Low-temperature fabrication of dye-sensitized solar cells by transfer. of composite porous layers supplementary material

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1 Low-temperature fabrication of dye-sensitized solar cells by transfer of composite porous layers supplementary material Michael Dürr, Andreas Schmid, Markus Obermaier, Silvia Rosselli, Akio Yasuda, and Gabriele Nelles Materials Science Laboratory, Sony Deutschland GmbH, Stuttgart, Germany Supplementary material on some of the topics discussed only shortly in the main manuscript are given below in the order of appearance in the main manuscript: Dissolving the gold layer applicability of lift-off process for mass production: Although the lift-off method is not restricted to the use of gold as the only material for the spacer layer, one might raise the questions if in principle the strategy of dissolving a thin gold layer is (a) cheap and (b) fast enough for a future production process. To measure the time needed to dissolve the gold in an iodine/iodide electrolyte, a cell consisting of a gold substrate on glass and a second glass plate, separated by a 30-µm-thick gap, was assembled. The absorbance of such a cell at = 800 nm was measured as a function of time (Fig. S1). With its absorption maximum at about 400 nm, the electrolyte does not change the absorbance of the cell at = 800 nm other than by reducing the reflection at the two interfaces between the glass plates and the gap material, i.e. either air or electrolyte. This leads to the initial fast drop of the signal after injection of the electrolyte. The subsequent drop of the absorbance is then attributed to the dissolving of the gold which occurs within 10 s for the 10-nmthick gold layer. We found a linear dependence on layer thickness. There is also a 1

2 dependence on the concentration of the redox pair in the electrolyte, however this dependence was found to be less pronounced in the investigated range of concentrations. The time needed for the dissolving of the gold is therefore much shorter than, e.g. the time required to stain the porous layers [S1]. With a layer thickness of 10 nm, the gold price is calculated to be lower than the costs for the transparent conductive oxide substrates. Additionally, the gold can be recycled by means of electrochemical methods. Whereas in lab-style fabrication of the lift-off cells the porous layers were manually moved from one substrate to another one, they can be also transferred be means of being sandwiched between the two substrates. This method is then also applicable for a roll-to-roll process as one out of several possibilities to implement the lift-off process into future mass production processes. Optical properties of layers comprising TiO 2 nanorods: The enhanced light scattering of the porous layers which comprise not only nanospheres but also nanorods can be seen the best from measurements of the incident-photon-to-current-efficiency (IPCE). In Fig. S2, IPCE data for a 8-µm-thick layer composed of only nanospheres are compared to IPCE data measured for a porous layer comprising a 5-µm-thick layer of nanospheres with an additional, approximately 3-µm-thick layer of nanorods. In the longer wavelength region, the IPCE values are higher for the more scattering films with nanorods. J V characteristics of the cells assembled on plastic substrates: Analogous to the lift-off cells prepared on glass substrates, the J V characteristics of cells fabricated on plastic substrates have been measured. In Fig. S3, 2

3 such an J V curve is shown for illumination with simulated sunlight at an intensity of 100 mw/cm 2. With a short circuit current density J SC = 10.8 ma/cm 2, an open circuit voltage V OC = 785 mv, and a fill factor FF = 0.68, the maximum power conversion efficiency was calculated to be = 5.8 %. The lower short circuit current density is mainly explained by the reduced transmission of the ITO-covered PET in the visible range of the solar spectrum, and indeed this has been confirmed by reference measurements with the PET substrate used as an additional filter for cells with glass substrates and vice versa. We therefore conclude that the quality of the porous TiO 2 layers is similar both on rigid and flexible substrates. Transport characteristics of porous layers fabricated according to the compression method and comparison with standard and lift-off layers: When cells fabricated by the standard or lift-off method are compared to cells fabricated by the application of the compression method only, the latter ones show comparable efficiency at low light intensities but significantly lower efficiencies when illuminated with full sunlight. This behavior is generally attributed to less efficient electron transport through the pressed layers, whereas the lift-off layers are expected to show the same transport properties as standard layers since they are prepared following the same sintering protocol. Another reason could be a difference in the contact between the TCO and the porous TiO 2 layer. This latter possibility seems to be unlikely from the experiments presented in the main text: because we make use of an adhesion layer similar to the porous layer used for the pressed films, both layers should then exhibit the same contact properties to the TCO. Moreover, one observes a decreasing efficiency with increasing thickness of the adhesion layer, although the 3

4 quality of the contact between the TCO and the adhesion layer should not change with the adhesion layer s thickness. However, to make this point even clearer, the transport properties through the porous layers were compared for hot-sintered and pressed layers. The experimental set-up is shown in the inset of Fig. S4 and is similar to the procedure reported by Nelson and coworkers [S2]. Thin stripes of colored porous layers were prepared on glass substrates and contacted from both sides with conductive silver paste. Such symmetric devices allow for the measurement of the photoconductivity parallel to the substrate. In contrast to the procedure in [S2], contacting the porous layer after its processing ensures the same contact quality for both hot-sintered and pressed layers. I V curves are then measured via those contacts for different levels of illumination intensity. To avoid rapid degradation of the dye molecules, a mild vacuum (15 mbar) was applied. For all the layers, we observe linear I V curves (Fig. S4) with a strong dependence of conductivity on light intensity (Fig. S5). Since the intrinsic charge carrier concentration n i is negligible in the wide band-gap semiconductor TiO 2, the conductivity = e (n i + n ph ), with e being the elementary charge and the charge carrier mobility, relies mainly on the photo-injected charge carrier density n ph. Under the assumption of constant mobility, the conductivity is thus a linear function of light intensity, as indeed is shown for the hot-sintered films in Fig. S5. The pressed layers also show such a dependence on light intensity, however, the measured conductivity is well below the value of the hot-sintered films. This could be either due to a lower charge carrier concentration or due to lower charge carrier mobility. For the former case, again two possibilities exist: either a lower injection rate or a higher recombination rate might be responsible for the lower charge carrier concentration. However, since at low light intensity, the pressed-layer cells show results similar to 4

