PCDTBT: en route for low cost plastic solar cells

Hexylation Stabilises Twisted Backbone Configurations in the Prototypical Low-Bandgap Copolymer PCDTBT

Conjugated donor-acceptor copolymers hold great potential as materials for high-performance organic photovoltaics, organic transistors and organic thermoelectric devices. Their low optical bandgap is achieved by alternation of donor and acceptor moieties along the polymer chain, leading to a pronounced charge-transfer character of electronic excitations. However, the influence of appended side chains and of chemical defects of the backbone on their photophysical and conformational properties remains largely unexplored on the level of individual chains. Here, we employ room temperature single-molecule photoluminescence spectroscopy on four compounds based on the prototypical copolymer PCDTBT with systematically changed chemical structure. Our results show that an increasing density of statistically added hexyl chains to the TBT comonomer distorts the molecular conformation, likely through the increase of average dihedral angles along the backbone. We find that, although the conformation becomes more twisted with high hexyl density, the side chains appear to stabilize the backbone in this twisted conformation. In addition, we demonstrate that homocoupling defects along the backbone barely influence the PL spectra of single chains, and thus intra-chain electronic properties.

Understanding the Microscopic Origin of the Contact Resistance at the Polymer-Electrode Interface

Contact resistance (RC) in organic devices originates from a mismatch in energy levels between injecting electrodes and organic semiconductors (OSCs). However, the microscopic effects governing charge transfer between electrodes and the OSCs have not been analyzed in detail. We fabricated transistors with different OSCs (PTB7, PCDTBT, and PTB7-Th) and electrodes (MoO3, Au, and Ag) and measured their contact resistance. Regardless of the electrodes, devices with PTB7-Th exhibit the lowest values of RC. To explain the trends observed, first-principles computations were performed on contact interfaces based on the projector operator diabatization method. Our results revealed that differences in energy levels and the electronic couplings between OSCs' highest occupied molecular orbitals and vacant states on the electrodes influence device RC. Further, based on values obtained from the first-principles, the rate of charge transfer between OSCs and electrodes is calculated and found to correlate strongly with trends in RC for devices with different OSCs. We thus show that device RC is governed by the feasibility of charge transfer at the contact interface and hence determined by energy levels and electronic coupling among orbitals and states located on OSCs and electrodes.

Design of ternary additive for organic photovoltaics: a cautionary tale

Silicon phthalocyanines as ternary additives are a promising way to increase the performance of organic photovoltaics. The miscibility of the additive and the donor polymer plays a significant role in the enhancement of the device performance, therefore, ternary additives can be designed to better interact with the conjugated polymer. We synthesized N-9′-heptadecanyl-2,7-carbazole functionalized SiPc ((CBzPho)₂-SiPc), a ternary additive with increased miscibility in poly[N-90-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT). The resulting additive was included into PCDTBT and [6,6]-phenyl C₇₁ butyric acid methyl ester as bulk (PC₇₁BM) heterojunction OPV devices as a ternary additive. While the (CBzPho)₂-SiPc demonstrated strong EQE >30% contribution in the range of 650–730 nm, the overall performance was reduced because (CBzPho)₂-SiPc acted as a hole trap due to its high-lying HOMO energy level. This study demonstrates the importance of the solubility, miscibility, and energy level engineering of the ternary additive when designing organic photovoltaic devices.

Silicon phthalocyanines with carbazole axial functional groups were synthesized to improve the miscibility in PCDTBT and for use as ternary additives in organic photovoltaics.

Durable polymer solar cells produced by the encapsulation of a WSe2 hole-transport layer and β-carotene as an active layer additive

WSe2 nanoflakes are obtained by liquid-phase exfoliation. Polymer solar cells with NF-WSe2 as the hole transport layer (HTL) are realized with superior photovoltaic characteristics.

