| Great interest in organic photovoltaic (OPV) cells has been generated since the demonstration of a bilayer cell by Tang in1986. Compared with traditional inorganic solar cells, OPV cells have some potential advantages such as their low manufacturing cost, light-weight and flexibility. Within the research area of OPV cells, the performances of small molecular weight based devices have improved steadily and rapily. The organic heterojunction allows for efficient dissociation of excitions at the donor-accepter (D/A) interface. At the same time, it reduces the recombination due to the electron and hole transferring in different materirals.Based on these reasons, it has large advantage compare to single device. Pentacene has been studied as a promising p-type photoactive material for organic solar cells due to its relatively large exciton diffusion length (65±16nm), high hole mobility and appreciable absorption in the visible spectrum. C60has became the most common material to be used as acceptor for organic small molecular since Sariciftci reported that there is ultrafast photoinduced electron transfer from donor to fullerene molecule C60.So all the works in this thesis are focused on the pentacene/C60based devices.The detailed contents and the main results are given below:Firstly, we have fabricated ITO/Pentacene/C60/Al heterojunctions organic small molecular solar cells using vacuum thermal evaporation method, choosing ITO for the bottom electrode, Al as the top electrode and small molecule pentacene and C60as organic semiconductor spacer. But the performance of the device is poor and it shows rapid degradation in subsequent measurement.Secondly, devices with the structure of ITO/Pentacene/C60/BCP/Al are studied, in which BCP is used as the buffer layer. By comparing the device without buffer layer, the increase in performance is observed after inserting a BCP layer. The improvement can be explained by three aspects.(ⅰ) The BCP layer prevents not only cathode atoms from diffusing but also water/oxygen from permeating into the active organic films, which will result in a smaller resistance of the devices and reducing the conductivity decline of C60.(ⅱ) As the hole blocking layer, BCP can reduce the recombination of electrons and holes near the cathode.(ⅲ) BCP can reduce the recombination of exciton near the cathode as the exciton blocking layer.Thirdly,8-hydroxy-quinoline aluminium (Alq3) is adopted to replaced BCP as the buffer layer. The configuration of the device is ITO/Pentacene/C6o/Alq3/AI. The efficiency improves from0.18%to0.37%. The improvement can be explained by two aspects. One is that the electron mobility of Alq3is higher than than that of BCP, which improve the transportation of the photogenerated electrons through the buffer layer to the Al cathode. The other is that the water/oxygen blocking effect of Alq3is more effective.Finally, further improvements on the properties of the devices are achieved by the effect of UV-ozone treatment on ITO before the fabrication of devices with the structure of ITO/Pentacene/C6o/Alq3/Al. The improvement can be contributed to the increase of work function and better morphology of ITO after UV-ozone treatment.The optimal solar cell’s parameters are as follow Jsc=3.6mA/cm, Foc=385mV, FF=0.40,η=0.55%. |
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