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.     

Synthesis and photovoltaic studies on novel fluorene based cross-conjugated donor-acceptor type polymers

Direct arylation polymerization (DAP) is emerging as a promising green, cheap, simple, and efficient environment friendly method for synthesizing conjugated polymers without involving any organometallic reagent. We report fluorene based novel cross-conjugated alternate and random copolymers for polymer solar cells (PSCs), which were synthesized by DAP and/or Yamamoto polymerization under appropriate reaction conditions to obtain high molecular weight. These cross-conjugated polymers possess absorption maxima in the range of 490–520 nm and have narrow band gap (1.7–2.05 eV) which is suitable for bulk heterojuntion (BHJ) type organic solar cells. Among the synthesized polymers, the highest number average molecular weight (M_n) i.e. 43.1 kg mol⁻¹ was obtained for polymer P2b (poly((9H-fluoren-9-ylidene)methylene)bis((2-ethylhexyl)sulfane)-alt-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)), and so good polymeric films were formed for P2b. Thus, BHJ films were prepared for P2b for device performance studies and the morphology of these films was studied by atomic force microscopy (AFM). Polymer P2b was blended with the fullerene derivative [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) in different ratios and under the illumination of solar simulator with Air Mass global (AM 1.5G) irradiated at 100 mW cm⁻². Power conversion efficiency (PCE) of 1.4% has been achieved for BHJs in ratio of 1:2 of P2b: PC₇₁BM in simply processed devices. This result indicates that cross-conjugated polymers can be tapped as potential donors for BHJs as the PCE obtained is the highest among this type of cross-conjugated polymers.     

Fluorination as an effective tool to increase the photovoltaic performance of indacenodithiophene-alt-quinoxaline based wide-bandgap copolymers

To investigate the effect of the fluoride phenyl side-chains into quinoxaline (PQx) unit on the photovoltaic performances of polymers, we demonstrated the synthesis and characterization of two novel wide-bandgap (WBG) copolymers, PIDT-DTPQx and PIDT-DTFPQx, in which indacenodithiophene (IDT), 2,3-diphenylquinoxaline (PQx) (and/or 2,3-bis(4-fluorophenyl)quinoxaline (FPQx)) and thiophene (T) were used as the donor (D) unit, acceptor (A) unit and π-bridge, respectively. Compared to the non-fluorine substituted PIDT-DTPQx, fluorine substituted PIDT-DTFPQx presents a deep HOMO energy level and a high hole mobility. Obviously, improved the V_oc, J_sc, and FF simultaneously, giving rise to overall efficiencies in the PIDT-DTFPQx/PC₇₁BM-based PSCs. A highest PCE of 5.78% was obtained with a V_oc of 0.86 V, J_sc of 10.84 mA cm⁻² and FF of 61.7% in the PIDT-DTFPQx/PC₇₁BM-based PSCs, while PIDT-DTPQx based devices also demonstrated a PCE of 5.11%, under the illumination of AM 1.5G (100 mW cm⁻²). Note that these PCE values were achieved for PSCs without any extra treatments. Furthermore, these optimal devices have a film thickness of about 175 nm for the polymer/PC₇₁BM-based active layers. The results provide that introduction of the fluorine atom into quinoxaline unit by side-chain engineering is one of the effective strategies to construct the promising polymer donor materials for future application of large-area polymer solar cells.     

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 thermal-treatment-free tandem polymer solar cells with high fill factors

It is an effective way to enhance device performance of polymer solar cells (PSCs) by using a tandem structure that combines two or more solar cells. For tandem PSCs, the buffer layer plays an important role in determining the device performance. The most commonly used buffer layers, such as PEDOT:PSS, TiO_x, and ZnO, need thermal treatments that are not beneficial for reducing the fabrication complexity and cost of tandem PSCs. It is necessary to develop tandem PSCs fabricated by a thermal-treatment-free process. In this paper, we report high performance thermal-treatment-free tandem PSCs by developing PFN as buffer layers for both subcells. A power conversion efficiency (PCE) of 10.50% and a high fill factor of 72.44% were achieved by stacking two identical PTB7:PC₇₁BM subcells. When adopting a rear PTB7-Th:PC₇₁BM subcell, the highest PCE of 10.79% was further obtained for the tandem devices. The thermal-treatment-free process is especially applicable to flexible devices, in which plastic substrates are usually used.     

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.     

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-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,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.     