Theoretical studies of optoelectronic and photovoltaic properties of D-A polymer monomers by Density Functional Theory (DFT)

In this research article, the new donor–acceptor (D–A) monomers developed using 4-methoxy-9-methyl-9 H-carbazole (MMCB) as electron donors and various electron acceptors. DFT and TD-DFT methods at the level of B3LYP with a 6–311 G basis set in a gas and chloroform solvent were used to calculate electronic and optoelectronic properties. To dissect the relationship between the molecular and optoelectronic structures, the impacts of specific acceptors on the geometry of molecules and optoelectronic properties of these D–A monomers were discussed. The calculations are also carried out on HOMO–LUMO, atomic orbital densities. The calculated band gap E_g of the monomers considered increases 3,6-MMCB-OCP ≈ 3,6-MMCB-BCO < 3,6-MMCB-SDP < 3,6-MMCB-SCP < 3,6-MMCB-TCP < 3,6-MMCB-TDP < 3,6-MMCB-BCS < 3,6-MMCB-BCT in both in the gas and solvent phases. Subsequently, the optoelectrical properties of E_HOMO, E_LUMO, E_opt, and E_B energies were critically updated. Compared to different monomers, the far lower E_g of the 3,6-MMCB-OCP and 3,6-CB-BCO has shown optoelectronic applications in organic solar cells like BHJ.

p-Cymene: A Sustainable Solvent that is Highly Compatible with Direct Arylation Polymerization (DArP)

For over a decade, Direct Arylation Polymerization (DArP) has been demonstrated to be an eco-friendly, facile, and low-cost alternative to conventional methodologies such as Stille polymerization for conjugated polymer synthesis. By accessing through a C-H activation pathway, DArP offers a reduction of synthetic steps while eliminating the generation of stoichiometric, highly toxic organotin byproducts. However, as the major component in these reactions, the solvents most prevalently employed for DArP are hazardous and produced from unsustainable sources, such as dimethylacetamide (DMA), tetrahydrofuran (THF), and toluene. Although the use of sustainable alternative solvents such as 2-MeTHF and cyclopentyl methyl ether (CPME) has recently emerged, drawbacks of ethereal solvents include the need for a pressurized reaction setup as well as potential peroxide formation. While aromatic solvents are superior in solubilizing conjugated polymers, very little has been done in searching for more sustainable, benign alternatives for this class of solvent. Herein, we report the application of a sustainable, naturally sourced, high-boiling aromatic solvent, p-cymene, to DArP for the first time. p-Cymene was found to display excellent solubilizing ability in the synthesis of a broad scope of alternating copolymers with M_n up to 51.3 kg/mol and yields up to 96.2%, outperforming those prepared using CPME and toluene. Structural analysis revealed the exclusion of defects in these polymers prepared using p-cymene as the solvent which, in the case of a 2,2'-bithiophene monomer, is challenging to access through the use of conventional solvents for DArP, such as DMA and toluene.

Theoretical design of low bandgap donor-acceptor (D-A) monomers for polymer solar cells: DFT and TD-DFT study

Endeavors have been made to construct new donor-acceptor (D-A) monomers utilizing 9 H-carbazole (CB) as electron donors and different electron acceptors. All estimations were finished using DFT and TD-DFT, and B3LYP level with a 6-311 G basis set in the gas and chloroform solvent. The impacts of the distinctive acceptors on the geometry of molecules and optoelectronic properties of these D-A monomers were discussed to dissect the connection connecting the molecular structures and the optoelectronic properties. Likewise, the HOMO - LUMO energies, atomic orbital densities are calculated theoretically. Notwithstanding the charge transfer measure between the carbazole electron donor unit and the electron acceptor one is upheld by breaking down the optical spectra of the acquired monomers and the restriction of involved HOMO and LUMO. The outcomes show that the D-A monomers, CB-ODP, CB-TDP, and CB-SDP, are acceptable for optoelectronic applications in organic solar cells like BHJ.

Copolymers based on trialkylsilylethynyl-phenyl substituted benzodithiophene building blocks for efficient organic solar cells

Trialkylsilylethynyl-phenyl was explored as a side chain to construct a benzodithiophene-containing polymer that demonstrated a PCE of 7.81% in organic solar cells.

