A low temperature route towards hierarchically structured titania films for thin hybrid solar cells
Solar energy as a renewable energy source has drawn great attention over last decades due to the depletion and environmental impact of fossil fuels. Many technologies have been developed to harvest solar energy, among which solar cells are a promising technology to directly convert energy into electricity. To date, the photovoltaic market is dominated by silicon-based solar cells as they give reasonable high power conversion efficiencies. However, silicon solar cell panels are heavy, brittle and costly. Therefore, extensive research is dedicated to find alternative solar cell systems. Among them, dye-sensitized solar cells (DSSCs) feature low costs, use of an easy manufacturing process and efficiency values up to 12 %. Nevertheless, manufacturing of DSSCs requires high-temperatures typically above 400˚C, which challenges the fabrication of flexible devices. Moreover, the use of dye molecules complicates the DSSC manufacturing process and increases the energy payback time. Thus, fabrication of dye-free hybrid solar cells at low temperatures is a promising approach to optimize current DSSC technology.
In the present work a low-temperature route for photovoltaic devices is realized by a combination of n-type titania films and p-type poly(3-hexylthiophene) (P3HT) in the active layer of the hybrid solar cells. In order to achieve more efficient titania photoanodes, artificial super-structures are introduced to mesoporous titania films at the low temperature. The sketch of the super-structured titania film integrated in to the bottom part of a hybrid solar cell is shown in Figure 1a. The hierarchically structured film is fabricated via the polymer template assisted sol-gel synthesis in combination with nano-imprint lithography (NIL). In this approach, the sol-gel chemistry gives rise to the mesopores (the size is in the range of 10 to 20 nm), whereas the NIL provides periodic superstructure in submicrometer length scale on the top of the mesoporous titania films. The surface morphology is characterized by scanning electron microscopy (SEM) as illustrated in Figure 1b, where the artificial superstructures and film mesoporous nature are clearly demonstrated. The inner film morphology is probed with grazing incidence small-angle X-ray scattering (GISAXS) measurements performed at the Austrian SAXS beamline at Elettra.
An example of the obtained 2D GISAXS data is depicted in Figure 1c. We find that NIL produces ordered superstructures efficiently and does not has any negative influence on the formation of mesopores, which are essential for exciting splitting.
Figure 1. a) Schematic of bottom part of hybrid solar cells based on a super-structured mesoporous titania film produced at low temperatures. b) Cross-section SEM image and c) 2D GISAXS data of a nano-imprinted mesoporous titania film.
The nano-imprinted mesoporous titania films are filled with P3HT to form the active layer of hybrid solar cells. Through the study of the active layers, we find that more light is absorbed by the nano-imprinted active layers as compared to the original active layers (without super-structures).
Finally, the dye-free hybrid solar cells are fabricated with our experimental route at low temperature to give a proof of practicability. The cells are measured at various angles of light incidence. The obtained power conversion efficiency (PCE) , short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF) are shown in Figure 2 as function of different light incidence angles for nano-imprinted and original cells. In general, the angle-dependent PCE and Jsc of the nano-imprinted solar cells are higher than those of the reference solar cells. This phenomenon is caused by the super-structures enhancing light-harvesting in the active layer and patterned gold contacts increasing the light back-reflection. However, the Voc and FF remain almost constant, suggesting that they are not significantly affected by the artificial super-structures.
Figure 2. a) PCE, b) Jsc, c) Voc, and d) FF normalized to the values that measured at zero incident angle for original solar cell. The pink triangles and the blue squares represent nano-imprinted and original solar cells, respectively. The dashed lines are guides to the eyes.
This research was conducted by the following research team:
Lin Song1, Amr Abdelsamie1, Christoph J. Schaffer1, Volker Körstgens1, Weijia Wang1,Tianyi Wang1, Efi Dwi Indari1, Thomas Fröschl2, Nicola Hüsing2, Tobias Haeberle3, Paolo Lugli3, Sigrid Bernstorff4, Peter Müller-Buschbaum1
1 Physik-Department, Technische Universität München, Garching, Germany
2 Materialchemie, Chemie und Physik der Materialien, Universität Salzburg, Salzburg, Austria
3 Department of Electrical Engineering and Information Technology, Technische Universität München, München, Germany
4 Elettra – Sincrotrone Trieste, Trieste, Italy
Contact person: Peter Müller-Buschbaum, email: muellerb [at] ph [dot] tum [dot] de
