Interdiffusion and Doping Gradients at the Buffer/Absorber Interface in Thin-Film Solar Cells

An accurate determination of the net dopant concentration in photovoltaic absorbers is critical for understanding and optimizing solar cell performance. The complex device structure of multilayered thin-film solar cells poses challenges to determine the dopant concentration. Capacitance–voltage (C–V) measurements of Cu(In,Ga)Se2 thin-film solar cells typically yield depth-dependent apparent doping profiles and are not consistent with Hall measurements of bare absorbers. We show that deep defects cannot fully explain these discrepancies. We instead find that the space charge region capacitance follows the model of a linearly graded junction in devices containing a CdS or Zn(O,S) buffer layer, indicating that elemental intermixing at the absorber/buffer interface alters the dopant concentration within the absorber. For absorbers covered with MgF2, C–V measurements indeed agree well with Hall measurements. Photoluminescence measurements of Cu(In,Ga)Se2 absorbers before and after deposition of a CdS layer pro...