A Dual-Polythiophene Blending Strategy to Reduce the Efficiency-Stability-Cost Gap of Solar Cells.

Benefiting from the photovoltaic material innovation and delicate device optimization, high-efficiency solar cells employing polymeric materials are thriving. Reducing the gap of cost, efficiency, and stability is the critical challenge faced by the emerging solar cells such as organics, quantum dots and perovskites. Poly(3-alkylthiophene) demonstrates great potential in organic solar cells and quantum dot solar cells as the active layer or the hole transport layer due to its large scalability, excellent photoelectric performance, and favorable hydrophobicity. The present low efficiency and insufficient stability, restrict its commercial application. In this work, a facile strategy of blending two simple polythiophenes is put forward to manipulate the film microstructure and enhance the device efficiency and thermal stability of solar cells. The introduction of P3PT can improve the power conversion efficiency (PCE) of a benchmark cost-effective blend P3HT:O-IDTBR to 7.41%, and the developed ternary solar cells also exhibit increased thermal stability. More strikingly, the quantum dot solar cells with the dual-polythiophene hole transport layer achieve the highest PCE of 10.51%, which is among the topmost efficiencies for quantum dots/polythiophene solar cells. Together, this work provides an effective route to simultaneously optimize the device efficiency and thermal stability of solar cells.

Design rules for minimizing voltage losses in high-efficiency organic solar cells.

The open-circuit voltage of organic solar cells is usually lower than the values achieved in inorganic or perovskite photovoltaic devices with comparable bandgaps. Energy losses during charge separation at the donor-acceptor interface and non-radiative recombination are among the main causes of such voltage losses. Here we combine spectroscopic and quantum-chemistry approaches to identify key rules for minimizing voltage losses: (1) a low energy offset between donor and acceptor molecular states and (2) high photoluminescence yield of the low-gap material in the blend. Following these rules, we present a range of existing and new donor-acceptor systems that combine efficient photocurrent generation with electroluminescence yield up to 0.03%, leading to non-radiative voltage losses as small as 0.21 V. This study provides a rationale to explain and further improve the performance of recently demonstrated high-open-circuit-voltage organic solar cells.

A Wide Band Gap Polymer with a Deep Highest Occupied Molecular Orbital Level Enables 14.2% Efficiency in Polymer Solar Cells.

To simultaneously achieve low photon energy loss ( Eloss) and broad spectral response, the molecular design of the wide band gap (WBG) donor polymer with a deep HOMO level is of critical importance in fullerene-free polymer solar cells (PSCs). Herein, we developed a new benzodithiophene unit, i.e., DTBDT-EF, and conducted systematic investigations on a WBG DTBDT-EF-based donor polymer, namely, PDTB-EF-T. Due to the synergistic electron-withdrawing effect of the fluorine atom and ester group, PDTB-EF-T exhibits a higher oxidation potential, i.e., a deeper HOMO level (ca. -5.5 eV) than most well-known donor polymers. Hence, a high open-circuit voltage of 0.90 V was obtained when paired with a fluorinated small molecule acceptor (IT-4F), corresponding to a low Eloss of 0.62 eV. Furthermore, side-chain engineering demonstrated that subtle side-chain modulation of the ester greatly influences the aggregation effects and molecular packing of polymer PDTB-EF-T. With the benefits of the stronger interchain π-π interaction, the improved ordering structure, and thus the highest hole mobility, the most symmetric charge transport and reduced recombination are achieved for the linear decyl-substituted PDTB-EF-T (P2)-based PSCs, leading to the highest short-circuit current density and fill factor (FF). Due to the high Flory-Huggins interaction parameter (χ), surface-directed phase separation occurs in the P2:IT-4F blend, which is supported by X-ray photoemission spectroscopy results and cross-sectional transmission electron microscope images. By taking advantage of the vertical phase distribution of the P2:IT-4F blend, a high power conversion efficiency (PCE) of 14.2% with an outstanding FF of 0.76 was recorded for inverted devices. These results demonstrate the great potential of the DTBDT-EF unit for future organic photovoltaic applications.

Molecular Design of Benzodithiophene-Based Organic Photovoltaic Materials.

Advances in the design and application of highly efficient conjugated polymers and small molecules over the past years have enabled the rapid progress in the development of organic photovoltaic (OPV) technology as a promising alternative to conventional solar cells. Among the numerous OPV materials, benzodithiophene (BDT)-based polymers and small molecules have come to the fore in achieving outstanding power conversion efficiency (PCE) and breaking 10% efficiency barrier in the single junction OPV devices. Remarkably, the OPV device featured by BDT-based polymer has recently demonstrated an impressive PCE of 11.21%, indicating the great potential of this class of materials in commercial photovoltaic applications. In this review, we offered an overview of the organic photovoltaic materials based on BDT from the aspects of backbones, functional groups, alkyl chains, and device performance, trying to provide a guideline about the structure-performance relationship. We believe more exciting BDT-based photovoltaic materials and devices will be developed in the near future.

