Effect of Alkylsilyl Side‐Chain Structure on Photovoltaic Properties of Conjugated Polymer Donors

Side‐chain engineering is an important strategy for optimizing photovoltaic properties of organic photovoltaic materials. In this work, the effect of alkylsilyl side‐chain structure on the photovoltaic properties of medium bandgap conjugated polymer donors is studied by synthesizing four new polymers J70 , J72 , J73 , and J74 on the basis of highly efficient polymer donor J71 by changing alkyl substituents of the alkylsilyl side chains of the polymers. And the photovoltaic properties of the five polymers are studied by fabricating polymer solar cells (PSCs) with the polymers as donor and an n‐type organic semiconductor (n‐OS) m‐ITIC as acceptor. It is found that the shorter and linear alkylsilyl side chain could afford ordered molecular packing, stronger absorption coefficient, higher charge carrier mobility, thus results in higher J_sc and fill factor values in the corresponding PSCs. While the polymers with longer or branched alkyl substituents in the trialkylsilyl group show lower‐lying highest occupied molecular orbital energy levels which leads to higher V_oc of the PSCs. The PSCs based on J70 :m‐ITIC and J71 :m‐ITIC achieve power conversion efficiency (PCE) of 11.62 and 12.05%, respectively, which are among the top values of the PSCs reported in the literatures so far.     

Improvement of Photovoltaic Performance of Polymer Solar Cells by Rational Molecular Optimization of Organic Molecule Acceptors

Two n‐type organic semiconductor (n‐OS) small molecules m‐ITIC‐2F and m‐ITIC‐4F with fluorinated 2‐(2,3‐dihydro‐3‐oxo‐1H‐inden‐1‐ylidene)propanedinitrile (IC) terminal moieties are prepared, for the application as an acceptor in polymer solar cells (PSCs), to further improve the photovoltaic performance of the n‐OS acceptor 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene) indanone) ‐5,5,11,11‐tetrakis(3‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐sindaceno[1,2‐b:5,6‐b′]‐dithiophene (m‐ITIC). Compared to m‐ITIC, these two new acceptors show redshifted absorption, higher molecular packing order, and improved electron mobilities. The power conversion efficiencies (PCE) of the as‐cast PSCs with m‐ITIC‐2F or m‐ITIC‐4F as an acceptor and a low‐cost donor–acceptor (D–A) copolymer PTQ10 as a donor reach 11.57% and 11.64%, respectively, which are among the highest efficiency for the as‐cast PSCs so far. Furthermore, after thermal annealing treatment, improved molecular packing and enhanced phase separation are observed, and the higher PCE of 12.53% is achieved for both PSCs based on the two acceptors. The respective and unique advantage with the intrinsic high degree of order, molecular packing, and electron mobilities of these two acceptors will be suitable to match with different p‐type organic semiconductor donors for higher PCE values, which provide a great potential for the PSCs commercialization in the near future. These results indicate that rational molecular structure optimization is of great importance to further improve photovoltaic properties of the photovoltaic materials.     

Side‐Chain Impact on Molecular Orientation of Organic Semiconductor Acceptors: High Performance Nonfullerene Polymer Solar Cells with Thick Active Layer over 400 nm

A new n‐type organic semiconductor (n‐OS) acceptor IDTPC with n‐hexyl side chains is developed. Compared to side chains with 4‐hexylphenyl counterparts (IDTCN), such a design endows the acceptor of IDTPC with higher electron mobility, more ordered face‐on molecular packing, and lower band gap. Therefore, the IDTPC‐based polymer solar cells (PSCs) with a newly developed wide bandgap polymer PTQ10 as donor exhibit the maximum power conversion efficiency (PCE) of 12.2%, a near 65% improvement in PCE relative to the IDTCN‐based control device. Most importantly, the IDTPC‐based device is insensitive to the thickness of the active layer from 70 to 505 nm, which still gives a PCE of 10.0% with the active‐layer thickness of 400 nm. To the best of the authors' knowledge, a PCE of 10.0% is the highest value for the nonfullerene PSCs with an active layer thicker than 400 nm. These results reveal that the blend of PTQ10 and IDTPC exhibits great potential for highly efficient nonfullerene PSCs and large‐area device fabrication.     

