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A three-terminal, monolithically integrated InP photovoltaic (PV) device with an InGaAs/InAlAs electroabsorption modulator for simultaneous power generation and data modulation has been designed and experimentally validated. A discrete modulator with a data rate approaching 1 Mbps with an on/off ratio of 1.20 was demonstrated. The InP power generating junction of the monolithic device attained an AM0 efficiency of 14.1% for a 0.25 cm2 area, with the efficiency loss primarily coming from the expected loss in the lateral conduction layer that was designed to minimize fractional power loss in the PV device and to minimize insertion loss at 1.55 $\mu$m. The monolithically integrated device demonstrated the potential for data modulation up to 0.5 Mbps at 1.55 $\mu$m. Simultaneous power generation and data modulation were observed in this monolithic device, and the device has application in areas such as low size, weight, power (SWaP) satellites, debris tagging, medical applications, and high altitude, long endurance unmanned aerial systems.

The addition of lower energy-gap InGaAs quantum wells to the depletion region of GaAs solar cells is an established approach to enhance photovoltaic performance by extending infrared collection. However, maintaining a high open-circuit voltage (Voc) when including quantum wells has proven more challenging. In this article, we report on high-efficiency (η > 23.5% AM0) strain-balanced quantum well (SBQW) solar cells with increased current output and efficiency while maintaining the same high open-circuit voltage of baseline devices without quantum wells (Voc > 1.02 V). The single-junction GaAs-based device structures discussed herein employ a radiation-tolerant, n-i-p front-junction architecture and include both an InGaP heterojunction (HJ) emitter and in most cases an underlying AlGaAs distributed Bragg reflector. The impact of well number on photovoltaic device characteristics is described using an analytical model that assumes current collection, radiative recombination, and nonradiative recombination all increase with well number and are additive to the baseline cell. The highest Voc and efficiency SBQW devices employ shallow 9.2 nm In0.07Ga0.93As quantum wells and 11.5 nm GaAs0.90P0.10 barrier layers to maintain strain balancing. Increasing the indium composition in the wells from 7% to 10% to form deeper wells requires a thicker strain-balancing barrier layer and results in an apparent increase in radiative recombination and decreased Voc. Single-junction high-efficiency SBQW device performance is also demonstrated in thinner base-layer structures with graded HJs—device structures suitable for inclusion in radiation-tolerant multi-junction structures.

This work investigates the optical performance of a textured back surface reflector (BSR) produced by a maskless solution-based wet chemical etch in Al$_\text{0.3}$Ga$_\text{0.7}$As for a 1.1-$\mu$m-thick GaAs solar cell. Combining the maskless texture with the 94% reflective flat mirror resulted in a high haze in reflectance near 80% across the GaAs absorbing region. The increased diffuse reflectance from the maskless BSR provided a 19.7% increase in the integrated short-circuit current density from the base region of the 1.1-$\mu$m GaAs solar cell with the maskless BSR compared to the simulated 1.1-$\mu$m solar cell with no light trapping structures. Based on the principle concepts for the Fabry–Pérot etalon, the lifetime enhancement factor (LEF), defined as the extended photon lifetime in an optical cavity due to light trapping, was derived for the BSR devices. The LEF for the maskless BSR device was measured to be 4.3 times greater than the single-pass photon lifetime, agreeing with the high haze in reflectance and improved photoabsorption near the GaAs band edge. This value agrees with the F factor in the analytical propagation model that fits the experimental external quantum efficiency curves and justifies using the LEF to quantitatively represent the optical performance of light trapping structures in sub-$\mu$m thick solar cells.

