Thin Film a-Si/poly-Si Multibandgap Tandem Solar Cells With Both Absorber Layers Deposited by Hot Wire Cvd
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3-04-01
THIN FILM a-Si/poly-Si MULTIBANDGAP TANDEM SOLAR CELLS WITH BOTH ABSORBER LAYERS DEPOSITED BY HOT WIRE CVD
R.E.I. SCHROPP, C.H.M. VAN DER WERF, M.K. VAN VEEN, P.A.T.T. VAN VEENENDAAL, R. JIMENEZ ZAMBRANO, Z. HARTMAN, J. LÖFFLER, and J.K. RATH Utrecht University, Debye Institute, Physics of Devices, P.O.Box 80000, 3508 TA Utrecht, The Netherlands
ABSTRACT The first competitive a-Si/poly-Si multibandgap tandem cells have been made in which the two intrinsic absorber layers are deposited by Hot Wire Chemical Vapor Deposition (HWCVD). These cells consist of two stacked n-i-p type solar cells on a plain stainless steel substrate using plasma deposited n- and p-type doped layers and Hot-Wire deposited intrinsic (i) layers, where the i-layer is either amorphous (band gap 1.8 eV) or polycrystalline (band gap 1.1 eV). In this tandem configuration, all doped layers are microcrystalline and the two intrinsic layers are made by decomposing mixtures of silane and hydrogen at hot filaments in the vicinity of the substrate. For the two layers we used individually optimized parameters, such as gas pressure, hydrogen dilution ratio, substrate temperature, filament temperature, and filament material. The solar cells do not comprise an enhanced back reflector, but feature a natural mechanism for light trapping, due to the texture of the (220) oriented poly-Si absorber layer and the fact that all subsequent layers are deposited conformally. The deposition rate for the throughput limiting step, the poly-Si i-layer, is ≈ 5-6 Å/s. This layer also determines the highest substrate temperature required during the preparation of these tandem cells (500 °C). The initial efficiency obtained for these tandem cells is 8.1 %. The total thickness of the silicon nip/nip structure is only 1.1 µm. INTRODUCTION Microcrystalline silicon (µc-Si:H) is an attractive alternative for a-SiGe:H as the lowbandgap (LBG) absorbing component in tandem cells or triple cells, because (i) a band gap lower than 1.3 – 1.4 eV can easily be obtained without sacrificing electronic transport properties, (ii) the use of the quite expensive and highly toxic GeH4-containing gases is avoided, and (iii) cells incorporating µc-Si:H are not susceptible to light-induced degradation as a result of the StaeblerWronski effect. The concept of combining µc-Si:H and a-Si:H in a thin film tandem solar cell, taking advantage of the optimum bandgap combination for a dual junction cell with respect to the terrestrial solar spectrum (AM1.5), was first introduced by the group at the University of Neuchâtel and was referred to as the micromorph solar cell concept. Good results have been obtained in the p-i-n/p-i-n configuration, that is, in superstrate cells constructed on TCO-coated glass substrates [1,2]. The essential modification of the Plasma Enhanced CVD method, necessary for µc-Si:H deposition, is either the elevation of the discharge frequency to the Very High Frequency range (VHFCVD) [3] or the use of high processing pressures at the commonly used frequency of 13.56 MHz. Usi
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