Billy

The promise of wearable and implantable devices has made stretchable organic semiconductors highly desirable. Though there are increasing attempts to design intrinsically stretchable conjugated polymers, their performance in terms of charge carrier mobility and maximum fracture strain is still lacking behind extrinsic approaches (i.e., buckling, Kirigami interconnects). Here, polymer crosslinking with flexible oligomers is applied as a strategy to reduce the tensile modulus and improve fracture strain, as well as fatigue resistance for a high mobility diketopyrrolopyrrole polymer. These polymers are crosslinked with siloxane oligomers to give stretchable films stable up to a strain ε = 150% and 500 strain-and-release cycles of 100% strain without the formation of nanocracks. Organic field-effect transistors are prepared to assess the electrical properties of the crosslinked film under cyclic strain loading. An initial average mobility (μ^avg) of 0.66 cm² V⁻¹ s⁻¹ is measured at 0% strain. A steady μ^avg above 0.40 cm² V⁻¹ s⁻¹ is obtained in the direction perpendicular to the strain direction after 500 strain-and-release cycles of 20% strain. The μ^avg in the direction parallel to strain, however, is compromised due to the formation of wrinkles.

Improved elastic property in diketopyrrolopyrrole polymer is achieved by crosslinking with a flexible siloxane oligomer. An enhancement in fracture strain and yielding point and a decrease in tensile modulus with film crystalinity are observed. The improved fatigue resistance is attributed to the covalent crosslinks that prevent irreversible sliding between polymer chains during cyclic loading.

Doping of organic bulk heterojunction solar cells has the potential to improve their power conversion efficiency (PCE). Deconvoluting the effect of doping on charge transport, recombination, and energetic disorder remains challenging. It is demonstrated that molecular doping has two competing effects: on one hand, dopant ions create additional traps while on the other hand free dopant-induced charges fill deep states possibly leading to V _OC and mobility increases. It is shown that molar dopant concentrations as low as a few parts per million can improve the PCE of organic bulk heterojunctions. Higher concentrations degrade the performance of the cells. In doped cells where PCE is observed to increase, such improvement cannot be attributed to better charge transport. Instead, the V _OC increase in unannealed P3HT:PCBM cells upon doping is indeed due to trap filling, while for annealed P3HT:PCBM cells the change in V _OC is related to morphology changes and dopant segregation. In PCDTBT:PC70BM cells, the enhanced PCE upon doping is explained by changes in the thickness of the active layer. This study highlights the complexity of bulk doping in organic solar cells due to the generally low doping efficiency and the constraint on doping concentrations to avoid carrier recombination and adverse morphology changes.

Ultralow level doping (≈ppm) can increase the power conversion efficiency of organic solar cells. Trap states filling by free charges and trap creation by dopant ions have competing effects on carrier mobility and open circuit voltage thereby imposing constraints on the effectiveness of doping. Measurements are performed to study what electronic process dominates in different materials or fabrication conditions.

Bimolecular recombination in bulk heterojunction organic solar cells is the process by which nongeminate photogenerated free carriers encounter each other, and combine to form a charge transfer (CT) state which subsequently relaxes to the ground state. It is governed by the diffusion of the slower and faster carriers toward the electron donor–acceptor interface. In an increasing number of systems, the recombination rate constant is measured to be lower than that predicted by Langevin's model for relative Brownian motion and the capture of opposite charges. This study investigates the dynamics of charge generation, transport, and recombination in a nematic liquid crystalline donor:fullerene acceptor system that gives solar cells with initial power conversion efficiencies of >9.5%. Unusually, and advantageously from a manufacturing perspective, these efficiencies are maintained in junctions thicker than 300 nm. Despite finding imbalanced and moderate carrier mobilities in this blend, strongly suppressed bimolecular recombination is observed, which is ≈150 times less than predicted by Langevin theory, or indeed, more recent and advanced models that take into account the domain size and the spatial separation of electrons and holes. The suppressed bimolecular recombination arises from the fact that ground-state decay of the CT state is significantly slower than dissociation.

A detailed study of bimolecular recombination in a high efficiency organic solar cell, comprised of a liquid crystalline donor and PC71BM, is presented. Using multiple techniques, it is shown that the bimolecular recombination is nearly 150 times suppressed with respect to that predicted by Langevin theory. This reduction is attributed to an equilibrium between charge transfer states and free charges.

