Acceptors For Polymer Solar Cells Research Articles

Multi Adducts of Diphenylmethanofullerenes as Electron Acceptors for Polymer Solar Cells: A Quantum Chemical Study

In the present study, quantum chemical methods were applied to investigate the energy levels of the frontier orbital of mono and multi adducts of diphenylmethanofullerenes (DPMs). Recently these molecules have been used as electron acceptors in polymer solar cells showing interesting properties compared to the widely used [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM). The geometries of all the investigated compounds were optimized with the density functional theory at the B3LYP/3-21G level of calculation. We studied the effect of electron-donating (methoxy and methyl thioether) and electron-withdrawing (fluorine) substituents (placed on the diphenylmethano addend in ortho, meta and para position of the phenyl rings), on the lowest unoccupied molecular orbital (LUMO) level of DPM. Afterward, we analysed all the possible isomers arising from bis and tris additions to fullerene cage. The LUMO level is significantly raised compared to the mono adduct but different isomers have different LUMO levels. Since the open circuit voltage (V(oc)) for polymer solar cells with ohmic contacts is proportional to the difference between the highest occupied molecular orbital (HOMO) of the donor polymer and the LUMO of the acceptor (fullerene), these bisadducts have the potential to increase the V(oc) of the corresponding devices.

High LUMO energy level C60(OCH3)4 derivatives: Electronic acceptors for photovoltaic cells with higher open-circuit voltage

Two regioselective C60(OCH3)4 derivatives, C60(OCH3)4-PCBM (a methanofullerene derivative of C60(OCH3)4, PCBM=[6,6]-phenyl-C61-butyric acid methyl ester) and C60(OCH3)4-APCBM (an aziridinofullerene derivative of C60(OCH3)4) were synthesized from C60Cl6 and used as acceptor for polymer solar cells. Revealed by cyclic voltammetry, the LUMO energy levels of C60(OCH3)4-APCBM and C60(OCH3)4-PCBM are 0.2 and 0.3eV higher than that of PCBM, respectively. For the polymer photovoltaic cells with fullerene (PCBM, C60(OCH3)4-APCBM or C60(OCH3)4-PCBM) acceptor in combination with poly(3-hexylthiophene) (P3HT) donor, the open-circuit voltage is increased from 0.58V (for PCBM) to 0.63V (for C60(OCH3)4-APCBM) and 0.72V (for C60(OCH3)4-PCBM). The higher open-circuit voltages are reasonably attributed to the higher LUMO levels of the C60(OCH3)4 derivatives because of four electron-donating methoxy groups attached. The photovoltaic performance of C60(OCH3)4-PCBM-based device is higher than that involving C60(OCH3)4-APCBM, largely due to the structural changeability of C60(OCH3)4-APCBM resulting from the rotatable N–C bond bridge therein. This work demonstrates that fullerene derivatives with higher LUMO level can be functionalized from multi-addition of electron-donating groups, and exemplifies that photovoltaic performances of fullerene-based solar cells are sensitive even to trivial bridge between functional group and fullerene core.

Fullerene derivatives as acceptors for polymer solar cells

Polymer solar cells (PSCs) have drawn broad attention because of their advantages of low cost, light weight, and flexibility. Tremendous progress has been made on polymer solar cells to achieve high power conversion efficiency (PCE), and PCE of PSCs has already reached to 8%. The fullerene derivative is used as the excellent electron acceptor for PSCs, and the development of highly efficient fullerene acceptors has become the hot research field in recent years. In this paper, we reviewed the recent research progress on several types of fullerene derivative acceptors and heir relevant photovoltaic device properties, which could afford some reference for the development of high performance fullerene derivative acceptors.

Effects of Alkoxy Chain Length in Alkoxy-Substituted Dihydronaphthyl-Based [60]Fullerene Bisadduct Acceptors on Their Photovoltaic Properties