Insights into the influence of fluorination positions on polymer donor materials on photovoltaic performance

To explore the influence of fluoro substitution position and number on optical, electrochemical and photovoltaic properties, three novel donor-acceptor (D-A) alternative copolymers (PHF, PFH and PFF) were synthesized by Stille polycondensation of 2,3-diphenyl-5,8-di(thiophen-2-yl)quinoxaline (DTQx) acceptor unit and indacenodithiophene (IDT) donor unit. As films, PHF and PFF comprising two fluoro substituents on the lateral phenyl groups displayed a broad absorption ranging from 350 to 700 nm; whereas PFH containing two fluorine atoms on the polymer main chain exhibited a slightly narrower absorption ranging from 350 to 650 nm. In addition, fluoro substitution on the polymer main chain can lower the HOMO level of the resulted polymers. As expected, PFH and PFF possess deeper HOMO energy level than PHF. Polymer solar cells (PSCs) were fabricated with these three polymers as donor materials and PC₇₁BM as acceptor material. PHF based PSCs gave a power conversion efficiency (PCE) of 7.2% with a V_oc of 0.84 V, a J_sc of 12.46 mA/cm² and an FF of 0.69. And PFH based PSCs showed a PCE of 6.19% with a V_oc of 0.93 V, a J_sc of 9.57 mA/cm² and an FF 0.70. However, a PCE of only 2.9% with a V_oc of 0.92 V, a J_sc of 4.61 mA/cm² and an FF of 0.68 was obtained for PFF based PSCs. Transmission electron microscopy (TEM) and resonant soft X-ray scattering (R-SoXS) studies indicated that the introduction of four fluorine atoms at each repeating unit can spoil the morphology of active layer. These results highlight the importance of fluorination position and number to the performance of PSCs.     

Efficient and ultraviolet durable inverted polymer solar cells using thermal stable GZO-AgTi-GZO multilayers as a transparent electrode

The optical and electrical properties of GZO/AgTi/AZO (GATG) multilayer transparent conducting films fabricated by magnetron sputtering method were investigated. The sheet resistance and maximum optical transmittance of GATG films are 5 Ω/sq and 86%, respectively. The sheet resistance of GATG still retains stable under annealing at 400 °C, which shows better thermal stability compared to GZO/Ag/AZO (GAG) film. The enhanced thermal stability of GATG is attributed to the formation of TiO_X in Ti doped Ag nanostructure film, which can inhibit Ag atom diffusion and aggregation. PTB7-TH:PC₇₁BM based inverted polymer solar cells (PSCs) with GATG electrode gave PCE of 9.20%, which is comparable to PCE (9.23%) of the control PSCs with ITO electrode. The PCE of PSCs with GATG and ITO electrodes respectively remain 59% and 23% of the original PCE values after UV exposure for 20 min with relativize humidity of 68% in air, indicating that PSCs with GATG show better UV durability. Our results suggest that GATG as an alternative to ITO electrode can obtain efficient inverted PSCs and have stronger anti-UV ability due to its low UV transparency.     

Efficient ternary blend polymer solar cells with a bipolar diketopyrrolopyrrole small molecule as cascade material

We present a ternary strategy to enhance the power conversion efficiency (PCE) of bulk heterojunction polymer solar cells (PSCs) with a bipolar small molecule as cascade material. A bipolar diketopyrrolopyrrole small molecule (F(DPP)₂B₂), as the second electron acceptor, was incorporated into poly(3-hexylthiophene) (P3HT): [6,6]-phenyl-C₆₁-butyric-acidmethyl-ester (PC₆₁BM) blend to fabricate ternary blend PSCs. The introduction of the bipolar compound F(DPP)₂B₂ can not only broaden the light absorption of the active layer because of its absorption in near infrared region but also play a bridging role between P3HT and PC₆₁BM due to the cascaded energy level structure, thus improving the charge separation and transportation. The optimized ternary PSC with 5 wt% F(DPP)₂B₂ content delivered a high PCE of 3.92% with a short-circuit current density (J_sc) of 9.63 mA cm⁻², an open-circuit voltage (V_oc) of 0.62 V and a fill factor (FF) of 64.90%, showing an 23% improvement of PCE as compared to the binary systems based on P3HT:PC₆₁BM (3.18%) or P3HT:F(DPP)₂B₂ (3.17%). The results indicate that the ternary PSCs with a bipolar compound have the potential to surpass high-performance binary PSCs after carefully device optimization.     

