Chlorophyll Derivative-based Solar Cells And Their Charge Transfer Mechanism

Due to the global warming issue caused by the consumption of fossil fuels,the development of new clean energy has become more and more urgent for researchers of both industrial and academic communities.Plenty of solar cell technologies have been developed to alleviate energy shortages,including inorganic silicon solar cells,thin film solar cells,polymer solar cells,and perovskite solar cells.However,the traditional inorganic silicon solar cells have the problem of high energy consumption during preparation process,resulting in high production costs;Thin film solar cells suffer from low efficiency;As for the polymer solar cells,their synthesis is very complicate,thereby causing a relative high cost.Perovskite solar cells as emerging technologies exhibit major problems of intrinsic instability of perovskite and toxicity of Pb.The above-mentioned drawbacks of each types of solar cells limit their practical application value.The natural photosynthetic organisms have evolved over a billion years to form a perfect transformation system from light energy to chemical energy.The core system is a pigment-protein complex based on chlorophyll molecules that functions from light energy capture to energy transfer and ultimately to charge separation.The whole working process is much more complicated than the existing solar cell systems,but the design is very subtle.The chlorophyll and its derivatives,which are extracted from natural green plants,algae,or photosynthetic bacteria,can function as the most abundant and environmentally friendly functional organic semiconductor materials.As a developing solar energy application,artificial photosynthesis-based solar cells that are inspired from natural photosynthesis has attained extensive attention.Such bio-solar cells can not only realize the effective usage of cheap renewable natural resources,but also likely to achieve potentially high-power conversion efficiency（PCE）by mimicking the light-to-energy conversion processes in natural systems.Their increasing PCE makes them promising candidates for practical application.Firstly,as a preliminary attempt,two kinds of self-aggregation chlorophyll and bacteriochlorophyll derivatives are applied as donor materials to organic solar cells in conjunction with fullerene C₇₀ as acceptor materials.The optimized PCE value of1.43%under standard AM1.5 m W/cm² sunlight was achieved with the BChl-1:C₇₀planar-heterojunction based organic solar cells.What’s more the absorbed sunlight of the BChl aggregates at the near-infrared region could be transferred into photocurrent,which can be proved from the external quantum efficiency.This indicates the energy and electron transferring between BChl aggregates are efficient.To further improve the photovoltaic performance of chlorophyll based organic solar cells,a chlorophyll aggregate with high carrier mobility and ambipolar performance is applied in this organic solar cell system and two different preparation methods were employed to prepare the active layer:（i）two-step spin-coating the self-aggregated CHL and PC₇₁BM solutions sequentially and（ii）one-step spin-coating the solution of CHL and PC₇₁BM blends,forming the“bilayer”（BL）and traditional bulk heterojunction（BHJ）configurations,respectively.Based on the abovementioned two kinds of active layer preparation methods,both inverted and regular types of organic solar cells were successfully investigated.All four types of devices can work normally,which is likely due to the ambipolar characteristics of CHL aggregate.Unexpectedly,the BL-based devices gave PCEs of 5.17%for regular type and 5.19%for inverted type,which were higher than those of the BHJ-based devices 3.96%for regular type and 3.50%for inverted type.Such an efficiency is the highest one for the Chl derivative based organic solar cells until now.The Z-scheme process of natural photosynthesis shows the pathway of photoinduced electron transport from photosystem II to photosystem I through an electron transfer chain.Inspired by the interesting Z-scheme of oxygenic photosynthesis,we imitated the dual photosynthesis systems into bio-solar cells.Due to higher HOMO/LUMO energy levels of Chl-A than those of Chl-D,the sublayer Chl-A corresponds to PSI and the upper layer Chl-D are equivalent to PSII,leading to double photoexcited electron transfer from Chl-D to Chl-A.The energy alignment of the photoactive layers here is in conflict with the traditional comprehension of photovoltaic devices.Interestingly,such an uncommon device can still work well and the well-worked device proves a new design idea of bio-solar cells containing photosystems for future sustainable energy production.In order to set out a better guidance for the design of the Chl-derivatives based solar cells,we have systemically applied sub-nanosecond pump-probe time-resolved absorption spectroscopic measurements to the Chl-A and Chl-D both in solutions and in solid-film states.The triplet excited states of Chl derivatives in solutions were confirmed by the deoxygenation of the solution samples（introducing nitrogen gas to the solution）,while those of the film samples were confirmed by the introduction of all-trans-β-carotene to the films as a triplet state scavenger.In addition,the radical species of the Chl derivatives in solutions were identified by introducing hydroquinone as a cation radical scavenger and/or anion radical donor.These radical species（either cation or anion）can be the carriers in the Chl-derivatives based solar cells.Interestingly,the introduction of the hydroquinone to the film samples enhanced the carrier lifetimes.This unexpected finding enhanced 20%photovoltaic performance of the Z-scheme process of photosynthesis inspired bio-solar cells.Finally,to further reveal the interface charge transfer mechanism of the chlorophyll based solar cells,sub-picosecond transient absorption spectroscopies were employed to investigate their excitation dynamics.Duel charge separation mechanism giving by TiO₂^-–H₂Chl⁺and H₂Chl^-–ZnChl-Ar⁺species is proposed and a H₂Chl/ZnChl-Ar intermediate effectively bridge the charge transfer of P3HT.This proves that P3HT layer only functions as a hole transporter rather than photoactive layer in such a Chl derivatives-based BSCs.Besides,further device design concepts are envisioned to improve current photovoltaic performance.The above exploration about chlorophyll derivatives-based photovoltaics gradually deepen our understanding about their further efficiency improvement and mechanism investigation.It also expands the application scope of artificial photosynthesis and provides a preliminary application of semi-synthetic Chl derivatives towards the solar energy application.