Enhanced π-π Interactions of Nonfullerene Acceptors by Volatilizable Solid Additives in Efficient Polymer Solar Cells

Fine-tuning of the nanoscale morphologies of the active layers in polymer solar cells (PSCs) through various techniques plays a vital role in improving the photovoltaic performance. However, for emerging nonfullerene (NF) PSCs, the morphology optimization of the active-layer films empirically follows the methods originally developed in fullerene-based blends and lacks systematic studies. In this work, two solid additives with different volatilities, SA-4 and SA-7, are applied to investigate their influence on the morphologies and photovoltaic performances of NF-PSCs. Although both solid additives effectively promote the molecular packing of the NF acceptors, due to the higher volatility of SA-4, the devices processed with SA-4 exhibit a power conversion efficiency of 13.5%, higher than that of the control devices, and the devices processed with SA-7 exhibit poor performances. Through a series of detailed morphological analyses, it is found that the volatilization of SA-4 after thermal annealing is beneficial for the self-assembly packing of acceptors, while the residuals due to the incomplete volatilization of SA-7 have a negative effect on the film morphology. The results delineate the feasibility of applying volatilizable solid additives and provide deeper insights into the working mechanism, establishing guidelines for further material design of solid additives.

Unraveling Sunlight by Transparent Organic Semiconductors toward Photovoltaic and Photosynthesis

Because the visible and the infrared (IR) regions take up ∼47% and ∼51% of the energy in the solar spectrum (AM 1.5G standard), respectively, utilizing the visible light for plant growth and the IR light for power generation is potentially extremely exciting. IR-absorbing organic semiconductors, with localized IR absorption and visible-light transmittance, would be promising materials for this purpose. Here, flexible transparent organic photovoltaics (TOPVs) based on IR-absorbing organic materials were proposed, which can be a simple, low-cost, and promising way to utilize the IR light for electricity generation, and the penetrated visible light will be utilized for photosynthesis in plants. A power-conversion efficiency of ∼10% with an average visible transmittance of 34% was achieved for TOPV devices. Meanwhile, the side-by-side comparison showed that plants grown under the TOPVs filtered light, and those under normal sunlight yielded very similar results. These outcomes demonstrated the results from TOPV devices beyond simple photovoltaic applications.

Enabling low voltage losses and high photocurrent in fullerene-free organic photovoltaics

Despite significant development recently, improving the power conversion efficiency of organic photovoltaics (OPVs) is still an ongoing challenge to overcome. One of the prerequisites to achieving this goal is to enable efficient charge separation and small voltage losses at the same time. In this work, a facile synthetic strategy is reported, where optoelectronic properties are delicately tuned by the introduction of electron-deficient-core-based fused structure into non-fullerene acceptors. Both devices exhibited a low voltage loss of 0.57 V and high short-circuit current density of 22.0 mA cm-2, resulting in high power conversion efficiencies of over 13.4%. These unconventional electron-deficient-core-based non-fullerene acceptors with near-infrared absorption lead to low non-radiative recombination losses in the resulting organic photovoltaics, contributing to a certified high power conversion efficiency of 12.6%.

Unique Energy Alignments of a Ternary Material System toward High-Performance Organic Photovoltaics

Incorporating narrow-bandgap near-infrared absorbers as the third component in a donor/acceptor binary blend is a new strategy to improve the power conversion efficiency (PCE) of organic photovoltaics (OPV). However, there are two main restrictions: potential charge recombination in the narrow-gap material and miscompatibility between each component. The optimized design is to employ a third component (structurally similar to the donor or acceptor) with a lowest unoccupied molecular orbital (LUMO) energy level similar to the acceptor and a highest occupied molecular orbital (HOMO) energy level similar to the donor. In this design, enhanced absorption of the active layer and enhanced charge transfer can be realized without breaking the optimized morphology of the active layer. Herein, in order to realize this design, two new narrow-bandgap nonfullerene acceptors with suitable energy levels and chemical structures are designed, synthesized, and employed as the third component in the donor/acceptor binary blend, which boosts the PCE of OPV to 11.6%.

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.

