Optimizing the efficiency of optoelectronic devices is challenging at low currents, even with high-quality materials, due to the dominance of non-radiative Shockley–Read–Hall recombination at low carrier densities. In this study, we nearly eliminate the typical non-radiative recombination current in a GaAs/GaInP double-heterojunction light-emitting diode (LED) by shifting the pn-junction 200 nm into the GaInP barrier layer on the n-side. This involves reducing the doping in the n-barrier to below the background p-type doping level to relocate the built-in electric field. As a result, the space charge recombination current with the ideality factor of two is strongly suppressed and remains concealed in our experimental dark current density–voltage measurements. The experimental results, coupled with our physics-based model, indicate the potential for considerable efficiency gains at current densities below ∼ 1 A/cm2. The findings prompt to carefully optimize the doping profiles of high efficiency LEDs and to reconsider the validity of using dark saturation currents as a metric for their performance.

Alternative types of artificial cooling techniques are of large interest for multiple applications. Here, we develop a framework for studying the role of electro-optical coupling in the analysis of solid-state refrigerators based on electroluminescent cooling (ELC) by combining device measurements with optical simulations. The studied device consists of a light-emitting diode (LED) epitaxially connected to a photodetector (PD) in a double-diode structure (DDS). Previous results of the DDS have indicated that the LED side already operates at conditions corresponding to ELC, but Ohmic losses and imperfect photodetection of the LED light in the PD have prevented observing the effect directly. Here, to break down the detection losses of the DDS, we report on the electro-optical response of the LED and the PD in detail, as well as the role of the spectral coupling from the LED to the PD. We present a detailed framework for combining measurements and simulations of the DDS to gain quantitative insight of the electro-optical response of the LED and PD, as well as the coupling between them, including the analysis of effects that are not directly accessible by standard measurements. The developed approach allows identifying the different photodetection loss mechanisms from the current-voltage and electroluminescence measurements and thereby gives guidance for designs toward a direct demonstration of ELC at practically relevant cooling powers. Somewhat surprisingly, the results show that an imperfect spectral absorption efficiency of the PD, in addition to its below unity quantum efficiency, are together required to explain the previously observed low photodetection efficiency of the DDS even for several microns thick PD structures. In comparison, the LED top mirror introduces only a minuscule drop in photodetection efficiency. Put in plain numbers, our analysis reveals that in the current DDS designs, there is headroom by 14% in the spectral matching between the LED and the PD, 5% in the charge collection efficiency of the PD, and 4% in the efficiency at which photons emitted from the LED reach the PD.

Thermophotovoltaic (TPV) power generators offer great possibilities for thermal energy conversion when thermal sources with temperatures nearing or exceeding 1000 K are available. While the power density of conventional TPV systems is generally determined by Planck's law in the far field, their fundamental performance is known to be dramatically affected by near field effects between the thermal emitter and the photovoltaic cell. Another potentially disruptive enhancement to the performance may be reached by transforming the thermal emitter to exploit electroluminescence. Taking advantage of an electroluminescent emitter as the source of radiation fundamentally alters the thermodynamics of the system. This allows boosting the achievable power densities by orders of magnitude, and also provides access to electroluminescent coolers, thermophotonic (TPX) heat pumps, and TPX power generation devices that can outperform both TPV and thermoelectric heat engines, especially at the low-grade waste heat (LGWH) temperature range (300- 500 K) containing in total the majority of recoverable energy. In reality, functional TPX devices are yet to be demonstrated experimentally, due to several material and design bottlenecks. Here, we discuss the thermodynamics, ideal characteristics and advantages of TPX heat engines, and quantify how non-idealities such as non-radiative recombination, optical and resistive losses affect their performance. Our results suggest that, at LGWH temperatures, TPX heat engines start to outperform the best TPV systems when reaching quantum efficiencies of the order of 90%; beyond this threshold, TPX systems become increasingly efficient and powerful.

Conventional photovoltaic devices are currently made from relatively thick semiconductor layers, ~150 µm for silicon and 2–4 µm for Cu(In,Ga)(S,Se)2, CdTe or III–V direct bandgap semiconductors. Ultrathin solar cells using 10 times thinner absorbers could lead to considerable savings in material and processing time. Theoretical models suggest that light trapping can compensate for the reduced single-pass absorption, but optical and electrical losses have greatly limited the performances of previous attempts. Here, we propose a strategy based on multi-resonant absorption in planar active layers, and we report a 205-nm-thick GaAs solar cell with a certified efficiency of 19.9%. It uses a nanostructured silver back mirror fabricated by soft nanoimprint lithography. Broadband light trapping is achieved with multiple overlapping resonances induced by the grating and identified as Fabry–Perot and guided-mode resonances. A comprehensive optical and electrical analysis of the complete solar cell architecture provides a pathway for further improvements and shows that 25% efficiency is a realistic short-term target. Ultrathin solar cells having thicknesses below 1 µm can still reach efficiencies comparable to their thicker counterparts, but require less material to manufacture. By exploiting light-trapping nanostructures, Chen and colleagues achieve GaAs solar cells with 20% efficiency at just 205 nm thicknesses.

