High-molecular-weight insulating polymers can improve the performance of molecular solar cells

Insulator-donor electron wavefunction coupling in pseudo-bilayer organic solar cells achieving a certificated efficiency of 19.18

The incorporation of polymeric insulators has led to notable achievements in the field of organic semiconductors. By altering the blending concentration, polymeric insulators exhibit extensive capabilities in regulating molecular configuration, film crystallinity, and mitigation of defect states. However, current research suggests that the improvement in such physical properties is primarily attributed to the enhancement of thin film morphology, an outcome that seems to be an inevitable consequence of incorporating insulators. Herein, we report a general and completely new effect of polymeric insulators in organic semiconductors: the insulator-donor electron wavefunction coupling effect. Such insulators can couple with donor polymers to reduce the energy barrier level and facilitate intramolecular electron transport. Besides the morphological effects, we observed that this coupling effect is another mechanism that can significantly enhance electron mobility (up to 100 times) through the incorporation of polymeric insulators in a series of donor systems. With this effect, we proposed a polymeric insulator blending approach to fabricate state-of-the-art pseudo-bilayer organic solar cells, and the PM6/L8-BO device exhibits a high efficiency of 19.50% (certificated 19.18%) with an improved interfacial electron transport property. This work not only offers a novel perspective on the quantum effect of polymeric insulators in organic semiconductors, but also presents a simple yet effective method for enhancing the performance of organic solar cells.

Solid Additive Engineering for Next-generation Organic Photovoltaics

Solution-processed bulk heterojunction (BHJ) organic solar cells (OSCs) have emerged as a promising next-generation photovoltaic technology. In this emerging field, there is a growing trend of employing solid additives (SAs) to fine-tune the BHJ morphology and unlock the full potential of OSCs. SA engineering offers several significant benefits for commercialization, including the ability to i) control film-forming kinetics to expedite high-throughput fabrication, ii) leverage weak noncovalent interactions between SA and BHJ materials to enhance the efficiency and stability of OSCs, and iii) simplify procedures to facilitate cost-effective production and scaling-up. These features make SA engineering a key catalyst for accelerating the development of OSCs. Recent breakthroughs have shown that SA engineering can achieve an efficiency of 19.67% in single-junction OSCs, demonstrating its effectiveness in promoting the commercialization of organic photovoltaic devices. This review provides a comprehensive overview of significant breakthroughs and pivotal contributions of emerging SAs, focusing on their roles in governing film-forming dynamics, stabilizing phase separation, and addressing other crucial aspects. The rationale and design rules for SAs in highly efficient and stable OSCs are also discussed. Finally, the remaining challenges are summarized, and perspectives on future advances in SA engineering are offered.

Eliminating the Imbalanced Mobility Bottlenecks via Reshaping Internal Potential Distribution in Organic Photovoltaics

The imbalanced carrier mobility remains a bottleneck for performance breakthrough in even those organic solar cells (OSCs) with recorded power conversion efficiencies (PCEs). Herein, a counter electrode doping strategy is proposed to reshape the internal potential distribution, which targets to extract the low mobility carriers at far end. Device simulations reveal that the key of this strategy is to partially dope the active layer with a certain depth, therefore it strengthens the electric field for low mobility carriers near counter electrode region while avoids zeroing the electric field near collection electrode region. Taking advantage of these, PCE enhancements are obtained from 15.4% to 16.2% and from 16.9% to 18.0%, respectively, via cathode p-doping and anode n-doping. Extending its application from opaque to semitransparent devices, the PCE of dilute cell rises from 10.5% to 12.1%, with a high light utilization efficiency (LUE) of 3.5%. The findings provide practical solutions to the core device physical problem in OSCs.

Crystallinity and Molecular Packing of Small Molecules in Bulk-Heterojunction Organic Solar Cells

Crystallinity has played a major role in organic solar cells (OSCs). In small molecule (SM) bulk-heterojunction (BHJ) OSCs, the crystallinity and crystalline packing of SM donors have been shown to have a dramatic impact on the formation of an optimum microstructure leading to high-power conversion efficiency (PCE). Herein we describe how crystallinity differs from polymers to SMs, and how the packing habits of SMs (particularly donors) in active layers of BHJ devices can be described as following two different main modes: a single crystal-like and a liquid crystal-like packing type. This notion is reviewed from a chronological perspective, emphasising milestone donor structures and studies focusing on the crystallinity in SM-BHJ OSCs. This review intends to demonstrate that a shift towards a liquid crystalline-like packing can be identified throughout the history of SM-BHJ, and that this shift can be associated with an increase in overall PCE.

Dual Interface Protection for High Performance and Excellent Long-Term Stability of Organic Solar Cells

Stability is still the main barrier to the commercial application of organic solar cells (OSCs), although the maximal power conversion efficiency (PCE) value has exceeded 19%. The encapsulation technique is an effective and vital way to guarantee the long-term stabilities of OSCs, but it can only avoid the penetration of water and oxygen from the environment. Herein, we introduced a structure that provides dual interface protection by using commercially available and chemically stable polyvinylidene fluoride (PVDF) as the cathode interface protection layer working as the cathode interlayer (CIL) and poly(styrene-comethyl-methacrylate) (PS-r-PMMA) as the anode interface protection layer between the poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) (PEDOT:PSS) and the active layer. With this structure, both the migration of impurities caused by degradation of the interfacial layer and the infiltration of oxygen and water in the air can be prevented. PVDF can effectively provide optimal electron transfer by improving the surface potential of active layers and lowering the work function of the Al electrode. PS-r-PMMA can improve the hydrophobicity of PEDOT:PSS and induce optimized phase separation, facilitating charge transfer. After storage in an air environment with a humidity of approximately 60% for 3600 h, the device based on the PM6:IT-4F blend film with dual interface protection showed a decrease in its PCE value from 13.43 to 10.90%, retaining 81.2% of its original PCE value, in contrast to the sharp decrease in the PCE value from 13.66 to 0.74% of the device without dual interface protection. The dual interface protection design could also be useful in the high-performance PM6:Y6 system, which shows a champion PCE of 15.39% and shows potential for the effective fabrication of stable OSCs in the future.

