Interlayer Crystallization And Active Layer Aggregation In Organic Solar Cells: Structure Regulation And Device Performance

Fossil energy has limited reserves,which also causes serious environmental pollution.It is urgent and imperative to develop renewable clean energy.Solar energy is abundant and distributed around the world,which is considered to replace fossil fuels in the near future.As the third-generation photovoltaic technology,organic solar cells（OSCs）owns the advantages of light-weight,flexibility and translucency,which complements the commercial silicon solar cells well.However,there is still a long way to realize practical applications by improving the power conversion efficiency and stability further on the premise of controlling cost.n-Type metal oxide is a low-cost cathode interface.Constrained by solution processing methods,the electron mobility needs improvement to adapt the demand of high-performance non-fullerene OSCs.By optimizing the preparation process and taking interface modification with organic molecule,the film crystallization and morphology are able to be controlled,which contributes to highly conductive interface.It is an effective way to improve the competitiveness of OSCs considering both efficiency and cost.Similarly,tuning the aggregation morphology of active layer to balance the efficient exciton separation and charge transport is one of the important factors for achieving excellent photoelectric conversion.To obtain the best phase separation scale,donors and acceptors with strong intermolecular interaction are considered as the key point,which generate continuous single-carrier transporting channels in blend film.Temperature-dependent polymer is a special one in those materials,whose pre-aggregation structure in solution can be controlled by adjusting the temperature.However,there are few processing methods based on this feature for the aggregation regulation now.Besides,the relationship between temperature-dependent performance and molecular structure is still unclear.Deep research is needed to provide ideas for designing efficient OSCs.In Chapter 2,the crystallization and aggregation morphology of titanium dioxide（Ti O₂）films were regulated by different preparation processes.Among them,as-synthesized Ti O₂,prepared by anhydrous sol-gel method,had the shortest light-soaking time,high crystallinity,smooth surface and low defect density,which was best choice for cathode interface in OSCs.It was difficult to form an interaction between perylene bisimide molecule and Ti O₂,which should be responsible for the failed interface modification.There was not photo-induced charge transfer any more and thus increased conductivity can’t be observed.By comparing the interface modification on the top and bottom of metal oxide,it was found that the bottom modification can fully utilize the perylene bisimide layer.Because there was a weak interface dipole between the perylene bisimide molecule and the ITO electrode,which was beneficial to reduce work function and made electron extraction more efficient.In Chapter 3,in-situ induced crystallization of zinc oxide（ic-Zn O）with high crystallinity and low defect density was prepared by introducing as-synthesized zinc oxide（as-Zn O）into the precursor solution for sol-gel zinc oxide（sg-Zn O）as the nucleating agent.During the particle growth process,hydroxyl groups on the surface of Zn O were consumed.As a result,structural defects were reduced and the average crystal length was increased,which improved the electron mobility of Zn O film effectively.Using this high-quality Zn O film as the electron transport layer,the inverted OSCs based on PM6:Y6 blend system achieved an energy conversion efficiency of16.62%,which was much higher than those based on sg-Zn O and as-Zn O（15.72%and14.98%,respectively）.It was also proved in fullerene OSCs based on PTB7-Th:PC₇₀BM,which exhibited better result than sg-Zn O and as-Zn O,achieving a device efficiency of 9.72%.Transient photovoltage showed that the carrier lifetime was effectively improved and the charge recombination in the device was suppressed.This work provided an easy,economic and effective way for cathode modification.In Chapter 4,two D-n-A type polymers,Pff BT4T-NDI and Pff BT4T-PBI,were designed and synthesized,in which Pff BT4T served as the polymer backbone and donor unit.Naphthalimide and perylene bisimide were used as acceptor units on the sidechain.The single crystal of polymer model molecule showed that the backbone had a slip-packed J-aggregated structure.Fluorescence spectroscopy at room temperature revealed there was an efficient charge transfer between donor and acceptor units.After the heating-slow cooling treatment,the polymer chains were disentangled,which improved the molecular fluidity.Under the traction of intermolecular interaction,the backbone pre-aggregated orderly,driving the sidechain same way.The hole mobility and electron mobility of the film were simultaneously improved.Atomic force microscopy and transmission electron microscopy showed that a better phase separation morphology was formed.It was applied in single-component OSCs and the inverted photovoltaic device based on Pff BT4T-PBI performed a 40%improvement compared to the untreated film.These results showed that pre-aggregation structure in solution had a profound impact on the optoelectronic properties of the film.Molecular fluidity,as well as the relaxation time,were the key factors controlling the ordered pre-aggregation of temperature-dependent polymers.