Polymer solar cells based low bandgap A1-D-A2-D terpolymer based on fluorinated thiadiazoloquinoxaline and benzothiadiazole acceptors with energy loss less than 0.5 eV

We synthesized an ultra low bandgap terpolymer denoted as P containing fluorinated-fluorene attached thiadiazoloquinoxaline and benzothiadiazole acceptors and thiophene as donor in its backbone and investigated its optical and electrochemical properties. This terpolymer is used for as donor along with PC₇₁BM as electron acceptor in solution processed polymer solar cells (PSCs). The P showed a shows strong absorption band from 650 nm to 1100 nm with an optical bandgap of 1.12 eV and highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of −5.25 eV and −3.87 eV, respectively. After the optimization of P to PC₇₁BM weight ratio, the optimized weight ratio 1:2 in chlorobenzene (CB) solution, the PSC showed overall power conversion efficiency of 4.10% (J_sc of 10.96 mA/cm², V_oc of 0.68 V and FF of 0.55). After the solvent additive (3 v% DIO) followed by subsequent thermal annealing (SA-TA) the PCE has been increased up to 7.54% with J_sc of 16.12 mA/cm², V_oc of 0.65 V and FF of 0.72. The increase in the PCE is related with the enhancement in the both J_sc and FF, attributed optimized nanoscale morphology of the active layer for both efficient exciton dissociation and charge transport towards the electrodes and balanced charge transport in the device, induced by the TSA treatment of the active layer. This is the highest PCE of PSCs with an energy loss about 0.47 eV with the low bandgap of 1.12 eV.     

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.     

New ultra low bandgap thiadiazolequinoxaline-based D-A copolymers for photovoltaic applications

In this communication, we designed two low bandgap D-A copolymers with same fluorinated thiadiazoloquinoxaline (TDQ) as acceptor and different donor units benzo[2,1-b;3,4-b′]dithiophene (P1) and benzo[1,2-b:4,5-b′]dithiophene (P2). P1 and P2 exhibit broad absorption profiles covering from 350 nm to 1150 nm and 350–950 nm, respectively with optical bandgaps of 1.06 eV and 1.18 eV, respectively. Both copolymers showed deep highest occupied molecular orbitals (HOMO), i.e. −5.38 eV and −5.26 eV, for P1 and P2. Their photovoltaic properties were evaluated using conventional devices with a structure of ITO/PEDOT:PSS/copolymer:PC₇₁BM/Al. After the optimizations of the copolymer to PC₇₁BM weight ratios, and concentration of the solvent additive (DIO), the devices showed overall power conversion efficiencies of 4.03% and 5.42% for the P1 and P2 based devices, respectively. The higher value of PCE of the P2 based device is attributed to the higher values of J_sc and FF, that is related to the higher hole mobility and better exciton dissociation efficiency. Although the PCEs of these devices are moderate, these ultra low band gap copolymers can be used for their potential application in tandem polymers solar cells. Finally, methanol treatment of the active layer was adopted to increase the PCE of the P2:PC₇₁BM based polymer solar cells that resulted in an improved PCE up to 6.93%.     

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.     

Synthesis and photophysical properties of semiconductor molecules D1-A-D2-A-D1-type structure based on derivatives of quinoxaline and dithienosilole for organics solar cells

A novel small molecule with D1-A-D2-A-D1 structure denoted as DTS(QxHT₂)₂ based on quinoxaline acceptor and dithienosilone donor units was synthesized and its optical and electrochemical properties were investigated. The thin film of DTS(QxHT₂)₂ showed a broad absorption profile covering the solar spectrum from 350 nm to 780 nm with an optical bandgap of 1.63 eV. The energy levels estimated from the cyclic voltammetry indicate that this small molecule is suitable as donor along with PC₇₁BM as acceptor for the fabrication solution processed bulk heterojunction solar cells for efficient exciton dissociation and high open circuit voltage. The organic solar cells based on optimized DTS(QxHT2)2:PC₇₁BM active layers processed with chloroform and DIO/CF showed overall power conversion efficiency of 3.16% and 6.30%, respectively. The higher power conversion efficiency of the solar cell based on the DIO/CF processed active layer is attributed to enhanced short circuit photocurrent and fill factor may be related to better phase separation between donor and acceptor in the active layer and more balanced charge transport, induced by the solvent additive. The power conversion efficiency of the organic solar cell was further improved up to 7.81% based on active layer processed with solvent additive, using CuSCN as hole transport layer instead of PEDOT:PSS and mainly attributed to increased fill factor and open circuit voltage due the formation of better Ohmic contact between the active layer and the CuSCN layer.     

