| The power conversion efficiency (PCE) of organic solar cells (OSCs) is mainly affected by the interfacial modification and the morphology of active layer. In this paper, a series of OSCs based on PTB7-Th:PC71BM were prepared to investigated the effects of interfacial modification and morphology of active layer on performance of OSCs.The effect of 1,8-diiodooctane (DIO) on the charge generation, transportation, recombination and their correlate parameters with the morphology of BHJ films were investigated. The addition of DIO to the casting solvent as a processing additive, results in improved crystallization of the film, smaller domains and a more finely interpenetrating BHJ morphology, relative to blend films cast without DIO, thus increase of exciton dissociation efficiency. The results of time-of-flight secondary ion mass spectrometer (TOF-SIMS) indicate that PTB7-Th migrates or diffuses toward the top surface of the BHJ films after the addition of DIO, which does not benefit to charge transport. In summary, experimental study indicates that the effect of DIO on charge generation and charge transport are conflicted. The introduction of DIO could decrease domain size and increase exciton dissociation efficiency with sacrifice of bi-continuous percolating network.Solvent-fluxing treatment was introduced to solve the conflicted effect of DIO on charge generation and charge transport. Experimental study proves that the cooperation of solvent additives and solvent flux treatment is a successful method to form graded three-dimensional BHJ structure with donor (p-type) rich layer near the high work function anode and an acceptor (n-type) rich layer near the low work function cathode. Furthermore, we improve the morphology of gradient three-dimensional BHJ by the following ways:(1) to tune the spin speed of methanol. The volume of methanol should match the thickness of the active layer so that DIO near the PEDOT:PSS side can also be extracted out. However, if DIO is too little, the DIO near the PEDOT:PSS side can’t be extracted out; if DIO is too much, the extraction extent will be enhanced; (2) to adjust time intervals between deposition of blend films and methanol fluxing. It is expected that performing the solvent-fluxing process too early interrupts the polymer crystallization, while performing the solvent-fluxing process too late is not effective in extracting fullerene-derivatives because the framework of the polymer will be too rigid; (3) to optimize the volume fraction of DIO. DIO helps the phase separation of BHJ. In this paper the superiority of gradient three-dimensional BHJ structure to regular BHJ structure was also investigated. Study shows that gradient three-dimensional BHJ structure is good to nanophase separation for charge generation and continuous pathways for charge transport, simultaneously.To obtain higher device performance, the ideal hole extraction layer (HEL) should possess both appropriate energy levels to improve charge selectivity for corresponding electrodes and sufficient conductivity to reduce resistive losses and have high transmittance in the Vis-NIR wavelengths to minimize optical losses. Herein, solvent-fluxing (DMSO-fluxing) method was introduced to enhance the conductivity and transmittance of PEDOT:PSS, synchronously. Experimental studies confirm the improvement of photoelectric properties of PEDOT:PSS:(1) the conductivity increase from 9.64×104 to 4.64×10-2 s/cm; (2) the transmittance of solvent-fluxing treated PEDOT:PSS was improved significantly which induced the increase of absorption of active layer. With solvent-fluxing treated PEDOT:PSS, the PCE of device was significantly increased from 7.35% to 9.48% due to the increased short circuit current density (Jsc) from 13.41 to 21.86 mA/cm2.Furthermore, we find that DMSO-fluxing treatment could decrease the work function of PEDOT:PSS, then it will increase hole extraction barrier. We present a smart two-step treated method to simultaneously improve the work function, conductivity and transmittance of PEDOT:PSS. The two-step treated method is shown as follows:firstly, PEDOT:PSS:PFI with different volume ratios were spin-coated on the ITO substrates to enhance the work function of PEDOT:PSS; secondly, the PEDOT:PSS:PFI was fluxed by DMSO to improve the conductivity and transmittance of PEDOT:PSS. With the two-step treated PEDOT:PSS, the PCE of OSCs was increased from 7.35% to 10.10%. Furthermore, this two-step treated method may provide a new sight to improve the performance of OSCs by adjusting the photoelectrical properties of hole extraction layer. |
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