Improvements to solar cell efficiency and radiation hardness that are compatible with low cost, high volume manufacturing processes are critical for power generation applications in future long-term NASA and DOD space missions. In this paper, we provide the results of numerical simulation of the radiation effects in a novel, ultra-thin (UT), Si photovoltaic cell technology that combines enhanced light trapping (LT) and absorption due to nanostructured surfaces, separation of photogenerated carriers by carrier selective contacts (CSC), and increased carrier density due to multiple exciton generation (MEG). Such solar cells have a potential to achieve high conversion efficiencies while shown to be rad-hard, lightweight, flexible, and low–cost, due to the use of Si high volume techniques.
Space exploration missions and space electronic equipment require improvements in solar cell efficiency and radiation hardness. Triple-junction photovoltaic (TJ PV) cell is one of the most widely used PV for space missions due to it high efficiency. A proper models and simulation techniques are needed to speed-up the development on novel solar cell devices and reduce the related expenses. In this paper we have developed a detailed 3D TCAD model of a TJ PV cell, and calibrated the various (not accurately known) physical parameters to match experimental data, such as dark and light JV, external quantum efficiency (EQE) . A detailed model of triple-junction InGaP/GaAs/Ge solar cell has been developed and implemented in CFDRC’s 3D NanoTCAD simulator. The model schematic, materials, layer thicknesses, doping levels and meshing are discussed. This triple-junction model is based on the experimental measurements of a Spectrolab triple-junction cell by  with material layer thicknesses provided by Rochester Institute of Technology . This model of the triple-junction solar cell is primarily intended to simulate the external quantum efficiency, JV and other characteristics of a physical cell. Simulation results of light JV characteristics and EQE are presented. The calculated performance parameters compare well against measured experimental data . Photovoltaic performance parameters (Jsc, Voc, Jm, Vm, FF, and Efficiency) can also be simulated using the presented model. This TCAD model is to be used to design an enhanced TJ PV with increased efficiency and radiation tolerance. Keywords: photovoltaic cell, triple-junction, numerical modeling, TCAD, dark and light JV.
A predictive computational approach that limits use of DLTS experiments is presented, developed using the experimental data and proposed physics based models. Three-dimensional NanoTCAD simulations are used for physicsbased prediction of space radiation effects in III-V solar cells, and validated with experimentally measured characteristics of a p+n GaAs solar cell with AlGaAs window. The computed dark and illuminated I-V curves as well as corresponding performance parameters matched very well experimental data for 2 MeV proton irradiation at various fluences. We analyze the role of majority vs. minority and deep vs. shallow carrier traps in the solar cell performance degradation. The traps/defects parameters used in the simulations were derived from Deep Level Transient Spectroscopy (DLTS) data obtained at NRL. It was noticed that the degradation caused by deep traps observed in single-trap numerical tests exhibit a very similar trend to the degradation caused by a full spectrum of defect traps, but to a lesser degree. This led to the development of a method to accurately simulate the degradation of a solar cell by using only a single deep level defect whose density is calculated by the Stopping and Range of Ions in Matter (SRIM) code. Using SRIM, we calculated the number of vacancies produced by 2 MeV proton irradiation for fluences ranging from 6x1010 cm-2 to 5x1012 cm-2. Based on the SRIM results, we applied trap models in NanoTCAD and performed full I-V simulations from which the amount of degradation of performance parameters (Isc, Voc, Pmax) was calculated. The physics-based models using SRIM allowed obtaining good match with experimental data.
We present a predictive computational approach that may reduce the need for extensive inputs from Deep Level Transient Spectroscopy (DLTS) experiments. Three-dimensional NanoTCAD simulations are used for physics-based prediction of space radiation effects in a p+n GaAs solar cell with AlGaAs window, and validated with experimental data. The computed dark and illuminated I-V curves, as well as corresponding performance parameters, matched experimental data very well for 2 MeV proton irradiation at various fluence levels. We analyze the role of majority vs. minority and deep vs. shallow carrier traps in the solar cell performance degradation. The defects level parameters used in the simulations were taken from DLTS data obtained at NRL. It was determined from numerical simulations that the degradation of the photovoltaic parameters could be modeled and showed similar trends when a only a single deep level defect was considered compared to a spectrum of defect levels. This led to the development of an alternate method to simulate the degradation of a solar cell by using only a single deep level defect whose density is calculated by the Stopping and Range of Ions in Matter (SRIM) code. Using SRIM, we calculated the number of vacancies produced by 2 MeV proton irradiation for fluence levels ranging from 6x1010 cm-2 to 5x1012 cm-2. Based on the SRIM results, we applied trap models in NanoTCAD and performed I-V simulations from which the degradation of the photovoltaic parameters (Isc, Voc, FF, Pmax) was calculated. The simulations using SRIM-derived defect concentrations showed reasonable agreement with simulations using parameters determined from DLTS.
