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Perylene monoimide based electron acceptors have great properties for use in organic solar cells, like thermal stability, strong absorption, and simple synthesis. However, they typically exhibit low values for the dielectric permittivity. This hinders efficient exciton dissociation, limiting the achievable power conversion efficiencies. In this work, we present the synthesis and utilization of two new acceptor–donor-acceptor (A-D-A) molecules, comprising perylene monoimide as electron withdrawing A unit. Oligo ethylene glycol side chain modified carbazole (PMI-[C-OEG]) and fluorene (PMI-[F-OEG]) linkers were used as electron rich D units, respectively. The polar side chains are expected to increase the polarizability of the molecules and, thus, their permittivity according to the Clausius–Mossotti relationship. We found that the incorporation of glycol chains improved the dielectric properties of both materials in comparison to the reference compounds with alkyl chains. The permittivity increased by 18% from 3.17 to 3.75 for the carbazole-based non-fullerene acceptor PMI-[C-OEG] and by 12% from 3.10 to 3.47 for the fluorene-based acceptor PMI-[F-OEG]. The fabricated solar cells revealed power conversion efficiencies of 3.71 ± 0.20% (record 3.92%) with PMI-[C-OEG], and 1.21 ± 0.06% (record 1.51%) with PMI-[F-OEG]. [ABSTRACT FROM AUTHOR]

A model of the photocurrent generation of charge carriers in blends of donor (D) and acceptor (A) materials structured on the nanoscale is considered. The absorption of a quantum of light in one of these materials creates a molecular exciton, which can reach the interface between the D and A phases and form an interfacial charge transfer (CT) exciton on this interface, which dissociates into an electron-hole pair. The probabilities of the dissociation of CT excitons into free current carriers are calculated as a function of the electric field and the thermalization length of the electron-hole pair. [ABSTRACT FROM AUTHOR]

Highlights: Two series of terpolymers with improved photostability were realized by the introduction of appropriate ratio of antioxidant butylated hydroxytoluene unit containing side chains. All-polymer solar cells and organic photodetectors (OPDs) with these terpolymers as both donors and acceptors have been prepared with simultaneously improved efficiency and stability. A feasible approach to develop terpolymers with antioxidant efficacy for improving the lifetime of organic solar cells and OPDs is proposed. It is of vital importance to improve the long-term and photostability of organic photovoltaics, including organic solar cells (OSCs) and organic photodetectors (OPDs), for their ultimate industrialization. Herein, two series of terpolymers featuring with an antioxidant butylated hydroxytoluene (BHT)-terminated side chain, PTzBI-EHp-BTBHTx and N2200-BTBHTx (x = 0.05, 0.1, 0.2), are designed and synthesized. It was found that incorporating appropriate ratio of benzothiadiazole (BT) with BHT side chains on the conjugated backbone would induce negligible effect on the molecular weight, absorption spectra and energy levels of polymers, however, which would obviously enhance the photostability of these polymers. Consequently, all-polymer solar cells (all-PSCs) and photodetectors were fabricated, and the all-PSC based on PTzBI-EHp-BTBHT0.05: N2200 realized an optimal power conversion efficiency (PCE) approaching ~ 10%, outperforming the device based on pristine PTzBI-EHp: N2200. Impressively, the all-PSCs based on BHT-featuring terpolymers displayed alleviated PCEs degradation under continuous irradiation for 300 h due to the improved morphological and photostability of active layers. The OPDs based on BHT-featuring terpolymers achieved a lower dark current at − 0.1 bias, which could be stabilized even after irradiation over 400 h. This study provides a feasible approach to develop terpolymers with antioxidant efficacy for improving the lifetime of OSCs and OPDs. [ABSTRACT FROM AUTHOR]

With the rapid increase in photoelectric conversion efficiency of organic photovoltaics (OPVs), prolonging the operational lifetime of devices becomes one of the critical prerequisites for commercial applications. Guided by the theoretical calculations of molecular stacking and miscibility, we proposed an effective approach to simultaneously improve device performance and thermal stability of high-efficiency OPVs by refining the aggregation of Y-series acceptors. The key to this approach is deliberately designing an asymmetric Y-series acceptor, named Y6-CNO, which acts as a third component regulator to finely tune the degree of acceptor aggregation and crystallization in the benchmark PM6:Y6-BO system. Strikingly, a champion photovoltaic efficiency of 18.0% was achieved by introducing 15 wt% Y6-CNO into the PM6:Y6-BO system, significantly higher than the control binary cell (16.7%). Moreover, annealing at 100 °C for over 1,200 h does not markedly affect the photovoltaic performance of the optimal ternary devices, maintaining above 95% of the initial performance and exhibiting an exceptionally high T80 lifetime of 9,000 h under continuous thermal annealing. By contrast, binary devices suffer from excessive crystallization of acceptors with long-term annealing. Additionally, mixing thermodynamics combined with morphological characterizations were employed to elucidate the microstructure-thermal stability relationships. The ternary OPVs consisting of symmetric and asymmetric homologous acceptors form better charge transport channels and can effectively suppress excessive aggregation of acceptors under long-term annealing. This work demonstrates the effectiveness of refining acceptor aggregation via molecular design for highly efficient and stable nonfullerene-based OPVs. [ABSTRACT FROM AUTHOR]

