Effective Electron Diffusion Length Research Articles

Electrochemical impedance spectroscopy analysis of dye-sensitized solar cells composed of electrospun composite photoanodes: A comparative study of natural and synthetic sensitizers

In this study, a transmission line model is applied to the electrochemical impedance spectroscopy (EIS) data of the fabricated dye-sensitized solar cells (DSSCs) to evaluate the charge transfer mechanism through the cells. Natural dye from black plum (Syzygium cumini) fruit was used as a cell sensitizer (SC-DSSC) and compared its photovoltaic and electron transport capabilities to those of a cell using a synthetic sensitizer (N719-DSSC). TiO2/ZnO electrospun composite nanofibers were used as the semiconductor layer of the photoanode to enhance electron transfer. The EIS analysis revealed the role of electron resistances through shant, interfaces, and electrolyte solution by measuring the electron transfer kinetic parameters of each element. Based on the results, the SC-DSSC and N719-DSSC are appropriate photovoltaic cells because their ratios of effective electron diffusion length to photoanode thickness are 12.5 and 2.8, respectively. The EIS analysis showed that the electrospun composite nanofiber coated on the photoanode reduces the semiconductor layer's electrical resistance to the cell's total resistance. The extracted natural dye also boosted electron lifetime to 3.68 ms and diffusion coefficient to 54.3×10−6 m2/s while minimizing back-electron recombination at the semiconductor-electrolyte interface. Moreover, the semiconductor-electrolyte interface resistance is over 85% of the overall resistance for both DSSCs and controls electron transport through the cells, which is due to the dye-semiconductor binding intensity. Based on the photovoltaic data, the SC-DSSC cell efficiency was lower than N719-DSSC which is attributed to its higher electron transfer rate-controlling element. Thus, enhancing dye-semiconductor interactions will decrease the rate-controlling impedance and enhance cell performance.

Two-temperature model for ultrafast melting of Au-based bimetallic films interacting with single-pulse femtosecond laser: Theoretical study of damage threshold

Ultrashort-pulse laser excitation and associated thermal damage of single-layer metallic films have been extensively studied both theoretically as well as experimentally. However, study of ultrafast heat transfer mechanism in bimetallic films, a topic of immense current interest for designing next-generation optical components with better thermal resilience, is limited to a low fluence regime. Moreover, available results for damage threshold (DT) of bimetallic films have several ambiguities. The current work is an attempt towards development of comprehensive theory for the DT of Au-based bimetallic films, analyzing its dependence on associated physical parameters and bridging the gap in existing literature. This has been achieved by developing fully implicit two-temperature model-based Python code. Excellent agreement of the predicted temperature profiles with recently reported [E. L. Gurevich, Y. Levy, S. V. Gurevich, and N. M. Bulgakova, Phys. Rev. B 95, 054305 (2017)] femtosecond laser irradiated Au film validates the code. Subsequently, the code is utilized to investigate the influence of different metallic substrates, $M$ ($M$ = Ni, Cr, Cu) on ultrafast melting damage of $\mathrm{Au}\text{/}M$ films excited by single-pulse femtosecond laser. Simulations carried out for increasing Au layer thickness revealed a nonmonotonic variation of DT for all substrates. This study also brought out an important observation that for every substrate metal, there exists an optimum Au thickness for maximizing the incipient and the complete melting threshold. Maximum achievable enhancement for $\mathrm{Au}\text{/}M$ films over pure Au are found to be 30%, 22%, and 14% with Cu, Ni, and Cr substrates, respectively. The observed distinction in maximum DT among three different $\mathrm{Au}\text{/}M$ targets is explained by developing appropriate thermophysical models. Finally, an empirical function is proposed here for expressing nonmonotonic variation of two-layer DTs. Very good agreement between the substrate-specific peak positions predicted by the proposed function and effective electron diffusion length arising from thermophysical analysis makes the modeling robust and applicable for a wide range of laser wavelengths as well as arbitrary combinations of bimetallic films.

