Photovoltaic Polymers Research Articles

Regulating Intermolecular Interactions and Film Formation Kinetics for Record Efficiency in Difluorobenzothiadizole‐Based Organic Solar Cells

AbstractThe difluorobenzothiadizole (ffBT) unit is one of the most classic electron‐accepting building blocks used to construct D‐A copolymers for applications in organic solar cells (OSCs). Historically, ffBT‐based polymers have achieved record power conversion efficiencies (PCEs) in fullerene‐based OSCs owing to their strong temperature‐dependent aggregation (TDA) characteristics. However, their excessive miscibility and rapid aggregation kinetics during film formation have hindered their performance with state‐of‐the‐art non‐fullerene acceptors (NFAs). Herein, we synthesized two ffBT‐based copolymers, PffBT‐2T and PffBT‐4T, incorporating different π‐bridges to modulate intermolecular interactions and aggregation tendencies. Experimental and theoretical studies revealed that PffBT‐4T exhibits reduced electrostatic potential differences and miscibility with L8‐BO compared to PffBT‐2T. This facilitates improved phase separation in the active layer, leading to enhanced molecular packing and optimized morphology. Moreover, PffBT‐4T demonstrated a prolonged nucleation and crystal growth process, leading to enhanced molecular packing and optimized morphology. Consequently, PffBT‐4T‐based devices achieved a remarkable PCE of 17.5 %, setting a new record for ffBT‐based photovoltaic polymers. Our findings underscore the importance of conjugate backbone modulation in controlling aggregation behavior and film formation kinetics, providing valuable insights for the design of high‐performance polymer donors in organic photovoltaics.

Efficient Derivatization of a Thienobenzobisthiazole-Based π-Conjugated Polymer Through Late-Stage Functionalization Towards High-Efficiency Organic Photovoltaic Cells.

Derivatization is essential for optimizing organic material properties. However, because functional groups are often introduced at an early stage of the synthesis, similar intermediates have to be repeatedly synthesized to produce derivatives, which amounts to a daunting and time-consuming task. Using thienobenzobisthiazole (TBTz) as a building unit of donor polymers for organic photovoltaics (OPVs), we demonstrate an efficient derivatization of a TBTz-based π-conjugated polymer by late-stage functionalization. In the developed synthetic route, functional groups are introduced at the last step of monomer synthesis, enabling us to easily synthesize several derivatives from a common intermediate. Ester and acyl groups are introduced into the polymer instead of the alkyl group, giving rise to deep HOMO energy levels and resulting in OPV cells with high open-circuit voltage even in the absence of halogen substituents that are typically introduced into the donor polymers. Notably, the ester-functionalized TBTz-based polymer shows a small nonradiative voltage loss (ΔVnr) of 0.19 V and has one of the highest charge generation efficiencies among the halogen-free donor polymers with similar ΔVnr, improving the critical trade-off relationship between voltage loss and charge generation. Our results provide an important guideline for the efficient development of high-performance polymers for OPVs.

Efficient Derivatization of a Thienobenzobisthiazole‐Based π‐Conjugated Polymer Through Late‐Stage Functionalization Towards High‐Efficiency Organic Photovoltaic Cells

AbstractDerivatization is essential for optimizing organic material properties. However, because functional groups are often introduced at an early stage of the synthesis, similar intermediates have to be repeatedly synthesized to produce derivatives, which amounts to a daunting and time‐consuming task. Using thienobenzobisthiazole (TBTz) as a building unit of donor polymers for organic photovoltaics (OPVs), we demonstrate an efficient derivatization of a TBTz‐based π‐conjugated polymer by late‐stage functionalization. In the developed synthetic route, functional groups are introduced at the last step of monomer synthesis, enabling us to easily synthesize several derivatives from a common intermediate. Ester and acyl groups are introduced into the polymer instead of the alkyl group, giving rise to deep HOMO energy levels and resulting in OPV cells with high open‐circuit voltage even in the absence of halogen substituents that are typically introduced into the donor polymers. Notably, the ester‐functionalized TBTz‐based polymer shows a small nonradiative voltage loss (ΔVnr) of 0.19 V and has one of the highest charge generation efficiencies among the halogen‐free donor polymers with similar ΔVnr, improving the critical trade‐off relationship between voltage loss and charge generation. Our results provide an important guideline for the efficient development of high‐performance polymers for OPVs.

