Donghyeop Shin

Earth-Abundant Chalcogenide Photovoltaic Devices with over 5% Efficiency Based on a Cu2BaSn(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.

Additive engineering for high-performance room-temperature-processed perovskite absorbers with micron-size grains and microsecond-range carrier lifetimes

Perovskite photovoltaics have attracted remarkable attention recently due to their exceptional power conversion efficiencies (PCE). State-of-the-art perovskite absorbers typically require thermal annealing steps for high film quality. However, the annealing process adds cost and reduces yield for device fabrication and may also hinder application in tandem photovoltaics and flexible/ultra-low-cost optoelectronics. Herein, we report an additive-based room-temperature process for realizing high-quality methylammonium lead iodide films with micron-sized grains (>2 μm) and microsecond-range carrier lifetimes (τ1 = 931.94 ± 89.43 ns; τ2 = 320.41 ± 43.69 ns). Solar cells employing such films demonstrate 18.22% PCE with improved current–voltage hysteresis and stability without encapsulation. Further, we reveal that room-temperature-processed perovskite film grain size strongly depends on the precursor aggregate size in the film-deposition solution and that additive-based tuning of aggregate properties enables enlarging grains to the micron scale. These results offer a new pathway for more versatile, cost-effective perovskite processing.

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

UNLABELLED 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%.

Na Doping Properties of Cu(In,Ga)Se 2 Absorber Layer Using NaF Interlayer on Mo Substrate

In high-efficiency Cu(In,Ga) solar cells, Na is doped into a Cu(In,Ga) light-absorbing layer from sodalime-glass substrate through Mo back-contact layer, resulting in an increase of device performance. However, this supply of sodium is limited when the process temperature is too low or when a substrate does not supply Na. This limitation can be overcome by supplying Na through external doping. For Na doping, an NaF interlayer was deposited on Mo/glass substrate. A Cu(In,Ga) absorber layer was deposited on the NaF interlayer by a three-stage co-evaporation process As the thickness of NaF interlayer increased, smaller grain sizes were obtained. The resistivity of the NaF-doped CIGS film was of the order of indicating that doping was not very effective. However, highest conversion efficiency of 14.2% was obtained when the NaF thickness was 25 nm, suggesting that Na doping using an NaF interlayer is one of the possible methods for external doping.

Grain-resolved ultrafast photophysics in Cu2BaSnS4−xSex 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.

Annealing and In Interlayer Effects on the Photovoltaic Properties of CBD-In 2 S 3 /CIGS Solar Cells

In this study, chemical bath deposited (CBD) indium sulfide buffer layers were investigated as a possible substitution for the cadmium sulfide buffer layer in CIGS thin film solar cells. The performance of the /CIGS solar cell dramatically improved when the films were annealed at in inert gas after the buffer layer was grown on the CIGS film. The thickness of the indium sulfide buffer layer was 80 nm, but decreased to 60 nm after annealing. From the X-ray photoelectron spectroscopy it was found that the chemical composition of the layer changed to indium oxide and indium sulfide from the as-deposited indium hydroxide and sulfate states. Furthermore, the overall atomic concentration of the oxygen in the buffer layer decreased because deoxidation occurred during annealing. In addition, an In-thin layer was inserted between the indium sulfide buffer and CIGS in order to modify the /CIGS interface. The /CIGS solar cell with the In interlayer showed improved photovoltaic properties in the and FF values. Furthermore, the /CIGS solar cells showed higher quantum efficiency in the short wavelength region. However, the quantum efficiency in the long wavelength region was still poor due to the thick buffer layer.