Room-temperature blue-light-emitting liquid crystalline materials based on phenanthroimidazole-substituted carbazole derivatives

We report disc-shaped materials with the A–Π–D molecular architecture based on the phenanthroimidazole-linked carbazole group as blue-light-emitting materials with room-temperature liquid crystalline properties.

End Group Engineering on the Side Chains of Conjugated Polymers toward Efficient Non-Fullerene Organic Solar Cells

Side chains properties of conjugated polymers, such as the length, branching point, and heteroatom, have been widely studied for application in organic solar cells (OSCs), but the end groups of side chains have been rarely reported. In this work, we systematically explored a series of new conjugated polymers with distinct side-chain end groups for high performance non-fullerene OSCs. The key components for the polymers contained functionalized units as the end groups of side chains, such as Br, alkyloxy (OMe), and alkylthienyl (T) groups. We found that the new conjugated polymers have similar absorption spectra and crystallinity with the polymer without substitution, but they showed distinct photovoltaic performance in solar cells. When the polymer without functionalized units had a power conversion efficiency (PCE) of 9.94%, the modified conjugated polymers provided high PCEs of over 13% with significantly enhanced photocurrent and fill factors. In addition, they also show additive-free and highly stable characteristics. These results demonstrate that end group engineering on side chains is a promising strategy to design new conjugated polymers toward efficient OSCs.

Two competing acceptors: Electronic structure of PNDITBT probed by time-resolved electron paramagnetic resonance spectroscopy

Balanced charge transport is particularly important for transistors. Hence, ambipolar organic semiconductors with comparable transport capabilities for both positive and negative charges are highly sought-after. Here, we report detailed insights into the electronic structure of PNDITBT, which is an alternating copolymer of naphthalene diimide (NDI), thiophene, benzothiodiazole (B), and thiophene (T) units, as gained by time-resolved electron paramagnetic resonance (TREPR) spectroscopy combined with quantum-chemical calculations. The results are compared to those obtained for PNDIT2 and PCDTBT, which are derivatives without B and NDI acceptor units, respectively. These two polymers show dominant n- and p-channel behavior in organic field-effect transistors. The TBT moiety clearly dominates the electronic structure of PNDITBT, although less so than in PCDTBT. Furthermore, the triplet exciton most probably delocalizes along the backbone, exhibits a highly homogeneous environment, and planarizes the polymer backbone. Obtaining the zero-field splitting tensors of these triplet states by means of quantum-chemical calculations reveals the triplet energy sublevel associated with the molecular axis parallel to the backbone to be preferentially populated, while the one perpendicular to the aromatic plane is not populated at all, consistent with the spin-density distribution. PNDITBT consisting of two acceptors (NDI and B) has a complex electronic structure, as evident from the two charge-transfer bands in its absorption spectrum. TREPR spectroscopy provides a detailed insight on a molecular level not available by and complementing other methods.

Impact of Side Chains of Conjugated Polymers on Electronic Structure: A Case Study

Processing from solution is a crucial aspect of organic semiconductors, as it is at the heart of the promise of easy and inexpensive manufacturing of devices. Introducing alkyl side chains is an approach often used to increase solubility and enhance miscibility in blends. The influence of these side chains on the electronic structure, although highly important for a detailed understanding of the structure-function relationship of these materials, is still barely understood. Here, we use time-resolved electron paramagnetic resonance spectroscopy with its molecular resolution to investigate the role of alkyl side chains on the polymer PCDTBT and a series of its building blocks with increasing length. Comparing our results to the non-hexylated compounds allows us to distinguish four different factors determining exciton delocalization. Detailed quantum-chemical calculations (DFT) allows us to further interpret our spectroscopic data and to relate our findings to the molecular geometry. Alkylation generally leads to more localized excitons, most prominent only for the polymer. Furthermore, singlet excitons are more delocalized than the corresponding triplet excitons, despite the larger dihedral angles within the backbone found for the singlet-state geometries. Our results show TREPR spectroscopy of triplet excitons to be well suited for investigating crucial aspects of the structure-function relationship of conjugated polymers used as organic semiconductors on a molecular basis.

