Investigation Of Impact Of Additive On The Performance Of High Efficiency Organic Solar Cells

Due to their unique advantages,photovoltaic technologies are attracting great attention as a potential solution to overcome the issues of depleting energy sources,e.g.,fossil and coal,and global warming.Among the different types of photovoltaic technologies,organic solar cells?OSCs?,whose bulk heterojunctions?BHJ?are naturally formed,have attracted much attention for their unique advantages of light weight,low cost,less energy consumption,wide material source,easy to be processed by solution and easy to be prepared into flexible large area.The power conversion efficiency?PCE?and stability of OSCs are still far from that of inorganic solar cells,which hinders the application of the large scale and low cost commercialization.Thus,it becomes a hot research topic to further improve the efficiency and stability of OSCs.It has been reported that,effectively controlling morphology is crucial for further improving the performance of OSCs,hence reducing gap in PCE between silicon-based solar cells and OSCs.Therefore,several methods have been employed for tuning the nanoscale morphology of the donor-acceptor active layer of OSCs,including 1)controlling the polymer film fabrication conditions,2)adjusting the molar ratio between the donors and the acceptors,3)adding solvent additives and 4)optimizing the annealing conditions.Among these methods,adding proper solvent additives have been intensively investigated and found to be simple but powerful ways to alter the film morphology to a more favorable state,thus significantly improving the performance of BHJ based OSCs.For adding solvent additives,there is a problem of additive residue after large scale and low cost fabrication of OSCs for industrialization,which can accelerate the photodegradation of photoelectronic materials and devices under light illumination.In order to improve the conversion efficiency and prolong the service life of OSCs,further research for solvent additive is very necessary.In this paper,we mainly study on?1?the impact of alkyl chain length of diiodoalkanes on PC₇₁ BM distribution in both bulk and surface of PTB7:PC₇₁BM film,?2?impact of additive residue on the photodegradation of high performance polymer solar cells.This paper mainly discusses the above two parts:?1?We systematically investigate the impact of alkyl chain length of diiodoalkane?1,4-diiodobutane?DIB?,1,6-diiodohexane?DIH?,1,8-diiodooctane?DIO?and 1,10-diiododecane?DID??on the performance of PTB7:PC₇₁BM based polymer solar cells.Among these additives,DIO is found to have the optimum alkyl chain length that maximizes the performance of PTB7:PC₇₁BM based polymer solar cells,attaining a PCE as high as 8.84%,which is almost four times higher than that without any additives.For DID additives?longer alkyl chain length than DIO?,a drop in efficiency to 7.91% was observed.Experimental investigations show that the microstructure of the bulk and the surface layer,components on the surface as well as the surface morphology of the PTB7:PC₇₁BM polymer film can be controlled simultaneously by varying the alkyl chain length of additives.Results show that the substantial improvement in performance is mainly attributed to the improved 1)PC₇₁BM distribution uniformity in the bulk of the PTB7:PC₇₁BM film which affects photo-generated exciton dissociation,2)phase segregation which affects electron transport.Finally,we explain that PC₇₁ BM molecules are electron-deficient,resulting in strong inter-molecular interactions between the PC₇₁ BM molecules,and hence,molecule aggregation,while iodine atoms are electron abundant.By adding the diiodoalkane,the electron-deficient PC₇₁ BM and the electronabundant iodine atom possibly attract each other,meaning that DIO molecule can hold PC₇₁ BM molecules on both ends of the chain and separate.?2?Due to the chemical and physical similarity of DID and DIO,we find that morphology,optical properties and electrical properties are similar for the film processing with DIO or DID under the same conditions.X-ray photoelectron spectroscope?XPS?measurements detect iodine in the active layer film spincoated from solution with DID additive,indicating the presence of residual DID after processing at high vacuum.However,the device with DIO does not exist in the final device,so the device with DIO can be used as reference to elucidate the impact of residual additive on the device degradation when it is exposed under light illumination since other degradation sources except the residual additive are the same.Compared with DIO,we find that device performance with the additive residue begins to accelerate the decline after 2 hours irradiation,declines the fastest after 6 hours,and begins to slow after 12 hours.In order to study the above phenomenon,we measure the electrochemical impedance spectroscopy?EIS?,finding that the carrier recombination rate is the most serious after six hours illumination.In order to study the nature of the carrier recombination,we analyze the structure of the material under different irradiation times.Structural changes in the films upon illumination are probed from Fourier Transform Infrared Spectrometer?FTIR?.However,we find that the absorption of the film with additive residue is unchanged under light illumination,which indicates that the conjugation effect is not affected by the residue.Due to DID residue accelerating the generation of PTB7 free radical after 2 hour illumination,forming a large number of-C-O-OH,causing a new peak position in 3200 cm^-1 after 6 hours.Since-CO-OH is the unstable intermediate product under the light and can continue to react to generate the final stable product without-OH,so that the-OH peak disappeared after 12 hours illumination.