Investigating the stability and photophysics of organic solar cell active layers based on multicomponent polymer materials

Abstract

Organic solar cells (OSCs) are considered strong contenders for next-generation renewable energy solutions due to their cost-effectiveness and flexible design. Significant advancements in the synthesis of innovative active-layer materials and enhancements in device fabrication have brought their power conversion efficiency (PCE) up to approximately 20% in both single- and multi-junction configurations. While some studies suggest OSCs may achieve a lifespan of up to 20 years, their current stability remains a barrier to full commercial deployment. Random terpolymerization has emerged as a promising approach to enhance the photovoltaic performance and stability of polymer donors. The molecular engineering of these terpolymers has allowed for simple morphological control in binary devices over ternary blends. However, the stability and photophysics of terpolymers have seldom been investigated, despite the promise of terpolymer materials in addressing the morphological instability found in bulk heterojunctions (BHJ) OSCs.

This thesis presents a systematic investigation into the stability and photophysical properties of a series of terpolymers, aimed at uncovering specific underlying molecular mechanisms. Stability tests on terpolymers were conducted under thermal stress, and the terpolymers demonstrated exceptional stability under elevated temperatures of 85 degrees Celsius.

In the first part, we explored the thermal stability of three terpolymers (P1, P2, and P3) made from one donor (thienyl-substituted benzodithiophene, BDTT) and different ratios of two types of electron acceptors, namely fluorobenzotriazole (FTAZ) and thienothiophene-capped diketopyrrolopyrrole (TTDPP), blended with the fullerene acceptor PC71BM (regioregular [6,6]-phenyl-C71-butyric acid methyl ester). The study found that the terpolymers exhibited broad absorbance from 350 nm to 900 nm, and thermal degradation had minimal effect on the pristine films. Incorporation of the FTAZ acceptor in the terpolymerization approach served as a suitable strategy in enhancing the thermal stability of the active layers. However, the BHJ films showed significant morphological changes due to PC71BM aggregation. Prolonged annealing resulted in PC71BM aggregation and terpolymer decomposition, but without affecting their molecular structure. The results highlight that controlled annealing can regulate PC71BM diffusion, improving the nanostructure crucial for efficient OSCs.

In the second part, we investigated the photophysics of the three DPP-based terpolymers with varying acceptor ratios (FTAZ and TTDPP), blended with PC71BM. Increasing the TTDPP ratio in P1 improved its molecular structure, leading to better intermolecular interactions, enhanced pi-conjugation, and a red shift in absorption by 13 nm into the near-infrared range. Transient absorption spectroscopy showed efficient charge carrier dynamics and intermolecular charge transfer in the P1 blend. However, thermal annealing reduced long-lived charge carriers across all blends due to aggregation of the fullerene acceptor, disrupting phase separation.