Interface Optimization And Performance Control Of ?-? Cadmium Quantum Dots-Based Photovoltaic Devices

?-? semiconductor quantum dots(QDs)have unique intrinsic characteristics including narrow and direct band gap,symmetric band structure and low Auger recombination rate,and more effective interaction between photons and electrons as compared to the bulk materials.They have great application potential in the fields of photovoltaic devices,light-emitting diodes and biological imaging medical treatments.However,high trap density due to the high surface-to-atom ratio and the mismatch problems between the interfaces in QD devices leading to much lower device performance as compared to the theoretical value.In this research,we focused on the surface modification of ?-? QDs,optimization and control of the device interfaces to enhance the performance of quantum dot solar cells(QDSCs)and photovoltaic detectors.We first proposed a double-function surface modification layer for QDs to enhance the performance of CdS QDSCs.The P-type Cu,S layer was introduced on the surface of the CdS QDs by in-situ cation interaction reaction method.CLuCl2 precursor solution was used to interact with CdS QDs.Cd2+could be replaced by Cu2+in situ on the surface of CdS QDs to form a CuxS modification layer.The P-type CuxS formed a PN junction with the N-type CdS,which could not only reduce surface defects,but also promote the electron-hole pairs separation and the charge transmission.In addition,CuxS is a narrow-band gap semiconductor that can also enhance the light absorption.Based on the double-function surface modification layer,the light absorption efficiency of CdS/CuxS QDSCs was increased by more than 15%and the short-circuit current density was double.The charge collection efficiency of the CdS/CuxS QDSCs increased from 80%to 92%.As a result,compared with the CdS QDSCs,the power conversion efficiency of CdS/CuxS QDSCs increased from 1.2 1%to 2.78%.In order to further improve the performance of QDSCs,CdSe QDs was deposited on the surface of CdS QDs in situ by chemical bath deposition method to form CdS/CdSe co-sensitized quantum dot active layer.Meanwhile,in order to solve the problem of high defect concentration at the interface between quantum dots and carriers(TiO2 electron transport layer),an ultrathin and high quality TiO2 modification layer was deposited on the surface of the mesoporous TiO2 layer by atomic layer deposition(ALD).As the material of the modification layer was the same as the TiO2 photoanode,the interface mismatch problem could be avoided.After ALD modification,the specific surface area and surface energy of the TiO2 electron transport layer increased by 7.2%and 7.9%,respectively,which significantly increased the quantum dot loading.Therefore,the light absorption was significantly improved.In addition,the ultrathin TiO2 layer increased the continuity between particles,reduced surface defects,promoted the transport of electrons,and decreased the recombination,thus the charge collection efficiency was increased from 97%to more than 99%.Finally,the power conversion efficiency of CdS/CdSe QDSCs modified by ALD on TiO2 carriers was increased from 4.03%to 5.07%.In order to achieve the light absorption and the detection ranging from ultraviolet(UV)to near infrared(NIR),Te element was introduced into CdSe QDs to form CdSeTe ternary QDs.The self-powered QD detector with wide-band response(UV-NIR)was fabricated.The device was composed of TiO2 electron transport layer,CdSeTe QDs,organic hole transport layer and silver electrode.The long chain oleic acid(OA)ligand was coated on the QDs surface to stabilize CdSeTe QDs,which can seriously hinder the charge transfer process.To solve this problem,short-chain mercaptoacetic acid(TGA)and conductive ion halogen(TBAI)ligands which can strongly bind with QDs were applied to replace OA ligands.After that,the charge transfer and photocurrent of the device were obviously improved.The results also showed that,the detection performance of TBAI-QDs device was better than that of OA-QDs and TGA-QDs devices.The detectivity of CdSeTe QD photodetector with TBAI modification was above 5�1012 Jones in the range of 350800 nm,which was superior to the reported QD detectors.We introduced N,N-diphenyl-N,N-bis(4-methylphenyl)-4,4-biphenylenediamine(P-TPD)layer onto the PEDOT:PSS hole transport layer to form a double-hole transport layer in this work.The introduction of P-TPD with a high lowest unoccupied molecular orbital(LUMO)solved the mismatch between the QDs and the hole transport layer,which could effectively act as a barrier to suppress the reverse flow of electrons,and improve the photocurrent density of the device(68%).Compared with single-layer device,double-layer device with flexible structure exhibited higher performance in the UV-NIR region,the detectivity increased from 2.5 x1011 to 1 x1012 Jones in 500 nm monochromatic light.Moreover,the performance of the detector with double-hole transport layer structure was superior to the flexible quantum dot detector reported recently.In addition,the double-hole transport layer structure detector showed excellent response to weak light,and the photocurrent reached 196 nA/cm2 under 5?W/cm2 light excitation.After bending different angles(0 degree.20 degrees.40 degrees,60 degrees)and 150 cycles,the flexible device didn't have any attenuate,showing that it has good flexural performance.