The effect of rigid-block length in elastomer-containing photoactive block copolymers on the photovoltaic and mechanical properties of polymer solar cells

Abstract

Organic solar cells (OSCs) used in wearable devices should exhibit mechanical robustness in addition to high power conversion efficiencies (PCEs). Achieving this goal involves integrating polymer donors (P_Ds) with elastomers via block copolymerization. However, the underlying design principles, particularly the effect of block length within the P_D-elastomer block copolymers (BCPs), are not well understood. In this work, we synthesize three D18-b-poly(dimethylsiloxane) (PDMS) BCPs with different block lengths—namely, D18_L-b-PDMS, D18_M-b-PDMS, and D18_H-b-PDMS—by varying the numberaverage molecular weight (M_n = 8.6, 14.4, and 22.6 kg mol⁻¹, respectively) of the D18 conjugated segments. An increase in the M_n of the D18 blocks leads to the formation of distinct crystalline structures in the photoactive layer, which facilitates charge transport and diminishes charge recombination in OSCs. Consequently, the PCE of OSCs is improved from D18_L-b-PDMS (15.1%), to D18_M-b-PDMS (16.4%), and to D18_H-b-PDMS (17.3%). The presence of PDMS segments in the D18_x-b-PDMS BCPs increases the degree of polymer-chain entanglement, resulting in the high toughness (>1.8 MJ m⁻³) of D18_x-b-PDMS:L8-BO blend films. Therefore, the OSCs based on these BCPs not only achieve PCEs (15.1–17.3%) that surpass those of the OSCs based on random copolymers (PCE = 12.30%) but also exhibit mechanical robustness (toughness = 1.8–2.6 MJ m⁻³) exceeding those of the OSCs based on the reference D18 (toughness = 0.5 MJ m⁻³) and D18:PDMS physical blends (PCE = 8.60% and toughness = 0.1 MJ m⁻³). Thus, this study demonstrates the effectiveness of the P_D-elastomer BCP design and underscores the significance of controlling the molecular weight of the block segments within the BCPs for simultaneously achieving high photovoltaic efficiency and mechanical properties in OSCs.