Origin of Interface Limitation in Zn(O,S)/CuInS₂-Based Solar Cells

Abstract

Copper indium disulfide (CuInS 2 ) grown under Cu-rich conditions exhibits high optical quality but suffers predominantly from charge carrier interface recombination, resulting in poor solar cell performance. An unfavorable "cliff"-like conduction band alignment at the buffer/CuInS 2 interface could be a possible cause of enhanced interface recombination in the device. In this work, we exploit direct and inverse photoelectron spectroscopy together with electrical characterization to investigate the cause of interface recombination in chemical bath-deposited Zn(O,S)/co-evaporated CuInS 2 -based devices. Temperature-dependent current− voltage analyses indeed reveal an activation energy of the dominant charge carrier recombination path, considerably smaller than the absorber bulk band gap, confirming the dominant recombination channel to be present at the Zn(O,S)/CuInS 2 interface. However, photoelectron spectroscopy measurements indicate a small (0.1 eV) "spike"like conduction band offset at the Zn(O,S)/CuInS 2 interface, excluding an unfavorable energy-level alignment to be the prominent cause for strong interface recombination. The observed band bending upon interface formation also suggests Fermi-level pinning not to be the main reason, leaving nearinterface defects (as recently observed in Cu-rich CuInSe 2 ) as the likely reason for the performance-limiting interface recombination.

Investigation of co-evaporated polycrystalline Cu(In,Ga)S2thin film yielding 16.0 % efficiency solar cell

“…This issue can be tackled by using the alternative (CBD)Zn(O,S) buffer layer; which bandgap is greater than 3 eV thus optically less harmful in the short wavelengths. In addition, this buffer is reported as yielding more adapted band configuration at the CIGS/ buffer interface [17] and thereby possibly leads to higher V oc relative to CdS-buffered device.…”

Sulfide Chalcopyrite Solar Cells––Are They the Same as Selenides with a Wider Bandgap?

“…[102] This can be mitigated by using buffers with a higher conduction band edge energy. [19,72] ZnOS has been shown to form a spike with CuInS 2 [103] and was also found to mitigate interface recombination in Cu-poor Cu(In,Ga)S 2 . [19] Other buffers that form a favorable interface with Cu-poor Cu(In,Ga)S 2 have been shown to be ZnMgO [70] or ZnSnO.…”

“…[ 19,60,72 ] Fermi‐level pinning can be excluded based on photoemission spectroscopy, where interface‐induced band bending is always observed. [ 102,103 ] The reason for the dominating interface recombination in Cu‐rich Cu(In,Ga)S 2 has been discussed earlier: it is due to a defective layer near the surface of the absorber, with defects related to chalcogen vacancies generated by the oxidation behavior and the necessary etching step of the stoichiometric chalcopyrite obtained in Cu‐rich growth. This cause of interface recombination can only be avoided by using Cu‐poor absorbers.…”

Chemical and Electronic Structure at the Interface between a Sputter‐Deposited Zn(O,S) Buffer and a Cu(In,Ga)(S,Se)2Solar Cell Absorber

“…[ 14 ] Recently, it was proposed that near‐interface defects might be present in devices with poor performance. [ 15 ]…”

“…[14] Recently, it was proposed that near-interface defects might be present in devices with poor performance. [15] Previously, CIGSSe absorbers made by AVANCIS GmbH contained a composition gradient of sulfur and selenium but showed only trace amounts of Ga at the surface. [5,[16][17][18][19] For the present study, the gallium gradient was modified, such that Ga is now also present at the surface in quantities easily detectable by photoelectron spectroscopy.…”