Fast and Selective Post-polymerization Modification of Conjugated Polymers Using Dimethyldioxirane

Modification of functional groups attached to conjugated polymer backbones can drastically alter the material properties. Oxidation of electron-donating thioalkyl substituents to electron-withdrawing sulfoxides or sulfones is a particularly effective modification. However, so far, this reaction has not been studied for the modification of conjugated polymers used in organic electronics. Crucial questions regarding selectivity and reaction time waited to be addressed. Here, we show that the reaction is highly selective and complete within just a few minutes when using dimethyldioxirane (DMDO) for the oxidation of thioalkyl substituents attached to the well-investigated conjugated polymers poly(9-(1-octylnonyl)carbazole-alt-4,7-dithienylbenzothiadiazole) (PCDTBT) and poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT). The selectivity was confirmed by comparison with polymers obtained from pre-oxidized monomers and by control experiments using related polymers without thioalkyl substituents. Using DMDO, the oxidation yields acetone as the only side-product, which reduces the work-up to mere evaporation of solvents and excessive reagent. Our results show that this oxidation is an exciting method for the preparation of electron-deficient conjugated polymers. It may even allow the preparation of electron acceptors for solar cells directly from the electron donors.

Structure-Function Relationship of Organic Semiconductors: Detailed Insights From Time-Resolved EPR Spectroscopy

Organic photovoltaics (OPV) is a promising technology to account for the increasing demand for energy in form of electricity. Whereas the last decades have seen tremendous progress in the field witnessed by the steady increase in efficiency of OPV devices, we still lack proper understanding of fundamental aspects of light-energy conversion, demanding for systematic investigation on a fundamental level. A detailed understanding of the electronic structure of semiconducting polymers and their building blocks is essential to develop efficient materials for organic electronics. Illuminating conjugated polymers not only leads to excited states, but sheds light on some of the most important aspects of device efficiency in organic electronics as well. The interplay between electronic structure, morphology, flexibility, and local ordering, while at the heart of structure-function relationship of organic electronic materials, is still barely understood. (Time-resolved) electron paramagnetic resonance (EPR) spectroscopy is particularly suited to address these questions, allowing one to directly detect paramagnetic states and to reveal their spin-multiplicity, besides its clearly superior spectral resolution compared to optical methods. This article aims at giving a non-specialist audience an overview of what EPR spectroscopy and particularly its time-resolved variant (TREPR) can contribute to unraveling aspects of structure-function relationship in organic semiconductors.

Influence of an ester directing-group on defect formation in the synthesis of conjugated polymers via direct arylation polymerization (DArP) using sustainable solvents

We report the application of green solvents in DArP and the structure-dependent β-defect formation due to an ester directing group.

Ternary organic solar cells with a phase-modulated surface distribution via the addition of a small molecular luminescent dye to obtain a high efficiency over 10.5

Incorporation of a ternary organic component is an effective strategy to enhance the performance of bulk heterojunction (BHJ) organic solar cells (OSCs). In this study, a small molecule luminescent dye, C545T, was first doped into blends of PTB7-Th/PC71BM and PTB7/PC71BM as a third component to fabricate ternary OSCs. It is demonstrated that C545T can disrupt the severe vertical distribution in the binary blend and effectively modulate the novel surface chemical configuration by improving the self-assembly process of the polymer donor, as a result of the good miscibility among the active layer materials and the π-π interactions between PC71BM and C545T. The obtained homogeneously bicontinuous BHJ with numerous interpenetrating nanofibers optimizes the domain size of exciton diffusion and the length of charge transfer. The energy transfer between C545T and polymers changes the transmission path of photo-generated excitons, which together improve the exciton dissociation process and reduce the recombination loss. Champion power conversion efficiencies (PCEs) of 10.69% and 9.42% were achieved by the ternary blends of PTB7-Th/C545T/PC71BM and PTB7/C545T/PC71BM, respectively, which correspond to a nearly 20% enhancement over their binary counterparts.

