Interface Optimization For High Efficiency Quantum Dot Solar Cell With Electron Transport Layer Free Configuration

Lead sulfide（PbS）colloidal quantum dots solar cells（CQDSCs）have attracted much attention due to the surface-chemical-tuned energy band of colloidal quantum dots（CQDs）.High-performance CQDSCs require a n-type electron transport layer（ETL）and p-type hole transport layer,that construct space charge region to ensure the photogenerated carrier collection.However,the band mismatching at ETL/PbS CQDs interface impedes the photogenerated electrons collection,and the instable ETL that absorbs high-energy ultraviolet photons limits the performance of CQDSCs.Therefore,this manuscript focuses on the ETL-free CQDSCs,which only formed by n-and p-type PbS CQDs.The band alignment,absorption and electrical properties of quantum junction forming by n-and p-type CQDs can be flexibly adjusted,enabling the ETL-free CQDSCs to make full use of the size effect and surface effect of CQDs.The ETL-free CQDSCs not only can utilize the wide solar spectrum by adjusting the size of CQDs on both sides of junction,but also can flexibly and extensively adjust the band matching of n-p junction by adjusting the ligand type of CQDs.However,due to the defects of n-and p-type CQDs,the unoptimized band alignment matching,and the interfacial issues caused by the direct contact at the FTO/CQDs interface in the ETL-free devices,the performance of ETL-free CQDSCs is greatly limited,much lower than that of Zn O-based n⁺-n-p CQDSCs.In this manuscript,the optimization of n-p junction in ETL-free CQDSCs devices and the modification of the interface between quantum dots and electrodes are studied.The specific research content is as follows:1.The carrier separation and transport of ETL-free CQDSCs were optimized,and the power conversion efficiency（PCE）exceeded 10%.We constructed ETL-free CQDSCs using deep-energy-band PbI₂ capped PbS CQDs（PbS-PbI₂）and shallow-energy-band 1,2-ethanedithiol（EDT）capped PbS CQDs（PbS-EDT）.A large depletion width（220 nm）was obtained in ETL-free CQDSCs,which enables effective carrier separation and transport.PbS-PbI₂ has a Fermi level lower than that of the FTO,which can form band bending between FTO and PbS-PbI₂,which is beneficial to electrons extraction by FTO.Compared with Zn O-based devices,ETL-free CQDSCs have higher UV photon utilization efficiency,which provides a convenient device structure for exploring the multi-exciton effect in PbS CQDSCs.In addition,an excellent light-socking stability is showed in ETL-free CQDSCs,that exhibit 95%of the initial efficiency after continuous 11 h exposure to solar simulator.2.Suppressing the interfacial recombination using an ultra-thin SnO_x buffer layer,further increasing the efficiency of ETL-free CQDSCs to 11.55%.In ETL-free CQDSCs,the directly contact between high-carrier-density(～10²⁰-10²¹ cm^-3)FTO and low-carrier-density(10¹⁶-10¹⁷cm^-3)PbS-PbI₂ induces electron back diffusion due to the large carrier density drop,leading to the series carrier recombination at the FTO/PbS-PbI₂ interface.Therefore,we reduced the carrier density drop at the FTO/PbS-PbI₂ interface using the atomic layer deposited（ALD）SnO_xbuffer layer.The ALD SnO_x layer effectively buffers the interface carrier recombination.As a consequence,we further improved the ETL-free device efficiency to 11.55%.To the best of our knowledge,this is the highest efficiency reported in ETL-free CQDSCs.In addition,the internal quantum efficiency of SnO_x device can reach 100%in the 430-470 nm（photon energy is more than 2 times of PbS CQDs bandgap）,which indicates that the SnO_x-modified ETL-free CQDSCS have potential in the study of multi-exciton effect in PbS CQD photovoltaic devices.3.Investigating the reason for J-V hysteresis in CQDSCs.In ETL-free CQDSCs,the large carrier density drop at the FTO/PbS interface leads to a large interfacial capacitance,inducing a large capacitance current released by the charge and discharge of capacitance which is the main reason for the J-V hysteresis effect in ETL free CQDSCs.The ultra-thin SnO_x buffer layer can alleviate the carrier density drop at the FTO/PbS interface,effectively inhibit the generation of interfacial capacitance,reducing the J-V hysteretic effect.In addition,we investigate the hysteresis behavior of Zn O-based heterojunction CQDSCs,showing that in heterojunction CQDSCs,serious J-V hysteresis behavior will also occur if the capacitance increase caused by carrier density drop.This work investigate the J-V hysteresis caused by capacitance effect in CQDSCs,which provides a new mechanism for understanding the J-V hysteresis behavior in CQD-based devices.