High-performance Non-fullerene Polymer Solar Cells Research Articles

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.

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.

Simple methoxy-substituted quinoxaline-based D-A type polymers for nonfullerene polymer solar cells

Two quinoxaline-based D-A-type conjugated polymer donors were designed and synthesized in this study with the aim of developing high-performance nonfullerene polymer solar cells (PSCs). The combination of the electron-donating indacenodithiophene (IDT) and indacenodithieno[3,2-b]thiophene (IDTT) units with the electron-accepting methoxy-substituted 2,3-diphenylquinoxaline moiety (QxM) under the Stille coupling condition produces the target polymers, denoted as PIDT-QxM and PIDTT-QxM, respectively. In these specific molecular architectures, the beneficial two-dimensional arrangement of the electron-donating components in both the horizontal and vertical directions of the central quinoxaline unit is readily achieved. The photovoltaic properties of the polymers were examined by fabricating nonfullerene PSCs with the structure of ITO/ZnO/Polymer:Y6BO/MoO3/Ag. The replacement of the electron-donating IDT in PIDT-QxM with IDTT in PIDTT-QxM could increase the power conversion efficiency (PCE) of the corresponding device from 9.49% to 10.40%. The prominent enhancement in the PCE of the device based on PIDTT-QxM can be primarily ascribed to the significant increase in its short-circuit current density. Moreover, the simple synthetic methods used in this study for manufacturing p-type polymers can provide the additional advantages of cost-effectiveness and scalability.

Investigation of Optical and Dielectric Constants of Benzobis(thiazole)- based Copolymer Films

Introduction: Copolymer PBB-T with benzo[1,2-d:4,5-d']bis(thiazole) (BBT) as the accepting unit and benzodithiophene (BDT) as the donor unit is a promising candidate for highperformance non-fullerene polymer solar cells (PSCs). So far optical and dielectric constants of the PBB-T are not fully known. Method: PBB-T was synthesized and thin films of PBB-T were prepared. By using the Kramers-Kronig relations and the transmission spectra of the PBB-T films, the optical and dielectric constants, including in absorption coefficient (α(λ)), extinction coefficient (κ(λ)), refractive index (n(λ)), dielectric constant (ε1(λ),ε2(λ)), band gap (Eg) and mobility of the PBB-T films were calculated and analyzed. Result: At 500 nm, α, κ, n, ε1 and ε2 are 1.65×105 cm-1, 0.46, 1.8163, 3.0 and 1.65 respectively. Eg is 2.111 eV. The hole mobility of PBB-T are 2.41×10-5 cm2 V-1 s-1 and 1.71×10-4 cm2 V-1 s-1 for the as-cast film and for the solvent vapor annealed film respectively. The results show that these optical and dielectric constants of the PBB-T films are almost independent on the thicknesses of the films, indicating our results are reliable. The features of the optical and dielectric constants show the PBB-T films are very promising candidates for high-performance non-fullerene PSCs and potential cut-off filter only permitting red and near-infrared light passing. Conclusion: These results are significant for designing optoelectronic devices related to the PBBT thin films.

Molecular design towards two-dimensional electron acceptors for efficient non-fullerene solar cells

Non-fullerene polymer solar cells (NF-PSCs) have gained wide attention recently. Molecular design of non-fullerene electron acceptors effectively promotes the photovoltaic performance of NF-PSCs. However, molecular electron acceptors with 2-dimensional (2D) configuration and conjugation are seldom reported. Herein, we designed and synthesized a series of novel 2D electron acceptors for efficient NF-PSCs. With rational optimization on the conjugated moieties in both vertical and horizontal direction, these 2D electron acceptors showed appealing properties, such as good planarity, full-spectrum absorption, high absorption extinction coefficient, and proper blend morphology with donor polymer. A high PCE of 9.76% was achieved for photovoltaic devices with PBDB-T as the donor and these 2D electron acceptors. It was also found the charge transfer between the conjugated moieties in two directions of these 2D molecules contributes to the utilization of absorbed photos, resulting in an exceptional EQE of 87% at 730 nm. This work presents rational design guidelines of 2D electron acceptors, which showed great promise to achieve high-performance non-fullerene polymer solar cells.

