Non-fullerene Polymer Research Articles

Quinoxaline based A-A Type Polymer Donors for Non-Fullerene Polymer Solar Cells

Quinoxaline based A-A Type Polymer Donors for Non-Fullerene Polymer Solar Cells

Cost-effective polymer donors based on pyridine for efficient nonfullerene polymer solar cells

Developing cost-effective polymer donors that can match nonfullerene acceptors is very important for practical applications of polymer solar cells (PSCs). In this work, low-cost 3-fluoropyridine and pyridine were employed as acceptor unit cores to construct cost-effective polymers PBDT-FPy, PBDT-Py and PBDF-Py by copolymerizing with commercially available donor units. PBDT-FPy, PBDT-Py and PBDF-Py show wide-bandgap of 2.08, 2.15 and 2.05 eV, respectively. Due to the strong electronegativity of nitrogen atom in pyridine and Nδ− … Hδ+ noncovalent dipole-dipole interactions, all polymers exhibit deep-lying highest-occupied molecular orbital (HOMO) energy levels, strong intermolecular π-π interaction and good planarity. The PBDT-FPy: Y6 based device exhibits a power conversion efficiency (PCE) of 8.34%, while the performance of PBDT-Py: Y6 and PBDF-Py: Y6 devices was significantly improved, with PCE of 10.02% and 10.00%. The results indicate that pyridine with simple structure and low synthetic cost is a promising building block to construct high-performance cost-effective polymer donors for application in PSCs.

High-performance nonfullerene polymer solar cells based on chlorinated quinoxaline copolymer with a high short-circuit current density

In organic solar cells, the absorption range and extinction coefficient of the active layer not only affect the charge separation and carrier transfer efficiency of excitons, but also influence the JSC, and thus the device efficiency. Herein, an efficient nonfullerene polymer solar cells (NF–PSCs) based on a medium-bandgap (MBG) polymer donor PBDTTS-TClQx comprising chlorinethiophene quinoxaline (Qx) unit and a small molecule nonfullerene acceptor (SM-NFA) Y6 is developed. The PBDTTS-TClQx shows a strong absorption in the wavelength region of 330∼750 nm with an optical band gaps (Egopt) of 1.68 eV, which is well complementary with that of Y6 (1.33 eV) and facilitates achieving of high short-circuit current (JSC) in PSCs. As a result, the PBDTTS-TClQx:Y6-based PSCs achieved a power conversion efficiency (PCE) of 14.28% with a JSC of 25.9 mA cm−2. The JSC of 25.9 mA cm−2 achieved is among the highest reported for Qx-based polymer donors in PSCs.

Charge photogeneration dynamics in non-fullerene polymer solar cells with fluorinated and non-fluorinated acceptors.

In this work, charge photogeneration and recombination processes of PM6:IDIC-4F and PM6:IDIC blend films were investigated by the steady-state and time-resolved spectroscopies, as well as the time-dependent density functional theory calculations. The peaks in absorption and photoluminescence (PL) spectra of IDIC and IDIC-4F solutions were assigned by combining the experiment and the simulation of UV-vis absorption and PL spectra. For neat acceptor films, the exciton diffusion length of neat IDIC and IDIC-4F films was estimated as ∼28.9 and ∼19.9nm, respectively. For PM6-based blend films, we find that the fluorine substitution engineering on the IDIC acceptor material can increase the phase separate size of acceptor material in blend films, resulting in the reduction of dissociation efficiencies of acceptor excitons. In addition, we find that the charge recombination in PM6:IDIC-4F is dominated by bimolecular recombination, in comparison to geminate type carrier recombination in PM6:IDIC blend films. In addition, we find that thermal annealing treatment has a weak influence on carrier recombination but slightly reduces the exciton dissociation efficiency of acceptor in PM6:IDIC blend films, leading to a slightly reduced power conversion efficiency of PM6:IDIC solar cells. These results may shed light on the design of high-performance semiconductor molecules for application in solar cells.

