The influence of donor material on achieving high photovoltaic response for organic bulk heterojunction cells with small ratio donor component

Authors demonstrated impact of series small ratio donors in C60 matrix on photovoltaic (PV) performance. A series of donor materials such as N′,N′-Di-1-naphthyl-N′,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB), 4,-4′-Bis(carbazol-9-yl) (CBP), 4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amine)triphenyl-amine (m-MTDATA), copper phthalocyanine (CuPc) and 4,4,4-tris(n-carbazolyl-triphenyl-amine) (TCTA) were blended with fullerene (C60) by different ratio. It was found that although donor–acceptor (DA) interface in planar heterojunction (PHJ) structure increased charge separation probability at the near interface section, the PV response was stronger for bulk heterojunction (BHJ) with low-ratio donor doping into C60 matrix in which exciton dissociation can take place immediately after photon absorption without a diffusion progress. The power conversion efficiency (PCE) of BHJ-PV cell based on NPB donor reaches 2.25%, which is double of that of the PHJ cell. In terms of our series results we obtained that ΔEHOMO (HOMOC60–HOMOdonor) between C60 acceptor and donors would provide a maximal influence on achievement of a maximal PCE and an optimal ΔEHOMO locates around 0.8 eV, which implies that dissociation of photo-exciton at C60 matrix needs feasible driving force. More detail mechanism was also argued.

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► A series of donor materials were doped into C60 with small ratio to fabricate bulk heterojunction OPV cells.
► The bulk heterojunction cells show stronger PV response compared with planar heterojunction ones.
► Influence of the donor material on the PV performance was investigated.
► HOMO energy level offset between C60 and the donor plays an important role in determining the PV response.
► The optimized HOMO energy level offset is found to be at around 0.8 eV.