Organic Photovoltaic Applications Research Articles

Morphology optimization of photoactive layers in organic solar cells

AbstractOrganic solar cells (OSCs) have unique advantages of light weight, low‐cost solution processing, and capability to be fabricated into flexible and semitransparent devices, which are widely recognized as a promising photovoltaic technology. Photoactive layers of the OSCs are composed of a blend of a p‐type organic semiconductor as a donor (D) and an n‐type organic semiconductor as acceptor (A). The morphology of the active layer with D/A nano‐scaled aggregation and face‐on π‐conjugated packing, and D/A interpenetrating network is crucial for achieving high photovoltaic performance of the OSCs. Therefore, great efforts have been devoted to control and optimize morphology of the active layers. This perspective focuses on the morphological control by solvent/solid processing additives and the morphology optimization by postdeposition treatment with thermal annealing and/or solvent vapor annealing, which have been extensively adopted and exhibit promising positive effect in optimizing the morphology. Representative examples are given and discussed to understand the foundation of the postdeposition treatments on tuning the morphology. Insights into the role of the postdeposition treatments and additive treatments on the morphology optimization will be beneficial to further improvement in morphology optimization for practical organic photovoltaic application.

Molecular Insights into the Mechanical Properties of Polymer–Fullerene Bulk Heterojunctions for Organic Photovoltaic Applications

We investigate the mechanical properties of $\pi$-conjugated polymeric materials composed of regioregular poly(3-hexylthiophene) (P3HT) and fullerene C$_{60}$ using coarse-grained molecular dynamics simulations. Specifically, we perform tensile simulations of P3HT:C$_{60}$ composites with varied degrees of polymerization and C$_{60}$ mass fractions to obtain their stress-strain responses. Decomposition of stress tensor into kinetic energy and virial contributions indicates that the tensile moduli of the pure P3HT samples are greatly dependent on non-bonded interactions and on bonded interactions associated with bond-stretching, while the addition of C$_{60}$ leads to an increase in the tensile modulus originating from enhanced non-bonded interactions associated with C$_{60}$. Additionally, the tensile strength of the P3HT:C$_{60}$ samples correlates well with molecular chain entanglements, which are characterized by the average number of kinks per chain obtained from primitive path analysis. We also find that the upper and lower yield points characterizing strain softening become more pronounced with increasing C$_{60}$ mass fraction. Persistent homology analysis indicates that the emergence of the yield points correlates well with the coalescence of microvoids in the course of tensile deformation, resulting in the generation of larger voids. These results provide a fundamental understanding of the molecular determinants of the mechanical properties of $\pi$-conjugated polymer-fullerene composites, which can also help to interpret and predict the mechanical properties of other polymer composites containing fullerene.

Rational design and crystal structure prediction of ring-fused double-PDI compounds as n-channel organic semiconductors: a DFT study.

In this study, we present an effective molecular design strategy to develop the n-type charge transport characteristics in organic semiconductors, using ring-fused double perylene diimides (DPDIs) as the model compounds. These dimeric-PDIs are formed by joining two separate PDI-units along their bay positions through ring fusion with pyrene, coronene and their N-doped counterparts. The bridging type has a significant steric effect at the annulation positions and controls the molecular geometry, mostly imposing buckling in the structure. The crystal structures of the designed compounds are also theoretically predicted. Thereafter, electronic structure parameters, molecular packing motifs, charge coupling strength and anisotropic mobilities were investigated to understand the charge transport efficiency of these systems. Among all the studied molecules, the 4N-coronene-fused DPDI (DPDI-6) is found to possess a lower LUMO level and a high EA, suggesting air-stable electron injection. Besides, DPDI-6 shows strong intermolecular electron coupling and possesses high electron mobility (μe = 5.31 × 10-2 cm2 V-1 s-1), which is better as compared with the other DPDI-compounds reported here. The DPDIs also possess optical absorption in the UV-visible region, opening up possible applications in organic photovoltaics. Besides, from the non-linear optical (NLO) analysis, DPDI-3 is found to possess the highest first-order hyperpolarizability, which is even better as compared with the reference compound urea, making it a promising candidate for NLO applications.

