Macromolecular architectures for organic photovoltaics

Research in the field of organic photovoltaics has gained considerable momentum in the last two decades owing to the need for developing low-cost and efficient energy harvesting systems. Elegant molecular architectures have been designed, synthesized and employed as active materials for photovoltaic devices thereby leading to a better molecular structure-device property relationship understanding. In this perspective, we outline new macromolecular scaffolds that have been designed within the purview of each of the three fundamental processes involving light harvesting, charge separation and charge transport.     

Small molecule semiconductors for high-efficiency organic photovoltaics

Organic photovoltaic cells (OPVs) are a promising cost-effective alternative to silicon-based solar cells, and possess light-weight, low-cost, and flexibility advantages. Significant progress has been achieved in the development of novel photovoltaic materials and device structures in the last decade. Nowadays small molecular semiconductors for OPVs have attracted considerable attention, due to their advantages over their polymer counterparts, including well-defined molecular structure, definite molecular weight, and high purity without batch to batch variations. The highest power conversion efficiencies of OPVs based on small molecular donor/fullerene acceptors or polymeric donor/fullerene acceptors are up to 6.7% and 8.3%, respectively, and meanwhile nonfullerene acceptors have also exhibited some promising results. In this review we summarize the developments in small molecular donors, acceptors (fullerene derivatives and nonfullerene molecules), and donor-acceptor dyad systems for high-performance multilayer, bulk heterojunction, and single-component OPVs. We focus on correlations of molecular chemical structures with properties, such as absorption, energy levels, charge mobilities, and photovoltaic performances. This structure-property relationship analysis may guide rational structural design and evaluation of photovoltaic materials (253 references).     

New benzodithiophene-pyrrolopyrroledione-thienopyrazine random terpolymers for organic photovoltaics

New random ternary copolymers containing fragments of pyrrolo[3,4-c]pyrrole-1,4-dione, 2,3-bis[6-fluoro-9-(2-octyl- dodecyl)-9H-carbazol-3-yl]-3,3′-[5,7-di(thiophen-2-yl)- thieno[3,4-b]pyrazine as electron-withdrawing fragments and benzodithiophene as an electron-donor block have been synthesized. The best power conversion efficiency of 9.18% was achieved for the terpolymer with pyrrolopyrroledione/ thienopyrazine ratio of 1:3. The introduction of the third thieno[3,4-b]pyrazine acceptor block into the structure of the ‘parent’ binary polymer can significantly adjust energy levels and the light absorption range of irregular copolymers, and can enhance photovoltaic properties due to improved absorption capacity and charge transfer.     

Alkyl-functionalized organic dyes for efficient molecular photovoltaics

We designed and synthesized new alkyl-functionalized organic dyes, MK-1 and MK-2, for dye-sensitized solar cells (DSSCs). Based on the MK-2 dye, a high performance of efficiency (eta, 7.7%; short-circuit current density Jsc = 14.0 mA cm-2, open-circuit voltage Voc = 0.74 V, and fill factor FF = 0.74) was achieved under AM 1.5 G irradiation (100 mW cm-2). Remarkably, the relatively higher Voc for DSSCs based on MK-1 and MK-2 dyes, which have long alkyl chains, were observed among the organic dyes caused by the increasing of the electron lifetime in the conduction band of TiO2. Our molecular design of alkyl-functionalized dyes strongly suggests the promising performance of molecular photovoltaics based on organic dyes.     

On the morphology of polymer-based photovoltaics

We review the morphologies of polymer-based solar cells and the parameters that govern the evolution of the morphologies and describe different approaches to achieve the optimum morphology for a BHJ OPV. While there are some distinct differences, there are also some commonalities. It is evident that morphology and the control of the morphology are important for device performance and, by controlling the thermodynamics, in particular, the interactions of the components, and by controlling kinetic parameters, like the rate of solvent evaporation, crystallization and phase separation, optimized morphologies for a given system can be achieved. While much research has focused on P3HT, it is evident that a clearer understanding of the morphology and the evolution of the morphology in low bad gap polymer systems will increase the efficiency further. While current OPVs are on the verge of breaking the 10% barrier, manipulating and controlling the morphology will still be key for device optimization and, equally important, for the fabrication of these devices in an industrial setting. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Understanding organic reactions in water: from hydrophobic encounters to surfactant aggregates

A crucial factor in realising a green chemical process in solution involves the choice of a safe, non-toxic and cheap solvent. Water is the obvious choice. Despite solubility problems, considerable interest has developed recently in organic chemistry in water. This interest also results from the fact that association and chemical reactions often benefit noticeably from the special properties of water, resulting mainly from its small molecular size, its three-dimensional hydrogen-bond network and hydrophobic interactions which are so unique for liquid water. Here we discuss organic reactions and assembly processes in water, largely taken from experiments performed in the authors' laboratories. We show that non-covalent interactions in water can be utilised for fine tuning organic reactions in aqueous media.     

