Alexandre W. Walker

Estimating cell temperature in a concentrating photovoltaic system

A temperature calibrated equivalent circuit model of a high efficiency CPV solar cell is used to simulate a measured six-cell module J-V curve to estimate its average operating temperature. The simulation is based on a two diode equivalent circuit model for each subcell of a representative triple junction cell. Module J-V curves in a real CPV system were measured with a test station that performs continuous voltage sweeps allowing cells to reach a well defined thermal equilibrium during measurement. The average electrical power extracted during measurement is then used to determine the cell temperature when they are operating at their maximum power point. It is shown that the cells would operate at 42 ± 2 C° above ambient (32 ± 2°C abs.) given the ambient conditions during the measurement.

Temperature dependent external quantum efficiency simulations and experimental measurement of lattice matched quantum dot enhanced multi-junction solar cells

The external quantum efficiency (EQE) of a high efficiency lattice matched multi-junction solar cell (MJSC) and a quantum dot enhanced MJSC are numerically simulated. An effective medium is developed and integrated into the model to simulate the absorption characteristics of the quantum dots in the latter device. A calibration of the model is carried out using room temperature EQE measurements of both MJSC designs. The numerical model is further generalized through the development of a novel temperature dependent absorption model based on the Varshni relation for bandgap narrowing due to temperature. Integrating this model into the numerical simulation environment accurately reproduced the experimentally observed shifts in the EQE edge of each sub-cell as a function of temperature, including the shift in the quantum dot peak. The current — voltage characteristics are discussed under the AM1.5D spectrum for concentrated illumination and realistic temperatures in concentrator systems. The development of this temperature dependent absorption model is an important addition to the set of design tools used to optimize high efficiency MJSC under realistic temperatures and spectral conditions experienced in concentrated photovoltaic systems.

Modeling a monocrystalline Cu(In,Ga)Se2 single junction solar cell grown on a GaAs substrate

A polycrystalline Cu(In,Ga)Se2 (CIGS) single junction solar cell model is developed with dependencies on the molar fraction of In and Ga with a 0.8 eV Shockley-Read-Hall (SRH) trap level above the valence band. The simulated performance of this solar cell over molar fraction compares well to data published in the literature using the SRH minority carrier lifetime to fit the trend in open circuit voltage. The material parameters are then used as a foundation for a numerical model of a monocrystalline CIGS solar cell grown on a GaAs substrate with an emphasis on modeling the CIGS/GaAs interface where a molar fraction gradient in CIGS forms due to lattice mismatch induced inter-diffusion of Ga and In from the substrate and CIGS layers. Without strain effects due to the lattice mismatch, the CIGS monocrystalline solar cell has an efficiency of 18.6% under the AM1.5G spectrum (1000 W/m2) with a short circuit current density of 36.5 mA/cm2, an open circuit voltage of 0.66 V and a fill factor of 77.4%. However, when reasonable strain effects are considered, such as the formation of strained induced interface defects and threading dislocation densities (TDD), the efficiency degrades to 6% for TDD < 1x107 cm-2. The models are able to reproduce a similar structure’s measured performance using a TDD of 1.5×107 cm-2 and a surface recombination velocity of 108 cm/s at the CdS/CIGS interface.

Modeling intermediate band solar cells: a roadmap to high efficiency

Intermediate band (IB) photovoltaics have the potential to be highly efficient and cost effective solar cells. When the IB concept was proposed in 1997, there were no known intermediate band materials. In recent years, great progress has been made in developing materials with intermediate bands, though power conversion efficiencies have remained low. To understand the material requirements to increase IB device efficiencies, we must develop good models for their behavior under bias and illumination. To evaluate potential IB materials, we present a figure of merit, consisting of parameters that can be measured without solar cell fabrication. We present a new model for IB devices, including the behavior of their junctions with n- and p-type semiconductors. Using a depletion approximation, we present analytic approximations for the boundary conditions of the minority carrier diffusion equations. We compare the analytic results to Synopsys Sentaurus device models. We use this model to find the optimal thickness of the IB region based on material parameters. For sufficiently poor IB materials, the optimal thickness is zero – i.e., the device is more efficient without the IB material at all. We show the minimum value of the figure of merit required for an IB to improve the efficiency of a device, providing a clear goal for the quality of future IB materials.

