Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells

Fluoro-Substituted n-Type Conjugated Polymers for Additive-Free All-Polymer Bulk Heterojunction Solar Cells with High Power Conversion Efficiency of 6.71

Fluorinated n-type conjugated polymers are used as efficient electron acceptor to demonstrate high-performance all-polymer solar cells. The exciton generation, dissociation, and charge-transporting properties of blend films are improved by using these fluorinated n-type polymers to result in enhanced photocurrent and suppressed charge recombination.

Amphiphilic fullerene/ZnO hybrids as cathode buffer layers to improve charge selectivity of inverted polymer solar cells

Two types of novel fullerene derivative/ZnO hybrids were prepared by physically blending amphiphilic fullerene-end-capped poly(ethylene glycol) (C60-PEG) with ZnO nanocrystals (ZnO/C60-PEG) and by in situ grown ZnO from C60-PEG (ZnO@C60-PEG) at relatively low temperatures. The C60-PEG could act as n-doping on the ZnO while the PEG side chain of C60-PEG could passivate the defects of the ZnO at the same time, consequently increasing the lowest unoccupied molecular orbital (LUMO) level. Compared with the ZnO/C60-PEG by the physical blend approach, the ZnO@C60-PEG by the growth approach showed a more favorable morphology and higher electron mobility by developing a homogeneous network. As a consequence, the efficiency of the inverted polymer solar cells based on thieno[3,4-b]-thiophene/benzodithiophene (PTB7):[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) is raised to 8.0% for the ZnO@C60-PEG cathode buffer layer and to 7.5% for the ZnO/C60-PEG cathode buffer layer with improved long-term stability.

Mapping fullerene crystallization in a photovoltaic blend: an electron tomography study

The formation of fullerene crystals represents a major degradation pathway of polymer/fullerene bulk-heterojunction thin films that inexorably deteriorates their photovoltaic performance. Currently no tools exist that reveal the origin of fullerene crystal formation vertically through the film. Here, we show that electron tomography can be used to study nucleation and growth of fullerene crystals. A model bulk-heterojunction blend based on a thiophene-quinoxaline copolymer and a fullerene derivative is examined after controlled annealing above the glass transition temperature. We image a number of fullerene nanocrystals, ranging in size from 70 to 400 nanometers, and observe that their center is located close to the free-surface of spin-coated films. The results show that the nucleation of fullerene crystals predominately occurs in the upper part of the films. Moreover, electron tomography reveals that the nucleation is preceded by more pronounced phase separation of the blend components.

Hydrophilic Conjugated Polymers with Large Bandgaps and Deep-Lying HOMO Levels as an Efficient Cathode Interlayer in Inverted Polymer Solar Cells

Two hydrophilic conjugated polymers, PmP-NOH and PmP36F-NOH, with polar diethanol-amine on the side chains and main chain structures of poly(meta-phenylene) and poly(meta-phenylene-alt-3,6-fluorene), respectively, are successfully synthesized. The films of PmP-NOH and PmP36F-NOH show absorption edges at 340 and 343 nm, respectively. The calculated optical bandgaps of the two polymers are 3.65 and 3.62 eV, respectively, the largest ones so far reported for hydrophilic conjugated polymers. PmP-NOH and PmP36F-NOH also possess deep-lying highest occupied molecular orbital levels of -6.19 and -6.15 eV, respectively. Inserting PmP-NOH and PmP36F-NOH as a cathode interlayer in inverted polymer solar cells with a PTB7/PC71 BM blend as the active layer, high power conversion efficiencies of 8.58% and 8.33%, respectively, are achieved, demonstrating that the two hydrophilic polymers are excellent interlayers for efficient inverted polymer solar cells.

Enhanced high-open circuit voltage in fluorinated benzoselenadiazole-based polymer solar cells

Three new donor–acceptor copolymers (PCDTBSe, PCDTFBSe, and PCDTDFBSe) were designed and synthesized with 2,7-carbazole as the donor (D) unit and benzoselenadiazole (BSe), monofluoro-benzoselenadiazole, and difluoro-benzoselenadiazole as the acceptor (A) units, respectively. The structure–property relationship of these polymers was elucidated in bulk heterojunction polymer solar cells. All the polymers were fully characterized and exhibited good thermal stability and broad absorption. The highest occupied molecular orbitals (HOMOs) of the PCDTBSe (−5.29 eV), PCDTFBSe (−5.32 eV), and PCDTDFBSe (−5.35 eV) were decreased by incorporating fluorine atoms on the polymer backbone. The low-lying HOMO energy level suggested that the polymers would exhibit high open circuit voltage ( VOC) when blended with fullerene as the electron acceptor. Solar cell based on PCDTFBSe displayed a power conversion efficiency of 1.09% with a short-circuit current density ( JSC) of 4.29 mA cm−2, a VOC of 0.78 V, and a fill factor (FF) of 32.51%, under the illumination of AM1.5G, 100 mW cm−2.

