Higher LUMO Energy Research Articles

Acceptor molecules with different central and terminal units for P3HT based solar cells: Effects of the molecular structures on their properties

The design strategy of acceptor materials matched with poly(3-hexylthiophene) (P3HT) is highly desired for the development of low-cost organic solar cells (OSCs). Herein, we introduced strong electron-donating central units and weak electron-withdrawing end groups to construct the acceptors with high HOMO and LUMO, modulated the molecular symmetry and conjugation length by changing the structures of central units, and synthesized six acceptor molecules, namely, TDTC-O2F, TTDTC-O2F, TTTDTC-O2F, TDTC-ThBr, TTDTC-ThBr and TTTDTC-ThBr. The high LUMO energy level of the acceptors endowed the P3HT based OSCs with relatively high Voc values. Molecular symmetry, conjugation length of central units and the terminal structure jointly affect the molecular crystallinity and miscibility of the blends. Owing to the proper film morphology, the P3HT:TTDTC-ThBr based devices achieved the champion power conversion efficiency of 6.15 %.

Synthesis and Properties of a Fused Antiaromatic Hydrocarbon By Oxidative Transformation of Carbacorrole Analogue

Polycyclic aromatic hydrocarbons (PAHs) containing fused pyrrole moieties are known as unique nitrogen-doped pi-conjugated molecules, e.g., azacoronenes, which possess a large π-conjugated planar structure and show rich redox properties. The related molecules are thus expected to be applied to redox-responsible optoelectronic materials. Furthermore, PAH-based compounds containing [4n]π-electron antiaromatic segments, such as pentalene and indacene are contributed to the ambipolar semiconducting material applications due to the distinct high HOMO and low LUMO energy levels.In this work, we have synthesized and characterized a novel N-doped poly-antiaromatic hydrocarbon (PAH) by the oxidative transformation of a dibenzocarbacorrole isomer, namely, dibenzocorrorin (1). X-ray crystal structure of the fused product (2) revealed a highly planar pi-conjugated structure containing a unique diaza-dicyclopentaazulene moiety. The distinct 12pi antiaromatic resonance structure led to the emergence of a near-infrared band beyond 1000 nm in the absorption spectrum and facile redox properties determined in the cyclic voltammetry. Furthermore, crystalline compound 2 showed carrier mobility in the single-crystal organic field effect transistor.

Organic solar cells based on non-fullerene acceptors containing thiophene [3,2-b] pyrrole

In this work, thiophene[3,2-b] pyrrole (TP) was introduced to construct two novel fused-ring conjugated non-fullerene acceptors (NFAs), named as PTBTP-2F and PTBTP-4F. By fusing two pyrrole rings on both sides of an indacenodithiophene (IDT) unit, a brand-new central core based on TP block (denoted as PTBTP) was achieved to serve as the D core to construct an A-D-A type NFA together with two accepting units (IC–F and IC-2F). Benefitting from the nitrogen atom in the pyrrole ring, both PTBTP-2F and PTBTP-4F exhibited great film forming property as well as high LUMO energy levels of −4.09 eV and −4.11 eV for PTBTP-2F and PTBTP-4F, respectively. Since IC-2F presents stronger electron-withdrawing capability than IC-F, PTBTP-4F demonstrated red-shifted absorptions in solution/film in comparison with PTBTP-2F. When blending with donor PBDB-T, the PTBTP-2F fabricated devices achieved an average power conversion efficiency (PCE) of 11.14% with a VOC of 0.91 V, a JSC of 17.89 mA cm−2 and an FF of 68.66%, while the PTBTP-4F fabricated devices afforded a PCE of 12.33% with a VOC of 0.86 V, a JSC of 20.74 mA cm−2 and an FF of 69.02%.

High-performance ternary solar cells by introducing a medium bandgap acceptor with complementary absorption, reducing energy disorder and enhancing glass transition temperature

By introducing TPIIC – novel NFA with a high LUMO energy, efficient light absorption in visible region, and high crystallinity into PM6:Y6 binary system as the third component, all photovoltaic parameters were improved resulting in PCE of 17.7%.