Solvothermal synthesis and structural characterization of three polyoxotitanium-organic acid clusters.

Three new titanium oxo-clusters Ti4O2(OiPr)10(OOCPhMe)2 (I), Ti6O4(OEt)8(OOCPhMe)8 (II) and Ti6O6(OEt)6(OOCCHPh2)6 (III) were obtained by easy one-step solvothermal reactions of titanium(iv) isopropoxide, alcohols and carboxylic acids. The three compounds were characterized by single-crystal and powder X-ray diffraction, TGA/DSC, optical and electron microscopy, and FTIR and NMR spectroscopy. X-ray powder diffraction and spectroscopy confirmed the purity of the compounds. Structural analysis indicates that in all compounds the titanium(iv) ions are six-coordinated (distorted octahedra). (I) is a tetranuclear complex containing a Ti4(μ4-O)(μ2-O) core, which is linked by two (μ2-OOCPhMe), four (μ2-OiPr) and six OiPr ligands. (II) and (III) are hexanuclear complexes with different cores, respectively Ti6(μ3-O)2(μ2-O)2 and Ti6(μ3-O)6. The coordination sphere of the Ti atoms is filled by eight (μ2-OOCPhMe), two (μ2-OEt) and six OEt in (II) and six (μ2-OOCHPh2) and six OEt in (III). Different steric hindrance of substituents attached to the carboxyl group or different concentrations lead to three main different cluster geometries with two ligands. The tetranuclear and the hexanuclear clusters were obtained with the OOCPhMe ligand, while the hexagonal prism cluster was obtained with the OOCCHPh2 ligand. Hirshfeld surface calculations indicated that the packing is driven by C–O⋯H–C weak hydrogen bonds. The clusters can be used as molecular models of organic molecules bonded to titania surface, used in organic photovoltaic (dye sensitized solar cells) or other optoelectronic applications.

Singlet fission from upper excited singlet states and polaron formation in rubrene film.

Femtosecond fluorescence up-conversion and transient absorption pump-probe setups are applied to study the relaxation dynamics of the lower and upper excited singlet electronic states in easy-to-make rubrene films. Upon 250 nm (4.96 eV) excitation, singlet fission was observed directly from S2 state bypassing S1 state within 30 fs i.e. breaking the classical Kasha rule. From the transient absorption measurements, polaron formation was also detected on the same time scale. Both singlet fission and polaron formation are accelerated from upper excited states compared with S1 state. Our work shows that rubrene films with low degree of crystallinity could display efficient singlet fission, notably in the case of excitation to upper lying electronic states. This can strongly expand the applications of rubrene in organic electronics. Moreover, our results will provide a new direction for synthesizing novel materials with optimized excited state properties for organic photovoltaic applications.

Molecular engineering of α and β peripherally tri-halogenated substituted boron subphthalocyanines as mixed alloys to control physical and electrochemical properties for organic photovoltaic applications

The relative impact of position and type of halogens on the boron subphthalocyanine periphery has been established as well as their ability to form mixed alloys and the impact on organic electronic properties utilizing organic solar cell base data.

High-Efficiency Organic Photovoltaic Cells With an Antimony Quantum Sheet Modified Hole Extraction Layer

2D materials with unique opto-electronic properties have shown immense potential for application in organic photovoltaics (OPVs). In this article, we reported a facile and efficient strategy to enhance the device performance of OPVs by doping 2D material of antimonene quantum sheets (AMQSs) into PEDOT:PSS emulsion. The interfacial properties between the hole extraction layer (HEL) and PBDB-T:ITIC-based active layer were properly meliorated, achieving boosted power conversion efficiency ( PCE ). Results indicate that the introduced AMQSs could help suppress the recombination loss and improve charge extraction efficiency. Over 8.09% enhancement of PCE is observed compared to that of the control cell with pure PEDOT:PSS. This work provides a simple and valid strategy of HEL modification to realize high efficiency OPVs.

