This thesis aims to contribute to the development of improved window layer structures for chalcopyrite thin-film solar cells, with an emphasis on the buffer layer, to assist future reductions of the levelized cost of energy. This is realized by exploring the potential of existing materials and deposition processes, as well as developing new buffer layer processes based on atomic layer deposition (ALD).

Ternary compound ALD processes are more complicated to control than when depositing binary compounds and the composition can be significantly different at the absorber interface as compared to the bulk. A method based on in-situ quartz crystal microbalance that can measure these compositional variations is demonstrated in the thesis. Furthermore, the addition of alkali-metal fluoride post-deposition treatments (PDTs) can further complicate ALD of buffer layers, due to residual salts that are formed on the absorber surface during a PDT process. When applying ALD ZnO_1-xS_x to KF-treated CIGS absorbers, competitive solar cell efficiencies could only be obtained after performing additional wet-chemical treatments prior to ALD processing.

It is shown that the performance of wide-bandgap solar cells can be greatly enhanced by improving the conduction band alignment between the absorber and buffer layers. By applying ALD Zn_1-xSn_xO_y buffer layers in CuGaSe₂ solar cells, record efficiency (η = 11.9%) and open-circuit voltage (V_oc = 1017 mV) values are demonstrated.

In search of a new buffer layer suitable for a wide range of absorber materials (and surface bandgaps), amorphous tin-gallium oxide grown by ALD is evaluated as a new buffer layer material. This material exhibits a highly variable bandgap (and electron affinity) the absorber/buffer conduction band alignment can be controlled by adjusting the cation composition and deposition temperature. The potential of Sn_1-xGa_xO_y as a buffer layer was studied in combination with low-bandgap (Ag,Cu)(In,Ga)Se₂ absorbers (E_g,surface ≈ 1.1 eV). A best cell efficiency of 17.0% was achieved, which was lower than the efficiency of 18.6% obtained for the corresponding CdS reference due to slightly lower V_oc and higher series resistance. However, the full potential of Sn_1-xGa_xO_y as a buffer layer remains to be revealed.