Medium Bandgap Polymer Donor Based on Bi(trialkylsilylthienyl‐benzo[1,2‐b:4,5‐b′]‐difuran) for High Performance Nonfullerene Polymer Solar Cells

A new 2D‐conjugated medium bandgap donor–acceptor copolymer, J81 , based on benzodifuran with trialkylsilyl thiophene side chains as donor unit and fluorobenzothiazole as acceptor, is synthesized and successfully used in nonfullerene polymer solar cells (PSCs) with low bandgap n‐type organic semiconductor (n‐OS) 3,9‐bis(2‐methylene‐ (3‐(1,1‐dicyanomethylene)‐indanone)‐5,5,11,11‐tetrakis(4‐ hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]‐ dithiophene (ITIC) and m ‐ITIC as acceptor. J81 possesses a lower‐lying highest occupied molecular orbital (HOMO) energy level of ?5.43 eV and medium bandgap of 1.93 eV with complementary absorption in the visible–near infrared region with the n‐OS acceptor. The PSCs based on J81 :ITIC and J81 :m ‐ITIC yield high power conversion efficiency of 10.60% and 11.05%, respectively, with high V _oc of 0.95–0.96 V benefit from the lower‐lying HOMO energy level of J81 donor. The work indicates that J81 is another promising polymer donor for the nonfullerene PSCs.     

Minimal ensembles of side chain conformers for modeling protein–protein interactions

The goal of this article is to reduce the complexity of the side chain search within docking problems. We apply six methods of generating side chain conformers to unbound protein structures and determine their ability of obtaining the bound conformation in small ensembles of conformers. Methods are evaluated in terms of the positions of side chain end groups. Results for 68 protein complexes yield two important observations. First, the end‐group positions change less than 1 Å on association for over 60% of interface side chains. Thus, the unbound protein structure carries substantial information about the side chains in the bound state, and the inclusion of the unbound conformation into the ensemble of conformers is very beneficial. Second, considering each surface side chain separately in its protein environment, small ensembles of low‐energy states include the bound conformation for a large fraction of side chains. In particular, the ensemble consisting of the unbound conformation and the two highest probability predicted conformers includes the bound conformer with an accuracy of 1 Å for 78% of interface side chains. As more than 60% of the interface side chains have only one conformer and many others only a few, these ensembles of low‐energy states substantially reduce the complexity of side chain search in docking problems. This approach was already used for finding pockets in protein–protein interfaces that can bind small molecules to potentially disrupt protein–protein interactions. Side‐chain search with the reduced search space will also be incorporated into protein docking algorithms. Proteins 2012. © 2011 Wiley Periodicals, Inc.     

Effects of Shortened Alkyl Chains on Solution‐Processable Small Molecules with Oxo‐Alkylated Nitrile End‐Capped Acceptors for High‐Performance Organic Solar Cells

Solution‐processable small molecules are significant for producing high‐performance bulk heterojunction organic solar cells (OSCs). Shortening alkyl chains, while ensuring proper miscibility with fullerene, enables modulation of molecular stacking, which is an effective method for improving device performance. Here, the design and synthesis of two solution‐processable small molecules based on a conjugated backbone with a novel end‐capped acceptor (oxo–alkylated nitrile) using octyl and hexyl chains attached to π–bridge, and octyl and pentyl chains attached to the acceptor is reported. Shortening the length of the widely used octyl chains improves self‐assembly and device performance. Differential scanning calorimetry and grazing incidence X‐ray diffraction results demonstrated that the molecule substituted by shorter chains shows tighter molecular stacking and higher crystallinity in the mixture with 6,6‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) and that the power conversion efficiency (PCE) of the OSC is as high as 5.6% with an open circuit voltage (V_oc) of 0.87 V, a current density (J_sc) of 9.94 mA cm^‐2, and an impressive filled factor (FF) of 65% in optimized devices. These findings provide valuable insights into the production of highly efficient solution‐processable small molecules for OSCs.     

Significant Influence of the Methoxyl Substitution Position on Optoelectronic Properties and Molecular Packing of Small‐Molecule Electron Acceptors for Photovoltaic Cells

