Nanostructured Channel for Improving Emission Efficiency of Hybrid Light-Emitting Field-Effect Transistors.

We report on the mechanism of enhancing the luminance and external quantum efficiency (EQE) by developing nanostructured channels in hybrid (organic/inorganic) light-emitting transistors (HLETs) that combine a solution-processed oxide and a polymer heterostructure. The heterostructure comprised two parts: (i) the zinc tin oxide/zinc oxide (ZTO/ZnO), with and without ZnO nanowires (NWs) grown on the top of the ZTO/ZnO stack, as the charge transport layer and (ii) a polymer Super Yellow (SY, also known as PDY-132) layer as the light-emitting layer. Device characterization shows that using NWs significantly improves luminance and EQE (≈1.1% @ 5000 cd m-2) compared to previously reported similar HLET devices that show EQE < 1%. The size and shape of the NWs were controlled through solution concentration and growth time, which also render NWs to have higher crystallinity. Notably, the size of the NWs was found to provide higher escape efficiency for emitted photons while offering lower contact resistance for charge injection, which resulted in the improved optical performance of HLETs. These results represent a significant step forward in enabling efficient and all-solution-processed HLET technology for lighting and display applications.

Traps and transport resistance are the next frontiers for stable non-fullerene acceptor solar cells.

Stability is one of the most important challenges facing material research for organic solar cells (OSC) on their path to further commercialization. In the high-performance material system PM6:Y6 studied here, we investigate degradation mechanisms of inverted photovoltaic devices. We have identified two distinct degradation pathways: one requires the presence of both illumination and oxygen and features a short-circuit current reduction, the other one is induced thermally and marked by severe losses of open-circuit voltage and fill factor. We focus our investigation on the thermally accelerated degradation. Our findings show that bulk material properties and interfaces remain remarkably stable, however, aging-induced defect state formation in the active layer remains the primary cause of thermal degradation. The increased trap density leads to higher non-radiative recombination, which limits the open-circuit voltage and lowers the charge carrier mobility in the photoactive layer. Furthermore, we find the trap-induced transport resistance to be the major reason for the drop in fill factor. Our results suggest that device lifetimes could be significantly increased by marginally suppressing trap formation, leading to a bright future for OSC.

Single-junction organic solar cells with over 19% efficiency enabled by a refined double-fibril network morphology.

In organic photovoltaics, morphological control of donor and acceptor domains on the nanoscale is the key for enabling efficient exciton diffusion and dissociation, carrier transport and suppression of recombination losses. To realize this, here, we demonstrated a double-fibril network based on a ternary donor-acceptor morphology with multi-length scales constructed by combining ancillary conjugated polymer crystallizers and a non-fullerene acceptor filament assembly. Using this approach, we achieved an average power conversion efficiency of 19.3% (certified 19.2%). The success lies in the good match between the photoelectric parameters and the morphological characteristic lengths, which utilizes the excitons and free charges efficiently. This strategy leads to an enhanced exciton diffusion length and a reduced recombination rate, hence minimizing photon-to-electron losses in the ternary devices as compared to their binary counterparts. The double-fibril network morphology strategy minimizes losses and maximizes the power output, offering the possibility of 20% power conversion efficiencies in single-junction organic photovoltaics.

Using Ligand Engineering to Produce Efficient and Stable Pb-Sn Perovskite Solar Cells with Antioxidative 2D Capping Layers.

Pb-Sn binary halide perovskites are a promising photovoltaic material due to their low toxicity and optical absorption spectrum well matched to the solar spectrum. However, the ready oxidation of Sn2+ to Sn4+ makes the material system currently too unstable to commercialize. Herein, ligand engineering based on antioxidative tyramine (hydrochloride, TACl) is presented for the first time to increase the stability of this material system. Using this strategy, we generate a two-dimensional (2D) capping layer on top of a standard three-dimensional Pb-Sn film. After capping, the surface defects can be passivated and the TACl-based 2D perovskite effectively protected Sn2+ from oxidation, which stabilized the Sn-Pb perovskite composition, avoiding the Pb-based perovskite formation. It is further found that the TACl treatment suppressed the halide segregation and improved the perovskite film photostability. Cell efficiency increases from 16.25 to 18.28% and device lifetime (T80) increases from less than 100 to over 1000 h. Our finding suggests that tuning ligand form/function represents a potentially highly productive direction to explore when trying to produce stable tin-based perovskite devices.