Synchronous construction of a porous intramolecular D-A conjugated polymer via electron donors for superior photocatalytic decontamination

The development of conjugated polymers with intramolecular donor–acceptor (D-A) units has the capacity to enhance the photocatalytic performance of carbon nitride (g-C3N4) for the removal of antibiotics from ambient ecosystems. This strategy addresses the challenge of narrowing the band gap of g-C3N4 while maintaining its high LUMO position. For this study, we introduced the above donor units into g-C3N4 to construct intramolecular D-A structures through the copolymerization of dicyandiamide with creatinine, which strategically extended light absorption into the green region and expedited photoelectron separation. The introduction of electron donor blocks kept the LUMO distributed on the melem, which maintained the high LUMO energy level of the copolymer with the potential to generate oxygen radicals. The as-prepared porous D-A conjugated polymer enhanced the photocatalytic degradation of sulfisoxazole with kinetic constants 5.6 times higher than that of g-C3N4 under blue light and 15.3 times higher under green light. Furthermore, we surveyed the degradation mechanism including the effective active species and degradation pathways. This study offers a new perspective for the synchronous construction of a porous intramolecular D-A conjugated polymer to enhance water treatment and environmental remediation capacities.

Development of a new family of a hole transport layer based on Pm1−xZnxTe QDs for flexible fluorine derivative solar cell

Fluorine derivative solar cells (FDSCs) are considered a promising candidate for next-generation photovoltaics because they hold promise for the realization of lightweight and flexible devices that can be fabricated by room-temperature solution approach. Herein, a new family of hole transport materials with high LUMO and HOMO energy levels was prepared. A series of Pm1−xZnxTe QDs were synthesized via N-naphthyl-N-phenylaminobiphenyl assisted colloidal approach and used as an effective hole transport layer of fluorine derivative solar cells. The use of Pm1−xZnxTe QDs brings unprecedentedly high voltage of 0.78 V, which is over 25% higher than that of 0.6 V for devices based on NiO and PEDOT:PSS. The device shows a remarkable power conversion efficiency of 7.8%. The enhanced photovoltaic performance by using the Pm1−xZnxTe QDs as HTL into FDSCs should be ascribed to the fast dissociation of the photoinduced excitons, which significantly reduces interface carriers recombination. This novel HTL may pave the way for the realization of lightweight and mechanically flexible solar cell devices with high power conversion efficiency.

Influence of the dipole moment on the photovoltaic performance of polymer solar cells employing non-fullerene small molecule acceptor

A new narrow bandgap non-fullerene acceptor with a DPP core and dicyanorhodanine terminals linked by selenophene spacers has been synthesized. Optical and electrochemical properties of MPU5 were examined and compared to that of its comparable MPU2, which contains thiophene linkers instead of selenophene. MPU5 has a lower optical bandgap (1.38 eV) than MPU2 (1.41 eV), this may be due to the higher electron richness and lower aromatici character of selenophene, comparative to thiophene, favoring a quinoidal structure and intramolecular charge transfer between push–pull moieties. MPU5 was employed as the electron acceptor in conjunction with a conjugated polymer (PCDTBT) as donor with a complementary absorption (350–650 nm) and high HOMO and low LUMO energy levels (–5.4 eV and –3.6 eV, respectively) for the fabrication of polymer solar cells using a bulk heterojunction active layer. The MPU5-based polymer solar cell enabled an overall power conversion efficiency (PCE) of 10.10% with an Eloss of 0.43 eV with an optimized bulk heterojunction active layer. These values are better than those for the MPU2 counterpart (PCE = 8.04%, Eloss = 0.56 eV). The higher PCE obtained for the MPU5-based polymer can be correlated with better balanced charge transport, appropriate active layer nanoscale morphology, efficient charge extraction and longer charge carrier lifetime, resulting in an improvement in the Jsc and FF values. The superior Voc value for the MPU5-based polymer may be related to the lower energy loss caused by the larger dipole moment and dielectric constant of MPU5, which result in a smaller exciton binding energy in the active layer.

Biologically active dihydropyridines: An efficient green synthesis, antimicrobial properties, machine aided results and SARs

In the present study, by using green solvent and ecofriendly catalyst, a series of dihydropyridine and their analogs were synthesized. The total synthetic path proceeded under ecofriendly environment and a reusable heterogeneous catalyst was utilized, consequently, products formed with good yield. The prepared compounds examined using proton NMR, carbon NMR and mass spectroscopies studies. In addition, all final moieties were screened for their microbial resistance properties against four bacterial pathogens and two fungi. Most of the products demonstrated higher to moderate antimicrobial actions. Compounds 5e and 5n were noted the prominent microbial remedial behavior. Further, compounds electronic parameters such as HOMO energy, LUMO energy, their energy gap and other were calculated to investigate any structure activity relationships (SARs). There was some correlation existed between the simulations results and biological results. From SARs, the high LUMO energy value contained compound showed high antimicrobial property vice versa. Thus, the synthetic method in this research may judge as a model for further environmentally benign reactions and existed biological consequences stands for additional studies interrelated to biotic drugs. On the other hand, simulation results also an important descriptor to support the biological experiments.