Molecular engineering of nonfullerene electron acceptors is of great importance for the development of organic photovoltaics. In this study, a series of methoxyl‐modified dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐based small‐molecule acceptor (SMA) isomers are synthesized and characterized to determine the effect of substitution position of the terminal group in these acceptor–donor–acceptor‐type SMAs. Minor changes in the substitution position are demonstrated to greatly influence the optoelectronic properties and molecular packing of the isomers. Note that SMAs with planar molecular backbones show more ordered molecular packing and smaller π–π stacking distances, thus dramatically higher electron mobilities relative to their counterparts with distorted end‐groups. By utilizing polymer poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophen)‐co‐(1,3‐di(5‐thiophene‐2‐yl)‐5,7‐bis(2‐ethylhexyl)benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione)] (PBDB‐T) as an electron donor, an optimum power conversion efficiency (PCE) of 11.9% is achieved in the device based on PBDB‐T:IT‐OM‐2, which is among the top efficiencies reported as of yet. Moreover, the PCE stays above 10% as the film thickness increases to 250 nm, which is very advantageous for large‐area printing. Overall, the intrinsic molecular properties as well as the morphologies of blends can be effectively modulated by manipulating the substituent position on the terminal groups, and the structure–property relationships gleaned from this study will aid in designing more efficient SMAs for versatile applications.     

Regulation of protein function: crystal packing interfaces and conformational dimerization

The accepted view of interprotein electron transport involves molecules diffusing between donor and acceptor redox sites. An emerging alternative hypothesis is that efficient long-range electron transport can be achieved through proteins arranged in supramolecular assemblies. In this study, we have investigated the crystal packing interfaces in three crystal forms of plastocyanin, an integral component of the photosynthetic electron transport chain, and discuss their potential relevance to in vivo supramolecular assemblies. Symmetry-related protein chains within these crystals have Cu-Cu separations of <25 A, a distance that readily supports electron transfer. In one structure, the plastocyanin molecule exists in two forms in which a backbone displacement coupled with side chain rearrangements enables the modulation of protein-protein interfaces.     

Structure of a cellulose I–ethylenediamine complex

The structure of a crystalline cellulose I–ethylenediamine complex has been determined by x-ray diffraction methods as part of an investigation of cellulose–solvent interaction. The complex studied is that formed when native ramie fibers are swollen in ethylenediamine and then vacuum-dried. The unit cell is monoclinic with dimensions a = 12.87 Å, b = 9.52 Å, c = 10.35 Å, and γ = 118.8°, and it contains disaccharide segments of two chains, with one ethylenediamine per glucose residue. The refined model contains parallel cellulose chains that are linked by hydrogen-bonded ethylenediamine molecules. The chains along the b-axis are packed in register, leading to stacks of chains analogous to those in chitin. All the hydroxyl groups are satisfactorily hydrogen-bonded and each ethylenediamine forms four donor and two acceptor hydrogen bonds. From this work it can be seen that the interaction of cellulose I with ethylenediamine involves scission of the intermolecular hydrogen bonds followed by disruption of the stacks of quarter-staggered chains.     

Side‐Chain Engineering for Enhancing the Properties of Small Molecule Solar Cells: A Trade‐off Beyond Efficiency

Three small molecules with different substituents on bithienyl‐benzo[1,2‐b:4,5‐b′]dithiophene (BDTT) units, BDTT‐TR (meta‐alkyl side chain), BDTT‐O‐TR (meta‐alkoxy), and BDTT‐S‐TR (meta‐alkylthio), are designed and synthesized for systematically elucidating their structure–property relationship in solution‐processed bulk heterojunction organic solar cells. Although all three molecules show similar molecular structures, thermal properties and optical band gaps, the introduction of meta‐alkylthio‐BDTT as the central unit in the molecular backbone substantially results in a higher absorption coefficient, slightly lower highest occupied molecular orbital level and significantly more efficient and balanced charge transport property. The bridging atom in the meta‐position to the side chain is found to impact the microstructure formation which is a subtle but decisive way: carrier recombination is suppressed due to a more balanced carrier mobility and BDTT based devices with the meta‐alkylthio side chain (BDTT‐S‐TR) show a higher power conversion efficiency (PCE of 9.20%) as compared to the meta‐alkoxy (PCE of 7.44% for BDTT‐TR) and meta‐alkyl spacer (PCE of 6.50% for BDTT‐O‐TR). Density functional density calculations suggest only small variations in the torsion angle of the side chains, but the nature of the side chain linkage is further found to impact the thermal as well as the photostability of corresponding devices. The aim is to provide comprehensive insight into fine‐tuning the structure–property interrelationship of the BDTT material class as a function of side chain engineering.     