High-Efficiency Thickness-Insensitive Organic Solar Cells with an Insulating Polymer

Achieving high-efficiency thick-film bulk heterojunction (BHJ) organic solar cells (OSCs) with thickness-independent power conversion efficiencies (PCEs) in a wide thickness range is still a challenge for the roll-to-roll printing techniques. The concept of diluting the transport sites within BHJ films with insulating polymers can effectively eliminate charge trapping states and optimize the charge transport. Herein, we first adopted the concept with insulating polypropylene (PP) in the efficient non-fullerene system (PM6:Y6) and demonstrated its potential to fabricate thick-film OSCs. The PP can form an insulating matrix prior to PM6 and Y6 within the BHJ film, resulting in an enhanced molecular interaction and isolated charge transport by expelling Y6 molecules. We thus observed reduced trap state density and improved charge transport properties in the PP-blended device. At around 300 nm, the PM6:Y6:PP device enjoys a high PCE of 15.5% and achieves over 100% of the efficiency of the optimal thin-film device, which is significantly improved compared to the binary PM6:Y6 counterpart. This research promotes an effective strategy with insulating polymers and provides knowledge of commercial production with response to the roll-to-roll technique demands.

Improving the Photostability of Small-Molecule-Based Organic Photovoltaics by Providing a Charge Percolation Pathway of Crystalline Conjugated Polymer

Photostability of small-molecule (SM)-based organic photovoltaics (SM-OPVs) is greatly improved by utilizing a ternary photo-active layer incorporating a small amount of a conjugated polymer (CP). Semi-crystalline poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT) and amorphous poly[(2,5-bis(2-decyltetradecyloxy)phenylene)-alt-(5,6-dicyano-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2CNBT) with similar chemical structures were used for preparing SM:fullerene:CP ternary photo-active layers. The power conversion efficiency (PCE) of the ternary device with PPDT2FBT (Ternary-F) was higher than those of the ternary device with PPDT2CNBT (Ternary-CN) and a binary SM-OPV device (Binary) by 15% and 17%, respectively. The photostability of the SM-OPV was considerably improved by the addition of the crystalline CP, PPDT2FBT. Ternary-F retained 76% of its initial PCE after 1500 h of light soaking, whereas Ternary-CN and Binary retained only 38% and 17% of their initial PCEs, respectively. The electrical and morphological analyses of the SM-OPV devices revealed that the addition of the semi-crystalline CP led to the formation of percolation pathways for charge transport without disturbing the optimized bulk heterojunction morphology. The CP also suppressed trap-assisted recombination and enhanced the hole mobility in Ternary-F. The percolation pathways enabled the hole mobility of Ternary-F to remain constant during the light-soaking test. The photostability of Ternary-CN did not improve because the addition of the amorphous CP inhibited the formation of ordered SM domains.

The Critical Role of Materials' Interaction in Realizing Organic Field-Effect Transistors Via High-Dilution Blending with Insulating Polymers

High-performance low-band-gap polymer semiconductors are visibly colored, making them unsuitable for transparent and imperceptible electronics without reducing film thickness to the nanoscale range. Herein, we demonstrate polymer/insulator blends exhibiting favorable miscibility that improves the transparency and carrier transport in an organic field-effect transistor (OFET) device. The mesoscale structures leading to more efficient charge transport in ultrathin films relevant to the realization of transparent and flexible electronic applications are explored based on thermodynamic material interaction principles in conjunction with optical and morphological studies. By blending the commodity polymer polystyrene (PS) with two high-performing polymers, PDPP3T and P (NDI2OD-T2) (known as N2200), a drastic difference in morphology and fiber network are observed due to considerable differences in the degree of thermodynamic interaction between the conjugated polymers and PS. Intrinsic material interaction behavior establishes a long-range intermolecular interaction in the PDPP3T polymer fibrillar network dispersed in the majority (80%) PS matrix resulting in a ca. 3-fold increased transistor hole mobility of 1.15 cm² V^-1 s^-1 (highest = 1.5 cm² V^-1 s^-1) as compared to the pristine material, while PS barely affects the electron mobility in N2200. These basic findings provide important guidelines to achieve high mobility in transparent OFETs.

Polystyrene enhanced crystallization of perovskites towards high performance solar cells

Perovskite solar cells have attracted significant attention due to their high efficiency and low cost. In the research on methylammonium lead-iodide (CH₃NH₃PbI₃), a lot of work has been devoted to optimize the film morphology and crystallinity resulting in an enhancement of the power conversion efficiency (PCE). A good surface coverage and uniform perovskite films are highly desirable along with a smooth and pinhole-free contact between the hole and electron extraction layers. Overall, this affects the charge transport and collection, and reduces charge recombination. Herein, we demonstrate a facile route to control perovskite crystallization by inserting an optimal amount of insulating polystyrene in the perovskite precursor solutions. The incorporation of a small amount of polystyrene results in much better surface coverage, and a smoother and uniform perovskite thin film leading to improved crystallization and larger grain size. Via careful optimization, and easy and low temperature solution-based processing, below 100 °C, we realize a device with PCE exceeding 13% along with significantly reduced leakage current.