Enhanced Photocatalytic and Photovoltaic Performance Arising from Unconventionally Low Donor-Y6 Ratios.

Development of both organic photovoltaics (OPVs) and organic photocatalysts has focused on utilizing the bulk heterojunction (BHJ). The BHJ promotes charge separation and enhances the carrier lifetime, but may give rise to increased charge traps, hindering performance. Here, high photocatalytic and photovoltaic performance is displayed by electron donor-acceptor (D-A) nanoparticles (NPs) and films, using the nonfullerene acceptor Y6 and polymer donor PIDT-T8BT. In contrast to conventional D-A systems, the charge generation in PIDT-T8BT:Y6 NPs is mainly driven by Y6, allowing a high performance even at a low D:A mass ratio of 1:50. The high performance at the low mass ratio is attributed to the amorphous behavior of PIDT-T8BT. Low ratios are generally thought to yield lower efficiency than the more conventional ≈1:1 ratio. However, the OPVs exhibit peak performance at a D:A ratio of 1:5. Similarly the NPs used for photocatalytic hydrogen evolution show peak performance at the 1:6.7 D:A ratio. Interestingly, for the PIDT-T8BT:Y6 system, as the polymer proportion increases, a reduced photocatalytic and photovoltaic performance is observed. The unconventional D:A ratios provide lower recombination losses and increased charge-carrier lifetime with undisrupted ambipolar charge transport in bulk Y6, enabling better performance than conventional ratios. This work reports novel light-harvesting materials in which performance is reduced due to unfavorable morphology as D:A ratios move toward conventional ratios of 1:1.2-1:1.

Impact of length of branched alkyl side chains on thiazolothiazole-based small molecular acceptors in non-fullerene polymer solar cells.

It has been reported that the length of branched alkyl side chains on fused-ring electron acceptors confers different impacts on properties versus solubility of BJH blends. However, because this impact on a non-fused acceptor backbone has rarely been studied, we examined the impact of molecular optimization from alkyl chain tuning based on non-fused thiazolothiazole small-molecule acceptors. The length of the side chain on the thiophene bridge was modified from 2-butyloctyl to 2-ethylhexyl, which corresponds to small molecules TTz3(C4C6) and TTz3(C2C4), respectively. Compared with the reported TTz3(C6C8) with long alkyl side chains, TTz3(C4C6) and TTz3(C2C4) exhibited stronger molecular aggregation, higher absorption coefficients, and greater redshifted UV absorption. Unexpectedly, after the alkyl chain was slightly shortened in this type of acceptor system, devices were successfully fabricated, but it was necessary to reduce the blending concentration at low rotation speeds due to the sharp decrease in the solubility of corresponding acceptor materials. Thus, the obtained unfavorable thickness and morphology of the active layer caused a decrease in Jsc and FF. As a consequence, TTz3(C4C6)- and TTz3(C2C4)-based devices showed an unsatisfactory power conversion efficiency of 6.02% and 2.71%, respectively, when donors were paired with the wide bandgap donor J71, which is inferior to that of TTz3(C6C8)-based devices (8.76%). These results indicate that it is challenging to determine the limit of the adjustable range of side chains to modify non-fused thiazolothiazole small-molecule acceptors for high-performance non-fullerene solar cells.

Recent progress in side chain engineering of Y-series non-fullerene molecule and polymer acceptors

Recent progress in side chain engineering of Y-series non-fullerene molecule and polymer acceptors

Effect of Chlorine Substituents on the Photovoltaic Properties of Monocyanated Quinoxaline-Based D–A-Type Polymers

