Single-junction polymer solar cells with over 10% efficiency by a novel two-dimensional donor-acceptor conjugated copolymer

Effect of External Magnetic Field on Bulk Heterojunction Polymer Solar Cells

Polymer solar cells (PSCs) with a bulk heterojunction (BHJ) device structure have incredible advantages, such as low-cost fabrication and flexibility. However, the power conversion efficiency (PCE) of BHJ PSCs needs to be further improved to realize their practical applications. In this study, boosted PCEs from PSCs based on BHJ composites incorporated with Fe₃ O₄ magnetic nanoparticles (MNPs), aligned by an external magnetic field (EMF), are reported. It is found that the coercive electric field within the Fe₃ O₄ MNPs generated by the EMF has a strong and positive influence on the charge generation, which results in a more than 10% increase in free charge carriers. Moreover, the coercive electric field speeds up the charge carrier transport and suppresses charge carrier recombination within PSCs. In addition, a shortened extraction time makes charge carriers more likely to make it to the electrodes. As a result, more than 15% enhancement in PCE is observed from the PSCs based on the BHJ composite incorporated with the Fe₃ O₄ MNPs and the EMF as compared with that based on the BHJ composite thin film. This work indicates that the incorporation of MNPs and the EMF is a facile way to enhance the PCEs of PSCs.

Engineered Bacteriorhodopsin Film with Oriented Patterns for the Improvement of the Photoelectric Response

The use of photosensitive proteins has become a competitive solar energy solution, owing to its pollution-free nature, high conversion efficiency, and good biocompatibility. Bacteriorhodopsin (bR) is an important light-sensitive protein that is widely used in the fabrication of photoelectronic devices. However, research on the optimization and comparison of the immobilization techniques is lacking. In this study, in order to obtain bR films with a high energy conversion efficiency, three immobilization techniques, namely dropcasting, electrophoretic sedimentation, and Langmuir-Blodgett deposition, were used to fabricate films, and their topographical and photoelectrical characteristics were compared. All three immobilization techniques can transfer bR molecules to substrates, forming functional photosensitive bR films. The absorption of the bR films at 568 nm reached the highest value of 0.3 under the EPS technique. The peak photocurrent for the EPS technique reached 5.03 nA. In addition, the EPS technique has the highest efficiency factor of 13.46, indicating that it can generate the highest value of photocurrent under the same light conditions, owing to the improved orientation, and no significant decrease in the peak photocurrent was observed after three weeks, which indicates the stability of the photoelectric response. These results indicate that the EPS technique has a great potential for the photoelectrical device fabrication and solar-energy conversion.

Cellulose Nanocrystals (CNC)-Based Functional Materials for Supercapacitor Applications

The growth of industrialization and the population has increased the usage of fossil fuels, resulting in the emission of large amounts of CO_2. This serious environmental issue can be abated by using sustainable and environmentally friendly materials with promising novel and superior performance as an alternative to petroleum-based plastics. Emerging nanomaterials derived from abundant natural resources have received considerable attention as candidates to replace petroleum-based synthetic polymers. As renewable materials from biomass, cellulose nanocrystals (CNCs) nanomaterials exhibit unique physicochemical properties, low cost, biocompatibility and biodegradability. Among a plethora of applications, CNCs have become proven nanomaterials for energy applications encompassing energy storage devices and supercapacitors. This review highlights the recent research contribution on novel CNC-conductive materials and CNCs-based nanocomposites, focusing on their synthesis, surface functionalization and potential applications as supercapacitors (SCs). The synthesis of CNCs encompasses various pretreatment steps including acid hydrolysis, mechanical exfoliation and enzymatic and combination processes from renewable carbon sources. For the widespread applications of CNCs, their derivatives such as carboxylated CNCs, aldehyde-CNCs, hydride-CNCs and sulfonated CNC-based materials are more pertinent. The potential applications of CNCs-conductive hybrid composites as SCs, critical technical issues and the future feasibility of this endeavor are highlighted. Discussion is also extended to the transformation of renewable and low-attractive CNCs to conductive nanocomposites using green approaches. This review also addresses the key scientific achievements and industrial uses of nanoscale materials and composites for energy conversion and storage applications.