Effect of seven different terthiophene π-spacers on dye performance in dye-sensitized solar cells

To ensure high photocurrents from dye-sensitized solar cells (DSSC), it is important that the dye absorbs as much of the energy from the sunlight as possible. To achieve a wide absorption and cover larger parts of the solar spectrum, dyes are frequently fitted with oligothiophene π-spacers. We wish to examine the terthiophene motif as π-spacers in triarylamine dyes and evaluate the suitability of using these in modern copper based DSSC devices. The foundation for this analysis will be the series of seven novel dyes (DMA-6 – DMA-12) which all are fitted with the same donor and acceptor, but linked with seven different terthiophene motifs. The photovoltaic performance of the dyes show that the smallest π-spacer in the series, dithieno[3,2-b:2′,3′-d]thiophene, was the most efficient. The DSSC device sensitized by this dye achieved a power conversion efficiency of 4.4% (Jsc = 7.0 mA cm−2, Voc = 0.95 V, FF = 0.66). The photovoltaic performance of the dyes was tested in devices with two different TiO2 thicknesses, and we established that the thinner TiO2 yielded the best DSSC performance for all the dyes. Electrochemical impedance spectroscopy revealed that the thicker semiconductor layer resulted in a reduction of the effective electron diffusion length.

Electrostatic potential fluctuations and light‐soaking effects in Cu(In,Ga)Se2 solar cells

AbstractIn Cu(In,Ga)Se2 (CIGS) thin‐film solar cells, laterally inhomogeneous distributions of point defects may induce electrostatic potential fluctuations and thus reduce the open‐circuit voltage (Voc). In the present work, we investigate possible origins of fluctuating potentials and estimate the amplitude of fluctuations and Voc losses in solar cells with various [Ga] in the CIGS absorber, with different buffer layers and with different durations of an RbF postdeposition treatment (PDT). Electron‐beam‐induced current measurements were employed to study the local difference in the width of the space‐charge region (wSCR). It is shown that the amplitude of fluctuations in the wSCR depends significantly on the choice of buffer system and on the duration of the RbF PDT. In addition, energy‐dispersive X‐ray spectroscopy and cathodoluminescence measurements reveal that band‐gap fluctuations do not have substantial impact on the device performance. Finally, some of the investigated cells were exposed to light soaking, which was found to be a means to reduce the detected electrostatic potential fluctuations and also to increase the effective electron diffusion length in the CIGS absorber for a part of the investigated cells.

Electrochemical performance of a self-assembled two-dimensional heterostructure of rGO/MoS2/h-BN.

We report the preparation and electrochemical performance evaluation of a two-dimensional (2D) self-assembled heterostructure of graphene oxide (rGO), molybdenum disulphide (MoS2), and hexagonal boron nitride (h-BN). In the present study, the rGO–MoS2–h-BN (GMH) multi-layered GMH heterostructure is fabricated via an in situ chemical route. Based on material analysis, the composite consists of bond conformations of C–B–C, Mo–S, C–N, B–N, and Mo–C, indicating the layered stacks of rGO/h-BN/MoS2. In electrochemical analysis, the composite showed superior performance in the aqueous medium of cobalt sulphate (CoSO4) over other samples. CV measurements, performed over the range 10 to 100 mV s−1, showed a change in specific capacitance (Csp) from 800 to 100 F g−1. GMH showed almost no degradation up to 20 000 cycles @ 100 mV s−1. The calculated Csp, energy density (ED), and power density (PD) are discussed in light of Nyquist, Bode, and Ragone analysis. An equivalent circuit is simulated for the cell and its discrete electronic components are discussed. Due to its larger effective electron diffusion length > 1000 μm, broadly, the composite showed battery-like characteristics, as supported by radical paramagnetic resonance and transport response studies. The symmetric electrodes prepared in one step are facile to fabricate, easy to integrate and involve no pre or post-treatment. They possess superior flat cell character, are cost effective, and are favourable towards practicality at an industrial scale, as demonstrated on the laboratory bench. The details are presented.

