First Synthesis of the Inherently Chiral Trans-4' Bisadduct of C59 N Azafullerene by Using Cyclo-[2]-dodecylmalonate as a Tether

The multiaddition chemistry of azafullerene C₅₉ N has been scarcely explored, and the isolation of pure bisadducts is in its infancy. Encouraged by the recent regioselective synthesis of the inherently chiral equatorial_face bisadduct of C₅₉ N, we focused on the isolation of the first trans-4 bisadduct in a simple two-step approach. The first regioselective synthesis of the trans-4 bisadduct of C₅₉ N by using cyclo-[2]-dodecylmalonate as a tether is now reported. The newly synthesized bisadduct has C₁ symmetry, as evidenced by ¹³ C NMR, while X-ray crystallography validated the trans-4' addition pattern. Furthermore, the inherently chiral trans-4' C₅₉ N bisadduct was enantiomerically resolved, and the mirror-image relation of the two enantiomers was probed by circular dichroism spectroscopy. Finally, UV-Vis and redox assays suggested that the addition pattern has a reflection in the light-harvesting and redox properties of the bisadduct.

A Cross-Linkable Electron-Transport Layer Based on a Fullerene-Benzoxazine Derivative for Inverted Polymer Solar Cells

The synthesis, optoelectronic characterization and device properties of a cross-linkable fullerene derivative, [6,6]-phenyl-C₆₁ -butyric benzoxazine ester (PCBB) is reported. PCBB shows all the basic photophysical and electrochemical properties of the parent compound [6,6]-phenyl-C₆₁ -butyric methyl ester (PCBM). Thermal cross-linking of the benzoxazine moiety in PCBB resulted in the formation of cross-linked, solvent resistive adhesive films (C-PCBB). Atomic force microscopy (AFM) and optical microscopic studies showed dramatic reduction in the roughness and aggregation behaviour of P3HT-PCBM polymer blend film upon incorporation of C-PCBB interlayer. An inverted bulk heterojunction solar cell based on the configuration ITO/ZnO/C-PCBB/P3HT-PCBM/V₂ O₅ /Ag achieved 4.27 % power conversion efficiency (PCE) compared to the reference device ITO/ZnO/P3HT-PCBM/V₂ O₅ /Ag (PCE=3.28 %). This 25 % increase in the efficiency is due to the positive effects of C-PCBB on P3HT/C-PCBB and PCBM/C-PCBB heterojunctions.

Comparative density functional theory-density functional tight binding study of fullerene derivatives: effects due to fullerene size, addends, and crystallinity on band structure, charge transport and optical properties

We present a systematic comparative density functional theory-density functional tight binding study of multiple derivatives of C60 and C70 with different addends, in molecular as well as solid state. In particular, effects due to fullerene size, type and number of addends, and of crystallinity on band structure, charge transport, and optical properties are investigated. These are important, in particular, for rational selection of fullerene derivatives as acceptor and electron transport layers in organic as well as planar inverted perovskite solar cells. We find that by the choice of type and number of addends, one can modulate the LUMO within 0.4 eV. Changes in the HOMO can reach 0.7 eV. Substituting C70 for C60 results in destabilization of the HOMO by about 0.1 eV for indene and quinodimethane addends and by a less significant amount for PCBM addends. The effect of C70-C60 substitution on the LUMO is of similar magnitude. A more significant change in HOMO-LUMO energy is seen for the aryl addends. On the other hand, all C70 based molecules have strong visible absorption. For most addends, the crystal packing leads to a stabilization of both the LUMO and HOMO by about ∼0.2 and ∼0.1 eV, respectively, vs. single molecules. When using bis-addends, it is also possible to enhance the visible absorption. Electron and hole transport rates are computed to vary vastly depending on the addends chosen; specifically, we compute that indene and quimodimethane addends can enhance charge transport rates while the aryl addend is predicted to result in substantially smaller mobilities of electrons and holes, vs. PC₆₀BM. Furthermore, the -CH₂ and bisaddend addition can significantly enhance the charge transfer rates for the PCBM addend.

