Nima E. Gorji

Numerical Simulation of Carbon Nanotubes/GaAs Hybrid PV Devices with AMPS-1D

The performance and characteristics of a hybrid heterojunction single-walled carbon nanotube and GaAs solar cell are modelled and numerically simulated using AMPS-1D device simulation tool. The device physics and performance with different junction parameters are analysed. The results suggest that the open-circuit voltage changes very slightly by changing the electron affinity, acceptor and donor density while the other electrical parameters reach an optimum value. Increasing the concentration of a discrete defect density in the absorber layer decreases the electrical parameters. The current-voltage characteristics, quantum efficiency, band gap, and thickness variation of the photovoltaic response will be quantitatively considered.

Electrical and Optical Characterization of R.F.-Sputtered CdTe Thin Films

Four sets of R.F.-sputtered CdS/CdTe thin-film solar cells are activated through comparable treatment procedures. Two samples were rinsed into deionized water for an hour before CdCl2 treatment and then activated in Ar or air ambient. The two others are normally treated and activated in Ar and air, respectively. The influence of water rinsing and annealing ambient on the electrical and optical properties of the films is investigated by current-voltage, field-emission scanning electron microscope, X-ray diffraction, and optical transmission spectroscopy. We examined water rinsing as a cheap and simple pretreatment step to modify the semiconductor surface and to obtain quality device. In this way, the fill factor of the cells that were rinsed into water before CdCl2 activation was comparable with the one prepared without rinsing. There is a challenge between the effect of water and Ar on the electrical parameters of the cells. Water-rinsed cells annealed in Ar showed better properties than those that received no water rinsing. It seems that water reduces the negative effect of Ar annealing or parasitic diffusion of Cu into the semiconductor. The higher shunt resistance of the rinsed samples represents that water rinsing could passivate the surface defects and pinholes. The grain size, phase, and band gap of the thin films are extracted, analyzed, and compared with the as-deposited devices.

Modeling the degradation and recovery of perovskite solar cells

Degradation and recovery in the device parameters of perovskite-based solar cells are modeled for the devices aged by exposing to air humidity/moisture. Several devices are considered with the same CH3NH3PbI3−xClx absorber layer but with different windows, hole transport layers and metallic back contacts. We proposed several models to fit the experimental data, reported in the literature, on the degradation of fill factor, efficiency and short-circuit current density. The kinetics proposed for the modeling of degradation/recovery in the fill factor are based on variation in the fast and slow metastable defect states in the respective layer. The degradation/recovery of the short-circuit current density is investigated by proposing a new model which accounts for the density of metastable defects before/after aging. Finally, the degradation of the device efficiency is modeled by fitting the data with proper functions mostly following the Gaussian bi-exponential shape. It is shown that instability of the device parameters might be due to variation in metastable defects in any component of a perovskite solar cell. However, the fill factor, or consequently the series resistance, is the main degradation source in perovskite solar cells.

Modeling of depletion width variation over time in thin film photovoltaics

The performance degradation of a hybrid solar cell is modelled considering the variation of depletion width over time. The p-i-n structure of a TiO2/perovskite/HTL photovoltaic is investigated. Several different time-dependent approaches are compared and a new model is introduced based on the variation of defect density over time in depletion region. This phenomenon consequently manifests itself in the device degradation. Our approach leads to rather complicated time-dependent equation for the defect density which takes into account also the non-uniformity of electric field in the depletion region. The thickness of TiO2 nano layer is taken 50 nm and perovskite layer is 330 nm. The nanoscale thickness of TiO2 layer warrants the carrier transport through tunneling mechanism.

CONSTRUCTION COMPONENTS ENGINEERING IN INTERMEDIATE BAND SOLAR CELLS

The parameter electron filling factor can be taken as a scale for the electronic states in the intermediate band which should be de-localized and thus the unconfined electrons at the quantum dots. For three different value of electron filling factor, the sunlight concentration effect on the efficiency of a quantum dot solar cell is calculated. The maximum point of efficiency and optimum thickness of the cell obtained under three different sunlight concentrations. We show the importance of electron filling factor as a parameter to be more considered. This parameter can be controlled by the quantum dots size and distance between quantum dot layers in the active region. Analysis of above mentioned parameters suggest that to attain a maximum efficiency, the size of the quantum dots and the distance between the periodically arrayed dot layers have to be optimized. In addition, sunlight concentration is recommended as an effective approach to have high efficiency and low cost level solar cells.