Regulation of Molecular Packing and Blend Morphology by Finely Tuning Molecular Conformation for High-Performance Nonfullerene Polymer Solar Cells.

The asymmetric thienobenzodithiophene (TBD) structure is first systematically compared with the benzo[1,2-b:4,5-b']dithiophene (BDT) and dithieno[2,3-d:2',3'-d']benzo[1,2-b:4,5-b']dithiophene (DTBDT) units in donor-acceptor (D-A) copolymers and applied as the central core in small molecule acceptors (SMAs). Specific polymers including PBDT-BZ, PTBD-BZ, and PDTBDT-BZ with different macromolecular conformations are synthesized and then matched with four elaborately designed acceptor-donor-acceptor (A-D-A) SMAs with structures comparable to their donor counterparts. The resulting polymer solar cell performance trends are dramatically different from each other and highly material-dependent, and the active layer morphology is largely governed by polymer conformation. Because of its more linear backbone, the PTBD-BZ film has higher crystallinity and more ordered and denser π-π stacking than those of the PBDT-BZ and PDTBDT-BZ films. Thus, PTBD-BZ shows excellent compatibility with and strong independence on the SMAs with varied structures, and PTBD-BZ-based cells deliver high power conversion efficiency (PCE) of 10-12.5%, whereas low PCE is obtained by cells based on PDTBDT-BZ because of its zigzag conformation. Overall, this study reveals control of molecular conformation as a useful approach to modulate the photovoltaic properties of conjugated polymers.

Impact of the Siloxane-Terminated Side Chain on Photovoltaic Performances of the Dithienylbenzodithiophene-Difluorobenzotriazole-Based Wide Band Gap Polymer Donor in Non-Fullerene Polymer Solar Cells.

To thoroughly disclose the role of the siloxane-terminated side chain with different substituent positions, three difluorobenzotriazole-dithienylbenzodithiophene (FTAZ-BDTT)-based polymers PBZ-1Si, PBZ-2Si, and PBZ-3Si with the siloxane-terminated side chain on the FTAZ unit (PBZ-1Si), on the BDTT unit (PBZ-2Si), and both on BDTT and FTAZ units (PBZ-3Si), respectively, were synthesized. The different side chain substitutions have slight influences on absorption behavior, thermal stability, and frontier molecular orbitals but have shown a great effect on the aggregation of the polymers. Grazing-incidence wide-angle X-ray scattering measurements reveal that, relative to PBZ-1Si with branched alkyl on the BDTT unit, polymers PBZ-2Si and PBZ-3Si, bearing the siloxane-terminated side chains on the BDTT unit, exhibit smaller π-π stacking distances and larger crystal coherence lengths, suggesting that adopting the siloxane-terminated side chain on the BDTT unit can promote the interchain π-π interaction and the ordering of molecular packing. With IT-M as the non-fullerene acceptor, among the three polymers, the PBZ-2Si-based active layer possesses the highest ordered crystals for both polymers and IT-M as well as the purest domain, which affords efficient exciton dissociation, the most balanced hole-electron transport, and reduced recombination, leading to the highest short-circuit current density (Jsc) and fill factor (FF) and then the highest power conversion efficiency (PCE) of 11.14%. In contrast, PBZ-1Si- and PBZ-3Si-based devices show lower PCEs of 8.98 and 9.92%, respectively. Moreover, PBZ-2Si:IT-M also exhibits good thickness tolerance, and its thick active layer of 240 nm shows the most limited decrease of efficiency after 77 days of storage, supplying good potential for mass fabrication. Our work suggests that the fine pairing of a siloxane-terminated side chain and an alkyl side chain is beneficial for the optimizing of a conjugated polymer donor toward high-performance non-fullerene polymer solar cells.

