Influence of the Carboxylic Acid Additive Structure on the Properties of Poly(3-hexylthiophene) Prepared via Direct Arylation Polymerization (DArP)

Carboxylic acids are known to have a profound effect on the rate of direct arylation. Despite this, little attention has been paid to them as additives for direct arylation polymerization (DArP). Here we report the influence of the carboxylic acid chemical structure on the properties of poly­(3-hexylthiophene) (P3HT) prepared via DArP. We study the effect that acid p<i>K</i>_a, steric bulk, and backbone cyclization have on the reactivity of the DArP catalytic system. We found that p<i>K</i>_a values do not correlate with DArP reactivity in the p<i>K</i>_a range of 4.76–5.05. The increase of acid size in the classes of linear, secondary, and tertiary acids leads to a continuous increase in polymer yield and molecular weight, whereas for the case of cyclic secondary acids the trend is reversed and a <i>decrease</i> of acid size leads to an <i>increase</i> in polymer yield and molecular weight in an alternating (zigzag) fashion depending on whether the acid contains an even or odd number of carbons. A profound enhancement of reactivity was found for the case of the smallest cyclopropanecarboxylic acid, which yields P3HT with remarkably high molecular weights (<i>M</i>_n 33.4 kDa and <i>M</i>_w 207.1 kDa), which is expected to benefit the development of direct arylation protocols for inherently less reactive substrates

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 the Acceptor Composition on Physical Properties and Solar Cell Performance in Semi-Random Two-Acceptor Copolymers

Five novel semi-random poly­(3-hexylthiophene) (P3HT) based donor–acceptor copolymers containing either thienopyrroledione (TPD) or both diketopyrrolopyrrole (DPP) and TPD acceptors were synthesized by Stille copolymerization, and their optical, electrochemical, charge transport, and photovoltaic properties were investigated. Poly­(3-hexylthiophene-thiophene-thienopyrroledione) polymers P3HTT-TPD-10% and P3HTT-TPD-15% with either 10% or 15% acceptor content were synthesized as a point of reference. Two-acceptor polymers containing both TPD and DPP were synthesized with varying acceptor ratios to fine-tune electrooptical properties, namely, P3HTT-TPD-DPP (1:1) (7.5% TPD and 7.5% DPP), P3HTT-TPD-DPP (2:1) (10% TPD and 5% DPP), and P3HTT-TPD-DPP (1:2) (5% TPD and 10% DPP). The two-acceptor copolymers have broad and uniformly strong absorption profiles from 350–850 nm with absorption coefficients up to 8 × 10⁴ cm^–1 at ∼700 nm for P3HTT-TPD-DPP (1:2). This is reflected in the photocurrent responses of polymer:fullerene bulk heterojunction solar cells with PC₆₁BM as an acceptor where P3HTT-TPD-DPP (1:1) and P3HTT-TPD-DPP (1:2) have peak external quantum efficiency (EQE) values of 61% and 68% at 680 nm, respectively, and at 800 nm show impressive EQE values of 29% and 40%. Power conversion efficiencies in solar cells of P3HTT-TPD-10% and P3HTT-TPD-15% are moderate (2.08% and 2.22%, respectively), whereas two-acceptor copolymers achieve high efficiencies between 3.94% and 4.93%. The higher efficiencies are due to a combination of very large short-circuit current densities exceeding 16 mA/cm² for P3HTT-TPD-DPP (1:2), which are among the highest published values for polymer solar cells and are considerably higher than those of previously published two-acceptor polymers, as well as fill factors over 0.60. These results indicate that semi-random copolymers containing multiple distinct acceptor monomers are a very promising class of polymers able to achieve large current densities and high efficiencies due to favorable properties such as semicrystallinity, high hole mobility, and importantly broad, uniform, and strong absorption of the solar spectrum

Influence of the Ethylhexyl Side-Chain Content on the Open-Circuit Voltage in rr-Poly(3-hexylthiophene-<i>co</i>-3-(2-ethylhexyl)thiophene) Copolymers

