Structure Design And Performances Optimization Of Small Molecule Organic Solar Cells

Organic solar cells?OSCs?have attracted much attention due to their potential of flexibility,low weight,and low-cost mass production.Though the power conversion efficiency?PCE?has improved gradually over the past few years,which is passed 10%,it is still significantly lower than that of inorganic counterparts and needs to be further improved for commercial applications.In this thesis,the efficiency of the organic solar cells is improved by means of understanding their physical mechanism and using new device structure.The main research work focused on the following aspects:1.SubPc is adopted as the interlayer in bulk heterojunction?BHJ?small molecule organic solar cells?OSCs?with a small donor doping concentration.For the device with a structure of ITO /MoO₃ /5wt%TAPC:C70 /Bphen /Al,when subPc is inserted in the MoO₃/5wt%TAPC:C70 interface,a PCE of 4.95% is obtained,which is about 28% higher than the reference device.Then a tendem device with a structure of ITO /Mo O₃ /SubPc /5wt%TAPC:C70 /Bphen /Ag /MoO₃ /SubPc /5wt%TAPC:C70 /Bphen /Al is fabricated.This device shows a high PCE of 7.27%,which is about 47% higher than the reference device.It is found that band bending is observed in both the MoO₃ /SubPc ? SubPc /C70 interfaces,which results in improved hole collection efficiency and hence the PCE of the devices.2.Suface plasmon enhanced OSCs are demonstrated with Au nanoparticles?NPs?.Au NPs are fabricated on indium-tin-oxide substrates by a thermal evaporation method and incorporated to the small molecule OSCs with a sutructure of ITO /Au NPs /MoO₃ /tetraphenyldibenzoperiflanthene?DBP?/C70 /Bphen /Al.The thicknesses of Au layer and MoO₃ layer play important roles in determining the performance of the devices.Through optiming the thickness of Au layer and MoO₃ layer,a highest PCE of 3.29% is obtained,which is 21.4% higher than that of the reference device without Au NPs.The improvement is mainly contributed to the increased short-circuit current which resulted from the enhanced light harvesting due to localized surface plasmon resonance of Au NPs and the increased conductivity of the device.3.Cooperative plasmon enhanced small molecule organic solar cells are demonstrated based on thermal coevaporated Au and Ag nanoparticles?NPs?.A series of devices with the structure of ITO /Au: Ag /MoO₃ /DBP /C70 /Bphen /Al are constructed.Through optiming the ratio of Au:Ag NPs and the thickness of MoO₃ layer,a highest PCE of 3.32% is obtained,which is 22.5% higher than that of the reference device without any NPs.The improvement is also mainly contributed to the enhanced light harvesting due to localized surface plasmon resonance of Au:Ag NPs and the increased conductivity of the device.Due to the different relative permittivity of Au and Ag NPs,the Au:Ag composite NPs will produce a surface plasmon resonance with a wider absorption band.As a result,the device with such a composite NPs will shows an absorption enhancement in a wider wavelength region.Besides,a large difference between the diameters of the Au and Ag NPs is found.Thus the thickness of MoO₃ layer cannot be simultaneously optimized for Au and Ag NPs,and the advantage of the composite NPs is not fully ultilized.