Organic Solar Cell Materials toward Commercialization

Processing the Interlayer and Optimizing the Active Layer by One-Step Dissolution Compensation in Organic Solar Cells

Optimized morphology of the active layer and electrode interface is critical for obtaining high-performance organic solar cells. However, achieving this typically involves a multifaceted, sequential process that renders outcomes unpredictable. Here, by exploiting the dissolution compensation, we propose a one-step method that integrates interlayer fabrication and a controllable morphology optimization. Taking an "out of the box" approach, we incorporate the good solvent of the active layer into the interlayer solution to act as dissolution compensation, breaking the orthogonal solvent principles to allow the morphology of the active layer to evolve to an optimized state while the interface layer is being processed. Using two commercially available material systems, D18:Y6 and D18:L8-BO, as examples, it was found that the JSC and fill factor (FF) device can be improved by using an appropriate ratio of the compensation solvent chloroform in the interlayer solution. As a result, the power conversion efficiency of the device based on the two state-of-the-art systems can be increased by about 7.5% (D18:Y6, from 17.04 to 18.31%; D18:L8-BO, from 17.97 to 19.31%). This one-step strategy has been shown to be universally applicable to other diverse systems and provides a simple yet reliable method for accurately depositing high-quality interlayers with an optimized active layer morphology in high-performance organic solar cells and other solution-processable organic electronics.

High Polymer Molecular Weight Yields Solar Cells with Simultaneously Improved Performance and Thermal Stability

Simple synthetic routes, high active layer thickness tolerance as well as stable organic solar cells are relentlessly pursued as key enabling traits for the upscaling of organic photovoltaics. Here, the potential to address these issues by tuning donor polymer molecular weight is investigated. Specifically, the focus is on PTQ10, a polymer with low synthetic complexity, with number average molecular weights of 2.4, 6.2, 16.8, 52.9, and 54.4 kDa, in combination with three different non-fullerene acceptors, namely Y6, Y12, and IDIC. Molecular weight, indeed, unlocks a threefold increase in power conversion efficiency for these blends. Importantly, efficiencies above 10% for blade coated devices with thicknesses between 200 and 350 nm for blends incorporating high molecular weight donor are shown. Spectroscopic, GIWAXS and charge carrier mobility data suggest that the strong photocurrent improvement with molecular weight is related to both, improved electronic transport and polymer contribution to exciton generation. Moreover, it is demonstrated that solar cells based on high molecular weight PTQ10 are more thermally stable due to a higher glass transition temperature, thus also improving device stability.

Bioinspired Light-Driven Proton Pump: Engineering Band Alignment of WS2 with PEDOT:PSS and PDINN

Bioinspired two-dimensional (2D) nanofluidic systems for photo-induced ion transport have attracted great attention, as they open a new pathway to enabling light-to-ionic energy conversion. However, there is still a great challenge in achieving a satisfactory performance. It is noticed that organic solar cells (OSCs, light-harvesting device based on photovoltaic effect) commonly require hole/electron transport layer materials (TLMs), PEDOT:PSS (PE) and PDINN (PD), respectively, to promote the energy conversion. Inspired by such a strategy, an artificial proton pump by coupling a nanofluidic system with TLMs is proposed, in which the PE- and PD-functionalized tungsten disulfide (WS2) multilayers construct a heterogeneous membrane, realizing an excellent output power of ≈1.13 nW. The proton transport is fine-regulated due to the TLMs-engineered band structure of WS2. Clearly, the incorporating TLMs of OSCs into 2D nanofluidic systems offers a feasible and promising approach for band edge engineering and promoting the light-to-ionic energy conversion.

Recent advances in integrated solar cell/supercapacitor devices: Fabrication, strategy and perspectives

BACKGROUND

Solar cell/supercapacitor integrated devices (SCSD) have made some progress in terms of device structure and electrode materials, but there are still many key challenges in controlling electrode performance and improving the efficiency of integrated devices.

AIM OF REVIEW

It is necessary to study how to balance the photoelectric conversion process and the storage process. From the microscopic mechanism of different functional unit materials to the mechanism of macroscopic devices, it is essential to conduct in-depth research.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Here, the structures and preparation methods of various types of integrated SCSD were introduced. Then, the strategies for improving the overall performance of integrated devices were evaluated. Finally, the key objectives of reducing the cost of materials, increasing the stability and sustainability of devices were highlighted. Better matching of different functional units of devices was also prospected.

Organic Photovoltaic Stability: Understanding the Role of Engineering Exciton and Charge Carrier Dynamics from Recent Progress

Benefiting from the synergistic development of material design, device engineering, and the mechanistic understanding of device physics, the certified power conversion efficiencies (PCEs) of single-junction non-fullerene organic solar cells (OSCs) have already reached a very high value of exceeding 19%. However, in addition to PCEs, the poor stability is now a challenging obstacle for commercial applications of organic photovoltaics (OPVs). Herein, recent progress made in exploring operational mechanisms, anomalous photoelectric behaviors, and improving long-term stability in non-fullerene OSCs are highlighted from a novel and previously largely undiscussed perspective of engineering exciton and charge carrier pathways. Considering the intrinsic connection among multiple temporal-scale photocarrier dynamics, multi-length scale morphologies, and photovoltaic performance in OPVs, this review delineates and establishes a comprehensive and in-depth property-function relationship for evaluating the actual device stability. Moreover, this review has also provided some valuable photophysical insights into employing the advanced characterization techniques such as transient absorption spectroscopy and time-resolved fluorescence imagings. Finally, some of the remaining major challenges related to this topic are proposed toward the further advances of enhancing long-term operational stability in non-fullerene OSCs.

