IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 51, NO. 6, JUNE 2004 877 Thin-Film Organic Polymer Phototransistors Michael C. Hamilton, Student Member, IEEE, Sandrine Martin, Member, IEEE, and Jerzy Kanicki, Senior Member, IEEE Abstract—We have studied the electrical performance of or- ganic polymer thin-film transistors (OP-TFTs) under steady-state white-light illumination, as well as the performance of these devices as photodetectors. The off-state drain current of the OP-TFT is significantly increased due to the illumination, while a smaller relative effect is observed on the drain current in the strong-accumulation regime. The illumination effectively decreases the threshold voltage of the device and increases the apparent subthreshold swing, while the field-effect mobility of the charge carriers in the polymer channel is unchanged. We have observed full recovery of our devices after the illumination is removed at room temperature. These observations are explained in terms of the photogeneration of excitons due to the absorbed photons. The photogenerated excitons subsequently diffuse and dissociate into free charge carriers, thereby enhancing the carrier density in the channel of the device. We have found broadband responsivities of approximately 0.7 mA/W for devices biased in the strong-accumulation regime and gate-to-source voltage-inde- pendent photosensitivities of approximately for devices in the off-state. We also determine, for the first time, the flatband voltage of these devices to be about V. Index Terms—Conjugated organic polymer, photodetector, pho- tosensitivity, phototransistor, responsivity, thin-film transistor. I. INTRODUCTION T HIN-FILM transistors (TFTs) based on conjugated organic materials, both small-molecules and polymers, have shown promise for use in large-area low-cost applications [1], [2]. Several groups have proposed or reported successful integration of such devices with light-emitting devices to demonstrate the possibility of all-organic broad-area flat-panel displays. These applications include pentacene-based organic TFTs (OTFTs) integrated with organic light-emitting devices (OLEDs) [3], poly(hexylthiophene) (P3HT) based organic polymer TFTs (OP-TFTs) integrated with OLEDs [4], copper phthalocyanine (CuPc) based organic static induction transis- tors (SITs) that may be suitable for integration into display devices [5], OTFT- and OP-TFT-driven active-matrix polymer dispersed liquid-crystal displays (PD-LCDs) [6]–[8], and organic electrophosphorescent devices (OELDs) driven by all-organic pentacene-based OTFTs [9]. On the other hand, a number of other groups have described the use of polymer-based devices as photodetectors. These de- Manuscript received December 5, 2003; revised March 22, 2004. This work was supported in part by the National Institute of Standards and Technology Ad- vanced Technology Program and in part by the Department of Defense under a National Defense Science and Engineering Graduate Fellowship sponsored by the U.S. Air Force office of Scientific Research and administered by the Amer- ican Society of Engineering Education. The review of this paper was arranged by Editor L. Lunardi. The authors are with the Department of Electrical Engineering and Com- puter Science, The University of Michigan, Ann Arbor, MI 48109 USA (e-mail: kanicki@eecs.umich.edu). Digital Object Identifier 10.1109/TED.2004.829619 tectors can be classified into one of two main groups: the two- terminal photodiode and the three-terminal phototransistor. Sev- eral groups have discussed various organic photodiode struc- tures [10]–[19]. The motivated reader will find useful reviews of recent work on organic photodetectors in [20]–[22]. A much smaller number of groups have published work on organic pho- totransistors, which is the subject of this paper. Zukawa et al. have demonstrated an organic heterojunction-based phototran- sistor [23]. Schön and Klock have shown the use of a pen- tacene-based metal–semiconductor field-effect transistor as a phototransistor [24]. Additionally, Narayan et al. have described an organic polymer field-effect transistor that responds to light [25], [26]. Since in optoelectronic applications, the device will either be integrated with light-emitting devices (as a switching device) or be used to detect light itself (as a light sensor), it is important to understand the effects of illumination on the electrical per- formance of these devices, as well as the underlying physics of these effects. In the past, our group has described the ef- fect of white-light illumination on the electrical performance of amorphous silicon TFTs with a device structure similar to that described in this paper, and we have shown that these devices can function effectively as photosensors [27]. In this paper, we present the results of our investigation of the performance of our organic polymer TFTs under steady-state illumination from a broadband source and the performance of these devices as pho- todetectors. II. DEVICE STRUCTURE A schematic cross-section of the device used in this study is shown in Fig. 1(a). The device is an inverted, defined-gate, gate-planarized, coplanar thin-film transistor that has been pre- viously described [28]–[30]. Indium tin oxide (ITO) was used for the source and drain contacts. Benzocyclobutene (BCB) was used as the gate-planarization layer, and it also functions as a gate insulator. PECVD hydrogenated amorphous silicon ni- tride (a-SiN:H) was used as a second gate insulator layer, and chromium (Cr) was used for the patterned gate electrode. For the case of these devices, the usefulness of the BCB planariza- tion layer is limited. However, if a thicker gate electrode is to be used, the planarization will allow for better step coverage of the subsequently deposited layers [31]. The devices were fab- ricated on a silicon substrate (with a thick thermally grown sil- icon dioxide layer) to facilitate processing in standard micro- electronic fabrication equipment; however, this device struc- ture could easily be fabricated on a glass substrate. We used a 1-wt % solution of the organic semiconductor F8T2 [poly(9, 9-dioctylfluorene-co-bithiophene)] alternating copolymer dis- solved in either xylenes or mesitylenes. The polymer film was deposited by spin-coating and was cured in a vacuum oven at 0018-9383/04$20.00 © 2004 IEEE