Invited Paper TF+AS-WeA3
Investigation of Recrystallization in Low-Temperature Grown CdTe Solar Cells in Substrate and Superstrate Configuration

Wednesday, October 31, 2012, 2:40 pm, Room 11

CdTe solar cells and modules on glass substrates have already shown high performance and low cost. Production costs and energy payback time can be further reduced by minimizing the thermal budget of the production process, increasing the throughput and by the use of low-cost substrates. We developed a process for the conventional superstrate configuration which involves substrate temperatures below 450°C. The low temperatures enable the growth on flexible polyimide foil. Efficiencies up to 15.6% and 13.8% on glass and polyimide were achieved, respectively.

In the conventional superstrate configuration sputtered ZnO:Al/ZnO was used as transparent front electrical contact. CdS and CdTe were evaporated at low temperatures of 160 and 350°C followed by an annealing treatment in the presence of CdCl₂ at 420°C and the cells were finished with a metallic electrical back contact. The annealing treatment is essential for highly efficient CdTe solar cells as it leads to grain growth of CdTe, improves electronic properties of CdTe and leads to an intermixing between CdTe and CdS.

For the growth on opaque substrates like flexible metal foils, we developed a growth process of CdTe solar cells in substrate configuration, where light does not need to pass the substrate. It enabled efficiencies of 11.3% and 8.7% on glass and flexible steel foil, respectively.

A combination of Mo, MoO₃ and Te was deposited as back contact and in some cases Cu was added. Evaporated MoO₃ grew with low crystallinity and recrystallized during subsequent processing. CdTe was deposited by vacuum evaporation while CdS was grown by chemical bath deposition. In substrate configuration, the CdTe and CdS layers were annealed separately as a combined annealing step would lead to excessive CdS-CdTe intermixing. The annealing treatment of the CdTe layer leads to similar grain growth as in superstrate configuration. The CdCl₂ treatment after deposition of CdS was optimized, resulting in increased grain size and wurtzite structure. CdS-CdTe intermixing, which is commonly observed in superstrate configuration was less pronounced in substrate configuration. The effects of the recrystallization treatments in substrate configuration are compared to the conventional superstrate configuration.