Molecular Optimization Enables over 13% Efficiency in Organic Solar Cells.

A new polymer donor (PBDB-T-SF) and a new small molecule acceptor (IT-4F) for fullerene-free organic solar cells (OSCs) were designed and synthesized. The influences of fluorination on the absorption spectra, molecular energy levels, and charge mobilities of the donor and acceptor were systematically studied. The PBDB-T-SF:IT-4F-based OSC device showed a record high efficiency of 13.1%, and an efficiency of over 12% can be obtained with a thickness of 100-200 nm, suggesting the promise of fullerene-free OSCs in practical applications.

Kornblum Oxidation Reaction-Induced Collective Transformation of Lead Polyhalides for Stable Perovskite Photovoltaics.

The iodide vacancy defects generated during the perovskite crystallization process are a common issue that limits the efficiency and stability of perovskite solar cells (PSCs). Although excessive ionic iodides have been used to compensate for these vacancies, they are not effective in reducing defects through modulating the perovskite crystallization. Moreover, these iodide ions present in the perovskite films can act as interstitial defects, which are detrimental to the stability of the perovskite. Here, an effective approach to suppress the formation of vacancy defects by manipulating the coordination chemistry of lead polyhalides during perovskite crystallization is demonstrated. To achieve this suppression, an α-iodo ketone is introduced to undergo a process of Kornblum oxidation reaction that releases halide ions. This process induces a rapid collective transformation of lead polyhalides during the nucleation process and significantly reduces iodide vacancy defects. As a result, the ion mobility is decreased by one order of magnitude in perovskite film and the PSC achieves significantly improved thermal stability, maintaining 82% of its initial power conversion efficiency at 85 °C for 2800 h. These findings highlight the potential of halide ions released by the Kornblum oxidation reaction, which can be widely used for achieving high-performance perovskite optoelectronics.

The Role of Solution Aggregation Property toward High-Efficiency Non-Fullerene Organic Photovoltaic Cells.

In organic photovoltaic cells, the solution-aggregation effect (SAE) is long considered a critical factor in achieving high power-conversion efficiencies for polymer donor (PD)/non-fullerene acceptor (NFA) blend systems. However, the underlying mechanism has yet to be fully understood. Herein, based on an extensive study of blends consisting of the representative 2D-benzodithiophene-based PDs and acceptor-donor-acceptor-type NFAs, it is demonstrated that SAE shows a strong correlation with the aggregation kinetics during solidification, and the aggregation competition between PD and NFA determines the phase separation of blend film and thus the photovoltaic performance. PDs with strong SAEs enable earlier aggregation evolutions than NFAs, resulting in well-known polymer-templated fibrillar network structures and superior PCEs. With the weakening of PDs' aggregation effects, NFAs, showing stronger tendencies to aggregate, tend to form oversized domains, leading to significantly reduced external quantum efficiencies and fill factors. These trends reveal the importance of matching SAE between PD and NFA. The aggregation abilities of various materials are further evaluated and the aggregation ability/photovoltaic parameter diagrams of 64 PD/NFA combinations are provided. This work proposes a guiding criteria and facile approach to match efficient PD/NFA systems.

Facile Modification of a Noncovalently Fused-Ring Electron Acceptor Enables Efficient Organic Solar Cells.

Electron acceptors with nonfused aromatic cores (NCAs) have aroused increasing interest in organic solar cells due to the low synthetic complexity and flexible chemical modification, but the corresponding device performance still lags behind. Herein, we designed and synthesized two new quinoxaline-based NCAs, namely, QOC6-4H and QOC6-4Cl. Although both NCAs show good backbone coplanarity, QOC6-4Cl with chlorinated end groups exhibits higher extinction coefficient, enhanced crystallinity, and more compact π-π stacking, which is correlated with the stronger intermolecular interactions induced by chlorine atoms. Benefiting from the broader and stronger optical absorption, improved carrier mobilities, and suppressed charge recombination, a notable power conversion efficiency (PCE) of 12.32% with a distinctly higher short-current density (Jsc) of 22.91 mA cm-2 and a fill factor (FF) of 69.01% could be obtained for the PBDB-T:QOC6-4Cl-based device. The PCEs of PBDB-T:QOC6-4H were only lower than 8%, which could mainly be attributed to the unsymmetric charge transport. Our work proves that the chlorination of end groups is a facile and effective strategy to enhance the intermolecular interactions and thus the photovoltaic performance of NCAs, and a careful modulation of the intermolecular interactions plays a vital role in further developing both high-performance and low-cost organic photovoltaic materials.