Competition between recombination and extraction of free charges determines the fill factor of organic solar cells

Among the parameters that characterize a solar cell and define its power-conversion efficiency, the fill factor is the least well understood, making targeted improvements difficult. Here we quantify the competition between charge extraction and recombination by using a single parameter θ, and we demonstrate that this parameter is directly related to the fill factor of many different bulk-heterojunction solar cells. Our finding is supported by experimental measurements on 15 different donor:acceptor combinations, as well as by drift-diffusion simulations of organic solar cells in which charge-carrier mobilities, recombination rate, light intensity, energy levels and active-layer thickness are all varied over wide ranges to reproduce typical experimental conditions. The results unify the fill factors of several very different donor:acceptor combinations and give insight into why fill factors change so much with thickness, light intensity and materials properties. To achieve fill factors larger than 0.8 requires further improvements in charge transport while reducing recombination.

Enhanced Efficiency in Fullerene-Free Polymer Solar Cell by Incorporating Fine-designed Donor and Acceptor Materials

Among the diverse nonfullerene acceptors, perylene bisimides (PBIs) have been attracting much attention due to their excellent electron mobility and tunable molecular and electronic properties by simply engineering the bay and head linkages. Herein, guided by two efficient small molecular acceptors, we designed, synthesized, and characterized a new nonfullerene small molecule PPDI with fine-tailored alkyl chains. Notably, a certificated PCE of 5.40% is realized in a simple structured fullerene-free polymer solar cell comprising PPDI as the electron acceptor and a fine-tailored 2D-conjugated polymer PBDT-TS1 as the electron donor. Moreover, the device behavior, morphological feature, and origin of high efficiency in PBDT-TS1/PPDI-based fullerene-free PSC were investigated. The synchronous selection and design of donor and acceptor materials reported here offer a feasible strategy for realizing highly efficient fullerene-free organic photovoltaics.

Effects of alkyl chain length on the optoelectronic properties and performance of pyrrolo-perylene solar cells

While the impact of alkyl side-chain length on the photovoltaic properties of conjugated polymers and their performance in bulk heterojunction (BHJ) solar cells has been studied extensively, analogous studies on pyrrolo-perylene-based polymers have not received adequate attention. To explore these effects, we synthesized two copolymers based on N-annulated pyrrolo-perylene and consisting of cyano group substituents on thiophene vinylene thiophene units with two different alkyl groups of 2-decyltetradecyl and 7-decylnonadecyl, and we studied them with regard to chemical structure and photovoltaic performance. UV-vis spectroscopy and cyclic voltammetry studies showed that variations in alkyl chain length affect crystallization, light absorption, and the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. These factors have a pronounced impact on the morphology of BHJ thin films and their charge carrier separation and transportation characteristics, which, in turn, influences photovoltaic properties.

Impact of charge transport on current-voltage characteristics and power-conversion efficiency of organic solar cells

This work elucidates the impact of charge transport on the photovoltaic properties of organic solar cells. Here we show that the analysis of current-voltage curves of organic solar cells under illumination with the Shockley equation results in values for ideality factor, photocurrent and parallel resistance, which lack physical meaning. Drift-diffusion simulations for a wide range of charge-carrier mobilities and illumination intensities reveal significant carrier accumulation caused by poor transport properties, which is not included in the Shockley equation. As a consequence, the separation of the quasi Fermi levels in the organic photoactive layer (internal voltage) differs substantially from the external voltage for almost all conditions. We present a new analytical model, which considers carrier transport explicitly. The model shows excellent agreement with full drift-diffusion simulations over a wide range of mobilities and illumination intensities, making it suitable for realistic efficiency predictions for organic solar cells.

