The Effects Of Interface Modification On The Performances Of SnO₂-based Hybrid Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells（PSCs）have been rapidly developed in the past ten years,due to their low manufacturing cost and high photovoltaic conversion efficiency.The highest certified efficiency of perovskite solar cells reported so far has reached25.5%.One of the main factors for excellent performance is the interface properties of each functional layer,especially the charge transport performance between the electron transport layer and the light collection layer and the hole transport layer（ETL/PVK,PV/HTL）.In the planar structure of perovskite solar cells,SnO₂ as an electron transport layer has been widely used.Because it has many advantages,such as high electron mobility and low temperature preparation.However,the high density of SnO₂ surface defect states and the mismatch with the energy level of the perovskite layer limit the efficiency and stability of the device to a certain extent,and hinder the further commercial development of perovskite solar cells.In order to improve the properties of the ETL/HTL interface and improve the PCE and long-term stability of the device,this paper uses SnO₂ as the ETL,FAMAPbI₃ as the perovskite light-absorbing layer,and uses PDTA and PDTA-OH and chlorine-containing quaternary ammonium salt respectively for SnO₂ ETL,ETL/PVK interface and perovskite.The mineral layer is modified to reduce grain boundary defects and slow down interface carrier recombination,thereby improving the PCE and stability of the device.The main research contents are as follows:（1）The low-cost sodium gluconate complexed with SnO₂ colloidal nanoparticles was used as the ETL.The effect of different sodium gluconate concentration（5,10,15,20 mg/mL）on the crystal structure and micro morphology of SnO₂ film was studied by XRD and SEM.For the planar structure of the PSC,using PL and TRPL to study the charge transport dynamics process of the ETL/PVK interface of the device,the electron transport life and the SnO₂ electron transport layer’s ability to extract electrons.The results show that sodium gluconate can undergo a chelation reaction with SnO₂,thereby slowing down the growth rate of SnO₂ crystal grains,making the prepared SnO₂ film increase in grain size,and the film surface is flat and compact,which in turn contributes to the homogeneous deposition of the upper perovskite layer,promoting the growth of perovskite grains and reducing of grain boundary defects.The results of PL and TRPL show that after adding sodium gluconate,the carrier lifetime is shortened and the charge recombination process at the ETL/PVK interface is inhibited.At the same time,due to the complexation of sodium gluconate and SnO₂ colloid,the conductivity of the SnO₂ film is improved,thereby increasing the electron transport layer’s ability to extract electrons and accelerating the electron transport at the interface.The photovoltaic performance test results of the assembled device show that when the complex concentration is 10 mg/mL,the PCE of the device reaches 20.65%,which is much higher than the 15.72%of the device without sodium gluconate.With the addition of sodium gluconate,the hysteresis factor（HI）of the device calculated under the reverse and forward scan test is reduced from 11.64%to 4.05%.And in the air,humidity of 40%,25℃,the device performance can still maintain more than 85%of the initial value after being stored in the dark state for 600 h,while the control group has decayed to less than 60%of the initial value.（2）In order to further improve the properties of the ETL/PV interface and reduce the non-radiative recombination,PDTA and PDTA-OH complexing agents were used to modify the surface of the SnO₂ETL,while adjusting the growth process of the perovskite film.XRD and SEM were used to analyze the crystallinity and microstructure of the perovskite film modified by PDTA and PDTA-OH.The results show that compared with the control group,the crystallinity and flatness of the perovskite film modified by PDTA and PDTA-OH have been improved to varying degrees,and the grain size of the perovskite film based on PDTA-OH is the largest,the PCE of the assembled device is the best.Therefore,the effects of PDTA-OH of different concentrations（0 mg/mL,0.2 mg/mL,0.4 mg/mL,0.6 mg/mL and 0.8 mg/mL）on the quality of the perovskite film and the interface properties and photovoltaic performance of the device have been deeply studied.The SEM results showed that when the concentration of modifier was 0.4 mg/mL,the perovskite film had the largest grain size and the smallest grain boundary.On this basis,the device was further assembled and the highest PCE of 20.9%was obtained.This is mainly because PDTA-OH at a suitable concentration can be used as the nucleation site of perovskite grains,which can induce the rapid nucleation and slow growth of perovskite;When the concentration is too large,the surface of the modified layer will be uneven,causing the homogeneous deposition process to be destroyed,so that the grain size is not uniform,and the surface roughness of the film is increased.Therefore,the optimal modifier concentration was determined to be 0.4 mg/mL.On this basis,a PCE of 20.9%of the assembled device was obtained.Compared with the control group,the open circuit voltage(V_oc)and fill factor（FF）of the device after PDTA-OH are significantly improved.This is mainly due to the introduction of-OH in PDTA-OH,which can effectively reduce the PDTA-OH molecules.Anchored on the surface of SnO₂,it accelerates the transport of electrons from the perovskite layer to the SnO₂ electron transport layer.At the same time,the double bond oxygen in the PDTA-OH structure has a lone pair of electrons,which can be used as a Lewis base and PbI₂in the perovskite precursor.A weak bond is formed between them,thereby regulating the growth rate of the perovskite film,promoting the growth of perovskite grains,and finally obtaining a perovskite film with fewer grain boundary defects,uniformity and flatness,and at the same time accelerating the charge transport of the ETL/PV interface Through the process,PSCs devices with high photovoltaic performance are finally obtained.（3）On the basis of the above two works,we further introduce bifunctional ionic additives to modify the perovskite layer to obtain high-quality perovskite film.By adding chlorine-containing quaternary ammonium salts with different molecular structures（dodecyl dimethyl benzyl ammonium chloride（DDBAC）,dodecyl trimethyl ammonium chloride（DTAC）,three Methylbenzyl ammonium chloride（TMBAC））,the perovskite film was prepared by a two-step solution method.In-depth exploration of the passivation method of the chloride-containing quaternary ammonium salt with different functional group structures on the grain boundary defects of the perovskite film,and the intrinsic correlation between the mechanical process of charge transport at the device interface and the photovoltaic performance of the device were revealed in detail.The analysis by XRD and SEM shows that compared with TMBAC containing only benzene ring and DTAC additive containing only long alkyl chain,DDBAC with both benzene ring and long alkyl chain obtains a smoother and denser perovskite film and high-performance battery devices.This is because the benzene ring in DDBAC will accumulateπ-πto accelerate the charge transport at the interface;on the other hand,the long alkyl chain helps to regulate the growth process of the PbI₂ film and obtain a loose and porous PbI₂ film,so that The organic amine salt in the second step reacts sufficiently with PbI₂ to reduce the residue of the reaction substrate,thereby reducing the defect state density of the perovskite film.At the same time,because the additive contains Cl^-,it can promote the crystallization of perovskite,increase the size of perovskite grains,thereby reducing grain boundary defects.Finally,based on the devices assembled by DDBAC,a short-circuit current of 23.14 mA cm^-2,an open voltage of 1.05 V,a fill factor of 77.5%,and an energy conversion efficiency of 18.8%were obtained.And due to the passivation effect of DDBAC on the grain boundary,it effectively blocks the intrusion of water and oxygen in the air,so that the stability of the battery is greatly improved.In the dark,room temperature and 40%humidity conditions,after 480h,it can still maintain 80%of its initial efficiency.