The Effect Of The Third Component On The Performance Of Active Layer And Organic Solar Cells

How to improve the photoelectric conversion efficiency of organic solar cells is an important problem to be solved in the industrialization of organic solar cells.The bulk heterojunction structure is currently the most effective organic solar cell structure.In this structure,the active layer is the core part,and the light absorption of the organic active layer material is mainly concentrated in the visible light range.The absorption rate of ultraviolet and infrared sunlight is low,and it is basically unused,and this part of the light accounts for more than 50%of the total solar spectrum.In addition,the carrier mobility of the organic active layer is very low,and these two factors limit the further improvement of the performance of organic solar cells.In this paper,different materials were added into the active layer to expand the absorption of sunlight and improve the charge transfer.The effects of different physical mechanisms in the active layer were studied,and the effects of different additives on light absorption and carrier transfer characteristics were discussed.First,In order to increase the utilization of ultraviolet light and improve the electron transport,three Cd Se quantum dots（QDs）with high fluorescence quantum efficiency（PLQY）were doped into the organic active layer of PTB7-Th:PC₇₁BM to prepare solar cells.The fluorescence emission peaks of these three Cd Se QDs were located at 623 nm,525 nm and 462 nm,respectively.The results of UV-visible absorption spectra show that the absorption of the active layer film increases in the short wavelength（350-400 nm）after doping Cd Se QDs and the device doped with Cd Se QDs at 623 nm has the strongest absorption.The photovoltaic properties of the three devices were measured under 350 nm monochromatic light.It was found that the device doped with 623 nm Cd Se QDs had the best performance,indicating that this device had the best utilization efficiency of ultraviolet light.In addition,the results of external quantum efficiency（EQE）spectra indicated that Cd Se QDs doping promoted the charge transport and improved the performance of the solar cells.We further analyzed the effect of Cd Se QDs doping on the carrier transport properties of active layer and device.The test results of AFM and SKPM show that Cd Se QDs do not have much effect on the morphology of the active layer film but the surface potential（SP）decreases from-420 m V to-540 m V,-521 m V and-500m V,respectively.The reduction of surface potential indicates that the work function of the active layer surface decreases.Besides,the calculated charge neutrality level(E_CNL)of the organic blend film is-4.58e V,which is higher than the work function of Cd Se QDs,higher SP means that the doped Cd Se QDs are enriched on the surface of the active layer,which can form a ladder barrier with the cathode and is beneficial to electron collection.Furthermore,the electron mobility of Cd Se QDs is higher than PTB7-Th and PC₇₁BM,which enhances the electron transfer from active layer to cathode.J_ph-V_eff curves and IMVS measurents showed that Cd Se QDs had no obvious promotion on the exciton dissociation in the device.According to the results of IMPS and IMVS measurements,Cd Se QDs can all improve the charge diffusion distance and enhance the charge collection efficiency in the devices.However,Cd Se QDs with the emission wavelength of 623 nm have relatively weak charge diffusion and collection.Therefore,the three kinds of QDs have similar effects on the device efficiency.In addition to short-wavelength sunlight,the utilization efficiency of near-infrared sunlight for organic solar cells is also relatively low.In order to increase the utilization efficiency of near-infrared sunlight,Pb S QDs with different contents were incorporated to the bulk heterojunction active layer PTB7:PC₇₁BM.According to the results of UV absorption spectra,Pb S QDs did not improve the overall optical absorption of the devices,especially in the infrared band,but performance of the devices was improved.Atomic force microscopy（AFM）and time-of-flight secondary ion mass spectrometry（To F-SIMS）were used to investigate the surface morphology change of the active layer and the distribution of Pb S QDs in the active layer.The results showed that since the molar mass of Pb S QDs was much lower than that of the organic material for the acceptor,an ultra-thin quantum dot inter-layer would be formed on the surface of the active layer and the thickness of the ultra-thin Pb S QDs layer was different according to the different mass fractions of the doped Pb S QDs.When the doping