Brush Printing Creates Polarized Green Fluorescence: 3D Orientation Mapping and Stochastic Analysis of Conductive Polymer Films

Brush printing is a unique method used to obtain uniaxially oriented films, whereby a polymer solution is brushed onto a substrate. However, there have been only a few reports on the brush-printing method. Here, we report the preparation of a uniaxially oriented film of a green light-emitting conductive polymer, poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT). The fluorescence polarization ratio of the oriented F8BT films was as high as 11.3, and the average orientation factor reached 0.74 ± 0.06. The orientation factor and the torsion angle of F8BT were visualized by two mappings of fluorescence and Raman spectral measurements by confocal spectromicroscopy, respectively. These two x-y mapping data with many pixels (∼750 pixels) were evaluated by x-y-z mapping of the film thickness at a single position and were used to reveal the three-dimensional (3D) orientation mechanism from a stochastic approach. Polarized green fluorescence originates from polymer chains uniaxially oriented along the brush direction. The high orientation for a film thickness < 100 nm is established by shear stress, faster capillary flow, and flow-induced chain extension for a thin solution film on a substrate. The high orientation factor was also demonstrated by a high brushing speed, whereas an optimized brushing speed existed. We found that this optimization is attributed to the property of a non-Newtonian fluid. By applying this brush-printing method to the fabrication of an optoelectrical device, polarized green electroluminescence was preliminarily demonstrated by the OLED assembled from an oriented F8BT film.

Single graphene derivative layer as a hole transport in organic solar cells based on PBDB-T:ITIC

A layer of fluorinated reduced graphene oxide (FrGO), as an alternative hole transport (HTL) in organic solar cells (OSCs) based on a PBDB-T:ITIC active layer, is reported. OSC configuration is ITO/HTL/PBDB-T:ITIC/PFN/FM; FM is Field's metal, a eutectic alloy deposited at room atmosphere. PEDOT:PSS, FrGO/PEDOT:PSS, and FrGO are tested as HTLs; the average efficiencies of 8.8, 8.2, and 5.3%, respectively, are reached. Inhomogeneity of the FrGO layer is determined as the main factor that affects the photovoltaic behavior and stability. Device stability is very acceptable, sometimes with a superior behavior than data previously reported; FM also could potentially contribute to this enhanced stability.

Scanning Probe Microscopy Analysis of Nonfullerene Organic Solar Cells

In this work, scanning probe microscopies (SPMs) are used for the analysis of PBDB-T, ITIC, and PBDB-T:ITIC layers of solar cells (OSCs). Scanning tunneling microscopy (STM) images of PBDB-T reveal that thin films (<1 nm) tend to form worm-like pattern (amorphous type) domains with an average chain-to-chain distance of 950 pm; likewise, STM images of ITIC show that side arms form chain-like patterns. STM images of PBDB-T:ITIC blend suggest why PBDB-T domains could facilitate charge dissociation. Further, a strong interchain π-π interaction of the ITIC molecules could promote self-organization, and under the mutual interaction with the PBDB-T polymer, it could influence the pathway formation for electron transport. Moreover, when correlating electrostatic force microscopy (EFM) and photoconductive atomic force microscopy (pc-AFM), the blend morphology and its electrical/electronic properties are determined; the ideal domain size of PBDB-T:ITIC blend phases for maximizing the generated photocurrent is 15-35 nm. Furthermore, phase contrast and surface electric potential characteristics with Kelvin probe force microscopy (KPFM) are measured to examine additional details about the surface and potential changes due to the domain differences in the active layer. OSCs based on the nonfullerene PBDB-T:ITIC active layer reach an average power conversion efficiency (PCE) of 9.1% (best 9.2%).

Fluorination Effect for Highly Conjugated Alternating Copolymers Involving Thienylenevinylene-Thiophene-Flanked Benzodithiophene and Benzothiadiazole Subunits in Photovoltaic Application

Two two-dimensional (2D) donor-acceptor (D-A) type conjugated polymers (CPs), namely, PBDT-TVT-BT and PBDT-TVT-FBT, in which two ((E)-(4,5-didecylthien-2-yl)vinyl)- 5-thien-2-yl (TVT) side chains were introduced into 4,8-position of benzo[1,2-b:4,5-b']dithiophene (BDT) to synthesize the highly conjugated electron-donating building block BDT-TVT, and benzothiadiazole (BT) and/or 5,6-difluoro-BT as electron-accepting unit, were designed to systematically ascertain the impact of fluorination on thermal stability, optoelectronic property, and photovoltaic performance. Both resultant copolymers exhibited the lower bandgap (1.60 ~ 1.69 eV) and deeper highest occupied molecular orbital energy level (EHOMO, -5.17 ~ -5.37 eV). It was found that the narrowed absorption, deepened EHOMO and weakened aggregation in solid film but had insignificant influence on thermal stability after fluorination in PBDT-TVT-FBT. Accordingly, a PBDT-TVT-FBT-based device yielded 16% increased power conversion efficiency (PCE) from 4.50% to 5.22%, benefited from synergistically elevated VOC, JSC, and FF, which was mainly originated from deepened EHOMO, increased μh, μe, and more balanced μh/μe ratio, higher exciton dissociation probability and improved microstructural morphology of the photoactive layer as a result of incorporating fluorine into the polymer backbone.

