Effect of Blend Composition on Binary Organic Solar Cells Using a Low Band Gap Polymer

Area of Science:

• Materials Science
• Organic Electronics
• Photovoltaics

Background:

• Bulk heterojunction organic solar cells (OSCs) are promising for renewable energy.
• The performance of OSCs is highly dependent on the morphology and composition of the active layer.
• Polymer:fullerene blends are commonly used in OSCs, with poly(2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H- cyclopenta[2,1-b:3,4-b'dithiophene-2,6-diy]]) (PCPDTBT) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) being a common pair.

Purpose of the Study:

• To investigate the effect of solution blend composition on the performance of PCPDTBT:PC71BM based organic solar cells.
• To determine the optimal blend ratio for maximizing photovoltaic performance.
• To understand the correlation between blend composition, morphology, and device efficiency.

Main Methods:

• Fabrication of organic solar cells using varying ratios of PCPDTBT and PC71BM, ranging from 1:1 to 2:9 (polymer:fullerene by weight).
• Characterization of the blend films, including absorption measurements.
• Electrical characterization of the fabricated devices, including measurement of short circuit current density (Jsc), fill factor (FF), and internal quantum efficiency (IQE).
• Analysis of electron mobility and charge transport properties as a function of blend composition.

Main Results:

• Increasing the polymer (PCPDTBT) content enhanced overall absorption due to the donor material's strong absorption.
• However, high polymer content resulted in poor photovoltaic performance.
• The optimal blend polymer:fullerene composition was determined to be 2:7.
• Increasing fullerene (PC71BM) content significantly improved internal quantum efficiency and electron mobility.
• Improved electron transport led to more balanced charge transport, significantly boosting Jsc and FF.

Conclusions:

• The blend composition of PCPDTBT:PC71BM is a critical factor in determining the performance of organic solar cells.
• An optimal blend ratio of 2:7 (polymer:fullerene) was identified for this specific material combination.
• Enhanced electron mobility and balanced charge transport, achieved by increasing fullerene content, are key to improving the efficiency of these OSCs.