Utilizing Solvent Repulsion between Dimethylformamide and Isopropanol to Manipulate Sn Distribution for Bifacial...

Area of Science:

• Materials Science
• Renewable Energy
• Thin-Film Photovoltaics

Background:

• Scalable solution-based methods are essential for cost-effective manufacturing of thin-film photovoltaic (PV) devices.
• The chemical interactions within precursor solutions significantly influence the structural, morphological, and electronic properties of the resulting thin films.
• Copper Zinc Tin Sulfide Selenide (Cu₂ZnSn(S,Se)₄ or CZTSSe) is a promising kesterite absorber material for PV applications.

Purpose of the Study:

• To investigate the impact of dimethylformamide (DMF) and isopropanol (IPA) solvent mixtures on the properties of CZTSSe precursor solutions and thin films.
• To correlate precursor solution characteristics (cation complexation, rheology) with the resulting CZTSSe film's morphology, composition, and photovoltaic performance.
• To rationalize the role of solvent-substrate interactions in the development of bifacial and other PV device architectures.

Main Methods:

• Preparation of CZTSSe precursor solutions using varying DMF:IPA ratios.
• Characterization of precursor solution rheology and dynamic viscosity.
• Analysis of thin-film morphology, composition, and structure using Raman spectroscopy, scanning electron microscopy (SEM), secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS), and energy-filtered photoemission electron microscopy (EF-PEEM).
• Fabrication and performance evaluation of complete photovoltaic devices.

Main Results:

• Increasing IPA proportion led to a nonlinear increase in precursor solution viscosity due to strong DMF-IPA repulsion (interaction cohesion parameter of 3.06).
• Optimal film homogeneity and composition were achieved with 25% IPA in DMF, leading to improved PV performance.
• Higher IPA concentrations resulted in Sn surface segregation and secondary phases, negatively impacting the electronic properties of the CZTSSe absorber layer.

Conclusions:

• Solvent mixture composition critically controls precursor rheology, film morphology, composition, and ultimately, the performance of CZTSSe-based solar cells.
• A DMF:IPA ratio of 75:25 offers a pathway to highly homogeneous CZTSSe thin films for efficient photovoltaic applications.
• Understanding and controlling solvent interactions are vital for optimizing scalable, solution-processed thin-film PV devices.