Regulating Intermolecular Interactions and Film Formation Kinetics for Record Efficiency in Difluorobenzothiadizole-based Organic Solar Cells.

The difluorobenzothiadizole (ffBT) unit is one of the most classic electron-accepting building blocks used to construct D-A copolymers for applications in organic solar cells (OSCs). Historically, ffBT-based polymers have achieved record power conversion efficiencies (PCEs) in fullerene-based OSCs owing to their strong temperature-dependent aggregation (TDA) characteristics. However, their excessive miscibility and rapid aggregation kinetics during film formation have hindered their performance with state-of-the-art non-fullerene acceptors (NFAs). Herein, we synthesized two ffBT-based copolymers, PffBT-2T and PffBT-4T, incorporating different π-bridges to modulate intermolecular interactions and aggregation tendencies. Experimental and theoretical studies revealed that PffBT-4T exhibits reduced electrostatic potential differences and miscibility with L8-BO compared to PffBT-2T. This facilitates improved phase separation in the active layer, leading to enhanced molecular packing and optimized morphology. Moreover, PffBT-4T demonstrated a prolonged nucleation and crystal growth process, leading to enhanced molecular packing and optimized morphology. Consequently, PffBT-4T-based devices achieved a remarkable PCE of 17.5%, setting a new record for ffBT-based photovoltaic polymers. Our findings underscore the importance of conjugate backbone modulation in controlling aggregation behavior and film formation kinetics, providing valuable insights for the design of high-performance polymer donors in organic photovoltaics.

Potential of Photoelectric Stimulation with Ultrasmall Carbon Electrode on Neural Tissue: New Directions in Neurostimulation Technology Development

AbstractNeuromodulation technologies have gained considerable attention for their clinical potential in treating neurological disorders and advancing cognition research. However, traditional methods like electrical stimulation and optogenetics face technical and biological challenges that limit their therapeutic and research applications. A promising alternative, photoelectric neurostimulation, uses near‐infrared light to generate electrical pulses and thus enables stimulation of neuronal activity without genetic alterations. This study explores various design strategies to enhance photoelectric stimulation with minimally invasive, ultrasmall, untethered carbon electrodes. Employing a multiphoton laser as the near‐infrared (NIR) light source, benchtop experiments are conducted using a three‐electrode setup and chronopotentiometry to record photo‐stimulated voltage. In vivo evaluations utilize Thy1‐GCaMP6s mice with acutely implanted ultrasmall carbon electrodes. Results highlighted the beneficial effects of high duty‐cycle laser scanning and photovoltaic polymer interfaces on the photo‐stimulated voltages by the implanted electrode. Additionally, the promising potential of carbon‐based diamond electrodes are demonstrated for photoelectric stimulation and the application of photoelectric stimulation in precise chemical delivery by loading mesoporous silica nanoparticles (SNPs) co‐deposited with polyethylenedioxythiophene (PEDOT). Together, these findings on photoelectric stimulation utilizing ultrasmall carbon electrodes underscore its immense potential for advancing the next generation of neurostimulation technology.

Molecular Design and Organic Photovoltaic Applications of Carboxylate‐Functionalized P‐type Polymers

AbstractThe significant progress of p‐type and n‐type active layer materials in the past several years has pushed the power conversion efficiency (PCE) of organic solar cells (OSCs) toward 19%. Due to the relatively low synthesis cost and simple synthesis method of carboxylate‐containing building blocks, including thiophene, thieno[3,2‐b]thiophene, thieno[3,4‐b]thiophene, furan, pyrazine, benzodithiophene, benzothiazole, quinoxaline, etc., are widely used to construct p‐type photovoltaic polymers. These resulting carboxylate‐bearing polymers present downward energy levels, high absorption coefficient, narrow bandgap, high hole mobility, and strong aggregation behavior, which have dabbled in the fabrication of mechanically stretchable, semitransparent, indoor, and tandem OSCs, etc., and produce excellent photovoltaic performance. The low‐cost carboxylate‐containing copolymers exhibit a satisfying PCE approaching 17%, and the random terpolymer systems achieve a high PCE over 19%. This review focuses on the progress of carboxylate‐containing photovoltaic polymers, summarizes the molecular characteristics, discusses their structure‐performance relationship, and offers a summary and outlook on the challenges for future molecular development.

