Benzodithiophene Donor Unit Research Articles

A Green Solvent Processable Wide‐Bandgap Conjugated Polymer for Organic Solar Cells

Herein, a novel wide‐band‐gap donor polymer, PBTVT, which comprises a bis(carboxylic ester)‐substituted thiophene–vinylene–thiophene (TVT) acceptor unit and benzodithiophene donor unit, is designed and synthesized. PBTVT displays good solubility in halogen‐free green solvents such as o‐xylene at elevated temperature. Grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) measurement revealed that PBTVT can form the desired face‐on orientation in the blend films. Organic solar cells are fabricated with PBTVT as the donor and ITOIC‐2F as the acceptor. Although the highest occupied molecular orbital (HOMO) level offset between PBTVT and 2,2′‐((2Z,2′Z)‐(((4,4,9,9‐tetrakis(4‐hexylphenyl)‐4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐2,7‐diyl)bis(3,4‐bis(hexyloxy)thiophene‐5,2‐diyl))bis(methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile (ITOIC‐2F) is only 0.08 eV, devices still exhibit an overall power conversion efficiency (PCE) of 11.04%. Especially, devices fabricated with the green solvent o‐xylene outperform those fabricated with halogenated solvents such as 1,2‐dichlorobenzene (o‐DCB).

Introducing an identical benzodithiophene donor unit for polymer donors and small-molecule acceptors to unveil the relationship between the molecular structure and photovoltaic performance of non-fullerene organic solar cells

We designed an acceptor ITIC-SF by fluorinating the thiophene ring in the benzodithiophene segment of ITIC-S and investigated its effect on the morphology and performance.

Synthesis, characterization and photovoltaic properties of low band gap donor-acceptor polymers containing benzodithiophene donor unit with fluorenylthiophene as 2D-conjugated side for organic solar cell application

ABSTRACTTwo donor−acceptor low band gap polymers P1 (octyl as solubilizing group) and P2 (ethylhexyl as solubilizing group) containing fluorenylthiophene-substituted benzoditihiophene as an electron-rich unit and 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione as an electron-deficient unit are designed and synthesized for polymer solar cells application. Compared with P2 based on ethyl hexyl group, P1 with octyl group displays well resolved vibronic shoulder peak in absorption spectra, stronger intermolecular interactions, and higher hole mobility. Polymer solar cells based on P1 and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) exhibit a maximum power conversion efficiency of 1.78% under AM 1.5G illumination (100 mW/cm2).

The effect of with/without resonance-mediated interactions on the organic solar cell performance of new 2D π-conjugated polymers

Bis-tolane as an integrated part of the benzodithiophene donor unit for OSCs.

Photochemical stability of high efficiency PTB7:PC70BM solar cell blends

Thieno[3,4 b]thiophene-alt-benzodithiophene (PTB7) is a promising donor–acceptor copolymer that has achieved high efficiencies (7–9%) in organic solar cells but suffers from poor stability and degrades when exposed to light and oxygen. Using resonant Raman spectroscopy to examine the nature of this photo-oxidation, three main changes to the vibrations of the conjugated backbone are observed: (1) shift of the benzodithiophene (BDT) CC stretch peak at ∼1489 cm−1 up to ∼1499 cm−1; (2) increase in the relative intensity of coupled fused thiophene and benzene CC stretch peaks at ∼1535 and ∼1575 cm−1; (3) appearance of a new peak at ∼1650 cm−1; which suggest oxidation takes place on the BDT unit without loss of conjugation. In situ accelerated photo-degradation reveals that the observed oxidation is the initial step of degradation, which is followed by reductions in absorption and Raman scattering intensities that indicate the loss of chromophores by a second, more extensive oxidation step. Blending PTB7 with PC70BM is found to accelerate the polymer's degradation, and further shift the BDT peak to ∼1509 cm−1. Using density functional theory to simulate Raman spectra for several possible oxidised products, the initial oxidation is best described by hydroxylation of 3rd and 7th positions on the BDT donor unit.

Synthesis and Photovoltaic Properties of 2D π-Conjugated Polymers Based on Alkylbenzothiophene Substituted Benzodithiophene Donor Unit with Titanium Sub-Oxide (TiOX) as an Interlayer in the Bulk Heterojunction Device Structure

