EXPERIMENTAL MEASUREMENT OF RESTRICTED RADIATIVE EMISSION IN QUANTUM WELL SOLAR CELLS J.G.J. Adams 1 , W. Elder 1 , P.N. Stavrinou 1 , J.S. Roberts 2 , M. Gonzalez 3 , J.G. Tischler 4 , R.J. Walters 4 , J. Abell 4 , I. Vurgaftman 4 , J.R. Meyer 4 , P. Jenkins 4 , K.W.J. Barnham 1 and N.J. Ekins-Daukes 1 1 Department of Physics, Imperial College London, London SW7 2AZ, UK. 2 EPSRC National Centre for III-V Technologies, University of Sheffield, Sheffield S1 3JD, UK. 3 Global Strategies Group, 2200 Defense Highway, Suite 405, Crofton MD, 21114, USA. 4 US Naval Research Laboratory, 4555 Overlook Ave., SW, Washington, DC 20375 USA. ABSTRACT We present the first experimental data showing a restriction of the radiative recombination in strain- balanced quantum well solar cells. This arises due to the combined effects of quantum confinement and strain splitting the valence band in the quantum wells into heavy and light hole sub-bands. Under increasing compressive strain, the light hole sub-band moves further in energy from the conduction band, suppressing the conduction-band-to-light-hole recombination which couples primarily to photons emitted in the plane of the quantum wells. As a solar cell fundamentally need only emit in the angular range of absorption, the strain- balanced quantum well solar cell operates more closely to this optimal regime than a bulk solar cell. INTRODUCTION InGaP/GaAs-based III-V multi-junction solar cells are the dominant technology for space satellite power, and are increasingly finding application in terrestrial concentrator systems [1]. In order to match the cell for optimal conversion of the incident spectrum it is necessary to be able to tune the band gap, or band gaps of the subcells in the case of the multi-junction solar cell. One technique for doing this without introducing dislocations is to grow strained quantum well (QW) layers in the bulk material. Figure 1. Band structure of the strain-balanced quantum well solar cell. InxGa1-xAs QW layers in compressive strain are alternated with GaAs1-yPy barrier layers in tensile strain in the i-region of a p-i- n device. The strain-balanced quantum well solar cell (SB-QWSC) is a GaAs p-i-n structure with lower band gap InxGa1-xAs QW layers incorporated into the field-bearing intrinsic region as shown in Figure 1. Increasing the indium composition increases the QW depth, pushing the band gap further into the infrared and thus closer towards the optimal 1.1 eV value for single-junction solar cells under the AM1.5d spectrum for both low and high concentrations [2]. The drop in open-circuit voltage due to the inclusion of lower band-gap material can be more than compensated by the increased photocurrent from the QWs under concentration, leading to a potential efficiency enhancement over bulk GaAs cells [3]. The carriers escape from the QWs with near unity efficiency at room temperature via thermally-assisted tunneling [4, 5]. The best power conversion efficiency for a SB- QWSC to date is 28.3 % under AM1.5d 500X [6]. As the InxGa1-xAs QW layers have a larger bulk lattice constant than the GaAs regions, they are compressively strained while the alternating GaAs1-yPy barriers are under tensile strain. Using this strain-balancing technique, more than 65 QW layers have been grown without the dislocations [7, 8] that would otherwise lead to a catastrophic drop in voltage [9]. At the high concentrator cell operating bias, the dominant loss mechanism in the SB-QWSC is radiative recombination from the QWs [10]. Further improvements to the single-junction cell efficiency can be achieved via optical approaches. For example, the inclusion of a distributed Bragg reflector can increase the short-circuit current by reflecting incident photons not absorbed on the first pass through the QWs, as well as increasing the open-circuit voltage by recycling photons lost to radiative recombination in the QWs. This has been found to increase the efficiency on the order of ~1 % absolute [11, 12]. Whilst prior research efforts to reduce the dominant radiative recombination loss in SB-QWSCs have focused on the use of back-reflectors and light trapping schemes to harness the photon recycling effect, in this paper we present experimental evidence that strain in the QWs can be used to prevent one mode of radiative recombination from even occurring. This leads to a fundamentally higher detailed balance limiting efficiency for the SB-QWSC than for equivalent bulk cells [13, 14]. MODELING THE STRAIN-BALANCED QUANTUM WELL SOLAR CELL The quantum efficiency (QE) and dark current of a SB- QWSC can be modeled by calculating the QW absorption coefficient [15] and solving one-dimensional drift-diffusion equations with bulk materials parameters from the literature [15-17]. Establishing the carrier