Scalable CIGS Solar Cells Employing a New Device Design of Nontoxic Buffer Layer and Microgrid Electrode.

The efficiency of copper indium gallium selenide (CIGS) solar cells that use transparent conductive oxide (TCO) as the top electrode decreases significantly as the device area increases owing to the poor electrical properties of TCO. Therefore, high-efficiency, large-area CIGS solar cells require the development of a novel top electrode with high transmittance and conductivity. In this study, a microgrid/TCO hybrid electrode is designed to minimize the optical and resistive losses that may occur in the top electrode of a CIGS solar cell. In addition, the buffer layer of the CIGS solar cells is changed from the conventional CdS buffer to a dry-processed wide-band gap ZnMgO (ZMO) buffer, resulting in increased device efficiency by minimizing parasitic absorption in the short-wavelength region. By optimizing the combination of ZMO buffer and the microgrid/TCO hybrid electrode, a device efficiency of up to 20.5% (with antireflection layers) is achieved over a small device area of 5 mm × 5 mm (total area). Moreover, CIGS solar cells with an increased device area of up to 20 mm × 70 mm (total area) exhibit an efficiency of up to 19.7% (with antireflection layers) when a microgrid/TCO hybrid electrode is applied. Thus, this study demonstrates the potential for high-efficiency, large-area CIGS solar cells with novel microgrid electrodes.

Atom-Scale Chemistry in Chalcopyrite-Based Photovoltaic Materials Visualized by Atom Probe Tomography.

Chalcopyrite-based materials for photovoltaic devices tend to exhibit complex structural imperfections originating from their polycrystalline nature; nevertheless, properly controlled devices are surprisingly irrelevant to them in terms of resulting device performances. The present work uses atom probe tomography to characterize co-evaporated high-quality Cu(In,Ga)Se2 (CIGS) films on flexible polyimide substrates either with or without doping with Na or doping with Na followed by K via a post-deposition treatment. The intent is to elucidate the unique characteristics of the grain boundaries (GBs) in CIGS, in particular the correlations/anti-correlations between matrix elements and the alkali dopants. Various compositional fluctuations are identified at GBs irrespective of the presence of alkali elements. However, [Cu-poor and Se/In,Ga-rich] GBs are significantly more common than [Cu-rich and Se/In,Ga-poor] ones. In addition, the anti-correlations between Cu and the other matrix elements are found to show not only regular trends among themselves but also the association with the degree of alkali segregation at GBs. The Na and K concentrations exhibited a correlation at the GBs but not in the intragrain regions. Density functional theory calculations are used to explain the compositional fluctuations and alkali segregation at the GBs. Our experimental and theoretical findings not only reveal the benign or beneficial characteristics of the GBs of CIGS but also provide a fundamental understanding of the GB chemistry in CIGS-based materials.

Grain-Resolved Ultrafast Photophysics in Cu2BaSnS4- xSe x Semiconductors Using Pump-Probe Diffuse Reflectance Spectroscopy and Microscopy.

In this paper, we analyze fundamental photoexcitation processes and charge carrier kinetics in Cu2BaSnS4- xSe x (CBTSSe), a recently introduced alternative to Cu(In,Ga)(S,Se)2 and Cu2ZnSnS4- xSe x (CZTSSe) photovoltaic/photoelectrochemical absorbers, using advanced laser spectroscopy and microscopy techniques. The broadband pump-probe diffuse reflectance spectroscopy technique facilitates monitoring the ultrafast processes in opaque CBTSSe films deposited on Mo-coated glass substrates, similar to the configuration found in functional devices. We spectrally resolve a sharp ground-state bleaching (GSB) peak for CBTSSe films, formed around the band edge transition, which is spectrally narrower than the GSB and stimulated emission in corresponding CZTSSe films. The presence of sharp electronic transitions is further deduced from the ensemble pump-probe spectroscopy and steady-state UV-vis diffuse reflectance spectra. Furthermore, using pump-probe diffuse reflectance scanning microscopy, we monitor the charge carrier formation and excited state pattern within the film grains at few hundred nanometer resolution and localize the kinetics of photogenerated carriers in each grain. The unique sensitivity of pump-probe microscopy and sharp electronic transitions allow for detection of small S/Se stoichiometry variations, Δ x ≤ 0.3, in CBTSSe grains-i.e., features that are largely unresolved for ensemble spectroscopy or luminescence measurements. By noting the sharp band edge transition, we show that the band tailing issue (prevalent for CZTSSe) is largely resolved for CBTSSe; however, other issues may remain, such as deep defects and fast carriers relaxations, which may still impact the photocurrent and open circuit voltage of the CBTSSe devices/films examined.