Inkjet printed highly porous TiO2 films for improved electrical properties of photoanode

The aim of presented work is to show the improvements obtained in the properties of TiO2 films for dye sensitized solar cells fabricated by inkjet printing using an innovative methodology. We describe the development and properties of TiO2-based inks used in a lab-scale printer, testing various commercial TiO2 pastes. The porosity of the deposited inkjet printed TiO2 films is much higher than using the conventional “doctor blade” deposition technique, as the ink solvent evaporates during the droplet fly from the nozzle to the substrate due to its picoliter volume and the applied heating of a printing stage (70°C). Thanks to higher surface area, the dye sensitized solar cells incorporating inkjet printed TiO2 film gave higher efficiencies (ηmax≈3.06%) than the more compact films obtained by the “doctor blade” method (ηmax≈2.56%). Furthermore, electrochemical analysis indicates that for whole tested thickness range, the inkjet printed layers have higher effective electron diffusion length indicating their better transport properties.

Effect of Isonicotinate derivatives as additive on the photovoltaic performance of Carbazole-dye sensitized nanostructured TiO2 solar cells

The effect of three Isonicotinate derivatives as additives in iodide/triiodide redox electrolyte are investigated on the performance of 3-(4-carbazole-9-yl-phenyl)-2-cyano-acrylic acid (TC301) and 2-cyano-3-(4-(3,6-di-tert-butyl-9H-carbazole-9-yl)phenyl) acrylic acid (TC302) dye-sensitized TiO2 solar cell. Addition of 0.5M additive into the electrolyte, leads to an increase of the open-circuit potential (Voc) and short current density (Jsc), consequently improving the energy conversion efficiency (η). Using these additives in the electrolyte shift the TiO2 band edge toward negative potentials. The electrochemical impedance spectroscopy (EIS) shows when an additive is added to electrolyte solution in comparison to an electrolyte without additive, a low recombination rate constant (keff), a higher recombination resistance (Rct) and a high effective electron-diffusion length (Ln) in TiO2 films are observed. In addition, cyclic voltammetry (CV) confirms the impedance results. Finally, we found that Isonicotinate derivatives are promising additives which can be used for high efficient and low cost dye-sensitized solar cells.

Structural, morphological, electrical and electron transport studies in ZnO–rGO (wt% = 0.01, 0.05 and 0.1) based dye-sensitized solar cell

Zinc oxide–reduced graphene oxide (ZnO–rGO) thin films were fabricated on a fluorine-doped tin oxide glass substrate by a chemical bath deposition method. The thin films were immersed in the Eosin Y dye for 24 and 48 h to be fabricated as a dye-sensitized solar cell. ZnO hexagonal wurtzite structures were analyzed by X-ray diffraction analysis. Field effect scanning electron microscope showed the images of ZnO nanoparticles and nanobranches. The atomic force microscopy analysis estimated the average roughness of ZnO–rGO films doped with 0.01, 0.05 and 0.1 wt% which varied from 178 to 267 nm. The electrical or photovoltaic performance of ZnO–rGO were measured and compared accordingly by considering their power conversion efficiency, η, photocurrent density, J sc , open-circuit voltage, V oc and fill factor, FF. The cell’s efficiency of ZnO with 0.01 wt% rGO, 0.05 wt% rGO and 0.1 wt% rGO reached η = 2.36, 0.78 and 0.38 %, respectively. EIS analysis estimated the effective electron lifetime, τ eff , effective electron chemical diffusion coefficient, D eff , effective electron diffusion length of the photoanode, L n , charge transport resistance, R ct and transport resistance, R t .

Relationship between Molecular Structure and Electron Transfer in a Polymeric Nitroxyl-Radical Energy Storage Material

In recent years, stable organic radical functional groups have been incorporated into a variety of polymeric materials for use as electrodes within energy storage devices, for example, batteries and capacitors. With the complex nature of the charge-transfer processes in a polymer matrix, the morphologies of the polymer films can have a significant impact on the redox behavior of the organic-based radical. To elucidate possible effects of packing on electron-transport mechanisms, theoretical modeling of the well-characterized cathode material poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA) was conducted. Polymer morphologies were modeled using classical molecular dynamics simulations, and subsequently, the electronic-coupling matrix element between each radical site was calculated. Building on a previously derived treatment of diffusion in inhomogeneous materials, an expression for an effective electron diffusion length and an effective electron diffusion rate was derived in terms of an electro...

