Synthesis and Photovoltaic Properties of a Donor‐Acceptor Copolymer of Dithienosilole and 5,6‐Bis(octyloxy)benzo[1,2,5]thiadiazole

A donor‐acceptor (D‐A) copolymer, PDTSDOBT , based on dithienosilole and 5,6‐bis(octyloxy)benzo[1,2,5]thiadiazole is synthesized by Pd‐catalyzed Stille coupling reaction for application as a donor material in polymer solar cells (PSCs). The polymer shows good thermal stability, strong absorption in the visible region, and a relatively low bandgap of 1.63 eV. The hole mobility of PDTSDOBT as measured by SCLC is 5.58 × 10^?4 cm² V^?1 s^?1. The power conversion efficiency of a PSC based on PDTSDOBT : PC₇₀BM (1:2 w/w) is 3.51% with a short‐circuit current density of 8.96 mA cm^?2, an open‐circuit voltage of 0.69 V, and a fill factor of 0.568, under the 100 mW cm^?2 AM1.5G illumination.     

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.

     

Thermochromic and Photovoltaic Properties of an Alternating Copolymer of Dithieno[3,2‐b:2′,3′‐d]thiophene and Thieno[3,4‐c]pyrrole‐4,6‐dione

The synthesis of a new alternating conjugated polymer, PDTTTPD, based on electron‐donating dithieno[3,2‐b:2′,3′‐d]thiophene (DTT) and electron‐withdrawing thieno[3,4‐c]pyrrole‐4,6‐dione (TPD) units is reported. This polymer shows strong thermochromic effect in chlorobenzene with a peak shift up to 170 nm. This phenomenon is studied with fluorescence spectroscopy and related to the steric hindrance along the polymer main chain. Polymer solar cells are fabricated from its blends with PC₇₁BM, and shows modest power conversion efficiency up to 2.1%. The low efficiency is due to the low short‐circuit current, which is also attributed to the steric effect. Based on these results, a general design rule for polymer photovoltaic material with controlled backbone conformation is proposed.     

Narrow Bandgap Donor–Acceptor Conjugated Polymer: Synthesis,Properties and Photovoltaic Application

New soluble, narrow bandgap bithiophene‐based conjugated polymer poly[3‐(1,4‐dihydro‐5,8‐didodecanoxy‐1,4‐methano‐naphthalen‐6‐yl)‐2,2′‐bithiophene‐5,5′‐diyl(2‐cyano‐1,2‐ethenediyl)‐1,4‐diphenylene(1‐cyano‐1,2‐ethenediyl)] ( PBT‐PDCN ) has been synthesized by Knoevenagel polycondensation method. PBT‐PDCN exhibits high solubility in common organic solvents. Optical studies show that the higher‐energy absorption band of PBT‐PDCN is red‐shifted with increasing polarity of solvents from toluene to THF, and from solution to solid film. Electrochemical characterizations revealed that polymer PBT‐PDCN has a low bandgap. The hole mobility was found to be 1.4 · 10^?4 cm² V^?1 s^?1. Bulk heterojunction solar cells have been fabricated using blends of PBT‐PDCN with PCBM in different ratios. The best solar cell performance obtained was based on the blend of PBT‐PDCN with PCBM in 1:3 weight ratio with the power conversion efficiency reaching 0.71%. The photocurrent action spectrum showed a broad curve between 400 nm and 700 nm, with an external quantum efficiency (EQE) maximum of 20% at 570 nm, matching the PBT‐PDCN absorption spectrum very closely.

     

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.

     

Carbazole‐Substituted Triphenylamine and Diketopyrrolopyrrole Alternating Copolymer for Photovoltaic Cells

A new alternating conjugated polymer (PTCDPP) of carbazole‐substituted triarylamine and diketopyrrolopyrrole is prepared and characterized in detail. The polymer exhibits two strong absorption bands at 345 and 600 nm. With highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of ?5.13 eV and ?3.67 eV, PTCDPP displays an energy gap of 1.66 eV. PTCDPP‐based bulk heterojunction solar cells with a structure of fluorinated tin oxide (FTO)/TiO₂/PTCDPP:[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM)/MoO₃/Ag are fabricated. The devices are optimized by adjusting the composition of the PTCDPP:PCBM active layer, thermal treatment, and addition of processing additives. The device based on PTCDPP:PCBM (1:4, w/w) shows a power conversion efficiency (PCE) of 2.31%, with a short‐circuit current of 4.17 mA cm^?2, an open‐circuit voltage of 0.79 V, and a fill factor of 0.35. The best cell performance (2.65% PCE) is achieved by using 1,8‐diiodooctane (3%, v/v) as a processing additive and annealing the active layer at 80 °C.

