Kinetics of Light Induced Changes in Protocrystalline Thin Film Materials and Solar Cells
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Kinetics of Light Induced Changes in Protocrystalline Thin Film Materials and Solar Cells Randy Koval, Xinwei Niu, Joshua Pearce, Lihong Jiao, Gautam Ganguly1, Jeffrey Yang2, Subhendu Guha2, Robert W. Collins, Christopher R. Wronski Center For Thin Film Devices, The Pennsylvania State University, University Park, PA, 16801 1 BP Solarex, 3601 Lagrange Parkway, Toano, VA 23168 2 United Solar Systems Corp., 1100 West Maple Road, Troy, MI 48084 ABSTRACT Studies have been carried out on the kinetics of light induced changes in thin films and solar cells fabricated in different laboratories. Unlike the many studies on undiluted a-Si:H films the kinetics reported here are for 1 sun illuminations carried out to the degraded steady state (DSS). The light induced changes at temperatures from 25 to 100oC were characterized with electron mobility lifetime products and subgap absorption in films and fill factors in solar cells. The significant changes in the degradation kinetics that occur in these materials in this temperature range have further confirmed the importance of charged defects, not only in the annealed state, but also in the degraded steady states. Results are also presented on corresponding solar cell structures, which also indicate that defects other than neutral dangling bonds have to be taken into account if any meaningful evaluation is to be made of their solar cell properties. INTRODUCTION Recently, progress has been made in establishing correlations between properties of protocrystalline materials and solar cells fabricated using silane (SiH4) diluted with hydrogen (H2) [1,2]. Such protocrystalline materials, which are prepared close to but not at the amorphousto-microcrystalline phase boundary, exhibit not only excellent initial properties and stability but also degradation kinetics that are more amenable to detailed studies [2,3,4]. We report here studies on kinetics of light induced changes carried out with 1 sun illumination on such a-Si:H thin film materials and solar cells fabricated by BP Solarex, United Solar, Electrotechnical Laboratory, and The Pennsylvania State University. Because the degree of ordering and film thickness at which the amorphous to microcrystalline transitions depend on R≡[H2]/[SiH4]; the substrates, and deposition conditions, spectroscopic ellipsometry (SE) and atomic force microscopy (AFM) measurements were used to confirm that the materials, were still a-Si:H. Results are presented and discussed here on the kinetics of light induced changes carried out to the degraded steady state (DSS) with 1 sun illuminations at temperatures from 25 to 100˚C. EXPERIMENTAL DETAILS The thin films of a-Si:H studied were prepared by PECVD at substrate temperatures between 200˚C and 300˚C, with R=10 or with “low” values of R (United Solar and BP Solarex). The deposition conditions of the PSU materials and cells are reported elsewhere [1,2]. The optical absorption obtained from T&R measurements and their protocrystalline (amorphous) state established with SE and AFM measurements. The thin films were
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