Energy Level Tuning of Poly(phenylene‐alt‐dithienobenzothiadiazole)s for Low Photon Energy Loss Solar Cells

Six poly(phenylene‐alt‐dithienobenzothiadiazole)‐based polymers have been synthesized for application in polymer–fullerene solar cells. Hydrogen, fluorine, or nitrile substitution on benzo­thiadiazole and alkoxy or ester substitution on the phenylene moiety are investigated to reduce the energy loss per converted photon. Power conversion efficiencies (PCEs) up to 6.6% have been obtained. The best performance is found for the polymer–fullerene combination with distinct phase separation and crystalline domains. This improves the maximum external quantum efficiency for charge formation and collection to 66%. The resulting higher photocurrent compensates for the relatively large energy loss per photon (E _loss = 0.97 eV) in achieving a high PCE. By contrast, the poly­mer that provides a reduced energy loss (E _loss = 0.49 eV) gives a lower photocurrent and a reduced PCE of 1.8% because the external quantum efficiency of 17% is limited by a suboptimal morphology and a reduced driving force for charge transfer.

     

A Conjugated Random Copolymer of Benzodithiophene‐Difluorobenzene‐Diketopyrrolopyrrole with Full Visible‐Light Absorption for Bulk‐Heterojunction Solar Cells

Stille coupling polymerization is used to synthesize a new donor–acceptor (D–A) conjugated random copolymer (PBDT‐DFB‐DPP) that comprises an electron‐rich benzodithiophene (BDT) unit in conjugation with electron‐deficient 2,5‐difluorobenzene (DFB) and diketopyrrolopyrrole (DPP) moieties, which have complementary light‐absorption behavior. The thermal, photophysical, and electrochemical properties of the polymer are investigated using thermogravimetric analysis, UV–vis absorption spectroscopy, and cyclic voltammetry. The random copolymer exhibits both broad light absorption and a low‐lying highest occupied molecular orbital (HOMO) level, which contribute to enhancement of the short‐circuit current density (J_sc) and open‐circuit voltage (V_oc). A bulk‐heterojunction polymer solar cell fabricated from PBDT‐DFB‐DPP and PC₆₀BM exhibits a promising power conversion efficiency (PCE) of 2.50%.

     

Efficient Polymer Solar Cells Employing Solution‐Processed Conjugated Polyelectrolytes with Differently Charged Side Chains

Poly(6‐(4,7‐dimethyl‐2H‐benzo[d][1,2,3]triazol‐2‐yl)‐N,N,N‐trimethylhexan‐1 aminium iodide) (PBTz‐TMAI) and poly(sodium 4‐(4,7‐dimethyl‐2H‐benzo[d] [1,2,3]triazol‐2‐yl)butane‐1‐sulfonate) (PBTz‐SO₃Na) based on the same benzotriazole‐conjugated backbone but with ammonium and sulfonated side chains are designed and synthesized through side‐chain functionalization and Yamamoto polymerization, respectively, and are used as the cathode interlayers in fullerene‐ and non‐fullerene‐based polymer solar cells. The interfacial modification of PBTz‐TMAI and PBTz‐SO₃Na onto the active layer achieves good energy alignment at cathode electrodes and optimized exciton‐dissociation efficiency from the active layer. Consequently, the power conversion efficiencies (PCEs) of 7.8% and 9.6% are obtained for the fullerene PTB7:PC₇₁BM‐based and non‐fullerene PBDB‐T:ITIC‐based polymer solar cells (PSCs) with PBTz‐SO₃Na interlayer. The PCS devices based on PTB7:PC₇₁BM and PBDB‐T:ITIC active layers with PBTz‐TMAI interlayer achieved a remarkably improved performance with PCEs of 8.2% and 10.2%, respectively.     

