Germanium Quantum Dot Grätzel‐Type Solar Cell

Abstract

Solar cells fabricated from sustainable quantum dot materials are currently not commercially available, but ongoing research provides a steady increase in efficiency and stability of laboratory devices. In this work, the first germanium quantum dot solar cell made with a gas aggregation nanoparticle source is presented. UV–vis spectroscopy reveals quantum confinement, and the spectral response of the germanium quantum dot Grätzel‐type solar cell confirms the presence of large and small band gap optical absorption due to a mix of particle sizes. Some of the particles are small enough to have substantial quantum confinement while others are so large that they have bulk‐like properties. The efficiency of the germanium quantum dot solar cells is very low but could reach 1% if the formation of germanium oxide layers is avoided in future experiments. This first quantum dot solar cell made with a gas aggregation nanoparticle source demonstrates, as a proof of concept, the technological potential for research and applications combining the fields of photovoltaics and gas aggregation nanoparticle sources.

Quantum Confinement in the Spectral Response of n-Doped Germanium Quantum Dots Embedded in an Amorphous Si Layer for Quantum Dot-Based Solar Cells

“…AFM characterization of the Ge QD a-Si layers as shown in Figure (b) and (c) clearly shows the individual germanium cauliflower particles covered by the silicon layer. The particles have a height, which corresponds to their size, of about 50 nm, in agreement with the previous study of Cardoso et al The 50 nm germanium cauliflower particles consist of much smaller particles in <10 nm range, which will be quantum confined. The top layer of the device is highly porous because of the assembling of nanoparticles, which likely absorbs the liquid silver contact very well.…”

“…Because the band gap of a-Si is only 1.4 eV, it is likely that the a-Si in this study is affected by the boron doping, which can widen the band gap. In this work the measured band gap of the germanium quantum dots is in good agreement with previous studies. , However, a valid Tauc plot of the Ge QD a-Si solar cell was only possible by using an exponential of 2, which corresponds to direct optical transitions, and provided a value of 1.80 ± 0.01 eV, between that of a-Si and the germanium QDs. This means that the germanium QDs, within the cauliflower structure, and possibly also the a-Si are quantum confined with direct optical transitions.…”

“…Since XPS probes the VBM only, the valence band levels which are positioned at lower energy levels due to quantum confinement are therefore obscured . Moreover, because germanium has an indirect optical transition, and the optical transition of the quantum confined parts are direct, this results in a much stronger optical absorption effect of the quantum confined regions, in agreement with Cardoso et al It is also possible that the n-doping of germanium influences the VBM as shown in studies on silicon nanocrystals. , The XPS sensitivity of our experiments was only ∼0.1 at. %, which was not sufficient to detect doping with the doping concentrations used here.…”

“…The TEM investigation of identical particles was published in Cardoso et al, providing structural information as well as a size distribution. An XRD study on the Ge QD a-Si device in this work revealed no Bragg peaks, indicating that both germanium and silicon are amorphous.…”

“…Since commercial quantum dot solar cells based on sustainable materials such as germanium are currently not available, a novel approach is needed. Although various methods have been employed to fabricate germanium nanoparticles, − the gas aggregation nanoparticle source was recently proposed as a promising device to produce semiconductor high quality (doped) quantum dots, which shortly after was followed by the realization of a germanium quantum dot Grätzel type solar cell made with the same technique. In this work the germanium QDs are again fabricated with a gas aggregation nanoparticle source, and this time, combined with an a-Si layer to form an all solid-state solar cell.…”