Synthesis of organic molecule donor for efficient organic solar cells with low acceptor content

A new planar A-D-A structured organic small molecule semiconductor (O-SMS) with dialkyl-thiophene substituted benzodithiophene (BDT) as central electron-rich core flanked by relatively electron-deficient units of [1,2,5]thiadiazolo[3,4-c]pyridine (PTz) and terminated with alkyl-bithiophene as π-conjugated end-caps, BDTDPTz, was designed and synthesized for the application as donor material in organic solar cells (OSCs). BDTDPTz possesses wider absorption spectra with an optical bandgap of 1.65 eV, lower the highest occupied molecular orbital (HOMO) energy level of −5.42 eV and highly crystalline structures in solid films. The OSCs based on BDTDPTz:PC₇₁BM blend film with a lower PC₇₁BM content of 40% demonstrate a power conversion efficiency (PCE) of 6.28% with a relatively higher open-circuit voltage of 0.868 V and short circuit current density of 12.83 mA cm⁻². These results indicate that highly coplanar and crystalline structure of BDTDPTz can effectively reduce the content of fullerene acceptor in the active layer and then enhance the absorption and PCE of the OSCs.     

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.     

Two-dimensional conjugated copolymers composed of diketopyrrolopyrrole,thiophene, and thiophene with side chains for binary and ternary polymer solar cells

Two new two-dimensional conjugated copolymers (named r-PTTDPP50 and r-PTTDPP75) consisting of a diketopyrrolopyrrole (DPP) derivative, thiophene with a conjugated side chain, and 2,5-bis(trimethylstannyl)thiophene were designed and synthesized via Stille cross-coupling reactions for use in bulk heterojunction (BHJ) polymer solar cells (PSCs); the feed-in ratios were varied to obtain the copolymers. It was found that the content of DPP units in the copolymer main chain significantly affected the molecular weight, absorption range, electronic energy level, and morphology of thin films of the copolymers. In the thin-film state, both copolymers exhibited a broad absorption band with two obvious peaks and a vibronic shoulder, as well as an absorption edge for wavelengths of up to 1000 nm. The vibronic shoulder in the absorption spectrum of r-PTTDPP75 was more intense than that in the spectrum of r-PTTDPP50, owing to the presence of a greater number of coplanar DPP units in the former. Electrochemical measurements indicated that the highest occupied molecular orbital (HOMO) energy levels for r-PTTDPP50 and r-PTTDPP75 were −5.16 and −5.19 eV, respectively, while their lowest unoccupied molecular orbital (LUMO) energy levels were −3.89 and −3.99 eV, respectively. On increasing the number of electron-deficient DPP segments in r-PTTDPP75, the LUMO energy level was lowered. Further, its HOMO energy level was also affected. BHJ PSCs composed of the electron-donor copolymers blended with an electron acceptor, namely [6,6],-phenyl-C61-butyric acid methyl ester (PC₆₁BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM), in 1:2 wt ratio were fabricated and characterized. The power conversion efficiency (PCE) of the r-PTTDPP50/PC₇₁BM-based (w/w = 1:2) PSC reached 2.32% for an open-circuit voltage of 0.632 V, short-circuit current of 9.81 mA/cm², and fill factor of 37.4%, under the illumination of AM 1.5G (100 mW/cm²). Ternary blend BHJ solar cells formed by doping r-PTTDPP50 into the common binary blend of P3HT and PC₆₁BM were also investigated. The optimized r-PTTDPP50:P3HT:PC₆₁BM device exhibited a PCE of 3.85%, which was significantly higher than that of the P3HT:PC₆₁BM device (2.97%).     

