Novel Guanidine Molecular Additive Engineering For Efficient And Stable Perovskite Solar Cells

In recent years,perovskite solar cells（PSCs）based on three-dimensional（3D）organic-inorganic hybrid perovskite materials(ABX₃,A=Cs⁺,CH₃NH₃⁺（MA⁺）,CH（NH₂）₂⁺（FA⁺）;B=Pb²⁺,Sn²⁺,Ge²⁺;X=Cl^-,Br^-,I^-)as the light-absorbing layers have become a research hotspot in the third-generation new photovoltaic technology due to their direct band gap,adjustable band gap,high absorption coefficient,low exciton binding energy and long carrier life,as well as the easy solution processing,low cost and high power conversion efficiency（PCE）,etc.After optimization,the certified efficiency of PSCs has increased from 3.8%in 2009 to 25.5%in 2020,which is close to the energy conversion efficiency of commercial silicon-based solar cells and Cu（In,Ga）Se₂（CIGS）thin-film solar cells,showing the promising research and application prospects.Nevertheless,there are still many issues to be solved for the commercialization of PSCs.High-quality perovskite films are the key to achieve highly efficient and stable PSCs.However,currently,most of perovskite films are prepared via the low-temperature solution process,which cannot effectively control the nucleation and crystal growth of perovskites.As a result,most of the prepared perovskite films are polycrystalline films,which inevitably contain a large number of crystal defects,such as uncoordinated ions and dangling bonds at the surface and grain boundaries.These defects will cause the non-radiative recombination during the transfer process of carriers and thus degrade the device performance.Moreover,due to the low formation energy,unstable crystal structure,as well as the strong hydrophilicity and volatility of organic cations,organic-inorganic hybrid perovskite materials are easily decomposed by environmental factors,such as light,heat,water and oxygen,etc,which will lead to the poor stability of PSCs and hence limit their further large-scale commercial application.In order to improve the performance and stability of PSCs,researchers have adopted many strategies,such as component engineering,additive engineering and interface engineering.Of these,additive engineering,as a kind of facial and simple approach,has drawn more and more attentions.Very recently,additives containing the guanidine groups have attracted extensive attention among numerous additives owing to the highly symmetrical chemical structure with amino groups.However,up to now,the guanidine-based additives used in PSCs only contain a guanidine functional group and the application of guanidine molecule with bifunctional groups in PSCs has not been reported.It can be expected,the novel guanidine molecular additives containing bifunctional groups can be employed in PSCs,which can not only make full use of the role of the guanidine group,but also can exert the synergistic effect of bifunctional groups.Based on the above considerations,we have designed and developed two novel guanidino molecules,which are used as additives for the perovskite active layers to improve the PCE and stability of PSCs.The main contents are as follows:1.The novel guanidine additive,GuaBF₄,has been synthesized and introduced into the perovskite precursor solution to improve the PCE and stability of MAPb I₃inverted planar heterojunction PSCs via taking full advantage of the synergistic effect of Gua⁺and BF₄^-.The results show that,compared with Gua I and NH₄BF₄,the incorporation of the GuaBF₄ additive in the MAPb I₃ perovskite precursor solution can effectively regulate the crystal growth behavior of perovskites and acquire the high-quality perovskite films with full-coverage,large grain size,low roughness and high crystallinity,which is mainly attributed to the strong interaction between GuaBF₄ and MAPb I₃.Meanwhile,the introduction of GuaBF₄ can effectively passivate defects in the perovskite film and reduce carrier non-radiative recombination.When the doping amount of GuaBF₄ is 1mg/m L,the PCE of 18.60%can be obtained,which is significantly higher than the control device（15.26%）,and the device hysteresis effect is effectively suppressed simultaneously.In addition,upon introducing GuaBF₄,the humidity stability of the perovskite film and device is significantly improved.The PCE of the unencapsulated PSC with GuaBF₄ can maintain 82%of the initial efficiency after being placed in an environment with the relative humidity of 60%at room temperature for 20 days.2.A guanidine molecule,N,N-Dimethylimidodicarbonimidic diamide（DMID）,has been incorporated into the Cs_0.05(FA_0.85MA_0.15)_0.95Pb(I_0.85Br_0.15)₃ perovskite precursor solution as additives.The influences of biguanide groups in DMID on the properties of perovskite films and the performance of PSCs are explored.It is found that the application of the DMID additive can improve the crystallinity of perovskite films,as well as increase the grain size and reduce surface roughness.Meanwhile,DMID can interact with the perovskites to effectively passivate the defects in the perovskite films,thereby,improving the device performance.The PCE has been increased from 17.55%for the control device to 19.83%for the device with DMID.Additionally,the DMID additive can slow down the degradation rate of the perovskite films and boost the stability of PSCs.As a result,the PCE of the DMID device without encapsulation can retain 85%of the original efficiency after storage in an environment with the relative humidity of 50%at room temperature for 30 days.In this work,the novel guanidine molecules containing bifunctional groups have been designed and developed,which provides a new additive strategy for efficient and stable PSCs.