Real Time and Mapping Spectroscopic Ellipsometry of Hydrogenated Amorphous and Nanocrystalline Si Solar Cells

Abstract

Real time spectroscopic ellipsometry (SE) has been applied to characterize the structural evolution and optical properties of the critical p-type doped and intrinsic hydrogenated silicon (Si:H) layers that comprise the nanocrystalline Si:H bottom cell of tandem photovoltaic (PV) devices. The tandem PV devices under study are fabricated in the amorphous/nanocrystalline Si:H (a-Si:H/nc-Si:H) p-i-n superstrate configuration in which the nc-Si:H cell is at the bottom of the device structure due to its narrower bandgap relative to that of the top cell a-Si:H. SE data collected in real time during Si:H solar cell fabrication by plasma enhanced chemical vapor deposition (PECVD) enable identification of Si crystallite development in the bottom cell p and i-layers through the evolution of surface roughness , as well as through variations in the optical properties in the form of the complex dielectric function $(\varepsilon = \varepsilon_{1} - {\text{i}}\varepsilon_{2} )$. Analysis of the dielectric function permits quantification of the relative amounts of the a-Si-H and nc-Si-H components that exist during the growth of mixed-phase Si:H layers. Based on these real time SE analysis results, a PECVD growth evolution diagram has been developed for the bottom cell i-layer of the tandem PV cell in order to guide fabrication in this device configuration. Correlations between the p and i-layer structures and device performance are evident and can be understood on the basis of the growth evolution diagram. A second growth evolution diagram has been developed to characterize PECVD of n-type Si-H thin films for use as the n-layer component of p-i-n a-Si:H top cells in the same superstrate configuration. This growth evolution diagram has been established to provide guidance for PECVD of the n-layers over the 15 cm × 15 cm areas of glass/TCO/p/i superstrates, where TCO represents the transparent conducting oxide layer serving as the topmost contact. The goal of this study is to correlate the structural characteristics provided by the diagram with the performance parameters of single-junction a-Si:H solar cells that can serve as the top cell of the tandem device. A 16 × 16 array of p-i-n dot cells has been fabricated over the 15 cm × 15 cm area of the TCO coated glass superstrate, and this same area has been studied by mapping SE. Analysis of the SE data collected over the full area provides maps of the p-layer effective thickness , i-layer thickness and bandgap , and n-layer thickness and nanocrystalline volume fraction for spatial correlation with the performance parameters from current density-voltage (J-V) measurements of the 16 × 16 array of dot cells. The goal of the correlations that exploit mapping SE is to identify and understand the relationships between the variations in the basic materials properties and in the thin film solar cell performance over large areas. The results also enable analysis of the impacts of spatial non-uniformities on PV module performance.