Functional Materials for Perovskite Solar Cells

Time: Fri 2020-02-28 13.00

Location: K2, Teknikringen 28, Stockholm (English)

Subject area: Chemistry

Doctoral student: Wei Zhang , Tillämpad fysikalisk kemi

Opponent: Professor Neil Robertson, University of Edinburgh

Supervisor: Professor Lars Kloo, Molekylär elektronik, CMD, Tillämpad fysikalisk kemi

Abstract

Energy plays a significant role in our daily lives, but most energy provided by fossil fuels causes serious environmental problems including air pollution, global warming, and ecological damage. In addition, it has been estimated that all of our fossil fuels will run out in 2088 and thus it is highly important to study and apply renewable energy sources. Among all the alternatives, solar energy is clean, sustainable, and abundant. It is estimated that the amount of power from the sun that strikes the earth in 90 minutes is more than the entire world consumes in one year. The perovskite solar cell (PSC) is one of the strongest tools to utilize solar energy because of its high power conversion efficiency and easy fabrication process. However, the lead that is normally used in the perovskite layer is considered harmful to the environment and to human health. Moreover, the low conductivity and hole mobility of the hole-transport material (HTM) Spiro-OMeTAD and the low overall device stability against humidity are all issues that might hinder the further development of PSC technology. This thesis concerns all of these aspects, with a general focus on different functional materials.

The aim of this thesis was to develop environmentally friendly and low-cost functional materials in order to solve existing problems while at the same time revealing insights into carrier transport, molecular doping, and surface passivation.

In Chapter 1 and Chapter 2, the current status of PSCs and the experimental and theoretical methods used in this thesis are presented.

In Chapter 3, the properties of coordination complexes, including molybdenum clusters and polyiodide-linked gold complexes, and their potential application in solar cells as lead-free light absorbers are discussed.

In Chapter 4, the synthesis of four coordination complexes with different metal cores and ligands and their application as HTMs in PSCs is discussed. Their oxidation potential, hole mobility, conductivity, and packing methods are presented.

In Chapter 5, two p-type dopants – Cu(bpcm)2 and (MeO-TPD)TFSI – are introduced for the organic HTM Spiro-OMeTAD. Both of these could significantly increase the conductivity of Spiro-OMeTAD films. In addition, (MeO-TPD)TFSI could work separately without hygroscopic LiTFSI at high doping amounts thus potentially increasing the device’s stability. The structure of oxidized Spiro-OMeTAD on the base of the Spiro(TFSI)2 is also discussed.

In Chapter 6, density functional theory modeling of four different functional groups – including amino (−NH2), phosphine (−PH2), hydroxyl (−OH), and thiol (−SH) groups – in combination with polyhedral oligomeric silsesquioxane is discussed in terms of estimating the adsorption energy with respect to different perovskite surface models. The amino functional group showed the strongest adsorption energy and was further compared with the thiol group in experiments.

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