Study On The Performance Improvement Of Polymer Solar Cells Through Interfacial Modification

ABSTRACT:In this thesis, we improve the performance of polymer solar cells (PSCs) by employing a more suitable material or a bilayer/multi-stacked film as the interfacial layer, when considering that the commonly used interfacial layer in PSCs has some drawbacks.A widely used cathode buffer layer in PSCs is LiF. However, the thickness of LiF is limited to less than2nm due to its insulating property. Such a thin layer of LiF cannot efficiently protect the underlying active layer from damage during the evaporation of hot cathode metal atoms, and also cannot provide sufficient protection against the diffusion of oxygen and water. Therefore, it is necessary to adopt much thicker buffer layers to replace LiF. Firstly, we introduced three small-molecule materials (Alq3, BCP and Bphen), which are commonly used in small-molecule organic solar cells (OSCs), as the cathode interfacial layers in PSCs. By optimizing the thickness of these three materials, we find that the device with Bphen buffer layer (the optimum thickness is12nm) exhibits the best performance, mainly attributed to the high electron mobility and large bandgap of Bphen. Secondly, considering that LiF can retard the diffusion of oxygen into the active materials, we expect a very thick LiF in the cathode buffer layer, and simultaneously ensure that the buffer layer possesses a good electrical conductivity. After introducing C60,5nm thick LiF is allowed to be used. The devices with the C6o/LiF (5nm) buffer layer show a peak power conversion efficiency (PCE) of3.65%, indicating the high electrical conductivity of C6o/LiF (5nm) films due to the formation of the mixed morphology between C60and LiF layers. However, the device with C6o (25nm)/LiF (8nm) buffer layer exhibits a rather low PCE. To further increase the LiF thickness, we prepared multi-stacked C6o/LiF film by alternating deposition of C6o and LiF layers. The devices with five stacks of C6o/LiF ((C6o/LiF)5) buffer layer show a maximum PCE of3.58%with a FF as high as70.2%, and most importantly, the high efficiency is almost independent of the thickness ratio between C6o and LiF, indicating the possibility to use a very thick LiF in the whole cathode buffer layer. This results in superior air stability of the devices with (C6o/LiF)s buffer layer.PEDOT.PSS is the commonly employed anode interfacial layer in PSCs. However, aqueous PEDOT:PSS dispersion tends to corrode ITO due to its high acidity (pH～1). Therefore, we use graphene oxide (GO) as the anode interfacial layer (also called hole transport layer, HTL) to replace PEDOT:PSS. GO annealed at an elevated temperature exhibits good electrical conductivity due to the removal of oxygen functional groups from the graphene sheet to a much-higher level, resulting in the better recovery of π-conjugated structure. The devices with high-temperature reduced GO as HTL have a maximum PCE of2.75%, which is～26%higher than that of the reference device without any HTL.Finally, we introduce Ag nanoparticles in PSCs, anticipating that the light absorption within the active layer can be improved without the need of a thicker active layer. However, it is found that Ag nanoparticles not only induce absorption enhancement of the active layer owing to the excitation of localized surface plasmon resonance (LSPR), but also induce exciton quenching, which leads to the decrease in shunt resistance (Rsh) of PSC devices. In order to prevent exciton quenching on the Ag nanoparticle surface and simultaneously utilize the high electric field strength around Ag nanoparticles as much as possible, we doped the Ag nanocubes (Ag NCs) into the PEDOT:PSS HTL and controlled the thickness of the PEDOT:Ag composite film to match with the Ag NCs size. The device with PEDOT:Ag HTL has higher short-circuit current density (Jsc) and PCE as compared to the device with pristine PEDOT:PSS HTL, mainly attributable to the absorption enhancement in the active layer arising from the excitation of LSPR. Additionally, Ag nanoplates (Ag NPs) were incorporated in P3HT:PCBM solar cells, and no obvious enhancement of Jsc is observed, which can be ascribed to the mismatch between the absorption peak (-500nm) of P3HT:PCBM and the resonance peak (603nm) of Ag NPs.