The Novel Additive 1‐Naphthalenethiol Opens a New Processing Route to Efficiency‐Enhanced Polymer Solar Cells

Polymer solar cells (PSCs) based on fullerene derivatives often require additives to optimize active layer morphology. Here, the novel additive 1‐naphthalenethiol (SH‐na) is proposed for processing the PSC active layer of PTB7:PC₇₁BM. Spin‐casting with SH‐na as additive achieves a power conversion efficiency (PCE) of 7.3%, compared to 6.7% for preparations containing the conventional 1,8‐diiodooctane additive. Dipping of the active layer into a methanol solution of critical SH‐na concentration increases the PCE further to 8.75%. This is mainly due to an improved open‐circuit voltage (from 0.72 to 0.79 V) together with a high achieved fill factor of 0.70. The improved PCE is correlated to the morphology optimization according to measurements of grazing incidence small/wide‐angle X‐ray scattering, neutron reflectivity, atomic force microscopy, Fourier transform infrared spectroscopy, and X‐ray photoelectron spectroscopy. The integrated results suggest that the halogen‐free additive SH‐na can form hydrogen bonds with both PTB7 and PC₇₁BM, resulting in substantially improved PTB7 crystallization and multi‐length‐scale PC₇₁BM dispersion for appropriate aggregation and networks. The subsequent dipping treatment with SH‐na further modifies the active layer morphology for a more PC₇₁BM‐enriched surface and better PC₇₁BM networks in the bulk film for an optimized electron‐to‐hole mobility ratio of 2.04, hence resulting in improved device performance.     

High efficiency arrays of polymer solar cells fabricated by spray‐coating in air

We present bulk heterojunction organic solar cells fabricated by spray‐casting both the PEDOT:PSS hole‐transport layer (HTL) and active PBDTTT‐EFT:PC₇₁BM layers in air. Devices were fabricated in a (6 × 6) array across a large‐area substrate (25 cm²) with each pixel having an active area of 6.45 mm². We show that the film uniformity and operational homogeneity of the devices are excellent. The champion device with spray cast active layer on spin cast PEDOT:PSS had an power conversion efficiency (PCE) of 8.75%, and the best device with spray cast active layer and PEDOT:PSS had a PCE of 8.06%. The impacts of air and light exposure of the active layer on device performance are investigated and found to be detrimental. Copyright © 2015 John Wiley & Sons, Ltd.     

Highly Efficient Indoor Organic Photovoltaics with Spectrally Matched Fluorinated Phenylene‐Alkoxybenzothiadiazole‐Based Wide Bandgap Polymers

The unique electro‐optical features of organic photovoltaics (OPVs) have led to their use in applications that focus on indoor energy harvesters. Various adoptable photoactive materials with distinct spectral absorption windows offer enormous potential for their use under various indoor light sources. An in‐depth study on the performance optimization of indoor OPVs is conducted using various photoactive materials with different spectral absorption ranges. Among the materials, the fluorinated phenylene‐alkoxybenzothiadiazole‐based wide bandgap polymer—poly[(5,6‐bis(2‐hexyldecyloxy)benzo[c][1,2,5]thiadiazole‐4,7‐diyl)‐alt‐(5,50‐(2,5‐difluoro‐1,4‐phenylene)bis(thiophen‐2‐yl))] (PDTBTBz‐2F_anti)‐contained photoactive layer—exhibits a superior spectrum matching with indoor lights, particularly a light‐emitting diode (LED), which results in an excellent power absorption ratio. These optical properties contribute to the state‐of‐the‐art performance of the PDTBTBz‐2F_anti:[6,6]‐phenyl‐C₇₁ butyric acid methyl ester (PC₇₁BM)‐based OPV with an unprecedented high power‐conversion efficiency (PCE) of 23.1% under a 1000 lx LED. Finally, its indoor photovoltaic performance is observed to be better than that of an interdigitated‐back‐contact‐based silicon photovoltaic (PCE of 16.3%).     