Effect of PVP-Capped ZnO Nanoparticles with Enhanced Charge Transport on the Performance of P3HT/PCBM Polymer Solar Cells

We attempted surface modification in ZnO nanoparticles (NPs) synthesized by the sol–gel process with polyvinyl pyrrolidone (PVP) applied to bulk-heterojunction polymer solar cells (PSCs) as an electron transport layer (ETL). In general, ZnO NPs have trap sites due to oxygen vacancies which capture electrons and degrade the performance of the PSCs. Devices with six different PVP:Zn ratios (0.615 g, 1.230 g, 1.846 g, 2.460 g, 3.075 g, and 3.690 g) were fabricated for surface modification, and the optimized PVP:Zn ratio (2.460 g) was found for PSCs based on P3HT/PCBM. The power conversion efficiency (PCE) of the fabricated PSCs with PVP-capped ZnO exhibited a significant increase of approximately 21% in PCE and excellent air-stability as compared with the uncapped ZnO-based PSCs.

Preparation and Physical Characterization of Pyrene and Pyrrolo[3,4-c]pyrrole-1,4-dione-Based Copolymers

Two narrow band-gap copolymers consisting of 2,7-bis(5-(trimethylstannyl)thiophen-2-yl)-4,5,9,10-tetrakis(2-ethylhexyloxy)-pyrene (M1) as an electron-rich unit and diketopyrrolopyrrole (DPP) as an electron-deficient unit have been synthesized and characterized for polymer solar cells. The two polymers were prepared by Stille coupling reactions. Two solubilizing alkyl chains (ethylhexyl and octlydodecyl) were attached to the DPP unit in order to evaluate their impact upon the optical and electrochemical characteristics of the two polymers. Poly[4,5,9,10-tetrakis[(2-ethylhexyl)oxy]pyrene-alt-3,6-bis(thiophen-2-yl)-2,5- bis(2-octyldodecyl)pyrrole[3,4-c]pyrrole- 1,4(2H,5H)-dione] (PPEHDT-DPPODo ) and poly[4,5,9,10-tetrakis((2-ethylhexyl)oxy)pyren-alt-3,6-bis(thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrole[3,4-c]pyrrole-1,4(2H,5H)-dione] (PPEHDT-DPPEH ) exhibited high thermal stability with decomposition temperatures over 300 °C. Optical properties showed that PPEHDT-DPPODo and PPEHDT-DPPEH have optical band gaps of around 1.40 eV. It is believed that both polymers adopt high planar structures in the thin film, leading to more electronic conjugation along the backbone of the conjugated polymers. Powder X-ray diffraction revealed that PPEHDT-DPPODo and PPEHDT-DPPEH seem to have an amorphous nature. The HOMO energy levels of the two polymers are clearly affected by changing alkyl chains. The HOMO levels of PPEHDT-DPPODo and PPEHDT-DPPEH were found to be at -5.27 and -5.38 eV, respectively. PPEHDT-DPPODo showed a HOMO energy level approximately 0.11 eV shallower than that of PPEHDT-DPPEH , which is probably a consequence of attaching a larger alkyl chain to the DPP moiety reducing its electron accepting ability.

Photostability of Fullerene and Non-Fullerene Polymer Solar Cells: The Role of the Acceptor

Recently, the advent of non-fullerene acceptors (NFAs) made it possible for organic solar cells (OSCs) to break the 10% efficiency barrier hardly attained by fullerene acceptors (FAs). In the past five years alone, more than hundreds of NFAs with applications in organic photovoltaics (OPVs) have been synthesized, enabling a notable current record efficiency of above 15%. Hence, there is a shift in interest toward the use of NFAs in OPVs. However, there has been little work on the stability of these new materials in devices. More importantly, there is very little comparative work on the photostability of FA versus NFA solar cells to ascertain the pros and cons of the two systems. Here, we show the photostability of solar cells based on two workhorse acceptors, in both conventional and inverted structures, namely, ITIC (as NFA) and [70]PCBM (as FA), blended with either PBDB-T or PTB7-Th polymer. We found that, irrespective of the polymer, the cell structure, or the initial efficiency, the [70]PCBM devices are more photostable than the ITIC ones. This observation, however, opposes the assumption that NFA solar cells are more photochemically stable. These findings suggest that complementary absorption should not take precedence in the design rules for the synthesis of new molecules and there is still work left to be done to achieve stable and efficient OSCs.

