Halogen-Free π $\upi$ -Conjugated Polymers Based on Thienobenzobisthiazole for Efficient Nonfullerene Organic Solar Cells: Rational Design for Achieving High Backbone Order and High Solubility

In π $\upi$ -conjugated polymers, a highly ordered backbone structure and solubility are always in a trade-off relationship that must be overcome to realize highly efficient and solution-processable organic photovoltaics (OPVs). Here, it is shown that a π $\upi$ -conjugated polymer based on a novel thiazole-fused ring, thieno[2',3':5,6]benzo[1,2-d:4,3-d']bisthiazole (TBTz) achieves both high backbone order and high solubility due to the structural feature of TBTz such as the noncovalent interlocking of the thiazole moiety, the rigid and bent-shaped structure, and the fused alkylthiophene ring. Furthermore, based on the electron-deficient nature of these thiazole-fused rings, the polymer exhibits deep HOMO energy levels, which lead to high open-circuit voltages (V_OC s) in OPV cells, even without halogen substituents that are commonly introduced into high-performance polymers. As a result, when the polymer is combined with a typical nonfullerene acceptor Y6, power conversion efficiencies of reaching 16% and V_OC s of more than 0.84 V are observed, both of which are among the top values reported so far for "halogen-free" polymers. This study will serve as an important reference for designing π $\upi$ -conjugated polymers to achieve highly efficient and solution-processable OPVs.

Morphological design strategies to tailor out-of-plane charge transport in conjugated polymer systems for device applications

The transport of charge carriers throughout an active conjugated polymer (CP) host, characterized by a heterogeneous morphology of locally varying degrees of order and disorder, profoundly influences the performance of CP-based electronic devices, including diodes, photovoltaics, sensors, and supercapacitors. Out-of-plane charge carrier mobilities (μ_out-of-plane) across the bulk of the active material host and in-plane mobilities (μ_in-plane) parallel to a substrate are highly sensitive to local morphological features along their migration pathways. In general, the magnitudes of μ_out-of-plane and μ_in-plane are very different, in part because these carriers experience different morphological environments along their migration pathways. Suppressing the impact of variations in the morphological order/disorder on carrier migration remains an important challenge. While much is known about μ_in-plane and its optimization for devices, the current challenges are associated with μ_out-of-plane and its optimization for device performance. Therefore, this review is devoted to strategies for improving μ_out-of-plane in neat CP films and the implications for more complex systems, such as D:A blends which are relevant to OPV devices. The specific strategies discussed for improving μ_out-of-plane include solvent/field processing methods, chemical modification, thickness confinement, chemical additives, and different post-annealing strategies, including annealing with supercritical fluids. This review leverages the most recent fundamental understanding of mechanisms of charge transport and connections to morphology, identifying robust design strategies for targeted improvements of μ_out-of-plane.

Mobility Relaxation of Holes and Electrons in Polymer:Fullerene and Polymer : Non-Fullerene Acceptor Solar Cells

A non-fullerene small molecular acceptor (NFA) is a prominent molecule that shows moderate electron mobility and a narrow bandgap complementary to middle-bandgap p-type conjugated polymers, which leads to great improvement in the performance of organic photovoltaic (OPV) cells. However, little is known about the relaxation of charge carriers, which is key to efficient charge transport. Simultaneous time-of-flight (TOF) and time-resolved microwave conductivity (TRMC) measurements have been carried out on benzodithiophene-based polymer (PBDB-T):soluble C₇₀ -fullerere (PCBM) and PBDB-T:NFA (ITIC or Y6) blends, as benchmark systems. In addition to the conventional TOF mobilities, relaxation of the hole and electron mobility are evaluated by TRMC under an external electric field. PBDB-T : ITIC exhibits much faster relaxation than PBDB-T : PCBM, whereas that in PBDB-T : Y6 is moderate. This is consistent with the energetic disorder estimated from the photoabsorption onset. Interestingly, the slower relaxation of the electrons compared to the holes in PBDB-T : Y6 is in line with the preferred normal device structure. Our work deepens the understanding of the energetics of polymer : NFA blends and offers a basis for achieving efficient NFA properties.

Evaluation-oriented exploration of photo energy conversion systems: from fundamental optoelectronics and material screening to the combination with data science

Light is a form of energy that can be converted to electric and chemical energies. Thus, organic photovoltaics (OPVs), perovskite solar cells (PSCs), photocatalysts, and photodetectors have evolved as scientific and commercial enterprises. However, the complex photochemical reactions and multicomponent materials involved in these systems have hampered rapid progress in their fundamental understanding and material design. This review showcases the evaluation-oriented exploration of photo energy conversion materials by using electrodeless time-resolved microwave conductivity (TRMC) and materials informatics (MI). TRMC with its unique options (excitation sources, environmental control, frequency modulation, etc.) provides not only accelerated experimental screening of OPV and PSC materials but also a versatile route toward shedding light on their charge carrier dynamics. Furthermore, MI powered by machine learning is shown to allow extremely high-throughput exploration in the large molecular space, which is compatible with experimental screening and combinatorial synthesis.

