The Interconnection Between Efficiency and Morphology of Two Component Systems in Plastic Solar Cells
- PDF / 207,159 Bytes
- 6 Pages / 432 x 647 pts Page_size
- 93 Downloads / 298 Views
ABSTRACT Using the ultrafast photoinduced electron transfer with long-living charge separation in the conjugated polymer/fullerene thin films, photovoltaic devices have been fabricated. The photoinduced charge separation happens with internal quantum efficiency near unity. The performance of such "bulk heterojunction" photovoltaic devices is critically dependent on the transport properties of the interpenetrating network. The influence of the variation of different donor / acceptor materials on the sample morphology is monitored by atomic force microscopy (AFM), while I/V characteristics have been studied to evaluate the conversion efficiency.
INTRODUCTION The technological advantages for the fabrication of polymer based organic solar cells, like roll to roll production of large areas, may lead to a possible reduction of the production costs. The mechanical flexibility as well as the tunability of the bandgap offer interesting perspectives of polymer based solar cells as compared to solar cells based on inorganic materials. Because of these advantages, the development of polymer solar cells would have a major impact, even if the efficiencies of these types of photovoltaic devices up to now are smaller than the efficiencies achieved in inorganic solar cells. Conjugated polymers such as derivatives of poly(para-phenylenevinylenes), or polythiophenes exhibit an ultrafast photoinduced electron transfer to C60 with forward transfer faster than 200 femtoseconds in the solid state [1, 2]. Thus, the quantum yield of the charge separation in such D/A blends is near unity. The charge separated state, however, is metastable at low temperatures. This photophysical process, resembling the primary steps of photosynthesis, has been utilized to fabricate solar cells [3, 4, 5]. Studies of conjugated polymer/fullerene photovoltaic devices showed that the energy conversion efficiency is limited by the collection of the charges at the electrodes [6].
C8H17
O
O
S
n
MDMO-PPV
C60
PCBM
n
P3OT
Figure 1: Chemical structure of MDMO-PPV, C60, PCBM and P3OT. While the excellent photovoltaic response of alkoxy-PPVs with different fullerene-based acceptors has been shown, the class of substituted polythiophenes is known to show a higher tendency of phase separation with fullerenes [7]. It is therefore interesting, to compare these two different classes of conjugated polymers with respect to their photovoltaic behavior in composites with fullerenes. The influence of the different acceptors on the morphology of the interpenetrating network is investigated separately for various substituted fullerenes.
EXPERIMENTAL The active area of the devices investigated was 4 times 4 cm2 on substrates with 6 cm by 6 cm. First the PEDOT-PSS (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (Baytron – Bayer AG) was spin coated (thickness approximately 100 nm) on transparent ITO (indium tin oxide) -coated polyester substrates (surface resistance of 55 ohm/square). P3OT cells were prepared by spin coating from ~1 wt.% xylene solutions on the t
Data Loading...
