Jihye Gwak

Determination of band gap energy (Eg) of Cu2ZnSnSe4 thin films: On the discrepancies of reported band gap values

We demonstrate experimental data to elucidate the reason for the discrepancies of reported band gap energy (Eg) of Cu2ZnSnSe4 (CZTSe) thin films, i.e., 1.0 or 1.5 eV. Eg of the coevaporated CZTSe film synthesized at substrate temperature (Tsub) of 370 °C, which was apparently phase pure CZTSe confirmed by x-ray diffraction (XRD) and Raman spectroscopy, is found to be around 1 eV regardless of the measurement techniques. However, depth profile of the same sample reveals the formation of ZnSe at CZTSe/Mo interface. On the other hand, Eg of the coevaporated films increases with Tsub due to the ZnSe formation, from which we suggest that the existence of ZnSe, which is hardly distinguishable from CZTSe by XRD, is the possible reason for the overestimation of overall Eg.

CuInSe2 (CIS) Thin Films Prepared from Amorphous Cu–In–Se Nanoparticle Precursors for Solar Cell Application

CuInSe(2) (CIS) absorber layers for thin film solar cells were formed via a nonvacuum route using nanoparticle precursors. A low-temperature colloidal process was used to prepare nanoparticles by which amorphous Cu-In-Se nanoparticles were formed within 1 min of reaction without any external heating. Raman spectra of the particles revealed that they were presumably mixtures of amorphous Cu-Se and In-Se binaries. Selenization of the precursor film prepared by doctor blade coating of the Cu-In-Se nanoparticles resulted in a facile growth of the particles up to micrometer scale. However, it also left large voids in the final film, which acted as short circuiting paths in completed solar cells. To solve this problem, we applied a solution-filling treatment in which a solution containing Cu and In ions was additionally coated onto the precoated nanoparticles, resulting in a complete infiltration of the filler solution into the pores in the nanoparticles based film. By this approach, short circuiting of the device was significantly mitigated and a conversion efficiency of up to 1.98% was obtained.

A hybrid ink of binary copper sulfide nanoparticles and indium precursor solution for a dense CuInSe2 absorber thin film and its photovoltaic performance

A newly developed hybrid ink of binary CuS nanoparticles and In precursor solution was prepared to form a CuInSe2 (CIS) thin film. Previously, we have shown a CIS thin film solar cell with 4.19% conversion efficiency using the hybrid ink of Cu2−xSe nanoparticles and In precursor. Deposition using hybrid ink offers advantages including the provision of stress-relief and crack-deflection centers by pure material based nanoparticles and effective binding with the nanoparticles by precursor solutions without other organic binders. Here, we demonstrate volume expansion of a thin film for forming a well-grown absorber layer using CuS nanoparticles instead of Cu2−xSe. Binary nanoparticles were synthesized by a low temperature colloidal process and a precursor solution was prepared by using a non-toxic chelating agent to disperse the In component stably. The band gap of the CIS thin film was 1.08 eV, as determined by external quantum efficiency (EQE) measurements, and the reproducible conversion efficiency of the fabricated device was 6.23%.

Enhanced exciton separation through negative energy band bending at grain boundaries of Cu2ZnSnSe4 thin-films

Local surface potential of Cu2ZnSnSe4 thin-films was investigated by Kelvin probe force microscopy. The surface potential profile across grain boundaries (GBs) shows a rise of 200–600 meV at GBs in a Cu-poor and Zn-poor film with 3.8% efficiency, which means positively charged GBs. In contrast, the GBs in a Cu-poor and Zn-rich film with 2% efficiency exhibit lowering of surface potential by 40 meV. The results indicate that GBs of Cu2ZnSnSe4 films play a role for exciton separation and governing defects for high efficiency could be not only CuZn but also VCu as explained theoretical predictions.

CuInSe2 Thin-Film Solar Cells with 7.72 % Efficiency Prepared via Direct Coating of a Metal Salts/Alcohol-Based Precursor Solution

A simple direct solution coating process for forming CuInSe₂ (CIS) thin films was described, employing a low-cost and environmentally friendly precursor solution. The precursor solution was prepared by mixing metal acetates, ethanol, and ethanolamine. The facile formation of a precursor solution without the need to prefabricate nanoparticles enables a rapid and easy processing, and the high stability of the solution in air further ensures the precursor preparation and the film deposition in ambient conditions without a glove box. The thin film solar cell fabricated with the absorber film prepared by this route showed an initial conversion efficiency of as high as 7.72 %.

Comprehensive review on material requirements, present status, and future prospects for building-integrated semitransparent photovoltaics (BISTPV)