MODELING THE IMPEDANCE OF NANOSTRUCTURED PV IN SIMULINK/MATLAB

Impedance measurement is a common method to study the electrical properties of thin film photovoltaics. For the first time, we use the MATLAB/Simulink environment to extract the complex impedance of the nanostructured heterojunction solar cells. The impedance magnitude, phase and Nyquist plot of the PV are simulated in LTI Viewer and Impedance versus Frequency analysis tools of SimPower GUI block of Simulink. We examined a variety of the equivalent circuits consisting of capacitance, series and shunt resistances representing the solar cell structure. The model uses the parameters with values reported in the literature at room temperature and zero bias. The effect of the additional capacitance and resistances in the equivalent circuits on the impedance components of the cells is considered by Simulink environment.

Review on the application of nanostructure materials in solar cells

In recent years, nanostructure materials have opened a promising route to future of the renewable sources, especially in the solar cells. This paper considers the advantages of nanostructure materials in improving the performance and stability of the solar cell structures. These structures have been employed for various performance/energy conversion enhancement strategies. Here, we have investigated four types of nanostructures applied in solar cells, where all of them are named as quantum solar cells. We have also discussed recent development of quantum dot nanoparticles and carbon nanotubes enabling quantum solar cells to be competitive with the conventional solar cells. Furthermore, the advantages, disadvantages and industrializing challenges of nanostructured solar cells have been investigated.

Peculiar Role of Holes and Electrons in the Degradation of CdTe Thin Films

For the first time, the degradation rate of the electrical parameters in thin-film solar cells based on CdS/CdTe materials is simulated numerically using the Analysis of Microelectronics and Photonic Structures (AMPS-1D) program. This time-dependent approach attributes the defect generation rate to the excess carrier concentration profile. The degradation rate is analyzed for the devices stressed under the open-circuit, short-circuit, illumination, and dark conditions. Illuminated open-circuited devices showed a faster degradation rate than the short-circuited and dark-rested ones. This instability was mostly driven by the loss in the fill factor relevant to the increased series resistance. A separate analysis of the degradation behavior arisen from the holes and electrons indicates that the holes degrade the device slightly stronger than the electrons. In the CdTe thin films, the drift mobility of the holes is an order of magnitude lower than that of the electrons, which allows a longer interaction of the holes with the semiconductor lattice. Starting from the simplest device structure, e.g., Gloeckler model, the calculations are extended to the defect increment at the very thin layers placed at the front and back regions of the device. Both layers caused an almost similar degradation trend but a slightly faster rate when the thin defective layer was placed at the junction. This time-dependent approach can be extended to simulate the degradation behavior of the electrical parameters in other thin-film devices, such as CIGS and CZTS materials, under the different stress conditions leading to the different defect distribution across the device thickness.

AN EQUIVALENT CIRCUIT MODEL PROPOSED FOR THE INTERMEDIATE BAND NANOSTRUCTURED QUANTUM DOT SOLAR CELLS

The equivalent circuit of the p-i-n structure has been considered and developed for the quantum dot intermediate band solar cells where the nanoparticles are inserted in the active region of the diode. The admittance of the circuits are calculated consisting of frequency dependent capacitance and conductance. The presence of quantum dot layers in the active region of the diode increases the capacitance and conductance of the cell in lower frequencies. However, the number of QD cannot be increased and has an optimum.

Review on the degradation and device physics of quantum dot solar cells

Briefly, we reviewed the latest progress in energy conversion efficiency and degradation rate of the quantum dot (QD) solar cells. QDs are zero dimension nanoparticles with tunable size and accordingly tunable band gap. The maximum performance of the most advanced QD solar cells was reported to be around 10%. Nevertheless, majority of research groups do not investigate the stability of such devices. QDs are cheaper replacements for silicon or other thin film materials with a great potential to significantly increase the photon conversion efficiency via two ways: (i) creating multiple excitons by absorbing a single hot photon, and (ii) formation of intermediate bands (IBs) in the band gap of the background semiconductor that enables the absorption of low energy photons (two-step absorption of sub-band gap photons). Apart from low conversion efficiency, QD solar cells also suffer from instability under real operation and stress conditions. Strain, dislocations and variation in size of the dots (under pressure of the other layers) are the main degradation resources. While some new materials (i.e. perovskites) showed an acceptable high performance, the QD devices are still inefficient with an almost medium rate of 4% (2010) to 10% (2015).