Quasi‐Solid‐State Dye‐Sensitized Solar Cells Using Nanocomposite Gel Polymer Electrolytes Based on Poly(propylene carbonate)

Poly(propylene carbonate) (PPC) is synthesized from the copolymerization of CO₂ and propylene oxide. Novel nanocomposite gel polymer electrolytes (GPEs) are prepared by utilizing PPC, organic solvents containing a redox couple, and aluminum oxide nanoparticles for application in dye‐sensitized solar cells (DSSCs). The quasi‐solid‐state DSSC assembled with optimized nanocomposite GPEs exhibits a relatively high conversion efficiency of 6.16% at 100 mW cm^?2 and better stability than DSSC with liquid electrolyte.     

Effect of Alkyl Side Chains on the Photovoltaic Performance of 2,1,3‐Benzoxadiazole‐Based (‐X‐DADAD‐)n‐Type Copolymers

A family of novel (X‐TATAT)_n‐type conjugated polymers based on the carbazole (X), thiophene (T), and benzoxadiazole (A) moieties is designed and explored as electron‐donor materials for organic bulk heterojunction solar cells. Incorporation of the branched side chains of different size and shape affects significantly the optoelectronic properties of the materials, particularly frontier energy levels of polymers translated to the open circuit voltages of the photovoltaic cells. The revealed unprecedented correlation between the parameters of the solar cells (V _OC, fill factor (FF), J _SC) and bulkiness of the alkyl side chains provides useful guidelines for rational design of novel materials for organic photovoltaics.

     

Synthesis of New Conjugated CNPPV Derivatives Containing Different Lengths of Oligothiophene Units for Organic Solar Cells

A new series of CNPPV derivatives that contain oligothiophenes in the polymer backbone, have been synthesized by Knoevenagel polycondensation. Different lengths of oligothiophene units have been introduced to study the relationship between polymer structure and electronic properties. Thermal stability, UV‐vis absorption spectra, and electrochemical properties of the copolymers were studied. The bandgaps estimated from UV‐vis spectra varied from 1.77 to 1.83 eV. BHJ photovoltaic cells from different copolymers and a soluble fullerene derivative have been fabricated. The best PCE of 1.62% was obtained for the polymer CNPPV‐7T, with a short‐circuit current of 5.68 mA · cm^?2, open‐circuit voltage of 0.66 V, and fill factor of 43.2%.

     

A High‐Mobility Low‐Bandgap Copolymer for Efficient Solar Cells

A new high‐mobility low‐bandgap polymer, PBDT‐DODTBT, based on benzodithiophene and 5,6‐bis(octyloxy)‐4,7‐di(thiophen‐2‐yl)benzothiadiazole has been synthesized through a standard Stille coupling reaction. The polymer is soluble in common organic solvents, such as chloroform, tetrahydrofuran, and chlorobenzene and has excellent film forming properties. Preliminary studies of the copolymer showed the charge mobility as high as 7.15 × 10^?3 cm² · V^?1 · s^?1 from SCLC measurement. Initial photovoltaic cells based on the composite structure of ITO/PEDOT:PSS/PBDT‐DODTBT: ^α PC₇₁BM (1:2)/Ca/Al showed an open‐circuit voltage of 0.76 V, a power conversion efficiency of 4.02%, and a short‐circuit current of 8.96 mA · cm^?2.

     

Rational Design of 2D p–π Conjugated Polysquaraines for Both Fullerene and Nonfullerene Polymer Solar Cells

So far, squaraine‐based polymer donors have been less explored for the bulk heterojunction (BHJ) polymer solar cells. In this work, two new p–π conjugated polysquaraines ( PASQ‐BDT1 and PASQ‐BDT2 ) with different electron‐rich subunits on the squaraine skeleton are rationally developed as new polymer donors based on the 2D structure design concept. PASQ‐BDT2 with N,N‐diisobutylaniline subunits shows superior device performances in both fullerene and nonfullerene PSCs compared to PASQ‐BDT1 containing triphenylamine subunits, with power conversion efficiencies (PCEs) of 4.34% and 3.72%, respectively, owing to increased light‐harvesting ability and more favorable nanoscale morphology in the BHJ films. Moreover, its demonstrated that solvent effects can play an effective role in elevating the device performance. For the PASQ‐BDT2 /PC₇₁BM blend, the PCE is improved from 3.19% to 4.34% after solvent vapor annealing treatment, mainly attributed to the optimized film morphology and increased hole mobility. More interestingly, when the processing solvent for nonfullerene devices is changed from chlorobenzene to chloroform, a significant enhancement on PCE from 1.96% to 3.72% is yielded for the PASQ‐BDT2 /ITIC blend, due to suppressed charge recombination and enhanced crystallinity in the chloroform‐processed BHJ films.     