A bilayer actuator made of carbon nanotubes (CNTs) and boron nitride (BN) is developed that can withstand high temperatures. The bilayer actuator can be powered quickly to a temperature up to 2000 K within 100 ms and can operate at frequencies from sub-Hertz to about 30 Hz due to the low heat capacity of the thin CNT layer.

Well-defined small molecule (SM) donors can be used as alternatives to π-conjugated polymers in bulk-heterojunction (BHJ) solar cells with fullerene acceptors (e.g., PC₆₁/₇₁BM). Taking advantage of their synthetic tunability, combinations of various donor and acceptor motifs can lead to a wide range of optical, electronic, and self-assembling properties that, in turn, may impact material performance in BHJ solar cells. In this report, it is shown that changing the sequence of donor and acceptor units along the π-extended backbone of benzo[1,2-b:4,5-b′]dithiophene–6,7-difluoroquinoxaline SM donors critically impacts (i) molecular packing, (ii) propensity to order and preferential aggregate orientations in thin-films, and (iii) charge transport in BHJ solar cells. In these systems (SM1-3), it is found that 6,7-difluoroquinoxaline ([2F]Q) motifs directly appended to the central benzo[1,2-b:4,5-b′]dithiophene (BDT) unit yield a lower-bandgap analogue (SM1) with favorable molecular packing and aggregation patterns in thin films, and optimized BHJ solar cell efficiencies of ≈6.6%. ¹H-¹H DQ-SQ NMR analyses indicate that SM1 and its counterpart with [2F]Q motifs substituted as end-group SM3 possess distinct self-assembly patterns, correlating with the significant charge transport and BHJ device efficiency differences observed for the two analogous SM donors (avg. 6.3% vs 2.0%, respectively).

Changing the sequence of donor and acceptor units along the π-extended backbone of benzo[1,2-b:4,5-b′]dithiophene–6,7-difluoroquinoxaline small molecule (SM) donors critically impacts (i) molecular packing, (ii) propensity to order and preferential aggregate orientations in thin-films, and (iii) charge transport in bulk-heterojunction (BHJ) solar cells. The lower-bandgap analogue (SM1) achieves distinct local packing and aggregation patterns in thin films, and optimized BHJ solar cell efficiencies of ≈6.6%.

The increasing human demand for clean and renewable energy and a cleaner environment has stimulated various materials related research and development for efficient solar energy conversion and utilization, photocatalytic environmental remediation, and photoassisted selective conversion of organic compounds. Among the various candidate solutions to improving the efficiency of these processes, semiconductor nanowire (NW) and nanorod (NR) array structures have been offering a unique tool box combining desirable characteristics in structure, function, and applicability. Efficient photocatalytic energy conversion requires excellent light absorption, readily charge separation, transport, and collection, as well as fast kinetics of interfacial reactions and mass transport of reactants. The long axis of the NW enables adequate light absorption, while the radical axis provides short electron–hole separation distance. Additionally, the NW arrays have relatively high surface area to facilitate interfacial kinetics while their open pore structure allows good mass transport properties. This study reviews the current status of research on NW and NR array structures of some most popular semiconducting materials for photocatalytic energy conversion and utilization including photocatalytic water splitting and CO₂ reduction/fixation, dye/quantum dot sensitized solar cells, and other photoassisted reactive applications such as pollutant degradation, selective conversion of organic compounds, and biological disinfection.

The semiconductor nanowire/nanorod array structures are promising candidates for the practical energy and environmental applications. An overview of recent advances in the applications of the nano-array structures for the photocatalytic energy conversion and utilization is reviewed with an emphasis on the rational design, assembly and integration, and function enabling of materials and structures.

The process of accurately gauging lifetime improvements in organic photovoltaics (OPVs) or other similar emerging technologies, such as perovskites solar cells is still a major challenge. The presented work is part of a larger effort of developing a worldwide database of lifetimes that can help establishing reference baselines of stability performance for OPVs and other emerging PV technologies, which can then be utilized for pass-fail testing standards and predicting tools. The study constitutes scanning of literature articles related to stability data of OPVs, reported until mid-2015 and collecting the reported data into a database. A generic lifetime marker is utilized for rating the stability of various reported devices. The collected data is combined with an earlier developed and reported database, which was based on articles reported until mid-2013. The extended database is utilized for establishing the baselines of lifetime for OPVs tested under different conditions. The work also provides the recent progress in stability of unencapsulated OPVs with different architectures, as well as presents the updated diagram of the reported record lifetimes of OPVs. The presented work is another step forward towards the development of pass-fail testing standards and lifetime prediction tools for emerging PV technologies.