RETRACTED: Study of dielectric and optical constants of Benzobis(thiazole)-based copolymer thin films for designing optoelectronic devices

Abstract A new series of benzobisthiadiazole (BBT)-based donor–acceptor copolymers, namely, PBBT-FT, PBBT-T-FT, and PBBT-Tz-FT, with different π-conjugated bridges have been developed for polymer thin film transistors (TFTs). It was found that inserting different π-conjugated bridges into the backbone of the polymer allowed tailoring of opto-electrical properties, molecular organizations, and accordingly, ambipolar transport of TFTs. Copolymer PBB-T with benzo[1,2-d:4,5-d’]bis(thiazole) (BBT) as the accepting unit and benzodithiophene (BDT) as the donor unit is a promising candidate for high-performance non-fullerene polymer solar cells (PSCs). As much as we know the optical and dielectric constants of the PBB-T are not fully known. PBB-T was synthesized and subsequently thin films were properly prepared. By using the Kramers-Kronig relations and the transmission spectra of these thin films, the dielectric and optical constants were calculated and analyzed, including the absorption coefficient, dielectric constant, extinction coefficient, and refractive index of the PBB-T thin films. The results show that these dielectric and optical constants of the PBB-T thin films are quasi-independent on the thicknesses of the thin films, indicating our results are reliable. The features of the dielectric and optical constants show the PBB-T thin films are very promising candidates for high-performance non-fullerene PSCs and potential cut-off filter only permitting red and near-infrared light pass. These results are significant for designing optoelectronic devices related to the PBB-T thin films.

High-performance non-fullerene polymer solar cells based on naphthobistriazole wide bandgap donor copolymers

The non-fullerene polymer solar cells (NF-PSCs) devices based on the PFBTZNT:m-ITIC system exhibited a power conversion efficiency of up to 11.02%.

Wide bandgap donor-acceptor conjugated polymers with alkylthiophene as side chains for high-performance non-fullerene polymer solar cells

Abstract Four wide bandgap donor-acceptor (D-A) conjugated polymers, i.e. P-diF2T, P-TT, P-BDT and P-NDT, with bis(5-(2-decyltetradecyl)-[2,3′-bithiophen]-2′-yl)arylenes as D-units and 2-propyl-5,6-difluorobenzo[d][1,2,3]triazole (ffTAZ) as A unit were synthesized for photovoltaic applications. In P-diF2T, P-TT, P-BDT and P-NDT, 3,3′-difluoro-2,2′-bithiophene (diF2T), thieno[3,2-b]thiophene (TT), benzo[1,2-b:4,5-b']dithiophene (BDT) and naphtho[1,2-b:5,6-b']dithiophene (NDT) were used as aryl central units, respectively, to adjust the properties of the polymers. The optical bandgap is 1.83 eV for P-diF2T, 1.85 eV for P-TT, 1.90 eV for P-BDT and 2.04 eV for P-NDT. All four polymers show temperature-dependent aggregation behavior in solution, and their absorption spectra are complementary with that of non-fullerene acceptor (NFA) 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(5-hexylthiophen-2-yl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC-Th). Polymer solar cells (PSCs) based on these polymers and ITIC-Th were fabricated and characterized. Power conversion efficiencies (PCEs) higher than 7% were demonstrated for all polymers. The devices based on P-diF2T exhibited the highest PCE up to 10.08% with a high fill factor (FF) of 0.741. The good photovoltaic performance of P-diF2T is attributed to the synergies of the low-lying highest occupied molecular orbital (HOMO) energy level of the polymer and the favorable microstructures and morphology and thereby high and balanced charge carrier mobilities of the blend films. This study suggested that adjusting aromatic D units together with using alkylthiophene as side chains is an effective approach to design donor polymers for high performance PSCs based on NFA.

Perylene Diimide-Based Zwitterion as the Cathode Interlayer for High-Performance Nonfullerene Polymer Solar Cells.

