Projects
Postdoctoral Research (National Renewable Energy Laboratory)
Custom High-Throughput Optical Mapping Instrument for Accelerated Stress Testing of PV Module Materials
The performance and integrity of photovoltaic (PV) modules and materials may be assessed through optical characterizations.
Developed a custom optical mapping instrument (OMI) to improve measurement throughput time, facilitate automated measurements, allow for localized specimen examination, obtain spatial information relative to commercial benchtop spectrophotometer instruments, and provide automated data analysis.
The measurement precision for the OMI is benchmarked for hemispherical transmittance relative to a previous round-robin study for commercial spectrophotometers.
MgxZn1-xO contact to CuGa3Se5 absorbers for PV and PEC devices
The wide band gap semiconductor CuGa3Se5 (Eg = 1.84 eV) has potential applications as a top-cell for both solar-cell / photovoltaic (PV) and photoelectrochemical (PEC) tandem devices. MgxZn1-xO (MZO) with a tunable conduction band position as a function of Mg content is an attractive n-type contact layer.
Investigated combinatorial sputter deposition of MgxZn1-xO contact/buffer layer for CuGa3Se5 absorber based PV and PEC device. Structural, optical and electrical properties of MZO thin films were studied as a function of different experimental conditions. MZO deposition and the surface pre-treatment (Cd2+ partial electrolyte) conditions were optimized. Cd2+ removed surface oxidation, led to Cd incorporation, and possibly changed the surface conductivity type.
Demonstrated a 20% increase in open-circuit voltage for MgxZn1-xO/ CuGa3Se5 solid-state solar cell device.
Co-optimization of the elemental compositions in MgxZn1-xO contact and CdSeyTe1-y absorbers for thin-film solar cells
CdTe is one of the major commercial thin-film photovoltaic (PV) technologies with over 25 GW installation and record laboratory efficiency of 22.1%. CdTe-based PV technology has been improved in the past decade mainly due to two factors: (1) integration of MgxZn1-xO (MZO) as the contact and window material improving the carrier collection in the short wavelengths. (2) Employing CdSeyTe1-y (CdSeTe) as the absorber material allowed more carrier collection in the longer wavelengths. The result is enhanced short-circuit current, improving the overall efficiency. However, Mg in MZO and Se in CdSeTe both changes the conduction band positions. Despite the output current gain due to these alloys, an unoptimized MZO/CdSeTe interface can detrimentally impact open-circuit voltage due to the conduction band offset. In this study, the effect of the elemental compositions in these alloys is investigated in determining the PV device performance.
Combinatorial libraries of PV devices were fabricated with orthogonal composition gradients in MZO and CdSeTe. MZO was deposited by combinatorial RF sputtering. CdSeTe was implemented by sequential evaporation of CdSe/CdTe and CdCl2 heat treatment, leading to a graded bandgap with Se diffusion.
Solar cell performance parameters were a strong function of both the elemental compositions. The device data facilitated identifying the optimal composition range for MZO (x = 0.10 - 0.15) and CdSeTe (y = 0.20 - 0.25), which resulted in PV device efficiency up to 17.7% with VOC = 831 mV, fill factor = 71% and JSC = 29.8 mA/cm2 .
ZnSnN2 device fabrication
Studied combinatorial synthesis and characterization of novel ternary nitride (ZnSnN2) thin films
Investigated the feasibility of fabricating PV device with ZnSnN2 absorber and different p-type transparent conductive materials (e.g. NiO, MoO3, SnO, NiCoO, ZnCoO, a-Si:H)
Devices with Al2O3 as buffer/surface passivation layer exhibited diode-like current-voltage behavior
Sr3Al2O6 as sacrificial lift-off layer for GaAs
The high cost of single-crystal substrates presently makes GaAs PV too expensive for terrestrial PV without significant substrate reuse. A water-soluble lift-off layer could dramatically lower the costs. Sr3Al2O6 (SAO) is a water-soluble cubic oxide, lattice constant 15.84Å, matching (2√2)aGaAs = 15.99Å; close lattice match between SAO and GaAs.
