The unique electronic features of highly mismatched alloys such as III-V GaNAs are suitable for the intermediate band solar cell (IBSC) application, in which an intermediate band (IB) acts as a stepping stone to generate additional photocarriers across the host semiconductor bandgap through sequential two-step below-bandgap photon absorption (TSPA). However, the collection of photocarriers in a realistic GaNAs IBSC is much lower and often accompanies S-shape kink features in the current–voltage (J–V) curves under illumination for which a coherent picture is lacking. Based on the solar cell characterization of GaNAs IBSC devices grown with and without barriers, with and without antimony, and with and without indium using molecular beam epitaxy, and also with the photocarrier collection analysis using equivalent circuit models, it was identified that the TSPA and the S-shape J–V of this system depend on two critical factors: (1) high carrier recombination currents (I0CI) across the GaNAs sub-gap between the conduction- and intermediate bands (EgCI) and (2) the counterdiode effect of the AlGaAs IB electron barrier. Dramatic improvements in the S-shape J–V feature of the solar cell characteristics were achieved when lattice-strain was compensated in GaInNAsSb epitaxial layers.
Epitaxially grown quantum well and quantum dot solar cells suffer from weak light absorption, strongly limiting their performance. Light trapping based on optical resonances is particularly relevant for such devices to increase light absorption and thereby current generation. Compared to homogeneous media, the position of the quantum layers within the device is an additional parameter that can strongly influence resonant absorption. However, this effect has so far received little attention from the photovoltaic community. We develop a theoretical framework to evaluate and optimize resonant light absorption in a thin slab with multiple quantum layers. Using numerical simulations, we show that the position of the layers can make the difference between strong absorption enhancement and completely suppressed absorption, and that an optimal position leads to a resonant absorption enhancement two times larger than average. We confirm these results experimentally by measuring the absorption enhancement from photoluminescence spectra in InAs/GaAs quantum dot samples. Overall, this work provides an additional degree of freedom to substantially improve absorption, encouraging the development of quantum wells and quantum dots-based devices such as intermediate-band solar cells.
Quantum-dot solar cells are a promising high-efficiency concept, but suffer from low absorption. Resonant light trapping can enable to absorb most of the incident light while maintaining good device quality. In this paradigm, the absorption depends critically on the vertical position of the quantum dot layers, but this has been largely ignored so far (this also applies to quantum wells). Here, we show the importance of the position of 10 InAs layers in a GaAs Fabry-Perot cavity. We then extend this approach to multi-resonant absorption, showing the potential absorption gain from optimizing the position of quantum dots in full devices.
A hot-carrier solar cell (HCSC) is a high-efficiency photovoltaic concept where electrons and holes are at a higher temperature than the lattice, allowing an additional thermoelectric energy conversion. There are two requirements for a HCSC: establishing a hot-carrier population and converting the temperature into extra voltage through energy-selective contacts. We focus on the generation of hot carriers, and the design of absorbers that can make this generation easier. Fundamentally, this requires to increase the power density absorbed per volume unit, so the photocarriers cannot fully thermalize (phonon bottleneck). Beyond simply increasing the light intensity, the main control knobs to favor hot carriers include reducing the thickness of the absorber, increasing its absorptivity, and reducing its bandgap. In this proceeding, we report the fabrication of structures that aim at measuring the influence of these different parameters. We justify our choices for sample structure and fabrication method from the need for high thermal conductivity, in order to prevent lattice heating. We characterize our structures in order to determine precisely the final thickness of all layers, and the absorptivity of the absorber layer. These samples are to be used for an analysis of the temperature with many variable parameters, in order to better understand the thermalization mechanisms and design better absorbers. Ultimately, our objective is to implement all solutions together in order to evidence a hot carrier population under concentrated sunlight illumination.