Synthesis of Alkyl‐Substituted Quinoxaline‐Based Copolymers Along with Photophysical Property Modulation for Polymer Solar Cells

A series of alkyl‐substituted quinoxaline‐based copolymers are prepared and their properties are studied. Stille copolymerization of 2,5‐bis(trimethylstannyl)thiophene (M3) with different ratios of 2,7‐dibromo‐9,9‐dioctyl‐9H‐fluorene (M4) and 5,8‐dibromo‐6,7‐difluoro‐2,3‐didodecylquinoxaline (M1) affords five new random copolymers, labeled P1–P5. Suzuki copolymerization of 4,5,5‐tetramethyl‐2‐[2‐(4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolan‐2‐yl)‐9,9‐dioctyl‐9H‐fluoren‐7‐yl]‐1,3,2‐dioxaborolane (M5) and 5,8‐bis(5‐bromothiophen‐2‐yl)‐2,3‐didodecyl‐6,7‐difluoroquinoxaline (M2) yielded a new alternating copolymer, labeled P6. All copolymers show high thermal stability, and the 5% weight loss temperature is above 400 °C. The estimated optical bandgap (E _g) values of random copolymers P1–P5 (E _g ≈ 1.93, 1.97, 1.97, 2.02, and 2.08 eV, respectively) are found to be relatively lower than that of alternating copolymer P6 (E _g ≈ 2.14 eV). All copolymers P1–P6 exhibit deep highest occupied molecular orbital (HOMO) energy levels, and the determined HOMO levels are ?5.65, ?5.67, ?5.67, ?5.60, ?5.59, and ?5.66 eV, respectively. The maximum power conversion efficiencies of polymer solar cells made with individual copolymers P1–P6 as a donor materials and PC₇₀BM as an acceptor are 3.98, 2.91, 3.33, 3.55, 3.07, and 2.78%, respectively.     

Efficient Polymer Solar Cells Based on New Random Copolymers with Porphyrin‐Incorporated Side Chains

Two new wide bandgap block copolymers (PL1 and PL2) with porphyrin‐incorporated side chains are designed and used as electron donors for solution‐processed bulk heterojunction polymer solar cells. The photophysical, electrochemical, and photovoltaic properties, charge transport mobility and film morphology of these two block copolymers are investigated. Detailed investigations reveal that the different alkyl groups and electron‐withdrawing substituents on the porphyrin pendant units have significant influence on the polymer solubility, absorption energy level, band gap, and charge separation in the bulk‐heterojunction thin films, and thus the overall photovoltaic performances. Organic photovoltaic devices derived from these copolymers and ([6,6]‐phenyl‐C₇₁‐butyric acid methyl ester) (PC₇₁BM) acceptor show the best power conversion efficiencies of 5.83% and 7.14%, respectively. These results show that the inclusion of a certain proportion of side chain porphyrin group as a pendant in the traditional donor‐acceptor (D‐A) type polymer can broaden the molecular absorption range and become a full‐color absorbing molecule. The size of the porphyrin pendant also has an obvious effect on the properties of the molecule.     

