Optimized Triple-Junction Solar Cells Using Inverted Metamorphic Approach

Record efficiencies with triple-junction inverted metamorphic designs, modeling useful to optimize, and consider operating conditions before choosing design

Component integration strategies in metamorphic 4-junction III-V concentrator solar cells

Progressing beyond 3-junction inverted-metamorphic multijunction solar cells grown on GaAs substrates, to 4-junction devices, requires the development of high quality metamorphic 0.7 eV GaInAs solar cells. Once accomplished, the integration of this subcell into a full, Monolithic, series connected, 4J-IMM structure demands the development of a metamorphic tunnel junction lattice matched to the 1eV GaInAs subcell. Moreover, the 0.7 eV junction adds about 2 hours of growth time to the structure, implying a heavier annealing of the subcells and tunnel junctions grown first. The final 4J structure is above 20 Pm thick, with about half of this thickness used by the metamorphic buffers required to change the lattice constant throughout the structure. Thinning of these buffers would help reduce the total thickness of the 4J structure to decrease its growth cost and the annealing time. These three topics: development of a metamorphic tunnel junction for the 4th junction, analysis of the annealing, and thinning of the structure, are tackled in this work. The results presented show the successful implementation of an antimonide-based tunnel junction for the 4th junction and of pathways to mitigate the impact of annealing and reduce the thickness of the metamorphic buffers

On the stepwise nature of self-supervised learning

We present a simple picture of the training process of self-supervised learning methods with joint embedding networks. We find that these methods learn their high-dimensional embeddings one dimension at a time in a sequence of discrete, well-separated steps. We arrive at this conclusion via the study of a linearized model of Barlow Twins applicable to the case in which the trained network is infinitely wide. We solve the training dynamics of this model from small initialization, finding that the model learns the top eigenmodes of a certain contrastive kernel in a stepwise fashion, and obtain a closed-form expression for the final learned representations. Remarkably, we then see the same stepwise learning phenomenon when training deep ResNets using the Barlow Twins, SimCLR, and VICReg losses. Our theory suggests that, just as kernel regression can be thought of as a model of supervised learning, \textit{kernel PCA} may serve as a useful model of self-supervised learning

Experimental and modeling analysis of internal luminescence in III-V solar cells

In high quality solar cells, the internal luminescence can be harnessed to enhance the overall performance. Internal confinement of the photons can lead to an increased open-circuit voltage and short-circuit current. Alternatively, in multijunction solar cells the photons can be coupled from a higher bandgap junction to a lower bandgap junction for enhanced performance. We model the solar cell as an optical cavity and compare calculated performance characteristics with measurements. We also describe how very high luminescent coupling alleviates the need for top-cell thinning to achieve current-matching

Enhanced external radiative efficiency for 20.8 efficient single-junction GaInP solar cells

We demonstrate 1.81 eV GaInP solar cells approaching the Shockley-Queisser limit with 20.8% solar conversion efficiency, 8% external radiative efficiency, and 80–90% internal radiative efficiency at one-sun AM1.5 global conditions. Optically enhanced voltage through photon recycling that improves light extraction was achieved using a back metal reflector. This optical enhancement was realized at one-sun currents when the non-radiative Sah-Noyce-Shockley junction recombination current was reduced by placing the junction at the back of the cell in a higher band gap AlGaInP layer. Electroluminescence and dark current-voltage measurements show the separate effects of optical management and non-radiative dark current reduction

Metamorphic III-V solar cells: recent progress and potential

Metamorphic semiconductor devices are commonly considered to have inferior electronic quality. However, recent development of compositionally graded buffers and junction structures have led to the achievement of high quality metamorphic solar cells exhibiting internal luminescence efficiencies over 90%. Optimizing the optical design of the solar cell becomes important in order to enhance photon recycling and open circuit voltage in these cells. In this paper we first present recent performance results for 1eV and 0.7eV GaInAs solar cells grown on GaAs substrates. Then an electro-optical model is used to assess the potential performance improvements in current metamorphic solar cells under different realizable design scenarios. The results show that significant improvements can be achieved by improving both the electronic quality and optics of these cells

Impurity-Band Model for GaP1-xNx

Low-temperature absorption studies on free-standing GaP1-xNx films provide direct experimental evidence that the host conduction-band minimum (CBM) near X1C does not plunge downward with increased nitrogen doping, contrary to what has been suggested recently; rather, it remains stationary for x up to 0.1%. This fact, combined with the results of earlier studies of the CBM at ..GAMMA.. and conduction-band edge near L, confirms that the giant bandgap lowering observed in GaP1-xNx results from a CBM that evolves purely from nitrogen impurity bands

