Distinguishing the Importance of Fullerene Phase Separation from Polymer Ordering in the Performance of Low Band Gap Polymer:Bis‐Fullerene Heterojunctions

One way to improve power conversion efficiency (PCE) of polymer based bulk‐heterojunction (BHJ) photovoltaic cells is to increase the open circuit voltage (V _oc). Replacing PCBM with bis‐adduct fullerenes significantly improves V _oc and the PCE in devices based on the conjugated polymer poly(3‐hexyl thiophene) (P3HT). However, for the most promising low band‐gap polymer (LBP) system, replacing PCBM with ICBA results in poor short‐circuit current (J _sc) and PCE although V _oc is significantly improved. The optimization of the morphology of as‐cast LBP/bis‐fullerene BHJ photovoltaics is attempted by adding a co‐solvent to the polymer/fullerene solution prior to film deposition. Varying the solubility of polymer and fullerene in the co‐solvent, bulk heterojunctions are fabricated with no change of polymer ordering, but with changes in fullerene phase separation. The morphologies of the as‐cast samples are characterized by small angle neutron scattering and neutron reflectometry. A homogenous dispersion of ICBA in LBP is found in the samples where the co‐solvent is selective to the polymer, giving poor device performance. Aggregates of ICBA are formed in samples where the co‐solvent is selective to ICBA. The resultant morphology improves PCE by up to 246%. A quantitative analysis of the neutron data shows that the interfacial area between ICBA aggregates and its surrounding matrix is improved, facilitating charge transport and improving the PCE.     

Enhanced Thermal Stability and Efficiency of Polymer Bulk‐Heterojunction Solar Cells by Low‐Temperature Drying of the Active Layer

This study addresses two key issues, stability and efficiency, of polymer solar cells based on blended poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) by demonstrating a film‐forming process that involves low‐temperature drying (?5 °C) and subsequent annealing of the active layer. The low‐temperature process achieves 4.70% power conversion efficiency (PCE) and ～1250 h storage half‐life at 65 °C, which are significant improvements over the 3.39% PCE and ～143 h half‐life of the regular room‐temperature process. The improvements are attributed to the enhanced nucleation of P3HT crystallites as well as the minimized separation of the P3HT and PCBM phases at the low drying temperature, which upon post‐drying annealing results in a morphology consisting of small PCBM‐rich domains interspersed within a densely interconnected P3HT crystal network. This morphology provides ample bulk‐heterojunction area for charge generation while allowing for facile charge transport; moreover, the P3HT crystal network serves as an immobile frame at heating temperatures less than the melting point (T_m) of P3HT, thus preventing PCBM/P3HT phase separation and the corresponding device degradation.     

Three‐Dimensional Bulk Heterojunction Morphology for Achieving High Internal Quantum Efficiency in Polymer Solar Cells

Here, an investigation of three‐dimensional (3D) morphologies for bulk heterojunction (BHJ) films based on regioregular poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) is reported. Based on the results, it is demonstrated that optimized post‐treatment, such as solvent annealing, forces the PCBM molecules to migrate or diffuse toward the top surface of the BHJ composite films, which induces a new vertical component distribution favorable for enhancing the internal quantum efficiency (η_IQE ) of the devices. To investigate the 3D BHJ morphology, novel time‐of‐flight secondary‐ion mass spectroscopy studies are employed along with conventional methods, such as UV‐vis absorption, X‐ray diffraction, and high‐resolution transmission electron microscopy studies. The η_IQE of the devices are also compared after solvent annealing for different times, which clearly shows the effect of the vertical component distribution on the performance of BHJ polymer solar cells. In addition, the fabrication of high‐performance P3HT:PCBM solar cells using the optimized solvent‐annealing method is reported, and these cells show a mean power‐conversion efficiency of 4.12% under AM 1.5G illumination conditions at an intensity of 100 mW cm^?2.     