Melt-processing of small molecule organic photovoltaics via bulk heterojunction compatibilization

Melt-processing of organic semiconductors (OSCs) is a promising environmentally-friendly technique that can alleviate dependence on toxic chlorinated solvents. While melt-processed single-component OSC devices (e.g. field-effect-transistors) have been demonstrated, multi-component bulk heterojunctions (BHJs) for organic photovoltaics (OPVs) remain a challenge. Herein, we demonstrate a strategy that affords tunable BHJ phase segregation and domain sizes from a single-phase homogeneous melt by employing strongly-crystalline small-molecule OSCs together with a customized molecular compatibilizing (MCP) additive. An optimized photoactive BHJ with 50 wt% MCP achieved a device power conversion efficiency of ca. 1% after melting the active layer at 240 °C (15 min, followed by slow cooling) before deposition of the top electrode. BHJ morphology characterization using atomic force and Kelvin probe microscopy, X-ray diffraction, and photo-luminescence measurements further demonstrate the trade-off between free charge generation and transport with respect to MCP loading in the BHJ. In addition, a functional OPV was also obtained from the melt-processing of dispersed micron-sized solid BHJ particles into a smooth and homogeneous thin-film by using the MCP approach. These results demonstrate that molecular compatibilization is a key prerequisite for further developments towards true solvent-free melt-processed BHJ OPV systems.

High-Performance Organic Photodetectors from a High-Bandgap Indacenodithiophene-Based π-Conjugated Donor-Acceptor Polymer

A conjugated donor-acceptor polymer, poly[4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro- s-indaceno[1,2- b:5,6- b']dithiophene-2,7-diyl- alt-5-(2-ethylhexyl)-4 H-thieno[3,4- c]pyrrole-4,6(5 H)-dione-1,3-diyl] (PIDT-TPD), is blended with the fullerene derivative [6,6]phenyl-C61-butyric acid methyl ester (PC₆₁BM) for the fabrication of thin and solution-processed organic photodetectors (OPDs). Systematic screening of the concentration ratio of the blend and the molecular weight of the polymer is performed to optimize the active layer morphology and the OPD performance. The device comprising a medium molecular weight polymer (27.0 kg/mol) in a PIDT-TPD:PC₆₁BM 1:1 ratio exhibits an external quantum efficiency of 52% at 610 nm, a dark current density of 1 nA/cm², a detectivity of 1.44 × 10¹³ Jones, and a maximum 3 dB cutoff frequency of 100 kHz at -5 V bias. These results are remarkable among the state-of-the-art red photodetectors based on conjugated polymers. As such, this work presents a functional organic active material for high-speed OPDs with a linear photoresponse at different light intensities.

Can the Dielectric Constant of Fullerene Derivatives Be Enhanced by Side-Chain Manipulation? A Predictive First-Principles Computational Study

The low efficiency of organic photovoltaic (OPV) devices has often been attributed to the strong Coulombic interactions between the electron and hole, impeding the charge separation process. Recently, it has been argued that by increasing the dielectric constant of materials used in OPVs, this strong interaction could be screened. In this work, we report the application of periodic density functional theory together with the coupled perturbed Kohn–Sham method to calculate the electronic contribution to the dielectric constant for fullerene C₆₀ derivatives, a ubiquitous class of molecules in the field of OPVs. The results show good agreement with experimental data when available and also reveal an important undesirable outcome when manipulating the side chain to maximize the static dielectric constant: in all cases, the electronic contribution to the dielectric constant decreases as the side chain increases in size. This information should encourage both theoreticians and experimentalists to further investigate the relevance of contributions to the dielectric constant from slower processes like vibrations and dipolar reorientations for facilitating the charge separation, because electronically, enlarging the side chain of conventional fullerene derivatives only lowers the dielectric constant, and consequently, their electronic dielectric constant is upper bound by the one of C₆₀.