Non-fullerene electron acceptors constructed by simple electron-withdrawing core: Distinct effect of bithiazole vs thiazolothiazole core on photovoltaic properties

Developing simple-structure and high-efficiency non-fullerene small molecule acceptors (NF-SMAs) are crucial for application of organic solar cells (OSCs). To deeply understand the effect of electron-withdrawing central building block on photo-electronic properties and better design simple-structure NF-SMAs, in this work, a novel simple SMA of BTz1 with an A (π-A′-A″)2 framework was synthesized and characterized, in which a unfused-ring weak acceptor (A) of bithiazole and two adjacent strong acceptors of A′-A″ are used as molecular central and terminal units, respectively. And the UV absorption, energy level, crystallinity, mobility, morphology and photovoltaic per-formance of BTz1 were studied systematically. It is found that BTz1 exhibits similar molecular conformations and photophysical properties, but a low HOMO and high LUMO energy level in comparison with its analogue of TTz1 based on thiazolothiazole center. Furthermore, significant different photovoltaic properties are observed in the BTz1 and TTz1 based PSCs using polymer J71 as donor in an inverted device structure. The BTz1-based OSCs exhibit an ascending open-circuit voltage (Voc) of 0.96 V, but TTz1-based devices display a significantly increasing power conversion efficiency (PCE) of 8.77%. The better film morphology of photoactive layer makes the TTz1-based OSCs exhibit higher PCE than the BTz1-based OSCs. This work indicates that tuning electron-withdrawing central structure is crucial to construct simple-structure and high-efficiency NF-SMAs.

Dibenzofuran derivatives with meta- and para-triphenylamine substituents as hole-transporting materials in organic light-emitting devices

Three novel hole-transporting materials, 3,3'-(dibenzo[b,d]furan-2,8-diyl)bis(N,N-diphenylaniline) (BF-m-TPA), 4,4'-(dibenzo[b,d]furan-2,8-diyl)bis(N,N-diphenylaniline) (BF-p-TPA) and 4,4'-(dibenzo[b,d]furan-2,6-diyl)bis(N,N-diphenylaniline) (BF-2,6-TPA), were designed and synthesized. Owing to the rigid dibenzofuran core, these BF-TPA derivatives exhibited high thermal decomposition temperatures of over 395 °C and very high LUMO energy levels. Electroluminescent (EL) devices were fabricated using these three hole-transporting materials. The best device performance was obtained for BF-m-TPA, with the maximum luminance (L) of 15,230 cd/m2, luminance efficiency (LE) of 56.5 cd/A, power efficiency (PE) of 13.3 lm/W, and external quantum efficiency (EQE) of 16.3%.

N-type organic field effect transistors based on naphthalene tetracarboxylic diimides derivatives containing halogen elements

Naphthalene tetracarboxylic diimides (NDIs) derivatives, functionalized with p-fluorophenyl (NDI-FAN), p-chlorophenyl (NDI-ClAN), p-fluorobenzyl (NDI-FBN), were synthesized and used to fabricated organic field effect transistors (OFETs) through vacuum evaporation. All materials display high decomposition temperature and low LUMO energy level facilitating air stable electron transport. OFETs devices based on N-octadecylphosphonic acid (ODPA) treated SiO2/Si substrate affords electron mobility up to 1.8 × 10−1 cm2v−1s−1 with high on/off ratio of 6.7 × 103 in air. Replaced the p-fluorophenyl by p-fluorobenzyl also leads to a comparable mobility reaching 1.1 × 10−1 cm2v−1s−1 with a low threshold voltage of 1.8 V. Microstructure and morphology of thin films were investigated to shed light on the reason for distinct electrical performance. These results demonstrate that fluorinated N-substituent is an alternative method to optimize molecular arrangement and device performance.