Recent advances in plasmonic organic photovoltaics

Light trapping based on the localized surface-plasmon resonance(LSPR)effect of metallic nanostructures is a promising strategy to improve the device performance of organic solar cells(OSCs).We review recent advances in plasmonic-enhanced OPVs with solution-processed metallic nanoparticles(NPs).The different types of metallic NPs(sizes,shapes,and hybrids),incorporation positions,and NPs with tunable resonance wavelengths toward broadband enhancement are systematically summarized to give a guideline for the realization of highly efficient plasmonic photovoltaics.     

Nature-inspired light-harvesting liquid crystalline porphyrins for organic photovoltaics

A new class of nanoscale light-harvesting discotic liquid crystalline porphyrins, with the same basic structure of the best photoreceptor in nature (chlorophyll), was synthesized. These materials can be exceptionally aligned into a highly ordered architecture in which the columns formed by intermolecular π-π stacking are spontaneously perpendicular to the substrate. The homeotropic alignment, well confirmed by synchrotron X-ray diffraction, could not only provide the most efficient pathway for hole conduction along the columnar axis crossing the device thickness, but also offer the largest area to the incident light for optimized light harvesting. Their preliminary photocurrent generation and photovoltaic performances were also demonstrated. The results provide new and efficient pathways to the development of organic photovoltaics by using homeotropically aligned liquid crystal thin films.     

Effect of solvent additives on bulk heterojunction morphology of organic photovoltaics and their impact on device performance

Morphology of the active layer in an organic photovoltaic (OPV) device is known to have a significant impact on the device performance. It is, however, difficult to characterize nanoscale morphologies in detail, especially at the ensemble level. Herein, we report the utilization of small angle neutron scattering (SANS) to investigate variations in the nanoscale morphologies of the active layer of poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction OPV depending on the composition of casting solvent. Both the power law and the poly hard sphere model were utilized to characterize the state of the donor and acceptor components, respectively, from the obtained SANS data. Furthermore, the relationship between the nanoscale morphology and device performance is outlined. It was found that the use of 2-chlorophenol, a poor solvent for P3HT and, at the same time, a very good solvent for PCBM, leads to nanomorphology featuring ordered, highly crystalline P3HT and small (15.2 nm) PCBM domains. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 128–134     

Special topic on organic and perovskite photovoltaics

<正>Photovoltaics, as a green technology for converting solar energy into electricity, have shown great promise to address the energy and environmental challenges for the sustainability of human society. In recent years, two types of photovoltaics, consisting of organic semiconductors or perovskites as light absorbers, have experienced a rapid development. The development of organic solar cells (OSC)can be dated back to 1986, since the discovery of a bilayer     

Diketopyrrolopyrrole (DPP)-based materials for organic photovoltaics

Diketopyrrolopyrrole (DPP) pigments have been widely used in inks, paints, and plastics since it was first developed back in the early 1970s. The DPP-based materials were gradually exploited by optical and electrical applications for the excellent fluorescent properties and good charge carrier mobility. In recent years, great interest has been focused on developing organic photovoltaic functional materials containing a DPP core and attractive efficiencies have been achieved. This feature article describes the application of DPP-based materials, highlighting the applications in optoelectrical fields.     

Over 16.7% efficiency of ternary organic photovoltaics by employing extra PC71BM as morphology regulator

Ternary organic photovoltaics(OPVs) are fabricated with PBDB-T-2 Cl:Y6(1:1.2, wt/wt) as the host system and extra PC_(71)BM as the third component. The PBDB-T-2 Cl:Y6 based binary OPVs exhibit a power conversion efficiency(PCE) of 15.49% with a short circuit current(J_(SC)) of 24.98 m A cm~(-2), an open circuit voltage(V_(OC)) of 0.868 V and a fill factor(FF) of 71.42%. A 16.71%PCE is obtained in the optimized ternary OPVs with PBDB-T-2 Cl:Y6:PC_(71)BM(1:1.2:0.2, wt/wt) active layer, resulting from the synchronously improved J_(SC) of 25.44 m A cm~(-2), FF of 75.66% and the constant V_(OC)of 0.868 V. The incorporated PC_(71)BM may prefer to mix with Y6 to finely adjust phase separation, domain size and molecular arrangement in ternary active layers, which can be confirmed from the characterization on morphology, 2 D grazing incidence small and wide-angle X-ray scattering, as well as Raman mapping. In addition, PC_(71)BM may prefer to mix with Y6 to form efficient electron transport channels, which should be conducive to charge transport and collection in the optimized ternary OPVs. This work provides more insight into the underlying reasons of the third component on performance improvement of ternary OPVs, indicating ternary strategy should be an efficient method to optimize active layers for synchronously improving photon harvesting, exciton dissociation and charge transport, while keeping the simple cell fabrication technology.     

