Ara Cho

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%.

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 %.

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).

Cu(In,Ga)Se2 thin films without Ga segregation prepared by the single-step selenization of sputter deposited Cu-In-Ga-Se precursor layers

We report a new approach to fabricating Cu(In,Ga)Se2 (CIGSe) light absorbing layers for thin film solar cells without Ga segregation using a sputtering and single-step selenization process. To mitigate Ga segregation at the CIGSe/back-contact region, which has frequently been observed in the selenization of metal/alloy precursor layers, we used Se-containing precursor layers (Cu-In-Ga-Se) to capture Ga in covalently bonded structures and investigated the effects of Se content in the precursor layers on the properties of the selenized CIGSe films and the devices. As the Se content in the precursor layer increased, Ga segregation was significantly mitigated, resulting in a completely homogenized Ga distribution when the Se/metal ratio of the precursor films is over 0.8. Finally, a thin CIGSe film (∼670 nm) with a uniform Ga distribution was processed to fabricate a solar cell, and the device exhibited a conversion efficiency of 11.7% with an open circuit potential of 0.6 V. An increase of the CIGSe film thickness to 1.55 μm resulted in a device efficiency of up to 13.16%.

Amorphous Cu–In–S Nanoparticles as Precursors for CuInSe2 Thin‐Film Solar Cells with a High Efficiency

CuInSe2 (CISe) absorber layers for thin-film solar cells were fabricated through the selenization of amorphous Cu-In-S nanoparticles, which were prepared by using a low-temperature colloidal process within one minute without any external heating. Two strategies for obtaining highly dense CISe absorber films were used in this work; the first was the modification of nanoparticle surface through chelate complexation with ethanolamine, and the second strategy utilized the lattice expansion that occurred when S atoms in the precursor particles were replaced with Se during selenization. The synergy of these two strategies allowed formation of highly dense CISe thin films, and devices fabricated using the absorber layer demonstrated efficiencies of up to 7.94% under AM 1.5G illumination without an anti-reflection coating.

Carbon-Impurity Affected Depth Elemental Distribution in Solution-Processed Inorganic Thin Films for Solar Cell Application

A common feature of the inorganic thin films including Cu(In,Ga)(S,Se)2 fabricated by nonvacuum solution-based approaches is the doubled-layered structure, with a top dense inorganic film and a bottom carbon-containing residual layer. Although the latter has been considered to be the main efficiency limiting factor, (as a source of high series resistance), the exact influence of this layer is still not clear, and contradictory views are present. In this study, using a CISe as a model system, we report experimental evidence indicating that the carbon residual layer itself is electrically benign to the device performance. Conversely, carbon was found to play a significant role in determining the depth elemental distribution of final film, in which carbon selectively hinders the diffusion of Cu during selenization, resulting in significantly Cu-deficient top CISe layer while improving the film morphology. This carbon-affected compositional and morphological impact on the top CISe films is a determining factor for the device efficiency, which was supported by the finding that CISe solar cells processed from the precursor film containing intermediate amount of carbon demonstrated high efficiencies of up to 9.15% whereas the performances of the devices prepared from the precursor films with very high and very low carbon were notably poor.

Development of semitransparent CIGS thin-film solar cells modified with a sulfurized-AgGa layer for building applications

Different from conventional photovoltaics, building-integrated photovoltaics needs not only high performance but also a high degree of transparency. Nevertheless, the Cu(In,Ga)Se2 (CIGS) solar cell has advantages in terms of the highest conversion efficiency and stability among all thin-film-based solar cells. The semitransparent (ST) CIGS solar cell using an ultrathin CIGS absorber on a transparent conducting oxide (TCO) experiences loss in fill factor and open circuit voltage due to the poor grain morphology and CIGS/TCO interface. Thus, these issues must be addressed to realize ST CIGS solar cells. Wide-bandgap (1.5 eV) submicron CIGS-based solar cells were prepared in this study unlike conventional CIGS with a bandgap of 1.2 eV, in order to enhance see-throughness. But such cells demonstrated low conversion efficiency due to the poor grain morphology and absence of back grading. Therefore, for the sake of improving grain morphology and to create back grading, a sulfurized-AgGa (AGS) layer was deposited between CIGS and the indium tin oxide (ITO) back contact. Ag from the AGS layer diffused throughout the absorber and thus ameliorated the grain morphology. However, Ga and S in the AGS layer remained confined near the back contact, therefore resulting in the creation of back grading. Consequently, a solar cell based on 230 nm thick CIGS modified with a 45 nm thick AGS layer exhibited efficiency of 5.94% with averaged visible transmittance over 25%. This is the highest reported efficiency for a ST CIGS solar cell with over 20% visible transparency. The CIGS solar cell based on this novel approach can be a competent candidate for building-integrated semitransparent photovoltaics applications.

Iron pyrite thin films deposited via non-vacuum direct coating of iron-salt/ethanol-based precursor solutions

We report a new approach for fabricating pyrite (cubic FeS2) thin films via a non-vacuum direct solution coating route using iron-acetate dissolved in ethanol as a precursor solution. The precursor ink is deposited by spin coating and annealed in air at 300 °C to produce amorphous iron oxide films. Subsequent heat treatment of the iron oxide films in a sulfur environment at 450 °C results in the formation of phase-pure, carbon-free, large-grained pyrite films. In particular, the phase evolution during sulfurization is systematically investigated, focusing on the effects of the principal experimental variables such as temperature and pressure. As the temperature increases, iron oxide first begins to transform into marcasite (orthorhombic FeS2) and then is completely converted to pyrite. Further increasing the temperature above 500 °C induces the decomposition of pyrite into pyrrhotite (Fe1−xS) and sulfur vapor. The decomposition is also strongly dependent on the partial pressure of sulfur: as the pressure decreases, the decomposition occurs at lower temperature. The synthesized single-phase pyrite films show an indirect band gap of 0.94 eV, a strong anodic photocurrent based on photo-electrochemical measurements, and n-type semiconducting properties based on Mott–Schottky analysis. Thus, the films demonstrate great potential for use as absorbing layers in solar cells.