Study Of The Interfacial Processes In Organic Solar Cells

Organic solar cells (OSCs) have attracted a lot of attention in recent years due to their low fabrication cost and compatibility for large-scale manufacturing. The process of converting excitons to free carriers in OSCs is the essential step for light-to-electricity conversion. The performance of the OSCs can be further improved by modifying the interface. The interfacial processes at the interface of organic solar cells, including the exciton-electron interaction and the interfacial exciton recombination and dissociation under a built-in electric field in organic solar cells, and the charge transfer at the counter electrode/electrolyte interface in dye-sensitysed solar cells have been studied in our research. The thesis is divided into the following six parts:Chapter1. Firstly, the background and significant of the soalr cells are briefly introduced. Secondly, silicon solar cells, inorganic film solar cells and organic solar cells are briefly introduced separately. In addition, the materials, structures and the modifying layers in organic soalr cells are reviewed in detail. The challenges of interfacial processes in organic solar cells are discussed. At last, the purposes and the innovations of this thesis are given.Chapter2. The preparation and measurements of organic solar cells. The methods for preparing and measuring organic solar cells in our lab are mentioned. The ITO substrates are washed before device fabrication. The equipment for device fabrication is then introduced. The current-voltage (I-V) measurements by keithley2400under illumination of the Newport solar simulators with AM1.5G filter. For external quantum efficiency (EQE) measurement, the lock-in technique is used to measure the photocurrent signal of the device generated by chopped monochromatic light. The transient photovoltage (TPV) characteristics are measured through recording the photovoltage variation with time with an oscilloscope after a burst of illumination from a pulsed laser. The UV absorptions are measured by the ultraviolet-visible spectrophotometer.Chapter3. The role of the interfacial layer between the anode and the active layer in an single layer organic solar cell. OSCs with the structure of ITO/NPB (x)/C60/Alq3/Al with (x≠O) and without (x=0) NPB modification layer are fabricated in a high vacuum chamber. Excitons of C6o at the ITO/C60interface cannot be dissociated efficiently when C60is directly in contact with ITO. We demonstrated that the performance of this all-C60device can be improved by modifying ITO with an interfacial layer of NPB to avoid the charge recombination at the ITO/C60interface. The insertion of a thin NPB layer is used to guide the electron flowing, avoiding the energy dissipation through charge recombination via conducting band of ITO. The large improvement of this all-C60device by inserting an ultra thin NPB layer between the ITO and C60layers is ascribed to the enhancement of the exciton dissociation at the modified ITO/C6o interface.Chapter4. The electron-exciton interaction at the Donor/Acceptor interface in organic solar cells. The electrons achieved from the exciton dissociation and the excitons coexist at the interface of CuPc/C60. Photocurrent generation through electron-exciton interaction, different from traditional exciton dissociation method, is found and discussed in this capter by systematically measuring a device with structure of ITO/CuPc/C6o/MoO3/Al. The energy level and Ⅰ-Ⅴ curves show that without considering the electron-exciton interaction a photocurrent flowing from ITO to Al can not be obtained. The Ⅰ-Ⅴ characteristics, TPV and EQE of the OSCs provide evidences for the photocurrent produced through the interaction between electrons and excitons at CuPc/C6o interface. Our results show that by simply changing the built-in electric field, the electron-exciton interaction can dominate in the photocurrent generation as compared to the exciton dissociation process in traditional OSCs.Chapter5. Co-exsisting of exciton dissociation and recombination at the same interface in an organic solar cell. The D/A interface is normally used for exciton dissociation in conventional planar or bulk heterojunction. In this chapter, the photocurrent generation by the exciton recombination across F16ZnPc/C60interface has been confirmed by superlinear dependence of the photocurrent to the light intensity. The coexisting of exciton recombination and dissociation at the same F16ZnPc/C6o interface is proved by the displacement current. And the exciton dissociation is believed to be much more efficient than the exciton recombination for generating photocurrent. Both processes contribute to the photocurrent in dissociation devices while only the exciton recombination generates measureable photocurrent in recombination devices. For organic solar cells with two n-type materials (such as F16ZnPc and C60), the process (exciton dissociation or recombination) dominance for photocurrent generation could be adjusted by simply changing the direction of built-in electric field in the devices.Chapter6. The improved catalysis at the graphene quantum dots-doped Polypyrrole/electrolyte interface. Graphene quantum dots-doped Polypyrrole (GQDs doped PPy) on FTO glass are fabricated via electrochemical deposition method. GQDs could electrostatically absorb py as nucleation sites to promote the PPy growth, resulting in many "nano-islands" for a highly porous structure and active sites, as confirmed by the SEM. Higher charge transfer rate and the catalysis toward reduction of I3-/I at the GQDs doped PPy/electrolyte interface have be proved by electrochemical measurements and device performance characterization under simulated sunlight. GQDs doping with10%GQDs can greatly enhance the performance of DSSCs to achieve the highest power conversion efficiency of5.27%, which is～20%higher than that of plain PPy based DSSCs and is comparable to that of Pt counter electrode based device (6.02%). The GQDs doped PPy provides a promising candidate to replace Pt as an inexpensive counter electrode for high-performance DSSCs.