It is urgent to address climate change by radically changing our energy sources. Organic photovoltaics (OPVs) are a competitive clean energy emerging technology and will undoubtedly have a market niche in a world that needs to take advantage of every possible type of renewable energy. Recent studies have brought relevant improvements on internal efficiency, focusing on two properties at the interface: energetic disorder and bending. However, how positional disorder affects internal efficiency is still an open question. Here, we show that positional disorder is desired at the interface, but only up to a threshold value of 0.2 nm for poly p-phenylene vinylene. Using a kinetic Monte Carlo simulator, we realized that not enough excitons were reaching the interface, and introduced the Beer-Lambert law of attenuance to correct it. Furthermore, we realized that the same disorder that facilitates charge separation at the interface diminishes exciton and charge mobility in bulk, so we propose here a new morphology for the active layer of OPVs. Our suggestion implicates in better overall performance, improving not just the internal but the overall cell efficiency. [ABSTRACT FROM AUTHOR]

A novel two-dimensional A-D-A acceptor named as CH8 with four electron-withdrawing end units has been successfully designed and synthesized. The enlarged conjugation in two directions renders CH8 exhibit an extremely low electron reorganization energy of 98 meV, which makes CH8 a potential candidate for outstanding organic semiconductor material. When blended with PM6, a considerate power conversion efficiency of 9.37% along with a high open-circuit voltage (Voc) 0.889 V and low energy loss (Eloss) below 0.6 eV is achieved. These results indicate that the two-dimensional A-D-A molecule with four electron-withdrawing end units is an effective molecular design strategy to achieve lower voltage loss and also possible high performance for organic photovoltaics if ideal morphology could be achieved. [ABSTRACT FROM AUTHOR]

A novel two-dimensional A-D-A acceptor named as CH8 with four electron-withdrawing end units has been successfully designed and synthesized. The enlarged conjugation in two directions renders CH8 exhibit an extremely low electron reorganization energy of 98 meV, which makes CH8 a potential candidate for outstanding organic semiconductor material. When blended with PM6, a considerate power conversion efficiency of 9.37% along with a high open-circuit voltage (Voc) 0.889 V and low energy loss (Eloss) below 0.6 eV is achieved. These results indicate that the two-dimensional A-D-A molecule with four electron-withdrawing end units is an effective molecular design strategy to achieve lower voltage loss and also possible high performance for organic photovoltaics if ideal morphology could be achieved. [ABSTRACT FROM AUTHOR]

Interfacial regulation, serving multiple roles, is critical for the fabrication of stable and efficient organic photovoltaics (OPVs). Herein, a multifunctional cathode interlayer PDINO (15 nm) is prepared by regulating film thickness, which is inserted between active components and stable silver electrode to align work function, and maintain good interfacial contact and device stability. The thick film can help to reduce interfacial surface defects, keep stable surface morphology, and block the silver diffusion into the active layer. Consequently, the optimal PM6:Y6 device records an impressive power conversion efficiency (PCE) of 17.48% with minimized non-radiative recombination loss of 0.239 V. More importantly, the unencapsulated device maintains 91% of the original PCE after aging for over 60 days at 25 °C and 10% relative humidity in dark conditions. Meanwhile, the PM6:eC9 device achieves a remarkable PCE of 18.22% with the enhancement of open-circuit voltage (Voc). Furthermore, the 1 cm2 device-based PDINO (15 nm)/Ag shows a high PCE of 15.2% while only 12.6% for PDINO (9 nm)/Al, indicating the good compatibility of PDINO (15 nm) interlayer with the R2R coating processes used in large-area OPVs fabrication. This work highlights the promise of interfacial regulation to simultaneously stabilize and enhance the efficiency of organic photovoltaics. [ABSTRACT FROM AUTHOR]

We build a general multi-length-scale morphology model with mixing phase and pure phase fibril structure, and simulate corresponding organic solar cells performance. Systematical multi-length-scale morphology optimization process by changing the proportion of mixing phase and pure phase in different period width cases shows a clear correlation between period width and device performance that a smaller period width with appropriate proportion of mixing phase and fibril structure is advantageous to achieve high-performance devices. Experiments on multiple donor/acceptor blends have been carried out by varying the composition and processing condition, which afford good structure-performance correlation that supports the model prediction. It is demonstrated that building such a multi-length-scale morphology merging the synergistic effects of mixing and pure phases is indeed an imperative avenue to improve device efficiency. [ABSTRACT FROM AUTHOR]

This manuscript describes the use of nitrophenyl-functionalized single-walled carbon nanotubes (NP-SWNT) in organic photovoltaic cells for the first time. We mainly focus on these innovative materials to scout for the replacement or doping of the universal buckminsterfullerene PCBM acceptor in the active layer. An optimization of the functional degree was first conducted for the position of energy bands and then experimentally confirmed. The morphology of the active layer was optimized with the NP-SWNT concentrations up to 20 wt% using additives and processing methods. The key step to avoid short circuits resulting from both high conductivity and high aspect ratio is controlling the NT functional degree and the film morphology. Using a specific functional degree (49 wt%), Organic photovoltaic (OPV) devices exhibit a maximum unprecedented efficiency of 0.13%, at an optimal nanotube loading of around 2 wt%, in BHJ with P3HT as the p-type semiconductor. In a leading precedent, our results constitute important progress for the application of NP-SWNTs in large area solution processed photovoltaic cells and provide insight into the role of structural organization in the device performance. [ABSTRACT FROM AUTHOR]

