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

Advancing Integration of Direct C-H Arylation-Derived Star-Shaped Oligomers as Second Acceptors for Ternary Organic Solar Cells

Organic solar cells (OSCs) could benefit from the ternary bulk heterojunction (BHJ), a method that allows for fine-tuning of light capture, cascade energy levels, and film shape, in order to increase their power conversion efficiency (PCE). In this work, the third components of PM6:Y6 and PM6:BTP-eC9 BHJs are a set of four star-shaped unfused ring electron acceptors (SSUFREAs), i.e., BD-IC, BFD-IC, BD-2FIC, and BFD-2FIC, that are facilely synthesized by direct C-H arylation. The four SSUFREAs all show complete complementary absorption with PM6, Y6, and BTP-eC9, which facilitates light harvesting and exciton collection. When BFD-2FIC is added as a third component, the PCEs of PM6:Y6 and PM6:BTP-eC9 binary BHJs are able to be improved from 15.31% to 16.85%, and from 16.23% to 17.23%, respectively, showing that BFD-2FIC is useful for most effective ternary OSCs in general, and increasing short circuit current (JSC) and better film morphology are two additional benefits. The ternary PM6:Y6:BFD-2FIC exhibits a 9.7% percentage of increase in PCE compared to the PM6:Y6 binary BHJ, which is one of the highest percentage increases among the reported ternary BHJs, showing the huge potential of BFD-2FIC for ternary BHJ OSCs.

Organic laser power converter for efficient wireless micro power transfer

Wireless power transfer with collimated power transmission and efficient conversion provides an alternative charging mode for off-grid and portable micro-power electronics. However, charging micro-power electronics with low photon flux can be challenging for current laser power converters. Here we show laser power converters with organic photovoltaic cells with good performance for application in laser wireless power transfer. The laser selection strategy is established and the upper limit of efficiency is proposed. The organic laser power converters exhibit a 36.2% efficiency at a 660 nm laser with a photon flux of 9.5 mW cm-2 and achieve wireless micro power transfer with an output of 0.5 W on a 2 meter scale. This work shows the good performance of organic photovoltaic cells in constructing organic laser power converters and provides a potential solution for the wireless power transfer of micro-power electronics.

Observation of an Exciton-Plasma Transition in a Molecular Semiconductor

The nonfullerene electron acceptors (NFAs) for organic solar cells are attracting intense research efforts due to their impressive performance. Understanding the temporal evolution of the excited states in NFAs is essential to gain insights into the working mechanism of these state-of-the-art devices. Here we characterized the photoconductivities of a neat Y6 film and a Y6:PM6 blend film using time-resolved terahertz spectroscopy. Three different types of excited states were identified based on their distinct terahertz responses, i.e., plasma-like carriers, weakly bound excitons, and spatially separated carriers. Under high-intensity excitation, the many-body interaction of excitons in the Y6 film leads to the plasma-like state, giving rise to a terahertz response characteristic for a dispersive charge transport. This transient state decays quickly into exciton gas due to fast Auger annihilation. Under low-intensity excitation, only isolated excitons are created and the plasma state is absent.

Effects of the diphenyl ether additive in halogen-free processed non-fullerene acceptor organic solar cells

The development of an environmentally friendly fabrication process for non-fullerene acceptor organic solar cells is an essential condition for their commercialization. However, devices fabricated by processing the active layer with green solvents still struggle to reach, in terms of efficiency, the same performance as those fabricated with halogenated solvents. The reason behind this is the non-optimal nanostructure of the active layer obtained with green solvents. Additives in solution have been used to fine-tune the nanostructure and improve the performance of organic solar cells. Therefore, the identification of non-halogenated additives and the study of their effects on the device performance and stability are of primary importance. In this work, we propose the use of diphenyl ether (DPE) as additive, in combination with the non-halogenated solvent o-xylene, to fabricate organic solar cells with a completely halogen-free process. Thanks to the addition of DPE, a best efficiency of 11.7% have been obtained for the system TPD-3F:IT-4F, an increase over 15% with respect to the efficiency of devices fabricated without additive. Remarkably, the stability under illumination of the solar cells is also improved when DPE is used. The addition of DPE has effects on the molecular organization in the active layer, with an enhancement in the donor polymer ordering, showing a higher domain purity. The resulting structure improves the charge carrier collection, leading to a superior short-circuit current and fill factor. Furthermore, a reduction of the non-radiative recombination losses and an improved exciton diffusion, are the results of the superior molecular ordering. With a comprehensive insight of the effects of DPE when used in combination with a non-halogenated solvent, our study provides an approach to make the fabrication of organic solar cell environmentally friendlier and more suitable for large scale production.

Over 19.2% Efficiency of Organic Solar Cells Enabled by Precisely Tuning the Charge Transfer State Via Donor Alloy Strategy

The large energy loss (E_loss ) is one of the main obstacles to further improve the photovoltaic performance of organic solar cells (OSCs), which is closely related to the charge transfer (CT) state. Herein, ternary donor alloy strategy is used to precisely tune the energy of CT state (E_CT ) and thus the E_loss for boosting the efficiency of OSCs. The elevated E_CT in the ternary OSCs reduce the energy loss for charge generation (ΔE_CT ), and promote the hybridization between localized excitation state and CT state to reduce the nonradiative energy loss (ΔE_nonrad ). Together with the optimal morphology, the ternary OSCs afford an impressive power conversion efficiency of 19.22% with a significantly improved open-circuit voltage (V_oc ) of 0.910 V without sacrificing short-cicuit density (J_sc ) and fill factor (FF) in comparison to the binary ones. This contribution reveals that precisely tuning the E_CT via donor alloy strategy is an efficient way to minimize E_loss and improve the photovoltaic performance of OSCs.

