Hybrid Solar Cells from a Blend of Poly(3‐hexylthiophene) and Ligand‐Capped TiO2 Nanorods

Hybrid bulk heterojunction solar cells based on nanocrystalline TiO₂ (nc‐TiO₂) nanorods capped with trioctylphosphine oxide (TOPO) and regioregular poly(3‐hexylthiophene) (P3HT) are processed from solution and characterized in order to relate the device function (optical absorption, charge separation, and transport and photovoltaic properties) to active‐layer properties and device parameters. Annealing the blend films is found to greatly improve the polymer–metal oxide interaction at the nc‐TiO₂/P3HT interface, resulting in a six‐fold increase of the charge separation yield and improved photovoltaic device performance under simulated solar illumination. In addition, the influence of the organic ligand at the nc‐TiO₂ particle surface is found to be crucial for charge separation. Ligand‐exchange procedures applied on the TOPO‐capped nc‐TiO₂ nanorods with an amphiphilic ruthenium‐based dye are found to further improve the charge‐separation yield at the polymer–nanocrystal interface. However, the poor photocurrents generated in the hybrid blend devices, before and after ligand exchange, suggest that transport within or between nanoparticles limits performance. By comparison with other donor–acceptor bulk heterojunction systems, we conclude that charge transport in the nc‐TiO₂:P3HT blend films is limited by the presence of an intrinsic trap distribution mainly associated with the nc‐TiO₂ particles.     

Tuning Conversion Efficiency in Metallo Endohedral Fullerene‐Based Organic Photovoltaic Devices

Here the influence that 1‐(3‐hexoxycarbonyl)propyl‐1‐phenyl‐[6,6]‐Lu₃N@C₈₁, Lu₃N@C₈₀–PCBH, a novel acceptor material, has on active layer morphology and the performance of organic photovoltaic (OPV) devices using this material is reported. Polymer/fullerene blend films with poly(3‐hexylthiophene), P3HT, donor material and Lu₃N@C₈₀–PCBH acceptor material are studied using absorption spectroscopy, grazing incident X‐ray diffraction and photocurrent spectra of photovoltaic devices. Due to a smaller molecular orbital offset the OPV devices built with Lu₃N@C₈₀–PCBH display increased open circuit voltage over empty cage fullerene acceptors. The photovoltaic performance of these metallo endohedral fullerene blend films is found to be highly impacted by the fullerene loading. The results indicate that the optimized blend ratio in a P3HT matrix differs from a molecular equivalent of an optimized P3HT/[6,6]‐phenyl‐C₆₁‐butyric methyl ester, C₆₀–PCBM, active layer, and this is related to the physical differences of the C₈₀ fullerene. The influence that active layer annealing has on the OPV performance is further evaluated. Through properly matching the film processing and the donor/acceptor ratio, devices with power conversion efficiency greater than 4% are demonstrated.     

High‐Performance Metal‐Free Solar Cells Using Stamp Transfer Printed Vapor Phase Polymerized Poly(3,4‐Ethylenedioxythiophene) Top Anodes

The use of vapor phase polymerized poly(3,4‐ethylenedioxythiophene) (VPP‐PEDOT) as a metal‐replacement top anode for inverted solar cells is reported. Devices with both i) standard bulk heterojunction blends of poly(3‐hexylthiophene) (P3HT) donor and 1‐(3‐methoxycarbonyl)‐propyl‐1‐phenyl‐(6,6)C₆₀ (PCBM) soluble fullerene acceptor and ii) hybrid inorganic/organic TiO₂/P3HT acceptor/donor active layers are studied. Stamp transfer printing methods are used to deposit both the VPP‐PEDOT top anode and a work function enhancing PEDOT:polystyrenesulphonate (PEDOT:PSS) interlayer. The metal‐free devices perform comparably to conventional devices with an evaporated metal top anode, yielding power conversion efficiencies of 3% for bulk heterojunction blend and 0.6% for organic/inorganic hybrid structures. These encouraging results suggest that stamp transfer printed VPP‐PEDOT provides a useful addition to the electrode materials tool‐box available for low temperature and non‐vacuum solar cell fabrication.     