A Designed Ladder‐Type Heteroarene Benzodi(Thienopyran) for High‐Performance Fullerene‐Free Organic Solar Cells

To meet the imminent requirement of fullerene‐free electron acceptors, a new ladder‐type heteroarene benzodi(thienopyran) (BDTP) is designed and synthesized by embedding pyran rings in [2,2‐(2,5‐dialkyloxy‐1,4‐phenylene)dithiophene] (PDT) for organic photovoltaic (OPV) applications. Originating from the strong p–π conjugation of the oxygen atom in the pyran rings, electron‐rich BDTP results in the BDTP‐cored molecular acceptor, NBDTP, with strong intramolecular charge transfer effect. With sp3 carbons in pyran rings, NBDTP have the flexibility of targeted modification to resolve the solubility as well as the morphological tuning by handy alkyl chain attachment. As a result, the fusion of pyran rings in BDTP means that the acceptor exhibits a narrow optical band gap of 1.58 eV and a high LUMO energy level of −3.79 eV. Meanwhile, blend morphology of NBDTP‐based OPVs shows fine tunability via thermal annealing. Owing to the beneficial optical absorption, suitable energy‐level alignment, favorable molecule packing, and proper morphology, the optimized NBDTP‐based OPVs achieve a power conversion efficiency of up to 10.1%, which exemplifies the potential of utilizing molecular acceptors featuring ladder‐type heteroarenes for high‐performance OPVs.

Synthesis, characterization, photophysical properties and stability of bay-substituted tetrachloro-perylene diimide dianion salt by alkali treatment

Bay-substituted tetrachloro-perylene diimide dianion salt was synthesized facilely by using K2CO3 as reducing agent. The structure was confirmed by using a very thorough analysis based on the data from 1H NMR, 13C NMR, FT-IR and UV–vis spectra. Perylene diimide dianion salt shows high stable characteristic in the air or in some specific solutions such as acidic solution or oxidizing metal ion dissolved in deionized water under solid condition. However, the dianion converts slowly into corresponding radical and neutral perylene diimide in alkali solutions. Furthermore, the perylene diimide dianion salt has high HOMO and LUMO energy levels in comparison with that of corresponding neutral perylene diimide derivatives.

A-π-D-π-A type oligomer based on carbazole and benzothiadiazole for organic solar cells

ABSTRACTAn acceptor-donor-acceptor (A-p-D-p-A) type oligomer based on benzothiadiazole and 2,7 carbazole, 2,7-Bis(5-{7–2[-(4-decyloxy-phenyl)-vinyl]-benzo[1,2,5]thiadiazol-4-yl}-thiophen-2-yl)-9-(2-octyl-dodecyl)-9 H-carbazole (CzBT-DP), is synthesized by the Suzuki Coupling reaction. In addition, to extend the effective of conjugated length, we introduce thiophene (T) ring as a p-bridge and stilbene in the end of the conjugated chain. Optical, electrical and thermal properties and photovoltaic performances of CzBT-DP is systematically investigated. UV-vis absorption spectra and cyclic voltammograms (CV) reveal that HOMO/LUMO level of CzBT-DP is −5.37 eV/−3.38eV with an optical band gap of 1.99 eV, respectively, due to the addition of conjugated chain using stilbene compound system. The HOMO energies of CzBT-DP were calculated from the measured onset potential of oxidation by assuming the energy level of ferrocene (Fc) as −4.8 eV by CV. Addition of thiophene affects the HOMO level which makes CzBT-DP has high-lying HOMO level. Due to higher LUMO energy values above 0.3 eV than PC71 BM, CzBT-DP could be applied to BHJ OSCs as the donor [1]. Inverted type organic solar cells (OSCs) with a configuration of ITO/ZnO/CzBT-DP:PC71BM/MoO3/Al are fabricated. Figure 1 shows the current density-voltage (J-V) characteristics of the optimized BHJ OSCs based on CzBT-DP:PC71BM blend ratios from 3:2 to 3:6 (w/w). Maximum power conversion efficiency (PCE) is 0.64% with a short-circuit current density (Jsc) of 1.78 mA/cm2, Voc of 0.94 V, and fill factor (FF) of 38.3%. The best blend ratio between CzBT-DP and PC71BM having the best PCE appeared at 3:3 (w/w).