Photovoltaic properties of ZnO photoanode incorporating with CNTs for dye-sensitized solar cell application

Zinc oxide carbon nanotube (ZnO-CNTs) thin films were prepared by a chemical bath deposition (CBD) method and immersed in N719 dye for 24 h. The structure and surface morphology of the samples was captured by X-ray diffraction (XRD) and field effect scanning electron microscopy (FESEM) unit, respectively. The photovoltaic properties of ZnO- and ZnO-CNT-based dye-sensitized solar cells (DSSCs) were measured by considering the power conversion efficiency (η), photocurrent density (J sc), open-circuit voltage (V oc), and fill factor (FF). The cell's efficiency doped with single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) reached 0.65 and 0.28 %, respectively. ZnO-based DSSC generated only η = 0.003 %. The electrochemical impedance spectroscopy (EIS) unit was employed to investigate the electron transport properties such as effective electron lifetime (τ eff), effective electron chemical diffusion coefficient (D eff), and effective electron diffusion length (L n ). The addition of CNTs has enhanced the photovoltaic properties of the DSSCs and reduced the recombination effect inside the solar cell.

Photovoltaic characteristics and stability of flexible dye-sensitized solar cells on ITO/PEN substrates

Improving the stability and understanding the degradation mechanisms of dye-sensitized solar cells (DSSCs) are essential for their commercialization and wide-spread use. Degradation mechanisms of DSSCs built on conductive glass substrates have been studied extensively, but very few studies have been carried out on cells built on flexible plastic substrates. Here we report the photovoltaic characteristics of flexible DSSCs, using TiO2 working electrodes on ITO/PEN films prepared by cold isostatic compression at different pressures, and their analysis using electrochemical impedance spectroscopy. The degradation mechanisms of the devices were investigated through long-term aging tests by exposing the DSSCs to indoor light at room temperature. Aging of these flexible DSSCs resulted in a widened potential difference between the conduction band potential of the TiO2 (Ec) and the Nernst potential of the redox couple (Ered), and reductions in recombination resistance and effective electron diffusion length due to gradual loss of particle contacts. In general, it was found that the compression technique can only improve the short-term photovoltaic performance of the devices. Their long-term stability does not depend on the compression pressure used and all the devices were gradually degraded due to reduced particle–particle contacts and particle-conductive layer contacts. These findings provide insights and directions towards the development of more stable, flexible DSSCs.

Electron transport analysis in zinc oxide-based dye-sensitized solar cells: A review

Abstract This research review highlights the use of zinc oxide (ZnO) nanostructure as a photoanode in the fabrication of dye-sensitised solar cells (DSSC). ZnO nanostructure thin films offer large surface area, direct electron pathways and effective light scattering centre. The fabrication work can produce a variety of ZnO nanostructures, from nanotubes, nanoporous, nanosheets, nanoflowers, nanoflakes, nanobranches and nanolipsticks. The internal mechanism inside the cell gives useful information on the electron transport properties and efficiency of ZnO-based DSSC. The review ends with an outlook highlighting the electron transport parameters analysed by electrochemical impedance spectroscopy (EIS) unit. Such parameters including charge transport resistance (Rct), transport resistance (Rt), chemical capacitance (Cμ), effective electron lifetime (τeff), effective electron chemical diffusion coefficient (Deff), effective rate constant for recombination (keff), effective electron diffusion length of the photoanode (Ln) and finite Warburg impedance in the electrolyte (ZD). Monitoring the electron transport properties may improve the photovoltaic performances of the ZnO-based DSSC such as open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF) and power conversion efficiency (η).