Machine learning property prediction for organic photovoltaic devices

Organic photovoltaic (OPV) materials are promising candidates for cheap, printable solar cells. However, there are a very large number of potential donors and acceptors, making selection of the best materials difficult. Here, we show that machine-learning approaches can leverage computationally expensive DFT calculations to estimate important OPV materials properties quickly and accurately. We generate quantitative relationships between simple and interpretable chemical signature and one-hot descriptors and OPV power conversion efficiency (PCE), open circuit potential (Voc), short circuit density (Jsc), highest occupied molecular orbital (HOMO) energy, lowest unoccupied molecular orbital (LUMO) energy, and the HOMO–LUMO gap. The most robust and predictive models could predict PCE (computed by DFT) with a standard error of ±0.5 for percentage PCE for both the training and test set. This model is useful for pre-screening potential donor and acceptor materials for OPV applications, accelerating design of these devices for green energy applications.

Nonfullerene acceptors from thieno[3,2-b]thiophene-fused naphthalene donor core with six-member-ring connection for efficient organic solar cells

Comprehensive design ideas on the fused-ring donor-core in state-of-the-art acceptor-donor-acceptor (A-D-A) nonfullerene acceptors (NFAs) are still of great importance for regulating the electron push-pull effect for the sake of optimal light-harvesting, frontier molecular orbital levels, and finally their photovoltaic properties. Herein, thieno[3,2-b]thiophenes were fused in bay-area of naphthalene via six-member-ring connection, resulting in the formation of dihydropyrenobisthieno[3,2-b]thiophene based octacyclic ladder-type donor core, which was flanked by two 1,1-dicyanomethylene-3-indanone (IC) acceptor motifs with and without 5,6-diflourination, namely PTT-IC and PTT-2FIC, respectively, as novel efficient A-D-A fused-ring electron acceptors (FREAs). Compared with PTT-IC, fluorinated PTT-2FIC possesses narrower optical bandgap of 1.48 eV, better π-π stacking, and its PBDB-T:PTT-2FIC blend film exhibited better morphology, and better hole and electron mobility. As a result, nonfullerene solar cells using PBDB-T:PTT-2FIC as the active layer achieved a decent PCE of 10.40%, with an open-circuit voltage (VOC) of 0.87 V, a fill factor (FF) of 0.65, and a much higher short-circuit current (JSC) of 18.26 mA/cm2. Meanwhile, the PBDB-T:PTT-IC cells delivered a lower JSC of 12.58 mA/cm2 but a higher VOC of 0.99 V, thus resulting in a PCE of 7.39% due to its wider optical bandgap of 1.58 eV and higher LUMO energy level. These results demonstrated that NFAs based on fused-ring donor core from fusing thieno[3,2-b]thiophenes with naphthalene via six-member-ring connection are promising for organic photovoltaic applications.

Organic Semiconductors for Vacuum-Deposited Planar Heterojunction Solar Cells.

Relative to widely used solution-processed bulk heterojunction organic solar cells (OSCs), planar heterojunction (PHJ) OSCs by vacuum-depositing active layers sequentially avoid tedious control of the blend film morphology, and it is easy to understand the physical process at the donor/acceptor interface. Here we summarize the developments of electron donor and acceptor materials for vacuum-deposited PHJ OSCs in the past decades and discuss the relationship between molecular structure and device performance. Finally, the challenges and prospects for the development of vacuum-deposited PHJ OSCs are also proposed. In addition to some basic requirements for high performance organic photovoltaic materials, such as broad and strong absorption, matched energy levels between donors and acceptors, and high charge carrier mobility, we suggest that extending the exciton diffusion length of organic photovoltaic materials should boost PHJ OSCs gradually become another option for organic photovoltaic application.