In this work, we present a Quantum Dot Intermediate Band Solar Cell (QD-IBSC) photogeneration model that is based
on detailed balance principles. The 3D Schrödinger equation is solved for a regimented array of cubic quantum dots
known as a Quantum Dot Crystal (QDC). Energy levels used in the simulation are derived from the dispersion relation.
We consider only the dispersion relation along the  quasi-crystallographic direction. Absorption coefficients used
were assumed to be constant and non-overlapping for each energy transition. Various JV curves were simulated for
different dot sizes for the InAs0.9N0.1/GaAs0.98Sb0.02 dot/host system. This material system was chosen due to its property
of a negligible valance band offset. The negligible valance band offset offers more feasibility for the isolation of the
intermediate band. Simulations were done under a non-concentrated 6000K black body spectrum at a cell temperature of
300K. Performance parameters for each IV curve were calculated in order to ascertain the effect of dot size on
performance from a fundamental level. Results show that for a fixed dot separation of 2nm, cell efficiency increases to
36.7% as the dot size is increased to 3.5 nm, but begins to decrease for larger dot sizes.
For applications in space systems, devices based on novel nanomaterials offer significant advantages over traditional
technologies in terms of light-weight and efficiency. Examples of such novel devices include quantum dot (QD) based
solar cells and photodetectors. However, the response of these devices to radiation effects is not well understood, and
radiation effects modeling tools are not yet available. In this paper we review our numerical models and experimental
investigation of radiation effects in quantum dot based solar cells. In the natural, high-radiation environment of space all
solar cells suffer from degradation. Although some studies have been conducted, and test data collected, on the
performance of solar cells in a radiation environment, the mechanisms of radiation-induced degradation of quantum dot
superlattices (QDS) has yet to be established. We have conducted proton irradiation experiments to provide a direct
comparison of radiation hardness of quantum dot based cells and regular solar cells. An approach to the development of
Nano-scale Technology Computer Aided Design (NanoTCAD) simulation software for simulation of radiation effects in
QDS-based photovoltaic (PV) devices is presented. The NanoTCAD tools are based on classical drift-diffusion and
quantum-mechanical models for the simulation of QD PV cells.
Intermediate-band (IB) solar cells were predicted to have the photovoltaic (PV) efficiency exceeding the Shockley-Queisser limit for a single junction cell. A possible practical implementation of the IB solar cells can be based on the
quantum dot superlattices (QDS). The parameters of such QDS structure have to be carefully tuned in order to achieve
the desired charge carrier dispersion required for the IB solar cell operation. We used the first-principle theoretical
models for calculating the carrier states and light absorption in QDS. This approach allowed us to determine the actual
dimensions of the quantum dots and the inter-dot spacing for inducing the carrier miniband in the band-gap region where
the miniband can play the role of the IB. Using the Shockley-Queisser detailed balance theory we determined that the
upper-bound PV efficiency of such IB solar cells can be as high as ~ 51%. The required QDS dimensions for the IB
implementation on the basis of InAsN/GaAsSb are technologically challenging but feasible: ~ 2 - 6 nm. The proposed
computational design approach may help with implementation of other solar cell concepts for advanced light-to-energy
conversion enabled by nanostructures.
Space electronic equipment, and NASA future exploration missions in particular, require improvements in solar cell
efficiency and radiation hardness. Novel nano-engineered materials and quantum-dot array based photovoltaic devices
promise to deliver more efficient, lightweight solar cells and arrays which will be of high value to long term space
missions. In this paper, we describe issues related to the development of the quantum-dot based solar cells and
comprehensive software tools for simulation of the nanostructure-based photovoltaic cells. Some experimental results
used for the model validation are also reviewed. The novel modeling and simulation tools for the quantum-dot-based
nanostructures help to better understand and predict behavior of the nano-devices and novel materials in space
environment, assess technologies, devices, and materials for new electronic systems as well as to better evaluate the
performance and radiation response of the devices at an early design stage. The overall objective is to investigate and
design new photovoltaic structures based on quantum dots (QDs) with improved efficiency and radiation hardness. The
inherently radiation tolerant quantum dots of variable sizes maximize absorption of different light wavelengths, i.e.,
create a "multicolor" cell, which improves photovoltaic efficiency and diminishes the radiation-induced degradation.
The QD models described here are being integrated into the advanced photonic-electronic device simulator NanoTCAD,
which can be useful for the optimization of QD superlattices as well as for the development and exploring of new solar