Morphological and electron transport studies in ZnO dye-sensitized solar cells incorporating multi- and single-walled carbon nanotubes

Dye-sensitized solar cells (DSSCs) incorporating zinc oxide (ZnO) nanostructures and carbon nanotubes (CNTs) were fabricated using a chemical bath deposition method. The nanoflake structures captured by a field-emission scanning electron microscopy analysis traced the appearance of multi-walled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs) in the photoanode thin film. The photovoltaic performance of the photoanode was quantified by means of an electrochemical impedance spectroscopy (EIS) unit with GAMRY-Physical Electrochemistry. The ZnO–SWCNT-based DSSC exhibited good photovoltaic performance with power conversion efficiency (η), photocurrent density (Jsc), open-circuit voltage (Voc) and fill factor (FF) of 1.31%, 15.31 mA cm−2, 0.224 V and 0.36, respectively. The EIS unit was also employed to quantify the charge transport resistance (Rct), transport resistance (Rt) and effective electron lifetime (τeff) of the DSSC. The impedance analysis of the ZnO–SWCNT-based DSSC also determined greater highly efficient electron transport due to long effective electron diffusion length than the film thickness of the photoanode.

Responsivity Characteristics of InP/InGaAs Heterojunction Phototransistor with a Strained InAs/InGaAs Multiquantum Well Absorption Layer in the Base or Collector

The responsivity characteristics of heterojunction phototransistors (HPTs) with a strained InAs/InGaAs multiquantum well (MQW) absorption layer inserted in the base or collector are investigated. It is shown that although the hetero-emitter injects hot electrons into the base, the effective electron diffusion length in the base with MQWs becomes five times lower than that of a base without MQWs. This results in higher current gain for HPTs with MQWs in the collector. In addition, enhanced absorption coefficient due to excitonic absorption is observed only on HPT with MQW in the collector. Due to these two factors, a high responsivity of more than 10 A/W is realized at a wavelength around 2.35 µm for the device with MQWs in the collector.

Fabrication of TiO2/CuSCN Bulk Heterojunctions by Profile-Controlled Electrodeposition

We demonstrate a facile bottom-up growth of CuSCN — a widely used hole conducting material, in mesoscopic TiO2 films by electrodeposition in the presence of TiO2 compact layer. The profile-controlled growth of CuSCN in the nanopores of mesoscopic TiO2 is realized by judiciously optimizing the electrolyte concentration and applied potential, so that the effective electron diffusion length in TiO2 is much shorter than film thickness and the electrodeposition is strictly confined within very narrow region from the conducting substrate. The TiO2/CuSCN heterojunctions exhibit a pore filling ratio of ∼80% and show good rectifying properties, which is anticipated to be an important step forward to the facile fabrication of bulk heterojunction for nanostructured solar cells or 3D batteries.

Evidence for enhancing charge collection efficiency with an alternative cost-effective binary ionic liquids electrolyte based dye-sensitized solar cells

To mitigate the mass transfer limitations of a highly viscous room temperature ionic liquid (RTIL) electrolyte, systematic characterizations of viscosity, ionic conductivity, apparent triiodide diffusion coefficient and photovoltaic response of an alternative cost-effective and highly conductive binary RTIL mixture of 1-ethyl-3-methylimidazolium trifluoroacetate (EMIATF) and 1-methyl-3-propylimidazolium iodide (PMII) were investigated. An emphasis was placed on the dynamics of electron transport and charge recombination processes; specifically, a focus was placed on the effective electron diffusion length and charge collection efficiency in the devices. Notably, the introduction of perfluorinated anions, with a strong delocalization of the negative charge over the anion backbone, was able to weaken the hydrogen bonding with the imidazolium cations and, thus, decrease the viscosity and increase the ionic conductivity of the electrolyte. A sealed device that employed the binary RTIL achieved an overall conversion efficiency of 5.22% under irradiation of 100 mW cm −2, which increased the value by 30% compared to a device with a blank PMII-based RTIL electrolyte. Electrochemical impedance spectroscopy and intensity-modulated photovoltage/photocurrent spectroscopy analysis revealed that employment of this alternative binary RTIL electrolyte system was able to significantly decrease the diffusion resistance of triiodide species in the electrolyte and retard the charge recombination between the injected electrons with triiodide anions in the electrolyte, thus improving the effective electron diffusion length and charge collection efficiency in the device. The charge collection efficiency was certified to be the dominant parameter governing the short-circuit photocurrent density and the overall conversion efficiency of devices with RTIL electrolyte.