A technique for increasing the degree of dispergation of TUBALL (OCSiAl) single-walled carbon nanotubes (SWCNTs) and for obtaining individual nanotubes was developed. A combination of steps including preliminary purification of the starting SWCNTs from residues of iron-containing catalyst, ultrasonic dispergation of SWCNTs, chemical shortening of SWCNTs, mild fluorination of SWCNTs in BrF3 vapors, centrifugation of a dispersion of SWCNTs in o-dichlorobenzene, and isolation of individual SWCNTs during supernatant filtration was used. This procedure led to a decrease in the average length of SWCNTs and a noticeable decrease in the diameter of their bundles. A considerable portion of a dispersion of SWCNTs separated in this manner consisted of individual nanotubes with diameters of 1–2 nm, in addition, there were bundles with diameters of up to 6 nm. This technique made it possible to obtain photovoltaic cells based on a composite of a conjugated polymer of poly-3-hexylthiophene (P3HT) and fluorinated SWCNTs with a reproducible photovoltaic effect. The energy of the boundary LUMO of fluorinated SWCNTs was determined to be equal to −4.3 eV by cyclic voltammetry. This is suitable for light-induced electron transfer from P3HT and most other donor polymers to fluorinated SWCNTs. [ABSTRACT FROM AUTHOR]

Two acceptor-donor-acceptor (A-D-A) single-component (SC) photovoltaic triad molecules, P3T4Rh-C6-PC61BM and P3T4Rh-C10-PC61BM, were synthesized. A conformation-locked planar conjugated core, 1,4-bis(thiophenylphenylthiophene)-2,5-difluorophenylene (P3T4), with intrachain noncovalent coulombic interactions was coupled with two fullerene derivatives, [6,6]-phenyl-C61 butyric acid propargyl ester, via copper (I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition. The D-A separation was varied by modulating the spacer alkyl chain length (C6 and C10). Both SC triads exhibited maximum absorption by the P3T4 core at λabs = 507-510 nm, as well as absorption by PC61BM at ∼300 nm. Because of the broken conjugation between the P3T4 core and PC61BM termini, the highest occupied molecular orbital (−5.58 to −5.59 eV) was determined by the P3T4 moiety, and the lowest unoccupied molecular orbital (−3.89 to −3.92 eV) was determined by PC61BM in the SC structures. In diluted solution, both SC triads showed significant photoluminescence quenching, indicating efficient intramolecular charge transfer between the P3T4 and PC61BM moieties. However, the semicrystalline packing of the P3T4 core was severely disrupted by the incorporation of a bulky PC61BM moiety at each terminus, which degraded the carrier transport and diode characteristics of SC organic solar cells (SCOSCs) based on P3T4Rh-C6-PC61BM and P3T4Rh-C10-PC61BM, as indicated by poor power conversion efficiency (∼0.4%). No clear spacer length effect was observed. To improve the performance of SCOSCs, a design strategy is needed that enhances the intermolecular packing and ordering of the D and A moieties, which are important prerequisites for the development of optimal SC photoactive molecules. [ABSTRACT FROM AUTHOR]

In the field of organic photovoltaics, a full electron donor (D) and deficient electron acceptor (A) are used to prepare fragments of conjugated polymers called D–A-type polymers, the efficiency of which is fundamentally affected by the substituents. In this study, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed to elucidate the effects of substituents (methoxy (–OCH3) and fluorine (–F)) on an ethyl 4-(2-ethoxyacetyl)-5-(3-(2-ethylhexyl)-[2,2':5',2''-terthiophen]-5-yl)-2-(4-(2-ethylhexyl)thiophen-2-yl)benzoate polymer. Energy levels, optical absorption, electron–hole distribution, and charge transfer were then calculated on the basis of the optimized structure with different substituents, ranging from monomer to tetramer, and periodic boundary conditions. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels from DFT calculations were in good agreement with the experimental oxidation and reduction potentials. In conclusion, this study demonstrated that –OCH3 helps increase the HOMO energy level and induces a decrease in the band gap; moreover, it can enhance intramolecular charge transfer and the concentration of electrons. The functionalized polymer is suitable for use in the active layer of organic photovoltaics, and functionalization is an effective approach for improving electron transfer. [ABSTRACT FROM AUTHOR]

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 inorganic-based 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. [ABSTRACT FROM AUTHOR]

The fabrication of nanoparticle (NP) organic photovoltaic (OPV) solar cells has attracted great interest due to the possibility of controlling the morphology of the active layer and eliminating harmful solvents. However, NP OPV devices exhibit low efficiency compared with bulk heterojunction devices. The solar cell capacitance simulator (SCAPS) simulation package using a framework based on a one-dimensional drift–diffusion model is applied in the current work to improve the efficiency of NP OPV devices with a poly(3-hexylthiophene) (P3HT):phenyl C61 butyric acid methyl ester (PCBM) active layer. The optimization results confirm that the best thickness for the active layer is 100 nm to 120 nm and that the device reaches its highest performance in this thickness range but low efficiency for thicknesses above 120 nm. Changing the electron mobility (in the range of 1 × 10−3 cm2/V-s < µe < 1 × 10−4 cm2/V-s) is found to have no impact on the device efficiency, whereas the device performance is improved by 27% when the hole mobility is varied in this range (1 × 10−3 cm2/V-s < µh < 1 × 10−4 cm2/V-s). Additionally, the donor and acceptor density (ND and NA) are optimized, revealing that the best values of 1 × 1015 cm−3 and 1 × 1018 cm3, respectively, lead to an improvement in the device efficiency by about 60%. The current findings indicate that the performance of NP OPV devices can indeed be enhanced by optimizing the morphology of the active layer. [ABSTRACT FROM AUTHOR]