4,5‐Ethylene‐2,7‐Carbazole‐Based Medium‐Bandgap Conjugated Polymers with Low‐Lying HOMO Levels Toward Efficient Polymer Solar Cells with High Open‐Circuit Voltage

Three medium‐bandgap polymers based on a 4,5‐ethylene‐2,7‐dithienyl carbazole as the electron‐donating unit and different 5,6‐dialkoxy‐2,1,3‐benzothiadiazoles as the electron‐accepting units, are synthesized as polymer donors for photovoltaic applications. The three copolymers possess highest occupied molecular oribital (HOMO) levels around ?5.47 eV and medium bandgaps of about 1.94 eV. The solar cells with polymer:[6,6]‐phenyl C71‐butyric acid methyl ester (PC₇₁BM) = 1:4 as the active layer, show an especially high open‐circuit voltage (V_oc) of 0.95 V and attain good power conversion efficiency up to 5.91%. The hole mobilities of the active layer films, measured by space‐charge‐limited current (SCLC), are up to 3.5 × 10^?4 cm² V^?1 s^?1. Given the favorable medium bandgaps, low‐lying HOMO levels, and good hole mobilities, these copolymers are promising candidates for the construction of a highly efficient front cell to harvest the shorter wavelength band of the solar radiation in a tandem solar cell with high V_oc.

     

A New Dithienylbenzotriazole‐Based Poly(2,7‐carbazole) for Efficient Photovoltaics

A new dithienyl benzotriazole‐based conjugated polymer was synthesized by Suzuki coupling reaction. The polymer was found to be soluble in common organic solvents, such as chloroform, tetrahydrofuran and chlorobenzene, with excellent film‐forming properties. The structure of the polymer was confirmed by ¹H NMR, the molecular weights determined by GPC and the thermal properties investigated by TGA and DSC. The polymer films exhibited an absorption band in the wavelength range 300 to 610 nm. Preliminary photovoltaic cells based on the composite structure of indium tin oxide (ITO)/PEDOT:PSS/ PCDTBTz:PC ₆₀ BM (1:2 w/w)/Al showed an open‐circuit voltage of 0.92 V, a power conversion efficiency of 2.2% and a short circuit current of 5.33 mA cm^?2.

     

Effect of Aryl Substituents and Fluorine Addition on the Optoelectronic Properties and Organic Solar Cell Performance of a High Efficiency Indacenodithienothiophene‐alt‐Quinoxaline π‐Conjugated Polymer

A series of donor‐acceptor (D‐A) π‐conjugated polymers, based on indacenodithienothiophene (IDTT) as an electron‐donating unit and quinoxaline as an electron‐deficient moiety, are synthesized via a Pd‐catalyzed Stille cross‐coupling polymerization. Molecular characteristics, photovoltaic parameters, and optoelectronic properties are examined through structural differences corresponding to thienyl versus phenyl side group substitutions on the IDTT and the non‐fluorinated versus the monofluoro quinoxaline derivatives. One of the most important outcome is that the power conversion efficiency (PCE) in the studied polymers is more device architecture dependent (conventional vs inverted) rather than chemical structure dependent. From single junction solar cells based on bulk heterojunction polymer:[6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) systems as the active layer, a maximum PCE of 5.33% has been achieved from the polymer containing the thienyl substituent on the IDTT and one fluorine atom on the quinoxaline. This demonstrates that finding the optimum molecular weight of ThIDTT‐QF or introducing the monofluoro‐quinoxaline in a regioregular motif in the polymer backbone significantly higher PCE can be expected versus the fully optimized high performance PhIDTT‐Q conjugated polymer.     

High‐Performance Non‐Fullerene Organic Solar Cells Based on a Pair of Medium Band Gap Polymer Donor and Perylene Bisimide Derivative Acceptor

Herein, high performance solution‐processed non‐fullerene organic solar cells employing a medium band gap conjugated polymer (PBDTTS‐TTffBT) and a bay‐linked perylene bisimide derivative (di‐PBI) as an electron donor and acceptor, respectively, have been demonstrated. Owing to the complementary absorption range and well‐matched energy levels, PBDTTS‐TTffBT and di‐PBI have great potential for constructing high‐performance non‐fullerene organic solar cells. The facile control of blend film morphology by careful choice of a processing solvent achieves a promising power conversion efficiency of the device up to 6.51% with high open‐circuit voltage (0.92 V), short circuit current density (10.89 mA cm^?2), and a fill factor (0.65). Furthermore, the blend film exhibits high thermal stability, retaining >80% of its initial efficiency at 100 °C up to 120 min. This work demonstrates that the medium band gap is a promising candidate as a donor material for constructing high‐performance non‐fullerene organic solar cells.