Donor-Acceptor Copolymers with 9-(2-Ethylhexyl)carbazole or Dibenzothiophene-5,5-dioxide Donor Units and 5,6-Difluorobenzo[c][1,2,5]thiadiazole Acceptor Units for Photonics

Semiconducting polymers, particularly of the third generation, including donor-acceptor (D-A) copolymers, are extensively studied due to their huge potential for photonic and electronic applications. Here, we report on two new D-A copolymers, CP1 and CP2, composed of different electron-donor (D) units: 9-(2-ethylhexyl)carbazole or dibenzothiophene-5,5-dioxide, respectively, and of 4,7-bis(4'-(2-octyldodecyl)thiophen-2'-yl)-5,6-difluorobenzo[c][1,2,5]thiadiazole building block with central 5,6-difluorobenzo[c][1,2,5]thiadiazole electron-acceptor (A) units, which were synthesized by Suzuki coupling in the high-boiling solvent xylene and characterized. The copolymers exhibited very good thermal and oxidation stability. A copolymer CP1 with different molecular weights was prepared in order to facilitate a comparison of CP1 with CP2 of comparable molecular weight and to reveal the relationship between molecular weight and properties. The photophysical, electrochemical, and electroluminescence properties were examined. Intense red photoluminescence (PL) with higher PL efficiencies for CP1 than for CP2 was observed in both solutions and films. Red shifts in the PL thin film spectra compared with the PL solution spectra indicated aggregate formation in the solid state. X-ray diffraction measurements revealed differences in the arrangement of molecules in thin films depending on the molecular weight of the copolymers. Light-emitting devices with efficient red emission and low onset voltages were prepared and characterized.

Suppressing Optical Losses in Solar Cells via Multifunctional and Large-Scale Geometric Arrays

The occurrence of optical loss on the surface of solar cells is inevitable due to the difference in the refractive index between air and glass, as well as the insufficient absorption of the active layer. To address this challenge, micron-sized geometry arrays, such as hemispheres and hemisphere pits, are prepared on quartz glass through the advanced indirect patterning technology of UV-LIGA. These geometric arrays exhibit multiple mechanisms for controlling light waves, including multiple rebounds, diffraction scattering, and total internal reflection. These synergistic effects suppress optical losses at the device's surface and prolong the photon propagation path in the active layer. After being patterned with this structure, the average transmittance and haze of the quartz glass reach 93.91% and 75%, respectively. Compared to their flat counterpart, the decorated monocrystalline silicon solar cells demonstrated an apparent improvement in photocurrent and produced a 7.2% enhancement in power conversion efficiency.

An additive manufacturing approach based on electrohydrodynamic printing to fabricate P3HT:PCBM thin films

Additive manufacturing (AM) enables the production of high value and high performance components with applications from aerospace to biomedical fields. We report here on the fabrication of poly(3-hexylthiophene): phenyl-C61-butyric acid methyl ester (P3HT:PCBM) thin films through the electrohydrodynamic atomization (EHDA) process and its integration as absorber layer for organic solar cells. Prior to the film fabrication, the optimization of the process was carried out by developing the operating envelope for the P3HT:PCBM ink to determine the optimal flow rate and the appropriate applied voltage to achieve a stable-cone deposition mode. The EHDA printed thin-film's topography, morphology and optical properties were systematically analyzed. The root-mean-square roughness was found to vary significantly with the annealing temperature and the flow rate and ranged from 1.938 to 3.345 nm. The estimated film mass and thickness were found between 3.235 and 23.471 mg and 597.5 nm to 1.60 µm, respectively. The films exhibited a broad visible absorption spectrum ranging from ~ 340 to ~ 600 nm, with a maximum peak λmax located at ~ 500 nm. As the annealing temperature and the flow rate were increased, discernible alterations in the PCBM clusters were consequently observed in the blends of the film and the size of the PCBM clusters has decreased by 3% while the distance between them was highly reduced by as much as 82%.

A 3-Fluoropyridine Manipulating the Aggregation and Fibril Network of Donor Polymers for Eco-Friendly Solution-Processed Versatile Organic Solar Cells

The development of eco-friendly solvent-processed organic solar cells (OSCs) suitable for industrial-scale production should be now considered the imperative research. Herein, asymmetric 3-fluoropyridine (FPy) unit is used to control the aggregation and fibril network of polymer blends. Notably, terpolymer PM6(FPy = 0.2) incorporating 20% FPy in a well-known donor polymer poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione)] (PM6) can reduce the regioregularity of the polymer backbone and endow them with much-enhanced solubility in eco-friendly solvents. Accordingly, the excellent adaptability for fabricating versatile devices based on PM6(FPy = 0.2) by toluene processing is demonstrated. The resulting OSCs exhibit a high power conversion efficiency (PCE) of 16.1% (17.0% by processed with chloroform) and low batch-to-batch variation. Moreover, by controlling the donor-to-acceptor weight ratio at 0.5:1.0 and 0.25:1.0, semi-transparent OSCs (ST-OSCs) yield significant light utilization efficiencies of 3.61% and 3.67%, respectively. For large-area (1.0 cm2 ) indoor OSC (I-OSC), a high PCE of 20.6% is achieved with an appropriate energy loss of 0.61 eV under a warm white light-emitting diode (3,000 K) with the illumination of 958 lux. Finally, the long-term stability of the devices is evaluated by investigating their structure-performance-stability relationship. This work provides an effective approach to realizing eco-friendly, efficient, and stable OSCs/ST-OSCs/I-OSCs.

Revealing the Molecular Weight Effect on Highly Efficient Polythiophene Solar Cells

Polythiophenes (PTs) are promising electron donors in organic solar cells (OSCs) due to their simple structures and excellent synthetic scalability. Benefiting from the rational molecular design, the power conversion efficiency (PCE) of PT solar cells has been greatly improved. Herein, five batches of the champion PT (P5TCN-F25) with molecular weights ranging from 30 to 87 kg mol^-1 were prepared, and the effect of the molecular weight on the blend film morphology and photovoltaic performance of PT solar cells was systematically investigated. The results showed that the PCEs of the devices improved first and then maintained a high value with the increase of molecular weight, and the highest PCE of 16.7% in binary PT solar cells was obtained. Further characterizations revealed that the promotion in photovoltaic performance mainly comes from finer phase separation structures and more compact molecular packing in the blend film. The best device stabilities were also achieved by polymers with high molecular weights. Overall, this study highlights the importance of optimizing the molecular weight for PTs and offers directions to further improve the PCE of PT solar cells.