Ammonia sensing properties of perylene diimides: Effects of core-substituted chiral groups

The detection of ammonia vapor has strong implications for health, safety and environment. Herein, we reported on an high efficient chemiresistive sensor based on perylenediimide derivatives (PDIs) substituted by (S)-1-octyn-3-ol (SOT), (S)-3-butyn-2-ol (STB) and (S)-1-phenyl-2-propyn-1-ol (SPP) on 1, 7- bay positions. These PDIs devices exhibited high sensitivity, excellent selectivity and low detection limit towards ammonia vapor. In these materials, SPP showed the most efficient sensing performance, and the increment of current is 4 and 1.2 orders of magnitude higher than SOT and STB due to its lowest LUMO energy, uniform and continuous crystalline phase with π-π stacking distance of 3.28Å. While the highest LUMO energy, poor crystallinity, and smaller π-π overlap as well as largest π-π distance of 3.35Å resulted in the lowest performance of SOT device. It is expected that excellent sensing parameters and detailed analysis of the roles of core-substituted chiral groups will provide a basis protocol for the design of simple, low-power and high efficient PDIs sensors.

A novel A-D-A small molecule with 1,8-naphthalimide as a potential non-fullerene acceptor for solution processable solar cells

In this work, the structure-property relationship of fused-ring electron acceptors is briefly discussed and the principle of molecular design is presented. Based on this principle, perylene diimide (PDI) was cracked by inserting thiophene and thieno[3,2-b]thiophene (TT) to design and synthesize a novel A-D-A (acceptor-donor-acceptor) small molecule DNIT-TT2T, which served as a non-fullerene acceptor material in our fabricated organic solar cells (OSCs). DNIT-TT2T shows excellent thermal stability, it possesses a broad absorption by covering the wavelength range of 300–600 nm and relatively high LUMO energy level of −3.75 eV, which is close to the theoretically calculated value. The power conversion efficiency (PCE) of OSCs based on the blend of P3HT donor and DNIT-TT2T acceptor (1:1, w/w) is found to be 1.25%, with a high open-circuit voltage (Voc) of 0.88 V, indicating that 1,8-naphthalimide (NI) based molecules are promising acceptors for non-fullerene polymer solar cells and excellent photovoltaic properties can be achieved by rationally designing the molecules.

Diphenylamino-substituted bicarbazole derivative: Hole-transporting material with high glass-transition temperature, good electron and triplet exciton blocking capabilities and efficient hole injection

A diphenylamino-substituted bicarbazole derivative (BCZDA) with high glass-transition temperature (170°C) has been developed. The introduction of the strongly electron-donating diphenylamino group endows this compound with high HOMO (−4.94eV), LUMO (−1.94eV) and triplet energy (2.65eV) levels which are beneficial for hole injection and electron/triplet exciton blocking. By adopting this compound as the hole-transporting layer, both fluorescent and phosphorescent devices with good performance have been realized. Through the device study, the performance of this compound is proved to be comparable to that of NPB. The utility of this compound as a host has also been evaluated.

A Thieno[3,4-b]thiophene-Based Non-fullerene Electron Acceptor for High-Performance Bulk-Heterojunction Organic Solar Cells.

A thieno[3,4-b]thiophene-based electron acceptor, ATT-1, is designed and synthesized. ATT-1 exhibits a planar conjugated framework, broad absorption with a large absorption coefficient, and a slightly high LUMO energy level. Bulk-heterojunction (BHJ) solar cells based on PTB7-Th electron donor and ATT-1 electron acceptor delivered power conversion efficiencies of up to 10.07%, which is among the best performances reported for non-fullerene BHJ solar cells using PTB7-Th as the electron donor.