Carrier Dynamics in Quantum-Dot Multijunction Solar Cells Under Concentration

The key performance metrics of quantum-dot (QD)- lattice-matched multijunction solar cells (MJSCs) composed of InGaP/(In)GaAs/Ge with InAs/GaAs QDs are explored under high-concentration illumination with a focus on the carrier dynamics in the QD layers of the middle subcell. An effective medium approach is used to describe generation and recombination in the QD system, including carrier escape and capture from the weakly confining quantum well and the QD states. At a concentration of 1000 suns, simulations indicate that the specific QD MJSC studied outperforms a standard MJSC by 1.1% in relative efficiency operating at 25 °C. However, this gain in efficiency is highly dependent on the confinement potentials of the wetting layer, as well as the resulting current mismatch between the top and middle subcells when carrier escape rates from within the wetting layer confinements are reduced.

Numerical modeling of silicon nanocrystal down-shifting layers for enhanced CIGS solar cell performance

The performance effects of silicon nanocrystals (SiNC) embedded in a silicon dioxide matrix to act as a downshifting (DS) layer mounted on the top surface of a polycrystalline Cu(In, Ga)Se2 solar cell are explored numerically. The DS layers are modeled by modifying the incident AM1.5G spectrum based on the absorption and emission properties of the SiNC. The effects of the DS layers as an anti-reflection coating leads to an 11.4% relative improvement in short-circuit current density under 1-sun illumination (0.1 W/cm2). Comparatively, the effect of down-shifting high-energy photons to lower energy photons showed a 4% relative short-circuit current density improvement, albeit for an optical conversion efficiency of 80%.

CURRENT–VOLTAGE MEASUREMENTS WITHIN THE NEGATIVE DIFFERENTIAL RESISTANCE REGION OF AlGaAs/AlGaAs TUNNEL JUNCTIONS FOR HIGH CONCENTRATION PHOTOVOLTAICS

The current–voltage characteristics of AlGaAs/AlGaAs tunnel junctions for use in multi-junction solar cells are studied experimentally, where tunneling current peaks of 1100 A/cm2 and specific contact resistivities of 0.3 × 10-4Ω⋅cm2 at 7 A/cm2 (typical concentrated photovoltaic operating current) are measured. This represents an ideal tunnel junction design, with a very low resistance and one of the highest tunneling peak currents reported for solar cells. Normally, solar cell current–voltage characteristics are measured using time-averaged methods, which, in this study, reveal a tunneling peak current density of ~950 A/cm2. Due to nonlinear oscillations within the measurement circuit, the precise locations and magnitudes of the tunneling peak and valley current densities are obscured when using time-average measurement methods. Here we present an alternative method to determine the tunneling peak current density, in which the nonlinear oscillations in the current and voltage are recorded over time and a current density–voltage curve is reconstructed. This time-dependent method results in a measured tunneling peak current density of ~ 1100 A/cm2. The nonlinear oscillations of the experimental circuit are reproduced by modeling an equivalent circuit, resulting in qualitative agreement with the observed oscillations. This model predicts the capacitance and inductance of the equivalent circuit to be approximately 3 nF and 3.5 μH, respectively. This numerical model can be used to determine the inductance and the capacitance of any circuit having a negative differential resistance region.

