Subtle Balance Between Length Scale of Phase Separation and Domain Purification in Small-Molecule Bulk-Heterojunction Blends under Solvent Vapor Treatment

Organic Solar Cells Based on Non-Fullerene Low Molecular Weight Organic Semiconductor Molecules

The development of narrow bandgap A-D-A- and ADA'DA-type non-fullerene small molecule acceptors (NFSMAs) along with small molecule donors (SMDs) have led to significant progress in all-small molecule organic solar cells. Remarkable power conversion efficiencies, nearing the range of 17-18 %, have been realized. These efficiency values are on par with those achieved in OSCs based on polymeric donors. The commercial application of organic photovoltaic technology requires the design of more efficient organic conjugated small molecule donors and acceptors. In recent years the precise tuning of optoelectronic properties in small molecule donors and acceptors has attracted considerable attention and has contributed greatly to the advancement of all-SM-OSCs. Several reviews have been published in this field, but the focus of this review concerns the advances in research on OSCs using SMDs and NFSMAs from 2018 to the present. The review covers the progress made in binary and ternary OSCs, the effects of solid additives on the performance of all-SM-OSCs, and the recently developed layer-by-layer deposition method for these OSCs. Finally, we present our perspectives and a concise outlook on further advances in all-SM-OSCs for their commercial application.

In Situ Transformation of an Amorphous Supramolecular Coating to a Hydrogen-Bonded Organic Framework Membrane to Trigger Selective Gas Permeation

We introduce a "solution-processing-transformation" strategy, deploying solvent vapor as scaffolds, to fabricate high-quality hydrogen-bonded organic framework (HOF) membranes. This strategy can overcome the mismatch in processing conditions and crystal growth thermodynamics faced during the facile solution processing of the membrane. The procedure includes the vapor-trigged in situ transformation of dense amorphous supramolecules to crystalline HOF-16, with HOF-11 as the transient state. The mechanism involves a vapor-activated dissolution-precipitation equilibrium shifting and hydrogen bonding-guided molecule rearrangement, elucidated through combined experimental and theoretical analysis. Upon removal of the molecular scaffolds, the resulting HOF-16 membranes showcase significant improvement in hydrogen separation performance over their amorphous counterparts and previously reported HOF membranes. The method's broad applicability is evidenced by successfully extending it to other substrates and HOF structures. This study provides a fundamental understanding of guest-induced ordered supramolecular assembly and paves the way for the advanced manufacture of high-performance HOF membranes for gas separation processes.

Optimization of Charge Management and Energy Loss in All-Small-Molecule Organic Solar Cells

All-small-molecule organic solar cells (ASM-OSCs) have received tremendous attention in recent decades because of their advantages over their polymer counterparts. These advantages include well-defined chemical structures, easy purification, and negligible batch-to-batch variation. Remarkable progress with a power conversion efficiency (PCE) of over 17% has recently been achieved with improved charge management (FF × JSC) and reduced energy loss (Eloss). Morphology control is the key factor in the progress of ASM-OSCs, which remains a significant challenge because of the similarities in the molecular structures of the donors and acceptors. In this review, the effective strategies for charge management and/or Eloss reduction from the perspective of effective morphology control are summarized. The aim is to provide practical insights and guidance for material design and device optimization to promote further development of ASM-OSCs to a level where they can compete with or even surpass the efficiency of polymer solar cells.

Organic Photovoltaics Utilizing Small-Molecule Donors and Y-Series Nonfullerene Acceptors

The emerging Y-series nonfullerene acceptors (Y-NFA) has prompted the rapid progress of power conversion efficiency (PCE) of all-small-molecule organic solar cells (ASM-OSCs) from around 12% to 17%. The excellent PCE improvement benefits from not only the outstanding properties of Y-series acceptors but also the successful development of small-molecule donors. The short-circuit current density, fill factor, and nonradiative recombination are all optimized to the unprecedented values, providing a scenery that is obviously different from the ITIC-series based ASM-OSCs. In this review, OSCs utilizing small-molecule donors and Y-NFA are summarized and classified in order to provide an up-to-date development overview and give an insight on structure-property correlation. Then, the characteristics of bulk-heterojunction (BHJ) formation of ASM-OSCs are discussed and compared with that of polymer-based OSCs. Finally, the challenges and outlook on designing ground-breaking small-molecule donor and forming an ideal BHJ morphology are discussed.

