Guo Research Group

Organic Electronics and Photonics

 

Organic semiconductors are known to have low carrier mobilities. For thin film transistor (TFT) applications, one way of remedy the problem is to make shorter channels. Fabrication of OTFTs also require low cost fabrication methods. Our group developed a polymer inking and stamping technique to directly pattern conductive polymers on a varied of substrates with high-throughput, eliminating the use of any chemical processes that may damage the material functionality (MRS’2005). Excellent pentacene OTFTs have been demonstrated (Appl. Phys. Lett. 88, 63513-1, 2006), and the transistor channel length can be sub-micron by using this technique. Traditional ITO electrodes are dominating in OLEDs and organic photovotaics, but In and Oxygen are known to diffuse into organics and affect the device long time reliability; ITO is a brittle thin film not fully compatible with flexible substrate. Additionally the ITO are becoming a very costly commodity. A thin nanostructured metallic electrodes have been proposed and developed in our lab, and demonstrated excellent transparency (approaching that of ITO) and superb conductivity (MRS’2005). The structure has been implemented in OLED on glass and flexible substrates (Adv. Mater. 2007, and JVST. B. 2007). It further has the potential of generating polarized emission, making it very attractive for flat panel displays. Currently organic solar cells suffer from low energy conversion efficiency which is a result of insufficient light absorption, poor charge separation and limited charge transport. To simultaneous solve all these issues, we and our collaborators have demonstrated significantly improved the efficiency of organic photovoltaic devices by controlled nanoscale interfacial morphology created by nanoimprinting (Appl. Phys. Lett. 90, 123113, 2007).

 

Polymer inking and stamping technique can be extended to roll-to-roll printing of conductive polymers and other organic polymer materials for organic electronics. The new nanostructured metal electrodes could also be fabricated on a roll-to-roll basis for both OLED and organic solar cells and for making tandem cells for higher efficiency. The demonstration of nanoscale morphology control provides an effective method to systematically understand the device physics for  improving the energy efficiency.

 

 

(a) Schematic of polymer inking and stamping process for patterning PEDOT conducting polymer. (b) Cross sectional AFM image of PEDOT on PDMS stamp (c) Concentric ring patterns transferred on PES flexible substrate. (d) PEDOT electrode patterns transferred on SiO₂ substrate for BC OTFTs, gap width from 2µm to 10µm. (e) PEDOT electrode patterns transferred on pentacene thin film surface for TC OTFTs, W/L = 140 µm/2µm. Pentacene outside of the PEDOT electrode pads adhered to the stamp and are removed when the PMDS stamp is pealed off from the substrate.

 

 

 

(a) Electrical output characteristics and (b) transfer characteristics of top contact pentacene TFTs (W/L=5, channel length of 10µm) with PEDOT electrodes. Characterization was carried out by applying V_DS and V_GS of less than -20V.

 

        

                 

 

A photograph of a flexible conjugated polymer solar cell with controlled nanoscale heterojunctions. Inset and right: TDPTD on a flexible ITO-coated PET, imprinted with a 700 nm period grating mold, diffracting ambient light.

 

 

        

(a)                                                                                (b)

 

Characterization of the device performance. (a) J-V curves of the flexible conjugated polymer solar cells when illuminated by a solar simulator (intensity = 56mW/cm2). (b) The increase in short circuit current correlates with the increase in interfacial area between the donor and acceptor layer produced by nanoimprinting with grating molds of different periodicity.

Photograph and SEM image of semitransparent metal electrode