Composition-Morphology Correlation in PTB7-Th/PC71BM Blend Films for Organic Solar Cells

From a morphological perspective, the understanding of the influence of the [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) content on the morphology of the active layer is not complete in organic solar cells (OSCs) with bulk heterojunction (BHJ) configuration based on the low-bandgap polymer poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2- b;4,5- b']dithiophene-2,6-diyl- alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4- b]thiophene-)-2-carboxylate-2-6-diyl] (PTB7-Th). In this work, we obtain the highest power conversion efficiency (PCE) of 10.5% for BHJ organic solar cells (OSCs) with a PTB7-Th/PC₇₁BM weight ratio of 1:1.5. To understand the differences in PCEs caused by the PC₇₁BM content, we investigate the morphology of PTB7-Th/PC₇₁BM blend films in detail by determining the domain sizes, the polymer crystal structure, optical properties, and vertical composition as a function of the PC₇₁BM concentration. The surface morphology is examined with atomic force microscopy, and the inner film morphology is probed with grazing incidence small-angle X-ray scattering. The PTB7-Th crystal structure is characterized with grazing incidence wide-angle X-ray scattering and UV/vis spectroscopy. X-ray reflectivity is employed to yield information about the film vertical composition. The results show that in PTB7-Th/PC₇₁BM blend films, the increase of PC₇₁BM content leads to an enhanced microphase separation and a decreased polymer crystallinity. Moreover, a high PC₇₁BM concentration is found to decrease the polymer domain sizes and crystal sizes and to promote polymer conjugation length and formation of fullerene-rich and/or polymer-rich layers. The differences in photovoltaic performance are well explained by these findings.

Synthesis and photophysical properties of water-soluble fluorinated poly(aryleneethynylene)s

Reported are the synthesis of water-soluble fluorinated conjugated polymers, and photophysical properties, and fluorescence quenching response to arylamines.

Benzodithiophene-Dithienylbenzothiadiazole Copolymers for Efficient Polymer Solar Cells: Side-Chain Effect on Photovoltaic Performance

A new series of low band gap D-A alternating polymers based on 4,5-bis((2-ethylhexyl)oxy)benzo[2,1- b:3,4- b']dithiophene (BDT) and 5-fluoro-4,7-bis(4-alkylthien-2-yl)benzo[ c][1,2,5]thiadiazole bearing different size of lateral alkyl substituents, namely, PfBB- n, n = 8, 10, 12, 14, and 16, was designed and synthesized for high-performance bulk heterojunction (BHJ) polymer solar cells (PSCs). PfBB- n-bearing linear alkyl side chains exhibited strong and controllable aggregation in both solution and solid states, which gives rise to a significant bathochromic shift of the absorption cut-off down to ∼780 nm in thin film. In addition, the strong and wide absorption (350-800 nm) of PfBB- n polymers can compensate for the relatively weak absorption of PC₇₁BM, particularly in the 300-400 range nm to enhance light harvesting of such an active blend. BHJ solar cells based on PfBB- n:PC71BM blends as an active layer showed power conversion efficiency (PCE) in the range 7.8-9.7%. Because of the strong stacking interchain interactions, PfBB-12-based PSC exhibited aggregation-induced spectral broadening, superior structural order, higher exciton dissociation, higher and more balanced charge carrier mobilities, as well as reduced recombination losses. As a result, PfBB-12-based device afforded the best PCE of 9.7%, with the highest short-circuit current density ( J_sc) of 16.6 mA cm^-2 and open-circuit voltage ( V_oc) of 0.92 V among devices fabricated. These results demonstrate that the alkyl side chain of the polymer significantly affects the absorption, morphology, and electronic properties of the active blend of PfBB- n/PC71BM, which would provide an alternative useful tool to fine-tune the device performance. Our results also highlight that the electron-rich benzo[2,1- b:3,4- b']dithiophene building block, BDT, is highly useful for the construction of low band gap D-A polymer for highly efficient PSCs.

Ternary organic solar cells with a phase-modulated surface distribution via the addition of a small molecular luminescent dye to obtain a high efficiency over 10.5

Incorporation of a ternary organic component is an effective strategy to enhance the performance of bulk heterojunction (BHJ) organic solar cells (OSCs). In this study, a small molecule luminescent dye, C545T, was first doped into blends of PTB7-Th/PC71BM and PTB7/PC71BM as a third component to fabricate ternary OSCs. It is demonstrated that C545T can disrupt the severe vertical distribution in the binary blend and effectively modulate the novel surface chemical configuration by improving the self-assembly process of the polymer donor, as a result of the good miscibility among the active layer materials and the π-π interactions between PC71BM and C545T. The obtained homogeneously bicontinuous BHJ with numerous interpenetrating nanofibers optimizes the domain size of exciton diffusion and the length of charge transfer. The energy transfer between C545T and polymers changes the transmission path of photo-generated excitons, which together improve the exciton dissociation process and reduce the recombination loss. Champion power conversion efficiencies (PCEs) of 10.69% and 9.42% were achieved by the ternary blends of PTB7-Th/C545T/PC71BM and PTB7/C545T/PC71BM, respectively, which correspond to a nearly 20% enhancement over their binary counterparts.

