Fabrication of High-performance Solar Cells Based on Imide-functionalized Benzotriazole Derivatives and the Study of Charge Transfer Mechanism

Author:Zheng Zuo Zuo

Supervisor:cao zuo


Degree Year:2019





Non-fullerene organic solar cells have attracted more and more attention because of the adjustable absorption and energy level.The absorption of non-fullerene acceptors can be designed to be overlapping with the solar irradiation spectrum in both visible and near-infrared regions which guarantee the light harvesting capability of the active layer.The adjustability of energy levels also increases the selectivity of match of donor and acceptor.The efficiency of single junction non-fullerene organic solar cells has exceeded16%.We can use two kinds of methods to enhance the performance of organic solar cells,on one hand improving the orientation and alignment of molecules in the device by suitable methods can increase the generation of excitons,transfer and extraction of charge.On the other hand,changing the structure of the device by adding a third component to form the ternary solar cells or using two subcells to make tandem device to get the efficient and stable non-fullerene organic solar cells.In addition,The contribution of non-fullerene acceptor to exciton generation can not be negligible like fullerene due to the absorption.The process of hole transfer need to be studied.Recently,more and more systems with little energy loss have emerged in non-fullerenes organic solar cells.The driving force of charge separation is no longer applicable to non-fullerenes solar cells because of the limitation of energy level between donor and acceptor.The theory also needs to be updated.This paper is devoted to optimize the organic solar cells based on imide-functionalized benzotriazole derivatives.And we investigated the transient absorption spectra of two non-fullerene systems which have different energy loss.We also optimized the fabrication process of the device with one-step method to obtain high-performance non-fullerene organic solar cells.In the first chapter of this dissertation,we fabricated both fullerene and non-fullerene organic solar cells based on a same donor PTzBI-2FP and various acceptors of PC71BM,ITIC,ITIC-2F,and ITIC-4F.Due to the limitation of absorption for fullerene acceptors,the PTzBI-2FP:PC71BM-device gives a modest efficiency of 2.42%,while the non-fullerene solar cell based on PTzBI-2FP:ITIC-4F exhibits a decent PCE of 9.8%.Further optimization of the non-fullerene organic solar cells by solvent annealing and thermal annealing leads to an optimal device efficiency exceeding 12%for the PTzBI-2FP:ITIC-4F system.Space-charge limited current(SCLC)method was used to estimate the charge mobility of the blend film before and after annealing.One can note that the combination of solvent annealing and thermal annealing can increase the electron mobility of the device,thus enabling more balanced transportation of holes and electrons.Morphology analysis was performed by AFM and TEM,where we observe that PTzBI-2FP:ITIC-4F bulk film has a smoother surface and a more suitable phase-separation size,contributing to improved charge separation.Importantly,the bimolecular recombination is weak in PTzBI-2FP:ITIC-4F-based devicesIn the second part,we used transient spectroscopy(TA)to explore the kinetic processes,i.e.,the generation and extinction of excited states,in both PTzBI-2FP:ITIC and PTzBI-2FP:ITIC-4F systems.The donors and acceptors were excited at different wavelength to distinguish the process of hole-and electron-transfer in the BHJ.The PTzBI-2FP neat film and blend film were excited at 480 nm.Two electron-transfer processes were observed from the TA spectrum,and the corresponding time scales were100 fs and 1 ps,respectively.ITIC and ITIC-4F were excited by 700 nm pump,and the components of the TA spectrum were analyzed.A significant hole transfer process was observed,and the hole transfer in the PTzBI-2FP:ITIC-4F system was quicker.We also observed the charge-separation(CS)states and the polarons in the TA results.In the PTzBI-2FP:ITIC-4F system,the CS state is generated in a shorter time,and the polaron has a longer lifetime,suggesting that the recombination is lower and the charge transfer and extraction are quicker in this system although the offset of HOMO level in two non-fullerene systems is less than the theoretical minimum driving force of charge separation of 0.3eV,the hole transfer between the two non-fullerene systems is still efficient.In the third section,5%N2200 was incorporated as the third component to increase the device efficiency from 11.72%to 13%for the PTzBI-2FP:ITIC-4F system,which is mainly due to the increase of short-circuit current(JSC).Considering the overlapping of absorption for N2200 and ITIC-4F in the visible region,the contribution of JSCC from N2200 is negligible.The orientation of the molecules in the blend film were measured by GIWAXS.It can be seen that the introduction of a tiny amount of N2200 did not change the original face-on orientation of both PTzBI-2FP and ITIC-4F.AFM and TEM results show that N2200 can form fibrous crystals as electron transport channels outside the PTzBI-2FP and ITIC-4F microcrystal regions.This can be verified form the SCLC results,where one note the electron mobility of the ternary solar cell is elevated by the introduction of N2200.This enables more balanced transport of electrons and holes in the ternary system.Meantime,the fluorescence spectrum indicates that the charge transfer between N2200 and ITIC-4F is efficient.These combined results contribute to the increased JSC of the ternary device.Moreover,the addition ofN2200 obviously improved the thermal stability of the resulting ternary device.In the last chapter,we used a small-molecule(POF-N)-doping method to fabricate highly efficient inverted organic solar cells.The results of etching XPS and TEM-EDX showed that POF-N self-assembled onto the ITO surface during spin-coating to spontaneously form a electron-transporting layer.The low solubility of POF-N in CB and CF and its high surface energy can drive the PON-F to move downward to the ITO substrate.Inverted organic solar cells based on fullerene,non-fullerene,and all polymers were fabricated by this method.An improved device efficiency of 10.0%,11.7%,and 7.0%was obtained for PTB7-Th:PC71BM,PTzBI-2FP:ITIC-4F,and PTzBI:N2200 system,respectively.This strategy simplifies the fabrication process of the device and provides new routes for future large-scale production.