Exploring the impact of solvent additives on dielectric properties in polymer photovoltaics: A deconvolution and in-depth study of electrical modulus, complex conductivity, and impedance responses

Impedance and dielectric spectroscopy is a well-known technique for characterizing interfaces, especially donor/receiver interfaces in bulk heterojunction (BHJ) solar cells. This technique was employed to investigate the interface of poly (4,8-bis-alkyloxybenzo(1,2-b:4,5-b0) dithio-phene-2,6-diyl-alt- (alkyl thieno (3,4-b) thiophene-2-carboxylate)-2,6-diyl) (PBDTTT-C): PC70BM in bulk heterojunction with DIO as additive. For this reason, a simulation was carried out to generate the complex impedance, modulus, and complex conductivity data based on the electrical parameters extracted from the equivalent circuit. Therefore, a new approach based on the coupling of complex functions such as impedance, modulus, and conductivity has been proposed. In addition, an extrapolation and deconvolution approaches are used to highlight the relaxation processes, the BHJ layer process, the two electrical contacts of the interfaces between PEDOT: PSS/PBDTTT-C: PC70BM and PBDTTT-C: PC70BM/Ca process, and the electrode polarization process. In the next step, the remove the low-frequency process enhances the visibility of other processes, particularly observable in the evolution of the electrical modulus and the conduction parameters associated with each process. In the complex functions, there was a good correlation between all parameters derived from both processes. The characteristics of the conduction processes, such as conductivity values at low and high frequencies and ionic strength were then extracted. All these parameters were evaluated and discussed as a function of the thickness of the active layer. Finally, we demonstrate that the electrical modulus and complex conductivity can be perfect tools to characterize the donor/acceptor interfaces.

Wide-bandgap donor polymers from organic photovoltaics as dopant-free hole transport layers for perovskite solar cells

Dopant-free polymer hole transport materials (HTMs) exhibit high thermal stability, hydrophobicity and film-processing capabilities, demonstrating excellent device efficiency and stability in perovskite solar cells (PSCs). Continued innovation of wide-bandgap polymers in organic photovoltaics (OPV) provides a valuable toolbox for developing polymeric HTMs. Here, we propose an effective molecule design for selecting structurally relevant polymers (D18, D18-Cl, PBQx-TCl) available for commercialization. We discover that the highly planar conjugated backbones play a crucial role in regulating the packing orientation of the film relative to the perovskite. The moderate aggregation with face-on packing orientation is conducive to the high-quality film, which is responsible for better contact with perovskite and superior charge extraction and transport. Simultaneously, these polymers with robust passivation enhanced the open circuit voltage (VOC) without additional passivation layers, streamlining the device process. Consequently, a PSC using dopant-free PBQx-TCl HTL demonstrated an efficiency of 24.12 % with a high VOC of 1.20 V and good operational stability (T90 > 600 h). This work reveals transparent structure–function-performance relationships between molecules and devices, paving the way for the subsequent development of high-performance HTMs.

Conjugated D–π–A photovoltaic polymers containing thieno[3,2-b]thiophene π-bridge

We review the development of TT-bridged D–π–A copolymers and emphasize the role played by TT-bridge on tuning polymer properties and improving device performance.

Chain conformation and aggregation structure regulation of an efficient acceptor–acceptor type photovoltaic polymer

Two novel acceptor1–acceptor2 (A1–A2) typed polymer donors with strong electron affinity were finely designed and tuned through the alkyl side chains. Their different H/J-aggregation behaviors and conformations obviously affected the corresponding nano morphology device performance.

Benzotriazole‐Based D–π–A‐Type Photovoltaic Polymers Break Through 17% Efficiency

