Keeshik Shin

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.

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.

Development of flexible silicon thin-film solar cells on nanotextured Ag/Al:Si bilayers

Highly textured Ag/Al:Si back reflectors were used to improve the short-circuit current density of flexible nanocrystalline silicon thin-film solar cells by means of enhanced light trapping. A nanotextured topography with a root-mean-square (σrms) surface roughness of 60.1 nm was induced at low substrate temperature of 100°C by abnormal grain growth of dc-sputtered Al:Si films. After depositing highly reflective Ag films on nanotextured Al:Si films, effective light scattering of the long wavelength over 500 nm was achieved, resulting in enhanced absorption of weakly absorbing, long-wavelength light in the solar cells. The effects of nanotextured surfaces of the bilayers on the light scattering properties and on the resultant cell performance were investigated.