Annealing‐Free High Efficiency and Large Area Polymer Solar Cells Fabricated by a Roller Painting Process

Polymer solar cells are fabricated by a novel solution coating process, roller painting. The roller‐painted film – composed of poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) – has a smoother surface than a spin‐coated film. Since the roller painting is accompanied by shear and normal stresses and is also a slow drying process, the process effectively induces crystallization of P3HT and PCBM. Both crystalline P3HT and PCBM in the roller‐painted active layer contribute to enhanced and balanced charge‐carrier mobility. Consequently, the roller‐painting process results in a higher power conversion efficiency (PCE) of 4.6%, as compared to that for spin coating (3.9%). Furthermore, annealing‐free polymer solar cells (PSCs) with high PCE are fabricated by the roller painting process with the addition of a small amount of octanedi‐1,8‐thiol. Since the addition of octanedi‐1,8‐thiol induces phase separation between P3HT and PCBM and the roller‐painting process induces crystallization of P3HT and PCBM, a PCE of roller‐painted PSCs of up to 3.8% is achieved without post‐annealing. A PCE of over 2.7% can also be achieved with 5 cm² of active area without post‐annealing.     

A Simple and Effective Modification of PCBM for Use as an Electron Acceptor in Efficient Bulk Heterojunction Solar Cells

[6,6]‐phenyl‐C‐61‐butyric acid methyl ester (PCBM) and poly(3‐hexylthiophene) (P3HT) are the most widely used acceptor and donor materials, respectively, in polymer solar cells (PSCs). However, the low LUMO (lowest unoccupied molecular orbital) energy level of PCBM limits the open circuit voltage (V_oc) of the PSCs based on P3HT. Herein a simple, low‐cost and effective approach of modifying PCBM and improving its absorption is reported which can be extended to all fullerene derivatives with an ester structure. In particular, PCBM is hydrolyzed to carboxylic acid and then converted to the corresponding carbonyl chloride. The latter is condensed with 4‐nitro‐4’‐hydroxy‐α‐cyanostilbene to afford the modified fullerene F . It is more soluble than PCBM in common organic solvents due to the increase of the organic moiety. Both solutions and thin films of F show stronger absorption than PCBM in the range of 250–900 nm. The electrochemical properties and electronic energy levels of F and PCBM are measured by cyclic voltammetry. The LUMO energy level of F is 0.25 eV higher than that of PCBM. The PSCs based on P3HT with F as an acceptor shows a higher V_oc of 0.86 V and a short circuit current (J_sc) of 8.5 mA cm^?2, resulting in a power conversion efficiency (PCE) of 4.23%, while the PSC based on P3HT:PCBM shows a PCE of about 2.93% under the same conditions. The results indicate that the modified PCBM, i.e., F , is an excellent acceptor for PSC based on bulk heterojunction active layers. A maximum overall PCE of 5.25% is achieved with the PSC based on the P3HT: F blend deposited from a mixture of solvents (chloroform/acetone) and subsequent thermal annealing at 120 °C.     

Difluorobenzothiadiazole‐Based Small‐Molecule Organic Solar Cells with 8.7% Efficiency by Tuning of π‐Conjugated Spacers and Solvent Vapor Annealing

The synthesis of a series of tetrafluorine‐substituted, wide‐bandgap, small molecules consisting of various π‐conjugated spacers (furan, thiophene, selenophene) between indacenodithiophene as the electron‐donating core and the electron‐deficient difluorobenzothiadiazole unit is reported and the effect of the π‐conjugated spacers on the photovoltaic properties is investigated. The alteration of the π‐conjugated spacer enables fine‐tuning of the photophysical properties and energy levels of the small molecules, and allows the adjustment of the charge‐transport properties, the morphology of the photoactive films, as well as their photovoltaic properties. Moreover, most of these devices exhibit superior device performances after CH₂Cl₂ solvent annealing than without annealing, with a high fill factor (0.70–0.75 for all cases). Notably, the devices based on the new molecule BIT4FTh (with thiophene as the spacer) show an outstanding PCE of 8.7% (with an impressive FF of 0.75), considering its wide‐bandgap (1.81 eV), which is among the highest efficiencies reported so far for small‐molecules‐based solar cells. The morphologies of the photoactive layers with/without CH₂Cl₂ solvent annealing are characterized by atomic force microscopy, transmission electron microscopy and two‐dimensional grazing incidence X‐ray diffraction analysis. The results reported here clearly indicate that highly efficient small‐molecules‐based solar cells can be achieved through rational design of their molecular structure and optimization of the phase‐separated morphology via an adapted solvent–vapor annealing process.     