Progress and Future Potential of All-Small-Molecule Organic Solar Cells Based on the Benzodithiophene Donor Material

Organic solar cells have obtained a prodigious amount of attention in photovoltaic research due to their unique features of light weight, low cost, eco-friendliness, and semitransparency. A rising trend in this field is the development of all-small-molecules organic solar cells (ASM-OSCs) due to their merits of excellent batch-to-batch reproducibility, well-defined structures, and simple purification. Among the numerous organic photovoltaic (OPV) materials, benzodithiophene (BDT)-based small molecules have come to the fore in achieving outstanding power conversion efficiency (PCE) and breaking the 17% efficiency barrier in single-junction OPV devices, indicating the significant potential of this class of materials in commercial photovoltaic applications. This review specially focuses on up-to-date information about improvements in BDT-based ASM-OSCs since 2011 and provides an outlook on the most significant challenges that remain in the field. We believe there will be more exciting BDT-based photovoltaic materials and devices developed in the near future.

Achieving Record-Efficiency Organic Solar Cells upon Tuning the Conformation of Solid Additives

Volatile solid additives (SADs) are considered as a simple yet effective approach to tune the film morphology for high-performance organic solar cells (OSCs). However, the structural effects of the SADs on the photovoltaic performance are still elusive. Herein, two volatilizable SADs were designed and synthesized. One is SAD1 with twisted conformation, while the other one is planar SAD2 with the S···O noncovalent intramolecular interactions (NIIs). The theoretical and experimental results revealed that the planar SAD2 with smaller space occupation can more easily insert between the Y6 molecules, which is beneficial to form a tighter intermolecular packing mode of Y6 after thermal treatment. As a result, the SAD2-treated OSCs exhibited less recombination loss, more balanced charge mobility, higher hole transfer rate, and more favorable morphology, resulting in a record power conversion efficiency (PCE) of 18.85% (certified PCE: 18.7%) for single-junction binary OSCs. The universality of this study shed light on understanding the conformation effects of SADs on photovoltaic performances of OSCs.

Modulation of the Fluorination Site on Side-Chain Thiophene Improved Efficiency in All-Small-Molecule Organic Solar Cells

Fine-tuning the phase-separated morphology is of great importance to achieve efficient all-small-molecule organic solar cells (ASM-OSCs). In this work, a pair of isomers are designed and synthesized, namely, BDT-UF and BDT-DF, in which the fluorine atom in BDT-UF is close to the alkyl chain of side-chain thiophene, while that in BDT-DF is close to the center core. Owing to the noncovalent interaction between fluorine and hydrogen, BDT-DF shows a smaller dihedral angle between the thiophene side chain and the BDT core, which causes better molecular planarity. When mixed with N3, BDT-UF shows better miscibility, higher crystallinity, and more ordered molecule stacking in the blend film. Finally, the device of BDT-DF:N3 gains a power conversion efficiency (PCE) of 14.5%, while that of BDT-UF:N3 shows an increase in Voc, Jsc, and FF and gains a PCE of 15.1%. Our work exhibits a way of adjusting the substitution site of fluorine atoms to improve the PCE of ASM-OSCs.

Immobilization of Conjugated Polymer Domains for Highly Stable Non-Fullerene-Based Organic Solar Cells

To commercialize organic solar cells (OSCs), changes in the optimized morphology of the photoactive layer caused by external stimuli that cause degradation must be addressed. This work improves OSC stability by utilizing the cross-linking additive 1,8-dibromooctane (DBO) and a sequential deposition process (XSqD) to fabricate the photoactive layer. The cross-linking additive in the donor polymer (PTB7-Th) improves polymer crystallinity and immobilizes the crystalline morphology by partial photo-cross-linking. Ellipsometry experiments confirm the increase in the glass transition temperature of cross-linked PTB7-Th. The polymer crystallinity is further improved after removal of non-cross-linked polymer and residual additive by chlorobenzene. The cross-linked polymer layer forms an efficient and stable heterojunction with a nonfullerene acceptor (IEICO-4F) layer via an XSqD process. The OSC based on the immobilized PTB7-Th exhibits excellent stability against light soaking and thermal aging.