Recent Advances in Morphology Optimization for Organic Photovoltaics

Organic photovoltaics are an important part of a next-generation energy-harvesting technology that uses a practically infinite pollutant-free energy source. They have the advantages of light weight, solution processability, cheap materials, low production cost, and deformability. However, to date, the moderate photovoltaic efficiencies and poor stabilities of organic photovoltaics impede their use as replacements for inorganic photovoltaics. Recent developments in bulk-heterojunction organic photovoltaics mean that they have almost reached the lower efficiency limit for feasible commercialization. In this review article, the recent understanding of the ideal bulk-heterojunction morphology of the photoactive layer for efficient exciton dissociation and charge transport is described, and recent attempts as well as early-stage trials to realize this ideal morphology are discussed systematically from a morphological viewpoint. The various approaches to optimizing morphologies consisting of an interpenetrating bicontinuous network with appropriate domain sizes and mixed regions are categorized, and in each category, the recent trends in the morphology control on the multilength scale are highlighted and discussed in detail. This review article concludes by identifying the remaining challenges for the control of active layer morphologies and by providing perspectives toward real application and commercialization of organic photovoltaics.

Identifying Molecular Orientation in a Bulk Heterojunction Film by Infrared Reflection Absorption Spectroscopy

The molecular orientation of organic molecules of zinc phthalocyanine (ZnPc) in single-component films on copper iodide (CuI) substrates can be controlled to achieve a molecular orientation lying flat on the substrate (flat-on) owing to π-d orbital interactions between the ZnPc molecules and the CuI. A 3-fold enhancement in the performance of organic photovoltaic cells has been reported by introducing a CuI interlayer between a ZnPc:fullerene (C60) bulk heterojunction (BHJ) film and the substrate. However, the mechanism underpinning the resultant solar cell performance enhancement was unclear. Herein, we report on the results of using in situ reflection absorption spectroscopy measurements during the vacuum deposition of coevaporated ZnPc:C60 BHJ films on various substrates to investigate the ZnPc molecular orientation. Our results revealed that the flat-on molecular orientation of ZnPc molecules in ZnPc:C60 BHJ films on CuI interlayers and flat-on ZnPc substrates can be successfully identified via the strong π-π interactions between the BHJ film and the substrate. The π-π interactions between individual ZnPc molecules are stronger than the π-d interactions between ZnPc molecules and CuI in coevaporated ZnPc:C60 films, as is evident from the molecular orientation of ZnPc, as determined by in situ reflection absorption spectroscopy. Our findings demonstrate that precisely controlling the molecular orientations of the films could enhance organic photovoltaic (OPV) performance. The present work provides important insights that will enable the design of higher performance OPV cells.

A PTB7-based narrow band-gap conjugated polyelectrolyte as an efficient cathode interlayer in PTB7-based polymer solar cells

A PTB7-based cationic narrow band-gap polyelectrolyte, named PTB7-NBr, has been designed and synthesized as a cathode interfacial material for polymer solar cells. PTB7-NBr exhibits excellent cathode interfacial modification in solar cells with PTB7 and PTB7-Th as donor polymer and a high PCE of 9.24% was achieved.

Conjugated Polymers for Flexible Energy Harvesting and Storage

Since the discovery of conjugated polymers in the 1970s, they have attracted considerable interest in light of their advantages of having a tunable bandgap, high electroactivity, high flexibility, and good processability compared to inorganic conducting materials. The above combined advantages make them promising for effective energy harvesting and storage, which have been widely studied in recent decades. Herein, the key advancements in the use of conjugated polymers for flexible energy harvesting and storage are reviewed. The synthesis, structure, and properties of conjugated polymers are first summarized. Then, their applications in flexible polymer solar cells, thermoelectric generators, supercapacitors, and lithium-ion batteries are described. The remaining challenges are then discussed to highlight the future direction in the development of conjugated polymers.

Improving Photovoltaic Properties of P3HT:IC60BA through the Incorporation of Small Molecules

We investigated the role of a functional solid additive, 2,3-dihydroxypyridine (DHP), in influencing the optoelectronic, morphological, structural and photovoltaic properties of bulk-heterojunction-based polymer solar cells (BHJ PSCs) fabricated using poly(3-hexylthiophene): indene-C60 bisadduct (P3HT:IC60BA) photoactive medium. A dramatic increase in the power conversion efficiency (~20%) was witnessed for the BHJ PSCs treated with DHP compared to the pristine devices. A plausible explanation describing the alignment of pyridine moieties of DHP with the indene side groups of IC60BA is presented with a view to improving the performance of the BHJ PSCs via improved crystalline order and hydrophobicity changes.

New iridium-containing conjugated polymers for polymer solar cell applications

The highest value of power conversion efficiency is 1.74% for the P3 based polymer.

