Quantum Efficiency Analysis of Double-Graded High Efficiency CIGS Solar Cells

M. TROVIANO; K. TARETTO

Valencia

Conferencia; 25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion; 2010

Double-graded solar cells offer advantages such as enhanced carrier collection and reduced surface recombination compared to traditional pn?junction cells, at the cost of increased complexity, specially during the analysis of characteristic curves. Here, we apply a model for the internal quantum efficiency (IQE) of double?graded bandgap CIGS solar cells, containing sub?gap absorption as well as drift?assisted collection due to the grading. Analyzing IQE measurements of high efficiency CIGS cell samples, the model enables the extraction of characteristic Urbach energies of electronic band?tails, and the minimum bandgap in the grading profile. Typical Urbach energies between 18 and 23.5 meV are found, and minimum gaps between 1.08 and 1.10 eV placed 0.3 μm from the CdS/CIGS interface. The extracted values permit a full simulation of the measured curves with our model, yielding diffusion lengths between 0.35 μm to 1.0 μm, which correlate to energy conversion efficiencies that go from 15.2 % to 18.0 %, respectively. The importance of the grading fields for the high efficiencies is discussed.