SeJin Ahn

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.

An 8.2% efficient solution-processed CuInSe2 solar cell based on multiphase CuInSe2 nanoparticles

Multiphase CuInSe2 (CISe) nanoparticles including the CuSe phase are synthesized by the microwave-assisted solvothermal method. Without additional processing, multiphase CISe nanoparticles facilitate the solution-processed CISe absorber layer with a dense microstructure, large grains, high crystallinity, and composition controllability, which are essential for acceptable thin-film solar cell performance. The high performance, solution-processed CISe solar cell, with a conversion efficiency of 8.2%, is obtained through phase transformation, microstructural evolution, and composition adjustment by selenization (annealing under a Se atmosphere) at 530 °C.

Effects of selenization conditions on densification of Cu(In,Ga)Se2 (CIGS) thin films prepared by spray deposition of CIGS nanoparticles

Spray deposited porous CIGS nanoparticle-derivedthin films were selenized in a two zone rapid thermal annealing furnace and effects of various selenization parameters including Se evaporation temperature, flow rate of carrier gas, and substrate temperature on densification of the CIGS layers were investigated. It was found that higher Se supply to CIGS nanoparticles either by increasing Se evaporation temperature or by increasing the flow rate of carrier gas resulted in larger CIGS grains with higher degree of crystallinity, while it also induced formation of a thicker MoSe 2 layer in-between CIGS and Mo which resulted in partial detachment of CIGS / MoSe 2 / Mo layers from the glass substrate. Densification of CIGS layer by growth of nanoparticles and formation of thick MoSe 2 were explained by a liquid Se assisted reaction rather than by a vapor phase Se assisted reaction.

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.

Enhanced Photoelectrochemical Solar Water Splitting Using a Platinum-Decorated CIGS/CdS/ZnO Photocathode.

A Cu(InGa)Se2 film was modified with CdS/ZnO for application to solar water splitting. Platinum was electrodeposited on the ZnO layer as a hydrogen evolution catalyst. The effects of the electroplating time and acidity level of the electrolyte on the photocurrent density were studied. The highest photocurrent density of -32.5 mA/cm(2) under 1.5 AM illumination was achieved with an electroplating time of 30 min at a pH of 9. This photocurrent density is higher than those reported in previous studies. The markedly high performance of the CIGS/CdS/ZnO photocathode was rationalized in terms of its type II cascade structure that facilitated efficient charge separation at the interface junction.

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 %.

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.

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.