Utilisation of Green Solvents in Organic Photovoltaic Devices for Large-Scale Fabrication Process

This dissertation presents innovative techniques for fabricating solution-processed bulk heterojunction solar cells, utilising eco-friendly green solvents by replacing conventional halogenated equivalent.

A key aspect of this study involves leveraging advanced Atomic Force Microscopy (AFM) phase analysis, and Raman mapping techniques. These approaches enabled a detailed examination of the constituent materials, resulting in a comprehensive three-dimensional profile of the electron donor and acceptor materials within the blend. Detailed analysis indicate that the acceptor material, PC71BM1,
predominantly accumulates at the blended film's surface. Further investigations concerning donor/acceptor distribution throughout the bulk revealed a systematic reduction of PC71BM, with an equilibrium of electron donor and acceptor presence approximately 50 nm below the film surface. These observations suggest a consistent relationship between PC71BM content and its distribution at various depths, exhibiting a funnel-like distribution pattern.

In further investigation, depth-specific electrical analyses offered valuable insights into fullerene migration's effects on device performance. While surface migration enhances charge collection at the cathode, could potentially impede exciton dissociation and disrupt charge carrier pathways due to reduced PC71BM concentration in the bulk. A higher PC71BM concentration at the film surface could potentially improve parameters like series resistance (Rs), parallel resistance (Rsh) and fill factor but could hinder charge carrier pathways.

Among the evaluated green solvents, 2-Methyl Anisole (2MA) demonstrated the most promising results, achieving higher power conversion efficiency (PCE) and closely matching the performance of the samples mixed with halogenated solvents (dissolved in ODCB:CB2) at a blend ratio of [1:2] for polymer/small organic molecule systems. Grazing

Incidence X-ray Diffraction (GIXRD) studies on pristine EFT3 (electron donor material) and EFT:PC71BM donor/acceptor systems further indicated that 2MA affects the intensity of the peak associated with the d100 plane of EFT molecules at lower 2θ values.

Beyond solvent optimisation, the current work reports on green solvent applications in passivation patterns for large-scale devices. Prototype samples utilising the EFT:PC71BM donor/acceptor system dissolved in 2MA were fabricated and analysed for their electrical parameters. The prototype cells' parallel configuration demonstrated the highest efficiency, achieving a PCE of 8.7-8.9% when utilising the selected green solvent, 2MA.