concentration is less than or higher than 5%weight ratio,the number of Pb S QDs is not sufficient to fully cover the surface of the active layer to form a complete inter-layer or some Pb S QDs overlap to suppress the electron transport as a result leading to a worse performance.When the doping concentration was 5%,Pb S QDs were paved on the surface of the active layer to form an ultra-thin layer with appropriate thickness,which modified the surface of the active layer,reduced the roughness,reduced the interfacial resistance,and promoted the transport and extraction of charges.The results of TEM measurements showed that the active layer doped with 5%Pb S QDs have uniform and compact in bulk morphology,indicating that the aggregation of PC₇₁BM decreases.Pb S QDs are uniformly dispersed throughout the active layer,which is conducive to the formation of a better interpenetrating network structure in the BHJ and promotes the charge transport.Finally,J_sc increased from 13.83m A cm^-2 to 14.81 m A cm^-2,FF increased from 68.70%to 70.85%,and PCE increased from 7.03%to 7.87%.The use of organic donor or acceptor materials with complementary absorption is also an effective means to expand the spectrum of the active layer and improve efficiency of organic solar cells,but the effect of the interaction of different components in the active layer on the carrier transport performance must be considered.We used the classic non-fullerene acceptor material ITIC-M as the third component with the complementary absorption spectrum,which can also form a cascade energy level,to add into the PM6:Y6active layer and prepared the solar cells.The absorption results prove that ITIC-M broadens the spectrum of PM6 and Y6 and increases the absorption of sunlight by organic solar cells.However,the performance of the solar cells decreases after the addition of ITIC-M.It indicates that there are other effects among the three components.We further analyze the physical mechanism of the device after the addition of the third component by combining film morphology and luminescence results.The results indicated that after the incorporation of ITIC-M,the internal recombination of the three components increases and the carrier mobility decreases.One of the reasons is that ITIC-M increases the phase domain area in the active layer,leading to the aggregation of the acceptor and the increase of the surface roughness of the film.The transmission mechanism and specific reasons for the decrease of efficiency in the ternary solar cells were investigated by electroluminescence（EL）and photoluminescence（PL）.According to the results,there are three charge transfer mechanisms in PM6:Y6:ITIC-M ternary solar cells:（1）Energy transfer:According to the absorption spectrum of pure PM6 and PL spectrum of pure ITIC-M thin films,it can be seen that the PL spectrum of PM6overlapped with the absorption spectrum of ITIC-M.Compared with the PL spectrum of PM6:Y6,PM6:Y6:ITIC-M thin films,it is found that the PL of PM6 is quenched after adding ITIC-M,which proves that there is energy transfer between PM6 and ITIC-M.（2）Alloy model:Comparing the EL spectra and PL spectra of PM6:Y6:ITIC-M,it was found that a new CT state electroluminescence peak（1.53 e V）appeared in the EL spectra of PM6:Y6:ITIC-M devices.According to the energy level analysis of the material,it is believed that this luminescence originates from the charge transfer states（CTs）between the alloy acceptor formed by Y6 and ITIC-M and donor PM6.（3）Parallel-connected transmission:the EL spectra of ternary solar cells doped with different contents of ITIC-M are fitted by Gaussian multi-peaks and the two peaks are located at 1.46 e V and 1.53 e V,which indicated that Y6 and ITIC-M can only form an alloy acceptor in a certain ratio.When the proportion of Y6 is high,Y6 will dissociate and transfer excitons with PM6 alone,but will not affect each other with the alloy acceptor,forming a parallel-like charge transfer mode.The reduced efficiency of the device is mainly attributed to the smaller IE caused by the smaller HOMO energy difference between ITIC-M and PM6,which affects the exciton dissociation on ITIC-M.At the same time,the small electron mobility of ITIC-M causes the charge mobility imbalance in the three-component device,which will lead to the accumulation of electrons in the device.The accumulated electrons form an additional electric field,which hinders the extraction of carriers and further Increase radiation recombination.This shows that in a ternary solar cells,in addition to complementary absorption spetrum and matched energy level,carriers dynamic mechanism also has a decisive influence on the efficiency of the organic solar cells.