Two new D–A conjugated polymers P(PTQD-Th) and P(PTQD-2Th) with same 9-(2-octyldodecyl)-8H-pyrrolo[3,4-b]bisthieno[2,3-f:3′,2′-h]quinoxaline-8,10(9H)-dione acceptor and different donor units for BHJ polymer solar cells application

We report the synthesis, characterization and photovoltaic properties of bulk heterojunction polymer solar cells of new donor–acceptor conjugated copolymers P(PTQD-Th) and P(PTQD-2Th) that incorporate same strong 9-(2-octyldodecyl)-8H-pyrrolo[3,4-b]bisthieno[2,3-f:3′,2′-h]quinoxaline-8,10(9H)-dione as strong acceptor and different weak thiophene (Th) and bi-thiophene (2Th) as donors, respectively. Both the copolymers showed suitable unoccupied lowest molecular orbital (LUMO) energy levels, compatible with the LUMO of PC₇₁BM for efficient electron transfer from copolymer to PC₇₁BM in the blended copolymer: PC₇₁BM thin films. Moreover the deeper highest occupied molecular orbital (HOMO) energy levels of both copolymers ensures the high open circuit voltage (V_oc) of the BHJ polymer solar cells. The optimized P(PTQD-Th):PC₇₁BM and P(PTQD-2Th):PC₇₁BM with weight ratio of 1:2 processed with chloroform solvent showed PCE of 3.65% and 3.96%, respectively. The higher value of J_sc for the device processed with P(PTQD-2Th):PC₇₁BM as compared to that for P(PTQD-Th):PC₇₁BM, attributed to narrower optical bandgap and broader absorption profile for P(PTQD-2Th) as compared to P(PTQD-Th). The PCE values of polymer solar cells were further improved (5.54% and 5.67% for P(PTQD-Th):PC₇₁BM and P(PTQD-2Th):PC₇₁BM, respectively) when small amounts of solvent additive, i.e. 1,8-diiodoctane (DIO) were used for the processing of active layers. The improved PCE has been attributed to both the enhanced values of short circuit current (J_sc) and fill factor (FF) due to the better nanomorphology and charge transport, induced by the high boiling point of solvent additive.     

High performance airbrush spray coated organic solar cells via tuning the surface tension and saturated vapor pressure of different ternary solvent systems

In this work, thieno [3,4-b] thiophene/benzodithiophene (PTB7): [6,6]-phenyl C71-butyric acid methyl ester (PC₇₁BM) based organic solar cell (OSC) with a new record of power conversion efficiency (PCE) of ∼7.62% has been realized using airbrush spray (AS) coating method in air ambient which can be well compatible with large-scale fabrication. By investigating the physical mechanism of AS coated blend films, a series of ternary solvent systems (TSS) are used to simultaneous optimize the surface tension and the saturated vapor pressure of solution. Therefore, different TSS further controls the morphology of PTB7:PC₇₁BM blend films precisely and systematically. It is elucidated that the chlorobenzene (CB)/o-Xylene (o-Xy)/1, 8-diiodoctane (DIO) TSS with a ratio of 37:60:3 vol.% could lead to a homogeneous surface morphology with a decreased aggregation domain size of active layer. In addition, the high fill factor, increased PC₇₁BM absorption and internal quantum efficiency indicate the formation of bicontinuous interpenetrating and fully percolated networks with nanostructured phase separation in BHJ blend films. Ultimately, the AS coated OSCs based on the TSS of CB/o-Xy/DIO gains a 34% enhancement in PCE, compared with the conventional CB/DIO solvent based OSCs.     

A universal roll‐to‐roll slot‐die coating approach towards high‐efficiency organic photovoltaics

This work develops a combinational use of solvent additive and in‐line drying oven on the flexible organic photovoltaics to improve large‐area roll‐to‐roll (R2R) slot‐die coating process. Herein, addition of 1,8‐diiodooctane (DIO) in the photoactive layer is conducted to yield a performance of 3.05% based on the blending of poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl C₆₁‐butyric acid methyl ester (PC₆₁BM), and a very promising device performance of 7.32% based on the blending of poly[[4,8‐bis[(2‐ethylhexyl)oxy] benzo[1,2‐b:4,5‐b’] dithiophene‐2,6‐diyl] [3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl]] (PTB7) and [6,6]‐phenyl C₇₁‐butyric acid methyl ester (PC₇₁BM). Based on this R2R slot‐die coating approach for various polymers, we demonstrate the high‐performance result with respect to the up‐scaling from small high‐PCE cell to large‐area module. This present study provides a route for fabricating a low‐cost, large‐area, and environmental‐friendly flexible organic photovoltaics.     