Comparative study of spectral and morphological properties of blends of P3HT with PCBM and ICBA

We report a comparative study on spectral and morphological properties of two blend systems for polymer solar cells: the donor material poly(3-hexylthiophene) (P3HT) in combination with the acceptor material of either [6,6]-phenyl-C₆₁ butyric acid methyl ester (PCBM) or indene-C₆₀ bisadduct (ICBA) that was reported to enhance efficiencies of polymer solar cells. Optical microscopy and grazing incidence X-ray scattering reveal the stronger tendency of PCBM to from larger and more ordered domains/grains than ICBA either in pure or blend films. Compared to PCBM, the presence of ICBA also substantially perturbs the organization and longer-range ordering of P3HT in increasing the ICBA ratio in blends. With larger and more ordered phase-separated domains, the P3HT/PCBM blend films exhibit significant optical scattering at higher PCBM ratios. Yet, such optical scattering is not significant for P3HT/ICBA blends (even with high ICBA ratios). Overall, results here suggest the reported higher efficiencies of P3HT/ICBA solar cells (vs. P3HT/PCBM cells) cannot be attributed to larger and/or more ordered phase-separated donor–acceptor domains and other characteristics play more important roles in this case.     

Photoconductivity of a Low‐Bandgap Conjugated Polymer

The photoconductive properties of a novel low‐bandgap conjugated polymer, poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b′]dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole)], PCPDTBT, with an optical energy gap of E_g ～ 1.5 eV, have been studied. The results of photoluminescence and photoconductivity measurements indicate efficient electron transfer from PCPDTBT to PCBM ([6,6]‐phenyl‐C61 butyric acid methyl ester, a fullerene derivative), where PCPDTBT acts as the electron donor and PCBM as the electron acceptor. Electron‐transfer facilitates charge separation and results in prolonged carrier lifetime, as observed by fast (t > 100 ps) transient photoconductivity measurements. The photoresponsivities of PCPDTBT and PCPDTBT:PCBM are comparable to those of poly(3‐hexylthiophene), P3HT, and P3HT:PCBM, respectively. Moreover, the spectral sensitivity of PCPDTBT:PCBM extends significantly deeper into the infrared, to 900 nm, than that of P3HT. The potential of PCPDTBT as a material for high‐efficiency polymer solar cells is discussed.     

Abrupt Morphology Change upon Thermal Annealing in Poly(3‐Hexylthiophene)/Soluble Fullerene Blend Films for Polymer Solar Cells

The in situ morphology change upon thermal annealing in bulk heterojunction blend films of regioregular poly(3‐hexylthiophene) (P3HT) and 1‐(3‐methoxycarbonyl)‐propyl‐1‐phenyl‐(6,6)C₆₁ (PCBM) is measured by a grazing incidence X‐ray diffraction (GIXD) method using a synchrotron radiation source. The results show that the film morphology—including the size and population of P3HT crystallites—abruptly changes at 140 °C between 5 and 30 min and is then stable up to 120 min. This trend is almost in good agreement with the performance change of polymer solar cells fabricated under the same conditions. The certain morphology change after 5 min annealing at 140 °C is assigned to the on‐going thermal transition of P3HT molecules in the presence of PCBM transition. Field‐emission scanning electron microscopy measurements show that the crack‐like surface of blend films becomes smaller after a very short annealing time, but does not change further with increasing annealing time. These findings indicate that the stability of P3HT:PCBM solar cells cannot be secured by short‐time annealing owing to the unsettled morphology, even though the resulting efficiency is high.     

The Effect of Thermal Treatment on the Morphology and Charge Carrier Dynamics in a Polythiophene–Fullerene Bulk Heterojunction

The influence of various thermal treatment steps on the morphology and the photoconductive properties of a non‐contacted, 50 nm thick blend (50:50 wt.‐%) of [6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM) and poly(3‐hexyl thiophene) (P3HT) spin‐coated from chloroform has been studied using transmission electron microscopy (TEM) and the electrodeless time‐resolved microwave conductivity technique. After annealing the film for 5 min at 80 °C, TEM images show the formation of crystalline fibrils of P3HT due to a more ordered packing of the polymer chains. The thermal treatment results in a large increase of the photoconductivity, due to an enhancement of the hole mobility in these crystalline P3HT domains from 0.0056 cm² V^–1 s ^–1 for the non‐annealed sample to 0.044 cm² V^–1 s ^–1 for the sample annealed at 80 °C. In contrast, the temporal shape of the photoconductivity, with typical decay half‐times, τ_1/2, of 1 μs for the lowest excitation intensities, is unaffected by the temperature treatment. Further annealing of the sample at 130 °C results in the formation of three different substructures within the heterojunction: a PCBM:P3HT blend with PCBM‐rich clusters, a region depleted of PCBM, and large PCBM single crystals. Only a minor increase in the amplitude, but a tenfold rise of the decay time of the photoconductivity, is observed. This is explained by the formation of PCBM‐rich clusters and large PCBM single crystals, resulting in an increased diffusional escape probability for mobile charge carriers and hence reduced recombination.     

