Toward Annealing Stable Molybdenum Oxide Based Hole Selective Contacts For Silicon Photovoltaics

Molybdenum oxide MoOX combines a high work function with broadband optical transparency. Sandwiched between a hydrogenated intrinsic amorphous silicon passivation layer and a transparent conductive oxide, this material allows a highly efficient hole selective front contact stack for crystalline silicon solar cells. However, hole extraction from the Si wafer and transport through this stack degrades upon annealing at 190 C, which is needed to cure the screen printed Ag metallization applied to typical Si solar cells. Here, we show that effusion of hydrogen from the adjacent layers is a likely cause for this degradation, highlighting the need for hydrogen lean passivation layers when using such metal oxide based carrier selective contacts. Pre MoOX deposition annealing of the passivating a Si H layer is shown to be a straightforward approach to manufacturing MoOX based devices with high fill factors using screen printed metallization cured at 190

Strategy for large???scale monolithic Perovskite/Silicon tandem solar cell: A review of recent progress

For any solar cell technology to reach the final mass-production/commercialization stage, it must meet all technological, economic, and social criteria such as high efficiency, large-area scalability, long-term stability, price competitiveness, and environmental friendliness of constituent materials. Until now, various solar cell technologies have been proposed and investigated, but only crystalline silicon, CdTe, and CIGS technologies have overcome the threshold of mass-production/commercialization. Recently, a perovskite/silicon (PVK/Si) tandem solar cell technology with high efficiency of 29.1% has been reported, which exceeds the theoretical limit of single-junction solar cells as well as the efficiency of stand-alone silicon or perovskite solar cells. The International Technology Roadmap for Photovoltaics (ITRPV) predicts that silicon-based tandem solar cells will account for about 5% market share in 2029 and among various candidates, the combination of silicon and perovskite is the most likely scenario. Here, we classify and review the PVK/Si tandem solar cell technology in terms of homo- and hetero-junction silicon solar cells, the doping type of the bottom silicon cell, and the corresponding so-called normal and inverted structure of the top perovskite cell, along with mechanical and monolithic tandemization schemes. In particular, we review and discuss the recent advances in manufacturing top perovskite cells using solution and vacuum deposition technology for large-area scalability and specific issues of recombination layers and top transparent electrodes for large-area PVK/Si tandem solar cells, which are indispensable for the final commercialization of tandem solar cells

Amorphous SixC1-x:H single layers before and after thermal annealing: Correlating optical and structural properties

Amorphous SixC1-x:H single layers with varying stoichiometry were prepared using plasma enhanced chemical vapour deposition (PECVD). The as-deposited layers were annealed at different temperatures (T-a) from 500 to 1100 degrees C. The influence of annealing on layers with varying composition x was investigated by a variety of analytical techniques including photoluminescence (PL) spectroscopy, reflectance and transmittance measurements, grazing incidence X-ray diffraction (GIXRD) and Fourier transform infrared spectrometry (FTIR). Before annealing, PL measurements show a shift of the spectra to higher energies with increase in carbon content. Calculations of the optical bandgap by other groups confirm this trend. The FTIR investigations show in addition to the expected Si-C bonds also the formation of oxygen and hydrogen related bonds within the layers. After annealing, solely a broad PL signal appears at energies around 2.0 eV. This might be due to luminescence of defects, created by the incorporated amount of carbon and the evolution of crystalline phases in the layers. GIXRD measurements confirmed the formation of both Si NCs after 700 degrees C and SiC NCs after annealing at 1000 degrees C. The FTIR spectra exhibit a shift of the SiC absorption band and an increase in the Si-C bond density due to the crystallization of SiC. After annealing a hydrogen passivation was performed, which leads to a strong decrease in the PL intensity. However, annealed Si-rich films show a peak around 1.2 eV after H-passivation, which might originate from quantum confinement effects in Si NCs

Modeling upconversion of erbium doped microcrystals based on experimentally determined Einstein coefficients

