Performance limiting factors in flexible dye solar cells

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TKK dissertations, 167
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 current-voltage 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.
solar cell, nanoparticle, dye, spray deposition, electron diffusion
Other note
  • [Publication 1]: Janne Halme, Jaakko Saarinen, and Peter Lund. 2006. Spray deposition and compression of TiO2 nanoparticle films for dye-sensitized solar cells on plastic substrates. Solar Energy Materials & Solar Cells, volume 90, numbers 7-8, pages 887-899. © 2006 Elsevier Science. By permission.
  • [Publication 2]: Janne Halme, Minna Toivola, Antti Tolvanen, and Peter Lund. 2006. Charge transfer resistance of spray deposited and compressed counter electrodes for dye-sensitized nanoparticle solar cells on plastic substrates. Solar Energy Materials & Solar Cells, volume 90, numbers 7-8, pages 872-886. © 2006 Elsevier Science. By permission.
  • [Publication 3]: Kati Miettunen, Janne Halme, Minna Toivola, and Peter Lund. 2008. Initial performance of dye solar cells on stainless steel substrates. The Journal of Physical Chemistry C, volume 112, number 10, pages 4011-4017.
  • [Publication 4]: Kati Miettunen, Janne Halme, Paula Vahermaa, Tapio Saukkonen, Minna Toivola, and Peter Lund. 2009. Dye solar cells on ITO-PET substrate with TiO2 recombination blocking layers. Journal of The Electrochemical Society, volume 156, number 8, pages B876-B883. © 2009 The Electrochemical Society (ECS). By permission.
  • [Publication 5]: Janne Halme, Gerrit Boschloo, Anders Hagfeldt, and Peter Lund. 2008. Spectral characteristics of light harvesting, electron injection, and steady-state charge collection in pressed TiO2 dye solar cells. The Journal of Physical Chemistry C, volume 112, number 14, pages 5623-5637.
  • [Publication 6]: Janne Halme, Kati Miettunen, and Peter Lund. 2008. Effect of nonuniform generation and inefficient collection of electrons on the dynamic photocurrent and photovoltage response of nanostructured photoelectrodes. The Journal of Physical Chemistry C, volume 112, number 51, pages 20491-20504.