The present disclosure relates to organic electronics, in particular to organic photo-detecting devices, more in particular to organic photo-detecting field-effect devices. The present disclosure is furthermore related to a method for operating such devices. Organic photo-detecting field-effect devices in accordance with this disclosure may for example be used in large area image sensor arrays or in other large area electronic devices or circuits.
In the last decade, a lot of progress has been made in the development of electronic devices based on organic semiconductors. Organic semiconductors and organic electronic devices are gaining interest for a number of reasons. Because of their ease of manipulation and processing at low temperatures, organic semiconductors can be processed on large area, flexible and transparent substrates. Because of the relatively low processing cost and the relatively low material cost of organic semiconductors, organic devices offer potentially a cost advantage over inorganic devices. In addition, the wide range in the molecular structure of organic semiconductors offers a substantial flexibility in their functionality. For example, the wavelength at which an organic emissive molecule emits light may generally be tuned by changing the side groups of the molecule. Currently, organic molecular and polymer semiconductors are used to realize, for example, field-effect transistors, light emitting devices, photovoltaic cells and photodetectors.
Large area image sensor arrays may be obtained by providing an array of organic photodetectors on a large area substrate. The substrate may be flexible to enable imaging of curved structures. An all-organic image sensor array may be achieved by integrating on the same substrate organic field-effect transistors, able to process the read-out signals of the photodetectors.
Organic photodetectors can be classified in two main groups: two-terminal photodiodes and three-terminal phototransistors.
Organic photodiodes are realized by sandwiching one or more appropriate organic layers between two conductive electrodes. Incoming light is absorbed, giving rise to the generation of excitons, being coulombic bound electron-hole pairs. Excitons are then split into free electrons and free holes, whereby the free electrons move towards the positively biased electrode, and the free holes move towards the negatively biased electrode. The organic layers are optimized to maximize the absorption of photons, the splitting of photo-generated excitons into free charge carriers and the collection of the free charge carriers at the electrodes. In these structures there is no amplification of the photocurrent and the optical-to-electrical gain is limited.
Phototransistors are based on the same principles as photodiodes, but they combine light detection and signal amplification in a single device. Due to their amplifying action, they have a higher optical-to-electrical gain and thus a higher light sensitivity than photodiodes.
A first type of organic phototransistors is the organic bipolar phototransistor as for example reported by Zukawa T., Naka S., Okada H. and Onnagawa H., in “Organic heterojunction phototransistor”, J. Appl. Phys. 91, 1171 (2002), where the photocurrent is amplified by a current-to-current amplification. These devices have at least three organic layers: an emitter layer, a base layer and a collector layer, between two conductive electrodes: a collector electrode and an emitter electrode. A forward voltage is applied to the collector with respect to the emitter. Light absorption gives rise to the generation of electron-hole pairs in the emitter, base and collector layers. These electron-hole pairs are split into free charge carriers under the influence of the electric field. Accumulation of one type of charge carriers in the base layer is obtained by an appropriate energy band design, e.g. by means of appropriate doping profiles or by means of organic heterojunctions. This accumulation of charge carriers facilitates and enhances the carrier injection from emitter to base and amplifies the current going through all three organic layers. Optimization of these devices includes the maximization of the enhanced injection from emitter to base.
A second type of organic phototransistors is the organic field-effect phototransistor. In organic field-effect phototransistors the photocurrent is amplified by the transconductance of the field-effect transistor. Typical organic field-effect phototransistors have a structure with three conductive electrodes, a source electrode, a drain electrode and a gate electrode, and one organic semi-conducting material (Narayan K. S. and Kumar N., “Light responsive polymer field-effect transistor”, Appl. Phys. Lett. 79, 1891 (2001); Hamilton M. C., Kanicki J., “Organic polymer thin-film transistor photosensors”, IEEE J. Select. Topics Quantum Electron. 10, 840 (2004); Noh Y. Y., Kim D. Y. and Yase K., “Highly sensitive thin-film organic phototransistors: effect of wavelength of light source on device performance”, J. Appl. Phys. 98, 074505 (2005)). Organic field-effect phototransistors, when operated in the off-state of the field-effect transistor, show higher gain and signal-to-noise ratio as compared to organic photodiodes and organic bipolar phototransistors. Moreover, integration of organic field-effect phototransistors and organic field-effect transistors in a circuit is more straightforward compared to other organic photodetectors thanks to their similar structure. Pixel design in an image sensor array can be simplified for the same reason.
When using the organic field-effect transistor for example as an image sensor, its operation is characterized by a charging period, a read-out period and a reset period. During the charging period the device is illuminated such that excitons are created in the organic material and subsequently split into free charge carriers. Once generated, these free charge carriers move under the influence of the applied electric fields through the channel of the transistor. One type of charge carriers is trapped in deep charge carrier traps within the structure of the device. This accumulation of trapped charge carriers causes a shift in the threshold voltage of the field-effect phototransistor, which is translated into an amplified current through the device between source and drain. During the reset period, after switching off the photo-excitation, the trapped charges are removed to prepare the device to be recharged during the next charging period.
To reach a high gain, an efficient photo-generation of free charge carriers, an excellent field-effect transistor characteristic and an efficient trapping of one type of charge carriers are required. However, controlled operation of organic field-effect phototransistors has been problematic due to the difficult control of the (deep) charge carrier traps which define the photo-response characteristics of the device. Furthermore, as charge carriers are trapped in deep charge carrier traps, removing the trapped charges during the reset period is a time consuming process. It has been observed that, after switching off the photo-excitation, the current persists at a higher value than the initial dark current, and the recovery to the initial dark current is characterized by an extremely slow relaxation process that may take several hours.
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