Alkoxy-Substituted Quadrupolar Fluorescent Dyes

Polar and polarizable π-conjugated organic molecules containing push-pull chromophores have been investigated extensively in the past. Identifying unique backbones and building blocks for fluorescent dyes is a timely exercise. Here, we report the synthesis and characterization of a series of fluorescent dyes containing quadrupolar A-D-A constitutions (where A = acceptor and D = donor), which exhibit fluorescence emission at a variety of different wavelengths. We have investigated the effects of different electron-withdrawing groups, located at both termini of a para-terphenylene backbone, by steady-state UV/vis and fluorescence spectroscopy. Pyridine and substituted pyridinium units are also introduced during the construction of the quadrupolar backbones. Depending on the quadrupolarity, fluorescence emission wavelengths cover from 380 to 557 nm. Time-resolved absorption and emission spectroscopy reveal that the photophysical properties of those quadrupolar dyes result from intramolecular charge transfer. One of the dyes we have investigated is a symmetrical box-like tetracationic cyclophane. Its water-soluble tetrachloride, which is non-cytotoxic to cells up to a loading concentration of 1 μM, has been employed in live-cell imaging. When taken up by cells, the tetrachloride emits a green fluorescence emission without any hint of photobleaching or disruption of normal cell behavior. We envision that our design strategy of modifying molecules through the functionalization of the quadrupolar building blocks as chromophores will lead to future generations of fluorescent dyes in which these A-D-A constitutional fragments are incorporated into more complex molecules and polymers for broader photophysical and biological applications.

Molecular engineering of indenoindene-3-ethylrodanine acceptors with A2-A1-D-A1-A2 architecture for promising fullerene-free organic solar cells

Considering the increased demand and potential of photovoltaic devices in clean, renewable electrical and hi-tech applications, non-fullerene acceptor (NFA) chromophores have gained significant attention. Herein, six novel NFA molecules IBRD1-IBRD6 have been designed by structural modification of the terminal moieties from experimentally synthesized A2-A1-D-A1-A2 architecture IBR for better integration in organic solar cells (OSCs). To exploit the electronic, photophysical and photovoltaic behavior, density functional theory/time dependent-density functional theory (DFT/TD-DFT) computations were performed at M06/6-311G(d,p) functional. The geometry, electrical and optical properties of the designed acceptor molecules were compared with reported IBR architecture. Interestingly, a reduction in bandgap (2.528-2.126 eV), with a broader absorption spectrum, was studied in IBR derivatives (2.734 eV). Additionally, frontier molecular orbital findings revealed an excellent transfer of charge from donor to terminal acceptors and the central indenoindene-core was considered responsible for the charge transfer. Among all the chromophores, IBRD3 manifested the lowest energy gap (2.126 eV) with higher λmax at 734 and 745 nm in gaseous phase and solvent (chloroform), respectively due to the strong electron-withdrawing effect of five end-capped cyano groups present on the terminal acceptor. The transition density matrix map revealed an excellent charge transfer from donor to terminal acceptors. Further, to investigate the charge transfer and open-circuit voltage (Voc), PBDBT donor polymer was blended with acceptor chromophores, and a significant Voc (0.696-1.854 V) was observed. Intriguingly, all compounds exhibited lower reorganization and binding energy with a higher exciton dissociation in an excited state. This investigation indicates that these designed chromophores can serve as excellent electron acceptor molecules in organic solar cells (OSCs) that make them attractive candidates for the development of scalable and inexpensive optoelectronic devices.

Enhanced Static and Dynamic Properties of Highly Miscible Fullerene-Free Green-Selective Organic Photodetectors

We developed p-n junction organic photodetectors (OPDs) composed of a polymer donor and a nonfullerene acceptor (NFA) to increase both the responsivity (R) and detectivity (D*) while maintaining a narrow wavelength selectivity. The selection of the polymer donor and NFA with similar green (G) absorption is important for achieving G-wavelength selectivity in these OPDs, which differentiates them from current fullerene-based OPDs and NFA-based panchromatic absorption OPDs. In addition, mixing the polymer donor and asymmetric NFA was efficient toward increasing the miscibility and decreasing the interfacial energy difference of the blended films, resulting in the formation of a uniform and well-mixed nanomorphology in the photoconductive layer. Two-dimensional (2D) grazing incidence X-ray diffraction and Fourier-transform infrared spectroscopy revealed that the lamellar ordering of the polymer donor was enhanced in the blend film prepared with an asymmetric NFA, whereas the aggregation of a symmetric NFA in the blend films did not increase the lamellar ordering of the polymer donor. Consequently, we achieved an R value of 0.31 A/W and D* value of 2.0 × 10¹³ Jones with a full width at half-maximum value of 230 nm at -2 V and fast response time of 27 μs without any external bias in the asymmetric NFA-based OPDs. The enhancement in the lamellar ordering and miscibility of the blended films are crucial toward increasing the static and dynamic properties of OPDs.