Ternary organic photovoltaic (OPV) strategy is an effective but facile approach to enhance the photovoltaic performance for single-junction devices. Herein, a series of ternary OPVs were fabricated by employing a wide bandgap donor (PBDB-TF) and two acceptor-donor-acceptor (A-D-A)-type nonfullerene small molecule acceptors (NF-SMAs, called F-2Cl and 3TT-OCIC). As the third component, the near-infrared SMA, 3TT-OCIC, has complementary absorption spectrum, narrow bandgap and well-compatible crystallization property to the host acceptor (F-2Cl) for efficient ternary OPVs. With these, the optimal ternary devices yield significantly enhanced power conversion efficiency of 15.23%, one of the very few examples with PCE higher than 15% other than Y6 systems. This is mainly attributed to the increased short-circuit current density of 24.92 mA cm−2 and dramatically decreased energy loss of 0.53 eV. This work presents a successful example for simultaneously improving current, minimizing energy loss and together with modifying the morphology of active layers in OPVs, which will contribute to the further construction of high performance ternary OPVs. [ABSTRACT FROM AUTHOR]

Two benzotriazole-based polymer donors, poly[4-(5-(2,5-bis((2-hexyldecyl]oxy]-4-(thiophene-2-yl]-phenyl]thiophene-2-yl]-5,6-difluoro-2-octyl-2H-benzo [d][1,2,3]triazole] (PPBTA-2HD) and poly[4-(5-(2,5-bis((2-hexyltridecyl]oxy]-4-(thiophene-2-yl]-phenyl]thiophene-2-yl]-5,6-difluoro-2-octyl-2H-benzo[d][1,2,3] triazole] (PPBTA-5HD), were synthesized, and their physical, optical, and electrochemical properties were characterized. The backbones of two polymers contain fluorine in the benzotriazole and alkoxy substituents in the phenylene units, resulting in molecular planarity by exploiting S⋯O and S⋯F noncovalent interactions. PPBTA-2HD and PPBTA-5HD were used as electron donors and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM] as an electron acceptor in the active layer of the bulk-hetero-junction solar cells. PPBTA-5HD exhibited better device performance than PPBTA-2HD, with a power conversion efficiency of 2.82% and a short-circuit current of 6.52 mA/cm2. Analysis of the microstructural film morphologies of two polymers by 2D-grazing incident X-ray diffraction revealed that PPBTA-5HD had higher crystallinity than PPBTA-2HD, leading to better device performance. [ABSTRACT FROM AUTHOR]

Three types of tail-to-tail (PDCBTTT), head-to-tail (PDCBTHT), and head-to-head (PDCBTHH) regioisomeric polythiophene copolymers were synthesized by modification of the substitution positions of their alkoxycarbonyl side-chains. Chain conformation, and optical, electrochemical, morphological, and charge-transport characteristics of the polymers were significantly influenced by their regiochemistry. PDCBTHH showed an amorphous morphology due to strong steric hindrance between its head-to-head alkoxycarbonyl side-chains, resulting in the poorest performance in organic solar cells and transistors among three types. PDCBTHT had a regiorandom structure with both head-to-tail and tail-to-head linkages in a polymer chain. As determined by field-effect mobility measurements, a highly regioregular and planar PDCBTTT exhibited the highest hole mobility (μ = 0.065 cm2 V-1s-1), which was two orders of magnitude higher than that of the regiorandom PDCBTHT. The highest photovoltaic performance was also measured for PDCBTTT:a fullerene acceptor (PC71BM), by maintaining the packed structures of pristine PDCBTTT polymers in the blend with PC71BM. Altering the topology of alkoxycarbonyl side-chains of the copolymers resulted in significant differences in crystalline morphologies and electrical properties and thus should be carefully considered in the molecular design of organic semiconductors. [ABSTRACT FROM AUTHOR]

Ternary organic photovoltaics (OPVs) are fabricated with PBDB-T-2Cl:Y6 (1:1.2, wt/wt) as the host system and extra PC71BM as the third component. The PBDB-T-2Cl:Y6 based binary OPVs exhibit a power conversion efficiency (PCE) of 15.49% with a short circuit current (JSC) of 24.98 mA cm−2, an open circuit voltage (VOC) 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-2Cl:Y6:PC71BM (1:1.2:0.2, wt/wt) active layer, resulting from the synchronously improved JSC of 25.44 mA cm−2, FF of 75.66% and the constant VOC of 0.868 V. The incorporated PC71BM 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, 2D grazing incidence small and wide-angle X-ray scattering, as well as Raman mapping. In addition, PC71BM 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. [ABSTRACT FROM AUTHOR]

In this study, isophorone derivatives, which are single step aldol products, were designed and synthesized. Our overview of the synthesis of solar cell application and biological applications of series vinyl substituted isophorone compounds is reported. All synthesized compounds were characterized both through experimental techniques and calculations with density functional theory. Additionally, we investigated the photophysical properties of small organic compounds for organic bulk heterojunction photovoltaic cells. The solar cell efficiency results were compared and compound 4 was found to be more efficient than the other compounds. It was found that the optical quantum yield values were very close. Isophorone derivatives showed better antioxidant activities, which were DPPH, metal chelating, reducing activity, and antimicrobial activities against Salmonella typhimurium, Klebsiella pneumonia, Candida albicans, Staphylococcus aureus, Bacillus subtilis, and Escherichia coli. Compound 7 (which is substituted quinoline) and compound 5 (which is substituted cyno) had the highest antioxidant effect. The 4-brom phenyl, 4-cyno phenyl were used against gram negative bacteria and the 4-ter-butyl phenyl group was indicated for gram positive bacteria and yeast. [ABSTRACT FROM AUTHOR]

