Molecular helices as electron acceptors in high-performance bulk heterojunction solar cells

Despite numerous organic semiconducting materials synthesized for organic photovoltaics in the past decade, fullerenes are widely used as electron acceptors in highly efficient bulk-heterojunction solar cells. None of the non-fullerene bulk heterojunction solar cells have achieved efficiencies as high as fullerene-based solar cells. Design principles for fullerene-free acceptors remain unclear in the field. Here we report examples of helical molecular semiconductors as electron acceptors that are on par with fullerene derivatives in efficient solar cells. We achieved an 8.3% power conversion efficiency in a solar cell, which is a record high for non-fullerene bulk heterojunctions. Femtosecond transient absorption spectroscopy revealed both electron and hole transfer processes at the donor−acceptor interfaces. Atomic force microscopy reveals a mesh-like network of acceptors with pores that are tens of nanometres in diameter for efficient exciton separation and charge transport. This study describes a new motif for designing highly efficient acceptors for organic solar cells

Electronic Structure and Transition Energies in Polymer–Fullerene Bulk Heterojunctions

© 2014 American Chemical Society. Photocurrent spectroscopy is used to measure both the charge transfer and exciton optical absorption spectra of various bulk heterojunction organic solar cells. The energy difference between the polymer HOMO energy and the fullerene LUMO energy is obtained from the spectra, along with the disorder energy. Combining information from cells with several different polymers and fullerenes allows measurements of the energy differences between HOMO or LUMO energies for about 10 different polymers and fullerenes, with an estimated uncertainty of 50 meV. Heterojunction band offsets are obtained for the various cells, distinguishing between the excitonic and the single-carrier band offsets. The cell open-circuit voltage is shown to be closely correlated with the interface band gap. The exciton disorder energy is directly correlated to the band-tail disorder and we also consider the effects of exciton thermalization on the charge generation mechanism. The data indicate that an energy offset between the polymer exciton and the charge transfer ground state below about 0.25 eV adversely affects the cell performance, while a HOMO band offset below about 0.2-0.3 eV also degrades cell performance but by a different mechanism

Crystalline Intermediates and Their Transformation Kinetics during the Formation of Methylammonium Lead Halide Perovskite Thin Films

The morphology of methylammonium lead halide perovskite thin films significantly affects the performance of opto-electronic devices that comprise them. Using X-ray diffraction studies, we elucidated the mechanisms of thin-film formation to complete the complex picture of structural development of perovskites under an inert atmosphere. The presence of excess methylammonium iodide during perovskite crystallization leads to the formation of layered intermediates and low-dimensional perovskites; these intermediates are correlated with the formation of large and continuous grains in the final films. When the precursors are present in stoichiometric equivalence, initial stages of crystallization instead involve the formation of solvates; the fully crystallized films have poor surface coverage and comprise needle-like structures. The activation energy of crystallization in films with stoichiometric excess methylammonium iodide is higher than that for films comprising stoichiometrically equivalent precursors; the higher energy barrier is consistent with the need to sublime excess methylammonium iodide during film formation. Replacing lead iodide with lead chloride does not qualitatively alter the crystallization process or the final morphology; it lowers the activation energy for crystallization, presumably because sublimation of methylammonium chloride is less energetic than that of methylammonium iodide. Our study suggests the necessity of layered structures and low-dimensional perovskites for the formation of technologically relevant continuous thin films

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

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

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

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

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