Although recently considerable attention has been paid to the impact of polymer alkyl side chains on conjugated-polymer:fullerene solar cell performance, and especially the <i>V</i>_oc and <i>J</i>_sc, a clear and comprehensive picture of the effect of side-chain positioning, length, and branching has yet to evolve. In order to address some of these questions, we designed a simple and modular model system of random copolymers based on rr-P3HT. The influence of increasing amounts of branched 2-ethylhexyl side chains (10, 25, and 50%) in rr-poly­(3-hexylthiophene-<i>co</i>-3-(2-ethylhexyl)­thiophene) copolymers on properties such as UV–vis absorption, polymer crystallinity, HOMO energy levels, polymer:PC₆₁BM solar cell performance, and especially the <i>V</i>_oc was studied and compared to the corresponding homopolymers P3HT and poly­(3-(2-ethylhexyl)­thiophene) (P3EHT). Polymers with 50% or less 2-ethylhexyl side chains (P3HT₉₀-<i>co</i>-EHT₁₀, P3HT₇₅-<i>co</i>-EHT₂₅, P3HT₅₀-<i>co</i>-EHT₅₀) have the same band gap and similar absorption properties and also retain the semicrystalline nature of P3HT, whereas P3EHT has a higher band gap and lower absorption coefficient. Polymer HOMO levels were determined by electrochemistry in solution and thin film and are virtually identical for all polymers in solution, whereas in the solid state an increase in the amount of 2-ethylhexyl side chains leads to marked and correlated decrease in the HOMO levels. This decrease is directly reflected in the <i>V</i>_oc measured in polymer:PC₆₁BM solar cells which increases with increasing 2-ethylhexyl side-chain content, indicating a relatively straightforward HOMO_DONOR–LUMO_ACCEPTOR dependence of the <i>V</i>_oc for this family of polymers. P3HT₇₅-<i>co</i>-EHT₂₅ benefits from an increased <i>V</i>_oc (0.69 V), a <i>J</i>_sc (9.85 mA/cm²) on the same order of P3HT, and a high FF and ultimately achieves an efficiency of 3.85% exceeding that measured for P3HT (<i>V</i>_oc = 0.60 V, <i>J</i>_sc = 9.67 mA/cm², efficiency = 3.48%). The observed efficiency increase suggests that the random incorporation of branched alkyl side chains could also be successfully used in other polymers to maximize the <i>V</i>_oc while maintaining the band gap and improve the overall polymer:fullerene solar cell performance

Compositional Dependence of the Open-Circuit Voltage in Ternary Blend Bulk Heterojunction Solar Cells Based on Two Donor Polymers

Ternary blend bulk heterojunction (BHJ) solar cells containing as donor polymers two P3HT analogues, high-band-gap poly­(3-hexylthiophene-<i>co</i>-3-(2-ethylhexyl)­thiophene) (P3HT₇₅-<i>co</i>-EHT₂₅) and low-band-gap poly­(3-hexylthiophene–thiophene–diketopyrrolopyrrole) (P3HTT-DPP-10%), with phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) as an acceptor were studied. When the ratio of the three components was varied, the open-circuit voltage (<i>V</i>_oc) increased as the amount of P3HT₇₅-<i>co</i>-EHT₂₅ increased. The dependence of <i>V</i>_oc on the polymer composition for the ternary blend regime was linear when the overall polymer:fullerene ratio was optimized for each polymer:polymer ratio. Also, the short-circuit current densities (<i>J</i>_sc) for the ternary blends were bettter than those of the binary blends because of complementary polymer absorption, as verified using external quantum efficiency measurements. High fill factors (FF) (>0.59) were achieved in all cases and are attributed to high charge-carrier mobilities in the ternary blends. As a result of the intermediate <i>V</i>_oc, increased <i>J</i>_sc and high FF, the ternary blend BHJ solar cells showed power conversion efficiencies of up to 5.51%, exceeding those of the corresponding binary blends (3.16 and 5.07%). Importantly, this work shows that upon optimization of the overall polymer:fullerene ratio at each polymer:polymer ratio, high FF, regular variations in <i>V</i>_oc, and enhanced <i>J</i>_sc are possible throughout the ternary blend composition regime. This adds to the growing evidence that the use of ternary blends is a general and effective strategy for producing efficient organic photovoltaics manufactured in a single active-layer processing step

Random Poly(3-hexylthiophene-<i>co</i>-3-cyanothiophene) Copolymers via Direct Arylation Polymerization (DArP) for Organic Solar Cells with High Open-Circuit Voltage

A family of four poly­(3-hexylthiophene) (P3HT) based copolymers containing 5, 10, 15, and 20% of 3-cyanothiophene (CNT) incorporated in a random fashion with a regioregular linkage pattern (P3HT-CNT) were successfully synthesized via direct arylation polymerization (DArP). Unique reaction conditions, previously reported for P3HT, were used, which employ very low loadings of Pd­(OAc)₂ as a catalyst and an inexpensive bulky carboxylic acid (neodecanoic acid) as an essential part of the palladium catalytic center. The chemical structures and optoelectronic properties of DArP P3HT-CNT polymers were found to be similar to those of previously investigated P3HT-CNT polymers synthesized via Stille polycondensation. All polymers are semicrystalline with high hole mobilities and UV–vis absorption profiles that resemble P3HT, while the polymer highest occupied molecular orbital (HOMO) level decreases with increasing content of cyanothiophene in both DArP and Stille P3HT-CNT polymers. In photovoltaic devices with a PC₆₁BM acceptor, DArP P3HT-CNT copolymers showed slightly lower open-circuit voltages (<i>V</i>_oc) than their Stille P3HT-CNT analogues but similar fill factors (FF) and significantly enhanced short-circuit current densities (<i>J</i>_sc), leading to overall power conversion efficiencies for the DArP polymers that rivaled or exceeded those of the Stille polymers. This work further emphasizes the generality and relevance of DArP for the synthesis of conjugated polymers for use in organic solar cells and the attractive simplicity and ease of synthesis of random conjugated polymers

Contrasting Performance of Donor–Acceptor Copolymer Pairs in Ternary Blend Solar Cells and Two-Acceptor Copolymers in Binary Blend Solar Cells

Here two contrasting approaches to polymer–fullerene solar cells are compared. In the first approach, two distinct semi-random donor–acceptor copolymers are blended with phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) to form ternary blend solar cells. The two poly­(3-hexylthiophene)-based polymers contain either the acceptor thienopyrroledione (TPD) or diketopyrrolopyrrole (DPP). In the second approach, semi-random donor–acceptor copolymers containing both TPD and DPP acceptors in the same polymer backbone, termed two-acceptor polymers, are blended with PC₆₁BM to give binary blend solar cells. The two approaches result in bulk heterojunction solar cells that have the same molecular active-layer components but differ in the manner in which these molecular components are mixed, either by physical mixing (ternary blend) or chemical “mixing” in the two-acceptor (binary blend) case. Optical properties and photon-to-electron conversion efficiencies of the binary and ternary blends were found to have similar features and were described as a linear combination of the individual components. At the same time, significant differences were observed in the open-circuit voltage (<i>V</i>_oc) behaviors of binary and ternary blend solar cells. While in case of two-acceptor polymers, the <i>V</i>_oc was found to be in the range of 0.495–0.552 V, ternary blend solar cells showed behavior inherent to organic alloy formation, displaying an intermediate, composition-dependent and tunable <i>V</i>_oc in the range from 0.582 to 0.684 V, significantly exceeding the values achieved in the two-acceptor containing binary blend solar cells. Despite the differences between the physical and chemical mixing approaches, both pathways provided solar cells with similar power conversion efficiencies, highlighting the advantages of both pathways toward highly efficient organic solar cells

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

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

Electronic States in Dilute Ternary Blend Organic Bulk Heterojunction Solar Cells

Electronic states and electronic excitations in a molecular solid such as an organic bulk heterojunction solar cell either may reflect the properties of individual molecules or may be delocalized over several molecules, exhibiting alloy properties of the average composition. Measurements of a variety of dilute ternary blend organic solar cells based on either two polymer donors and one fullerene acceptor or one polymer donor and two fullerene acceptors provide information about the degree of localization in different situations. In the two polymer case, where the polymers are well intermixed, excitons have molecular characteristics. Despite their localization, excitons from the dilute low band gap component readily diffuse to the heterojunction interface and generate mobile charge, and their diffusion is attributed to rod percolation. Mobile holes are delocalized, and the blend concentration dependence suggests delocalization over about 10 polymer molecules. In contrast, with poorly intermixed polymers, low band gap excitons are unable to diffuse and exhibit no charge generation. With fullerene mixtures, two different behaviors are also observed. Mixtures of PC₆₁BM and ICBA exhibit delocalized alloy states, while dilute PC₈₄BM in PC₆₁BM mixtures exhibits localized trap states. The difference is attributed to the size mismatch of the larger PC₈₄BM molecule