Thin film Cu(In,Ga)Se2 solar cells processed from solution pastes with polymethyl methacrylate binder

A non-vacuum, solution-based method is investigated to deposit thin layers of Cu(In,Ga)Se2, which is the core component of CIGS thin film solar cells. The concept is based on paste coating of Cu, In, and Ga salt solution with an organic binder and a subsequent annealing of the paste in selenium atmosphere. Previous experiments with ethylcellulose as the binder resulted in photo conversion efficiencies up to 6.7%, although further improvements were hindered by a limited thickness of the CIGS layer and a residual carbon layer between the CIGS and metal back contact.In the present work, polymethyl methacrylate (PMMA) is tested as an alternative binder material, which theoretically should leave no char residues upon thermal degradation at temperatures higher than 350 °C. A series of pastes with different solvents are prepared and the resulting CIGS layers are investigated systematically by microscopy, SEM, EDX, XRF, and XRD. Absorber layers are processed into Mo/CIGS/CdS/ZnO solar cells and their I–V characteristics are measured. PMMA can be used as the organic binder alternative to cellulose and more complete evaporation of the organic matrix is achieved as compared to the reference cellulose-containing recipe. However, maximum solar cell efficiencies are limited to 3% because the obtained CIGS layers are porous and contain traces of parasitic oxide phases when heated above 330 °C in ambient atmosphere.