Eco-friendly fabrication of PBDTTPD:PC71BM solar cells reaching a PCE of 3.8% using water-based nanoparticle dispersions

In this work we report on the eco-friendly processing of PBDTTPD:PC₇₁BM organic solar cells using water-based nanoparticle (NP) dispersions. The polymer:fullerene NPs are prepared using the miniemulsion-solvent evaporation method, despite employing high-boiling solvents. Polymer solar cells are fabricated from these blend NPs and the device characteristics are studied in function of annealing time and temperature. The photoactive layer formation is carefully analyzed using atomic force microscopy (AFM). Annealing for longer times significantly increases the power conversion efficiency (PCE), up to 3.8%, the highest value reported for surfactant based NP solar cells. Our work shows that the low bandgap polymer PBDTTPD has the ability to afford reasonable efficiencies in NP solar cells in combination with PC₇₁BM and paves the way to a truly eco-friendly processing of organic photovoltaics (OPVs).     

Rational Design of Small Molecular Donor for Solution‐Processed Organic Photovoltaics with 8.1% Efficiency and High Fill Factor via Multiple Fluorine Substituents and Thiophene Bridge

A series of tetrafluorine‐substituted small molecules with a D₁‐A‐D₂‐A‐D₁ linear framework based on indacenodithiophene and difluorobenzothiadiazole is designed and synthesized for application as donor materials in solution‐processed small‐molecule organic solar cells. The impacts of thiophene π‐bridge and multiple fluorinated modules on the photophysical properties, the energy levels of the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO), charge carrier mobility, the morphologies of blend films, and their photovoltaic properties as electron donor material in the photoactive layer are investigated. By incorporating multiple fluorine substituents of benzothiadiazole and inserting two thiophene spacers, the fill factor (FF), open‐circuit voltage, and short‐circuit current density are dramatically improved in comparison with fluorinated‐free materials. With the solvent vapor annealing treatment, further enhancement in charge carrier mobility and power conversion efficiency (PCE) are achieved. Finally, a high PCE of 8.1% with very‐high FF of 0.76 for BIT‐4F‐ T/PC₇₁BM is achieved without additional additive, which is among one of the highest reported for small‐molecules‐based solar cells with PCE over 8%. The results reported here clearly indicate that high PCE in solar cells based small molecules can be significantly increased through careful engineering of the molecular structure and optimization on the morphology of blend films by solvent vapor annealing.     

Impact of alkyl chain length of 1,n-diiodoalkanes on PC71BM distribution in both bulk and air surface of PTB7:PC71BM film

The impact of alkyl chain length of different additives, such as 1,4-diiodobutane (DIB), 1,6-diiodohexane (DIH), 1,8-diiodooctane (DIO) and 1,10-diiododecane (DID), on the PC₇₁BM distribution in PTB7:PC₇₁BM-based polymer solar cells, is systematically investigated, for the first time. Among these additives, DIO is found to have the optimum alkyl chain length that maximizes the performance of PTB7:PC₇₁BM based polymer solar cells, attaining a power conversion efficiency as high as 8.84%, which is almost four times higher than that without any additives. For DID additives (longer alkyl chain length than DIO), a drop in efficiency to 7.91% was observed. Experimental investigations show that the microstructure of the bulk and the surface layer as well as the surface morphology of the PTB7:PC₇₁BM polymer film can be controlled simultaneously by varying the alkyl chain length of additives. Results also show that the substantial improvement in performance is attributed to the improved 1) phase segregation, 2) PC₇₁BM distribution uniformity in the bulk of the PTB7:PC₇₁BM film, 3) surface smoothness and 4) high PTB7 content at the interface between the active layer and the top electrode.     

Formation of charge‐transfer complexes significantly improves the performance of polymer solar cells based on PBDTTT‐C‐T: PC71BM

By adding appropriate proportions of nitrobenzene (C₆H₅NO₂) to the blended solution of poly{[4,8‐bis‐(2‐ethyl‐hexyl‐thiophene‐5‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl]‐alt‐[2‐(2′‐ethyl‐hexanoyl)‐thieno[3,4‐b]thiophen‐4,6‐diyl]}:[6,6]‐phenyl‐C71‐butyric acid methyl ester (PBDTTT‐C‐T: PC₇₁BM), we substantially improved the power conversion efficiency from the best reported value of 7.48–8.88%. Experiments and simulations show that nitrobenzene and PBDTTT‐C‐T form stable coplanar charge‐transfer complexes through hydrogen bonds. Formation of the PBDTTT‐C‐T‐C₆H₅NO₂ complex simultaneously increases the external quantum efficiency. The underlying mechanisms of increased external quantum efficiency are attributed to the following: (i) higher lowest unoccupied molecular orbital (LUMO) of PBDTTT‐C‐T‐C₆H₅NO₂ for more efficient photoinduced electron transfer to the LUMO of PC₇₁BM and (ii) efficient quenching of fluorescence in the active layers due to formation of the PBDTTT‐C‐T‐C₆H₅NO₂ complex. This discovery clearly illustrates the potential of hydrogen‐bonded complexes as a new route for efficient polymer‐based photovoltaic devices. Copyright © 2014 John Wiley & Sons, Ltd.     