Synthesis and correlation between structure and photovoltaic performance of two-dimensional BDT-TPD polymers

Four BDT-TPD polymers (PA–PD) were synthesized by modifying the alkylthienyl chains on BDT, placing spacer group between BDT and TPD, and installing extended conjugated side chains on the BDT of the polymer to investigate the correlation between structure and photovoltaic performance for these polymers. The molecular weight of PA–PD polymers ranged from the highest (Mn = 80 kDa for PA) to the lowest (Mn = 7.9 kDa for PD), and their decomposition temperatures at 5% weight loss were in the range 401–435 °C. PA, PB, and PC showed similar UV–vis absorption spectra; however, PD showed much broader absorption spectrum in the entire UV–vis region, because of the extended conjugated side chains. The HOMO levels of the polymers were −5.72, −5.63, −5.48, and −5.61 eV for PA, PB, PC, and PD, respectively, indicating very low-lying HOMO energy levels. The bandgaps of these polymers were calculated and found to be in the range 1.85–1.88 eV. The theoretical calculations clearly show that the torsional angles between the alkylthienyl group and BDT unit of the simplified dimer correlated to the π-orbital delocalization, suggesting that the HOMO π-electrons of vertically aligned conjugated side chains do not delocalize well in the polymers such as PA, PB, and PC bearing high torsional angles. The optimized weight ratios of the polymer to PC₆₁BM were determined to be 1:1, 1:1.5, and 1:1 for PA, PC, and PD, respectively, and the average PCEs of the devices were 5.36%, 4.62%, and 2.74% for PA, PC, and PD, respectively, after optimization with 1,8-diiodooctane (DIO). A relatively small amount of DIO as an additive was necessary to reach the optimal PCEs of the devices, and the device incorporating PC needed only 0.5% DIO to obtain the best PCE. The AFM study reveals that the blend films after adding DIO showed much smooth morphologies, and the blend film of PA exhibited more crystalline property, as shown by the XRD analysis.     

Efficient solution processed D1-A-D2-A-D1 small molecules bulk heterojunction solar cells based on alkoxy triphenylamine and benzo[1,2-b:4,5-b′]thiophene units

Two molecules denoted as VC96 and VC97 have been synthesized for efficient (η = 6.13% @ 100 mW/cm² sun-simulated light) small molecule solution processed organic solar cells. These molecules have been designed with the D₁-A-D₂-A-D₁ structure bearing different central donor unit, same benzothiadiazole (BT) as π-acceptor and end capping triphenylamine. Moreover, the optical and electrochemical properties (both experimental and theoretical) of these molecules have been systematically investigated. The solar cells prepared from VC96:PC₇₁BM and VC97:PC₇₁BM (1:2) processed from CF (chloroform) exhibit a PCE (power conversion efficiency) of η = 4.06% (J_sc = 8.36 mA/cm², V_oc = 0.90 V and FF = 0.54) and η = 3.12% (J_sc = 6.78 mA/cm², V_oc = 0.92 V and FF = 0.50), respectively. The higher PCE of the device with VC96 as compared to VC97 is demonstrated to be due to the higher hole mobility and broader IPCE spectra. The devices based on VC96:PC₇₁BM and VC97:PC₇₁BM processed with solvent additive (3 v% DIO, 1,8-diiodooctane) showed PCE of η = 5.44% and η = 4.72%, respectively. The PCE device of optimized VC96:PC₇₁BM processed with DIO/CF (thermal annealed) has been improved up to 6.13% (J_sc = 10.72 mA/cm², V_oc = 0.88 V and FF = 0.61). The device optimization results from the improvement of the balanced charge transport and better nanoscale morphology induced by the solvent additive plus the thermal annealing.     

Synthesis and characterization of a new perylene bisimide (PBI) derivative and its application as electron acceptor for bulk heterojunction polymer solar cells

A symmetrical perylene bisimide derivative (PBI) with 2-(4-nitrophenyl)acrylonitrile groups at the 1,7 bay positions of perylene and solubilizing cyclohexyl units was synthesized and characterized. The absorption spectrum of PBI was broad with the most prominent peak at 655 nm and optical band gap of 1.72 eV. The electrochemical investigation indicates that PBI has a LUMO energy level of −3.9 eV which is similar to that of PCBM or PC₇₀BM. Bulk heterojunction solar cell fabricated using a blend of poly(3-hexylthiophene) (P3HT) and PBI (1:1 w/w) as active layer cast from THF exhibited power conversion efficiency (PCE) at 1.56%. However, the device with P3HT:PBI blend deposited from mixed solvent (DIO/THF) improved the PCE to 2.78% which further increased to 3.17% on using the thermal annealed active layer. The improvement in the PCE is attributed to the enhanced crystallinity of the blend (particularly P3HT) and increase in hole mobility leading to balanced charge transport.     