Oxygen-Induced Degradation in C60-Based Organic Solar Cells: Relation Between Film Properties and Device Performance

Fullerene-based molecules are the archetypical electron-accepting materials for organic photovoltaic devices. A detailed knowledge of the degradation mechanisms that occur in C60 layers will aid in the development of more stable organic solar cells. Here, the impact of storage in air on the optical and electrical properties of C60 is studied in thin films and in devices. Atmospheric exposure induces oxygen-trap states that are 0.19 eV below the LUMO of the fullerene C60. Moreover, oxygen causes a 4-fold decrease of the exciton lifetime in C60 layers, resulting in a 40% drop of short-circuit current from optimized planar heterojunction solar cells. The presence of oxygen-trap states increases the saturation current of the device, resulting in a 20% loss of open-circuit voltage. Design guidelines are outlined to improve air stability for fullerene-containing devices.

Manganese powder promoted highly efficient and selective synthesis of fullerene mono- and biscycloadducts at room temperature

Discovery of an efficient, practical, and flexible synthetic method to produce various important electron acceptors for low-cost organic photovoltaics (OPVs) is highly desirable. Although the most commonly used acceptor materials, such as PC₆₁BM, PC₇₁BM, IC₆₀BA, bisPC₆₁BM have been proved to be promising for the OPVs, they are still very expensive mainly due to their low production yields and limited synthetic methods. Herein, we report an unprecedented and innovative synthetic method of a variety of fullerene mono- and biscycloadducts by using manganese powder as a promotor. The reaction of fullerenes with various dibromides proceeds efficiently and selectively under very mild conditions to give the corresponding cycloadducts in good to excellent yields. The combination of manganese power with DMSO additive is crucial for the successful implementation of the present cycloaddition. Notably, the standard OPV acceptors, such as PCBMs, have been obtained in extraordinarily high yields, which cannot be achieved under the previously reported methods.

The dramatic effect of the annealing temperature and dielectric functionalization on the electron mobility of indene-C60 bis-adduct thin films

The effect of annealing temperature/duration and surface functionalization is explored for indene-C₆₀ bis-adduct (ICBA) films. Electron mobility approaches 0.1 cm² V⁻¹ s⁻¹.

Facile synthesis and photovoltaic applications of a new alkylated bismethano fullerene as electron acceptor for high open circuit voltage solar cells

In this work, a bis-adduct C₆₀ derivative was facilely synthesized using an alkyl solubilizing group. This semiconductor offers a higher LUMO level compared to PCBM, which resulted in a significantly enhanced V_oc of 0.73 V in organic solar cells.

Fulleropyrrolidines derived from dioxa- and trioxaalkyl-tethered diglycines

The synthesis, characterization, morphological, electrochemical and antioxidant properties of fulleropyrrolidines bridged by polyoxaalkyl chains are described.

Efficient polymer solar cells based on poly(3-hexylthiophene) and indene-C₆₀ bisadduct fabricated with non-halogenated solvents