Effects of Alkyl Side Chains of Small Molecule Donors on Morphology and the Photovoltaic Property of All-Small-Molecule Solar Cells

Unraveling the relationship between nanoscale morphology of active layers and chemical structures of organic semiconductor photovoltaic materials is crucially important for further advancing the development of all-small-molecule organic solar cells (SM-OSCs). Here, in order to delve into the effect of flexible side chains of small molecule donors on the photovoltaic properties of SM-OSCs, we synthesized two new small molecule donors substituted by different flexible alkyl chains (iso-octyl chains for SM1-EH and n-octyl chains for SM1-Oct). As a result, the two small molecules present different absorption properties, energy levels, and stacking characteristics. When blending with Y6 as an acceptor, the SM1-Oct-based SM-OSC demonstrated a higher PCE value of 11.73%, while the SM1-EH-based device presents a relatively poorer PCE value of 8.42%. In addition, the morphology analysis demonstrated that, compared with the SM1-EH:Y6 blend, the SM1-Oct:Y6 blend film displayed better molecular stacking properties with stronger multilevel diffraction and preferable phase separation, resulting in the higher hole mobility, more efficient charge separation efficiency, and better device performance. These results underline that reasonably adjusting the flexible alkyl chains of small molecule donors can be an effective approach to further advance the development of the SM-OSCs field.

Resolving Different Physical Origins toward Crystallite Imperfection in Semiconducting Polymers: Crystallite Size vs Paracrystallinity

The crystallization and aggregation behaviors of semiconducting polymers play a critical role in determining the ultimate performance of optoelectronic devices based on these materials. Due to the soft nature of polymers, crystallite imperfection exists ubiquitously. To this aspect, crystallinity is often used to represent the degree of crystallite imperfection in a reciprocal relation. Despite of the importance, the discussion on crystallinity is still on the phenomenological level and ambiguous in many cases. As two major contributors to crystallite imperfection, crystallite size and paracrystallinity are highly intertwined and hardly separated, hindering more accurate and trustworthy structural analysis. Herein, with the aid of synchrotron-based X-ray diffraction, combined with environmentally controlled heating capability, the evolution of crystallite size and paracrystallinity of two prototypical polythiophene-based thin films have been successfully measured. Strikingly, the paracrystallinity of poly(3-hexylthiophene-2,5-diyl) (P3HT) crystallites remains unchanged with annealing, while the paracrystallinity of poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) becomes diminished with crystallite growth. This work delivers a promising gesture to semiconducting polymers community, confirming that it is possible to experimentally separate crystallite size and paracrystallinity, both of which are highly intertwined. With this progress, investigation on the correlation between further detailed microstructural parameters and device performance can be achieved.

Differently Linked Perylene Bisimide Dimers with Various Twisting and Phase Structures for Nonfullerene All-Small-Molecule Organic Solar Cells

Nonfullerene all-small-molecule organic solar cells (NF all-SMSCs) are an important classification in the organic solar cell system. However, the application and research of NF all-SMSCs are limited due to the easy aggregation of small molecules to form large-phase domains. Perylene bisimides (PBIs) have been widely used as nonfullerene acceptors. Simply changing the link position of the PBI dimer can control the accumulation of molecules to regulate the size of the phase domain. Herein, the bay-linked, ortho-linked, and hydrazine-linked PBI dimers as nonfullerene acceptors, named as B-SdiPBI, O-SdiPBI, and H-SdiPBI, respectively, were chosen. The link position of the PBI dimer can lead to diverse molecular torsion and planarity, which affects the film-forming ability, phase separation, and thus optoelectronic properties. NF all-SMSCs based on B-SdiPBI, O-SdiPBI, and H-SdiPBI as nonfullerene acceptors and a small molecule DR3TBDTT as the donor achieve the best power conversion efficiencies of 1.93, 3.30, and 4.05%, respectively. The result is consistent with the sequence of inter-PBI twist and phase domain size of the corresponding blend films in the device. The DR3TBDTT:H-SdiPBI system has the best efficiency with the largest dihedral angle of H-SdiPBI (ψ = 90°) and an appropriate phase size (10-40 nm), whereas the smaller dihedral angle of O-SdiPBI (ψ = 86°) produces a larger phase size (20-50 nm) and the smallest dihedral angle of B-SdiPBI (ψ = 71°) gives the largest phase size (30-80 nm). This illustrates that the twist angle can effectively increase the phase separation between the acceptor and donor to obtain an effective phase size in this system. The work provides a guide for designing the acceptors and controlling phase domains of high-performance NF all-SMSCs.