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.

Recent Advances in Non-Fullerene Acceptors of the IDIC/ITIC Families for Bulk-Heterojunction Organic Solar Cells

The introduction of the IDIC/ITIC families of non-fullerene acceptors has boosted the photovoltaic performances of bulk-heterojunction organic solar cells. The fine tuning of the photophysical, morphological and processability properties with the aim of reaching higher and higher photocurrent efficiencies has prompted uninterrupted worldwide research on these peculiar families of organic compounds. The main strategies for the modification of IDIC/ITIC compounds, described in several contributions published in the past few years, can be summarized and classified into core modification strategies and end-capping group modification strategies. In this review, we analyze the more recent advances in this field (last two years), and we focus our attention on the molecular design proposed to increase photovoltaic performance with the aim of rationalizing the general properties of these families of non-fullerene acceptors.

Metal-Organic Framework Materials for Perovskite Solar Cells

Metal-organic frameworks (MOFs) and MOF-derived materials have been used for several applications, such as hydrogen storage and separation, catalysis, and drug delivery, owing to them having a significantly large surface area and open pore structure. In recent years, MOFs have also been applied to thin-film solar cells, and attractive results have been obtained. In perovskite solar cells (PSCs), the MOF materials are used in the form of an additive for electron and hole transport layers, interlayer, and hybrid perovskite/MOF. MOFs have the potential to be used as a material for obtaining PSCs with high efficiency and stability. In this study, we briefly explain the synthesis of MOFs and the performance of organic and dye-sensitized solar cells with MOFs. Furthermore, we provide a detailed overview on the performance of the most recently reported PSCs using MOFs.

Efficient Exciton Diffusion in Micrometer-Sized Domains of Nanographene-Based Nonfullerene Acceptors with Long Exciton Lifetimes in Blend Films with Conjugated Polymer

Phase-separated structures in photoactive layers composed of electron donors and acceptors in organic photovoltaics (OPVs) generally exert a profound impact on the device performance. In this study, nonfullerene acceptors (NFAs) where a heteronanographene central core was furnished with branched alkoxy chains of different lengths, TACIC-EH, TACIC-BO, and TACIC-HD, were prepared to adjust the aggregation tendency and systematically probe the relationships of film structures with photophysical and photovoltaic properties. The side-chain length showed negligible effects on the absorption properties and energy levels of TACICs. In addition, regardless of the chain length, all TACIC films exhibited characteristically long singlet exciton lifetimes (1330-2330 ps) compared to those in solution (≤220 ps). Using a conjugated polymer donor, PBDB-T, the best OPV performance was achieved with TACIC-BO that contained medium-length chains, exhibiting a power conversion efficiency (PCE) of 9.92%. TACIC-HD with the longest chains showed deteriorated electron mobility due to the long insulating alkoxy groups. Therefore, the PBDB-T:TACIC-HD-based device revealed a low charge collection efficiency and PCE (8.21%) relative to the PBDB-T:TACIC-BO-based device, but their film morphologies were analogous. Meanwhile, TACIC-EH with the shortest chains showed low solubility and formed micrometer-sized large aggregates in the blend film with PBDB-T. Although the charge collection efficiency of PBDB-T:TACIC-EH was lower than that of PBDB-T:TACIC-BO, the efficiencies of exciton diffusion to the donor-acceptor interface were sufficiently high (>98%) owing to the elongated singlet exciton lifetime of TACIC-EH. The PCE of the PBDB-T:TACIC-EH-based device remained moderate (7.10%). Therefore, TACICs with the long singlet exciton lifetimes in the films provide a clear guideline for NFAs with low sensitivity of OPV device performance to the blend film structures, which is advantageous for large-scale OPV production with high reproducibility.

Crystallography, Morphology, Electronic Structure, and Transport in Non-Fullerene/Non-Indacenodithienothiophene Polymer:Y6 Solar Cells

Emerging nonfullerene acceptors (NFAs) with crystalline domains enable high-performance bulk heterojunction (BHJ) solar cells. Thermal annealing is known to enhance the BHJ photoactive layer morphology and performance. However, the microscopic mechanism of annealing-induced performance enhancement is poorly understood in emerging NFAs, especially regarding competing factors. Here, optimized thermal annealing of model system PBDB-TF:Y6 (Y6 = 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3'':4',5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]-thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) decreases the open circuit voltage (V_OC) but increases the short circuit current (J_SC) and fill factor (FF) such that the resulting power conversion efficiency (PCE) increases from 14 to 15% in the ambient environment. Here we systematically investigate these thermal annealing effects through in-depth characterizations of carrier mobility, film morphology, charge photogeneration, and recombination using SCLC, GIXRD, AFM, XPS, NEXAFS, R-SoXS, TEM, STEM, fs/ns TA spectroscopy, 2DES, and impedance spectroscopy. Surprisingly, thermal annealing does not alter the film crystallinity, R-SoXS characteristic size scale, relative average phase purity, or TEM-imaged phase separation but rather facilitates Y6 migration to the BHJ film top surface, changes the PBDB-TF/Y6 vertical phase separation and intermixing, and reduces the bottom surface roughness. While these morphology changes increase bimolecular recombination (BR) and lower the free charge (FC) yield, they also increase the average electron and hole mobility by at least 2-fold. Importantly, the increased μ_h dominates and underlies the increased FF and PCE. Single-crystal X-ray diffraction reveals that Y6 molecules cofacially pack via their end groups/cores, with the shortest π-π distance as close as 3.34 Å, clarifying out-of-plane π-face-on molecular orientation in the nanocrystalline BHJ domains. DFT analysis of Y6 crystals reveals hole/electron reorganization energies of as low as 160/150 meV, large intermolecular electronic coupling integrals of 12.1-37.9 meV rationalizing the 3D electron transport, and relatively high μ_e of 10^-4 cm² V^-1 s^-1. Taken together, this work clarifies the richness of thermal annealing effects in high-efficiency NFA solar cells and tasks for future materials design.

