Advanced Functional Materials and Devices (AFMD) Group

Abstract:

Organic solar cells have the potential to become the cheapest form of electricity, beating even silicon photovoltaics. This article summarizes the state of the art in the field, highlighting research challenges, mainly the need for an efficiency increase as well as an improvement in long‐term stability. It discusses possible current and future applications, such as building integrated photovoltaics or portable electronics. Finally, the environmental footprint of this renewable energy technology is evaluated, highlighting the potential to be the energy generation technology with the lowest carbon footprint of all.

Abstract:

<jats:title>Abstract</jats:title><jats:p>Organic semiconductors still lag behind their inorganic counterparts in terms of mobility due to their lower structural order, in particular in thin films. Here, the highly ordered phase of triclinic rubrene – characterized by high vertical hole mobility – grown from a vacuum‐deposited thin film is used by post‐annealing and implemented into organic photodetectors. Since the triclinic rubrene exhibits a high roughness with a peak‐to‐valley value of 250 nm, which is detrimental to the dark current, strategies to control the crystal growth are developed. These investigations show that a suppression layer of 20 nm C<jats:sub>60</jats:sub> is the most promising approach to successfully reduce the surface roughness while maintaining the triclinic phase, proven by grazing‐incidence wide‐angle X‐ray scattering (GIWAXS). With the smoothened active layer, the dark current density is reduced by three orders of magnitude compared to the neat rubrene layer. It is as low as 2.5 × 10<jats:sup>−10 </jats:sup>A cm<jats:sup>−2</jats:sup> at −0.1 V bias, reflected in an overall specific detectivity of 6 × 10<jats:sup>11</jats:sup> Jones at zero bias (based on noise measurements) and a high linear dynamic range of 170 dB. This strategy using a suppression layer thus proves successful and is very promising to be applied to other crystalline materials.</jats:p>

Abstract:

Free charge generation after photoexcitation of donor or acceptor molecules in organic solar cells generally proceeds via (1) formation of charge transfer states and (2) their dissociation into charge separated states. Research often either focuses on the first component or the combined effect of both processes. Here, we provide evidence that charge transfer state dissociation rather than formation presents a major bottleneck for free charge generation in fullerene-based blends with low energetic offsets between singlet and charge transfer states. We investigate devices based on dilute donor content blends of (fluorinated) ZnPc:C60 and perform density functional theory calculations, device characterization, transient absorption spectroscopy and time-resolved electron paramagnetic resonance measurements. We draw a comprehensive picture of how energies and transitions between singlet, charge transfer, and charge separated states change upon ZnPc fluorination. We find that a significant reduction in photocurrent can be attributed to increasingly inefficient charge transfer state dissociation. With this, our work highlights potential reasons why low offset fullerene systems do not show the high performance of non-fullerene acceptors.

Abstract:

The efficiency and longevity of metal-halide perovskite solar cells are typically dictated by nonradiative defect-mediated charge recombination. In this work, we demonstrate a vapor-based amino-silane passivation that reduces photovoltage deficits to around 100 millivolts (>90% of the thermodynamic limit) in perovskite solar cells of bandgaps between 1.6 and 1.8 electron volts, which is crucial for tandem applications. A primary-, secondary-, or tertiary-amino–silane alone negatively or barely affected perovskite crystallinity and charge transport, but amino-silanes that incorporate primary and secondary amines yield up to a 60-fold increase in photoluminescence quantum yield and preserve long-range conduction. Amino-silane–treated devices retained 95% power conversion efficiency for more than 1500 hours under full-spectrum sunlight at 85°C and open-circuit conditions in ambient air with a relative humidity of 50 to 60%.

Abstract:

Carbon nanomaterials are expected to be bright and efficient emitters, but structural disorder, intermolecular interactions and the intrinsic presence of dark states suppress their photoluminescence. Here, we study synthetically-made graphene nanoribbons with atomically precise edges and which are designed to suppress intermolecular interactions to demonstrate strong photoluminescence in both solutions and thin films. The resulting high spectral resolution reveals strong vibron-electron coupling from the radial-breathing-like mode of the ribbons. In addition, their cove-edge structure produces inter-valley mixing, which brightens conventionally-dark states to generate hitherto-unrecognised twilight states as predicted by theory. The coupling of these states to the nanoribbon phonon modes affects absorption and emission differently, suggesting a complex interaction with both Herzberg–Teller and Franck– Condon coupling present. Detailed understanding of the fundamental electronic processes governing the optical response will help the tailored chemical design of nanocarbon optical devices, via gap tuning and side-chain functionalisation.