A series of alkoxy-substituted dihydronaphthyl-based [60]fullerene bisadduct derivatives (Cn-NCBA, n = 1-6), with the alkoxy chain length from 1 to 6 carbon atoms, were synthesized as acceptors for polymer solar cells (PSCs), for the purpose of systematically investigating the effects of fullerene derivative structures on the photovoltaic properties of PSCs. Although the absorption spectra and electrochemical properties of Cn-NCBA are almost the same, the PSCs based on P3HT:Cn-NCBA showed different photovoltaic properties. The device based on the P3HT:C3-NCBA blend demonstrated the highest power-conversion efficiency (PCE) of ca. 4.1%, while those with shorter or longer alkoxy-substituted dihydronaphthyl-based [60]fullerene bisadduct derivatives showed relatively lower PCE values. C5-NCBA and C6-NCBA with longer alkoxy chain length showed relatively low electron mobilities, leading to relatively poor photovoltaic performance. More importantly, we found that the alkoxy chain length changes the hydrophobicity of Cn-NCBA and, thus, the interfacial interaction and miscibility with P3HT, which were analyzed by interfacial tension and atomic force microscopy (AFM) measurements. The hydrophobicity of Cn-NCBA increased as the alkoxy chain length increased. A distinct phase separation for the P3HT:C1-NCBA blend film due to the large interfacial tension and poor miscibility between P3HT and C1-NCBA could be one reason for the low PCE value of the C1-NCBA-based devices. C3-NCBA may provide the most appropriate combination of electron mobility and miscibility with P3HT to achieve optimal photovoltaic properties. The current study provides the molecular structure-device performance relationship, especially with respect to the alkoxy chain length of Cn-NCBA and their interfacial interactions with P3HT, and suggests a design rule for high-performance fullerene bisadduct acceptors for PSC applications.

Indene-C60/C70 Bisadduct as Acceptor in Polymer Solar Cells

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Functionalized dihydronaphthyl-C60 derivatives as acceptors for efficient polymer solar cells with tunable photovoltaic properties

A class of alkoxy substituted dihydronaphthyl-C60 derivatives, such as 2-methoxy-(5,8)-dihydronaphthyl-(6,7)- [6,6]–C60 (MDNC) and its bisadduct (Bis-MDNC) with high-lying LUMO level, were synthesized and evaluated as electron acceptors in polymer solar cells. Influence of substituent groups (methyloxy, benzyloxy, benzoyloxy, hexyloxy, and 3-ethoxycarbonylpropyloxy) on photovoltaic and electrochemical properties regarding these fullerene derivatives were studied. The polymer bulk heterojunction (BHJ) solar cell using Bis-MDNC blend with poly (3-hexylthiophene) (P3HT) as photoactive layer showed a power conversion efficiency (PCE) of 4.58% with an open-circuit voltage (Voc) of 0.83V, which is superior to the P3HT: PCBM control device (3.25% PCE with Voc of 0.59V). This work exemplifies a facile functionalization and an alterable photovoltaic behavior susceptible to substituent groups in the dihydronaphthyl-C60 derivatives.

Highly Efficient and Thermally Stable Polymer Solar Cells with Dihydronaphthyl‐Based [70]Fullerene Bisadduct Derivative as the Acceptor

AbstractThe efficiency of polymer solar cells (PSCs) can be essentially enhanced by improving the performance of electron‐acceptor materials, including by increasing the lowest unoccupied molecular orbital (LUMO) level, improving the optical absorption, and tuning the material solubility. Here, a new soluble C70 derivative, dihydronaphthyl‐based C70 bisadduct (NC70BA), is synthesized and explored as acceptor in PSCs. The NC70BA has high LUMO energy level that is 0.2 eV higher than [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM), and displays broad light absorption in the visible region. Consequently, the PSC based on the blend of poly(3‐hexylthiophene) (P3HT) and NC70BA shows a high open‐circuit voltage (Voc = 0.83 V) and a high power conversion efficiency (PCE = 5.95%), which are much better than those of the P3HT:PCBM‐based device (Voc = 0.60 V; PCE = 3.74%). Moreover, the amorphous nature of NC70BA effectively suppresses the thermally driven crystallization, leading to high thermal stability of the P3HT:NC70BA‐based solar cell devices. It is observed that the P3HT:NC70BA‐based device retains 80% of its original PCE value against thermal heating at 150 °C over 20 h. The results unambiguously indicate that the NC70BA is a promising acceptor material for practical PSCs.

Effect of the Active Layer Thickness on the Device Performance of Polymer Solar Cells Having [60]PCBM and [70]PCBM as Electron Acceptor

Recently it has been shown that the use of [70]PCBM as electron acceptor in polymer solar cells can improve the current output of such devices because C70 derivatives have a stronger and broader absorption compared to C60 ones. In this work we studied the influence of the fullerene on the optical behaviour of the photoactive blend film of a polymer solar cell. We simulated the optical absorption of the active layer inside the device structure and we calculated the maximum achievable short circuit current density with the aim to correlate the variation of the optical constants to the output current of the device. Afterward, we realized several polymer solar cells and we proved the theoretical findings.

