Vijila Chellappan

Imbalanced charge mobility in oxygen treated polythiophene/fullerene based bulk heterojunction solar cells

The effect of oxygen induced traps on charge mobility in bulk heterojunction solar cells using poly(3-hexylthiophene) (P3HT):l-(3-methoxycarbonyl)-propyl-l-phenyl-(6, 6) methanofullerene (PCBM) blend have been studied using photoinduced charge extraction by linearly increasing voltage (PhotoCELIV) technique. The solar cells exposed to oxygen exhibit dual PhotoCELIV peaks, whereas the solar cell without oxygen treatment show single PhotoCELIV peak with the charge mobility of the order of 10−4 cm2/V s. It is demonstrated that the oxygen treatment imbalance the charge mobility in the P3HT/PCBM photoactive layer, which affects the power conversion efficiency and lifetime of the solar cell. The single PhotoCELIV peak for the device without oxygen treatment indicates that the charge mobility is balanced, that causes the overlapping of electron and hole transients.

Bridged-triarylamine starburst oligomers as hole transporting materials for electroluminescent devices

Two bridged-triphenylamine starburst molecules were designed and synthesized. Their electrochemical properties were studied in detail using cyclic voltammetry (CV) and square wave voltammetry (SWV) methods. The bridged molecular structure stabilizes the oxidative state and decreases the oxidation potentials, resulting in an increased HOMO energy level. Double-layer sandwich electroluminescence devices were fabricated. The bridged triphenylamine starburst 4 has shown significantly enhanced performance compared with the triphenylamine analog 3 and its ethenyl-linked derivative 6.

Hole transport in Poly[2,7-(9,9-dihexylfluorene)-alt-bithiophene] and high-efficiency polymer solar cells from its blends with PCBM.

We report herein a detailed study of the thermal and hole-transport properties of poly[2,7-(9,9-dihexylfluorene)-alt-bithiophene] (F6T2) and its photovoltaic performance in a bulk-heterojunction (BHJ) solar cell. This crystalline polymer has a high weight-average molecular weight (M(w) = 52 400) with a polydispersity index of 1.99. With a band gap of 2.36 eV, F6T2 exhibits strong absorption in the 300-500 nm region. BHJ solar cells blending F6T2 with [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) (1:3 weight ratio) as the active layer present a high open-circuit voltage (V(oc) approximately 0.9 V) and a promising power conversion efficiency of 2.4% under simulated solar light AM1.5G (100 mW/cm(2)). Furthermore, F6T2 shows sufficient hole mobility [ca. 8.4 x 10(-5) cm(2)/(V s) at 310 K and 2.5 x 10(5) V/cm applied electric field] by a time-of-flight transient photocurrent technique, allowing efficient charge extraction and a good fill factor for solar cell application. Nanoscale phase separation was observed in F6T2/PCBM films with a surface roughness lower than 60 nm.

Electrospun ZnO Nanowire Plantations in the Electron Transport Layer for High-Efficiency Inverted Organic Solar Cells

Inverted bulk heterojunction organic solar cells having device structure ITO/ZnO/poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61 butyric acid methyl ester (PCBM) /MoO3/Ag were fabricated with high photoelectric conversion efficiency and stability. Three types of devices were developed with varying electron transporting layer (ETL) ZnO architecture. The ETL in the first type was a sol-gel-derived particulate film of ZnO, which in the second and third type contained additional ZnO nanowires of varying concentrations. The length of the ZnO nanowires, which were developed by the electrospinning technique, extended up to the bulk of the photoactive layer in the device. The devices those employed a higher loading of ZnO nanowires showed 20% higher photoelectric conversion efficiency (PCE), which mainly resulted from an enhancement in its fill factor (FF). Charge transport characteristic of the device were studied by transient photovoltage decay and charge extraction by linearly increasing voltage techniques. Results show that higher PCE and FF in the devices employed ZnO nanowire plantations resulted from improved charge collection efficiency and reduced recombination rate.

Biopolymer as an electron selective layer for inverted polymer solar cells

In this work, a solution-processable electron selective layer is introduced for inverted polymer solar cells (PSCs). Cationic biopolymer poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) is used as a solution-processable work function modifier of indium-tin-oxide transparent conducting electrode to yield efficient inverted PSCs of 3.3% under AM1.5G illumination, with poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester as the active layer. Devices using PDMAEMA exhibit greater stability in ambient “working conditions” as compared to devices using ZnO, retaining 90% of peak power conversion efficiency after 8 weeks. Therefore, PDMAEMA has great potential as a universal work function modifier material with high robustness.

Impact of molybdenum out diffusion and interface quality on the performance of sputter grown CZTS based solar cells

We have investigated the impact of Cu2ZnSnS4-Molybdenum (Mo) interface quality on the performance of sputter-grown Cu2ZnSnS4 (CZTS) solar cell. Thin film CZTS was deposited by sputter deposition technique using stoichiometry quaternary CZTS target. Formation of molybdenum sulphide (MoSx) interfacial layer is observed in sputter grown CZTS films after sulphurization. Thickness of MoSx layer is found ~142 nm when CZTS layer (550 nm thick) is sulphurized at 600 °C. Thickness of MoSx layer significantly increased to ~240 nm in case of thicker CZTS layer (650 nm) under similar sulphurization condition. We also observe that high temperature (600 °C) annealing suppress the elemental impurities (Cu, Zn, Sn) at interfacial layer. The amount of out-diffused Mo significantly varies with the change in sulphurization temperature. The out-diffused Mo into CZTS layer and reconstructed interfacial layer remarkably decreases series resistance and increases shunt resistance of the solar cell. The overall efficiency of the solar cell is improved by nearly five times when 600 °C sulphurized CZTS layer is applied in place of 500 °C sulphurized layer. Molybdenum and sulphur diffusion reconstruct the interface layer during heat treatment and play the major role in charge carrier dynamics of a photovoltaic device.

