Ashraf Uddin - Academia.edu (original) (raw)

Papers by Ashraf Uddin

Nanomaterials, 2020

Semi-transparent organic solar cells (ST-OSCs) have attracted significant research attention, as ... more Semi-transparent organic solar cells (ST-OSCs) have attracted significant research attention, as they have strong potential to be applied in automobiles and buildings. For ST-OSCs, the transparent top electrode is an indispensable component, where the dielectric/metal/dielectric (D/M/D) structured electrode displayed a promising future due to its simplicity in the fabrication. In this work, by using the MoO3-/Ag-/MoO3-based D/M/D transparent electrode, we fabricated ST-OSCs based on the PM6:N3 active layer for the first time. In the device fabrication, the D/M/D transparent electrode was optimised by varying the thickness of the outer MoO3 layer. As a result, we found that increasing the thickness of the outer MoO3 layer can increase the average visible transmittance (AVT) but decrease the power conversion efficiency (PCE) of the device. The outer MoO3 layer with a 10 nm thickness was found as the optimum case, where its corresponding device showed the PCE of 9.18% with a high AVT o...

Solar RRL, 2021

Opaque and semitransparent organic solar cells (ST‐OSCs) have made tremendous progress in recent ... more Opaque and semitransparent organic solar cells (ST‐OSCs) have made tremendous progress in recent years. Efficiencies over 18% and 13% have been demonstrated for opaque ST‐OSCs, respectively. OSCs do not contain unfavorable elements such as lead, which makes them available for broader potential applications when compared with other lead‐contained thin‐film solar cells. There has also been tremendous progress in the ST‐OSCs, which makes them extremely appealing for promising emerging applications such as building‐integrated photovoltaics. Herein, a progress review in the field is presented for helping the researchers better understand ST‐OSCs and further realize their potentials. Recent strategies in ST‐OSCs based on three perspectives are summarized, including electrode engineering, active layer engineering, and device engineering. A wide range of applications where ST‐OSCs can be used is discussed and challenges for future developments of ST‐OSCs are pointed out. Finally, the outloo...

Materials Chemistry Frontiers, 2019

The ternary organic solar cell is a promising technology towards high power conversion efficiency.

ACS Omega, 2018

Inorganic cesium lead triiodide (CsPbI 3) perovskite materials are becoming increasingly attracti... more Inorganic cesium lead triiodide (CsPbI 3) perovskite materials are becoming increasingly attractive for use in perovskite/silicon tandem solar cells, due to their almost ideal band gap energy (E g) of about 1.7 eV. To be useful as photovoltaic absorbers, the CsPbI 3 must form the cubic or black phase (α-CsPbI 3). To do so at relatively low temperatures, hydroiodic acid (HI) is required as a solution additive. This paper demonstrates CsPbI 3 perovskite solar cells with an efficiency of 6.44%, formed using a HI concentration of 36 μL/mL. This value is higher than the previous most commonly used HI additive concentration. Herein, by undertaking a systematic study of the HI concentration, we demonstrate that the structural, morphological, optical, and electrical properties of CsPbI 3 solar cells, processed with this HI additive concentration, are superior.

Photovoltaic is one of the promising renewable sources of power to meet the future challenge of e... more Photovoltaic is one of the promising renewable sources of power to meet the future challenge of energy need. Organic and perovskite thin film solar cells are an emerging cost-effective photovoltaic technology because of low-cost manufacturing processing and a light-weight. The main barrier of commercial use of organic and perovskite solar cells is the poor stability of devices. Encapsulation of these photovoltaic devices is one of the best ways to address this stability issue and enhance the device lifetime by employing materials and structures that possess high barrier performance for oxygen and moisture. The aim of this review paper is to find different encapsulation materials and techniques for perovskite and organic solar cells according to the present understanding of reliability issues. It discusses the available encapsulate materials and their utility in limiting chemicals such as water vapour, oxygen penetration. It also covers the mechanisms of mechanical degradation within...

ACS Applied Energy Materials, 2018

Tin Oxide (SnOR 2 R) has been reported as a promising electron transport layer (ETL) for planar h... more Tin Oxide (SnOR 2 R) has been reported as a promising electron transport layer (ETL) for planar heterojunction perovskite solar cells (PSCs). This work reports a low temperature solutionprocessed bilayer SnOR 2 R as an efficient ETL in gas-quenched planar-heterojunction methylammonium lead iodide (MAPbIR 3 R) perovskite solar cells. SnOR 2 R nanoparticles were employed to fill the pin-holes of sol-gel SnOR 2 R layer and form a smooth and compact bilayer structure. The PCE of bilayer devices has increased by 30% compared with sol-gel reference device and the JR sc R, VR oc R and FF has been improved simultaneously. The superior performance of bilayer SnOR 2 R is attributed to the reduced current leakage, enhanced electron extraction characteristics, and mitigated the trap-assisted interfacial recombination, via X-Ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS) and space-charge limited current-voltage (SCLC) analysis.