Enhanced photovoltaic performance in TiO2/P3HT hybrid solar cell by interface modification

A TiO2/P3HT hybrid solar cell was fabricated by infiltrating P3HT into the pores of TiO2 nanorod arrays. To further enhance the photovoltaic performance, anthracene-9-carboxylic acid was employed to modify the interface of TiO2/P3 HT before P3HT was coated. Results revealed that the interface treatment significantly enhances the photovoltaic performance of the cell. The efficiency of the hybrid solar cells reaches 0.28% after interface modification, which is three times higher compared with the un-modified one. We find that except for the increased exciton dissociation efficiency recognized by the previous reports, the suppressing of electron back recombination is another important factor leading to the enhanced photovoltaic performance.     

Influence of the Relative Humidity on the Performance of Polymer/TiO2 Photovoltaic Cells

Hydrolysis of titanium(IV ) isopropoxide (TTIP) is a well‐known method for the fabrication of TiO₂. Normally it is made via a sol–gel reaction in the presence of water. In this paper we report on the preparation of flat TiO₂ films for conjugated polymer/TiO₂ photovoltaic cells, from a TTIP/isopropanol solution. It is shown that the morphological structure of the TiO₂ film is strongly dependent on the relative humidity during spin‐coating of the TTIP/isopropanol solution. In bilayer devices consisting of TiO₂/poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene] (MDMO‐PPV), a low relative humidity (< 25 %, room temperature) is needed in order to form smooth, transparent TiO₂ films. Increasing the relative humidity results in porous TiO₂ films with a high surface roughness, which leads to shunted devices. Apart from bilayer devices, bulk‐heterojunction (BHJ) hybrid TiO₂:MDMO‐PPV photovoltaic cells have been made, by spin‐coating a mixture of TTIP and MDMO‐PPV in toluene. Again a strong relation was found between the relative humidity during spin‐coating and the current–voltage characteristics of the devices. However, in contrast to the bilayer devices, the best BHJ devices were made at higher relative humidity. The observed performance dependence on relative humidity is discussed in relation to the TiO₂ morphology.     

Band‐Offset Engineering for Enhanced Open‐Circuit Voltage in Polymer–Oxide Hybrid Solar Cells

The power conversion efficiency of organic and hybrid solar cells is commonly reduced by a low open‐circuit voltage (V_OC). In these cases, the V_OC is significantly less than the energy of the lowest energy absorbed photon, divided by the elementary charge q. The low photovoltage originates from characteristically large band offsets between the electron donor and acceptor species. Here a simple method is reported to systematically tune the band offset in a π‐conjugated polymer–metal oxide hybrid donor–acceptor system in order to maximize the V_OC. It is demonstrated that substitution of magnesium into a zinc oxide acceptor (ZnMgO) reduces the band offset and results in a substantial increase in the V_OC of poly(3‐hexylthiophene) (P3HT)–ZnMgO planar devices. The V_OC is seen to increase from 500 mV at x = 0 up to values in excess of 900 mV for x = 0.35. A concomitant increase in overall device efficiency is seen as x is increased from 0 to 0.25, with a maximum power‐conversion efficiency of 0.5 % obtained at x = 0.25, beyond which the efficiency decreases because of increased series resistance in the device. This work provides a new tool for understanding the role of the donor–acceptor band offset in hybrid photovoltaics and for maximizing the photovoltage and power‐conversion efficiency in such devices.     