How the Mixed Cations (Guanidium, Formamidinium, and Phenylethylamine) in Tin Iodide Perovskites Affect Their Charge Carrier Dynamics and Solar Cell Characteristics

Despite recent interest in lead-free Sn iodide perovskite (ASnI₃) solar cells, the role of mixed A-site cations is yet to be fully understood. Here, we report the effect of the ternary mixing of organic A-site cations (guanidium, GA; formamidinium, FA; and phenylethylamine, PEA) on the solar cell performance and charge carrier dynamics that are evaluated using time-resolved microwave conductivity (TRMC). (GA_xFA_1-x)_0.9PEA_0.1SnI₃ exhibits the maximum power conversion efficiency (PCE) of 7.90% at x = 0.15 and a drastic decrease with increasing GA content. Notably, our TRMC measurements of ASnI₃ with/without a hole transport layer reveal the same trend with the devices. From the analyses, we suggest that a variation of electron mobility affected by the location of the GA cation in the grains significantly impacts the PCE. Our work sheds light on the role of mixed A-site cations and directs a route toward the further development of Sn perovskite solar cells.

Regioregularity and Electron Deficiency Control of Unsymmetric Diketopyrrolopyrrole Copolymers for Organic Photovoltaics

Manipulating the electron deficiency and controlling the regioregularity of π-conjugated polymers are important for the fine-tuning of their electronic and electrochemical properties to make them suitable for an organic solar cell. Here, we report such a molecular design of unsymmetric diketopyrrolopyrrole (DPP) based copolymers with different aromatic side units of either thiophene (Th), pyridine (Py), or fluorobenzene (FBz). The unsymmetric electron acceptors of Th-DPP-Py and Th-DPP-FBz were polymerized with the electron donor of two-dimensional benzobisthiophene (BDT-Th), affording two regiorandom DPP copolymers. They exhibited contrasting molecular orbital levels and bulk heterojunction morphology in methanofullerene-blended films, leading to power conversion efficiencies of 3.75 and 0.18%, respectively. We further synthesized a regioregular DPP copolymer via sandwiching the centrosymmetric BDT-Th unit by two Th-DPP-Py units in an axisymmetric manner. The extensive characterization through morphology observation, X-ray diffraction, and space-charge-limited current mobilities highlight the case-dependent positive/negative effects of regioregularity and electron deficiency control.

Anomalous Photoinduced Hole Transport in Type I Core/Mesoporous-Shell Nanocrystals for Efficient Photocatalytic H2 Evolution

Controlling the carrier dynamics in a semiconductor nanoparticulate photocatalyst is the key to developing catalytic activity. Generally, type I band alignment is unsuitable for photocatalysts because the photoinduced carriers accumulate in the narrow bandgap semiconductor. To avoid the termination of reactions and/or photocorrosion of materials caused by carrier accumulation, it is common to employ type II band alignment for photoenergy conversion systems instead of type I. However, CdS/ZnS core/mesoporous-shell heterostructures show superior photocatalytic activity despite having type I band alignment that is generally unfavorable for photocatalytic reactions. Transient absorption spectroscopy and time-resolved microwave conductivity revealed efficient photoinduced hole transfer from the CdS phase to the ZnS phase. The defect-mediated hole transfer from the CdS to the ZnS phase resulted in long-lived charge separation (>2.4 ms) leading to high photocatalytic performance. This study provides insight into defect-mediated carrier transfer in nanoparticulate photocatalysts, which could be used as a guideline for the design of highly active and stable nanoparticulate photocatalysts.

Computer-Aided Screening of Conjugated Polymers for Organic Solar Cell: Classification by Random Forest

Owing to the diverse chemical structures, organic photovoltaic (OPV) applications with a bulk heterojunction framework have greatly evolved over the last two decades, which has produced numerous organic semiconductors exhibiting improved power conversion efficiencies (PCEs). Despite the recent fast progress in materials informatics and data science, data-driven molecular design of OPV materials remains challenging. We report a screening of conjugated molecules for polymer-fullerene OPV applications by supervised learning methods (artificial neural network (ANN) and random forest (RF)). Approximately 1000 experimental parameters including PCE, molecular weight, and electronic properties are manually collected from the literature and subjected to machine learning with digitized chemical structures. Contrary to the low correlation coefficient in ANN, RF yields an acceptable accuracy, which is twice that of random classification. We demonstrate the application of RF screening for the design, synthesis, and characterization of a conjugated polymer, which facilitates a rapid development of optoelectronic materials.