Boron subphthalocyanine chloride is an electron donor material used in small-molecule organic photovoltaics with an unusually large molecular dipole moment. Using first-principles calculations, we investigate how to control the electronic and optical properties of boron subphthalocyanine chloride by substituting the boron and chlorine atoms with other trivalent and halogen atoms and thereby modifying the molecular dipole moment. Gas-phase molecular structures and properties are predicted using hybrid functionals. Using positions and orientations of the known compounds as the starting coordinates for these molecules, stable crystalline structures are derived following a procedure that involves perturbation and accurate total energy minimization. Electronic structure and photonic properties of the predicted crystals are computed using the GW method and the Bethe–Salpeter equation, respectively. Finally, a simple transport model is used to quantitatively demonstrate the effects of the strength and orientation of molecular dipole moments at interfaces on device performance. [ABSTRACT FROM AUTHOR]

Short-axis substitution, as an effective way to change the optical and electronic properties of the organic semiconductors for organic photovoltaics (OPVs), is a readily approach to modify non-fullerene acceptors, especially for the linear fused rings system. Here, two new fused-ring electron acceptors (CBT-IC and SBT-IC) were designed and developed by short-axis modification based on the dithienyl[1,2-b:4,5-b′]benzodithiophene (BDCPDT) system. Combined with a medium bandgap polymer donor J71, both of the OPV devices exhibit high power conversion efficiency (PCE) over 11%, and ~70% external quantum efficiencies. To better understand how this kind of substitution affects the BDCPDT based acceptors, a comparative analysis is also made with the the plain acceptor BDT-IC without this modification. We believe this work could disclose the great potential and the versatility of BDCPDT block and also enlighten other ladder-type series for further optimization. [ABSTRACT FROM AUTHOR]

Evolution of favourable nanomorphology which can withstand external stimuli is a critical issue for efficient and stable organic solar cells. Here, we demonstrate a novel strategy for the stabilization of nanomorphology of organic solar cells by inducing polymeric nanowire network structures. Thermal annealing of poly(3- hexylthiophene-2,5-diyl) nanowires, highly crystalline, 1-dimensional structures held together through interchain π-π stacking, led to the formation of nanowire network structures confirmed through small angle neutron scattering measurements. The physically interconnected network structures form robust electron donor domains and impose confinement which suppresses the aggregation of the electron acceptor, [6,6]-phenyl-C61-butylric acid methyl ester. Organic solar cells having the nanowire network structures showed increased power conversion efficiencies and dramatically enhanced thermal stability compared to bulk heterojunction (BHJ) and non-network nanowire-based devices. Furthermore, the performance of the nanowire network-based devices was inversely related to the size of the networks, attesting to the significance of nanoconfined geometry formed within nanowire network structures. [ABSTRACT FROM AUTHOR]

Organic photovoltaic (OPV) cells and organic light-emitting diodes (OLEDs) are energy harvesting and generation devices that have attracted great attention these years because of their low costs, thin film structures, flexibility and environment-friendly manufacturing processes. For such thin film devices, photon management methods that increase the light absorption of OPV cells and the light extraction from OLEDs are highly desirable. Here, we report the experimental efforts to fabricate geometrically tunable and spontaneously formed nano-wrinkle structures with large areas, and the theoretical results on photon management with the nano-wrinkles. [ABSTRACT FROM AUTHOR]

The low-cost patterning of poly(3,4-ethylenedioxythiophene) poly(4-styrenesulfonate) (PEDOT:PSS) interfacial layers inserted between indium tin oxide and poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C-butyric acid ester blends leads to an improvement in organic photovoltaics (OPV) device performance. Significantly, improvements in all device parameters, including the open-circuit voltage, are achieved. The nanoimprinted devices improved further as the pattern period and imprinting depth was reduced from 727 nm and 42 nm to 340 nm and 10 nm, respectively. A residue of poly(dimethylsiloxane) (PDMS) is found on the interfacial PEDOT:PSS film following patterning and can be used to explain the increase in OPV performance. Ultraviolet photoelectron spectroscopy measurements of the PEDOT:PSS interfacial layer demonstrated a reduction of the work function of 0.4 eV following nanoimprinting which may originate from chemical modification of the PDMS residue or interfacial dipole formation supported by x-ray photoelectron spectroscopy analysis. Ultimately, we have demonstrated a 39% improvement in OPV device performance via a simple low-cost modification of the anode interfacial layer. This improvement can be assigned to two effects resulting from a PDMS residue on the PEDOT:PSS surface: (1) the reduction of the anode work function which in turn decreases the hole extraction barrier, and (2) the reduction of electron transfer from the highest occupied molecular orbital of PCBM to the anode. [ABSTRACT FROM AUTHOR]

In this work, we explored industrially relevant pigments quinacridones as organic semiconductors with respect to their applicability in air-stable homojunction (single material-single layer) organic photovoltaic cells. We studied quinacridone and two linear transquinacridones, i.e., Pigment Red 122 (2,9′-dimethylquinacridone) and Pigment Red 202 (2,9′-dichloroquinacridone) in vacuum-processible OPV films and compared their performance to OPV films made from their solution-processed counterparts. We show that this class of materials generates photocurrent without the need of creating a donor-acceptor junction. Stable homojunctions were created both via vacuum-processible and solution-processible routes, with the latter method based on the chemical functionalization of the respective molecules with the thermolabile group, t-BOC, that affords solubility in various organic solvents and conversion back to the parent molecule via exposure to temperatures around 170 °C for periods not exceeding 30 min. We show that power conversion efficiencies around 0.15% are reachable without scrupulous optimization of the homojunction cells in terms of electrode surface functionalization or film deposition conditions. This class of materials holds promise for the further development of a new generation of air-stable organic photovoltaic cells based on simple fabrication technologies. Graphical abstract: [ABSTRACT FROM AUTHOR]

