TA – modeling of photo-induced dynamics in CZTS kesterite nanoparticles

Abstract

The interest in energy sources has shifted over the years toward sustainable energy, which has popularized the research in solar cells (among other energy sources). One of the key topics in solar cell research is the power conversion efficiency, and to understand how this can be improved one must understand the photo-induced dynamics of the solar cell material. In this thesis, we modeled the conduction band dynamics in CZTS kesterite nanoparticles using TA – data from a previous experiment. The modeling was performed using the optimization functions in the plot_func project. We managed to create a model of 6 states and 7 processes, assigned the states to time-and wavelength region in the spectra. Based on our results combined with results from literature, we also identified these states and processes. Of these 7 processes, we modeled one as Auger recombination which has not been observed directly before, to the author’s knowledge. Due to confinement effects, the Auger recombination could be written as a second order process (instead of a third order) allowing us to only consider the electron concentration. The same could be said about the recombination from the bottom of the conduction band, reducing it into a first order reaction instead of a second order. We also modeled shallow and deep trap states and tried to model the filling of these, and calculated the number of charge carriers in deep-and shallow trap states. The trap-assisted recombination from shallow traps was also reduced to a first order process probably due to localization of electrons in these states. To reliably model the main processes in CZTS kesterite with only TA – data is quite an achievement if done successfully, as it reduces the number of required measurements, and we see this attempt as a first step towards this goal.

Popular Abstract

Green and renewable energy sources have over the years seen an increased interest in research, where solar cells have received special attention. Solar cells consist of several layers, or parts, with different functions and this thesis focuses on the material that makes up the so-called active layer. The active layer is where sunlight is absorbed, exciting electrons to then generate a current to the other layers. Therefore, it is important for the active layer to have a high absorption and power conversion efficiency (efficiency to generate electrons with the energy of the sunlight).

To improve this efficiency, we need to first understand what actually happens in the active layer when light is absorbed. Generating a current is one process, that we want, but other processes
that we do not want, occur too. To gain this understanding, we rely on experimental data from transient absorption (TA) measurements, from which we can see the generation of electrons and the processes these participate in. As the processes occur, the absorption of light in the material changes in strength and in wavelength, and since different processes occur at different speeds, the absorption difference signal will appear at different wavelength-and time regions that consist of different strengths.

With the knowledge of signal strength, wavelength-and time region we can use the data from the TA – measurement to create a model, a representation, of what happens in the active layer. This way, we point out the sources (processes) of any reduction in power conversion efficiency. In this master thesis, we use TA – data to model the processes in Cu2ZnSnS4 (CZTS) of the kesterite structure. With this model, we manage to observe 7 different processes involving 6 different states that we assign to a wavelength-and time region in the spectra.

Popular Abstract (Swedish)

Gröna och förnybara energikällor har under åren sett ökad uppmärksamhet inom forskning och utveckling, där solceller har fått särskild uppmärksamhet. Solceller består av flera lager, eller komponenter, med olika funktioner och den här tesen fokuserar på material som bygger upp det aktiva lagret. Det aktiva lagret är det som absorberar solljus vilket exciterar elektroner till att bilda en elektrisk ström till de andra lagren. Därför är det viktigt att det aktiva lagret har en hög absorption och effektivitet med att omvandla solljus till en elektrisk ström.

För att förbättra denna effekt, behöver vi först förstå vad som händer i det aktiva lagret när ljus absorberas. Att bilda en elektrisk ström är en process, som vi vill ha, men det sker även andra processer som vi inte vill ha. För att få denna förståelse, använder vi experimentell data från transient absorptionsmätningar som låter oss se hur solljuset genererar elektroner och vilka processer dessa elektroner deltar i. Allt eftersom processer sker, absorptionen av ljus ändras i styrka och våglängd, och eftersom olika processer sker olika fort ser vi signalen för ändring i absorption vid olika våglängds-och tids regioner som också har olika styrkor.

Med kunskapen om signalstyrka, våglängds-och tidsregion kan vi använda datan från transient absorptionsmätningarna till att skapa en modell, en representation, av vad som händer i det aktiva lagret. På det här sättet, ser vi vilka processer som orsakar minskningen i effektivitet. I det här masterarbetet, använder vi transient absorptionsdata till att modellera processerna i Cu2ZnSnS4 (CZTS) av kesteritstrukturen. Med den här modellen lyckas vi identifiera 7 olika processer som involverar 6 olika tillstånd som vi tilldelar en våglängds-och tidsregion i spektrumen.

• author: Eriksson, Axl
• supervisor: Jens Uhlig
• organization: Department of Chemistry
• course: KEMR45 20211
• year: 2021
• type: H2 - Master's Degree (Two Years)
• subject: Chemistry
• keywords: CZTS, kesterite, transient absorption, dynamics, chemical physics
• language: English
• id: 9056921
• date added to LUP: 2021-06-30 10:32:09
• date last changed: 2021-06-30 10:32:09