Inter-crosslinking through both donor and acceptor with unsaturated bonds for highly efficient and stable organic solar cells

Abstract

A novel approach for thermally stable bulk heterojunction (BHJ) solar cells based on the inter-crosslinking of a vinyl-functionalized low band-gap copolymer donor poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-alt-4,6-thieno[3,4-b]thiophen-2-yl-2-ethylhexan-1-one-alt-4,8-bis(oct-7-en-1-yloxy)benzo[1,2-b:4,5-b′]dithiophene} (PBDTTT-V), and vinyl-functionalized fullerene derivative acceptor [6,6]-phenyl C₆₁ butyric acid dec-9-en-1-yl ester (PCBD), is developed. Compared with non-crosslinked corresponding heterojunctions and mono-crosslinked corresponding heterojunctions, inter-crosslinked heterojunctions with and without the addition of [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) showed well-maintained and stable morphology after thermal annealing at 150 °C for 8 hours, in which the large-scale phase separation caused by the aggregation of fullerenes was dramatically suppressed. Although the mono-crosslinked PBDTTT-V : PCBM blend efficiently improved the stability of device with respect to a non-crosslinked corresponding heterojunction device, the power conversion efficiency (PCE) still decreased to an half of its initial efficiency value after annealing at 150 °C for 8 h. On the contrary, a remarkably enhanced stability was obtained in inter-crosslinked devices with performance maintained at 60% for PBDTTT-V : PCBM : PCBD and at 79% for PBDTTT-V : PCBD. It is worthy to note that the device based on PBDTTT-V : PCBM : PCBD showed comparable performance to the non-crosslinked one, indicating that the inter-crosslinking of both donor and acceptor with unsaturated bonds is a promising approach to ensure a device with long-term thermal stability, and maintain the good performance at the same time.