High Peformance PbS Quantum Dot Solar Cells Using Organic Hole Transport Layesrs

During the past decade,the power conversion efficiency(PCE)of PbS quantum dot(QD)solar cells has increased rapidly through optimizing both the electron transport layer(ETL)and the QD absorbing active layer.In contrast,less attention was paid to the hole transport layer(HTL).The widely adopted HTL 1,2-ethanedithiol-treated PbS(PbS-EDT)exhibits lots of drawbacks like surface defects,complicated fabrication process and high materials cost,which is not ideal for large-scale fabrication.In order to overcome these critical issues,organic conjugated polymeric hole conductors have been used as alternative HTLs in PbS QD solar cells.However,the efficiency of current QD solar cells still lagged behind PbS-EDT based devices.Therefore,it is urgent to explore more efficient organic polymers to further improve the photovoltaic efficiency.In this project,we introduced lots of functional organic conjugated polymers,together with synergistically QD interfacial engineering towards scalable,stable and efficient PbS QD solar cells.The project can be divided into the following three parts:(1)High performance PbS QD solar cells using tailored polymeric hole conductors.Here we propose a series of conjugated polymers(PBDB-T,PBDB-T(Si),PBDB-T(S),PBDB-T(F))for PbS CQD solar cells as HTLs.Through polymer side-chain engineering,we optimized the model polymer PBDB-T to tune the energy levels,increase hole mobility,improve solid-state ordering,and increase free carrier density.PbS QD solar cells based on modified polymer PBDB-T(F)exhibit a best efficiency PCE of 11.2%,which outperforms the devices based on conventional PbS-EDT HTL(10.6%),and is the highest PCE for PbS solar cells based on organic HTLs currently.These results may provide a new insight into the selection of organic HTLs for efficient QD solar cells.(2)Interfacial engineering towards efficient and stable PbS QD solar cells based on organic polymeric HTL.Based on the device long-term stability characterization,we found that iodide ion(I")migration into the electrode layer is the main reason of the poor device stability.To solve this critical issue,we further introduce a thin PbS-BST(1-butyl mercaptan treated PbS QD)as an ion blocking layer to insert between QD active layer and polymeric HTL.The optimized devices based on the improved device structure exhibits a best efficiency of 10.4%,which is comparable to that of the control devices(10.6%).Importantly,the device stability is significantly improved,maintaining over 94%of the initial value after 1200 h aging under the atmosphere without any encapsulation.(3)Morphology optimization towards efficient and stable PbS QD solar cells based on organic polymeric HTL.Finally,we reported a simple yet efficient thermal annealing treatment to optimize the morphology of the QD active layer that can reduce I-migration,and eventually improve the device air stability with a simple fabrication process.Through optimizing the thermal annealing process,the PbS QD device after 60? annealing for 60 mins exhibits a highest efficiency of 10.3%,and excellent stability after 360 h aging.It is a new train of thought to introduce organic hole transport materials for PbS QD solar cells with high efficiencies and good stability.Organic semiconductors are environmentally-friendly with several advantages of controllable band gap,energy levels,molecular structures and so on.PbS QD solar cells based on organic hole transport materials have a great development potential and it is expected to break through the current highest efficiency.