Effect of Light Soaking on the Spectrally and Spatially Resolved Collection Efficiency of a-Si:H Solar Cells
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EFFECT OF LIGHT SOAKING ON THE SPECTRALLY AND SPATIALLY RESOLVED COLLECTION EFFICIENCY OF a-Si:H SOLAR CELLS M. B. VON DER LINDEN, R. E. I. SCHROPP, 0. P. LEKKERKERKER, J. DAEY OUWENS AND W. F. VAN DER WEG Debye Institute, Dept. of Atomic & Interface Physics, Utrecht University, P.O. Box 80.000, NL - 3508 TA Utrecht, The Netherlands. ABSTRACT Light soaking of a-Si:H solar cells gives rise to a decrease in the spatially resolved collection efficiency. In the internal collection efficiency distributions measured under bias light conditions two crossing points appear during light soaking. The existence and the shift of these points could be explained by a combined bulk i-layer light soaking effect and a more pronounced collection loss at the p+/i-interface. This spatially non-uniform degradation was confirmed by the Dynamic Inner Collection (DICE) method. INTRODUCTION Knowledge of the spatial distribution of defect creation due to light soaking is important to understand the degradation mechanics of a-Si:H solar cells. From simulations of the spectral response, measured without bias light, it was concluded that a defect rich interface layer (5nm thick) exists between the buffer and i-layer. Light soaking of the devices results in an increase of the defect density in this buffer/interface layer and a less significant enhancement of bulk i-layer defects [1, 2]. To study the light soaking behavior of the spectrally and spatially resolved collection efficiency (CE), determined under operating conditions of the device (bias light), is even more important. In this case the photogenerated and trapped charge carriers cause a rearrangement of the electric field as well as the drift and diffusion currents of the charge carriers. In this paper we will investigate the spectral and spatial effect of degradation on high efficiency a-Si:H solar cells under bias light conditions. INTERNAL COLLECTION EFFICIENCY AND DICE From spectral response measurements the external quantum efficiency of the entire device-structure can be deduced, i.e. including reflection and absorption effects of the substrate and back contact. Normalizing the spectral response Q(A, V) with Q(A, -IV) at a sufficient reverse bias voltage results in the so called internal collection efficiency qA(V). The penetration depth of blue light is smaller compared to long wavelength light and the spectral response is therefore an indirect depth-profiler. The Dynamic Inner Collection method (DICE) [3] is based on this concept to extract the i-layer depth dependent DICEvalue qDICE(xi) from the internal collection efficiency: =o (x,) qA(V)= =) Z;( (xWA) i=o Z qDICE(Xi) . W(xi,A)
(1)
with xm is the i-layer thickness. The two-dimensional matrix IQ(xi, A) represents the number of electron-hole pairs generated at depth Xm for wavelength A and is calculated from exponential absorption functions for the different wavelengths. With Singular Value Decomposition (SVD) the matrix equation (1) is inverted to obtain the DICE-value. The i-layer grid distribution is determined in such a wa
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