Durian-Shaped CdS@ZnSe Core@Mesoporous-Shell Nanoparticles for Enhanced and Sustainable Photocatalytic Hydrogen Evolution

In artificial photosynthesis, the establishment of design guidelines for nanostructures to maximize the photocatalytic performance remains a great challenge. In contrast with the intense research into band-offset tuning for photocatalysts, the relationship between nanostructures and photoinduced carrier dynamics has still been insufficiently explored. We synthesized durian-shaped CdS@ZnSe core@mesporous-shell nanoparticles ( d-CdS/ZnSe NPs) and investigated the carrier dynamics in photocatalytic hydrogen evolution. The cocatalyst-free d-CdS/ZnSe NPs exhibited high photocatalytic activity for H₂ evolution (14.8% apparent quantum yield at 420 nm) and excellent stability (maintaining 80% activity after 72 h) under visible-light irradiation (>422 nm). The transient absorption measurement and flash photolysis time-resolved microwave conductivity unveiled that the ultra-long-lived charge separation (>6.2 ms) and swift hole transfer to the surfaces of ZnSe shell (11 ns) contribute the high catalytic activity and stability. The present work provides a novel insight into designing nanoparticulate photocatalysts with optimized performance.

Organic photovoltaics of diketopyrrolopyrrole copolymers with unsymmetric and regiorandom configuration of the side units

Diketopyrrolopyrrole (DPP) is a representative electron acceptor incorporated into narrow-bandgap polymers for organic photovoltaic cells (OPV). Commonly, identical aromatic units are attached to the sides of the DPP unit, forming symmetric DPP polymers. Herein we report the synthesis and characterization of DPP copolymers consisting of unsymmetric configurations of the side aromatics. The unsymmetric DPP copolymer with thienothiophene and benzene side moieties exhibits good solubility owing to the twisted dihedral angle at benzene and regiorandom configuration. A significant shallowing of the highest occupied molecular orbital level is observed in accordance with the electron-donating nature of the side units (benzene, thiophene, and thienothiophene). The overall power conversion efficiencies of the unsymmetric DPPs (2.3–2.4%) are greater than that of the centrosymmetric analogue (0.45%), which is discussed in view of bulk heterojunction morphology, polymer crystallinity, and space-charge-limited current mobilities. This comparative study highlights the effect of unsymmetric design on the molecular stacking and OPV performance of DPP copolymers.

Diketopyrrolopyrroles with unsymmetric side aromatics of benzene and (thiophene or thienothiophene) were copolymerized with 2-dimensional benzodithiophene, and their solar cell devices were characterized.

Exploring Alkyl Chains in Benzobisthiazole-Naphthobisthiadiazole Polymers: Impact on Solar-Cell Performance, Crystalline Structures, and Optoelectronics

The shapes and lengths of the alkyl chains of conjugated polymers greatly affect the efficiencies of organic photovoltaic devices. This often results in a trade-off between solubility and self-organizing behavior; however, each material has specific optimal chains. Here we report on the effect of alkyl side chains on the film morphologies, crystallinities, and optoelectronic properties of new benzobisthiazole-naphthobisthiadiazole (PBBT-NTz) polymers. The power conversion efficiencies (PCEs) of linear-branched and all-branched polymers range from 2.5% to 6.6%; the variations in these PCEs are investigated by atomic force microscopy, two-dimensional X-ray diffraction (2D-GIXRD), and transient photoconductivity techniques. The best-performing linear-branched polymer, bearing dodecyl and decyltetradecyl chains (C12-DT), exhibits nanometer-scale fibers along with the highest crystallinity, comprising predominant edge-on and partial face-on orientations. This morphology leads to the highest photoconductivity and the longest carrier lifetime. These results highlight the importance of long alkyl chains for inducing intermolecular stacking, which is in contrast to observations made for analogous previously reported polymers.

Organic Planar Heterojunctions: From Models for Interfaces in Bulk Heterojunctions to High-Performance Solar Cells

Recent progress regarding planar heterojunctions (PHJs) is reviewed, with respect to the fundamental understanding of the photophysical processes at the donor/acceptor interfaces in organic photovoltaic devices (OPVs). The current state of OPV research is summarized and the advantages of PHJs as models for exploring the relationship between organic interfaces and device characteristics described. The preparation methods and the characterization of PHJ structures to provide key points for the appropriate handling of PHJs. Next, we describe the effects of the donor/acceptor interface on each photoelectric conversion process are reviewed by examining various PHJ systems to clarify what is currently known and not known. Finally, it is discussed how we the knowledge obtained by studies of PHJs can be used to overcome the current limits of OPV efficiency.

