UC Berkeley

Transparent electronics has been considered as a critical key to solve problems exhibited by virtually all flat panel displays and solar cells. Transparent electronics is expected to allow for the realization of displays with high aperture ratio for high brightness. This will enable the embedding of system-level electronics directly onto the display glass. The minimization of light lost when going through multi-layer stack structures is a major opportunity for solar cells and particularly for transparent solar cells consisting of transparent photovoltaic units. Over the last decade, amorphous phase and poly crystalline metal oxides, including ZnO, In2O3, and SnO2 and their ternary and quaternary alloys, have been considered as candidates for transparent electronics. Using conventional vacuum-based deposition techniques and lithography technology, metal oxide based transistors with field effect mobility values high enough for the simultaneous operation of integrated circuits, pixel drivers and peripheral drivers have been demonstrated. In addition, by combining transparent source, drain, and gate electrodes with a transparent insulator, fully transparent circuits can be fabricated. Normally, hetero- junction solar cells consist of an absorber layer, its metal contacts, window layers, and their metal contacts. Transparent metal oxide n-type conducting window layers can be fabricated with the aforementioned semiconductors (ZnO, In2O3 or SnO2). These layers provide a large band gap, excellent electronic transport properties, and easy metal contact formation. Among the aforementioned metal oxides SnO2 has its own promising characteristics. It has a larger band-gap, lower melting point and higher bulk mobility Thus, a high-quality SnO2 layer can potentially be created at a relatively low sintering temperature resulting in high performance transistor characteristics.

The main goals of this field are to improve process throughput for large area panels, to lower fabrication cost and to improve device performance. Combining the aforementioned attractive properties of SnO2 with the process benefits of inkjet printing, fully ink-jet printed SnO2 TFTs were demonstrated as a good candidate to achieve these main goals. To realize this, SnO2, ZrO2, and Sb-doped SnO2 were selected for the transparent semiconductor, the insulator and the electrodes respectively. Solution phase

sol-gel precursors were used as inks for inkjet printing to form these materials. To fabricate uniform and coffee ring less layers, the spreading speed and evaporation rate were controlled by addition of a high-viscosity solvent mixture and with increased substrate temperatures. Using these innovations, transparent metal-oxide-based transistors were fabricated on glass substrates. These devices consisted of a SnO2 semiconductor layer, a ZrO2 insulator layer and Sb-doped SnO2 electrodes formed through ink-jet printing. They exhibited excellent transistor characteristics, and thus represent a promising step towards making printed transparent electronic systems a reality.