Study Ofthe Stability And Photovoltaic Performance Of Spiro-core Hole Transport Materials In Photovoltaic Devices

Perovskite solar cells(PSCs)is a new type of the 3rd generation thin film solar cells with organic-inorganic metal halide perovskite material MAPbX3(X = I,Br,Cl)as photosensitive agent.PSCs have demonstrated a good application foreground because of their advantages such aseasy processing,low cost and high efficiency,and become a hotspot in photovoltaic field.From 2009 to date,the power-conversion efficiency(PCE)of PSCs based on MAPbX3 has been rapidly increased from 3.8% to 22.1%.As an critical component in PSC,the hole transportmaterial(HTM)is responsible for extraction of photo-generated holes from the perovskite and transmission these charges to the back electrode,simultaneously,playing an important role in minimizing charge recombination losses at the interface,and thus achieving high performances.The most efficient HTM used in PSCs is currently Spiro-OMeTAD.The excellent performance of Spiro-OMeTAD is believed to result from the structural advantages of the spirobifluorene-based core structure,which prevents molecular aggregation and confers good solubility in organic solvents.Nevertheless,the harsh conditions and high cost of Spiro-OMeTAD synthesis are also disadvantageous for future large-scale applications.So developing simple structure,low cost and high performance novel HTM is imperative.Furthermore,as an excellent photovoltaic device,the most critical issue for PSCs is the poor long-term stability.The intrinsic stability of HTM is a key factor influencing the stability of PSCs.In this thesis,we investigate the stability of Spiro-OMeTAD commonly used as HTM in PSCs.The effects of humidity and oxygen on the chemical structure and optical properties of Spiro-OMeTADare studied in detail;found that water and oxygen have little effecton the properties of Spiro-OMeTAD.However,the cost of Spiro-OMeTAD is too high,so we retain its core structure and focus on synthesizing efficient and simplified structure HTM.Firstly,the results of thermogravimetric analysisshow that Spiro-OMeTAD possesses a good thermal stability,and the decomposition temperature is ca.400 oC.In addition,after exposed in high humidity and oxygen atmosphere for a long time,neither the molecular structures nor the optical properties of Spiro-OMeTAD change significantly,indicating that this material has good chemical stability in ambient air.The PSCs based on theSpiro-OMeTADtreated under different conditions exhibit comparativepower conversion efficiencies.These results indicate that Spiro-OMeTAD has a good stability in the air.Then,we report the synthesis of a new,high-yielding spiro-type HTM,Spiro-027.The replacement of two 4,4'-dimeoxyl-diphenylamine at the 2' and 7' positions of conventional Spiro-OMeTAD with two tert-butyl in Spiro-027 results in a greatly simplified molecular structure with unaffected thermal stability or highest occupied molecular orbital(HOMO)and lowest unoccupied molecular orbital(LUMO).In addition,enhanced hole mobility is observed in Spiro-027.Thus,the PSC devices based on Spiro-027 demonstrated a high power conversion efficiency(PCE)of 16.60%,which was higher than the 16.12% PCE achieved by those based on Spiro-OMeTAD.By combining these results with theoretical calculations,our work confirmed the feasibility of simplifying the molecular structure of Spiro-OMeTAD,which contributed to our knowledge of spiro-type optoelectronic functional materials.Moreover,our results offer a novel strategy for the replacement of expensive HTMs and the enhancement of PSCs performance.