Carbazole-based green and blue-BODIPY dyads and triads as donors for bulk heterojunction organic solar cells

Two BODIPY derivatives with one (B2) and two (B3) carbazole moieties were synthesized and applied as electron-donor materials in organic photovoltaic cells (OPV). Their optical and electrochemical properties were systematically investigated. These BODIPY dyes exhibit excellent solubility in organic solvents and present high molar extinction coefficients (1.37-1.48 × 10⁵ M^-1 cm^-1) in solutions with absorption maxima at 586 nm for mono-styryl groups and at 672 nm for di-styryl groups. The introduction of the styryl moieties results in a large bathochromic shift and a significant decrease in the HOMO-LUMO energy-gaps. The BODIPY dyes show relatively low HOMO energies ranging from -4.99 to -5.16 eV as determined from cyclic voltammetry measurements. Cyclic voltammetry measurements and theoretical calculations demonstrate that the frontier molecular orbital levels of these compounds match with those of PC₇₁BM as the acceptor, supporting their application as donor materials in solution-processed small molecule bulk heterojunction (BHJ) organic solar cells. After the optimization of the active layer, B2:PC₇₁BM and B3:PC₇₁BM based organic solar cells showed an overall power conversion efficiency of 6.41% and 7.47%, respectively. The higher PCE of the B3-based OSC is ascribed to the more balanced charge transport and exciton dissociation, better crystallinity and molecular packing.

13.7% Efficiency Small-Molecule Solar Cells Enabled by a Combination of Material and Morphology Optimization

Compared with the quick development of polymer solar cells, achieving high-efficiency small-molecule solar cells (SMSCs) remains highly challenging, as they are limited by the lack of matched materials and morphology control to a great extent. Herein, two small molecules, BSFTR and Y6, which possess broad as well as matched absorption and energy levels, are applied in SMSCs. Morphology optimization with sequential solvent vapor and thermal annealing makes their blend films show proper crystallinity, balanced and high mobilities, and favorable phase separation, which is conducive for exciton dissociation, charge transport, and extraction. These contribute to a remarkable power conversion efficiency up to 13.69% with an open-circuit voltage of 0.85 V, a high short-circuit current of 23.16 mA cm^-2 and a fill factor of 69.66%, which is the highest value among binary SMSCs ever reported. This result indicates that a combination of materials with matched photoelectric properties and subtle morphology control is the inevitable route to high-performance SMSCs.

Efficient and thermally stable organic solar cells based on small molecule donor and polymer acceptor

Efficient organic solar cells (OSCs) often use combination of polymer donor and small molecule acceptor. Herein we demonstrate efficient and thermally stable OSCs with combination of small molecule donor and polymer acceptor, which is expected to expand the research field of OSCs. Typical small molecule donors show strong intermolecular interactions and high crystallinity, and consequently do not match polymer acceptors because of large-size phase separation. We develop a small molecule donor with suppressed π-π stacking between molecular backbones by introducing large steric hindrance. As the result, the OSC exhibits small-size phase separation in the active layer and shows a power conversion efficiency of 8.0%. Moreover, this OSC exhibits much improved thermal stability, i.e. maintaining 89% of its initial efficiency after thermal annealing the active layer at 180 °C for 7 days. These results indicate a different kind of efficient and stable OSCs.

Enhanced π-π Interactions of Nonfullerene Acceptors by Volatilizable Solid Additives in Efficient Polymer Solar Cells

Fine-tuning of the nanoscale morphologies of the active layers in polymer solar cells (PSCs) through various techniques plays a vital role in improving the photovoltaic performance. However, for emerging nonfullerene (NF) PSCs, the morphology optimization of the active-layer films empirically follows the methods originally developed in fullerene-based blends and lacks systematic studies. In this work, two solid additives with different volatilities, SA-4 and SA-7, are applied to investigate their influence on the morphologies and photovoltaic performances of NF-PSCs. Although both solid additives effectively promote the molecular packing of the NF acceptors, due to the higher volatility of SA-4, the devices processed with SA-4 exhibit a power conversion efficiency of 13.5%, higher than that of the control devices, and the devices processed with SA-7 exhibit poor performances. Through a series of detailed morphological analyses, it is found that the volatilization of SA-4 after thermal annealing is beneficial for the self-assembly packing of acceptors, while the residuals due to the incomplete volatilization of SA-7 have a negative effect on the film morphology. The results delineate the feasibility of applying volatilizable solid additives and provide deeper insights into the working mechanism, establishing guidelines for further material design of solid additives.

High-Crystallinity π-Conjugated Small Molecules Based on Thienylene-Vinylene-Thienylene: Critical Role of Self-Organization in Photovoltaic, Charge-Transport, and Morphological Properties

Narrow-band-gap small molecules with π-extended backbones are promising donor materials for solution-processed bulk-heterojunction (BHJ) organic solar cells (OSCs). Herein, a series of acceptor-donor-acceptor (A-D-A) photovoltaic small molecules incorporating thienylene-vinylene-thienylene (TVT) as a central D unit and alkyl-substituted rhodanine or 2-(1,1-dicyanomethylene)rhodanine as terminal A units are designed and synthesized. Their physical properties including photoabsorption, electronic energy levels, hole mobility, and morphological characteristics are systematically investigated. Using solvent vapor annealing (SVA), the morphologies of the BHJ photoactive layers composed of these small-molecule donors and a [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) acceptor can be properly modulated. As a result of increased crystallinity of the donors and desired phase segregation between the donors and PC₇₁BM upon rapid SVA treatment, the photovoltaic performances of the resultant OSC devices undergo drastic enhancement. The results reported here indicate that high-efficiency small-molecule OSCs can be achieved through rational design of the TVT-based molecular framework and optimization of the nanoscale phase-segregated morphology via proper SVA treatment.

