Study On Preparation Of Organic Solar Cells Based On Narrow Bandgap Non-fullerene Acceptors

In recent years,scientists have gradually shifted their goals to new energy utilization equipment with the emergence of energy crises around the world.Organic solar cells(OSCs)have received widespread attention from the academic community since their advent.Compared with fullerene acceptors,non-fullerene acceptors have developed rapidly,mainly due to their simple molecular designs,simple synthetic methods,and easy controlling of the film morphologies,especially in the past two years.The syntheses of narrow bandgap star molecules and near-infrared non-fullerene absorbing materials,such as ITIC,ITIC-TH,ITCC,IT-M,COi8DFIC,IEICO-4F and Y6,lead to break the efficiency record again and again,which have aroused widespread concern in academia and industry.However,there is a lack of research on the photoelectric properties of these new materials or the preparation methods and working mechanism for new photoelectric devices.Therefore,it is necessary for us to do systematic research on the device structures and the optoelectronic properties of these new narrow bandgap materials.The innovation of this paper is to study the preparation and device physics of tandem organic solar cells,ternary organic solar cells,integrated organic solar cells based on high-efficiency narrow bandgap non-fullerene acceptors.The work function(WF)of the film was detected by Ultraviolet Photoelectron Spectroscopy(UPS)and Kelvin Probe Force Microscope(KPFM).The Atomic Force Microscope(AFM)and Scanning Electron Microscope(SEM)were used to analyze the change of film morphology.Electroluminescence(EL)spectrum and Photoluminescence(PL)spectrum was used to study the energy distribution and transfer in the ternary OSC.The Transient Absorption Spectra(TAS)of the thin film were used to characterize the carrier dynamics of the film.Besides,we quantitatively calculate carrier concentration of films,the width of the depletion region,and the energy loss of the radiative and non-radiative recombination parts in the open circuit voltage of the ternary organic solar cell.Finally,the fabrication methods and working principles of different devices based on non-fuller narrow bandgap acceptors were revealed.The following are the main contents of this paper.(1)PBDB-T and narrow bandgap non-fullerene acceptor ITIC were used to fabricate a symmetric tandem organic solar cell(OSC).However,it is still challenging to prepare a solution-processed interconnecting layer(ICL)with an appropriate work function and ideal conductivity in tandem OSC.In this work,a novel ICL based on modified TiOx and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS)is successfully utilized,where an n-type dipole material poly[(9,9-bis(3'-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)](PFN-Br)is doped into the TiOx layers via a subsequent hybrid doping(SHD)method.This allows linear tuning of electron density in the TiOx nanoparticle layer.Moreover,the difference in interfacial potential between TiOx and PEDOT:PSS can be further increased owing to dipole molecular orientation of PFN-Br.Consequently,a further enhancement in electron and hole recombination between TiOx and PEDOT:PSS was realized by reducing the WF of TiOx and thus boosting quantum tunnelling,making the TiOx nanoparticle a consummate n-type material for tandem application.Regarding device performance,the open-circuit voltage in all solution-processed symmetric tandem OSCs exhibits perfect superposition of each sub-cell.Notably,the fill factor is maintained at the same level as that of the single cell.Device stability is also improved.(2)High fluorescence quantum yield non-fullerene material BDT-IC was successfully introduced into a common PBDB-T:ITIC-TH system to prepare narrow bandgap ternary OSCs and to investigate the voltage loss of ternary OSCs.The electroluminescence(EL)spectrum of the device indicates that the EL spectrum of the blended device is red-shifted relative to the EL spectrum of pure phase,which indicates that there is a charge transfer(CT)state in the OSCs.It is interesting that the EL peak of the narrow bandgap binary cell of PBDB-T:BDT-IC is blue-shifted relative to the peak of PBDB-T:ITIC-TH,and the EL spectrum of the ternary cell is between the two binary cells,which indicates that there are different CT states in the ternary system with parallel cells,that is a higher CT state exists in narrow bandgap cell of PBDB-T:BDT-IC and a lower CT state stays in PBDB-T:ITIC-TH,simultaneously.The electroluminescent quantum yield(EQEEL)of the device indicates that there is a low non-radiative energy loss in the narrow bandgap solar cell of PBDB-T:BDT-IC,while the non-radiative energy loss in PBDB-T:ITIC-TH is higher.Therefore,in the ternary system in which the two types of cells coexist,the open circuit voltage(Voc)that can be changed with the doping ratio.At the same time,the charge mobility of the cells can also be adjusted by doping BDT-IC.Our work has further deepened the carrier dynamics in ternary organic solar energy and has certain guiding significance for improving the Voc of OSCs.(3)A perovskite cell in the ultraviolet-visible light absorption range and a near-infrared OSC are connected as a "integrated solar cell".That is,the electron transport layer of the narrow bandgap OSC is replaced with a wide bandgap perovskite cell with high mobility.This simple and easy method expands the absorption range of near-infrared non-fullerene organic solar cells,without affecting the charge transfer and material energy levels in the integrated cell.Perovskite solar cells can be used as the electron transport layer of organic solar cells.Appropriate proportions of OSCs in the integrated solar cell can optimize the morphology of hybrid OSC films.In addition,OSC can also be used as an excellent charge transport layer for perovskite cell,which can enhance electrical conductivity,balance charge transport,and suppress non-radiative recombination losses of the films.Thereby,the short-circuit current and fill factor of integrated solar cell are higher than the single cell,and the open circuit voltage of the integrated device is consistent with that of the perovskite cells.