Linked-Acceptor Type Conjugated Polymer for High Performance Organic Photovoltaics with an Open-Circuit Voltage Exceeding 1 V

A linked-acceptor type conjugated polymer is designed and sythesized based on 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene (BDTT) and linked-thieno[3,4-c]pyrrole-4,6-dione (LTPD). This polymer uses alkyl-substituted thiophene as a bridge. The PBDTT-LTPD includes two TPD units in one repeating unit, which can enhance acceptor density in the polymer backbone and lower the highest occupied molecular orbital (HOMO) level. By contrast, variable alkyl substitutions in the thiophene-bridges ensure the subtle regulation of polymer properties. The solar cells based on PBDTT-LTPD display an open-circuit voltage (Voc) that exceeds 1 V, and a maximum power conversion efficiency (PCE) of 7.59% is obtained. This PCE value is the highest for conventional single-junction bulk heterojunction solar cells with Voc values of up to 1 V. Given that PBDTT-LTPD exhibits a low HOMO energy level and a band gap equivalent to that of poly(3-hexylthiophene), PBDTT-LTPD/phenyl-C61-butyric acid methyl ester may be a promising candidate for the front cell in tandem polymer solar cells.

A Tetraperylene Diimides Based 3D Nonfullerene Acceptor for Efficient Organic Photovoltaics

A nonfullerene acceptor based on a 3D tetraperylene diimide is developed for bulk heterojunction organic photovoltaics. The disruption of perylene diimide planarity with a 3D framework suppresses the self-aggregation of perylene diimide and inhibits excimer formation. From planar monoperylene diimide to 3D tetraperylene diimide, a significant improvement of power conversion efficiency from 0.63% to 3.54% can be achieved.

A series of simple oligomer-like small molecules based on oligothiophenes for solution-processed solar cells with high efficiency

A series of acceptor-donor-acceptor simple oligomer-like small molecules based on oligothiophenes, namely, DRCN4T-DRCN9T, were designed and synthesized. Their optical, electrical, and thermal properties and photovoltaic performances were systematically investigated. Except for DRCN4T, excellent performances were obtained for DRCN5T-DRCN9T. The devices based on DRCN5T, DRCN7T, and DRCN9T with axisymmetric chemical structures exhibit much higher short-circuit current densities than those based on DRCN6T and DRCN8T with centrosymmetric chemical structures, which is attributed to their well-developed fibrillar network with a feature size less than 20 nm. The devices based on DRCN5T/PC71BM showed a notable certified power conversion efficiency (PCE) of 10.10% under AM 1.5G irradiation (100 mW cm(-2)) using a simple solution spin-coating fabrication process. This is the highest PCE for single-junction small-molecule-based organic photovoltaics (OPVs) reported to date. DRCN5T is a rather simpler molecule compared with all of the other high-performance molecules in OPVs to date, and this might highlight its advantage in the future possible commercialization of OPVs. These results demonstrate that a fine and balanced modification/design of chemical structure can make significant performance differences and that the performance of solution-processed small-molecule-based solar cells can be comparable to or even surpass that of their polymer counterparts.

Highly efficient low-bandgap polymer solar cells with solution-processed and annealing-free phosphomolybdic acid as hole-transport layers

We demonstrate a novel solution-processed method to fabricate a stable anode buffer layer without any annealing process. As we know, buffer layers in polymer solar cells (PSCs) are always prepared by the traditional high-vacuum thermal evaporation or annealing-treated spin-coating methods, but the fabricating processes are complicated and time-consuming. Here, a solution method without any annealing to fabricate phosphomolybdic acid (PMA) as anode buffers is presented, which brings an obvious improvement of power conversion efficiency (PCE) from 1.75% to 6.57% by optimizing the PMA concentrations and interface pretreatment with device structure shown as ITO/TiO2/PCDTBT:PC70BM/PMA/Ag. The improvement is ascribed to the fine energy-level matching and perfect surface modification. This annealing-free method greatly simplifies the device fabrication process and supplies a wide way to achieve a large area fabrication for PSCs.

A new application area for fullerenes: voltage stabilizers for power cable insulation

Fullerenes are shown to be efficient voltage-stabilizers for polyethylene, i.e., additives that increase the dielectric strength of the insulation material. Such compounds are highly sought-after because their use in power-cable insulation may considerably enhance the transmission efficiency of tomorrow's power grids. On a molal basis, fullerenes are the most efficient voltage stabilizers reported to date.

Tuning the physical properties of pyrrolo[3,4-c]pyrrole-1,3-dione-based highly efficient large band gap polymers via the chemical modification on the polymer backbone for polymer solar cells

The property modulation of DPPD-based high energy converting large band gap polymer (PBDT–DPPD) was studied via the incorporation of strong electron accepting TPD, TT or DPP units on the polymer backbone.

