Manuel Schnabel

A Membrane Device for Substrate‐Free Photovoltaic Characterization of Quantum Dot Based p‐i‐n Solar Cells

Silicon nanocrystals (Si NCs) embedded in Si-based high band gap matrices show promise as building blocks for all crystalline silicon (c-Si) tandem solar cells. [ 1–3 ] Careful control of the Si nanocrystal size permits a high and adjustable effective band gap due to quantum confi nement. [ 4 , 5 ] The effi ciency limit of a silicon solar cell could be increased from 29% to 42.5% with the addition of a 1.7 eV band gap top solar cell to the 1.12 eV bulk Si bottom solar cell. [ 6 ] Several other types of tandem solar cells have already been realized, but either suffer from degradation and low overall conversion effi ciencies [ 7 , 8 ] or can be used only in concentrator systems due to their expensive material and process costs. [ 9 ] Si NCs embedded in a high band gap matrix provide a material class which is nontoxic, abundant and compatible with Si technology. Size controlled homo genously distributed [ 4 ] and highly luminescent nanocrystals have been achieved by a superlattice approach and high-temperature annealing. Evidence for quantum confi ned states in Si NC embedded in silicon dioxide (SiO 2 ) has been given by photoluminescence [ 10 ] and luminescence quantum yields of up to 25% were already obtained. [ 11 ]

Silicon nanocrystals embedded in silicon carbide: Investigation of charge carrier transport and recombination

An illumination-dependent analysis of silicon nanocrystal p-i-n solar cells is presented within the framework of the constant field approximation. Silicon nanocrystals in silicon carbide were prepared by solid-phase crystallization and contacted with doped a-SixC1−x:H. This paper aims at determining the fundamental transport and recombination properties, i.e., the effective mobility lifetime product, of the nanocrystal layer at device level. Illumination-dependent current-voltage curves are modelled with a voltage-dependent collection function with only two free parameters, and excellent agreement is found between theory and experiment. An effective mobility lifetime product of 10−10 cm2/V is derived and confirmed independently from an alternative method.

Absence of quantum confinement effects in the photoluminescence of Si3N4–embedded Si nanocrystals

Superlattices of Si-rich silicon nitride and Si3N4 are prepared by plasma-enhanced chemical vapor deposition and, subsequently, annealed at 1150 °C to form size-controlled Si nanocrystals (Si NCs) embedded in amorphous Si3N4. Despite well defined structural properties, photoluminescence spectroscopy (PL) reveals inconsistencies with the typically applied model of quantum confined excitons in nitride-embedded Si NCs. Time-resolved PL measurements demonstrate 105 times faster time-constants than typical for the indirect band structure of Si NCs. Furthermore, a pure Si3N4 reference sample exhibits a similar PL peak as the Si NC samples. The origin of this luminescence is discussed in detail on the basis of radiative defects and Si3N4 band tail states in combination with optical absorption measurements. The apparent absence of PL from the Si NCs is explained conclusively using electron spin resonance data from the Si/Si3N4 interface defect literature. In addition, the role of Si3N4 valence band tail states as...

Charge transport and electroluminescence of silicon nanocrystals/SiO2 superlattices

Charge transport and electroluminescence mechanisms in Si-rich Si oxynitride/silicon oxide (SRON/SiO2) superlattices deposited on p-type Si substrate are reported. The superlattice structures were deposited by plasma-enhanced chemical-vapor deposition and subsequently annealed at 1150 °C to precipitate and crystallize the Si excess into Si nanocrystals. The dependence of the electrical conduction on the applied voltage and temperature was found to be well described by a Poole-Frenkel transport mechanism over a wide voltage range. On the other hand, the observed dependence of the electroluminescence on the SRON layer thickness is a clear proof of quantum confinement and was attributed to an excitonic radiative recombination taking place in the confined states within the Si quantum dots. A model is proposed based on thermal hopping of electrons between the quantum dots acting as trap states (Poole-Frenkel). A correlation between carrier transport and electroluminescence has been established considering impa...

Absorption and emission of silicon nanocrystals embedded in SiC: Eliminating Fabry-Pérot interference

Silicon nanocrystals embedded in SiC are studied by spectrophotometry and photoluminescence (PL) spectroscopy. Absorptivities are found to be affected by residual Fabry-Perot interference arising from measurements of reflection and transmission at locations of different film thickness. Multiple computational and experimental methods to avoid these errors in thin film measurements, in general, are discussed. Corrected absorptivity depends on the quantity of Si embedded in the SiC but is independent of the Si crystallinity, indicating a relaxation of the k-conservation criterion for optical transitions in the nanocrystals. Tauc gaps of 1.8–2.0 and 2.12 eV are determined for Si nanoclusters and SiC, respectively. PL spectra exhibit a red-shift of ∼100 nm per nm nominal Si nanocluster diameter, which is in agreement with quantum confinement but revealed to be an artifact entirely due to Fabry-Perot interference. Several simple experimental methods to diagnose or avoid interference in PL measurements are devel...

Electrical and Optical Characterisation of Silicon Nanocrystals Embedded in SiC

Silicon nanocrystals (Si NCs) are a promising candidate for the top cell of an all-Si tandem solar cell with a band gap from 1.3-1.7 eV, tuneable by adjusting NC size. They are readily produced within a Si-based dielectric matrix by precipitation from the Si excess in multilayers of alternating stoichiometric and silicon-rich layers. Here we examined the luminescence and transport of Si NCs embedded in SiC. We observed luminescence that redshifts from 2.0 to 1.5 eV with increasing nominal NC size. Upon further investigation, we found that this redshift is to a large extent due to Fabry-Pérot interference. Correction for this effect allows an analysis of the spectrum emitted from within the sample. We also produced p-i-n solar cells and found that the observed I-V curves under illumination could be well-fitted by typical thin-film solar cell models including finite series and parallel resistances, and a voltage-dependent current collection function. A minority carrier mobility-lifetime product on the order of 10-10 cm2/V was deduced, and a maximum open-circuit voltage of 370 mV achieved.

