Dipjyoti Das

Improved molecular architecture of D–π–A carbazole dyes: 9% PCE with a cobalt redox shuttle in dye sensitized solar cells

Two new D–π–A dyes (SK2 & SK3) based on carbazole, and vinylene-phenylene (π-bridge) with rhodanine-3-acetic acid and cyanoacrylic acid as electron withdrawing–injecting as well as anchoring groups were designed and synthesised under conditions that were free from precious metal-catalysts and well characterized for dye-sensitized solar cell (DSSC) applications. A high power conversion efficiency (PCE) of 9% (AM 1.5 G, 100 mW cm−2) has been achieved using cyanoacrylic acid as an acceptor in D–π–A carbazole dye (SK3) with a cobalt based redox shuttle, while a PCE of 7.1% was exhibited in triiodide based redox mediators. A short-circuit current density, Jsc of ∼18.2 mA cm−2, an open-circuit voltage, Voc of ∼725 mV, and a fill factor, FF of ∼67% have been afforded by the SK3 based DSSC incorporating the Co2+/Co3+ electrolyte as the one-electron redox mediator. In contrast, SK2 dye based DSSCs with a cobalt based redox mediator have shown a Jsc ≈ 8.4 mA cm−2, a Voc ≈ 587 mV, and a FF ≈ 48%, yielding a PCE of 2.4%. The devices based on SK3 showed outstanding stability performance without significant degradation even after 1000 h of illumination under standard conditions in the Co2+/Co3+ electrolyte.

Electroluminescent room temperature columnar liquid crystals based on bay-annulated perylene tetraesters

Room temperature columnar liquid crystals with a wide thermal range, based on bay-annulated perylene tetraesters are reported. Through the incorporation of heteroatoms like N, S and Se in the bay position of the perylene tetraester the emission behavior in the solution state has been effectively tuned, where the N, S and Se-annulated derivatives showed bright green, blue and weak yellowish green fluorescence. The electroluminescence behavior of these compounds has been explored as emissive layers in organic light emitting diodes. Further, a remarkable improvement in the emission was achieved, when these molecules were doped in a matrix of polyvinyl carbazole (PVK), which is due to a combination of Forster resonance energy transfer from the host to the guest and charge carrier trapping in the emissive layer.

Effect of Dual Cathode Buffer Layer on the Charge Carrier Dynamics of rrP3HT:PCBM Based Bulk Heterojunction Solar Cell

In bulk heterojunction (BHJ) solar cells, the buffer layer plays a vital role in enhancing the power conversion efficiency (PCE) by improving the charge carrier dynamics. A comprehensive understanding of the contacts is especially essential in order to optimize the performance of organic solar cells (OSCs). Although there are several fundamental reports on this subject, a proper correlation of the physical processes with experimental evidence at the photoactive layer and contact materials is essential. In this work, we incorporated three different additional buffer layers, namely, tris(8-hydroxyquinolinato) aluminum (Alq3), bathophenanthroline (BPhen) or bathocuproine (BCP) with LiF/Al as conventional cathode contact in both rrP3HT:PC61BM and rrP3HT:PC71BM blend BHJ solar cells and their corresponding photovoltaic performances were systematically correlated with their energy level diagram. The device with dual cathode buffer layer having ITO/PEDOT:PSS/blend polymer/BCP/LiF/Al configuration showed the best device performance with PCE, η = 4.96%, Jsc = 13.53 mA/cm(2), Voc = 0.60 V and FF= 61% for rrP3HT:PC71BM and PCE, η = 4.5% with Jsc = 13.3 mA/cm(2), Voc = 0.59 V and FF = 59% for rrP3HT:PC61BM. This drastic improvement in PCE in both the device configurations are due to the combined effects of better hole-blocking capacity of BCP and low work function provided by LiF/Al with the blend polymer. These results successfully explain the role of dual cathode buffer layers and their contribution to the PCE improvement and overall device performance with rrP3HT:PCBM based BHJ solar cell.

Combined influence of plasmonic metal nanoparticles and dual cathode buffer layers for highly efficient rrP3HT:PCBM-based bulk heterojunction solar cells

The combined influence of plasmon-induced metallic nanoparticles (NPs), which increase the photoabsorption capability, and dual cathode buffer layers for enhanced charge collection is presented, which significantly improves the efficiency of organic photovoltaic (OPV) devices. Two different types of metal NPs, viz. citrate capped gold (Au) and silver (Ag) NPs, were blended (20 v/v%) separately into a PEDOT:PSS hole transport layer. For the dual cathode buffer layer, we chose two different hole blocking layers, BPhen and BCP, with a LiF/Al cathode contact. The combined influence of both the NPs and the dual cathode buffer layers was investigated with two blend polymers, rrP3HT:PC61BM and rrP3HT:PC71BM. It was observed that for both the blend polymer systems the power conversion efficiency (PCE) increases significantly in the presence of PEDOT:PSS + AuNPs and PEDOT:PSS + AgNPs with BCP/LiF/Al as a cathode contact compared to the bare PEDOT:PSS layer. In particular, in the presence of PEDOT:PSS + AuNPs and BCP/LiF/Al, the highest PCE was observed for both the blend polymers because of the better band alignment of BCP/LiF/Al with the active layers and the superior surface plasmon resonance of the AuNPs in the visible spectrum compared to AgNPs. The device with an ITO/PEDOT:PSS + AuNPs/rrP3HT:PC61BM/BCP/LiF/Al configuration showed a PCE (η) of 4.99% with Jsc = 13.9 mA cm−2, Voc = 0.59 V and FF = 62%, whereas for ITO/PEDOT:PSS + AuNPs/rrP3HT:PC71BM/BCP/LiF/Al, the device showed a PCE (η) of 5.65% with Jsc = 16.1 mA cm−2, Voc = 0.58 V and FF = 61%. These results conclusively explain the combined influence of the dual cathode buffer layer and the plasmonic metal NPs to remarkably improve the PCE and overall device performance of rrP3HT:PCBM based BHJ solar cells.