A string of monocyanated quinoxaline (Qx)-based D-A-type polymers systematically decorated with electron-attracting chlorine (Cl) atoms was created for use in non-fullerene polymer solar cells (PSCs). First, coupling of the benzodithiophene (BDT) donor and Qx acceptor with the strong electron-attracting cyano (CN) unit at its 5-position yielded the monocyanated reference polymer PB-CNQ. Subsequently, the additional Cl atoms were separately or simultaneously incorporated into the thiophene side groups of the BDT donor and Qx acceptor to create other objective polymers, PBCl-CNQ, PB-CNQCl, and PBCl-CNQCl. The Cl substituents on the BDT donor and Qx acceptor are represented by the names of the polymers. Owing to the favorable contributions of Cl substituents, the inverted-type non-fullerene PSCs based on partially chlorinated PBCl-CNQ (12.80%) and PB-CNQCl (13.93%) exhibited better power conversion efficiencies (PCEs) than the device based on unchlorinated reference PB-CNQ (11.19%). However, a significantly reduced PCE of 9.84% was observed for the device based on PBCl-CNQCl, in which Cl atoms were loaded on both the BDT donor and Qx acceptor at the same time. Hence, these results reveal that optimization of the number and position of Cl substituents in monocyanated Qx-based polymers is essential for enhancing their photovoltaic nature through the synergistic effects between two strong electron-attracting CN and Cl substituents.

Impact of metal oxide diffusion and materials design on thermal stabilities of non-fullerene polymer solar cells

This study demonstrates that metal oxide diffusion is a critical degradation pathway in non-fullerene acceptor-based polymer solar cells.

Asymmetric fluorine and chlorine side-chain engineered quinoxaline-based D–A copolymer for both fullerene and nonfullerene polymer solar cells

A novel copolymer donor (Qx) based quinoxaline with asymmetric side chains, namely PBDTTS-2FClQx, is synthesized. The PCE of optimized ternary OSC (PBDTTS-2FClQx:Y6:PC71BM = 1 : 0.9 : 0.1) is 13.59%, but binary PBDTTS-2FClQx:Y6 (12.41%).

A polymer donor based on difluoro-quinoxaline with a naphthalimide substituent unit exhibits a low-lying HOMO level for efficient non-fullerene polymer solar cells.

The design and synthesis of polymer donors with a low-lying highest occupied molecular orbital (HOMO) level are crucial for increasing open-circuit voltages (VOC) and achieving high-performance non-fullerene polymer solar cells. Here, we developed two copolymers using non-fluorinated or fluorinated thienyl-conjugated benzodithiophenes as electron donor units, and difluoro-quinoxaline with a naphthalimide substituent (DNB) as the electron acceptor unit. These copolymers, namely PDNB and PDNB-2F, exhibited deep HOMO levels owing to the strong electron-withdrawing ability of the naphthalimide substituent. Density-functional theory calculations demonstrated that the skeletons of the two copolymers featured good coplanarity. Owing to the fluorination, PDNB-2F displayed an increased absorption coefficient and deeper HOMO level than PDNB. Moreover, the blended film based on PDNB-2F:Y6 demonstrated enhanced carrier mobility, decreased bimolecular recombination as well as favorable phase-separation regions. Consequently, the PDNB-2F:Y6-based device yielded a superior power conversion efficiency (PCE) of 12.18%, whereas the device based on PDNB:Y6 showed a comparatively lower PCE of 8.83%. These results indicate that difluoro-quinoxaline with a naphthalimide substituent is a prospective electron-deficient building block to develop donor polymers with low-lying HOMO levels to achieve efficient non-fullerene polymer solar cells.

Incorporating a weak acceptor unit into PTB7-Th to tune the open circuit voltage for non-fullerene polymer solar cells

