Light Induced Changes in PIN Solar Cells: Beyond the Staebler-Wronski Effect
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Light Induced Changes in PIN Solar Cells: Beyond the Staebler-Wronski Effect Ka-Hyun Kim1,2, Erik V. Johnson2, Samir Kasouit1 and Pere Roca i Cabarrocas2 1
TOTAL S.A., Gas & Power – R&D Division, Courbevoie, France Laboratoire de Physique des Interfaces et des Couches Minces (UMR 7647 CNRS), Ecole Polytechnique, Palaiseau, France 2
ABSTRACT Hydrogenated polymorphous silicon (pm-Si:H) is one of the most promising candidates for a stable top cell material in multi-junction thin film solar cells. Solar cells using pm-Si:H as their absorbing layer show very interesting degradation kinetics when compared to hydrogenated amorphous silicon (a-Si:H), summarized by macroscopic structural changes and irreversible changes in solar cell characteristics, while nevertheless preserving a higher stabilized efficiency. Notably, pm-Si:H solar cells, once degraded, respond to neither annealing nor further lightsoaking. Such results suggest a device degradation mechanism including structural changes, active hydrogen motion, and interface delamination mediated by fast hydrogen diffusion and accumulation at the interface. Interestingly, a similar behavior was reported for a-Si:H solar cells under severe light soaking conditions (at 350 °C or under 50 suns) while pm-Si:H solar cells show such behavior under 1 sun at 40 °C. INTRODUCTION The aim of this study is to understand the "device-versus-material" debate concerning the light-induced degradation of thin film silicon solar cells. For such devices, the stability of intrinsic material rarely translates into the stability of solar cells [1]. We have studied the degradation kinetics of a-Si:H and pm-Si:H solar cells, and we propose a qualitative lightinduced degradation model of PIN solar cells. Despite the fact that pm-Si:H provides excellent stability as a material [2], pm-Si:H PIN solar cells show very interesting degradation kinetics when compared to a-Si:H, summarized by macroscopic changes at the micron scale and irreversible changes in the solar cell characteristics. EXPERIMENT The PIN solar cells were deposited in the ARCAM reactor, a multi-plasma, monochamber RF-PECVD reactor with an excitation frequency of 13.56 MHz [3]. The a-Si:H i-layer was deposited by the dissociation of pure silane at a pressure of 50 mTorr and RF power density of 5 mW/cm2. Intrinsic pm-Si:H films are deposited under carefully controlled plasma conditions using highly hydrogen-diluted silane gas mixture, a total pressure of 2 Torr and an RF power density of 30 mW/cm2. After deposition, the cell areas (0.126 cm2) were defined by evaporation of aluminum back-contacts. Various substrates, such as textured SnO2, flat ZnO, Corning glass and intrinsic float-zone (FZ) c-Si wafer are used. For the observation of lightinduced degradation, sets of current-density-voltage (J(V)) measurements at various stages of light-soaking (LS) were performed. Between each J(V) measurement, the PIN solar cells were light-soaked under open-circuit condition using an Oriel-Apex Xe lamp. The PIN solar cells were both light-soaked an
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