     

Synthesis and Photovoltaic Properties of a Poly(2,7‐carbazole) Derivative Based on Dithienosilole and Benzothiadiazole

A low bandgap poly(2,7‐carbazole) derivative PCDSBT containing dithienosilole and benzothiadiazole is synthesized by Stille polymerization. The incorporation of dithienosilole and carbazole causes PCDSBT to have a broad absorption from 350 to 800 nm and a relatively low HOMO level. Solar cell devices fabricated by blending PCDSBT with [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) at a 1:2 weight ratio in a mixed solvent of 3% (by volume), 1,8‐diiodooctane, and 97% dichlorobenzene lead to a power conversion efficiency of 1.65% under the illumination of AM 1.5G (100 mW · cm^?2).

     

Synthesis and Photovoltaic Properties of a D–A Copolymer Based on the 2,3‐Di(5‐hexylthio­phen‐2‐yl)quinoxaline Acceptor Unit

A new D–A copolymer ( PBDT‐DTQx) based on the 2,3‐di(5‐hexylthiophen‐2‐yl)quinoxaline acceptor unit and a bithienyl‐substituted benzodithiophene (BDT) donor unit is designed and synthesized for application as the donor material in polymer solar cells (PSCs). The polymer film shows a broad absorption band covering the wavelength range from 300 to 720 nm and a low highest occupied molecular orbital (HOMO) energy level at ?5.35 eV. A device based on PBDT‐DTQx :PC₇₀BM ([6,6]‐phenyl‐C71‐butyric acid methyl ester) (1:2.5, w/w) with chloronaphthalene as a solvent additive displays a power conversion efficiency (PCE) of 3.15%. With methanol treatment, the PCE of the PSCs is further improved to 3.90% with a significant increase of the short‐circuit current density, J_sc, from 10.10 mA cm^?2 for the device without the methanol treatment to 11.71 mA cm^?2 for the device with the methanol treatment.

     

N‐Acylisoindigo Derivatives as Polymer Acceptors for “All‐Polymer” Bulk‐Heterojunction Solar Cells

Polymer acceptors are a class of “non‐fullerene” electron‐transporting materials that can replace [6,6]‐phenyl C61/71 butyric acid (PC61/71BM) in bulk‐heterojunction (BHJ) solar cells. In this contribution is reported the use of N‐acyl‐substituted isoindigo (IID) motifs (IID(CO)) as electron‐deficient units in the design of polymer acceptors for “all‐polymer” BHJ solar cells. While IID motifs are commonly used in the design of polymer donors for efficient BHJ solar cells with the fullerene PC61/71BM acceptors, here it is shown that IID(CO) building units used in conjunction with suitable co‐monomers represent a practical avenue for polymer acceptors with electron affinity values comparable to that of PC61/71BM (i.e., 4.2 eV).     

Synthesis and Characterization of Wide‐Bandgap Conjugated Polymers Consisting of Same Electron Donor and Different Electron‐Deficient Units and Their Application for Nonfullerene Polymer Solar Cells

