Research On PEDOT:PSS Buried Interface Modification Of Tin-Based Perovskite Solar Cells

Tin（Sn）-based perovskite materials have many advantages as the photoactive layer of perovskite solar cells（PSCs）,such as suitable optical bandgap,large light absorption coefficient,low exciton binding energy,low toxicity and environmental protection,etc.However,the highest power conversion efficiency（PCE）of tin-based PSCs is only about 14%,which is a large gap compared with lead-based PSCs,mainly due to the poor crystallization quality of tin-based perovskite films,divalent tin(Sn²⁺)is easy to oxidize,and there is a mismatch of interface energy levels in the device structure.In particular,there are a large number of defects at the grain boundaries of tin-based perovskite films and their interfaces with other functional layers,which not only cause severe carrier recombination but also induce Sn²⁺oxidation,thereby reducing the efficiency and stability of tin-based PSCs.Although a lot of research has been carried out on the regulation of tin-based perovskite crystallization and Sn²⁺anti-oxidation,there are relatively few reports on improving the film quality and device performance of tin-based perovskites by poly（3,4-ethylenedioxythiophene）:poly（styrenesulfonate）（PEDOT:PSS）buried interface engineering.Therefore,this paper first optimized the preparation process of tin-based perovskite films,and then prepared high-quality tin-based perovskite films by introducing two phosphoric acid-containing interfacial materials at the PEDOT:PSS/perovskite layer interface,which further improved the efficiency of tin-based PSCs.The research contents of this paper are as follows:（1）Optimization of the annealing process of tin-based perovskite films to prepare high-quality tin-based perovskite films.The effect of one-step direct annealing（OS）and gradient multi-step annealing（MS）on the quality of tin-based perovskite films and device performance was studied.The results show that MS annealing is beneficial to the preparation of tin-based perovskite films with uniform surface morphology and good crystallinity.The PCE of tin-based PSCs treated by MS method increased from 8.34%to 9.11%,in which the short circuit current(J_SC)of the device increased from 20.36 m A cm^-2 to 21.82 m A cm^-2,and the open circuit voltage(V_OC)increased from0.57 V to 0.58 V.Compared to the devices under OS conditions,the devices under MS optimal conditions have higher recombination resistance(R_rec),indicating that charge recombination is effectively suppressed.Moreover,the smaller dark current in the device indicates fewer defects in the film,thus improving the V_OC and J_SC of the device.Finally,the improvement of the film morphology leads to better device stability.（2）Modification of the PEDOT:PSS buried interface with 2-chloroethylphosphonic acid（ETH）to study their effect on the quality of tin-based perovskite films and device performance.Since the buried perovskite heterointerface has a higher number of submicron-sized defects than the top perovskite heterointerface,the poor surface quality of the as-prepared films limits the device performance.Therefore,we chose small molecule ETH with phosphonic acid groups to modify the PEDOT:PSS/perovskite interface,and tin-based PSCs were prepared based on the optimal MS conditions.The ETH interface modification can tune the work function（WF）of the PEDOT:PSS film at the heterointerface,and the WF increases from 5.09 e V to 5.24 e V,which closely matches with the energy level of the perovskite layer.The molecular interaction between ETH and perovskite increases the grain size of the perovskite film,improves the crystallinity and light absorption ability of the film,and effectively inhibits the oxidation of Sn²⁺in the film.Meanwhile,the defect density of the film decreased from 3.35×10¹⁶ cm^-3 to 2.51×10¹⁶ cm^-3 after ETH interface modification,and the surface roughness was reduced from 22.7 nm to 20.1 nm.Finally,the PCE of the device after interface modification increased from 8.95%to 9.61%.Due to the hydrophobicity of the ETH interfacial layer,the stability of PSCs is significantly improved.（3）Modification of the PEDOT:PSS buried interface with phosphomolybdic acid（PMA）to study their effect on the quality of tin-based perovskite films and device performance.Due to the energy level mismatch between PEDOT:PSS and the perovskite layer,the low WF of PEDOT:PSS leads to high V_OC loss in PSCs.Therefore,we chose PMA,a polyoxometalates,to regulate the WF and film surface quality of PEDOT:PSS to improve the efficiency of Sn-PSCs.The results show that the molecular interaction between PMA and Sn I₂ in the perovskite effectively improves the crystallinity of the perovskite film,giving it a preferential orientation along the（100）crystal plane.The grain size of the perovskite film is relatively large,and the surface is denser and smoother,while the roughness of the film is reduced.The PEDOT:PSS/PMA double layer can adjust the WF of the PEDOT:PSS film,the energy level of the PEDOT:PSS/PMA thin film is better matched with the perovskite layer to improve the hole injection efficiency.Correspondingly,the defect density of the film decreased from 1.92×10¹⁶ cm^-3 to 1.08×10¹⁶ cm^-3,and the nonradiative recombination in the film was effectively suppressed.Finally,the PCE of the device after interface modification increased from 9.32%to 10.21%,and the V_OC increased from 0.58 V to 0.59 V.