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The integration of III-V multi-junction solar cells on Si substrates is currently one of the most promising possibilities to combine high photovoltaic performance with a reduction of the manufacturing costs. In this work, we propose a prospective study for the realization of an InP/InGaAs tandem solar cell lattice-matched to InP on a commercially available Si template by direct MOVPE growth. The InP top cell and the InGaAs bottom cell were firstly separately grown and optimized using InP substrates, which exhibited conversion efficiencies of 13.5% and 11.4%, respectively. The two devices were then combined in a tandem device by introducing an intermediate InP/AlInAs lattice-matched tunnel junction, showing an efficiency of 18.4%. As an intermediate step towards the realization of the tandem device on Si, the InP and InGaAs single junction solar cells were grown on top of a commercial InP/GaP/Si template. This transitional stage enabled to isolate and evaluate the effects of the growth of III-V on Si on the photovoltaic performance through the comparison with the aforementioned devices on InP. Each cell was electrically characterized by external quantum efficiency and dark and illuminated current-voltage under solar simulator. The material quality was also analyzed by means of X-ray diffraction, Atomic-Force Microscopy, Transmission Electron and Scanning Electron Microscopy. The III-V on Si devices showed efficiencies of 3.6% and 2.0% for the InP and InGaAs solar cells, respectively.

In this study we analysed halide perovskite films deposited directly on crystalline silicon by means of two set-ups using different operating modes of the surface photovoltage (SPV) methods, i.e., the Kelvin probe force microscopy (KPFM) and the metal-insulator-semiconductor (MIS) technique. The KPFM allowed to visualize surface potential distribution on a microscale while MIS technique allowed to study SPV spectral dependence. We studied wavelength dependent SPV of these samples, which allowed us to effectively vary the probe depth in the sample and discern the contribution from each interface to the overall effect measured under white light illumination. Depending on where the photocarriers are generated, different SPV signals are observed: at the perovskite/Si interface, the signal depends on Si doping type, while at the surface the SPV is always negative indicating downward surface band bending. This is confirmed by analysing SPV phase measured in the AC MIS mode. In addition, distinction between slow and fast processes contributing to measured SPV was possible. It has been observed, that with decreasing the illumination wavelength, the processes causing SPV become slower, which can indicate that high energy photons not only generate electronic photocarriers but can also induce chemical changes with creation of defects or ionic species that also modify the measured SPV.

Electroluminescence allows rapid characterization of an entire photovoltaic solar cell and visualization of defects at the micrometer scale. Here we focus on the optoelectronic properties of silicon interdigitated back contact cells characterized by electroluminescence. The spatially resolved electroluminescence helps us control the quality of interdigitated back contact structures used in silicon bottom subcells in a three-terminal tandem perovskite on silicon solar cell. Local variations in minority carrier diffusion length, surface recombination velocity and, the impact of resistive and optical losses were analyzed by electroluminescence mapping. In addition, we quantify the radiative saturation current density and the radiative open circuit voltage using the electroluminescence spectrum of the cell. This step allows us to accurately assess the performance limits induced in the device due to the non-radiative recombination.

Solar cells are complex devices, being constituted of many layers and interfaces. The study and the comprehension of the mechanisms that take place at the interfaces is crucial for efficiency improvement. This paper applies Kelvin probe force microscopy (KPFM) to study materials and interfaces with nanometer scale imaging of the surface potential in the dark and under illumination. KPFM measurements are highly sensitive to surface states and to the experimental measurement environment influencing the atomic probe operating conditions. Therefore, in order to develop a quantitative understanding of KPFM measurements, we have prepared a dedicated structured sample with alternating layers of InP:S and InP:Fe whose doping densities were determined by secondary-ion mass spectroscopy. We have performed KPFM measurements and shown that we can spatially resolve 20 nm thick InP layers, notably when performed under illumination which is well-known to reduce the surface band-bending.

Epitaxial silicon layers were grown on highly doped c-Si substrates using the plasma-enhanced chemical vapour deposition process (PECVD) at low temperature (175 °C). The transport and defect-related properties of these epi-Si layers were characterized by current density-voltage (J–V) and capacitance–voltage (C–V) techniques. The results show that the epi-Si layers exhibit a non-intentional n-type doping with a low apparent doping density of about 2 × 1015 cm−3. The admittance spectroscopy technique is used to investigate the presence of deep-level defects in the structure. An energy level at 0.2 eV below the conduction band has been found with a density in the range of 1015 cm−3 which may explain the observed apparent doping profile.

Dilute nitrides lattice-matched to GaP were studied to explore the possibilities to improve their properties by additional indium or arsenic content in the GaPN alloy for further utilization in solar cells. Admittance spectroscopy shows that intrinsic layers of GaPNAs and InP/GaPN grown by molecular-beam epitaxy have unintentional background silicon donor doping. Deep-level transient spectroscopy allowed us to reveal several defect levels. In GaPNAs, two defect levels were detected at Ec − 0.58 eV and Ev + 0.44 eV, with respective concentrations of 4 × 1015 cm−3 and 2 × 1015 cm−3. After thermal annealing, these could be reduced by a factor of two and by more than one order of magnitude, respectively, leading to an increase of external quantum efficiency and open-circuit voltage of solar cells. The InP/GaPN layer exhibits a defect level at Ec − 0.44 eV (with a concentration of 2 × 1014 cm−3), which is of similar nature as the one at Ec − 0.58 eV in GaPNAs. Furthermore, unlike in GaPNAs, defect levels close to midgap were also detected in the InP/GaPN layer. These non-radiative recombination centers lead to poorer photoelectric properties of solar cells based on InP/GaPN as compared to those based on GaPNAs. Therefore, the introduction of arsenic in the compound and post-growth thermal annealing allowed us to reduce the defect concentrations in dilute nitrides and improve photoelectrical properties for photovoltaic applications. [ABSTRACT FROM AUTHOR]

A method for the analysis of Kelvin probe force microscopy (KPFM) characterization of semiconductor devices is presented. It enables evaluation of the influence of defective surface layers. The model is validated by analysing experimental KPFM measurements on crystalline silicon samples of contact potential difference (VCPD) in the dark and under illumination, and hence the surface photovoltage (SPV). It is shown that the model phenomenologically explains the observed KPFM measurements. It reproduces the magnitude of SPV characterization as a function of incident light power in terms of a defect density assuming Gaussian defect distribution in the semiconductor bandgap. This allows an estimation of defect densities in surface layers of semiconductors and therefore increased exploitation of KPFM data.

Photovoltaics is amongst the most important technologies for renewable energy sources, and plays a key role in the development of a society with a smaller environmental footprint. Key parameters for solar cells are their energy conversion efficiency, their operating lifetime, and the cost of the energy obtained from a photovoltaic system compared to other sources. The optimization of these aspects involves the exploitation of new materials and development of novel solar cell concepts and designs. Both theoretical modeling and characterization of such devices require a comprehensive view including all scales from the atomic to the macroscopic and industrial scale. The different length scales of the electronic and optical degrees of freedoms specifically lead to an intrinsic need for multiscale simulation, which is accentuated in many advanced photovoltaics concepts including nanostructured regions. Therefore, multiscale modeling has found particular interest in the photovoltaics community, as a tool to advance the field beyond its current limits. In this article, we review the field of multiscale techniques applied to photovoltaics, and we discuss opportunities and remaining challenges.