Dye-sensitized solar cells
 

pictures

Juan Bisquert
Professor of Applied Physics
Departament de Física
Universitat Jaume I
12071 Castelló de la Plana


bisquert@fca.uji.es

Homepage

 

 

 

 

 

 

 

 

 

 


DSC prepared at UJI-Castelló

Schematic rendering of the TiO2 (titanium dioxide) nanoparticulate structure sensitized with dye molecules that forms the core of a DSC

 

Introduction

Dye-sensitized solar cells (DSC) were invented by Michael Grätzel and Brian O'Regan [B. O' Regan, M. Grätzel, Nature, 353, 737 (1991)] .


photo courtesy of Hanne Lauritzen
The DSC is formed by a combination of organic and inorganic components that could be produced at a low cost. The DSC offers the prospect of a cheap and versatile technology for large scale production of solar cells.
The basic element of a DSC is a nanostructured material, an assembly of titanium dioxide nanoparticles about 20 nm diameter, well connected to their neighbors. TiO2 is the prefered material since its surface is higly resistant to the continued electron transfer. However, TiO2 only absorbs a small fraction of the solar photons (those in the UV). Molecular sensitizers (dye molecules) attached to the semconductor surface, are used to harvest a great portion of the solar light. The main dye molecules consist on one Ru metal atom and a large organic structure that provides the required properties (wide absorption range, fast electron injection, and stability).

The dye is sensible to the visible ligth. The light creates and excitation in the dye that consists on a highly energetic electron, which is rapidly injected to the TiO2 particles.
The nanoparticulate semiconductor functions as the transporter of light induced electrons towards the external contact, a transparent conductor that lies at the basis of the TiO2 film.
Because the dye attaches well to the whole TiO2 surface, the nanostructure enhances the area that is used for collecting photons by a factor 100-1000, with respect to the device area.
When a dye molecule injects an electron, it becomes oxidized. It is very rapidly regenerated by a hole transporting material, usually a redox electrolyte. It may also be a solid hole conductor that fills the pores of the semiconductor nanostructure.


scheme courtesy of Imperial College London

In the complete DSC, electrons and holes created at the dye molecules, are successfully transfered, separately, to the external electrodes, and electricity is generated from sunlight. Maximum conversion efficiencies are currently about 11%, and long term stability has been improved using viscous solvents.
 
Photovoltaic principles
The appearance of new classes of solar cells, that are very different from the conventional silicon pn junction cells, but realize the same type of photovoltaic conversion of sunlight energy to electricity, has lead to a reappraisement of the general photovoltaic principles governing the different classes of solar cell devices.
In essence, a solar cell is constituted by a light absorber material where photon energy is converted into excess carriers with associated electrochemical potentials (Fermi levels) that are maintained away from the equilibrium values by the impinging photons. The number of excess carriers is determined by equliibrium between their rates of generation and recombination.
For converting the available energy in the absorber to electricity, it is needed to extract the carriers with selective contacts. Each selective contact will be transparent to the transference of one carrier, and impenetrable to the other one. Therefore, a potential difference will be obtained between the contacts. It can be used to generate a photocurrent, depending on the external load.

In many accounts, and based on the operation of pn junctions, it is supposed that electrical fields at a junction realize the separation of positive and negative carriers in the photovoltaic effect. However, electrical fields are not generally needed for the operation of selective contacts. A definite proof is given by P. Würfel, Physics of Solar Cells. From Principles to New Concepts (Wiley, Weinheim, 2005), and the DSC is a manifest example.

In a DSC, the light absorber is the dye. TiO2 and the hole conductor realize the function of selective contacts to the dye. These contacts also transport the separate carrier along distances of the order of micrometer towards the external contacts.
In 2008, dye-sensitized solar cells are entering large-scale production at G24 Innovations
Review article

J. Bisquert, D. Cahen, G. Hodes, S. Rühle, A. Zaban
Physical chemical principles of photovoltaic conversion with nanoparticulate, mesoporous dye-sensitized solar cells 
Journal of Physical Chemistry B, 108, 8106-8118 (2004)