An investigation of the role acceptor side chains play in the processibility and efficiency of organic solar cells fabricated from small molecular donors featuring 3,4-ethylenedioxythiophene cores

Organic photovoltaic devices fabricated from small molecular donors continue to receive significant interest due to their desirable properties such as convenient synthesis, purification and batch-to-batch reproducibility. In this study, we have synthesized two small molecules based on an alternating A–D–A structure, utilizing a central EDOT donor moiety and either 2-ethylhexyl cyanoacetate (SAM-72) or N-(2-ethylhexyl)cyanoacetamide (SAM-80) units as acceptor termini. The small molecules were incorporated into bulk heterojunction solar cells with PC₇₁BM. Our investigations have shown that the side chains utilized for SAM-80 only allow for solution processing using volatile solvents, such as chloroform, which limits the reproducibility of device fabrication. However, SAM-72 displays better solubility and devices fabricated using a SAM-72:PC₇₁BM active layer reached average power conversion efficiencies of 1.9%, with fill factors reaching 60%. Post-processing methods such as thermal and solvent vapor annealing were found to significantly increase the stability of devices, but were not able to improve overall device performance.

The chemical nature of the acceptor side chain plays an important role in the processability and photovoltaic performance of EDOT-based small molecule donors.

Corrole-BODIPY Dyad as Small-Molecule Donor for Bulk Heterojunction Solar Cells

Dyes based on charge-transfer (CT) characteristics are attractive candidates for organic photovoltaics due to their intense and broad absorption window. In these molecular frameworks, electron-rich donors and electron-deficient acceptors are covalently linked to achieve an effective CT process. Corrole, a tetrapyrrolic congener of porphyrin, is an excellent example of an electron-rich molecule with a large molar extinction coefficient. BODIPY, on the other hand, is a well-known electron-deficient bypyrrolic boron difluoride complex with intense absorption complementary to the corrole. A combination of these two structural motifs should result in a dyad having a wide absorption window, which will be suitable for organic photovoltaics. Herein, a corrole derivative has been envisaged as an efficient donor for solution-processed bulk heterojunction solar cells with PC₇₁BM as an acceptor for the first time. The current molecule exhibits broad absorption in the visible range in solution as well as in thin films, with a high molar extinction coefficient and a low band gap of 1.79 eV. Frontier molecular orbital energy levels were found to be complementary to those of the well-known acceptor PC₇₁BM. The optimized devices based on Cor-BODIPY:PC₇₁BM showed a high power conversion efficiency (PCE) of 6.6% with J_sc = 11.46 mA/cm², V_oc = 0.90 V, and FF = 0.61. A remarkable value of incident photon-to-current conversion efficiency (IPCE) of 61% has also been observed.

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.

Two Novel Small Molecule Donors and the Applications in Bulk-Heterojunction Solar Cells

Two novel small molecules DTRDTQX and DTIDTQX, based on ditolylaminothienyl group as donor moiety and quinoxaline as middle acceptor moiety with different terminal acceptor groups were synthesized and characterized in this work. In order to study the photovoltaic properties of DTRDTQX and DTIDTQX, bulk-heterojunction solar cells with the configuration of FTO/c-TiO₂/DTRDTQX(or DTIDTQX):C₇₀/MoO₃/Ag were fabricated, in which DTRDTQX and DTIDTQX acted as the donors and neat C₇₀ as the acceptor. When the weight ratio of DTRDTQX:C₇₀ reached 1:2 and the active layer was annealed at 100°C, the optimal device was realized with the power conversion efficiency (PCE) of 1.44%. As to DTIDTQX:C₇₀-based devices, the highest PCE of 1.70% was achieved with the optimal blend ratio (DTIDTQX:C₇₀ = 1:2) and 100°C thermal annealing treatment. All the experimental data indicated that DTRDTQX and DTIDTQX could be employed as potential donor candidates for organic solar cell applications.