(Invited) Novel Fullerenes and Their Photovoltaic Properties

Thousands fullerenes with even-numbered Cn (n > 20 but with exception of 22) were produced in the original gas-phase laser vaporization experiment conducted by Kroto, Curl and Smalley.[1] However, only a few fullerenes survive the ambient conditions and were isolated in the form of bare hollow fullerene. In recent years, tens labile fullerenes with novel cage configuration have been stabilized by exohedral chlorination or hydrogenation using carbon arc or combustion method in our lab, such as #271C50, #540C54, #864C56, #913C56, #916C56, #1,809C60, #1,804C 60, #4,169C66, #11,188C 72 and #23,863C 78.[2 -6 ] The availability of these unprecedented fullerenes provides experimental opportunities for scientists from a wide of variety of disciplines to expand their insight into the new world of these fullerenes. We have paid special attention to the possibility of using these novel fullerenes as electron acceptors in polymer solar cells. Cyclic voltammetric analysis revealed that C50H10 exhibited reversible cathodic behavior and useful reduced potentials for possible application in photovoltaic devices. The first reduction potential of C50H10 (-1.41 V vs Fc/Fc+ in acetonitrile/o-DCB) is about 0.34 V more negative than the corresponding value of C60 as well as its hydride C60H2. The LUMO of C50H10 (-3.37 eV) is about 0.34 eV higher than that of C60, with implication to utilize C50H10 as promising electron acceptor for making polymer solar cells with higher open circuit voltage.[ 7 ] In addition, two classes of novel C60 derivatives with higher LUMO energy levels were synthesized, including alkoxy substituted dihydronaphthyl-C60 derivatives and PCBM-C60 derivatives (PCBM = [6,6]- phenyl- C61- butyric acid methyl ester). Photovoltaic devices using each of these fullerene derivatives as acceptor in combination with poly(3-hexylthiophene) as donor were fabricated. Power conversion efficiencies of the device based on 2-methoxy-(5,8)-dihydronaphthyl-(6,7)-[6,6]-C60 is 4.58%, better than PCBM-based polymer solar cell.[ 8 ] The higher power conversion efficiency implies that photovoltaic performance of fullerene can be improved by chemically modified with electron-donating group(s), and lends credit to a bright future of more efficient polymer (or even perovskite) solar cells based on novel fullerenes.[9-11] [1] Kroto H. W., Heath J. R., O'Brien S. C., Curl R. F. and Smalley R. E., “C60: Buckminsterfullerene”, Nature, 318 (1985) 162-163. [2] Xie S.Y., Gao F., Lu X., Huang R. B., Wang C. R., Zhang X., Liu M. L., Deng S. L. and Zheng L. S., “Capturing the labile fullerene[50] as C50Cl10”, Science, 304 (2004) 699-699. [3] Tan Y.Z., Liao Z. J., Qian Z. Z., Chen R. T., Wu X., Liang H., Han X., Zhu F., Zhou S.J., Zheng Z. P., Xie S. Y, Lu X., Huang R.B. and Zheng L. S. “Two Ih-symmetry-breaking C60 isomers stabilized by chlorination”, Nature Mater ., 7 (2008) 790-794. [4] Tan Y.Z., Xie S.Y., Huang R.B. and Zheng, L.S., “The stabilization of fused-pentagon fullerene molecules”, Nature Chem., 1 (2009) 450-460. [5] Tan Y.Z., Li J., Zhu F., Han X., Jiang W. S., Huang R.B., Zheng Z. P., Qian Z. Z., Chen R. T., Liao Z. J., Xie S. Y, Lu X. and Zheng L. S. “Chlorofullerenes featuring triple sequentially fused pentagons”, Nature Chem., 2 (2010) 269-273. [6] Tan Y.Z., Chen R.T., Liao Z.J., Li J., Zhu F., Lu X., Xie S.Y., Li J., Huang R.B. and Zheng L.S. “Carbon arc production of heptagon-containing fullerene[68]”, Nature C ommun ., 2 (2011) 240. [7] Chen J.H., Gao, Z.Y., Weng Q.H., Jiang W.S., He Q., Liang H, Deng L.L., Xie S.L., Huang, H.Y., Lu X., Xie S.Y., Shi K., Huang R.B. and Zheng, L.S., “Combustion Synthesis and Electrochemical Properties of the Small Hydrofullerene C50H10”, Chem. Eur. J.,18 (2012) 3408-3415. [8] Deng L.L., Feng J., Sun L.C., Wang S., Xie S.L., Xie S.Y., Huang R.B. and Zheng, L.S., “Functionalized dihydronaphthyl-C60 derivatives as acceptors for efficient polymer solar cells with tunable photovoltaic properties” Sol. Energy Ma ter. Sol. Cells, 104 (2012) 113-120. [9] Deng L.L., Xie S.L., Yuan C., Liu R.F., Feng J., Sun L.C., Lu X., Xie S.Y., Huang R.B. and Zheng, L.S., “High LUMO energy level C60(OCH3)4 derivatives: Electronic acceptors for photovoltaic cells with higher open-circuit voltage” Sol. Energy Mater. Sol. Cells, 111 (2013) 193-199. [10] Tian C.B., Deng, L.L., Zhang Z.Q., Dai S.M., Gao C.L., Xie S.Y., Huang R.B. and Zheng, L.S., “Bis-adducts of Benzocyclopentane- and Acenaphthene-C60 Superior to Mono-adducts as Electron Acceptors in Polymer Solar Cells”, Sol. Energy Mater. Sol. Cells, 125 (2014) 198-205. [11] Chen W.Y., Deng, L.L., Dai S.M., Wang X., Tian C.B., Zhan X.X., et al. “Low-cost solution-processed copper iodide as an alternative to PEDOT:PSS hole transport layer for efficient and stable inverted planar heterojunction perovskite solar cells”, J. Mater. Chem. A, 3 (2015) 19353.

