MXene Back Contact for CdTe Photovoltaic Devices

The patent introduces an intermediate layer (such as Cu-doped ZnTe or other buffer layers) between the high work-function metal contact and the CdTe absorber. This intermediary layer serves as a mediator that provides good band alignment with CdTe while allowing the metal contact to maintain its high conductivity, thus eliminating the direct harmful interaction between the metal and CdTe that causes downward band-bending.

The patent modifies the electronic parameters of the back contact interface by using Cu-doping to adjust the dopant density and work function of the contact layer. By changing the doping concentration and composition, the patent optimizes the band alignment and reduces Schottky barrier formation while maintaining electrical conductivity.

The patent optimizes the Cu-doping concentration to suboptimal levels (10^14-10^15 cm^-3) rather than using high doping densities. This parameter optimization prevents self-compensation effects and maintains device stability while still achieving adequate carrier extraction. The patent also controls the timing and method of Cu-introduction to minimize degradation.

The patent introduces Cu-doping at controlled stages during the fabrication process, such as during the ZnTe buffer layer deposition or through subsequent annealing steps. This preliminary and controlled doping approach ensures uniform distribution and prevents excessive local concentration that would lead to degradation and self-compensation.

The patent optimizes the local properties of the Cu-doped ZnTe buffer layer by controlling the Cu-distribution and doping concentration specifically at the interface region. By creating a gradient or localized doping profile, the patent maintains good band alignment and copper immobilization at the contact interface while minimizing interfacial states that cause recombination in the bulk of the buffer layer.

The patent adjusts the ZnTe layer thickness, Cu-doping concentration, and annealing temperature to optimize the balance between band alignment and surface recombination. By carefully controlling these parameters, the patent reduces the density of interfacial states while maintaining the beneficial band alignment properties.

The patent employs composite structures combining multiple materials with complementary properties. For example, combining Cu-doped ZnTe buffer layer with specific metal contacts, or using hybrid organic-inorganic interfaces. This composite approach allows each layer to address specific constraints (band alignment, conductivity, stability) while the combination achieves all requirements simultaneously.

The patent designs the back contact structure to perform multiple functions simultaneously: the Cu-doped ZnTe layer provides both band alignment and copper ion immobilization, while the metal contact provides conductivity and mechanical stability. This multi-functional design ensures all constraints are met within a single integrated back contact system.