Critical Role of Vertical Phase Separation in Small-Molecule Organic Solar Cells

An inverted device structure is a more stable configuration than a regular device structure for solution-processed organic solar cells (OSCs). However, most of the solution-processed small-molecule OSCs (SM-OSCs) reported in the literature used the regular device structure, and a regular device normally exhibits a higher efficiency than an inverted device. Herein, a representative small-molecule DR3TBDTT was selected to figure out the reason for photovoltaic performance differences between regular and inverted devices. The mechanisms for a reduced open-circuit voltage ( V_oc) and fill factor (FF) in the inverted device were studied. The reduced V_oc and FF is due to the vertical phase separation with excess [6,6]-phenyl-C₇₁-butyric acid methyl ester near the air/blend surface, which leads to a reduction in build-in voltage and unbalanced charge transport in the inverted device. Another reason for the reduced FF is the unfavorable DR3TBDTT crystallite orientation distribution along the film thickness, which is preferential edge-on crystallites in the top layer of the blend film and the increased population of face-on crystallites in the bottom layer of the blend film. This study illustrates that the morphology plays a key role in photovoltaic performance difference between regular and inverted devices and provides useful guidelines for further optimization of the morphology of solution-processed SM-OSCs.

Impact of helical organization on the photovoltaic properties of oligothiophene supramolecular polymers

Higher order structures of semiconducting supramolecular polymers have a huge impact on their BHJ-OPV device performance.

Helical self-assembly of functional π-conjugated molecules offers unique photochemical and electronic properties in the spectroscopic level, but there are only a few examples that demonstrate their positive impact on the optoelectronic device level. Here, we demonstrate that hydrogen-bonded tapelike supramolecular polymers of a barbiturated oligo(alkylthiophene) show notable improvement in their photovoltaic properties upon organizing into helical nanofibers. A tapelike hydrogen-bonded supramolecular array of barbiturated oligo(butylthiophene) molecules was directly visualized by STM at a liquid–solid interface. TEM, AFM and XRD revealed that the tapelike supramolecular polymers further organize into helical nanofibers in solution and bulk states. Bulk heterojunction solar cells of the helical nanofibers and soluble fullerene showed a power conversion efficiency of 4.5%, which is markedly high compared to that of the regioisomer of butyl chains organizing into 3D lamellar agglomerates.

Origin of Reduced Open-Circuit Voltage in Highly Efficient Small-Molecule-Based Solar Cells upon Solvent Vapor Annealing

In this study, we demonstrate that remarkably reduced open-circuit voltage in highly efficient organic solar cells (OSCs) from a blend of phenyl-C₆₁-butyric acid methyl ester and a recently developed conjugated small molecule (DPPEZnP-THD) upon solvent vapor annealing (SVA) is due to two independent sources: increased radiative recombination and increased nonradiative recombination. Through the measurements of electroluminescence due to the emission of the charge-transfer state and photovoltaic external quantum efficiency measurement, we can quantify that the open-circuit voltage losses in a device with SVA due to the radiative recombination and nonradiative recombination are 0.23 and 0.31 V, respectively, which are 0.04 and 0.07 V higher than those of the as-cast device. Despite of the reduced open-circuit voltage, the device with SVA exhibited enhanced dissociation of charge-transfer excitons, leading to an improved short-circuit current density and a remarkable power conversion efficiency (PCE) of 9.41%, one of the best for solution-processed OSCs based on small-molecule donor materials. Our study also clearly shows that removing the nonradiative recombination pathways and/or suppressing energetic disorder in the active layer would result in more long-lived charge carriers and enhanced open-circuit voltage, which are prerequisites for further improving the PCE.

Dimeric Porphyrin Small Molecules for Efficient Organic Solar Cells with High Photoelectron Response in the Near-Infrared Region

Small molecules (SMs) with elongated backbones are promising for achieving a higher photovoltaic performance. Herein, a dimeric porphyrin small molecule, ZnP2-DPP, consisting of two porphyrin units linked with an ethynylene as the core and two diketopyrrolopyrrole (DPP) units as the arms is designed and synthesized as an electron donor for solution-processed bulk-heterojunction (BHJ) organic solar cells (OSCs). A significantly enhanced power conversion efficiency of 8.45% with an impressive short-circuit current density (J_sc) up to 19.65 mA cm^-2 is achieved for the BHJ OSCs based on ZnP2-DPP under AM 1.5G irradiation (100 mW cm^-2) compared to that for the OSCs based on the dimeric porphyrin linked with bis-ethynylenes reported previously. Furthermore, the devices show broad photoelectron responses up to 1000 nm with high near-infrared external quantum efficiency up to 66% at 780 nm. This is the first study reporting SM OSCs displaying such a large J_sc of about 20 mA cm^-2 simultaneously with a considerably high and deep photoelectron response of up to 1000 nm.

