Significance of miscibility in multidonor bulk heterojunction solar cells

Recent Advances of Furan and Its Derivatives Based Semiconductor Materials for Organic Photovoltaics

The state-of-the-art bulk-heterojunction (BHJ)-type organic solar cells (OSCs) have exhibited power conversion efficiencies (PCEs) of exceeding 18%. Thereinto, thiophene and its fused-ring derivatives play significant roles in facilitating the development of OSCs due to their excellent semiconducting natures. Furan as thiophene analogue, is a ubiquitous motif in naturally occurring organic compounds. Driven by the advantages of furan, such as less steric hindrance, good solubility, excellent stacking, strong rigidity and fluorescence, biomass derived fractions, more and more research groups focus on the furan-based materials for using in OSCs in the past decade. To systematically understand the developments of furan-based photovoltaic materials, the relationships between the molecular structures, optoelectronic properties, and photovoltaic performances for the furan-based semiconductor materials including single furan, benzofuran, benzodifuran (BDF) (containing thienobenzofuran (TBF)), naphthodifurans (NDF), and polycyclic furan are summarized. Finally, the empirical regularities and perspectives of the development of this kind of new organic semiconductor materials are extracted.

Efficient Nonhalogenated Solvent-Processed Ternary All-Polymer Solar Cells with a Favorable Morphology Enabled by Two Well-Compatible Donors

The rational design of the morphology of ternary all-polymer solar cells (all-PSCs) having broadened photon harvesting is crucial to achieve high device performance. However, multicomponent blends often illustrated an unfavorable morphology such as large-sized phase separation due to their complicated interaction. Herein, we proposed to solve these problems by employing two donors with good miscibility (J51 and PTB7-Th), which also have similar compatibility with the acceptor (N2200). The resultant ternary blend films of J51:PTB7-Th:N2200 feature a uniform phase separation morphology due to the reduced competitive effect of intermolecular interactions. As an additional polymer donor, PTB7-Th could not only enhance the absorption of the binary blend but also act as a crystallization regulator to boost the face-on orientation in ternary blends. Accordingly, the J51:PTB7-Th:N2200 ternary blends exhibited improved sunlight absorption and higher and well-balanced carrier mobility accompanied by enhanced carrier extraction. With the nonhalogenated cyclopentyl methyl ether as the processing solvent, the ternary all-PSCs showed outstanding power conversion efficiency (PCE) higher than 9% when varying the PTB7-Th weight ratio in donors from 20 to 50%. Due to the PTB7-Th content holding a 30% weight ratio in donors, the ternary all-PSCs demonstrated the optimal PCE of 9.60%, which perform better than those of binary all-PSCs (PCE = 7.58 or 5.63%).

Facile Method for the Investigation of Temperature-Dependent C60 Diffusion in Conjugated Polymers

We developed a novel all-optical method for monitoring the diffusion of a small quencher molecule through a polymer layer in a bilayer architecture. Experimentally, we injected C₆₀ molecules from a C₆₀ layer into the adjacent donor layer by stepwise heating, and we measured how the photoluminescence (PL) of the donor layer becomes gradually quenched by the incoming C₆₀ molecules. By analyzing the temporal evolution of the PL, the diffusion coefficient of C₆₀ can be extracted, as well as its activation energy and an approximate concentration profile in the film. We applied this technique to three carbazole-based low-bandgap polymers with different glass temperatures with a view to study the impact of structural changes of the polymer matrix on the diffusion process. We find that C₆₀ diffusion is thermally activated and not driven by WFL-type collective motion above T_g but rather by local motions mediated by the sidechains. The results are useful as guidance for material design and device engineering, and the approach can be adapted to a wide range of donor and acceptor materials.