Improving the Stability of Non-fullerene-Based Organic Photovoltaics through Sequential Deposition and Utilization of a Quasi-orthogonal Solvent

The development of organic photovoltaic (OPV) devices based on non-fullerene acceptors (NFAs) has led to a rapid improvement in their efficiency. Despite these improvements, significant performance degradation in the early stages of operation, known as burn-in, remains a challenge for NFA-based OPVs. To address this challenge, this study demonstrates a stable NFA-based OPV fabricated using sequential deposition (SqD) and a quasi-orthogonal solvent. The quasi-orthogonal solvent, which is prepared by incorporating 1-chloronaphthalene (1-CN) into dichloromethane (DCM), reduces the vapor pressure of the solvent and allows for the efficient dissolution and penetration of the Y6 (one of efficient NFAs) into a PM6 polymer-donor layer without damaging the latter. The resulting bulk heterojunction (BHJ) is characterized by a higher degree of crystallinity in the PM6 domains than that prepared using a conventional single-step deposition (SD) process. The OPV fabricated using the SqD process exhibits a PCE of 14.1% and demonstrates superior thermal stability to the SD-processed OPV. This study conclusively reveals that the formation of a thermally stable interface between the photoactive layer and the electron-transport layer (ETL) is the primary factor contributing to the high thermal stability observed in the SqD-processed OPV.

Broadband Photon Harvesting in Organic Photovoltaic Devices Induced by Large-Area Nanogrooved Templates

Thin-film organic photovoltaic (OPV) devices represent an attractive alternative to conventional silicon solar cells due to their lightweight, flexibility, and low cost. However, the relatively low optical absorption of the OPV active layers still represents an open issue in view of efficient devices that cannot be addressed by adopting conventional light coupling strategies derived from thick PV absorbers. The light coupling to thin-film solar cells can be boosted by nanostructuring the device interfaces at the subwavelength scale. Here, we demonstrate broadband and omnidirectional photon harvesting in thin-film OPV devices enabled by highly ordered one-dimensional (1D) arrays of nanogrooves. Laser interference lithography, in combination with reactive ion etching (RIE), provides the controlled tailoring of the height and periodicity of the silica grooves, enabling effective tuning of the anti-reflection properties in the active organic layer (PTB7:PCBM). With this strategy, we demonstrate a strong enhancement of the optical absorption, as high as 19% with respect to a flat device, over a broadband visible and near-infrared spectrum. The OPV device supported on these optimized nanogrooved substrates yields a 14% increase in short-circuit current over the corresponding flat device, highlighting the potential of this large-scale light-harvesting strategy in the broader context of thin-film technologies.

Solution-Processable NiOx:PMMA Hole Transport Layer for Efficient and Stable Inverted Organic Solar Cells

For organic solar cells (OSCs), nickel oxide (NiO_x) is a potential candidate as the hole transport layer (HTL) material. However, due to the interfacial wettability mismatch, developing solution-based fabrication methods of the NiO_x HTL is challenging for OSCs with inverted device structures. In this work, by using N, N-dimethylformamide (DMF) to dissolve poly(methyl methacrylate) (PMMA), the polymer is successfully incorporated into the NiO_x nanoparticle (NP) dispersions to modify the solution-processable HTL of the inverted OSCs. Benefiting from the improvements of electrical and surface properties, the inverted PM6:Y6 OSCs based on the PMMA-doped NiO_x NP HTL achieves an enhanced power conversion efficiency of 15.11% as well as improved performance stability in ambient conditions. The results demonstrated a viable approach to realize efficient and stable inverted OSCs by tuning the solution-processable HTL.

The impact of plasmonic electrodes on the photocarrier extraction of inverted organic bulk heterojunction solar cells

Nano-patterning the semiconducting photoactive layer/back electrode interface of organic photovoltaic devices is a widely accepted approach to enhance the power conversion efficiency through the exploitation of numerous photonic and plasmonic effects. Yet, nano-patterning the semiconductor/metal interface leads to intertwined effects that impact the optical as well as the electrical characteristic of solar cells. In this work we aim to disentangle the optical and electrical effects of a nano-structured semiconductor/metal interface on the device performance. For this, we use an inverted bulk heterojunction P3HT:PCBM solar cell structure, where the nano-patterned photoactive layer/back electrode interface is realized by patterning the active layer with sinusoidal grating profiles bearing a periodicity of 300 nm or 400 nm through imprint lithography while varying the photoactive layer thickness (L _PAL ) between 90 and 400 nm. The optical and electrical device characteristics of nano-patterned solar cells are compared to the characteristics of control devices, featuring a planar photoactive layer/back electrode interface. We find that patterned solar cells show for an enhanced photocurrent generation for a L _PAL above 284 nm, which is not observed when using thinner active layer thicknesses. Simulating the optical characteristic of planar and patterned devices through a finite-difference time-domain approach proves for an increased light absorption in presence of a patterned electrode interface, originating from the excitation of propagating surface plasmon and dielectric waveguide modes. Evaluation of the external quantum efficiency characteristic and the voltage dependent charge extraction characteristics of fabricated planar and patterned solar cells reveals, however, that the increased photocurrents of patterned devices do not stem from an optical enhancement but from an improved charge carrier extraction efficiency in the space charge limited extraction regime. Presented findings clearly demonstrate that the improved charge extraction efficiency of patterned solar cells is linked to the periodic surface corrugation of the (back) electrode interface.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s00339-023-06492-6.

Learning Conductance: Gaussian Process Regression for Molecular Electronics

Experimental studies of charge transport through single molecules often rely on break junction setups, where molecular junctions are repeatedly formed and broken while measuring the conductance, leading to a statistical distribution of conductance values. Modeling this experimental situation and the resulting conductance histograms is challenging for theoretical methods, as computations need to capture structural changes in experiments, including the statistics of junction formation and rupture. This type of extensive structural sampling implies that even when evaluating conductance from computationally efficient electronic structure methods, which typically are of reduced accuracy, the evaluation of conductance histograms is too expensive to be a routine task. Highly accurate quantum transport computations are only computationally feasible for a few selected conformations and thus necessarily ignore the rich conformational space probed in experiments. To overcome these limitations, we investigate the potential of machine learning for modeling conductance histograms, in particular by Gaussian process regression. We show that by selecting specific structural parameters as features, Gaussian process regression can be used to efficiently predict the zero-bias conductance from molecular structures, reducing the computational cost of simulating conductance histograms by an order of magnitude. This enables the efficient calculation of conductance histograms even on the basis of computationally expensive first-principles approaches by effectively reducing the number of necessary charge transport calculations, paving the way toward their routine evaluation.

