The behaviour of charge distributions in dielectric media

Flexible Miniaturized Multispectral Detector Derived from Blade-Coated Organic Narrowband Response Unit Array

Multispectral sensing is extremely desired in intelligent systems, e.g., autonomous vehicles, encrypted information communication, and health biometric monitoring, due to its highly sensitive spectral discrimination ability. Nevertheless, rigid bulky optics and delicate optical paths in devices significantly increase their complexity and size, which subsequently impede their integration in smart optoelectronic chips for universal applications. In this work, a filterless miniaturized multispectral photodetector is realized with an organic narrowband response unit array. With the manipulation of Frenkel exciton dissociation in active layers, a series of narrowband organic sensing units with full-width-at-half-maximum (fwhm) narrowing to ∼50 nm are achieved from 700 to 1050 nm with a laudable performance of responsivity of over 60 mA/W, -3 dB bandwidth over 10 kHz, linear dynamic range (LDR) reaching ∼120 dB, and a low noise current of less than 4 × 10^-14 A·Hz^-0.5. Furthermore, a 6 × 8 multispectral sensing array on a flexible substrate was fabricated with blade-coating. Assisted by a computational process, we successfully demonstrate the spectral recognition with a resolution of ∼50 nm and a mismatch of ∼10 nm. Finally, the function of matter identification is successfully achieved with our multispectral detector array.

Development of low bandgap polymers for red and near-infrared fullerene-free organic photodetectors

Two low bandgap donor polymers, PDTPTT and PCPDTTT, were synthesized and their photodetecting properties were investigated under a 680 nm red LED.

Self-filtering narrowband high performance organic photodetectors enabled by manipulating localized Frenkel exciton dissociation

The high binding energy and low diffusion length of photogenerated Frenkel excitons have long been viewed as major drawbacks of organic semiconductors. Therefore, bulk heterojunction structure has been widely adopted to assist exciton dissociation in organic photon-electron conversion devices. Here, we demonstrate that these intrinsically “poor” properties of Frenkel excitons, in fact, offer great opportunities to achieve self-filtering narrowband organic photodetectors with the help of a hierarchical device structure to intentionally manipulate the dissociation of Frenkel excitons. With this strategy, filter-free narrowband organic photodetector centered at 860 nm with full-width-at-half-maximum of around 50 nm, peak external quantum efficiency around 65% and peak specific detectivity over 10¹³ Jones are obtained, which is one the best performed no-gain type narrowband organic photodetectors ever reported and comparable to commercialized silicon photodetectors. This novel device structure along with its design concept may help create low cost and reliable narrowband organic photodetectors for practical applications.

Narrowband organic photodetectors (OPDs) are attractive for emerging applications. Here, the authors report a simple strategy to produce filter-free narrowband OPDs with outstanding performances by manipulating exciton dissociation with a hierarchical device structure.

Hole Transfer Promoted by a Viscosity Additive in an All-Polymer Photovoltaic Blend

Viscosity modifiers are widely applied to improve the mechanical compliance of organic optoelectronic devices. However, the effect of the viscosity additives on the charge dynamics remains poorly understood. Here, we report the observation of markedly different effects of a high-viscosity polymeric additive on the electron- and hole-transfer dynamics in all-polymer organic photovoltaic blends. By using ultrafast transient absorption spectroscopy, we determine that hole transfer from charge-transfer excitations in the acceptor is markedly promoted while the electron transfer from local excitations in the donor remains nearly unchanged upon introduction of viscosity additives into the blends. We argue that the modification of dielectric screening is the mechanism underlying the effect of the additive on the charge dynamics. This finding suggests a new strategy for designing high-performance flexible organic photovoltaic devices by manipulating the dielectric environment.

Can the Dielectric Constant of Fullerene Derivatives Be Enhanced by Side-Chain Manipulation? A Predictive First-Principles Computational Study

The low efficiency of organic photovoltaic (OPV) devices has often been attributed to the strong Coulombic interactions between the electron and hole, impeding the charge separation process. Recently, it has been argued that by increasing the dielectric constant of materials used in OPVs, this strong interaction could be screened. In this work, we report the application of periodic density functional theory together with the coupled perturbed Kohn–Sham method to calculate the electronic contribution to the dielectric constant for fullerene C₆₀ derivatives, a ubiquitous class of molecules in the field of OPVs. The results show good agreement with experimental data when available and also reveal an important undesirable outcome when manipulating the side chain to maximize the static dielectric constant: in all cases, the electronic contribution to the dielectric constant decreases as the side chain increases in size. This information should encourage both theoreticians and experimentalists to further investigate the relevance of contributions to the dielectric constant from slower processes like vibrations and dipolar reorientations for facilitating the charge separation, because electronically, enlarging the side chain of conventional fullerene derivatives only lowers the dielectric constant, and consequently, their electronic dielectric constant is upper bound by the one of C₆₀.

On the Efficiency Limit of Conjugated Polymer:Fullerene-Based Bulk Heterojunction Solar Cells

The power conversion efficiency potential of eight high-performance polymer-fullerene blends is investigated. All studied absorbers show the typical organic solar cell losses limiting their performance to ≈13%.

Promising Strategy To Improve Charge Separation in Organic Photovoltaics: Installing Permanent Dipoles in PCBM Analogues

A multidisciplinary approach involving organic synthesis and theoretical chemistry was applied to investigate a promising strategy to improve charge separation in organic photovoltaics: installing permanent dipoles in fullerene derivatives. First, a PCBM analogue with a permanent dipole in the side chain (PCBDN) and its reference analogue without a permanent dipole (PCBBz) were successfully synthesized and characterized. Second, a multiscale modeling approach was applied to investigate if a PCBDN environment around a central donor-acceptor complex indeed facilitates charge separation. Alignment of the embedding dipoles in response to charges present on the central donor-acceptor complex enhances charge separation. The good correspondence between experimentally and theoretically determined electronic and optical properties of PCBDN, PCBBz, and PCBM indicates that the theoretical analysis of the embedding effects of these molecules gives a reliable expectation for their influence on the charge separation process at a microscopic scale in a real device. This work suggests the following strategies to improve charge separation in organic photovoltaics: installing permanent dipoles in PCBM analogues and tuning the concentration of these molecules in an organic donor/acceptor blend.

Exciton dispersion in molecular solids

The investigation of the exciton dispersion (i.e. the exciton energy dependence as a function of the momentum carried by the electron-hole pair) is a powerful approach to identify the exciton character, ranging from the strongly localised Frenkel to the delocalised Wannier-Mott limiting cases. We illustrate this possibility at the example of four prototypical molecular solids (picene, pentacene, tetracene and coronene) on the basis of the parameter-free solution of the many-body Bethe-Salpeter equation. We discuss the mixing between Frenkel and charge-transfer excitons and the origin of their Davydov splitting in the framework of many-body perturbation theory and establish a link with model approaches based on molecular states. Finally, we show how the interplay between the electronic band dispersion and the exchange electron-hole interaction plays a fundamental role in setting the nature of the exciton. This analysis has a general validity holding also for other systems in which the electron wavefunctions are strongly localized, as in strongly correlated insulators.