Reduced Nonradiative Recombination Energy Loss Enabled Efficient Polymer Solar Cells via Tuning Alkyl Chain Positions on Pendent Benzene Units of Polymers.

Nonradiative recombination energy loss (ΔE3) plays a key role in enhancing device efficiencies for polymer solar cells (PSCs). Until now, there is no clear resolution for reducing ΔE3 via molecular design. Herein, we report two conjugated polymers, PBDB-P-p and PBDB-P-m, which are integrated from benzo[1,2-b:4,5-b']dithiophene with alkylthio chain substituted at para- or meta-position on pendent benzene and benzo[1,2-c:4,5-c']dithiophene-4,8-dione. Both the polymers have different temperature-dependent aggregation properties but similar molecular energy levels. When BO-4Cl was used as an acceptor to fabricate PSCs, the device of PBDB-P-p:BO-4Cl displayed a maximal power conversion efficiency (PCE) of 13.83%, while the best device of PBDB-P-m:BO-4Cl exhibited a higher PCE of 14.12%. The close JSCs and fill factors in both PSCs are attributed to their formation of effective nanoscale phase separation as confirmed by atomic force microscopy measurements. We find that the PBDB-P-m-based device has 1 order of magnitude higher electroluminescence quantum efficiency (EQEEL) than in the PBDB-P-p-based one, which could arise from the relatively weak aggregation in the PBDB-P-m-based film. Thus, the PBDB-P-m-based device has a remarkably enhanced VOC of 0.86 V in contrast to 0.80 V in the PBDB-P-p-based device. This study offers a feasible structural optimization way on the alkylthio side chain substitute position on the conjugated polymer to enhance VOC by reducing nonradiative recombination energy loss in the resulting PSCs.

A Self-Organized Poly(vinylpyrrolidone)-Based Cathode Interlayer in Inverted Fullerene-Free Organic Solar Cells.

Herein, poly(vinylpyrrolidone) (PVP) is used as the cathode interlayer (CIL) through the self-organization method in inverted organic solar cells (OSCs). By coating a solution of PVP and active layer materials onto a glass/indium tin oxide (ITO) substrate, the PVP can segregate to the near ITO side due to its high surface energy and strong intermolecular interaction with the ITO electrode. The power conversion efficiency (PCE) of the obtained OSC device reaches 13.3%, much higher than that of the control device with a PCE of only 10.1%. The improvement results from the increased exciton dissociation efficiency and the depressed trap-assisted recombination, which can be attributed to the reduced work function of the cathode by the self-organized PVP. Additionally, the molecular weight of the PVP has almost no influence on the device performance, and the PVP-modified device presents superior stability. This method can also be applied in other highly efficient fullerene-free OSCs, and with a fine selection of the active layer, a high PCE of 14.0% is obtained. Overall, this work demonstrates the great potential of the PVP-based CIL in inverted OSCs fabricated via the self-organization method.

p-Doped Conducting Polyelectrolyte as an Anode Interlayer Enables High Efficiency for 1 cm2 Printed Organic Solar Cells.

Manufacturing large-area devices through a low-cost and large batch printing technique is the key to the commercialization of organic solar cells (OSCs). However, the lack of printable anode interlayer (AIL) materials severely impedes the development of high-efficiency printed OSCs. Herein, we synthesize three p-type self-doped conjugated polyelectrolytes (CPEs), namely, PCP-B, PCP-2B, and PCP-3B, as printable AIL materials for fabricating high-performance and large-area OSCs. By increasing the number of benzene units in the polymer backbone, the work function of the CPEs was enhanced from 4.57 to 5.01 eV, and the optical transparency was also improved because of the enlarged polymer band gap. The improved photoelectronic properties as well as a good film-forming capacity make the PCP-3B an ideal AIL material to be processed by the printing technique. By using PCP-3B, a 1 cm2 printed device was fabricated in which all the functional layers, including the AIL, active layer, and cathode interlayer were processed by blade-coating, achieving a power conversion efficiency (PCE) of 9.67%. The PCE belongs to the highest efficiency at present for printable large-area OSCs, showing a promising prospect for the OSC mass production.

A High-Efficiency Organic Solar Cell Enabled by the Strong Intramolecular Electron Push-Pull Effect of the Nonfullerene Acceptor.

