Sungyoung Yun

Control of naturally coupled piezoelectric and photovoltaic properties for multi-type energy scavengers†

In this paper, we present a simple, low-cost and flexible hybrid cell that converts individually or simultaneously low-frequency mechanical energy and photon energy into electricity using piezoelectric zinc oxide (ZnO) in conjunction with organic solar cell design. Since the hybrid cell is designed by coupled piezoelectric and photoconductive properties of ZnO, this is a naturally hybrid architecture without crosstalk and an additional assembling process to create multi-type energy scavengers, thus differing from a simple integration of two different energy generators. It is demonstrated that the behavior of a piezoelectric output is controlled from alternating current (AC) type to direct current (DC)-like type by tailoring mechanical straining processes both in the dark and under light illumination. Based on such controllability of output modes, it is shown that the performance of the hybrid cell is synergistically enhanced by integrating the contribution made by a piezoelectric generator with a solar cell under a normal indoor level of illumination. Our approach clearly demonstrates the potential of the hybrid approach for scavenging multi-type energies whenever and wherever they are available. Furthermore, this work establishes the methodology to harvest solar energy and low-frequency mechanical energies such as body movements, making it possible to produce a promising multi-functional power generator that could be embedded in flexible architectures.

Dipolar donor–acceptor molecules in the cyanine limit for high efficiency green-light-selective organic photodiodes

We report on two novel p-type small molecules with a donor–acceptor molecular structure for application to green-light-selective organic photodiodes (OPDs). To achieve the requirement of high light selectivity and sensitivity, an electron-donating aryl amino moiety is combined with two respective electron-accepting heterocycles so that the molecules approach cyanine-like character, characterized by intense and sharp absorption. Molecular stacking is controlled by the addition of bulky aryl functional groups to the main backbone to further control the electrical charge transport properties. With this molecular design, a maximum external quantum efficiency close to 61% (λmax = 550 nm) and a dark-current density below 1.6 nA cm−2 (or specific detectivity D* = 1.19 × 1013 cm Hz1/2 W−1) at an applied reverse bias of 3 V are obtained when mixed with fullerene (C60) in an inverted-structure bulk heterojunction OPD composed of two transparent electrodes. The potential construction of a full-color photodetector or an image sensor is demonstrated by combining the green-light-selective OPD with a silicon photodiode containing solely blue and red color filters in a stacked architecture.

Autocatalytic effect of amine-terminated precursors in mixed self-assembled monolayers

We investigated the formation of mixed self-assembled monolayers (SAMs), comprising two silanes terminated with an amine and a non-reactive functional group, to demonstrate the autocatalysis of the mixed SAMs by the amine-terminated precursors. Measurements of surface energy and angle-resolved X-ray photoelectron spectroscopy on the mixed SAMs revealed that the final composition and the surface coverage of the mixed SAMs after deposition strongly depended on the presence and the concentration of the aminesilane precursor in solution. To explain the observed dependence, we further suggested a mechanism based on the autocatalytic effect of the aminesilane precursor on the mixed SAM. These results highlight the complex role of the aminesilane, which behaves simultaneously as a component of the mixed SAM and a catalyst. An important implication of this is that the compositions of mixed SAMs can be significantly influenced by the autocatalytic effect of the component carrying a reactive functional group such as amine.

Auger electron nanoscale mapping and x-ray photoelectron spectroscopy combined with gas cluster ion beam sputtering to study an organic bulk heterojunction

The lateral and vertical distributions of organic p/n bulk heterojunctions for an organic solar cell device are, respectively, investigated using nanometer-scale Auger electron mapping and using X-ray photoelectron spectroscopy (XPS) with Ar gas cluster ion beam (GCIB) sputtering. The concentration of sulfur, present only in the p-type material, is traced to verify the distribution of p-type (donor) and n-type (acceptor) materials in the blended structure. In the vertical direction, a considerable change in atomic sulfur concentration is observed using XPS depth profiling with Ar GCIB sputtering. In addition, Auger electron mapping of sulfur reveals the lateral 2-dimensional distribution of p- and n-type materials. The combination of Auger electron mapping with Ar GCIB sputtering should thereby allow the construction of 3-dimensional distributions of p- and n-type materials in organic photovoltaic cells.

Enhancement of the power conversion efficiency in a polymer solar cell using a work-function-controlled TimSinOx interlayer

Work-function-adjustable TimSinOx provides an opportunity to optimize the energy-level alignment at the photoactive/cathode interface in the polymer bulk heterojunction solar cell. The work function of TimSinOx is engineered by adjusting the Si mol% during the sol–gel reaction. The controlled work function provides an energetically downhill cascade pathway for electrons from the electron acceptor to the cathode, which contributes to improvement in electron collection at the cathode. The valence band maxima of TimSinOx also become deeper as the Si mol% increases and the hole-blocking ability of TimSinOx is enhanced as a result. Accordingly, polymer solar cells fitted with the optimized TimSinOx exhibit enhanced performance.

Recent developments in green light sensitive organic photodetectors for hybrid CMOS image sensor applications(Conference Presentation)

Typical CMOS colour image sensors consist of Si-based photodetectors (PDs) attached with colour filter arrays (i.e., the Bayer pattern). Recent trends on the development of high resolution image sensors, however, require downsizing the pixel dimension, which inevitably results in the loss of sensitivity due to the reduction in the photon acquisition. Very recently, hybrid stacks of organic photodetectors (OPDs) on conventional CMOS technologies have been proposed as one of the promising approaches to realise highly sensitive image sensors by doubling the light detecting area in the limited pixel size. Specifically, OPDs with orthogonal photosensitivity to green light and Si-based PDs with red and blue colour filters serve as the top and bottom photo-conversion layers, respectively. In this presentation, we will introduce the recent development of high performance green light sensitive OPDs and the demonstration of colour images from hybrid CMOS image sensors proposed. OPDs consisting of small molecule organic bulk heterto-junction structures, hole/electron buffer layers, and transparent top/bottom ITO electrodes exhibited peak external quantum efficiencies of 60-65% at 550-560 nm wavelengths and full width at half maximum of ~120 nm at reverse bias of 3 V. Extremely low dark current densities in the range of 0.2-0.5 nA/cm2 at reverse bias of 3V and consequently high specific detectivities over 2×10^13 Jones were obtained from the developed OPD system. Further investigations in terms of the molecular structures of organic light absorbing materials, buffer materials, layer sequences, and even integration issues of the OPD on the CMOS will be described in detail.