Prakash R. Somani

Electrochromic materials and devices: present and future

Abstract The increase in the interaction between man and machine has made display devices indispensable for visual communication. The information which is to be communicated from a machine can be often in the form of color images. Electrochromic display device (ECD) is one of the most powerful candidate for this purpose and has various merits such as multicolor, high contrast, optical memory, and no visual dependence on viewing angle. A large number of electrochromic materials are available from almost all branches of synthetic chemistry. In this review, we have tried to describe the fundamentals of such electrochromic materials and their use in EDDs. The most important examples from major classes of electrochromic materials namely transition metal oxides, Prussian blue, phthalocyanines, viologens, fullerenes, dyes and conducting polymers (including gels) are described. Examples of their use in both prototype and commercial electrochromic devices are given.

Study and development of conducting polymer-based electrochromic display devices

Abstract The electrochromic response of conducting polypyrrole (PPy)/Prussian blue (PB) and conducting polyaniline (PANI)/Prussian blue (PB) composite films has been studied in different electrolytes such as KCl, LiClO 4 , K 2 SO 4 , KNO 3 , KI/I 2 , etc. by depositing the PB films on top of the conducting PPy and PANI films—all prepared by electrochemical methods. The results show that the use of PB on PPy and PANI not only yields high contrast but also extends the electrochromic response to a wider region of the visible spectrum, thus working as a sensitizer for improving the electrochromic response of the conducting PPy and PANI. These electrochromic films have been characterized by other physicochemical characterization techniques such as cyclic voltammetry, SEM, XPS, XRD, etc. The electrochromic behaviour of the PPy/PB composite system has been described in detail. Electrochromic display devices using solid polymer electrolytes, which are directly useful for commercial production of such devices are currently under study.

Toward organic thick film solar cells: Three dimensional bulk heterojunction organic thick film solar cell using fullerene single crystal nanorods

Fullerene single crystal nanorods (C60-nanorods) are synthesized by simple liquid/liquid interface precipitation method. Three dimensional bulk heterojunction donor-acceptor-type organic thick film solar cell is demonstrated having an active layer made from C60-nanorods and regioregular poly(3-octylthiophene). C60-nanorods seem to be promising material for organic solar cell applications. Our preliminary results opens an area of “organic thick film solar cells” which was almost thought to be impossible due to lower mobility of charge carriers and small exciton diffusion lengths in most of the organic materials, including small molecules and conjugated polymers.

Application of metal nanoparticles decorated carbon nanotubes in photovoltaics

Carbon nanotubes decorated with metal nanoparticles are introduced to photovoltaic application. The introduction of metal nanoparticles in the organic/organic-inorganic heterojunction solar cells is expected to improve the exciton dissociation (due to strong electric field at the metal-organics interface) and hence photovoltaic action. An improved photovoltaic action is indeed observed in n-Si/poly(3-octylthiophene) heterojunction solar cells incorporating multiwalled carbon nanotubes (MWCN) decorated with platinum metal nanoparticles (Pt: 20wt%, 10–15nm) as compared to pristine MWCN. The incorporation of metal nanoparticles should provide an alternative strategy to improve the photovoltaic performance of organic/organic-inorganic solar cells.

Improving photovoltaic response of poly„3-hexylthiophene…/n-Si heterojunction by incorporating double walled carbon nanotubes

Poly(3-hexylthiophene)/n-Si heterojunction solar cells were studied with and without incorporation of double walled carbon nanotubes (DWCNs) in the polymer layer. Performance of the device improves by manyfold by incorporation of DWCN. The authors report power conversion efficiency, open circuit voltage, short-circuit current density, and fill factor of 0.026%, 0.446V, 0.3398mA∕cm2, and 0.17, respectively, for an unoptimized cell containing DWCN. Reference cells without DWCNs show much lower performance. DWCN incorporation yields better hole transport, easy exciton splitting, and suppression of charge recombination, thereby improving photovoltaic action. DWCN seems promising materials for improving hole transport in organic solar cells.

Electrochromic response in polypyrrole sensitized by Prussian blue

Abstract The electrochromic response of polypyrrole (PPy)/Prussian blue (PB) composite films has been studied in different electrolytes such as KCl, LiClO4, K2SO4, KNO3, KI/I2, etc., by depositing the Prussian blue films on top of the conducting polypyrrole films, both being prepared by an electrochemical method. The results show that the use of PB on PPy not only yields high contrast but also extends the electrochromic response to a wider region of the visible spectrum, thus working as a sensitizer for improving the electrochromic contrast of the conducting polypyrrole.

Improving the photovoltaic response of a poly(3-octylthiophene)/n-Si heterojunction by incorporating double-walled carbon nanotubes

Poly(3-octylthiophene)/n-Si heterojunction solar cells were studied with and without incorporation of double-walled carbon nanotubes (DWCNs) in the polymer layer. The performance of the device improves significantly by the incorporation of DWCNs. We report a power conversion efficiency, open circuit voltage, short-circuit current density and fill factor of 0.49%, 0.53 V, 5.9 mA cm−2 and 0.15 respectively for an un-optimized cell containing DWCNs. Reference cells without DWCNs show a much lower performance. DWCN incorporation yields better hole transport, easy exciton splitting and suppression of charge recombination, thereby improving photovoltaic action. DWCN seems a promising material for improving hole transport in organic solar cells.

Effect of moisture (in solid polymer electrolyte) on the photosensitivity of conducting polypyrrole sensitized by prussian blue in solid-state photocells

Abstract The effect of small concentration of dispersed prussian blue (PB) [in solid polymer electrolyte viz polyvinyl alcohol (PVA) with phosphoric acid] on the photocurrents were studied (in wet state and in dry state of solid polymer electrolyte) in solid state photocells fabricated using conducting polypyrrole (PPy). These exhibit good photoresponse to visible light: photocurrents being 20 to 25 times greater than the dark currents depending upon the concentration of the sensitizer. The current–voltage ( I – V ) characteristics in such cells reveals that the charge transport is mainly governed by the space charge effects and the sensitization effects are due to the lowering of the potential barrier formed at the polypyrrole (PPy)/polyvinyl alcohol (PVA) interface under photoexcitation. Also, sensitization effects were studied in the wet state and in dry state of solid polymer electrolyte (PVA). It has been observed that the moisture content in the solid polymer electrolyte reduces the photosensitivity factor ( S = I l / I d , where I l is the light current and I d is the dark current), since it increases the ionic conductivity in the solid polymer electrolyte thereby increasing the dark current values which results in overall decrease in the photosensitivity factor ( S ). The importance of the other factors such as dye concentration, dye aggregation etc. have been explained on the basis of the experimental observations. The sensitization effects are explained on the basis of the energy band diagram of the materials forming the photocells.

Sensitization of photocurrents in solid-state electrochemical cells using conducting polypyrrole

Abstract The effect of small concentration of methylene blue on the photocurrents were studied in solid-state photoelectrochemical cells fabricated using conducting polypyrrole (PPy) coated electrodes sandwiched with solid polymer electrolyte viz. polyvinyl alcohol (PVA) with phosphoric acid. These exhibit excellent photo-response to visible light: photocurrent being 3 to 4 orders of magnitude greater than dark current depending on the concentration of the sensitizer. The current–voltage characteristics in such cells reveal that the charge transport is mainly governed by the space charge effects and the sensitization effects are due to lowering of potential barrier formed at the PPy/PVA interface under photoexcitation.

Charge transport processes in conducting polypyrrole/Prussian Blue bilayers

Abstract The electronic transport properties of electrochemically deposited conducting polypyrrole (PPy)/Prussian Blue (PB) bilayer films have been studied by investigating the current–voltage ( I – V ) characteristics in dark and under white light illumination. The I – V characteristics observed are highly non-linear, both in dark and under illumination. Space charge limited conduction (SCLC) appears to be the dominant process governing the charge transport in such composite film structures which is also seen when these films are being used in photoelectrochemical or electrochromic mode. The sensitization effect appears due to relative ease of transport of the photogenerated carriers as compared the carrier transport in dark.