Enhanced Performance Using an SU-8 Dielectric Interlayer in a Bulk Heterojunction Organic Solar Cell

The effect of inserting an SU-8 dielectric interlayer into inverted bulk heterojunction (BHJ) organic solar cells (OSCs) was studied. Insertion of an ultrathin layer of SU-8 between the zinc oxide (ZnO) electron transport layer and the photoactive layer resulted in a smoother interface and a 14% enhancement in power conversion efficiency. The properties of devices with and without an SU-8 interlayer were investigated using transient photovoltage (TPV) and double injection (DoI) techniques, and it was found that devices with SU-8 show longer carrier lifetimes and greater mobility-lifetime (μ-τ) products than those without. Devices with SU-8 were also found to have improved stability. The results indicate that the insertion of an SU-8 interlayer reduces the recombination rate for photogenerated carriers without affecting the charge transport properties, improving overall performance and stability.

Bovine Serum Albulmin Protein‐Templated Silver Nanocluster (BSA‐Ag13): An Effective Singlet Oxygen Generator for Photodynamic Cancer Therapy

This paper reports a novel synthesis approach of bovine serum albumin (BSA) protein-templated ultrasmall (<2 nm) Ag nanocluster (NC) with strong singlet oxygen generation capacity for photodynamic therapy (PDT). An atomically precise BSA-Ag13 NC (i.e., 13 Ag atoms per cluster) is successfully synthesized for the first time by using NaOH-dissolved NaBH4 solution as the controlling reducing agent. The ubiquitous size of BSA-Ag13 NC results in unique behaviors of its photoexcited states as characterized by the ultrafast laser spectroscopy using time-correlated single photon counting and transient absorption techniques. In particular, triply excited states can be largely present in the excited BSA-Ag13 NC and readily sensitized molecular oxygen to produce singlet oxygen (1 O2 ) with a high quantum efficiency (≈1.26 using Rose Bengal as a standard). This value is much higher than its Au analogue (i.e., ≈0.07 for BSA-Au25 NC) and the commonly available photosensitizers. Due to the good cellular uptake and inherent biocompatibility imparted by the surface protein, BSA-Ag13 NC can be applied as an effective PDT agent in generating 1 O2 to kill cancer cell as demonstrated in this study.

Effect of low temperature on charge transport in operational planar and mesoporous perovskite solar cells

Low-temperature optoelectrical studies of perovskite solar cells using MAPbI3 and mixed-perovskite absorbers implemented into planar and mesoporous architectures reveal fundamental charge transporting properties in fully assembled devices operating under light bias. Both types of devices exhibit inverse correlation of charge carrier lifetime as a function of temperature, extending carrier lifetimes upon temperature reduction, especially after exposure to high optical biases. Contribution of bimolecular channels to the overall recombination process should not be overlooked because the density of generated charge surpasses trap-filling concentration requirements. Bimolecular charge recombination coefficient in both device types is smaller than Langevin theory prediction, and its mean value is independent of the applied illumination intensity. In planar devices, charge extraction declines upon MAPbI3 transition from a tetragonal to an orthorhombic phase, indicating a connection between the trapping/detrapping mechanism and temperature. Studies on charge extraction by linearly increasing voltage further support this assertion, as charge carrier mobility dependence on temperature follows multiple-trapping predictions for both device structures. The monotonously increasing trend following the rise in temperature opposes the behavior observed in neat perovskite films and indicates the importance of transporting layers and the effect they have on charge transport in fully assembled solar cells. Low-temperature phase transition shows no pattern of influence on thermally activated electron/hole transport.

Morphological dependance of charge transport in nanostructured ZnO-based dye sensitized solar cells

Zinc oxide (ZnO) photoanodes of two different morphologies — nanorods and nanosheets are used to fabricate dye sensitised solar cells in order to study the influence of morphology on device performance. Characteristics such as dye loading capacity and light scattering ability of the photoanodes depend on the morphology to a large extent. This leads to change in the electron transport properties, which in turn affects power conversion efficiency. Here we report the difference in the charge transport properties of these morphologies evaluated using the time resolved photocurrent measurements. The photocurrent transients for ZnO nano-rod film exhibited dual peak behaviour, whereas transients for ZnO nano-sheet film exhibited only one peak as the time delayed second peak corresponding to slow electron diffusion was totally suppressed. The transient photocurrent decay also indicates that the charge transport rate is much faster in the ZnO nano-sheets, which allows the efficient charge collection over much larger thickness compared to other ZnO nano-rod film. The power conversion efficiency (PCE) of the devices was found to be 1.6% and 1.5 % for the ZnO-nano-rod and nano-sheet based dye sensitized solar cells respectively. A comparative study is performed and the photovoltaic parameters obtained in each case are correlated with the photocurrent transients enabling better understanding of the impact of morphology.