Interdependence of absorber composition and recombination mechanism in Cu(In,Ga)(Se,S)2 heterojunction solar cells

Abstract

Temperature-dependent current-voltage measurements are used to determine the dominant recombination path in thin-film heterojunction solar cells based on a variety of Cu(In,Ga)(Se,S)2 alloys. The activation energy of recombination follows the band gap energy of the respective Cu(In,Ga)(Se,S)2 alloy as long as the films are grown with a Cu-poor final composition. Thus, electronic loss in these devices is dominated by bulk recombination. In contrast, all devices based on absorber alloys with a Cu-rich composition prior to heterojunction formation are dominated by recombination at the heterointerface, with activation energies smaller than the band gap energy of the absorber material. These activation energies are independent from the S/Se ratio but increase with increasing Ga/In ratio.

Numerical simulation of carrier collection and recombination at grain boundaries in Cu(In,Ga)Se2 solar cells

“…First, there is ample evidence that the Cu-poor region providing the internal valence band offset is present at the surface of the absorber. [34][35][36] It is therefore reasonable to conclude that a Cu-poor region at the GBs is more likely to be present toward the surface of the absorber than toward the back contact. Second, measurements of the majority-carrier transport properties of CIGS films in coplanar geometry, i.e., perpendicular to many GBs, exhibit activation energies in a range between 60 and 120 meV, 41 i.e., much smaller than an effective internal band offset.…”

“…[13][14][15] The effect of this internal offset at the GBs could be of similar importance as it is at the surface of the absorber, 33 whereas, there is ample experimental evidence for the Cu-poor surface layer 34,35 and its beneficial consequences for the performance of CIGS solar cells as long as the overall film composition is Cu-poor, 36 the question whether or not such a Cu-poor layer is a general positive feature of GBs in CIGS is still under discussion. [37][38][39][40] The present study concentrates on investigating the effect of such an internal band offset and on finding the conditions that have to be fulfilled if the Cu-poor layer should have a decisive beneficial effect on the performance of polycrystalline CIGS solar cells.…”

Electrical Characterization of Defects in Cu-Rich Grown CuInSe2Solar Cells

“…Furthermore Hall measurements on CuGaSe 2 (CGS) reveal increased carrier mobilities for Cu/In ratios above stoichiometry [3], which if it translates to CIS can improve carrier collection, resulting in better shortcircuit currents (J SC ) than for the Cu deficient material. But although Cu-poor devices reach efficiencies up to 15 % [4], in general Cu-rich devices show much lower efficiencies, because they suffer from interface recombination [5]. Nonetheless, it The authors are with the Laboratory for Photovoltaics, Physics and Material Science Research Unit, University of Luxembourg, L-4422 Belvaux, Luxembourg e-mail: (tobias.bertram@uni.lu; validep@hotmail.com; susanne.siebentritt@uni.lu).…”

Quasi Fermi level splitting of Cu-rich and Cu-poor Cu(In,Ga)Se2 absorber layers

“…3 CIGS layers grown under copper excess have a lower defect density but solar cells are dominated by recombination near the interface with a CdS buffer layer and thus show a strong decrease in the open circuit voltage (V oc ). 3,4 With absolute calibrated photoluminescence (PL) experiments on etched absorber layers at room temperature, it is possible to measure the quasi Fermi level splitting (qFLs) l which is an upper limit for the open circuit voltage. [5][6][7][8] A material with a high recombination rate will show a low qFLs due to a lower density of photo-generated charge carriers.…”