Although methods for harvesting subbandgap solar photons have been demonstrated, present approaches still face substantial challenges. We evaluate carrier escape mechanisms in an InAs/GaAs quantum dot (QD) intermediate band photovoltaic (PV) device using photocurrent measurements under subbandgap illumination. We show that subbandgap photons can generate photocurrent through a two-photon absorption process, but that carrier trapping and retrapping limit the overall photocurrent regardless of whether the dominant carrier escape mechanism is optical, tunneling, or thermal. We introduce a new design for an InAs QD-based nanostructured material that can efficiently upconvert two low-energy photons into one high-energy photon. Efficiency is enhanced by intentionally sacrificing a small amount of photon energy to minimize radiative and nonradiative loss. Upconversion PV devices based on this approach separate the absorption of subbandgap photons from the current-harvesting junction, circumventing the carrier-trapping problems.

The effects of delta-doping InAs quantum-dot (QD)-enhanced GaAs solar cells were studied both through modeling and device experimentation. Delta doping of two, four, and eight electrons per QD, as well as nine holes per QD, was used in this study. It was observed that QD doping reduced Shockley-Read-Hallrecombination in the QDs, which results in a reduced dark current and an improved open-circuit voltage over undoped QD devices. A voltage recovery of 121 mV was observed for the eight-electron sample compared with the undoped sample. QD doping had no positive effects on subbandgap photon collection but actually degraded bulk and QD response as doping levels were increased by limiting minority carrier collection through the QD region. Despite this, an absolute AM0 efficiency improvement of 1.41% was observed for the four-electron sample over the undoped QD device while maintaining a current enhancement.

A comparison of quantum dot (QD) triple-junction solar cells (TJSCs) with the QD superlattice under tensile strain are compared with those under compressive strain and baseline devices to examine the effects of strain induced by the InAs QD layers in the middle junction. Theoretical results show samples with tensile-strained InAs QDs have lower defect formation energy while compressive-strained QDs have the greatest. Experimentally, it is found that tensile strain leads to degradation of i-region material at values of -706 ppm. Irradiating with 1-MeV electrons, TJSCs with tensile strain exhibit a faster degradation in Isc of the QD samples and slower degradation in Voc but overall faster degradation in efficiency compared with baseline TJSCs, regardless of the magnitude of tensile strain. Compressively strained QD TJSCs have similar degradation in Isc and slower degradation in Voc compared with baseline TJSCs. From this study, it is determined that a slightly compressive strain in the QD superlattice allows for the best performance pre- and postirradiation for QD TJSCs based upon AM0 IV and quantum efficiency measurements and analysis. Fabricating devices with improvements determined from samples with varying strain leads to QD TJSCs with better radiation tolerance in terms of power output for 5, 10, 15, and 20 layers of QDs.

Ten-, 20-, and 40-layer InAs/GaAs quantum-dot (QD)-embedded superlattice solar cells were compared with a baseline GaAs p-i-n solar cell. Proper strain balancing and a reduction of InAs coverage value in the superlattice region of the QD embedded devices enabled the systematic increase in short-circuit current density with QD layers (0.02-mA/cm$^2$/QD layer) with minimal open-circuit voltage loss (∼50 mV). The improvement in voltage was found to be due to a reduced nonradiative recombination resulting from a reduced density of larger defective QDs and effective strain management. The 40-layer device exceeded the baseline GaAs cell by 0.5% absolute efficiency improving efficiency relative to the baseline by 3.6%.

The operation of solar cells incorporating multiple repeat units of InAs quantum dot structures, as well as those with and without -doping of 4 and 8 electrons per quantum dot, were studied. Room temperature measurements of these samples revealed high quality devices, but insignificant differences between -doped samples and undoped samples. An IR-pumped quantum efficiency measurement was performed at 6 K to probe the extraction of quantum confined carriers through a two-photon process while shutting off phonon-assisted extraction. No two-photon signal rose above the noise, but additional sub-bandgap illuminated IV curves revealed current generation in the quantum dot devices, suggesting the dominant carrier removal mechanism is through tunneling. Finally, dark-diode data was taken and fit to determine ideality factor as a function of temperature. Control devices had an overall larger ideality, while QD devices exhibited variations as a function of temperature, which were attributed to kinetic barriers in the first QD layers, as well as possible Auger recombination at very low temperature.