Operating Principles of Zero-Bias Retinomorphic Sensors

Zero bias retinomorphic sensors (ZBRSs) are a new type of optical sensor which produce a signal in response to changes in light intensity, but not to constant illumination. For this reason, they are hoped to enable much faster identification of moving objects than conventional sensing strategies. While recent proof-of-principle experimental demonstrations are significant, there does not yet exist a robust quantitative model for their behaviour, which represents an impediment for effective progress to be made in this field. Here I report a mathematical framework to quantify and predict the behaviour of ZRBSs. A simple device-level model and a more detailed carrier-dynamics model are derived. Both models are tested computationally, yielding equivalent behaviour consistent with experimental observations. A figure of merit, Λ_0, was identified which is hoped to enable facile comparison of devices between different research groups. This work is hoped to serve as the foundation for a consistent description of ZBRSs

The effect of substrate curvature on capacitance and transfer characteristics for thin film transistors on the surface of spheres

Conformable, flexible, and stretchable thin film transistors hold promise for ubiquitous and low-cost electronics. As part of the research endeavor toward this goal, the challenges associated with compatible materials and growth processes have been intensely studied. What is seldom considered, however, is how device electrostatics change as the physical form of devices change. In this report, we study how one would expect the current–voltage characteristics of thin film transistors to change as they are deformed on the surface of a sphere. We derive analogous equations to those derived in the gradual channel approximation to relate current to applied voltage for various spherical geometries. Combined with a finite-difference strategy to evaluate geometric capacitance, example current–voltage characteristics are calculated. The results demonstrate for certain deformations in this geometry, the behavior deviates from what one would expect using just the gradual channel approximation. For flexible electronics to be commercially viable, it must be predictable in any physical form. These results represent some of the first steps in a broader effort to quantify the relationship between device geometry and electrical behavior

Role of blend ratio in bulk heterojunction organic retinomorphic sensors

Conventional image sensors are designed to digitally reproduce every aspect of the visual field; in general representing brighter regions of a scene as brighter regions in an image. While the benefits of detecting and representing light in this way are obvious, limitations imposed by processing power and frame rate place a cap on the speed at which moving objects can be identified. An emerging alternative strategy is to use sensors which output a signal only in response to changes in light intensity, hence inherently identifying movement by design. These so-called retinomorphic sensors are hoped to outperform conventional sensors for certain tasks, such as identification of moving objects. In this report, the working mechanism of retinomorphic sensors based on organic semiconductors as the active layer is probed. It is observed that the sign of the voltage signal is changed when electrode connections are reversed, suggesting our previous description of device behaviour was incomplete. By systematically varying the ratio of poly(3-hexylthiophene-2,5-diyl) (P3HT) to phenyl-C61-butyric acid methyl (PCBM) in the absorption layer, a maximum performance was observed when the ratio was 1 : 2 P3HT : PCBM, while pure P3HT and pure PCBM exhibited very weak signals

Role of A‐Site Composition in Charge Transport in Lead Iodide Perovskites

As the power conversion efficiency and stability of solar cells based on metal halide perovskites continue to improve, the community increasingly relies on compounds formed of mixed cations and mixed halides for the highest performing devices. The result is that device engineers now have a potentially infinite number of compositions to choose from. While this has provided a large scope for optimization, it has increased complexity of the field, and the rationale for choosing one composition over another remains somewhat empirical. Herein, the distribution of electronic properties for a range of lead iodide perovskite thin films is mapped. The relative percentages of methylammonium, formamidinium, and cesium are varied, and the electronic properties are measured with time-resolved microwave conductivity, a contactless technique enabling extraction of electronic properties of isolated films of semiconductors. It is found a small amount of Cs leads to larger carrier mobilities and longer carrier lifetimes and that compositions with a tolerance factor close to 0.9 generally show lower performance that those closer to 0.8 or 1.0

Self-assembly and charge transport properties of a benzobisthiazole end-capped with dihexylthienothiophene units

The synthesis of a new conjugated material is reported; BDHTT–BBT features a central electron-deficient benzobisthiazole capped with two 3,6-dihexyl-thieno[3,2-b]thiophenes. Cyclic voltammetry was used to determine the HOMO (−5.7 eV) and LUMO (−2.9 eV) levels. The solid-state properties of the compound were investigated by X-ray diffraction on single-crystal and thin-film samples. OFETs were constructed with vacuum deposited films of BDHTT–BBT. The films displayed phase transitions over a range of temperatures and the morphology of the films affected the charge transport properties of the films. The maximum hole mobility observed from bottom-contact, top-gate devices was 3 × 10−3 cm2 V−1 s−1, with an on/off ratio of 104–105 and a threshold voltage of −42 V. The morphological and self-assembly characteristics versus electronic properties are discussed for future improvement of OFET devices

Predicting solar cell performance from terahertz and microwave spectroscopy

Mobilities and lifetimes of photogenerated charge carriers are core properties of photovoltaic materials and can both be characterized by contactless terahertz or microwave measurements. Here, the expertise from fifteen laboratories is combined to quantitatively model the current-voltage characteristics of a solar cell from such measurements. To this end, the impact of measurement conditions, alternate interpretations, and experimental inter-laboratory variations are discussed using a (Cs,FA,MA)Pb(I,Br)3 halide perovskite thin-film as a case study. At 1 sun equivalent excitation, neither transport nor recombination is significantly affected by exciton formation or trapping. Terahertz, microwave, and photoluminescence transients for the neat material yield consistent effective lifetimes implying a resistance-free JV-curve with a potential power conversion efficiency of 24.6 %. For grainsizes above ≈20 nm, intra-grain charge transport is characterized by terahertz sum mobilities of ≈32 cm2 V−1 s−1. Drift-diffusion simulations indicate that these intra-grain mobilities can slightly reduce the fill factor of perovskite solar cells to 0.82, in accordance with the best-realized devices in the literature. Beyond perovskites, this work can guide a highly predictive characterization of any emerging semiconductor for photovoltaic or photoelectrochemical energy conversion. A best practice for the interpretation of terahertz and microwave measurements on photovoltaic materials is presented

Charge-Carrier Dynamics and Crystalline Texture of Layered Ruddlesden–Popper Hybrid Lead Iodide Perovskite Thin Films

Solution-processable organic metal halide Ruddlesden–Popper phases have shown promise in optoelectronics because of their efficiencies in solar cells along with increased material stability relative to their three-dimensional counterparts (CH₃NH₃PbI₃). Here, we study the layered material butylammonium methylammonium lead iodide (C₄H₉NH₃)₂(CH₃NH₃)_<i>n</i>−1Pb_<i>n</i>I_3<i>n</i>+1 for values of <i>n</i> ranging from 1 to 4. Thin films cast from solution show a gradual change in the crystalline texture of the two-dimensional lead iodide layers from being parallel to the substrate to perpendicular with increasing <i>n</i>. Contactless time-resolved microwave conductivity measurements show that the average recombination rate order increases with <i>n</i> and that the yield–mobility products and carrier lifetimes of these thin films are much lower than that of CH₃NH₃PbI₃, along with increased higher-order recombination rate constants