Investigation Of Electrolyte In Self-Powered Electrochromic Device And Development Of Device Structure

Electrochromism refers to the phenomenon of the reversible change of a material's optical properties under an applied voltage,usually displayed by the appearance of color change.This phenomenon is essentially the change of redox state of material under external potential,while the material has different optical properties in the oxidized and reduced states.Self-powered electrochromic devices emerged as an ingenious combination of electrochromic technology and dye-sensitized solar cell(DSSC),which can achieve optical modulation under illumination only by changing the external state(such as the circuit state between two electrodes,the light intensity etc.)with no need for external power supply.This property makes self-powered electrochromic devices have a broad development prospect in the field of building energy efficiency.The research work of this dissertation can be divided into two sections.The first section is the investigation of influence of redox electrolyte on photoelectrochromic device(PECD)performance.PECD is the most classic type in self-powered electrochromic devices;however,present PECDs can barely obtain both high transmittance modulation and fast response time,especially when using organic electrochromic materials,PECD optical modulation performance is often unsatisfactory.Moreover,the most widely used I-/I3-electrolyte in DSSC is not suitable for PECD due to its deep color.In this dissertation,we aim to improve the optical modulation properties of PECD.We fabricated the PECD based on ruthenium-based dye N719 and polymer electrochromic material poly(3,4-(2,2-dimethyl-propylenedioxy)thiophene)(PProDOT-Me2),and then studied the impact of different redox electrolytes on the performance of PECD.These redox electrolytes include:Br/Br3-and I-/I3-electrolytes that both belong to halogen;three kinds of cobalt complexes that are[Co(bpy)3]n+,[Co(phen)3]n+ and[Co(NO2-phen)3]n+;three kinds of organic small molecular materials.We found that the redox potential of electrolyte had a great influence on the transmittance modulation of PECD.The electrolyte with high redox potential had the ability of oxidizing PProDOT-Me2 film completely,which could greatly improve the transmittance of PECD in the bleached state,leading to an enhanced transmittance modulation of device.On the other hand,if the electrons infecting into PProDOT-Me2 film are not sufficient to counteract the oxidation of redox electrolyte to PProDOT-Me2 film,the coloring process of PECD would hardly proceed.Therefore,when using the electrolyte of high redox potential,it is still necessary for the device to achieve a suitable short-circuit current value for coloration.In addition,the electron transfer resistance between the electrochromic film and redox electrolyte affected the bleaching time of PECD.The smaller the resistance value,the faster the bleaching time.The PECD using Br-/Br3-electrolyte eventually obtained a maximum transmittance modulation of more than 40%and a coloring/bleaching response time of less than 3 seconds;meanwhile,the device also showed excellent cycling stability.The second section is the development of self-powered electrochromic device structure.Since the electrochromic part and the TiO2/dye part are arranged in the "face-to-face" state in PECD structure,the device will present the deep color of the dye molecules in the bleached state.For this case,we first designed two kinds of self-powered electrochromic devices that can reveal color change between colorless state and deep blue:(1)by rearranging the dye part and the electrochromic part in the thickness direction of device,and by using completely colorless tetrabutylammonium bromide(TBABr)electrolyte,the final device could achieve coloring in short circuit state and bleaching in open circuit state under illumination with no need for external potential;(2)by integrating the electrochromic element and the photovoltaic element on one conductive substrate,while simultaneously using LiBr/Br2 electrolyte with high transmittance in the visible region,the final device was deep blue under illumination,and it presents a colorless and transparent state in low light or dark condition.Additionally,we proposed a self-powered electrochromic system composed of DSSCs and a self-erasing electrochromic device in series.When connected with the photovoltaic part in the system,the self-erasing electrochromic device turned into colored state;when disconnected with the photovoltaic part in the system,the self-erasing electrochromic device automatically returned to the transparent state.