Hole-Transfer Dependence on Blend Morphology and Energy Level Alignment in Polymer: ITIC Photovoltaic Materials

Bulk-heterojunction organic photovoltaic materials containing nonfullerene acceptors (NFAs) have seen remarkable advances in the past year, finally surpassing fullerenes in performance. Indeed, acceptors based on indacenodithiophene (IDT) have become synonymous with high power conversion efficiencies (PCEs). Nevertheless, NFAs have yet to achieve fill factors (FFs) comparable to those of the highest-performing fullerene-based materials. To address this seeming anomaly, this study examines a high efficiency IDT-based acceptor, ITIC, paired with three donor polymers known to achieve high FFs with fullerenes, PTPD3T, PBTI3T, and PBTSA3T. Excellent PCEs up to 8.43% are achieved from PTPD3T:ITIC blends, reflecting good charge transport, optimal morphology, and efficient ITIC to PTPD3T hole-transfer, as observed by femtosecond transient absorption spectroscopy. Hole-transfer is observed from ITIC to PBTI3T and PBTSA3T, but less efficiently, reflecting measurably inferior morphology and nonoptimal energy level alignment, resulting in PCEs of 5.34% and 4.65%, respectively. This work demonstrates the importance of proper morphology and kinetics of ITIC → donor polymer hole-transfer in boosting the performance of polymer:ITIC photovoltaic bulk heterojunction blends.

Realizing Over 13% Efficiency in Green-Solvent-Processed Nonfullerene Organic Solar Cells Enabled by 1,3,4-Thiadiazole-Based Wide-Bandgap Copolymers

Two novel wide-bandgap copolymers, PBDT-TDZ and PBDTS-TDZ, are developed based on 1,3,4-thiadiazole (TDZ) and benzo[1,2-b:4,5-b']dithiophene (BDT) building blocks. These copolymers exhibit wide bandgaps over 2.07 eV and low-lying highest occupied molecular orbital (HOMO) levels below -5.35 eV, which match well with the typical low-bandgap acceptor of ITIC, resulting in a good complementary absorption from 300 to 900 nm and a low HOMO level offset (≤0.13 eV). Compared to PBDT-TDZ, PBDTS-TDZ with alkylthio side chains exhibits the stronger optical absorption, lower-lying HOMO level, and higher crystallinity. By using a single green solvent of o-xylene, PBDTS-TDZ:ITIC devices exhibit a large open-circuit voltage (V_oc ) up to 1.10 eV and an extremely low energy loss (E_loss ) of 0.48 eV. At the same time, the desirable high short-circuit current density (J_sc ) of 17.78 mA cm^-2 and fill factor of 65.4% are also obtained, giving rise to a high power conversion efficiency (PCE) of 12.80% without any additive and post-treatment. When adopting a homotandem device architecture, the PCE is further improved to 13.35% (certified as 13.19%) with a much larger V_oc of 2.13 V, which is the best value for any type of homotandem organic solar cells reported so far.

Phenothiazine-based small-molecule organic solar cells with power conversion efficiency over 7% and open circuit voltage of about 1.0 V using solvent vapor annealing

We have used two unsymmetrical small molecules with a D–A–D–π–D configuration as small molecule donors, along with PC₇₁BM as an acceptor, for solution processed bulk heterojunction solar cells.

Small-Molecule Organic Photovoltaic Modules Fabricated via Halogen-Free Solvent System with Roll-to-Roll Compatible Scalable Printing Method

For the first time, the photovoltaic modules composed of small molecule were successfully fabricated by using roll-to-roll compatible printing techniques. In this study, blend films of small molecules, BTR and PC₇₁BM were slot-die coated using a halogen-free solvent system. As a result, high efficiencies of 7.46% and 6.56% were achieved from time-consuming solvent vapor annealing (SVA) treatment and roll-to-roll compatible solvent additive approaches, respectively. After successful verification of our roll-to-roll compatible method on small-area devices, we further fabricated large-area photovoltaic modules with a total active area of 10 cm², achieving a power conversion efficiency (PCE) of 4.83%. This demonstration of large-area photovoltaic modules through roll-to-roll compatible printing methods, even based on a halogen-free solvent, suggests the great potential for the industrial-scale production of organic solar cells (OSCs).