Ternary System with Controlled Structure: A New Strategy toward Efficient Organic Photovoltaics

Recently, a new type of active layer with a ternary system has been developed to further enhance the performance of binary system organic photovoltaics (OPV). In the ternary OPV, almost all active layers are formed by simple ternary blend in solution, which eventually leads to the disordered bulk heterojunction (BHJ) structure after a spin-coating process. There are two main restrictions in this disordered BHJ structure to obtain higher performance OPV. One is the isolated second donor or acceptor domains. The other is the invalid metal-semiconductor contact. Herein, the concept and design of donor/acceptor/acceptor ternary OPV with more controlled structure (C-ternary) is reported. The C-ternary OPV is fabricated by a sequential solution process, in which the second acceptor and donor/acceptor binary blend are sequentially spin-coated. After the device optimization, the power conversion efficiencies (PCEs) of all OPV with C-ternary are enhanced by 14-21% relative to those with the simple ternary blend; the best PCEs are 10.7 and 11.0% for fullerene-based and fullerene-free solar cells, respectively. Moreover, the averaged PCE value of 10.4% for fullerene-free solar cell measured in this study is in great agreement with the certified one of 10.32% obtained from Newport Corporation.

Effect of fullerene acceptor on the performance of solar cells based on PffBT4T-2OD

We have studied organic solar cells composed of PffBT4T-2OD as electron donor and three different electron accepting fullerenes, in order to understand the impact of different fullerenes on the morphology and efficiency of the corresponding photovoltaic devices.

Polymer semiconductors incorporating head-to-head linked 4-alkoxy-5-(3-alkylthiophen-2-yl)thiazole

Head-to-head linked bithiophenes with planar backbones hold distinctive advantages for constructing organic semiconductors, such as good solubilizing capability, enabling narrow bandgap, and effective tuning of frontier molecular orbital (FMO) levels using minimal thiophene numbers. In order to realize planar backbone, alkoxy chains are typically installed on thiophene head positions, owing to the small van der Waals radius of oxygen atom and accompanying noncovalent S⋯O interaction. However, the strong electron donating alkoxy chains on the electron-rich thiophenes lead to elevated FMO levels, which are detrimental to material stability and device performance. Thus, a new design approach is needed to counterbalance the strong electron donating property of alkoxy chains to bring down the FMOs. In this study, we designed and synthesized a new head-to-head linked building block, 4-alkoxy-5-(3-alkylthiophen-2-yl)thiazole (TRTzOR), using an electron-deficient thiazole to replace the electron-rich thiophene. Compared to previously reported 3-alkoxy-3′-alkyl-2,2′-bithiophene (TRTOR), TRTzOR is a weaker electron donor, which considerably lowers FMOs and maintains planar backbone through the noncovalent S⋯O interaction. The new TRTzOR was copolymerized with benzothiadiazoles with distinct F numbers to yield a series of polymer semiconductors. Compared to TRTOR-based analogous polymers, these TRTzOR-based polymers have broader absorption up to 950 nm with lower-lying FMOs by 0.2–0.3 eV, and blending these polymers with PC₇₁BM leads to polymer solar cells (PSCs) with improved open-circuit voltage (V_oc) by ca. 0.1 V and a much smaller energy loss (E_loss) as low as 0.59 eV. These results demonstrate that thiazole substitution is an effective approach to tune FMO levels for realizing higher V_ocs in PSCs and the small E_loss renders TRTzOR a promising building block for developing high-performance organic semiconductors.

A new head-to-head linked thienylthiazole was synthesized, enabling polymer semiconductors with low energy loss of 0.59 eV in solar cells.

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

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

Solution-Phase Conformation and Dynamics of Conjugated Isoindigo-Based Donor-Acceptor Polymer Single Chains

Conjugated polymers are the key material in thin-film organic optoelectronic devices due to the versatility of these molecules combined with their semiconducting properties. A molecular-scale understanding of conjugated polymers is important to the optimization of the thin-film morphology. We examine the solution-phase behavior of conjugated isoindigo-based donor-acceptor polymer single chains of various chain lengths using atomistic molecular dynamics simulations. Our simulations elucidate the transition from a rod-like to a coil-like conformation from an analysis of normal modes and persistence length. In addition, we find another transition based on the solvent environment, contrasting the coil-like conformation in a good solvent with a globule-like conformation in a poor solvent. Overall, our results provide valuable insights into the transition between conformational regimes for conjugated polymers as a function of both the chain length and the solvent environment, which will help to accurately parametrize higher level models.

