Interface Modification And Performance Optimization Of Polymer Solar Cells

With the rapid development of the global economy,human demand for energy is increasing.The burning of fossil fuels can cause a greenhouse effect and cause great damage to the environment.In order to solve environmental and energy problems,we urgently need to develop new types of renewable and clean energy.As a kind of green renewable energy,solar energy has many advantages,such as wide distribution,clean and environmental protection,high utilization value and inexhaustible.It is expected to become the ideal energy source for the most potential application.One of the main forms of utilizing solar energy is the conversion of solar energy into electrical energy,and the main component that achieves this form is solar cells.At present,silicon-based solar cells have entered the stage of mass production and practical application.However,due to high preparation cost,complicated process,and harsh environment,the commercialization process of inorganic silicon solar cells is seriously restricted.Polymer solar cells have attracted extensive research attention due to their wide range of sources,light weight,good mechanical flexibility,low cost and the ability to prepare large-area flexible devices.In recent years,polymer solar cells have developed rapidly in terms of preparation processes,photovoltaic material synthesis and device structure,and the highest photoelectric conversion efficiency has exceeded 14%.However,in order to meet the needs of commercial production,the efficiency and stability of the device still need to be further optimized and improved.In this work,aiming at the design and development of high-efficiency and stable polymer solar cells,the devices based on metal oxide cathode interfacial layer and anode interfacial layer were constructed from the preparation and development of new interface modification materials.By selecting the type of interface modification materials,we regulated the energy level arrangement,and constructed excellent interface characteristics to improve the photoelectric conversion efficiency of the polymer solar cells.Combining with a series of photoelectric performance measurements,the intrinsic correlation between the device performance improvement and the interface between the active layer and the conductive electrode is studied.The surface morphology,wettability,mobility,electrical conductivity and photo-generated carrier recombination mechanism in the device are discussed in detail.The main research contents are shown as follows:?1?We deposited different thickness of strontium fluoride?SrF₂?layers on ZnO nanoparticle films synthesized by sol-gel method,and acted them as cathode interface modification materials for inverted polymer solar cells.The device structure is ITO/ZnO/SrF₂/PTB7-Th:PC₇₁BM/MoO₃/Ag.By changing the thickness of SrF₂,the effect on the performance of polymer solar cells was studied.It was found that the efficiency of photovoltaic devices was as high as 10.46%when the thickness of SrF₂was 1.5 nm,which was about 21%higher than that of reference devices.ZnO film acted an effective electron transport layer,and ultra-thin SrF₂ adjusted the energy level by forming the interface dipole on the surface of ZnO film and increased the built-in potential,thus promoting the extraction and transport of photogenerated electrons.In addition,the ultra-thin SrF₂ layer reduces the inherent incompatibility between the hydrophilic ZnO film and the hydrophobic photoactive layer.?2?Polymer solar cells based on SnO₂ cathode interfacial layers were prepared.Firstly,Mg-doped SnO₂ nanoparticles were synthesized by low-temperature solution-processed and used as the cathode interface modification layer of the inverted polymer solar cell.The device structure was FTO/Mg:SnO₂/P3HT:PC₆₀BM/MoO₃/Ag.The efficiency of the device is further optimized by changing the doping ratio of the Mg element.When 7.5 at%Mg-doped SnO₂ is used as the interfacial layer,the photovoltaic performance parameters are significantly improved,and the conversion efficiency can reach 4.08%.The improved photovoltaic performance is ascribed to the increased electron mobility,elevated electrical conductivity and optimized surface morphology,which makes it an excellent growth platform for a flat and high quality photoactive layer.Furthermore,we show the Mg:SnO₂ interfacial layers to dramatically improve the electron extraction and effectively suppress the photogenerated carrier recombination.Secondly,the SnO₂ nanoparticles synthesized by the low-temperature solution-processed were inserted between the ZnO electron transport layer and the FTO cathode.The device structure was FTO/SnO₂/ZnO/P3HT:PC₆₀BM(PCDTBT:PC₇₁BM)/MoO₃/Ag.The photovoltaic performance of the solar cell are optimized by changing the concentration of the SnO₂ precursor solution.The device efficiency is the best when the precursor concentration of SnO₂ is 0.2 M.The device efficiencies based on P3HT:PC₆₁BM and PCDTBT:PC₇₁BM systems are 4.25%and 7.16%,respectively.The improved device performance is attributed to enhanced optical transmission,reduced surface roughness,energy level barriers,and carrier recombination at the interface.At the same time,SnO₂ film can effectively promote electron extraction and block hole injection.?3?Polymer solar cells based on NiO_x anode interfacial layer were prepared.Firstly,Cu-doped NiO_x nanoparticles were synthesized by sol-gel method and used as the anode interface modification material for the PCDTBT:PC₇₁BM-based device.The device structure is FTO/Cu:NiO_x/PCDTBT:PC₇₁BM/Alq₃/Al.When 5.0 at%Cu-doped NiO_x is used as the anode interfacial layer,the efficiency of the PCDTBT:PC₇₁BM-based device can reach 7.05%,which is about 30%higher than that of the undoped reference device.The Cu-doped NiO_x interlayer enhances the conductivity of the material and improves the interfacial contact with the active layer,thus promoting the hole extraction and reducing the photo-generated carrier recombination at the interface.Secondly,an ultra-thin insulating organic layer of polytetrafluoroethylene?PTFE?is introduced between the photoactive layer and the hole transport layer?NiO_x?to further improve the efficiency of the polymer solar cell.The device structure is FTO/NiO_x/PTFE/PCDTBT:PC₇₁BM/Alq₃/Al.The effect on cell efficiency was investigated by depositing different thicknesses of PTFE interfacial layers.When the thickness of the PTFE layer was 1.5 nm,the photovoltaic performance of the device was significantly improved,and the efficiency reached7.11%.The PTFE layer not only improves the energy level alignment by forming the interface dipole,but also reduces the barriers of hole injection and transport.Furthermore,it also improves the incompatibility between the hydrophobic active layer and the hydrophilic NiO_x layer,thereby facilitating the charge extraction and transport in polymer solar cells.