Isomery-Dependent Miscibility Enables High-Performance All-Small-Molecule Solar Cells

Nonfullerene polymer solar cells develop quickly. However, nonfullerene small-molecule solar cells (NF-SMSCs) still show relatively inferior performance, attributing to the lack of comprehensive understanding of the structure-performance relationship. To address this issue, two isomeric small-molecule acceptors, NBDTP-F_out and NBDTP-F_in , with varied oxygen position in the benzodi(thienopyran) (BDTP) core are designed and synthesized. When blended with molecular donor BDT3TR-SF, devices based on the two isomeric acceptors show disparate photovoltaic performance. Fabricated with an eco-friendly processing solvent (tetrahydrofuran), the BDT3TR-SF:NBDTP-F_out blend delivers a high power conversion efficiency of 11.2%, ranked to the top values reported to date, while the BDT3TR-SF:NBDTP-F_in blend almost shows no photovoltaic response (0.02%). With detailed investigations on inherent optoelectronic processes as well as morphological evolution, this performance disparity is correlated to the interfacial tension of the two combinations and concludes that proper interfacial tension is a key factor for effective phase separation, optimal blend morphology, and superior performance, which can be achieved by the "isomerization" design on molecular acceptors. This work reveals the importance of modulating the materials miscibility by interfacial-tension-oriented molecular design, which provides a general guideline toward efficient NF-SMSCs.

Understanding the loss mechanisms in high-performance solution-processed small molecule bulk heterojunction solar cells doped with a PFN impurity

A small amount of PFN, introduced as an impurity, can have a dramatic impact on the performance of small molecule-based BHJ solar cells.

Achieving Balanced Crystallinity of Donor and Acceptor by Combining Blade-Coating and Ternary Strategies in Organic Solar Cells

As a prototype tool for slot-die coating, blade-coating exhibits excellent compatibility with large-area roll-to-roll coating. A ternary organic solar cell based on PBDB-T:PTB7-Th:FOIC blends is fabricated by blade-coating and exhibits a power conversion efficiency of 12.02%, which is one of the highest values for the printed organic solar cells in ambient environment. It is demonstrated that blade-coating can enhance crystallization of these three materials, but the degree of induction is different (FOIC > PBDB-T > PTB7-Th). Thus, the blade-coated PBDB-T:FOIC device presents much higher electron mobility than hole mobility due to the very high crystallinity of FOIC. Upon the addition of PTB7-Th into the blade-coated PBDB-T:FOIC blends, the crystallinity of FOIC decreases together with the corresponding electron mobility, due to the better miscibility between PTB7-Th and FOIC. The ternary strategy not only maintains the well-matched crystallinity and mobilities, but also increases the photocurrent with complementary light absorption as well as the Förster resonant energy transfer. Furthermore, small domains with homogeneously distributed nanofibers are observed in favor of the exciton dissociation and charge transport. This combination of blade-coating and ternary strategies exhibits excellent synergistic effect in optimizing morphology, showing great potential in the large-area fabrication of highly efficient organic solar cells.

Towards high-bandwidth organic photodetection based on pure active layer polarization

Organic photodetectors offer distinct advantages over their inorganic analogues, most notably through optical transparency and flexibility, yet their figures-of-merit still lag behind those of inorganic devices, and optimization strategies generally encounter a trade-off between device responsivity and bandwidth. Here we propose a novel photodetector architecture in which an organic photoactive semiconductor layer (S) is sandwiched between two thick insulating layers (I) that separate the semiconductor from the metallic contacts (M). In this architecture a differential photocurrent response is generated purely from the polarization of the active layer under illumination. Especially for an asymmetric MISIM design, where one insulating layer is a high-k ionic liquid I_IL and the other a low-k polymer dielectric I_p, the responsivity/bandwidth trade-off is broken, since the role of the I_IL in efficient charge separation is maintained, while the total device capacitance is reduced according to I_p. Thus the benefits of single insulating layer differential photodetectors (MISM) using either I_IL or I_p are combined in a single device. Further improvements in device performance are also demonstrated by decreasing the series resistance of the photoactive layer through semiconductor:metal blending and by operation under strong background light.

Fluorination Triggered New Small Molecule Donor Materials for Efficient As-Cast Organic Solar Cells

Solution-processable small molecules (SMs) have attracted intense attention due to their definite molecular structures, less batch-to-batch variation, and easier structure control. Herein, two new SM donors based on substituted isatin unit (DI3T, DI3T-2F) are synthesized and applied as electron donors with the mixture of PC₇₁ BM to construct organic photovoltaics. As a result, 5,6-difluoro isatin derivative (DI3T-2F) obtains a power conversion efficiency of 7.80% by a simple solution spin-coating fabrication process without any additives, solvent, or thermal annealing process. More intuitively, due to stronger intermolecular interaction and higher hole mobility after the incorporation of fluorine atoms in end units, the devices present good tolerance to active layer thickness. The results indicate that DI3T-2F shows promising potential for large-scale printing processes and flexible application of efficient small molecule organic solar cells.

