Sergey V. Dayneko

Applying direct heteroarylation synthesis to evaluate organic dyes as the core component in PDI-based molecular materials for fullerene-free organic solar cells

Direct heteroarylation has emerged as a versatile and powerful tool to access π-conjugated materials through atom-economical Pd-catalyzed carbon–carbon bond forming reactions. Employing this synthetic protocol has enabled the facile evaluation of a series of organic dyes in a PDI-dye-PDI framework. Material properties are largely dictated by the PDI components, but the incorporation of either thienoisoindigo, diketopyrrolopyrrole or isoindigo has been shown to influence the ionization potential and absorption profiles of the final materials. Solution-processable organic solar cell devices were fabricated to investigate the influence of the different dye cores on photovoltaic performance when paired with the donor polymer PTB7-Th. It was found that the diketopyrrolopyrrole-based material out-performed the other organic dyes, demonstrating energy losses of less than 0.6 eV, promising efficiencies when cast from non-halogenated solvents and the ability to dictate self-assembly induced by small volume fractions of the high-boiling solvent additive 1,8-diiodooctane to reach best device efficiencies of 4.1%.

N-Annulated perylene diimide dimers: acetylene linkers as a strategy for controlling structural conformation and the impact on physical, electronic, optical and photovoltaic properties

The geometry of organic π-conjugated small molecules can impact the morphology of blended-thin films and subsequent performance in opto-electronic devices. In this report, we investigate the role of molecular conformation of perylene diimide (PDI) dimers designed to act as non-fullerene acceptors in organic solar cells. A series of three PDI dimers is presented in which the PDI chromophores are directly linked via the bay position (PDI2, 3) or separated by one (PDI2Ac, 4) or two (PDI2Ac2, 5) acetylene spacers. In all cases, the exo-position of the PDI dimers is N-annulated. New compounds 4 and 5 were synthesized via an optimized and facile synthetic pathway. Directly linked PDI dimers adopted a highly twisted conformation whereas adding two acetylene spacers rendered the PDI chromophores coplanar. 1H NMR spectroscopic analysis of each dimer revealed a highly sensitive electronic structure that is strongly influenced by the acetylene spacers. It was found that compounds 4 and 5 with less twisted structures exhibited similar electron affinities but lower ionization potentials, lower organic solvent solubility, and red-shifted optical absorption spectra when compared to the highly twisted dimer 3. In addition, 4 and 5 showed a stronger tendency to aggregate in both solution and the solid state. This had a large impact on the performance of organic solar cells using these materials as electron acceptors. Bulk-heterojunction solar cells based upon a PTB7-Th:3 active layer could reach high power conversion efficiencies of 5.23%. In contrast, PTB7-Th:4 and PTB7-Th:5 based devices had ∼5 times lower performance owing to the formation of unfavourable active layer morphologies.

N-annulated perylene diimide dimers: the effect of thiophene bridges on physical, electronic, optical, and photovoltaic properties

In this work, we report on the synthesis, characterization, and photovoltaic properties of two new N-annulated PDI derivatives connected with one (M1) or two (M2) thiophene bridging units and directly compare to a twisted PDI dimer with no thiophene units (tPDI-Hex). Compounds M1 and M2 were synthesized using an optimized direct heteroarylation carbon–carbon bond forming method. A combination of optical absorption and emission spectroscopy, cyclic voltammetry, polarized optical microscopy, X-ray diffraction, and density functional theory were used to characterize all compounds. Introduction of thiophene units into the PDI dimer scaffold served to red-shift the optical absorption and lower electron affinities. A decrease in the fluorescence intensity of M1 and M2 relative to tPDI-Hex was observed and attributed to photoinduced electron transfer. Notably, both M1 and M2 exhibited less structured thin-films as evidenced by optical microscopy and thin film X-ray diffraction. Photovoltaic properties were evaluated by fabricating bulk heterojunction solar cells with the inverted architecture ITO/ZnO/BHJ/MoOx/Ag and using PTB7-Th as the donor polymer. All solar cells were made and tested in air, and only ‘as-cast’ active layers were evaluated to ensure a direct comparison amongst the three acceptors. Solar cells with PTB7-Th:tPDI-Hex gave power conversion efficiencies of ∼5% with Vocs of 0.95 V, akin to our previous results. Comparatively, solar cells with PTB7-Th:M1 and PTB7-Th:M2 active layers exhibited higher Voc values upwards of 1.05 V but only had PCE of 1.1–1.2% when cast from chloroform, which we attributed to the formation of large domain sizes. A change in casting solvent to o-xylene resulted in improved bulk heterojunction morphology by reducing domain size and a doubling of the PCE values to ∼2% was realized for devices using M1 and M2 as electron acceptors. Use of the high boiling solvent, trimethylbenzene, resulted in further improvements in PCE for M1 and M2, increasing the PCE for devices using these acceptors to ∼3%. Importantly, devices made using the new materials exhibit low energy losses in the range of 0.5–0.7 eV.

