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Small-molecule-based organic solar cells (OSCs) are a recurrent alternative to polymer-based OSCs. Due to the higher purity and definition of small molecules compared to polymers, the morphological requirements can be more relaxed. Here, we present a series of novel rhodanine-based small-molecule electron donors and blend them with the standard acceptor PC70BM. By performing a target analysis on femtosecond spectroscopy data, we quantify the rates of geminate charge recombination. We are able to reproduce these rates by applying the Marcus–Levich–Jortner equation, using results from quantum chemical calculations. This shows that in a series of differently substituted compounds, one can correctly predict trends in geminate recombination rates by relying only on quantities that are easy to measure (cyclic voltammetry, optical spectra) or that can be calculated by relatively inexpensive methods such as (TD)DFT. Our method should thus accelerate the search for high-performance small-molecule photovoltaic blends.

Ultrafast photochromic molecules are being actively investigated to meet the demand for fast optical switching systems. Inspired on the irreversible cyclization of 9-phenylphenalenone plant metabolites to yield highly-coloured naphthoxanthenes for the purpose of defense against pathogens, aryl-substituted phenalenones have been developed that undergo a similar but reversible photochromic reaction. The lifetime of the naphthoxanthene photoisomer spans nine orders of magnitude, ranging from tens of picoseconds to tens of milliseconds depending on the electronic properties of the 9-aryl group.

The synthesis and characterization of three novel HTMs with different highest occupied molecular orbital (HOMO) energy levels and their performances in MAPbI3-based devices in comparison with Spiro-OMeTAD is reported. Without systematic optimization, the HTMs performed well. The devices delivered fill factors comparable to the one with Spiro-OMeTAD but suffered from short-circuit current (JSC). Interestingly, despite the significant differences in HOMO energy levels, all three HTMs generated the same open-circuit voltage (VOC). We explored the performance limiting factors of the HTMs by simple transient photovoltage/photocurrent (TPV/TPC) measurements along with drift-diffusion simulations. We found no correlation between HOMO energy levels of the HTMs and VOC of the devices. Performances of the devices are limited by high trap density as well as low carrier mobility of the HTMs, and by the shunts present in the devices. Furthermore, the high trap density and low carrier mobility of the HTM are found to induce ion migration effect in the device causing slow decaying components in TPV/TPC. Nevertheless, it is confirmed that JSC and VOC, measured at steady state, are not influenced by the ion migration effect. These HTMs can be improved further by optimizing their conductivity, trap density, morphology, and can be used as alternatives to Spiro-OMeTAD or other expensive, synthetically challenging HTMs. The simple and inexpensive approach presented in this work can also be applied for effectively evaluating charge transporting materials for perovskite solar cells.

The device preconditioning dependent hysteresis and the consequential performance degradation hinder the actual performance and stability of the perovskite solar cells. Ion migration and charge trapping in the perovskite with large contribution from grain boundaries are the most common interpretations for the hysteresis. Yet, the high performing devices often include intermediate hole and electron transporting layers, which can further complicate the dynamical process in the device. Here, by using Kelvin Probe Force Microscopy and Confocal Photoluminescence Microscopy, we elucidate the impact of charge-transporting layers and excess MAI on the spatial and temporal variations of the photovoltage on the MAPbI3-based solar cells. By studying the devices layer by layer, we found that the light-induced ion migration occurs predominantly in the presence of an imbalanced charge extraction in the solar cells, and the charge transporting layers play crucial role in suppressing it. Careful selection and processing of the electron and hole-transporting materials are thus essential for making perovskite solar cells free from the ion migration effect.

Self-assembled molecules for outstanding hydrogen evolution rate and durability should promise practical water splitting due to the versatile visible light absorption, low production cost, and ease...

A highly efficient organic photocatalyst (OPC) for photoinduced electron/energy-transfer reversible addition–fragmentation chain-transfer (PET-RAFT) polymerization was identified through a systemat...

Solution-processable all-small-molecule organic solar cells (OSCs) have shown dramatic progress in improving stability and photovoltaic efficiency. However, knowledge of photoexcitation dynamics in this novel class of materials is very limited. To fully exploit the design capacities inherent in small molecule chemistry, the elementary processes and branching yields must be known in detail. Here, we present a combined computational–experimental study of photoexcitation dynamics of a prototypical all-small-molecule photovoltaic blend, p-DTS(FBTTh2)2 as a donor and NIDCS-MO as an acceptor. Femtosecond spectroscopy data show that excitonic coupling is small and that the charge transfer states are localized, at first glance contradicting the high internal quantum efficiency (IQE) and open circuit voltage (VOC) of this material. A target analysis of the femtosecond spectra yields exciton dissociation rates of 1/(25 ps) and 1/(100 ps) for the as-deposited and annealed blend, respectively. These rates are far slower than in typical polymer based organic solar cells. Still, internal quantum yields are high because parasitic quenching processes are found to be even slower. In the framework of semiclassical Marcus theory, we demonstrate that our system shows near-optimum energy conversion and charge separation yields, due to negligible activation energy for charge generation but high activation energy for charge recombination, allowing enough time to separate localized charge transfer states. We thus justify both the high internal quantum yields and the high open circuit voltage found in this system. Finally, we predict that highly efficient and stable low-optical bandgap systems can be realized by reducing the electronic coupling between the donor and acceptor.

Single-walled carbon nanotubes (SWNTs) show high aspect ratio, thermal and chemical stability as well as charge mobility, and therefore appear ideally suited for improving charge extraction in organic photovoltaic (OPV) devices. Since typical charge extraction times in OPV devices are in the microsecond range, the interplay of the desired charged states with long-lived neutral states in SWNTs such as triplet excitons becomes important. Triplet excitons have recently been investigated in (6,5) SWNTs with an optical yield close to 32%. Here, we present transient absorption (TA) dynamics of (6,5)- and (7,5)-rich SWNT networks from the femtosecond to microsecond time scale. Comparing our TA spectra to results from a recent spectroelectrochemical study allows us to distinguish between contributions from charged and neutral photoexcitations. For long pump–probe delay times we identify an excess photobleach which we use as a chirality specific probe for the density of triplet excitons. We show that triplet energ...