Si solid-state quantum dot-based materials for tandem solar cells

The concept of third-generation photovoltaics is to significantly increase device efficiencies whilst still using thin-film processes and abundant non-toxic materials. A strong potential approach is to fabricate tandem cells using thin-film deposition that can optimise collection of energy in a series of cells with decreasing band gap stacked on top of each other. Quantum dot materials, in which Si quantum dots (QDs) are embedded in a dielectric matrix, offer the potential to tune the effective band gap, through quantum confinement, and allow fabrication of optimised tandem solar cell devices in one growth run in a thin-film process. Such cells can be fabricated by sputtering of thin layers of silicon rich oxide sandwiched between a stoichiometric oxide that on annealing crystallise to form Si QDs of uniform and controllable size. For approximately 2-nm diameter QDs, these result in an effective band gap of 1.8 eV. Introduction of phosphorous or boron during the growth of the multilayers results in doping and a rectifying junction, which demonstrates photovoltaic behaviour with an open circuit voltage (VOC) of almost 500 mV. However, the doping behaviour of P and B in these QD materials is not well understood. A modified modulation doping model for the doping mechanisms in these materials is discussed which relies on doping of a sub-oxide region around the Si QDs

Assessment of the composition of Silicon-Rich Oxide films for photovoltaic applications by optical techniques

Abstract The deposition of sub-stoichiometric silicon rich oxide (SRO) is the first step to obtain well ordered silicon Quantum Dots (QDs) in a dielectric matrix. This structure is used also for third generation photovoltaic devices operating in a tandem architecture. A precise control and assessment of the stoichiometry of these films is crucial to tune the electrical and optical properties of the device. In this paper we discuss two optical techniques to assess the composition of such films and we compare their results

Impacts of Post-metallisation Processes on the Electrical and Photovoltaic Properties of Si Quantum Dot Solar Cells

As an important step towards the realisation of silicon-based tandem solar cells using silicon quantum dots embedded in a silicon dioxide (SiO2) matrix, single-junction silicon quantum dot (Si QD) solar cells on quartz substrates have been fabricated. The total thickness of the solar cell material is 420 nm. The cells contain 4 nm diameter Si quantum dots. The impacts of post-metallisation treatments such as phosphoric acid (H3PO4) etching, nitrogen (N2) gas anneal and forming gas (Ar: H2) anneal on the cells’ electrical and photovoltaic properties are investigated. The Si QD solar cells studied in this work have achieved an open circuit voltage of 410 mV after various processes. Parameters extracted from dark I–V, light I–V and circular transfer length measurement (CTLM) suggest limiting mechanism in the Si QD solar cell operation and possible approaches for further improvement

Optical characterisation of silicon nanocrystals embedded in SiO2/Si3N4 hybrid matrix for third generation photovoltaics

Silicon nanocrystals with an average size of approximately 4 nm dispersed in SiO2/Si3N4 hybrid matrix have been synthesised by magnetron sputtering followed by a high-temperature anneal. To gain understanding of the photon absorption and emission mechanisms of this material, several samples are characterised optically via spectroscopy and photoluminescence measurements. The values of optical band gap are extracted from interference-minimised absorption and luminescence spectra. Measurement results suggest that these nanocrystals exhibit transitions of both direct and indirect types. Possible mechanisms of absorption and emission as well as an estimation of exciton binding energy are also discussed

A Method to Overcome the Time Step Limitation of PC1D in Transient Excitation Mode

AbstractThis paper presents a method to overcome the time step limitation of PC1D in the transient excitation mode without changing the source code of PC1D. By comparing the results obtained from PC1D using the proposed method with results obtained from a finite element analysis, the correctness of the method is confirmed. The proposed method allows simulations over long times with high time precision, which is currently not possible in PC1D in transient excitation mode

Optical evaluation of doping concentration in SiO2 doping source layer for silicon quantum dot materials

We have investigated and proposed a simple method to correlate optical absorption with high B doping concentrations in thin SiO2 films that offer a potential doping source for Si quantum dots. SiO2 films with boron and phosphorus were deposited using a computer controlled co-sputtering system. By assessing the absorption coefficients, it was observed that the doping can dramatically increase the absorption of the transparent SiO2. Additionally, the highly doped SiO2 films have a very broad Urbach like absorption tail and the absorption corresponds well with the doping level