Integrated Opv-Otft System for Self-Powered, Solution Processable Organic Electronics

Abstract: Integrated OPV-OTFT System for Self-Powered, Solution Processable Organic Electronics

Renata Hanna, Doctor Yan Zhou, Professor Zhenan Bao

Bao Group, Department of Chemical Engineering, Stanford University

The field of organic electronics has attracted much attention from academia and industry for its flexible, light, low-cost, and solution processable devices. In recent years, efficient organic photovoltaics (OPVs), organic diodes (OLEDs), and organic transistors (OTFTs) have been studied and optimized. Going beyond the development of individual devices, in our study we propose a more complex circuit integrating OTFTs and OPVs.

Transistors are widely employed as amplifiers and control units in current devices. In TFTs, the gate serves as a control and the channel is responsible for the amplified response. In OTFTs, the channel is a thin-film of organic semiconductors.

Photovoltaics (PV) are a promising technology for attending global energy challenges. In PVs, the active layer is composed by an electron donor component and an electron acceptor component and is responsible for absorbing light and converting it into electricity. In OPVs, the active layer is a film composed of a blend of organic semiconductors.

The integrated circuit (IC) we propose moves away from individual devices and towards fully organic, complex circuitry that can be processed on a single substrate without the need for lithography, a limiting factor in inorganic electronics. In the IC, OPV cells generate the voltage potential necessary to drive the OTFT in the saturation region, in which the transistor achieves best amplification conditions. The circuit structure developed is solution processable, and could be applied onto flexible substrates for specific applications. In this way, the IC proposed is a self-powered, solution processable, low-cost, and potentially flexible organic system.

The proposed structure was developed with the intent to enable OPV characterization under solution environments, which is required for the study of many applications of the technology. Besides the original intent, further applications have also been devised for the proposed system. One simple application would be to apply it to flexible and portable devices that require low-weight and independent power generation. Another application would be to combine the already proposed structure to more OFET devices such as to create what are called current mirrors, through which it would be possible to amplify the current depending on the relations between device sizes. A further application of this structure would be as a type of photodetector for what is known as smart windows, which change coloration and the amount of light passing through depending on the intensity of the incident light. These applications would require further inquiry, but seem reasonable with further study, optimization, testing, and data collection.

Another application that would have great market impact would be the use of this structure, coupled with other circuit elements, as an alternative form of radio-frequency identification (RFID) active transmitter. This configuration is advantageous over other forms of RFID transmitters for several reasons, and currently, the most apparent is that it is less costly in terms of materials and production methods, since organic electronics require cheap materials that can be processed without the high temperatures and vacuum levels that inorganic electronics require.

The integrated circuit was prepared from solution through spin-coating techniques. The integration was achieved through a complex planning such that the production steps did not conflict with one another. This is important with the preparation of layers through spin-coating because the solution that is the precursor to a following layer cannot be able to dissolve previously formed layers. Moreover, it is important to have layer-planning such that the steps taken between layer-deposition steps, such as curing and annealing, do not conflict with the needs of other layers.

The development of the correct sequence of layering and processing revealed the need for further study of the thickness of the dielectric layer. Optimization of the solvent and concentration of the dielectric solution was necessary for the formation of a clear solution and of a proper thin-film with reduced number of defects and pin-holes. Moreover, the study of the influence of the concentration over the thickness of the layer indicated a possible trade-off between high mobilities and low threshold voltages. Lower dielectric solution concentrations yielded thinner layers with higher mobilities (µ) and threshold voltages (V_TH), under the condition that the layers were thick enough to prevent shunting due to pin-holes.  In opposition to these results, more concentrated solutions yielded lower mobilities and threshold voltages.

Consequently, there is a need for compromising between high mobilities and low threshold voltage, both of which are desirable characteristics for the proposed IC. Low threshold voltages are essential so that the OPVs are capable of driving the OTFTs, whereas high mobilities are more desirable because they ensure better device performances.

The study of polymer blends for the OPV active layer indicated the need for further optimization of the threshold voltage of the OTFTs since on average the OPVs could only drive up to approximately 0.9V. Considering the IC structure, higher driving voltages would allow better performances and amplification capabilities for the transistors.

Therefore, although this study has achieved an efficient process for the preparation of the IC, much needs to be done in order for the system to be operational. The main points that continue to be studied are the optimization of the dielectric layer of the OTFT and of the active layer of the OPV.