An extended database of lifetime for organic photovoltaics is presented. The data is utilized for establishing lifetime baselines for the samples tested under different aging conditions. The baselines can serve as a reference for gauging the improvements in device lifetimes in future reports. Encapsulated and unencapsulated samples with the best lifetimes reported in literature so far are listed as well.

Recently, the rapid and significant progress in the development of various stretchable electronics has triggered intense research interest. Although the remarkable features of transfer printing processes have enabled the use of inorganic crystalline semiconductors in various types of stretchable devices, including solar cells, light-emitting diodes, circuits, and photodetectors, there are few examples of stretchable electronics using thin film semiconductors. Transfer printing of inorganic amorphous thin film semiconductors remains a challenge because no suitable sacrificial layer is available. To meet this challenge, a water-soluble germanium oxide sacrificial layer is developed. Stretchable inorganic amorphous thin film solar cells are produced using a transfer printing process with a water-soluble sacrificial layer. This first attempt to fabricate stretchable solar cells with inorganic amorphous thin film semiconductors significantly broadens the scope of solar cell applications. Moreover, the germanium oxide sacrificial layer can be used in other thin film electronics applications.

Water-soluble germanium oxide sacrificial layer for transfer printing of thin film solar cells is developed. The main advantages of a germanium oxide sacrificial layer are no use of corrosive reagents in the etching process, compatibility with high-temperature processes. Stretchable thin film solar cells are produced for the first time using this transfer printing process with a water-soluble sacrificial layer.

In this work, the detailed morphology studies of polymer poly(3-hexylthiophene-2,5-diyl) (P3HT):fullerene(PCBM) and polymer(P3HT):polymer naphthalene diimide thiophene (PNDIT) solar cell are presented to understand the challenge for getting high performance all-polymer solar cells. The in situ X-ray scattering and optical interferometry and ex situ hard and soft X-ray scattering and imaging techniques are used to characterize the bulk heterojunction (BHJ) ink during drying and in dried state. The crystallization of P3HT polymers in P3HT:PCBM bulk heterojunction shows very different behavior compared to that of P3HT:PNDIT BHJ due to different mobilities of P3HT in the donor:acceptor glass. Supplemented by the ex situ grazing incidence X-ray diffraction and soft X-ray scattering, PNDIT has a lower tendency to form a mixed phase with P3HT than PCBM, which may be the key to inhibit the donor polymer crystallization process, thus creating preferred small phase separation between the donor and acceptor polymer.

The morphology development of polymer:fullerene and polymer:polymer solar cells is characterized by real-time X-ray scattering and interferometry. Polymer:fullerence blends show a smaller phase-separation due to high glass-transition temperature of PCBM acceptors, compared to polymer:polymer blends.

The incorporation of multiple donors into the bulk-heterojunction layer of organic polymer solar cells (PSCs) has been demonstrated as a practical and elegant strategy to improve photovoltaics performance. However, it is challenging to successfully design and blend multiple donors, while minimizing unfavorable interactions (e.g., morphological traps, recombination centers, etc.). Here, a new Förster resonance energy transfer-based design is shown utilizing the synergistic nature of three light active donors (two small molecules and a high-performance donor–acceptor polymer) with a fullerene acceptor to create highly efficient quaternary PSCs with power conversion efficiencies (PCEs) of up to 10.7%. Within this quaternary architecture, it is revealed that the addition of small molecules in low concentrations broadens the absorption bandwidth, induces cocrystalline molecular conformations, and promotes rapid (picosecond) energy transfer processes. These results provide guidance for the design of multiple-donor systems using simple processing techniques to realize single-junction PSC designs with unprecedented PCEs.

A viable strategy to realize highly efficient quaternary blend solar cells is introduced that breaks efficiency above 10% with complementary squaraine small molecules–low band-gap polymer combinations. Our quaternary design demonstrates several advantages: (i) broader light absorption, (ii) improved surface morphology, (iii) enhanced cocrystallization packing, (iv) multiple energy and charge transfer pathways to reduce recombination, and (v) increased charge mobility.

