Wide-bandgap Polymer Donor Research Articles

Design of a New Fused-Ring Electron Acceptor with Excellent Compatibility to Wide-Bandgap Polymer Donors for High-Performance Organic Photovoltaics.

Fused-ring electron acceptors (FREAs) have recently received intensive attention. Besides the continuing development of new FREAs, the demand for FREAs featuring good compatibility to donor materials is becoming more and more urgent, which is highly desirable for screening donor materials and achieving new breakthroughs. In this work, a new FREA is developed, ZITI, featuring an octacyclic dithienocyclopentaindenoindene central core. The core is designed by linking 2,7-dithienyl substituents and indenoindene with small methylene groups, in which the indeno[1,2-b]thiophene-2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile part provides a large and unoccupied π-surface. Most notably, ZITI possesses an excellent compatibility with commercially available polymer donors, delivering very high power conversion efficiencies of over 13%.

Silaindacenodithiophene‐Based Fused‐Ring Non‐Fullerene Electron Acceptor for Efficient Polymer Solar Cells

AbstractIn this work, a new A‐D‐A type nonfullerene small molecular acceptor SiIDT‐IC, with a fused‐ring silaindacenodithiophene (SiIDT) as D unit and 2‐(3‐oxo‐2,3‐dihydroinden‐1‐ylidene)malononitrile (INCN) as the end A unit, was design and synthesized. The SiIDT‐IC film shows absorption peak and edge at 695 and 733 nm, respectively. The HOMO and LUMO of SiIDT‐IC are of −5.47 and −3.78 eV, respectively. Compared with carbon‐bridging, the Si‐bridging can result in an upper‐lying LUMO level of an acceptor, which is benefit to achieve a higher open‐circuit voltage in polymer solar cells (PSCs). Complementary absorption and suitable energy level alignment between SiIDT‐IC and wide bandgap polymer donor PBDB‐T were found. For the PBDB‐T:SiIDT‐IC based inverted PSCs, a D/A ratio of 1: 1 was optimal to achieve a power conversion efficiency (PCE) of 7.27%. With thermal annealing (TA) of the blend film, a higher PCE of 8.16% could be realized due to increasing of both short‐circuit current density and fill factor. After the TA treatment, hole and electron mobilities were elevated to 3.42 × 10−4 and 1.02 × 10−4 cm2·V−1·s−1, respectively. The results suggest that the SiIDT, a Si‐bridged fused ring, is a valuable D unit to construct efficient nonfullerene acceptors for PSCs.

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.

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.

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.

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.

Polymer Solar Cells: Eco‐Friendly Solvent‐Processed Fullerene‐Free Polymer Solar Cells with over 9.7% Efficiency and Long‐Term Performance Stability (Adv. Energy Mater. 19/2017)

In article number 1700566, Min Ju Cho, Dong Hoon Choi, and co-workers report a new conjugated widebandgap donor polymer, 3MT-Th, harmonized with an ITIC acceptor to enable the production of a polymer solar cell (PSC) with high efficiency of 9.73% under eco-friendly conditions using a non-halogenated solvent. This PSC also exhibits excellent shelf-life stability in air and good operational stability under continuous light illumination.

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).

A new small molecule acceptor based on indaceno[2,1-b:6,5-b’]dithiophene and thiophene-fused ending group for fullerene-free organic solar cells

A new small molecule acceptor of IDTCN with indaceno[2,1-b:6,5-b’]dithiophene as electron-donating core and CPTCN (2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-lidene)malononitrile) as electron-withdrawing end-capping group was synthesized and investigated in organic solar cells. The IDTCN shows strong and broad absorption with a high extinction coefficient and appropriate energy levels matching with wide bandgap donor polymer. The best device based on PBT1-EH:IDTCN exhibits an open circuit voltage (Voc) of 0.93 V, a short circuit current (Jsc) of 13.31 mA/cm2, a FF of 0.702, achieving a PCE of 8.69%.

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%).

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.

Two Well-Miscible Acceptors Work as One for Efficient Fullerene-Free Organic Solar Cells.

High-performance ternary organic solar cells are fabricated by using a wide-bandgap polymer donor (bithienyl-benzodithiophene-alt-fluorobenzotriazole copolymer, J52) and two well-miscible nonfullerene acceptors, methyl-modified nonfullerene acceptor (IT-M) and 2,2'-((2Z,2'Z)-((5,5'-(4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydros-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(4-((2-ethylhexyl)oxy)thiophene-5,2-diyl))bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IEICO). The two acceptors with complementary absorption spectra and similar lowest unoccupied molecular orbital levels show excellent compatibility in the blend due to their very similar chemical structures. Consequently, the obtained ternary organic solar cells (OSC) exhibits a high efficiency of 11.1%, with an enhanced short-circuit current density of 19.7 mA cm-2 and a fill factor of 0.668. In this ternary system, broadened absorption, similar output voltages, and compatible morphology are achieved simultaneously, demonstrating a promising strategy to further improve the performance of ternary OSCs.

A Wide-Bandgap Donor Polymer for Highly Efficient Non-fullerene Organic Solar Cells with a Small Voltage Loss.

