Urbach energy in CIGS extracted from quantum efficiency analysis of high performance CIGS solar cells

M. TROVIANO; K. TARETTO

Hamburg

Conferencia; 24th European Photovoltaic Solar Energy Conference; 2009

We develop a method to extract the characteristic energy of the Urbach energy band tails in thin film CIGS. By modeling the temperature-dependent internal quantum efficiency (IQET) of CIGS solar cells including sub?gap absorption, we identify a linear region in a semilogarithmic IQET vs. photon energy plot, which yields the Urbach energy EU. Measurements of high efficiency (18 %) CIGS solar cells prepared at the Institut für PhysikalischeElektronik (Stuttgart, Germany), yield room temperature Urbach energies around 30 meV in cells with a Ga/(In+Ga) ratio of 0.35. Cryostat temperature-dependent IQET measurements yield an increasing value of EU from 31 meV at 146 K up to 40 meV at 293 K. Additionally, a conversion from IQET to electroluminescence reveals defect bands obtained by other groups with spectroscopic measurements.