Mathews1,2, Weiwei Yu1, Declan Gordon2, Donagh O’Mahony1, Nicolas Cordero1, Brian Corbett1, Alan P. Morrison1,2 1Tyndall National Institute UCC, Lee Maltings, Prospect Row, Cork Ireland 2Department of Electrical and Electronic Engineering, University College Cork, Cork, Ireland Simulation Method Model based on the one-dimensional semiconductor transport properties and absorption profiles of GaAs, Si, Ge, GaSb and GaInAs. Introduction Monolithically-integrated triple-junction solar cells are currently the state of the art in photovoltaic devices with light-to-electricity conversion efficiencies of circa 40%. The advancement of monolithic solar cells beyond 40% efficient devices is proving very difficult due to lattice and current matching constraints. Mechanically stacked solar cells (MSSC) are a multi-junction alternative which potentially offer a route to higher efficiencies due to the relaxation of these constraints. The main challenges of an MSSC will be to integrate the various sub-cells into a reliable stacked configuration with minimal optical loss and cost. Here, we investigate the power output of promising MSSC material combinations incorporating GaAs single junction top-cells and Si, Ge, GaSb or Ga0.47In0.53As (GaInAs) bottom cells. The effect of material thickness on power output has been considered. Model conditions Spectrum: AM1.5 Direct Incident Power: 1000 W/m2 Cell Temperature: 300 K Ideal Diode Equation: J(V) = Jo(eqV/kT– 1) – JL where: JL = ∫qPF(λ)(1 – e-α(λ)x)Δλ Jo = qni2(De/NALe + Dh/NDLh) Challenges Monolithic multi-junction devices • Crystal lattice mismatch (between optimum cell materials) • Current matching (current reduction from bottom junction) • Reliability (Tunnel Junctions) • Mechanically stacked solar cells • Optical losses (high reflection losses at interfaces) • Cost (Substrate and processing/assembly costs) • Can MSSC cost be reduced by using thin cells without significantly reducing output power? Results: Group IV Cell Materials Optimum Bandgap Combination Results: Group III-V Cell Materials Photovoltaic conversion efficiency (%) as a function of bandgap for an ideal dual-junction mechanical stack calculated using. EtaOpt [1]. Model conditions: AM1.5d, x500 suns, T = 300 K Acknowledgements This work has been supported by Enterprise Ireland and the European Regional Development Fund. References [1] G. Letay, A. Bett, EtaOpt – a program for calculating limiting efficiency and optimum bandgap structure for multi-bandgap solar cells and TPV cell, Proc. Of 17th EU-PVSEC Munich (2001) Conclusion Ge and GaInAs bottom cells provide an additional photovoltaic conversion efficiency of 6% when mechanically stacked under single-junction GaAs top cells. A Si bottom cell gives an additional conversion efficiency of 3-4%. The greatest boost to device efficiency is given by GaSb which provides an additional conversion efficiency of 7%. The cost of the bottom cell must also be considered and in this regard, Si bottom cells would be preferable given their wide availability, maturity and low cost compared with Ge, GaInAs and GaSb.