Metamorphic Ga0.76In0.24As/GaAs0.75Sb0.25 tunnel junctions grown on GaAs substrates

Lattice-matched and pseudomorphic tunnel junctions have been developed in the past for application in a variety of semiconductor devices, including heterojunction bipolar transistors, vertical cavity surface-emitting lasers, and multijunction solar cells. However, metamorphic tunnel junctions have received little attention. In 4-junction Ga0.51In0.49P/GaAs/Ga0.76In0.24As/Ga0.47In0.53As inverted-metamorphic solar cells (4J-IMM), a metamorphic tunnel junction is required to series connect the 3rd and 4th junctions. We present a tunnel junction based on a metamorphic Ga0.76In0.24As/GaAs0.75Sb0.25 structure for this purpose. This tunnel junction is grown on a metamorphic Ga0.76In0.24As template on a GaAs substrate. The band offsets in the resulting type-II heterojunction are calculated using the first-principles density functional method to estimate the tunneling barrier height and assess the performance of this tunnel junction against other material systems and compositions. The effect of the metamorphic growth on the performance of the tunnel junctions is analyzed using a set of metamorphic templates with varied surface roughness and threading dislocation density. Although the metamorphic template does influence the tunnel junction performance, all tunnel junctions measured have a peak current density over 200 A/cm2. The tunnel junction on the best template has a peak current density over 1500 A/cm2 and a voltage drop at 15 A/cm2 (corresponding to operation at 1000 suns) lower than 10 mV, which results in a nearly lossless series connection of the 4th junction in the 4J-IMM structure.The authors thankfully acknowledge the invaluable support by W. Olavarria and M. Young growing and processing the semiconductor devices. I. Garcıa holds an IOF grant from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7/ 2007-2013) under REA grant agreement No. 299878. This work is supported by the U.S. Department of Energy under Contract No. DE-AC36-08-GO28308 with the National Renewable Energy Laboratory

Effects of internal luminescence and internal optics on V-oc and J(sc) of III-V solar cells

For solar cells dominated by radiative recombination, the performance can be significantly enhanced by improving the internal optics. Internally radiated photons can be directly emitted from the cell, but if confined by good internal reflectors at the front and back of the cell they can also be re-absorbed with a significant probability. This so-called photon recycling leads to an increase in the equilibrium minority carrier concentration and therefore the open-circuit voltage, Voc. In multijunction cells, the internal luminescence from a particular junction can also be coupled into a lower bandgap junction where it generates photocurrent in addition to the externally generated photocurrent, and affects the overall performance of the tandem. We demonstrate and discuss the implications of a detailed model that we have developed for real, non-idealized solar cells that calculates the external luminescent efficiency, accounting for wavelength-dependent optical properties in each layer, parasitic optical and electrical losses, multiple reflections within the cell and isotropic internal emission. The calculation leads to Voc, and we show data on high quality GaAs cells that agree with the trends in the model as the optics are systematically varied. For multijunction cells the calculation also leads to the luminescent coupling efficiency, and we show data on GaInP/GaAs tandems where the trends also agree as the coupling is systematically varied. In both cases, the effects of the optics are most prominent in cells with good material quality. The model is applicable to any solar cell for which the optical properties of each layer are well-characterized, and can be used to explore a wide phase space of design for single junction and multijunction solar cells

Generalized optoelectronic model of series-connected multijunction solar cells

The emission of light from each junction in a series-connected multijunction solar cell both complicates and elucidates the understanding of its performance under arbitrary conditions. Bringing together many recent advances in this understanding, we present a general 1-D model to describe luminescent coupling that arises from both voltage-driven electroluminescence and voltage-independent photoluminescence in nonideal junctions that include effects such as Sah-Noyce-Shockley (SNS) recombination with n ≠ 2, Auger recombination, shunt resistance, reverse-bias breakdown, series resistance, and significant dark area losses. The individual junction voltages and currents are experimentally determined from measured optical and electrical inputs and outputs of the device within the context of the model to fit parameters that describe the devices performance under arbitrary input conditions. Techniques to experimentally fit the model are demonstrated for a four-junction inverted metamorphic solar cell, and the predictions of the model are compared with concentrator flash measurements