Devices solution-processed

However, only small changes have a pronounced impact on the morphology of bulk heterojunction thin films as well as their charge carrier separation and transportation. This in turn influences the photovoltaic properties of the small molecule-based bulk heterojunction devices. Solution-processed organic solar cells based on the molecules with the shortest methyl end groups exhibit a high short circuit current and fill factor (128). 

In Section 13.2, we introduce the materials used in OLEDs. The most obvious classification of the organic materials used in OLEDs is small molecule versus polymer. This distinction relates more to the processing methods used than to the basic principles of operation of the final device. Small molecule materials are typically coated by thermal evaporation in vacuum, whereas polymers are usually spin-coated from solution. Vacuum evaporation lends itself to easy coaling of successive layers. With solution processing, one must consider the compatibility of each layer with the solvents used for coating subsequent layers. Increasingly, multilayered polymer devices arc being described in the literature and, naturally, hybrid devices with layers of both polymer and small molecule have been made. 

Although high-performance TFTs are needed for several electronic applications, the potential for printed, inorganic electronics encompasses other devices and applications. A major opportunity is in optoelectronic applications, which impose different requirements, challenges, and opportunities (see Chapters 6, 7, 9, and 11 for discussion of solution-processed solar cells and other printed optical devices). 

Figure 4.9. Log(/D)-VGS (VDS = 20V) and (inset) /D Vds characteristics for an RF-sputtered ZnO TFT on a spin-coated A1PO dielectric annealed at 300 °C. VGS is stepped from 0 to 40V in 5-V increments for the ID-VDS curves device W/L = 5 L = 100 pm. [Reproduced with permission. Meyers, S. T. Anderson, J. T. Hong, D. Hung, C. M. Wager, J. F. Keszler, D. A. 2007. Solution processed aluminum oxide phosphate thin-film dielectrics. Chem. Mater. 19 4023-4029. Copyright 2007 American Chemical Society.]...

Figure 5.14 and Table 5.4 show the electrical characteristics of the fabricated TFTs (W/L = lOpm/lOpm). TFT-4 and 5 (Gox, UDL and channel Si are solution-processed) have the mobility values, 23,0cm2/Vs and 9.9cm2/Vs, respectively. They are lower than that of TFT-6 (only the channel silicon was solution-processed). In this experiment, however, the mobility of the reference TFT (TFT-6) is also relatively poor, as expected, because the laser power and other conditions under which the channel silicon was solution-processed were not optimized. Thus, the mobilities of TFT-4 and TFT-5 were also affected by the channel silicon and were much lower than the mobilities of TFT-1 and TFT-2. With optimization of the conditions under which the channel silicon is deposited, we believe that higher mobility values can be achieved in the devices with solution-processed Gox, UDL, and channel Si. 

Finally, silicon-based polymers, especially with hydrogen lateral groups, are very interesting, but they are not yet explored sufficiently. There are many unknown properties in these materials, including the details of the photopolymerization process and a-Si formation from polysilane. Additional academic work in this field is expected and necessary to make the solution processing of silicon devices more convenient and reliable. 

Kuo, C. Payne, M. Anthony, I Jackson, T. 2003. Solution processed OTFTs with lcm2/V-s mobility. 62nd DRC Device Research Conference. Conference Digest. 2 8-9. 

---