High-efficiency organic solar cells processed from a real green solvent

The fabrication of organic solar cells (OSCs) depends heavily on the use of highly toxic chlorinated solvents, which are incompatible with industrial manufacturing. The reported alternative solvents such as non-halogenated aromatic hydrocarbons and cyclic ethers are also not really "green" according to the "Globally Harmonized System of Classification and Labelling of Chemicals" of the United Nations. Therefore, processing from real green solvents such as water, alcohols, or anisole will constitute a big breakthrough for OSCs. However, it is fundamentally challenging to obtain high-performance photovoltaic materials processable from these solvents. Herein, we propose the incorporation of a B-N covalent bond, which has a dipole moment of 1.84 Debye, into the conjugated backbone of polymer donors to fabricate high-efficiency OSCs from anisole, a real green and eco-compatible solvent recommended by the United Nations. Based on a newly developed B-N-based polymer, the OSCs with a record-high efficiency of 15.65% in the 0.04 cm² device and 14.01% in the 1.10 cm² device have thus been realized via real green processing.

Phosphotungstate-Based Anode Interfacial Material for Constructing High-Performance Polymer Solar Cells with a Fill Factor over 80

In the field of organic solar cells (OSCs), the interfacial layer plays the role of enhancing carrier extraction/transportation, inhibiting their recombination, etc. In contrast to the wide variety of cathode interfacial materials with good modification ability, much less effort has been reported for anode interfacial materials. In this study, we report a polyoxometalate-based inorganic molecular cluster, zinc phosphotungstate (Zn₃P₂W₂₄O₈₀, denoted ZnPW), as an anode interfacial layer. Based on the PM6/EH-HD-4F/L8-BO-F ternary system, the device with ZnPW modification achieved a high power conversion efficiency (PCE) and a fill factor of up to 18.67 and 80.29%, respectively, which are higher than the counterpart device (PCE of 18.01%) with PEDOT/PSS as the anode interfacial layer. Detailed studies revealed that under the modification of ZnPW, the devices obtained promoted light absorption and suitable energy level matching between the active layer and the electrode, reduced contact resistance, and suppressed charge recombination. In addition, the ZnPW-modified devices had improved photostability and storage stability compared to PEDOT/PSS-modified devices. Our work shows that the polyoxometalate-based inorganic nanocluster ZnPW has great advantages in enhancing the device performance and stability of OSCs.

New Benzotriazole and Benzodithiophene-Based Conjugated Terpolymer Bearing a Fluorescein Derivative as Side-Group: In-Ternal Förster Resonance Energy Transfer to Improve Organic Solar Cells

A new benzodithiophene and benzotriazole-based terpolymer bearing a fluorescein derivative as a side group was synthesized and studied for organic solar cell (OSC) applications. This side group was covalently bounded to the backbone through an n-hexyl chain to induce the intramolecular Förster Resonance Energy Transfer (FRET) process and thus improve the photovoltaic performance of the polymeric material. The polymer exhibited good solubility in common organic chlorinated solvents as well as thermal stability (TDT10% > 360 °C). Photophysical measurements demonstrated the occurrence of the FRET phenomenon between the lateral group and the terpolymer. The terpolymer exhibited an absorption band centered at 501 nm, an optical bandgap of 2.02 eV, and HOMO and LUMO energy levels of −5.30 eV and −3.28 eV, respectively. A preliminary study on terpolymer-based OSC devices showed a low power-conversion efficiency (PCE) but a higher performance than devices based on an analogous polymer without the fluorescein derivative. These results mean that the design presented here is a promising strategy to improve the performance of polymers used in OSCs.

Bihalogenated Thiophene-Based Terpolymers for High-Performance Semitransparent Organic Solar Cells Processed by an Eco-Friendly Solvent and Layer-by-Layer Deposition

The development of photoactive materials simultaneously satisfying high performance, low cost, and eco-friendly processability remains challenging in organic solar cells (OSCs). Herein, a synergistic strategy is proposed to design three terpolymers (PM7(ClCl = 0.2), PM7(ClBr = 0.2), and PM7(BrBr = 0.2)) based on bihalogenated thiophenes with relatively low cost, for improving the optical and electrochemical properties, solubility in nontoxic solvents, and crystallinity and miscibility balance. In summary, a bulk-heterojunction (BHJ)-processed device based on PM7(ClCl = 0.2) with 20% dichlorinated thiophene achieves the highest power conversion efficiency (PCE) of 15.2% using toluene (best PCE ≈ 15.8% on the ternary blend). Moreover, high-performance semitransparent OSCs (ST-OSCs) were fabricated by a combination of layer-by-layer (LBL) and sequential dynamic and static spin-coating techniques according to the molecular weight of PM7(ClCl = 0.2). Using this unique LBL strategy, the PM7(ClCl = 0.2)-MW (H; high molecular weight)-processed ST-OSCs yield a high PCE of 11.5% and an average visible transmittance (AVT) of 27.1% with outstanding tolerance to device reproducibility. By optimizing ST-OSCs with tungsten trioxide as a distributed Bragg reflector, a light utilization efficiency (LUE) of 3.61% is realized with a PCE of 10.8% and an AVT of 33.4% (certified PCE ≈ 11.157%; LUE ≈ 3.73%). This study provides a novel perspective for designing and developing actual photoactive materials for OSC commercialization.

Concepts and principles of self-n-doping in perylene diimide chromophores for applications in biochemistry, energy harvesting, energy storage, and catalysis

Self-doping is an essential method of increasing carrier concentrations in organic electronics that eliminates the need to tailor host-dopant miscibility, a necessary step when employing molecular dopants. Self-n-doping can be accomplished using amines or ammonium counterions as an electron source, which are being incorporated into an ever-increasingly diverse range of organic materials spanning many applications. Self-n-doped materials have demonstrated exemplary and, in many cases, benchmark performances in a variety of applications. However, an in-depth review of the method is lacking. Perylene diimide (PDI) chromophores are an important mainstay in the semiconductor literature with well-known structure-function characteristics and are also one of the most widely utilized scaffolds for self-n-doping. In this review, we describe the unique properties of self-n-doped PDIs, delineate structure-function relationships, and discuss self-n-doped PDI performance in a range of applications. In particular, the impact of amine/ammonium incorporation into the PDI scaffold on doping efficiency is reviewed with regard to attachment mode, tether distance, counterion selection, and steric encumbrance. Self-n-doped PDIs are a unique set of PDI structural derivatives whose properties are amenable to a broad range of applications such as biochemistry, solar energy conversion, thermoelectric modules, batteries, and photocatalysis. Finally, we discuss challenges and the future outlook of self-n-doping principles.

