High performance p-type molecular electron donors for OPV applications via alkylthiophene catenation chromophore extension

Resolving conjugated polymer film morphology with polarised transmission and time-resolved emission microscopy

The alignment of chromophores plays a crucial role in determining the optoelectronic properties of materials. Such alignment can make interpretation of fluorescence anisotropy microscopy (FAM) images somewhat ambiguous. The time-resolved emission behaviour can also influence the fluorescence anisotropy. This is particularly the case when probing excitation energy migration between chromophores in a condensed phase. Ideally information concerning the chromophoric alignment, emission decay kinetics and fluorescence anisotropy can be recorded and correlated. We report on the use of polarised transmission imaging (PTI) coupled with both steady-state and time-resolved FAM to enable accurate identification of chromophoric alignment and morphology in thin films of a conjugated polydiarylfluorene. We show that the combination of these three imaging modes presents a comprehensive methodology for investigating the alignment and morphology of chromophores in thin films, particularly for accurately mapping the distribution of amorphous and crystalline phases within the thin films, offering valuable insights for the design and optimization of materials with enhanced optoelectronic performance.

Efficient all-small-molecule organic solar cells based on a fluorinated small-molecule donor

A fluorinated donor with a deep HOMO energy level enables efficient all-small-molecule organic solar cells.

Morphological Requirements for Nanoscale Electric Field Buildup in a Bulk Heterojunction Solar Cell

The morphology of organic semiconductors is critical to their function in optoelectronic devices and is particularly crucial in the donor-acceptor mixture that comprises the bulk heterojunction of organic solar cells. Here, energy landscapes can play integral roles in charge photogeneration, and recently have been shown to drive the accumulation of charge carriers away from the interface, resulting in the buildup of large nanoscale electric fields, much like a capacitor. In this work we combine morphological and spectroscopic data to outline the requirements for this interdomain charge accumulation, finding that this effect is driven by a three-phase morphology that creates an energetic cascade for charge carriers. By adjusting annealing conditions, we show that domain purity, but not size, is critical for an electro-absorption feature to grow-in. This demonstrates that the energy landscape around the interface shapes the movement of charges and that pure domains are required for charge carrier buildup that results in reduced recombination and large interdomain nanoscale electric fields.

Experimental Evidence Relating Charge-Transfer-State Kinetics and Strongly Reduced Bimolecular Recombination in Organic Solar Cells

Significantly reduced bimolecular recombination relative to the Langevin recombination rate has been observed in a limited number of donor-acceptor organic semiconductor blends. The strongly reduced recombination has been previously attributed to a high probability for the interfacial charge-transfer (CT) states (formed upon charge encounter) to dissociate back to free charges. However, whether the reduced recombination is due to a suppressed CT-state decay rate or an improved dissociation rate has remained a matter of conjecture. Here we investigate a donor-acceptor material system that exhibits significantly reduced recombination upon solvent annealing. On the basis of detailed balance analysis and the accurate characterization of CT-state parameters, we provide experimental evidence that an increase in the dissociation rate of CT states upon solvent annealing is responsible for the reduced recombination. We attribute this to the presence of purer and more percolated domains in the solvent-annealed system, which may, therefore, have a stronger entropic driving force for CT dissociation.

Reduced Recombination and Capacitor-like Charge Buildup in an Organic Heterojunction

Organic photovoltaic (OPV) efficiencies continue to rise, raising their prospects for solar energy conversion. However, researchers have long considered how to suppress the loss of free carriers by recombination-poor diffusion and significant Coulombic attraction can cause electrons and holes to encounter each other at interfaces close to where they were photogenerated. Using femtosecond transient spectroscopies, we report the nanosecond grow-in of a large transient Stark effect, caused by nanoscale electric fields of ∼487 kV/cm between photogenerated free carriers in the device active layer. We find that particular morphologies of the active layer lead to an energetic cascade for charge carriers, suppressing pathways to recombination, which is ∼2000 times less than predicted by Langevin theory. This in turn leads to the buildup of electric charge in donor and acceptor domains-away from the interface-resistant to bimolecular recombination. Interestingly, this signal is only experimentally obvious in thick films due to the different scaling of electroabsorption and photoinduced absorption signals in transient absorption spectroscopy. Rather than inhibiting device performance, we show that devices up to 600 nm thick maintain efficiencies of >8% because domains can afford much higher carrier densities. These observations suggest that with particular nanoscale morphologies the bulk heterojunction can go beyond its established role in charge photogeneration and can act as a capacitor, where adjacent free charges are held away from the interface and can be protected from bimolecular recombination.

A Double Support Layer for Facile Clean Transfer of Two-Dimensional Materials for High-Performance Electronic and Optoelectronic Devices

Clean transfer of two-dimensional (2D) materials grown by chemical vapor deposition is critical for their application in electronics and optoelectronics. Although rosin can be used as a support layer for the clean transfer of graphene grown on Cu, it has not been usable for the transfer of 2D materials grown on noble metals or for large-area transfer. Here, we report a poly(methyl methacrylate) (PMMA)/rosin double support layer that enables facile ultraclean transfer of large-area 2D materials grown on different metals. The bottom rosin layer ensures clean transfer, whereas the top PMMA layer not only screens the rosin from the transfer conditions but also improves the strength of the transfer layer to make the transfer easier and more robust. We demonstrate the transfer of monolayer WSe₂ and WS₂ single crystals grown on Au as well as large-area graphene films grown on Cu. As a result of the clean surface, the transferred WSe₂ retains the intrinsic optical properties of the as-grown sample. Moreover, it does not require annealing to form good ohmic contacts with metal electrodes, enabling high-performance field effect transistors with mobility and ON/OFF ratio ∼10 times higher than those made by PMMA-transferred WSe₂. The ultraclean graphene film is found to be a good anode for flexible organic photovoltaic cells with a high power conversion efficiency of ∼6.4% achieved.

Inverted P3HT:PC61BM organic solar cells incorporating a π-extended squaraine dye with H- and (or) J-aggregation

In this report, we investigated the impact of adding a large π-extended squaraine dye to the classic bulk heterojunction P3HT:PC61BM system. The compound, SQIQ-A, is a bis-indole squaraine dye appended with acetylene-indoloquinoxaline end-capping units that exhibits strong optical absorption in thin film from 650 to 750 nm, beyond the absorption wavelength cut-off for P3HT:PC61BM films (ca. ∼650 nm). The dye SQIQ-A can form H- or J-aggregates (blue or red shifting the optical absorption profiles) via solvent vapour annealing with CHCl3 or THF, respectively, thus providing a simple method to control the morphology and optical properties of the molecule. Ternary blended films composed of P3HT:PC61BM:SQIQ-A (1:1:0.3 mass ratio) were investigated as-cast, after solvent vapour annealing, and after thermal annealing. Films were characterized using optical absorption spectroscopy, X-ray diffraction, atomic force microscopy, and polarized light microscopy. Solar cells were fabricated using an inverted architecture in air and analysed using current–voltage and external quantum efficiency measurements. It was found that solar cells thermally annealed at 130 °C for 10 min gave good power conversion efficiencies of 3%, similar to the control P3HT:PC61BM but with improved fill factors and noticeable photocurrent generation from 650 to 760 nm where the SQIQ-A molecule absorbs. Exposure to solvent vapour (CHCl3 or THF) results in aggregation of all components but specifically leads to the formation of micron-sized domains, lowering the overall photovoltaic performance.