Hybrid heterostructures based on aromatic polyimide and semiconductor CdSe quantum dots for photovoltaic applications

A nanohybrid photoactive material based on aromatic polyimide (PI) doped with CdSe quantum dots (QDs) has been developed to be used in photovoltaic solar cells. The solar cell is based on a heterostructure of an ITO electrode covered with a layer of Cu–phthalocyanine and a layer of a PI–QD composite. The photovoltaic properties of the CuPc/PI:CdSe hybrid heterostructure at various QD concentrations in the PI matrix have been studied. Luminescent and transmission electron microscopy analyses have shown that the optimal QD mass concentration is 60%. The efficiency of the solar cell based on optimized PI:CdSe structures approaches those for the structures based on conventional MEH-PPV organic semiconductor. Moreover, the photovoltaic characteristics of the solar cell remain stable in the air for a long time (120 h). This is expected to considerably simplify the technology of manufacturing these hybrid solar cells. The mechanisms of the excitation and charge transfer from QDs to the organic semiconductors and...

A non-fullerene acceptor with a diagnostic morphological handle for streamlined screening of donor materials in organic solar cells

Utilizing the N-annulated PDI acceptor PDI–DPP–PDI, a simple air-processed and air-tested organic photovoltaic device fabrication procedure has been established to streamline the screening of donor materials. Post-deposition solvent vapour annealing of the active layer blend results in a preferential re-organization dictated by PDI–DPP–PDI and is responsible for significant increases in device performance. Employing this method, a series of low-cost and scalable donor polymers were screened to help select promising candidates as alternatives to the expensive high-performance donor polymer PTB7-Th. Universal improvements in performance upon solvent vapour annealing with a range of donor polymers highlighted the advantages of PDI–DPP–PDI and this post-deposition treatment. This facile screening protocol identified PDTT-BOBT as the most suitable PTB7-Th alternative in the surveyed series, with the best device efficiencies reaching 4.5% PCE compared to control devices with PTB7-Th at 4.6% PCE.

Effect of surface ligands on the performance of organic light-emitting diodes containing quantum dots

Quantum dots (QDs) have numerous applications in optoelectronics due to their unique optical properties. Novel hybrid organic light-emitting diodes (OLEDs) containing QDs as an active emissive layer are being extensively developed. The performance of QD–OLED depends on the charge transport properties of the active layer and the degree of localization of electrons and holes in QDs. Therefore, the type and the density of the ligands on the QD surface are very important. We have fabricated OLEDs with a CdSe/ZnS QD active layer. These OLEDs contain hole and electron injection layers consisting of poly(9-vinyl carbazole) and ZnO nanoparticles, respectively. The energy levels of these materials ensure efficient injection of charge carriers into the QD emissive layer. In order to enhance the charge transfer to the active QD layer and thereby increase the OLED efficiency, the QD surface ligands (tri-n-octyl phosphine oxide, TOPO) were replaced with a series of aromatic amines and thiols. The substituents were expected to enhance the charge carrier mobility in the QD layer. Surprisingly, the devices based on the original TOPO-coated QDs were found to have the best performance, with a maximum brightness of 2400 Cd/m2 at 10 V. We assume that this was due to a decrease in the charge localization within QDs when aromatic ligands are used. We conclude that the surface ligands considerably affect the performance of QD–OLEDs, efficient charge localization in QD cores being more important for good performance than a high charge transfer rate.

