Pierre Rale

Monolithic Integration of Diluted-Nitride III–V-N Compounds on Silicon Substrates: Toward the III–V/Si Concentrated Photovoltaics

Abstract GaAsPN semiconductors are promising material for the development of high-efficiency tandem solar cells on silicon substrates. GaAsPN diluted-nitride alloy is studied as the top-junction material due to its perfect lattice matching with the Si substrate and its ideal bandgap energy allowing a perfect current matching with the Si bottom cell. The GaP/Si interface is also studied in order to obtain defect-free GaP/Si pseudo-substrates suitable for the subsequent GaAsPN top junctions growth. Result shows that a double-step growth procedure suppresses most of the microtwins and a bi-stepped Si buffer can be grown, suitable to reduce the anti-phase domains density. We also review our recent progress in materials development of the GaAsPN alloy and our recent studies of all the different building blocks toward the development of a PIN solar cell. GaAsPN alloy with energy bandgap around 1.8 eV, lattice matched with the Si substrate, has been achieved. This alloy displays efficient photoluminescence at room temperature and good light absorption. An early-stage GaAsPN PIN solar cell prototype has been grown on a GaP(001) substrate. The external quantum efficiency and the I–V curve show that carriers have been extracted from the GaAsPN alloy absorber, with an open-circuit voltage above 1 eV, however a low short-circuit current density obtained suggests that GaAsPN structural properties need further optimization. Considering all the pathways for improvement, the 2.25% efficiency and IQE around 35% obtained under AM1.5G is however promising, therefore validating our approach for obtaining a lattice-matched dual-junction solar cell on silicon substrate.

Material challenges for solar cells in the twenty-first century: directions in emerging technologies

Abstract 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.

Quantitative optical measurement of chemical potentials in intermediate band solar cells

Abstract. Having shown some demonstration of two photon absorption and multiband emission processes in quantum dots (QD), multiquantum wells (MQW), and highly mismatched alloys, intermediate band solar cells are currently the subject of numerous studies. To better understand the underlying mechanisms, our objective is to experimentally probe the multiband operation of this device. We used photoluminescence recorded with a calibrated hyperspectral imager which provides spectrally resolved images with a spatial resolution of 2  μm and spectral resolution of 2 nm on proof of concept QD and MQW solar cells samples. Device emission can be described with the generalized Planck’s law from which the quasi-Fermi level splitting of the three bands can be determined. The advantage of the technique is that it can be used to investigate the intermediate band material without the need to make contacts or a full device structure. We also discuss the usefulness of a dual-beam method.

Multijunction photovoltavics: integrating III–V semiconductor heterostructures on silicon

Gallium arsenide phosphide nitride shows promise for developing highefficiency tandem solar cells on low-cost silicon substrates

GaAsPN-based PIN solar cells MBE-grown on GaP substrates: toward the III-V/Si tandem solar cell.

GaAsPN semiconductors are promising material for the elaboration of high efficiencies tandem solar cells on silicon substrates. GaAsPN diluted nitride alloy is studied as the top junction material due to its perfect lattice matching with the Si substrate and its ideal bandgap energy allowing a perfect current matching with the Si bottom cell. We review our recent progress in materials development of the GaAsPN alloy and our recent studies of some of the different building blocks toward the elaboration of a PIN solar cell. A lattice matched (with a GaP(001) substrate, as a first step toward the elaboration on a Si substrate) 1μm-thick GaAsPN alloy has been grown by MBE. After a post-growth annealing step, this alloy displays a strong absorption around 1.8-1.9 eV, and efficient photoluminescence at room temperature suitable for the elaboration of the targeted solar cell top junction. Early stage GaAsPN PIN solar cells prototypes have been grown on GaP (001) substrates, with 2 different absorber thicknesses (1μm and 0.3μm). The external quantum efficiencies and the I-V curves show that carriers have been extracted from the GaAsPN alloy absorbers, with an open-circuit voltage of 1.18 V, while displaying low short circuit currents meaning that the GaAsPN structural properties needs a further optimization. A better carrier extraction has been observed with the absorber displaying the smallest thickness, which is coherent with a low carriers diffusion length in our GaAsPN compound. Considering all the pathways for improvement, the efficiency obtained under AM1.5G is however promising.

In situ passivation of GaAsP nanowires

We report on the structural and optical properties of GaAsP nanowires (NWs) grown by molecular-beam epitaxy. By adjusting the alloy composition in the NWs, the transition energy was tuned to the optimal value required for tandem III-V/silicon solar cells. We discovered that an unintentional shell was also formed during the GaAsP NW growth. The NW surface was passivated by an in situ deposition of a radial Ga(As)P shell. Different shell compositions and thicknesses were investigated. We demonstrate that the optimal passivation conditions for GaAsP NWs (with a gap of 1.78 eV) are obtained with a 5 nm thick GaP shell. This passivation enhances the luminescence intensity of the NWs by 2 orders of magnitude and yields a longer luminescence decay. The luminescence dynamics changes from single exponential decay with a 4 ps characteristic time in non-passivated NWs to a bi-exponential decay with characteristic times of 85 and 540 ps in NWs with GaP shell passivation.

Determination of n-Type Doping Level in Single GaAs Nanowires by Cathodoluminescence

We present an effective method of determining the doping level in n-type III-V semiconductors at the nanoscale. Low-temperature and room-temperature cathodoluminescence (CL) measurements are carried out on single Si-doped GaAs nanowires. The spectral shift to higher energy (Burstein-Moss shift) and the broadening of luminescence spectra are signatures of increased electron densities. They are compared to the CL spectra of calibrated Si-doped GaAs layers, whose doping levels are determined by Hall measurements. We apply the generalized Plancks law to fit the whole spectra, taking into account the electron occupation in the conduction band, the bandgap narrowing, and band tails. The electron Fermi levels are used to determine the free electron concentrations, and we infer nanowire doping of 6 × 1017 to 1 × 1018 cm-3. These results show that cathodoluminescence provides a robust way to probe carrier concentrations in semiconductors with the possibility of mapping spatial inhomogeneities at the nanoscale.

