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.

Lithium-Doping Effects in Cu(In,Ga)Se2 Thin-Film and Photovoltaic Properties.

The beneficial effects of heavy alkali metals such as K, Rb, and Cs in enhancing Cu(In,Ga)Se2 (CIGS) photovoltaic efficiencies are widely known, though the detailed mechanism is still open for discussion. In the present work, the effects of the lightest alkali metal, Li, on CIGS thin-film and device properties are focused upon and compared to the effects of heavy alkali metals. Till date, the beneficial effects of elemental Li on Cu2ZnSnS4 photovoltaic devices in enhancing efficiencies have been reported. On the other hand, it is shown in the present work that the beneficial effects of Li on CIGS are not so significant. In contrast to the effects of Na or Rb in enhancing CIGS(112) growth orientation, Li was revealed not to affect CIGS growth orientation. The most distinctive feature observed between Li and other alkali metals was the elemental depth profile in CIGS films. Namely, Na and heavier alkali metals show a concentration peak near the surface (relatively Cu-poor) region of CIGS films, whereas elemental Li showed no such trend, suggesting that Li has no significant effect on CIGS surface modification. Nonetheless, Li was found to have some effect in enhancing the PL peak intensity and photovoltaic performance of CIGS, though the effect is relatively small in comparison to that obtained with other alkali metals.

Si-Doping Effects in Cu(In,Ga)Se2 Thin Films and Applications for Simplified Structure High-Efficiency Solar Cells.

We found that elemental Si-doped Cu(In,Ga)Se2 (CIGS) polycrystalline thin films exhibit a distinctive morphology due to the formation of grain boundary layers several tens of nanometers thick. The use of Si-doped CIGS films as the photoabsorber layer in simplified structure buffer-free solar cell devices is found to be effective in enhancing energy conversion efficiency. The grain boundary layers formed in Si-doped CIGS films are expected to play an important role in passivating CIGS grain interfaces and improving carrier transport. The simplified structure solar cells, which nominally consist of only a CIGS photoabsorber layer and a front transparent and a back metal electrode layer, demonstrate practical application level solar cell efficiencies exceeding 15%. To date, the cell efficiencies demonstrated from this type of device have remained relatively low, with values of about 10%. Also, Si-doped CIGS solar cell devices exhibit similar properties to those of CIGS devices fabricated with post deposition alkali halide treatments such as KF or RbF, techniques known to boost CIGS device performance. The results obtained offer a new approach based on a new concept to control grain boundaries in polycrystalline CIGS and other polycrystalline chalcogenide materials for better device performance.

Interfacial alkali diffusion control in chalcopyrite thin-film solar cells.

Alkali elements, specifically sodium (Na), are key materials to enhance the energy conversion efficiencies of chalcopyrite and related thin-film photovoltaic solar cells. Recently, the effect of potassium (K) has also attracted attention because elemental K has unique effects different from Na as well as a similar beneficial effect in improving device performance. In this study, the control of selective alkali K and Na diffusion into chalcopyrite thin-films from soda-lime glass substrates, which serve as the monolithic alkali source material and contain both K and Na, is demonstrated using ternary CuGaSe2. Elemental K is found to be incorporated in the several ten nanometer thick Cu-deficient region, which is formed on the CuGaSe2 film surface, while Na is ejected, although both K and Na diffuse from the substrate to the CuGaSe2 film surface during growth. The alkali [K]/[Na] concentration ratio in the surface region of CuGaSe2 films strongly depends on the film structure and can be controlled by growth parameters under the same substrate temperature conditions. The results we present here offer new concepts necessary to explore and develop emerging new chalcopyrite and related materials and optimize their applications.