The effect of polymer solubilizing side-chains on solar cell stability.

Area of Science:

• Materials Science
• Organic Electronics
• Photovoltaics

Background:

• Organic solar cells (OSCs) offer potential for low-cost renewable energy.
• Photothermal stability is a critical challenge limiting the operational lifetime of OSCs.
• Polymer:fullerene bulk heterojunctions (BHJs) are common active layers in OSCs.

Purpose of the Study:

• To investigate the impact of polymer side-chain variations on the photothermal stability of OSCs.
• To understand the degradation mechanisms in polymer:fullerene blends under illumination and elevated temperature.
• To identify side-chain structures that enhance the stability of the BHJ active layer.

Main Methods:

• Synthesis of four benzodithiophene (BDT) polymers with different side-chains, copolymerized with diketopyrrolopyrrole (DPP) via Stille polymerization.
• Fabrication of inverted photovoltaic devices using polymer:phenyl-C61-butyric acid methyl ester (PCBM) active layers.
• Assessment of photothermal stability under AM1.5 illumination at 50 °C, correlated with morphological analysis (AFM, XRD, UV-Vis).

Main Results:

• Device degradation is governed by PCBM crystallization and dimerization, with dimerization causing initial rapid performance loss.
• Polymer side-chain structure dictates blend morphology and influences the extent of degradation processes.
• Linear alkyl side-chains provided the most effective stabilization of the BHJ, followed by no side-chain, alkoxy, and branched side-chains.
• Lowering fullerene concentration reduced degradation rates, dependent on polymer side-chains.

Conclusions:

• Side-chain engineering of BDT-DPP polymers is a viable strategy to enhance the photothermal stability of organic solar cells.
• Linear alkyl side-chains are optimal for stabilizing the BHJ morphology against light and heat-induced degradation.
• Reducing fullerene concentration offers a general approach to improve polymer:PCBM blend stability.