Time‐Dependent Morphology Evolution by Annealing Processes on Polymer:Fullerene Blend Solar Cells

Changes in the nanoscale morphologies of the blend films of poly (3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM), for high‐performance bulk‐heterojunction (BHJ) solar cells, are compared and investigated for two annealing treatments with different morphology evolution time scales, having special consideration for the diffusion and aggregation of PCBM molecules. An annealing condition with relatively fast diffusion and aggregation of the PCBM molecules during P3HT crystallization results in poor BHJ morphology because of prevention of the formation of the more elongated P3HT crystals. However, an annealing condition, accelerating PCBM diffusion after the formation of a well‐ordered morphology, results in a relatively stable morphology with less destruction of crystalline P3HT. Based on these results, an effective strategy for determining an optimized annealing treatment is suggested that considers the effect of relative kinetics on the crystallization of the components for a blend film with a new BHJ materials pair, upon which BHJ solar cells are based.     

Cover Picture: Tuning the Dimensions of C60‐Based Needlelike Crystals in Blended Thin Films (Adv. Funct. Mater. 6/2006)

A new ordered structure of the C₆₀ derivative PCBM is obtained in thin films based on the blend PCBM:P3HT, as detailed by Swinnen, Manca, and co‐workers on p. 760. Needlelike crystalline PCBM structures, whose dimensions and spatial distribution ca be tuned by adjusting the blend ratio and annealing conditions, are formed. In typical solar‐cell applications of these blended films, these results indicate that during long‐term operation under normal conditions (50–70 °C) morphology changes and a decrease in cell performance could occur. A new ordered structure of the C₆₀ derivative PCBM ([6‐6]‐phenyl C₆₁‐butyric acid methyl ester) is obtained in thin films based on the blend PCBM:regioregular P3HT (poly(3‐hexylthiophene)). Rapid formation of needlelike crystalline PCBM structures of a few micrometers up to 100 μm in size is demonstrated by submitting the blended thin films to an appropriate thermal treatment. These structures can grow out to a 2D network of PCBM needles and, in specific cases, to spectacular PCBM fans. Key parameters to tune the dimensions and spatial distribution of the PCBM needles are blend ratio and annealing conditions. The as‐obtained blended films and crystals are probed using atomic force microscopy, transmission electron microscopy, selected area electron diffraction, optical microscopy, and confocal fluorescence microscopy. Based on the analytical results, the growth mechanism of the PCBM structures within the film is described in terms of diffusion of PCBM towards the PCBM crystals, leaving highly crystalline P3HT behind in the surrounding matrix.     

Charge Transport and Photocurrent Generation in Poly(3‐hexylthiophene): Methanofullerene Bulk‐Heterojunction Solar Cells

The effect of controlled thermal annealing on charge transport and photogeneration in bulk‐heterojunction solar cells made from blend films of regioregular poly(3‐hexylthiophene) (P3HT) and methanofullerene (PCBM) has been studied. With respect to the charge transport, it is demonstrated that the electron mobility dominates the transport of the cell, varying from 10^–8 m² V^–1 s^–1 in as‐cast devices to ≈3 × 10^–7 m² V^–1 s^–1 after thermal annealing. The hole mobility in the P3HT phase of the blend is dramatically affected by thermal annealing. It increases by more than three orders of magnitude, to reach a value of up to ≈ 2 × 10^–8 m² V^–1 s^–1 after the annealing process, as a result of an improved crystallinity of the film. Moreover, upon annealing the absorption spectrum of P3HT:PCBM blends undergo a strong red‐shift, improving the spectral overlap with solar emission, which results in an increase of more than 60 % in the rate of charge‐carrier generation. Subsequently, the experimental electron and hole mobilities are used to study the photocurrent generation in P3HT:PCBM devices as a function of annealing temperature. The results indicate that the most important factor leading to a strong enhancement of the efficiency, compared with non‐annealed devices, is the increase of the hole mobility in the P3HT phase of the blend. Furthermore, numerical simulations indicate that under short‐circuit conditions the dissociation efficiency of bound electron–hole pairs at the donor/acceptor interface is close to 90 %, which explains the large quantum efficiencies measured in P3HT:PCBM blends.     