Scientific Reports, 2017

Optically transparent, smooth, defect-free, chemically inert and with good adhesion to a variety ... more Optically transparent, smooth, defect-free, chemically inert and with good adhesion to a variety of substrates, plasma polymers from plant-derived secondary metabolites have been identified as promising encapsulating materials for organic electronics and photovoltaics. Here, we demonstrate that an encapsulating layer of plasma polymerized γ-terpinene reduces degradation-related loss in conversion efficiency in PCPDTBT:PC70BM solar cells under ambient operating conditions. The stability of γ-terpinene films was then investigated under extreme UV irradiation conditions as a function of deposition power. When exposed to ambient air, prolonged exposure to UV–A and UV–B light led to notable ageing of the polymer. Photooxidation was identified as the main mechanism of degradation, confirmed by significantly slower ageing when oxygen was restricted through the use of a quartz cover. Under unnatural high-energy UV–C irradiation, significant photochemical degradation and oxidation occurred e...

Energies, 2016

Organic-inorganic hybrid perovskite solar cells (PSCs) have emerged as a new class of optoelectro... more Organic-inorganic hybrid perovskite solar cells (PSCs) have emerged as a new class of optoelectronic semiconductors that revolutionized the photovoltaic research in the recent years. The perovskite solar cells present numerous advantages include unique electronic structure, bandgap tunability, superior charge transport properties, facile processing, and low cost. Perovskite solar cells have demonstrated unprecedented progress in efficiency and its architecture evolved over the period of the last 5-6 years, achieving a high power conversion efficiency of about 22% in 2016, serving as a promising candidate with the potential to replace the existing commercial PV technologies. This review discusses the progress of perovskite solar cells focusing on aspects such as superior electronic properties and unique features of halide perovskite materials compared to that of conventional light absorbing semiconductors. The review also presents a brief overview of device architectures, fabrication methods, and interface engineering of perovskite solar cells. The last part of the review elaborates on the major challenges such as hysteresis and stability issues in perovskite solar cells that serve as a bottleneck for successful commercialization of this promising PV technology.

Nanomaterials and Nanotechnology, 2015

Zinc oxide (ZnO) tetrapods have received much interest due to their unique morphology, that is, f... more Zinc oxide (ZnO) tetrapods have received much interest due to their unique morphology, that is, four arms connected to one centre. Tetrapod networks possess the excellent electronic properties of the ZnO semiconductor, which is attractive for photoelectrode materials in energy-conversion devices because of their advantages in electron extraction and transportation. In this review, we have discussed recent advancements in the field of ZnO tetrapod synthesis, including vapour transport synthesis and the wet chemical method, together with their advantages and disadvantages in terms of morphology control and yield regulation. The developments and improvements in the applications of ZnO nanotetrapods in photovoltaics, including dye-sensitized solar cells and polymer solar cells, are also described. Our aim is to give readers a comprehensive and critical overview of this unique morphology of ZnO, including synthesis control and growth mechanism, and to understand the role of this particul...

Materials for Renewable and Sustainable Energy, 2015

The present review rationalizes the significance of the metal oxide semiconductor (MOS) interface... more The present review rationalizes the significance of the metal oxide semiconductor (MOS) interfaces in the field of photovoltaics and photocatalysis. This perspective considers the role of interface science in energy harvesting using organic photovoltaics (OPVs) and dye-sensitized solar cells (DSSCs). These interfaces include large surface area junctions between photoelectrodes and dyes, the interlayer grain boundaries within the photoanodes, and the interfaces between photoactive layers and the top and bottom contacts. Controlling the collection and minimizing the trapping of charge carriers at these boundaries is crucial to overall power conversion efficiency of solar cells. Similarly, MOS photocatalysts exhibit strong variations in their photocatalytic activities as a function of band structure and surface states. Here, the MOS interface plays a vital role in the generation of OH radicals, which forms the basis of the photocatalytic processes. The physical chemistry and materials science of these MOS interfaces and their influence on device performance are also discussed.