Morphology plays a vital role on the performance of organic photovoltaics. However, our understanding of the morphology-performance relationships for organic photovoltaics remains lacking. Specifically, it is still an open question why some bulk-heterojunction blends exhibit electric field dependent J–V curves, while others do not. Through detailed fs-μs transient absorption spectroscopy and morphology studies on the representative bulk-heterojunction type small molecule (SM) donor system, a picture of different J–V behaviors from morphology aspects and excited dynamics is revealed. Our findings reveal that amorphous morphology in the lack of percolated pathways leads to the formation of strongly bound charge transfer states (CTSs), which accounts for about one third of the photoexcited species. Therefore, field-dependent J–V curves are obtained as these CTSs mainly undergo geminate recombination or function as interfacial traps for nongeminate recombination. On the other hand, the CTSs are totally suppressed after post-treatment owning to the formation of bicontinuous morphology, which results in very high efficiencies from exciton generation, diffusion, dissociation to charge extraction, thus contributes to field-independent J–V characteristics. The insights gained in this work provide the effective guidelines to further optimize the performance of bulk-heterojunction type SM-organic photovoltaics through judicious morphology control and engineering.

A picture of different J–V behaviors from morphology aspects and excited dynamics is revealed through fs-μs transient absorption measurements. The amorphous donor morphology without percolated pathways facilitates the formation of strongly bound charge transfer states, which results in the field-dependent J–V curves as these charge transfer states can only be dissociated and extracted by applying very large reverse voltages.

A crosslinked organic hole-transporting layer (HTL) is developed to realize highly efficient and stable perovskite solar cells via a facile thiol-ene thermal reaction. This crosslinked HTL not only facilitates hole extraction from perovskites, but also functions as an effective protective barrier. A high-performance (power conversion efficiency: 18.3%) device is demonstrated to show respectable photo and thermal stability without encapsulation.

The photophysical properties and solar cell performance of the classical donor–acceptor copolymer PCDTBT

(poly(N-9′-heptadecanyl-2,7-carbazole-alt -5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole))) in relation to unintentionally formed main chain defects are investigated. Carbazole–carbazole homocouplings (Cbz hc) are found to significant extent in PCDTBT made with a variety of Suzuki polycondensation conditions. Cbz hc vary between 0 and 8 mol% depending on the synthetic protocol used, and are quantified by detailed nuclear magnetic resonance spectroscopy including model compounds, which allows to establish a calibration curve from optical spectroscopy. The results are corroborated by extended time-dependent density functional theory investigations on the structural, electronic, and optical properties of regularly alternating and homocoupled chains. The photovoltaic properties of PCDTBT:fullerene blend solar cells significantly depend on the Cbz hc content for constant molecular weight, whereby an increasing amount of Cbz hc leads to strongly decreased short circuit currents J_SC. With increasing Cbz hc content, J_SC decreases more strongly than the intensity of the low energy absorption band, suggesting that small losses in absorption cannot explain the decrease in J_SC alone, rather than combined effects of a more localized LUMO level on the TBT unit and lower hole mobilities found in highly defective samples. Homocoupling-free PCDTBT with optimized molecular weight yields the highest efficiency up to 7.2% without extensive optimization.

Electronic main chain defects in conjugated polymers caused by carbazole homocouplings (Cbz hc) significantly deteriorate the performance of organic photovoltaics as shown for poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) (PCDTBT):PC₇₁BM devices. Cbz hc occur frequently, are quantified for a variety of Suzuki conditions, and are correlated with photophysical properties. PCDTBT without main chain defects and optimized molar mass gives the highest power conversion efficiencies exceeding 7%.

It is observed that degradation of organic bulk heterojunction is directly associated with degradation of solvent additive, 1,8-diiodooctane. The iodide impurities released from 1,8-diiodooctane react with fullerenes, making fullerene-iodide intermediate compounds which lead to fullerene oxidation and efficiency drop, irrespective of photooxidation of polymer in polymer/fullerene bulk heterojunctions. Replacing the additive with an iodine-free reagent significantly elongates a device lifetime.

All-polymer solar cells with 7.57% power conversion efficiency are achieved via a new perylenediimide-based polymeric acceptor. Furthermore, the device processed in ambient air without encapsulation can still reach a high power conversion efficiency (PCE) of 7.49%, which is a significant economic advantage from an industrial processing perspective. These results represent the highest PCE achieved from perylenediimide-based polymers.

