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

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