Unique Energy Alignments of a Ternary Material System toward High-Performance Organic Photovoltaics

Incorporating narrow-bandgap near-infrared absorbers as the third component in a donor/acceptor binary blend is a new strategy to improve the power conversion efficiency (PCE) of organic photovoltaics (OPV). However, there are two main restrictions: potential charge recombination in the narrow-gap material and miscompatibility between each component. The optimized design is to employ a third component (structurally similar to the donor or acceptor) with a lowest unoccupied molecular orbital (LUMO) energy level similar to the acceptor and a highest occupied molecular orbital (HOMO) energy level similar to the donor. In this design, enhanced absorption of the active layer and enhanced charge transfer can be realized without breaking the optimized morphology of the active layer. Herein, in order to realize this design, two new narrow-bandgap nonfullerene acceptors with suitable energy levels and chemical structures are designed, synthesized, and employed as the third component in the donor/acceptor binary blend, which boosts the PCE of OPV to 11.6%.

Recent Advances in Morphology Optimization for Organic Photovoltaics

Organic photovoltaics are an important part of a next-generation energy-harvesting technology that uses a practically infinite pollutant-free energy source. They have the advantages of light weight, solution processability, cheap materials, low production cost, and deformability. However, to date, the moderate photovoltaic efficiencies and poor stabilities of organic photovoltaics impede their use as replacements for inorganic photovoltaics. Recent developments in bulk-heterojunction organic photovoltaics mean that they have almost reached the lower efficiency limit for feasible commercialization. In this review article, the recent understanding of the ideal bulk-heterojunction morphology of the photoactive layer for efficient exciton dissociation and charge transport is described, and recent attempts as well as early-stage trials to realize this ideal morphology are discussed systematically from a morphological viewpoint. The various approaches to optimizing morphologies consisting of an interpenetrating bicontinuous network with appropriate domain sizes and mixed regions are categorized, and in each category, the recent trends in the morphology control on the multilength scale are highlighted and discussed in detail. This review article concludes by identifying the remaining challenges for the control of active layer morphologies and by providing perspectives toward real application and commercialization of organic photovoltaics.

Solvent Additives: Key Morphology-Directing Agents for Solution-Processed Organic Solar Cells

Organic photovoltaics (OPV) have the advantage of possible fabrication by energy-efficient and cost-effective deposition methods, such as solution processing. Solvent additives can provide fine control of the active layer morphology of OPVs by influencing film formation during solution processing. As such, solvent additives form a versatile method of experimental control for improving organic solar cell device performance. This review provides a brief history of solution-processed bulk heterojunction OPVs and the advent of solvent additives, putting them into context with other methods available for morphology control. It presents the current understanding of how solvent additives impact various mechanisms of phase separation, enabled by recent advances in in situ morphology characterization. Indeed, understanding solvent additives' effects on film formation has allowed them to be applied and combined effectively and synergistically to boost OPV performance. Their success as a morphology control strategy has also prompted the use of solvent additives in related organic semiconductor technologies. Finally, the role of solvent additives in the development of next-generation OPV active layers is discussed. Despite concerns over their environmental toxicity and role in device instability, solvent additives remain relevant morphological directing agents as research interests evolve toward nonfullerene acceptors, ternary blends, and environmentally sustainable solvents.

Thiophene-Based Organic Semiconductors

Thiophene-based π-conjugated organic small molecules and polymers are the research subject of significant current interest owing to their potential use as organic semiconductors in material chemistry. Despite simple and similar molecular structures, the hitherto reported properties of thiophene-based organic semiconductors are rather diverse. Design of high performance organic semiconducting materials requires a thorough understanding of inter- and intra-molecular interactions, solid-state packing, and the influence of both factors on the charge carrier transport. In this chapter, thiophene-based organic semiconductors, which are classified in terms of their chemical structures and their structure-property relationships, are addressed for the potential applications as organic photovoltaics (OPVs), organic field-effect transistors (OFETs) and organic light emitting diodes (OLEDs).

A new strategy to engineer polymer bulk heterojunction solar cells with thick active layers via self-assembly of the tertiary columnar phase

We report that the addition of a non-photoactive tertiary polymer phase in the binary bulk heterojunction (BHJ) polymer solar cell leads to a self-assembled columnar nanostructure, enhancing the charge mobilities and photovoltaic efficiency with surprisingly increased optimal active blend thicknesses over 300 nm, 3-4 times larger than that of the binary counterpart. Using the prototypical poly(3-hexylthiophene) (P3HT):fullerene blend as a model BHJ system, we discover that the inert poly(methyl methacrylate) (PMMA) added in the binary BHJ blend self-assembles into vertical columns, which not only template the phase segregation of electron acceptor fullerenes but also induce the out-of-plane rotation of the edge-on-orientated crystalline P3HT phase. Using complementary interrogation methods including neutron reflectivity, X-ray scattering, atomic force microscopy, transmission electron microscopy, and molecular dynamics simulations, we show that the enhanced charge transport originates from the more randomized molecular stacking of the P3HT phase and the spontaneous segregation of fullerenes at the P3HT/PMMA interface, driven by the high surface tension between the two polymeric components. The results demonstrate a potential method for increasing the thicknesses of high-performance polymer BHJ solar cells with improved photovoltaic efficiency, alleviating the burden of stringently controlling the ultrathin blend thickness during the roll-to-roll-type large-area manufacturing environment.

