Surface photovoltage investigation of recombination at the a-Si/c-Si heterojunction L. Korte a, ⁎, A. Laades a,b , K. Lauer b,c , R. Stangl a , D. Schaffarzik a , M. Schmidt a a Helmholtz-Zentrum Berlin für Materialien und Energie, Abteilung Silizium-Photovoltaik, Kekuléstr.5, D-12489 Berlin, Germany b CiS Institut für Mikrosensorik GmbH, SolarZentrum Erfurt, Konrad-Zuse-Str. 14, D-99099 Erfurt, Germany c TU Ilmenau, Institut für Physik, Weimarer Str. 32, 98693 Ilmenau, Germany abstract article info Available online 21 February 2009 Keywords: Surface photovoltage Band bending Interface defects Amorphous/crystalline heterojunction We investigate the use of time-resolved surface photovoltage (SPV) transients as a means to determine band bending and recombination properties at amorphous/crystalline silicon (a-Si:H/c-Si) heterojunctions. Experimentally, it is shown that for a-Si:H film thicknesses above ~6 nm, SPV transients do not depend on the film thickness anymore. On this basis, a simple numerical model is proposed that consists of a single rechargeable gap state on the c-Si wafer surface, into which the properties of the a-Si:H/c-Si interface and the adjacent a-Si:H are lumped. It is shown that this model can reproduce all principal features of high excitation SPV transients, i.e. an initial fast decay shown to be due to Auger recombination, a plateau region for high injection conditions and a fast decay when the sample returns into low injection and the defect states are recharged. Under sufficiently high excitation, the SPV saturates at a value that is determined by the a-Si:H/c-Si interface band bending in the dark. From the slope of the transient decay, defect parameters (density, energetic position) can be extracted. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Amorphous/crystalline silicon heterojunctions produced by plasma enhanced chemical vapour deposition (PECVD) of ultrathin (5–10 nm) hydrogenated amorphous silicon layers, a-Si:H, on crys- talline silicon substrates, c-Si, are an interesting concept for high- efficiency solar cells on monocrystalline wafers, and to realize large area heterojunction cell concepts on for low-cost thin-film silicon. For those cell types, in order to achieve high efficiencies, it is necessary to suppress interface recombination at the a-Si:H/c-Si interface. We have shown previously [1], that this can be achieved by decreasing the density of interface defect states D it (E) of the a-Si:H/c-Si inter- face and/or by increasing the c-Si band bending qφ s0 . Low D it can be obtained by reducing the c-Si surface defect density prior to a-Si:H deposition, and by choosing optimized a-Si:H deposition conditions [2], while the band bending is adjusted by the a-Si:H doping. The a-Si:H doping level has to be chosen as a trade-off between high band bending and high density of a-Si:H gap states at increased doping levels, which lead to enhanced recombination in the a-Si:H [3]. All those optimizations profit greatly from a tool to characterize the a-Si:H/c-Si interface recombination. To this end, we have been using surface photovoltage measurements (SPV), especially in the time- dependent and intensity-dependent modes (TD-SPV/ID-SPV) [4,5] and references therein], as a “fingerprinting” method to monitor the electronic properties of the a-Si:H/c-Si effective interface. With the present paper, using numerical simulations, we aim at a refined understanding of the mechanisms that determine the transient decrease of photovoltage over time after the short illumination light pulse. This will allow to extract a-Si:H/c-Si interface recombination properties that can be compared, for adequate sample structures, to those obtained by other means such as quasi-steady state and microwave photoconductance decay (QSSPC and μPCD). 2. Sample preparation Phosphorous-doped n-type amorphous silicon (a-Si:H) layers were deposited by PECVD (base pressure 10 −7 mbar) on high quality p-doped (resistivity 1–3 Ω cm) monocrystalline Si (c-Si) wafers with b111N surface orientation and bulk diffusion lengths of several 100 μm. For experimental details, see e.g. [3,4]. a-Si:H/c-Si solar cells produced using the same PECVD system have yielded maximum efficiencies of 19.8% [6]. The investigated set of samples consists of (n + )a-Si:H/(p)c-Si (a-Si:H doping 10 3 ppm PH 3 in the gas phase) with increasing a-Si:H layer thickness (0.4 – 120 nm). 3. Experimental methods 3.1. Surface photovoltage Surface photovoltage (SPV) is an electrical characterization technique that is well-established for the determination of the density of surface states especially at the silicon/silicon oxide interface [7]. The principle is described elsewhere [4,8] and will be briefly outlined here: The sample under test is sandwiched in a structure consisting of a transparent conductive front contact (TCO — typically zinc oxide), an Thin Solid Films 517 (2009) 6396–6400 ⁎ Corresponding author. Tel.: +49 30 80621351. E-mail address: korte@helmholtz-berlin.de (L. Korte). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.02.090 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf