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

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

Quantifying Charge Recombination in Solar Cells Based on Donor–Acceptor P3HT Analogues

The creation of semi-random donor–acceptor analogues of poly­(3-hexylthiophene) (P3HT) yields polymers that exhibit pan-chromatic absorption spectra extending into the near-infrared. Despite this extended absorption however, different semi-random polymers exhibit markedly different photovoltaic performance when blended as a bulk-heterojunction with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). To understand the physical origin of these differences, we performed transient absorption (TA) measurements and device characterization of blends of two representative semi-random polymers, poly­(3-hexylthiophene-thiophene-thienopyrazine) (P3HTT-TP-10%) and poly­(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP-10%), with PCBM. Although both polymers absorb strongly throughout the visible and near-infrared, devices based on P3HTT-DPP-10%:PCBM exhibit a power conversion efficiency of ∼6%, while films consisting of P3HTT-TP-10%:PCBM blends display values under 1%. TA experiments reveal that polarons generated upon excitation of a P3HTT-TP-10%:PCBM blend undergo a high degree of geminate recombination (survival percentage, ϕ_S ∼45%) independent of excitation wavelength, explaining its lower efficiency. In contrast, P3HTT-DPP-10%:PCBM blends show excitation wavelength-dependent polaron recombination dynamics. While excitation of the polymer in the visible region leads to less geminate recombination (ϕ_S ∼65%) compared to P3HTT-TP-10%:PCBM, this loss process is ∼1.5 times more deleterious following near-infrared (NIR) excitation. Despite this observation, a significant fraction (ϕ_S ∼ 45%) of the charges formed following NIR excitation escape recombination, partly explaining the high performance of P3HTT-DPP-10%:PCBM devices

Annealing-Induced Changes in the Molecular Orientation of Poly-3-hexylthiophene at Buried Interfaces

The molecular organization at interfaces of organic semiconducting materials plays a crucial role in the performance of organic photovoltaics and field effect transistors. Vibrational sum-frequency generation (VSFG) was used to characterize the molecular orientation at interfaces of regioregular poly-3-hexylthiophene (rrP3HT). Polarization-selected VSFG spectra of the CC stretch of the thiophene ring yield the orientation of the conjugated backbone of P3HT, which is directly relevant to the electronic properties at the interface. The molecular orientation at buried polymer–substrate interfaces was compared for films spin-cast on SiO₂ and AlO_X substrates, before and after thermal annealing at 145 °C. On SiO₂, annealing results in the thiophene rings adopting a more edge-on orientation, tilting away from the surface plane by Δθ = +(3–10)°. In contrast, an opposite change is observed for films deposited on AlO_<i>x</i>, Δθ = −(3–26)°, where annealing leads to a more face-on orientation of the thiophene rings of the polymer. Although subtle, such orientational changes may significantly affect the electron transfer rates across interfaces and hence the overall photovoltaic efficiency