Fabrication And Working Mechanism Of Chlorophyll-based Solar Cells

The development and utilization of clean and renewable energy have become crucial issues of our society because of the energy and environmental crisis.The solar energy has been favored by many researchers due to it could convert luminous energy into thermal energy and electric energy efficiently.However,the practical application of conventional photovoltaic devices based on expensive and harmful inorganic or organic materials are limited to some extent.Photosynthesis is the most fundamental way to convert renewable solar energy into electricity,chemical energy and bioenergy on earth.It maintains the carbon-oxygen equilibrium in nature,and provides necessary energy and environment for lives.Chlorophylls are essential natural pigment in photosynthesis,playing key roles of light-harvesting as well as photo-induced charge separation and transfer in natural photosynthetic apparatus.It provides new thinking styles and research methods for utilization of clean and renewable energy.In the paper,from the photophosphorylation process in green plants,we synthesized a series of chlorophyll derivatives and applied them to artificial photosynthetic systems for the conversion of solar energy to electrical energy.We tried to improve the photovoltaic performance of devices via various methods such as solvent engineering,and further explored the excited-state dynamics of the artificial photosynthetic system.Furthermore,inspired by Z?scheme process of oxygenic photosynthesis,we had fabricated trilayer chlorophyll cascade biosolar cells and researched their photovolatic performance.Therefore,these studies not only definitely deepen our knowledge about the photoelectrical and physicochemical properties of biological materials,but also provide a new possibility of using bioresources and photosynthesis for electricity production.Firstly,considering the safety hazards and the charge separation and transfer in chlorophyll photovoltaicdevices,we synthesizedbiocompatibilityand self-aggregation chlorophyll derivatives and applied them to chlorophyll based solid-state solar cells.As a preliminary attempt,although the photo to electricity conversion efficiency of the solar cell is low,only about 0.25%,it also proved that chlorophyll aggregates could realize charge transfer.It is worthy noting that the chlorophyll aggregates not only could transfer charge,but also could harveste light and separate charge effectively to form photocurrent from the photo-to-electron conversion efficiency spectra.Therefore,the photovoltaic devices with two chlorophyll H₂Chl-1 and?ZnChl-1?_n which could form photo-generated current may be a new artificial photosynthetic system.In order to further improve the photovoltaic performance of chlorophyll based solar cells,we tried to introduce chlorophyll aggregates with high carrier mobility,and control the morphologies and hole carrier mobilities of them via solvent engineering.The power conversion efficiency of the solar cell had enhanced to 2.13%through solvent engineering.Meanwhile,the aggregation morphologies was dependent on the different solvents would influence the function and charge separation of the?ZnChl-2?_n in the solar cells.Both chlorophylls can be excited to generate photocurrent in the solar cells which are prepared by mixed solvent.However,the device prepared with pure chlorobenzene contains major spectral components from the H₂Chl-1 sensitizer,in the case,the exciton generated in?ZnChl-2?_n must be less dissociated,?ZnChl-2?_n only could act as hole transport materials.In addition,through the spectral analysis of the devices and films,we found that TiO₂-H₂Chl-1/?ZnChl-2?_n can form an organic-inorganic heterojunction which is low cost and easy degradation.Subsequently,we successfully employed high-molecular polymer P3HT as a hole transport material in chlorophyll based solar cells to improve the charge collection efficiency at the interface of device.Although the amount of?ZnChl-2?_n is slightly reduced,the photon-to-electron conversion in both 300–540 nm and 660–725 nm wavelength regions have enhanced significantly.Thus,the power conversion efficiency has increased to 3.06%.The monochrome photo-to-electron conversion efficiency spectra and electrochemical impedance data further illustrate that the presence of P3HT could improve charge transfer and suppress the charge recombination at the interfaces of solar cell effectively,thereby promoting the charge collection in the device.For further study the working mechanism of the"green"solar cells and the processes occurring at the interface between the two chlorophylls,we try to explore the charge transfer dynamics in excitation of chlorophylls in the device via subpicosecond time-resolved absorption spectroscopy.A charge transfer state between TiO₂-H₂Chl-1 and?ZnChl-1?_n was observed at 640 nm after excitation at 680nm.This charge transfer?CT?state is entirely different from the CT states observed for either TiO₂-H₂Chl-1⁺and TiO₂-?ZnChl-1?_n⁺systems.Meanwhile,excitation of?ZnChl-1?_n at 720 nm can also generate the CT state between the two Chl species together absorption of monomer ZnChl-1.Therefore,effective charge dissociation and transfer can take place at the interface between the two chlorophylls due to the presence of CT state,and the chlorophyll based solar cells work coefficiently with these two chlorophyll species.Finally,in order to further enhance the possibility of artificial photosynthesis,we tried to prepare a chlorophyll based trilayer cascade biosolar cell.In addition to employing ZnChl-1 and H₂Chl-3 as reaction center simulators of photosysterm I and II to simulate Z-scheme process of oxygenic photosynthesis,H₂Chl-1/2 sensitized mesoporous TiO₂ layer was introduced to act as photoactive layer and charge extraction layer to simulate the primary electron acceptor in photosysterm.As expected,all chlorophyll in trilayer cascade biosolar cells could be excited and induced to generate photocurrent.In addition,the power conversion efficiency of the devices with trilayer structure of TiO₂-H₂Chl-1/ZnChl-1/H₂Chl-3 is significantly improved compared with the bilayer chlorophyll solar cell.Moreover,the performance enhancement is mainly due to the improvement of light-harvasting and charge collection.In addition,by co-sensitizing H₂Chl-1 with 1%H₂Chl-2,the electron injection efficiency at the interface of TiO₂-H₂Chl-1 can be further improved,so that the power conversion efficiency of co-sensitized trilayer cascade biosolar cells can be increased to 4.14%.At the same time,the devices display good reproducibility and stability.These researches on chlorophyll solar cells have deepened our understanding of artificial photosynthesis.Furthermore,the photo-to-electron conversion performance and excitation dynamics of the chlorophyll solar cells provide new research ideas and directions for the artificial application of photosynthesis.Meanwhile,it is possible to realize the application of solar energy,a clean and renewable energy,through artificial photosynthesis.