ORGANICPV
light is absorbed in an organic semiconductor, the energy produces a neutral quasiparticle, or exciton, rather than free charge carriers. In most organic solar cells, the exciton is typically dissociated into free charges at the interface between two different organic semiconductors with different electron affinities, hence the widespread use of donor/acceptor blends.
However, while the active layer of an organic solar cell needs to be ~100-200 nm thick to absorb most of the incident light, the diffusion length of an exciton is ~10 nm,7, 8
and thus the donor and
acceptor materials must be mixed on this length scale to yield an efficient device.
Analysis of this nanoscale morphology requires high-resolution spatial mapping of the active layer, particularly using scanning probe techniques such as atomic force microscopy (AFM) and its various extensions. Scanning probe microscopy is especially useful because of the ability to image at
50
resolutions approaching the ~10-100 nm scale of the domains observed in common OPV materials. Several groups have, for example, analyzed OPV systems using AFM,9, 10
electrostatic force microscopy (EFM),11
conducting AFM,11, 12 and
scanning Kelvin probe microscopy (SKPM).13-16 Optical variations such as near-field scanning optical microscopy (NSOM)17, 18 luminescence-based AFM19 probe OPV blend morphology.
and tunneling have also been used to
We have recently reviewed the broad use of scanning probe microscopy in the field of organic electronics20
and identified specific areas of
nanoscale physics that are important to OPV operation.21
In this article, we take a more practical turn and discuss the experimental challenges and opportunities associated with two different AFM techniques, photoconductive AFM (pcAFM)22, 23 time-resolved EFM (trEFM),24
that have been used
help understand how morphology impacts OPV performance.
Figure 1. (A)
Schematic of a typical bulk heterojunction organic photovoltaic device. Schematic diagrams of the (B) trEFM and (C) pcAFM experimental setups based on Asylum Research’s MFP 3D- AFM system
trEFM is a non-contact technique that utilizes time- resolved measurements on OPV layers to analyze the local variations in photoinduced charge generation, collection, and discharge, while pcAFM is a contact-mode method that measures the photocurrent directly to obtain useful information of the local morphology and its relation to the local photoresponse. Figure 1b and 1c show a schematic diagram of the instrumentation used in trEFM and pcAFM. In our lab, we have implemented both techniques using Asylum Research’s MFP-3D-BIO AFM coupled to a Nikon TE2000-U inverted optical microscope and controlled using a custom code suite written in Igor (WaveMetrics, Inc.), the scientific software environment used by Asylum Research’s AFMs . The system is mounted on an optical breadboard to accommodate external optics (Thor Labs) and is supported by a passive vibration isolation stage (Minus K Technology). Neutral density (ND) filters allow us to adjust the illumination intensity in both techniques. The entire system is housed within a plywood box with copper mesh shielding and acoustic-damping foam. In order to protect potentially air-sensitive samples from photo- oxidation, we use an Asylum Research sealed fluid cell. Originally designed for imaging biological samples under liquid, the design permits the loading of sensitive samples into the cell in a glovebox and allows us to perform measurements while purged with dry nitrogen. This approach obviates the need to install the entire AFM/optical system in a glovebox.
and
www.solar-pv-management.com Issue II 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 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104