# Impact of Different π-Bridges on the Photovoltaic Performance of A-D-D′-D-A Small Molecule-Based Donors

## TL;DR

This study explores how different π-bridges in small donor molecules affect solar cell performance, showing that thiophene π-bridges improve efficiency and voltage.

## Contribution

The novel contribution is demonstrating that thiophene π-bridges with alkyl chains enhance photovoltaic performance in small molecule donors.

## Key findings

- Using a thiophene π-bridge increases open circuit voltage to 0.75 V and achieves 3% power conversion efficiency.
- Adding alkyl chains to the π-bridge improves solubility, leading to higher current, filling factor, and 4.25% PCE.

## Abstract

Three small donor molecule materials (S1, S2, S3) based on dithiophene [2,3-d:2′,3′-d′]dithiophene [1,2-b:4,5-b′]dithiophene (DTBDT) utilized in this study were synthesized using the Vilsmeier–Haack reaction, traditional Stille coupling, and Knoevenagel condensation. Then, a variety of characterization methods were applied to study the differences in optical properties and photovoltaic devices among the three. By synthesizing S2 using a thiophene π-bridge based on S1, the blue shift in ultraviolet absorption can be enhanced, the band gap and energy level can be reduced, the open circuit voltage (VOC) can be increased to 0.75 V using the S2:Y6 device, and a power conversion efficiency (PCE) of 3% can be achieved. Also, after developing the device using Y6, S3 introduced the alkyl chain of thiophene π-bridge to S2, which improved the solubility of tiny donor molecules, achieved the maximum short-circuit current (JSC = 10.59 mA/cm2), filling factor (FF = 49.72%), and PCE (4.25%). Thus, a viable option for future design and synthesis of small donor molecule materials is to incorporate thiophene π-bridges into these materials, along with alkyl chains, in order to enhance the device’s morphology and charge transfer behavior.