Dye Sensitized Cs₂AgBiBr₆ Double Perovskite Solar Cells

The development and utilization of renewable energy become the key to the sustainable development of human society.Perovskite solar cells（PSCs）exhibit comparable power conversion efficiency（PCE）and lower production cost compared to silicon-based solar cells.However,the large scale application of the traditional lead（Pb）-based PSCs are limited by the toxicity of Pb and stability issues.Although packaging and other technologies can partly solve the problem of Pb leakage,developing new lead-free perovskite materials is the fundamental way to get rid of lead pollution to the environment.As an important branch of lead-free perovskite materials,solar cells based on double perovskite materials exhibit high PCE and great stability.Cs₂Ag Bi Br₆,one of them,has been proved to be a potential photovoltaic materials.However,this material can only absorb photons with wavelength less than550 nm due to the large band gap,limiting the photocurrent density and PCE of Cs₂Ag Bi Br₆ based PSCs.Herein,we choose several small organic molecular materials which can absorb photons in the long wavelength range as the charge transport layer and to sensitize the perovskite photosensitive layer,and prepared a series of double perovskite solar cells in different structures with enhanced photons absorption range,higher photocurrent density and higher PCE.The solar cells exhibit good stability and environmental friendliness and broken the PCE record of double perovskite solar cells for several times.Firstly,we select a chlorophyll derivative with zinc as the center metal（Zn-Chl）.This chlorophyll derivative shows good optical absorption at around 400 nm,which can complement the weak absorption of double perovskite Cs₂Ag Bi Br₆ near 420 nm,and exhibits higher the highest occupied molecular orbit（HOMO）level than Cs₂Ag Bi Br₆.Therefore,we use it as hole transport material to prepare a planar double perovskite solar cell based on Sn O2 as electron transport layer（ETL）.Compared with the solar cells based on other traditional hole transport layers（HTLs）,the optical absorption and quantum response of double perovskite solar cells based on Zn-Chl have been significantly improved.Zn-Chl makes up for the quantum response defects of double perovskite solar cells near 420 nm,acting as both a HTL to transport holes and a photosensitive layer to provide photocurrent,which is extremely rare in traditional HTLs.We found that the photogenerated electrons in Zn-CHL can return to the Cs₂Ag Bi Br₆ layer due to the large energy level difference between the LUMO level of Cs₂Ag Bi Br₆ and the Soret level of Zn-Chl.Compared with the reference group,the solar cell based on Zn-CHL shows higher extinction coefficient.The short-circuit current density(J_SC)increases by 22-27%,reaching 3.83 m A cm^-2,and the PCE reaches 2.79%for the first time,which is the highest efficiency when this work publication.In this study,chlorophyll derivatives with photosensitive properties were used to sensitize double perovskite for the first time,which provides a new way to improve the photocurrent density and PCE of double perovskite solar cells.Subsequently,in order to broaden the absorption spectra of Cs₂Ag Bi Br₆ double perovskite solar cells,we used a chlorophyll derivative with carboxyl group（C-Chl）to sensitized mesoporous Ti O₂ as the ETL to prepare the dye-sensitized double perovskite solar cells.Due to the absorption of C-Chl in the long wavelength range,the J_SC reached 4.09 m A cm^-2 and a PCE exceeding 3%was achieved for the first time,breaking the PCE record of double perovskite solar cells again.We found that the charge transfer path in the dye-sensitized PSCs can be divided into two parts:when Cs₂Ag Bi Br₆ was excited,due to the low adsorption capacity of C-Chl,Cs₂Ag Bi Br₆ is in direct contact with the surface of mesoporous Ti O₂ and the excited electrons can be directly injected into Ti O₂ ETL and the holes transfer into HTL.When C-Chl is excited,electrons transfer into Ti O₂ rapidly.As the LUMO level of Cs₂Ag Bi Br₆ is slightly lower than that of C-Chl,the holes in C-Chl can obtain the electrons excited from Cs₂Ag Bi Br₆.Such charge transfer pathway similar to Z-type oxygen-containing photosynthesis in nature has been confirmed by our previous works.Finally,in order to further improve and broaden the optical absorption of Cs₂Ag Bi Br₆ double perovskite solar cells and make the raw materials available,we used three commercial indole dyes D102,D131 and D149 to sensitize mesoporous Ti O₂ as ETLs to prepare indole-based dye-sensitized Cs₂Ag Bi Br₆ PSCs.Due to the excellent optical absorption ability of the indole dyes in the long wavelength range,the solar cells show good optical response at 300-700 nm.By comparing the dye-sensitized double perovskite solar cells based on the indole dyes,we found that the solar cells based on D149 shows the best photovoltaic performance,with a J_SCexceeding 8 m A cm^-2,which is higher than the dye-sensitized solar cells based on D149,and a PCE up to 4.23%for the first time.In addition,we found that due to the high extinction coefficient of indole dyes,the total photons incident into the perovskite film decreases,resulting in decreased photocurrent from the perovskite layer.The open-circuit voltage(V_OC)of the cells is lower than that of the original double perovskite solar cells,but higher than that of the corresponding dye-sensitized solar cells.In addition,as the crystallization process of Cs₂Ag Bi Br₆ needs high annealing temperature,the indole dye will partly decompose,which also affects the overall photocurrent density of the cell.In spite of this,the dye-sensitized double perovskite solar cells break the highest PCE record of double perovskite solar cells.Our research provides a simple and effective way to improve the photovoltaic performances of double perovskite solar cells.These researches on dye-sensitized Cs₂Ag Bi Br₆ PSCs have deepened our understanding of the working mechanism of double perovskite solar cells.On the other hand,the combination of organic small molecular photosensitizers and double perovskite can further improve the PCE of double perovskite solar cells,which is beneficial to realize the commercial application of perovskite solar cells.