Fullerene-free Organic Solar Cells Research Articles

Development of n-Type Porphyrin Acceptors for Panchromatic Light-Harvesting Fullerene-Free Organic Solar Cells.

The development of n-type porphyrin acceptors is challenging in organic solar cells. In this work, we synthesized a novel n-type porphyrin acceptor, PZn-TNI, via the introduction of the electron withdrawing naphthalene imide (NI) moiety at the meso position of zinc porphyrin (PZn). PZn-TNI has excellent thermal stability and unique bimodal absorption with a strong Soret band (300–600 nm) and weak Q-band (600–800 nm). The weak long-wavelength absorption of PZn-TNI was completely covered by combining the low bandgap polymer donor, PTB7-Th, which realized the well-balanced panchromatic photon-to-current conversion in the range of 300–800 nm. Notably, the one-step reaction of the NI moiety from a commercially available source leads to the cheap and simple n-type porphyrin synthesis. The substitution of four NIs in PZn ring induced sufficient n-type characteristics with proper HOMO and LUMO energy levels for efficient charge transport with PTB7-Th. Fullerene-free organic solar cells based-on PTB7-Th:PZn-TNI were investigated and showed a promising PCE of 5.07% without any additive treatment. To the best of our knowledge, this is the highest PCE in the porphyrin-based acceptors without utilization of the perylene diimide accepting unit.

Conjugated Polymers Containing Sulfonic Acid Fluorene Unit for Achieving Multiple Interfacial Modifications in Fullerene-free Organic Solar Cells

Conjugated polymers (CPs) as interfacial layer materials have played a crucial role in improving the power conversion efficiency (PCE) of organic solar cells (OSCs). Sulfonic acid-functionalized fl...

Polyamino acid interlayer facilitates electron extraction in narrow band gap fullerene-free organic solar cells with an outstanding short-circuit current

In this communication, we present a study to investigate interfacial characteristics modulated by electron extraction layers (EEL) comprising of a polyamino acid α-poly-L-lysine (α-PLL) in fullerene-free organic solar cells (OSC) based on narrow-band-gap PTB7-Th:IEICO-4 heterojunctions. Benefitting from effective interfacial modifications with the α-PLL EEL, PTB7-Th:IEICO-4F solar cells receive a considerable boost in the short-circuit current (26.6 mA/cm2) and ultimate power conversion efficiencies exceeding 12%, which is a result of promoted charge extraction. Aided by combinatorial analyses using electron spin resonance spectroscopy, angle-resolved X-ray photoelectron spectroscopy together with charge transport measurements, we found that the interfacial doping occurs at the α-PLL/IEICO-4F interface, possibly due to the low steric hindrance of the bared primary amines and non-conjugated backbones in α-PLL. The interface strategy with α-PLL displays a universal applicability, enabling to produce a series of high efficiency PCBM or non-fullerene OSCs with different device architectures. This work enriches fundamental insights into key interfacial processes and provides a promising pathway utilizing wide-accessible biomaterials to attain high performance OSCs with emerging fullerene-free photovoltaic materials.

N-Type core effect on perylene diimide based acceptors for panchromatic fullerene-free organic solar cells

Perylene diimide (PDI) based high bandgap acceptors, DTBTP, DTF2BTP, and DTF2TZP, are synthesized for use in fullerene-free organic solar cells. The two PDI rings are connected to the end of the n-type core, forming a PDI-n-type core-PDI structure. Several n-type core materials, 4,7-dithieno-2,1,3-benzothiadiazole (DTBT), 5,6-difluoro-4,7-dithieno-2,1,3-benzothiadiazole (DTF2BT), and 4,6-difluoro-2H-benzo[d][1,2,3]triazole (DTF2TZ), are incorporated in the PDI acceptors and the n-type core effect on photovoltaic properties is studies. The introduction of alkyl side chains onto the core structure weakened the intermolecular interaction, whereas fluorination of the core structure improved the backbone planarity and intermolecular ordering. DTF2BTP having a planar core structure without bulky alkyl chains yielded the best power conversion efficiency, 4.41%, when mixed with PTB7-Th donor. The n-type core structure was beneficial in terms of increasing the electron accepting properties and the absorption in the high bandgap region of non-fullerene acceptors.

A perylenediimide dimer containing an asymmetric π-bridge and its fused derivative for fullerene-free organic solar cells

Recent success stories of perylenediimide (PDI) derivatives have stimulated tremendous interest.

Hydrogen bond induced high performance ternary fullerene-free organic solar cells with increased current density and enhanced stability

We demonstrated that introducing hydrogen bonds is a promising strategy to fabricate high performance and stable organic solar cells.

Fluorination increases the electron mobility of zinc azadipyrromethene-based electron acceptors and enhances the performance of fullerene-free organic solar cells

Strategic fluorination of non-planar electron acceptors reduces bimolecular recombination in OPVs and significantly enhances the electron mobility to ∼10−3 cm2 V−1 s−1 in diodes.