Stability,encapsulation and large-area fabrication of organic photovoltaics

Organic photovoltaics(OPVs) have become a timely research topic for their advantages of light weight, low cost, low toxicity,environmental adaptability, flexibility, and large-area manufacture, especially after non-fullerene acceptor ITIC reported in 2015.The highest power conversion efficiency(PCE) is currently above 18% for OPV. However, there are still imparities in the efficiency of OPVs when compared with silicon-based photovoltaics, as well as in their shelf life. Compared with inorganicbased photovoltaics, the efficiency of large-area OPVs is lower and the life time of OPVs is shorter. Therefore, such inferior performance of large-area OPVs restricts the commercial development. Based on these constraints, this paper reviews the research work regarding OPVs into three aspects: stability, encapsulation technology, and recent large-area preparation technologies.     

Conjugated polymer consisting of dibenzosilole and quinoxaline as donor materials for organic photovoltaics

The new D–A type polymers poly(dibenzosilole-diphenylquinoxaline) (PSiPDTQ) and dibenzosilole-dibenzophenazine) (PSiFDTQ), both of which adopted benzosilole as a donor, were polymerized through a Suzuki coupling reaction. PSiPDTQ and PSiFDTQ were able to be dissolved in organic solvents and exhibited high thermal stability. Due to the appropriate LUMO energy levels, an effective charge transport was observed in PSiPDTQ and PSiFDTQ. According to X-ray diffraction measurements, a single broad diffraction peak was detected at approximately 20.5°. The π–π stacking distances (d_π) for PSiPDTQ and PSiFDTQ were 4.4 and 4.3 Å, respectively. When PSiPDTQ and PC₇₁BM were blended in a 1:3 ratio and used as the active layer in a solar cell, the resulting V_oc, J_sc, FF and PCE were 0.89 V, 5.1 mA/cm², 30.2% and 1.4%, respectively. For solar cells using a 1:6 ratio of PSiFDTQ to PC₇₁BM, the resulting V_oc, J_sc, FF and PCE were 0.98 V, 3 mA/cm², 52.8% and 1.6%, respectively. In addition, for a PSiPDTQ and PC₇₁BM blended film (1:3 ratio) with an additional layer of PFN, the PCE of the resulting solar cells was improved (relative to solar cells without PFN) to 2.1% due to the interfacial adhesion of PFN.     

Designs and understanding of small molecule-based non-fullerene acceptors for realizing commercially viable organic photovoltaics

Organic photovoltaics (OPVs) have emerged as a promising next-generation technology with great potential for portable, wearable, and transparent photovoltaic applications. Over the past few decades, remarkable advances have been made in non-fullerene acceptor (NFA)-based OPVs, with their power conversion efficiency exceeding 18%, which is close to the requirements for commercial realization. Novel molecular NFA designs have emerged and evolved in the progress of understanding the physical features of NFA-based OPVs in relation to their high performance, while there is room for further improvement. In this review, the molecular design of representative NFAs is described, and their blend characteristics are assessed via statistical comparisons. Meanwhile, the current understanding of photocurrent generation is reviewed along with the significant physical features observed in high-performance NFA-based OPVs, while the challenging issues and the strategic perspectives for the commercialization of OPV technology are also discussed.

This review describes the current understandings and the significant features observed in NFA-based OPVs, with a particular focus on photophysical, electrical, and morphological characteristics.     

Van der Waals effects on structure and optical properties in organic photovoltaics

We have recently reported that dispersion forces can have drastic effects on the structure and charge separation in organic photovoltaics (OPVs) (Martinez et al., J. Phys. Chem. C 121, 20 134 [2017]). Here we investigate dimer complexes formed by the polymer P3HT and the fullerene derivative PCBM. We show how van der Waals (vdW) interactions affect the geometrical structure, which has strong effects on the electronic structure and UV-Vis absorption spectrum. Time-dependent density functional theory calculations demonstrate that the experimentally observed blue-shift of the absorption maximum of P3HT/PCBM OPV cells with respect to pure P3HT results mainly from distortions in P3HT due to vdW interactions between donor and acceptor fragments. Reduced absorption in the red region of the UV-Vis spectrum results from distortions of P3HT and small charge transfer between P3HT and PCBM. These results are in qualitative agreement with experiments and recent theoretical results on the corresponding solid-state films.     