Time-dependent analysis of AlGaAs/AlGaAs tunnel junctions for high efficiency multi-junction solar cells

The current density-voltage characteristic of an AlGaAs/AlGaAs tunnel junction is determined by taking a time-averaged measurement across the device. A tunnelling peak of ~950A/cm2 is recorded by this method. Measurements of the tunnelling peak and valley currents by the time averaging method are obscured due to the unstable nature of the negative differential resistance region of the current density-voltage characteristic. This AlGaAs/AlGaAs tunnel junction is then biased inside the negative differential resistance region of the current density-voltage characteristic, causing the current and the voltage to oscillate between the peak and the valley. The current and voltage oscillations are measured over time and then currents and voltages corresponding to the same time stamps are plotted against each other to form a timedependent curve from which a tunnelling peak of a value larger than 1100A/cm2 is determined. The peak determined by this method is 11-20% larger than previously determined using the time averaged measurement. An AlGaAs/InGaP tunnel junction having no negative differential resistance region is also presented.

Simulation, modeling, and comparison of III-V tunnel junction designs for high efficiency metamorphic multi-junction solar cells

Simulations of AlxGa1-xAs/GaAs (x = 0.3) and AlxGa1-xAs/AlxGa1-xAs (x < 0.2) tunnel junction J-V characteristics are studied for integration into a 2D metamorphic multi-junction solar cell model composed of GaInP/GaAs/InGaAs. A comparison of the simulated solar cell J-V characteristics under AM1.5D spectrum is discussed in terms of short circuit current density (Jsc), open circuit voltage (VOC), fill factor (FF) and efficiency (η) for both tunnel junction designs. Using AlxGa1-xAs/GaAs top and bottom tunnel junctions, the metamorphic solar cell obtained values of Jsc = 12.3 mA/cm2, VOC = 2.56 V, FF = 0.81 and η = 25.5%, whereas the solar cell with the AlxGa1-xAs/AlxGa1-xAs top and bottom tunnel junctions reported values of Jsc = 12.3 mA/cm2, VOC = 2.22 V, FF = 0.81 and η = 22.1%. At open circuit voltage, energy band diagrams show minimal curvature in the electron and hole quasi Fermi levels; furthermore, the difference between the top sub-cell electron quasi Fermi level and the bottom sub-cell hole quasi Fermi level is shown to be equal to qVOC for both designs. The energy band diagram of the complete structure is compared for both tunnel junction designs, showing the difference in energy levels that correspond to the difference in measured open circuit voltage. The observed decrease in open circuit voltage was ΔVOC = 0.34 V, which was attributed to the difference in tunnel junction material band parameters such as bandgap, valence and conduction band offsets at heterojunctions and Fermi level degeneracies due to doping concentration differences.

Inverted Metamorphic III–V Triple-Junction Solar Cell with a 1 eV CuInSe2 Bottom Subcell

A new triple-junction solar cell (3J) design exploiting the highly absorptive I–III–VI chalcopyrite CuInSe2 material is proposed as an alternative to III–V semiconductor 3J solar cells. The proposed structure consists of GaInP (1.9 eV)/Ga(In)As (1.4 eV)/CuInSe2 (1 eV) which can be grown on a GaAs substrate in an inverted manner using epitaxial lift-off techniques. To lattice-match epitaxial CuInSe2 to Ga(In)As, a compositionally graded buffer region composed of GaxIn1−xP is used. The modeling and simulation of the device include the effects of threading dislocations on minority carrier lifetimes in the metamorphic buffer and bottom subcell active region. Studies focus on device performance under standard testing conditions and concentrated illumination. The results are compared to a reference lattice mismatched 3J composed of GaInP (1.9 eV)/Ga(In)As (1.4 eV)/GaInAs (1 eV) and to a lattice matched 3J composed of GaInP (1.9 eV)/Ga(In)As (1.4 eV)/Ge (0.67 eV). The advantage of CuInSe2 is its higher absorption coefficient, which requires only 1 μm of active material compared to 4 μm of GaInAs in the bottom subcell of the reference lattice mismatched cell. The proposed design reaches an efficiency of 32.6% under 1 sun illumination at 300 K with 105 cm−2 threading dislocations and 39.6% at 750 suns.