PTB7-Th is one of the most effective polymer donors which have drawn huge attention in organic solar cells. However, the highest occupied molecular orbital (HOMO) level of PTB7-Th is relatively high, which limit open circuit voltage (Voc) of the cell devices. In this work, to develop the new members of PTB7-Th family materials with higher Vocs, two new random terpolymers PTB7-Th(0.9) and PTB7-Th(0.8) have been synthesized, in which contains 10% or 20% benzo[1,2-b:4,5-b′]dithiophene-2,6-dicarboxylate (VBDTC, a weak acceptor unit) moiety in PTB7-Th. It was found that the incorporating of VBDTC unit has great influence on optical and electronic properties of the related polymers. Comparison with PTB7-Th, the PTB7-Th(0.9) and PTB7-Th(0.8) exhibit blue shift in the UV−visible spectra, which forming a more complementary absorption with the low band gap non-fullerene acceptors, like ITIC and Y6. Increasing the percentage of VBDTC moiety, it can sufficiently deepen the low-lying HOMO level of the related polymers, which would expect to an enhancement in Voc. Non-fullerene solar cells blended with ITIC, PTB7-Th(0.9) exhibited a Voc of 0.839 V, with power conversion efficiency (PCE) of 4.86%. Interestingly, PTB7-Th(0.8) showed superior photovoltaic performance with a Voc of 0.862 V, and PCE of 5.24%. And when blended with Y6, the PCE of PTB7-Th(0.8) based device was further improved to 6.13%, due to increasing the short-circuit current (Jsc). The comprehensive study reveals that it is an effective strategy to tune the absorption spectra and deepens the HOMO energy level of the PTB7-Th related polymers by incorporation of a weak acceptor unit, thus, achieving higher Voc as well as photovoltaic performance.

Charge Photogeneration and Recombination Dynamics in PTQ10:Y6 Solar Cells

In this work, charge photogeneration dynamics in PTQ10:Y6 solar cells were studied by steady-state and time-resolved spectroscopies. For neat donor and acceptor films, we determined the exciton diffusion coefficients of PTQ10 and Y6 as 1.3 × 10−3 cm2·s−1 and 6.8 × 10−3 cm2·s−1, respectively. Furthermore, we find the LUMO and HOMO level offsets of 0.14 eV and 0.11 eV are sufficient for the dissociation of donor and acceptor excitons, respectively. For PTQ10:Y6 blend films, we find DIO additive could increase the scales of acceptor and donor phases. The acceptor phase increased slightly from 17.2 nm to 20.0 nm, while the donor phase increased from 2.3 nm to 5.8 nm. In addition, we find bimolecular recombination is a critical form for carrier recombination and DIO additive can significantly suppress the carrier recombination rate of PTQ10:Y6 active layer in an ultrafast time scale. This work is helpful for understanding the charge photogeneration processes in non-fullerene polymer solar cells.

Flexible Polymer Solar Cells with High Efficiency and Good Mechanical Stability

Single-junction polymer solar cells have demonstrated exceptional power conversion efficiency. Interlayer adhesion will be critical in building flexible polymer solar cells since inorganic conveyance layers would surely break. Aluminium-doped zinc oxide modified by polydopamine has emerged as a viable electron transportation layer in polymer solar cells, enhancing mechanical qualities by offering a high degree of flexibility and adhesion to the active layer. Power conversion efficiency of 12.7% is achieved in nonfullerene polymer solar cells built on PBDB-T2F:IT-4F with aluminium-doped zinc oxide 1.5% polydopamine electron transporting layer. Furthermore, the device based on Ag-mesh wire-wound electrodes has a power conversion efficiency of 11.5% and retains more than 90% of original power conversion efficiency afterward 1500 cycles of bending. For implantable and adaptable polymer solar cells for wide areas, roll-to-roll fabrication of inorganic electron transport layers is advantageous because of their mechanical resilience and thickness insensitivity.

Efficient Non-Fullerene Solar Cells Enabled by a Temperature-Dependent Terpolymer with Controlled Aggregation and Orientation

Light harvesting, molecular aggregation, and orientation of photoactive materials are crucial for non-fullerene polymer solar cells. Random copolymerization is a feasible and effective way to modify the molecular structure of polymer donors. However, it is very challenging to transfer the edge-on orientation of oligothiophene-based polymers to face-on orientation. Herein, we employed random copolymerization strategy by incorporating a specific ratio of a benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD)-based monomer into the backbone of a temperature-dependent polymer PFBT4T to improve the device performance. The resulting terpolymer donor PFBT4T-T20 with a comparable synthetic complexity possesses a dominant face-on orientation and weakened aggregations in solution and film, which are different from the parent polymer PFBT4T. When blended with acceptor Y14, the PFBT4T-T20-based device exhibits a more balanced carrier transport and optimal morphology and then achieved a higher efficiency of 15.60%. Our results reveal that the terpolymer strategy involving an extra component of the C2v symmetric structure can revive the oligothiophene-based conjugated polymer toward efficient non-fullerene solar cells.

