ORGANICSOLAR
from chlorobenzene solution exhibits less phase separation than that of the films prepared from toluene solution (Fig 5a,b).
BM films results in more interfaces for exciton dissociation. The average short-circuit photocurrent (Isc
The smooth surface of chlorobenzene-cast MDMO- PPV:PC71
) under white light illumination is
more efficient than that in toluene-cast films.9 This result can be confirmed by current-voltage characteristics averaged from different locations as shown in Fig 5c.
By using tunable monochromatic lights, we are able to reveal the large phase regions within toluene-cast films (Fig 5b) are PC71
BM rich- 42
Fig 5. Morphology of MDMO-PPV:PC71
BM
films spun cast from chlorobenzene (CB) (a) and toluene (Tol) (b). The topography is shown in z-scale of 30nm for (a) and 200nm for (b), while the color is short- circuit photocurrent overlaid on the surface. (Asylum Research).
The α value is suggestive of the degree of recombination processes. For example, the α in location C is 0.42 as compared with 0.72 in location A, suggesting that the free carrier loss due to charge recombination in location C is much larger than that in location A. Further study, such as external quantum efficiency measurements, can examine the blend composition which results in high photogenerated carrier recombination in C. Comparing this investigation to processing conditions would be the best way for enhancing the efficiency of photovoltaics.
Spectral Analysis
A significant advance in the development of pcAFM for organic solar cell characterization is to move from spatial imaging to spectral analysis. Because the photoactive layer is made from a solution of donor and acceptor materials, its morphology is very complex and sensitive to processing conditions.
Using the pcAFM technique, complex nanoscale morphology and photocurrent generation can be visualized. For example, the photoactive layer of poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4- phenylenevinylene (MDMO-PPV) and [6,6] phenyl- C71
-butyric acid methyl ester (PC71 BM) spun cast
Currently, pcAFM is the only technique that can provide insights into nanoscale morphology and electrical properties simultaneously and hence elucidate structure-property-performance relationships.
However, pcAFM is still in its infancy and requires further development. It is possible to push the limit of the technique to obtain useful information that is relevant to device performance. Together with the complexity of solar cell morphology and photophysics, the quantitative analysis and interpretation of the obtained pcAFM data still remain a challenge.
Acknowledgements We thank the Department of Energy, Office of Naval Research, and the California NanoSystems Institute for the seed grant for financial support and the Camille Dreyfus Teacher Scholar Award for financial support. We thank Monteith G. Heaton of Asylum Research for reading of the manuscript and corrections..
All data for this work was obtained using the MFP-3D™ Atomic Force Microscope from Asylum Research
domains and small particles are distributed by MDMO-PPV polymer. Detailed analysis of external quantum efficiency spectra has elucidated both charge transfer mechanisms existing in the polymer solar cells (Fig 5d); i.e. the holes transfer from the photoexcited acceptor to the donor, and electrons transfer from the photoexcited donor to the acceptor.
Summary In summary, we have briefly described the applicability of the pcAFM technique for analyzing solution-processed, polymer and small molecule bulk heterojunction solar cells. Due to the nature of charge generation, transport and collection occurring at the nanometer scale, the useful information on device operation can be lost from macroscopic measurements.
www.solar-pv-management.com Issue VI 2010
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64