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New Photoelectrochromic Device with Chromatic Silica/Tungsten Oxide/ Copper Hybrid Film and Photovoltaic Polymer/Quantum Dot Sensitized Anode Ankita Kolay, Aparajita Das, Partha Ghosal, and Melepurath Deepa ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00765 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018
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ACS Applied Energy Materials
New Photoelectrochromic Device with Chromatic Silica/Tungsten Oxide/Copper Hybrid Film and Photovoltaic Polymer/Quantum Dot Sensitized Anode Ankita Kolay,a Aparajita Das,a Partha Ghosalb , Melepurath Deepa a,* a
Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi-502285, Sangareddy, Telangana (India)
b
Defence Metallurgical Research Laboratory, Defence Research & Development Organisation (DRDO), Hyderabad500058, Telangana (India) KEYWORDS: Electrochromic; solar; photoelectrochromic; tungsten oxide; copper film
ABSTRACT: A unique SiO2/WO3/Cu counter electrode (CE) coupled with a TiO2/CdS/P3HT photoanode is employed to design a cost-effective photoelectrochromic energy conversion device for self-powered smart window application. The main objective behind the inclusion of Cu in this device as a complement to the previously researched WO 3 electrochromic counter electrode is to augment the photoelectrochromic response of the latter. The electrons made available due to hot electron injection from Cu to WO3 boost the intensity of coloration while also promoting better chromatic switching in the SiO2/WO3/Cu CE. The transmission modulation (ΔT) under 0.25 sun at 465 nm wavelength is 50.6% for the SiO2/WO3/Cu film with a coloration efficiency of 30.6 cm2 C-1, whereas it is 39.9% for SiO2/WO3 with much lower coloration efficiency of 22.4 cm2 C-1. The ΔTVIS of the complete liquid photoelectrochromic assembly with SiO2/WO3/Cu CE is 41.6%, which is much higher as compared to the value of 33.6% that is obtained when the device uses only SiO2/WO3 as counter electrode. Under 1 sun irradiance, the power conversion efficiency (PCE) of the TiO 2/CdS/P3HT-S2--SiO2/WO3/Cu cell, at 5.88%, is a noticeable enhancement over the value of 3.77% attained by using a SiO 2/WO3 CE (without Cu) with the same photoanode. The high electrical conductivity of Cu allows facile charge propagation, making SiO2/WO3/Cu a proficient CE. Channelized movement of the photogenerated electrons is responsible for the vast improvement in the photoelectrochromic as well as photovoltaic performances of this novel assembly, as has been rationalized in detail in this study.
smart window application can be effectively implemented in modern energy-saving buildings.
Introduction A photoelectrochromic device (PECD) synergistically merges a photovoltaic (PV) part and an electrochromic (EC) part, enabling it to display a dual behavior of simultaneous energy harvesting and energy saving, with only light as the stimulus. PECDs are self-powered devices in which the photovoltaic segment of the assembly converts solar radiation into electrical energy, which in turn acts as the stimulus for the electrochromic material to vary its chromatic characteristics.1 On illumination, excited electrons ejected from the photoanode are supplied to the electrochromic film along with the electrochemical reversible ion insertion from the electrolyte, which then exists in the reduced state and becomes colored. In absence of light, i.e., under open circuit conditions, the reverse process is initiated with loss of electrons from the EC film and extraction of ions, resulting in bleaching or discoloration.2 PECDs may find application in a host of technologies, ranging from ‘smart windows’ and skins of future buildings, to optical displays, glare-control devices and rear-view mirrors in automotives.3,4 PECD with integrated photovoltaic and electrochromic components enables an adjustable transparency glazing where the photovoltaic part supplies the power to drive the coloration. Such stand-alone, self-powered, wireless devices are of commercial interest for integration into glass windows and surfaces of buildings and automotive or aeronautic vehicles. These devices with
Various photovoltaic units have been investigated for PECDs, ranging from dye sensitized solar cells (DSSCs) to organic photovoltaic devices (OPVs) and silicon solar cells.5,6 The electrochromic material is essentially the photo-responsive counter electrode (CE) of the assembly that is capable of switching between a bleached state and a colored state, or between two colored states. Two major classes of common electrochromic materials include transition metal oxides (TMOs) and conjugated polymers (CPs). Works dealing with TMOs having electrochromic characteristics are largely based on tungsten and nickel.7,8 The principle of a PEC cell was first illustrated in the pioneering work by Bechinger with a ruthenium polypyridinesensitized nanocrystalline TiO2 electrode in the photovoltaic component and tungsten oxide (WO3) film on the CE.9 A wide range of conjugated polymers have been utilized in the construction of PECDs, including polyaniline (PANI),10 poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3,4propylenedioxythiophene) (PProDOT).11,12 Investigations on PECDs incorporating dye-sensitized TiO2 electrodes highlight various possible innovations that can yield high optical contrast.13,14,15 While the color tunability of DSSCs is of great advantage in its use as the PV component of PECDs, organic dye-sen-
