APPROACH TOWARD HIGH-EFFICIENCY CDTE CDS HETEROJUNCTION SOLAR-CELLS/

CdTe solar cells were fabricated by depositing MOCVD grown CdTe films on CdS/SnO2/glass substrates with varying Te:Cd mole ratios in the gro wth ambient. Cells grown in Te-rich ambient showed increased atomic in terdiffusion at the CdS/CdTe interface and produced high-efficiency ce lls (11.9%) with an open-circuit voltage (V-oc) of 780 mV. Cd-rich cel ls were < 6% efficient. Carrier transport analysis showed that the tra nsport mechanism switches from tunneling/interface recombination in th e Cd-rich cells to depletion region recombination limited transport in the Te-rich cells. This suggests that the enhanced interdiffusion is beneficial for these cells and leads to reduced lattice mismatch or gr adual transition from CdS to CdTe with fewer interface states. In orde r to understand the loss mechanisms associated with grain boundaries i n polycrystalline CdTe cells, an attempt was made to fabricate thin fi lm CdTe/CdS device structures using an epitaxial lift-off (ELO) proces s. Single crystal CdTe and CdTe/CdS epitaxial layers were separated fr om the GaAs substrate by selective etching and were then transferred a nd bonded to a SnO2/glass substrate. SIMS analysis of CdTe/CdS cells, with Au/Cu ohmic contacts to CdTe, showed much less Cu diffusion in th e single crystal CdTe films due to the absence of grain boundaries. X- ray diffraction measurements showed that the CdTe/CdS lattice structur e and quality does not change appreciably after the lift-off process. A new methodology was developed, using XRD and the lift-off technique, to assess the lattice mismatch induced strain and defects at the hete rojunction interface. The lattice constant of CdTe at the interface wi th GaAs was 6.317 Angstrom, which is smaller than the ideal lattice co nstant of 6.481 Angstrom for the CdTe bulk.