Enhanced open-circuit voltage of two side chain conjugated polythiophene derivatives blended with indene-C 60 bisadduct

Two side chain conjugated polythiophene copolymer: poly[3-(5′-octylthienylenevinyl) thiophene]-thiophene (P1) and poly{3-[4-(2-ethyl-hexyloxy)-phenylvinyl]-thiophene}-thiophene (P2) were synthesized. Polymer photovoltaic solar cells using P1 and P2 as donor and a high LUMO energy level fullerene derivative indene-C 60 bisadduct (IC 60BA) as acceptor were demonstrated. The polymer solar cells (PSCs) based P1/IC 60BA and P2/IC 60BA showed enhanced open circuit voltage ( V oc) of 0.89 and 0.88 V respectively, in comparison with that of P1/PC 61BM and P2/PC 61BM as 0.58 and 0.56 V, respectively. The improved V oc of IC 60BA as acceptor in PSCs is benefited from the higher LUMO energy levels of the C 60 derivatives. The power conversion efficiency of the PSCs based on P1/IC 60BA and P2/IC 60BA were 0.32% and 0.35%, respectively.

Self-assembled mesogens modified fullerene for efficiently stable bulk heterojunction solar cells

A facile approach to develop a novel soluble liquid-crystalline fullerene derivative, N-methyl-2- [(4-(4′-cyano)biphenyloxy)hexyloxyphenyl)]-3,4-fulleropyrrolidine, C 60 -bp-CN is reported. Compared to C 60 , the modified C 60 -bp-CN shows broadened light harvesting and lower LUMO level (−4.2 eV), ascribing to the strong intermolecular interaction induced by self-assembled cyanobiphenyl mesogens, especially after thermal annealing from liquid crystalline states. When blended with poly(3-hexylthiophene) ( P3HT), the absorption of the blend film is red-shifted to 700 nm, and the intensity is also remarkably increased. The results indicate that the modified C 60 -bp-CN is a promising acceptor for polymer solar cells (PSCs) based on BHJ active layers. Therefore, solar cell devices based on an ITO/PEDOT:PSS/ P3HT:C 60 -bp-CN/LiF/Al configuration are fabricated. Clearly, the performances of all the devices are dramatically enhanced relative to corresponding species based on P3HT/C 60 active layers. Among all the thermal treatments the mesophase annealing achieves the best performance with a short-circuit current density ( J sc) of 5.5 mA/cm 2, an open-circuit voltage ( Voc) of 0.52 V, a calculated fill factor (FF) of 0.23, and a power conversion efficiency (PCE) of 0.65%, which is 3 times higher than that of the untreated one.

Synthesis and photovoltaic properties of biindene-C 70 monoadduct as acceptor in polymer solar cells

A new fullerene derivative, biindene-C 70 monoadduct (BC 70MA), was synthesized by [4+2] cycloaddition reaction between 1,1′-biindene and C 70, for the application as acceptor in polymer solar cells (PSCs). BC 70MA is soluble in common organic solvents such as tetrahydrofuran, chloroform, toluene, o-dichlorobenzene, etc., and shows stronger absorption in the visible region and a slightly up-shifted lowest unoccupied molecular orbital (LUMO) energy level than that of PCBM. PSCs were fabricated with BC 70MA as acceptor and poly(3-hexylthiophene) (P3HT) as donor for investigating the photovoltaic properties of BC 70MA. The power conversion efficiency of the PSC based on P3HT/BC 70MA (1:1, w/w) with the additive of 3% octane-1,8-dithiol and thermal annealing at 110 °C for 10 min reached 3.44% with open circuit voltage of 0.64 V, short circuit current of 8.02 mA/cm 2 and fill-factor of 0.67, under the illumination of AM1.5, 100 mW/cm 2.