A newbenzo[1,2-b:4,5-b’]dithiophene (BDT) derivative with conjugated side chain substituent, 5-ethylhexylbenzothiophene group, was synthesized as donor unit (D) and copolymerized with two acceptor units (A), 3,3′-diethylhexyl-3,4-propylenedioxythienyl-bisbenzothiadiazole and 3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP), respectively, using Stille coupling reaction to afford two new copolymers, P1 and P2. Low bandgaps were successfully accomplished for P1 (1.64 eV) and P2 (1.37 eV) and were attributed to the strong intramolecular charge transfer within the D–A alternating structure. Both P1 and P2 exhibited highest occupied molecular orbital (HOMO) energy levels that were deeper due to the conjugated benzothiophene substituent. The photovoltaic performances of the synthesized polymers were investigated using the device configuration ITO/PEDOT:PSS/polymer:PC71BM/(with or without)TiOx/Al. We employed titanium sub-oxide (TiOx) as an effective interlayer in the bulk heterojunction device structure. The subsequent photovoltaic performances showed high open-circuitvoltages (Voc) ranging from 0.78 to 0.81 V, whereas the power conversion efficiencies (PCEs) depended strongly on the blend morphologies. The polymer solar cell devices based on the blend of P2 and PC71BM gave the best photovoltaic performance among the series, with a Voc of 0.78 V, short-circuit current density (Jsc) of 5.69 mA/cm2, fill factor (FF) of 58.09 % and PCE of 2.60 %.

Conjugated polymers based on benzodithiophene and fluorinated quinoxaline for bulk heterojunction solar cells: thiophene versus thieno[3,2-b]thiophene as π-conjugated spacers

Two conjugated donor–acceptor copolymers based on a benzodithiophene donor unit and a fluorinated quinoxaline acceptor unit, spaced with either thiophene or thieno[3,2-b]thiophene π-bridges, were designed and synthesized. The effect of different π-bridges and of the processing conditions on optical, electrical, morphological and photovoltaic properties of the polymer:fullerene blend films were investigated. The polymer containing the thieno[3,2-b]thiophene π-bridge (PBDTFQ-TT) showed a red-shifted absorption and an enhanced charge carrier mobility, as compared to its analogue with the thiophene π-bridge (PBDTFQ-T), due to its narrower optical gap (by ∼0.1 eV) and stronger inter-chain interactions, favored by the structural planarity and increased linearity of the polymer backbone, as also supported by DFT calculations. The blend of PBDTFQ-TT and PC61BM ([6,6]-phenyl-C61-butyric acid methyl ester), compared to the PBDTFQ-T:PC61BM one processed under the same conditions (by blade-coating technique), showed greatly enhanced photovoltaic performance, with more than doubled power conversion efficiency (PCE up to 5.60% for the best device) due to the increased short-circuit current density and fill factor. However, similar PCEs were also achieved for PBDTFQ-T:PC61BM-based devices by optimizing the processing conditions through the addition of 1,8-diiodooctane (DIO) as the solvent additive. Through morphological and electrical analysis of the films, produced with and without additive, it was observed that the addition of DIO greatly enhances the self-organization, and consequently the charge mobility, of the thiophene π-bridge-based polymer, while it was detrimental for the nanoscale morphology and photovoltaic performances of the thieno[3,2-b]thiophene π-bridge-based polymer in the corresponding blend.

Effect of Incorporated Nitrogens on the Planarity and Photovoltaic Performance of Donor–Acceptor Copolymers

Systematic control of the chemical structure of conjugated polymers is critically important to elucidate the relationship between the conjugated polymer structures and properties and to optimize their performance in bulk heterojunction (BHJ) polymer solar cell (PSC) devices. Herein, we synthesized three new copolymers, i.e., P0, P1, and P2; these copolymers contain the same benzodithiophene donor unit but have different acceptor units with different numbers of nitrogen atoms in the range of 0–2. The effects of the introduced nitrogen atoms on the structural, optical, electrical, and photovoltaic properties of the conjugated polymers were investigated; the structural properties of the polymers, in particular, were studied using both experimental (grazing-incidence X-ray scattering (GIXS) measurements) and computational methods (molecular simulation). As the number of introduced nitrogen atoms increased, the planarity of the main chain conformation increased in the order of P0 < P1 < P2. Additionally, the P0, P1, and P2 polymers showed increased interlayer domain spacings of 1.61, 1.72, and 1.78 nm, respectively, with increased intermolecular ordering. These results were in excellent agreement with the simulation results. In addition, the enhanced planarity resulted in a red-shifting at the onset of absorption in the polymer film from 544 to 585 nm, a downshift in the lowest unoccupied molecular orbital (LUMO) energy level from −3.02 to −3.26 eV, and an increase in the hole mobility from 2.33 × 10–6 to 3.78 × 10–5 cm2/(V s). As a result, we observed dramatically enhanced performance of the PSCs in the order of P0 < P1 < P2. For example, the P2:PC61BM device exhibited a 3.5-fold improvement in power conversion efficiency (PCE) compared to that of P0:PC61BM. The further optimization of P2 with PC71BM showed the PCE of 3.22%.

