In this work, nanoscale electrical and optical properties of n-GaN nanowires (NWs) containing GaN/AlN multiple quantum discs (MQDs) grown by molecular beam epitaxy are investigated by means of single wire I(V) measurements, electron beam induced current microscopy (EBIC) and cathodoluminescence (CL) analysis. A strong impact of non-intentional AlN and GaN shells on the electrical resistance of individual NWs is put in evidence. The EBIC mappings reveal the presence of two regions with internal electric fields oriented in opposite directions: one in the MQDs region and the other in the adjacent bottom GaN segment. These fields are found to coexist under zero bias, while under an external bias either one or the other dominates the current collection. In this way EBIC maps allow us to locate the current generation within the wire under different bias conditions and to give the first direct evidence of carrier collection from AlN/GaN MQDs. The NWs have been further investigated by photoluminescence and CL analyses at low temperature. CL mappings show that the near band edge emission of GaN from the bottom part of the NW is blue-shifted due to the presence of the radial shell. In addition, it is observed that CL intensity drops in the central part of the NWs. Comparing the CL and EBIC maps, this decrease of the luminescence intensity is attributed to an efficient charge splitting effect due to the electric fields in the MQDs region and in the GaN base.

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.

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 determine the grain-boundary (GB) recombination velocity, SGB, and grain-interior (GI) lifetime, τGI, 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 τGI, SGB, grain size, and the surface recombination velocity, Ssurf. TRPL measurements provide an estimate of Ssurf 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 SGB and τGI values for the samples: SGB(τGI)= 2.6 × 106 cm/s (68–250 ps), SGB(τGI)= 4.1 × 105 cm/s (1.5–3.3 ns), and SGB(τGI)= 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 SGB and τGI 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.

Semiconductor nanostructures open new perspectives for light trapping, lattice-mismatched crystal growth (III-V on Si for instance) and novel transport phenomenon for next generation photovoltaics. The characterization of material properties at the nanoscale remains challenging. In particular, doping is a key parameter in the design and fabrication of solar cells. Here, we show that cathodoluminescence mapping can be used to determine both n-type and p-type doping levels of GaAs with nanometer resolution. n-type semiconductor shows characteristic blueshift emission from the electron filling, while ptype semiconductor exhibits redshift emission due to dominant bandgap narrowing at high concentrations. The generalized Planck's law is used to fit the whole spectra and extract electron Fermi levels (n-type) and effective bandgap (p-type). Quantitative doping assessment is achieved by systematic spectral analysis, and is demonstrated on planar GaAs layers, and on single GaAs nanowires. This method can be extended to other semiconductors and nanostructures.

In this work, we combine quantitative cathodoluminescence (CL) with time-resolved photoluminescence (TRPL) and numerical simulations to determine grain-boundary, grain-interior, and surface recombination parameters in standard CdTe thin films. CL intensities from thousands of grains are analyzed to accumulate statistics and chart variations with grain size. Grain-boundary contrast results for small grains indicate that the grain-boundary recombination velocity, S GB , decreases significantly with CdCl 2 treatment, but S GB is increased by subsequent Cu-diffusion. Furthermore, within a given sample, data suggests that S GB is nearly independent of grain size. The back-surface recombination velocity, S, is extracted from TRPL measurements incident on the back surface, and CL profiles are simulated to determine the grain-interior lifetime, τ GI . Finally, CL intensity vs. grain size relationships are simulated to check for self-consistency of the S GB , S, and τ GI values.

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 Planck's 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.

We present a new method to determine the doping level of n-type semiconductors at the nanoscale. Lowtemperature and room-temperature cathodoluminescence (CL) measurements are carried out on single Si-doped GaAs nanowires. The spectral shift and the broadening of luminescence spectra are a signature of an increased density of electrons. They are compared to CL spectra of well-calibrated planar Si-doped GaAs layers whose doping levels are determined by Hall measurements and compared to yrevious experimental studies. We infer a n-type doping of 1×10 10 cm-3 to 2×10 18 cm -3 , with a high spatial homogeneity along the nanowire. These results show that cathodoluminescence provides an alternative way to probe carrier concentration in nanostructured and polycrystalline semiconductors, and to map the spatial inhomogeneity of dopants.

With 25.6% conversion efficiency, solar cells based on the c-Si generation reach their limits. However, the third generation solar cells, such as multijunction solar cells and hot-carrier solar cells (HCSCs), allow to overcome the intrinsic limit of a single junction. Firstly, we show the first stage developments of a GaAsPN/Si tandem cell on Si, which benefit from the low cost and technological maturity of Si cells. The GaAsPN dilute-nitride compound, grown by MBE, is quasi lattice-matched with Si, and displays the required the required 1.7 eV pseudo-bandgap. Secondly, HCSCs aim to reduce the thermalization process which is a major loss in a classical PV cells. This can be done by the reduction of the electron-phonons interactions in quantum wells (QWs) structures. Therefore, we have investigated a InGaAsP multi-QWs heterostructure epitaxially grown on a InP(001) substrate and demonstrate its potential to work as a hot carrier cell absorber. Finally, some outstanding properties of the hybrid organic perovskites, a very new technology which has quickly reached conversion efficiency larger than 20%, will be presented, along with results from a collaboration between FOTON laboratory, Los Alamos National Laboratory, Rutgers University New Jersey, Purdue university, Brookhaven National Laboratory and Chemical Sciences Institute of Rennes.

The development of a III‐V on silicon tandem cell is reported. GaAsPN material is considered for top cell absorber because of its quasi‐lattice matching with silicon and its pseudo‐direct bandgap at 1.7 eV. First, GaAsPN single cells are grown on both GaP and Si substrates, and their electrical performances are compared. It is found that the performances of the cell are limited by the low carrier mobility induced by the use of dilute nitrides. Then the first‐stage development of a tandem photovoltaic solar cell is reported, including a tunnel junction (TJ) made of Si.

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.

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 mu m 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. (C) 2015 Society of Photo-Optical Instrumentation Engineers (SPIE)

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