Experimental indication for band gap widening of chalcopyrite solar cell absorbers after potassium fluoride treatment

Abstract

The implementation of potassium fluoride treatments as a doping and surface modification procedure in chalcopyrite absorber preparation has recently gained much interest since it led to new record efficiencies for this kind of solar cells. In the present work, Cu(In,Ga)Se2 absorbers have been evaporated on alkali containing Mo/soda-lime glass substrates. We report on compositional and electronic changes of the Cu(In,Ga)Se2 absorber surface as a result of a post deposition treatment with KF (KF PDT). In particular, by comparing standard X-ray photoelectron spectroscopy and synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES), we are able to confirm a strong Cu depletion in the absorbers after the KF PDT which is limited to the very near surface region. As a result of the Cu depletion, we find a change of the valence band structure and a shift of the valence band onset by approximately 0.4 eV to lower binding energies which is tentatively explained by a band gap widening as expected for Cu deficient compounds. The KF PDT increased the open circuit voltage by 60–70 mV compared to the untreated absorbers, while the fill factor deteriorated.

Injection Current Barrier Formation for RbF Postdeposition‐Treated Cu(In,Ga)Se2‐Based Solar Cells

“…A surface bandgap widening was observed after a KF PDT also experimentally. [8,9] These results hint to the fact that similar to a KF PDT, a RbF PDT introduces a surface layer, which might be responsible for the blocking of the diode current. That Rb-containing surface layer is subsequently called RIS layer.…”

“…[25] It was found that similar to a KF PDT, a surface layer forms. [5][6][7][8][9]26,30] Additionally, as presented here for a RbF PDT, with the introduction of a KF PDT, a blocking of the diode current was observed. [15] Recently, Malitckaya et al calculated the bandgap energies of AlkInSe 2 (Alk = Li, Na, K, Rb, Cs) secondary phases, which are expected to segregate on the surface of the CIGS absorber for K, Rb, and Cs.…”

NaF/KF post-deposition treatments and their influence on the structure of Cu(In, Ga)Se<inf>2</inf> absorber surfaces

“…[2]- [5] A controlled incorporation of alkali elements in the absorber by means of a post-deposition treatment (PDT) employing alkali-fluorides helps to further enhance performance. [6]- [8] While it was shown that a combined NaF/KF-PDT significantly enhances the device efficiency [7], its impact on the chemical, electronic, and topographical absorber structure is poorly understood.…”

Chalcopyrite compound semiconductors for thin film solar cells

“…The Cu-depleted layer was found to have a wider surface band gap than untreated absorbers [16,19] [24e26]). Whether the ion exchange reaction is limited to the grain boundaries or appears also in the bulk is not known currently.…”

“…(i) The potassium treatment creates a completely Cudepleted surface layer, which can be continuous [3,16] or structured [15], depending on the exact treatment conditions. The Cu-depleted layer was found to have a wider surface band gap than untreated absorbers [16,19] [24e26]).…”