The photovoltaic performance of poly(3-hexylthiophene) (P3HT) has been improved greatly by using indene-C60 bisadduct (ICBA) as acceptor instead of phenyl-C61-butyric acid methyl ester (PCBM). However, the solvent of dichlorobenzene (DCB) used in fabricating polymer solar cells (PSCs) limited the application of the PSCs, because of the environmental problem caused by the harmful halogenated solvent. In this work, we fabricated the PSCs based on P3HT/ICBA processed with four low-harmful non-halogenated solvents of toluene, o-xylene, m-xylene, and p-xylene. The PSCs based on P3HT/ICBA (1:1, w/w) with toluene as the solvent exhibit the optimized power conversion efficiency (PCE) of 4.5% with open-circuit voltage (Voc) of 0.84 V, short circuit current density (Jsc) of 7.2 mA/cm(2), and fill factor (FF) of 71%, under the illumination of AM 1.5G at 100 mW/cm(2). Upon using 1% N-methyl pyrrolidone (NMP) as a solvent additive in the toluene solvent, the PCE of the PSCs was greatly improved to 6.6% with a higher Jsc of 10.3 mA/cm(2) and a high FF of 75%, which is even higher than that of the devices fabricated with halogenated DCB solvent. The X-ray diffraction (XRD) measurement shows that the crystallinity of P3HT increased with the NMP additive. The investigations on morphology of the active layers by atomic force microscopy (AFM) and transmission electron microscopy (TEM) indicate that the NMP additive promotes effective phase separation and formation of nanoscaled interpenetrating network structure of the active layer, which is beneficial to the improvement of Jsc and PCE for the PSCs fabricated with toluene as the solvent.

Quinoxaline-functionalized C60 derivatives as electron acceptors in organic solar cells

Two new quinoxaline-functionalized C₆₀ derivatives with high LUMO levels have been synthesized and applied in organic solar cells. BHJ-OSCs incorporating P3HT as donor and TQMA (or TQBA) as acceptor exhibit an open-circuit voltage (V_OC) of 0.76 V (or 0.84 V), which is about 0.12 V (or 0.20 V) higher than PCBM as electron acceptor.

Influence of charge carrier mobility and morphology on solar cell parameters in devices of mono- and bis-fullerene adducts

Herein, we analyze charge carrier mobility and morphology of the active blend layer in thin film organic solar cells and correlate them with device parameters. A low band gap donor-acceptor copolymer in combination with phenyl-C61-butyric acid methyl ester (PCBM) or two bis-adduct fullerenes, bis-PCBM and bis-o-quino-dimethane C60 (bis-oQDMC), is investigated. We study the charge transport of polymer:fullerene blends in hole- and electron-only devices using the space-charge limited current method. Lower electron mobilities are observed in both bis-adduct fullerene blends. Hole mobility, however, is decreased only in the blend containing bis-oQDMC. Both bis-adduct fullerene blends show very high open circuit voltage in solar cell devices, but poor photocurrent compared to the standard PCBM blend for an active layer thickness of 200 nm. Therefore, a higher short circuit current is feasible for the polymer:bis-PCBM blend by reducing the active layer thickness in order to compensate for the low electron mobility, which results in a PCE of 4.3%. For the polymer:bis-oQDMC blend, no such improvement is achieved due to an unfavorable morphology in this particular blend system. The results are supported by external quantum efficiency measurements, atomic force microscopy, transmission electron microscopy and UV/vis spectroscopy. Based on these results, the investigations presented herein give a more scientific basis for the optimization of solar cells.

Fullerene-bisadduct acceptors for polymer solar cells

Polymer solar cells (PSCs) have drawn great attention in recent years for their simple device structure, light weight, and low-cost fabrication in comparison with inorganic semiconductor solar cells. However, the power-conversion efficiency (PCE) of PSCs needs to be increased for their future application. The key issue for improving the PCE of PSCs is the design and synthesis of high-efficiency conjugated polymer donors and fullerene acceptors for the photovoltaic materials. For the acceptor materials, several fullerene-bisadduct acceptors with high LUMO energy levels have demonstrated excellent photovoltaic performance in PSCs with P3HT as a donor. In this Focus Review, recent progress in high-efficiency fullerene-bisadduct acceptors is discussed, including the bisadduct of PCBM, indene-C60 bisadduct (ICBA), indene-C70 bisadduct (IC70BA), DMPCBA, NCBA, and bisTOQC. The LUMO levels and photovoltaic performance of these bisadduct acceptors with P3HT as a donor are summarized and compared. In addition, the applications of an ICBA acceptor in new device structures and with other conjugated polymer donors than P3HT are also introduced and discussed.