Besides broadening of the absorption spectrum, modulating molecular energy levels, and other well-studied properties, a stronger intramolecular electron push-pull effect also affords other advantages in nonfullerene acceptors. A strong push-pull effect improves the dipole moment of the wings in IT-4F over IT-M and results in a lower miscibility than IT-M when blended with PBDB-TF. This feature leads to higher domain purity in the PBDB-TF:IT-4F blend and makes a contribution to the better photovoltaic performance. Moreover, the strong push-pull effect also decreases the vibrational relaxation, which makes IT-4F more promising than IT-M in reducing the energetic loss of organic solar cells. Above all, a power conversion efficiency of 13.7% is recorded in PBDB-TF:IT-4F-based devices.

Design and application of volatilizable solid additives in non-fullerene organic solar cells.

Most of the high-performance organic solar cells are fabricated with the assistance of high-boiling-point solvent additives to optimize their charge transport properties; this has adverse effects on the OSCs' stability and reproducibility in large-scale production. Here, we design volatilizable solid additives by considering the molecular structure feature of an acceptor-donor-acceptor-type non-fullerene acceptor. The application of solid additives can enhance the intermolecular π-π stacking of the non-fullerene acceptor and thus facilitate the charge transport properties in the active layers, leading to improved efficiencies of OSCs. Importantly, devices fabricated using volatilizable solid additives exhibit higher stability and reproducibility when compared with the OSCs processed with solvent additives. Our results not only demonstrate an approach of applying volatilizable solid additives to benefit the large-scale production of OSCs but also provide a potential direction for designing specific solid additives for different active layers.

Environmentally Friendly Solvent-Processed Organic Solar Cells that are Highly Efficient and Adaptable for the Blade-Coating Method.

The power conversion efficiencies (PCEs) of state-of-the-art organic solar cells (OSCs) have increased to over 13%. However, the most commonly used solvents for making the solutions of photoactive materials and the coating methods used in laboratories are not adaptable for future practical production. Therefore, taking a solution-coating method with environmentally friendly processing solvents into consideration is critical for the practical utilization of OSC technology. In this study, a highly efficient PBTA-TF:IT-M-based device processed with environmentally friendly solvents, tetrahydrofuran/isopropyl alcohol (THF/IPA) and o-xylene/1-phenylnaphthalene, is fabricated; a high PCE of 13.1% can be achieved by adopting the spin-coating method, which is the top result for OSCs. More importantly, a blade-coated non-fullerene OSC processed with THF/IPA is demonstrated for the first time to obtain a promising PCE of 11.7%; even for the THF/IPA-processed large-area device (1.0 cm2 ) made by blade-coating, a PCE of 10.6% can still be maintained. These results are critical for the large-scale production of highly efficient OSCs in future studies.

Surpassing 10% Efficiency Benchmark for Nonfullerene Organic Solar Cells by Scalable Coating in Air from Single Nonhalogenated Solvent.

The commercialization of nonfullerene organic solar cells (OSCs) critically relies on the response under typical operating conditions (for instance, temperature and humidity) and the ability of scale-up. Despite the rapid increase in power conversion efficiency (PCE) of spin-coated devices fabricated in a protective atmosphere, the efficiencies of printed nonfullerene OSC devices by blade coating are still lower than 6%. This slow progress significantly limits the practical printing of high-performance nonfullerene OSCs. Here, a new and relatively stable nonfullerene combination is introduced by pairing the nonfluorinated acceptor IT-M with the polymeric donor FTAZ. Over 12% efficiency can be achieved in spin-coated FTAZ:IT-M devices using a single halogen-free solvent. More importantly, chlorine-free, blade coating of FTAZ:IT-M in air is able to yield a PCE of nearly 11% despite a humidity of ≈50%. X-ray scattering results reveal that large π-π coherence length, high degree of face-on orientation with respect to the substrate, and small domain spacing of ≈20 nm are closely correlated with such high device performance. The material system and approach yield the highest reported performance for nonfullerene OSC devices by a coating technique approximating scalable fabrication methods and hold great promise for the development of low-cost, low-toxicity, and high-efficiency OSCs by high-throughput production.

Potential of Nonfullerene Small Molecules with High Photovoltaic Performance.

