Performance Enhancement Of Efficient Planar Single Junction And Tandem Perovskite Solar Cells

Organic-inorganic hybrid perovskite materials have unique light and electronic properties,including high light absorption coefficient and longer carrier lifetime as well as low exciton separation energy.Perovskite solar cells(PVSCs)with low fabrication cost and high efficiency have attracted lots of research interests,and developed quickly with the best power conversion efficiencies exceeding 23%just in several years.The bandgap can be tuned from 1.17 to 2.3 eV through compositional changing the in corresponding perovskites.By stacking one wide bandgap semitransparent top cell with a low bandgap bottom cell,all-perovskite tandem solar cells could break the Shockley-Queisser(SQ)radiative efficiency limits for single-junction cells.Here,we investigated the performance enhancement of planar structure PVSCs based on SnO2 electron selective layers and mixed tin-lead low bandgap PVSCs with inverted structure.The corresponding power conversion efficiencies have reached 21.01%and 18.66%,respectively.Moreover,we fabricated effcienct four terminal tandem solar cells by combining a semitransparent device with a low bandgap device and reached 23.66%.Firstly,we fabricated high quality SnO2 electron selective layers(ESLs)on FTO substrates with plasma enhanced atomic layer deposition technique at 100 degree C,this SnO2 can conformally coated on FTO with excellent coverage.The PVSCs based on MAPbI3 absorbers reached 19.03%through interface modification with a fullerene self assembled monolayer(C60-SAM).However,a big issue of large current density-voltage(J-V)hysteresis was occurred in these device.Through detailed device characterization using cross-sectional Kelvin probe force microscopy(KPFM)and trap density of states measurements,we identify that the J-V hysteresis seen in planar organic-inorganic hybrid PVSCs using SnO2 ESLs synthesized by low-temperature PEALD method is mainly caused by the imbalanced charge transportation between the ESL/perovskite and the hole selective layer(HSL)/perovskite interfaces.We deleloped a post treatment method to improve the electron mobility.As a result,the hysteresis in PVSCs was sufficiently reduced and the PCEs of corresponding devices were enhanced impressively.The best PVSC based on MA0.7FA0.3PbI3 absorber has no hysteresis behavior and give a stable output of 20.3%.In order to further reduced hysteresis of flexible PVSCs fabricated on ITO/PET substrates,we use water vapor to treat low temperature processed SnO2 ESLs in order to increase the conductivity,and the best performing flexible PVSC with a PCE of 18.36%was achieved.Secondly,we mixed FAPbI3 solution with MAPbI3 to reduce the bandgap of corresponding perovskite absorber,and found the MA0.7FA0.3PbI3 perovskite presented the highest efficiency.However,the mixed perovskite layer has very small grain size.Therefore we employed lead thioyanate(Pb(SCN)2)additive to enlarge the crystal size and film quality,and achieved high performance PVSCs with PCEs over 20%.However,in order to improve the poor illuminiation stability of MAFA mixed perovskite,we further introduced Cs cation into perovskite material to fabricate triple cation materials.The performance as well as illumination stability of CsMAFA triple cation based PVSCs have been significately improved.After 20000 seconds irradiation,PVSCs still remain 97%of their initial outputs.Furthermore,we introduced polymer into perovskite materials to enhance the envoiremental stability.Combined with modified annealing process,planar structure PVSCs fabricated on rigid and flexible substrates reached 21%and18.71%PCE,respectively.Thirdly,we use mixed tin-lead low bandgap perovskite materials to fabricate efficient low bandgap single junction solar cells.After compositional optimization,MA0.4FA0.6Pbo.4Sn0.6I3 with a bandgap of 1.25 eV reached a best PCE over 17%.However,the grain size of this material is still very tiny.We used close space annealing process to improve the quality of perovskite film and prolong the carrier lifetime.At last,perovskite film with crystal size over 1 micrometer and carrier lifetime exceeding 300 ns was achieved,and the corresponding PVSC has a PCE of 18.66%.Lastly,we fabricated large bandgap PVSCs through Br incorporation and modified film annealing process.Combined with a precisely selected transparent electrode,wide bandgap PVSC reached a PCE of 16.02%with good transmission over 70%at longer wavelength range.Finally,we made four terminal all perovskite tandem solar cells with a 23.66%PCE by stacking a semitransparent top cell and a low bandgap bottom cell.