Synthesis,Morphology, and Properties of Poly(3‐hexylthiophene)‐block‐Poly(vinylphenyl oxadiazole) Donor–Acceptor Rod–Coil Block Copolymers and Their Memory Device Applications

Novel donor–acceptor rod–coil diblock copolymers of regioregular poly(3‐hexylthiophene) ( P3HT )‐block‐poly(2‐phenyl‐5‐(4‐vinylphenyl)‐1,3,4‐oxadiaz‐ole) ( POXD ) are successfully synthesized by the combination of a modified Grignard metathesis reaction ( GRIM ) and atom transfer radical polymerization ( ATRP ). The effects of the block ratios of the P3HT donor and POXD pendant acceptor blocks on the morphology, field effect transistor mobility, and memory device characteristics are explored. The TEM, SAXS, WAXS, and AFM results suggest that the coil block fraction significantly affects the chain packing of the P3HT block and depresses its crystallinity. The optical absorption spectra indicate that the intramolecular charge transfer between the main chain P3HT donor and the side chain POXD acceptor is relatively weak and the level of order of P3HT chains is reduced by the incorporation of the POXD acceptor. The field effect transistor (FET) hole mobility of the system exhibits a similar trend on the optical properties, which are also decreased with the reduced ordered P3HT crystallinity. The low‐lying highest occupied molecular orbital (HOMO) energy level (–6.08 eV) of POXD is employed as charge trap for the electrical switching memory devices. P3HT‐ b ‐POXD exhibits a non‐volatile bistable memory or insulator behavior depending on the P3HT / POXD block ratio and the resulting morphology. The ITO/ P3HT₄₄ ‐b‐ POXD₁₈ /Al memory device shows a non‐volatile switching characteristic with negative differential resistance (NDR) effect due to the charge trapped POXD block. These experimental results provide the new strategies for the design of donor‐acceptor rod‐coil block copolymers for controlling morphology and physical properties as well as advanced memory device applications.     

Charge Generation at Polymer/Metal Oxide Interface: from Molecular Scale Dynamics to Mesoscopic Effects

The correlation between molecular scale morphology and charge generation across hybrid photovoltaic interfaces made of metal oxides (ZnO and TiO₂) and a prototypical electron donor polymer, P3HT, is investigated. Device characterization and UV‐NIR transient absorption spectroscopy are used to demonstrate that the local disorder of the polymer chains on the surface of the metal–oxide film provides better electron injection efficiencies than the crystalline phases, though the latter are essential for energy and charge transport. An unambiguous spectroscopic tool is also demonstrated to probe the occupation of the conduction band of ZnO following the electron injection from the polymer through the ultrafast tracking of the Burstein‐Moss effect.     

Template‐Directed Liquid ALD Growth of TiO2 Nanotube Arrays: Properties and Potential in Photovoltaic Devices

Dense and well‐aligned arrays of TiO₂ nanotubes extending from various substrates are successfully fabricated via a new liquid‐phase atomic layer deposition (LALD) in nanoporous anodic alumina (AAO) templates followed by alumina dissolution. The facile and versatile process circumvents the need for vacuum conditions critical in traditional gas‐phase ALD and yet confers ALD‐like deposition rates of 1.6–2.2 Å cycle^?1, rendering smooth conformal nanotube walls that surpass those achievable by sol–gel and Ti‐anodizing techniques. The nanotube dimensions can be tuned, with most robust structures being 150–400 nm tall, 60–70 nm in diameter with 5–20 nm thick walls. The viability of TiO₂ nanotube arrays deposited on indium tin oxide (ITO)–glass electrodes for application in model hybrid poly(3‐hexylthiophene) (P3HT):TiO₂ solar cells is studied. The results achieved provide platforms and research directions for further advancements.     

Efficient Polymer Solar Cells Based on Poly(3‐hexylthiophene):Indene‐C70 Bisadduct with a MoO3 Buffer Layer