Solution processed bulk heterojunction polymer solar cells with low band gap DPP-CN small molecule sensitizer

The improvement of near infrared wavelength sensitivity in the bulk heterojunction organic polymer solar cell based on poly (3-hexylthiophene) (P3HT) and PC₇₀BM, by the addition of soluble DPP-CN small molecule is reported. By incorporating DPP-CN, the photosensitivity in the longer wavelength region was improved and the power conversion efficiency (PCE) has been reached to 4.37% as compared to 3.23% for the device based on P3HT:PC₇₀BM blend. The increase in the PCE is attributed to the increase in light harvesting property of the blend and efficient dissociation of excitons into free charge carriers due to the increased number of D–A sites. The PCE has been further enhanced to 4.70%, when mixed solvent cast P3HT:DPP-CN:PC₇₀BM blend is used as photoactive layer. The optical absorption spectra of the blend showed that the blend film cast from mixed solvent broadened the absorption wavelength range. This occurred as result of a large red shift of P3HT absorption peak and same time a widening and small red shift of DPP-CN absorption peak in the blend film. The improved light harvesting property of thermally annealed film is considered to the factor responsible for the improvement in the PCE.     

New soluble porphyrin bearing a pyridinylethynyl group as donor for bulk heterojunction solar cells

Bulk heterojunction solar cells were fabricated using the blend films of a porphyrin bearing pyridinylethynyl group (POR) as electron donor and PC₆₀BM or PC₇₀BM as electron acceptor. Photoluminescence measurement of the blend films of POR with fullerene derivatives indicated that charge transfer was possible between POR and fullerene derivatives. The solution processed bulk heterojunction solar cells using the blend of POR with PC₆₀BM and PC₇₀BM exhibit overall power conversion efficiency (PCE) of 1.96% and 2.54%, respectively. The PCE of the BHJ solar cell has been further improved up to 3.27% when thermally annealed POR:PC₇₀BM was used as active layer, which is attributed mainly to the increase in short circuit current. The increase in J_sc is attributed to the enhanced crystallinity of the blend (particularly POR) and efficient π-electron conjugation of POR, resulting to an improvement in hole mobility, leading to more balance charge transport. The PCE of the device based on as cast POR:PC₇₀BM has been further improved up to 4.06% when DMF treated PEDOT:PSS buffer layer was used.     

Synthesis,optical and electrochemical properties new donor–acceptor (D–A) copolymers based on benzo[1,2-b:3,4-b′:6,5-b″] trithiophene donor and different acceptor units: Application as donor for photovoltaic devices

Two new conjugated D–A polymers P3 (PBTT-d-BTT) and P4 (PBTT-d-TPD) based on same benzo[1,2-b:3,4-b′:6,5-b″] trithiophene (BTT) donor and different acceptors monomers 5,8-dibromo-2-dodecanoylbenzo[1,2-b:3,4-b′:6,5-b″] trithiophene (d-BTT), and 1,3-dibromo-5-(2-ethylhexyl)thieno[3,4]pyrrol-4,6-dione (d-TPD) respectively, were synthesized by Stille cross-coupling reaction and characterized by gel permeation chromatography (GPC), ¹H NMR, UV–Vis absorption, thermal analysis and electrochemical cyclic voltammetry (CV) tests. Photovoltaic properties of the polymers were studied by using the polymers as donor and PC₇₁BM as acceptor with a weight ratio of polymer:PC₇₁BM 1:1, 1:2 and 1:2.5. The optimized photovoltaic device was fabricated with an active layer of a blend P3:PC₇₁BM and P4:PC₇₁BM with a blend ratio of 1:2 showed PCE 3.16% and 2.42%, respectively under illumination of AM 1.5 at 100 mW/cm² with solar simulator. The PCE of the device based on P3:PC₇₁BM processed with DIO/o-DCB has been further improved up to 4.64% with J_sc of 10.52 mA/cm² and FF of 0.58 attributed to the increase in crystalline nature of active layer and more balanced charge transport in the device, induced by DIO additive.     

