Position Influence of Sputtered Zn1-xMgxO/Zn1-xMgxO:Al Layers in Flexible and Cd-Free Cu(In,Ga)(S,Se)2 Solar Cells.

Flexible, Cd-free, and all-dry process Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells on stainless steel (SUS) substrates are fabricated, and their structure consists of SUS/glass (SiO2)/Mo/CIGSSe absorber/sputtered Zn0.84Mg0.16O/sputtered Zn1-xMgxO:Al transparent conductive oxide (TCO). The effect of the sample position during the sputtering of Zn0.84Mg0.16O buffer and Zn1-xMgxO:Al TCO layers of the solar cells is examined to avoid intense plasma exposure. The sample position plays a vital role in improving the cell performance. Namely, the sample position close to the material targets of the sputtering system causes severe exposure of the sample to the intense plasma, giving rise to low and nonuniform local external quantum efficiency (EQE) with very weak electroluminescence (EL) imaging, thereby reducing photovoltaic performance. On the other hand, the deviation of the sample position from material targets helps to avoid the intense plasma, thus resulting in high and uniform local EQE with bright EL imaging as well as reducing carrier recombination rates (or carrier lifetimes) throughout the solar cells. Ultimately, the conversion efficiency of flexible, Cd-free, and all-dry process CIGSSe solar cells is enhanced to 16.5% under the optimized sample position deviation from material targets to avoid intense plasma exposure.

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.

Transparent Electrode and Buffer Layer Combination for Reducing Carrier Recombination and Optical Loss Realizing over a 22%-Efficient Cd-Free Alkaline-Treated Cu(In,Ga)(S,Se)2 Solar Cell by the All-Dry Process.

The structures of K or Cs alkaline-treated Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells are developed, and their carrier recombination rates are scrutinized. It is determined that short-circuit current density (JSC) is enhanced (decreased optical loss), when ZnS(O,OH), (Cd,Zn)S, and Zn0.8Mg0.2O buffers with a large band gap energy (Eg) are applied as a replacement of CdS buffer. The JSC is further increased, reducing the optical loss more, when Zn0.9Mg0.1O:B is used as the transparent conductive oxide (TCO) with a larger Eg and lower free carrier absorption than those of ZnO:Al. Furthermore, all carrier recombination rates throughout the devices with K or Cs treatment, especially at the buffer/absorber interface and in the quasi neutral region, are reduced, thereby reducing open-circuit voltage deficit (VOC,def), well consistent with the simulated ones. The carrier recombination rate at the buffer/absorber interface is further decreased, when the CdS and (Cd,Zn)S buffers, deposited by chemical bath deposition, are applied, leading to the greater reduction of the VOC,def and the high conversion efficiency (η) of about 21%. Under the trade-off between VOC,def and optical loss, the highest η of 22.6% is attained with the lowest power loss (or the highest VOC × JSC) in the Cs-treated Cd-free CIGSSe solar cell with an optimized structure of glass/Mo/CIGSSe/Zn0.8Mg0.2O/Zn0.9Mg0.1O:B, fabricated by the all-dry process, where the Zn0.8Mg0.2O buffer is prepared by the sputtering method. This occurs because the JSC is the highest attributable to the larger Eg of Zn0.8Mg0.2O buffer than those of the CdS and (Cd,Zn)S.

Characterization of Cd-Free Zn1- xMg xO:Al/Zn1- xMg xO/Cu(In,Ga)(S,Se)2 Solar Cells Fabricated by an All Dry Process Using Ultraviolet Light Excited Time-Resolved Photoluminescence.

Cd-free Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells with a structure of glass/Mo/CIGSSe/Zn1- xMg xO (buffer)/Zn1- xMg xO:Al (TCO), fabricated by an all dry process, are characterized using ultraviolet light excited time-resolved photoluminescence (UV-TRPL). The impact of bandgap energy ( Eg) values of buffer and transparent conductive oxide (TCO) layers, denoted by Eg of buffer and TCO, is examined. The Eg values of buffer and TCO layers are kept almost similar and varied from 3.30 to 3.94 eV. In this work, UV-TRPL measurement is performed to examine the UV-TRPL carrier lifetimes near the Zn1- xMg xO buffer/CIGSSe interface in the solar cell structure. It is revealed that the UV-TRPL carrier lifetimes near the Zn1- xMg xO buffer/CIGSSe interface in Cd-free solar cells are increased upon enhancing the Eg of buffer and TCO from 3.30 to 3.94 eV, thus increasing the open-circuit voltage and fill factor. Additionally, short-circuit current density is enhanced up to about 38 mA/cm2 owing to the highly transparent Zn1- xMg xO/Zn1- xMg xO:Al layers. Ultimately, an 18.5%-efficient Cd-free solar cell with the Eg of buffer and TCO of 3.94 eV, prepared by an all dry process, is fabricated, which has the same level of 18.3% for the reference solar cell (glass/Mo/CIGSSe/CdS/ZnO/ZnO:Al).