Fine Tuning of Polymer Properties by Incorporating Strongly Electron-Donating 3‑Hexyloxythiophene Units into Random and Semi-random Copolymers

Two series of copolymers containing varying percentages of strongly electron-donating 3-hexyloxythiophene units (3HOT), namely the random poly­(3-hexylthiophene<i>-co</i>-3-hexyloxythiophene)­s (P3HT-<i>co-</i>3HOTs) and the semi-random poly­(3-hexylthiophene–thiophene–3-hexyloxythiophene–diketopyrrolopyrrole)­s (P3HTT-HOT-DPPs), were synthesized via Stille polymerization. The influence of 3HOT content on UV–vis absorption, HOMO energy levels, polymer crystallinity, and polymer:PC₆₁BM solar cell performance, especially the open-circuit voltage (<i>V</i>_oc), was investigated. Importantly, introduction of the strong donor 3HOT results in regularly decreased band gaps and broadened absorption compared to the corresponding parent polymers, regioregular poly­(3-hexylthiophene) (P3HT) and semi-random poly­(3-hexylthiophene–thiophene–diketopyrrolopyrrole) (P3HTT-DPP). The HOMO energies of both random P3HT-<i>co-</i>3HOT and semi-random P3HTT-HOT-DPP copolymers increase significantly with 3HOT incorporation, from −5.2 eV to around −4.95 eV with half of the 3-hexylthiophene units (3HT) being replaced by 3HOT, and the trend is directly reflected in the <i>V</i>_oc measured in polymer:PC₆₁BM solar cells. High absorption coefficient and semicrystallinity are retained for all of the copolymers. The semi-random P3HTT-HOT-DPP copolymers with low percentage of 3HOT (up to 15%) show <i>J</i>_sc of above 10 mA/cm², which is comparable to the parent P3HTT-DPP. Importantly, this study demonstrates that significant changes in polymer electronic properties can be induced with only small percentage of comonomers in random and semi-random conjugated polymers

Influence of Surface Energy on Organic Alloy Formation in Ternary Blend Solar Cells Based on Two Donor Polymers

The compositional dependence of the open-circuit voltage (<i>V</i>_oc) in ternary blend bulk heterojunction (BHJ) solar cells is correlated with the miscibility of polymers, which may be influenced by a number of attributes, including crystallinity, the random copolymer effect, or surface energy. Four ternary blend systems featuring poly­(3-hexylthiophene-<i>co</i>-3-(2-ethylhexyl)­thiophene) (P3HT₇₅-<i>co</i>-EHT₂₅), poly­(3-hexylthiophene-<i>co</i>-(hexyl-3-carboxylate)), herein referred to as poly­(3-hexylthiophene-<i>co</i>-3-hexylesterthiophene) (P3HT₅₀-<i>co</i>-3HET₅₀), poly­(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP-10%), and an analog of P3HTT-DPP-10% with 40% of 3-hexylthiophene exchanged for 2-(2-methoxyethoxy)­ethylthiophen-2-yl (3MEO-T) (featuring an electronically decoupled oligoether side-chain), referred to as P3HTTDPP-MEO40%, are explored in this work. All four polymers are semicrystalline and rich in rr-P3HT content and perform well in binary devices with PC₆₁BM. Except for P3HTTDPP-MEO40%, all polymers exhibit similar surface energies (∼21–22 mN/m). P3HTTDPP-MEO40% exhibits an elevated surface energy of around 26 mN/m. As a result, despite the similar optoelectronic properties and binary solar cell performance of P3HTTDPP-MEO40% compared to P3HTT-DPP-10%, the former exhibits a pinned <i>V</i>_oc in two different sets of ternary blend devices. This is a stark contrast to previous rr-P3HT-based systems and demonstrates that surface energy, and its influence on miscibility, plays a critical role in the formation of organic alloys and can supersede the influence of crystallinity, the random copolymer effect, similar backbone structures, and HOMO/LUMO considerations. Therefore, we confirm surface energy compatibility as a figure-of-merit for predicting the compositional dependence of the <i>V</i>_oc in ternary blend solar cells and highlight the importance of polymer miscibility in organic alloy formation

Fine Tuning Surface Energy of Poly(3-hexylthiophene) by Heteroatom Modification of the Alkyl Side Chains

Recent work has pointed to polymer miscibility and surface energy as key figures of merit in the formation of organic alloys and synergistic behavior between components in ternary blend solar cells. Here, we present a simple model system and first report of poly­(3-hexylthiophene)-based random copolymers featuring either a semifluoroalkyl (P3HT<i>-<i>co</i>-</i>FHT) or oligoether (P3HT<i>-<i>co</i>-</i>MET) side chain, prepared via Stille polycondensation. Water drop contact angle measurements demonstrated that P3HT<i>-<i>co</i>-</i>FHT polymers reached a minimum surface energy of 14.2 mN/m at 50% composition of comonomers, while in contrast, P3HT<i>-<i>co</i>-</i>MET polymers increased as high as 27.0 mN/m at 50% composition, compared to P3HT at 19.9 mN/m. Importantly, the surface energy of the copolymers was found to vary regularly with comonomer composition and exhibited fine-tuning. Optical and electronic properties of the polymers are found to be composition independent as determined by UV–vis and CV measurements; HOMO energy levels ranged from 5.25 to 5.30 eV; and optical band gaps all measured 1.9 eV. Following this model, surface energy modification of state-of-the-art polymers, without altering desirable electronic and optical properties, is proposed as a useful tool in identifying and exploiting more alloying polymer pairs for ternary blend solar cells