Fundamental Study of a Organic Semiconductor Device for Radiation Distribution Measurement

半导体辐射探测器的优势在于其从辐射沉积的能量到电荷的转换效率高。然而，由于它们是由无机材料组成的，一直存在着难以经济地制造它们的问题。在本研究中，我们尝试使用已经发展并应用于各个领域的有机材料来制作辐射传感器。在ITO电极上制备了α-NPD (500A)/td-PTC (500A)/A1 （700A）结构的有机光电二极管（PD）器件。材料在4.5x10^<-6>[Torr]的真空条件下气相沉积在衬底上。阳极和阴极电极的宽度为2mm，因此，有效面积为4mm^2。为了估计其对可见光的响应，我们用1[kW/cm^2]的可见光照射它。通过对器件施加反向电压，感应电流比暗电流大10^5倍。用这种类型的装置进行了x射线发生器的实验。在各器件上施加逆偏置电压，以710[mSv/h]左右的剂量率照射x射线。用静电计测量暗电流和感应电流。当x射线照射时，与暗电流相比，电流增加了约40[pA]至50[pA]。感应电流随偏置电压的增大而增大。这可以解释为电压的增加导致器件内部更大的耗尽层。随着加速电压的增大，感应电流也随之增大。可以说，这些结果证明了有机PD可以作为辐射探测器，但存在偏置电压与暗电流关系不稳定的问题，需要对制作工艺和器件结构进行优化。我们将探讨用有机发光二极管制造辐射成像器件的可能性。

Semiconductor radiation detectors are advantageous in their high transforming efficiency from energy deposited by radiation to electrical charges. However, as they are composed of inorganic materials, there has been a problem that it is difficult to fabricate them in economical ways. In this study, we tried to fabricate radiation sensors using organic materials which have been developed and applied to various areas.We fabricated a organic photodiode (PD) device with a structure of α-NPD (500A)/td-PTC (500A)/A1 (700A) on a ITO electrode. The materials were vapor deposited on the substrate at vacuum of 4.5x10^<-6>[Torr]. The anode and cathode electrodes had a dimension of 2mm-width and, consequently, the effective area was 4mm^2. In order to estimate its response to visual light, we irradiated it with visible light of 1[kW/cm^2]. By applying inverse voltage to the device, induced current became 10^5 times larger than the dark current.Experiments with an X-ray generator have been carried out with this type of devices. With inverse bias voltage on each device, X-ray was irradiated on it with the dose rate of about 710[mSv/h]. Dark and induced current was measured with an electrometer.When irradiated with X-rays, the current increased with about 40[pA] to 50[pA] compared with the dark current. The induced current increased with increasing the bias voltage. This can be explained that the increased voltage resulted in larger depleted layer inside the device. Also when the acceleration voltage was increased, the induced current increased. It can be said that these results have demonstrated that the organic PD can work as radiation detectorAs there is a problem that the relation between bias voltage and dark current is unstable, fabrication process and device structure should be optimized. We are going to examine the possibility of fabricating radiation imaging device with organic PDs.