Charge Recombination Of Organic Bulk Heterojunction And Organic/Silicon Heterojunction Photovoltaics

The third-generation heterojunction photovoltaics present promising strategies for achieving cost-effective power supply.However,their performances and applications still have obvious limitations,compared with that of traditional silicon photovoltaics.Since the carrier recombination causes adverse energy loss,reducing recombination is inevitable to achieve a high power conversion efficiency(PCE).In this thesis,organic bulk heterojunction and organic/silicon heterojunction photovoltaics were fabricated.And the PCE of devices is enhanced through controling the adverse carrier recombination.In organic photovoltaics(OPVs),a tandem structure was fabricated to reduce the monomolecular and bimolecular recombination of carriers.After reducing the recombination,the sunlight harvesting property was enhanced and consequently a PCE of 14.1%was achieved.Meanwhile,a silicon hybrid photovoltaic was fabricated by deposition of a conductive polymer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS)onto the silicon surface.A reduced adverse surface recombination was achieved through optimizing the performances of PEDOT:PSS and controlling the trap states on silicon surface,A PCE of 14.3%was achieved in silicon-PEDOT:PSS hybrid photovoltaics.To demonstrate the potential application of third-generation photovoltaic devices,a semitransparent OPV was proposed to serve as the"roof and window of a greenhouse",where the infrared light can be absorbed by OPVs for power supply,and the penetrated visible light can be utilized for photosynthesis for plants.The main work includes:1.A tandem structure was applied to reduce the recombination of carriers and then improve the efficiency of OPVs.The monomolecular recombination of sub-cells in the tandem structure was reduced due to the change of the light intensity.Concurrently,the thinner active layer of the front cell decreases the bimolecular recombination.When the two sub-cells are stacked into a tandem structure,a PCE of 14.1%was achieved in tandem OPVwith an optimized thickness of active layers.2.The heterojunction properties of silicon-PEDOT:PSS photovoltaics were investigated.After enhancing the work function of the PEDOT:PSS film,a strong inversion layer was formed on a silicon surface,which induced a quasi p-n junction.The thickness of the quasi p-n junction is 100 nm,and the diffusion length of minority carriers in the junction region reaches 140 ?m.The quasi p-n junction results in a higher built-in potential of solar cells,thus increases carrier collection and decreases the adverse recombination.A PCE of 13.4%was achieved in silicon-PEDOT;PSS photovoltaics.3.Sulfonated graphene was chosen as a modifying material for PEDOT:PSS film.The sulfonated graphene was added into PEDOT:PSS solution to decrease the amount of insulating PSS in PEDOT:PSS film,since sulfonated graphene replaces the PSS that interacts with PEDOT.After decreasing the insulating PSS,the resistance of carriers transport between PEDOT reduces,which favors the carriers collection.Meanwhile,the minority carrier lifetime was enhanced,which decreases the adverse recombination loss.The output properties of silicon-PEDOT:PSS photovoltaic was improved and a PCE of 13.8%was achieved.4.The rear surface of the silicon was chemically passivated by titanium oxide to reduce surface recombination.Chemical bonds of silicon-oxygen-titanium were formed on silicon surface,which effectively reduced the density of defect states on the silicon surface.The reduced defect states suppressed the adverse recombination of carriers,improving the utilization of long-wavelength photons and the open-circuit voltage.In addition,the titanium oxide reduces the contact resistance between the silicon and aluminum electrodes hence enlarges the fill factor.A PCE of 14.3%is achieved in silicon-PEDOT:PSS photovoltaics.5.A semitransparent OPV fabricated with infrared light absorbing organic materials was developed as the "roof and window for greenhouses".The infrared lightwas utilized to generate electricity by the photovoltaic process of OPV.The visible light traveled through the OPVsupported photosynthesis of plants.A PCE of 10%with an average visible transmittance of 34%was achieved in a semitransparent OPV.Meanwhile,the plants grown under the semitransparent OPV yielded a healthy growth tendency.