Quantum Dot Solar Cells: Towards Environmentally Friendly Materials
Abstract: To decrease the world’s energy dependence on fossil fuels, energy production and specifically electricity production needs to shift to more sustainable alternatives. One such alternative is solar cells as they directly convert energy emitted from the sun into electricity. Silicon and thin film solar cells which are commercialized today are either expensive to make and rigid, or utilizes rare and toxic materials. This has resulted in an increase in the field of solar cell research to find cheaper alternatives which are also based on abundant materials.Colloidal quantum dot (CQD) solar cells are third generation solar cells which has received much focus in the last decade due to the property of the quantum confinement effects. This makes it possible to increase the band gap by decreasing the size of the crystallites.CQDs made of PbS were in this thesis developed for high PCE (power conversion efficiency), low weight and flexible CQD solar cells. By applying the PbS CQDs on flexible and durable substrates, light weight solar cells with a high power-per-weight of 15.2 W g-1 value were accomplished. Other PbS CQD solar cells were covered and passivated by a mid-high band gap perovskite semiconductor with good lattice matching to facilitate improved stability and PCE, with the latter reaching 10.7 %.By shifting the focus towards more low-toxic and more environmentally friendly materials, materials such as PbS could be phased out. In this thesis, Ag2S and AgBiS2 were investigated as alternatives. By utilizing a heat-up synthesis method for the production of Ag2S CQDs, a proof-of-concept PCE of 0.34 % was accomplished for solar cells based on these CQDs. To improve the PCE, a hot-injection method was utilized to produce Ag2S CQDs with a surface better suited for solar cell applications. This resulted in an improved PCE of 2.2 %. In a final study, AgBiS2 CQDs were investigated by changing the composition of the precursor ratio Ag:Bi:S. The final composition was linearly dependent on the Ag precursor concentration. The best device with the highest PCE had a composition close to the stoichiometric ratio of 1:1:2 which was achieved by a precursor composition of 0.72:0.9:1. This resulted in a PCE of 3.3 %.The understanding of PbS CQD solar cells and how they can be further improved and ap-plying the relevant information and research to low-toxic alternatives is necessary for the im-provements of these more environmentally friendly CQD materials in solar cell applications.
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