Impact of Aggregation on the Photochemistry of Fullerene Films: Correlating Stability to Triplet Exciton Kinetics

The photochemistry and stability of fullerene films is found to be strongly dependent upon film nanomorphology. In particular, PC₆₁BM blend films, dispersed with polystyrene, are found to be more susceptible to photobleaching in air than the more aggregated neat films. This enhanced photobleaching correlated with increased oxygen quenching of PC₆₁BM triplet states and the appearance of a carbonyl FTIR absorption band indicative of fullerene oxidation, suggesting PC₆₁BM photo-oxidation is primarily due to triplet-mediated singlet oxygen generation. PC₆₁BM films were observed to undergo photo-oxidation in air for even modest (≤40 min) irradiation times, degrading electron mobility substantially, indicative of electron trap formation. This conclusion is supported by observation of red shifts in photo- and electro-luminescence with photo-oxidation, shown to be in agreement with time-dependent density functional theory calculations of defect generation. These results provide important implications on the environmental stability of PC₆₁BM-based films and devices.

Effect of Annealing on Exciton Diffusion in a High Performance Small Molecule Organic Photovoltaic Material

Singlet exciton diffusion was studied in the efficient organic photovoltaic electron donor material DTS(FBTTh₂)₂. Three complementary time-resolved fluorescence measurements were performed: quenching in planar heterojunctions with an electron acceptor, exciton-exciton annihilation, and fluorescence depolarization. The average exciton diffusivity increases upon annealing from 1.6 × 10^-3 to 3.6 × 10^-3 cm² s^-1, resulting in an enhancement of the mean two-dimensional exciton diffusion length (L_D = (4Dτ)^1/2) from 15 to 27 nm. About 30% of the excitons get trapped very quickly in as-cast films. The high exciton diffusion coefficient of the material leads to it being able to harvest excitons efficiently from large donor domains in bulk heterojunctions.

Butanedithiol Solvent Additive Extracting Fullerenes from Donor Phase To Improve Performance and Photostability in Polymer Solar Cells

In this work, we demonstrated that the excited 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)] (PTB7-Th) will be degraded by [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) or photolysis fragment of 1,8-diiodooctane (DIO) in the presence of oxygen and under irradiation of red light. From the previous reports, the fragment of DIO may be involved in the reaction directly. Our work indicates the PC₇₁BM is not directly involved in the reaction, but is acting as a catalyst to promote the reaction of excited donors with oxygen. Thus, PTB7-Th urgently needs a kind of nonresidual iodine-free additive to replace DIO and remove the fullerene from the donor phase at the same time. Taking into consideration PC₇₁BM solubility and boiling point difference between solvent additives and host solvents, 1,4-butanedithiol solvent was selected to fabricate PTB7-Th:PC₇₁BM-based solar cells achieving a best power conversion efficiency (PCE) of 10.2% (8.5% for PTB7:PC₇₁BM). Iodine-free butanedithiol can not only avoid excited polymer reacting with the photolysis fragment of DIO but also suppress the degradation of the excited PTB7-Th caused by synergistic effect between the fullerene and oxygen via extracting the free/trapped PC₇₁BM from the donor phase. Eventually, the film prepared with 1,4-butanedithiol shows higher stability than the film prepared without any additives and much better than the film with DIO in macro-/micromorphology, light absorption, and device performance.

Reduced Bimolecular Recombination in Blade-Coated, High-Efficiency, Small-Molecule Solar Cells

To realize the full promise of solution deposited photovoltaic devices requires processes compatible with high-speed manufacturing. We report the performance and morphology of blade-coated bulk heterojunction devices based on the small molecule donor p-DTS(FBTTh2)2 when treated with a post-deposition solvent vapor annealing (SVA) process. SVA with tetrahydrofuran improves the device performance of blade-coated films more than solvent additive processing (SA) with 1,8-diiodooctane. In spin-coating, SA and SVA achieve similar device performance. Our optimized, blade coated, SVA devices achieve power conversion efficiencies over 8 % and maintain high efficiencies in films up to ≈ 250 nm thickness, providing valuable resilience to small process variations in high-speed manufacturing. Using impedance spectroscopy, we show that this advantageous behavior originates from highly suppressed bimolecular recombination in the SVA-treated films. Electron microscopy and grazing-incidence X-ray scattering experiments show that SA and SVA both produce highly crystalline donor domains, but SVA films have a radically smaller domain size compared to SA films. We attribute the different behavior to variations in initial nucleation density and relative ability of SVA and SA to control subsequent crystal growth.

A Solution-Doped Polymer Semiconductor:Insulator Blend for Thermoelectrics

Poly(ethylene oxide) is demonstrated to be a suitable matrix polymer for the solution-doped conjugated polymer poly(3-hexylthiophene). The polarity of the insulator combined with carefully chosen processing conditions permits the fabrication of tens of micrometer-thick films that feature a fine distribution of the F4TCNQ dopant:semiconductor complex. Changes in electrical conductivity from 0.1 to 0.3 S cm^-1 and Seebeck coefficient from 100 to 60 μV K^-1 upon addition of the insulator correlate with an increase in doping efficiency from 20% to 40% for heavily doped ternary blends. An invariant bulk thermal conductivity of about 0.3 W m^-1 K^-1 gives rise to a thermoelectric Figure of merit ZT ∼ 10^-4 that remains unaltered for an insulator content of more than 60 wt%. Free-standing, mechanically robust tapes illustrate the versatility of the developed dopant:semiconductor:insulator ternary blends.

