Low-temperature Processed Flexible PbS Colloidal Quantum Dot Solar Cells With ZnO NWs As Electron Transport Layer

Colloidal quantum dots(CQDs)have attracted much attention in the field of low-cost thin-film flexible photovoltaic research because of their size-tunable bandgap,solution-processing,and multiple exciton generation.In the past decade,the power conversion efficiency(PCE)and device stability of colloidal quantum dot solar cells(CQDSCs)have been rapidly improved through advancements in the surface treatment of CQDs and device engineering.Recently,several groups have attempted to fabricate flexible CQDSCs with a planar depleted heterojunction(PDH)structure using planar electron transport layers(ETLs).In the planar heterostructure devices,the thickness of the absorbing layer is not enough to limit the improvement of the photocurrent,which makes the device more efficient.When the film deposited by inorganic nanoparticles is an electron transport layer,it is easy to cause the loss of the device performance.Solving these problems is an important direction for further development of flexible devices.In contrast with planar ETLs,well-aligned ZnO nanowire(NW)arrays have recently been employed as three-dimensional(3D)ETLs to fabricate CQDSCs with an ordered bulky depleted heterojunction(OBDH)structure.This OBDH structure is expected to further enhance the efficiency of carrier collection in CQDSCs by the orthogonalization of light absorption and carrier separation processes,thus increasing the photovoltaic performance of cells.Conversely,well-aligned NWs are thought to be able to efficiently release the bending stress by adjusting the gaps between NWs,which is beneficial for preventing crack formation in the ETL and improving the bendability of CQDSCs.In this paper,the seed layer were synthesized by spin coating ZnO nanoparticles,and then the three dimensional structure of Zn O nanowire arrays were prepared by hydrothermal method.We could easy control the morphology and length to get a ETL with good crystal quality,less surface defects and good optical transmittance.We constructed the PbS CQDSCs on flexible substrates using ZnO nanoparticles and ZnO nanoparticles as electron transport layer,respectively.It is found that the three-dimensional device has a higher short-circuit current,but a part of the open circuit voltage is lost.Furthermore,CQDSCs with ZnO NW arrays revealed higher bending tolerance than those with ZnO NPs,which is proved by the variation in the extent of the decrease of the initial PCE for two solar cells(7% for the cell with ZnO NWs and 23% for that with ZnO NPs)at the same bending angle of 160°.The microstructure analysis of the ZnO NP layer and ZnO NW array after the bending test demonstrated that the origin of this PCE reduction for CQDSCs with ZnO NPs is largely derived from the fracture of the ZnO NP ETL.The good bending stability of ZnO NWs array provides possibility for its application to other flexible devices to provide a reference and direction.