Conformational Disorder Enhances Solubility and Photovoltaic Performance of a Thiophene–Quinoxaline Copolymer

The side‐chain architecture of alternating copolymers based on thiophene and quinoxaline (TQ) is found to strongly influence the solubility and photovoltaic performance. In particular, TQ polymers with different linear or branched alkyloxy‐phenyl side chains on the quinoxaline unit are compared. Attaching the linear alkyloxy side‐chain segment at the meta‐ instead of the para‐position of the phenyl ring reduces the planarity of the backbone as well as the ability to order. However, the delocalisation across the backbone is not affected, which permits the design of high‐performance TQ polymers that do not aggregate in solution. The use of branched meta‐(2‐ethylhexyl)oxy‐phenyl side‐chains results in a TQ polymer with an intermediate degree of order. The reduced tendency for aggregation of TQ polymers with linear meta‐alkyloxy‐phenyl persists in the solid state. As a result, it is possible to avoid the decrease in charge‐transfer state energy that is observed for bulk‐heterojunction blends of more ordered TQ polymers and fullerenes. The associated gain in open‐circuit voltage of disordered TQ:fullerene solar cells, accompanied by a higher short‐circuit current density, leads to a higher power conversion efficiency overall. Thus, in contrast to other donor polymers, for TQ polymers there is no need to compromise between solubility and photovoltaic performance.     

Hexylthiophene‐Featured D–A–π–A Structural Indoline Chromophores for Coadsorbent‐Free and Panchromatic Dye‐Sensitized Solar Cells

For a sensitizer with a strong π‐conjugation system, a coadsorbent is needed to hinder dye aggregation. However, coadsorption always brings a decrease in dye coverage on the TiO₂ surface. Organic ‘‘D–A–π–A’’ dyes, WS‐6 and WS‐11, are designed and synthesized based on the known WS‐2 material for coadsorbent‐free, dye‐sensitized solar cells (DSSCs). Compared with the traditional D–π–A structure, these D–A–π–A indoline dyes, with the additional incorporated acceptor unit of benzothiadiazole in the π‐conjugation, exhibit a broad photoresponse, high redox stability, and convenient energy‐level tuning. The attached n‐hexyl chains in both dyes are effective to suppress charge recombination, resulting in a decreased dark current and enhanced open‐circuit voltage. Electrochemical impedance spectroscopy studies indicate that both the resistance for charge recombination and the electron lifetime are increased after the introduction of alkyl chains to the dye molecules. Without deoxycholic acid coadsorption, the power‐conversion efficiency of WS‐6 (7.76%) on a 16 μm‐thick TiO₂ film device is 45% higher than that of WS‐2 (5.31%) under the same conditions. The additional n‐hexylthiophene in WS‐11 extends the photoresponse to a panchromatic spectrum but causes a low incident photon‐to‐current conversion efficiency.     

Understanding the Impact of Oligomeric Polystyrene Side Chain Arrangement on the All‐Polymer Solar Cell Performance

The introduction of oligomeric polystyrene (PS) side chains into the conjugated backbone is proven to enhance the processability and electronic properties of semiconducting polymers. Here, two series of donor and acceptor polymers are prepared with different molar percentages of PS side chains to elucidate the effect of their substitution arrangement on the all‐polymer solar cell performance. The observed device performance is lower when the PS side chains are substituted on the donor polymer and higher when on the acceptor polymer, indicating a clear arrangement effect of the PS side chain. The incorporation of PS side chains to the acceptor polymer contributes to the decrease in phase separation domain size in the blend films. However, the reduced domain size was still an order of magnitude larger than the typical exciton diffusion length. A detailed morphological study together with the estimation of solubility parameter of the pristine PS, donor, and acceptor polymers reveals that the relative value of solubility parameter of each component dominantly contributes to the purity of the phase separated domain, which strongly impacts the amount of generated photocurrent and overall solar cell performance. This study provides an understanding of the design strategies to improve the all‐polymer solar cells.     

Free‐energy function based on an all‐atom model for proteins

We have developed a free‐energy function based on an all‐atom model for proteins. It comprises two components, the hydration entropy (HE) and the total dehydration penalty (TDP). Upon a transition to a more compact structure, the number of accessible configurations arising from the translational displacement of water molecules in the system increases, leading to a water‐entropy gain. To fully account for this effect, the HE is calculated using a statistical‐mechanical theory applied to a molecular model for water. The TDP corresponds to the sum of the hydration energy and the protein intramolecular energy when a fully extended structure, which possesses the maximum number of hydrogen bonds with water molecules and no intramolecular hydrogen bonds, is chosen as the standard one. When a donor and an acceptor (e.g., N and O, respectively) are buried in the interior after the break of hydrogen bonds with water molecules, if they form an intramolecular hydrogen bond, no penalty is imposed. When a donor or an acceptor is buried with no intramolecular hydrogen bond formed, an energetic penalty is imposed. We examine all the donors and acceptors for backbone‐backbone, backbone‐side chain, and side chain‐side chain intramolecular hydrogen bonds and calculate the TDP. Our free‐energy function has been tested for three different decoy sets. It is better than any other physics‐based or knowledge‐based potential function in terms of the accuracy in discriminating the native fold from misfolded decoys and the achievement of high Z‐scores. Proteins 2009. © 2009 Wiley‐Liss, Inc.     