Organic Semiconductors: A New Future of Nanodevices and Applications

Organic materials with attractive electronic and optoelectronic properties are in high demand for functional electro-optical device applications. Research on synthetic polymer and organic small molecules has gained a great deal of attention due to their potential applications in low cost, ultra-thin, and flexible products and are expected to have a revolutionary role in modern day life. Basically, organic semiconductors are imperative for a range of optoelectronic applications including organic photovoltaic devices, light-emitting diodes, organic light-emitting transistor and organic field effect transistor-driven photonics since they possess photoluminescence, electroluminescence, and nonlinear optical properties in addition to liquid crystalline, structural patterning, printing and solution casting abilities. Electronic devices based on organic materials can be fabricated by simple processing techniques and are under intense investigation in academic and industrial laboratories because of their potential for mass production of flexible and cost-effective devices models. Organic devices also possess light-weight and transparent advantages compared to silicon-based devices. Significant progress has been achieved in the development of novel materials and new device engineering in the last decade. In this regard, arylene diimide families have shown high promise as useful building blocks for the fabrication of next-generation electronic and optoelectronics devices, providing deeper insight into their transistor characteristics at the molecular scale to practical applications ranging from medicine to high end security systems. These organic compounds are being developed for improved resistance to thermal and environmental stresses, which is one of the major challenges in the field. Several other classes of compounds based on fullerene (C60) and chemically modified thiophenes are also being explored and developed for use in a plethora of devices encompassing interdisciplinary research areas. These devices, owing to their low-cost, ease of synthesis and processing, are expected to pave the way for next-generation electronics with energy-efficient security systems, sensors, photonics, and spintronic memories. In this chapter we present a brief review on the different types of organic devices, their fabrication processes, and recent applications by citing examples of oligomers and polymers.

High-Performance ZnPc Thin Film-Based Photosensitive Organic Field-Effect Transistors: Influence of Multilayer Dielectric Systems and Thin Film Growth Structure

The key impact and significance of a multilayer polymer-based dielectric system on the remarkable photoresponse properties of zinc phthalocyanine (ZnPc)-based photosensitive organic field-effect transistors (PS-OFETs) have been systematically analyzed at various incident optical powers. A combination of inorganic aluminum oxide (Al2O3) and organic nonpolar poly(methyl methacrylate) (PMMA) is used as the bilayer dielectric configuration, whereas in the trilayer dielectric system, a bilayer polymer dielectric, consisting of PMMA, as the low-k dielectric polymer, on top of poly(vinyl alcohol) (PVA), the high-k polar dielectric, has been fabricated along with Al2O3 as the third layer. Before fabricating the OFETs, a systematic optimization of the nature of growth of the ZnPc molecules, deposited on PMMA-coated glass substrates at different substrate temperatures (Ts) was performed and examined by atomic-force microscopy, field-emission scanning electron microscopy, X-ray diffraction, and Raman analysis. At 90 °C, the fabricated PS-OFETs with the Al2O3/PVA/PMMA trilayer dielectric configuration showed the best p-channel behavior, with an enhanced and remarkable photoresponsivity of R ∼ 9689.39 A W–1 compared to that of the Al2O3/PMMA bilayer dielectric system (R ∼ 2679.40 A W–1) due to the polarization of the dipoles inside the polar PVA dielectric, which increases the charge transport through the channel. The charge carrier mobility of the device also improved by one order (μh ∼ 1.3 × 10–2 cm2 V–1 s–1) compared to that of the bilayer dielectric configuration (μh ∼ 3.5 × 10–3 cm2 V–1 s–1). The observed specific detectivity (D*) and NEP values of the bilayer dielectric system were 6.01 × 1013 Jones and 2.655 × 10–17 W Hz–1/2, whereas for the trilayer dielectric system, the observed D* and NEP values were 5.13 × 1014 Jones and 1.043 × 10–17 W Hz–1/2, respectively. Additionally, the operating voltage of each of the fabricated devices was also very low (−10 V) due to the influence of the inorganic high-k Al2O3 dielectric layer. The electrical stability of all of the fabricated devices was also investigated by bias stress analysis under both light and dark conditions in vacuum. To the best of our knowledge, the photoresponsivity (R) reported here with an Al2O3/PVA/PMMA trilayer dielectric configuration is the highest reported value for thin film-based PS-OFETs, at a remarkably low operating voltage of −10 V, on low-cost glass substrates without indium tin oxide or/and Si/SiO2.