ORGANICPV
As with trEFM, we can assess the quantitative relationship between the pcAFM current information in characterizing OPV efficiency by correlating the spatially-averaged photocurrent in pcAFM data with EQE measurements on the same materials
54
-butyric acid methyl ester (PCBM). We observed microscopic heterogeneity in both topography and short-circuit photocurrent, even in the most efficient MDMO-PPV/PCBM solar cells.22
The
Acknowledgments
The authors thank Kevin Noone for assistance with the first figure and Obadiah Reid for helpful discussion. This review is based in part on work
supported by the NSF (DMR-0120967 and DMR-0449422), AFOSR, DOE, and ONR. D.S.G. also thanks the Camille Dreyfus Teacher- Scholar Awards program and the Alfred P. Sloan foundation for support.
variation in photocurrent in otherwise topographically similar areas implies different sub- surface compositions. More recently, we used further pcAFM measurements on P3HT:PCBM samples with different processing conditions to study the underlying morphological contribution to changes in device performance. Annealing a deposited film is a common processing step to improve the efficiency of the device.29
Using
pcAFM, we were able to directly observe the relationship between photocurrent distribution and annealing, namely the increase in both average and peak photocurrent with increased annealing time.23
For example, in Figure 4a and 4b we show the topography and corresponding short-circuit photocurrent for a P3HT:PCBM film annealed for 10 minutes. As with the MDMO-PPV:PCBM samples, local variations in photocurrent are evident within topographically featureless areas.
As with trEFM, we can assess the quantitative relationship between the pcAFM current information in characterizing OPV efficiency by correlating the spatially-averaged photocurrent in pcAFM data with EQE measurements on the same materials. As can be seen in Figure 4c, photocurrent measurements derived via pcAFM follow the same qualitative trend as the efficiencies obtained from the macroscopic devices. This
With pcAFM, the photocurrent measured at a given location reflects the local charge generation properties. At 0 V, this current represents the short- circuit current; it is also possible to record local I-V curves at each point by varying the voltage. We have performed pcAFM on typical polymer/fullerene active layers such as poly(2- methoxy-5-(3’,7’-dimethyloctyl-oxy)-1,4-phenylene vinylene) (MDMO-PPV) or poly(3-hexylthiophene) (P3HT) mixed with fullerene derivative (6,6)-phenyl-
C61
result suggests that pcAFM can probe the microscopic underpinnings of macroscopic device performance. The pcAFM data acquired can then be useful to extract electron and hole current and mobility from OPV devices and could even be used as a tool to select optimal blends and processing conditions. However, it is important to emphasize that while the qualitative trends between the pcAFM data and EQE are generally in good agreement, we often find quantitative differences between the local pcAFM averages and the bulk device properties. This is not entirely surprising given that we are using high work function tips (Au, Pt) and that contact effects are expected to play some role in the current extraction. Indeed, it is for this reason that we believe pcAFM data generally correlates better with macroscopic performance for a fixed blend ratio than it does across a wide range of donor/acceptor concentrations,22
and the
importance of the tip workfunction and any associated injection/extraction barrier should always be kept in mind when analyzing pcAFM data.
Summary and Outlook
in area will implicitly involve a great deal of averaging of the device properties and much of the local microscopic detail will subsequently be lost.
The BHJ morphology upon which OPVs rely is extremely complex. The mixing of an electron donor and acceptor in a common solution, followed by spin coating, yields a morphology that has features on a variety of length scales. These features in turn affect the ability of the device to split excitons and the ability of the resulting charges to navigate a route through the film to emerge as useful photocurrent. As a result, the performance of OPVs is inherently a local property. Measurements on bulk devices that are several mm2
The techniques we have described here are extensions to the conducting-AFM measurement made with an Asylum Research MFP-3D-BIO. These in turn have allowed us to make measurements of the morphology, electrical and optical properties of BHJs all on the nanoscale, and, crucially, on the same area of the device.
As a result we have been able to make significant steps forward in our understanding of how even well-characterized OPV systems operate in terms of the local morphology.
It seems that there is much room for further advancement. For example improvements in time resolution in trEFM beyond the current 100 µs limit, for example using newer generation equipment with higher bandwidth, could open up further time- resolved experiments on different systems such as
www.solar-pv-management.com Issue II 2010
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