Photovoltaic Device Applications of Porous Silicon
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PHOTOVOLTAIC DEVICE APPLICATIONS OF POROUS SILICON Y.S. TSUO*, M.J. HEBEN*, X. WU*, Y. XIAO*, C.A. MOORE**, P. VERLINDEN***, AND S.K. DEB* *National Renewable Energy Laboratory, Golden, CO 80401 "**Universityof Denver, Denver, CO 80208 ***SunPower Corp., Sunnyvale, CA 94086 ABSTRACT We report on the results of our investigation of using porous Si to enhance the performance of crystalline silicon photovoltaic solar cells. Possible approaches include using the porous Si for (1) surface texturing to enhance light trapping, (2) front or back surface fields because of its wider bandgap, and (3) photon color conversion of blue light to longer wavelengths that have higher quantum efficiency in a Si solar cell. In our surface texturing study, a porous-Si-covered single-crystal Si wafer showed an integrated reflectance of only 1.4% at 500-nm wavelength compared to about 40% for a polished Si surface. For our solar cell study, we used a point-contact cell structure with diffused p+ and n" point contacts on the back of the cell. This cell structure allows us to form the porous Si on the front surface after both the junction formation and the evaporation and alloying of metal contacts. INTRODUCTION While the majority of research on porous Si has been directed toward preparing lightemitting layers and understanding the physical mechanisms underlying the luminescence processes, experimental progress to date is sufficiently advanced to begin examining Si-based optoelectronic devices, such as light-emitting diodes and photovoltaic solar cells. Electroluminescence in porous Si structures has been reported for p- and n-type substrates with top contacts of Au and ITO [1-31. Generally speaking, materials that exhibit photoluminescent and electroluminescent properties should also be good candidates for solar cells and photodetectors. Zheng et al. [4] fabricated Al/porous Si/Si photodetectors and found that they have high sensitivity with a spectral response curve similar to that of conventional single-crystal Si photodetectors. They concluded that the porous surface was "perfect" in trapping light and the surface was "well passivated with very low surface recombination". Smestad et al. [5] studied the photoresponse of a Pt probe/porous Si/Si structure and also showed that the spectral response was characteristic of the single-crystal Si substrate itself. Moreover, from their time-resolved photoconductivity measurements the authors concluded that the porous Si/Si interface was poorly passivated and that the porous Si/Si interface had a higher initial surface recombination rate than either the SiO 2/Si or air/Si interface. We investigated the possibilities of using porous Si to enhance the performance of crystalline Si photovoltaic solar cells. Potential advantages of using porous Si in solar cell structures include: (i) The highly textured morphology of porous Si may be used to enhance light trapping as noted by Zheng et al. [4]. (ii) The bandgap of porous Si may be adjusted for optimum sun light absorption; the maximum theoretical sola
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