Polymer solar cells (PSCs) with poly(3‐hexylthiophene) (P3HT) as a donor, an indene‐C₇₀ bisadduct (IC₇₀BA) as an acceptor, a layer of indium tin oxide modified by MoO₃ as a positive electrode, and Ca/Al as a negative electrode are presented. The photovoltaic performance of the PSCs was optimized by controlling spin‐coating time (solvent annealing time) and thermal annealing, and the effect of the spin‐coating times on absorption spectra, X‐ray diffraction patterns, and transmission electron microscopy images of P3HT/IC₇₀BA blend films were systematically investigated. Optimized PSCs were obtained from P3HT/IC₇₀BA (1:1, w/w), which exhibited a high power conversion efficiency of 6.68%. The excellent performance of the PSCs is attributed to the higher crystallinity of P3HT and better a donor–acceptor interpenetrating network of the active layer prepared under the optimized conditions. In addition, PSCs with a poly(3,4‐ethylenedioxy‐thiophene):poly(styrenesulfonate) (PEDOT:PSS) buffer layer under the same optimized conditions showed a PCE of 6.20%. The results indicate that the MoO₃ buffer layer in the PSCs based on P3HT/IC₇₀BA is superior to that of the PEDOT:PSS buffer layer, not only showing a higher device stability but also resulting in a better photovoltaic performance of the PSCs.     

Diphenylmethanofullerenes: New and Efficient Acceptors in Bulk‐Heterojunction Solar Cells

A novel fullerene derivative, 1,1‐bis(4,4′‐dodecyloxyphenyl)‐(5,6) C₆₁, diphenylmethanofullerene (DPM‐12), has been investigated as a possible electron acceptor in photovoltaic devices, in combination with two different conjugated polymers poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐para‐phenylene vinylene] (OC₁C₁₀‐PPV) and poly[3‐hexyl thiophene‐2,5‐diyl] (P3HT). High open‐circuit voltages, V_OC = 0.92 and 0.65 V, have been measured for OC₁C₁₀‐PPV:DPM‐12‐ and P3HT:DPM‐12‐based devices, respectively. In both cases, V_OC is 100 mV above the values measured on devices using another routinely used fullerene acceptor, [6,6]‐phenyl‐C₆₁ butyric acid methyl ester (PCBM). This is somewhat unexpected when taking into account the identical redox potentials of both acceptor materials at room temperature. The temperature‐dependent V_OC reveals, however, the same effective bandgap (HOMO_Polymer–LUMO_Fullerene; HOMO = highest occupied molecular orbital, LUMO = lowest unoccupied molecular orbital) of 1.15 and 0.9 eV for OC₁C₁₀‐PPV and P3HT, respectively, independent of the acceptor used. The higher V_OC at room temperature is explained by different ideality factors in the dark‐diode characteristics. Under white‐light illumination (80 mW cm^–2), photocurrent densities of 1.3 and 4.7 mA cm^–2 have been obtained in the OC₁C₁₀‐PPV:DPM‐12‐ and P3HT:DPM‐12‐based devices, respectively. Temperature‐dependent current density versus voltage characteristics reveal a thermally activated (shallow trap recombination limited) photocurrent in the case of OC₁C₁₀‐PPV:DPM‐12, and a nearly temperature‐independent current density in P3HT:DPM‐12. The latter clearly indicates that charge carriers traverse the active layer without significant recombination, which is due to the higher hole‐mobility–lifetime product in P3HT. At the same time, the field‐effect electron mobility in pure DPM‐12 has been found to be μ_e = 2 × 10^–4 cm² V^–1 s^–1, that is, forty‐times lower than the one measured in PCBM (μ_e = 8 × 10^–3 cm² V^–1 s^–1).     

The Effect of Polymer Optoelectronic Properties on the Performance of Multilayer Hybrid Polymer/TiO2 Solar Cells