Efficient carbazole-based small-molecule organic solar cells with an improved fill factor

In this study, a new acceptor–donor–acceptor (A–D–A) small molecule, DI3TCz, with carbazole as the central unit and 1,3-indanedione as the end group, was designed and synthesized for application in organic solar cells. In contrast to the molecule DR3TCz with rhodanine as end groups, DI3TCz exhibited deep a LUMO energy level and a nearly unchanged HOMO energy level with a narrow optical band gap of 1.75 eV and red shifted absorption. Compared with DR3TCz, the DI3TCz device showed a PCE of 6.46% with a remaining high V_oc value of 0.97 V, improved J_sc of 10.40 mA cm⁻¹ and a notable FF of 0.65, which is the highest PCE value reported to data for carbazole-based small molecules OPVs.

In this study, a new acceptor–donor–acceptor (A–D–A) small molecule, DI3TCz, with carbazole as the central unit and 1,3-indanedione as the end group, was designed and synthesized for application in organic solar cells.

Medium Bandgap D-A Type Photovoltaic Polymers Based on an Asymmetric Dithienopyran Donor and a Benzotriazole Acceptor

Conjugated polymers based on the donor of an asymmetric 5H-dithieno[3,2-b:2',3'-d]pyran (DTPa) and the acceptors of benzo[d][1,2,3]triazole (BTA) or di-fluorinated benzo[d][1,2,3]triazole (ffBTA) with thiophene as π-bridge were designed and synthesized. Two asymmetric-building-block-containing polymers (ABC-polymers) possess a strong and broad absorption in the range of 300⁻750 nm and medium optical bandgap of 1.73 and 1.77 eV for PDTPa-TBTA and PDTPa-TffBTA, respectively. Polymer solar cells using PDTPa-TBTA as donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an acceptor exhibited power conversion efficiencies (PCE) of 2.22% with a Voc of 0.58 V, a Jsc of 6.04 mA/cm², and an FF of 63.41%. The introduction of fluorine substituents on the BTA unit evidently influenced the optical and photovoltaic properties. Interestingly, although the HOMO energy level indeed decreased, PDTPa-TffBTA showed a decreased Voc of 0.52 V in solar cells. Combined with an obviously enhanced Jsc of 10.23 mA/cm², and an outstanding FF of 0.64, the PCE of solar cells based on PDTPa-TffBTA was improved by nearly 55%, reached 3.43%. Our results indicate that the BTA unit can be used to construct ABC polymers with a medium bandgap, and the introduction of fluorine on the BTA unit is also effective in improving the photovoltaic performance.

High-Performance Polymer Solar Cells Employing Rhodamines as Cathode Interfacial Layers

The development of simple and water-/alcohol-soluble interfacial materials is crucial for the cost-effective fabrication process of polymer solar cells (PSCs). Herein, highly efficient PSCs are reported employing water-/alcohol-soluble and low-cost rhodamines as cathode interfacial layers (CILs). The results reveal that rhodamine-based CILs can reduce the work function of the Al cathode and simultaneously increase the open-circuit voltage, current density, fill factor, and power conversion efficiency (PCE) of PSCs. The solution-processed rhodamine-based PSCs demonstrated a remarkable PCE of 10.39%, which is one of the best efficiencies reported for thieno[3,4-b]thiophene/benzodithiophene:[6,6]-phenyl C₇₁-butyric acid methyl ester-based PSCs so far. The efficiency is also 42.3% higher than that of the vacuum-deposited Ca-based device (PCE of 7.30%) and 21.5% higher than that of the complicated solution-processable polymeric electrolyte poly[(9,9-bis(3-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]-based device (PCE of 8.55%). Notably, rhodamines are very economical and have been extensively used as dyes in industries. Our work indicates that rhodamines have shown a strong potential as CILs compared to their counterparts in the large-area fabrication process of PSCs.

Efficient Organic Solar Cells with Polymer-Small Molecule: Fullerene Ternary Active Layers

In this study, we report organic solar cells (OSCs) fabricated by a polymer-small molecule: the fullerene ternary active layer. It is found that a significantly enhanced power conversion efficiency contributed to the enhanced short-circuit current density and fill factor (FF). Investigation of absorption spectra and external quantum efficiency spectra indicate that the enhancement in photocurrent originates from the improved light absorption attributed to the small molecule. Further investigations by grazing-incidence wide-angle X-ray scattering, transmission electron microscopy, and atomic force microscopy reveal that charge transport within the ternary active layer is facilitated by a reduced π-π distance between the adjacent polymer chains along the out-of-plane direction, good miscibilities between ternary components, and the rougher surface of the resultant thin film. As a result, the hole mobility of the polymer electron donor and electron mobility of the fullerene electron acceptor are considerably increased, resulting in enhanced FFs. Our studies provide a facile route to realize efficient OSCs.