Tailoring the Morphology's Microevolution for Binary All-Polymer Solar Cells Processed by Aromatic Hydrocarbon Solvent with 16.22% Efficiency

Herein, we report a systematic solvent selection for eco-friendly processed binary all-polymer solar cells (APSCs) with decent power conversion efficiencies (PCEs). Three typical solvents, toluene, o-xylene, and 1,2,4-trimethylbezene, are chosen and compared. The device enabled by o-xylene exhibits the most outstanding PCE of 16.22%, thanks to its favorable morphology, which is to say a well-formed face-on orientation packing motif and a suitable crystallinity and size of phase segregation. Consequently, the solar cell affords sufficient charge generation, as well as efficient and balanced charge transport, which are all positive to pursuing high efficiency. This work offers an understanding of using complete solvent selection as the strategy to realize high-performance devices by sophisticatedly controlling the morphology.

Anion-Doped Thickness-Insensitive Electron Transport Layer for Efficient Organic Solar Cell

In organic solar cells, interfacial materials play essential roles in charge extraction, transportation, and collection. Currently, highly efficient and thickness-insensitive interfacial materials are urgently needed in printable large area module devices. Herein, water/alcohol-soluble conjugated polyelectrolyte PFNBT-Br, with medium bandgap based on benzothiadiazole, are doped by two alkali metal sodium salts, NaH₂ PO₂ , Na₂ C₂ O₄ with different counter anions, to pursue high efficiency and thickness-insensitive electron-transport layers. Results show that the doping of electron-transport material can effectively promote the performance of the devices. Moreover, electron-transport layers doped by these salts with different counter anions show different behaviors in performances. Among which, the salt with oxalate anion C₂ O₄ ^2- (also named Ox^2- ) shows much better device performance than the salt with hypophosphite anion (H₂ PO₂ ^- ), especially under the thick film condition (e.g., 50 nm). The greatly enhanced performances of interfacial material doped by Ox^2- are due to reduced R_s between the active layer material and the electrode, reduced dark-current, improved charge transport and extraction efficiency, and decreased charge recombination for the devices at thick-film condition. These results demonstrated that n-doping could be a great potential strategy for making thickness-insensitive interfacial layers, besides, the performances can be further improved by carefully selecting salts. This article is protected by copyright. All rights reserved.

Synthesis of a Low-Cost Thiophene-Indoloquinoxaline Polymer Donor and Its Application to Polymer Solar Cells

A simple wide-bandgap conjugated polymer based on indoloquinoxaline unit (PIQ) has been newly designed and synthesized via cheap and commercially available starting materials. The basic physicochemical properties of the PIQ have been investigated. PIQ possesses a broad and strong absorption band in the wavelength range of 400~660 nm with a bandgap of 1.80 eV and lower-lying highest occupied molecular orbital energy level of −5.58 eV. Polymer solar cells based on PIQ and popular acceptor Y6 blend display a preliminarily optimized power conversion efficiency of 6.4%. The results demonstrate indoloquinoxaline is a promising building unit for designing polymer donor materials for polymer solar cells.

Impact of the segment ratio on a donor-acceptor all-conjugated block copolymer in single-component organic solar cells

The development of single-component organic solar cells (SCOSCs) using only one photoactive component with a chemically bonded D/A structure has attracted increasing research attention in recent years. At represent, most relevant studies focus on comparing the performance difference between a donor-acceptor (D-A) conjugated block copolymer (CBC) and the commensurate blending systems based on the same donor and acceptor segments, and still there are no reports on the impact of the segment ratio for a certain D-A CBC on the resultant photovoltaic performance. In this study, we synthesized a D-A all-conjugated polymers based on an n-type PNDI2T block and a p-type PBDB-T donor block but with three different segment ratios (P1-P3) and demonstrate the significance of the D/A segment ratio on photovoltaic performance. Our results reveal that the n-type PNDI2T block plays a more critical role in the inter/intra-chain charge transfer. P1 with a higher content of PNDI2T delivers superior exciton dissociation and charge transfer behavior than P2 and P3, benefitting from its more balanced hole/electron mobility. In addition, a higher packing regularity associated with a more dominant face-on orientation is also observed for P1. As a result, SCOSC based on P1 exhibits the highest PCE among the synthesized CBCs. It also possesses a minimal energy loss due to the better suppressed non-radiative recombination loss. This work provides the first discussion of the impact of the segment ratio for a D-A all-conjugated block copolymer and signifies the critical role of the n-type segment in designing high-performance single-component CBCs.

Meniscus-Assisted Coating with Optimized Active-Layer Morphology toward Highly Efficient All-Polymer Solar Cells

Morphology control is the key to engineering highly efficient solution-processed solar cells. Focusing on the most promising application-oriented photovoltaic all-polymer solar cells (all-PSCs), herein a facile and effective meniscus-assisted-coating (MAC) strategy is reported for preparing high-quality blend films with enhanced crystallinity and an interpenetrating nanofiber network morphology. The all-PSCs based on MAC exhibit excellent optoelectronic properties with efficiencies exceeding 15%, which is the best performance of solution-printing-based all-PSCs, as well as better stability. The crystallization kinetics of the polymer blend film is investigated by in situ UV-vis absorption spectroscopy, and the result explains the linear relationship between the meniscus advance speed and the crystallinity (crystallization rate) of the polymer. To verify the compatibility and universality of this strategy, the MAC strategy is applied to the other three binary systems. By precisely controlling the meniscus advancing speed, 1 cm² all-PSC devices whose efficiencies exceed 12% are fabricated. Such progress demonstrates that the application of the MAC strategy is a promising approach for the fabrication of high-efficiency all-PSCs.