Low band-gap polymers based on easily synthesized thioester-substituted thieno[3,4-b]thiophene for polymer solar cells

The polymers based on thioester-substituted thieno[3,4-b]thiophene (TTS) exhibit a low band gap (∼1.5 eV) and desirable HOMO and LUMO energy levels relative to the fullerene acceptors.

Donor–acceptor conjugated polymers based on cyclic imide substituted quinoxaline or dibenzo[a,c]phenazine for polymer solar cells

A series of donor–acceptor type of conjugated polymers based on cyclic imide substituted quinoxaline or dibenzo[a,c]phenazine were synthesized, where the copolymers comprising dibenzo[a,c]phenazine showed much higher photovoltaic performances.

New advances in non-fullerene acceptor based organic solar cells

Non-fullerene organic molecules are alternative and competitive acceptor materials for high-efficiency organic solar cells.

Small molecular push–pull donors for organic photovoltaics: effect of the heterocyclic π-spacer

A series of (D–π–A) small push–pull molecules involving a triphenylamine electron-rich group (D) connected to a dicyanovinyl electron-deficient unit (A) through different chalcogenophene type π-connectors has been synthesized.

Highly efficient small molecule solar cells fabricated with non-halogenated solvents

Fabricated with non-halogenated solvents toluene and o-xylene, the bulk-heterojunction organic solar cells based on a porphyrin small molecule show high power conversion efficiencies up to 5.46% and 5.85%, respectively.

Effect of fluorine substitution on the photovoltaic performance of poly(thiophene-quinoxaline) copolymers

Three new conjugated polymers with or without fluorine substituents on quinoxaline ring were investigated in detail to assess the role of F in OPV performance.

Molecular orientation within thin films of isomorphic molecular semiconductors

Subtle structural variation of isomorphic molecular semiconductors leads to sharp contrast in electronic structures and molecular orientation in the bulk.

Improved performances of PCDTBT:PC71BM BHJ solar cells through incorporating small molecule donor

Incorporating SQ-BP into a PCDTBT:PC₇₁BM host blend not only broadens the absorption spectrum but also decreases the energy loss of excited PCDTBT.

Perylene and naphthalene diimide polymers for all-polymer solar cells: a comparative study of chemical copolymerization and physical blend

Five copolymers, having 4,4,9,9-tetrakis(4-hexylphenyl)-indaceno[1,2-b:5,6-b′]-dithiophene as a donor unit, and perylene diimide (PDI) and/or naphthalene diimide (NDI) as acceptor moieties, were synthesized, and used as electron acceptors in polymer solar cells.

Surface-initiated polymerization of A–A/B–B type conjugated monomers by palladium-catalyzed Stille polycondensation: towards low band gap polymer brushes

A surface-initiated Stille polycondensation from Pd catalyst-immobilized ZnO nanorods affords well-defined core–shell nanoparticles. For the first time, a low band gap polymer was anchored on ZnO nanorods to create hybrid materials with tunable photophysical properties.

Highly efficient photovoltaics and field-effect transistors based on copolymers of mono-fluorinated benzothiadiazole and quaterthiophene: synthesis and effect of the molecular weight on device performance

A D–A conjugated polymer based on mono-fluorinated benzothiadiazole (FBT) was designed and synthesized, and high performance photovoltaics and FETs were achieved.

Utilizing alkoxyphenyl substituents for side-chain engineering of efficient benzo[1,2-b:4,5-b′]dithiophene-based small molecule organic solar cells

A new alkoxyphenyl substituted benzo[1,2-b:4,5-b′]dithiophene-based small molecule was designed and synthesized for solution-processed organic solar cells.

Solution-processed new porphyrin-based small molecules as electron donors for highly efficient organic photovoltaics

A series of new A–D–A structural 5,15-dialkylated porphyrin-cored small molecules have been developed as donors in bulk heterojunction organic solar cells, and the highest power conversion efficiency of 6.49% has been achieved.

Diels–Alders adducts of C60 and esters of 3-(1-indenyl)-propionic acid: alternatives for [60]PCBM in polymer:fullerene solar cells

Two new, easily synthesized fullerene derivatives are introduced that give significantly higher device efficiency (PCE) than [60]PCBM in the standard P3HT:fullerene solar cells.