Rational Design of Small Molecular Donor for Solution‐Processed Organic Photovoltaics with 8.1% Efficiency and High Fill Factor via Multiple Fluorine Substituents and Thiophene Bridge

A series of tetrafluorine‐substituted small molecules with a D₁‐A‐D₂‐A‐D₁ linear framework based on indacenodithiophene and difluorobenzothiadiazole is designed and synthesized for application as donor materials in solution‐processed small‐molecule organic solar cells. The impacts of thiophene π‐bridge and multiple fluorinated modules on the photophysical properties, the energy levels of the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO), charge carrier mobility, the morphologies of blend films, and their photovoltaic properties as electron donor material in the photoactive layer are investigated. By incorporating multiple fluorine substituents of benzothiadiazole and inserting two thiophene spacers, the fill factor (FF), open‐circuit voltage, and short‐circuit current density are dramatically improved in comparison with fluorinated‐free materials. With the solvent vapor annealing treatment, further enhancement in charge carrier mobility and power conversion efficiency (PCE) are achieved. Finally, a high PCE of 8.1% with very‐high FF of 0.76 for BIT‐4F‐ T/PC₇₁BM is achieved without additional additive, which is among one of the highest reported for small‐molecules‐based solar cells with PCE over 8%. The results reported here clearly indicate that high PCE in solar cells based small molecules can be significantly increased through careful engineering of the molecular structure and optimization on the morphology of blend films by solvent vapor annealing.     

Entanglement of Conjugated Polymer Chains Influences Molecular Self‐Assembly and Carrier Transport

The influence of polymer entanglement on the self‐assembly, molecular packing structure, and microstructure of low‐M_w (lightly entangled) and high‐M_w (highly entangled) poly (3‐hexylthiophene) (P3HT), and the carrier transport in thin‐film transistors, are investigated. The polymer chains are gradually disentangled in a marginal solvent via ultrasonication of the polymer solution, and demonstrate improved diffusivity of precursor species (coils, aggregates, and microcrystallites), enhanced nucleation and crystallization of P3HT in solution, and self‐assembly of well‐ordered and highly textured fibrils at the solid–liquid interface. In low‐M_w P3HT, reducing chain entanglement enhances interchain and intrachain ordering, but reduces the interconnectivity of ordered domains (tie molecules) due to the presence of short chains, thus deteriorating carrier transport even in the face of improving crystallinity. Reducing chain entanglement in high‐M_w P3HT solutions increases carrier mobility up to ≈20‐fold, by enhancing interchain and intrachain ordering while maintaining a sufficiently large number of tie molecules between ordered domains. These results indicate that charge carrier mobility is strongly governed by the balancing of intrachain and interchain ordering, on the one hand, and interconnectivity of ordered domains, on the other hand. In high‐M_w P3HT, intrachain and interchain ordering appear to be the key bottlenecks to charge transport, whereas in low‐M_w P3HT, the limited interconnectivity of the ordered domains acts as the primary bottleneck to charge transport.     

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

The 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 (V_oc = 0.83 V) and a high power conversion efficiency (PCE = 5.95%), which are much better than those of the P3HT:PCBM‐based device (V_oc = 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.     

Effects of Photo‐oxidation on the Performance of Poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene]:[6,6]‐Phenyl C61‐Butyric Acid Methyl Ester Solar Cells

A study of the photo‐oxidation of films of poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene] (MDMO‐PPV) blended with [6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM), and solar cells based thereon, is presented. Solar‐cell performance is degraded primarily through loss in short‐circuit current density, J_SC. The effect of the same photodegradation treatment on the optical‐absorption, charge‐recombination, and charge‐transport properties of the active layer is studied. It is concluded that the loss in J_SC is primarily due to a reduction in charge‐carrier mobility, owing to the creation of more deep traps in the polymer during photo‐oxidation. Recombination is slowed down by the degradation and cannot therefore explain the loss in photocurrent. Optical absorption is reduced by photo‐bleaching, but the size of this effect alone is insufficient to explain the loss in device photocurrent.     