Improved Domain Size and Purity Enables Efficient All-Small-Molecule Ternary Solar Cells

An all-small-molecule ternary solar cell is achieved with a power conversion efficiency of 10.48% by incorporating phenyl-C₇₁ -butyric-acid-methyl ester (PC₇₁ BM) into a nonfullerene binary system. The addition of PC₇₁ BM is found to modulate the film morphology by improving the domain purity and decreasing the domain size. This modulation facilitates charge generation and suppresses charge recombination, as manifested by the significantly enhanced short-circuit current density and fill factor. The results correlate the domain characteristics with the device performance and offer new insight from the perspective of morphology modulation for constructing efficient ternary devices.

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).

Importance of Solvent Removal Rate on the Morphology and Device Performance of Organic Photovoltaics with Solvent Annealing

Solvent vapor annealing has been widely used in organic photovoltaics (OPV) to tune the morphology of bulk heterojunction active layer for the improvement of device performance. Unfortunately, the effect of solvent removal rate (SRR) after solvent annealing, which is one of the key factors that impact resultant morphology, on the morphology and device performance of OPV has never been reported. In this work, the nanoscale morphology of small molecule (SM):fullerene bulk heterojunction (BHJ) solar cell from different SRRs after solvent annealing was examined by small-angle neutron scattering and grazing incidence X-ray scattering. The results clearly demonstrate that the nanoscale morphology of SM:fullerene BHJ especially fullerene phase separation and concentration of fullerene in noncrystalline SM was significantly impacted by the SRR. The enhanced fullerene phase separation was found with a decrease of SRR, while the crystallinity and molecular packing of SM remained unchanged. Correlation to device performance shows that the balance between pure fullerene phase and mixing phase of SM and fullerene is crucial for the optimization of morphology and enhancement of device performance. Moreover, the specific interfacial area between pure fullerene phase and mixing phase is crucial for the electron transport and thus device performance. More importantly, this finding would provide a more careful and precise control of morphology of SM:fullerene BHJ and offers a guideline for further improvement of device performance with solvent annealing.

Applying Thienyl Side Chains and Different π-Bridge to Aromatic Side-Chain Substituted Indacenodithiophene-Based Small Molecule Donors for High-Performance Organic Solar Cells

A pair of linear tetrafluorinated small molecular donors, named as ThIDTTh4F and ThIDTSe4F, which are with tetrathienyl-substituted IDT as electron-rich central core, electron-deficient difluorobenzothiadiazole as acceptor units, and donor end-capping groups, but having differences in the π-bridge (thiophene and selenophene), were successfully synthesized and evaluated as donor materials in organic solar cells. Such π-bridge and core units in these small molecules play a decisive role in the formation of the nanoscale separation of the blend films, which were systematically investigated through absorption spectra, grazing incidence X-ray diffraction pattern, transmission electron microscopy images, resonant soft X-ray scattering profiles, and charge mobility measurement. The ThIDTSe4F (with selenophene π-bridge)-based device exhibited superior performance than devices based on ThIDTh4F (with thiophene π-bridge) after post annealing treatment owing to optimized film morphology and improved charge transport. Power conversion efficiency of 7.31% and fill factor of ∼0.70 were obtained by using a blend of ThIDTSe4F and PC₇₁BM with thermal annealing and solvent vapor annealing treatments, which is the highest PCE from aromatic side-chain substituted IDT-based small molecular solar cells. The scope of this study is to reveal the structure-property relationship of the aromatic side-chain substituted IDT-based donor materials as a function of π-bridge and the post annealing conditions.

Modifying the Chemical Structure of a Porphyrin Small Molecule with Benzothiophene Groups for the Reproducible Fabrication of High Performance Solar Cells

A porphyrin-based molecule DPPEZnP-BzTBO with bulky benzothiophene groups was designed and synthesized as an electron donor material for bulk heterojunction (BHJ) solar cells. The optimized devices under thermal annealing (TA) and then chloroform solvent vapor anneanling (SVA) for 80 s exhibited an outstanding power conversion efficiencie (PCE) of 9.08%. Contrasted with the smaller thienyl substituted analogues we reported previously, DPPEZnP-BzTBO-based BHJ solar cells exhibited a higher open circuit voltage due to the lower highest occupied molecular orbital energy level. The TA post-treatment of the active layers induced the formation of more crystallized components, and the subsequent SVA provided a driving force for the domain growth, resulting in more obvious phase segregation between the donor and the acceptor in nanoscale. Furthermore, the PCEs kept above 95% upon the further SVA treatment within the time range of 60 to 95 s probably because the bulky benzothiophene groups retard the too quick change of crystallinity, providing a wide processing window for the reproducible device fabrication.