Active Layer Materials Of Organic Solar Cells Based On Small Molecule Donor And Polymer Acceptor

Organic solar cell(OSC)is considered to be a promising photovoltaic technology for practical applications due to their great advantages of flexibility,light weight and solution processability.High-performance OSCs usually employ the blends of polymer donors and small molecule acceptors as the active layers.On the other hand,OSCs based on small molecular donor/polymer acceptor(MD/PA)blends have superior morphology thermal stability,but usually suffer from low power conversion efficiencies(PCE<6%).The origin is that MD/PA-type OSCs often exhibit large-size phase separation morphology of the active layers.The research topic of this thesis is to improve the photovoltaic efficiency of MD/PA-type OSCs.For addressing the issue of large-size phase separation morphology of the active layers,small molecular donors with large steric hindrance at the central core and polymer acceptors with adjustable aggregation tendencies in solution have been developed.As a result,the PCEs of OSC based on small molecular donors and polymer acceptors have been increased up to 9.5%,which is the highest value reported to date for MD/PA-type OSCs.Moreover,an OSC device with a very high operational temperature of 150?has been achieved in our works.The main research results are summarized as follows:1.Design and synthesis of small molecular donors with large steric hindrance at the central coreA series of acceptor-donor-acceptor(A-D-A)type small molecular donor materials have been designed and synthesized by modifying the substituents on the central D unit,the terminal group A unit and the alkyl chain substitution position on the ? bridging unit.The correlation of chemical structures of small molecular donors and opto-electronic properties,blend phase separation morphology,and device performance of MD/PA-type OSCs were investigated.a)The substituents on the central D unit.A series of small molecule donors,DR3TBDTT,DR3TBDBT,DR3TBDTC and DR3TBD2C were synthesized by introducing thienyl,benzothienyl,9-carbazolyl and 10-carbazolyl groups in the D units,respectively.These substituents in the D units can act as steric hindrance groups.The ?-? stacking capability and charge transport ability of small molecular donors are weakening with increasing the steric hindrance.When blending with polymer acceptor PBN-13,the crystallization of the small molecular donors with strong ?-?stacking capability dominates the blend film morphology,while the aggregation of polymer acceptor dominates the blend film morphology and crystallization of the small molecular donors with weak ?-? stacking capability is restricted,which contribute to the decrease of phase separation size from DR3TBDTT:PBN-13 blend to DR3TBD2C:PBN-13 blend.Owing to the suitable phase separation size and effective charge transport,the OSC device based on DR3TBDTC with 9-carbazolyl as the substituent exhibits high PCE of 8.05%.b)The terminal group(A unit).Based on DR3TBDTC with the central core bearing carbazolyl substituents,three small molecular donors,DC3TBDTC,DR3TBDTC and DI3TBDTC,were synthesized by using different terminal groups of methyl 2-cyanoacetate,3-ethylrhodanine and 2H-indene-1,3-dione.With increasing the electron-withdrawing capability of the terminal groups,the small molecular donors exhibit downshifted LUMO levels,decreased bandgaps and redshifted absorption spectra.Among the small molecular donors,the one with 3-ethylrhodanine terminal group exhibits suitable energy levels and molecular stacking,leading to the best device performance.c)The alkyl chain substitution position on the ? bridging unit.On the basis of the molecular backbone of DR3TBDTC,the small molecular donor,BD3T,is designed and synthesized by adjusting the alkyl chain substitution position on the ? bridging unit.The replacement leads to improved backbone planarity,enhanced the molecular crystallinity and charge transport of BD3T.When blending with polymer acceptor PBN-13,the corresponding PCE of the MD/PA-type OSCs is increased to 8.62%.2.The correlation between aggregation behavior of polymer acceptor in solution and phase separation morphology of the MD/PA blendDuring the film-forming process of MD/PA blend,the aggregation tendency of polymer acceptor in solution affects the phase separation process,and thus the phase separation morphology of the active layer.By introducing fluorine atoms into conjugated backbone,the aggregation tendency of the polymer acceptor in solution is adjusted and the phase separation morphology of blend film is controlled;by replacing thiophene unit of the polymer acceptor with selenophene unit,non-halogenated solvent-processed MD/PA-type OSC is realized.a)In order to adjust the aggregation tendency of the polymer acceptors in solution,we designed and synthesized the polymer acceptor PBN-15 by introducing two fluorine atoms into the backbone of the polymer acceptor PBN-14.During the film-forming process,the crystallinity of small molecular donor and polymer acceptor compete with each other.Due to the weak aggregation of PBN-14 in solution,BD3T crystallizes excessively during film-forming process,leading to large-size phase separation in the active layer.On the contrary,aggregation of PBN-15 is dominant and crystallization of BD3T is restricted when PBN-15 is blended with BD3T,which leads to small-size phase separation in the active layer.As a result,the MD/PA-type OSC based on the small molecular donor BD3T and the polymer acceptor PBN-15 exhibits a high photovoltaic performance with the PCE of 9.51%.Moreover,the BD3T:PBN-15 based device exhibits excellent thermal stability,i.e.maintaining 84%of its initial efficiency after thermal treating the device at 150? for 72 hours.b)Due to the strong aggregation property of polymer acceptor in solution,the active layer of BD3T:PBN-15 needs to be processed with halogenated solvent chlorobenzene,leading to the difficulty of large-scale solution processing.For the controllability and environmental friendliness of the device manufacturing process,it is necessary to prepare the active layer at low temperature.In this regard,we designed and synthesized the polymer acceptor PBN-15-Se by replacing thiophene unit of polymer acceptor PBN-15 with selenophene unit.Due to the large van der Waals radius of the selenium atom,PBN-15-Se exhibits weak ?-? stacking.The solubility of PBN-15-Se is improved and it can be dissolved in low boiling point solvents.When blending DR3TBDTC with PBN-15-Se,we use the non-halogenated solvent Tetrahydrofuran(THF)as the processing solvent to construct the active layer,and a photovoltaic performance with the PCE of 7.02%is achieved.This work demonstrates the realization of green solvent THF processed MD/PA-type OSCs.3.The crystallinity regulation of boron-nitrogen coordination bond(B?N)polymer acceptor materialsIn order to investigate the relationship of chemical structures of conjugated polymers and crystallinity,we develop a series of conjugated polymers with various backbone flexibility,P-nT(n=1-5),based on alternating double B?N bridged bipyridine(BNBP)unit and oligothiophene units with various length.As the length of oligothiophene unit increases,the polymer backbone becomes more flexible.The flexible polymer backbones give rise to improved crystallinity in solid state,and the crystallinity affects the phase separation of polymer donor/polymer acceptor blends and consequently affects the OSC device performance.