Towards See-Through Displays: Fully Transparent Thin-Film Transistors Driving Transparent Organic Light-Emitting Diodes**

Fully transparent computer displays have, until now, been the vision of science-fiction movies. Nevertheless, there are numerous applications for these devices ranging, for example, from head-mounted displays to their integration in automotive windshields. In this paper we demonstrate that this vision is on the verge of becoming reality. The realization of entirely transparent displays requires both transparent light-emitting devices, in our case organic light-emitting diodes (OLEDs) with transparent contacts, and a driving scheme based on transparent thin-film transistors (TTFTs). Since the early reports on electroluminescence from multilayer, thin-film devices composed of vacuum-sublimed small organic molecules [1] and spin-coated conjugated polymers, [2] substantial research has been devoted to the improvement of device efficiencies, color purity, and lifetime. Incitement of this development is the potential use of OLEDs in future commercial flat-panel displays. OLEDs usually consist of a layer sequence of organic functional materials (charge transporters/blockers/emitters) with an overall thickness of the order of 100 nm. Most of these materials absorb light in the deep-blue or ultraviolet spectral region and are nearly transparent in the visible part of the spectrum. Organic layers applied to emit visible light are often based on so-called guest–host systems, in which a wide-bandgap host material (absorbing in the UVonly) is doped with a few weight percent of a light-emitting dye. [3] As a result, the emitting layer appears transparent. Transparent conductive oxides, most prominently indium tin oxide (ITO) and aluminum-doped ZnO (AZO), may be used as electrical contacts to OLEDs. Therefore, OLEDs seem to be promising devices for the realization of entirely transparent visible-light emitters. [4–6] OLED displays driven in passive-matrix mode are based on conventional bottom-emitting OLEDs and are considered as an approach to fabricate small-sized, low-information-content displays with moderate pixel counts. To accomplish largerarea, high-resolution OLED displays an active-matrix-addressing scheme has been suggested. [7] Conventional a-Si:H or poly-Si TFT backplanes are not suitable as drivers for transparent displays because they are opaque in the visible part of the spectrum. Putting pixels and transistors next to each other would compromise the displays’ fill factor. Organic field-effect transistors (OFETs) as pixel drivers for OLEDs have been discussed by Sirringhaus et al. [8] Transparent OFETs have also been reported. [9] However, their performance is still poor; transparent active pixels using OFETs have not yet been demonstrated. Therefore, in this paper we focus on the production of TTFTs based on the wide-bandgap oxide semiconductor zinc tin oxide (ZnO)x(SnO2)1–x ,a s a viable alternative to previously fabricated devices. Recently, research on TTFTs with channels made from ox