Bromination of the benzothioxanthene Bloc: toward new π-conjugated systems for organic electronic applications

A selective and efficient method to afford a monobrominated benzothioxanthene (Br-BTXI) derivative is reported. Br-BTXI was extensively employed in common palladium catalyzed coupling reactions. Finally, as a proof of concept, a BTXI based molecular donor was synthesized and evaluated in bulk heterojunction solar cells.

Engineering of hybrid heterostructures from organic semiconductors and quantum dots for advanced photovoltaic applications

Semiconductor quantum dots (QDs) are characterized by high extinction coefficients adjustable by varying the nanoparticle size and a high quantum yield of charge generation. They have the advantage of efficient charge transfer from QDs to organic semiconductors. An advanced photovoltaic cell where a SnO2/ITO electrode is covered with layers of CdSe QDs integrated in a polyimide (PI) organic semiconductor (about 100 nm thick) and Cu–phthalocyanine (20–40 nm thick) has been developed.Laser-induced photoluminescence analysis has permitted the optimization of the QD concentration in the PI matrix. Special attention has been paid to the electrode surface quality, including the effect of oxygen-plasma treatment of the transparent SnO2/ITO electrode surface on the heterostructure photoconductivity. The mechanisms of excitation and charge transfers from QDs to the organic semiconductor and their effects on the efficiency of solar radiation conversion to electricity are discussed. Photovoltaic study of the structures developed has been performed, and the effect of the Cu–phthalocyanine layer on their photoconductivity has been estimated. The photovoltaic efficiency of optimized PI–CdSe hybrid structures approaches that of the best performing systems based on the MEH–PPV organic semiconductor. Incorporation of CdSe QDs in MEH–PPV has been demonstrated to increase the photovoltaic efficiency of the system by 50%, thus allowing the development of novel QD-based inorganic/organic hybrid materials with considerably improved photovoltaic properties.

Environment friendly solvent processed, fullerene-free organic solar cells with high efficiency in air

Fullerene-free organic solar cells (OSCs) have attracted significant interest in the research community over the past few years. Their efficiency has risen rapidly, with multiple reports of record power conversion efficiencies (PCEs) breaching 14%. While encouraging, these performance metrics are often achieved with the utilization of toxic halogenated solvents for the fabrication process, which is less attractive for large-scale manufacturing. Dimeric perylene diimide (PDI) electron transport materials are currently considered amongst the key candidates for the realization of low-cost, highefficiency “green-processed” OSCs. The low-cost synthetic versatility of the PDI skeleton allows for a range of chemical “fine-tuning” and the chromophore has excellent photochemical stability and strong light absorption in the visible region. This report will detail our research into OSCs using PDI dimers as the electron acceptors and active layer fabrication using non-halogenated solvents. PTB7-Th was chosen as the donor material, owing to its good solubility in nonhalogenated solvents, complementary light absorption and suitable energy level alignment for pairing with our PDI acceptors. Two different PDI dimers having linear and branched alkyl chains are studied. We have previously shown that PTB7-Th:PDI based solar cells with active layers processed from 2Me-THF, o-xylene, or 1,2,4-trimethylbenzene could reach PCEs from 5-6%. The processing solvent can be extended to toluene with solar cells exhibiting PCEs of 5%. Thus, this work highlights the many processing options for the PTB7-Th / N-annulated PDI dimer active layer combination.

Direct (Hetero)Arylation Polymerization of a Spirobifluorene and a Dithienyl-Diketopyrrolopyrrole Derivative: New Donor Polymers for Organic Solar Cells

The synthesis and preliminary evaluation as donor material for organic photovoltaics of the poly(diketopyrrolopyrrole-spirobifluorene) (PDPPSBF) is reported herein. Prepared via homogeneous and heterogeneous direct (hetero)arylation polymerization (DHAP), through the use of different catalytic systems, conjugated polymers with comparable molecular weights were obtained. The polymers exhibited strong optical absorption out to 700 nm as thin-films and had appropriate electronic energy levels for use as a donor with PC70BM. Bulk heterojunction solar cells were fabricated giving power conversion efficiencies above 4%. These results reveal the potential of such polymers prepared in only three steps from affordable and commercially available starting materials.