Nonfullerene polymer solar cells (PSCs) have earned widespread and intense interest on account of their properties such as tunable energy levels, potential for low-cost production processes, reduced energy losses, and strong light absorption coefficients. Here, a water-/alcohol-soluble zwitterion perylene diimide zwitterion (PDI-z) consisted of sulfobetaine ion as a terminal substituent and PDI as a conjugated core was synthesized. PDI-z was employed as an electron-transport layer (ETL) for nonfullerene PSC devices, obtaining an optimal power conversion efficiency (PCE) above 11.23%. Moreover, nonfullerene PSCs with the PDI-z cathode interlayer displayed an excellent performance on a large scale of interlayer thickness, which was compatible with printing fabrication techniques. Additionally, the PDI-z interlayer presented good ability of modifying high work function metals (for instance, Au, Cu, and Ag) in nonfullerene devices, and the Ag device displayed a PCE of 9.38%. This work provides a good alternative ETL for high-efficiency nonfullerene PSCs.

Ultrafast Channel II process induced by a 3-D texture with enhanced acceptor order ranges for high-performance non-fullerene polymer solar cells

The enhanced SMA order ranges in well-intermixed 3-D textures enabled an ultrafast Channel II process, affording a highest efficiency of 12.01%.

An effective strategy for controlling the morphology of high-performance non-fullerene polymer solar cells without post-treatment: employing bare rigid aryl rings as lever arms in new asymmetric benzodithiophene

Weakening the entanglements between polymer chains enables the creation of high-performance non-fullerene solar cells without post-treatment.

Achieving high short-circuit current and fill-factor via increasing quinoidal character on nonfullerene small molecule acceptor

Recently, the fused-ring based low band gap (LBG) small molecule acceptors (SMAs) have emerged as efficient nonfullerene acceptors. So far, these LBG SMAs are mainly designed with IC (2-methylene-(3-(1,1-dicyanomethylene)indanone)) or its analogs, the benzo-type electron-accepting (A) units. Compared to benzene, thiophene is less aromatic and thus the thiophene-involving semiconducting molecule has more quinoidal character, which effectively reduces the energy gap between the highest occupied molecular orbit (HOMO) and the lowest unoccupied molecular orbit (LUMO). Herein, we show that replacing the IC units in ITIC with the CT (cyclopenta[c]thiophen-4-one-5-methylene-6-(1,1-dicyanomethylene)), a thiophene-fused A unit, the quinoidal character is enhanced from 0.0353 on ITIC to 0.0349 on ITCT, the CT-ended SMA. The increase in the quinoidal character reduces the optical band gap and enhances the near IR absorptivity. When blended with the wide band gap (WBG) polymer donor, PBDB-T, an average power conversion efficiency of 10.99% is obtained with a short-circuit current-density (Jsc) of 17.88mA/cm2 and a fill-factor (FF) of 0.723. For comparisons, the Jsc is of 16.92mA/cm2, FF is of 0.655 and PCE is of 9.94% obtained from the ITIC:PBDB-T device. This case indicates that the replacement of the benzene ring on the IC unit with a more polarizable five-member ring such as thiophene is an effective way to enhance the absorption of the near IR solar photons towards designing high-performance nonfullerene polymer solar cells.

High-performance nonfullerene polymer solar cells with open-circuit voltage over 1 V and energy loss as low as 0.54 eV

In this work, we developed high efficiency nonfullerene PSCs based on a wide bandgap polymer donor PBPD-Th with a structure of meta-alkoxyphenyl benzodithiophene-alt-thienyl benzodithiophene-4,8-dione and a low bandgap small molecule acceptor ITIC. The PBPD-Th shows a strong absorption in the short wavelength of 300–650nm with a bandgap of 1.91eV, which is complementary with that of ITIC (1.55eV). Moreover, PBPD-Th possesses a deep HOMO level of −5.42eV, which is beneficial to obtain high open-circuit voltage (Voc) in PSCs. As a result, the optimal PSCs based on PBPD-Th: ITIC showed a power conversion efficiency (PCE) of 10.8% with a Voc of up to 1.01V, a high short-circuit current density (Jsc) of 18.3mAcm−2 and a fill factor (FF) of 59%, under the illumination of AM 1.5G, 100mWcm−2. Notably, the energy loss (Eloss) of the optimal PSCs is 0.54eV, which is smaller than the empirically low threshold of 0.6eV. The PCE of 10.8% is one of the highest values reported in the literatures for the PSCs with Voc over 1.0V.

