| The power conversion efficiencies of perovskite solar cells have been enormously improved in a few years due to the excellent opto-electrical properties of the organic-inorganic hybrid perovskite light absorber.Recently,a certified power conversion efficiency of 22.1%has been reported,demonstrating great potential in commercial application.Carbon electrode based hole transport material free perovskite solar cells exhibit both high power conversion efficiencies and excellent stability,thus attracting extensive attention.The perovskite material can be easily degraded by the moisture in the atmosphere,therefore,putting higher requirement on the infrastructure of the laboratory,such as glove box and super clean room.In order to ease the preparation process and fabrication cost of perovskite solar cells,we have focused on the preparation and optimization of carbon electrode based hole transport material free perovskite solar cells in ambient atmosphere with high humidity.With step by step optimization,the power conversion efficiency is successfully boosted to 13%(champion efficiency above 14%)from an initial value of below 10%.The details are summarized as follows:1.We have successfully fabricated perovskite solar cells with cell structure:FTO/cp-TiO2/meso-TiO2/perovskite/carbon using commercial P25 TiO2 nanoparticles as scaffold layer and a modified commercial conductive carbon paste as low temperature printable carbon electrode.Average power conversion efficiency of 9.3%is obtained.The low coverage of the perovskite layer on the underlying TiO2 layer might result in potential direct contact between the carbon electrode and TiO2 layer and thus high charge recombination rate.Therefore,we have introduced a mesoporous Sio2 insulating layer to effectively separate them.With optimized Sio2 thickness,average power conversion efficiency of 12%is achieved.2.Under high relative humidity,the conversion of PbI2 into MAPbI3 perovskite is incomplete and the residual PbI2 content is hard to control in the conventional two step sequential process,which will degrade the performance and worse the reproducibility of the perovskite solar cells.In order to solve these problems,a solvent treatment process is introduced.The spin-coated wet PbI2 films are treated with common solvents(ethanol,isopropanol,and chlorobenzene).Due to the extremely low solubility of PbI2 in these solvent,PbI2 nanoparticles will quickly precipitate and crystallize,leading to dendrite-like or flake-like structure with nano-sized pin holes,which facilitates the conversion of PbI2 into perovskite with negligible PbI2 amount.With ethanol treatment,power conversion efficiency of 11.22%is obtained.3.The commercial carbon paste is composed of micrometer-sized graphite and nano-sized carbon black.Due to the rough perovskite surface,the doctor-bladed low temperature carbon electrode exhibits a bad interfacial contact with the perovskite layer,resulting in a poor hole collecting efficiency.In order to improve the interfacial contact,multiwall carbon nanotube(MWCNT)is dispersed in PbI2 solution,and MAPbI3/MWCNT composite film is prepared with the two step method.MWCNT can act as charge transport high way for holes and facilitate the hole extraction by the carbon electrode.When the concentration of MWCNT is optimized,power conversion efficiency of 11.6%is achieved.4.Preparing a smooth and uniform perovskite film is essential for achieving high performance.Therefore,we have combined solvent treatment with a methylamine ethanol gas treatment process.First,ethanol treatment is adopted to lower down the residual PbI2 amount,then,methylamine ethanol gas treatment is applied to recover a smooth perovskite surface with enlarged perovskite crystals.With these two treatment processes,high quality perovskite films can be obtained.Power conversion efficiency of 12.84%is achieved in hole transport material free perovskite solar cells,which is boosted by 17.3%compared with the control cells.When a CuPc hole transport layer is further introduced,power conversion efficiency of 16.61%is achieved. |
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