Study of Band Alignment at CBD-CdS/Cu(In_1-xGa_x)Se₂ (x = 0.2 - 1.0) Interfaces by Photoemission and Inverse Photoemission Spectroscopy

Abstract

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<x<1.0 has been systematically studied by means of photoemission spectroscopy (PES) and inverse photoemission spectroscopy (IPES). In-rich CIGS samples had their conduction band minimum (CBM) lower than that of CdS. Their conduction band offset (CBO) was positive and approximately +0.3 ~ +0.4 eV. Almost flat conduction band alignment was realized at x = 0.4 ~ 0.5. On the other hand, at the interfaces over the Ga-rich CIGS, CBM of CIGS was higher than that of CdS, and CBO became negative. The present study reveals that the decrease of CBO with a rise of x presents over the wide rage of x, which results in the sign change of CBO around 0.4 ~ 0.45. In the Ga-rich interfaces, the minimum of band gap energy, which corresponded to energy spacing between CBM of CdS and valence band maximum of CIGS, was almost identical compared to the change of band gap energy of CIGS. Additionally, local accumulation of oxygen related impurities was observed at the Ga-rich samples, which might cause the local rise of band edges in central region of the interface.

Characterization of electronic structure of oxysulfide buffers and band alignment at buffer/absorber interfaces in Cu(In,Ga)Se2-based solar cells

“…The VBM of CBD-Zn(S,O,OH) is approximately ¹1.8 to ¹1.9 eV, which is rather close to that of CBD-CdS. 4,5,[23][24][25] Although the CBM of the original surface was approximately +1.7 eV, it showed an apparent shift with the progress of etching. The CBM of the 750-s etched surface, which was just above the top of CIGS, was about +0.8 eV.…”

“…Since the CBM of CIGS with a Ga substitution ratio for In of about 0.35 locates at +0.5 to +0.6 eV, the conduction band alignment at this interface should be type I with the conduction band discontinuity of about 0.3 eV, which is similar to that at the CBD-CdS/CIGS interface in high-performance cells. 5,25) The band gap of the Zn(S,O,OH) adjacent to CIGS, therefore, shrank to about 2.6 eV. Figures 1(c) and 1(d) show changes of the band edges and the compositional ratio of sulfur to sulfur plus oxygen in oxide states S/(S+O) with etching time, respectively.…”

“…The highest conversion efficiencies in thin film solar cells have been achieved in the Cu(In 1¹x Ga x )Se 2 (CIGS)-based cells, which have reached efficiencies above 20%. 1) Tuning of the band-gap energy E g of the CIGS absorber toward a wider and optimum region, [2][3][4][5][6][7][8][9] and the introduction of new buffer materials [10][11][12][13][14][15][16] instead of toxic elements involving CdS are major research trends for pursuing higher conversion efficiency and environmental friendliness in CIGS-based solar cells. Since the expansion of E g is mainly originated in the monotonic rise of the conduction band minimum (CBM) of CIGS, conduction band alignment at the buffer/CIGS interface should be simultaneously modified by the expansion.…”

“…[17][18][19][20][21][22] As experimentally revealed, the CBM of CIGS with E g ² 1.3 eV exceeds that of the conventional CdS buffer grown by CBD. 5) Under this so-called type-II alignment, the energy separation between electrons and holes in the interface region should be limited by the energy difference between the valence band maximum VBM of CIGS and the CBM of CdS, which causes a saturation of open-circuit voltage and serious degradation of cell performance. 17,18) Extensive studies have been carried out for surveying new buffer materials with a higher CBM than CdS.…”

Efficiency limitations of chalcopyrite and kesterite solar cells

“…On the other hand, Ea (for Cu-poor cells) continues to equal Eg,a [27]. For CuGaSe2/CdS, the interface band gap is 1.4 eV [28]. Thus, interface recombination for the Cu-poor CuGaSe2 cell must be ruled out because otherwise Ea would equal or be smaller than Eg,IF (see below).…”