Tuning the Dimensions of C60‐Based Needlelike Crystals in Blended Thin Films

A new ordered structure of the C₆₀ derivative PCBM ([6‐6]‐phenyl C₆₁‐butyric acid methyl ester) is obtained in thin films based on the blend PCBM:regioregular P3HT (poly(3‐hexylthiophene)). Rapid formation of needlelike crystalline PCBM structures of a few micrometers up to 100 μm in size is demonstrated by submitting the blended thin films to an appropriate thermal treatment. These structures can grow out to a 2D network of PCBM needles and, in specific cases, to spectacular PCBM fans. Key parameters to tune the dimensions and spatial distribution of the PCBM needles are blend ratio and annealing conditions. The as‐obtained blended films and crystals are probed using atomic force microscopy, transmission electron microscopy, selected area electron diffraction, optical microscopy, and confocal fluorescence microscopy. Based on the analytical results, the growth mechanism of the PCBM structures within the film is described in terms of diffusion of PCBM towards the PCBM crystals, leaving highly crystalline P3HT behind in the surrounding matrix.     

VOC enhancement in polymer solar cells with isobenzofulvene–C60 adducts

We report the use of isobenzofulvene–C₆₀ adducts in bulk heterojunction organic solar cells, synthesized via the [4 + 2] cycloaddition of C₆₀ with an in situ generated isobenzofulvene intermediate. The LUMO energy levels of these adducts are 20–180 meV higher than that of PCBM ([6,6]-phenyl-C₆₁-butyric acid methyl ester). This large increase of the LUMO level is attributed to cofacial π-orbital interactions between the fullerene surface and the isobenzofulvene π–system (aromatic ring and double bond). Raised LUMO levels of fullerenes, together with their desirably slow recombination dynamics, led to higher open-circuit voltages (V_OC) in bulk heterojunction polymer solar cells (up to 0.75 V for bisadducts) relative to cells tested in parallel using the well-known PCBM as the fullerene acceptor. In addition to enhanced V_OC, the short-circuit current densities (J_SC) were improved in the devices containing the epoxide analogs of the isobenzofulvene–C₆₀. Notably the epoxide derivative of the monoadduct (IBF–Ep) exhibited ∼20% enhancement of power conversion efficiency (PCE) compared to reference P3HT:PCBM solar cells. A combination of optical and electronic methods was used to investigate the origin of the PCE enhancement observed with these new fullerene acceptors with particular attention to the increased V_OCs.     

Synthesis of a Modified PC70BM and Its Application as an Electron Acceptor with Poly(3‐hexylthiophene) as an Electron Donor for Efficient Bulk Heterojunction Solar Cells

A simple and effective modification of phenyl‐C₇₀‐butyric acid methyl ester (PC₇₀BM) is carried out in a single step after which the material is used as electron acceptor for bulk heterojunction polymer solar cells (PSCs). The modified PC₇₀BM, namely CN‐PC₇₀BM, showed broader and stronger absorption in the visible region (350–550 nm) of the solar spectrum than PC₇₀BM because of the presence of a cyanovinylene 4‐nitrophenyl segment. The lowest unoccupied molecular energy level (LUMO) of CN‐PC₇₀BM is higher than that of PC₇₀BM by 0.15 eV. The PSC based on the blend (cast from tetrahydrofuran (THF) solution) consists of P3HT as the electron donor and CN‐PC₇₀BM as the electron acceptor and shows a power conversion efficiency (PCE) of 4.88%, which is higher than that of devices based on PC₇₀BM as the electron acceptor (3.23%). The higher PCE of the solar cell based on P3HT:CN‐PC₇₀BM is related to the increase in both the short circuit current (J_sc) and the open circuit voltage (V_oc). The increase in J_sc is related to the stronger light absorption of CN‐PC₇₀BM in the visible region of the solar spectrum as compared to that of PC₇₀BM. In other words, more excitons are generated in the bulk heterojunction (BHJ) active layer. On the other hand, the higher difference between the LUMO of CN‐PC₇₀BM and the HOMO of P3HT causes an enhancement in the V_oc. The addition of 2% (v/v) 1‐chloronapthalene (CN) to the THF solvent during film deposition results in an overall improvement of the PCE up to 5.83%. This improvement in PCE can be attributed to the enhanced crystallinity of the blend (particularly of P3HT) and more balanced charge transport in the device.