Nanocrystalline SiC formed by annealing of a-SiC:H on Si substrates: A study of dopant interdiffusion

Nanocrystalline silicon carbide (nc-SiC) is an interesting material for electronics applications, both in its own right and as a host matrix for silicon quantum dots. When synthesized by annealing of a-SiC:H on Si substrates, interdiffusion of dopants occurs if either the a-SiC:H or the Si substrate is doped. Annealing a-SiC:H on highly boron-doped substrates at 1100 °C leads to a fairly homogeneous doping level of ≥4 × 1019 cm−3 throughout the nc-SiC film. An unexpected anomaly in secondary ion mass spectroscopy quantification is observed and a method to circumvent it is shown. The nanostructure of the nc-SiC is only weakly affected as most of the diffusion occurs after the onset of crystallization. Annealing of doped a-SiC:H on Si substrates at 1100 °C leads to strong free carrier absorption at infrared wavelengths. This is demonstrated to originate from dopants that have diffused from the a-SiC:H to the Si substrate, and a method is developed to extract from it the doping profile in the Si substrate. T...

High-Bandgap Silicon Nanocrystal Solar Cells: Device Fabrication, Characterization, and Modeling

Silicon nanocrystals (Si NCs) embedded in Si-based dielectrics provide a Si-based high-bandgap material (1.7 eV) and enable the construction of crystalline Si tandem solar cells. This chapter focusses on Si NC embedded in silicon carbide, because silicon carbide offers electrical conduction through the matrix material. The material development is reviewed, and optical modeling is introduced as a powerful method to monitor the four material components, amorphous and crystalline silicon as well as amorphous and crystalline silicon carbide. In the second part of this chapter, recent device developments for the photovoltaic characterization of Si NCs are examined. The controlled growth of Si NCs involves high-temperature annealing which deteriorates the properties of any previously established selective contacts. A membrane-based device is presented to overcome these limitations. In this approach, the formation of both selective contacts is carried out after high-temperature annealing and is therefore not affected by the latter. We examine p-i-n solar cells with an intrinsic region made of Si NCs embedded in silicon carbide. Device failure due to damaged insulation layers is analyzed by light beam-induced current measurements. An optical model of the device is presented for improving the cell current. A characterization scheme for Si NC p-i-n solar cells is presented which aims at determining the fundamental transport and recombination properties, i.e., the effective mobility lifetime product, of the nanocrystal layer at device level. For this means, an illumination-dependent analysis of Si NC p-i-n solar cells is carried out within the framework of the constant field approximation. The analysis builds on an optical device model, which is used to assess the photogenerated current in each of the device layers. Illumination-dependent current–voltage curves are modelled with a voltage-dependent current collection function with only two free parameters, and excellent agreement is found between theory and experiment. An effective mobility lifetime product of 10−10 cm2/V is derived and confirmed independently from an alternative method. The procedure discussed in this chapter is proposed as a characterization scheme for further material development, providing an optimization parameter (the effective mobility lifetime product) relevant for the photovoltaic performance of Si NC films.

Phosphorus diffusion in nanocrystalline 3C-SiC

Phosphorus diffusion in nanocrystalline 3C silicon carbide (nc-SiC) with a grain size of 4–7 nm is studied using polycrystalline silicon (poly-Si) as the phosphorus source. Diffusion is much faster than in monocrystalline SiC and proceeds exclusively via grain boundaries (GBs). The poly-Si deposition step, alone or followed by a 1000 °C drive-in step, is sufficient to create a shallow phosphorus profile <100 nm deep, while drive-in steps above 1100 °C lead to phosphorus penetrating the 200 nm thick films and reaching the Si substrate. In the bulk of the films, GB diffusion is Fickian, and thermally activated with an activation energy of 5.2 ± 0.3 eV, which is substantially lower than in the monocrystalline case. Boltzmann-Matano analysis corroborates the analysis of the phosphorus profiles in the bulk of the films using error functions and shows that the high near-surface concentrations observed can be explained in terms of a concentration-dependent diffusivity. The concentration dependence is stronger an...

Boron diffusion in nanocrystalline 3C-SiC

The diffusion of boron in nanocrystalline silicon carbide (nc-SiC) films with a grain size of 4–7 nm is studied using a poly-Si boron source. Diffusion is found to be much faster than in monocrystalline SiC as it takes place within the grain boundary (GB) network. Drive-in temperatures of 900–1000 °C are suitable for creating shallow boron profiles up to 100 nm deep, while 1100 °C is sufficient to flood the 200 nm thick films with boron. From the resulting plateau at 1100 °C a boron segregation coefficient of 28 between nc-SiC and the Si substrate, as well as a GB boron solubility limit of 0.2 nm−2 is determined. GB diffusion in the bulk of the films is Fickian and thermally activated with DGB(T)=(3.1−5.6)×107exp(−5.03±0.16  eV/kBT) cm2s−1. The activation energy is interpreted in terms of a trapping mechanism at dangling bonds. Higher boron concentrations are present at the nc-SiC surface and are attributed to immobilized boron.