Polymer electron acceptors are the key materials in all-polymer solar cells (all-PSCs). In this review, we focused on introducing the principle of boron-nitrogen coordination bond (B←N), and summarizing our recent research on polymer electron acceptors containing B←N unit for efficient all-PSC devices. Two approaches have been reported to design polymer electron acceptors using B←N unit. The one is to replace a C-C unit by a B←N unit in conjugated polymers to transform a polymer electron donor to a polymer electron acceptor. The other approach is to construct novel electron-deficient building block based on B←N unit for polymer electron acceptors. The polymer electron acceptors containing B←N unit showed tunable lowest unoccupied molecular orbital (LUMO) energy levels and exhibited excellent all-PSC device performance with power conversion efficiency of exceeding 6%. These results indicate that organic boron chemistry is a new toolbox to develop functional polymer materials for optoelectronic device applications. [ABSTRACT FROM AUTHOR]

We provide an overview of the development and application of nonconventional techniques that allowed us to probe the molecular excited states in conjugated polymers with respect to their molecular orientation and the nanomechanical properties of the topmost surface layer. To probe triplet and charge-transfer excited states and their fate, we applied the optically detected magnetic resonance technique extended for angular resolution. The surface morphology and three-dimensional depth profiles of the topmost surface layer were obtained with multi-set point intermittent contact mode atomic force microscopy. These studies were performed on model systems such as poly(3-hexylthiophene) as well as on the novel, low-bandgap copolymer poly[(4,8-bis-(2-ethylhexyloxy)-benzo(1,2-b:4,5-b9)dithiophene)-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)]. [ABSTRACT FROM AUTHOR]

Small-molecule Π-systems bearing donor (D) and acceptor (A) groups constitute an interesting class of dyes because of their tunable strong absorption, which covers the visible and near-infrared range. The dipolarity associated with D–A structures directs antiparallel stacking arrangements in the solid state, thus reducing the dipolar disorder at the supramolecular level. Their straightforward synthesis and purification make them good candidates for photovoltaic application with power conversion efficiencies >6 %. This review summarizes the application of D–A dyes, and in particular merocyanines, in organic photovoltaics in recent years. [ABSTRACT FROM AUTHOR]

The photovoltaic properties of samples of films having free surface and containing isomeric symmetrical cationic polymethine dyes inserted into photoconducting polymers based on carbazole and thiophene were investigated. It was shown that isomerism of the dyes in the thiophene-containing polymer, which absorbs light in the spectral region of the investigated dyes, has a significant effect both on the magnitude and on the sign of the photovoltaic effect. The results were analyzed by DFT/B3LYP ab initio quantum-chemical calculations. [ABSTRACT FROM AUTHOR]

Commercial success of optoelectronical devices such as solar cells depend strongly on lifetime next to production costs and power conversion efficiency. While a steep decrease in production costs and significant increase in efficiency have been achieved, lifetime still plays often a limiting factor, in particular, in case of highly innovative new device types. Lifetime tests measuring the stability of devices without external influences (i. p. water) generally take very long time due to the required long lifetime of optoelectronical devices. We established a novel accelerated lifetime test (ALT) setup which may increase the test speed by a factor of several hundred. We verified the setup and the applicability of our ALT measurement routine with an experiment on a well-studied innovative thin-film solar cell type (P3HT:PCBM). [ABSTRACT FROM AUTHOR]

We propose and demonstrate the incorporation of porous microstructures on indium-doped zinc oxide (IZO) electron selective layer in inverted organic photovoltaics (OPV). Porosity was induced in the IZO layer with the addition of polyethylene glycol (PEG) organic template at the optimal IZO/PEG ratio of 4:1. When compared to the OPV device with non-porous IZO, the device employing porous IZO showed a 16 % improvement in current density and a 13 % improvement in efficiency. This is primarily due to the increased light scattering as substantiated by the haze factor studies. This PEG assisted method of introducing microporous structure is therefore shown to be compatible with the doped interlayer and is thus a portable method of enhancing light scattering in OPV devices. Graphical Abstract: [ABSTRACT FROM AUTHOR]

Guiding principles to design highly optimized organic photovoltaic devices that achieve high power conversion efficiencies (PCEs) are strongly desired. To obtain a useful guideline, we have investigated the photo- and electrochemical properties of four diketopyrrolopyrrole (DPP) derivatives containing double DPP blocks by ab initio calculations according to Yu's recent finding that changes in dipole moments of donor molecules by photoexcitation correlate with the short-circuit current (Carsten et al. in J Am Chem Soc 133(50):20468, 2011). We found that the short-circuit current correlates with changes in dipole moments of complexes of donor and acceptor molecules rather than with the changes in dipole moments of donor molecules only. This is because of photoinduced partial charge transfer from the HOMO on the donor to the charge-bridging orbital that is delocalized over both donor and acceptor molecules. Therefore, we suggest that the change in the dipole moment in the complex of donor and acceptor molecules is a promising guideline for designing new donor molecules to improve the short-circuit current and PCEs. [ABSTRACT FROM AUTHOR]

