Study On Polymer Solar Cells Based On ZnO Composite Electron Transport Layer

Nowadays,energy issues have increasingly become the focus of most countries,especially for clean,renewable,safe and low-cost new energy.As one of the clean and renewable new energy sources,solar energy has attracted wide attention.Making full use of solar energy will be promising in solving the problem of energy shortage fundamentally.Polymer solar cells(PSCs)have been playing an important role in solar energy utilization.Because of the advantages of low cost,short energy payback time and flexible fabrication in large scale,PSCs can be applied in some special occasions and have complementary advantages with traditional inorganic solar cells,thus showing a good application prospect.However,compared with inorganic solar cells,the energy conversion efficiency and stability of PSCs need to be improved urgently.PSCs based on inverted structure can well match the vertical phase separation in the active layer and possess top electrode self-encapsulation effect,thus the devices exhibit high efficiency and stability.Developing more efficient and stable electron transport layer(ETL)materials for this device structure is one of the important topics in the field of PSCs research.ZnO is considered to be a promising material for ETL because of its excellent solution processability and semiconductor properties.In order to effectively integrate on flexible substrates,ZnO thin films are usually prepared by low-temperature sol-gel or nanoparticle methods.However,the low temperature processing leads to the propensity of ZnO to form bulk and surface defects.These defects and corresponding trap levels can capture electrons,resulting in decreased carrier mobility and increased interfacial recombination losses.Although this can be improved by using ultraviolet or ultraviolet-ozone irradiation to excite ZnO to produce electrons and fill these unoccupied electronic states.However,this will in turn degrade organic active layer,which is not conducive to the long-term stability of the device.In this work,based on the concept of injecting extra carrier into ZnO to fill electrontrap states,we constructed two novel ZnO-based composite films as electron transport layer.One is a composite film composed of Au@CNTs and ZnO.The results show that the localized surface plasmon resonance(LSPR)effect of Au@CNTs nanostructures leads to the formation of “hot electrons”,which could transfer to ZnO and fill the trap states.Meanwhile,carbon nanotubes(CNTs)can also provide additional pathway for charge transport.Compared with pure ZnO devices,Au@CNTs:ZnO devices show improved properties in charge extraction,transport and recombination at the interface.The solar cell efficiency based on PTB7-Th:PC71BM active layer is increased from 8.52% to 10.67%.The other is designing an organic small molecule(BTTT-Br)and incorporating it into ZnO precursor.Experimental results show that after light absorbing,the photoelectrons produced by the small molecule can transfer to ZnO and fill the defect states.This molecular doping effectively improves the photoconductivity of the ZnO layer and inhibits the trap-assisted recombination rate during the charge transport process.The polymer solar cells based on PTB7:PC71BM active layer can still achieve nearly 10% device efficiency under a thick ETL(~90 nm).At the same time,all layers are obtained at low temperature(less than 150 C),which guarantee their good application potential in the preparation of large-area flexible devices.