Bulk-Heterojunction Organic Solar Cells: Five Core Technologies for Their Commercialization

The past two decades of vigorous interdisciplinary approaches has seen tremendous breakthroughs in both scientific and technological developments of bulk-heterojunction organic solar cells (OSCs) based on nanocomposites of π-conjugated organic semiconductors. Because of their unique functionalities, the OSC field is expected to enable innovative photovoltaic applications that can be difficult to achieve using traditional inorganic solar cells: OSCs are printable, portable, wearable, disposable, biocompatible, and attachable to curved surfaces. The ultimate objective of this field is to develop cost-effective, stable, and high-performance photovoltaic modules fabricated on large-area flexible plastic substrates via high-volume/throughput roll-to-roll printing processing and thus achieve the practical implementation of OSCs. Recently, intensive research efforts into the development of organic materials, processing techniques, interface engineering, and device architectures have led to a remarkable improvement in power conversion efficiencies, exceeding 11%, which has finally brought OSCs close to commercialization. Current research interests are expanding from academic to industrial viewpoints to improve device stability and compatibility with large-scale printing processes, which must be addressed to realize viable applications. Here, both academic and industrial issues are reviewed by highlighting historically monumental research results and recent state-of-the-art progress in OSCs. Moreover, perspectives on five core technologies that affect the realization of the practical use of OSCs are presented, including device efficiency, device stability, flexible and transparent electrodes, module designs, and printing techniques.

A PCBM Electron Transport Layer Containing Small Amounts of Dual Polymer Additives that Enables Enhanced Perovskite Solar Cell Performance

A polymer/PCBM hybrid electron transport layer is reported that enables high-performance perovskite solar cells with a high power conversion efficiency of 16.2% and with negligible hysteresis. Unlike previous approaches of reducing hysteresis by thermal annealing or fullerene passivation, the success of our approach can be mainly attributed to the doping of the PCBM layer using an insulating polymer (polystyrene) and an amine-containing polymeric semiconductor named PFNOX.

Significantly Increasing the Ductility of High Performance Polymer Semiconductors through Polymer Blending

Polymer semiconductors based on donor-acceptor monomers have recently resulted in significant gains in field effect mobility in organic thin film transistors (OTFTs). These polymers incorporate fused aromatic rings and have been designed to have stiff planar backbones, resulting in strong intermolecular interactions, which subsequently result in stiff and brittle films. The complex synthesis typically required for these materials may also result in increased production costs. Thus, the development of methods to improve mechanical plasticity while lowering material consumption during fabrication will significantly improve opportunities for adoption in flexible and stretchable electronics. To achieve these goals, we consider blending a brittle donor-acceptor polymer, poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]thiadiazolo[3,4-c]pyridine] (PCDTPT), with ductile poly(3-hexylthiophene). We found that the ductility of the blend films is significantly improved compared to that of neat PCDTPT films, and when the blend film is employed in an OTFT, the performance is largely maintained. The ability to maintain charge transport character is due to vertical segregation within the blend, while the improved ductility is due to intermixing of the polymers throughout the film thickness. Importantly, the application of large strains to the ductile films is shown to orient both polymers, which further increases charge carrier mobility. These results highlight a processing approach to achieve high performance polymer OTFTs that are electrically and mechanically optimized.

Blending of n-type Semiconducting Polymer and PC61BM for an Efficient Electron-Selective Material to Boost the Performance of the Planar Perovskite Solar Cell

The highly efficient CH3NH3PbI3 perovskite solar cell (PeSC) is simply achieved by employing a blended electron-transport layer (ETL) consisting of PC61BM and P(NDI2OD-T2). The high molecular weight of P(NDI2OD-T2) allows for a thinned ETL with a uniform morphology that optimizes the PC61BM ETL more effectively. As a result of this enhancement, the power conversion efficiency of a PC61BM:P(NDI2OD-T2)-based PeSC is 25% greater than that of the conventional PC61BM based-PeSC; additionally, the incorporation of P(NDI2OD-T2) into PC61BM attenuates the dependence of the PeSC on the ETL-processing conditions regarding its performance. It is revealed that, in addition to the desirable n-type semiconducting characteristics of PC61BM:P(NDI2OD-T2)-including a higher electron-mobility and a more-effective electron selectivity of a blended ETL for an efficient electron extraction-the superior performance of a PC61BM:P(NDI2OD-T2) device is the result of a thinned and uniformly covered ETL on the perovskite layer.

