Programm Photovoltaik Ausgabe 2009 ... - Bundesamt für Energie BFE

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Tasks of the collaboration partners
New materials (Ciba, TUE): Ciba’s competence in synthesizing tailor-made materials for a variety of
dye applications is crucial for this project. The challenge in the design of synthetic organic semiconductors
for solar cells is to optimize the absorption spectra (for matching the illumination), the transport
energy levels and mobilities (for low electrical losses) as well as the electron-hole binding energy (for
efficient dissociation). Solubility and printability concerns add additional constraints.
A new series of small band gap donor-type polymers with high intrinsic charge carrier mobility and energy
levels that are optimized for charge carrier generation in combination with PCBM need to be synthesized.
Of the most promising materials multi-gram quantities will be available for further studies and
printing at the other partners.
Cell optimization and new concepts (TUE, CSEM): Even though the excited state energy of the
absorbing solar cell material can be as high as 2 eV, the open-circuit voltage is typically as low as
0.6V. This represents a loss by a factor of 3 and is one example of the room for improvement. While
high efficiencies and innovative concepts were demonstrated with multi-cell stacks at TUE, in this project
we will focus on low-complexity cell concepts that are feasible for printing. Solution-processed organic
solar cells rely on bulk heterojunctions that must separate the charges effectively. These blends
must be optimized for high efficiency, open-circuit voltage and fill factor. Combinatorial device fabrication
by use of a high-throughput pipetting robot (cf. figure 3) and automatic characterization has
proven highly useful for OLED research and development at CSEM and shall be explored for solar
cells.
The active layers need to be fully characterized with respect to their morphology and optical absorption.
The IQE, EQE, and J-V characteristics of the devices will be characterized to determine the major
loss mechanisms and –hence – identify opportunities for further optimization.
Figure 2: CSEM’s ink-jet printer (left) and the high throughput fabrication robot (HTF-7) for polymer
devices based on a modified pipetting robot (right).
Cell characterization methods and tools (UJI, TUE, CSEM, ZHAW): Electrical characterization of
solar cells by impedance spectroscopy has been pioneered at UJI for dye sensitized solar cells. Equivalent
circuit models are physically motivated and able to reveal loss mechanisms. While impedance
spectroscopy has also been employed in the past to study OLEDs, little is known in the context of organic
solar cells and the numerical modelling thereof.
Cell modelling (ZHAW, UJI): A comprehensive device model for the study of operation mechanisms
and the interpretation of measured data will be developed. It will cover the whole process chain from
light absorption, exciton dissociation, charge carrier transport and collection by electrodes. The ZHAW
has expertise in numerical modelling of opto-electronic processes in transient and steady state in
OLEDs. The maximum achievable short circuit-current will be addressed with optical simulations that
provide the spatial exciton generation rate density. It is intended to distinguish the detrimental effects
of charge trap, recombination and collection losses by the use of drift-diffusion simulations.
APOLLO, B. Ruhstaller, ICP ZHAW
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