Surface and Coatings Technology, 2005

GaN has shown great potential for high-power high-frequency electronic devices and short-waveleng... more GaN has shown great potential for high-power high-frequency electronic devices and short-wavelength optical devices. To integrate GaNbased optoelectronic devices with Si-based electronic devices and reduce the cost, it is desirable to grow epitaxial GaN thin films and device structures on the Si substrate. However, a proper buffer layer is essential for epitaxial growth of GaN films on Si substrate due to large mismatch between them in the area of lattice constant, thermal expansion coefficient and chemistry feature. In the present work, the growth of AlN buffer layer was studied. Wurtzite aluminum nitride thin films were grown on Si (111) and Si (100) substrates using reactive sputtering deposition under different discharge powers. X-ray diffraction (XRD) patterns showed that full width at half maximum (FWHM) of AlN (0002) peak grown on Si (111) substrates was smaller than that grown on Si (100) substrates. Vibrational characterization by Fourier transform infrared spectroscopy (FTIR) revealed that the stress in the AlN films deposited on Si (111) substrates was also smaller than that deposited on Si (100) substrates. For Si (100) substrates, the large lattice mismatch between AlN (0001) and Si (100) is a main contribution to the large strain in the formed films. For Si (111) substrates, the strain in the films deposited on Si (111) largely depends on the discharge power in sputtering, and the strain due to defects and thermal mismatch contributes largely to the residual strain in the deposited films.

Solar RRL, 2020

Organic solar cells (OSCs) are considered to have reached a second golden age with profoundly imp... more Organic solar cells (OSCs) are considered to have reached a second golden age with profoundly improved power conversion efficiency (PCE) and device stability in recent years. The modification of the interface layer plays a significant role in achieving performance enhancement in OSCs. Herein, the use of the atomic layer deposition (ALD) ultrathin TiOx to modify the interface layer in OSCs is reported. The modification with only two TiOx ALD cycles not only effectively passivates the interface between the ZnO electron transport layer (ETL) and the active layer, but also reduces the series resistance and improves the charge transport process in the device. An absolute 1% increase in PCE with enhanced device stability for modified OSCs is achieved. Semitransparent OSCs are also fabricated by applying this interface modification strategy. The modification with two TiOx ALD cycles increases the electrical device performance without affecting the optical properties of the semitransparent ...

The International Conference on Electrical Engineering, 2008

Two-inch-diameter sapphire (0001) substrates were used for the growth of AlGaN/GaN structure by h... more Two-inch-diameter sapphire (0001) substrates were used for the growth of AlGaN/GaN structure by hydride vapor phase epitaxy (HVPE). The two-dimensional electron gas (2DEG) structure consisted of a 2 µm undoped GaN layer substrate, followed by 30 nm thick Al 0.3 Ga 0.7 N layer. Structural and optical properties of GaN layer were characterized by high resolution X-ray diffraction, transmission electron microscopy (TEM) and photoluminescence (PL) measurements. TEM images showed the dislocation density in the GaN layer was ~ 9 × 10 9 /cm 2. The full width at half maximum (FWHM) values of the x-ray rocking curve for GaN (0002) was 0.156 deg. PL spectra of our sample exhibited a predominant band-edge emission of the wurzite GaN epilayer near 3.36 eV with FWHM = 45 meV at 50 K and 75 meV at room temperature, respectively. The Hall effect measurements gave a sheet electron concentration of 7.8×10 12 cm-2 and a mobility of 850 cm 2 /Vs at 300K. The experimental data indicated that the HVPE grown GaN layers exhibited superior quality compared to GaN layers grown by metal organic chemical vapor deposition in prior art literatures.

ACS Applied Materials & Interfaces, 2017

The current work reports the Lithium (Li) doping of low-temperature processed zinc oxide (ZnO) el... more The current work reports the Lithium (Li) doping of low-temperature processed zinc oxide (ZnO) electron transport layer (ETL) for highly efficient, triple-cation based MA 0.57 FA 0.38 Rb 0.05 PbI 3 (MA: methyl ammonium, FA: formamidinium, Rb: Rubidium) perovskite solar cells (PSCs). Lithium intercalation in the host ZnO lattice structure is dominated by interstitial doping phenomena, which passivates the intrinsic defects in ZnO film. In addition, interstitial Li doping also downshifts the Fermi energy position of L-ZnO ETL by 30 meV, which contributes to the reduction of electron injection barrier from photoactive perovskite layer. Compared to the pristine ZnO, the power conversion efficiency (PCE) of the PSCs incorporating Lithium doped ZnO (L-ZnO) is raised from 14.07% to 16.14%. The superior performance is attributed to the reduced current leakage, enhanced charge extraction characteristics and mitigated trap-assisted recombination phenomena in L-ZnO devices, thoroughly investigated by means of electrochemical impedance spectroscopy (EIS) analysis. L-ZnO PSCs also exhibit lower photo-current hysteresis than ZnO devices, which is investigated with regard to electrode polarization phenomena of the fabricated devices.