We report a study of the effects of polymer optoelectronic properties on the performance of photovoltaic devices consisting of nanocrystalline TiO₂ and a conjugated polymer. Three different poly(2‐methoxy‐5‐(2′‐ethylhexoxy)‐1,4‐phenylenevinylene) (MEH‐PPV)‐based polymers and a fluorene–bithiophene copolymer are compared. We use photoluminescence quenching, time‐of‐flight mobility measurements, and optical spectroscopy to characterize the exciton‐transport, charge‐transport, and light‐harvesting properties, respectively, of the polymers, and correlate these material properties with photovoltaic‐device performance. We find that photocurrent is primarily limited by the photogeneration rate and by the quality of the interfaces, rather than by hole transport in the polymer. We have also studied the photovoltaic performance of these TiO₂/polymer devices as a function of the fabrication route and device design. Including a dip‐coating step before spin‐coating the polymer leads to excellent polymer penetration into highly structured TiO₂ networks, as was confirmed through transient optical measurements of the photoinduced charge‐transfer yield and recombination kinetics. Device performance is further improved for all material combinations studied, by introducing a layer of poly(ethylene dioxythiophene) (PEDOT) doped with poly(styrene sulfonic acid) (PSS) under the top contact. Optimized devices incorporating the additional dip‐coated and PEDOT:PSS layers produced a short‐circuit current density of about 1 mA cm^–2, a fill factor of 0.50, and an open‐circuit voltage of 0.86 V under simulated AM 1.5 illumination (100 mW cm^–2, 1 sun). The corresponding power conversion efficiency under 1 sun was ≥ 0.4 %.     

Photo‐induced Charge Transfer and Relaxation of Persistent Charge Carriers in Polymer/Nanocrystal Composites for Applications in Hybrid Solar Cells

The photo‐induced charge transfer and the dynamics of persistent charge carriers in blends of semiconducting polymers and nanocrystals are investigated. Regioregular poly(3‐hexylthiophene) (P3HT) is used as the electron donor material, while the acceptor moiety is established by CdSe nanocrystals (nc‐CdSe) prepared via colloidal synthesis. As a reference system, organic blends of P3HT and [6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM) are studied as well. The light‐induced charge transfer between P3HT and the acceptor materials is studied by photoluminescence (PL), photo‐induced absorption (PIA) and light‐induced electron spin resonance spectroscopy (LESR). Compared to neat P3HT samples, both systems show an intensified formation of polarons in the polymer upon photo‐excitation, pointing out successful separation of photogenerated charge carriers. Additionally, relaxation of the persistent charge carriers is investigated, and significant differences are found between the hybrid composite and the purely organic system. While relaxation, reflected in the transient signal decay of the polaron signal, is fast in the organic system, the hybrid blends exhibit long‐term persistence. The appearance of a second, slow recombination channel indicates the existence of deep trap states in the hybrid system, which leads to the capture of a large fraction of charge carriers. A change of polymer conformation due to the presence of nc‐CdSe is revealed by low temperature LESR measurements and microwave saturation techniques. The impact of the different recombination behavior on the photovoltaic efficiency of both systems is discussed.     

Solid‐State Photovoltaic Thin Films using TiO2, Organic Dyes,and Layer‐by‐Layer Polyelectrolyte Nanocomposites

We report photovoltaic devices consisting of patterned TiO₂, porphyrin dyes, and layer‐by‐layer (LBL) polyelectrolyte multilayer/oligoethylene glycol dicarboxylic acid (OEGDA) composite films. A composite polyelectrolyte LBL/OEGDA film was fabricated by formation of an alternating multilayer of linear polyethyleneimine (LPEI) and polyacrylic acid (PAA), followed by immersion of the LBL film into an OEGDA aqueous solution. The ionic conductivity attained in this LBL LPEI/PAA and OEGDA composite film was approximately 10^–5 S cm^–1 at room temperature and humidity. Investigations of dye‐sensitized photovoltaic devices constructed with the LBL (LPEI/PAA)/OEGDA composite films, TiO₂, and four types of porphyrin dyes resulted in optimization of the dye molecule and its orientation at the interface with the ionically conductive composite. The photocurrent value of photovoltaic devices constructed with the composite LBL/OEGDA film from illumination of a xenon white light source exhibited a nearly 1.5 times enhancement over the device without OEGDA. This enhancement of the photocurrent was due to the high room‐temperature ionic conductivity of the multilayer composite film. Further marked improvements of the photovoltaic performance were achieved by patterning the TiO₂ electrode using polymer stamping as a template for TiO₂ deposition. The device with patterned TiO₂ electrodes exhibited almost 10 times larger conversion efficiencies than a similar device without patterning.     