     

Synthesis and Photovoltaic Performance of a [1,2,3]Triazolo[4,5‐g]quinoxaline‐Based Low‐Bandgap Polymer

A narrow‐bandgap conjugated polymer, PFDTBTzQ‐2OC1, is prepared by alternating [1,2,3]triazolo[4,5‐g]quinoxaline and 9,9‐didodecyl‐fluorene. With a bandgap of 1.63 eV, this polymer has wide absorption ranging from 300–760 nm in film. Bulk heterojunction solar cells fabricated by blending PFDTBTzQ‐2OC1 with [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester exhibit a maximum power conversion efficiency of 1.31%, with a short‐circuit current density of 1.98 mA cm^–2, an open‐circuit voltage of 0.74 V, and a fill factor of 0.47.

     

Bisfuran‐s‐Tetrazine‐Based Conjugated Polymers: Synthesis,Characterization, and Photovoltaic Properties

Bisfuran‐s‐tetrazine (FTz) and its copolymers with cyclopenta[2,1‐b:3,4‐b′]dithiophene (CPDT) and benzo[1,2‐b:4,5‐b′]dithiophene (BDT) (PCPDTFTz and PBDTFTz) are prepared with the alternating s‐tetrazine and CPDT or BDT units bridged by a furan ring. Their optical and electrochemical properties are studied and compared with their thiophene analogs (PCPDTTTz‐out and PCPDTTTz‐in), in which the bridging unit is 4‐hexylthiophene or 3‐hexylthiophene, respectively. Bulk heterojunction (BHJ) solar cells fabricated from these polymers with PC₇₁BM have a power conversion efficiency (PCE) of 0.8%, which is much lower than that from the PCPDTTTz‐out analog (3.2%), due to the low steric hindrance of the furan polymers in the absence of alkyl substitution on the furan ring.     

Optimization of Polymer Solar Cells Based on the Conjugated Copolymer of Poly(phenylenevinylene‐alt‐4,7‐diphenyl‐2,1,3‐benzothiadiazole) (PP‐DBT)

Polymer solar cells based on poly(phenylenevinylene‐alt‐4,7‐diphenyl‐2,1,3‐benzothiadiazole) (PP‐DBT):[6,6]‐phenyl C₇₁ butyric acid methyl‐ester (PC₇₁BM) blend films are optimized. The results show that the solar cell has a better performance at a 1:4 blend ratio than at 1:1, 1:2, 1:3, and 1:5 blend ratios, due to the higher hole mobility of active layer and the optimized morphology. The device prepared from chlorobenzene (CB) has a higher power conversion efficiency (PCE) than the devices prepared from chloroform and o‐dichlorobenzene, because the former has a much higher short‐circuit current density (J_sc) resulting from the morphology with proper phase separation. The solar cell modified with two layers of ZnO nanocrystals achieves the best performance with an open‐circuit voltage of 0.9 V, a J_sc of 7.46 mW cm^?2, fill factor (FF) of 0.50, and PCE of 3.36%.

     

Synthesis and Photovoltaic Properties of New Multifused Anthradithiophene‐Based Narrow‐Bandgap D–A Copolymers

Three structurally novel donor–acceptor copolymers are synthesized and fully characterized using an anthradithiophene (ADT) derivative as the donor block. All of them exhibit broad absorption ranges and moderate hole and electron mobilities. The multifused ADT donor block makes them exhibit lower highest occupied molecular orbital (HOMO) energy levels than many reported benzodithiophene‐based polymers. Copolymer blends with [6,6]‐phenyl C71 butyric acid methyl ester (PC₇₁BM) as active layers are used to fabricate polymer solar cells (PSCs) and a variety of post‐treatment methods are employed to optimize the device performance. The conventional and inverted configuration devices are prepared to evaluate the photovoltaic properties. A maximum power conversion efficiency of 1.66% is achieved for the inverted‐configuration device. The improved efficiency is caused by the close energy alignment between the work function of MoO₃ and the HOMO energy levels of the copolymers, facilitating better charge transport and increased short‐circuit current density (J_sc) with the inverted devices.

     

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.

     

Donor/Spacer/Acceptor Block Copolymer Containing Poly(2,7‐carbazole) and Perylenetetracarboxydiimide Subunits

A straightforward synthesis of a conjugated rod/spacer/rod‐type block copolymer containing PCz electron‐donor and PDI electron‐acceptor blocks is described. Two chromophores are covalently connected through sebacate units as saturated spacer. The resulting donor/spacer/acceptor‐type block copolymer (PCz‐S‐PDI) can be applied to limit charge recombination between donor/acceptor interfaces and to control the scale length of nanostructure formation. PCz‐S‐PDI was used to produce a solar cell with the power conversion efficiency of 0.004%.

