© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2289 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Adv. Mater. 2012, 24, 2289–2293 Beatrice Fraboni,* Andrea Ciavatti, Francesco Merlo, Luca Pasquini, Anna Cavallini, Alberto Quaranta, Annalisa Bonfiglio, and Alessandro Fraleoni-Morgera Organic Semiconducting Single Crystals as Next Generation of Low-Cost, Room-Temperature Electrical X-ray Detectors Prof. B. Fraboni, A. Ciavatti, F. Merlo, Dr. L. Pasquini, Prof. A. Cavallini Dipartimento di Fisica Università di Bologna Viale Berti Pichat 6/2, 40127 Bologna BO, Italy E-mail: beatrice.fraboni@unibo.it Prof. A. Quaranta Dipartimento Ingegneria dei Materiali e Tecnologie Industriali (DIMTI) Università di Trento Via Mesiano 77, 38050 Povo TN, Italy Prof. A. Bonfiglio Dipartimento Ingegneria Elettrica ed Elettronica Università di Cagliari Piazza d’Armi, Cagliari CA, Italy Dr. A. Fraleoni-Morgera Sincrotrone Trieste Strada Statale 14, Km 163.5, 34012 Basovizza TS, Italy DOI: 10.1002/adma.201200283 Ionizing radiation can be detected by directly converting it into an electrical signal. Only a few, expensive inorganic sem- iconductors (e.g., CdTe, SiC) presently offer the possibility of realizing portable detectors that operate at room temperature. Organic semiconductors are very promising materials for sev- eral different electronic applications, ranging from thin-film transistors (TFTs) and light-emitting diodes (LEDs) to solar cells and sensors. [1–5] As detectors of ionizing radiation, organic semiconductors have so far been mainly proposed in the indi- rect conversion approach, for example, as scintillators, [6,7] which convert ionizing radiation into visible photons, or as photo- diodes, [8–10] which detect visible photons coming from a scin- tillator and convert them into an electrical signal. The direct conversion of ionizing radiation into an electrical signal within the same device is a more effective process than the indirect one, since it improves the signal-to-noise ratio and it reduces the device response time. However, the few examples of this approach reported up to now in the literature refer to semicon- ducting- or conducting-polymer thin films, or charge-transfer conducting organic crystals, which exhibit major stability problems and rely on the presence in the detector of metallic electrodes directly exposed to the ionizing radiation. [11–17] Here, we show that organic semiconducting single crystals (OSSCs) can be used as effective direct X-ray detectors. In par- ticular, devices based on solution-grown OSSCs (from two dif- ferent molecules: 4-hydroxycyanobenzene (4HCB) ( Figures 1a,b) and 1,8-naphthaleneimide (NTI, Figures 1c,d) have been fab- ricated and operated in air, under ambient light and at room temperature, at voltages as low as few volts, delivering well- reproducible performances and a stable linear response to the X-ray dose rate, with notable radiation hardness and resistance to aging. The role of high-atomic-number components (e.g., metals) in the device response has been elucidated, evidencing the intrinsic response to X-rays of the crystals, which allowed the fabrication of well-performing all-organic and optically transparent devices. We assess here the intrinsic conversion process of X-rays into an electrical signal, within the tested crys- tals, thus allowing all-organic optically transparent devices to be fabricated. The observed performance indicates that OSSCs are very promising candidates for a novel generation of low-cost, room-temperature X-ray detectors. Solution-grown 4HCB single crystals can be easily grown in tunable-size platelets at low cost and possess stable and reproducible three-dimensional electronic transport proper- ties, [18,19] evidencing a notable robustness to prolonged elec- trical probing. [20] In particular, average mobility values ranging around 5 × 10 -2 , 5 × 10 -3 , and 5 × 10 -6 cm 2 V -1 s -1 along the axes a, b, and c, respectively, were demonstrated [21,22] and reproduc- ible, anisotropic densities of states and trap distributions along the three crystallographic directions were described. [23] In the work reported here, we investigated the photoelectric response of 4HCB crystals under an X-ray beam in air at room tempera- ture and under ambient light. In these conditions the bulk “dark” currents I OFF , that is, those measured in the absence of the X-ray beam, along the two planar axes a and b are compa- rable ( Figures 2a,b), while that along the vertical axis c differs (in line with previously reported data [21] ); therefore, we will refer in the following only to the behavior along “vertical” and “planar” axes, without distinguishing between the two planar axes a and b. As shown in Figure 2b, the irradiation of 4HCB crystals with a 35 kV X-ray beam induces a significant increase in the photocurrent along both the planar and vertical axes, indicating the creation of photogenerated carriers. Interest- ingly, the normalized photocurrent ( I ON – I OFF )/ I OFF vs V curve presents a maximum at rather low voltages for both axes, sug- gesting that practical devices may be operated at voltages as low as 50 V, hence with low power requirements (Figure 2e). No hysteresis, and no appreciable current drift, is observed upon repeated X-ray beam on/off cycles (Figure 2f) for different bias voltages. The response time, shorter than 70 ms, is remarkably fast for organic electronic devices. [16] A similar behavior under X-ray irradiation has been observed for NTI crystals, whose