Synthesis and photovoltaic properties of D-A copolymers based on alkylthio-thiophene or alkylthio-selenophene-BDT donor unit and DPP acceptor unit

Two new 2D-conjugated D-A copolymers, PBDTT-S-DPP and PBDTSe-S-DPP, based on benzodithiophene (BDT) donor unit with alkylthio-thiophene or alkylthio-selenophene conjugated side chains and 2,5-bis(2-butyloctyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione) (DPP) acceptor unit, were synthesized for the application as donor materials in polymer solar cells (PSCs). The two polymers were characterized by absorption spectroscopy, cyclic voltammetry, thermogravimetric analysis, theoretical calculation with density functional theory, X-ray diffraction and photovoltaic measurements. The results show that the alkylthio-thiophene/selenophene side groups on BDT unit and intramolecular hydrogen bonding interaction in DPP acceptor unit play important roles in affecting the absorption, HOMO energy levels, molecular planarity and the crystallinity of the polymers. The PSCs based on PBDTT-S-DPP or PBDTSe-S-DPP as donor and PC₇₁BM as acceptor demonstrate power conversion efficiency (PCE) of 5.62% and 5.01%, with relatively higher V_oc of 0.79 V and 0.76 V, respectively.     

Investigations into inward positioned 3,3′-Dihexylditheinylbenzothiadiazole (DTBTh)-Benzodithiophene (BDT) based polymer solar cells by controlling molecular weight and alkyl side chain

In previous studies, PSCs based on polymers with an inward alkyl positioned DTBT unit showed poor power conversion efficiency mainly due to the greatly distorted polymer backbone structure caused by severe steric hindrance between the alkyl groups on the flanking thiophene of DTBT and the BT unit. In this study, PSCs based on polymers with an inward alkyl positioned DTBT unit are markedly improved by controlling the molecular weight and alkyl chain length. Two BDT-DTBTs and one BDT-BT polymers were synthesized by engineering alkylthienyl chains on BDT and by installing these with a short alkyl chain on the inward alkyl positioned DTBT. Extraordinary bathochromic shifts in the absorption maxima at 146 nm for PA and 165 nm for PB were observed going from solution to a solid film state, suggesting great differences in the polymer structures of the two states. Optical and electrochemical measurements were taken, and the HOMO levels of PA, PB, and PC were determined to be −5.76, −5.66, and −5.71 eV, respectively, indicating very low-lying HOMO energy levels. The optimized PSCs based on PA, PB, and PC exhibit power conversion efficiencies (PCEs) of 3.75%, 2.42%, and 2.30%, respectively, with V_oc (0.77–0.86 V), J_sc (6.9–8.7 mA/cm²), and FF (38–52%). We believe that the highest PCE for the PSCs based on PA may be attributed to the high molecular weight and improved processability relative to those of PB and PC. A theoretical study suggests that the polymer backbones of PA and PB are highly distorted between the donor unit and the acceptor unit, by as much as 49°, possibly by the steric hindrance between BT and the inward positioned methyl group on the flanking thiophene. Therefore, the conjugations for the HOMO p-orbitals of PA and PB are highly localized throughout the backbone while the conjugations for the HOMO p-orbitals of PC are well delocalized. The AFM study revealed that DIO additive greatly changed the morphology of the polymer blend from an amorphous state into distinct nanoscale phase separated states, leading to a great improvement in PCEs. The XRD study revealed that all polymers are amorphous.     

Influence of molar mass ratio,annealing temperature and cathode buffer layer on power conversion efficiency of P3HT:PC71BM based organic bulk heterojunction solar cell

In bulk heterojunction (BHJ) solar cells, the molar mass ratio of donor-acceptor polymers, the annealing temperature (T_an) and the cathode buffer layer plays very consequential role in improving the power conversion efficiency (PCE) by tuning the film morphology and enhancing the charge carrier dynamics. A comprehensive understanding of each of these factors is essential in order to optimize the performance of organic solar cells (OSCs). Albeit there are several fundamental reports regarding these factors, an altogether meticulous correlation of these physical processes with experimental evidence of the photo active layer are required. In this work, we systematically analyzed the influence of different molar mass ratio, the annealing temperature (T_an) and the cathode buffer layer of rrP3HT:PC₇₁BM based BHJ solar cells and their corresponding photovoltaic performances were correlated carefully with their thin film growth structure and energy level diagram. The device having 1:0.8 molar mass ratio of rrP3HT:PC₇₁BM and T_an = 150 °C annealing temperature with Bathocuproine (BCP) as the cathode buffer layer having ITO/PEDOT:PSS/rrP3HT:PC₇₁BM (molar mass ratio = 1:0.8; (T_an = 150 °C)/BCP/Al) configuration showed the best device performance with PCE, ɳ = 4.79%, Jsc = 14.21 mA/cm², V_oc = 0.58 V and FF = 57.8%. This drastic variation in PCE of the device having BCP/Al as the cathode contact compared to the other device configurations is due to the coalesced effects of better hole-blocking capacity of BCP along with Al and better phase separation of the active blend layer at 150 °C annealing temperature. These results explicate the cumulate role of all these physical parameters and their combined contribution to the PCE amendment and overall device performance with rrP3HT:PC₇₁BM based organic BHJ solar cell.     