Over the past decades, fullerene derivatives have become the most successful electron acceptors in organic solar cells (OSCs) and have achieved great progress, with power conversion efficiencies (PCEs) of over 11 %. However, fullerenes have some drawbacks, such as weak absorption, limited energy-level tunability, and morphological instability. In addition, fullerene-based OSCs usually suffer from large energy losses of over 0.7 eV, which limits further improvements in the PCE. Recently, nonfullerene small molecules have emerged as promising electron acceptors in OSCs. Their highly tunable absorption spectra and molecular energy levels have enabled fine optimization of the resulting devices, and the highest PCE has surpassed 12 %. Furthermore, several studies have shown that OSCs based on small-molecule acceptors (SMA) have very efficient charge generation and transport efficiency at relatively low energy losses of below 0.6 eV, which suggests great potential for the further improvement of OSCs. In this focus review, we analyze the challenges and potential of SMA-based OSCs and discuss molecular design strategies for highly efficient SMAs.

Ternary Polymer Solar Cells based on Two Acceptors and One Donor for Achieving 12.2% Efficiency.

Ternary polymer solar cells are fabricated based on one donor PBDB-T and two acceptors (a methyl-modified small-molecular acceptor (IT-M) and a bis-adduct of Bis[70]PCBM). A high power conversion efficiency of 12.2% can be achieved. The photovoltaic performance of the ternary polymer solar cells is not sensitive to the composition of the blend.

Two Well-Miscible Acceptors Work as One for Efficient Fullerene-Free Organic Solar Cells.

High-performance ternary organic solar cells are fabricated by using a wide-bandgap polymer donor (bithienyl-benzodithiophene-alt-fluorobenzotriazole copolymer, J52) and two well-miscible nonfullerene acceptors, methyl-modified nonfullerene acceptor (IT-M) and 2,2'-((2Z,2'Z)-((5,5'-(4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydros-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(4-((2-ethylhexyl)oxy)thiophene-5,2-diyl))bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IEICO). The two acceptors with complementary absorption spectra and similar lowest unoccupied molecular orbital levels show excellent compatibility in the blend due to their very similar chemical structures. Consequently, the obtained ternary organic solar cells (OSC) exhibits a high efficiency of 11.1%, with an enhanced short-circuit current density of 19.7 mA cm-2 and a fill factor of 0.668. In this ternary system, broadened absorption, similar output voltages, and compatible morphology are achieved simultaneously, demonstrating a promising strategy to further improve the performance of ternary OSCs.

Design of a New Small-Molecule Electron Acceptor Enables Efficient Polymer Solar Cells with High Fill Factor.

Improving the fill factor (FF) is known as a challenging issue in organic solar cells (OSCs). Herein, a strategy of extending the conjugated area of end-group is proposed for the molecular design of acceptor-donor-acceptor (A-D-A)-type small molecule acceptor (SMA), and an indaceno[1,2-b:5,6-b']dithiophene-based SMA, namely IDTN, by end-capping with the naphthyl fused 2-(3-oxocyclopentylidene)malononitrile is synthesized. Benefiting from the π-conjugation extension by fusing two phenyls, IDTN shows stronger molecular aggregation, more ordered packing structure, thus over one order of magnitude higher electron mobility relative to its counterpart. By utilizing the fluorinated polymer (PBDB-TF) as the electron donor, the corresponding device exhibits a high efficiency of 12.2% with a record-high FF of 0.78, which is approaching the theoretical limit of OSCs. Compared with the reference molecule, such a high FF in the IDTN system can be mainly attributed to the more ordered π-π packing of acceptor aggregates, higher domain purity and symmetric carrier transport in the blend. Hence, enlarging the conjugated area of the terminal-group in these A-D-A-type SMAs is a promising approach not only for enhancing the electron mobility, but also for improving the blend morphology, and both of them are conducive to the fill-factor breakthrough.

Role of Polymer Segregation on the Mechanical Behavior of All-Polymer Solar Cell Active Layers.

An all-polymer bulk heterojunction (BHJ) active layer that removes the use of commonly used small molecule electron acceptors is a promising approach to improve the thermomechanical behavior of organic solar cells. However, there has been limited research on their mechanical properties. Here, we report on the mechanical behavior of high-performance blade-coated all-polymer BHJ films cast using eco-friendly solvents. The mechanical properties considered include the elastic modulus, crack onset strain, and cohesive fracture energy. We show that the mechanical behavior of the blend is largely unaffected by significant changes in the segregation characteristics of the polymers, which was varied systematically through solvent formulation. In comparison to a polymer:fullerene BHJ counterpart, the all-polymer films were found to have lower stiffness and increased ductility. Yet, the fracture energy of the all-polymer films is not significantly improved compared to that of the polymer:fullerene films. This study highlights that improved mechanical behavior of all-polymer systems cannot be assumed, and that details of the molecular structure, molecular weight, and film morphology play an important role in both the optoelectronic and mechanical properties. Furthermore, we show that simple composite modeling provides a predictive tool for the mechanical properties of the polymer blend films, providing a framework to guide future optimization of the mechanical behavior.