High photo-conversion efficiency in double-graded Cu(In,Ga)(S,Se)₂thin film solar cells with two-step sulfurization post-treatment

Abstract

Sulfur is extensively used to increase the bandgap of Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells and to improve the open circuit voltage (VOC) in order to optimize the characteristics of the devices. This study uses a sulfurization process to obtain a double‐graded bandgap profile. Selenization was carried out on Cu(In,Ga) precursors, followed by one sulfurization process or two consecutive sulfurization processes on top of the CIGSe absorber layer surface. The optimum two‐step sulfurization process provides an increase of VOC of 0.05 V and an improvement of conversion efficiency of 1.17%. The efficiency of the 30 × 30 cm2 monolithic module, which has 64 CIGS cells connected in series (aperture area: 878.6 cm2), is 15.85%. The optical and electrical properties of the phase and the work function distribution were investigated using the depth profiles of the absorber layer as a function of the sulfurization conditions. The CIGSSe thin film formed by two‐step sulfurization with a high sulfur concentration exhibits a single work function peak, better crystallinity, and higher conversion efficiency than those of the thin film formed by two‐step sulfurization at low sulfur concentration. In terms of the Raman spectra depth profile, the phase areas for the CIGSSe thin film that underwent the optimized high sulfur concentration two‐step‐sulfurization appeared to have less of Cu2‐xSe phase than that with low sulfur concentration. Consequently, surface and interface phase analysis is an essential consideration to improve cell efficiency. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.

Toward High-Efficiency Cu(In,Ga)(S,Se)2 Solar Cells by a Simultaneous Selenization and Sulfurization Rapid Thermal Process

“…Thus, the band gap of the CIGSSe absorber fabricated with a higher S concentration increased gradually. A previous study reported that certain S treatment can improve the band gap and surface uniformity as well as the V oc of the devices . In addition, the performance of the CIGSSe solar cells can be improved by introducing S in the absorber layer using a sulfurization process with an increased S concentration at the top and rear side of the absorber layer .…”

“…The challenges relate with the increased thermal activity of selenium vapor under high temperatures; therefore, it tends to escape the reaction system during crystallization in the rapid annealing process, resulting in the nonuniform spatial distribution of selenium in the atmosphere . In addition to selenium, sulfur will be supplied appropriately during or after the selenization process to produce CIGSSe thin films, which can improve the V oc and performance of the device. , Replacing selenium with sulfur of CIGSSe absorbers, both surface and grain boundary defects can be effectively passivated. − …”

“…Typically, there are three methods to provide Se to react for the selenization/sulfurization process to fabricate a CIGS absorber layer. The first method is using hydrogen selenide (H 2 Se) as a selenium source during the high-temperature selenization process to react with the CuGa/In precursor, forming a CIGSe absorber. − H 2 Se is reported to fabricate high-efficiency CIGSe solar cells by improving the crystallinity of the absorber layer and reducing the grain boundary recombination. However, the H 2 Se gas is highly toxic, expensive, and corrosive, thus imposing strict requirements for selenization equipment.…”

Carbonized Bark by Laser Treatment for Efficient Solar-Driven Interface Evaporation

“…The carbonized bark greatly meets the requirement of mechanical stability in practical application of solar vapor generation (Figure S5). It can be the key to keep the carbonized bark moving along the water surface persistently and efficiently due to the excellent mechanical flexibility and the self-floating capability . (3) The hydrophilic ability is another important factor to ensure efficient water transport.…”

“…Solar-driven evaporation is a green, efficient, and low-cost technology to obtain pure water, enable electricity generation, and treat the wastewater by water desalination based on converting solar light into heat energy. − The interfacial evaporation is considered as an ideal approach to treat water through localizing heat at the liquid–air interface rather than all body of liquid. ,− The efficiency of solar-driven steam generation depends on various factors including broadening solar absorption, managing localized thermal, replenishing rapidly water, and transporting vapor. − Currently, the double-layer structure, consisting of upper photothermal conversion layer and bottom insulator layer, is known as an ideal design for solar-driven steam generation, which can efficiently inhibit the heat transferring to the bulk water. − …”