Beyond PCBM: Understanding the Photovoltaic Performance of Blends of Indene‐C60 Multiadducts with Poly(3‐hexylthiophene)

The effect of functionalization of the C₆₀ cage with multiple indene groups in relation to the dynamics of photogenerated species in blends with poly(3‐hexylthiophene) (P3HT) and the performance of P3HT:indene‐C₆₀ photovoltaic devices is reported. Despite the systematic decrease of the electron affinity of the acceptor with the number of additions, exciton dissociation is efficient in blends of P3HT with all three indene‐C₆₀ derivatives. By replacing the prototypical acceptor [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) with mono‐indene‐C₆₀ (ICMA) or a sample of a mixture of bis‐indene‐C₆₀ regioisomers (ICBA) the power conversion efficiency is enhanced, predominantly due to an increase in the open‐circuit voltage that originates from the lower electron affinity of the indene‐C₆₀ acceptor. The use of an acceptor sample that represents a mixture of tris‐indene‐C₆₀ (ICTA) regioisomers results in a reduction of the short‐circuit current density, fill factor, and open‐circuit voltage of the photovoltaic device. The electron mobility in ICTA domains is ca. a factor 10 lower than in ICMA and ICBA. Density functional theory calculations of the LUMO energies in ICTA isomers demonstrate that energetic disorder caused by the presence of regioisomers is unlikely to be responsible for the low electron mobility in ICTA. The observed deterioration in device performance is attributed to the formation of small ICTA clusters “coated” in insulating indene units that reduce electronic coupling between the molecules and cause the low electron mobility in ICTA domains. These findings indicate that while multiple additions to a fullerene cage provide a facile methodology for controlling the energy levels, they may have limited success in improving OPV device performance.     

Donor–Acceptor Poly(3‐hexylthiophene)‐block‐Pendent Poly(isoindigo) with Dual Roles of Charge Transporting and Storage Layer for High‐Performance Transistor‐Type Memory Applications

Field‐effect transistor memories usually require one additional charge storage layer between the gate contact and organic semiconductor channel. To avoid such complication, new donor–acceptor rod–coil diblock copolymers (P3HT₄₄‐b‐Piso_n) of poly(3‐hexylthiophene) (P3HT)‐block‐poly(pendent isoindigo) (Piso) are designed, which exhibit high performance transistor memory characteristics without additional charge storage layer. The P3HT and Piso blocks are acted as the charge transporting and storage elements, respectively. The prepared P3HT₄₄‐b‐Piso_n can be self‐assembled into fibrillar‐like nanostructures after the thermal annealing process, confirmed by atomic force microscopy and grazing‐incidence X‐ray diffraction. The lowest‐unoccupied molecular orbital levels of the studied polymers are significantly lowered as the block length of Piso increases, leading to a stronger electron affinity as well as charge storage capability. The field‐effect transistors (FETs) fabricated from P3HT₄₄‐b‐Piso_n possess p‐type mobilities up to 4.56 × 10^?2 cm² V^?1 s^?1, similar to that of the regioregular P3HT. More interestingly, the FET memory devices fabricated from P3HT₄₄‐b‐Piso_n exhibit a memory window ranging from 26 to 79 V by manipulating the block length of Piso, and showed stable long‐term data endurance. The results suggest that the FET characteristics and data storage capability can be effectively tuned simultaneously through donor/acceptor ratio and thin film morphology in the block copolymer system.     

Sensing of Vaporous Organic Compounds by TiO2 Porous Films Covered with Polythiophene Layers

This paper describes the detection of volatile organic compounds (VOCs) through analyses of two output signals from integrated microcantilever sensor arrays coated with organic‐inorganic hybrid sensing layers. The surface of TiO₂ porous films was modified by amphiphilic terthiophene monomers and the adsorbed monomers were polymerized at the surface of TiO₂ nanoparticles. The TiO₂ porous films covered with polythiophene layers worked as highly sensitive sensing interfaces to provide two output signals for weight and resistance changes during exposure to VOC vapor. When the TiO₂ porous films onto the sensor arrays were dyed with various kinds of amphiphilic monomers with different substituents, the resulting films provide exact information on VOC concentration from the mass changes as well as VOC classification from the analyses of response patterns.     