Donor-acceptor polymers based on 5,6-difluoro-benzo[1,2,5]thiadiazole for high performance solar cells

Two donor-acceptor polymers P8 and P9 based on 5,6-difluoro-benzo[1,2,5]thiadiazole unit have been prepared and applied as the donor materials in polymer solar cells. Due to the slight difference between electronic structures of thiophene and selenophene, P8 and P9 show similar absorption spectra and similar frontier energy levels. However, the pristine P8:PC₇₁BM and P9:PC₇₁BM blend films display distinct morphologies as revealed by AFM measurement. After the addition of DIO, both blend films feature a nanoscale interconnected-network structure, which leads to the enhancement in solar cells performance with PCE up to 6.73% and 6.84% for P8 and P9, respectively. Alternating current impedance spectrometry measurements revealed that high surface roughness could improve the PCE of P8-based PSCs, while in P9-based PSCs DIO can enhance hole and electron mobilities of the active layer.     

Donor–acceptor–acceptor–donor small molecules for solution processed bulk heterojunction solar cells

We report on the optical and electrochemical characterization (experimental and theoretical) of two donor substituted benzothiadiazole with different cyano based acceptor π-linkers, tetracyanobutadiene (TCBD) SM1 and dicyanoquinomethane (DCNQ) SM2, and explore them as the donor component for solution processed bulk heterojunction organic solar cells, along with PC₇₁BM as the electron acceptor. The solution bulk heterojunction (BHJ) solar cells based on dichloromethane (DCM) processed active layer with SM1 and SM2 as donor and PC₇₁BM as acceptor achieve power conversion efficiency (PCE) of 2.76% and 3.61%, respectively. The solar cells based on these two small molecules exhibit good Voc, which is attributed to their deep HOMO energy level. The higher PCE of the device based on SM2 compared to SM1 is attributed to the its small bandgap, broader absorption profile and enhanced hole mobility. Additionally, the PCE of the SM2:PC₇₁BM based solar cells processed with 1-chloronaphthalene CN (3 v%)/DCM is further improved reaching upto 4.86%. This increase in PCE has been attributed to the improved nanoscale morphology and more balanced charge transport in the device, due to the solvent additive.     

Molecular Orientation Unified Nonfullerene Acceptor Enabling 14% Efficiency As‐Cast Organic Solar Cells

Molecular orientation and π–π stacking of nonfullerene acceptors (NFAs) determine its domain size and purity in bulk‐heterojunction blends with a polymer donor. Two novel NFAs featuring an indacenobis(dithieno[3,2‐b:2?,3?‐d]pyrrol) core with meta‐ or para‐alkoxyphenyl sidechains are designed and denoted as m‐INPOIC or p‐INPOIC , respectively. The impact of the alkoxyl group positioning on molecular orientation and photovoltaic performance of NFAs is revealed through a comparison study with the counterpart ( INPIC‐4F ) bearing para‐alkylphenyl sidechains. With inward constriction toward the conjugated backbone, m‐INPOIC presents predominant face‐on orientation to promote charge transport. The as‐cast organic solar cells (OSCs) by blending m‐INPOIC and PBDB‐T as active layers exhibit a power conversion efficiency (PCE) of 12.1%. By introducing PC₇₁BM as the solid processing‐aid, the ternary OSCs are further optimized to deliver an impressive PCE of 14.0%, which is among the highest PCEs for as‐cast single‐junction OSCs reported in literature to date. More attractively, PBDB‐T: m‐INPOIC :PC₇₁BM based OSCs exhibit over 11% PCEs even with an active layer thickness over 300 nm. And the devices can retain over 95% of PCE after storage for 20 days. The outstanding tolerance to film thickness and outstanding stability of the as‐cast devices make m‐INPOIC a promising candidate NFA for large‐scale solution‐processable OSCs.     

Fluorinated and non-fluorinated conjugated polymers showing different photovoltaic properties in polymer solar cells with PFNBr interlayers

Photovoltaic properties of the two polymers (named as PBQ-0F and PBQ-4F) were investigated by employing [6, 6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as the acceptor and a water-alcohol-soluble polymer interlayer material (named as PFNBr) to fabricate photovoltaic devices. For the PBQ-0F:PC₇₁BM blend, the device using Ca/Al as cathode showed very similar efficiency as the device using PFNBr/Al as cathode, while for the PBQ-0F:PC₇₁BM blend, photovoltaic performance of the device can be distinctly improved by replacing Ca/Al with PFNBr/Al. As a result, the best PCE of the PBQ-4F:PC₇₁BM based devices reached 9.04%, which is much higher than that of the PBQ-0F:PC₇₁BM based devices. The results obtained from the quantum chemistry calculations and water contact angle measurements demonstrate that these two polymers are low polar materials, and also the films based on them have hydrophobic surfaces. Since PFNBr has an amphipathic structure (hydrophobic backbone and hydrophilic side chain) and the blend films of PBQ-4F:PC₇₁BM and PBQ-4F:PC₇₁BM have different surface energies, the PFNBr organization atop these two blend types of should be different, which will affect device photovoltaic performance.     