Solid-State Properties and Spectroscopic Analysis of Thin-Film TPBi

We characterized the prominent electron transport layer 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) via single-crystal X-ray diffraction, grazing incidence X-ray diffraction (GIXRD), infrared reflection absorption spectroscopy (IRRAS), and quantum mechanical calculations. The crystals generated via vapor diffusion are of the orthorhombic space group Pbca, with a unit cell [a = 19.3935(2) b = 12.81750(10) c = 28.5610(3) Å] containing eight TPBi molecules, and screw axes and glide planes along all three crystallographic axes. Thin-film analysis becomes viable with unit cell and symmetry data, and GIXRD measurements, which demonstrate that when the amorphous TPBi thin films are annealed, the molecules preferentially orient with the a–b crystallographic face exposed at the surface and with the central benzene rings oriented 29° from the surface normal. Changes in vibrational modes at the surface, studied via infrared reflection absorption spectroscopy (IRRAS), concur with the X-ray based assignments. A minor conformer of TPBi with C3 symmetry was also identified via computational methods, appearing 0.95 kcal/mol higher in energy at the MP2/6-31G*//B3LYP/6-31G* level of theory. The combined structural insight allows fine-tuning of a device structure for organic light-emitting diodes (OLEDs) and organic photovoltaic applications.

Novel Conjugated Polymers Containing 3-(2-Octyldodecyl)thieno[3,2-b]thiophene as a π-Bridge for Organic Photovoltaic Applications.

3-(2-Octyldodecyl)thieno[3,2-b]thiophen was successfully synthesized as a new π-bridge with a long branched side alkyl chain. Two donor-π-bridge-acceptor type copolymers were designed and synthesized by combining this π-bridge structure, a fluorinated benzothiadiazole acceptor unit, and a thiophene or thienothiophene donor unit, (PT-ODTTBT or PTT-ODTTBT respectively) through Stille polymerization. Inverted OPV devices with a structure of ITO/ZnO/polymer:PC71BM/MoO3/Ag were fabricated by spin-coating in ambient atmosphere or N2 within a glovebox to evaluate the photovoltaic performance of the synthesized polymers (effective active area: 0.09 cm2). The PTT-ODTTBT:PC71BM-based structure exhibited the highest organic photovoltaic (OPV) device performance, with a maximum power conversion efficiency (PCE) of 7.05 (6.88 ± 0.12)%, a high short-circuit current (Jsc) of 13.96 mA/cm2, and a fill factor (FF) of 66.94 (66.47 ± 0.63)%; whereas the PT-ODTTBT:PC71BM-based device achieved overall lower device performance. According to GIWAXS analysis, both neat and blend films of PTT-ODTTBT exhibited well-organized lamellar stacking, leading to a higher charge carrier mobility than that of PT-ODTTBT. Compared to PT-ODTTBT containing a thiophene donor unit, PTT-ODTTBT containing a thienothiophene donor unit exhibited higher crystallinity, preferential face-on orientation, and a bicontinuous interpenetrating network in the film, which are responsible for the improved OPV performance in terms of high Jsc, FF, and PCE.

First theoretical framework of Z-shaped acceptor materials with fused-chrysene core for high performance organic solar cells.

Chrysene core containing fused ring acceptor materials have remarkable efficiency for high performance organic solar cells. Therefore, present study has been carried out with the aim to design chrysene based novel Z-shaped electron acceptor molecules (Z1-Z6) from famous Z-shaped photovoltaic material FCIC (R) for organic photovoltaic applications. End-capped engineering at two electron-accepting end groups 1,1-dicyanomethylene-3-indanone of FCIC is made with highly efficient end-capped acceptor moieties and impact of end-capped modifications on structure-property relationship, photovoltaic and electronic properties of newly designed molecules (Z1-Z6) has been studied in detail through DFT and TDDFT calculations. The efficiencies of the designed molecules are evaluated through energy gaps, exciton binding energy along with transition density matrix (TDM) analysis, reorganizational energy of electron and hole, absorption maxima and open circuit voltage of investigated molecules. The designed molecules exhibit red-shift and intense absorption in near-infrared region (683-749nm) of UV-Vis-NIR absorption spectrum with narrowing of HOMO-LUMO energy gap from 2.31eV in R to 1.95 in eV in Z5. Moreover, reduction in reorganization energy of electron from 0.0071 (R) to 0.0049 (Z5), and enhancement in open circuit voltage from 1.08V in R to 1.20V in Z5 are also observed. Twisted Z-shape of designed molecules prevents self-aggregation that facilitates miscibility of donor and acceptor. Low values of binding energy, excitation energy, and reorganizational energy (electron and hole) suggest that novel designed molecules offer high charge mobilities as compared to FCIC. Our findings indicate that these novel designed molecules can display better photovoltaic parameters and are suitable candidates if used in organic solar cells.

