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Some GaAs solar cells degrade when exposed to high reverse currents. The degradation is significantly reduced when G e substrates are used instead of GaAs substrates for MOCVD growth of the GaAs cell layers. We compare the performance of GaAs/GaAs and GaAs/Ge cells before and after reverse current stress testing. Diode measurements (dark and illuminated), infrared thermography and crystal defect delineation were used to analyze the cell performance. A three-dimensional computer model for temperature distribution with the cells during testing added insight. The possible effect of the conclusions on advanced GaAs cell design is discussed. BACKGROUND

Recent advances in PV cell technology and in thermal emitters has led t o renewed interest in TPV electric power generation. We review thermal sources, theoretical analysis t o select materials with optimum bandgap, and fabrication of efficient TPV cells. Some experimental data are included. BACKGROUND

The paper descn-bes the factors which control ck surface reflectance (BSR). High BSR has been used to reduce operating temperatures for space cells and cells operated at high current, under concentrators r in TPV applimtioins. High BSR also increases current i high efficiency Si cells.i IINTRODUCTION There are several PV cell applications in which wavelengths in the incident spectrum which are not sorbed, (usually wavelengths longer than the intrinsic ndedge wavelength) can be redirected to improve cell rfomnce. The method used most is to reflect the n-absorbed wavelengths from the back surface of the 11, and we will discuss the advantages of high back rface reflectance (BSR).In the following sections we will analyze the cell ectance for various waveiengths, and will discuss the I properties which control the BSR. We will discribe use of high BSR to reduce the operating perature for cells used in space, to enhance the llected current in high eficiency Si cells, or in some V cells, and to reduce the heat generated in cells rating at high1 current densities, either under ncentrated light, or with themophotovottaic VPV)For each of these applications we will discuss adjustments required to combine high BSR with er necessary features to ensure high cell efficiency. dell Reflectance figure 1 shows the reflectance features of a cell. Part of the incident radiation is reflected at the tance r). Radiation is absorbed up to gth for the particular semiconductor, elengths are transmitted to the back ey are reflected. The value of rb 8 on the back surface finish of the r on the smoothness of the back contact rface, and on the reflectance of the with the semiconductor. The metals reflectance are Au, Ag, Cu and AI, and approaching 100% can be obtained out to velengths beyond 10 gm. If the PV cell is grown on a strate. there may also be a small reflection at the dace between the cell and the substrate.F B I 1 I ----I -x --Bulk and/or SubstrateFigure 1: Optical Rentctions in Pv CellThe properties of the substrate dictate the beyond-bandgap transmission and the back surface reflection.If an AR coating is used on the front surface, r depends on the wavelength, and the r-values at different wavelengths are incorporated in the measured values of reflectance.The radiation which is not absorbed is multiply reflected and the total reflectance at each wavelength is given by RT = r + (Cr)*rb (1) 1-r.rb RT is the value measured at each wavelength.Knowing the values of r at a given wavelength, rb can be calculated.If the cell and/or the substrate introduce beyond bandgap absorption (usually from free carrier absorption), RT is given by equation (2). RT = r + (I-r)2b exp (-2q.x) (2) l-r.rb exp (-2af.x) In (2) x is free carrier absorption length, usually the thickness of the cell or substrate. or in some mses the thickness of a highly doped back surface field layer. q is the free carrier absorption coefficient. I 109 0-7$03-3166-4/96/$5.100 0 1996 IEEE 25th PVSC;