Formulation Engineering Of Fullerene-based Electron Transport Materials For Perovskite Solar Cells

As is known to all,most of the world's energy consumption comes from the fossil fuels burning,which lead to a pernicious effect on climate and the environment in nowadays.Developing clean and renewable energy resources has become one of the most important themes of the world.Compared with other renewable energy resources,solar energy turns out to be an excellent choice for human being because of the advantages of clean and inexhaustible.In the past few decades,a lot of effort has been paid to develop many kinds of solar cells,such as Si solar cell,dye-sensitized solar cell and organic solar cell,to improve the utilization rate of solar energy.However,organolead halide perovskite photovoltaics has attracted much attention as one of the promising renewable energy to reduce the vast fossil fuel consumption,transcending organic and dye-sensitized solar cells in terms of power conversion efficiency(PCE)on a very short timescale.Since the seminal report of Miyasaka and co-workers in 2009,the PCEs of perovskite solar cells have been improved to over 20%in 5 years.As one of the vital parts of perovskite solar cells,the electron transport materials(ETMs)play a critical role in charge injection and separation.Fullerenes have been prevalently used as organic electron transport material with the advantages of low temperature solution-processing on flexible substrates,feasibly eliminating photocurrent hysteresis,and better charge injection/separation efficiency.Up to now,a large number of fullerene derivatives have been used as acceptors or ETMs in organic solar cells.However,only a few fullerenes have been employed as the ETMs in perovskite solar cells.Few research groups have investigated the factors influencing the PCE of perovskite solar cells with fullerene derivatives as the ETMs.It is urgent to develop the strategy to achieve the best fullerene ETMs and reduce the cost for the commercial applications of high performance perovskite solar.Herein,a series of typical fullerene C60 derivatives were employed as the ETMs in the inverted PHJ perovskite solar cells.The correlation between electrochemical properties,charge-transporting properties,and film-forming properties of these fullerene ETMs and the resulting device performance were also established.Largely due to better surface morphology,the EDNC device exhibited better performance(12.64%)than that of BDNC device(7.36%)despite of their similar LUMO level,electron mobility,optical properties and electrochemical properties.High electron mobility and good surface morphology have been demonstrated as critical factors to influence photovoltaic performance of fullerene-based perovskite solar cells.At this circumstance,we focus on searching for fullerene ETMs with high electron mobility and developing practical strategy to optimize the surface morphology of fullerene ETMs.Fullerene derivative[6,6]-phenyl-C71-butyric acid methyl ester(PC71BM)is the unreplaceable electron acceptor widely used in organic solar cells,perovskite solar cells to date,but commercially available sample of PC71BM is a "random" mixture of a-,?1 and ?2-PC71BM isomers typically.During the exploration on our experiment,we discovered that the purified isomer only,no matter each of ?-,?1-or ?2-PC71BM,would resulted in poor film of electron acceptor layer with discontinuous microcrystalline and,in turn,poor photovoltaic performance in the devices.Interestingly,we found that the surface morphology of PC71BM ETM can be greatly turned by mixing the three isomers with different formulations.This so-called formulation engineering concept has been prevalent in pharmacy but seldom discussed in photovoltaics.Taking planar heterojunction perovskite(CH3NH3PbI3)solar cell as an example,PCE can be flexible in the range of 0.38-17.56%depending on isomeric composition of PC71BM.Mixture of the three isomers of PC71BM with a ratio of 17:1:2(a:?1:?2)stands out with higher PCE and less photocurrent hysteresis.We show that the fullerene ETM as well as the PCE of solar cells can be optimized by the strategy of fullerene formulation engineering.Isomeric fullerene derivatives such as a-,?1-or ?2-PC71BM,having comparable photovoltaic-relative properties,LUMO energy level,bandgap and UV-Vis absorption,are thus established as the ideal starting materials for formulation engineering to optimize fullerene-based electron acceptors.Compared with PCnBM,pristine fullerenes such as C60 and C70,have lower cost and exhibit higher electron mobility.Reasonably pristine fullerenes can be ideal ETMs better than modified ones such as the typically used PCnBM for high-performance perovskite solar cells.Unsatisfactory,the poor solubility of pristine fullerenes in common organic solvents and their strong tendency to aggregate or crystallize during solvent removal make it difficult to deposit compact and continuous thin film for efficient perovskite solar cells.It is highly expected to develop low-cost solution-processed pristine fullerenes ETMs for mass-productive and cost-effective preparation of perovskite solar cells.Encouraged by "fullerene formulation engineering" concept,we mixed pristine fullerenes(C60 and C70)as the ETMs on inverted perovskite solar cells.Smooth and continuous thin films of pristine fullerenes were obtained by mixing solutions of C60 and C70 with different ratios.A PCE of 14.04%was obtained by using 1:1 ratio of C60 and C70 as the ETM with higher performance and better device stability by using simple low-cost solution processing method,which is comparable to that of PC61BM-involving device(PCE =13.74%).The strong tendency of pristine fullerene to aggregate or crystallize on solvent removal can be mitigated by using fullerene mixture,which is practical to fabricate compact and continuous thin ETMs film by low-cost solution processing for high-performance inverted PHJ perovskite solar cells.It should be noted that discovering and exploring the fullerene formulation engineering,as well as doeveloping device fabrication technology,have expended a mass of time and effort,the mechanism and precondition of the formulation engineering are still not quite clear,which remained to be further investigated along with knowledge and ability developing in the future.