Wide-bandgap Polymer Research Articles

Synthesis and photovoltaic properties of a simple non-fused small molecule acceptor

Abstract In this work, a new non-fused small molecule acceptor Ph-IC based on alkoxyl benzene and IC units was synthesized for photovoltaic applications. The small molecule Ph-IC has a very simple chemical structure and is easily synthesized. Benefited from the coplanar conjugated structure, the O-H and S-H non-covalent interactions, Ph-IC exhibits a narrow optical bandgap of 1.62 eV and a medium electron mobility of 1.84 × 10−4 cm2V−1s−1. When using Ph-IC as acceptor and a wide-bandgap polymer PBFTZ as donor, the devices show a maximum PCE of 5.68% with Voc of 0.89 V, Jsc of 12.37 mA cm−2 and FF of 51.5% under the illumination of AM 1.5 G at 100 mW/cm2. This work demonstrates that the simple-structured and low cost non-fused group can also be used as building blocks in high performation n-OS acceptors.

A 2-(trifluoromethyl)thieno[3,4-b]thiophene-based small-molecule electron acceptor for polymer solar cell application

A new TbT unit 2-(trifluoromethyl)thieno[3,4-b]thiophene is designed and synthesized for developing small-molecule acceptor. A small molecule acceptor ATT-8 with an indacenodithiophene (ITD) core, two rhodanine ending-groups, and two TbT linkers were constructed. A wide-bandgap polymer donor PBDB-T is selected to fabricate bulk-heterojunction solar cells with ATT-8 acceptor. The as-cast film device delivers a PCE of 4.80% with an open circuit voltage (Voc) of 0.955 V, a short-circuit current (Jsc) of 10.32 mA cm−2 and a fill factor (FF) of 48.71%. Further optimization via additive and thermal annealing (TA) promotes PCE of the device to 7.31% with a Voc of 0.804 V, a Jsc of 13.90 mA cm−2, and an FF of 65.42%.

8.0% Efficient all-polymer solar cells based on novel starburst polymer acceptors

The polymer N2200, with its π-conjugated backbone composed of alternating naphthalene diimide (NDI) and bithiophene (DT) units, has been widely used as an acceptor for all-polymer solar cells (all-PSCs) owing to its high electron mobility and suitable ionization potential and electron affinity. Here, we developed two naphthalene diimide derivatives by modifying the molecular geometry of N2200 through the incorporation of a truxene unit as the core and NDI-DT as the branches. These starburst polymers exhibited absorption spectra and molecular orbital energy levels that were comparable to N2200. These copolymers were paired with the wide-bandgap polymer donor PTzBI-O to fabricate all-polymer solar cells (all-PSCs), which displayed impressive power conversion efficiencies up to 8.00%. The improved photovoltaic performances of all-PSCs based on these newly developed starburst acceptors can be ascribed to the combination of increased charge carrier mobilities, reduced bimolecular recombination, and formation of more favorable film morphology. These findings demonstrate that the construction of starburst polymer acceptors is a feasible strategy for the fabrication of high-performance all-PSCs.

Fusing Benzo[c][1,2,5]oxadiazole Unit with Thiophene for Constructing Wide-bandgap High-performance IDT-based Polymer Solar Cell Donor Material.

Benzo[c][1,2,5]oxadiazole (BO) moiety is a strong electron-withdrawing unit compared to benzo[c][1,2,5]thiadiazole (BT). It is usually introduced as an acceptor to construct narrow band-gap donor-acceptor (D-A) materials. Herein, the π-extended conjugated moiety dithieno[3',2':3,4″;2,3″:5,6]benzo[1,2-c][1,2,5]oxadiazole (BOT) was adopted as the acceptor moiety to design D-A polymers. Considering the more extended π-conjugated molecular system of BOT compared to the BO unit, a narrower optical band-gap is expected for BOT-based IDT polymer (PIDT-BOT). Unexpectedly, the UV-vis absorption spectra of PIDT-BOT films display a great hypochromatic shift of about 60 nm compared to a BO-based analog (PIDT-BO). The optical band-gaps of the materials are broadened from 1.63 eV (PIDT-BO) to 2.00 eV (PIDT-BOT) accordingly. Although the range of external quantum efficiency (EQE) of PIDT-BOT-based polymer solar cell (PSC) devices is not as wide as for PIDT-BO-based devices, the EQE response intensities of the PIDT-BOT based device are evidently high. As a result, PSC devices based on PIDT-BOT reveal the best power conversion efficiency at 6.08%.