Tailoring microstructure and morphology via sequential fluorination to enhance the photovoltaic performance of low-cost polymer donors for organic solar cells

For utilizing the organic solar cells for commercial applications, reducing the overall cost of the photo absorbent materials is also very crucial, along with the realization of high power conversion efficiency (PCE) and excellent stability. Herein, we tried to address such challenge by synergistically controlling the amount of fluorine (F)-substituents (n = 2, 4) on easily scalable, low-cost wide-bandgap molecular design involving alternate fluorinated-thienyl benzodithiophene donor and 2,5-difluoro benzene (2FBn) or 2,3,5,6 tetrafluorobenzene (4FBn) to form two new polymer donors PBDT-2FBn and PBDT-4FBn, respectively. As expected, sequential fluorination causes lowering of the frontier energy levels and planarization of polymer backbone via F···S and C-H···F noncovalent molecular locks, which results in more pronounced molecular packing and enhanced crystallinity from PBDT-2FBn to PBDT-4FBn. By mixing with IT-4F acceptor, PBDT-2FBn:IT-4F-based blend demonstrated favorable molecular orientation with shorter π-π stacking distance, higher carrier mobilities with good trade-off ratio and desirable nanoscale morphology, hence delivered good PCE of 9.3% than PBDT-2FBn:IT-4F counterpart (8.6%). Furthermore, pairing PBDT-2FBn with BTP-BO-4Cl acceptor further improved absorption range and promoted privileged morphology with ideal domain sizes for efficient exciton dissociation and charge transport, resulting in further improvement of PCE to 10.2% with remarkably low energy loss of 0.46 eV, which is seldomly reported in NF-OSCs. Consequently, this study provides valuable guidelines for designing efficient and low-cost polymer donors for organic solar cell applications. This article is protected by copyright. All rights reserved.

Nonalloy Model-Based Ternary Organic Solar Cells

Ternary blending based on an alloy-like model has been proved as an efficient strategy for high-efficiency organic solar cells (OSCs). However, the third component that possesses excellent miscibility with host materials in the alloy-like model may trigger adverse effects for the active layer, especially at a high doping ratio. In this work, we propose a new concept of nonalloy model for the ternary OSCs in which the third component presents moderate miscibility with the acceptor and distributes at the interspace between donor and acceptor domains. The nonalloy model is constructed based on the PM6:Y6 system, and a Y6 analogue (BTP-MCA) is synthesized as the third component. The BTP-MCA can maintain initial excellent morphology of the active layer and enhance the morphological stability by acting as a frame around the host materials. As a result, ternary OSCs based on the PM6:Y6:BTP-MCA blend exhibit an impressive efficiency of 17.0% with a high open-circuit voltage of 0.87 V. Moreover, the devices present a high doping tolerance (keeping high efficiency with a doping ratio of 50%) and improved stability. This work indicates that the nonalloy model can be a promising method to fabricate efficient and stable ternary OSCs apart from the conventional alloy-like model.

Recent Progress in Organic Solar Cells: A Review on Materials from Acceptor to Donor

In the last few decades, organic solar cells (OSCs) have drawn broad interest owing to their advantages such as being low cost, flexible, semitransparent, non-toxic, and ideal for roll-to-roll large-scale processing. Significant advances have been made in the field of OSCs containing high-performance active layer materials, electrodes, and interlayers, as well as novel device structures. Particularly, the innovation of active layer materials, including novel acceptors and donors, has contributed significantly to the power conversion efficiency (PCE) improvement in OSCs. In this review, high-performance acceptors, containing fullerene derivatives, small molecular, and polymeric non-fullerene acceptors (NFAs), are discussed in detail. Meanwhile, highly efficient donor materials designed for fullerene- and NFA-based OSCs are also presented. Additionally, motivated by the incessant developments of donor and acceptor materials, recent advances in the field of ternary and tandem OSCs are reviewed as well.

Construction of Laterally Asymmetric Heterojunctions with Sub-Micrometer Resolution by Hierarchical Self-Assembly of Polythiophene Nanofibers

Polymeric semiconductors are crucial candidates for the construction of next-generation flexible and printable electronic devices. By virtue of the successful preparation of monodispersed colloidal solution in orthogonal solvent, poly(3-hexylthiophene) (P3HT) nanofibers are developed into versatile building blocks for nanoelectronics and their compatibilities are verified with photolithographic lift-off technology. Then, the joint efforts from both the bottom-up hierarchical self-assembly and top-down self-alignment technology have led to the realization of lateral asymmetric heterojunctions with resolution better than 1 µm. As a result, planar photovoltaic devices incorporating N,N'-dioctyl-3,4,9,10-perylenedicarboximide and P3HT supramolecular nanowires as active components are constructed with the cathode-to-anode distance being tuned from ≈0.1 to 1-2 µm. Based on such a novel device configuration, an interesting phenomenon of channel-length-dependent photovoltaic efficiency is observed for the first time, strongly suggesting the impact of near-field light intensity on the performance of nanophotonic devices.

Novel Pyrrolo [3,4-b] dithieno [3, 2-f:2",3"-h] quinoxaline-8,10 (9H)-dione Based Wide Bandgap Conjugated Copolymers for Bulk Heterojunction Polymer Solar Cells

Two D-A copolymers consisted of fused ring pyrrolo-dithieno-quinoxaline acceptor are synthesized with different donor units, i.e., benzodithiophene (BDT) with alkylthienyl (P134) and 2-ethylhexyloxy (P117) side chains. These copolymers are used as donor and a narrow bandgap acceptor Y6 to fabricate bulk heterojunction polymer solar cell devices. Owing to the strong electron-deficient fused ring pyrrolo-bithieno-quinoxaline and weak alkyl thienyl side chains in BDT, the polymer solar cells based on P134:Y6 attained the power conversion efficiency of 15.42%, which is higher than P117:Y6 counterpart (12.14%). The superior value of PCE for P134:Y6 could be associated with more well-adjusted charge transport, weak charge recombination, proficient exciton generation and dissociation into free charge carriers and their subsequent charge collection owing to the dense π-π stacking distance and more considerable crystal coherence length for the P134:Y6 thin films. This investigation confirms the great potential of a strong acceptor-weak donor tactic for developing efficient D-A copolymers consists of quinoxaline acceptor for polymer solar cells. This article is protected by copyright. All rights reserved.