Indene Addition of [6,6]-Phenyl-C61-butyric Acid Methyl Ester for High-Performance Acceptor in Polymer Solar Cells

Indene addition product of [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM), indene-PC60BM (IPC60BM), was synthesized by a one-pot reaction of indene and PC60BM at 180 °C for 12 h, for combining the advantages of PC60BM and indene-C60 bis-adduct in the application as acceptor in polymer solar cells (PSCs). The lowest unoccupied molecular orbital (LUMO) energy level of IPC60BM is 0.12 eV up-shifted than that of PC60BM. The PSC based on poly(3-hexylthiophene) (P3HT) as donor and IPC60BM as acceptor shows a higher Voc of 0.72 V and higher power conversion efficiency (PCE) of 4.39% under the illumination of AM1.5G, 100 mW/cm2, while the PSC based on P3HT/PC60BM displays Voc of 0.58 V and PCE of 3.49% only. The results indicate that IPC60BM is a better acceptor than PC60BM for the P3HT-based PSCs and could be a promising new acceptor instead of PC60BM for high-performance PSCs.

Fullerene derivative acceptors for high performance polymer solar cells

Polymer solar cells (PSCs) are composed of a blend film of a conjugated polymer donor and a soluble fullerene derivative acceptor sandwiched between a PEDOT : PSS coated ITO positive electrode and a low workfunction metal negative electrode. The conjugated polymer donor and the fullerene derivative acceptor are the key photovoltaic materials for high performance PSCs. For the acceptors, although [6,6]-phenyl-C(61)-butyric acid methyl ester (PC(60)BM) and its corresponding C(70) derivative PC(70)BM are dominantly used as the acceptors in PSC at present, several series of new fullerene derivatives with higher-lying LUMO energy level and better solubility were reported in recent years for further improving the power conversion efficiency of the PSCs. In this perspective paper, we reviewed the recent research progress on the new fullerene derivative acceptors, including various PC(60)BM-like C(60) derivatives, PC(60)BM bisadduct, PC(70)BM bisadduct, indene-C(60) bisadduct and indene-C(70) bisadduct, trimetallic nitride endohedral fullerenes and other C(60) derivatives with multi side chains. The synthesis and physicochemical properties of PC(60)BM and PC(70)BM were also introduced considering the importance of the two fullerene acceptors.

Enhanced photovoltaic performance of polymer solar cells by adding fullerene end-capped polyethylene glycol

A fullerene end-capped polyethylene glycol has been synthesized and used as an additive in organic solar cells based on poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (PCBM). It was found that an appropriate amount of the additive can dramatically improve both the photovoltaic performance and the lifetime of the devices, which have been attributed to the successful morphological control of PCBM by the additive. Under transmission electron microscopy, uniformly distributed and stable PCBM crystallites with an average size of only several nanometres in the active layer were observed. We speculate that the immobile fullerenes bonded to polyethylene glycol chains act as nucleation centers for PCBM crystallites and thus result in a stable and uniform PCBM distribution in the active layer, which is essential for high performance organic solar cells. Since PCBM is used as a universal acceptor in polymer solar cells, these results suggest that such an additive can be used in many other different polymer solar cells to control the morphology of PCBM and improve the device performance.

Biindene-C 60 adducts for the application as acceptor in polymer solar cells with higher open-circuit-voltage

Two C 60 derivatives, biindene-C 60 monoadduct (BC 60MA) and biindene-C 60 bisadduct (BC 60BA), were synthesized by an one-pot reaction of 1, 1′-biindene and C 60, for the application as acceptor materials in Polymer Solar Cells (PSCs). The two C 60 derivatives possess good solubility in toluene and o-dichlorobenzene, and the solubility of BC 60BA is even better than that of BC 60MA. The electrochemical properties and the LUMO energy levels of the two fullerene derivatives were investigated by cyclic voltammetry. The LUMO energy levels of BC 60MA and BC 60BA were 0.06 and 0.17 eV higher (up-shifted) than that of PCBM, respectively. The PSCs based on P3HT as donor and BC 60MA or BC 60BA as acceptor exhibited higher V oc of 0.68 and 0.82 V, respectively, which is benefited from the higher LUMO energy levels of the C 60 derivatives. The power conversion efficiency of the PSC based on P3HT/BC 60MA was 2.21% after annealing at 140 °C for 5 min.