Stepwise heating in Stille polycondensation toward no batch-to-batch variations in polymer solar cell performance

For a given π-conjugated polymer, the batch-to-batch variations in molecular weight (M_w) and polydispersity index (Ð) can lead to inconsistent process-dependent material properties and consequent performance variations in the device application. Using a stepwise-heating protocol in the Stille polycondensation in conjunction with optimized processing, we obtained an ultrahigh-quality PTB7 polymer having high M_w and very narrow Ð. The resulting ultrahigh-quality polymer-based solar cells demonstrate up to 9.97% power conversion efficiencies (PCEs), which is over 24% enhancement from the control devices fabricated with commercially available PTB7. Moreover, we observe almost negligible batch-to-batch variations in the overall PCE values from ultrahigh-quality polymer-based devices. The proposed stepwise polymerization demonstrates a facile and effective strategy for synthesizing high-quality semiconducting polymers that can significantly improve device yield in polymer-based solar cells, an important factor for the commercialization of organic solar cells, by mitigating device-to-device variations.

cis-1 Isomers of tethered bismethano[70]fullerene as electron acceptors in organic photovoltaics

Isomer-controlled [70]fullerene bis-adducts can achieve high performance as electron-acceptors in organic photovoltaics (OPVs) because of their stronger absorption intensities than [60]fullerene derivatives, higher LUMO energy levels than mono-adducts, and less structural and energetic disorder than random isomer mixtures. Especially, attractive are cis-1 isomers that have the closest proximity of addends owing to their plausible more regular close packed structure. In this study, propylene-tethered cis-1 bismethano[70]fullerene with two methyl, ethyl, phenyl, or thienyl groups were rationally designed and prepared for the first time to investigate the OPV performances with an amorphous conjugated polymer donor (PCDTBT). The cis-1 products were found to be a mixture of two regioisomers, α-1-α and α-1-β as major and minor components, respectively. Among them, the cis-1 product with two ethyl groups (Et₂-cis-1-[70]PBC) showed the highest OPV performance, encouraging us to isolate its α-1-α isomer (Et₂-α-1-α-[70]PBC) by high-performance liquid chromatography. OPV devices based on Et₂-cis-1-[70]PBC and Et₂-α-1-α-[70]PBC with PCDTBT showed open-circuit voltages of 0.844 V and 0.864 V, respectively, which were higher than that of a device with typical [70]fullerene mono-adduct, [70]PCBM (0.831 V) with a lower LUMO level. However, the short-circuit current densities and resultant power conversion efficiencies of the devices with Et₂-cis-1-[70]PBC (9.24 mA cm⁻², 4.60%) and Et₂-α-1-α-[70]PBC (6.35 mA cm⁻², 3.25%) were lower than those of the device with [70]PCBM (10.8 mA cm⁻², 5.8%) due to their inferior charge collection efficiencies. The results obtained here reveal that cis-1 [70]fullerene bis-adducts do not guarantee better OPV performance and that further optimization of the substituent structures is necessary.

cis-1 Isomers of [70]fullerene bis-adducts were utilized as electron-acceptors in organic photovoltaic devices for the first time.