Efficient organic ternary solar cells based on PTB7 and PC70BM with Bis-PC70BM

In recent years, solar cells, especially the bulk heterojunction (BHJ) polymer solar cells (PSCs), have attracted considerable attention. BHJ PSCs have several advantages such as easy fabrication, light weight, low cost and flexibility. The research on ternary BHJ PSCs will become a hot topic since incorporating near infrared region (NIR) low bandgap polymer materials into the donor/acceptor system can easily extend the absorption spectral range and improve the photon harvesting. In this paper, we investigate the ternary PSCs based on poly{4, 8-bis[(2-ethylhexyl)-oxy]benzo[1, 2-b:4, 5-b']dithiophene-2, 6-diyl-alt-3-fluoro-2-[(2-ethylhexyl) carbonyl]thieno[3, 4-b]thiophene-4, 6-diyl} (PTB7); Bis adduct of phenyl-C71-butyric acid methyl ester (Bis-PC70BM); [6, 6]-phenyl-C71-butyric-acid-methyl-ester (PC70BM). The performance of PSCs based on PTB7 and PC70BM may be improved by doping with Bis-PC70BM which is used as an electron-cascade acceptor material. Ternary blend PSCs with 3% Bis-PC70BM exhibit a power conversion efficiency (PCE) of 7.00%, higher than that (6.07%) of the PTB7 :PC70BM binary blend. The open-circuit voltage (VOC) is 0.77 V, the short-circuit current (JSC) is 13.92 mA cm-2 and the fill factor (FF) is 65%. However, in our research, the absorption spectra for the films with different amount of Bis-PC70BM are hardly changed, implying that doping with Bis-PC70BM would not improve the photon harvesting. The LUMO (HOMO) energy levels of PTB7, Bis-PC70BM and PC70BM are -3.49 eV (-5.31 eV), -3.80 eV (-6.10 eV) and -3.91 eV (-6.20 eV), respectively. Due to the higher LUMO energy levels of Bis-PC70BM relative to PC70BM, the VOC increases when Bis-PC70BM is used. The cascade-like energy levels of ternary blend PSCs can facilitate the charge transfer at the donor/acceptor interface owing to the bridging effect. There are three routes for charge transfer (PTB7-Bis-PC70BM, Bis-PC70BM-PC70BM and PTB7-PC70BM) in ternary PSCs, more than that one in the binary PTB7:PC70BM counterpart. Moreover, PC70BM can provide a driving force to transfer the electrons on the LUMO of Bis-PC70BM to a lower energy orbital (the LUMO of PC70BM), which can facilitate charge transfer from PTB7 to Bis-PC70BM. Atomic force microscopy (AFM) images show that when 3% Bis-PC70BM is used, the film of the ternary blend active layer becomes smoother and the root-mean-square (RMS) roughness decreases from 1.87 nm to 1.80 nm. The decreased roughness is likely good for the contact between the PEDOT:PSS and the active layer, improving the transport rate. We have fabricated hole-only devices using a high-work-function material (Au) as the cathode to block the back injection of electrons in order to investigate charge carrier transport and collection in the PSCs. Result shows that doping with Bis-PC70BM may not change the hole mobility in the device. Besides, the Jph-Veff characteristics shows that doping with 3% Bis-PC70BM can facilitate exciton dissociation and charge collection at a low voltage. Our results indicate that using Bis-PC70BM as an electron-cascade acceptor material in PTB7 :PC70BM blend to fabricate ternary blend PSCs is a promising way to improve the PCE.