Utilizing Benzotriazole and Indacenodithiophene Units to Construct Both Polymeric Donor and Small Molecular Acceptors to Realize Organic Solar Cells With High Open-Circuit Voltages Beyond 1.2 V.

Ailing Tang,Bo Xiao,Jianfeng Li,Erjun Zhou,Xiaochen Wang,Jing Yang,Fan Chen

doi:10.3389/fchem.2018.00147

Abstract

Devolopment of organic solar cells with high open-circuit voltage (VOC) and power conversion efficiency (PCE) simutaniously plays a significant role, but there is no guideline how to choose the suitable photovoltaic material combinations. In our previous work, we developed “the Same-Acceptor-Strategy” (SAS), by utilizing the same electron-accepting segment to construct both polymeric donor and small molecular acceptor. In this study, we further expend SAS to use both the same electron-accepting and electron-donating units to design the material combination. The p-type polymer of PIDT-DTffBTA is designed by inserting conjugated bridge between indacenodithiophene (IDT) and fluorinated benzotriazole (BTA), while the n-type small molecules of BTAx (x = 1, 2, 3) are obtained by introducing different end-capped groups to BTA-IDT-BTA backbone. PIDT-DTffBTA: BTAx (x = 1–3) based photovolatic devices can realize high VOC of 1.21–1.37 V with the very small voltage loss (0.55–0.60 V), while only the PIDT-DTffBTA: BTA3 based device possesses the enough driving force for efficient hole and electron transfer and yields the optimal PCE of 5.67%, which is among the highest value for organic solar cells (OSCs) with a VOC beyond 1.20 V reported so far. Our results provide a simple and effective method to obtain fullerene-free OSCs with a high VOC and PCE.

Highlights

As one of the most promising technique in photoelectric conversion, bulk-heterojunction (BHJ) organic solar cells (OSCs) have been extensively studied
The lowest unoccupied molecular orbital (LUMO) offsets between PITD-DTffBTA and three acceptors (BTA2, BTA1 and BTA3) ( ELUMOD−LUMOA ) are calculated to be 0.08, 0.27, and 0.49 eV, respectively, and the according highest occupied molecular orbital (HOMO) offsets ( EHOMOD−HOMOA ) are 0.38, 0.45, 0.65 eV, respectively. These results reveal that utilizing the same building blocks to construct both p-type polymeric donor and n-type small molecular acceptors has the potential to realize similar LUMO levels and give rise to high voltage
The shape of PL from the blend with BTA1 is close to the emission of the pristine BTA1 film but the polymer PL quenching efficiency significantly raises up to 88%, indicating that excitons initially generated on the PIDT-DTffBTA can transfer to BTA1 in high yield but they are inefficiently quenched by the heterojunction (Hoke et al, 2013)

Summary

Introduction

As one of the most promising technique in photoelectric conversion, bulk-heterojunction (BHJ) organic solar cells (OSCs) have been extensively studied. These results reveal that utilizing the same building blocks to construct both p-type polymeric donor and n-type small molecular acceptors has the potential to realize similar LUMO levels and give rise to high voltage. The device using the PIDT-DTffBTA: BTA1 and PIDT-DTffBTA: BTA2 blend (1:1 in wt %) with thermal annealing show nearly no performance with a PCE of 0.12 and 0.07%, respectively, which is likely due to their very high-lying LUMO, resulting in insufficient charge transfer from polymer to acceptor.

Results

Conclusion