AbstractBenzo[d][1,2,3]triazole (BTA) unit is one of the most classic electron‐accepting units (A) to construct donor (D)‐π‐A‐type photovoltaic polymers. However, the highest power conversion efficiency (PCE) of organic photovoltaics (OPVs) based on BTA‐containing polymers is restricted to 15–16%, lagging other promising polymers. Thus, investigating the structure‐performance relationship and breaking the efficiency bottleneck of BTA‐based polymers is challenging but critical. Herein, the effects of fusing two thiophene rings at D (PE52), π (PE4), and A (PE39) units of a classic D–π–A‐type BTA‐containing polymer J52‐Cl, respectively, on the backbone conformation, crystallinity, molecular stacking, and photovoltaic performance are systematically investigated. When blended with a BTA‐containing non‐fullerene acceptor (NFA), Y18, all three polymers with extending conjugated backbones can decrease the energy loss of photovoltaic devices. Notably, PE4, with a linear backbone conformation, realizes the champion PCE of 17.08%, with a short‐circuit current density (JSC) of 26.83 mA cm−2, a large breakthrough for BTA‐based photovoltaic polymers. What's more, the photovoltaic device based on PE4:Y18 combination fabricated by a non‐halogenated solvent of o‐xylene also displays an excellent PCE of 16.87%. The results indicate that fusing thiophene rings to BTA‐polymers, especially at π‐bridge, is a simple and effective method to improve the photovoltaic performance via modulating the molecular conformation and crystallinity.

A SHORT REVIEW OF ISOINDIGO ACCEPTOR FOR CONJUGATED POLYMERIC PHOTOVOLTAICS

This paper focussed on the recent development of conjugated polymers that contains isoindigo as acceptor moiety in the application of copolymeric solar cell. In the past decade, various modifications have been done either on the isoindigo acceptor itself or incorporated the isoindigo with different donor moieties. Recently, the power conversion efficiency (PCE) of this isoindigo-based polymeric photovoltaics have achieved up to ~7%. Hence, it is a promising acceptor for the photovoltaics and is expected to break through the recent PCE achievement in the future. This review briefly summarized the structures and properties of the isoindigo-based polymers that have been investigated by the past researches.

Dithienobenzothiadiazole (DTBT)-Based Polymers Enable Organic Solar Cells with Ultrahigh VOC of ≈1.3V.

Dithieno[3',2':3,4;2",3":5,6]benzo[1,2-c][1,2,5]thiadiazole (DTBT) is a newly emerging building block to construct effective photovoltaic polymers. Organic solar cells (OSCs) based on DTBT-based polymers have realized power conversion efficiency (PCEs) over 18%, despite their relatively low open-circuit voltage (VOC ) of 0.8-0.95V. To extend the application of DTBT-based polymers in high-voltage OSCs, herein, D18-Cl and PE55 are used to combine with a wide-bandgap non-fullerene acceptor (NFA), BTA3, and achieve ultrahigh VOC of 1.30 and 1.28V, respectively. Compared with D18-Cl based on tricyclic benzodithiophene (BDT) segment, PE55 containing the pentacyclic dithienobenzodithiophene (DTBDT) unit possesses better hole mobility, higher charge-transfer efficiency, and more desirable phase separation. Hence, PE55:BTA3 blend exhibits a higher efficiency of 9.36% than that of D18-Cl: BTA3 combination (6.30%), which is one of the highest values for OSCs at ≈1.3V VOC . This work attests that DTBT-based p-type polymers are ideal for the application in high-voltage OSCs.

(Z)-4-(thiophen-2-ylmethylene)-4H-thieno[2,3-b]pyrrol-5(6H)-one based polymers for organic photovoltaics

(Z)-4-(thiophen-2-ylmethylene)-4H-thieno[2,3-b]pyrrol-5(6H)-one based polymers for organic photovoltaics

Benzo[1,2‐b:4,5‐b′]Difuran‐Based Polymer for Organic Solar Cells with 17.5% Efficiency via Halogenation‐Mediated Aggregation Control

AbstractThe realization of high‐efficiency organic solar cells (OSCs) from renewable sources will bring a real green energy technology. Benzo[1,2‐b:4,5‐b′]difuran (BDF) is such a building block for photovoltaic polymers as it can be built from furfural, which is available from trees and vegetables. However, the device performance of BDF‐based polymers is limited by aggregation properties and unfavorable active layer morphology. Herein, two new BDF‐based wide bandgap‐conjugated polymers, PFCT‐2F and PFCT‐2Cl, are developed by copolymerizing the fluorinated or chlorinated BDF units with the 3‐cyanothiophene unit for use as electron donors in OSCs. Benefitting from the more rotatable nature of the side‐chain thiophene rings on the BDF unit, PFCT‐2Cl exhibits more adjustable aggregation, higher π–π stacking ordering, and appropriate miscibility with the electron acceptor. As a result, a high power conversion efficiency (PCE) of 17.2% is offered by PFCT‐2Cl, which is nearly two times higher than that of PFCT‐2F (8.9%). A more remarkable PCE up to 17.5% is further achieved by PFCT‐2Cl in a ternary OSC, which is the new efficiency record for BDF‐based polymers. This success proves the critical role of aggregation control in BDF‐based polymers and the bright future for constructing high‐performance OSCs from bio‐renewable sources.