Oligothiophene based small molecules with a new end group for solution processed organic photovoltaics

Two oligothiophene based small molecules (DINER5T and DINER7T) with a new end group INER were synthesized as the donors for organic solar cells, and their photovoltaic performance was studied and compared with the corresponding compounds (DRHD7T and DIN7T) with the same backbone structure but different end groups. Both of the new molecules exhibit broad and red shift absorption compared with DRHD7T and DIN7T, with very low band gaps of 1.47 eV and 1.34 eV, respectively. The devices based on DINER5T:PC₇₁BM and DINER7T:PC₇₁BM blend films gave PCEs of 4.22% and 4.02%, respectively, through a solvent vapor annealing (SVA) process with CH₂Cl₂.     

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.     

Morphological study of polymer/fullerene interfaces via benzene–PC70BM interaction

We designed and synthesized a series of highly soluble random copolymers (P(BDTT-PhC8TPD-TPD)) comprising a two-dimensional (2D) electron-rich 2-ethylhexylthiophene-substituted benzo[1,2-b:4,5-b′]dithiophene (M1) electron-rich unit and various ratios of two side chain-functionalized thieno[3,4-c]-pyrrole-4,6-diones (M2 and M3) as planar electron-deficient units. The morphologies and photovoltaic performances of the bulk heterojunction blend films were examined, revealing concentration-dependent competitive benzene–benzene and benzene–fullerene interactions. A bulk heterojunction solar cell prepared using 10 mol% M2 (RP1) and PC₇₀BM exhibited a power conversion efficiency (PCE) of 4.9%, representing a significant improvement over the PCE (1.1%) obtained from parent copolymer (P1). The PCE in these cells was strongly associated with the nanoscale morphology of the BHJ film, which depended on the higher miscibility of RP1 with PC₇₀BM compared to RP2 (M2: 20 mol%), and RP3 (M2: 30 mol%).     

Perylene diimide-benzodithiophene D-A copolymers as acceptor in all-polymer solar cells

Two n-type conjugated D-A copolymers with perylene diimide (PDI) as acceptor unit and benzodithiophene (BDT) as donor unit, P(PDI-BDT-Ph) and P(PDI-BDT-Th), were synthesized and applied as electron acceptor in all-polymer solar cells (all-PSCs). P(PDI-BDT-Ph) and P(PDI-BDT-Th) films exhibit similar absorption spectra in the visible region with optical bandgap (E_g) of 1.65 eV and 1.55 eV respectively, and the identical LUMO level of −3.89 eV. The all-PSCs based on P(PDI-BDT-Ph) as acceptor and PTB7-Th as donor demonstrated a power conversion efficiency (PCE) of 4.31% with a short-circuit current density (J_sc) of 11.94 mA cm⁻², an open-circuit voltage (V_oc) of 0.81 V, and a fill factor (FF) of 44.49%. By contrast, the corresponding all-PSCs with P(PDI-BDT-Th) as acceptor showed a relative lower PCE of 3.58% with a J_sc of 11.36 mA cm⁻², V_oc of 0.79 V, and FF of 40.00%.     

Fluorine functionalized asymmetric indo [2,3-b]quinoxaline framework based D-A copolymer for fullerene polymer solar cells

Innovating molecular structure of copolymer donor materials is still one of the prominent approach to obtain high-performance polymer solar cells (PSCs). In this paper, two novel wide bandgap (WBG) copolymers, namely PBDTTS-IQ and PBDTTS-DFIQ, based on asymmetric planar aromatic core indo [( Li et al., 2012; Wang et al., 2020) 2,32,3-b]quinoxaline (IQ) as acceptor unit through tuning side chains with fluorine (F) atom engineering and exemplary alkylthio-thienyl substituted benzodithiophene (BDTTS) donor group, are synthesized and finally employed as the photovoltaic donor materials for fullerene polymer solar cells (PSCs). After blending with PC₇₁BM acceptor, the PBDTTS-DFIQ:PC₇₁BM blend film presented better efficient exciton dissociation and charge extraction, more balanced electron/hole mobility (μ_h/μ_e), and nice morphology in comparison with PBDTTS-IQ:PC₇₁BM blend film. Encouragingly, the PBDTTS-DFIQ:PC₇₁BM based PSCs exhibits a higher power conversion efficiency (PCE) of 7.4% than that of the device based on the PBDTTS-IQ:PC₇₁BM blend with a PCE of 4.96%, which thanks to an enhancement of open-circuit voltage (V_oc) of 0.84 V, short current density (J_sc) of 13.26 mA cm⁻² and fill factor (FF) of 66.00% simultaneously. These results demonstrate that this asymmetric IQ framework is a wonderful acceptor moiety to build light-harvesting copolymers for highly efficient PSCs.     

