Performance Limiting Factors in Flexible Dye Solar Cells

OF DOCTORAL DISSERTATION HELSINKI UNIVERSITY OF TECHNOLOGY P.O. BOX 1000, FI-02015 TKK http://www.tkk.fi Author Janne Halme Name of the dissertation Performance limiting factors in flexible dye solar cells Manuscript submitted 10.3.2009 Manuscript revised 11.5.2009 Date of the defence 1.6.2009 Monograph Article dissertation (summary + original articles) Faculty Faculty of Information and Natural Sciences Department Department of Applied Physics Field of research Solar cells Opponent(s) Professor Laurence Peter Supervisor Professor Peter Lund Instructor Professor Peter Lund Abstract Photovoltaic cells convert the electromagnetic energy of sunlight to electrical energy. An example of this is the electrochemical dye solar cell (DSC). In this work, preparation of DSCs on flexible plastic and metal substrates was studied concentrating on the factors that limit their energy conversion efficiency. Flexible substrates enable manufacturing of solar cells with cost-effective roll-to-roll techniques, but set restrictions to their materials and fabrication processes. An improved method for the preparation of nanoparticle films on plastic was developed using spray deposition and mechanical pressing at room-temperature. With the method, deposition of nanostructured TiO2 photoelectrodes was fast but their photocurrent output was low. To investigate this, an improved experimental method was introduced that allows determining the quantum efficiencies of photocurrent generation in DSC by conventional optical spectroscopy and spectral response measurements. The reason was low electron collection efficiency due to too short electron diffusion length in the pressed TiO2 films. Stainless steel 304 was found to possess excellent electrochemical properties for its use as the photoelectrode substrate. 4.4 % cell efficiency was reached with a steel based DSC. When the effects of different cell components on the currentvoltage curve of the cell were studied using electrochemical impedance spectroscopy (EIS), it was found that the stainless steel decreased the electron recombination resistance of the TiO2 photoelectrode film. The performance of the plastic substrate based cells at low light intensities was significantly limited by electron recombination via the substrate. Preparing a 4 nm thick compact TiO2 layer on the substrate by atomic layer deposition (ALD) suppressed the recombination, but introduced an additional contact resistance that decreased the fill factor and cell efficiency. The dynamic photocurrent and photovoltage response of DSC is significantly affected by non-uniform generation profile and inefficient collection of electrons. This complicates the interpretation of the dynamic data and can lead to erroneous conclusions when a common approximate way of analysis is used. As a solution to this, a new dynamic performance characteristic was introduced, that allows also determining the effective electron diffusion coefficient and lifetime at the short circuit condition consistently with the steady state cell performance.