Dependence of band alignments at interfaces between CdS (grown by chemical bath deposition) and Cu(In 1-x Ga x )Se 2 (CIGS) (by conventional 3-stage co-evaporation) with 0.2
In-situ characterization of composition, electronic structure and their depth profiles of surface of Cu(In1-xGax)Se2 (CIGS) film grown by three stage co-evaporation has been carried out by means of photoemission and inverse photoemission spectroscopy (PES/IPES), for the purpose of investigating the formation mechanism of the CIGS-side wide band-gap region adjacent to CBD-CdS/CIGS interface in cell structure. Sample-transportation in vacuum below 1 x 10-8 Torr yielded almost contamination-free feature of the CIGS surface. The as-transferred surface of Cu0.93(In0.65Ga0.35)Se2 grown at the identical condition for the high performance solar cell exhibited seriously Cu and Ga deficient composition. Chemical formula of this region was inbetween Cu : (In+Ga): Se = 1 : 3 : 5 and 1 : 5 : 8. In-situ UPS/IPES measurements CIGS showed that the as-grown surface region of the CIGS already had expanded band gap energy up to 1.4 eV and n-type character. A gradual decrease of band energy and a rise of valence band maximum as a function of depth from the original surface were observed. These results have revealed that the surface of CIGS by the three stage method already has the wide band gap, which might originate in so-called Cu-vacancy ordered phase.
For understanding the origin of the improvements of properties of CIGS-based cells, in which the CIGS absorber has been fabricated by H 2 O-introduced co-evaporation [CIGS-H 2 O], the band alignment at the interfaces between chemical bath deposited CdS and CIGS-H 2 O with Ga substitution ratio ~ 40 % has been studied by photoemission and inverse photoemission spectroscopy. The CdS layer over the CIGS-H 2 O showed an identical electronic structure to that of CdS on the conventionally grown CIGS; band gap energy of 2.3 ~ 2.4 and the location of the conduction band minimum (CBM) and valence band maximum (VBM) relative to Fermi level were + 0.75 eV and -1.6 ~ -1.7 eV, respectively. In the interface region, decreases of CBM and a rise of VBM were observed. Total amount of the decrease of CBM over the interface was about 0.3 eV. Binding-energy shifts of the core-level signals over the interface showed a band bending correction for band offset of -0.1 eV. Consequently, the conduction band offset (CBO) and valence band offset (VBO) at the CBD-interface above the CIGS-H 2 O (Ga~40%) are about +0.2, and 0.8 ~ 0.9 eV, respectively. This positive CBO is in contrast with the almost zero or slightly negative CBO at the interface between CBD-CdS/conventionally grown CIGS (also with Ga ~ 40 % as measured previously. These results indicate that the H 2 O introduction is effective in extending the upper limit of the Ga substitution ratio where the Type-I conduction band alignment is maintained. The observed band alignments are consistent with the rise of V oc and efficiency in the CIGS-H 2 O based cells.
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