Non-fullerene Polymer Solar Cells 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.

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.

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.

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%).

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.

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.

Anode interfacial modification for non‐fullerene polymer solar cells: Recent advances and prospects

AbstractRecently, the power conversion efficiency (PCE) of single‐junction non‐fullerene polymer solar cells (NF‐PSCs) has surpassed 19% due to the fast development of novel donor polymers, NF‐acceptors, device engineering, and interlayer materials. The anode interlayer (AIL) plays a vital role in improving the efficiency and stability of PSCs. The challenges and opportunities in this research area encourage researchers to pursue great innovation in developing new materials and strategies for highly efficient NF‐PSCs. This review summarizes the recent development of AIL materials and their modification strategies in single‐junction NF‐PSCs. Firstly, a brief introduction, key functions, basic requirements, and peculiar features of AILs when employed in NF‐PSCs are discussed. Then, the impact of AIL materials (including organic, inorganic, and hybrid materials) on the PCE and the stability of NF‐PSCs are described. Afterward, the fabrication of large‐area devices and related issues are highlighted. The summary and the future challenges regarding efficient AIL are summarized in the last section of this review.image

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.

Enhanced efficiency of polymer solar cells via simple fluorination on the π-bridge of polymer donors

In this contribution, a novel fluorinated polymer based on the well-known J52, using the newly developed 3-fluorothiophene as π bridges on the conjugated main chain, named J52-FTh , was synthesized and was used as polymer solar cells (PSCs) donor material. The effects of fluorinated π bridge on the polymer's thermal stability, light absorption, energy level, molecular configuration and photovoltaic performance were systematically studied. It demonstrated that J52-FTh exhibited an enhanced absorption, a narrower optical band gap, a low-lying the highest occupied molecular orbitals (HOMO) energy level and better coplanar compared with J52 without any fluorine substituent on its π bridges. The non-fullerene PSCs based on J52-FTh :ITIC (1:0.8, w/w) achieved a high PCE of 13.32% with a V OC of 0.92 V, a J SC of 21.29 mA cm −2 and an FF of 68%, which was much better than the PSCs based on J52:ITIC with a PCE of 5.51%. The results indicated that fluorination on the polymer's main chain was a simple and efficient strategy toward high efficiency. A novel fluorinated polymer based on the well-known J52, using the newly developed 3-fluorothiophene as π bridges on the conjugated main chain, named J52-FTh , was synthesized and was used as polymer solar cells (PSCs) donor material. The PSCs based on J52-FTh :ITIC achieved a high PCE of 13.32% with a V OC of 0.92 V, a J SC of 21.29 mA cm −2 and a FF of 68%, which was superior to that of the binary PSCs based on J52:ITIC. • A novel fluorinated polymer based on the well-known J52, using the newly developed 3-fluorothiophene as π bridges, named J52-FTh , was synthesized. • The effects of fluorinated π bridge on the polymer’s thermal stability, light absorption, energy level, molecular configuration and photovoltaic performance were systematically studied. • J52-FTh exhibited an enhanced absorption, a narrower optical band gap, a low-lying HOMO energy level and better coplanar compared with J52 without any fluorine substituent on its π bridges. The non-fullerene PSCs based on J52-FTh :ITIC achieved a high PCE of 13.32%, which was much better than the PSCs based on J52:ITIC. • The results indicated that fluorination on the polymer’s main chain was a simple and efficient strategy toward high efficiency.

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.

Novel conjugated polymers based on cyclopenta[c]thiophene-4,6(5H)-dione for efficient polymer solar cells

A series of polymers based on indandione derivatives, TIND-HT-BDT, TIND-HT-BDTF, TIND-DHT-BDT, and TIND-DHT-BDTF, were synthesized as donor materials for polymer solar cells (PSCs). The polymers have a typical donor-acceptor configuration, where TIND-HT (5-((5-hexylthiophen-2-yl)methylene)-4H-cyclopenta[c]thiophene-4,6(5H)-dione) and TIND-DHT (5-((4,5-dihexylthiophen-2-yl)methylene)-4H-cyclopenta[c]thiophene-4,6(5H)-dione) are the electron acceptor units, while benzodithiophene (BDT) or fluorinated benzodithiophene (BDTF) are the electron donor units. Non-fullerene inverted-type PSCs with a configuration of ITO/ZnO/polymer:Y6BO/MoO3/Ag devices based on polymers as the donor and Y6BO as the acceptor were fabricated. The best power conversion efficiency (PCE) of 11.1% was achieved using the inverted-type PSCs based on TIND-DHT-BDTF with an open-circuit voltage (Voc) of 0.82 V and short-circuit current density (Jsc) of 22.6 mA/cm2. This study provides insights into novel donor materials for non-fullerene PSCs.

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.

New Medium Bandgap Donor D-A1 -D-A2 Type Copolymers Based on Anthra[1,2-b: 4,3-b":6,7-c"'] Trithiophene-8,12-dione Groups for High-Efficient Non-Fullerene Polymer Solar Cells.

A new acceptor unit anthra[1,2-b: 4,3-b': 6,7-c'']trithiophene-8,12-dione (А3Т) (A2) is synthesized and used to design D-A1 -D-A2 medium bandgap donor copolymers with same thiophene (D) and A2 units but different A1, i.e., fluorinated benzothiadiazole (F-BTz) and benzothiadiazole (BTz) denoted as P130 and P131, respectively. Their detailed optical and electrochemical properties are examined. The copolymers show good solubility in common organic solvents, broad absorption in the visible spectral region from 300 to 700 nm, and deeper HOMO levels of -5.45 and -5.34 eV for P130 and P131, respectively. Finally, an optimized polymer solar cell (PSC) based on P131 as the donor and narrow bandgap non-fullerene small molecule acceptor Y6 demonstrated a power conversion efficiency (PCE) of >11.13%. To further improve the efficiency of the non-fullerene PSC, the P130 is optimized by introducing a fluorine atom into the BTz unit, F-BTz acceptor unit, and PCE PSC based on P130: Y6 active layer increased to >15.28%, which is higher than that for the non-fluorinated analog P131:Y6. The increase in the PCE for former PSC is attributed to the more crystalline nature and compact π-π stacking distance, leading to more balanced charge transport and reduced charge recombination. These remarkable results demonstrate that A3T-based copolymer P130 with F-BTz as the second acceptor is a promising donor material for high-performance PSCs.