| The performance of polymer solar cells (PSCs) is really correlated with the polymer donor materials and the fabricating method of PSCs devices.Broadening the absorption spectra of polymers can improve the usage of sunlight. Optimizing the device fabricating method and accurately controling the morphology of active layer are two of the most effective ways to improve devices power conversion efficiency (PCE). Based on a benzodithiophen-quinoxaline copolymer photovoltaic materials which our team reported, we further study the way to broaden the absorption spectra of copolymer, and improve the device fabricating method to improve the PCE.(1)In order to broadening the absorption spctra in long wavelength direction and improving the usage of sunlight, the alkyl side chains on the previous polymers PBDT-DFQX-TT were replaced by branched and linear alkythio groups, respectively. Two new polymers were synthesized, named PBDT-DFQX-TTSEH with branched alkylthio group side chains and named PBDT-DFQX-TTSC8 with linear alkylthio groups. Compared two new polymers with their alkyl side chain analogue, alkylthio groups substituded polymers performed a lower HOMO levels and a slightly red-shift absortion peaks and onset. The PSCs device of PBDT-DFQX-TTSEH showed a higher Voc than of PBDT-DFQX-TTSC8 because of lower HOMO levels, while PBDT-DFQX-TTSC8 performed a higher hole mobilities and Jsc. In optimal situation,PBDT-DFQX-TTSEH got PCE=7.84% and PBDT-DFQX-TTSC8 was PCE=8.51%. To further investigating the effectives of diffrent treatment in building device, atomic force microscope (AFM) and grazing incidence X-ray diffraction were used. The result is that using additives and thermal annealing treatment would prompt the polymer and PC71BM self-assembling to form undesirable large size domain and be hamful of the PSCs performance.(2)Introduing and changing the ratio of third comonomer with strong absorption in long wavelength is a broaden polymer absorption spectra strategy.For improving the benzoditiophene-quinoxaline (BDT-Qx) serie polymers long wavelength absorption, we use diketopyrrolopyrrole (DPP) unit, possessing strong electronic absorbability, as third comonomer to design and synthesize two series of BDT-Qx-DPP like D-A1-D-A2 ternary copolymers. The ICT peak and near-infrared onset showed red-shift follow by increasing the DPP rate of charge. The best aborsption spectra of copolymers showed at the ratio of charge is Qx: DPP=30:70. But the device performances are drop dramatically by adding DPP as third comonomers. The electron cloud of HOMO and LUMO were concentrated on DPP unit and broke the continuity of cloud. The broken HOMO and LUMO would hinder the transfer of charge carrier and reduce the Jsc, FF and PCE of device.(3)We use two new non-fullerene small-molecule acceptor (NFSMA)materials with strong long wavelength absorption, named ITIC and O-IDTBR,to replace PC71BM in building PBDT-DFQX-TTSC8 PSCs device. The NFSMA blending films perfomed a broaden absorption in long wavelength direction than the PC71BM one. And we use SBS as morphology adjust additive to build PSCs device while the control group is using DIO as additive. All the Jsc of devices with additives showed a reduction than the relevant thermal annealing treatment only devices because the additive residual in active layer formed charge carrier traps. While the devices with SBS as additive show a increased FF. The PBDT-DFQX-TTSC8/O-IDTBR device with 1%SBS (v/v)additive performed a higher FF than thermal annealing only device and the PCE improved to 8.61%. But the Jsc of PBDT-DFQX-TTSC8/ITIC device with 1%SBS (v/v) performed a large and reduced the PCE. Using AFM and 2D-GIXS to investigate the films mopholaghy in different treatment showed one the reasons of the Jsc decreased was that the charge transfered abilities lower by molecular lamellar structure of O-IDTBR accepter changing from anisotropic to isotropic after treating with SBS. |
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