Regioisomerically Pure 1,7-Dicyanoperylene Diimide Dimer for Charge Extraction from Donors with High Electron Affinities

Perylene diimide (PDI) has attracted widespread interest as an inexpensive electron acceptor for photovoltaic applications; however, overcrystallization in the bulk heterojunction typically leads to low device performance. Recent work has addressed this issue by forming bay-linked PDI dimers and oligomers, where the steric bulk of adjacent PDI units forces the molecule to adopt a nonplanar structure. This disrupts the molecular packing and limits domain sizes in the bulk heterojunction. Unfortunately, the introduction of electron-donating/-withdrawing groups in the bay region is also the best way to fine-tune the frontier molecular orbitals (FMOs) of PDI, which is highly desirable from a device optimization standpoint. This competition for the bay region has made it difficult for PDI to keep pace with other non-fullerene acceptors. Here, we report the synthesis of regioisomerically pure 1,7-dicyanoperylene diimide and its dimerization through an imide linkage. We show that this is an effective strategy to tune the energies of the FMOs while simultaneously suppressing overcrystallization in the bulk heterojunction. The resulting acceptor has a low LUMO energy of -4.2 eV and is capable of accepting photogenerated electrons from donor polymers with high electron affinities, even when conventional acceptors such as PDI, PC₇₁BM, and ITIC cannot.

Ultrafast Hole Transfer and Carrier Transport Controlled by Nanoscale-Phase Morphology in Nonfullerene Organic Solar Cells

Nonfullerene acceptors (NFAs) have attracted great attention in high-efficiency organic solar cells (OSCs). While the effect of molecular properties including structures and energetics on charge transfer has been extensively investigated, the effect of macroscopic-phase properties is yet to be revealed. Here we have performed a correlation study of the nanoscale-phase morphology on the photoexcited hole transfer (HT) process and photovoltaic performance by combining ultrafast spectroscopy with high temporal resolution and photo-induced force microscopy (PiFM) with high spatial and chemical resolution. In PM6/IT-4F, we observe biphasic HT behavior with a minor ultrafast (<100 fs) interfacial process and a major diffusion-mediated HT process until ∼100 ps, which depends strongly on phase segregation. Because of the interplay between charge transfer and transport, a compromised domain size of 20-30 nm for NFAs shows the best performance. This study highlights the critical role of phase morphology in high-efficiency OSCs.

Nonfused Nonfullerene Acceptors with an A-D-A'-D-A Framework and a Benzothiadiazole Core for High-Performance Organic Solar Cells

Nonfullerene acceptors (NFAs) have contributed significantly to the progress of organic solar cells (OSCs). However, most NFAs feature a large fused-ring backbone, which usually requires a tedious multiple-step synthesis, and are not applicable to commercial applications. An alternative strategy is to develop nonfused NFAs, which possess synthetic simplicity and facile tunability in optoelectronic properties and solid-state microstructures. In this work, we report two nonfused NFAs, BTCIC and BTCIC-4Cl, based on an A-D-A'-D-A architecture, which possess the same electron-deficient benzothiadiazole central core but different electron-withdrawing terminal groups. The optical properties, energy levels, and molecular crystallinities were finely tuned by changing the terminal groups. Moreover, a decent power conversion efficiency of 9.3 and 10.5% has been achieved by BTCIC and BTCIC-4Cl, respectively, by blending them with an appropriate polymer donor. These results demonstrate the potential of A-D-A'-D-A type nonfused NFAs for high-performance OSCs. Further development of nonfused NFAs will be very fruitful by employing appropriate building blocks and via side-chain optimizations.

Efficient light-harvesting, energy migration, and charge transfer by nanographene-based nonfullerene small-molecule acceptors exhibiting unusually long excited-state lifetime in the film state

Electron-acceptor small-molecules possessing a long exciton lifetime and a narrow energy band gap, opposing the energy gap law, are highly desirable for high-performance organic photovoltaics (OPVs) by realizing their efficient light-harvesting ability (LH), exciton diffusion (ED), and charge transfer (CT). Toward this goal, we designed an acceptor–donor–acceptor (A–D–A) type nonfullerene acceptor (NFA), TACIC, having an electron-donating, self-assembling two-dimensional (2D) nanographene unit, thienoazacoronene, at the center with electron-withdrawing groups at both ends. The TACIC film exhibited a narrow band gap (1.59 eV) with excellent LH. Surprisingly, the TACIC film showed an extremely long exciton lifetime (1.59 ns), suppressing undesirable nonradiative decay by its unique self-assembling behavior. When combined with a conjugated polymer donor, PBDB-T, slow ED and CT were observed (60 ps) with the excitation of TACIC owing to the large TACIC domain sizes. Nevertheless, the unusually high efficiencies of ED and CT (96% in total) were achieved by the long TACIC exciton lifetime. Additionally, unusual energy transfer (EnT) from the excited PBDB-T to TACIC was seen, demonstrating its dual LH role. The OPV device with PBDB-T and TACIC showed a high incident photon-to-current efficiency (IPCE) exceeding 70% at up to 710 nm and a power conversion efficiency of ∼10%. This result will open up avenues for a rational strategy of OPVs where LH, ED, and CT from the acceptor side as well as LH, EnT, ED, and CT from the donor side can be better designed by using 2D nanographene as a promising building block for high-performance A–D–A type NFAs.