High‐Efficiency Organic Solar Cells Based on Preformed Poly(3‐hexylthiophene) Nanowires

Bulk heterojunction solar cells based on blends of poly(3‐hexylthiophene) (P3HT) and phenyl‐C61‐butyric acid methyl ester (PC₆₁BM) are fabricated using self‐assembled P3HT nanowires in a marginal solvent without post‐treatments. The interconnected network structures of self‐organized P3HT nanowires create continuous percolation pathways through the active layer and contribute to enhanced carrier mobility. The morphology and photovoltaic properties are studied as a function of ageing time of the P3HT precursor solution. Optimal photovoltaic properties are found at 60 h ageing time, which increases both light absorption and charge balance. Multilayered solar cells with a compositionally graded structure are fabriacted using preformed P3HT nanowires by inserting a pure P3HT donor phase onto the hole‐collecting electrode. Applying optimized annealing conditions to the P3HT buffer layer achieves an enhanced hole mobility and a power conversion efficiency of 3.94%. The introduction of a compositionally graded device structure, which contains a P3HT‐only region, reduces charge recombination and electron injection to the indium tin oxide (ITO) electrode and enhances the device properties. These results demonstrate that preformed semiconductor nanowires and compositionally graded structures constitute a promising approach to the control of bulk heterojunction morphology and charge‐carrier mobility.     

Comparative Studies on Optical,Redox, and Photovoltaic Properties of a Series of D–A–D and Analogous D–A Chromophores

A series of new symmetrical donor‐acceptor‐donor (D?A?D) dyes based on s‐indacene‐1,3,5,7(2H,6H)‐tetraone as an acceptor unit containing varying electron donating moieties and analogous donor‐acceptor (D?A) chromophores with indane‐1,3‐dione as an acceptor are synthesized. By employing these two sets of dyes, the influence of a scaffold change from unsymmetric push‐pull (D?A) to symmetrical (D?A?D) systems on optical, electrochemical, and photovoltaic properties are explored. Detailed comparative studies reveal favorable optical characteristics and considerably decreased bandgaps for the D?A?D dyes compared to those of the reference D?A chromophores. Accordingly, the evaluation of the present dyes as donor materials in bulk heterojunction (BHJ) solar cells in combination with fullerene derivatives PC₆₁BM or PC₇₁BM as acceptors afforded significantly improved performance for devices based on D?A?D blends (up to a factor of 4 compared to the respective D‐A reference) with power conversion efficiencies of up to 2.8%. In less polar solvents such as toluene, some of the novel D?A?D chromophores exhibit unexpectedly high fluorescence quantum yields Φ_em of up to unity, in striking contrast to their weakly fluorescent D‐A counterparts.     

Stable Inverted Polymer/Fullerene Solar Cells Using a Cationic Polythiophene Modified PEDOT:PSS Cathodic Interface

A cationic and water‐soluble polythiophene [poly[3‐(6‐pyridiniumylhexyl)thiophene bromide] (P3PHT⁺Br^?)] is synthesized and used in combination with anionic poly(3,4‐ethylenedioxythiophene):poly(p‐styrenesulfonate) (PEDOT:PSS)^? to produce hybrid coatings on indium tin oxide (ITO). Two coating strategies are established: i) electrostatic layer‐by‐layer assembly with colloidal suspensions of (PEDOT:PSS)^?, and ii) modification of an electrochemically prepared (PEDOT:PSS)^? film on ITO. The coatings are found to modify the work function of ITO such that it could act as a cathode in inverted 2,5‐diyl‐poly(3‐hexylthiophene) (P3HT)/[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) polymer photovoltaic cells. The interfacial modifier created from the layer‐by‐layer assembly route is used to produce efficient inverted organic photovoltaic devices (power conversion efficiency ～2%) with significant long‐term stability in excess of 500 h.