Mutual Diffusion of Model Acceptor/Donor Bilayers under Solvent Vapor Annealing as a Novel Route for Organic Solar Cell Fabrication

The fabrication of bulk heterojunction organic solar cells (OSCs) is primarily based on a phase demixing during solution deposition. This spontaneous process is triggered when, as a result of a decrease in the solvent concentration, interactions between donor and acceptor molecules begin to dominate. Herein, we present that interdiffusion of the same molecules is possible when a bilayers of donors and acceptors are exposed to solvent vapor. Poly(3-hexyl thiophene) (P3HT), and poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT) were used as donors and two types of fullerene derivatives were used as acceptors: phenyl-C61-butyric acid methyl ester (PC60BM) and phenyl-C71-butyric acid methyl ester (PC70BM), Secondary ion mass spectrometry depth profiling revealed that the interpenetration of donors and acceptors induced by solvent vapor annealing was dependent on solvent vapor and component compatibility. Exposure to chloroform vapor resulted in a complete intermixing of both components. The mutual mixing increased efficiency of inverted solar cells prepared by solvent vapor annealing of model donor/acceptor bilayers. These results provide a new means for mixing incompatible components for the fabrication of organic solar cells.

Sources of Thermal Power Generation and Their Influence on the Operating Temperature of Organic Solar Cells

Thermal stability, closely associated with the operating temperature, is one of the desired properties for practical applications of organic solar cells (OSCs). In this paper, an OSC of the structure of ITO/PEDOT:PSS/P₃HT:PCBM/ZnO/Ag was fabricated, and its current-voltage (J-V) characteristics and operating temperature were measured. The operating temperature of the same OSC was simulated using an analytical model, taking into consideration the heat transfer, charge carrier drift-diffusion and different thermal generation processes. The simulated results agreed well with the experimental ones. It was found that the thermalization of charge carriers above the band gap had the highest influence on the operating temperature of the OSCs. The energy off-set at the donor/acceptor interface in the bulk heterojunction (BHJ) was shown to have a negligible impact on the thermal stability of the OSCs. However, the energy off-sets at the electrode/charge-transporting layer and BHJ/charge-transporting layer interfaces had greater impacts on the operating temperature of OSCs at the short circuit current and maximum power point conditions. Our results revealed that a variation over the energy off-set range from 0.1 to 0.9 eV would induce an almost 10-time increase in the corresponding thermal power generation, e.g., from 0.001 to 0.01 W, in the cells operated at the short circuit current condition, contributing to about 16.7% of the total solar power absorbed in the OSC.

Recycling Heterogenous Catalysts for Multi-Batch Conjugated Polymer Synthesis via Direct Arylation Polymerization

Despite the inherent sustainability direct arylation polymerization (DArP) offers through a C-H activation pathway, the use of expensive homogeneous Pd catalysts remains problematic for large-scale conjugated polymer (CP) synthesis. Herein, the first report on the recycling of heterogeneous catalysts for CP synthesis using DArP is presented. We found SiliaCat Pd-DPP to be a highly efficient and recyclable catalyst for multi-batch CP synthesis providing CPs with molecular weights (M_n) up to 82 kg/mol even after being recycled three times. Batch-to-batch variations were further optimized to afford up to five batches of polymers with a M_n of 25 ± 2.5 kg/mol without structural disparity. Significantly, this work discloses among the most sustainable CP synthesis protocols to date and presents the critical concept of catalyst-recycling to the important field of organic semiconducting polymers, which potentially enables access to truly low-cost flow chemistry for industrial-scale CP synthesis.

A facile depolymerization reaction enables efficient feedstock recycling of polymerized small molecule acceptors in all-polymer solar cells

A facile depolymerization reaction of polymerized small molecule acceptors (PSMAs) is developed for efficiently recovering the polymeric monomers in various circumstances, showing potential on decreasing the cost of PSMA-based all-PSCs.

Pressure-induced polymerization and bandgap-adjustment of TPEPA

Organic solar cells have become an important development direction in solar cell materials because of their low cost, light weight, and good flexibility. However, the size of their bandgap is difficult to continuously regulate, resulting in a low power conversion efficiency. In this work, an organic molecule TPEPA was synthesized, and its luminescence performance and polymerization under high pressure were studied by performing in situ Raman, IR, fluorescence, and UV-vis spectroscopy. The Raman and IR spectroscopic results show that single bonds (C–H, C–Ph) and long chains (C–C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C–C) are more unstable and prone to amorphization under high pressure. At 10 GPa, the TPEPA molecule undergoes a transition of amorphization accompanied by a few polymerizations in the CC bond structure. After holding pressure at 20 GPa for one day and releasing to ambient pressure, the other peaks almost disappeared, while the new peak of C(sp³)–H from the polymerization of the benzene ring was observed, indicating that the irreversible amorphization and polymerization did occur. UV-vis spectra results show that the bandgap is reduced from 2.9 eV to 1.3 eV, which is just in the maximum conversion efficiency bandgap range (1.3–1.4 eV) of p–n junction solar cell materials. This pressure is within the working pressure range of a large volume press, which is favorable in applications of large-scale synthesis. Our strategy may provide a method for the large-scale synthesis of novel organic solar cell materials.

Organic solar cells have become an important development direction in solar cell materials because of their low cost, light weight, and good flexibility.

Insights into the Structural and Morphological Properties of Layer-by-Layer Processed Organic Photovoltaics

Recently, with the development of figure-of-merit non-fullerene acceptor materials combined with a ternary strategy and layer-by-layer (LbL) processing, the efficiency of single-junction organic solar cells has exceeded 18%. However, the structural properties of LbL-processed films have not been sufficiently elucidated. Herein, we systematically investigate films fabricated via LbL processing of three different systems, including a ternary system. In particular, we focus on the structural and morphological transitions associated with the diffusion process controlled by thermal annealing and an additive solvent. Different diffusion and crystal formation mechanisms were clearly identified, which were observed to be dependent on the characteristics of the upper layer formed during the LbL process. Based on this insight, the photovoltaic properties associated with various LbL conditions are elucidated, and an ideal path toward a better device is suggested.

