Md. Anower Hossain

Carrier Generation and Collection in CdS/CdSe-Sensitized SnO2 Solar Cells Exhibiting Unprecedented Photocurrent Densities

CdS/CdSe-sensitized nanostructured SnO(2) solar cells exhibiting record short-circuit photocurrent densities have been fabricated. Under simulated AM 1.5, 100 mW cm(-2) illumination, photocurrents of up to 17.40 mA cm(-2) are obtained, some 32% higher than that achieved by otherwise identical semiconductor-sensitized solar cells (SSCs) employing nanostructured TiO(2). An overall power conversion efficiency of 3.68% has been achieved for the SnO(2)-based SSCs, which compares very favorably to efficiencies obtained by the TiO(2)-based SSCs. The characteristics of these SSCs were studied in more detail by optical measurements, spectral incident photon-to-current efficiency (IPCE) measurements, and impedance spectroscopy (IS). The apparent conductivity of sensitized SnO(2) photoanodes is apparently too large to be measured by IS, yet for otherwise identical TiO(2) electrodes, clear electron transport features could be observed in impedance spectra, tacitly implying slower charge transport in TiO(2). Despite this, electron diffusion length measurements suggest that charge collection losses are negligible in both kinds of cell. SnO(2)-based SSCs exhibit higher IPCEs compared with TiO(2)-based SSCs which, considering the similar light harvesting efficiencies and the long electron diffusion lengths implied by IS, is likely to be due to a superior charge separation yield. The resistance to charge recombination is also larger in SnO(2)-based SSCs at any given photovoltage, and open-circuit photovoltages under simulated AM 1.5, 100 mW cm(-2) illumination are only 26-56 mV lower than those obtained for TiO(2)-based SSCs, despite the conduction band minimum of SnO(2) being hundreds of millielectronvolts lower than that of TiO(2).

CdSe-sensitized mesoscopic TiO2 solar cells exhibiting >5% efficiency: redundancy of CdS buffer layer

Semiconductor-sensitized TiO2 solar cells employing CdSe as a light absorber demonstrate superior photovoltaic performance to the best-performing cascaded CdS/CdSe cells with practically identical optical density in the study. A careful comparison between CdSe and CdS/CdSe sensitized cells reveals that while CdS can greatly promote the subsequent growth of CdSe in the cascade electrodes and hence light harvesting, the presence of a CdS buffer layer impedes the injection of electrons from CdSe to TiO2 and accelerates charge recombination at the TiO2/sensitizer interface. As a result, better performance was achieved with CdSe-sensitized solar cells when light absorption is identical to that of CdS/CdSe cells, making the CdS buffer layer redundant. CdSe-sensitized TiO2 solar cells incorporating light scattering layers and an aqueous polysulfide electrolyte yielded an unprecedented power conversion efficiency of up to 5.21% under simulated AM 1.5, 100 mW cm−2 illumination.

Characteristics of p-NiO Thin Films Prepared by Spray Pyrolysis and Their Application in CdS-sensitized Photocathodes

Compact nickel oxide (NiO) thin films were prepared on various substrates via a simple spray pyrolysis technique. Morphological and structural characterization indicates that these NiO films are very uniform in thickness (∼100 nm) and possess the bunsenite crystal structure. Optical measurements show that the NiO films are highly transparent with a band gap of 3.70 ± 0.05 eV. Mott-Schottky plots obtained from electrochemical impedance spectroscopy measurements reveal that the as-deposited NiO on fluorine-doped tin oxide (FTO) glass behaves as a p-type semiconductor. The flat band potential of NiO was estimated to be ∼0.36 V (vs. NHE) in 0.10 M tetrabutylammonium perchlorate/acetonitrile electrolytes. Cyclic voltammetric measurements of the NiO films on FTO in various redox electrolytes show that electrochemical reactions proceed in the accumulation region but are completely inhibited in the depletion region, indicating the NiO films effectively block the FTO substrate. Using these NiO blocking layers, a CdS-sensitized mesoscopic NiO photocathode operating in a polysulfide electrolyte is unambiguously demonstrated for the first time. It is anticipated that NiO thin films synthesized by spray pyrolysis could find important applications as stable and transparent electron barrier layers for various optoelectronic devices.