Photovoltaics (PV) is a potential alternative to the worlds quickly depleting fossil fuel and can be a mean of curtailing green-house gas emissions as well. In regions in which there is a shortage of free land and no fossil fuel reserves, building-integrated photovoltaics with an additional attribute of semitransparency (BISTPV) can help to realize sustainable economic growth. The net electricity savings in dwellings and commercial edifices can be substantially enhanced by using BISTPV shrewdly. In addition to building applications, semitransparent (ST) solar cells can be used in yacht hatches, signboards on express-ways, automobiles, electronic displays, and other applications. It is anticipated that in the foreseeable future, dye-sensitized solar cells (DSSCs), amorphous silicon (a-Si:H) solar cells, and chalcopyrite solar cells based BISTPV modules will replace aesthetically repulsive c-Si-based ST modules, which are currently the key player in the BIPV market. DSSCs are superior to a-Si:H and chalcopyrite-based PV in terms of the appropriate choice of dyes, for which wavelengths that have a low eye sensitivity factor can be selectively harnessed. The transparency, or ability to see-through, can be further increased by making use of highly transparent photoanodes and counter electrodes. However, the stability issues that are associated with DSSCs have halted their propagation to the BIPV market. In contrast, ST a-Si:H solar cells have been well developed on a laboratory scale, and ST panels made by various companies are available on the market. Apart from ST DSSCs and a-Si:H solar cells, which have evolved considerably, chalcopyrite STPV has only recently started the journey of its development on a laboratory scale although it has a large potential for BISTPV. Therefore, considerable efforts must be made for it to emerge in the BIPV market. This article will present a detailed review of the material requirements for fabricating ST dye-sensitized-, amorphous silicon (a-Si:H)-, and chalcopyrite-based solar cells, especially for BIPV applications. Furthermore, the present status and future prospects of semitransparent photovoltaics (STPV) and strategies to enhance photocurrent in STPV will also be discussed.

Electrostatic Spray Deposition of Copper–Indium Thin Films

Electrostatic spray deposition is an innovative coating technique that produces fine, uniform, self-dispersive (due to Coulombic repulsion), and highly wettable, atomized droplets. Copper–indium salts are dissolved in an alcohol-based solvent; this precursor is then electrostatically sprayed onto a moderately heated, molybdenum-coated substrate. Precursor flowrates range from 0.02 to 5 mL/h under applied voltages of 1–18 kV, yielding droplet sizes around a few hundred nanometers. Comparing scanning electron microscope images of the coated samples showed that the substrate temperature, applied voltage, and precursor flowrate were the primary parameters controlling coating quality. Also, the most stable electrostatic spray mode that reliably produced uniform and fine droplets was the cone-jet mode with a Taylor cone issuing from the nozzle.

Sputtering processed highly efficient Cu2ZnSn(S,Se)4 solar cells by a low-cost, simple, environmentally friendly, and up-scalable strategy

Earth abundant copper–zinc–tin chalcogenide (CZTSSe) is an emerging material for the development of low cost and sustainable thin film solar cells (TFSCs). A low cost, green, and up-scalable approach to the fabrication of TFSCs through a sputtering process is the main challenge to achieve high efficiency for Cu2ZnSn(S,Se)4 (CZTSSe) solar cells for industrialization. Based on a closed isothermal chamber annealing system, we could precisely calculate and control the chalcogenide partial vapour pressure during the annealing process. We designed, developed, and optimized an environmentally friendly strategy to synthesise a high quality CZTSSe absorber thin film and to fabricate a solar cell without using toxic H2Se and H2S gases as the Se and S sources or any other volatile compounds (SnS and Sn), and a chalcogenide diffusion barrier layer. We fabricated a CZTSSe TFSC with 9.24% efficiency, which is the highest performance for sputtering processed CZTSSe TFSC prepared without using toxic gases and additional processes. Based on this green strategy, we also fabricated the integrated submodule using CZTSSe absorber layers with efficiencies as high as η = 2.76% with eight interconnected cells (active area of 22.4 cm2). Our studies on this green synthesis strategy for CZTSSe solar cells could introduce a possible pathway to green fabrication for the low cost and highly efficient TFSC industrialization field.

Role of chelate complexes in densification of CuInSe2 (CIS) thin film prepared from amorphous Cu–In–Se nanoparticle precursors

CuInSe2 (CIS) absorber layers for thin film solar cells were fabricated via a non-vacuum route using amorphous Cu–In–Se nanoparticle precursors prepared by a low temperature colloidal process within one minute of reaction without any external heating. In particular, we intentionally added a chelating agent to the nanoparticle colloid in order to increase the density of the final films by enhancing the viscous flow of precursor materials during high temperature selenization. This is based on the decreased reactivity of precursor particles due to the formation of chelate complexes at particle surfaces. While the CIS films formed from the amorphous Cu–In–Se particles without surface modification were found to have large voids, those formed from surface modified particles showed flat and dense morphologies. In accordance with the improvements in the film morphology and density, efficiencies of the devices were also significantly increased from 0% (complete short circuit in the case without surface modification) to 4.41% (with surface modification).

Carbon- and Oxygen-Free Cu(InGa)(SSe)2 Solar Cell with a 4.63% Conversion Efficiency by Electrostatic Spray Deposition

We have demonstrated the first example of carbon- and oxygen-free Cu(In,Ga)(SSe)2 (CIGSSe) absorber layers prepared by electrospraying a CuInGa (CIG) precursor followed by annealing, sulfurization, and selenization at elevated temperature. X-ray diffraction and scanning electron microscopy showed that the amorphous as-deposited (CIG) precursor film was converted into polycrystalline CIGSSe with a flat-grained morphology after post-treatment. The optimal post-treatment temperature was 300 °C for annealing and 500 °C for both sulfurization and selenization, with a ramp rate of 5 °C/min. The carbon impurities in the precursor film were removed by air annealing, and oxide that was formed during annealing was removed by sulfurization. The fabricated CIGSSe solar cell showed a conversion efficiency of 4.63% for a 0.44 cm(2) area, with Voc = 0.4 V, Jsc = 21 mA/cm(2), and FF = 0.53.