Substantial development has been made in nonfullerene small molecule acceptors (NFSMAs) that has resulted in a significant increase in the power conversion efficiency (PCE) of nonfullerene‐based polymer solar cells (PSCs). In order to achieve better compatibility with narrow‐bandgap nonfullerene small molecule acceptors, it is important to design the conjugated polymers with a wide bandgap that has suitable molecular orbital energy levels. Here two donor–acceptor (D–A)‐conjugated copolymers are designed and synthesized with the same thienyl‐substituted benzodithiophene and different acceptors, i.e., poly{(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)‐alt‐(1,3‐bis(2‐octyldodecyl)‐1,3‐dihydro‐2H‐dithieno[3′,2′:3,4;2″,3″:5,6]benzo[1,2‐d]imidazol‐2‐one‐5,8‐diyl) } ( DTBIA , P1 ) and poly{(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)‐alt‐(2‐(5‐(3‐octyltridecyl)thiophen‐2‐yl)dithieno[3′,2′:3,4;2″,3″:5,6]benzo[1,2‐d]thiazole‐5,8‐diyl)} ( TDTBTA , P2 ) (and their optical and electrochemical properties are investigated). Both P1 and P2 exhibit similar deeper highest occupied molecular orbital energy level and different lowest unoccupied molecular orbital energy level. Both the copolymers have complementary absorption with a well‐known nonfullerene acceptor ITIC‐F. When blended with a narrow‐bandgap acceptor ITIC‐F, the PSCs based on P1 show a power conversion efficiency of 11.18% with a large open‐circuit voltage of 0.96 V, a J_sc of 16.89 mA cm^?2, and a fill factor (FF) of 0.69, which is larger than that for P2 counterpart (PCE = 9.32%, J_sc = 15.88 mA cm^?2, V_oc = 0.91 V, and FF = 0.645). Moreover, the energy losses for the PSCs based on P1 and P2 are 0.54 and 0.59 eV, respectively. Compared to P2, the P1‐ based PSCs show high values of incident photon to current conversion efficiency (IPCE) in the shorter‐wavelength region (absorption of donor copolymer), more balanced hole and electron mobilities, and favorable phase separation with compact π–π stacking distance.     

Synthesis of Ruthenium Complex Containing Conjugated Polymers and Their Applications in Dye‐Sensitized Solar Cells

The synthesis of two ruthenium terpyridine complexes containing conjugated polymers is reported and their application as photosensitizers in dye‐sensitized solar cells (DSSC) was studied. The polymers were synthesized by the palladium‐catalyzed coupling reaction between the ruthenium complex monomer and the fluorene‐based comonomer. One polymer was functionalized with carboxylic acid groups on the main chain, which strongly facilitate the anchorage of polymer dye molecules on the electrode surface. DSSCs based on TiO₂ nanoparticles and nanotubes were fabricated. It was found that the ruthenium complexes played an important role in the photosensitization process, and the power conversion efficiencies measured were in the order of 0.1%. Annealing of TiO₂ nanotubes in ammonia gas flow did not result in significant improvement in cell performance.

     

Improvement in Power Conversion Efficiency and Performance of P3HT/PCBM Solar Cells Using Dithiafulvalene‐Based π‐Conjugated Oligomers

The improvement in the power conversion efficiency (PCE) and performance of poly(3‐hexylthiophene) (P3HT)/[6,6]‐phenyl‐C₆₁butyric acid methyl ester (PCBM) solar cells are reported by using dithiafulvalene (DTF)‐based conjugated oligomers ((poly[2‐(9H‐fluoren‐9‐ylidene)‐4,5‐bis(hexylthio)‐1,3‐dithiole‐ran‐(2,1,3‐benzothiadiazole)] (PTBT) and poly[2,7‐(9,9‐dihexyl­fluorene)‐ran‐(2‐(9H‐fluoren‐9‐ylidene)‐4,5‐bis(hexylthio)‐1,3dithiole]‐ran‐(2,1,3‐benzothiadiazole)] (PFTBT)) that contain a DTF unit, which serves as an electron‐rich donor, and a benzothiadiazole group, which serves as an electron‐deficient acceptor in the main chain. A P3HT/PCBM device with 5 wt% PTBT exhibits an efficiency of up to 1.78%, which is higher than that of a device composed only of a P3HT/PCBM blend (1.01%). Introducing 2 wt% PTBT into the P3HT/PCBM blend substantially increases the charge‐carrier mobility from 2.01 × 10^?4 to 4.59 × 10^?4 cm² V^?1s^?1. The improvement of the PCE is attributed to improved charge transport in the device and an increased open‐circuit voltage, suggesting that blending PTBT increases the intermolecular interaction of the molecules. In addition, doping the oligomer in ambient atmospheric conditions enhances the stability of these devices.