Earth-Abundant Chalcogenide Photovoltaic Devices with over 5% Efficiency Based on a Cu2 BaSn(S,Se)4 Absorber.

In recent years, Cu2 ZnSn(S,Se)4 (CZTSSe) materials have enabled important progress in associated thin-film photovoltaic (PV) technology, while avoiding scarce and/or toxic metals; however, cationic disorder and associated band tailing fundamentally limit device performance. Cu2 BaSnS4 (CBTS) has recently been proposed as a prospective alternative large bandgap (~2 eV), environmentally friendly PV material, with ~2% power conversion efficiency (PCE) already demonstrated in corresponding devices. In this study, a two-step process (i.e., precursor sputter deposition followed by successive sulfurization/selenization) yields high-quality nominally pinhole-free films with large (>1 µm) grains of selenium-incorporated (x = 3) Cu2 BaSnS4-x Sex (CBTSSe) for high-efficiency PV devices. By incorporating Se in the sulfide film, absorber layers with 1.55 eV bandgap, ideal for single-junction PV, have been achieved within the CBTSSe trigonal structural family. The abrupt transition in quantum efficiency data for wavelengths above the absorption edge, coupled with a strong sharp photoluminescence feature, confirms the relative absence of band tailing in CBTSSe compared to CZTSSe. For the first time, by combining bandgap tuning with an air-annealing step, a CBTSSe-based PV device with 5.2% PCE (total area 0.425 cm2 ) is reported, >2.5× better than the previous champion pure sulfide device. These results suggest substantial promise for the emerging Se-rich Cu2 BaSnS4-x Sex family for high-efficiency and earth-abundant PV.

Synthesis and Characterization of an Earth-Abundant Cu2BaSn(S,Se)4 Chalcogenide for Photoelectrochemical Cell Application.

Cu2BaSnS4-xSex films consisting of earth-abundant metals have been examined for photocathode application. Films with different Se contents (i.e., Cu2BaSnS4-xSex with x ≤ 2.4) were synthesized using a cosputter system with post-deposition sulfurization/selenization annealing treatments. Each film adopts a trigonal P31 crystal structure, with progressively larger lattice constants and with band gaps shifting from 2.0 to 1.6 eV, as more Se substitutes for S in the parent compound Cu2BaSnS4. Given the suitable bandgap and earth-abundant elements, the Cu2BaSnS4-xSex films were studied as prospective photocathodes for water splitting. Greater than 6 mA/cm2 was obtained under illumination at -0.4 V versus reversible hydrogen electrode for Pt/Cu2BaSnS4-xSex films with ∼60% Se content (i.e., x = 2.4), whereas a bare Cu2BaSnS4-xSex (x = 2.4) film yielded ∼3 mA/cm2 at -0.4 V/RHE.

Solution-Processed Ag Nanowires + PEDOT:PSS Hybrid Electrode for Cu(In,Ga)Se2 Thin-Film Solar Cells.

To reduce the cost of the Cu(In,Ga)Se2 (CIGS) solar cells while maximizing the efficiency, we report the use of an Ag nanowires (NWs) + poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) ( PEDOT: PSS) hybrid transparent electrode, which was deposited using all-solution-processed, low-cost, scalable methods. This is the first demonstration of an Ag NWs + PEDOT: PSS transparent electrode applied to CIGS solar cells. The spin-coated 10-nm-thick PEDOT: PSS conducting polymer layer in our hybrid electrode functioned as a filler of empty space of an electrostatically sprayed Ag NW network. Coating of PEDOT: PSS on the Ag NW network resulted in an increase in the short-circuit current from 15.4 to 26.5 mA/cm(2), but the open-circuit voltage and shunt resistance still needed to be improved. The limited open-circuit voltage was found to be due to interfacial recombination that is due to the ineffective hole-blocking ability of the CdS film. To suppress the interfacial recombination between Ag NWs and the CdS film, a Zn(S,O,OH) film was introduced as a hole-blocking layer between the CdS film and Ag NW network. The open-circuit voltage of the cell sharply improved from 0.35 to 0.6 V, which resulted in the best cell efficiency of 11.6%.