Upconversion of infrared photons is a promising possibility to enhance solar cell efficiency by producing electricity from otherwise unused sub-band-gap photons. We present a rate equation model, and the relevant processes, in order to describe upconversion of near-infrared photons. The model considers stimulated and spontaneous processes, multi-phonon relaxation and energy transfer between neighboring ions. The input parameters for the model are experimentally determined for the material system \beta-NaEr0.2Y0.8F4. The determination of the transition probabilities, also known as the Einstein coefficients, is in the focus of the parameterization. The influence of multi-phonon relaxation and energy transfer on the upconversion are evaluated and discussed in detail. Since upconversion is a non-linear process, the irradiance dependence of the simulations is investigated and compared to experimental data of quantum efficiency measurements. The results are very promising and indicate that upconversion is physically reasonably described by the rate equations. Therefore, the presented model will be the basis for further simulations concerning various applications of upconversion, such as in combination with plasmon resonances in metal nanoparticles.Comment: 29 pages, 13 figure

Structural and optical characterization of Si quantumdots in a SiC matrix

Amorphous hydrogenated Si1-xCx / SiC multilayers consisting of alternating Si1-xCx and stoichiometric SiC layers were prepared using Plasma Enhanced Chemical Vapour Deposition (PECVD). Annealing at temperatures up to 1100°C was done targeting the size controlled crystallization of Si nanocrystals (NCs) in a SiC matrix. The influence of annealing temperature on the nanostructure of the multilayers was studied using Glancing Incidence X-ray Diffraction (GIXRD), Raman spectroscopy and Transmission Electron Microscopy (TEM). GIXRD reveal the crystallization of Si and SiC, when annealing temperatures exceed 900°C. The crystallization of Si and SiC was confirmed by TEM bright field imaging and electron diffraction. Annealing at 900°C, leads to the formation of Si NCs with a size of 3 nm, whereas the SiC NCs also have a size of 3 nm. However, a large amount of Si is still amorphous as shown by Raman spectroscopy. Annealing at temperatures exceeding 900°C reduces the amorphous phase and a further growth of Si NCs occurs

Experimental analysis of upconversion with both coherent monochromatic irradiation and broad spectrum illumination

Upconversion of sub-band-gap photons promises to increase solar cell efficiencies by making these photons useful. In this paper, we investigate the application of -NaYF4:20% Er3 to silicon solar cells. We determine the external quantum efficiency of an upconverter silicon solar cell, both under monochromatic excitation and, for the first time in the context of silicon solar cells, under broad spectrum illumination as it is relevant for the application to harvest solar energy. The investigated upconverter silicon solar cell responds under broad spectrum illumination with an average upconversion efficiency of 1.07±0.13% in the spectral range from 1460 to 1600 nm. The resulting efficiency increase for the used solar cell with an overall efficiency of 16.7% is calculated to be 0.014% relative

Electrical Properties of Recrystallised SiC Films from PECVD Precursors for Silicon Quantum Dot Solar Cell Applications

Silicon carbide (SiC) is a promising host material for silicon quantum dots (Si QDs), which are being investigated as absorber materials for tandem solar cells based solely on crystalline silicon. Amorphous silicon carbide (a-SiC) films are deposited by plasma-enhanced chemical vapour deposition (PECVD) and annealed under the same conditions usually used to precipitate Si QDs. During annealing, the films shrink by 20%, and some a-SiC transforms into SiC nanocrystals (nc-SiC) about 3 nm in size. P-type doping with boron is found to inhibit SiC crystallisation and lower conductance as compared to intrinsic films. N-type doping with phosphorus on the other hand promotes SiC crystallisation and leads to a higher conductance. The trends in conductance are ascribed solely to the effect of the dopant on crystallisation. Al, Ti, Cr, Ni20%Cr, and ITO are all found to form Ohmic contacts to the SiC films on deposition, with no change in contact properties brought about by sintering at 425°C. Temperature-dependent conductivity measurements on intrinsic SiC films reveal two distinct activation energies; 65 meV below 200 K, and 158 meV above 200 K