Research On The Interface Of Hole Transport Layer/Absorber In Perovskite Solar Cells

Since first successfully prepared in 2009,the efficiency of perovskite solar cells(PSCs)has been improved continuously,which implies great potential for industrial application.Different from the p-n homojunction structure of traditional silicon solar cells,PSCs are heterojunction composed of hole transport layer(HTL)/perovskite absorber/electron transport layer(ETL).As a result,the interface has very important effect on PSC's performance.In order to relieve the energy level mismatch and crystal defects at the interface of HTL/absorber in PSCs,corresponding interfacial optimization strategies are proposed here,and their influences on the properties of adjacent functional layers and on the interfacial carrier dynamics are further studied.Finally,the device performance is improved.The interfacial optimization strategies used in this work are listed as follows:(1)The energy level mismatch between HTL/perovskite will result in reduced open-circuit voltage(Voc)and efficiency.To address the issue,multifunctional organic molecules are used to modify the interface,including the following two contents.(a)The interfacial energy level mismatch is optimized by doping a bipolar polymer,Nafion,into PEDOT:PSS precursor.It is found that the introduction of Nafion can not only offset the energy level mismatch,but also improve the mobility of the PEDOT:PSS,which favors the efficient charge extraction and transport through the interface.After optimization,the Voc of the device increases from 0.97 V to 1.02 V,and the efficiency increases from 13.51%to 16.68%.In addition,a large proportion of Nafion self-assembles at the interface.The hydrophobic fluorocarbon skeleton of Nafion can protect perovskite crystals from the water in hygroscopic PEDOT:PSS.As a result,the device stability is improved.(b)The energy level mismatch at PEDOT:PSS/perovskite interface is improved by introducing a NPB interfacial modification layer.It is found that the introduction of NPB can alleviate the valence band shift of PEDOT:PSS and thus offset the energy level mismatch.Moreover,the introduction of NPB improves the morphology and crystallinity of perovskite films,and increases the carrier extraction at the interface.Finally,device Voc increases from 0.96 V to 1.05 V,fill factor(FF)increases from 72%to 78%,and efficiency increases from 15.40%to 18.40%.In addition,NPB layer also enhances the humidity and UV stability of device.Moreover,the NPB-based large-area(1 cm2)and flexible(0.1225 cm2)devices show a efficiency of 17.78%and 14.39%,respectively,which further proves the superiority of the developed method.(2)There are a large number of crystal defects at the HTL/perovskite interface,which will hinder carrier transfer,increase carrier recombination rate,even reduce device stability.To address the issue,the functional group,pyridine,is introduced into the molecular structure of HTM to passivate interfacial crystal defects,so as to improve the efficiency and stability of solar cells.The following two contents are included.(a)Pyridine is introduced into the arm of linear carbazole-triphenylamine-based small molecule to prepare new HTMs.It is found that the introduction of pyridine deepens the HOMO level and improves the thermal stability of HTMs,but decreases the hole mobility.Moreover,it is observed that pyridine group can passivate the crystal defects and promote the charge extraction at the interface.Finally,pyridine-HTM-based devices obtain the best efficiency of 18.45%,which is significantly improved compared with that of the devices based on non-pyridine-HTM(17.11%)and the device based on traditional spiro-OMeTAD HTM(17.81%).In addition,the device stability is also significantly improved.(b)In order to improve the conjugation of molecule and thus improve its hole transport ability,pyridine unit is further introduced into star-shape triphenylamine molecule as core.Both density functional theory analysis and experimental measurements indicate that the introduction of pyridine group can improve the molecular conjugation.The hole mobility increases and HOMO level shifts deeper after the introduction of pyridine.Similarly,the defect passivation effect of pyridine at the interface enhances carrier extraction and transfer and reduces carrier recombination rate.Finally,the efficiency of the optimized device is 17.09%,which is much higher than that of the devices with non-pyridine-HTM(11.56%)and traditional PEDOT:PSS devices(12.14%).In addition,the device stability is significantly improved compared with PEDOT:PSS-based device.After storage in ambient for 20 days,the device efficiency remains above 80%of the initial value.(3)Structure design of HTL/absorber interface for BiI3-based solar cell is also carried out.The toxicity and stability of PSCs have been concerned.Instead,BiI3 has great potential as non-toxic and stable absorber.The absorption edge of BiI3 is about 650 nm,leading to the waste of long-wavelength photons.To address the issue,a novel embedded nano-interface is designed.Firstly,the vertical orientation of BiI3 crystal is controlled by Li-TFSI doping process to obtain the nanoscale BiI3 polycrystalline template.Secondly,the polymer donor material,PTB7-Th,which can absorb long-wavelength photons,is embedded into the BiI3 nano-template and uniformly covers on the top,forming BiI3/PTB7-Th quasi-bulk heterojunction.Excitons in PTB7-Th can be timely transferred and separated at the embedded nano-interface before quenching.Therefore,compared with the planar interface,solar cells with embedded interface show a widened spectral response from 650 nm to 750 nm.Consequently,the short-circuit current(Jsc)is significantly increased from 1.30 to 7.80 mA·cm-2,which is the highest Jsc of BiI3 solar cells till now.In addition,after stored at ambient for 2 years,the efficiency of encapsulated device remains over 90%of the initial value,indicating the excellent long-term stability.