Effect of fullerene tris-adducts on the photovoltaic performance of P3HT:fullerene ternary blends

Fullerene tris-adducts have the potential of achieving high open-circuit voltages (V(OC)) in bulk heterojunction (BHJ) polymer solar cells (PSCs), because their lowest unoccupied molecular orbital (LUMO) level is higher than those of fullerene mono- and bis-adducts. However, no successful examples of the use of fullerene tris-adducts as electron acceptors have been reported. Herein, we developed a ternary-blend approach for the use of fullerene tris-adducts to fully exploit the merit of their high LUMO level. The compound o-xylenyl C60 tris-adduct (OXCTA) was used as a ternary acceptor in the model system of poly(3-hexylthiophene) (P3HT) as the electron donor and the two soluble fullerene acceptors of OXCTA and fullerene monoadduct (o-xylenyl C60 monoadduct (OXCMA), phenyl C61-butyric acid methyl ester (PCBM), or indene-C60 monoadduct (ICMA)). To explore the effect of OXCTA in ternary-blend PSC devices, the photovoltaic behavior of the device was investigated in terms of the weight fraction of OXCTA (W(OXCTA)). When W(OXCTA) is small (<0.3), OXCTA can generate a synergistic bridging effect between P3HT and the fullerene monoadduct, leading to simultaneous enhancement in both V(OC) and short-circuit current (J(SC)). For example, the ternary PSC devices of P3HT:(OXCMA:OXCTA) with W(OXCTA) of 0.1 and 0.3 exhibited power-conversion efficiencies (PCEs) of 3.91% and 3.96%, respectively, which were significantly higher than the 3.61% provided by the P3HT:OXCMA device. Interestingly, for W(OXCTA) > 0.7, both V(OC) and PCE of the ternary-blend PSCs exhibited nonlinear compositional dependence on W(OXCTA). We noted that the nonlinear compositional trend of P3HT:(OXCMA:OXCTA) was significantly different from that of P3HT:(OXCMA:o-xylenyl C60 bis-adduct (OXCBA)) ternary-blend PSC devices. The fundamental reasons for the differences between the photovoltaic trends of the two different ternary-blend systems were investigated systemically by comparing their optical, electrical, and morphological properties.

Efficient light trapping in inverted polymer solar cells by a randomly nanostructured electrode using monodispersed polymer nanoparticles

The randomly nanotextured back electrode provides a simple and efficient route for enhancing photocurrent in polymer solar cells (PSCs) by light trapping, which can increase light absorption within a finite thickness of the active layer. In this study, we incorporated mono-disperse 60 nm polystyrene nanoparticles (PS NPs) into a 50 nm thick poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) anode buffer layer (ABL) to create a randomly nanotextured back electrode with 10 nm height variations in inverted-type PSCs. The roughened interface between the PS NP-PEDOT:PSS ABL and the Ag electrode scatters light in the visible range, leading to efficient light trapping within the device and enhanced light absorption in the active layer. Inverted PSCs with randomly nanotextured electrodes (φ(NP) = 0.31) showed short-circuit current density (J(SC)) and power conversion efficiency (PCE) values that were 15% higher than those of control devices with flat electrodes. External quantum efficiency, reflectance, and optical light scattering as a function of ϕ(NP) were examined to determine the origin of the enhancement in J(SC) and PCE.

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.

