Tandem small molecule organic photovoltaic cells with broad spectral response up to 1 μm and a high open-circuit voltage

We report a series-connected small molecule tandem photovoltaic cell utilizing two donors with complementary photovoltaic characteristics, lead phthalocyanine (PbPc) in the front subcell and boron subphthalocyanine chloride (SubPc) in the back subcell, to achieve both near infrared (NIR) response up to 1 μm and high open-circuit voltage (V_OC) of more than 1.5 V in the same device. We find that the C₆₀layer thickness in the front subcell has a critical impact on the overall optical structure and photovoltaic performance of the tandem device. By combining transfer matrix calculations with subcell-selective spectral measurements, we are able to tune the optical field distribution inside the active layers and increase the photocurrent outputs from both subcells, leading to EQE > 30% over the wavelength range 400 nm < λ < 900 nm. This optimized tandem cell exhibits J_SC= (5.5 ± 0.1) mA/cm², fill factor = 0.54, V_OC= 1.53 V, and a power conversion efficiency of (4.5 ± 0.2)%.