Strategic structural evolution for enhancing the photovoltaic performance of quinoxaline-based polymers

In this study, strategic structural modifications of quinoxaline-based polymers were performed to improve their photovoltaic properties. First, an indacenodithiophene (IDT) unit was connected to methoxy-substituted quinoxaline via a thiophene linker to produce the reference polymer, PIDT-QxTM. The electron-accepting quinoxaline unit was surrounded by the electron-donating IDT and methoxy group in its horizontal and vertical directions, respectively, creating a beneficial two-dimensional configuration. Subsequently, two promising attempts to position the electron-pulling fluorine atom on the quinoxaline acceptor and replace IDT with a more extended indacenodithieno[3,2-b]thiophene unit were sequentially performed to afford the objective polymers PIDT-FQxTM and PIDTT-FQxTM, respectively. Because of the positive and synergistic effects, the photovoltaic characteristics of the polymers in an inverted-type non-fullerene polymer solar cell gradually enhanced in the order of PIDT-QxTM, PIDT-FQxTM, and PIDTT-FQxTM, with power conversion efficiencies of 6.50%, 7.82%, and 8.51%, respectively. The best performance from the device with PIDTT-FQxTM is primarily ascribed to the superior short-circuit current density to that of the others.

New wide band gap π-conjugated copolymers based on anthra[1,2-b: 4,3-b': 6,7-c''] trithiophene-8,12-dione for high performance non-fullerene polymer solar cells with an efficiency of 15.07 %

Developing efficient wide-bandgap copolymer donor materials to match with narrow bandgap non-fullerene acceptors is continuously ongoing for polymer solar cells. Herein, two new D-A copolymers are designed and synthesized by embedding the same anthra[1,2-b:4,3-b':6,7-c"] trithiophene-8,12-dione (A3T) acceptor unit and different donor units, i.e., BDTTZ ( P126 ) and BDTTh ( P127 ). These copolymers showed broad absorption from 350 to 680 nm and deeper HOMO energy level. We have used these two copolymers as donors and a narrow bandgap non-fullerene acceptor Y6 to prepare bulk heterojunction polymer solar cells (PSCs). After the optimization, P126 :Y6 and P127 :Y6 attained overall power conversion efficiency of 15.07% and 12.27%, respectively. The higher PCE for the P126 than P127 is associated with the more efficient photon harvesting and photogenerated excitons, balanced charge transport, and low energy loss. Our results may help to design new polymers with a deeper highest occupied molecular orbital level that will be well-matched with non-fullerene acceptors. • We have used the approach of weak donor and strong acceptor approach to design the D-A copolymers. • Two D-A copolymers were designed with same A3T acceptor and different donors, i.e., BDTTZ ( P126 ) and BDTTh (P127 ). • Both the copolymers showed deeper HOMO energy level to achieved high V OC . • P126 :Y6 and P127 :Y6 showed power conversion efficiency of 15.07% and 12.27%, respectively.

Fluorinated phenanthrenequinoxaline-based D-A type copolymers for non-fullerene polymer solar cells