Binary and Ternary Polymer Solar Cells Based on a Wide Bandgap D-A Copolymer Donor and Two Nonfullerene Acceptors with Complementary Absorption Spectral

A new wide-bandgap conjugated D-A polymer denoted as P106 with a medium acceptor dithieno [2,3-e;3'2'-g]isoindole-7,9 (8H) (DTID) unit and strong 2-dodecylbenzo[1,2-b:3,4-b':6,5-b"]trithiophene (3TB) donor units shows an optical bandgap of 2.04 and highest occupied molecular orbital energy level of -5.56 eV. P106 is used as the donor and two nonfullerene acceptors-medium bandgap DBTBT-IC and narrow band Y18-DMO-are used as acceptors for the construction of binary and ternary bulk heterojunction polymer solar cells. The optimized polymer solar cells based on P106 : DBTBT-IC and P106 : Y18-DMO exhibit power conversion efficiencies of 11.76 % and 14.07 %, respectively. The short-circuit current density (22.78 mA cm^-2 ) for the P106 : Y18-DMO device is higher than that for P106 : DBTBT-IC (18.56 mA cm^-2 ) one, which could be attributed to the more photon harvesting efficiency of the P106 : Y18-DMO active layer. In light of the high short-circuit current densities and fill factors for the Y18-DMO based device and the high value of open circuit voltage of the DBTBT-IC based device, ternary polymer solar cells are fabricated by using ternary active layer (P106 : DBTBT-IC : Y18-DMO) and achieve a power conversion efficiency of 16.49 % with low energy loss of 0.47 eV.

Multi-Functional Solid Additive Induced Favorable Vertical Phase Separation and Ordered Molecular Packing for Highly Efficient Layer-by-Layer Organic Solar Cells

Layer-by-layer (LBL) deposition strategy enabling favorable vertical phase distributions has been regarded as promising candidates for constructing high-efficient organic photovoltaic (OPV) cells. However, solid additives with the merits of good stability and reproducibility have been rarely used to fine-tune the morphology of the LBL films for improved efficiency and stability. Herein, hierarchical morphology control in LBL OPV is achieved via a dual functional solid additive. Series of LBL devices are fabricated by introducing the solid additive individually or simultaneously to the donor or acceptor layer to clarify the functions of additives. Additive in the donor layer can facilitate the formation of preferable vertical component distribution, and that in the acceptor layer will enhance the molecular crystallinity for better charge transport properties. The optimized morphology ultimately contributed to high PCEs of 16.4% and 17.4% in the binary and quaternary LBL devices. This reported method provides an alternative way to controllably manipulate the morphology of LBL OPV cells.

Achieving over 10 % Efficiency in Poly(3-hexylthiophene)-Based Organic Solar Cells via Solid Additives

The photovoltaic performance of organic solar cells (OSCs) based on poly(3-hexylthiophene) (P3HT) has been steadily improved by developing novel non-fullerene acceptors (NFAs) in recent years. Herein, to further improve the performance of P3HT-based OSCs, a solid additive (SA4) and a typical solvent additive [1,8-diiodooctane (DIO)] were employed to process P3HT:ZY-4Cl-based OSCs, respectively. In comparison with the DIO-processed device, the SA4-processed one exhibited a more ordered molecular packing and more favorable phase separation, leading to enhanced charge transport and reduced carrier recombination. As a result, the SA4-processed device delivered a power conversion efficiency (PCE) of 10.24 %, which was much higher than that of the DIO-processed counterpart (6.26 %). This work reported a PCE over 10 % in P3HT-based OSCs for the first time, indicating the promising development of P3HT-based OSCs by morphological modulation.

Suppressing Energetic Disorder Enables Efficient Indoor Organic Photovoltaic Cells With a PTV Derivative

Indoor organic photovoltaics (IOPVs) cells have attracted considerable attention in the past few years. Herein, two PTV-derivatives, PTVT-V and PTVT-T, were used as donor materials to fabricate IOPV cells with ITCC as the acceptor. The preferred orientation of the crystals changed from edge-on to face-on after replacing the ethylene in the backbones of PTVT-V by the thiophene in that of PTVT-T. Besides, it was found that, the energetic disorder of the PTVT-T:ITCC based system is 58 meV, which is much lower than that of PTVT-V:ITCC-based system (70 meV). The lower energetic disorder in PTVT-T:ITCC leads to an efficient charge transfer, charge transport, and thus the weak charge recombination. As a result, a PCE of 9.60% under AM 1.5 G and a PCE of 24.27% under 1,000 lux (LED 2,700 K) with a low non-radiative energy loss of 0.210 eV were obtained based on PTVT-T:ITCC blend. The results indicate that to improve the PTV-derivatives photovoltaic properties by suppressing the energetic disorder is a promising way to realize low-cost IOPV cells.

Studying the Effect of High Substrate Temperature on the Microstructure of Vacuum Evaporated TAPC: C60 Organic Solar Thin Films

Organic solar cells (OSCs), also known as organic photovoltaics (OPVs), are an emerging solar cell technology composed of carbon-based, organic molecules, which convert energy from the sun into electricity. Key for their performance is the microstructure of the light-absorbing organic bulk heterojunction. To study this, organic solar films composed of both fullerene C₆₀ as electron acceptor and different mole percentages of di-[4-(N,N-di-p-tolyl-amino)-phenyl]-cyclohexane (TAPC) as electron donor were evaporated in vacuum in different mixing ratios (5, 50 and 95 mol%) on an ITO-coated glass substrate held at room temperature and at 110 °C. The microstructure of the C₆₀: TAPC heterojunction was studied by grazing incidence wide angle X-ray scattering to understand the effect of substrate heating. By increasing the substrate temperature from ambient to 110 °C, it was found that no significant change was observed in the crystal size for the C₆₀: TAPC concentrations investigated in this study. In addition to the variation done in the substrate temperature, the variation of the mole percent of the donor (TAPC) was studied to conclude the effect of both the substrate temperature and the donor concentration on the microstructure of the OSC films. Bragg peaks were attributed to C₆₀ in the pure C₆₀ sample and in the blend with low donor mole percentage (5%), but the C₆₀ peaks became nondiscernible when the donor mole percentage was increased to 50% and above, showing that TAPC interrupted the formation of C₆₀ crystals.