Crystal and molecular structure of the amylose–DMSO complex

The crystal and molecular structure of the complex of amylose with dimethyl sulfoxide has been studied by a combination of stereochemical analysis, potential energy, and X-ray diffraction methods. The complex crystallizes in a pseudotetragonal unit cell with a = b = 19.17 Å and c (fiber axis) = 24.39 Å, with two antiparallel chains per unit cell and space group P2₁2₁2₁. The amylose chain is a left-handed 6₁(1.355) helix with three turns per crystallographic repeat. The O(6) rotational position is approximately gt. Dimethyl sulfoxide is located inside the helix with one DMSO molecule for every three glucose residues. An additional four DMSO molecules and eight water molecules each are located in the large interstices between chains, and it is the interaction of these molecules with the helix that results in the pseudotetragonal chain packing. The interstitial DMSO is the source of the previously reported additional layer lines, which are not consistent with the 8.13-Å amylose repeat distance. The final R factor for the layers with amylose contribution to the structure factors was 0.29, while the overall R factor was 0.35. The stereochemical packing analysis provided suitable phasing models for the subsequent X-ray refinement.     

A Facile Method to Fine‐Tune Polymer Aggregation Properties and Blend Morphology of Polymer Solar Cells Using Donor Polymers with Randomly Distributed Alkyl Chains

The device performance of polymer solar cells (PSCs) is strongly dependent on the blend morphology. One of the strategies for improving PSC performance is side‐chain engineering, which plays an important role in controlling the aggregation properties of the polymers and thus the domain crystallinity/purity of the donor–acceptor blends. In particular, for a family of high‐performance donor polymers with strong temperature‐dependent aggregation properties, the device performances are very sensitive to the size of alkyl chains, and the best device performance can only be achieved with an optimized odd‐numbered alkyl chain. However, the synthetic route of odd‐numbered alkyl chains is costly and complicated, which makes it difficult for large‐scale synthesis. Here, this study presents a facile method to optimize the aggregation properties and blend morphology by employing donor polymers with a mixture of two even‐numbered, randomly distributed alkyl chains. In a model polymer system, this study suggests that the structural and electronic properties of the random polymers comprising a mixture of 2‐octyldodecyl and 2‐decyltetradecyl alkyl chains can be systematically tuned by varying the mixing ratio, and a high power conversion efficiency (11.1%) can be achieved. This approach promotes the scalability of donor polymers and thus facilitates the commercialization of PSCs.     

Feasible D1–A–D2–A Random Copolymers for Simultaneous High‐Performance Fullerene and Nonfullerene Solar Cells

A series of PBDB‐TTn random donor copolymers is synthesized, consisting of an electron‐deficient benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione (BDD) unit and different ratios of two electron‐rich benzo[1,2‐b:4,5‐b′]dithiophene (BDT) and thieno[3,2‐b]thiophene (TT) units, with intention to modulate the intrachain and/or interchain interactions and ultimately bulk‐heterojunction morphology evolution. A comparative study using 4 × 2 polymer solar cell (PSC) performance maps and each of the [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) and the fused‐aromatic‐ring‐based molecule (m‐ITIC) acceptors are carried out. Given the similarities in their absorption ranges and energy levels, the PBDB‐TTn copolymers clearly reveal a change in the absorption coefficients upon optimization of the BDT to TT ratio in the backbone. Among the given acceptor combination sets, superior performances are observed in the case of PBDB‐TT5 blended with PC₇₁BM (8.34 ± 0.10%) or m‐ITIC (11.10 ± 0.08%), and the dominant factors causing power conversion efficiency differences in them are found to be distinctly different. For example, the performances of PC₇₁BM‐based PSCs are governed by size and population of face‐on crystallites, while intermixed morphology without the formation of large phase‐separated aggregates is the key factor for achieving high‐performance m‐ITIC‐based PSCs. This study presents a new sketch of structure–morphology–performance relationships for fullerene‐ versus nonfullerene‐based PSCs.     

