Dye sensitized solar cells : from liquid electrolytes to solid state hole transport materials

摘要

There is a pressing need to find alternatives for polluting and often non-sustainable fossiludfuels. An obvious alternative energy source is the sun, which sends more than enoughudenergy to Earth to satisfy mankind’s (current) demands. Modern commercial solar celludtechnology is still mainly based on silicon, which has to meet very high purity standardsudand is therefore expensive relative to fossil fuels. A viable alternative could be dye sensitizedudsolar cells (DSCs), which operate with the much cheaper semiconductor titanium dioxideud(TiO2). Since the light is absorbed by a dye rather than the bulk semiconductor materialuditself, a DSC is much more versatile then a silicon based solar cell. This thesis discusses udthe development of dye sensitized solar cells in terms of semiconductor fabrication as well udas dye development. Several types of dye sensitized solar cells have been fabricated and testedudfor various copper(I) and ruthenium(II) dyes. The emphasis was on solid state dye sensitized udsolar cells (ssDSCs) since liquid electrolyte type DSCs (leDSCs) have a stability disadvantageuddue to possible leakage and evaporation. SsDSCs rely on a solid hole conductor for charge udtransport rather than on a liquid electrolyte and charge transport is more dependent on chargeudhopping through the hole transport material (HTM) than on diffusion of charge carrying ions,udlike in leDSCs. Among the top performing dyes today are often transition metal complexes basedudon ruthenium, one of the standards being the ruthenium(II) dye N719. All cell development partsudof this thesis (Chapters 3-5) have been almost exclusively conducted with this dye. Since rutheniumudis not very abundant in the Earth’s crust compared to other metals, the focus in dye development udwas laid on the cheaper transition metal copper in the last chapter. Chapter 1 gives a general udintroduction about energy needs of mankind and why we need to find alternative ways of saturatingudthem. Chapter 2 gives an overview of the methods and materials used for solar cell fabricationudand characterization. In Chapter 3, liquid electrolyte DSCs (leDSCs) based on an iodide/triiodideudredox electrolyte have been fabricated and optimized with home made TiO2 particles and layers. udNot only the bare electrodes, but also scattering layers, whose function it is to retain theudlight longer in the DSC itself, have been fabricated and tested. Chapter 4 contains celluddevelopment on ssDSCs based on copper(I) iodide (CuI) as the HTM. In Chapter 5, ssDSCsudemploying polyethylenedioxythiophene (PEDOT) as the HTM have been assembled and tested udfor performance. In Chapter 6, several copper(I) metalorganic complex dyes incorporating udback to back ligands have been tested for leDSCs and PEDOT ssDSCs. Those dyes have been comparedudto the standard ruthenium dye (N719) in cell performance. Chapter 7 Is the conclusion of this thesis.udExperimental and specific cell fabrication details are included in each chapter separately.