Direct imaging of quasi Fermi level splitting in photovoltaic absorbers

We analyze photoluminescence (PL) and electroluminescence (EL) using a hyperspectral imager that records spectrally resolved luminescence images of solar cell absorbers. The system is calibrated allowing us to measure the luminescence flux in absolute values. This system gives the opportunity to quantitatively image physical parameters such as the voltage with an uncertainty of less than 30mV. The wide field illumination, low power excitation and fast acquisition brings new insights compare to classical setup such as confocal microscope. Several types of absorbers have been analyzed. For instance, we can investigate spatial fluctuations of the Quasi Fermi Levels splitting in CIGS polycristallines absorbers and link those fluctuations to transport properties. The method is general to the point that third generation PV cells absorbers can also be evaluated. We illustrate the great potential of our setup by imaging carriers Temperature in Hot Carriers Solar cells absorbers and quasi Fermi levels splitting in Intermediate Band Solar cells.

Nanoscale analyses of axial and radial III-V nanowires for solar cells (Conference Presentation)

The record in photovoltaic conversion efficiency is detained by multi-junction solar cells based on III-V semiconductors. However, the wide adoption of these devices is hindered by their high production cost, to a large extent due to the expensive III-V substrates. As an alternative, a hybrid geometry has been proposed [LaPierre JAP 2011], which combines a 2D Si bottom cell with a III-V nanowire top cell in a tandem device. This approach, which may reach theoretical efficiencies of approx. 34%, requires smaller amounts of expensive III-V materials compared to conventional III-V tandem cells and benefits from the nanowire light trapping effects. In this work, we report the fabrication and nanoscale characterization of two types of nanostructures for solar cells: radial GaAlAs and axial GaAsP p-n junction nanowires. Nanowires are grown by gallium-assisted molecular beam epitaxy using Be and Si as doping sources. The composition (probed by EDX and cathodoluminescence) was adjusted to tune the bandgap toward the optimal value for a III-V-on-Si tandem cell (approx. 1.7 eV). Local I-V characteristics and electron beam induced current (EBIC) microscopy under different biases are used to probe the electrical properties and the generation pattern of individual nanowires. For radial junction nanowires, EBIC mappings revealed a homogeneous collection of carriers on the entire nanowire length. For axial junction nanowires, the doping concentrations and the minority carrier diffusion lengths were extracted from the EBIC generation profiles. The effect of an epitaxial GaP passivating shell on the optical and generation properties was assessed.

Luminescence methodology to determine grain-boundary, grain-interior, and surface recombination in thin-film solar cells

We determine the grain-boundary (GB) recombination velocity, S G B, and grain-interior (GI) lifetime, τ G I, parameters in superstrate CdS/CdTe thin-film solar cell technology by combining cathodoluminescence (CL) spectrum imaging and time-resolved photoluminescence (TRPL) measurements. We consider critical device formation stages, including after CdTe deposition, CdCl2 treatment, and Cu diffusion. CL image analysis methods extract GB and GI intensities and grain size for hundreds of grains per sample. Concurrently, a three-dimensional CL model is developed to simulate the GI intensity as a function of τ G I, S G B, grain size, and the surface recombination velocity, S surf. TRPL measurements provide an estimate of S surf for the CL model. A fit of GI intensity vs. grain size data with the CL model gives a self-consistent and representative set of S G B and τ G I values for the samples: S G B ( τ G I ) = 2.6  × 106 cm/s (68–250 ps), S G B ( τ G I ) = 4.1  × 105 cm/s (1.5–3.3 ns), and S G B ( τ G I ) = 5.5  × 105 cm/s (1.0–3.8 ns) for as-deposited, CdCl2-treated, and CdCl2- and Cu-treated samples, respectively. Thus, we find that the CdCl2 treatment both helps to passivate GBs and significantly increase the GI lifetime. Subsequent Cu diffusion increases GB recombination slightly and has nuanced effects on the GI lifetime. Finally, as a partial check on the S G B and τ G I values, they are input to a Sentaurus device model, and the simulated performance is compared to the measured performance. The methodology developed here can be applied broadly to CdTe and CdSeTe thin-film technology and to other thin-film solar cell materials including Cu(In1-xGax)Se2, Cu2ZnSnS4, and perovskites.We determine the grain-boundary (GB) recombination velocity, S G B, and grain-interior (GI) lifetime, τ G I, parameters in superstrate CdS/CdTe thin-film solar cell technology by combining cathodoluminescence (CL) spectrum imaging and time-resolved photoluminescence (TRPL) measurements. We consider critical device formation stages, including after CdTe deposition, CdCl2 treatment, and Cu diffusion. CL image analysis methods extract GB and GI intensities and grain size for hundreds of grains per sample. Concurrently, a three-dimensional CL model is developed to simulate the GI intensity as a function of τ G I, S G B, grain size, and the surface recombination velocity, S surf. TRPL measurements provide an estimate of S surf for the CL model. A fit of GI intensity vs. grain size data with the CL model gives a self-consistent and representative set of S G B and τ G I values for the samples: S G B ( τ G I ) = 2.6  × 106 cm/s (68–250 ps), S G B ( τ G I ) ...