Improved Tandem All‐Polymer Solar Cells Performance by Using Spectrally Matched Subcells

AbstractAll‐polymer solar cells (all‐PSCs) are attractive as alternatives to fabricate thermally and mechanically stable solar cells, especially with recent improvements in their power conversion efficiency (PCE). In this work, efficient all‐PSCs with near‐infrared response (up to 850 nm) are developed using newly designed regioregular polymer donors with relatively narrow optical gap. These all‐PSCs systems achieve PCEs up to 6.0% after incorporating fluorine into the polymer backbone. More importantly, these polymers exhibit absorbance that is complementary to previously reported wide bandgap polymer donors. Thus, the superior properties of the newly designed polymers afford opportunities to fabricate the first spectrally matched all‐polymer tandem solar cells with high performance. A PCE of 8.3% is then demonstrated which is the highest efficiency so far for all‐polymer tandem solar cells. The design of narrow bandgap polymers provides new directions to enhance the PCE of emerging single‐junction and tandem all polymer solar cells.

A wide-bandgap polymer based on the alkylphenyl-substituted benzo[1,2-b:4,5-b′]dithiophene unit with high power conversion efficiency of over 11%

A novel wide bandgap polymer PTZP with Eoptg of 2.01 eV was designed and synthesized. PSCs based on PTZP exhibited high PCE of 11.8%. PCEs of over 10% were obtained with an active layer thickness of 200 nm or an area of 0.81 cm2. PTZP was shown to be a promising conjugated polymer for the fabrication of efficient large area PSCs.

Near-infrared absorbing non-fullerene acceptors with selenophene as π bridges for efficient organic solar cells

Two NIR-absorbing non-fullerene acceptors IDT2Se and IDT2Se-4F with selenophene as π bridges displayed high PCEs.

Synthesis and properties of wide bandgap polymers based on tetraphenylsilane and their applications as hosts in electrophosphorescent devices

By introducing the electron-withdrawing diphenylphosphine oxide in PFSCPO, its doped OLEDs achieve enhanced current and external quantum efficiencies than the PFSC-hosted device.

Efficient and thermally stable all-polymer solar cells based on a fluorinated wide-bandgap polymer donor with high crystallinity

Efficient all-PSCs based on PFBZ:N2200 were developed and achieved a high efficiency of 8.1%. The PSCs show high thermal stability and storability.

High-Performance As-Cast Nonfullerene Polymer Solar Cells with Thicker Active Layer and Large Area Exceeding 11% Power Conversion Efficiency.

In this work, a nonfullerene polymer solar cell (PSC) based on a wide bandgap polymer donor PM6 containing fluorinated thienyl benzodithiophene (BDT-2F) unit and a narrow bandgap small molecule acceptor 2,2'-((2Z,2'Z)-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IDIC) is developed. In addition to matched energy levels and complementary absorption spectrum with IDIC, PM6 possesses high crystallinity and strong π-π stacking alignment, which are favorable to charge carrier transport and hence suppress recombination in devices. As a result, the PM6:IDIC-based PSCs without extra treatments show an outstanding power conversion efficiency (PCE) of 11.9%, which is the record value for the as-cast PSC devices reported in the literature to date. Moreover, the device performances are insensitive to the active layer thickness (≈95-255 nm) and device area (0.20-0.81 cm2 ) with PCEs of over 11%. Besides, the PM6:IDIC-based flexible PSCs with a large device area of 1.25 cm2 exhibit a high PCE of 6.54%. These results indicate that the PM6:IDIC blend is a promising candidate for future roll-to-roll mass manufacturing and practical application of highly efficient PSCs.

Novel wide-bandgap copolymer bearing alkylthio-thiophene-substituted benzodithiophene and methyl thiophene-3-carboxylate for highly stable fullerene-free simple polymer solar cells

We successfully synthesized a wide-bandgap p-type copolymer, 3MT-Th-S, bearing alkylthio-substituted benzodithiophene (BDT) as a donor and methyl-3-thiophenecarboxylate (3MT) as a weak electron acceptor. The electron affinity of the sulfur atom in the 2-ethylhexylthio side chains tethered to the thiophene units induces a low-lying highest occupied molecular orbital (HOMO) energy level in the corresponding polymer. 3MT-Th-S displays a strong absorption band from 400 to 600 nm, resulting in a relatively wide optical bandgap. Thin films of 3MT-Th-S exhibit a prominent face-on orientation in the out-of-plane profile, which might be a favorable orientation for moving charge carriers in the vertical direction in polymer solar cells (PSCs). Non-fullerene PSCs based on 3MT-Th-S as the donor and an n-type small molecule (ITIC, ITIC-Me, or IMIDT) were fabricated, and their performance was investigated in detail. Among the PSC devices bearing three different acceptors, the 3MT-Th-S:ITIC-based PSC exhibits the highest power conversion efficiency of 7.86% with a high short-circuit current density of 16.11 mA cm−2 and an open circuit voltage of 0.96 V. The high PCE values of PSC using the as-cast blend films of 3MT-Th-S and ITIC as an active layer without solvent additive are attributed to the low-lying HOMO level of the donor polymer, the complementary absorption behavior of the blend film, the face-on arrangement of polymer chains on the substrate, and the well-formed morphology of the binary blend film. These results clearly demonstrate that 3MT-Th-S is a promising wide-bandgap p-type polymer for high-performance fullerene-free simple PSCs.