Synthesis of Cu(In,Ga)(S,Se)2 thin films using an aqueous spray-pyrolysis approach, and their solar cell efficiency of 10.5%

Semiconducting Cu(In,Ga)(S,Se)2 (CIGSSe) thin-film is prepared by the spray-pyrolysis of aqueous precursor solutions of copper (CuCl2), indium (InCl3), gallium (GaCl3), and sulphur (SC(NH2)2) sources. The non-vacuum approach of making the CIGSSe thin film using environmentally benign halide-based aqueous precursor solutions paves the way for fabricating solar cells at a much cheaper cost. Here, gallium (Ga) is incorporated into the host lattice of CuIn(S,Se)2 (CISSe) films grown on a Mo-coated soda-lime glass substrate to modify the optoelectronic properties of CIGSSe films. The bandgap engineered, Ga-doped CIGSSe film leads to better photovoltaic characteristics and shows one of the highest efficiency for CIGS thin film solar cell made by the non-vacuum deposition of environmentally-friendly precursors. The optimum efficiency of solar cells with the device configuration of glass/Mo/CIGSSe/CdS/i-ZnO/AZO show j–V characteristics of Voc = 0.621 V, jsc = 24.29 mA cm−2, FF = 69.84%, and a power conversion efficiency of 10.54% under simulated AM 1.5, 100 mW cm−2 illuminations, demonstrating its potential in making a cost-effective thin film solar cell.

Conformal growth of nanocrystalline CdX (X = S, Se) on mesoscopic NiO and their photoelectrochemical properties

Semiconductor-sensitized NiO photocathodes have been fabricated by successive ionic-layer adsorption and reaction (SILAR) deposition of CdS, CdSe and cascaded CdS/CdSe onto mesoscopic NiO films. Detailed morphological and structural characterization reveals that the growth of CdS and CdSe on mesoscopic NiO electrodes results in the formation of crystalline and conformal layers under ambient conditions. With a polysulfide redox electrolyte and a Pt counter electrode, CdX (X = S and Se)-sensitized p-NiO solar cells operating in a photocathodic mode are unambiguously demonstrated when NiO blocking layers are used, which are critical to prevent anodic photocurrent due to electron injection from CdX into the SnO2:F substrate. To decrease the recombination rate, a CdS barrier layer was deposited between NiO and a CdSe sensitizer which results in much enhanced cell performance. Front and rear spectral incident photon-to-current efficiency (IPCE) measurements were used to investigate charge collection and separation in the cells. The measurements indicate that charge collection in this system is limited by a short hole diffusion length.

Ecofriendly and Nonvacuum Electrostatic Spray-Assisted Vapor Deposition of Cu(In,Ga)(S,Se)2 Thin Film Solar Cells.

Chalcopyrite Cu(In,Ga)(S,Se)2 (CIGSSe) thin films have been deposited by a novel, nonvacuum, and cost-effective electrostatic spray-assisted vapor deposition (ESAVD) method. The generation of a fine aerosol of precursor solution, and their controlled deposition onto a molybdenum substrate, results in adherent, dense, and uniform Cu(In,Ga)S2 (CIGS) films. This is an essential tool to keep the interfacial area of thin film solar cells to a minimum value for efficient charge separation as it helps to achieve the desired surface smoothness uniformity for subsequent cadmium sulfide and window layer deposition. This nonvacuum aerosol based approach for making the CIGSSe film uses environmentally benign precursor solution, and it is cheaper for producing solar cells than that of the vacuum-based thin film solar technology. An optimized CIGSSe thin film solar cell with a device configuration of molybdenum-coated soda-lime glass substrate/CIGSSe/CdS/i-ZnO/AZO shows the photovoltaic (j-V) characteristics of Voc=0.518 V, jsc=28.79 mA cm(-2), fill factor=64.02%, and a promising power conversion efficiency of η=9.55% under simulated AM 1.5 100 mW cm(-2) illuminations, without the use of an antireflection layer. This demonstrates the potential of ESAVD deposition as a promising alternative approach for making thin film CIGSSe solar cells at a lower cost.

Interfacial Kinetics and Ionic Diffusivity of the Electrodeposited MoS2 Film

The transition-metal disulfide (MoS2) is a fantastic material used in diverse fields of applications. Ionic diffusivity and interfacial exchange current density are model parameters that play a crucial role for the optimization of device performances, which are not clearly known for this material. The additive-free dense film of MoS2 has been deposited by a facile electrodeposition approach and characterized by structural, morphological, and compositional analyses. This report provides the characterization of interfacial charge-transfer kinetics and diffusion of lithium ion in the MoS2 films as a function of lithium concentration at 25 °C temperature. The interfacial exchange current density is observed to be varied barely, ∼0.069-0.066 mA/cm2, with the change of lithium content, from x = 0.01-0.25, in Li xMoS2. The ionic diffusivity of the film is found to be in the range of ∼3 × 10-11-10-11 cm2 s-1 and does not vary much with the measured lithium concentration window. The electrochemical performances of the material are limited by the transport of lithium ion and interfacial kinetics over the measured state of lithium content. A submicron-size particle with high surface area is needed to be used as an electrode of the material for practical C-rates.