Effect of a symmetry breaking layer on the open circuit voltage of conventional bulk-heterojunction solar cells

Solution processable titanium suboxide (TiO_x) was introduced as an artificial symmetry breaking layer in bulk-heterojunction (BHJ) solar cells comprising a low band gap conjugated polymer, poly[(4,4′-bis(2-ethylhexyl) dithiene[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1, 3-benzothiadiazole)-5,5′-diyl] (Si-PCPDTBT), and a soluble fullerene, [6,6]-phenyl-C₇₁-butyric methyl ester (PC₇₁BM). The inserted TiO_x layer obviously extracted the same level of open circuit voltage (V_oc) regardless of metal work function. Ultraviolet photoelectron spectroscopy (UPS) results indicated that the formation of the interface dipole between the TiO_x symmetry breaking layer and metal electrode successfully modifies the effective work function of the cathode electrode, thereby leading to symmetry breaking in BHJ solar cells.