Scaling Considerations for Organic Photovoltaics for Indoor Applications

Organic semiconductor-based photovoltaic (OPV) devices have many properties that make them attractive for indoor applications, such as tailorable light absorption, low embodied energy manufacturing and cost, structural conformality, and low material toxicity. Compared to their use as organic solar cells (OSCs) for standard outdoor solar harvesting, indoor OPV (IOPV) devices operate at low light intensities, and thus demonstrate different area-scaling behavior. In particular, it appears as though the performance of large-area IOPV devices is much less affected by the sheet resistances of the transparent conductive electrodes (a major limit in OSCs), but instead by factors such as their shunt resistance at low light intensities. Herein, the key parameters for improving the efficiency of large-area IOPV using drift–diffusion and finite element modeling (FEM) are examined. The scaling behavior at low-light intensities is theoretically and experimentally probed and demonstrated using the model PM6:Y6 system. The implications for the fabrication of large-area devices and the requirements for high shunt resistances for low-light performance are examined. These new insights present a clear route toward realizing monolithic large-area organic photovoltaic cells for indoor applications – which is a necessary technical step to practical implementation

Mitigating Detrimental Effect of Self‐Doping Near the Anode in Highly Efficient Organic Solar Cells

Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) has been one of the most established hole transport layers (HTL) in organic solar cells (OSCs) for several decades. However, the presence of PSS− ions is known to deteriorate device performance via a number of mechanisms including diffusion to the HTL-active layer interface and unwanted local chemical reactions. In this study, it is shown that PSS− ions can also result in local p-doping in the high efficiency donor:non-fullerene acceptor blends – resulting in photocurrent loss. To address these issues, a facile and effective approach is reported to improve the OSC performance through a two-component hole transport layer (HTL) consisting of a self-assembled monolayer of 2PACz ([2-(9H-Carbazol-9-yl)ethyl]phosphonic acid) and PEDOT:PSS. The power conversion efficiency (PCE) of 17.1% using devices with PEDOT:PSS HTL improved to 17.7% when the PEDOT:PSS/2PACz two-component HTL is used. The improved performance is attributed to the overlaid 2PACz layer preventing the formation of an intermixed p-doped PSS− ion rich region (≈5–10 nm) at the bulk heterojunction-HTL contact interface, resulting in decreased recombination losses and improved stability. Moreover, the 2PACz monolayer is also found to reduce electrical shunts that ultimately yield improved performance in large area devices with PCE enhanced from 12.3% to 13.3% in 1 cm2 cells

Real-time detection of hepatitis B surface antigen using a hybrid graphene-gold nanoparticle biosensor

A hybrid biosensor based on a graphene resistor functionalized with self-assembled Graphene-AuNPs (Gold Nanoparticles) is demonstrated for the real-time detection of hepatitis B surface antigen (HBsAg). The hybrid biosensor consists of a ssDNA sequence attached to a graphene resistor device via π–π stacking interactions in combination with a ssDNA functionalized AuNP. The ssDNA has complementary sequences which through hybridization, yield the graphene-AuNP hybrid biosensor. Real-time 2-point resistance measurements, performed using varying concentrations of HBsAg, show a linear dependence of resistance change against the logarithm of HBsAg concentration (log[HBsAg]). A limit of detection of 50 pg ml−1 was observed. Moreover, the hybrid biosensor platform has potential to be applied to any biomarker of interest

Application of Molecular Vapour Deposited Al2O3 for Graphene-Based Biosensor Passivation and Improvements in Graphene Device Homogeneity

Graphene-based point-of-care (PoC) and chemical sensors can be fabricated using photolithographic processes at wafer-scale. However, these approaches are known to leave polymerresidues on the graphene surface, which are difficult to remove completely. In addition, graphenegrowth and transfer processes can introduce defects into the graphene layer. Both defects and resistcontamination can affect the homogeneity of graphene-based PoC sensors, leading to inconsistentdevice performance and unreliable sensing. Sensor reliability is also affected by the harsh chemicalenvironments used for chemical functionalisation of graphene PoC sensors, which can degrade partsof the sensor device. Therefore, a reliable, wafer-scale method of passivation, which isolates thegraphene from the rest of the device, protecting the less robust device features from any aggressive chemicals, must be devised. This work covers the application of molecular vapour depositiontechnology to create a dielectric passivation film that protects graphene-based biosensing devicesfrom harsh chemicals. We utilise a previously reported “healing effect” of Al2O3 on graphene toreduce photoresist residue from the graphene surface and reduce the prevalence of graphene defects to improve graphene device homogeneity. The improvement in device consistency allows formore reliable, homogeneous graphene devices, that can be fabricated at wafer-scale for sensing andbiosensing applications

A Facile Method for the Non-Covalent Amine Functionalization of Carbon-Based Surfaces for Use in Biosensor Development

Affinity biosensors based on graphene field-effect transistor (GFET) or resistor designs require the utilization of graphene’s exceptional electrical properties. Therefore, it is critical when designing these sensors, that the electrical properties of graphene are maintained throughout the functionalization process. To that end, non-covalent functionalization may be preferred over covalent modification. Drop-cast 1,5-diaminonaphthalene (DAN) was investigated as a quick and simple method for the non-covalent amine functionalization of carbon-based surfaces such as graphene, for use in biosensor development. In this work, multiple graphene surfaces were functionalized with DAN via a drop-cast method, leading to amine moieties, available for subsequent attachment to receptor molecules. Successful modification of graphene with DAN via a drop-cast method was confirmed using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and real-time resistance measurements. Successful attachment of receptor molecules also confirmed using the aforementioned techniques. Furthermore, an investigation into the effect of sequential wash steps which are required in biosensor manufacture, on the presence of the DAN layer, confirmed that the functional layer was not removed, even after multiple solvent exposures. Drop-cast DAN is thus, a viable fast and robust method for the amine functionalization of graphene surfaces for use in biosensor development

A Low-Temperature Batch Process for the Deposition of High-Quality Conformal Alumina Thin Films for Electronic Applications

High-quality, alumina thin films are extensively used as dielectrics, passivation layers and barrier layers in electronics and many other applications. However, to achieve optimum stoichiometry and thus performance, the layers are often grown at elevated temperatures (> 200 °C) using techniques such as Atomic Layer Deposition (ALD). This is problematic for substrates or structures with low thermal budgets. In this work, alumina thin films were grown on 200 mm silicon substrates employing a versatile deposition method known as Molecular Vapour Deposition (MVD) at low deposition temperatures (35-150 °C). The chemical composition of the resulting films was investigated post-deposition using X-ray Photoelectron Spectroscopy (XPS) and Variable Angle Spectroscopic Ellipsometry (VASE), with fully stoichiometric Al2O3 achieved at deposition temperatures as low as 100 °C. Dielectric measurements confirm outstanding dielectric properties compared to typical thermal ALD layers deposited at much higher temperatures. We rationalise and understand this low-temperature deposition performance by considering the MVD reactor design and the ‘pump-type’ regime of precursor delivery versus the ‘flow-type’ regime of ALD. Our results clearly demonstrate that alumina thin films grown with MVD are highly versatile for electronic applications and are of particular relevance and interest for the high-volume processing of dielectric, passivation, and barrier layers at low temperatures