Enhancing Electron Collection Efficiency and Effective Diffusion Length in Dye‐Sensitized Solar Cells

Intensity-modulated photocurrent spectroscopy and intensity-modulated photovoltage spectroscopy are employed to measure the dynamics of electron transport and recombination in the ZnO nanowire (NW) array-ZnO/layered basic zinc acetate (LBZA) nanoparticle (NP) composite dye-sensitized solar cells (DSSCs). The roles of the vertical ZnO NWs and insulating LBZA in the electron collection and transport in DSSCs are investigated by comparing the results to those in the TiO(2)-NP, horizontal TiO(2)-NW and vertical ZnO-NW-array DSSCs. The electron transport rate and electron lifetime in the ZnO NW/NP composite DSSC are superior to those in the conventional TiO(2)-NP cell due to the existence of the vertical ZnO NWs and insulating LBZA. It indicates that the ZnO NW/NP composite anode is able to sustain efficient electron collection over much greater thickness than the TiO(2)-NP cell does. Consequently, a larger effective electron diffusion length is available in the ZnO composite DSSC.

Efficient electron transport in tetrapod-like ZnO metal-free dye-sensitized solar cells

The tetrapod-like ZnO nanopowders are employed to construct an efficient electron transport network as the photoanode of the dye-sensitized solar cells (DSCs). Due to the high extinction coefficient of the metal-free D149 dye offering a high photocurrent through the ZnO network, the best performance of DSCs based on a 42 µm terapod-like ZnO film showed a high energy conversion efficiency of 4.9% with a high short-circuit photocurrent density of 12.3 mA cm−2, an open-circuit photovoltage of 0.6 V, and a fill factor of 0.65 under AM 1.5 irradiation. Highly efficient electron transport may also be ascribed by a long effective electron diffusion length of 46 µm determined from the electrochemical impedance spectroscopy which is consistent with thickness dependent JSC measurements.

Energy-Level and Molecular Engineering of Organic D-π-A Sensitizers in Dye-Sensitized Solar Cells

A series of organic D-π-A sensitizers composed of different triarylamine donors in conjugation with the thienothiophene unit and cyanoacrylic acid as an acceptor has been synthesized at a moderate yield. Through tuning the number of methoxy substituents on the triphenylamine donor, we have gradually red-shifted the absorption of sensitizers to enhance device efficiencies. Further molecular engineering by the substitution of two hexyloxy chains in place of the methoxy groups allows fabricating a solvent-free dye-sensitized solar cell with a power conversion efficiency of 7.05% measured under the air mass 1.5 global sunlight. Time- and frequency-domain photoelectrical techniques have been employed to scrutinize the aliphatic chain effects with a close inspection on effective electron lifetime, diffusion coefficient, and diffusion length.

Chemical vapour etching‐based porous silicon and grooving: Application in silicon solar cells processing

Sponge like porous silicon (PS) was formed by a simple and low cost chemical vapour etching (CVE) method and applied in polycrystalline silicon (mc-Si) solar cells processing. The CVE method consists of exposing Si wafers to HNO3/HF vapours. It was shown that 8 min of HNO3/HF CVE (volume ratio = 1/7) is sufficient to form optimized PS layers on the emitter of mc-Si cells. The CVE–based PS can simultaneously passivate the Si surface and serves as an effective antireflection coating (ARC). As a result, the reflectivity decreases by about 60% of its initial value and the internal quantum efficiency is improved, particularly in the short wavelength region. For acid vapours rich in HNO3 (HNO3/HF >1/4), the CVE method favours the formation of a (NH4)2SiF6 powder, which is highly soluble in water. These findings let us achieve anisotropic grooving that enables to groove mc-Si wafers locally and in depth using an adequate anti-acid mask. The CVE - based grooving technique was used to form buried metallic contacts on the rear and frontal surface of the Si wafer in order to improve the current collection in mc-Si solar cells. No alteration of the spectral response in the long wavelength range was observed in mc-Si cells with rear-buried contacts. Adjustments of theoretical spectral responses to experimental ones show an increase in the effective electron diffusion length (Ln), which was attributed to Al gettering (passivation) at grain boundaries and to the reduction of the effective thickness of the base of the cells. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)