Synthesis and applications of low‐bandgap conjugated polymers containing phenothiazine donor and various benzodiazole acceptors for polymer solar cells

AbstractA series of soluble donor‐acceptor conjugated polymers comprising of phenothiazine donor and various benzodiazole acceptors (i.e., benzothiadiazole, benzoselenodiazole, and benzoxadiazole) sandwiched between hexyl‐thiophene linkers were designed, synthesized, and used for the fabrication of polymer solar cells (PSC). The effects of the benzodiazole acceptors on the thermal, optical, electrochemical, and photovoltaic properties of these low‐bandgap (LBG) polymers were investigated. These LBG polymers possessed large molecular weight (Mn) in the range of 3.85−5.13 × 104 with high thermal decomposition temperatures, which demonstrated broad absorption in the region of 300−750 nm with optical bandgaps of 1.80−1.93 eV. Both the HOMO energy level (−5.38 to −5.47 eV) and LUMO energy level (−3.47 to −3.60 eV) of the LBG polymers were within the desirable range of ideal energy level. Under 100 mW/cm2 of AM 1.5 white‐light illumination, bulk heterojunction PSC devices containing an active layer of electron donor polymers mixed with electron acceptor [6,6]‐phenyl‐C61‐butyric acid methyl ester (PC61BM) or [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM) in different weight ratios were investigated. The best performance of the PSC device was obtained by using polymer PP6DHTBT as an electron donor and PC71BM as an acceptor in the weight ratio of 1:4, and a power conversion efficiency value of 1.20%, an open‐circuit voltage (Voc) value of 0.75 V, a short‐circuit current (Jsc) value of 4.60 mA/cm2, and a fill factor (FF) value of 35.0% were achieved. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010

Synthesis of new nanocrystal-polymer nanocomposite as the electron acceptor in polymer bulk heterojunction solar cells

A hydroxyl-coated CdSe nanocrystal (CdSe-OH) and a CdSe-polymer nanocomposite were synthesized and used as the electron acceptors in polymer solar cells (PSCs). The CdSe-polymer composite was prepared via atom transfer radical polymerization (ATRP) of N-vinylcarbazole on functionalized CdSe quantum dots. Physical properties and photovoltaic characteristics of the CdSe-poly( N-vinylcarbazole) (CdSe-PVK) nanocomposite have been investigated. Thermogravimetric analysis (TGA) results displayed higher thermal stability for CdSe-PVK nanohybrid in comparison with the linear-type PVK polymer. Differential scanning calorimetry (DSC) studies indicated that CdSe-PVK had a lower glass-transition temperature ( T g) in comparison with PVK due to the branch effect of the star-shaped polymer hybrid. Cyclic voltammetric (CV) measurements were performed to obtain HOMO and LUMO values of PVK and CdSe-PVK. TEM and SEM micrographs exhibited CdSe nanoparticles were well coated with PVK polymer. Both CdSe-OH and CdSe-PVK were blended with poly(3-hexylthiophene) (P3HT) and used as the active layer in bulk heterojunction solar cells. Polymer solar cell based on CdSe-PVK as acceptor revealed that the photovoltaic properties can be significantly improved when PVK polymer chains were grafted on surfaces of CdSe nanocrystals. In comparison with the P3HT:CdSe-OH system, PSC based on P3HT:CdSe-PVK showed an improved power conversion efficiency (0.02% vs. 0.001%). Film topography studied by AFM further confirmed the better device performance was due to the enhanced compatibility between P3HT and CdSe-PVK.

Higher fullerenes as electron acceptors for polymer solar cells: A quantum chemical study

In the present study, we have used quantum chemical methods to study the energy levels of the frontier orbitals of higher fullerene derivatives (from C 70 to C 84 and having the same addend as in [6,6]-phenyl C 61-butyric acid methyl ester) with the aim to understand if they can be used as electron acceptors in bulk heterojunction polymer–fullerene solar cells. Higher fullerenes have a stronger and broader absorption compared to C 60 and they can improve the current output of the corresponding devices. The geometries of all the compounds were optimized with the density functional theory at the B3LYP/3-21G* level of calculation. The lowest unoccupied molecular orbital (LUMO) levels of the investigated compounds correlate well with the reduction potentials (obtained by cyclic voltammetry) of the already prepared species. We found that the LUMO level depends not only on the fullerene size (number of carbons of the cage) and constitutional isomer, but also on the position and, in some cases, the addend orientation. This issue should be considered because for a proper device operation, a well-defined LUMO is required. The position of the LUMO level of some higher fullerene derivatives can be suitable for low-bandgap polymers.