     

Synthesis of Poly(N‐aryl‐2,7‐carbazole)s for Efficient Blue Electroluminescence Materials

Several poly(N‐phenyl‐2,7‐carbazole)s that have dialkoxy groups at the m‐ and p‐positions (PmpCzDC, PmpPhDC, PmpCBiDC, PmpEHC), a silyl group at the p‐position (PpPhDSiC), and a diphenylamino group (PmPhDAC, PmEHAC) at the m‐position of the N‐phenyl portion are synthesized, and their optical properties are characterized. These polymers have been used as emitting layer materials of organic light‐emitting diode (OLED) devices that have a configuration of ITO/PEDOT(PSS)/polymer/CsF/Al. The OLED devices embedded with PmpCzDC, PmpPhDC, and PmpEHC show intense luminance of about 15 000 cd · m^?2 with efficiencies of about 1 cd · A^?1, while the devices embedded with PpPhDSiC, PmPhDAC, and PmEHAC show less luminance but retain the color purity of blue emission under a wide range of applied voltages.

     

Extremely Color‐Stable Blue Light‐Emitting Polymers Based on Alternating 2,7‐Fluorene‐co‐3,9‐carbazole Copolymer

Novel alternating 2,7‐fluorene‐co‐3,9‐carbazole copolymer(3,9‐PFCz) was synthesized by Suzuki polycondensation between 3‐bromo‐9‐(2‐iodo‐9,9‐dioctyl‐9H‐fluoren‐7‐yl)‐9H‐carbazole and 2,7‐bis(4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolan‐2‐yl)‐9,9‐dioctylfluorene. Cyclic voltammetry studies showed a higher HOMO level (?5.23 eV) than the polyfluorene homopolymer(PFO). The glass transition temperature of the copolymer (110 °C) was much higher than that of the PFO (67 °C). After the copolymer had been annealed at 200 °C for 2 h, the PL spectra showed almost no change and no aggregation‐ or oxidation‐related undesirable long‐wavelength emission has been observed. The light‐emitting diode in configuration of ITO/PEDOT/EML/CsF/Al showed a maximum external quantum efficiency of 1.54% with a luminance of 435 cd · m^?2 and the maximum brightness of 2 630 cd · m^?2 with CIE coordinate (0.17, 0.14) was achieved at 8.4 V. The device prepared in configuration of ITO/PEDOT/PVK/polymer/Ba/Al exhibits an efficient, stable blue emission; it has a turn‐on voltage of 6 V and maximum external quantum efficiency of 1.1%. The EL emission remains stable at high current density and after thermal treatment at 150 °C for 30 min. Extremely color‐stable blue emission from alternating poly[(2,7‐fluorene)‐co‐(3,9‐carbazole)] copolymer makes it a promising candidate for flat‐panel display applications.

     

Effect of Branched Side Chains on the Physicochemical and Photovoltaic Properties of Poly(3‐hexylthiophene) Isomers

Two P3HT isomers with branched alkyl side chains, P3EBT and P3MPT, are synthesized. The HOMO energy levels of P3EBT and P3MPT are ?5.35 and ?5.24 eV, respectively, which are significantly lower than that of P3HT with a linear side chain. The absorption edges of the two P3HT isomer films, especially those of P3EBT, are blue‐shifted in comparison with that of P3HT. A PSC based on P3EBT:IC₆₀BA (2:1 w/w) shows a high open‐circuit voltage of 0.98 V, which is the highest V_oc reported so far for polythiophene‐based PSCs. A PSC based on P3MPT:IC₇₀BA (2:1 w/w) exhibits a power conversion efficiency of 3.62% with a V_oc of 0.91 V. P3MPT is suitable for the application in tandem PSCs.     

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.