Mechanical Properties of Solution-Processed Small-Molecule Semiconductor Films

Advantages of semiconducting small molecules-as opposed to semiconducting polymers-include synthetic simplicity, monodispersity, low cost, and ease of purification. One purported disadvantage of small-molecule films is reduced mechanical robustness. This paper measures the tensile modulus and crack-onset strain for pure films of the high-performance solution-processable small-molecule donors 7,7'-[4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dithiophene-2,6-diyl]bis[6-fluoro-4-(5'-hexyl-[2,2'-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole] (DTS(FBTTh2)2), 2,5-di(2-ethylhexyl)-3,6-bis(5″-n-hexyl-[2,2',5',2″]terthiophen-5-yl)-pyrrolo[3,4-c]pyrrole-1,4-dione (SMDPPEH), and 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), the acceptor 5,5'-(2,1,3-benzothiadiazole-4,7-diyldi-2,1-ethenediyl)bis[2-hexyl-1H-isoindole-1,3(2H)-dione] (HPI-BT), blends of DTS(FBTTh2)2 and SMDPPEH with [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) and with HPI-BT, and bulk heterojunction films processed with the additives 1,8-diiodooctane (DIO) and polystyrene (PS). The most deformable films of solution-processed organic semiconductors are found to exhibit tensile moduli and crack-onset strains comparable to those measured for conjugated polymers. For example, the tensile modulus of as-cast DTS(FBTTh2)2 is 0.68 GPa (i.e., comparable to poly(3-hexylthiophene) (P3HT), the common polymer), while it exhibits no cracks when stretched on an elastomeric substrate to strains of 14%. While this high degree of stretchability is lost upon the addition of PC71BM (4.2 GPa, 1.42%), it can be partially recovered using processing additives. Tensile modulus and crack-onset strain are highly correlated, which is typical of van der Waals solids. Increased surface roughness was correlated to increased modulus and brittleness within films of similar composition. Decreased stiffness for soluble molecular semiconductors can be rationalized by the presence of alkyl side chains, which decrease the van der Waals attraction between molecules in the crystalline grains. These measurements and observations could have important consequences for the stability of devices based on molecular semiconductors, especially those destined for stretchable or ultraflexible applications, or those demanding mechanical robustness during roll-to-roll fabrication or use in the outdoor environment.

Development of simple hole-transporting materials for perovskite solar cells

Three low-cost propeller-shaped small molecules based on a triphenylamine core and the high-performance donor molecule 7,7′-[4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl]bis[6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole] (DTS(FBTTh2)2) were investigated as hole-transporting materials in perovskite solar cells. Each hole-transporting material was designed with highly modular side arms, allowing for different bandgaps and thin-film properties while maintaining a consistent binding energy of the highest occupied molecular orbitals to facilitate hole extraction from the perovskite active layer. Perovskite solar cell devices were fabricated with each of the three triphenylamine-based hole-transporting materials and DTS(FBTTh2)2 and were compared to devices with 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) hole-transporting layers. Each of our triphenylamine hole-transporting materials and DTS(FBTTh2)2 displayed surface morphologies that were considerably rougher than that of spiro-OMeTAD; a factor that may contribute to lower device performance. It was found that using inert, insulating polymers as additives with DTS(FBTTh2)2 reduced the surface roughness, resulting in devices with higher photocurrents.

Integrated Effects of Two Additives on the Enhanced Performance of PTB7:PC71BM Polymer Solar Cells

Organic photovoltaics (OPVs) are fabricated with blended active layers of poly [[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl]]: [6,6]-phenylC71-butyric acid methyl ester (PTB7:PC71BM). The active layers are prepared in chlorobenzene (CB) added different additives of 1, 8-Diiodooctane (DIO) and polystyrene (PS) with different concentrations by spin coating. A small addition, 0.5%-5% by weight relative to the BHJ components, of inert high molecular weight PS is used to increase the solution viscosity and film thickness without sacrificing desirable phase separation and structural order. The effects of the PS are studied with respect of photovoltaic parameters such as fill factor, short circuit current density, and power conversion efficiency. Together with DIO, the device with 3.0 v% DIO and 1 wt % PS shows a high power conversion efficiency (PCE) of 8.92% along with an open-circuit voltage (Voc) of 0.76 V, a short-circuit current (Jsc) of 16.37 mA/cm², and a fill factor (FF) of 71.68%. The absorption and surface morphology of the active layers are investigated by UV-visible spectroscopy, atomic force microscopy (AFM) respectively. The positive effect of DIO and PS additives on the performance of the OPVs is attributed to the increased absorption and the charge carrier transport and collection.

Symmetrical and unsymmetrical triphenylamine based diketopyrrolopyrroles and their use as donors for solution processed bulk heterojunction organic solar cells

Two small molecules DPP3 (D–π–A) and DPP4 (D–π–A–π–D) with triphenylamine (TPA) donors and diketopyrrolopyrrole (DPP) acceptors linked with ethyne linkers were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction.

Surface treatment by binary solvents induces the crystallization of a small molecular donor for enhanced photovoltaic performance

The surface treatment of p-DTS(FBTTh₂)₂:PC₇₁BM films with binary solvents of methanol and 1-chloronaphthalene enhanced the PCE from 2.4% to 6.5%.

Efficient promotion of charge separation and suppression of charge recombination by blending PCBM and its dimer as electron transport layer in inverted perovskite solar cells

Admixing PCBM and its dimer as electron transport material significantly improves charge carrier dynamic behavior in inverted perovskite device.

Photoactive composite films prepared from mixtures of polystyrene microgel dispersions and poly(3-hexylthiophene) solutions

Whilst polystyrene microgels belong to the oldest family of microgel particles, their behaviours when deposited onto substrates or prepared as composites have received little attention. Because polystyrene microgels are solvent-swellable, and inherently colloidally stable, they are well suited to form composites with conjugated polymers. Here, we investigate the morphology and light absorption properties of spin coated composite films prepared from mixed dispersions of polystyrene microgels and poly(3-hexylthiophene) (P3HT) for the first time. We compare the morphologies of the composite films to spin coated microgel films. The films were studied using optical microscopy, SEM, AFM, wide-angle X-ray diffraction and UV-visible spectroscopy. The films contained flattened microgel particles with an aspect ratio of ∼10. Microgel islands containing hexagonally close packed particles were evident for both the pure microgel and microgel/P3HT composite films. The latter were electrically conducting. The composite film morphology was dependent on the microgel and P3HT concentration used for film preparation and a morphology phase diagram was constructed. The P3HT phase acted as an electrically conducting cement and increased the robustness of the films to solvent washing. The composite films were photoactive due to the P3HT component. The absorbance for the films was tuneable and increased linearly with both microgel and P3HT concentration. The results of the study should apply to other organic swellable microgel/conjugated polymer combinations and may lead to new colloidal composites for future optoelectronic applications.