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.     

2,2-Dicyanovinyl-end-capped oligothiophenes as electron acceptor in solution processed bulk-heterojunction organic solar cells

Three 2,2-dicyanovinyl (DCV) end-capped A-π-D-π-A type oligothiophenes (DCV-OTs) containing dithieno[3,2-b:2′,3′-d]silole (DTSi), cyclopenta[1,2-b:3,4-b′]dithiophene (DTCP) or dithieno[3,2-b:2′,3′-d]pyrrole (DTPy) unit as the central donor part, mono-thiophene as the π-conjugation bridge were synthesized. The absorption spectroscopies, cyclic voltammetry of these compounds were characterized. Results showed that all these compounds have intensive absorption band over 500–680 nm with a LUMO energy level around −3.80 eV, which is slightly higher than that of [6,6]phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM, E_LUMO = −4.01 eV), but lower than that of poly(3-hexylthiophene) (P3HT, E_LUMO = −2.91 eV). Solution processed bulk heterojunction “all-thiophene” solar cells using P3HT as electron donor and the above mentioned oligothiophenes as electron acceptor were fabricated and tested. The highest power conversion efficiency (PCE) of 1.31% was achieved for DTSi-cored compound DTSi(THDCV)₂, whereas PTB7:DTSi(THDCV)₂ based device showed slightly higher PCE of 1.56%. Electron mobilities of these three compounds were measured to be around 10⁻⁵ cm² V⁻¹ s⁻¹ by space charge limited current method, which is much lower than that of PC₆₁BM, and was considered as one of the reason for the low photovoltaic performance.     

High‐Yield Synthesis and Electrochemical and Photovoltaic Properties of Indene‐C70 Bisadduct

[6, 6]‐Phenyl‐C₆₁‐butyric acid methyl ester (PC₆₀BM) is the widely used acceptor material in polymer solar cells (PSCs). Nevertheless, the low LUMO energy level and weak absorption in visible region are its two weak points. For enhancing the solar light harvest, the soluble C₇₀ derivative PC₇₀BM has been used as acceptor instead of PC₆₀BM in high efficiency PSCs in recent years. But, the LUMO level of PC₇₀BM is the same as that of PC₆₀BM, which is too low for the PSCs based on the polymer donors with higher HOMO level, such as poly (3‐hexylthiophene) (P3HT). Here, a new soluble C₇₀ derivative, indene‐C₇₀ bisadduct (IC₇₀BA), is synthesized with high yield of 58% by a one‐pot reaction of indene and C₇₀ at 180 °C for 72 h. The electrochemical properties and electronic energy levels of the fullerene derivatives are measured by cyclic voltammetry. The LUMO energy level of IC₇₀BA is 0.19 eV higher than that of PC₇₀BM. The PSC based on P3HT with IC₇₀BA as acceptor shows a higher V_oc of 0.84 V and higher power conversion efficiency (PCE) of 5.64%, while the PSC based on P3HT/PC₆₀BM and P3HT/PC₇₀BM displays V_oc of 0.59 V and 0.58 V, and PCE of 3.55% and 3.96%, respectively, under the illumination of AM1.5G, 100 mW cm^?2. The results indicate that IC₇₀BA is an excellent acceptor for the P3HT‐based PSCs and could be a promising new acceptor instead of PC₇₀BM for the high performance PSCs based on narrow bandgap conjugated polymer donor.     

High efficiency polymer solar cells based on alkylthio substituted benzothiadiazole-quaterthiophene alternating conjugated polymers

We have designed and synthesized two alkylthio substituted benzothiadiazole-quaterthiophene based conjugated polymers (P1 and P2) and investigated their photovoltaic performances. Theoretical simulation has demonstrated that the introduction of alkylthio substituents can increase the planarity of the resulted conjugated polymers. The fluorinated polymer P1 possesses a deeper HOMO energy level than the non-fluorinated polymer P2 and can form well-developed fibril networks when blended with PC₇₁BM. PSCs based on P1:PC₇₁BM (1:1.2, by weight) gave a PCE of 7.76% with a V_oc of 0.69 V, a J_sc of 16.30 mA cm⁻² and an FF of 0.69. Our results have demonstrated that alkylthiothiophene could be a useful building block for the construction of high efficiency polymer donor materials used for PSCs.