A nonfullerene acceptor, TACIC, showed efficient light-harvesting, exciton diffusion, and charge transfer.

Color-Selective Printed Organic Photodiodes for Filterless Multichannel Visible Light Communication

Future lightweight, flexible, and wearable electronics will employ visible-light-communication schemes to interact within indoor environments. Organic photodiodes are particularly well suited for such technologies as they enable chemically tailored optoelectronic performance and fabrication by printing techniques on thin and flexible substrates. However, previous methods have failed to address versatile functionality regarding wavelength selectivity without increasing fabrication complexity. This work introduces a general solution for printing wavelength-selective bulk-heterojunction photodetectors through engineering of the ink formulation. Nonfullerene acceptors are incorporated in a transparent polymer donor matrix to narrow and tune the response in the visible range without optical filters or light-management techniques. This approach effectively decouples the optical response from the viscoelastic ink properties, simplifying process development. A thorough morphological and spectroscopic investigation finds excellent charge-carrier dynamics enabling state-of-the-art responsivities >10² mA W^-1 and cutoff frequencies >1.5 MHz. Finally, the color selectivity and high performance are demonstrated in a filterless visible-light-communication system capable of demultiplexing intermixed optical signals.

Photoinduced electron transfer from zinc meso-tetraphenylporphyrin to a one-dimensional perylenediimide aggregate: Probing anion delocalization effects

Organic photovoltaics incorporating non-fullerene acceptors based on perylenediimide (PDI) now rival fullerene acceptor-based devices in performance, although the mechanisms of charge generation in PDI-based devices are not yet fully understood. Fullerene-based systems are proposed to undergo electron transfer directly from the photoexcited donor into a band of delocalized acceptor states, thus increasing charge generation efficiency. Similarly, anion delocalization has been shown to enhance the rate of electron transfer from a photoexcited donor to two electronically coupled PDI acceptors. Here we investigate how additional electron acceptors may further increase the rate of electron transfer from the donor zinc meso-tetraphenylporphyrin (ZnTPP) to an aggregate of PDI acceptors (PDI[Formula: see text]. Femtosecond transient visible and mid-infrared absorption spectroscopies show that the rate of electron transfer from 1*ZnTPP to the PDI assembly ZnTPP2-PDI3 is statistically identical to that of the previously examined ZnTPP-PDI2. A Marcus theory analysis indicates that the parameters governing electron transfer are nearly identical for the two molecules, suggesting that the maximum electron transfer rate enhancement has been achieved in a cofacial PDI dimer because the ZnTPP directly couples to the first two PDI acceptors whereas the coupling to the third PDI is too weak.

Sub-picosecond charge-transfer at near-zero driving force in polymer:non-fullerene acceptor blends and bilayers

Organic photovoltaics based on non-fullerene acceptors (NFAs) show record efficiency of 16 to 17% and increased photovoltage owing to the low driving force for interfacial charge-transfer. However, the low driving force potentially slows down charge generation, leading to a tradeoff between voltage and current. Here, we disentangle the intrinsic charge-transfer rates from morphology-dependent exciton diffusion for a series of polymer:NFA systems. Moreover, we establish the influence of the interfacial energetics on the electron and hole transfer rates separately. We demonstrate that charge-transfer timescales remain at a few hundred femtoseconds even at near-zero driving force, which is consistent with the rates predicted by Marcus theory in the normal region, at moderate electronic coupling and at low re-organization energy. Thus, in the design of highly efficient devices, the energy offset at the donor:acceptor interface can be minimized without jeopardizing the charge-transfer rate and without concerns about a current-voltage tradeoff.

It has been commonly believed that the driving force at the donor-acceptor heterojunction is vital to efficient charge separation in organic solar cells. Here Zhong et al. show that the driving force can be as small as 0.05 eV without compromising the charge transfer rate and efficiency.

Regulating the Packing of Non-Fullerene Acceptors via Multiple Noncovalent Interactions for Enhancing the Performance of Organic Solar Cells

Three noncovalently fused-ring electron acceptors (FOC6-IC, FOC6-FIC, and FOC2C6-2FIC) are synthesized. Single crystals of FOC6-IC and FOC2C6-2FIC are prepared, and structure analyses reveal that the molecular backbone can be planarized via the formation of the intramolecular noncovalent interactions. These acceptor molecules can be packed closely in the solid state via π-π stacking and static interactions between the central phenylene unit and the terminal group with a distance of 3.3-3.4 Å. Besides, multiple intermolecular noncovalent interactions can be observed in the single crystal structure of the fluorinated acceptor FOC2C6-2FIC, which help increase the crystallinity of acceptors and the charge mobility of the blends. Photovoltaic devices based on FOC2C6-2FIC give a power conversion efficiency of 12.36%, higher than 12.08% for FOC6-FIC and 10.80% for FOC6-IC.

