2, Colorado State University, Fort Collins, Colorado, United States
CdTe-based thin film solar cells occupy the largest market share among the commercially produced thin-film solar cells. CdTe has the advantages of a direct bandgap of 1.45 eV at room temperature and a large absorption coefficient. A II-VI/Si based Tandem solar cell shows great promise, as modeling indicates that efficiency greater than 30% in practical cells is possible, while at a comparable cost per watt with CdTe and Si single-junction alternatives.
In this study we focus on the temperature effects of the interlayers that connect the II-VI top cell and the Si bottom cell in tandem solar cells. CdTe is used as the top cell in these experiments. We analyze how the high temperature deposition of CdTe affects each interconnection layer separately, and integrated in the full tandem stack, in terms of their electrical, optical characteristics and the morphology. In the tandem structure under study, the TCO acts as a vertical conduction layer between the Si and CdTe layers, and ZnTe acts as a back-surface field for the CdTe cell. Specific contact resistance(ρc) and sheet resistance(Rs) of the TCO layers are measured through TLM patterning measurements. Transmittance, reflectance, absorptance, refractive index, and haze will be reported before and after the thermal treatments that simulate the II-VI top cell growth on Si, in a substrate configuration rather than the conventional superstrate CdTe cell structure. Optical properties of individual interlayers as well as of the stacked layer combination will be discussed. Test structures used for the analysis of the TCO interlayer were n+ diffused n-type and p-type Si wafers. ITO layers of 50 nm were deposited in a TLM pattern through a shadow mask, followed by Ag contacts to the ITO formed through the same mask, to measure ρc between the ITO and Si layers. The same process was followed for the deposition of the IZO layer. Two runs were made with different O2 concentration in the DC sputtered TCO layers. The measured Rs for the ITO and IZO are 82 and 115 Ω/sq respectively. The samples were subjected to rapid thermal annealing at 450○C for 3 minutes. After annealing, the sheet resistances measured were 41 and 61 Ω/sq for the ITO and IZO respectively. Additionally, it was observed that the contacts did not exhibit ohmic behaviour after the anneal, and that the TCOs deposited under higher O2 concentration are more resistive after the anneal.
A full study of the I-V characteristics at ZnTe/TCO and TCO/doped Si interfaces will be presented, including fluorine-doped tin oxide (FTO), and their evolution with temperature exposures typical for II-VI cell deposition. The transmittance and absorptance of optical stacks from these interlayer materials will also be discussed as a function of the II-VI growth temperature.
Temperature studies of the interlayers in monolithic II-VI/Si tandem cells will facilitate a better understanding of this promising low-cost, high-efficiency multijunction cell architecture.