Alex Walker

The Effects of Absorption and Recombination on Quantum Dot Multijunction Solar Cell Efficiency

The key characteristics of quantum dot (QD)-enhanced multijunction solar cells (MJSC) are explored theoretically by focusing on the generation and recombination rates throughout the QD layers in the middle subcell. The quantum dots are modeled using an effective medium to describe light absorption, confinement, and recombination properties. We report an 8% increase in the short-circuit current density accompanied by a 3% drop in an open-circuit voltage for a QD- enhanced MJSC at 1 sun illumination (1 kW/m2) compared with a control MJSC without QD. The drop in an open-circuit voltage is due in part to the increased recombination rates in the depletion region, decreased carrier lifetimes in the QDs, and the increased recombination rates resulting from carrier escape and capture. Overall, these contribute to an absolute increase in efficiency of over 1% for the studied QD-enhanced MJSC design for a QD density of 125 QD/μm2.

Enhanced Efficiencies for High-Concentration, Multijunction PV Systems by Optimizing Grid Spacing under Nonuniform Illumination

The design of a triple junction solar cell’s front contact grid can significantly affect cell conversion efficiency under high concentration. We consider one aspect of grid design, choosing a linear grid within a distributed resistance cell model to optimize finger spacings at concentrations between 500 and 2500 suns under uniform and nonuniform illumination. Optimization for maximum efficiency under Gaussian irradiance profiles is done by SPICE analysis. Relative to the optimized uniform illumination designs, we find enhancements of 0.5% to 2% in absolute efficiencies for uniform spacing. Efficiency enhancement with nonuniform spacing under nonuniform illumination is also evaluated. Our model suggests that, at lower concentrations (<1000 suns), the penalty for using uniformly spaced fingers instead of nonuniformly spaced fingers is <0.1%. However, at a concentration of 2500 suns the penalty increases to 0.3%. Thus, relative to a uniform irradiance optimization, an absolute efficiency increase of 2.3% can be attained for an optimized nonuniform spacing given the Gaussian irradiance profile under consideration.

AlGaAs tunnel junction for high efficiency multi-junction solar cells: Simulation and measurement of temperature-dependent operation

AlGaAs tunnel junctions are shown to be well-suited to concentrated photovoltaics where temperatures and current densities can be dramatically higher than for 1-sun flat-panel systems. Detailed comparisons of AlGaAs/AlGaAs tunnel junction experimental measurements over a range of temperatures expected during device operation in concentrator systems are presented. Experimental and simulation results are compared in an effort to decouple the tunnel junction from the overall multi-junction solar cell. The tunnel junction resistance is experimentally studied as a function of the temperature to determine its contribution to overall efficiency of the solar cell. The current-voltage behavior of the isolated TJ shows that as the temperature is increased from 25°C to 85°C, the resistance decreases from ~4.7×10-4 ¿·cm2 to ~0.3×10-4 ¿·cm2 for the operational range of a multi-junction solar cell under concentration.

Tunnel-Junction-Limited Multijunction Solar Cell Performance Over Concentration

The simulation of tunnel junctions is performed by using nonlocal band-to-band and trap assisted tunneling models that are capable of reproducing the experimental current-voltage characteristics of p⁺⁺AlGaAs/ n⁺⁺AlGaAs and p⁺⁺AlGaAs/ n⁺⁺GaAs based devices. These simulated characteristics are then implemented within a lattice matched InGaP/(In)GaAs/Ge multijunction solar cell (MJSC) to assess the performance as a function of tunnel junction layer doping in the regime where the TJ limits the performance of the MJSC. At 500 suns, a 4.6% absolute drop in simulated efficiency is observed for an AlGaAs/GaAs bottom TJ corresponding to a degenerately p-doped layer of 2.5 × 10¹⁹ cm^-3 compared to a TJ with a doping of 4×10²⁰ cm^-3. A minimum p⁺⁺ doping level of 3.3 × 10 ¹⁹ cm^-3 is required in order to avoid bottom TJ limitation up to 1000 suns concentration for an n⁺⁺ doping of 2 × 10¹⁹ cm^-3 based on the calibrated models. Furthermore, the effects of the peak and valley current densities are shown to have a strong influence on the efficiency over concentration within the TJ limiting regime.