3D Structural Model of High-Performance Non-Fullerene Polymer Solar Cells as Revealed by High-Resolution AFM.

Rapid improvements in nonfullerene polymer solar cells (PSCs) have brought power conversion efficiencies to greater than 12%. To further improve device performance, a fundamental understanding of the correlations between structure and performance is essential. In this paper, based on a typical high-performance system consisting of J61(one donor-acceptor (D-A) copolymer of benzodithiophene and fluorine substituted benzotriazole) and ITIC (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']-dithiophene), a 3D structural model is directly imaged by employing high-resolution atomic force microscopy (AFM). Hierarchical morphologies ranging from fiberlike crystallites, several nanometers in size, to a bicontinuous morphology, having domains tens of nanometers in size, are observed. A fibrillar interpenetrating networks of J61-rich domains embedded in a matrix comprised of a J61/ITIC is seen, reflecting the partial miscibility of J61 with ITIC. These hierarchical nanostructural characteristics are coupled to significantly enhanced exciton dissociation, and further contribute to photocurrent and final device performance.

High-Performance Nonfullerene Polymer Solar Cells based on Imide-Functionalized Wide-Bandgap Polymers.

High-performance nonfullerene polymer solar cells (PSCs) are developed by integrating the nonfullerene electron-accepting material 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophne) (ITIC) with a wide-bandgap electron-donating polymer PTzBI or PTzBI-DT, which consists of an imide functionalized benzotriazole (TzBI) building block. Detailed investigations reveal that the extension of conjugation can affect the optical and electronic properties, molecular aggregation properties, charge separation in the bulk-heterojunction films, and thus the overall photovoltaic performances. Single-junction PSCs based on PTzBI:ITIC and PTzBI-DT:ITIC exhibit remarkable power conversion efficiencies (PCEs) of 10.24% and 9.43%, respectively. To our knowledge, these PCEs are the highest efficiency values obtained based on electron-donating conjugated polymers consisting of imide-functionalized electron-withdrawing building blocks. Of particular interest is that the resulting device based on PTzBI exhibits remarkable PCE of 7% with the thickness of active layer of 300 nm, which is among the highest values of nonfullerene PSCs utilizing thick photoactive layer. Additionally, the device based on PTzBI:ITIC exhibits prominent stability, for which the PCE remains as 9.34% after thermal annealing at 130 °C for 120 min. These findings demonstrate the great promise of using this series of wide-bandgap conjugated polymers as electron-donating materials for high-performance nonfullerene solar cells toward high-throughput roll-to-roll processing technology.

High-performance nonfullerene polymer solar cells based on a fluorinated wide bandgap copolymer with a high open-circuit voltage of 1.04 V

Nonfullerene polymer solar cells based on a polymer donor PM6 containing a fluorinated-thienyl benzodithiophene unit and a small molecule acceptor ITIC showed a PCE of 9.7% with a Voc of up to 1.04 V and an energy loss as low as 0.51 eV.

New M- and V-shaped perylene diimide small molecules for high-performance nonfullerene polymer solar cells

New M- and V-shaped perylene diimide (PDI)-based small molecules using a non-conjugated 1,1-diphenylcyclohexane linker (CP-M and CP-V, respectively) were designed and synthesized as new n-type acceptors for nonfullerene-based polymer solar cells. The blended film with poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT) and CP-V displayed a higher power conversion efficiency of 5.28% due to higher short circuit current and fill factor values.