The intermediate-band assisted hot-carrier solar cell (IB-HCSC) concept has been proposed in order to assist the extraction of hot carriers from an absorber that has an intermediate band (IB). In this study, we used a heterostructure based on ten layers of In(Ga)As quantum dots (QDs) embedded in an Al0.2Ga0.8As single-junction solar cell designed for an intermediate band solar cell (IBSC). QD-IBSCs have proven to be limited by the thermal escape of photo-carriers from QDs at room temperature. In such cases, fundamental improvements in conversion efficiency are only possible if carriers in the IB are not in thermal equilibrium with either conduction or valence bands. Additionally, the IB-HCSC concept provides a high-efficiency limit and enables us to work with all relaxation mechanisms (thermalization, carrier-carrier scattering, and thermal radiation). Under high irradiation, we confirmed the emergence of a hot carrier population in the QDs, limited mostly by thermionic emission, which assists the IBSC by providing a thermoelectric gain in voltage without hindering the possibility of sequential two-photon absorption. Absolute intensity calibrated photoluminescence spectroscopy indicated that the triggering mechanism happens when the QD ensemble is estimated to have a high carrier concentration that behaves as a metal-like IB. Experimental results suggested that the hot carrier effect also occurred in other solar cells based on quantum heterostructures and directions for improvements of hot-carrier assisted QD-IBSCs are proposed for further efficiency gain in optimized device architectures.

This work focuses on the characterization of GaInNAsSb solar cells whose substrates are removed via the epitaxial lift-off (ELO) technique. As a result of the substrate removal, increases in the photocurrent and the interference feature were clearly observed. This is clear evidence of the light-confinement effect, whereby some of the unabsorbed photons at the rear metal contact were reflected back towards the front side of the ELO thin-film cell. We successfully demonstrated that the ELO technique can be applied for the GaInNAsSb cell, and the light management should add flexibility in designing the cell structures. (C) 2018 The Japan Society of Applied Physics

Photovoltaic generation has stepped up within the last decade from outsider status to one of the important contributors of the ongoing energy transition, with about 1.7% of world electricity provided by solar cells. Progress in materials and production processes has played an important part in this development. Yet, there are many challenges before photovoltaics could provide clean, abundant, and cheap energy. Here, we review this research direction, with a focus on the results obtained within a Japan-French cooperation program, NextPV, working on promising solar cell technologies. The cooperation was focused on efficient photovoltaic devices, such as multijunction, ultrathin, intermediate band, and hot-carrier solar cells, and on printable solar cell materials such as colloidal quantum dots.

We review and propose light trapping strategies in ultra-thin solar cell. We demonstrate JSC=24.8 mA/cm2 in 205 nm-thick GaAs solar cell using a nanostructured TiO2/Ag back mirror fabricated by Nanoimprint lithography.

We have designed and fabricated ultrathin (200nm) GaAs solar cells. Multi-resonant light trapping is achieved with a nanostructured TiO2/Ag back mirror and the short-circuit current predicted by numerical calculations is Jsc= 25 mA/cm2. We have investigated the physical origin of the resonances using analytical models that can be used to optimize the geometry of nanostructured mirrors suited for ultrathin solar cells. For the fabricated solar cell, the nanostructured TiO2/Ag mirror is combined with localized ohmic contacts. The solar cells are patterned by Soft Nanoimprint Lithography and transferred on a glass substrate. A similar solar cell with Ag flat mirror was also fabricated. We have measured above 60% enhancement on short-circuit current compared to as-grown reference cell.

In this work we study the carrier population dynamics in an AlGaAs p-i-n single junction solar cell with 10 layers of InAs quantum dots (QDs) in the i-region. Light management and band engineering technics were used to optimize intermediate band solar cells (IBSCs) operation. The IBSC operations are investigated through quantitative luminescence based characterization methods based on photoluminescence spectra recorded in absolute values and two-color excitation. The results show that the sample is compatible with the IBSC operations at room temperature.

We study light management in a 430 nm-thick GaAs p-i-n single junction solar cell with 10 pairs of InGaAs/GaAsP multiple quantum wells (MQWs). The epitaxial layer transfer on a gold mirror improves light absorption and increases the external quantum efficiency below GaAs bandgap by a factor of four through the excitation of Fabry-Perot resonances. We show a good agreement with optical simulation and achieve around 10% conversion efficiency. We demonstrate numerically that this promising result can be further improved by anti-reflection layers. This study paves the way to very thin MQWs solar cells.

We report on the fabrication and characterization of ultrathin GaAs solar cells with a silver back mirror and absorber thicknesses of only t = 120 nm and t = 220 nm. The silver back mirror is combined with localized ohmic contacts. Without antireflection coating, Fabry-Perot resonances lead to strong enhancement over single-pass absorption (up to 4), and external quantum efficiency reaches 0.8 at resonance wavelengths. An analytical model is used to determine the resonance wavelengths and the absorption maxima. A short-circuit improvement of 27% results from the enhanced absorption induced by the Fabry-Perot resonances. By implementing an additional antireflection coating, short-circuit currents reach 16.3 mA/cm2 fort = 120 nm and 20.7 mA/cm2 for t = 220 nm, corresponding to efficiencies of 8.7 % and 12.9 %, respectively.

We study light management in a 430 nm-thick GaAs p-i-n single junction solar cell with 10 InGaAs/GaAsP multiple quantum wells (MQWs). The epitaxial layer transfer on a gold mirror improves light absorption and increases the quantum efficiency in the MQW region by a factor of 4 through the excitation of Fabry-Perot resonances. We show a good agreement with optical simulation and achieve around 9% efficiency. This promising result can be further improved by antireflection layers and additional light-trapping solutions.