Hot-carrier solar cells (HCSC) can potentially overcome the Shockley-Queisser limit, by having carriers at a higher temperature than the lattice. To this end, the carriers need to thermalize slower than power is generated by absorbing photons. In thin films, a hot-carrier distribution can only be achieved with very high incident power, by saturating the thermalization channels. Ultra-thin absorbers have a smaller thermalization rate, due to fewer channels. However, they typically absorb only a limited amount of light, which prevents them from reaching high efficiencies. Light trapping is an excellent way to increase significantly the amount of light absorbed in an ultra-thin material. Yet, studies on the coupling between light trapping and hot carriers are still lacking, due to the complexity of the whole system. We analyze numerically and experimentally how light trapping can enable high-efficiency HCSC. This manuscript presents the progress towards the experimental demonstration of the enhancement of the hot-carrier effect with light trapping. 280 nm-thick devices have successfully been reported on a gold mirror using epitaxial lift-off (ELO) and gold-gold bonding. These devices have been characterized by photoluminescence spectroscopy. Hot carriers with a temperature 37 K above lattice temperature were measured, in accordance with theoretical predictions. We are now working towards the ELO of absorbers 10 times thinner, on which we will implement light trapping to increase the carrier temperature.
In0.227GaAs/GaAsN0.011 was introduced as a 1.2-eV multiple quantum well (MQW) with a flat conduction band (FCB) in which a conduction band edge of GaAsN was adjusted to be equal to that of InGaAs. This MQW was established as a candidate material for a middle absorption layer of three-junction solar cell since electron confinement was eliminated and a short electron lifetime in GaNAs was compensated by InGaAs layer. The band alignment of MQW was characterized by the power-dependent photoluminescence (PL) measurement under low temperature. According to the band-anti crossing model, the FCB is possibly constructed since a small amount of incorporated N can drastically reduce the energy of the conduction band edge of GaAsN. The PL results demonstrated that In0.227GaAs/GaAsN MQW was a type-I structure when N content was below 1.1%, and became a type-II structure when N content was above 1.1%. The type-II MQW was characterized by the observation of blueshift of PL peak when increasing excitation power. This blueshift is a result of band-bending effect due to the accumulation of excited carriers at the interface between two materials, which is unique for the type-II MQW. In addition, it was observed that the activation energy estimated from the Arrhenius plot provided a minimum value in the structure with 1.1%N; the lowest activation energy indicated the weakest confinement energy of carriers in the structure. These results approved that a transition from type-I to type-II occurred when N content surpassed 1.1%, and our designed In0.227GaAs/GaAsN0.011 MQW was potentially the FCB structure.
For intermediate band solar cells, the control of the carrier filling ratio in intermediate band is important to achieve high efficiency. We have investigated the effect of carrier doping of InAs/GaAs quantum dots (QDs) with Si and sunlight concentration on the quantum dots solar cell (QDSC) characteristics. The prefilling by Si doping of InAs/GaAs QDs was performed using two methods: modulation or δ-doping and direct doping. A gradual recovery in the open-circuit voltage with increasing Si doping concentration was observed, and it suggested a decrease of recombination via Si-doped QD states. Under high-concentrated sunlight illumination, QD states were additionally filled with photocarriers, and the open-circuit voltage increased nonlinearly with concentration ratio in both the nondoped and Si-doped QDSCs.
The subband features E‒ and E+ for the conduction band of III-V dilute nitride alloys make them promising for intermediate band solar cell application. However, presence of bandgap states can limit the two-step photon absorption activity, a necessary requirement for IBSC functionality. A model analysis is performed to characterize the density of states. The sub-band tails states are characterized using a temperature-dependent map of photo-modulated reflectance spectroscopy for GaNAs thin films grown on GaAs substrates using molecular beam epitaxy. The effect of indium and antimony incorporation on the subband features were investigated. Marked improvements in the thin films were observed both for the lower (E‒) and the upper (E+) conduction bands (CB) when In was introduced with marginal enhancement by Sb. These improvements are associated with suppression of tail states below both the E‒ and E+ bands. Sb rather mainly plays a surfactant role improving the abruptness of the GaNAs/GaAs hetero-interface.
Double resonant tunneling barriers are considered for an application as energy selective contacts in hot carrier solar cells. Experimental symmetric and asymmetric double resonant tunneling barriers are realized by molecular beam epitaxy and characterized by temperature dependent current-voltage measurements. The negative differential resistance signal is enhanced for asymmetric heterostructures, and remains unchanged between low- and room-temperatures. Within Tsu-Esaki description of the tunnel current, this observation can be explained by the voltage dependence of the tunnel transmission amplitude, which presents a resonance under finite bias for asymmetric structures. This effect is notably discussed with respect to series resistance. Different parameters related to the electronic transmission of the structure and the influence of these parameters on the current voltage characteristic are investigated, bringing insights on critical processes to optimize in double resonant tunneling barriers applied to hot carrier solar cells.
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