High Performance Roll‐to‐Roll Produced Fullerene‐Free Organic Photovoltaic Devices via Temperature‐Controlled Slot Die Coating

Solution‐processed organic photovoltaics (OPVs) have continued to show their potential as a low‐cost power generation technology; however, there has been a significant gap between device efficiencies fabricated with lab‐scale techniques—i.e., spin coating—and scalable deposition methods. Herein, temperature‐controlled slot die deposition is developed for the photoactive layer of OPVs. The influence of solution and substrate temperatures on photoactive films and their effects on power conversion efficiency (PCE) in slot die coated OPVs using a 3D printer‐based slot die coater are studied on the basis of device performance, molecular structure, film morphology, and carrier transport behavior. These studies clearly demonstrate that both substrate and solution temperatures during slot die coating can influence device performance, and the combination of hot substrate (120 °C) and hot solution (90 °C) conditions result in mechanically robust films with PCE values up to 10.0% using this scalable deposition method in air. The efficiency is close to that of state‐of‐the‐art devices fabricated by spin coating. The deposition condition is translated to roll‐to‐roll processing without further modification and results in flexible OPVs with PCE values above 7%. The results underscore the promising potential of temperature‐controlled slot die coating for roll‐to‐roll manufacturing of high performance OPVs.     

10.8% Efficiency Polymer Solar Cells Based on PTB7‐Th and PC71BM via Binary Solvent Additives Treatment

In this work, polymer solar cells are fabricated based on the blend of PTB7‐Th: PC₇₁BM by using a mixed solvent additive of 1,8‐diiodooctane and N‐methyl pyrrolidone to optimize the morphology of the blend. A high power conversion efficiency (PCE) of 10.8% has been achieved with a simple conventional device. In order to deeply investigate the influence of the mixed solvent additives on the morphology and device performance, the variations of the molecular packing and bulk morphology of the blend film cast from ortho‐dichlorobenzene with single or binary solvent additives are measured. Although all the blend films exhibit similar domain size and nanoscale phase separation, the blend film processed with mixed solvent additive shows the highest domain purity, resulting in the least bimolecular recombination, relatively high J_sc and FF, and hence enhanced PCE. Therefore, the best photovoltaic performance with the V_oc of 0.82 V, J_sc of 19.1 mA cm^?2, FF of 69.1%, and PCE of 10.8% are obtained for the device based on the blend with binary solvent additive treatment.     

Benzothiadiazole-pyrrolo[3,4-b]dithieno[2,3-f:3′,2′-h]quinoxalindione-based random terpolymer incorporating strong and weak electron accepting [1,2,5]thiadiazolo[3,4g]quinoxalinefor polymer solar cells

We report the synthesis of a D-A random terpolymer denoted as P2 consists of one thiophene donor unit and three acceptor benzothiadiazole (BT), pyrrolodithienoquinoxalinedione (PDQD) and thiadiazoloquinoxaline (TDQ) units by Stille-coupling reaction and investigated its optical and electrochemical properties. We have compared its properties with the parent copolymer P1. The P2 exhibits bandgap of about 1.18 eV which is lower than that of P1 (1.50 eV), indicating strength of accepting units controls both the optical and electrochemical bandgap. We have used terpolymer P2 as electron donor along with [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) as electron acceptor for the fabrication of solution processed bulk heterojunction polymer solar cells (PSCs). PSC based on an optimized P2:PC₇₁BM (1:2 by weight) active layer processed with 3v % DIO/DCB solution, displayed a power conversion efficiency (PCE) of 7.22%, which is higher than that for P1 based polymer solar cell (PCE = 6.56%) processed under same conditions. The higher value of PCE for P2:PC₇₁BM may be related to more favorable phase separated morphology of active layer as compared to P1:PC₇₁BM, beneficial for the exciton dissociation and charge transport, as evidenced from the larger hole mobility.