Demonstrated pulsed laser deposition (PLD) growth of epitaxial SrTiO3/Sr3Al2O6 templates on SrTiO3 (100) and Ge (100) substrates.
Demonstrated growth of partially epitaxial GaAs on SrTiO3/Sr3Al2O6/SrTiO3 templates
Custom High-Throughput Optical Mapping Instrument for Accelerated Stress Testing of PV Module Materials
Photovoltaic (PV) module/material performance and integrity may be assessed through optical characterizations. A custom Optical Mapping Instrument (OMI) to improve measurement throughput time, facilitate automated measurements, and obtain spatial information is developed for contemporary PV packaging materials. The measurement precision for the OMI is benchmarked by comparing the hemispherical transmittance relative to a previous round-robin study for commercial spectrophotometers.
Goals of the present study include:
Benchmark the OMI against interlaboratory round-robin precision study to quantify repeatability and reproducibility.
Demonstrate the capability and advantages of the OMI in transmittance and reflectance measurements.
Providing new insights from the examination of specimens from previous studies.
Develop data analysis and visualization capability as well as a mean to share results through the DuraMAT Data Hub internet resource.
Provide feedback for future instrument use and development.
Ph.D. Research (University of South Florida)
CdTe Solar Cells
To improve open-circuit voltage for CdTe PV, the p-type carrier concentration and minority carrier lifetime of the CdTe absorber needs to be improved. Both these parameters are directly related to the point defect distribution of the semiconductor, which is a function of deposition stoichiometry, dopant incorporation and post-deposition treatments.
The effect of gas-phase Cd/Te deposition ratio on the minority carrier lifetime and doping concentration of polycrystalline CdTe thin films was investigated.
Extrinsic doping of polycrystalline CdTe by in situ incorporations of antimony (Sb) and phosphorus (P) was demonstrated. Ex situ incorporation of Cu and Cl, and it's effect on solar cell performance was also investigated.
Designed and implemented an Elemental Vapor Transport (EVT) deposition system for polycrystalline CdTe thin film deposition. Optimized and developed process parameters for EVT CdTe deposition, and Group V dopant incorporation.
Fabricated and characterized CdTe solar cells from bare substrates to the finished devices.
Investigated the deep defects in polycrystalline CdTe as a function of process parameters and various post-deposition treatments. Several majority and minority carrier traps were identified and assigned to different point defects.
Designed and implemented low-cost close-spaced sublimation system for CdTe thin film deposition.
MS Thesis (University of South Florida)
Dye-Sensitized Solar Cell
Different components and ambient process conditions for the fabrication of Dye-Sensitized Solar Cell (DSC) based on titanium dioxide were investigated.
Titanium dioxide substrate thickness and morphology was found to have a direct impact on solar cell efficiency. Scanning Electron Microscopy (SEM) was used to investigate the TiO2 nanostructure.
Different chemical treatments and electrolytes were also explored towards optimizing the cell performance. A group of porphyrin based organic dyes were synthesized and evaluated.
Solar cell efficiency up to 4.4% was obtained for the best condition.
Undergraduate Project (BUET)
Photovoltaic maximum power point tracking battery charge controller
Photovoltaic generators exhibit nonlinear I-V characteristics that vary with solar insolation. A technique was developed for real-time tracking of the maximum power point (MPPT) of a solar panel charging a battery.
A step-down switching regulator is used where the duty cycle is varied to attain the desired MPPT level. The performance of the MPPT is evaluated theoretically using ORCAD SPICE simulation.
A prototype of the MPPT is built using ATmega 32 microcontroller and tested to charge a 70Ah lead-acid battery with a 40W solar panel. Both simulation and experimental results validated the capability of the proposed technique to track the maximum power at different solar insolations.