Tuning Surface Energy of Conjugated Polymers via Fluorine Substitution of Side Alkyl Chains: Influence on Phase Separation of Thin Films and Performance of Polymer Solar Cells

Different contents of fluorine in side alkyl chains were incorporated into three conjugated polymers (namely, PBDTTT-f13, PBDTTT-f9, and PBDTTT-f5) whose backbones consist of benzodithiophene donors and thienothiophene acceptors. These three fluorinated polymers, in comparison with the well-known analogue PTB7-Th, show comparable energy levels and optical band gaps. However, the fluorination of side alkyl chains significantly changed the surface energy of bulk materials, which leads to distinctly different self-assembly behaviors and phase separations as being mixed with PC₇₁BM. The increased mismatch in surface energies between the polymer and PC₇₁BM causes larger scale phase domains, which makes a sound explanation for the difference in their photovoltaic properties.

Energy Transfer Kinetics in Photosynthesis as an Inspiration for Improving Organic Solar Cells

Clues to designing highly efficient organic solar cells may lie in understanding the architecture of light-harvesting systems and exciton energy transfer (EET) processes in very efficient photosynthetic organisms. Here, we compare the kinetics of excitation energy tunnelling from the intact phycobilisome (PBS) light-harvesting antenna system to the reaction center in photosystem II in intact cells of the cyanobacterium Acaryochloris marina with the charge transfer after conversion of photons into photocurrent in vertically aligned carbon nanotube (va-CNT) organic solar cells with poly(3-hexyl)thiophene (P3HT) as the pigment. We find that the kinetics in electron hole creation following excitation at 600 nm in both PBS and va-CNT solar cells to be 450 and 500 fs, respectively. The EET process has a 3 and 14 ps pathway in the PBS, while in va-CNT solar cell devices, the charge trapping in the CNT takes 11 and 258 ps. We show that the main hindrance to efficiency of va-CNT organic solar cells is the slow migration of the charges after exciton formation.

Hexafluoroquinoxaline Based Polymer for Nonfullerene Solar Cells Reaching 9.4% Efficiency

Through introducing six fluorine atoms onto quinoxaline (Qx), a new electron acceptor unit-hexafluoroquinoxaline (HFQx) is first synthesized. On the basis of this unit, we synthesize a new donor-acceptor (D-A) copolymer (HFQx-T), which is composed of a benzodithiophene (BDT) derivative donor block and an HFQx accepting block. The strong electron-withdrawing properties of fluorine atoms increase significantly the open-circuit voltage (V_oc) by tuning the highest occupied molecular orbital (HOMO) energy level. In addition, fluorine atoms enhance the absorption coefficient of the conjugated copolymer and change the film morphology, which implies an increase of the short-circuit current density (J_sc) and fill factor (FF). Indeed, the HFQx-T:ITIC blended film achieves an impressive power conversion efficiency (PCE) of 9.4% with large short-current density (J_sc) of 15.60 mA/cm², high V_oc of 0.92 V, and FF of 65% via two step annealing (thermal annealing (TA) and solvent vapor annealing (SVA) treatments). The excellent results obtained show that the new copolymer HFQx-T synthesized could be a promising candidate for organic photovoltaics.

Effect of Annealing on Exciton Diffusion in a High Performance Small Molecule Organic Photovoltaic Material

Singlet exciton diffusion was studied in the efficient organic photovoltaic electron donor material DTS(FBTTh₂)₂. Three complementary time-resolved fluorescence measurements were performed: quenching in planar heterojunctions with an electron acceptor, exciton-exciton annihilation, and fluorescence depolarization. The average exciton diffusivity increases upon annealing from 1.6 × 10^-3 to 3.6 × 10^-3 cm² s^-1, resulting in an enhancement of the mean two-dimensional exciton diffusion length (L_D = (4Dτ)^1/2) from 15 to 27 nm. About 30% of the excitons get trapped very quickly in as-cast films. The high exciton diffusion coefficient of the material leads to it being able to harvest excitons efficiently from large donor domains in bulk heterojunctions.

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.

