• Energy Research

We report the synthesis of a novel pentacyclic donor moiety, dithienosilolothiophene, and its incorporation into low bandgap semiconducting polymers. The unique geometry of this new donor allowed attaching four solubilizing side chains on the same side of the fused ring system, thus ensuring sufficient solubility when incorporated into conjugated polymers while simultaneously reducing the steric hindrance between adjacent polymer chains. The optoelectronic properties of three new polymers comprising the novel pentacyclic donor were investigated and compared to structurally similar thieno[3,2-b]thienobis(silolothiophene) polymers. Organic solar cells were fabricated in order to evaluate the new materials’ potential as donor polymers in bulk heterojunction solar cells and gain further insight into how the single-sided side-chain arrangement affects the active layer blend morphology.

The design, synthesis, and characterization of a series of diketopyrrolopyrrole-based copolymers with different chalcogenophene comonomers (thiophene, selenophene, and tellurophene) for use in field-effect transistors and organic photovoltaic devices are reported. The effect of the heteroatom substitution on the optical, electrochemical, and photovoltaic properties and charge carrier mobilities of these polymers is discussed. The results indicate that by increasing the size of the chalcogen atom (S < Se < Te), polymer band gaps are narrowed mainly due to LUMO energy level stabilization. In addition, the larger heteroatomic size also increases intermolecular heteroatom-heteroatom interactions facilitating the formation of polymer aggregates leading to enhanced field-effect mobilities of 1.6 cm(2)/(V s). Bulk heterojunction solar cells based on the chalcogenophene polymer series blended with fullerene derivatives show good photovoltaic properties, with power conversion efficiencies ranging from 7.1-8.8%. A high photoresponse in the near-infrared (NIR) region with excellent photocurrents above 20 mA cm(-2) was achieved for all polymers, making these highly efficient low band gap polymers promising candidates for use in tandem solar cells.

Photocurrent from fullerene excitons in polymer–fullerene solar cells is optimised by employing a series of low bandgap diketopyrrolopyrrole-containing polymers with differing molecular weights. The low LUMO level of this donor polymer prevents efficient charge generation from polymer excitons, allowing us to focus on charge and photocurrent generation specifically from fullerene excitons. We employ femtosecond transient absorption spectroscopy and transmission electron microscopy to show a correlation between fullerene domain size and the kinetics of polaron generation from fullerene excitons, and relate these observations to device efficiency. Charge generation from fullerene excitons is shown to occur on the 0.5 ns timescale, limited by the kinetics of diffusion of fullerene excitons to domain interfaces with donor polymers. For devices employing the highest molecular weight donor polymer, ∼7 mA cm−2 of photocurrent is observed from fullerene excitons, leading to an overall device efficiency of 5.2%.

The inferior short-circuit current density (JSC) of the conjugated block copolymers (CBCs) based single component organic solar cells (SCOSCs), compared with that of binary all-polymer systems, is the main reason for their limited power conversion efficiency (PCE). To improve the JSC of SCOSCs by enhancing the light-harvesting ability, we designed and synthesized three narrower bandgap acceptor monomers with different chlorine-substituted terminals, and incorporated them into the CBCs. As the position of chlorine substitution altered, the blend morphology, the device performance, and the physical mechanism changed accordingly. It is worth noting that the PB-b-PYCl-2-based SCOSCs exhibited a significantly improved JSC, accompanied by the best PCE of 13.42 %, and good thermal stability. This study illustrated the terminal chlorination strategy is a promising way to modify the device performance of the SCOSCs.

AbstractThe synthesis of a novel fused hexacyclic electron rich monomer incorporating thieno[3,2‐b]thiophene is reported and characterized by single crystal X‐ray diffraction. Suzuki co‐polymerization with benzothiadiazole (BT) afforded a novel low band‐gap polymer P4TBT with high molecular weights and good solution processability. Bulk heterojunction solar cell devices using the P4TBT and [70]PCBM gave power conversion efficiencies of 2.5%. Top‐gate, bottom‐contact field effect transistors (FETs) using P4TBT displayed high hole mobilities of 0.07 cm2 · Vs−1 demonstrating the suitability of the novel monomer and polymer for use in high performing organic electronic devices.magnified image

A series of donor–acceptor (D–A) conjugated polymers utilizing 4,4‐bis(2‐ethylhexyl)‐4H‐germolo[3,2‐b:4,5‐b′]dithiophene (DTG) as the electron rich unit and three electron withdrawing units of varying strength, namely 2‐octyl‐2H‐benzo[d][1,2,3]triazole (BTz), 5,6‐difluorobenzo[c][1,2,5]thiadiazole (DFBT) and [1,2,5]thiadiazolo[3,4‐c]pyridine (PT) are reported. It is demonstrated how the choice of the acceptor unit (BTz, DFBT, PT) influences the relative positions of the energy levels, the intramolecular transition energy (ICT), the optical band gap (Egopt), and the structural conformation of the DTG‐based co‐polymers. Moreover, the photovoltaic performance of poly[(4,4‐bis(2‐ethylhexyl)‐4H‐germolo[3,2‐b:4,5‐b′]dithiophen‐2‐yl)‐([1,2,5]thiadiazolo[3,4‐c]pyridine)] (PDTG‐PT), poly[(4,4‐bis(2‐ethylhexyl)‐4H‐germolo[3,2‐b:4,5‐b′]dithiophen‐2‐yl)‐(2‐octyl‐2H‐benzo[d][1,2,3]triazole)] (PDTG‐BTz), and poly[(4,4‐bis(2‐ethylhexyl)‐4H‐germolo[3,2‐b:4,5‐b′]dithiophen‐2‐yl)‐(5,6‐difluorobenzo[c][1,2,5]thiadiazole)] (PDTG‐DFBT) is studied in blends with [6,6]‐phenyl‐C70‐butyric acid methyl ester (PC70BM). The highest power conversion efficiency (PCE) is obtained by PDTG‐PT (5.2%) in normal architecture. The PCE of PDTG‐PT is further improved to 6.6% when the device architecture is modified from normal to inverted. Therefore, PDTG‐PT is an ideal candidate for application in tandem solar cells configuration due to its high efficiency at very low band gaps (Egopt = 1.32 eV). Finally, the 6.6% PCE is the highest reported for all the co‐polymers containing bridged bithiophenes with 5‐member fused rings in the central core and possessing an Egopt below 1.4 eV.

A bis‐azide‐based small molecule cross­linker is synthesized and evaluated as both a stabilizing and efficiency‐boosting additive in bulk heterojunction organic photovoltaic cells. Activated by a non­invasive and scalable solution processing technique, polymer:fullerene blends exhibit improved thermal stability with suppressed polymer skin formation at the cathode and frustrated fullerene aggregation on ageing, with initial efficiency increased from 6% to 7%.

Non-fullerene acceptors with optimized energy levels enable 10% efficient solar cells with reduced voltage losses <0.6 V.