     

Synthesis and Photovoltaic Properties of D–A Copolymers Based on 11,12‐Difluorodibenzo[a,c]phenazine Acceptor Unit

Four conjugated D‐π‐A copolymers, P1 – P4 , based on 11,12‐difluorodibenzo[a,c]phenazine as an acceptor unit, benzodithiophene (BDT) with an alkoxy side chain ( P1 and P2 ) or thiophene‐conjugated side chain ( P3 and P4 ) as a donor unit, and thiophene π‐bridges, are designed and synthesized to investigate the effect of the side‐chain nature and position on the photovoltaic performance of the conjugated polymer donor materials in polymer solar cells (PSCs). It is found that the copolymers of P3 and P4 with thiophene conjugated side chains on the BDT unit demonstrate broader absorption and better photovoltaic performance than that of P1 and P2 with alkoxy side chains on the BDT unit. The photovoltaic performance of P1 and P3 with an alkyl side chain on the thiophene π‐bridge close to the acceptor unit is improved over that of P2 and P4 with an alkyl side chain on the thiophene π‐bridge close to the donor unit, due to the lower steric hindrance of P1 and P3 . The results indicate that side‐chain engineering is very important for improving the photovoltaic performance of conjugated polymer donor materials.

     

Synthesis and Photovoltaic Properties of Alternating Conjugated Polymers Derived from Indeno[1,2‐b]fluorene and Bithiophene or Thieno[3,2‐b]thiophene‐Cored Benzothiadiazole

Two narrow bandgap copolymers derived from 6,6′,12,12′‐tetraoctyl‐indeno[1,2‐b]fluorene and bithiophene or thieno[3,2‐b]thiophene‐cored benzothiadiazole are synthesized and characterized. The copolymers show broad absorption in the range 350–700 nm. The application of the copolymers as photovoltaic cells with configurations ITO/PEDOT:PSS/blend/Al and ITO/PEDOT:PSS/blend/interlayer/Al is investigated. A power conversion efficiency (PCE) of approximately 3.0% is achieved under an AM 1.5G solar simulator (80 mW cm^?2) for the cells with ITO/PEDOT:PSS/polymer:PC₇₁BM([6,6]‐phenyl‐C₇₁ butyric acid methyl ester) (1:4)/interlayer/Al.

     

Thermally Stable Dibenzo[def,mno]chrysene‐Based Polymer Solar Cells: Effect of Thermal Annealing on the Morphology and Photovoltaic Performances

The photovoltaic performances of dibenzo[def,mno]chrysene‐based polymer (poly(2,2′‐thiophenevinylenthiophene‐4,10‐[6,12‐bis(2‐decyltetradecyloxy)‐dibenzo[def,mno]chrysene]) (PTVTC) or poly(4,10‐bithiophene‐6,12‐bis(2‐decyltetradecyloxy)‐dibenzo[def,mno]chrysene) (PTTC)) solar cells as a function of thermal annealing temperatures from 100 to 180 °C are reported. Interestingly, the solar cells with PTTC containing two thiophene units, have superior thermal stability compared to the PTVTC:[6,6]‐phenyl‐C71‐butyric acid methyl ester (PC₇₁BM) solar cells, in which the PTVTC has two thiophene and vinyl groups. Atomic force microscopy, transmission electron microscopy, and X‐ray diffraction demonstrate that morphological stability of PTTC:PC₇₁BM blend films conduces thermally stable photovoltaic performances of PTTC solar cells. Therefore, the PTTC:PC₇₁BM bulk heterojunction solar cells have highly stable efficiency, retaining 97% of its original power conversion efficiency value without PCBM clusters in the blend films even at elevated temperatures. There have been no previous reports on the thermal stability aspect of dibenzo[def,mno]chrysene‐based polymer solar cells so far.

     

Synthesis,Properties, and Solar Cell Performance of Poly(4‐(p‐alkoxystyryl)thiazole)s

A series of polythiazoles (PvTzs) featuring conjugated styryl sidechains equipped with different solubilizing p‐alkoxy‐groups (? OR, R = n‐octyl, n‐dodecyl, 2‐ethylhexyl, 2‐hexyldecyl) is prepared by Negishi‐coupling polycondensation. Soluble material with number‐average molecular weights of up to M_n = 8.5 kDa (polydispersity (PDI) = 1.3, degree of polymerization (DPn) ≈ 20) is obtained, with a head‐to‐tail content of the PvTzs of ≈77%, as estimated from comparison with reference polymers. The polymers exhibit optical absorption properties similar to their polythiophene analogues, while their electrochemical characterization shows a significant stabilization of their frontier orbital levels. Fluorescence measurements indicate that upon excitation of the electron rich alkoxystyryl side‐chains charge transfer onto the more electron deficient polythiazole backbone occurs. This finding is corroborated by density functional theory (DFT) calculations on oligomeric model systems, which also consistently reproduce the optical properties observed for the polymers. The potentialities of these materials for applications in organic electronics can be demonstrated by their use as donor materials in organic photovoltaic cells, which exhibit higher open circuit voltages (V_OC, up to 0.86 V) than P3HT‐ or analogous polythiophene‐based cells (V_OC = 0.5–0.6 V).