Abstract
We report novel photovoltaic (PV) switching based on the low exciton-binding energy property of an organic heptazole (C26H16N2) thin film after fabrication of an heptazole-based Schottky diode. The Schottky diode cell displayed an instantaneous voltage of 0.3 V as an open circuit voltage (VOC) owing to the work function difference between the Schottky and ohmic electrode under deep blue illumination. Four tandem diode cells therefore produced ~1.2 V. As a PV diode circuit can be formed using an even number of diodes, a photo-excited charge accumulation takes place, generating VOC in the central electrode of the tandem diode array by illuminating one half of the array. An electron–hole recombination then also takes place in that electrode by illuminating the other half, making the VOC decrease to 0 V. Utilizing this charge accumulation and recombination under deep blue illumination, we successfully demonstrated quite fast PV optical switching, logic gating and, ultimately, the gate switching of an organic field-effect transistor. We therefore concluded that our self-powered PV-induced switching was novel and promising enough to open a new door for energy harvest-related device applications in organics. Photovoltaic switching in organic thin films has been realized by a team in Korea. The photovoltaic effect is virtually synonymous with light harvesting using solar-cell technology, but researchers have now demonstrated another application of it — self-powered switching. Seongil Im of Yonsei University in Seoul and co-workers made Schottky diode cells that exploit the low exciton-binding energy of thin films made from heptazole (C26H16N2). Under deep-blue illumination, the cells generated a voltage of 0.3 volts. Furthermore, they exhibited relatively fast photovoltaic switching, logic gating and gate switching of an organic field-effect transistor. The team anticipates that this self-powered switching could be used for detection of ultraviolet and visible light, wireless healthcare devices and powerless communication. It is especially suited for devices designed to protect the eyes from ultraviolet light. Photovoltaic switching in organic heptazole Schottky diode circuits is successfully implemented under dynamic illumination. Blue energetic photons generate electron–hole pairs, and instantaneous voltage of 0.3 V is produced that is multiplied by tandem diode structure. Using the photovoltaic effects, NOT, OR and AND logics are demonstrated without any electric power.
Highlights
We chose an heptazole (C26H16N2) thin film, as the latter showed a rather small exciton-binding energy of ~ 40 meV, while pentacene (C22H14) and dinaphtho[2,3-b:2′,3′-f]thieno[3,2-B]thiophene (DNTT; C22H12S2) revealed somewhat larger binding energies of 100 and 120 meV, respectively. (See Supplementary Figure S1 and Supplementary Table S2 for a detailed comparison of the respective exciton-binding energy properties of heptazole, DNTT28 and pentacene thin films based on photo-excited charge collection spectroscopic results.29) Besides its small exciton-binding energy, our organic heptazole with 2.95 eV energy band gap sensitively and selectively detects deep blue/UV photons in PV switching operations, which would be promising toward smart eye protection glass applications.[27]
Because of the intrinsic potential energy difference between Al and indium-tin-oxide (ITO), our Schottky diode displayed an instantaneous voltage of 0.3 V by deep blue illumination from a laser diode, and four tandem diode cells could produce ~ 1.2 V when integrated onto the same glass substrate
A more interesting I–V curve behavior achieved from the circuit configuration of Figure 1c under the blue Light 1 (L1) and Light 2 (L2) combination can be observed in Figure 1e
Summary
Photovoltaic (PV) effects have long been studied for their main applications in solar energy and PV cells, which are energy-harvesting devices.[1,2,3,4,5,6] Organic and inorganic semiconductors have been used as active components of those energy-harvesting devices in a variety of diode forms, including Schottky diodes,[7,8,9,10] p-n diodes[11,12,13,14] and p-i-n diodes,[15,16,17,18] showing outstanding progress in enhancing their power efficiencies.[19,20] This study reports a rather different new application, dynamic PV switching and voltage generation by deep blue photons.Dynamic PV switching has so far been ignored in organic electronics and rarely attempted in inorganics despite its potential great usefulness as a dynamic energy-harvesting method.[13,21,22,23,24,25] dynamic PV switching has only been reported for its limited practical applications with inorganic nanostructure junctions, as seen in Supplementary Table S1.13,21–25 speaking, instantaneously built-in voltage can be used as a switching signal induced by energetic photons (ON/OFF: photo-induced charge collection/recombination), as any semiconductor thin film possesses electrodes for charge collection and recombination within a diode array.
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