High Efficiency Aqueous Solution Sprayed CIGSSe Solar Cells: Effects of Zr⁴⁺‐Alloyed In₂S₃ Buffer and K‐Alloyed CIGSSe Absorber

Abstract

Even though solution‐processed Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells have been developed for long time, their power conversion efficiency (PCE) is still much behind that achieved by vacuum‐based process, and some solvents used in solution‐based processes are not environmental friendly. High‐efficiency CIGSSe solar cells made by using water‐based spray deposition in an air environment are reported. Water is the most eco‐friendly and cheapest solvent compared to the other solvents used in solution‐based fabrication. In2S3‐based buffer (eco‐friendly Cd‐free buffer) and CdS buffer are tested as a buffer layer. For higher PCE with In2S3‐based buffer, a Zr4+‐alloyed In2S3 is used and Zr4+ alloying effects are investigated. It is found that Zr4+ alloying increases the electron concentration in the buffer and passivates defect states in the CIGSSe absorber, which enhance PCE significantly. On the other hand, it is found that K‐alloying into the CIGSSe absorber passivates defects in the CIGSSe absorber, which further increases PCE. Owing to the observed beneficial effects, highest PCE of 16.08% and 17.74% is achieved with Zr4+‐In2S3 buffer and CdS buffer, respectively. In addition, since fabricated solar cells are made with bandgap below 1.15 eV, they can be utilized as a bottom cell for high efficiency tandem cell.

Efficiency Improvement of Narrow Band Gap Cu(In,Ga)(S,Se)2 Solar Cell with Alkali Treatment via Aqueous Spray Pyrolysis Deposition

“…Comparing the surface and cross-sectional images, Rb-CIGSSe and Cs-CIGSSe absorbers show increased grain size, which also suggests that Rb-PDT and Cs-PDT enhance the crystallization as observed in XRD and Raman results. For the chalcogenide absorbers, alkali inclusion is known to increase the grain size during the crystallization process. , However, there are few in-depth studies on the role of alkali inclusion. The Tiwari group has proposed that the Na 2 Se x liquid phase induces grain growth by incorporating Na into the kesterite Cu 2 ZnSnSe 4 surface with Se .…”

“…This trend was also observed in the potential fluctuation analysis, as seen in Figure h, where the values were calculated to be 23.3, 21.7, and 22.1 meV, respectively. Although the decrease was small, the alkali treatment resulted in a slight reduction in both Urbach energy and potential fluctuation, which could improve device performance …”

“…The full device structure was then completed with the deposition of i-ZnO (∼50 nm) and tin-doped indium oxide (ITO) (∼300 nm) layers by radio frequency (RF) magnetron sputtering. Detailed information on the buffer and window layer fabrication processes can be found elsewhere. ,,− No additional antireflection coatings were applied to the fabricated devices. For measurement of device performance, we mechanically scribed the cell with an area of 0.20 cm 2 .…”

“…As the vacuum process requires expensive vacuum facilities, low-cost nonvacuum processes have been developed. Several nonvacuum processes such as electrodeposition, spin-coating with molecular ink solution and nanoparticle ink, , and spray pyrolysis , have been developed for the fabrication of CIGSe absorber. Comparing these techniques, spray pyrolysis is considered as the most promising alternative technique to vacuum-based deposition, which can yield a high throughput combined with roll-to-roll fabrication in an air environment.…”

“…In addition, alkali doping/alloying is easily achievable by spraying an alkali-dissolved aqueous solution. 16,17 For the narrow band gap solar cells (E g = ∼1.1 eV) on a flexible SUS substrate, spraying Na/K codissolved Cu-In-Ga-S precursor solution in an air environment (in situ alkali doping/alloying) provides a PCE of 11.45%. 17 In this work, we introduce a newly developed aqueous spray deposition for the fabrication of narrow band gap CIGSSe solar cells.…”