Effects of π-Conjugated Bridges on Photovoltaic Properties of Donor-π-Acceptor Conjugated Copolymers

A series of conjugated donor (D)-π-acceptor (A) copolymers, P(BDT-F-BT), P(BDT-T-BT), and P(BDT-TT-BT), based on benzodithiophene (BDT) donor unit and benzothiadiazole (BT) acceptor unit with different π-bridges, were designed and synthesized via a Pd-catalyzed Stille-coupling method. The π-bridges between the BDT donor unit and BT acceptor unit are furan (F) in P(BDT-F-BT), thiophene (T) in P(BDT-T-BT) and thieno[3,2-b]thiophene (TT) in P(BDT-TT-BT). It was found that the π-bridges significantly affect the molecular architecture and optoelectronic properties of the copolymers. With the π-bridge varied from furan to thiophene, then to thieno[3,2-b]thiophene, the shape of the molecular chains changed from z-shaped to almost straight line gradually. Band gaps of P(BDT-F-BT), P(BDT-T-BT) and P(BDT-TT-BT) were tuned from 1.96 to 1.82 to 1.78 eV with HOMO levels up-shifted from −5.44 to −5.35 to −5.21 eV, respectively. Bulk heterojunction solar cells with the polymers as donor and PC71BM as acceptor demonstrated power conversion efficiency varied from 2.81% for P(BDT-F-BT) to 3.72% for P(BDT-T-BT) and to 4.93% for P(BDT-TT-BT). Compared to furan and thiophene, thieno[3,2-b]thiophene π-bridge in the copolymers shows superior photovoltaic performance. The results indicate that the photovoltaic performance of some high efficiency D–A copolymers reported in literatures could be improved further by inserting suitable π-bridges.

A Copolymer of Benzodithiophene with TIPS Side Chains for Enhanced Photovoltaic Performance

A new conjugated copolymer PBDTTT-TIPS containing thiazolothiazole acceptor unit and triisopropylsilylethynyl (TIPS)-functionalized benzodithiophene donor unit was synthesized by Pd-catalyzed Stille coupling and compared with its alkoxy-substituted analogue PBDTTT-C12. PBDTTT-TIPS exhibits decomposition temperature of 377 °C, absorption maximum of 598 nm, HOMO level of −5.3 eV, and hole mobility as high as 1.2 × 10–3 cm2 V–1 s–1. Without any post-treatments, polymer solar cells based on the blend of PBDTTT-TIPS and PC71BM exhibited power conversion efficiency as high as 4.33%, which is 2 times that for device based on the PBDTTT-C12:PC71BM blend. TIPS and C12 side chains exhibit a little impact on absorption, HOMO level, and hole mobility of the polymers, but they significantly influence blend film morphology and photovoltaic performance. TIPS side chains induces excellent compatibility between PBDTTT-TIPS and PC71BM, and the PBDTTT-TIPS:PC71BM blend exhibits perfect phase separation scales around 10–20 nm, which is beneficial to charge separation and enhanced efficiency of the device, while C12 side chains leads to large phase separation, which is responsible for the lower efficiency.

Side Chain Engineering of Copolymers Based on Bithiazole and Benzodithiophene for Enhanced Photovoltaic Performance

Four new copolymers P1–P4 containing the same backbone of bithiazole acceptor unit and benzodithiophene donor unit but different side chain pattern were synthesized by Pd-catalyzed Stille coupling. The effect of the side chains on backbone conformation, solubility, absorption spectra, energy levels, charge transport, blend film morphology, and photovoltaic properties of the polymers were experimentally and theoretically investigated. The planarity of the main chain increases in the order P3 < P4 < P1 ≈ P2. Upon increasing the planarity of the main chain, the polymer exhibits red-shifting of absorption maximum in film from 448 to 544 nm, upshifting of HOMO from −6.0 to −5.4 eV, downshifting of LUMO from −2.6 to −2.9 eV, and increasing of hole mobility from 4.7 × 10–4 to 0.06 cm2 V–1 s–1. Upon increasing the planarity of the polymer main chain, the phase separation size in polymer:PC71BM blend increases. The polymer solar cells based on P4:PC71BM (1:1, w/w) exhibit highest power conversion efficiency of 2.54% under AM 1.5, 100 mW cm–2, which is attributed to combination of broad absorption, high mobility, and suitable phase separation benefited from moderate planarity of the main chain.

Synthesis and Photovoltaic Properties of Copolymers from Benzodithiophene and Thiazole

Two new D−A conjugated polymers containing a benzodithiophene donor unit and the acceptor unit of bithiazole (BTz) or thiazolothiazole (TzTz), P1 and P2, were synthesized by Stille cross-coupling polymerization. The thermal, optical, and electrochemical properties were well investigated. Bulk heterojunction polymer solar cell devices based on an active layer of electron donor copolymers, P1 and P2, blended with electron acceptor [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) in a weight ratio of 1:2 were explored; the external quantum efficiency (EQE) measurements showed a peak external quantum efficiency of 45% for P1 and P2. The first result of the PSC device with P2 and PC71BM in a weight ratio of 1:2 gave an overall power conversion efficiency (PCE) of 2.6%, an open-circuit voltage of 0.77 V, a short-circuit current of 6.68 mA/cm2, and a fill factor of 0.51, under the illumination of AM1.5, 100 mW/cm2.