Linear-type carbazoledioxazine-based organic semiconductors: the effect of backbone planarity on the molecular orientation and charge transport properties

We report the synthesis of a linear-type dibromocarbazoledioxazine (CZ) derivative as a new precursor for semiconducting polymers. The chemical structures of the CZ unit and its polymers with thiophene or thienothiophene spacers (namely, PCZT and PCZTT) were fully characterized. PCZT and PCZTT possessed similar medium optical band gap (E^opt_g) and electrochemical band gap (E^cv_g) of around 1.70 eV estimated from the onset absorption and electrochemical redox potentials of the thin films, respectively. Computational density functional theory (DFT) calculations suggested that the backbone of the PCZT might be highly twisted, while that of PCZTT could be very planar. The effect of different backbone geometries on the charge–transport properties was studied by using thin film transistors (TFTs). The TFT device based on PCZTT showed a four times higher hole mobility as compared to that based on PCZT. The superior TFT performances of PCZTT were reasonably attributed to its edge-on backbone packing orientations toward the Si substrate revealed by the grazing-incidence wide-angle X-ray scattering (GIWAXS), which was favorable for in-plane charge transport in the TFT devices.

A linear-type dibromocarbazoledioxazine (CZ) derivative and its two polymers are newly designed and synthesized. Structure–property relationship studies reveal that PCZTT shows a four times higher hole mobility than PCZT.

Fast oxygen diffusion and iodide defects mediate oxygen-induced degradation of perovskite solar cells

Methylammonium lead halide perovskites are attracting intense interest as promising materials for next-generation solar cells, but serious issues related to long-term stability need to be addressed. Perovskite films based on CH₃NH₃PbI₃ undergo rapid degradation when exposed to oxygen and light. Here, we report mechanistic insights into this oxygen-induced photodegradation from a range of experimental and computational techniques. We find fast oxygen diffusion into CH₃NH₃PbI₃ films is accompanied by photo-induced formation of highly reactive superoxide species. Perovskite films composed of small crystallites show higher yields of superoxide and lower stability. Ab initio simulations indicate that iodide vacancies are the preferred sites in mediating the photo-induced formation of superoxide species from oxygen. Thin-film passivation with iodide salts is shown to enhance film and device stability. The understanding of degradation phenomena gained from this study is important for the future design and optimization of stable perovskite solar cells.

Determining the degradation pathways in lead halide perovskites is key to its development as a viable solar technology. Aristidou et al. identify iodide vacancies as the preferred sites in mediating the photo-induced formation of superoxide species from intercalated oxygen.

Direct heteroarylation polymerization: guidelines for defect-free conjugated polymers

Direct (hetero)arylation polymerization (DHAP) has emerged as a valuable and atom-economical alternative to traditional cross-coupling methods for the synthesis of low-cost and efficient conjugated polymers for organic electronics. However, when applied to the synthesis of certain (hetero)arene-based materials, a lack of C-H bond selectivity has been observed. To prevent such undesirable side-reactions, we report the design and synthesis of new, bulky, phosphine-based ligands that significantly enhance selectivity of the DHAP process for both halogenated and non-halogenated electron-rich and electron-deficient thiophene-based comonomers. To better understand the selectivity issues, density functional theory (DFT) calculations have been performed on various halogenated and non-halogenated electron-rich and electron-deficient thiophene-based comonomers. Calculations showed that the presence of bromine atoms decreases the energy of activation (Ea) of the adjacent C-H bonds, allowing undesirable β-defects for some brominated aromatic units. Both calculations and the new ligands should lead to the rational design of monomers and methods for the preparation of defect-free conjugated polymers from DHAP.

Analysis and modification of defective surface aggregates on PCDTBT:PCBM solar cell blends using combined Kelvin probe, conductive and bimodal atomic force microscopy

Organic photovoltaic systems comprising donor polymers and acceptor fullerene derivatives are attractive for inexpensive energy harvesting. Extensive research on polymer solar cells has provided insight into the factors governing device-level efficiency and stability. However, the detailed investigation of nanoscale structures is still challenging. Here we demonstrate the analysis and modification of unidentified surface aggregates. The aggregates are characterized electrically by Kelvin probe force microscopy and conductive atomic force microscopy (C-AFM), whereby the correlation between local electrical potential and current confirms a defective charge transport. Bimodal AFM modification confirms that the aggregates exist on top of the solar cell structure, and is used to remove them and to reveal the underlying active layer. The systematic analysis of the surface aggregates suggests that the structure consists of PCBM molecules.

