양자점 태양전지의 광학 및 전기적 설계에 대한 연구

Abstract

Colloidal quantum dots (CQDs) are emerging photovoltaic materials displaying exclusive characteristics that can be adjusted by its size and surface chemistry. Their strong quantum confined excitonic features promise future-oriented solar cells having higher Shockley–Queisser limit. In addition, CQDs have absorption features related to the distinct size-dependent density-of-state, therefore, one can design a system where an in-depth study of the optical and electrical properties of materials or devices is feasible with regard to their band gaps. However, the reported CQD solar cells have lower efficiencies compared with other promising solar cells such as perovskite and silicon, due to the lack of knowledge on its optical and electrical features. A CQD based optoelectronic device design needs specialized approaches compared to conventional bulk-based solar cells. In this paper, I introduce the design manner for CQD thin-film solar cells from two different viewpoints, optics and electrics, and suggest new approaches to enhance the limitation of CQD solar cells. In optical design, the confined energy level of CQD contributes to adjust band alignment, adapting to a device purpose. However, selecting material properties for this energy adjustment can raise the light loss produced by interface reflection. Thus, adequate light path management is important for optical CQD solar cell design. On the other hand, the surface modification of CQD is a crucial issue for the electrical design of CQD solar cells. Thin-film solar cell architecture can be fabricated by heterojunction, and ligand exchange provides suitable doping states and enhanced carrier transfer for the junction. To reduce the trap-assisted recombination of CQD solar cells, superb surface treatment is essential. In Chapter 2, I demonstrate how the fluctuation of the current in the flat solar cell design disturbs estimating the optimal active layer thickness in the solar cells, using PbS CQD solar cell system. In detail, I display why the CQD solar cells have shown a shorter active layer thickness compared to the estimates derived from its depletion width and diffusion length. To demonstrate the effect of photocurrent and collection efficiency independently, I simulated two different zinc oxide structure-based solar cells with regard to various active layer thickness. The simulation results were in good agreement with the experimental results and showed how interference affects the optimization of device thickness. In Chapter 3, I found a route to reduce hydroxyl formation on the PbS CQD surface which is considered detrimental to device performance by creating in-gap trap energy states. Specifically, to prevent undesirable hydroxyl group formation on the surface of PbS nanocrystals, I developed the in-situ solution-phase halide exchange without PbS CQDs being exposed to protic molecules. Furthermore, my new process can eliminate the solvent purification process completely which has been considered a cumbersome obstacle to the commercialization of the CQD photovoltaics. The conventional purification steps require toxic solvent such as methanol and toluene as well as the complex centrifuge process. As a result, this new method can be saving the synthesis cost of 59.3% and the total manufacturing cost of 23.9% according to a reported study. In summary, I designed the high efficiency CQD solar cells based on the understanding of optical and electrical study during my Ph.D. course. Both results present world-class values in terms of current density and voltage, respectively, promising to go to beyond the widely-known knowledge and limitations. Therefore, I believe that my researches provide a good design strategy and inspiration for thin-film solar cells to other scientists, leading deep study and commercialization on the research field.