Utilization of small-molecule based organic photovoltaic (OPV) devices has received strong attention due to their easy preparation, purification, and batch-to-batch resemblance in properties. In this work, a series of simple benzothiadiazole and triphenylamine-containing molecules were synthesized, and their application to organic photovoltaic devices was investigated. The absorption spectra demonstrated that the absorption wavelength of the small molecules could be tuned dramatically by extension of molecular structure from donor-acceptor-acceptor (D-A-A') to A'-A-D-D-A-A' sequences. Due to the intramolecular energy transfer from the acceptor to donor and vice versa in D-A-A' structures in prepared molecules, the maximum emission wavelengths were red-shifted gradually with the increase of chain length. Bulk heterojunction (BHJ) type solar cell devices were fabricated by using the small molecules as donors and (6,6)-phenyl C61-butyric acid methyl ester (PCBM) as acceptor (2:1), gave maximal open circuit voltage of the photovoltaic cells of 0.59 V and the power conversion efficiencies of the devices were measured 0.83% under AM 1.5G irradiation (100 mWcm). [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Wood cellulose is a desirable inexpensive, abundant and biodegradable substrate material for flexible organic electronics. While paper-based conductors are easily achieved, their applicability for diode architectures is challenging. To diminish short circuits, smoothing fillers and coatings are used before diode deposition. In this report, an alternative approach for use of cellulose in organic photovoltaics is demonstrated, using natural, rough unmodified kraft pulp fiber networks as a carrier of a diode architecture. Silver nanowires are adsorbed by the hydroxyl groups of cellulose fiber, forming the semi-transparent anode. The follow-on coatings for the diode are simply deposited as a mantle on these fiber anodes. The potential of this approach is demonstrated by the first production of cellulose fiber-based P3HT:PCBM photovoltaic devices, showing clear diode behaviour in the dark and a significant photoresponse under illumination. Fill factors near 35 % and open-circuit voltages of 0.2 V were achieved without any optimization. Devices show even noticeable photocurrent at low unfavourable light conditions, indicating their advantageous capture of stray light within the paper fiber network. These results highlight that for 'paper' diodes, its natural properties can be turned into advantages, instead of seeking routes to suppress them. [ABSTRACT FROM AUTHOR]

In the study of bulk heterojunction solar cells based on poly(3-hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM), P3HT/PCBM device performance is strongly depends on the thickness of active region. In such devices, optoelectronic behaviors such as charge carrier generation and recombination, photocurrent generation and charge transport mechanism is different in devices of different thickness. An electrical model accounting for study the influences of active layer thickness on the device performance, is developed to achieve the device parameter for high performance of the polymer-fullerene bulk heterojunction solar cells. In this model, by solving the drift-diffusion equations by considering the boundary condition and uniform potential energy, the effects of the active region thickness on the performance of P3HT/PCBM bulk heterojunction solar cell has been studied. By using this model, we were able to show that the thickness dependence of electrical characteristics is responsible for a reduction of the overall performance of the solar cells. Simulated current-voltage characteristics as a function of active layer thickness reveals relatively good agreement between the model's predictions and published modeling and experimental reports. [ABSTRACT FROM AUTHOR]

The easy oxidation and surface roughness of Cu nanowire (NW) films are the main bottlenecks for their usage in transparent conductive electrodes (TCEs). Herein, we have developed a facile and scaled-up solution route to prepare Cu NW-based TCEs by embedding Cu NWs into pre-coated smooth poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films on poly(ethylene terephthalate) (PET) substrates. The so obtained Cu NW-PEDOT:PSS/PET films have low surface roughness (∼70 nm in height), high stability toward oxidation and good flexibility. The optimal TCEs show a typical sheet resistance of 15 Ω·sq at high transparency (76% at λ = 550 nm) and have been used successfully to make polymer (poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester) solar cells, giving an efficiency of 1.4%. The overall properties of Cu NW-PEDOT:PSS/PET films demonstrate their potential application as a replacement for indium tin oxide in flexible solar cells. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

We review some of the computational methodologies used in our research group to develop a better understanding of the geometric and electronic structures of organic-organic interfaces present in the active layer of organic solar cells. We focus in particular on the exciton-dissociation and charge-transfer processes at the pentacene-fullerene interface. We also discuss the local morphology at this interface on the basis of molecular dynamics simulations. [ABSTRACT FROM AUTHOR]

Five 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT)-based conjugated copolymers with controlled molecular weight were synthesized to explore their optical, energy level and photovoltaic properties. By tuning the positions of hexyl side chains on DTBT unit, the DTBT-fluorene copolymers exhibited very different aggregation properties, leading to 60 nm bathochromic shift in their absorptions and the corresponding power conversion efficiencies (PCEs) value of photovoltaic cells varied from 0.38%, 0.69% to 2.47%. Different copolymerization units, fluorene, carbazole and phenothiazine were also investigated. The polymer based on phenothiazine exhibited lower PCE value due to much lower molecular weight owing to its poor solubility, although phenothiazine units were expected to be a better electron donor. Compared with the fluorene-based polymer, the carbazole-DTBT copolymer showed higher short circuit current density ( Jsc) and PCE value due to its better intermolecular stacking. [ABSTRACT FROM AUTHOR]

In recent years, diketopyrrolopyrroles (DPPs) have become one of the extensively studied organic building blocks of oligomers and polymers having promising optoelectronic properties, especially in organic solar cells. There are a large number of reports on structural modification of DPP cores to improve their properties in devices. There is plenty of room left for research in the field of structural modification and bandgap engineering to maintain energy level alignment and to achieve optimum morphology/crystallinity leading to high performance in organic photovoltaic (OPV) devices. This article reviews the DPP containing materials and their variants as potential active layer materials for OPV devices. An attempt has been made to go through their synthesis, structure and optoelectronic properties in such a way that any future efforts to modify this versatile structure have benefited. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

The efficiency of conversion of light to electrical energy with the help of conjugated polymers and molecules is rapidly improving. The optical absorption properties of these materials can be designed, and implemented via molecular engineering. Full coverage of the solar spectrum is thus feasible. Narrow absorption spectra allow construction of tandem solar cells. The poor transport properties of these materials require thin devices, which limits optical absorption. Alternative device geometries for these flexible materials compensate for the optical absorption by light trapping, and allow tandem cells. [ABSTRACT FROM AUTHOR]

