The section deals with the surface analysis of thin film of P3HT organic semiconductor (45 nm) deposited on the silicon wafer. AFM pictures reveal that the film of P3HT gives homogeneously distributed individual crystalline grain like structures. Interestingly, the size of the grain increases with increasing P3HT concentration (also film thickness). Futher, the average grain size is calculated with Gwyddion software applying grain-correlation standard library package. The software applies water-shed algorithm to the grains producing average disc radius. The average grain size of 0.5 wt% and 45 nm very thin film of P3HT P200 semiconductor deposited on clean silicon wafer (above the oxide layer) is calculated to be 150 nm.
Sample preparation: A clean silicon wafer (with oxider thickness of 100 nm) was taken. Sample was first cleaned inside Piranha solution (4:1 ratio of 98% H2SO4 and H2O2 heated at 75 °C) for 20min. Cleaned sample was then treated inside the O3 chamber for 12min. Sample was immediately placed inside the HMDS (Hexamethyldisilizane) evaporated chamber for 14 hours (Pressure < 100 mbar). Sample was then ready for the deposition of P3HT solution.
Semiconductor preparation: Sepiolid P3HT (P200) semiconductor was prepared in 2 ml of Choloroform solvent. The prepared solution was stirred at 60°C for 30min. The solution was then ready for deposition.The samples were deposited with the following spin-coating parametres:
time = 20 s, velocity = 2000 /min and acceleration = 3000 m/s2
Samples are then immedately baked on the hot plate for 30 s at 90 °C. The height of thin-film of semiconductor was measured on the Dektak Profilometer and confirmed with AFM (Atomic Force Microscope) profile measurement.
Surface measurement in AFM: Different AFM pictures were taken for semiconductor solution preapred with 0.5 to 3 by weight percentage (wt.%) of P3HT (P200) in 2 ml of Chloroform solvent. The AFM measurements are carried out at the Nanoelectronics Laboratory, Jacobs University Bremen. The measurements are as follows:
Figure (1): AFM topography (left) and phase contrast (right) measurement of a sample DDT001. The sample is deposited with 0.3 wt.% of P3HT (P200) in 2 ml of Chloroform. Film is 20 nm thick.
Figure (2): AFM topography (left) and phase contrast (right) measurement of a sample DDT004. The sample is deposited with 0.5 wt.% of P3HT (P200) in 2ml of Chloroform. Film is 45 nm thick.
Figure (3): AFM topography (left) and phase contrast (right) measurement of a sample DDT003. The sample is deposited with 1.5 wt.% of P3HT (P200) in 2 ml of Chloroform. Film is 60 nm thick.
Figure (4): AFM topography (left) and phase contrast (right) measurement of a sample DDT002. The sample is deposited with 2.0 wt.% of P3HT (P200) in 2 ml of Chloroform. Film is 165 nm thick.
The AFM clearly reveals the increasing individual crystal (grains) size with increasing concentation of the P3HT semiconductor solution. Phase contrast images from above pictures are valuable in analysing such grain structures. Average grain size calculation would be discussed in the following section.
Calculation of average P3HT grain size in Gwyddion v2.2 software: First the phase contrast image is loaded in the sofware. Necessary image correctios are then performed. Watershed algorithm is then appled to the respective image. The software then applies water bubbles to the image above the threshold (figure 5).
Figure (4): Snapshot of Gwyddion v2.2 software and the image after making all necessay noise corrections. Image correction has been applied to Phase contrast image of figure (2)- Sample DDT004, 0.5 wt.% of P3HT (P200).
Figure(5): Snapshot of the AFM picture of sample DDT004 after applying watershed algorithm.
After applying watershed algorithm, the grain correlation function from the software is opted. The softwares calculates then number of water-bubbles (total number of 145 such individual bubbles has been found from the figure (5). Second step is to calculate grain-correlation length. Goto Dataprocess-Grains-Correlate. The software then calculates the average disc radius of each individual grains. In the next step, a histogram is plotted for all such grains with similar disc radii (figure 6).
Figure (6): Histogram of a equivalent disc radius of all individual grains.
Result: Hence, the histogram gives the average grain radius of 75 nm (and size of 150 nm, discarding the error) when 0.5 wt.% of Sepiolid P3HT P200 (45 nm thick) is deposited on the clean silicon wafer above silicondioxide layer.