Application Of Chlorophyll Hole Transport Materials In Perovskite Solar Cells

The primary task facing the current society is to develop and utilize renewable energy to solve energy and environmental issues,which is also the key to achieving sustainable development of economic social.As one of the widely used clean energy sources,solar energy can greatly alleviate the current situation of energy shortage,change the existing energy consumption system dominated by coal,oil and natural gas,and then realize energy replacement.Photovoltaic devices are an important method of solar energy development and application,which can efficiently convert light energy into electrical energy that can be used for production and life,ensure the quality of human life,and actively promote the large-scale development of solar energy.Perovskite solar cells stand out among photovoltaic devices due to their unique advantages such as wide spectral response,excellent photoelectric properties,and tunable crystal structure.Since the mystery veil of perovskite solar cells(PSCs)was uncovered in 2009,the power conversion efficiency(PCE)of devices has been soaring,showing amazing application prospects,PSCs are regarded as the most potential new renewable energy cells.Despite the rapid development of PSCs in recent years,there are still many challenges ahead for commercialization,such as the tolerance of devices to humidity and temperature,the performance of each functional layer,PCE,fabrication cost,and toxicity.The hole transport layer(HTL)is one of the important components of PSCs.It can be used to directionally transport the holes generated in the light absorption layer,so that they can be efficiently collected by the outer electrodes,reduces the energy loss caused by the recombination of photogenerated carriers at the interface,and plays a crucial role in improving the performance of cells.Chlorophylls,as indispensable natural pigments in nature,act a key role in photosynthesis.Their efficacy is mainly reflected in the efficient capture of sunlight and the separation and transfer of charges.Besides,the green material chlorophylls also have the characteristics of rich resources,simple structure and easy regulation.These properties are conducive to their excellent charge extraction and transfer capabilities in PSCs,thereby preparing high-efficiency perovskite photovoltaic devices.In view of this,in this paper,a series of chlorophyll derivatives are synthesized by molecular engineering of natural pigment chlorophylls,and are used as HTLs in PSCs,so as to design photovoltaic devices with excellent performance.First,using natural bacteriochlorophyll-a as raw material,we synthesized five bacteriochlorophyll derivatives BChls-1~5 with different substituents at C17 position,and used them as hole transport materials(HTMs)for CH3NH3 Pb I3-based PSCs.BChls-1~5 with various esterifying groups can form aggregate films with different surface morphologies through simple spin coating,and exhibit distinct charge extraction and transfer capabilities.Among them,dodecyl-based BChl-2 can provide the most efficient transport path and the best interface environment for charge transfer inside the device,achieving a PCE of 13.6%.This work contributes to the development of low-cost,environmentally friendly,effective green materials,and provides ideas and ways for sustainable development in photovoltaic field.In order to further explore the application potential of chlorophyll molecules in photovoltaic devices,we designed zinc-chlorophyll derivatives Zn Chls-1~4,which are similar in structure to BChls-1~5.The central zinc ion,31-hydroxy group,and 131-carbonyl group in their structure can produce strong intermolecular interaction and promote the self-assembly of chlorophyll molecules to form J-aggregation.This highly ordered structure is beneficial to the transport of carriers.In addition,Zn Chls-1~4 have hydrocarbon chains with different lengths at the C17 position that are not directly conjugated with the chlorophyll π system.This structural difference does not affect the electronic absorption spectra and energy levels of Zn Chls-1~4 monomers,but will make their aggregate films exhibit various photophysical and chemical properties,which are mainly reflected in the performance disparity of PSCs based on different materials.The devices using Zn Chls-2,3 as HTLs achieve the best photovoltaic performance,which can be attributed to the lower HOMO energy levels of Zn Chls-2,3 and the most efficient charge transport in PSCs.Besides,the existence of long-chain hydrocarbon groups improves the solubility of Zn Chls-2,3,thus forming aggregate films with smooth surfaces,which also have positive effects on device performance.In the previous work,we have been committed to developing and applying monolayer chlorophyll derivatives and investigating the impact of their molecular structure on device performance.However,with the in-depth research,the deficiencies of monolayer transport layer in structure and function are gradually exposed,and its film morphology and hole transport performance are not satisfactory.Therefore,we tried to use two chlorophyll derivatives(Zn Chl-1 and H2Chl)as bilayer HTL for planar PSCs,and prepared cells with ITO/Sn O2/MAPb I3/Zn Chl-1/H2Chl/Ag structure,aiming to improve devices efficiency by combining the performance merits of these two materials.Although H2 Chl hinders the interlayer transport of holes due to its too low HOMO energy level,resulting in a decrease in Jsc of the device based on bilayer structure,the bilayer HTL still shows its unique advantages,which can effectively suppress carrier recombination at the interface and reduce the loss of photogenerated carriers,thus achieving a maximum PCE of 14.1% and obtaining good stability.Subsequently,considering the mismatched energy level distribution and poor hole transport ability of H2 Chl molecule,we introduced pentacene with excellent photoelectric properties into the bilayer structure to replace H2 Chl,prepared a chlorophyll-based bilayer organic HTL(pentacene/Zn Chl-1),and used it in PSCs to further verify the possibility of chlorophyll-based bilayer HTL in photovoltaic devices.Among them,pentacene and Zn Chl-1 can be respectively prepared into ordered crystal film and aggregate film by different film-forming methods,which means that carriers can be efficiently transported inside them.Therefore,devices based on monolayer pentacene and monolayer Zn Chl-1 can obtain the PCE of 9.25% and11.5%,respectively.While the bilayer organic transport layer perfectly replicates the effect of bilayer chlorophyll derivatives in inhibiting charge recombination and reduces the energy loss at the interface.Finally,the PSCs based on pentacene/Zn Chl-1obtained the highest fill factor(FF)and PCE.The above research on chlorophyll-type HTL not only meets the development requirements of green economy,but also enables us to have a deeper understanding and cognition of the application potential of natural material chlorophyll in photovoltaic devices,which is conducive to realizing the efficient utilization of renewable clean energy.