Control of Nanomorphology in Fullerene-Free Organic Solar Cells by Lewis Acid Doping with Enhanced Photovoltaic Efficiency

Generating desired efficiency enhancements in organic solar cells (OSCs) by charge-transfer doping requires to obtain modified optoelectronic properties while retaining the favorable nanomorphology. We report a thermally assisted doping based on Lewis acid tris(pentafluorophenyl)-borane (BCF) as a p-dopant for two groups of OSCs comprising the PBDB-TF and PBDB-T donors and a nonfullerene acceptor IT-4F. We found that the face-on molecular packing in the PBDB-TF:IT-4F blend or neat PBDB-TF donor films is favorably modified with the formation of frustrated Lewis pairs (FLPs) in the donor, which is in contrast to the hampered π-π stacking in the doped PBDB-T film. The different impacts of BCF dopants on the morphology lead to contrasting photovoltaic behaviors where the PBDB-TF-based devices receive enhanced power conversion efficiencies (PCEs) in the presence of BCF, while reduction of efficiencies is observed in the PBDB-T device. In the best doping conditions with the proposed hot-film deposition, we achieve a boosted PCE of 14.1% in PBDB-TF:IT-4F solar cells at low BCF concentrations. Based on the same fluorinated donor, the described BCF doping also applies to NF-solar cells based on the NF-acceptor Y6, leading to an increase in the PCE to 16.0%. Our results suggest that controlling the degree of FLP formation in the donor component with the addition of BCF is key to obtaining desired improvements on nanomorphology and relevant photophysical properties in OSCs.

Subtle Molecular Tailoring Induces Significant Morphology Optimization Enabling over 16% Efficiency Organic Solar Cells with Efficient Charge Generation

Manipulating charge generation in a broad spectral region has proved to be crucial for nonfullerene-electron-acceptor-based organic solar cells (OSCs). 16.64% high efficiency binary OSCs are achieved through the use of a novel electron acceptor AQx-2 with quinoxaline-containing fused core and PBDB-TF as donor. The significant increase in photovoltaic performance of AQx-2 based devices is obtained merely by a subtle tailoring in molecular structure of its analogue AQx-1. Combining the detailed morphology and transient absorption spectroscopy analyses, a good structure-morphology-property relationship is established. The stronger π-π interaction results in efficient electron hopping and balanced electron and hole mobilities attributed to good charge transport. Moreover, the reduced phase separation morphology of AQx-2-based bulk heterojunction blend boosts hole transfer and suppresses geminate recombination. Such success in molecule design and precise morphology optimization may lead to next-generation high-performance OSCs.

Hole Transfer Originating from Weakly Bound Exciton Dissociation in Acceptor-Donor-Acceptor Nonfullerene Organic Solar Cells

The underlying hole-transfer mechanism in high-efficiency OSC bulk heterojunctions based on acceptor-donor-acceptor (A-D-A) nonfullerene acceptors (NFAs) remains unclear. Herein, we study the hole-transfer process between copolymer donor J91 and five A-D-A NFAs with different highest occupied molecular orbital energy offsets (ΔE_H) (0.05-0.42 eV) via ultrafast optical spectroscopies. Transient absorption spectra reveal a rapid hole-transfer rate with small ΔE_H, suggesting that a large energy offset is not required to overcome the exciton binding energy. Capacitance-frequency spectra and time-resolved photoluminescence spectra confirm the delocalization of an A-D-A-structured acceptor exciton with weak binding energy. Relative to the hole-transfer rate, hole-transfer efficiency is the key factor affecting device performance. We propose that holes primarily stem from weakly bound acceptor exciton dissociation, revealing a new insight into the hole-transfer process in A-D-A NFA-based OSCs.

Modulating Structure Ordering via Side-Chain Engineering of Thieno[3,4-b]thiophene-Based Electron Acceptors for Efficient Organic Solar Cells with Reduced Energy Losses

Nonfullerene-based organic solar cells (OSCs) have made a huge breakthrough in the recent years. Introducing a proper side chain on the π-conjugated backbone plays a vital role for further improving the power conversion efficiency (PCE) of OSCs due to easy tuning of the physical properties of the molecule such as absorption, energetic level, solid-state stacking, and charge transportation. More importantly, the side chain significantly affected the blend film's morphology and thus determined the PCEs of the devices. In this work, two low-band-gap nonfullerene acceptors, ATT-4 and ATT-5, with an alkyl or branched alkyl substitute on indacenodithiophene (IDT) and thieno[3,4-b]thiophene (TbT) backbone were synthesized for investigating the effect of the substituent on the performance of the nonfullerene acceptors (NFAs). In comparison to ATT-1 with p-hexylphenyl-substituted IDT and n-octyl-substituted TbT moieties, ATT-4 and ATT-5 exhibit better crystallinity with shorter interchain distance and ordered molecular structure in neat and the corresponding blend films. The tailored ATT-5 exhibits a high PCE of 12.36% with a V_oc of 0.93 V, J_sc of 18.86 mA cm^-2, and fill factor (FF) of 0.71, blending with a wide-band-gap polymer donor PBDB-T. Remarkably, although ATT-4 and ATT-5 exhibit broader light absorption, the devices obtained higher V_oc than that of ATT-1 mainly due to the reduced nonradiative recombination in the blend films. These results implied that side-chain engineering is an efficient approach to regulate the electronic structure and molecular packing of NFAs, which can well match with polymer donor, and obtain high PCEs of the OSCs with improved V_oc, J_sc, and FF, simultaneously.