Bulk Heterojunction Morphologies with Atomistic Resolution from Coarse-Grain Solvent Evaporation Simulations

Control over the morphology of the active layer of bulk heterojunction (BHJ) organic solar cells is paramount to achieve high-efficiency devices. However, no method currently available can predict morphologies for a novel donor–acceptor blend. An approach which allows reaching relevant length scales, retaining chemical specificity, and mimicking experimental fabrication conditions, and which is suited for high-throughput schemes has been proven challenging to find. Here, we propose a method to generate atom-resolved morphologies of BHJs which conforms to these requirements. Coarse-grain (CG) molecular dynamics simulations are employed to simulate the large-scale morphological organization during solution-processing. The use of CG models which retain chemical specificity translates into a direct path to the rational design of donor and acceptor compounds which differ only slightly in chemical nature. Finally, the direct retrieval of fully atomistic detail is possible through backmapping, opening the way for improved quantum mechanical calculations addressing the charge separation mechanism. The method is illustrated for the poly(3-hexyl-thiophene) (P3HT)–phenyl-C61-butyric acid methyl ester (PCBM) mixture, and found to predict morphologies in agreement with experimental data. The effect of drying rate, P3HT molecular weight, and thermal annealing are investigated extensively, resulting in trends mimicking experimental findings. The proposed methodology can help reduce the parameter space which has to be explored before obtaining optimal morphologies not only for BHJ solar cells but also for any other solution-processed soft matter device.

Realizing Small Energy Loss of 0.55 eV, High Open-Circuit Voltage >1 V and High Efficiency >10% in Fullerene-Free Polymer Solar Cells via Energy Driver

A new, easy, and efficient approach is reported to enhance the driving force for charge transfer, break tradeoff between open-circuit voltage and short-circuit current, and simultaneously achieve very small energy loss (0.55 eV), very high open-circuit voltage (>1 V), and very high efficiency (>10%) in fullerene-free organic solar cells via an energy driver.

Abnormal strong burn-in degradation of highly efficient polymer solar cells caused by spinodal donor-acceptor demixing

The performance of organic solar cells is determined by the delicate, meticulously optimized bulk-heterojunction microstructure, which consists of finely mixed and relatively separated donor/acceptor regions. Here we demonstrate an abnormal strong burn-in degradation in highly efficient polymer solar cells caused by spinodal demixing of the donor and acceptor phases, which dramatically reduces charge generation and can be attributed to the inherently low miscibility of both materials. Even though the microstructure can be kinetically tuned for achieving high-performance, the inherently low miscibility of donor and acceptor leads to spontaneous phase separation in the solid state, even at room temperature and in the dark. A theoretical calculation of the molecular parameters and construction of the spinodal phase diagrams highlight molecular incompatibilities between the donor and acceptor as a dominant mechanism for burn-in degradation, which is to date the major short-time loss reducing the performance and stability of organic solar cells.

Li et al. study degradation in organic photovoltaics from a morphological perspective. They find that donor and acceptor phases undergo excessive demixing via spinodal decomposition resulting in a reduction of charge generation. Demixing is due to the inherently low miscibility of both materials.

Simulating charge transport in organic semiconductors and devices: a review

Charge transport simulation can be a valuable tool to better understand, optimise and design organic transistors (OTFTs), photovoltaics (OPVs), and light-emitting diodes (OLEDs). This review presents an overview of common charge transport and device models; namely drift-diffusion, master equation, mesoscale kinetic Monte Carlo and quantum chemical Monte Carlo, and a discussion of the relative merits of each. This is followed by a review of the application of these models as applied to charge transport in organic semiconductors and devices, highlighting in particular the insights made possible by modelling. The review concludes with an outlook for charge transport modelling in organic electronics.

Impact of solution phase behaviour and external fields on thin film morphology: PCBM and RRa-P3HT model system

We report the impact of the ternary solution phase behaviour on the film morphology and crystallization of a model polymer:fullerene system. We employ UV-Vis absorption spectroscopy, combined with sequential filtration and dilution, to establish the phase diagram for regio-random poly(3-hexylthiophene-2,5-diyl) and phenyl-C61-butyric acid methyl ester (PCBM) in chlorobenzene. Films are systematically cast from one- and two-phase regions decoupling homogeneous and heterogenous nucleation, and the role of pre-formed aggregates from solutions. Increasing annealing temperature from 120 to 200 °C reveals a highly non-monotonic nucleation profile with a maximum at 170 °C, while the crystal growth rate increases monotonically. UV ozonolysis is employed to vary substrate energy, and found to increase nucleation rate and to promote a binary crystallization process. As previously found, exposure to light, under an inert atmosphere, effectively suppresses homogeneous nucleation; however, it has a considerably smaller effect on heterogeneous nucleation, either from solution aggregates or substrate-driven. Our results establish a quantitative link between solution thermodynamics, crystallization and provide insight into morphological design based on processing parameters in a proxy organic photovoltaic system.