Three polymers with indacenodithiophene (IDT) as the electron-rich unit and phenanthrenequinoxaline (phanQ) or fluorine substituted phenanthrenequinoxaline (FphanQ, 2FphanQ) as the electron-deficient units have been synthesized. The band gaps of PIDT-phanQ, PIDT-FphanQ, PIDT-2FphanQ were estimated to be 1.68, 1.68 and 1.71 eV with a HOMO energy level of −5.28, −5.33 and −5.41 eV, respectively. The photovoltaic performance of the three polymers in non-fullerene polymer solar cells with Y6 as the acceptor material was carefully investigated. The corresponding devices showed improved V oc as the fluorine atom increased in the phanQ unit, but the champion PCE was achieved in the PIDT-FphanQ:Y6 based device due to its higher J sc and FF. The best PCE was recorded to be 11.6% with a V oc of 0.79 V, a J sc of 21.4 mA cm −2 and a FF of 68.7% for the PIDT-FphanQ:Y6 blend without using additives. The results indicate that the proper design in donor and acceptor units of D-A conjugated polymer, i.e. introducing the moderate fluorine substitutes, will be able to simplify the processing for the efficient PSCs applications. Three donor polymers based on IDT unit and Phenanthrenequinoxaline derivatives were developed. The photovoltaic performance was improved by tuning the number of fluorine on the phenanthrenequinoxaline moiety. High efficiency was obtained without the use of additives, thus simplifying processing of efficient polymer solar cells. • Three polymers based on indacenodithiophene and phenanthrenequinoxaline units were designed for photovoltaic applications. • The mono-fluorinated polymer PIDT-FphanQ delivers a best PCE of 11.6% due to higher J sc and FF. • The high photovoltaic performance was achieved without solvent additives, simplifying the process for efficient PSC.

Fullerene-free, MoTe2 atomic layer blended bulk heterojunctions for improved organic solar cell and photodetector performance

This paper introduces a facile solution route to fabricate MoTe2 grain nano-domains for hybrid polymer heterojunction solar cells and photodetectors. Fabricated MoTe2 structures were investigated using Raman and XRD, and surface morphologies characterized using FE-SEM and TEM. We subsequently assembled a non-fullerene polymer heterojunction device using the PBDB-T:ITIC active layer for ITO/HTL/active-layer/LiF/Al architype solar cells and photodetectors. We also investigated effects from different MoTe2 nanostructure proportions embedded in the PBDB-T:ITIC polymer matrix as active layers to assemble solar cell and photodetector devices. Maximum power conversion efficiency ≈8.94% with improved carrier mobility ≈9.23 × 10−4 cm2 V−1 s−1 was achieved for 10 vol% MoTe2 nanostructure inclusion with PBDB-T:ITIC active layer. A fabricated photodetector using 10 vol%-MoTe2@PBDB-T:ITIC film achieved maximum detectivity ≈2.62 × 1010 Jones with 0.706 A W−1 responsivity at 755 μW cm−2 light density under 656 nm laser source. The proposed facile approach to fabricate 2D MoTe2 atomic layer infusion with polymer heterojunction offers a credible mechanism to progress interfacial characteristics for future organic semiconducting devices.

Study of charge transfer in D-A conjugated donor and non-fullerene small molecule acceptor for the application of non-fullerene polymer organic solar cell

The progress in Bulk Heterojunction-Polymer Solar Cells (BHJ-PSCs) has received much interest since the emergence of non-fullerene small molecule acceptors (NFSMAs), and power conversion efficiency (PCE) have reached up to 18%. In general, a mixer of conjugated polymer Donor and NFSM as Acceptor consists of the BHJ photoactive layer. Their optical bands and the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are of great importance to improve the resulting PSCs' efficiency and performance of the polymer solar cells. The HOMO and LUMO of Poly(4,6-bis(5-octylthiophen-2-yl)benzo[1,2-b:6,5-b':3,4-c'']terthiophene-2,8-diyl)-alt-(thiophen-2,5-diyl]-alt-(5,6-difluoro benzo[c][1,2,5]thiadiazol 4,7-diyl (denoted as P95) and NFSMAs ITIC-F, were measured using cyclic voltammetry(CV). Optical bandgap energy for each material's were calculated by UV–VIS absorbance spectra. Further charge transfer study was carried out by photoluminescence (PL) spectroscopy. It's results show that efficient electron transfer from P95 to ITIC-F and hole from ITIC-F to P95. On the basis of experimental outcomes these polymer materials can effectively produce viable organic solar cell (OSCs) devices.