Fabrication Processes to Generate Concentration Gradients in Polymer Solar Cell Active Layers

Polymer solar cells (PSCs) are considered as one of the most promising low-cost alternatives for renewable energy production with devices now reaching power conversion efficiencies (PCEs) above the milestone value of 10%. These enhanced performances were achieved by developing new electron-donor (ED) and electron-acceptor (EA) materials as well as finding the adequate morphologies in either bulk heterojunction or sequentially deposited active layers. In particular, producing adequate vertical concentration gradients with higher concentrations of ED and EA close to the anode and cathode, respectively, results in an improved charge collection and consequently higher photovoltaic parameters such as the fill factor. In this review, we relate processes to generate active layers with ED–EA vertical concentration gradients. After summarizing the formation of such concentration gradients in single layer active layers through processes such as annealing or additives, we will verify that sequential deposition of multilayered active layers can be an efficient approach to remarkably increase the fill factor and PCE of PSCs. In fact, applying this challenging approach to fabricate inverted architecture PSCs has the potential to generate low-cost, high efficiency and stable devices, which may revolutionize worldwide energy demand and/or help develop next generation devices such as semi-transparent photovoltaic windows.

The marriage of AIE and interface engineering: convenient synthesis and enhanced photovoltaic performance

As a promising option out of all of the well-recognized candidates that have been developed to solve the coming energy crisis, polymer solar cells (PSCs) are a kind of competitive clean energy source. However, as a convenient and efficient method to improve the efficiency of PSCs, the inherent mechanism of the interfacial modification was still not so clear, and interfacial materials constructed with new units were limited to a large degree. Here we present a new kind of interfacial material consisting of AIE units for the first time, with an efficiency of 8.94% being achieved by inserting TPE-2 as a cathode interlayer. This is a relatively high PCE for PC71BM:PTB7-based conventional PSCs with a single-junction structure. Different measurements, including TEM, AFM, SEM, GIXRD, UPS, SKPM, and SCLC, were conducted to investigate the properties in detail. All of the obtained experimental results confirmed the advantages of the utilization of new interfacial materials with AIE characteristics in polymer solar cells, thus providing an additional choice to develop new organic cathode interfacial layers with high performances.

Morphology Control in Films of Isoindigo Polymers by Side-Chain and Molecular Weight Effects

The performance of devices relying on organic electronic materials, such as organic field-effect transistors (OFET) and organic photovoltaics (OPV), is strongly correlated to the morphology of the conjugated material in thin films. For instance, several factors such as polymer solubility, weak intermolecular forces between polymers and fullerene derivatives, and film drying time impact phase separation in the active layer of a bulk heterojunction OPV device. In an effort to probe the influence of polymer assembly on morphology of polymer thin films and phase separation with fullerene derivatives, five terthiophene-alt-isoindigo copolymers were synthesized with alkyl side-chains of varying lengths and branching on the terthiophene unit. These P[T3(R)-iI] polymers were designed to have similar optoelectronic properties but different solubilities in o-dichlorobenzene and were predicted to have different tendencies for crystallization. All polymers with linear alkyl chains exhibit similar thin film morphologies as investigated by grazing-incidence wide-angle X-ray scattering (GIWAXS) and atomic force microscopy (AFM). The main differences in electronic and morphological properties arise when P[T3(R)-iI] is substituted with branched 2-ethylhexyl (2EH) side-chains. The bulky 2EH substituents lead to a blue-shifted absorption, a lower ionization potential, and reduced ordering in polymer thin films. The five P[T3-iI] derivatives span hole mobilities from 1.5 × 10^-3 to 2.8 × 10^-2 cm² V^-1 s^-1 in OFET devices. In OPV devices, the 2EH-substituted polymers yield open-circuit voltages of 0.88 V in BHJ devices yet low short-circuit currents of 0.8 mA cm^-2, which is explained by the large phase separation observed by AFM in blends of P[T3(2EH)-iI] with PC₇₁BM. In these P[T3(R)-iI] systems, the propensity for the polymers to self-assemble prior to aggregation of PC₇₁BM molecules was key to achieving fine phase separation and increased short-circuit currents, eventually resulting in power conversion efficiencies of 5% in devices processed using a single solvent.

Donor-Acceptor Interface Stabilizer Based on Fullerene Derivatives toward Efficient and Thermal Stable Organic Photovoltaics

