Impacts of KF Post-Deposition Treatment on the Band Alignment of Epitaxial Cu(In,Ga)Se2 Heterojunctions.

In this study, we investigated band alignments at CdS/epitaxial CuInxGa1-xSe2 (epi-CIGSe) and epi-CIGSe/GaAs heterointerfaces for solar cell applications using ultraviolet, inverse, and X-ray photoemission spectroscopy (UPS, IPES, and XPS) techniques. We clarified the impacts of KF postdeposition treatment (KF-PDT) at the CdS/epi-CIGSe front heterointerfaces. We found that KF-PDT changed the conduction band alignment at the CdS/epi-CIGSe heterointerface from a cliff to flat configuration, attributed to an increase in the electron affinity (EA) and ionization potential (IP) of the epi-CIGSe surface because of a decrease in Cu and Ga contents. Herein, we discuss the correlation between the impacts of KF-PDT and the solar cell performance. Furthermore, we also investigated the band alignment at the epi-CIGSe/GaAs rear heterointerface. Electron barriers were formed at the epi-CIGSe/GaAs interface, suppressing carrier recombination as the back surface field. Contrarily, a hole accumulation layer is formed by the valence band bending, which is like Ohmic contact.

Physical and chemical aspects at the interface and in the bulk of CuInSe2-based thin-film photovoltaics.

Chalcopyrite CuInSe2 (CISe)-based thin-film photovoltaic solar cells have been attracting attention since the 1970s. The technologies of CISe-based thin-film growth and device fabrication processes have already been put into practical applications and today commercial products are available. Nevertheless, there are numerous poorly understood areas in the physical and chemical aspects of the underlying materials science and interfacial and bulk defect physics in CISe-based thin-films and devices for further developments. In this paper, current issues in physical and chemical studies of CISe-based materials and devices are reviewed. Correlations between Cu-deficient phases and the effects of alkali-metals, applications to lightweight and flexible solar minimodules, single-crystalline epitaxial Cu(In,Ga)Se2 films and devices, differences between Cu(In,Ga)Se2 and Ag(In,Ga)Se2 materials, wide-gap CuGaSe2 films and devices, all-dry processed CISe-based solar cells with high photovoltaic efficiencies, and also fundamental studies on open circuit voltage loss analysis and the energy band structure at the interface are among the main areas of discussion in this review.

Efficient Narrow Band Gap Cu(In,Ga)Se2 Solar Cells with Flat Surface.

In this study, the influences of bromine-based etching (Br etching) of narrow band gap CuInSe2 (CIS) absorbers and Cu(In,Ga)Se2 absorbers with various single Ga gradings (CIS:Ga) on the properties of solar cells were investigated. Absorbers with narrow absorption edge energies (Eabs) of 1.0-1.02 eV, ideal for the application as a bottom cell in a tandem device, were fabricated using a modified three-stage process and subjected to Br etching. The evolution of surface flatness and their optical and electrical properties upon Br etching were investigated. Br etching typically reduced the root-mean-square deviation of the surface roughness height (Rq) for a CIS:Ga absorber from several hundreds to several tens of nanometers, whereas for some CIS absorbers, Rq reduction was limited by the remaining voids. Moreover, Br etching reduced the leakage current across the pn junction. The high shunt resistances (Rsh) typically up to >10 kΩ·cm2 were obtained by introduction of Br etching. However, etching sometimes adversely increased the VOC deficit. The investigation of the minority carrier lifetime and diode parameters revealed that back-surface recombination in CIS and low-Ga CIS:Ga solar cells increased as the absorber layer thickness decreased. A higher Ga grading significantly reduced back-surface recombination. Narrow band gap CIGS solar cells with improved surface flatness and high VOC were achieved by introducing Br etching and proper Ga grading.

Optical and Structural Properties of High-Efficiency Epitaxial Cu(In,Ga)Se2 Grown on GaAs.

Photovoltaic devices based on Cu(In,Ga)Se2 (CIGS) typically employ polycrystalline thin films as the absorber layer. This is because, to date, the highest conversion efficiencies have been attained with polycrystalline CIGS films. Recently, Nishinaga et al. presented an epitaxial CIGS thin-film solar cell grown on a GaAs (100) substrate with a conversion efficiency of 20.0%. In this contribution, we study the optical and structural properties of this high-efficiency epitaxial film, along with others with different compositions using cathodoluminescence spectrum imaging and transmission electron microscopy. A comparison of the high-efficiency epitaxial film and a traditional polycrystalline film with a similar global composition reveals significant differences in microstructure and uniformity of emission properties despite similar performance. The analysis of epitaxial films with a higher gallium concentration indicates that the emission characteristics and nature of extended defects in epitaxial CIGS films are strongly dependent on the gallium content. The results presented here provide evidence that, with further optimization, photovoltaic conversion efficiencies of epitaxial CIGS films could exceed those of polycrystalline CIGS.

Cu(In,Ga)Se2 Solar Cells with Amorphous In2O3-Based Front Contact Layers.

Amorphous (a-) In2O3-based front contact layers composed of transparent conducting oxide (TCO) and transparent oxide semiconductor (TOS) layers were proved to be effective in enhancing the short-circuit current density (Jsc) of Cu(In,Ga)Se2 (CIGS) solar cells with a glass/Mo/CIGS/CdS/TOS/TCO structure, while maintaining high fill factor (FF) and open-circuit voltage (Voc). An n-type a-In-Ga-Zn-O layer was introduced between the CdS and TCO layers. Unlike unintentionally doped ZnO broadly used as TOS layers in CIGS solar cells, the grain-boundary(GB)-free amorphous structure of the a-In-Ga-Zn-O layers allowed high electron mobility with superior control over the carrier density (N). High FF and Voc values were achieved in solar cells containing a-In-Ga-Zn-O layers with N values broadly ranging from 2 × 1015 to 3 × 1018 cm-3. The decrease in FF and Voc produced by the electronic inhomogeneity of solar cells was mitigated by controlling the series resistance within the TOS layer of CIGS solar cells. In addition, a-In2O3:H and a-In-Zn-O layers exhibited higher electron mobilities than the ZnO:Al layers conventionally used as TCO layers in CIGS solar cells. The In2O3-based layers exhibited lower free carrier absorption while maintaining similar sheet resistance than ZnO:Al. The TCO and TOS materials and their combinations did not significantly change the Voc of the CIGS solar cells and the mini-modules.