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.

Phthalimide-Based Wide Bandgap Donor Polymers for Efficient Non-Fullerene Solar Cells

Organic solar cells (OSCs) have achieved rapid progress, recently, due to the breakthrough of using fused-ring electron acceptors (FREAs), which show broad absorption and narrow bandgap compared to fullerene derivatives. To further improve the device performance of OSCs, it is highly desired to develop suitable donor polymers which feature complementary absorption and favorable energy levels to match the non-fullerene acceptors. We report here the synthesis of two phthalimide-based wide bandgap polymers TPhI-BDT and TffPhI-DBT. The TffPhI-BDT is based on a new electron acceptor unit, difluorophthalimide (ffPhI). The fluorine addition leads to TffPhI-DBT with comparable absorption but lower-lying frontier molecular orbitals versus the non-fluorinated analogue TPhI-BDT. When incorporated into non-fullerene OSCs, polymer TPhI-BDT shows a PCE of 8.31% with a Voc of 0.90 V, a Jsc of 14.07 mA cm–2, and a FF of 66.0%. The fluorine-containing analogue polymer TffPhI-BDT exhibits an improved PCE of 9.48% with a la...

Wide Band Gap Polymer Based on Indacenodithiophene and Acenaphthoquinoxaline for Efficient Polymer Solar Cells Application.

A new wide band gap polymer PIDT-AQx with indacenodithiophene (IDT) as the electron-rich unit and acenaphthoquinoxaline (AQx) as the electron-deficient unit has been designed and synthesized. The optical band gap of PIDT-AQx was 1.81 eV with a HOMO energy level of −5.13 eV. Polymer solar cells with the blend of PIDT-AQx/PC71BM as the active layer achieved a power conversion efficiency (PCE) of 4.56%, with an open-circuit voltage (Voc) of 0.84 V, a current density (Jsc) of 9.88 mA cm−2, and a fill factor (FF) of 55% without any solvent additives and pre- or post-treatments. The photovoltaic performance of PIDT-AQx could be slightly improved with a PCE up to 4.78% after thermal annealing due to enhanced Jsc. The results indicate that acenaphthoquinoxaline is a promising building block for developing conjugated polymers for efficient solar cells application.

Fullerene-Free Organic Solar Cells with an Efficiency of 10.2% and an Energy Loss of 0.59 eV Based on a Thieno[3,4-c]Pyrrole-4,6-dione-Containing Wide Band Gap Polymer Donor.

Although the combination of wide band gap polymer donors and narrow band gap small-molecule acceptors achieved state-of-the-art performance as bulk heterojunction (BHJ) active layers for organic solar cells, there have been only several of the wide band gap polymers that actually realized high-efficiency devices over >10%. Herein, we developed high-efficiency, low-energy-loss fullerene-free organic solar cells using a weakly crystalline wide band gap polymer donor, PBDTTPD-HT, and a nonfullerene small-molecule acceptor, ITIC. The excessive intermolecular stacking of ITIC is efficiently suppressed by the miscibility with PBDTTPD-HT, which led to a well-balanced nanomorphology in the PBDTTPD-HT/ITIC BHJ active films. The favorable optical, electronic, and energetic properties of PBDTTPD-HT with respect to ITIC achieved panchromatic photon-to-current conversion with a remarkably low energy loss (0.59 eV).

Ultrafast Spectroscopic Identification of Hole Transfer in All-Polymer Blend Films of Poly(1-{4,8-bis[5-(2-ethylhexyl)thiophen-2-yl]-benzo[1,2-b:4,5-b′]dithiophen-2-yl}-3-methyl-5-(4-octylphenyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione) and Poly[1,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)

The all-polymer solar cells composed of wide band gap polymer PTP8 as the donor and N2200 as the acceptor exhibit a broad absorbance in the range of 300-900 nm, thanks to the complementary absorption of the near-infrared light by N2200. Although N2200 shows reasonably high electron mobility, the contribution of the photo-generated excitons in N2200 to the power conversion of the PTP8/N2200 solar cell is insignificant. Here, the hole transfer from the N2200 to PTP8 in PTP8/N2200 blend films was investigated by utilizing the ultrafast transient absorption spectroscopy. The spectral fingerprints of the ground state bleaching and the hole polaron-induced absorption of the PTP8 are identified under selective excitation of the N2200 component, which unambiguously indicate the hole transfer from the N2200 to PTP8. The hole transfer is slow (~100 ps), comparable to the geminate exciton recombination rate, consequently limiting the transfer efficiency and carrier generation. The hole-transfer efficiency depends on...