An interface stabilizer based on alkylation-functionalized fullerene derivatives, [6, 6]-Phenyl-C61-butyric acid (3,5-bis(octyloxy)phenyl)methyl ester (PCB-C8oc), was successfully synthesized and applied for the active layer of Organic Photovoltaics (OPVs). The PCB-C8oc can replace part of the phenyl-C61-buty-ric acid methyl ester (PCBM) and be distributed on the interface of poly(3-hexylthiophene) (P3HT) and PCBM to form P3HT/PCBM/PCB-C8oc ternary blends, leading to thermally stable and efficient organic photovoltaics. The octyl groups of PCB-C8oc exhibit intermolecular interaction with the hexyl groups of P3HT, and the fullerene unit of PCB-C8oc are in tight contact with PCBM. The dual functions of PCB-C8oc will inhibit the phase separation between electron donor and acceptor, thereby improving the stability of devices under long-time thermal annealing at high temperature. When doped with 10 wt % PCB-C8oc, the power conversion efficiency (PCE) of the P3HT system decreased from 3.54% to 2.88% after 48 h of thermal treatment at 150 °C, whereas the PCE of the reference device without PCB-C8oc dramatically dropped from 3.53% to 0.73%. When doping 10 or 20 wt % PCB-C8oc, the unannealed P3HT/PCBM/PCB-C8oc device achieved a higher PCE than the P3HT/PCBM device without any annealing following the same fabricating condition. For the PTB7/PCBM-based devices, after adding only 5 wt % PCB-C8oc, the OPVs also exhibited thermally stable morphology and better device performances. All these results demonstrate that the utilization of alkyl interchain interactions is an effective and practical strategy to control morphological evolution.

Efficient Polymer Solar Cells by Lithium Sulfonated Polystyrene as a Charge Transport Interfacial Layer

In this paper, we report the highly efficient bulk heterojunction (BHJ) polymer solar cells (PSCs) with an inverted device structure via utilizing an ultrathin layer of lithium sulfonated polystyrene (LiSPS) ionomer to reengineer the surface of the solution-processed zinc oxide (ZnO) electron extraction layer (EEL). The unique lithium-ionic conductive LiSPS contributes to enhanced electrical conductivity of the ZnO/LiSPS EEL, which not only facilitates charge extraction from the BHJ active layer but also minimizes the energy loss within the charge transport processes. In addition, the organic-inorganic LiSPS ionomer well circumvents the coherence issue of the organic BHJ photoactive layer on the ZnO EEL. Consequently, the enhanced charge transport and the lowered internal resistance between the BHJ photoactive layer and the ZnO/LiSPS EEL give rise to a dramatically reduced dark saturation current density and significantly minimized charge carrier recombination. As a result, the inverted BHJ PSCs with the ZnO/LiSPS EEL exhibit an approximatively 25% increase in power conversion efficiency. These results indicate our strategy provides an easy, but effective, approach to reach high performance inverted PSCs.

Development of Spiro[cyclopenta[1,2-b:5,4-b']dithiophene-4,9'-fluorene]-Based A-π-D-π-A Small Molecules with Different Acceptor Units for Efficient Organic Solar Cells

Three acceptor-π-donor-π-acceptor (A-π-D-π-A) small molecules (STFYT, STFRDN, and STFRCN) with spiro[cyclopenta[1,2-b:5,4-b']dithiophene-4,9'-fluorene] (STF) as the central donor unit, terthiophene as the π-conjugated bridge, indenedione, 3-ethylrhodanine, or 2-(1,1-dicyanomethylene)rhodanine as the acceptor unit are designed, synthesized, and characterized as electron donor materials in solution-processing organic solar cells (OSCs). The effects of the spiro STF-based central core and different acceptors on the molecular configuration, absorption properties, electronic energy levels, carrier transport properties, the morphology of active layers, and photovoltaic properties are investigated in detail. The three molecules exhibit desirable physicochemical features: wide absorption bands (300-850 nm) and high molar absorption coefficients (4.82 × 10⁴ to 7.56 × 10⁴ M^-1 cm^-1) and relatively low HOMO levels (-5.15 to -5.38 eV). Density functional theory calculations reveal that the spiro STF central core benefits to reduce the steric hindrance effect between the central donor block and terthiophene bridge and suppress excessive intermolecular aggregations. The optimized OSCs based on these molecules deliver power conversion efficiencies (PCEs) of 6.68%, 3.30%, and 4.33% for STFYT, STFRDN, and STFRCN, respectively. The higher PCE of STFYT-based OSCs should be ascribed to its better absorption ability, higher and balanced hole and electron mobilities, and superior active layer morphology as compared to the other two compounds. So far, this is the first example of developing the A-π-D-π-A type small molecules with a spiro central donor core for high-performance OSC applications. Meanwhile, these results demonstrate that using spiro central block to construct A-π-D-π-A molecule is an alternative and effective strategy for achieving high-performance small molecule donor materials.

A New Nonfullerene Electron Acceptor with a Ladder Type Backbone for High-Performance Organic Solar Cells

Nonfullerene acceptor FDICTF (2,9-bis(2methylene-(3-(1,1-dicyanomethylene)indanone))-7,​12-​dihydro-​4,​4,​7,​7,​12,​12-​hexaoctyl-​4H-​cyclopenta[2″,​1″:5,​6;3″,​4″:5',​6']​diindeno[1,​2-​b:1',​2'-​b']dithiophene) modified by fusing the fluorene core in a precursor, yields 10.06% high power conversion efficiency, and demonstrates that the ladder and fused core backbone in A-D-A structure molecules is an effective design strategy for high-performance nonfullerene acceptors.