Nanosphere templated continuous PEDOT:PSS films with low percolation threshold for application in efficient polymer solar cells

Nanometer-sized monodisperse polystyrene nanospheres (PS NS) were designed as an opal template for the formation of three-dimensionally continuous poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films. The resultant films were successfully applied as the anode buffer layer (ABL) to produce highly efficient polymer solar cells (PSCs) with enhanced stability. The conductivity of the PS NS-PEDOT:PSS films was maintained up to ø(PS) = 0.75-0.80, indicating that the formation of continuous PEDOT:PSS films using PS NS templates was successful. To demonstrate the applicability of the PS NS-PEDOT:PSS film for organic electronics, the PS NS-PEDOT:PSS films were used as ABLs in two different PSCs: P3HT:PCBM and P3HT:OXCBA. The photovoltaic performances of both PSCs were maintained up to ø(PS) = 0.8. In particular, the power conversion efficiency of the P3HT:OXCBA PSC with a PS NS-PEDOT:PSS ABL (ø(PS) = 0.8) was greater than 5% and the air stability of the device was significantly enhanced.

Small molecule semiconductors for high-efficiency organic photovoltaics

Organic photovoltaic cells (OPVs) are a promising cost-effective alternative to silicon-based solar cells, and possess light-weight, low-cost, and flexibility advantages. Significant progress has been achieved in the development of novel photovoltaic materials and device structures in the last decade. Nowadays small molecular semiconductors for OPVs have attracted considerable attention, due to their advantages over their polymer counterparts, including well-defined molecular structure, definite molecular weight, and high purity without batch to batch variations. The highest power conversion efficiencies of OPVs based on small molecular donor/fullerene acceptors or polymeric donor/fullerene acceptors are up to 6.7% and 8.3%, respectively, and meanwhile nonfullerene acceptors have also exhibited some promising results. In this review we summarize the developments in small molecular donors, acceptors (fullerene derivatives and nonfullerene molecules), and donor-acceptor dyad systems for high-performance multilayer, bulk heterojunction, and single-component OPVs. We focus on correlations of molecular chemical structures with properties, such as absorption, energy levels, charge mobilities, and photovoltaic performances. This structure-property relationship analysis may guide rational structural design and evaluation of photovoltaic materials (253 references).

Tuning the energy level offset between donor and acceptor with ferroelectric dipole layers for increased efficiency in bilayer organic photovoltaic cells

Ultrathin ferroelectric polyvinylidene fluoride (70%)-tetrafluoroethylene (30%) copolymer film is inserted between the poly3(hexylthiophene) (P3HT) donor and [6,6]-phenyl-C61-butyric acid methylester (PCBM) acceptor layers as the dipole layer to tune the relative energy levels, which can potentially maximize the open circuit voltage of bilayer organic solar cells. In this work, the power conversion efficiency of P3HT/PCBM bilayer solar cells is demonstrated to be doubled with the inserted dipoles.

Controlling number of indene solubilizing groups in multiadduct fullerenes for tuning optoelectronic properties and open-circuit voltage in organic solar cells

The ability to tune the lowest unoccupied molecular orbital (LUMO)/highest occupied molecular orbital (HOMO) levels of fullerene derivatives used as electron acceptors is crucial in controlling the optical/electrochemical properties of these materials and the open circuit voltage (V(oc)) of solar cells. Here, we report a series of indene fullerene multiadducts (ICMA, ICBA, and ICTA) in which different numbers of indene solubilizing groups are attached to the fullerene molecule. The addition of indene units to fullerene raised its LUMO and HOMO levels, resulting in higher V(oc) values in the photovoltaic device. Bulk-heterojunction (BHJ) solar cells fabricated from poly(3-hexylthiophene) (P3HT) and a series of fullerene multiadducts-ICMA, ICBA, and ICTA showed V(oc) values of 0.65, 0.83, and 0.92 V, respectively. Despite demonstrating the highest V(oc) value, the P3HT:ICTA device exhibited lower efficiency (1.56%) than the P3HT:ICBA device (5.26%) because of its lower fill factor and current. This result could be explained by the lower light absorption and electron mobility of the P3HT:ICTA device, suggesting that there is an optimal number of the solubilizing group that can be added to the fullerene molecule. The effects of the addition of solubilizing groups on the optoelectrical properties of fullerene derivatives were carefully investigated to elucidate the molecular structure-device function relationship.