Morphological–Electrical Property Relation in Cu(In,Ga)(S,Se)₂ Solar Cells: Significance of Crystal Grain Growth and Band Grading by Potassium Treatment

Abstract

Solution‐processed Cu(In,Ga)(S,Se)2 (CIGS) has a great potential for the production of large‐area photovoltaic devices at low cost. However, CIGS solar cells processed from solution exhibit relatively lower performance compared to vacuum‐processed devices because of a lack of proper composition distribution, which is mainly instigated by the limited Se uptake during chalcogenization. In this work, a unique potassium treatment method is utilized to improve the selenium uptake judiciously, enhancing grain sizes and forming a wider bandgap minimum region. Careful engineering of the bandgap grading structure also results in an enlarged space charge region, which is favorable for electron–hole separation and efficient charge carrier collection. Besides, this device processing approach has led to a linearly increasing electron diffusion length and carrier lifetime with increasing the grain size of the CIGS film, which is a critical achievement for enhancing photocurrent yield. Overall, 15% of power conversion efficiency is achieved in solar cells processed from environmentally benign solutions. This approach offers critical insights for precise device design and processing rules for solution‐processed CIGS solar cells.

Solution-processed near-infrared Cu(In,Ga)(S,Se)2 photodetectors with enhanced chalcopyrite crystallization and bandgap grading structure via potassium incorporation

“…The (112) diffraction peak of K-solCIGS film can be deconvoluted with three peaks located at 1.900, 1.923, and 1.963 Å −1 . These peak positions are slightly lower than those of solCIGS films (1.904, 1.936, and 1.978 Å −1 , respectively), suggesting improved selenization in the K-solCIGS film 14 . Furthermore, the peak width (FWHM: full-width at half maximum) of the lowest angle peak (Peak 1) of K-solCIGS and solCIGS film is 0.0297 and 0.0324 Å −1 , respectively, where the apparent crystal size estimated by Scherrer equation 25 corresponds to 19.0 and 17.4 nm, respectively.…”

“…The K-incorporation process in the CIGS photodetector fabrication is schematically shown in Supplementary Fig. S1 14 . The Cu-In-Ga (CIG) precursor and potassium fluoride (KF) solution were formed by dissolving copper nitrate hydrate (Cu(NO 3 ) 2 ·xH 2 O), indium nitrate hydrate (In(NO 3 ) 3 ·xH 2 O), and gallium nitrate hydrate (Ga(NO 3 ) 3 ·xH 2 O), and KF in methanol, respectively.…”

“…S2 , in which the applied incidence angle was much higher than the critical angle of CIGS film (α c ≈ 0.13° at 18.986 keV) in order to obtain crystallographic information about the total thickness of the film. The characteristic diffraction peaks observed near 1.90, 3.10, and 3.65 Å −1 , corresponding to the (112), (220), and (312) plane, respectively, were identified as the chalcopyrite CIGS phases 12 , 14 , 24 . The circular averaged 1D profiles obtained from 2D GI-WAXS patterns at α i = 0.7°, especially enlarged around (112) diffraction peak, are shown in Fig.…”

“…This improved carrier mobility is attributed to the enhanced crystal and grain size in K-solCIGS by K-incorporation. The larger grains with reduced grain boundaries ensure the existence of percolation paths because the boundary and gaps suppress carrier transport and act as recombination centers 14 . In addition, the wide notch region and mitigated notch depth in the K-solCIGS bandgap grading structure also contributes to a decrease in the electron accumulation in the E g,min region, and thus leads to a decreasing charge carrier recombination and increasing charge transport and collection 20 , 21 .…”

“…However, the CIGS-based heterojunction films were fabricated using the co-evaporation method in vacuum-based equipment, which incurred a high production cost. Recently, considerable progress has been made in the development of multiple precursor coatings and post-chalcogenization processes based on benign solvents, providing manufacturability 12 – 14 . In addition, the power conversion efficiency of solution-processed CIGS photovoltaic devices has reached over 15.0% 13 – 18 .…”