Indeno[1,2-b]fluorene-based novel donor–acceptor conjugated copolymers

A series of conjugated copolymers based on indeno[1,2-b]fluorene as donor unit with different acceptor units have been synthesized to explore the influences of molecular backbone planarization and acceptor electronegativity on charge transport and photovoltaic properties. Polymer incorporating 2,3-diphenylquinoxaline acceptor moiety shows poor light-harvesting capacity and inferior photovoltaic efficiency of 0.5% due to twisted geometry. By introducing the stronger acceptor thiadiazolo[3,4-c]pyridine in polymer, intramolecular charge transfer is enhanced, giving rise to improved absorption property and photovoltaic efficiency of 1.39%. However, the polymer backbone is still twisted. When thiophene-flanked diketopyrrolopyrrole (DPP) is incorporated as electron acceptor, the polymer exhibits a more planar molecular geometry, yielding a broader and red-shifted absorption spectrum as well as a significantly improved hole mobility of 1.46E-2 cm2 V−1 s−1. However, the photovoltaic device efficiency is only enhanced to be 1.69%. The low-lying lowest unoccupied molecular orbital of −3.95 eV as a result of the strong electron deficiency of the DPP unit may lead to the inefficient charge dissociation and increase the charge recombination, which may give rise to the limit photovoltaic performance.

Core/Shell Conjugated Polymer/Quantum Dot Composite Nanofibers through Orthogonal Non-Covalent Interactions

Nanostructuring organic polymers and organic/inorganic hybrid materials and controlling blend morphologies at the molecular level are the prerequisites for modern electronic devices including biological sensors, light emitting diodes, memory devices and solar cells. To achieve all-around high performance, multiple organic and inorganic entities, each designed for specific functions, are commonly incorporated into a single device. Accurate arrangement of these components is a crucial goal in order to achieve the overall synergistic effects. We describe here a facile methodology of nanostructuring conjugated polymers and inorganic quantum dots into well-ordered core/shell composite nanofibers through cooperation of several orthogonal non-covalent interactions including conjugated polymer crystallization, block copolymer self-assembly and coordination interactions. Our methods provide precise control on the spatial arrangements among the various building blocks that are otherwise incompatible with one another, and should find applications in modern organic electronic devices such as solar cells.

Following the TRMC Trail: Optimization of Photovoltaic Efficiency and Structure-Property Correlation of Thiophene Oligomers

Semiconducting conjugated oligomers having same end group (N-ethylrhodanine) but different central core (thiophene: OT-T, bithiophene: OT-BT, thienothiophene: OT-TT) connected through thiophene pi-linker (alkylated terthiophene) were synthesized for solution processable bulk-heterojunction solar cells. The effect of the incorporation of an extra thiophene to the central thiophene unit either through C-C bond linkage to form bithiophene or by fusing two thiophenes together to form thienothiophene on the optoelectronic properties and photovoltaic performances of the oligomers were studied in detail. Flash photolysis time-resolved microwave conductivity (FP-TRMC) technique shows OT-TT has significantly higher photoconductivity than OT-T and OT-BT implying that the former can outperform the latter two derivatives by a wide margin under identical conditions in a bulk-heterojunction solar cell device. However, the initial photovoltaic devices fabricated from all three oligomers (with PC71BM as the acceptor) gave power conversion efficiencies (PCEs) of about 0.7%, which was counterintuitive to the TRMC observation. By using TRMC results as a guiding tool, solution engineering was carried out; no remarkable changes were seen in the PCE of OT-T and OT-BT. On the other hand, 5-fold enhancement in the device efficiency was achieved in OT-TT (PCE: 3.52%, VOC: 0.80 V, JSC: 8.74 mA cm(-2), FF: 0.50), which was in correlation with the TRMC results. The structure-property correlation and the fundamental reasons for the improvement in device performance upon solvent engineering were deduced through UV-vis absorption, atomic force microscopy, bright-field transmission electron microscopy, photoluminescence quenching analysis and two-dimensional grazing incidence X-ray diffraction studies.