Quantifying the influence of encapsulant and backsheet composition on PV‐power and electrical degradation

AbstractAlthough the technical and economic properties of the standard polymer photovoltaic (PV) materials (ethylene‐vinyl acetate (EVA) encapsulant and fluorine‐containing polyethylene terephthalate (PET) backsheet) meet the basic technical requirements, more sustainable polyolefin‐based encapsulants and backsheets have been developed. These new polyolefin materials have to prove their performance compared to the established standard materials in terms of the electrical performance of the modules and in terms of reliability. The long‐term stability of the new materials is tested and evaluated using accelerated aging tests and degradation modelling. Based on experimental results, the influence of the type of encapsulant and backsheet (i) on the electrical output power of PV test modules and (ii) on the aging‐related electrical and material degradation under accelerated stress tests was estimated using statistical modelling approaches. First results showing significant effects for encapsulant, backsheet and the combination of both on the initial power output are presented. In general, modules with polypropylene‐based backsheets have a higher initial power (PMPP) than those with PET‐based backsheets, with the combination of thermoplastic polyolefin (TPO) encapsulation material and polyolefin backsheet being superior to the other material combinations. A comparison of the material‐dependent degradation rates obtained from the mixed‐effects models clearly shows that the degradation rate upon damp heat exposure for modules with EVA is significantly larger than that using polyolefin encapsulants. The derived relations aim to provide valuable input for innovative material developments as well as predictive maintenance specifications.

Fine-tuning of the inner sidechain of donor polymers for efficient indoor organic photovoltaics

Recently, the performance of organic photovoltaics (OPVs) has increased with the advent of non-fullerene acceptors, and significant efforts have been devoted to improving the performance via the side-chain engineering of Y6 and its derivatives.

A newly designed benzodithiophene building block: tuning of the torsional barrier for non-halogenated and non-aromatic solvent-processible photovoltaic polymers

A new benzodithiophene (BDT)-based building block, 3-FBDT, was synthesized and incorporated into PBDB-T-2F to yield an eco-friendly (non-aromatic and non-halogenated) solvent-processible photovoltaic copolymer, PBDB-T-2F(3/4).

Recent advances in polymer and perovskite based third-generation solar cell devices

This review article begins with a comparative overview of the configurations, materials, fabrication methods, and energy conversion efficiency of polymer and perovskite solar cells' photovoltaic performances. Firstly, there has been a significant increase in the adoption of solar cells due to the growing need for renewable and clean energy. Among all photovoltaic technologies, polymer and perovskite solar cells have garnered considerable attention owing to their potential to be affordable, lightweight, easy to manufacture, and quick charging. Research on polymer solar cells has spanned over two decades, whereas the use of perovskite solar cells is just eight years old. This expanding topic offers a wealth of information to readers interested in polymers and perovskite. As a basic comparison, the best power conversion efficiency results are 21.6 percent for a 1 cm2 perovskite solar cell and 15.2 percent for polymer solar cells. Finally, this article recommends necessary improvements and future research areas in polymer and perovskite solar cells.

Resonant Enhancement of Polymer-Cell Optostimulation by a Plasmonic Metasurface.

Organic semiconductors have shown great potential as efficient bioelectronic materials. Specifically, photovoltaic polymers such as the workhorse poly(thiophene) derivatives, when stimulated with visible light, can depolarize neurons and generate action potentials, an effect that has been also employed for rescuing vision in blind rats. In this context, however, the coupling of such materials with optically resonant structures to enhance those photodriven biological effects is still in its infancy. Here, we employ the optical coupling between a nanostructured metasurface and poly(3-hexylthiophene) (P3HT) to improve the bioelectronic effects occurring upon photostimulation at the abiotic-biotic interface. In particular, we designed a spectrally tuned aluminum metasurface that can resonate with P3HT, hence augmenting the effective field experienced by the polymer. In turn, this leads to an 8-fold increase in invoked inward current in cells. This enhanced activation strategy could be useful to increase the effectiveness of P3HT-based prosthetic implants for degenerative retinal disorders.