A novel thiazole based acceptor for fullerene-free organic solar cells

In this work, an electron-withdrawing thiazole ring is used as a bridged unit in the design of the small molecular electron acceptor. The new electron acceptor, namely DC-IDT2Tz, was designed with indacenodithiophene as core and 1,1-dicyanomethylene-3-indanone as end-capped unit, and thiazole ring as spacers. Compared with the thiophene spacer, the thiazole bridge exhibited blue-shifted absorption and deep energy levels, which provides more complementary absorption and large diving-force for the exciton dissociation. Besides it has remained some advantages of the thiophene linked acceptors, such as strong and board absorption in visible light region and planate backbone. The device based on PTB7-Th as the donor material and DC-IDT2Tz as the acceptor material exhibited a high PCE of 5.81%.

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).

Influence of the replacement of alkoxyl with alkylthienyl on photovoltaic properties of two small molecule donors for organic solar cells

Two benzo[1,2-b:4,5-b ¢ ]dithiophene (BDT)-based small molecule (SM) donor materials with identical conjugated backbones but different substitution groups, named as DRTB-O and DRTB-T, were well explored to demonstrate the influence of the replacement of alkoxy with alkylthienyl on their photovoltaic properties in fullerene-based and fullerene-free organic solar cells (OSCs). The study shows that the two SM donors possess similar absorption spectra and energy levels but different crystalline structures in solid films. The carrier transport property and phase separation morphologies of the blend films have also been fully investigated. By employing PC71BM as the acceptor, the power conversion efficiency (PCE) of DRTB-O:PC71BM and DRTB-T:PC71BM based devices were 4.91% and 7.08%, respectively. However, by blending with IDIC, the two SM donors exhibited distinctly different photovoltaic properties in fullerene-free OSCs, and the PCE of DRTB-O:IDIC and DRTB-T:IDIC based devices were 0.15% and 9.06%, respectively. These results indicate that the replacement of alkoxyl with alkylthienyl in designing SM donor materials plays an important role in the application of fullerene-free OSCs.

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%.

PDI Derivative through Fine-Tuning the Molecular Structure for Fullerene-Free Organic Solar Cells.

A perylenediimide-based (PDI-based) small molecular (SM) acceptor with both an extended π-conjugation and a three-dimensional structure concurrently is critical for achieving high-performance PDI-based fullerene-free organic solar cells (OSCs). Herein, a novel PDI-based SM acceptor has been successfully synthesized through fusing PDI units with a spiro core 4,4'-spirobi[cyclopenta[2,1-b;3,4-b']dithiophene (SCPDT) together via β-position coupling with thiophene bridges. An enhanced absorption from 350 to 520 nm has been observed. Moreover, compared with previously reported acceptor SCPDT-PDI4, in which the PDI units and SCPDT are not fused together, the LUMO energy level of FSP (the new SCPDT-based molecule) increases. OSCs containing PTB7-Th as a donor and FSP as an acceptor have been demonstrated to show an excellent performance with a power conversion efficiency as high as 8.89%. This result might be attributed to the efficient and complementary photoabsorption, balanced carrier mobilities, and favorable phase separation in the blend film. This research could offer an effective strategy to design novel high-performance PDI-based acceptors.

2D/1A Strategy to Regulate Film Morphology for Efficient and Stable Nonfullerene Organic Solar Cells

Recently, the ternary blend method has been successfully applied to nonfullerene organic solar cells (OSCs) and enhanced the device performance by utilizing complementary optical absorption. Here we demonstrate the two polymer donors and one small-molecule acceptor (i.e., 2D/1A) strategy to finely regulate the blend film morphology in fullerene-free OSCs. One crystalline polymer donor, PffBT4T–2OD, can act as an effective morphology regulator for a benchmark blend of PTB7–Th and ITIC, leading to appropriate phase-separated morphology, suppressed charge recombination, efficient charge transport and high carrier mobility. The resulting solvent additive- and annealing-free fabricated bulk-heterojunction OSCs show the best power conversion efficiency (PCE) of 8.22% with a significant increase of fill factor compared to their binary counterparts. Importantly, such ternary OSCs when processed under ambient condition retain excellent device performance with a PCE of 7.57%, indicative of good air-stability.

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.

Environmentally-friendly solvent processed fullerene-free organic solar cells enabled by screening halogen-free solvent additives

Though the power conversion efficiencies (PCEs) of organic solar cells (OSCs) have been boosted to 12%, the use of highly pollutive halogenated solvents as the processing solvent significantly hinders the mass production of OSCs. It is thus necessary to achieve high-efficiency OSCs by utilizing the halogen-free and environmentally-friendly solvents. Herein, we applied a halogen-free solvent system ( o -xylene/1-phenylnaphthalene, XY/PN) for fabricating fullerene-free OSCs, and a high PCE of 11.6% with a notable fill factor (FF) of 72% was achieved based on the PBDB-T:IT-M blend, which is among the top efficiencies of halogen-free solvent processed OSCs. In addition, the influence of different halogen-free solvent additives on the blend morphology and device performance metrics was studied by synchrotron-based tools and other complementary methods. Morphological results indicate the highly ordered molecular packing and highest average domain purity obtained in the blend films prepared by using XY/PN co-solvent are favorable for achieving increased FFs and thus higher PCEs in the devices. Moreover, a lower interaction parameter ( χ ) of the IT-M:PN pair provides a good explanation for the more favorable morphology and performance in devices with PN as the solvent additive, relative to those with diphenyl ether and N -methylpyrrolidone. Our study demonstrates that carefully screening the non-halogenated solvent additive plays a vital role in realizing the efficient and environmentally-friendly solvent processed OSCs.