2016 International Conference on Numerical Simulation of Optoelectronic Devices (NUSOD), 2016

The optical properties of a novel semi-transparent organic solar cell were investigated to maximi... more The optical properties of a novel semi-transparent organic solar cell were investigated to maximize photocurrent generation. The effect of multilayer anode thickness and illumination direction was studied. Optimized device can provide ~2.5 fold enhancement in photocurrent.

Energy & Environmental Science, 2015

Non-fullerene organic solar cells with power conversion efficiencies of up to 6.3% are reported u... more Non-fullerene organic solar cells with power conversion efficiencies of up to 6.3% are reported using properly matched donor and acceptor.

International Journal of Computer Network and Information Security, 2013

Renewable Energy and Environmental Sustainability, 2017

Perovskite solar cell (PSCs) is considered as the game changer in emerging photovoltaics technolo... more Perovskite solar cell (PSCs) is considered as the game changer in emerging photovoltaics technology. The highest certified efficiency is 22% with high temperature processed (∼500°C) TiO 2 based electron transport layer (ETL). High temperature process is a rudimentary hindrance towards roll-to-roll processing of PSCs on flexible substrates. Low temperature solution process (<150°C) ZnO based ETL is one of the most promising candidate for large scale roll-to-roll fabrication of cells as it has nearly identical electron affinity (4.2 eV) of TiO 2. The mixed organic perovskite (MA 0.6 FA 0.4 PbI 3) devices with Al doped ZnO (AZO) ETL demonstrate average cell efficiency over 16%, which is the highest ever reported efficiency for this device configuration. The energy level alignment and related interfacial charge transport dynamics at the interface of ZnO and perovskite films and the adjacent charge transport layers are investigated. Significantly improved device stability, hysteresis free device photocurrent have been observed in MA 0.6 FA 0.4 PbI 3 cells. A systematic electrochemical impedance spectroscopy, frequency dependent capacitance spectra, surface morphology and topography characterization have been conducted to understand the role of interfacial electronic properties between perovskite and neighbouring layers in perovskite device. A standardized degradation study, interfacial electronic property and capacitive spectra analysis of aged device, have been measured to understand the enhanced device stability in mixed MA 0.6 FA 0.4 PbI 3 cells. Slow perovskite material decomposition rate and augmented device lifetime with AZO based devices have been found to be correlated with the more hydrophobic and acidic nature of AZO surface compared to pristine ZnO film.

Frontiers in Optics, 2005

The doping effect on charge carrier mobility in tris (8-hydroxyquinolinato) aluminum (Alq 3) was ... more The doping effect on charge carrier mobility in tris (8-hydroxyquinolinato) aluminum (Alq 3) was studied by time-of-flight (TOF) measurements. The polar dopant, 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) was used for this study. The DCM doped into Alq 3 films reduced the electron mobility. The electricfield dependence electron mobility in Alq 3 :DCM films was separated into two discrete regions of critical field c E. The value of c E was ranged from 360 to 405 (V/cm) 1/2 depending on DCM doping concentration up to 24 wt.% in Alq 3 films. The energetic disorder in Alq 3 :DCM films was increased from 0.01 eV to 0.09 eV with DCM doping concentration in Alq 3. The positional disorder in Alq 3 :DCM films was increased from 0.3 to 6.5 with DCM doping concentration up to 24 wt.%. These results indicated very strong Coulombic and dipole-dipole interactions between DCM and Alq 3 molecules. The codoped of rubrene with DCM into Alq 3 improved the carrier mobility compared to the single doped Alq 3 :DCM film. The carrier mobility in single doped Alq 3 :rubrene and Alq 3 :DCM films was not closely followed the Poole-Frenkel (PF) model. The mobility was in agreement with the PF model at two different ranges of electric fields separated by a critical field c E. The mobility in co-doped rubrene and DCM into Alq 3 followed the linear relationship with the PF model. The details of our investigations and the mechanism of the doping effects will be discussed in the presentation.