Molecular Doping Inhibits Charge Trapping in Low-Temperature-Processed ZnO toward Flexible Organic Solar Cells.

There has been a growing interest in the development of efficient flexible organic solar cells (OSCs) due to their unique capacity to provide energy sources for flexible electronics. To this end, it is required to design a compatible interlayer with low processing temperature and high electronic quality. In this work, we present that the electronic quality of the ZnO interlayer fabricated from a low-temperature (130 °C) sol-gel method can be significantly improved by doping an organic small molecule, TPT-S. The doped TPT-S, on the one hand, passivates uncoordinated Zn-related defects by forming N-Zn bonds. On the other hand, photoinduced charge transfer from TPT-S to ZnO is confirmed, which further fills up electron-deficient trap states. This renders ZnO improved electron transport capability and reduced charge recombination. By illuminating devices with square light pulses of varying intensities, we also reveal that an unfavorable charge trapping/detrapping process observed in low-temperature-processed devices is significantly inhibited after TPT-S doping. OSCs based on PBDB-T-2F:IT-4F with ZnO:TPT-S being the cathode interlayer yield efficiencies of 12.62 and 11.33% on rigid and flexible substrates, respectively. These observations convey the practicality of such hybrid ZnO in high-performance flexible devices.

Self-Assembly of Chiral-at-End Diketopyrrolopyrroles: Symmetry Dependent Solution and Film Optical Activity and Photovoltaic Performance.

Chiral thiophene-diketopyrrolopyrrole derivatives have been synthesised to investigate the potential of stereochemistry and symmetry as a means of modulating properties by influencing self-assembly of these purely organic materials. In particular, derivatives of diketopyrrolopyrrole were employed because of their proven interest as dyes, especially for organic solar cells. The natural product myrtenal was used as the source of stereochemistry, introduced through a Kröhnke reaction of a thiophene-bearing pyridinium salt and diketopyrrolopyrroles were prepared through Suzuki coupling with this chiral moiety at one end only as well as at both ends. Absorption spectroscopy and electrochemistry confirmed the potential suitability of the compounds for photovoltaic devices. The nanostructures formed by the compounds have been probed with circular dichroism spectroscopy in solution and in films. It is shown that a chiral C2 symmetric molecule assembles in solution giving a strong circular dichroic signal while as a film this optical activity is nulled, whereas an asymmetric homologue is most optically active as a thin film. The X-ray crystal structure of the asymmetric compound shows a polar order of the molecules that might explain this observation. The lack of optical activity in solution is very likely a result of the high solubility of the compound. The results reaffirm the sensitivity of circular dichroism spectroscopy to inter-chromophore organisation, whereas absorption spectroscopy in the visible region reveals only slight changes to the bands. The differing order in the compounds also affects their performance in bulk heterojunction photovoltaic devices. Atomic force microscopy of the blended thin films with the fullerene derivative usually employed (PC61 BM) showed that smooth and well mixed films were achieved, with the conditions required during spin coating depending greatly on the derivative, because of their differing solubility. The apparently better performance of the symmetrical compound (although with very low efficiency) is probably a result of the alignment of the molecules inferred by the circular dichroism experiments, whereas the asymmetric compound presumably adopts a twisted supramolecular organisation.

Increasing organic solar cell efficiency with polymer interlayers.

We demonstrate how organic solar cell efficiency can be increased by introducing a pure polymer interlayer between the PEDOT:PSS layer and the polymer:fullerene blend. We observe an increase in device efficiency with three different material systems over a number of devices. Using both electrical characterization and numerical modeling we show that the increase in efficiency is caused by optical absorption in the pure polymer layer and hence efficient charge separation at the polymer bulkheterojunction interface.

A numerical study of mobility in thin films of fullerene derivatives.

The effect of functional group size on the electron mobility in films of fullerene derivatives is investigated numerically. A series of four C(60) derivatives are formed by attaching saturated hydrocarbon chains to the C(60) cage via a methano bridge. For each of the derivatives investigated, molecular dynamics is used to generate a realistic material morphology. Quantum chemical methods are then used to calculate intermolecular charge transfer rates. Finally, Monte Carlo methods are used to simulate time-of-flight experiments and thus calculate the electron mobility. It is found that as the length of the aliphatic side chain increases, the configurational disorder increases and thus the mobility decreases.