聚合物/TiO_2分层光电导型器件的电荷传输特性

Charge Transportation Properties of Polymer/TiO_2 Bilayer Photoconductive Devices

研究了聚合物PVK与TiO2分层光电导器件的电荷传输特性,分别比较了两种器件:器件S1(ITO/TiO2/PVK/Al)和器件S2(ITO/PVK/TiO2/Al)。实验发现,器件S1的暗电流远小于器件S2的暗电流,S1的正向峰值光电流约是其反向峰值光电流的4倍,而S2的正向和反向峰值光电流都基本与S1的反向峰值光电流相近。这是由于PVK/TiO2界面处有效的电荷转移、恰当的电荷传输层、器件各层间能级匹配及其与电极功函数的匹配影响了光电流的强度大小。由此判断,器件S1的性能要优于器件S2。随电压的增大,S1结构的光电导响应谱在短波区域的拖尾增大,而S2结构几乎没有拖尾,这可能是两种结构的吸收和陷阱能级的差别造成的。

Instead of photovoltaic devices which attract much attention, photoconductive devices were objects of our work. Two kinds of PVK/TiO2 bilayer devices were prepared, including S1∶ITO/TiO2/PVK/ Al and S2∶ITO/PVK/TiO2/Al. The properties of charge transportation of organic/inorganic devices were investigated by dark current and photocurrent spectra. The small noises are better properties of PVK/TiO2 bilayer devices. For these two devices, both of photocurrent response peaks are at 340nm or so, which indicated that the main excited layer is PVK film and photogenerated excitons of PVK influenced on the production of photocurrent. The TiO2 film functioned as electron transporting and adjuvant excited layer. For dark current of two devices, the dark current of S1 was much smaller than that of S2. Dark current is resulted from: electron injection and transportation in PVK from cathode; hole injection and transportation in TiO2 from anode; hole transfer from TiO2 to the value band of PVK. These three points can be used to explain the smaller dark current of S1. For photocurrent of the two devices, the peak photocurrent of S1 at forward bias was 4 times of that of S1 at reversed bias, which was similar with the peak photocurrent of S2 at forward and reversed bias. The effective charge transfer at the interface of PVK/TiO2, proper charge transportation layers and the match of energy level and work function had been considered to be the main factors that influence on the intensity of photocurrent of the devices. The reasons induced different properties of two devices, including: different morphology of PVK/TiO2 interface caused by different fabricating sequences, the exchanged contacts near PVK film and TiO2 film and the different absorption because of excited light reaching the different film first which led to photocurrent response spectra of the two devices were different. As a result, the properties of device S1 is better than that of S2. With the increase of applied voltage, the tails of S1 photocurrent spectra at the short wavelength turned higher, but photocurrent spectra of S2 nearly had no tails, which may be attributed to the different absorption and trapping energy level.