To achieve efficient non-fullerene organic solar cells, it is important to reduce the voltage loss from the optical bandgap to the open-circuit voltage of the cell. Here we report a highly efficient non-fullerene organic solar cell with a high open-circuit voltage of 1.08 V and a small voltage loss of 0.55 V. The high performance was enabled by a novel wide-bandgap (2.05 eV) donor polymer paired with a narrow-bandgap (1.63 eV) small-molecular acceptor (SMA). Our morphology characterizations show that both the polymer and the SMA can maintain high crystallinity in the blend film, resulting in crystalline and small domains. As a result, our non-fullerene organic solar cells realize an efficiency of 11.6%, which is the best performance for a non-fullerene organic solar cell with such a small voltage loss.

Highly Efficient Parallel-Like Ternary Organic Solar Cells

Ternary bulk heterojunction (BHJ) blends have been demonstrated as a promising approach to increase the power conversion efficiencies (PCEs) of organic solar cells. Currently, most studies of ternary organic solar cells are based on blends of two donors and one acceptor, because of the limitation in acceptor materials. Here, we report that high-performance ternary solar cells have been fabricated with a wide-bandgap polymer donor (PDBT-T1) and two acceptor materials, phenyl-C70-butyric acid methyl ester (PC70BM), and nonfullerene acceptor (ITIC-Th). The addition of ITIC-Th into the BHJ blends dramatically increases the light absorption. Consequently, the champion ternary solar cell shows a high PCE of ∼10.5%, with an open-circuit voltage (Voc) of 0.95 V, a short-circuit current (Jsc) of 15.60 mA/cm2, and a fill factor (FF) of 71.1%, which largely outperforms their binary counterparts. Detailed studies reveal that the ternary solar cells work in a parallel-like device model (ITIC-Th and PC70BM form their o...

Single-Junction Binary-Blend Nonfullerene Polymer Solar Cells with 12.1% Efficiency.

A new fluorinated nonfullerene acceptor, ITIC-Th1, has been designed and synthesized by introducing fluorine (F) atoms onto the end-capping group 1,1-dicyanomethylene-3-indanone (IC). On the one hand, incorporation of F would improve intramolecular interaction, enhance the push-pull effect between the donor unit indacenodithieno[3,2-b]thiophene and the acceptor unit IC due to electron-withdrawing effect of F, and finally adjust energy levels and reduce bandgap, which is beneficial to light harvesting and enhancing short-circuit current density (JSC ). On the other hand, incorporation of F would improve intermolecular interactions through CF···S, CF···H, and CF···π noncovalent interactions and enhance electron mobility, which is beneficial to enhancing JSC and fill factor. Indeed, the results show that fluorinated ITIC-Th1 exhibits redshifted absorption, smaller optical bandgap, and higher electron mobility than the nonfluorinated ITIC-Th. Furthermore, nonfullerene organic solar cells (OSCs) based on fluorinated ITIC-Th1 electron acceptor and a wide-bandgap polymer donor FTAZ based on benzodithiophene and benzotriazole exhibit power conversion efficiency (PCE) as high as 12.1%, significantly higher than that of nonfluorinated ITIC-Th (8.88%). The PCE of 12.1% is the highest in fullerene and nonfullerene-based single-junction binary-blend OSCs. Moreover, the OSCs based on FTAZ:ITIC-Th1 show much better efficiency and better stability than the control devices based on FTAZ:PC71 BM (PCE = 5.22%).

A novel wide bandgap conjugated polymer (2.0 eV) based on bithiazole for high efficiency polymer solar cells

An efficient wide bandgap conjugated polymer (PTZ6) based on alkoxylphenyl substituted benzodithiophene as donor unit and bithiazole as acceptor unit was developed for polymer solar cells (PSCs). The polymer exhibited a wide bandgap of 2.0eV with strong absorption in the range of 300–620nm, and a low-lying highest occupied molecular orbital (HOMO) energy level of −5.36eV. The PSCs based on PTZ6: PC71BM show a PCE of 8.1% with a Voc of 0.96V, a Jsc of 10.9mAcm−2 and a high FF of 76.7%, which is among the highest values for the fullerene PSCs based on conjugated polymer donors with bandgap near to 2.0eV. Moreover, for this blend system, the photovoltaic performance of the devices changes little when the active layer thickness increases from 90nm to 220nm. More importantly, the non-fullerene PSCs based on PTZ6: ITIC exhibit a PCE of 10.3% with a high Voc of 1.01V, which should be the best value for the non-fullerene PSCs with the Eloss less than 0.6eV to date. Our results indicate that PTZ6 is a promising wide bandgap polymer donor for the photovoltaic application in PSCs.

Molecular design of a wide-band-gap conjugated polymer for efficient fullerene-free polymer solar cells

Two wide band gap polymer donors were developed for fullerene-free polymer solar cells. The photovoltaic performance of the PB3T:IT-M device processed by anisole achieved a high PCE of 11.9%.

Side-chain engineering for efficient non-fullerene polymer solar cells based on a wide-bandgap polymer donor

Non-fullerene polymer solar cells based on a wide-bandgap polymer, PSBZ, exhibited a PCE of up to 10.5% with a high Jsc of 19.0 mA cm−2.