Molecular Optimization Enables over 13% Efficiency in Organic Solar Cells.

A new polymer donor (PBDB-T-SF) and a new small molecule acceptor (IT-4F) for fullerene-free organic solar cells (OSCs) were designed and synthesized. The influences of fluorination on the absorption spectra, molecular energy levels, and charge mobilities of the donor and acceptor were systematically studied. The PBDB-T-SF:IT-4F-based OSC device showed a record high efficiency of 13.1%, and an efficiency of over 12% can be obtained with a thickness of 100-200 nm, suggesting the promise of fullerene-free OSCs in practical applications.

Molecular Engineering Strategy for High Efficiency Fullerene-Free Organic Solar Cells Using Conjugated 1,8-Naphthalimide and Fluorenone Building Blocks.

We designed, synthesized, and characterized a series of novel electron deficient small molecule nonfullerene acceptors based on 1,8-naphthalimide (NAI) and 9-fluorenone (FN) with different branched alkyl chains using various techniques. These molecules are based on an acceptor-donor-acceptor-donor-acceptor (A1-D-A2-D-A1) molecular design configuration with NAI as the end-capping acceptor (A1), FN as electron-withdrawing central (A2) group, and thiophene ring as a donor (D) unit. These materials are named as NAI-FN-NAI (BO) and NAI-FN-NAI (HD) where BO and HD represent butyloctyl and hexyldecyl alkyl groups, respectively. To further modify energy levels of these materials, we converted the weak electron withdrawing ketonic group (C═O) attached to the FN moiety of NAI-FN-NAI (BO) to a stronger electron withdrawing cyano group (C≡N) to obtain the compound NAI-FCN-NAI (BO) by keeping the same alkyl chain. The optical, electrochemical, and thermal properties of the new acceptors were studied. The materials exhibited higher to medium band gaps, low lowest unoccupied molecular orbital (LUMO) energy levels, and highly thermally stable properties. Organic solar cell devices employing conventional poly(3-hexylthiophene) (P3HT) a donor polymer and the newly designed small molecules as the acceptor were investigated. Among all new materials, organic solar cell devices based on NAI-FN-NAI (BO) as an acceptor exhibit the highest performance with an open circuit voltage (VOC) of 0.88 V, a short-circuit current density (JSC) of 9.1 mAcm-2, a fill factor (FF) of 45%, and an overall power conversion efficiency (PCE) of 3.6%. This is the first report of 9-fluorenone based nonfullerene acceptor with P3HT donor in organic solar cell devices with such a promising performance.

Fullerene-Free Organic Solar Cells with Efficiency Over 12% Based on EDTA–ZnO Hybrid Cathode Interlayer

Interlayer materials play an important role in the rapid development of organic solar cells (OSCs). State-of-the-art interlayers in fullerene OSCs may be not the best choice for fullerene-free OSCs, thus new interlayers need to be developed. Here, we report high performance fullerene-free OSCs with efficiency over 12% by using low temperature (150 oC) solution processed EDTA-ZnO hybrid interlayer. The performance improvement can be attributed to the dual nature of EDTA. First, EDTA itself shows good ability in interface modification as an interlayer, leading to 10% device efficiency. Second, when combined with ZnO, EDTA can passivize defects in ZnO film due to its chelation function, leading to balanced hole and electron mobility in OSCs. As a result, high internal quantum efficiency (IQE) approaching 100% is achieved in the range from 480 nm to 720 nm. The remarkable IQE means that in OSCs with EDTA-ZnO, every absorbed photon can generate a separated pair of charged carriers and that almost all photo-gen...

Non-fullerene organic solar cells with 7% efficiency and excellent air stability through morphological and interfacial engineering

We report on the fabrication of the poly{[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]} (PTB7) and 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)) active layer combination employing air brush spray coating technique in 2-methyl anisole. Optical absorption characteristics of the blend layer were examined utilizing UV–visible spectra in the wavelength sweep varying from 300 to 900 nm. Atomic force microscopy was utilized to analyze the surface characteristics of the fabricated active layer. Under the radiance of simulated solar light with 100 mW cm−2 (AM 1.5G), the current density voltage (J-V) characteristics were determined by employing a solar simulator. Fullerene-free organic solar cells were build using a combination of P(NDI2OD-T2) acceptor and a polymer donor PTB7 with SnO2 acting as an interlayer, which showed power conversion efficiency (PCEs) of more than 7.0%, which is considered as the best PCEs been reported for the chosen donor and the acceptor. The device is extremely stable, holding 75% of its unique effectiveness subsequent to being put away in air for 72 days even without encapsulation. These outcomes demonstrate that the spray-coated film is a feasible contrasting option to the vacuum-deposited ITO film in terms of cost for mass production and for roll-to-roll based organic solar cells.

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.