A computational protocol combining a heuristic search based on genetic algorithms (GAs) and quantum chemistry methods is implemented and applied to a family of acceptor compounds based on the 9,9′-bifluorenylidene backbone, to be coupled with the poly-3(hexylthiophene) polymer (donor) in a bulk heterojunction solar cell. Highly performing candidates are generated via GA from an initial generation, after a number of iterations (i.e., new generations), under the selective pressure of electronic constrains calculated at density functional theory level. The combination of heuristic search techniques and advanced electronic structure methodologies for characterization seems to be amenable to further applications in the field of molecular design. [ABSTRACT FROM AUTHOR]

The electronic and structural properties of pyrrolic ring derivatives were studied using density functional theory (DFT) in terms of their application as organic semiconductor materials in photovoltaic devices. The B3LYP hybrid functional in combination with Pople type 6-31G(d) basis set with a polarization function was used in order to determine the optimized geometries and the electronic properties of the ground state, while transition energies and excited state properties were obtained from time-dependent (TD)-DFT with B3LYP/6-31G(d) calculation. The investigation of pyrrolic derivatives formed by the arrangement of several monomeric units revealed that three-dimensional (3D) conjugated architectures in which the combination of a triphenylamine (TPA) core with π-conjugated rings attached to the core, present the best geometric and electronic characteristics for use as an organic semiconductor material. The highest occupied molecular orbital (HOMO) − lowest unoccupied molecular orbital (LUMO) energy gap was decreased in 3D-structures that extend the absorption spectrum toward longer wavelengths, revealing a feasible intramolecular charge transfer process in these systems. All calculations in this work were performed using the Gaussian 03 W software package. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

The potential of gold nanorods to act as 'antennae' in organic photovoltaic devices and improve performance by enhancing the optical field within the device has been examined. Gold nanorods were blended into the buffer layer of standard poly (3-hexothiopene) (P3HT): 6, 6-phenyl C61-butyric acid methyl ester (PCBM) photovoltaic cells. Increases in both the broad spectrum absorption and device fill factors (as compared to solar cells without nanorods) were observed. A comparison of the internal resistance and absorption allows estimation of what part in the change in performance can be attributed to greater absorption and increases in buffer layer conductivity. [ABSTRACT FROM AUTHOR]

Extensive research activities in synthesis and device engineering have been devoted to the development of donor–acceptor (D–A) bulk heterojunction solar cells in the last few years. Several photophysical processes occur in such devices which have to be optimized for an efficient device operation. First, excitons that are created upon light absorption need to reach the D/A interface within their short exciton diffusion length (10–20 nm), where they may dissociate into holes and electrons. Subsequent charge transport and charge collection can then take place at the electrodes, given that co-continuous pathways of donor and acceptor domains are provided. An active layer thickness of 100–200 nm is required to absorb most of the light, and vertically aligned pathways with a high aspect ratio of either phase should percolate through the film, taking into account the small exciton diffusion lengths. The morphologies resulting from this ideal situation resemble those of vertically oriented microphase separated block copolymer thin films, and hence suggest the exploitation of D–A block copolymers for organic photovoltaics. Furthermore, complex block copolymer architectures are not only desired in order to improve the morphological control but also to enhance the long term stability of the device. The potential of such block copolymers to microphase separate into well-defined structures several tens of nanometers in size thus addresses the morphological requirements mentioned above. This chapter gives an overview of the emerging field of D–A block copolymers. General synthetic efforts that have been undertaken towards this direction are summarized. The D–A block copolymers prepared in our group are reviewed and complemented with recent work on crystalline–crystalline block copolymers. [ABSTRACT FROM AUTHOR]

Recent developments on photovoltaic elements based on solid state composites of conjugated, semiconducting polymers mixed with buckminsterfullerene are reviewed. The photoinduced charge transfer from donor-type semiconducting conjugated polymers onto acceptor-type conjugated polymers or acceptor molecules such as buckminsterfullerene is reversible, ultrafast (within 100 fs) with a quantum efficiency approaching unity, and the charge separated state is metastable (up to ms at 80 K). This phenomenon of photoinduced electron transfer leads to a number of potentially interesting applications which include, among others, sensitization of the photoconductivity, reverse saturable absorption (optical limiting), and photovoltaic phenomena. Recent studies on the realization of photovoltaic elements with 3% power conversion efficiency are reported. [ABSTRACT FROM AUTHOR]

Organic photovoltaics (OPV) have been theoretically studied within the usual parameters: open circuit voltage (Voc), short circuit current (Jsc), and fill factor (FF). The first two refer mostly to electronic properties, whereas the last contains all other possible contributions to cell efficiency, importantly including molecular geometrical and topological structures, both within a single molecule as amongst a system of molecules. In order to study the effects of molecular morphology of the heterostructures used in OPVs, molecular dynamic simulations (MDS) are imperative, and therefore parameterization of force fields to account for the description of planarity between aromatic rings, both intra- and inter-molecules, is of key importance. In this work, we present quantum mechanical analysis of geometry for the ground, and singlet and triplet excited states, of two simple prototypical molecules used to parametrize the corresponding force-fields. Central for this parameterization is the assignment of local charges within molecules; we studied and compared six different methods to assign atomic charges. With the parameters obtained, we performed molecular dynamics simulations of nanosystems of these molecules. Planarity effects and comparison between different methods for deriving atomic charges are investigated. These results can be applied in future MDS to interpret and characterize charge-transfer models in molecules suitable for OPV design. [ABSTRACT FROM AUTHOR]