Green poly-lysine as electron-extraction modified layer with over 15% power conversion efficiency and its application in bio-based flexible organic solar cells

Abstract In this study, we first revealed a green material-Poly-lysine as an interfacial layer (IFL) of electron-extraction layer (EEL) and investigated evolution of morphologies of the active layers in organic photovoltaics (OPVs). The surface structure of the active layers was characterized by atomic force microscopy (AFM) and the molecular packing features were revealed by grazing-incidence wide-angle X-ray scattering (GIWAXS). By embedding poly-lysine as IFL, we evidenced the changes of the blend film phase segregation and observed an increase order of molecular packing in preferential face-on orientation. The optimized blend film morphology facilitated the carrier extraction, thereby significantly improved the OPV performance with the best power conversion efficiency (PCE) of 12.5% and 15.3% for the PBDB-T-2Cl:IT-4F and PBDB-T-2F:Y6 blends, respectively. Compared with the controlled devices, the poly-lysine-modified devices showed significantly improved thermal-and photo-stability which keeps 80% of its initial efficiency after 400-h heating and illumination. As the first discovery of its general applicability of the promising IFL material, poly-lysine was further applied to fabricate a flexible OPV device using 100% bio-based polyethylene furandicarboxylate (PEF) substrate and its derived OPV device exhibited a PCE greater than 7% and maintained the 80% efficiency under the high bending radius of 3 mm. The present study suggests the potential application of using bio-based materials for flexible OPV applications.

The development of conjugated polymers as the cornerstone of organic electronics

The expansion of the field of organic electronics has hinged upon structural and fundamental developments in the field of conjugated polymers. Conjugated polymers allow for the manufacture of low-cost, light-weight, flexible organic electronic devices, with notable applications in organic photovoltaics (OPV), organic field effect transistors (OFET), and organic light emitting diodes (OLED). This extraordinary breadth of applications is due to the remarkable diversity of structure, an extensive understanding of structure-function relationships, and the relative ease of structural tuning. Important advancements and the development of such applications is largely due to the evolution of polymer structure, enabled by a broad range of synthetic methods for the tailoring of structures and the development of improved polymerization conditions to limit defects. This perspective provides background from the early days of concerted and focused conjugated polymer research (1970s-1990) to the present, and describes how the tailoring of conjugated polymer structure has now become closely tied to application. A significant focus is also directed to how polymerization methods have evolved from the aggressive, unselective conditions used for oxidative polymerizations to finely tuned transition-metal catalyzed polymerizations that can provide incredibly high structural-fidelity and can proceed through C–H activation. Areas for future work and emerging areas are also discussed, such as high-performing polymers without added structural and synthetic complexity, identifying polymer structures with improved environmental stability, flexible and transient or bioresorbable conjugated polymers, mixed-ion conductors, and photocatalytic and select biological applications.

The Bulk Heterojunction in Organic Photovoltaic, Photodetector, and Photocatalytic Applications.

Organic semiconductors require an energetic offset in order to photogenerate free charge carriers efficiently, owing to their inability to effectively screen charges. This is vitally important in order to achieve high power conversion efficiencies in organic solar cells. Early heterojunction-based solar cells were limited to relatively modest efficiencies (<4%) owing to limitations such as poor exciton dissociation, limited photon harvesting, and high recombination losses. The development of the bulk heterojunction (BHJ) has significantly overcome these issues, resulting in dramatic improvements in organic photovoltaic performance, now exceeding 18% power conversion efficiencies. Here, the design and engineering strategies used to develop the optimal bulk heterojunction for solar-cell, photodetector, and photocatalytic applications are discussed. Additionally, the thermodynamic driving forces in the creation and stability of the bulk heterojunction are presented, along with underlying photophysics in these blends. Finally, new opportunities to apply the knowledge accrued from BHJ solar cells to generate free charges for use in promising new applications are discussed.

