Dominant Effect of p/i Interface on Dark J-V Characteristics in p-i-n Nano-crystalline Si Solar Cells

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A9.45.1

Dominant Effect of p/i Interface on Dark J-V Characteristics in p-i-n Nano-crystalline Si Solar Cells U. Das, A. Bozsa1 and A. Madan MVSystems Inc., 17301 West Colfax Ave, Suite 305, Golden, Colorado 80401 USA 1 Institut für Physikalische Elektronik, Universität Stuttgart, Pfaffenwaldring 47, 70569 Stuttgart, Germany ABSTRACT Nanocrystalline silicon (nc-Si) based p-i-n solar cells were fabricated onto various substrates using modified pulsed PECVD technique. Dark J-V characteristics of nc-Si p-i-n solar cells were found to depend strongly on the substrates and are studied at different i-layer thickness and varying the p/i interface structures. In this work, we report an almost constant diode quality factor (n = 1.2 – 1.3) up to the thickness of 3.8 µm for the devices grown on “suitably textured” ZnO substrates. The rather insensitive variation of n with i-layer thickness suggests that the dark J-V characteristics are not dominated by bulk recombination for the devices grown on textured ZnO, which prevents grain collision in the i-layer. In contrast to that, a significant change of n (1.8 – 1.3) was found while changing the p/i interface using various duration of H2 plasma treatment of nc-p surface (ST). The p/i interface structure in nc-Si p-i-n device plays the crucial role either by changing the p/i interface defects or the film structure at p/i interface determines the quality of subsequently grown nc-i layer and hence the whole device performance. INTRODUCTION The nc-Si:H p-i-n solar cells grown by plasma enhanced chemical vapor deposition (PECVD) / hot wire chemical vapor deposition (HWCVD) techniques have demonstrated device efficiency of > 9% [1,2]. Apart from the structural properties of nc-Si:H films such as the crystal orientation and passivation of grains, oxygen concentration, voids, crystalline fraction etc., there are some other critical issues from a device point of view, such as the minimization of the incubation layer, control of interfaces, effect of texture, etc. The p/i interface in a superstrate-type (p-i-n) amorphous silicon solar cells plays a crucial role in device performance and has been studied extensively in the past. Its role in nc-Si:H p-i-n type solar cells is even more critical for solar cells. The formation of p/i interface defects due to enhanced boron diffusion into the i-layer was found for p-i-n type nc-Si:H solar cells [3]. The film structure at p/i interface is made more complicated due to an additional fact that the nc-Si:H grown by PECVD technique often starts with an amorphous incubation phase (typically >10 nm), the extent of which sensitively varies with the film growth conditions and on the substrate material. Numerous efforts have been made to reduce / eliminate such an incubation layer for the films deposited on glass by various techniques like layer-by-layer deposition [4], flow modulation methods [5], triode type RF PECVD with high H2 dilution etc. [6]. From p-i-n device point of view, it is often believed that the nc-p layer with a sufficient amount of cryst