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sitized photoanodes generally suffer from certain limitations such as low transmittance modulation and deep color in the discolored state. These problems can be circumvented by employing quantum dots (QDs) as an alternative sensitizer in place of dyes in the photoanode architecture.16,17 QDs possess several excellent properties such as tunable bandgaps due to their size quantization property, multiple exciton generation, higher absorption coefficients than most organic dyes and relatively lower cost. The parameters defining the performance of an ECD include (i) Low Operating Potentials (˂ 5 V) (ii) High Contrast Ratio (∆T = Tb-Tc) (iii) Large Coloration Efficiency (η = ∆OD/Q) (iv) Fast Switching Kinetics (A / T vs. t) and (v) High Cycle Life. WO3 that has been extensively used in the past fulfils the aforementioned criteria and can therefore be considered an appropriate choice as the EC material. With suitable modifications, WO3 can show enhanced electrochromism without compromising its potential to serve as an efficient CEs in QDSCs due to its excellent electrocatalytic activity and high electrical conductivity, both of which are necessary prerequisites of a counter electrode for the photovoltaic part.18 Injection of electrons into the electrochromic WO3 layer with the concurrent intercalation of cation into the tungsten oxide lattice lead to the formation of the bluish colored tungsten bronze. (transparent) WO3 + x(M+ + e−) → MxWVI(1−x)WVxO3 (bluecolored) (M: Na+) (1) To further improve the electrochromic and counter electrode performance of a WO3 film, here, for the first time, WO3 is integrated with a metallic Cu thin film to yield a WO3/Cu hybrid film. The Cu film being highly conductive, enables rapid electron transfer to the redox electrolyte. The WO3/Cu hybrid film results in superior photovoltaic performances and enhancement of the overall optical modulation compared to a pristine WO3 film. The objective in this work is to harness the potential of these individual components through a WO3/Cu hybrid film, thereby augmenting the PCE of the device while also amplifying its electrochromic performance. A very thin coating of SiO2 over the WO3/Cu electrode induces a passivating effect to inhibit the etching of the electrochromic WO3 layer in the alkaline aqueous sulfide redox electrolyte. The sulfide electrolyte acts as a hole scavenger as well as an ionic electrolyte necessary for realizing the electrochromism in a WO3/Cu hybrid film. Coloration is feasible only with the intercalation of the cations from the electrolyte into the electrochromic film along with the electrons from the external circuit. Cadmium sulfide (CdS) QDs sensitized titania photoanode has received wide attention over the years.19 Similarly, the regioregular semiconducting polymer poly(3-hexylthiophene) (P3HT) has also been widely explored in organic solar cells as a donor material. In solid state dye/QDs sensitized solar devices, the hole conductive role of P3HT has been adequately highlighted over its contribution to light absorption as testified in a few reports.20,21,22However, there
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remains enough room for exploring the photosensitive property of P3HT especially in QDSCs. P3HT when incorporated in the photoanode, serves both as an assistant light harvester and a hole transporting material. Effective infiltration of P3HT may not only assist the charge injection from the QDs through the self-organizing interaction when P3HT is located on the QDs-sensitized TiO2 surface, but can also enhance the spectral response in the region of 400–700 nm.22 Here, P3HT co-sensitized TiO2/CdS photoanode in a liquid junction quantum dot solar cell enables broader solar spectrum utilization owing to a band gap in the visible region, while also facilitating unhindered charge propagation due to better hole diffusion. These factors cumulatively manifest in a high performance QDSC. This is a first report on a PECD with the following architecture: TiO2/CdS/P3HT-S2--SiO2/WO3/Cu, and a detailed analysis is provided to validate the potential of this PECD as a new high performance, efficient, low cost and scalable alternative to conventional and exhaustively studied dye sensitized TiO2 and solely WO3 based PECDs.
Experimental Chemicals Fluorine doped tin oxide (FTO) glass with a sheet resistance of ~25 Ω cm-2 was purchased from Pilkington and cleaned consecutively in soap solution, 10% HCl solution, 10% NaOH solution, distilled water, acetone/ethanol (v/v: 1:1) and iso-propanol. TiO2 (P25) and fumed silica were free gifts from Evonik and Cabosil respectively. Titanium chloride (TiCl4), Triton X-100, sodium hydroxide (NaOH, 99%), hydrochloric acid (HCl), methanol, ethanol, iso-propanol, hydrogen peroxide 30%, chloroform, tungsten (W) metal powder (