Crystallisation-enhanced bulk hole mobility in phenothiazine-based organic semiconductors

A series of three novel donor-acceptor systems based on C(3)-malononitrile-substituted phenothiazines was synthesised in good overall yields and their thermal, spectroscopic, and electrochemical properties were characterised. The compounds were prepared through a sequence of Ullmann-coupling, Vilsmeier-Haack formylation and Knoevenagel-condensation, followed by Suzuki-coupling reactions for introduction of aryl substitutents at C(7) position of the phenothiazine. The introduction of a donor unit at the C(7) position exhibited a weak impact on the optical and electrochemical characteristics of the compounds and led to amorphous films with bulk hole mobilities in the typical range reported for phenothiazines, despite the higher charge delocalisation as attested by computational studies. In contrast, highly ordered films were formed when using the C(7)-unsubstituted 3-malononitrile phenothiazine, exhibiting an outstanding mobility of 1 × 10⁻³ cm² V⁻¹ s⁻¹, the highest reported for this class of compounds. Computational conformational analysis of the new phenothizanes suggested that free rotation of the substitutents at the C(7) position suppresses the ordering of the system, thereby hampering suitable packing of the new materials needed for high charge carrier mobility.

The fate of electron-hole pairs in polymer:fullerene blends for organic photovoltaics

There has been long-standing debate on how free charges are generated in donor:acceptor blends that are used in organic solar cells, and which are generally comprised of a complex phase morphology, where intermixed and neat phases of the donor and acceptor material co-exist. Here we resolve this question, basing our conclusions on Stark effect spectroscopy data obtained in the absence and presence of externally applied electric fields. Reconciling opposing views found in literature, we unambiguously demonstrate that the fate of photogenerated electron–hole pairs—whether they will dissociate to free charges or geminately recombine—is determined at ultrafast times, despite the fact that their actual spatial separation can be much slower. Our insights are important to further develop rational approaches towards material design and processing of organic solar cells, assisting to realize their purported promise as lead-free, third-generation energy technology that can reach efficiencies over 10%.

Charge generation and transport are crucial to the performance of organic solar cells, but the mechanism remains controversial. Causa' et al. show that the phase morphology of polymer:fullerene blends determines the exciton dissociation at femtoseconds, although the spatial separation can occur at picoseconds.

Unraveling the Fundamental Mechanisms of Solvent-Additive-Induced Optimization of Power Conversion Efficiencies in Organic Photovoltaic Devices

The realization of controllable morphologies of bulk heterojunctions (BHJ) in organic photovoltaics (OPVs) is one of the key factors enabling high-efficiency devices. We provide new insights into the fundamental mechanisms essential for the optimization of power conversion efficiencies (PCEs) with additive processing to PBDTTT-CF:PC71BM system. We have studied the underlying mechanisms by monitoring the 3D nanostructural modifications in BHJs and correlated the modifications with the optical analysis and theoretical modeling of charge transport. Our results demonstrate profound effects of diiodooctane (DIO) on morphology and charge transport in the active layers. For small amounts of DIO (<3 vol %), DIO promotes the formation of a well-mixed donor-acceptor compact film and augments charge transfer and PCE. In contrast, for large amounts of DIO (>3 vol %), DIO facilitates a loosely packed mixed morphology with large clusters of PC71BM, leading to deterioration in PCE. Theoretical modeling of charge transport reveals that DIO increases the mobility of electrons and holes (the charge carriers) by affecting the energetic disorder and electric field dependence of the mobility. Our findings show the implications of phase separation and carrier transport pathways to achieve optimal device performances.

Tuning the driving force for exciton dissociation in single-walled carbon nanotube heterojunctions

Understanding the kinetics and energetics of interfacial electron transfer in molecular systems is crucial for the development of a broad array of technologies, including photovoltaics, solar fuel systems and energy storage. The Marcus formulation for electron transfer relates the thermodynamic driving force and reorganization energy for charge transfer between a given donor/acceptor pair to the kinetics and yield of electron transfer. Here we investigated the influence of the thermodynamic driving force for photoinduced electron transfer (PET) between single-walled carbon nanotubes (SWCNTs) and fullerene derivatives by employing time-resolved microwave conductivity as a sensitive probe of interfacial exciton dissociation. For the first time, we observed the Marcus inverted region (in which driving force exceeds reorganization energy) and quantified the reorganization energy for PET for a model SWCNT/acceptor system. The small reorganization energies (about 130 meV, most of which probably arises from the fullerene acceptors) are beneficial in minimizing energy loss in photoconversion schemes.