Concentration dependent energy levels shifts in donor-acceptor mixtures due to intermolecular electrostatic interaction

Recent progress in the improvement of organic solar cells lead to a power conversion efficiency to over 16%. One of the key factors for this improvement is a more favorable energy level alignment between donor and acceptor materials, which demonstrates that the properties of interfaces between donor and acceptor regions are of paramount importance. Recent investigations showed a significant dependence of the energy levels of organic semiconductors upon admixture of different materials, but its origin is presently not well understood. Here, we use multiscale simulation protocols to investigate the molecular origin of the mixing induced energy level shifts and show that electrostatic properties, in particular higher-order multipole moments and polarizability determine the strength of the effect. The findings of this study may guide future material-design efforts in order to improve device performance by systematic modification of molecular properties.

Photovoltaic Blend Microstructure for High Efficiency Post-Fullerene Solar Cells. To Tilt or Not To Tilt?

Achieving efficient polymer solar cells (PSCs) requires a structurally optimal donor-acceptor heterojunction morphology. Here we report the combined experimental and theoretical characterization of a benzodithiophene-benzothiadiazole donor polymer series (PBTZF4-R; R = alkyl substituent) blended with the non-fullerene acceptor ITIC-Th and analyze the effects of substituent dimensions on blend morphology, charge transport, carrier dynamics, and PSC metrics. Varying substituent dimensions has a pronounced effect on the blend morphology with a direct link between domain purity, to some extent domain dimensions, and charge generation and collection. The polymer with the smallest alkyl substituent yields the highest PSC power conversion efficiency (PCE, 11%), reflecting relatively small, high-purity domains and possibly benefiting from "matched" donor polymer-small molecule acceptor orientations. The distinctive morphologies arising from the substituents are investigated using molecular dynamics (MD) simulations which reveal that substituent dimensions dictate a well-defined set of polymer conformations, in turn driving chain aggregation and, ultimately, the various film morphologies and mixing with acceptor small molecules. A straightforward energetic parameter explains the experimental polymer domain morphological trends, hence PCE, and suggests strategies for substituent selection to optimize PSC materials morphologies.

Unraveling the Morphology in Solution-Processed Pseudo-Bilayer Planar Heterojunction Organic Solar Cells

The conventional bulk heterojunction (BHJ) structure is widely used for fabricating high-performance organic solar cells (OSCs) due to the nanometer-scale phase separation of the donor/acceptor component. However, the elaborate control of the BHJ morphology is difficult to carry out because the morphology evolution is such a complicated process. The compatibility requirement of materials in the same solvent restricts the structural diversity of the molecules to some extent. Meanwhile, the nanoscopic interpenetrating donor/acceptor domains reduce their crystallinity. The bilayer planar heterojunction (PHJ), by contrast, possesses complementary advantages that can make it an alternative candidate to achieve device fabrication and produce different vertical stratification in heterojunction films. However, the flat contact area limits the charge separation and transmission efficiency. The sequential solution processed approach was used to facilitate material diffusion in layers. Also, solvent additives were employed to further enhance the diffusion and thus the device performance. Nevertheless, the morphology of the formed pseudo-bilayer planar heterojunction (PPHJ) has not been fully revealed yet. Here, we carefully study the morphology of the nonfullerene-based PPHJ device in three dimensions. High hole mobility of 2.09 × 10^-4 cm² V^-1 s^-1 and electron mobility of 7.91 × 10^-5 cm² V^-1 s^-1 were obtained in the solution-processed PPHJ device. Meanwhile, a distinct phase separation size with a vertical rearrangement of donor and acceptor was observed, which enable the pseudo-bilayer devices to be equipped with a comparable spectral response to the BHJ devices. We demonstrate that a unique device architecture (ITO/ZnO/PBDB-T/ITIC/MoO₃/Ag) with a power conversion efficiency of 7% can be obtained from a larger molecular weight of PBDB-T without using extra additives. The solution-processed PPHJ films have much in common with the BHJ films. The results proposed that with appropriate molecular design and vertical phase separation optimization, the performance of the solution-processed PPHJ-based OSCs can be further improved.

Morphology-Dependent Hole Transfer under Negligible HOMO Difference in Non-Fullerene Acceptor-Based Ternary Polymer Solar Cells

In the field of organic solar cells, it has been generally accepted until recently that a difference in band energies of at least 0.3 eV between the highest occupied molecular orbital (HOMO) level of the donor and the HOMO of the acceptor is required to provide adequate driving force for efficient photoinduced hole transfer due to the large binding energy of excitons in organic materials. In this work, we investigate polymeric donor:non-fullerene acceptor junctions in binary and ternary blend polymer solar cells, which exhibit efficient photoinduced hole transfer despite negligible HOMO offset and demonstrate that hole transfer in this system is dependent on morphology. The morphology of the organic blend was gradually tuned by controlling the amount of ITIC and PC₇₀BM. High external quantum efficiency was achieved at long wavelengths, despite ITIC-to-PC₇₀BM ratio of 1:9, which indicates efficient photoinduced hole transfer from ITIC to the donor despite an undesirable HOMO energy offset. Transient absorption spectra further confirm that hole transfer from ITIC to the donor becomes more efficient upon optimizing the morphology of the ternary blend compared to that of donor:ITIC binary blend.