A small-molecular acceptor, tetraphenylpyrazine-perylenediimide tetramer (TPPz-PDI₄), which has a reduced extent of intramolecular twisting compared to two other small-molecular acceptors is designed. Benefiting from the lowest extent of intramolecular twisting, TPPz-PDI₄ exhibits the highest aggregation tendency and electron mobility, and therefore achieves a highest power conversion efficiency of 7.1%.

New indoline dyes (RK-1–4) were designed with a planar geometry and high molar extinction coefficient, which provided surprising power conversion efficiency (PCE) with a thin titanium dioxide film in dye-sensitized solar cells (DSCs). They had a difference in only alkyl chain length. Despite the same molecular structure, the performance of the respective DSCs varied significantly. Investigating the dye adsorption processes and charge transfer kinetics, the alkyl chain length was determined to affect the dye surface coverage as well as the recombination between the injected photoelectrons and the oxidized redox mediators. When applied to the DSCs as a light harvester, RK-3 with the dodecyl group exhibited the best photocurrent density, consequently achieving the best PCE of 9.1% with a 1.8 μm active and 2.5 μm scattering layer because of the most favorable charge injection. However, when increasing the active layer thickness, overall device performance deteriorated and the charge collection and regeneration played major roles for determining the PCE. Therefore, RK-2 featuring the highest surface coverage and moderate alkyl chain length obtained the highest PCEs of 8.8% and 7.9% with 3.5 and 5.1 μm active layers, respectively. These results present a promising perspective of organic dye design for thin film DSCs.

New indoline derivatives characterized by good planarity and high molar absorptivity are successfully applied to dye-sensitized solar cells (DSCs). By controlling the hydrocarbon chain length, noticeable photocurrent density is achieved at a very thin TiO₂ film, which is traced elementally, guiding the research of organic dyes for thin film-based DSCs.

A novel wide-bandgap conjugated polymer PBTA-FPh based on benzodithiophene-alt-benzo[1,2,3]triazole as the main chain and a polar pentafluorothiophenyl (FPh) group in the side chain has been designed and synthesized. In comparison to the pristine polymer PBTA-BO that consists of nonpolar alkyl side chains, the resulting PBTA-FPh exhibits less pronounced aggregation while possessing analogous optical and electrochemical bandgaps. Contact angle measurements demonstrate that the surface energy can be enhanced by incorporating FPh moiety, leading to a better miscibility of PBTA-BO with PC₇₁BM in the presence of a certain amount of PBTA-FPh. The photoactive layer of PBTA-BO:PC₇₁BM:PBTA-FPh with weight ratio of 1:1.2:0.02% exhibits a percolated network with the fibrous features, as revealed by transmission electron microscopy measurements. Of particular interest is the significantly improved photovoltaic performances of polymer solar cell devices for which the power conversion efficiency is enhanced from 6.46% for the control device to 7.91% for device processed with PBTA-FPh as the polymeric additive. These observations indicate that introducing donor–acceptor type of polymeric additive comprising of polar groups in the side chain can be a promising strategy for the fabrication of high-performance polymer solar cells.

A novel wide-bandgap conjugated polymer PBTA-FPh based on benzodithiophene-alt-benzo[1,2,3]triazole as the main chain and a polar pentafluorothiophenyl (FPh) group in a side chain has been designed and synthesized as a polymeric additive. The incorporation of PBTA-FPh can lead to improved miscibility and morphology of PBTA-BO:PC₇₁BM blend films, resulting in obviously improved power conversion efficiency of polymer solar cells from 6.46% to 7.91%.

Donor–acceptor–acceptor′ small-molecule donors are synthesized to investigate regioisomeric effects on organic photovoltaic device performance. Cross-conjugation in 2-((7-(N-(2-ethylhexyl)-benzothieno[3,2-b]thieno[3,2-d]pyrrol-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)methylene)malononitrile leads to an increased open-circuit voltage compared with its isomer 2-((7-(N-(2-ethylhexyl)-benzothieno[3,2-b]thieno[2,3-d]pyrrol-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)methylene)malononitrile. A correlation is then established between molecular conjugation length and orbital energies, and hence open-circuit voltage.

B. Q. Sun and co-workers develop a simple and effective method to enhance the adhesion force between crystalline Si and amorphous organic layers by covalent chemical mutual anchoring, as described on page 5035. This method dramatically improves the charge transport and suppresses charge recombination at the organic-inorganic interface simultaneously, resulting in high-efficiency solar cells. These findings suggest a promising approach to low-cost and simple fabrication for high-performance organic-inorganic heterojunction solar cells.