Pyrrole-Containing Semiconducting Materials: Synthesis and Applications in Organic Photovoltaics and Organic Field-Effect Transistors.

Organic semiconducting materials derived from π-electron-rich pyrroles have garnered attention in recent years for the development of organic semiconductors. Although pyrrole is the most electron-rich five-membered heteroaromatic ring, it has found few applications in organic photovoltaics and organic field-effect transistors due to synthetic challenges and instability. However, computational modeling assisted screening processes have indicated that relatively stable materials containing pyrrolic units can be synthesized without compromising their inherent electron-donating properties. In this work, we provide a complete, up-to-date review of pyrrole-containing semiconducting materials used for organic photovoltaics and organic field-effect transistors and highlight recent advances in the synthesis of these materials.

Plasmonic effects of copper nanoparticles in polymer photovoltaic devices for outdoor and indoor applications

The use of metal nanoparticles (NPs) that can trigger localized surface plasmon resonance (LSPR) is an effective method for improving the performance of organic photovoltaics (OPVs). Currently, most plasmonic NPs are based on noble metals, including gold and silver; their high cost limits their commercial applications in the cost-effective OPVs. Herein, copper (Cu) NPs, which are more abundant and cheaper, are adopted to fabricate OPVs. To avoid oxidation of Cu NPs, they are positioned at the cathode interface, so that their fabrication could be implemented in an inert environment. The resulting OPVs exhibited improved power conversion efficiencies (PCEs) under illumination at 1 sun, and the device enhancement could be attributed to the LSPR effects of Cu NPs. Further, their potential to enhance the performance of OPVs under indoor lighting conditions is evaluated. The enhancement factor of PCEs was higher, while the light source had a lower color temperature. It could be due to the fact that the main plasmonic band of the Cu NPs is localized in the red spectral range. The results reveal the consideration of matching between the LSPR spectral range and the emission spectra of the artificial light sources is very critical for indoor applications.

Development of Perylene-Based Non-Fullerene Acceptors through Bay-Functionalization Strategy.

Perylene has had a tremendous impact in the history of material research for the molecular semiconductors. Among numerous derivatives of this polyaromatic hydrocarbon, perylene diimide (PDI) represents a promising class of organic materials envisioned as non-fullerene acceptors (NFAs) for the practical organic photovoltaic (OPV) applications due to their enhanced photo- and thermal stability and remarkably high electron affinity, some of which realize band-like transport properties. The present review guides some of the representative achievements in the development of rationally designed PDI systems, highlighting synthetic methodologies based on bay-functionalization strategies for creating well-designed molecular nanostructures and structure-performance relationship of perylene-based small molecular acceptors (SMAs) for the photovoltaic outcomes.

Can Ferroelectricity Improve Organic Solar Cells?

Blends of semiconducting (SC) and ferroelectric (FE) polymers have been proposed for applications in resistive memories and organic photovoltaics (OPV). For OPV, the rationale is that the local electric field associated with the dipoles in a blend could aid exciton dissociation, thus improving power conversion efficiency. However, FE polymers either require solvents or processing steps that are incompatible with those required for SC polymers. To overcome this limitation, SC (poly(3-hexylthiophene)) and FE (poly(vinylidene fluoride-trifluoroethylene)) components are incorporated into a block copolymer and thus a path to a facile fabrication of smooth thin films from suitably chosen solvents is achieved. In this work, the photophysical properties and device performance of organic solar cells containing the aforementioned block copolymer consisting of poly(vinylidene fluoride-trifluoroethylene): P(VDF-TrFE), poly(3-hexylthiophene): P3HT and the electron acceptor phenyl-C61 -butyric acid methyl ester: [60]PCBM are explored. A decrease in photovoltaic performance is observed in blends of the copolymer with P3HT:[60]PCBM, which is attributed to a less favorable nanomorphology upon addition of the copolymer. The role of lithium fluoride (the cathode modification layer) is also clarified in devices containing the copolymer, and it is demonstrated that ferroelectric compensation prevents the ferroelectricity of the copolymer from improving photovoltaic performance in SC-FE blends.