Carbazole-based copolymers via direct arylation polymerization (DArP) for Suzuki-convergent polymer solar cell performance

Direct arylation polymerization (DArP) is used to synthesize a variety of carbazole-based copolymers for evaluation in solar cells.

To branch or not to branch: C–H selectivity of thiophene-based donor–acceptor–donor monomers in direct arylation polycondensation exemplified by PCDTBT

The possibility for unselective C–H activation of a thiophene-based, donor–acceptor–donor monomer during direct arylation polycondensation is investigated.

A study of fused-ring thieno[3,4-e]pyrazine polymers as n-type materials for organic supercapacitors

Conjugated polymer pseudocapacitors achieve high capacitances because they store charge through fast, reversible redox reactions.

Long-Term Stable Organic Photodetectors with Ultra Low Dark Currents for High Detectivity Applications

Printed organic photodetectors can transform plastic, paper or glass into smart surfaces. This innovative technology is now growing exponentially due to the strong demand in human-machine interfaces. To date, only niche markets are targeted since organic sensors still present reduced performances in comparison with their inorganic counterparts. Here we demonstrate that it is possible to engineer a state-of-the-art organic photodetector approaching the performances of Si-based photodiodes in terms of dark current, responsivity and detectivity. Only three solution-processed layers and two low-temperature annealing steps are needed to achieve the performance that is significantly better than most of the organic photodetectors reported so far. We also perform a long-term ageing study. Lifetimes of over 14,000 hours under continuous operation are more than promising and demonstrate that organic photodetectors can reach a competitive level of stability for successful commercialization of this new and promising technology.

Compatibilization of All-Conjugated Polymer Blends for Organic Photovoltaics

Compatibilization of an immiscible binary blend comprising a conjugated electron donor and a conjugated electron acceptor polymer with suitable electronic properties upon addition of a block copolymer (BCP) composed of the same building blocks is demonstrated. Efficient compatibilization during melt-annealing is feasible when the two polymers are immiscible in the melt, i.e. above the melting point of ∼250 °C of the semicrystalline donor polymer P3HT. To generate immiscibility at these high temperatures, the acceptor polymer PCDTBT is equipped with fluorinated side chains leading to an increased Flory-Huggins interaction parameter. Compatibilization in bulk and thin films is demonstrated, showing that the photovoltaic performance of pristine microphase separated and nanostructured BCPs can also be obtained for compatibilized blend films containing low contents of 10-20 wt % BCP. Thermodynamically stable domain sizes range between several tens of microns for pure blends and ∼10 nm for pure block copolymers. In addition to controlling domain size, the amount of block copolymer added dictates the ratio of edge-on and face-on P3HT crystals, with compatibilized films showing an increasing amount of face-on P3HT crystals with increasing amount of compatibilizer. This study demonstrates the prerequisites and benefits of compatibilizing all-conjugated semicrystalline polymer blends for organic photovoltaics.

Regioisomer effects of [70]fullerene mono-adduct acceptors in bulk heterojunction polymer solar cells

Regioisomer separations of [70]fullerene mono-adducts for polymer solar cell (PSC) applications were conducted for the first time.

Despite numerous organic semiconductors being developed during the past decade, C₇₀ derivatives are predominantly used as electron acceptors in efficient polymer solar cells (PSCs). However, as-prepared C₇₀ mono-adducts intrinsically comprise regioisomers that would mask individual device performances depending on the substituent position on C₇₀. Herein, we separate the regioisomers of C₇₀ mono-adducts for PSC applications for the first time. Systematic investigations of the substituent position effect using a novel symmetric C₇₀ mono-adduct ([70]NCMA) and a prevalent, high-performance one ([70]PCBM) reveals that we can control the structures of the blend films with conjugated polymers and thereby improve the PSC performances by regioisomer separation. Our approach demonstrates the significance of exploring the best-matching regioisomer of C₇₀ mono-adducts with high-performance conjugated polymers, which would achieve a remarkable progress in PSC devices.