Solution-Processed Small-Molecule Bulk Heterojunctions: Leakage Currents and the Dewetting Issue for Inverted Solar Cells

In organic photovoltaic (PV) devices based on solution-processed small molecules, we report here that the physicochemical properties of the substrate are critical for achieving high-performances organic solar cells. Three different substrates were tested: ITO coated with

PEDOT

PSS, ZnO sol-gel, and ZnO nanoparticles. PV performances are found to be low when the ZnO nanoparticles layer is used. This performance loss is attributed to the formation of many dewetting points in the active layer, because of a relatively high roughness of the ZnO nanoparticles layer, compared to the other layers. We successfully circumvented this phenomenon by adding a small quantity of polystyrene (PS) in the active layer. The introduction of PS improves the quality of film forming and reduces the dark currents of solar cells. Using this method, high-efficiency devices were achieved, even in the case of substrates with higher roughness.

Enhancing the Thermal Stability of Solution-Processed Small-Molecule Semiconductor Thin Films Using a Flexible Linker Approach

Using flexible aliphatic chains to link conjugated molecular semiconductors affords a polymeric material that possesses defined conjugated segments but extended covalent connectivity, which enhances crystallinity and thermal stability in field-effect transistors and bulk heterojunction solar-cell devices when used as an additive.

Schematic Studies on the Structural Properties and Device Physics of All Small Molecule Ternary Photovoltaic Cells

Although the field of ternary organic solar cells has seen much progress in terms of device performance in the past few years, limited understanding has restricted further development. For example, studies of the crystalline packing structure of ternary blends have rarely been reported in the solar cell field. Consequently, we chose two ternary blends of small molecules, two fullerene derivatives (small-molecule:PC71BM:PC61BM or small-molecule:PC71BM:ICBA), to investigate crystallization behavior and interactions among the three components. The crystalline structure of the ternary active blends was characterized using various techniques such as 2D-GIWAXS and AFM, and the relationship of the observed morphologies to device performance is discussed. Furthermore, the device physics associated with the charge generation, transport, and recombination dynamics of these ternary blend systems were investigated.

Synergistic Effects of Binary-Solvent Annealing for Efficient Polymer-Fullerene Bulk Heterojunction Solar Cells

Conjugated polymer-fullerene-based bulk-heterojunction (BHJ) organic solar cells (OSCs) have attracted tremendous attention over the past two decades because of their potential to develop low-cost and easy methods to produce energy from light. The complicated microstructure and morphology with randomly organized architecture of these polymer-fullerene-based active layers (ALs) is a key factor that limits photovoltaic performance. In this study, a binary-solvent annealing (BSA) approach was established to improve the poly(3-hexylthiophene):indene-C60 bisadduct-based AL for efficient BHJ-type OSCs by varying the second solvents with different boiling points (BP). Thus, we were able to change the evaporation behavior of cosolvents and consequently obtain the various microstructural properties of the AL. An in-depth study was conducted on the solvent-evaporation driven morphology of the active layer under various cosolvent conditions and its effect on the photovoltaic parameters of OSCs. Under the BSA processes, we found that the specimens with low-BP second solvents allows us to observe a more ideal AL for increasing photon absorption and efficient charge transport and collection at the respective electrodes, resulting in enhanced PCE of the corresponding OSCs. By contrast, the specimens with high-BP second solvents exhibit random microstructures, which are detrimental to charge transport and collection and lead to diminished PCE of the corresponding OSCs. By appropriately selecting the composition of a binary solvent, BSA can be employed as an easy method for the effective manipulation of the microstructures of ALs. BSA is a promising technique for the performance enhancement of not only OSCs but also other organic/polymeric-based electronic devices.

Improving the Efficiency of Organic Solar Cells upon Addition of Polyvinylpyridine

We report on the efficiency improvement of organic solar cells (OPVs) based on the low energy gap polyfluorene derivative, APFO-3, and the soluble C₆₀ fullerene PCBM, upon addition of a residual amount of poly (4-vinylpyridine) (PVP). We find that the addition of 1% by weight of PVP with respect to the APFO-3 content leads to an increase of efficiency from 2.4% to 2.9%. Modifications in the phase separation details of the active layer were investigated as a possible origin of the efficiency increase. At high concentrations of PVP, the blend morphology is radically altered as observed by Atomic Force Microscopy. Although the use of low molecular weight additives is a routine method to improve OPVs efficiency, this report shows that inert polymers, in terms of optical and charge transport properties, may also improve the performance of polymer-based solar cells.

All-organic and fully-printed semitransparent photodetectors based on narrow bandgap conjugated molecules

All-organic, fully-printed and semitransparent photodetectors with a broad wavelength band response, based on a ternary blend comprising narrow band-gap small molecules, are demonstrated. The ternary blend with a semiconducting polymer allows for the optimal printing of small molecules, suppressing strong phase segregation, and uncontrolled crystallization. The insertion of a suitable interlayer enables the adoption of polymer, transparent, top and bottom printed electrodes, thus making light detection possible from both device sides.