Minimizing Roughness Induced Optical Losses for a 4-Terminal CdTe/Si Solar Device

CdTe/Si four-terminal tandem solar cells are a techno-economically advantageous option for high efficiency, low-cost photovoltaic cells. The tandem design reduces thermalization losses and can increase efficiency above commercial Si cells, and both junctions can be mass produced at relatively low cost. A four-terminal configuration provides flexibility in the electrical design of the tandem module, as the top and bottom cells avoid the current matching requirement that can limit performance. However, this configuration introduces significant optical challenges, as three high-transparency conductive layers and an insulating interlayer are required that all lead to increased parasitic absorption and reflection losses. This study uses a combination of experiments and simulations to characterize and understand CdTe roughness induced optical losses and provide strategies to increase optical coupling in the CdTe/Si four-terminal module.<br/><br/>The CdTe device stack is comprised of a planar front transparent conductive oxide (TCO) deposited on plate glass with a CdSe_xTe_1-x absorber layer. A conformally grown back TCO serves as the rear electrode and adopts the roughness characteristics of the polycrystalline CdSe_xTe_1-x absorber layer. Transmission measurements of the devices measured with UV-Vis spectrophotometry demonstrated that the sub-bandgap CdSe_xTe_1-x module transmission was enhanced with reduced CdSe_xTe_1-x roughness.<br/><br/>Surface height data from atomic force microscopy (AFM) measurements were incorporated into finite-different time-domain (FDTD) simulations of the CdSe_xTe_1-x cell to capture the influence of roughness on the optical properties. The CdSe_xTe_1-x transmission was modeled into air and into ethylene-vinyl acetate (EVA). The reduced index contrast by encapsulating the back TCO with EVA increases the transmission and through the CdSe_xTe_1-x top cell by as much as 33%.<br/><br/>The dominant loss mechanisms preventing sub-bandgap CdSe_xTe_1-x transmission were backscattering, reflection, front TCO absorption, and back TCO absorption. The optical losses are mitigated through decreased roughness and inclusion of the interlayer. The reflection and front TCO absorption are both controlled by the increased backscattering from the rougher back surface of the CdSe_xTe_1-x. Spatially resolved maps of the back TCO absorption show a non-uniform absorption profile with regions of light localization and absorption enhancement controlled by the surface topography. Mesoscale sized surface features create a focusing effect from constructive interference of the scattered and diffracted optical plane waves. The roughness induced regions of local absorption enhancement leads to the global enhancement of the back TCO absorption. The reduced index contrast with inclusion of the EVA does not eliminate the focusing effect but reduces the magnitude of the intensity enhancement. Simulations on an ideally flat surface indicated that planarizing eliminates the light localization and parasitic absorption enhancement.<br/><br/>Higher index optical coatings were also simulated to increase optical coupling in the tandem device. 1.7 and 1.9 refractive index optical coatings with finite thickness did not significantly increase the CdSe_xTe_1-x transmission compared to just the EVA interlayer, but combining a 1.9 and 1.7 index film to create a graded index coating increased the sub-bandgap transmission by about 2% between 900 and 1000 nm.<br/><br/>This study describes how increased roughness and different interlayer materials influences the reflection, TCO absorption, and optical coupling in a CdTe/Si four-terminal tandem device, and demonstrates methods to mitigate losses and promote CdSe_xTe_1-x transmission to the Si cell.