Devices annealing

Kim Y., Choulis S. A., Nelson J., Bradley D. D. C., Cook S. and Durrant J. R. (2005), Device annealing effect in organic solar cells with blends of regioregular poly(3-hexylthiophene) and soluble fuUerene , Appl. Phys. Lett. 86, 063502. 

Figure 6.12 Field-effect mobilities and current on/off ratios of OFET devices annealed at 120 °C for different time intervals, using DHBTP-SC (circles) and DHPT-SC (rectangles) as the semiconductors. Open symbols depict the on/off current ratio whereas solid symbols the field-effect mobility.

As noted, solar cells fabricated with yrTCy BM active layers yielded devices with a long-standing record in PCE. Top-performing devices had an active layer that was a 3 2 7 PCy BM ratio cast from chloroform and were annealed at 110 °C. Chloroform is a relatively fast drying solvent and annealing was used to control domain size to lead from a PCE of 0.3% for as-cast devices to 4.4% for devices annealed at 110 °C. Similar PCEs could also be achieved for annealed 7 PCyiBM devices cast from other relatively low boiling point... 

Another problem in the construction of tlrese devices, is that materials which do not play a direct part in the operation of the microchip must be introduced to ensure electrical contact between the elecuonic components, and to reduce the possibility of chemical interactions between the device components. The introduction of such materials usually requires an annealing phase in the construction of die device at a temperature as high as 600 K. As a result it is also most probable, especially in the case of the aluminium-silicon interface, that thin films of oxide exist between the various deposited films. Such a layer will act as a banier to inter-diffusion between the layers, and the transport of atoms from one layer to the next will be less than would be indicated by the chemical potential driving force. At pinholes in the AI2O3 layer, aluminium metal can reduce SiOa at isolated spots, and form the pits into the silicon which were observed in early devices. The introduction of a tlrin layer of platinum silicide between the silicon and aluminium layers reduces the pit formation. However, aluminium has a strong affinity for platinum, and so a layer of clrromium is placed between the silicide and aluminium to reduce the invasive interaction of aluminium. 

Figure 16-41. Electrical characteristics lor single-layer ITO/Oocl-OI V.VAI devices with an as-deposited (O) or annealed (I20"C, 5 min 0) active layer. Inset luminance as a function of cell current for both thin-filin morphologies.

The substituted five-ring OPVs have been processed into poly crystal line thin films by vacuum deposition onto a substrate from the vapor phase. Optical absorption and photolumincscence of the films are significantly different from dilute solution spectra, which indicates that intermolecular interactions play an important role in the solid-state spectra. The molecular orientation and crystal domain size can be increased by thermal annealing of the films. This control of the microstruc-ture is essential for the use of such films in photonic devices. 

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