Organic Bistable Devices And Electrical Properties Of Organic Solar Cells

There has been a great deal of interest and much activities in studying organic semiconductor in the past twenties years,including the organic bistable devices and organic solar cells.Though there have been a lot of reports on organic bistable devices, the original mechanism is still not clear.For organic solar cells,their problems of stability and cost need great improvement to meet the requirement for practical application.In this thesis,small-molecular organic bistable devices are fabricated and its electrical properties and possible mechanisms are studied.Some work is also done to improve the stability or decrease the cost of the organic solar cells.The main work in this thesis is shown below:1.Due to different materials,device architectures,and fabrication conditions,the reported memory devices have showed different performances.However,almost all the reported OBDs exhibit a common feature;i.e.,the OFF to ON transition of each device always occurs above a so-called threshold voltage V_th.There has been no report so far that shows a slow switch from off to on state at voltages below the threshold voltage.Wwe report a delayed-switch-on effect in organic bistable devices.It is found that when biased at voltages below Vth,the devices will switch from off to on state in a certain period of time.The switching time increases from milliseconds to about 10⁴ s with decreasing voltage.Moreover, intermediate states can be obtained by applying a certain voltage in the region of V_th＜V＜V_max,where V_max is the voltage corresponding to the local current maximum.2.In previous reports,most research efforts focused on the voltage V within the switching range,0＜V＜V_min,while less attention was paid to the electrical properties beyond this range,i.e.,in the voltage regions of V>V_min.Tthere have been no reportso far to investigate the electrical properties of OBDs in the voltage range of V＞V_min to show the effect of such higher voltage on OBDs.We systematically investigate the effect of higher voltage V＞Vmin on OBDs.It is found that when the scan termination voltage VT exceeds Vmin,the bistable effect of OBDs gets worse and worse with the increasing VT in the following scan. When the voltage reaches a very high value of about 2Vmin,the device completely loses its bistability.However,it is found that the lost bistability can be recovered by applying a voltage pulse close to V_min for milliseconds or longer.3.Pentacene is a material with very high electron and hole mobility.The device fabricated with pentacene might have some difference between ordinary organic bistable devices.Actually,in our experiment,the device made with pentacene has different bistable effect in the positive bias direction.It can't switch from the ON state to the OFF state.But in the negative bias side,the bistable effect is the same as typical device.We inserted buffer layer between ITO and Pentacene interface, such as LiF,Al or Al₂O₃;the devices with the buffer layer show typical bistable effect as the ordinary organic bistable device again.We also used NPB 6nm as the buffer layer and it is found out that the device show both the two kind of bistable effect,the ratio is aobut 1:1.We propose that the reason for the different bistable effect of single layer pentacene device ITO/Pentacene/Al is the too large hole current under positive bias.4.The lifetime of organic solar cells to date is too short to meet the requirement for practical applications,especially when the cells are unencapsulated.For the cells based on CuPc)/(C₆₀) heterojunction structures,a very likely reason for their degradation is the decrease of C₆₀ conductivity upon oxygen permeation.If we invert the device structure,the fullerene layer will be protected by the CuPc layer and will have better stability.In this device,Al is not suitable as top anode due to its low work function.We used C₆₀/Al composite electrode as anode and fabricated inverted organic solar cells,achieving an efficiency of 0.78%and a shelf lifetime of over 950 h without encapsulation.5.To enable the commercial application,it is equally important that solar cell devices can be fabricated with a low-cost way to reduce the manufacturing cost. The roll-to-roll lamination process is one very promising technique to fulfill this requirement owing to its simplicity and low cost.This method involves no thermal evaporation process in the process,and each layer is formed by a low-cost and easy solution process,which is potentially easy to be applied for large-area device. We fabricated semitransparent and flexible solar cells with PET/ITO substrate and P3HT:PCBM as the active materials by a lamination process with a chemical bath deposited CdS film acted as the electron transport layer.The device structure is PET/ITO/PEDOT:PSS/P3HT:PCBM/CdS/ITO/PET.The properties of the CdS are characterized and the power conversion efficiency of the device reaches 1.2%under AM1.5G 100mw/cm².