Solution processed reduced graphene oxide electrodes for organic photovoltaics

Since the isolation of free standing graphene in 2004, graphene research has experienced a phenomenal growth. Due to its exceptional electronic, optical and mechanical properties, graphene is believed to be the next wonder material for optoelectronics. The enhanced electrical conductivity, combined with its high transparency in the visible and near-infrared regions of the spectrum, enabled graphene to be an ideal low cost indium-tin oxide (ITO) substitute. Solution-processed reduced graphene oxide combines the unique optoelectrical properties of graphene with large area deposition and flexible substrates rendering it compatible with roll-to-roll manufacturing methods. This paper provides an overview of recent research progress in the application and consequent physical-chemical properties of solution-processed reduced graphene oxide-based films as transparent conductive electrodes (TCEs) in organic photovoltaic (OPV) cells. Reduced graphene oxide (rGO) can be effectively utilized as the TCE in flexible OPVs, where the brittle and expensive ITO is incompatible. The prospects and future research trends in graphene-based TCEs are also discussed.

A "roller-wheel" Pt-containing small molecule that outperforms its polymer analogs in organic solar cells

A novel Pt-bisacetylide small molecule (Pt-SM) featuring "roller-wheel" geometry was synthesized and characterized. When compared with conventional Pt-containing polymers and small molecules having "dumbbell" shaped structures, Pt-SM displays enhanced crystallinity and intermolecular π-π interactions, as well as favorable panchromatic absorption behaviors. Organic solar cells (OSCs) employing Pt-SM achieve power conversion efficiencies (PCEs) up to 5.9%, the highest reported so far for Pt-containing polymers and small molecules.

Synergetic Solvent Engineering of Film Nanomorphology to Enhance Planar Perylene Diimide-Based Organic Photovoltaics

Solvent additive has proven as a useful protocol for improving the film nanomorphology of polymer donor (D): fullerene acceptor (A) blends in bulk heterojunction (BHJ) photovoltaic cells. By contrast, the effect of such solvent additive on nonfullerene BHJ cells based on perylene diimide acceptor, for instance, is less effective because of their highly planar structure and strong π-aggregation in solid state. Here we choose N,N'-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide (PDI) and thieno[3,4-b]thiophene-alt-benzodithiophene (PTB7) as a model D:A blend system to investigate how solvent engineering strategy synergistically impacts the blend film nanomorphology. Based on the differences of solvent volatility and solubility, various host solvents-chloroform (CF) and chlorobenzene (CB) and solvent additives-chloronaphthalene (CN) and 1,8-diiodooctane (DIO) are selected for comparative studies. It is found that the π-aggregation of PDIs can be largely suppressed by using low-boiling point (Tb) CF solvent, yet enlarged by using high-Tb CB. Moreover, CN additive provides good solubility of PDI molecules and hence reduces large PDI aggregates in CB system, while DIO exhibiting poor solubility works oppositely. By contrast, DIO that presents larger Tb difference with CF prolongs the film-forming, which assists in optimizing the PDI aggregation and increases the intermixed PTB7:PDI phases more significantly than CN in CF system, yielding the finest phase-separation morphology and balanced charge mobility. Consequently, the inverted BHJ cells based on CF-processed PTB7:PDI blend film with 0.4 vol % DIO exhibit the highest PCE of 3.55% with a fill factor of 56%, both of which are among the best performance for such a paradigm PTB7:PDI blend-based BHJ cells.

Graphene and Carbon Quantum Dot-Based Materials in Photovoltaic Devices: From Synthesis to Applications

Graphene and carbon quantum dots have extraordinary optical and electrical features because of their quantum confinement properties. This makes them attractive materials for applications in photovoltaic devices (PV). Their versatility has led to their being used as light harvesting materials or selective contacts, either for holes or electrons, in silicon quantum dot, polymer or dye-sensitized solar cells. In this review, we summarize the most common uses of both types of semiconducting materials and highlight the significant advances made in recent years due to the influence that synthetic materials have on final performance.