Effect of Solvents on the Electrical and Morphological Characteristics of Polymer Solar Cells

The nanoscale morphology of poly(3-hexylthiophene) (P3HT) and [⁶,6]-phenyl-C71. butyric acid methylester (PCBM) blend film is affected by various parameters such as the solvent, coating, and thermal annealing conditions. We investigated the effect of solvents on the performance of inverted solar cells based on the active layer of a P3HT:PCBM bulk heterojunction. P3HT and PCBM (weight ratio 1:0.8) were dissolved in chlorobenzene (CB) and dichlorobenzene (DCB). The difference in the volatility characteristics of the solvents resulted in different P3HT crystallite morphologies. The difference in the P3HT:PCBM film morphology was systemically investigated via atomic force microscopy, ultraviolet (UV)-visible absorption spectroscopy, X-ray diffraction, and electrical impedance spectroscopy. The DCB solvent lead to better P3HT crystallinity and device performance. For example, the short-circuit current density (JSC) and the power conversion efficiency (PCE) of the device using DCB (9.89 mA/cm² and 3.62%, respectively) were larger than those (9.12 mA/cm² and 3.01%) of the device using CB.

Charge-transfer state dynamics in all-polymer solar cells: formation, dissociation and decoherence

All-polymer solar cells have made substantial achievements in recent years, offering numerous unsettled subjects for mechanical researchers. In order to quantitatively study the influence of the molecular electrostatic potential on the charge generation proposed by the experimenter, we simulate the ultrafast dynamics of the charge-transfer (CT) state at the interface between two polymer chains, which are respectively regarded as the donor and acceptor in all-polymer solar cells. The formation of a stable CT state is found to be sensitive to the distance between two oppositely charged polarons and the relevant critical electrostatic potential is thus quantified, which is in good agreement with experiments. In order to get insight into the dependence of the dissociation of the CT state on the width of the interfacial layer, two quantities are calculated: one is the Coulomb capture radius between the two polarons and the other is the quantum trace distance which serves as the fingerprint of the quantum coherence between them. The dissociation of the CT state is found to take place within an ultrafast timescale for an optimum interfacial width. The classical spatial distance and the quantum trace distance manifest a converging trend, suggesting a decoherence scenario for the charge separation in all-polymer solar cells.

Ultrafast hole transfer mediated by polaron pairs in all-polymer photovoltaic blends

The charge separation yield at a bulk heterojunction sets the upper efficiency limit of an organic solar cell. Ultrafast charge transfer processes in polymer/fullerene blends have been intensively studied but much less is known about these processes in all-polymer systems. Here, we show that interfacial charge separation can occur through a polaron pair-derived hole transfer process in all-polymer photovoltaic blends, which is a fundamentally different mechanism compared to the exciton-dominated pathway in the polymer/fullerene blends. By utilizing ultrafast optical measurements, we have clearly identified an ultrafast hole transfer process with a lifetime of about 3 ps mediated by photo-excited polaron pairs which has a markedly high quantum efficiency of about 97%. Spectroscopic data show that excitons act as spectators during the efficient hole transfer process. Our findings suggest an alternative route to improve the efficiency of all-polymer solar devices by manipulating polaron pairs.

All-polymer solar cells have shown high efficiencies but the ultrafast charge transfer processes are less known. Here Wang et al. show that polaron pairs play vital role facilitating the hole transfer, which is quite different from the exciton dominated pathway in polymer-fullerene blends.

Hidden Structure Ordering Along Backbone of Fused-Ring Electron Acceptors Enhanced by Ternary Bulk Heterojunction

Fused-ring electron acceptors (FREAs), as a family of non-fullerene (NF) acceptors, have achieved tremendous success in pushing the power conversion efficiency of organic solar cells. Here, the detailed molecular packing motifs of two extensively studied FREAs-ITIC and ITIC-Th are reported. It is revealed for the first time the long-range structure ordering along the backbone direction originated from favored end group π-π stacking. The backbone ordering could be significantly enhanced in the ternary film by the mutual mixing of ITIC and ITIC-Th, which gives rise to an improved in-plane electron mobility and better ternary device performance. The backbone ordering might be a common morphological feature of FREAs, providing explanations to previously observed small open circuit voltage loss and superior performance of FREA-based devices and guiding the future molecular design of high-performance NF acceptors.

Synthesis and photovoltaic properties of a non-fullerene acceptor with F-phenylalkoxy as a side chain

A new acceptor–donor–acceptor (A–D–A) non-fullerene acceptor (ITIC-FOR) was synthesized and applied in non-fullerene solar cells with a PCE of up to 7.33%.

Synthesis of an indacenodithiophene-based fully conjugated ladder polymer and its optical and electronic properties

A fully conjugated ladder polymer (PFIDT) based on the indacenodithiophene unit was synthesized via a simple strategy.