Fig. 4

Charge-transfer state energies at bulk and planar heterojunctions: a Simplified sketch of the interface between donor and acceptor phase in a bulk heterojunction (BHJ). The black arrows sketch a representative dissociation process of a charge-transfer (CT) exciton. b Experimental change of E_CT as a function of the molar content of F₄ZnPc in the intermixed donor phase (ZnPc:F₄ZnPc), forming a ternary BHJ (purple squares) or a planar heterojunction (green circles) with the acceptor C₆₀. E_CT values for the solar cells are obtained from Gaussian fits to sensitively measured external quantum efficiency (see Supplementary Fig. 11) and electroluminescence spectra⁴⁴. The dashed black line is the shift of E_CT, as calculated from the change of molecular parameters (see Eq. 4 in the Methods section). The black arrow highlights the decrease of E_CT at large ZnPc contents, which we attribute to charge−quadrupole interactions. c Change of V_oc, measured at 1 sun illumination intensity, as a function of F₄ZnPc content for both solar cell architectures. Note that the voltage losses (ΔV_oc = E_CT/e − V_oc) are slightly lower for planar heterojunctions due to the reduced interfacial area⁵⁸