Identification and overcoming of loss mechanisms in nanostructured hybrid solar cells - pathways towards more efficient devices
Final Report Abstract
Promising candidates for low-cost alternatives to Si photovoltaics are based on organic compounds and metal-oxides. In this project we have focused on hybrid solar cells combining inorganic TiO2 and organic polymer such as P3HT. We have investigated the current limitations of such solar cells and were able to improve their performance. There are a number of reasons why hybrid solar cells show a lower performance than e.g. organic solar cells. In our project we were able to identify some of the reasons by careful analysis of the metal-oxide structure. We could show that interface modification can drastically change the charge transfer. The role of the interface engineering through dye layers has been systematically investigated and found that the system which allows energy transfer from the polymer to the dye is most efficient. It is currently still an open question, why in TiO2:dye:P3HT cells electron injection from the P3HT to the dye or TiO2 is very inefficient even though energetically allowed. However, if the excitation happened in the dye, we find efficient hole transfer to the polymer. The reason for this unbalance is still unclear and needs further investigations. We could show that core-shell nanostructures allow suppressing charge recombination at the organic/inorganic interface through cascading energy levels. Such cascading levels can be realized for example by creating a 1-2 nm thin anatase shell on rutile TiO2 nanowires. Advanced transmission electron microscopy studies revealed how defect states in hydrothermally grown TiO2 nanowires are developing and what influence they have in terms of device performance. We were able to improve the device performance through healing these defect states by thermal annealing and also by doping of the nanowires. All over there is not a single reason that limits the performance of hybrid solar cells, but there are many issues influencing the device performance. Therefore in such hybrid systems all effects need to be taken in account, which includes all materials, and their film/structure formation. Interfaces between such layers need to be carefully tuned and fully optimized. This project significantly helped to understand several aspects that currently lower the performance and therefore consecutively helped to improve the device performance. It is now possible to give further design rules for optimized hybrid solar cells.
Publications
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Nanoscale investigation on large crystallites in TiO2 nanotube arrays and implications for high-quality hybrid photodiodes. Journal of Material Science. 47 (17), 6459 (2012)
A. Wisnet, M. Thomann, J. Weickert, L. Schmidt-Mende, C. Scheu
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Synergistic Effects of Interfacial Modifiers Enhance Current and Voltage in Hybrid Solar Cells, APL Mater. 1 042109 (2013)
J. Weickert, E. Zimmermann, J.B. Reindl, T. Pfadler, J.A. Dorman, A. Petrozza, L. Schmidt- Mende
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Control of Recombination Pathways in TiO 2 Nanowire Hybrid Solar Cells using Sn 4+ Dopants, J. Phys. Chem. C. (2014)
J.A. Dorman, J. Weickert, J.B. Reindl, M. Putnik, A. Wisnet, M. Noebels, C. Scheu, L. Schmidt- Mende
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Model for Hydrothermal Growth of Rutile Wires and the Associated Development of Defect Structures, Crystal growth and design 14 (9) 4658 (2014)
A. Wisnet, S. Betzler, R. Zucker, J.A. Dorman, P. Wagatha, S. Matich, E. Okunishi, L. Schmidt- Mende, C. Scheu
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Template-free synthesis of novel, highly-ordered 3D hierarchical Nb3O7(OH) superstructures with semiconductive and photoactive properties, J Mater Chem A 2 12005 (2014)
S.B. Betzler, A. Wisnet, B. Breitbach, C. Mitterbauer, J. Weickert, L. Schmidt-Mende, C. Scheu
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A comparison of light-coupling into high and low index nanostructured photovoltaic thin films, APL Materials, 3, 066101(2015)
T. Pfadler, M. Stärk, E. Zimmermann, M. Putnik, J. Boneberg, J. Weickert, L. Schmidt-Mende
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Defeating loss mechanisms in 1D TiO 2- based hybrid solar cells, Adv Funct Mater 25 (17), 2601 (2015)
A. Wisnet, K. Bader, S. B. Betzler, M. Handloser, P. Ehrenreich, T. Pfadler, J. Weickert, A. Hartschuh, L. Schmidt-Mende, C. Scheu, J. A. Dorman
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Role of Charge Separation Mechanism and Local Disorder at Hybrid Solar Cell Interfaces, Phys Rev B 91, 035304 (2015)
P. Ehrenreich, T. Pfadler, F. Paquin, L.-I. Dion-Bertrand, O. Pare-Labrosse, C. Silva, J. Weickert, L. Schmidt-Mende
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Towards High-Efficiency Solution-Processed Planar Heterojunction Sb 2S3 Solar Cells, Adv Science 2 (5) (2015)
E. Zimmermann, T. Pfadler, J. Kalb, J.A. Dorman, D. Sommer, G. Hahn, J. Weickert, L. Schmidt- Mende