5 the results of the standard-layer cells, lower injection efficiency seems to be unlikely. With respect to the recombination rate, we measured the conductivity as a function of time for layers illuminated at constant light intensity. Surprisingly, we find a different behavior for the pressed and the hot-sintered cells. Whereas the latter ones show a slight initial decrease with time similar to what is reported in [S2] for their layers measured under ambient conditions, the conductivity of the pressed layers increases slowly with time, closer to the behavior of the layers of [S2] which have been measured in vacuum (10-1 mbar). The authors of [S2] attributed the observed difference to different recombination rates: whereas the recombination rate is high for the cells measured under ambient conditions due to efficient electron scavenging by the oxygen present, the recombination rate is low for the cells measured in vacuum. By considerations as to the reasons for the different time dependencies in the conductivity of the hot-sintered and pressed layers in these experiments we therefore conclude that there is a higher recombination rate for the hot-sintered films. As a further consequence, we exclude a higher recombination rate to be responsible for the lower absolute values of the conductivity of the pressed layers. This naturally leads us to conclude on a lower charge carrier mobility in these pressed films. It is likely due to an inferior connection between the single particles. On the other hand, lower charge carrier mobility leads to a higher internal resistance contributing to the overall serial resistance of the device. Such an increased serial resistance is known to reduce the fill factor of solar cells, especially at higher current densities, i. e. higher illumination intensities [S3]. The reduced charge carrier mobility may therefore, at least in part, account for the non-linear dependence of efficiency on light intensity as observed, e.g. in Ref. [4] and in our own experiments with cells prepared from such layers. 5

6 References: [S1] Späth, M. et al. Reproducible manufacturing of dye-sensitized solar cells on a semi-automated baseline. Prog. Photovolt: Res. Appl. 11, (2003). [S2] Nelson, J., Eppler, A. M. & Ballard, I. M. Photoconductivity and charge trapping in porous nanocrystalline titanium dioxide. J. Photochem. Photobio A 148, (2002). [S3] Böer, K. W. Survey of semiconductor physics, Volume II. Van Nostrand Reinhold, New York (1992). 6

7 Figure legends: Fig. S1: Dissolving a gold layer in iodine/iodide solution. Time dependence of the absorption of a thin gold layer at = 800 nm when being dissolved in an iodine/iodide solution with c(i - ) = 0.5 M and c(i 2 ) = 150 mm. After injection of the electrolyte into the 30 µm thick gap between the gold substrate and a second glass plate, the absorbance shows a rapid drop due to reduced reflectivity of the filled cell. The subsequent slower drop is attributed to the dissolving of the gold layer because the electrolyte shows no absorbance in this wavelength region. Inset: experimental set-up. Fig. S2: Incident-photon-to-current-efficiency of standard layers and layers consisting both of nanospheres and nanorods. Stronger light scattering in the longer wavelength region leads to more efficient absorption and in consequence to a better external quantum efficiency of the layers with nanorods. Fig. S3: Current density and efficiency as a function of cell voltage. Best result for a 0.24-cm 2 -area DSSC fabricated by means of lift-off techniques on an ITO-covered PET substrate. Irradiation intensity was 100 mw/cm 2 of simulated sunlight (air mass AM1.5). Fig. S4: Current-voltage characteristics for light-induced electrical transport in nanoporous TiO 2. Inset: set-up for the measurement of light-induced conductivity. The colored layers prepared on glass were about 1µm thick and the gap between the two electrodes was approx. 500 µm. Main panel: I V characteristics of hot-sintered 7

8 films for different light intensities. (Light intensity was measured outside the chamber and is not corrected for reflection and absorption losses.) Fig. S5: Light-induced conductivity as a function of light intensity. Filled symbols: hot-sintered layers, open symbols: pressed layers. Experimental conditions as in Fig. S4, the data have been corrected for the measured conductivity in dark. The experimental values for the hot-sintered films compare well with the data from Ref. [S2]. 8

9 Dürr et al. Low-temperature fabrication - supplementary material Figure S1: Absorbance [a.u.] injection of electrolyte 10 nm gold light = 800 nm Time [s] Au electrolyte

10 Dürr et al. Low-temperature fabrication - supplementary material Figure S2: Standard cells hot sintered d = 9 m IPCE [%] spheres spheres and rods Wavelength [nm]

11 Dürr et al. Low-temperature fabrication - supplementary material Figure S3: Current density J [ma/cm 2 ] current density efficiency Voltage [V] AM 1.5 max = 5.8 % Efficiency [%]

12 Dürr et al. Low-temperature fabrication - supplementary material Figure S4: 4 Current [10-9 A] 2 l light 0 dark 10 mw/cm mw/cm 2 50 mw/cm mw/cm Voltage [V]

13 Dürr et al. Low-temperature fabrication - supplementary material Figure S5: Specific conductivity [10-4 AV -1 m -1 ] sintered layers linear fit pressed layers linear fit Light intensity [mw/cm 2 ]