Synthesis and properties of the conjugated polymers with indenoindene and benzimidazole units for organic photovoltaics

A new electron deficient unit, dimethyl‐2H‐benzimidazole (MBI), and dihydroindeno[2,1‐a]indene (ININE) moiety as electron‐rich unit were coupled to synthesize the conjugated polymers containing electron donor–acceptor pair for organic photovoltaics. ININE, MBI, and thiophene (or bithiophene) units were incorporated using Stille and Suzuki polymerization to generate poly(2,7‐(5,5,10,10‐tetrakis(2‐ethylhexyl)‐5,10‐dihydro‐ indeno[2,1‐a]indene)‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2,2‐dimethyl‐2H‐benzimidazole)) (PININEDTMBIs) (or PININEBBTMBIs). In MBI, the sulfur at 2‐position of 2,1,3‐benzothiadiazole (BT) unit was replaced with dialkyl‐substituted carbon, whereas keeping the 1,2‐quinoid form, to improve the solubility of the polymers. The field‐effect hole mobility of PININEBBTMBI was 3.2 × 10^?4 cm²/Vs which was improved as compared to that of PININEDTMBI (2.7 × 10^?5 cm²/Vs) caused by the introduction of bithiophene units. In case of the most efficient polymer, PININEBBTMBI, the device with the configuration of indium tin oxide (ITO)/poly(3,4‐ethylenedioxythiophene) (PEDOT):polystyrene sulfonate (PSS)/polymer:PC₇₁BM(1:4 w/w)/Al, annealed at 100 °C for 10 min demonstrated a open circuit voltage of 0.78 V, a short‐circuit current density of 6.66 mA/cm², and a fill factor of 0.41, leading to the power conversion efficiency of 2.11%, under white‐light illumination (AM 1.5 G, 100 mW/cm²). © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Lifetimes of organic photovoltaics: photooxidative degradation of a model compound

A poly phenylene vinylene (PPV‐type) oligomer used in organic photovoltaics was designed to facilitate the interpretation of mass spectral data. A film of the oligomer was subjected to various degrees of illumination (1000 W m^?2, AM1.5) in air resulting in photooxidation of the material. The surface chemistry was monitored by TOF‐SIMS and XPS. The experiment described accelerated photooxidation without any contributions from interface processes. The photooxidative degradation mechanisms are described starting from the intact molecule through presumably, intermediate photooxidation products to small photooxidation products. The processes are described with various degrees of specificity and with varying degrees of detail. Copyright © 2006 John Wiley & Sons, Ltd.     

Impact of end-capped engineering on the optoelectronic characteristics of pyrene-based non-fullerene acceptors for organic photovoltaics

Pyrene-based molecules are being explored as prospective fullerene-free acceptors for organic solar cells (OSCs), due to their easy accessibility, structural planarity, and excellent electron delocalization. In this work, we successfully designed and analyzed pyrene-based acceptor materials (QL1–QL8) to investigate their photophysical and electro-optical parameters. Various geometric parameters were computed at the MPW1PW91/6-31G(d,p). Advanced quantum chemical approaches were employed to characterize the molecules. All the tailored molecules (QL1–QL8) exhibit a lower bandgap than the reference (R), signifying their superiority. Among these, QL8 was found to have a maximum absorption (λ_max) at 791.37 nm and an optical bandgap (E_LUMO − E_HOMO) minimum of 2.11 eV. Redshifted absorption spectra are observed in both gaseous and solvent phases for all the designed (QL1–QL8) molecules in contrast to R. Among these, QL4 exhibits the highest light harvesting efficiency (0.9826), and open-circuit voltage. A detailed donor–acceptor investigation of QL8/PBDB-T revealed the marvelous charge switching at the donor–acceptor interface. The approach used in this study is anticipated to facilitate the manufacturing of highly efficient OSC molecules.     

Fluorinated AB diblock copolymers and their aggregates in organic solvents

This investigation reported the preparation of fluorinated and nonamphiphilic well‐defined poly(styrene)‐block‐poly(2,2,3,3,4,4,4‐heptafluorobutyl methacrylate) (PS‐b‐PHFBMA) diblock copolymers via atom transfer radical polymerization (ATRP). Their chemical composition, structure, and bulk morphology were thoroughly investigated. In addition, their self‐assembly behavior in a dilute organic mixture solution was investigated. It was found that that the ATRP could be used to prepare the well‐defined fluorinated and nonamphiphilic PS‐b‐PHFBMA diblock copolymers in a controlled manner. The results also showed that abundant morphologies including sphere, worm‐like structure, and vesicle could be formed with different volume ratios of these two solvents, which proves that the nonamphiphilic fluorinated diblock copolymers can self‐assemble in a dilute solution, and the aforementioned reason for self‐assembly was also discussed preliminarily in this work. Finally, the effect of temperature on the aggregates was investigated to verify whether the self‐assembly behavior was to some extent temperature sensitive. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011