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 growth ambient. Cells grown in Te-rich ambient showed increased atomic interdiffusion at the CdS/CdTe interface and produced high-efficiency cells (11.9%) with an open-circuit voltage (V_oc) of 780 mV. Cd-rich cells were < 6% efficient. Carrier transport analysis showed that the transport mechanism switches from tunneling/interface recombination in the 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 gradual transition from CdS to CdTe with fewer interface states. In order to understand the loss mechanisms associated with grain boundaries in polycrystalline CdTe cells, an attempt was made to fabricate thin film CdTe/CdS device structures using an epitaxial lift-off (ELO) process. Single crystal CdTe and CdTe/CdS epitaxial layers were separated from the GaAs substrate by selective etching and were then transferred and 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 the single crystal CdTe films due to the absence of grain boundaries. X-ray diffraction measurements showed that the CdTe/CdS lattice structure 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 heterojunction interface. The lattice constant of CdTe at the interface with GaAs was 6.317 Å, which is smaller than the ideal lattice constant of 6.481 Å for the CdTe bulk.