Design and synthesis of low band gap non-fullerene acceptors for organic solar cells with impressively high Jsc over 21 mA cm_2

Three low bandgap non-fullerene acceptors based on thieno[3,2-b]thiophene fused core with different ending groups, named TTIC-M, TTIC, TTIC-F were designed and synthesized. Using a wide bandgap polymer PBDB-T as donor to form a complementary absorption in the range of 300–900 nm, high efficencies of 9.97%, 10.87% and 9.51% were achieved for TTIC-M, TTIC and TTFC-F based photovoltaic devices with impressively high short circuit current over 21 mA cm−2.

High-Efficiency fullerene-free organic solar cell based on P3HT: P(NDI2OD-T2) co-polymers through integrated molecular and interfacial engineering​

We explored the photophysics of an all polymeric solar system based on the combination of the wide bandgap polymers poly(3-hexylthiophene) (P3HT) as donor and low bandgap polymer poly{[N,N-9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)} (P(NDI2OD-T2)) as an acceptor blend active layer in 2-methyl anisole using airbrush spray coating method. Polyethyleneimine ethoxylated (PEIE) is used as the surface modifier and SnO2 as an anode, to minimize the chemical damage of the transparent conducting electrode. The molecular aggregations were investigated by means of absorbance, photoluminescence measurements and atomic force microscopy. The current density–voltage characteristics were examined using an Oriel 1000W solar simulator under the illumination condition through the simulated solar light with 100mWcm−2 (AM 1.5G). The power conversion efficiency (PCE) of about 5.6% is been reported for the photovoltaic device and this outcome indicates that these spray coating method can be a feasible technique when compared with other high-cost vacuum deposition techniques for mass production and low-cost roll to roll based fabrication.

Rational design of perylenediimide-based polymer acceptor for efficient all-polymer solar cells

Abstract Perylenediimide (PDI)-based small molecules have significantly contributed to the development of non-fullerene acceptors, whereas the development of PDI-based polymer acceptors is relatively lagging behind. In this study, we designed and synthesized two PDI-based n-type polymers named as PF-PDI and PBDT-PDI, in which PDI was used as electron-deficient unit and fluorene (F) or benzodithiophene (BDT) were used as electronrich components. The density functional theory (DFT) calculations and grazing incidence wide-angle X-ray scattering (GIWAXS) results indicate that the PF-PDI shows larger steric hindrance and relatively weaker lamellar packing than that of PBDT-PDI. Comparing with PBDT-PDI, PF-PDI shows red-shift absorption and lower-lying HOMO level, which agrees well with the DFT results. A well-known wide bandgap polymer donor, PDBT-T1 was employed to fabricate polymer solar cells (PSCs) with the two acceptors. The all polymer solar cells (all-PSCs) based on PDBT-T1:PF-PDI showed a high power conversion efficiency (PCE) of 4.47%, which is approximately 2-fold larger than that of devices with PDBT-T1:PBDT-PDI (PCE = 2.70%).

Efficient pyrrolo[3,4-c]pyrrole-1,3-dione-based wide band gap polymer for high-efficiency binary and ternary solar cells

Wide band gap pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione (PPD)-based alternating polymer, P(BDTT-ttPPD) was synthesized by combining electron-rich 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDTT) and electron-deficient 4,6-bis(2-bromothieno[3,2-b]thiophen-5-yl)-5-octyl-2-(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,3(2H, 5H)-dione (ttPPD). When the P(BDTT-ttPPD) polymer was applied to binary polymer solar cells (PSCs) as a donor material, the device exhibited a maximum power conversion efficiency (PCE) of 6.93% with an open-circuit voltage (Voc) of 0.87 V, a short-circuit current (Jsc) of 11.38 mA/cm2, and a fill factor (FF) of 70%. The photovoltaic performance was further improved to 9.08% (Voc ∼ 0.80 V, Jsc ∼ 16.05 mA/cm2, and FF ∼ 71%) when the polymer was used as a donor component for absorbing high-energy light in ternary PSCs with a blend of P(BDTT-ttPPD):PTB7-Th:PC70BM. The overall PCE of ternary PSCs was found to be higher than those of the binary PSCs made form P(BDTT-ttPPD) or PTB7-Th. We discuss the property modulation of PPD-based polymers via the replacement of thiophene π–spacer units with thieno[3,2-b]thiophene.