Harmonious Compatibility Dominates Influence of Side‐Chain Engineering on Morphology and Performance of Ternary Solar Cells

A growing number of recent studies have demonstrated the substantial impact of the alkyl side chains on the device performance of organic semiconductors. However, detailed investigation of the effect of side‐chain engineering on the blend morphology and performance of ternary organic solar cells (OSCs) has not yet been undertaken. In this study, the performance of ternary OSCs is investigated in a given poly(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b;4,5‐b′]dithiophene‐2,6‐diyl‐alt‐(4‐(2‐ethylhexyl)‐3‐fluorothieno[3,4‐b]thiophene‐)‐2‐carboxylate‐2‐6‐diyl)):[6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PTB7‐Th:PC₇₁BM) host set by introducing various small molecule donors (SMDs) with different terminal side‐chain lengths. As expected, the performance of binary OSCs with SMDs depends greatly on the side‐chain length. In contrast, it is observed that all SMD‐based ternary OSCs exhibit almost identical and high power‐conversion efficiencies of 12.0–12.2%. This minor performance variation is attributed to good molecular compatibility between the two donor components, as evidenced by in‐depth electrical and morphological investigations. These results highlight that the alloy‐like structure formed due to the high compatibility of the donor molecules has a more significant effect on the overall performance than the side‐chain length, offering a new guideline for pairing donor components for achieving high‐performance ternary OSCs.     

Alkyl Chain Length Effects of Polymer Donors on the Morphology and Device Performance of Polymer Solar Cells with Different Acceptors

The development of nonfullerene acceptors has brought polymer solar cells into a new era. Maximizing the performance of nonfullerene solar cells needs appropriate polymer donors that match with the acceptors in both electrical and morphological properties. So far, the design rationales for polymer donors are mainly borrowed from fullerene‐based solar cells, which are not necessarily applicable to nonfullerene solar cells. In this work, the influence of side chain length of polymer donors based on a set of random terpolymers PTAZ‐TPD10‐Cn on the device performance of polymer solar cells is investigated with three different acceptor materials, i.e., a fullerene acceptor [70]PCBM, a polymer acceptor N2200, and a fused‐ring molecular acceptor ITIC. Shortening the side chains of polymer donors improves the device performance of [70]PCBM‐based devices, but deteriorates the N2200‐ and ITIC‐based devices. Morphology studies unveil that the miscibility between donor and acceptor in blend films depends on the side chain length of polymer donors. Upon shortening the side chains of the polymer donors, the miscibility between the donor and acceptor increases for the [70]PCBM‐based blends, but decreases for the N2200‐ and ITIC‐based blends. These findings provide new guidelines for the development of polymer donors to match with emerging nonfullerene acceptors.     

Perylene Sensitization of Fullerenes for Improved Performance in Organic Photovoltaics

We present the addition of an energy relay dye to fullerenes resulting in increased light harvesting and significantly improved power conversion efficiency for organic photovoltaic (OPV) devices. Although exhibiting excellent properties as electron acceptors, visible light absorption of fullerenes is limited. Strongly light absorbing donor materials are needed for efficient light harvesting in the thin active layer of OPV devices. Therefore, photocurrent generation and thus power conversion efficiency of this type of solar cell is confined by the overlap of the relatively narrow absorption band of commonly used donor molecules with the solar spectrum. Herein the concept of fullerene dye sensitization is presented, which allows increased light harvesting on the electron acceptor side of the heterojunction. The concept is exemplarily shown for an UV absorbing small molecule and a near infrared absorbing polymer, namely hexa‐peri‐hexabenzocoronene (HBC) and Poly[2,1,3‐benzothiadiazole‐4,7‐diyl[4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b:3,4‐b']dithiophene‐2,6‐diyl]] (PCPDTBT), respectively. In both systems remarkably higher power conversion efficiency is achieved via perylene sensitization of the fullerene acceptor. Steady state photoluminescence, transient absorption and transient photocurrent decay studies reveal pathways of the additionally generated excited states at the sensitizer molecule. The findings suggest fluorescence resonance energy transfer from the photo‐excited dye to the fullerene enabling decoupling of light absorption and charge transport. The presented sensitization method is proposed as a viable new concept for performance enhancement in organic photovoltaic devices.