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.     

Synthesis and photovoltaic properties low bandgap D-A copolymers based on fluorinated thiadiazoloquinoxaline

In this communication, we report the design a low bandgap D-A copolymer consist of fluorinated thiadiazoloquinoxaline (TDQ) as strong acceptor and benzothiophene (BT), denoted as P(ffFlTDQ_x-BT) exhibit broad absorption profile covering from 350 nm to 1000 nm with optical bandgap of 1.26 eV. P(ffFlTDQ_x-BT) showed highest occupied molecular orbital (HOMO) energy level of −5.46 eV which is deeper than that for nonfluorinated counterpart copolymer. The photovoltaic properties were evaluated using conventional devices with a structure of ITO/PEDOT:PSS/P(ffFlTDQ_x-BT):PC₇₁BM/Al. After the optimizations of the P(ffFlTDQ_x-BT) to PC₇₁BM weight ratios, and concentration of the solvent additive (DIO), the devices showed overall power conversion efficiency of 7.27%. The higher value of PCE of this device is higher than that of nonfluorinated copolymer (5.80%) is attributed to the higher values of both J_sc and FF, related to the higher hole mobility and better exciton dissociation efficiency. Moreover, employing a low boiling point solvent additive, i.e. o-chlorobenzaldehyde (CBA) (boiling point 132 °C) for active layer deposition and after the optimization of concentration of CBA, the resulted PSC showed overall PCE of 8.10%, which is higher than the PSC based on active processed with DIO/CB, related to the better balanced charge transport, induced by the fast removal of residues of solvent. To our best of our knowledge, PCE of 8.10% is also the highest for the PSCs with low bandgap of below 1.30 eV.     

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.     

Highly efficient ITO-free polymer solar cells based on metal resonant microcavity using WO3/Au/WO3 as transparent electrodes

Indium tin oxide (ITO)-free polymer solar cells (PSCs) with the structure of Glass/tungsten trioxide (WO₃)/Au/WO₃/PCDTBT: PC₇₀BM/LiF/Al was fabricated and studied. The multilayer structure of WO₃/Au/WO₃ is used as the potential transparent electrode to replace ITO. Metal resonant microcavity, which can enhance light harvesting of active layers, was constructed between Au and Al electrodes. According to the J–V and IPCE characterization with 70 nm active layer, power conversion efficiency (PCE) of the ITO-free microcavity device is approaching 4.55%, which is higher than that of the ITO-based device. However, PCE of the ITO-free device is much lower than that of the ITO-based device when the thickness of active layer increases to 130 nm. The opposite experimental tendency leads to theoretical research toward the simulation of light absorption and optical electric field and the calculation of maximum short circuit current density (J_{sc max}) as a function of active layer thickness based on ITO-free and ITO-based devices. The research results show that microcavity effect is closely linked to intrinsic absorption of active layers.     

An asymmetric small molecule based on thieno[2,3-f]benzofuran for efficient organic solar cells

A new asymmetric small molecule, named R3T-TBFO, with 4,8-bis(2-ethylhexyloxy)-substituted thieno[2,3-f]benzofuran (TBF) as central donor block, has been synthesized and used as donor material in organic solar cells (OSCs). With thermal annealing (TA) and solvent vapor annealing (SVA) treatment, the blend of R3T-TBFO/PC₇₁BM shows a higher hole mobility of 1.37 × 10⁻⁴ cm² V⁻¹ s⁻¹ and a more balanced charge mobilities. Using a structure of ITO/PEDOT:PSS/R3T-TBFO:PC₇₁BM/ZrAcac/Al, the device with TA treatment delivered a moderate power conversion efficiency (PCE) of 5.63%, while device after TA + SVA treatment showed a preferable PCE of 6.32% with a high fill factor (FF) of 0.72.