Different Cathodic Buffer Layers Used To Improve The Performance Of Pentacene/C₆₀ Solar Cells

ABSTRACT:Interface between the organic layer and cathode plays an important role in the performance of small-molecule organic solar cells, so it is common to insert an organic buffer layer to modify this interface. Current researches are mainly focused on the CuPc/C6o based small-molecule organic solar cells, considering Pentacene has better crystalline and higher electron mobility than CuPc, all the works in this thesis are focused on the mechanism of buffer layer in the Pentacene/C6o based devices. Author's main contributions and innovations are as follows:1. Devices with the structure of ITO/PEDOT:PSS/pentacene/C60/BCP/Al are studied, in which bathocuproine (BCP) is used as the buffer material. By comparing the device without buffer layer, the increase in performance is observed after inserting a BCP layer. The change was the same with that of the devices based on CuPc/C6o which has been extensively studied. The improvement can be explained by two aspects. One is that the BCP buffer layer as the exciton blocking layer can reduce the recombination of exciton near the cathode; the other is the protection of active layer during the Al electrode deposition. BCP layer can reduce the conductivity decline of C6o caused by the invasive metal atoms, which results in the smaller resistance of the devices. In addition, we studied the device performance with different BCP thickness, we found, as the BCP thickness increases, the conductivity is decreased, which is the main reason for the lower device performance.2. When using bathophenanthroline (Bphen) instead of BCP, as the electron mobility of Bphen is two orders of magnitude higher than that of BCP, the efficiency is improved from 0.56% to 0.46%. Electrons transport through the buffer layer to the Al electrode after excitons dissociation at the donor/acceptor interface. We believe charge carrier transport the buffer layer by the defect state density which generated during the Al electrode deposition, but the electron mobility of the buffer layer material will also impact the performance of the devices. The much higher electron mobility in Bphen definitely leads to better electron transport through the buffer layer and thereby the electronic collection could be enhanced.3. We use the 3,4,9,10-Perylenetetracarb-oxylicdianhydride (PTCDA) which has large absorption in visible spectrum as the buffer layer in our devices. The performance of devices with PTCDA is much better than those using the same thickness of BCP and Bphen materials. Using 10 nm PTCDA the current density is increased to 5.97mA/cm2 and the efficiency to 0.87%. So if buffer layer is a good light harvesting material, it can not only modify the interface of the cell, but also increase the absorption efficiency.