Efficiency Measurements and Simulations of GaInP/InGaAs/Ge Quantum Dot Enhanced Solar Cells at up to 1000‐Suns Under Flash and Continuous Concentration

Quantum dot (QD) enhanced GaInP/InGaAs/Ge solar cells are presented and characterized under flash and continuous solar simulators. InAs QD within the middle sub‐cell increase the carrier generation due to absorption in the range 900–940 nm. These QD‐enhanced solar cells routinely achieve production efficiencies of ∼40%, and this set of research samples obtain a peak efficiency of >38% under flash solar simulators. Continuous solar simulator testing is performed to test the QD‐enhanced solar cells under thermal loads similar to concentrated photovoltaic systems, in which cells demonstrate excellent reliability. Numerical simulations of the QD‐enhanced solar cells are performed using an effective medium to model the additional absorption due to the QD layers. Temperature dependence of the QD‐enhanced solar cells are modeled, in which temperature‐dependent bandgap narrowing changes the dark current and the semiconductor absorption profiles. Comparison between the experimental results and numerical model show...

Modeling down-conversion and down-shifting for photovoltaic applications

The efficiency improvements achieved by adding idealized, top-mounted, down-conversion (DC) and luminescent down-shifting (LDS) layers to a commercial grade silicon solar cell are studied. A comparison is then made to silicon nanocrystals (Si-NC) LDS layer coupled to a silicon solar cell, where the optical properties of the Si-NC are based on measured data. Since the modeled DC and LDS layers are electrically isolated from the solar cell, the devices are studied by modifying the incident AM1.5G spectrum according to the bandgap, absorption and emission profiles, and global efficiency of the DC and LDS layers. Simulation results indicate that a minimum DC/LDS efficiency of 1% is required to enhance the solar cell efficiency, and that this threshold rises to 38% for a Si-NC based LDS layer. Additionally, the incorporation of an optimal, perfectly efficient DC layer (200%) is shown to enhance the photovoltaic efficiency from 14.1% to 16.6% as opposed to 16.3% for a perfect LDS layer (100%).

Design of the KMTNet large format CCD camera

We present the design for the 340 Mpixel KMTNet CCD camera comprising four newly developed e2v CCD290-99 imaging sensors mounted to a common focal plane assembly. The high performance CCDs have 9k x 9k format, 10 micron pixels, and multiple outputs for rapid readout time. The camera Dewar is cooled using closed cycle coolers and vacuum is maintained with a cryosorption pump. The CCD controller electronics, the electronics cooling system, and the camera control software are also described.

4 Junction dilute nitride solar cell optimization: Comparing current matching approaches in detailed balance algorithms

The optimization of quadruple junction solar cell designs in the detailed balance limit via an equivalent circuit model for each sub-cell is explored using spectral sharing from three perspectives: (i) current matching at short circuit current (as per etaOpt software), (ii) corrected current matching at short circuit current and (iii) unconstrained short-circuit currents. At 1-sun illumination (968 W/m2), we report an efficiency increase of 1.3% absolute for the second current matching approach over the first, solely due to an increase in fill factor. For concentrated illumination at 1000 suns, the efficiency increase becomes 1.6% absolute.

Quantum efficiency measurements of down-shifting using silicon nanocrystals for photovoltaic applications

Silicon nanocrystal (Si-NC) luminescent down-shifting materials for photovoltaic (PV) applications were fabricated by ion implantation and plasma-enhanced chemical vapor deposition (PECVD). The absolute optical conversion efficiency of the Si-NC-emitted photoluminescence was measured using conventional methods, and an optical set-up involving an integrating sphere. Modeling shows that down-shifting the light incident on a single-junction silicon cell (SJSC) can improve the cell performance if the optical conversion efficiency is sufficiently high. The measured conversion efficiency of the Si-NCs in a fused silica host was found to range from 0.8% to 1.84% and was compared with the efficiency required to maintain the performance of a SJSC.

Design constraints of n-p InGaAsN dilute nitride sub-cells for 3- and 4- junction solar cell applications under concentrated illumination

Solar cells with an n-p structure consisting of an InGaAsN dilute nitride emitter with n-type background doping and a p-type doped InGaAsN base are numerically simulated on GaAs and Ge substrates. The InGaAsN material parameters are chosen based on of structures reported in the literature. The short-circuit current of cells on GaAs substrates is 11% greater than similar designs on Ge substrates due to an increased photon path length. The current density to optical intensity ratio of 0.135 A/W, required to match the operating current density of an InGaP/InGaAs/InGaAsN 3-junction solar cell, is readily obtained for devices on GaAs substrates, but the required current density is not attainable when a Ge substrate is used. The InGaAsN sub-cell demonstrates enhanced performance for increasing levels of concentration up to 1000 suns. Open-circuit voltage increases near logarithmically with concentration, and the ratio of maximum-power point current to short-circuit current is constant at 82% between 10 and 1000 suns.