Researches On The Carrier Dynamics Process And Interfacial Engineering In Organic Solar Cells

Organic solar cells?OSCs?have drawn much attention in research field due to its advantages of light weight,low cost,flexibility and solution processability.In recent years,with the development of new materials and device engineering,the power conversion efficiency?PCE?of OSCs has exceeded 18%.However,the mechanism behind it is still unclear.Especially with the fast development of non-fullerene acceptors,the understand on the process of photo-induced hole transfer is still lacking,and the relationship between structure and performance needs to be further explored.In addition,interface engineering has long been considered as an integral part to boost the efficiency of OSCs.The studies on interfacial materials for application in large-band gap non-fullerene acceptors and the stability of interface materials in the OSCs are thus quite urgent and necessary.This thesis focuses on the active layer and the interface layer,and discusses the relationship between the structure and performance of the active layer materials from the perspective of charge generation and recombination.In addition,the adaptability and stability of the interface materials in OSCs are also discussed.The thesis is committed to providing theoretical and practical guidance for the development of OSCs in the future.In Chapter 3,the NT-based polymers were introduced to elucidate the influence of alkyl chain length on charge transfer process and morphology that determine the final device performance.Our results showed that the short-chain modified NT812 donor has better crystalline nature and charge transport ability.The transient absorption spectroscopy showed that more photogenerated excitons were observed in NT812 than that of NT1014 under the same lasing flux,which has a positive effect on photocurrent.When blended with PC₇₁BM,the NT812 blend films exhibited suppressed geminate recombination,faster charge separation,and slow recombination of polarons,resulting in superior device performance with PCE over 10%.However,due to the disorder structure of the long-chain in NT1014,the PC₇₁BM tended to form large aggregates in the blend films,which accelerated the geminate recombination,and inhibited the charge extraction and thus the poor device performance.In Chapter 4,The typical non-fullerene systems based on PBDB-T:ITIC and their fluorinated derivatives were introduced to analyze the effects of fluorine atoms on the device performance by studying their ultrafast spectra,morphology and device physics.The ultrafast spectra of photo-excited donor or acceptor showed that the fluorine on acceptors can accelerate hole transfer process and inhibit the recombination of separated charge.Morphological and other device physical tests revealed that the fluorine-substituted donor and acceptor have good miscibility,which helped to optimize the phase separation,and balance the carrier transport,and finally boost the device performance.Besides,the study found that the hole transfer has no direct relationship with?HOMO between donor and acceptor.In Chapter 5,we carefully investigated the influence of cathode interlayers?CILs?with different energy levels on the performance of non-fullerene OSCs.It was found that with the LUMO of the non-fullerene acceptor keeps shifting up,choosing a CIL with a high-lying LUMO is crucial for achieving high V_oc and efficient charge collection.In addition,we found that the electron extraction and transport abilities of a CIL can be tuned by blending two CIL materials together with different ratios,which offered us the opportunity to design a binary CIL with excellent electron extraction ability and high electron transport ability.Based on the findings,homo-junction tandem OSCs with a high V_oc of up to 2.39 V was obtained by introducing a binary interlayer into the interconnecting layer,which is the highest V_oc among reported double junction tandem solar cells,we further demonstrated its practical application in solar-energy-driven water splitting.This study guides the design of CIL materials for large bandgap non-fullerene acceptors.Besides,the binary interlayer offers a new strategy to extend the application of neat CILs and helps to develop efficient and thick CILs for printed large area non-fullerene OSCs.In Chapter 6,we focused on the stability of water/alcohol soluble polymer interface in OSCs.It was found that the devices modified with polymer interfaces exhibit superior stability than that of Zn O particles.Furthermore,we carefully investigated the photo-stability among the three devices modified with polymer interlayer,the results revealed that the absorption and the electron mobility of PIF-PTE-N declined after photo-aging,the device showed the worst stability among them.While the absorption of PIF-NDI-N and PIF-PDI-N did not change significantly,besides,the mobility showed increasing trend,and the devices exhibited better stability.We speculated that the possible reason of it is related to the LUMO level of the interlayer.When the interlayer is illuminated,the photon induced self-doping process may accompanied by the molecular structure destruction in the polymer interface with high LUMO level,which will hinder the charge extraction and ultimately reduce device stability.In addition,the thermal stability test suggested that it may improve the device stability when the surface energy of the active layer is closer to the polymer interlayer.In Chapter 7,we demonstrated a simple strategy in improving both the efficiency and stability of inverted PSCs by introducing the C-dots as cathode modifier layer on Zn O.The study found that the insertion of C-dots can further smooth the surface of Zn O and decrease the work function,which helped to enhance the charge extraction between active layer and interface layer.In addition,the devices modified by C-dots were better than that of pure Zn O devices in photo-stability and air storage stability.Moreover,C-dots modification showed a very good universality in improving the performance and stability of other OSCs,which providing a feasible solution for the development of efficient and stable devices.