Study On The Performance Of Eco-Friendly Silver Gold Selenide Quantum Dot Solar Cells

Colloidal quantum dots(CQDs)are promising candidates for exploiting the nextgeneration solar cells by virtue of their numerous merits,such as facile bandgap tenability,wide absorption range and adjustable electronic properties as well as solution processability.Compared with lead-based and perovskite CQDs solar cells containing toxic heavy metal elements,seeking suitable heavy-metal-free CQDs to construct the eco-friendly solar cells is more in line with the development trend of clean energy.However,taking the silver-based CQDs as example,a large number of photo-generated carriers are captured by their inherent defects,which leads to the unsatisfactory performance of their corresponding photovoltaic devices.Therefore,the design of CQDs with low defect density are crucial to improve the performance of environmentally friendly devices.In this paper,the surface and crystal defects of Ag2Se CQDs are effectively suppressed by surface chemical engineering and alloying treatment respectively,which obtain AgAuSe CQDs with low defect states.Subsquently,the AgAuSe CQDs are acted as light absorbers of active layer to construct eco-friendly solar cells for the first time.Then,through the interface modification and component regulation,the problem of charge transfer was improved and the device performance was further improved.Specific research progresses are summarized as follows:1.In order to obtain AgAuSe CQDs with ultra-low defect state,firstly,the effect of chain length of thiol ligands,including 1-hexadecanethiol,1-dodecanethiol and 1octanethiol,on the surface defects and photoluminescence(PL)properties of Ag2Se CQDs were systematically studied.By analyzing the binding states of CQD/ligand interface and the influence of ligand-ligand interaction,we found that the binding affinity between CQD and ligand increased with the shortening of the ligand chain length,accompanied with the disordered configuration of ligands on the CQD surface.Both of these varations were beneficial to the passivation of surface defects of Ag2Se CQDs,which enhanced its PL intensity.Then alloying the high-quality of parent Ag2Se CQs with Au+ precuror,the nearly defect-free AgAuSe QDs were obtained,possessing a record absolute PL quantum yield of 87.2%.The low defect properties of AgAuSe QDS also exhibited their potential as photovoltaic semiconductor materials.2.Based on the characteristics of AgAuSe CQDs like ultra-low defect state,ideal band gap width(1.2 eV-1.5 eV),long fluorescence lifetime(>4 μs)and high molar extinction coefficient(-105 cm-1),AgAuSe CQDs were employed as light absorbers of active layer for the first time.By optimizing exchange ligands,active layer thickness and charge transport materials,a basic n-i-p structure of AgAuSe CQD solar cell with SnO2/EDT-CQD/PCE10 was constructed,showing a power conversion efficiency(PCE)of 2.44%.Firstly,in order to endow the film with conductivity and modulate the electron-hole concentration,short chain thiol ligands and halogen ligands were selected as exchange ligands.The J-V characteristics exhibted that 1,2-ethanethiol(EDT)ligands were more conducive to the transport of electrons and holes in devices.Subsequently,by adjusting the CQD concentration and the times of spin-coating,the active layer thickness was optimized,and it was found that the active layer device with the thickness of～30 nm had the best performance.Finally,we carefully investigated the impact of charge transport materials on the performance of devices by screening a range of n-type metal-oxides and p-type polymers.The results indicated that SnO2 acted as the electron transport material given the best device performance contrast with ZnO or ZnMgO,while PCE 10 was the most suitable p-type hole transport material than other thiophene-rich polymers.This work has initially confirmed the feasibility of building environment-friendly photovoltaic devices using AgAuSe CQDs as the absorber of the active layer.3.Through the analysis of energy loss pathes,we found the above-mentioned n-ip architecture of AgAuSe CQD soalr cell still existed several limitations in the CQD/HTL interface,which hampered the improvement of device performance.For example,the charge extraction efficiency from CQD to PCE 10 was unsatisfactory because of their large surface energy difference,while the energy level mismatching between EDT-CQDs and PCE 10 was also non-negligible.Therefore,the Au NPs with intrinsic conspicuous conductivity were incorporated into the CQD/organic HTL interface,which significantly improving the PCE of the device from 2.44%to 3.32%by increasing the open-circuit voltage and fill factor.Different from the traditional absorption enhancement mechanism induced by surface plasmon effect,this incorporation of Au NPs significantly affected the energy level structures of CQD,which enhanced the hole extraction ability from PCE10 to CQD,and passivated the interface defects to a certain extent,resulting in enhanced device performance.Then,to relieve the phenomenon of charge accumulation and enhance the photocurrent response ability of the device in longer wavelength region,the PCE 10 layer was blended with BTP-4Cl to form a bulk-heterojunction(BHJ)structure and increase the short-circuit current density of device through complementary absorption and further increasing the PCE up to 4.14%with the synergistic effect of Au NPs modification.Additinally,the device also exhibited excellent stability.This work has laid a firm foundation for the future development of AgAuSe CQDs photovoltaic devices.