Multijunction organic photovoltaics with a broad spectral response.

We demonstrate series-integrated multijunction organic photovoltaics fabricated monolithically by vapor-deposition in a transposed subcell order with the near-infrared-absorbing subcell in front of the green-absorbing subcell. This transposed subcell order is enabled by the highly complementary absorption spectra of a near-infrared-absorbing visibly-transparent subcell and a visible-absorbing subcell and motivated by the non-spatially-uniform optical intensity in nanoscale photovoltaics. The subcell order and thicknesses are optimized via transfer-matrix formalism and short-circuit current simulations. An efficient charge recombination zone consisting of layers of BCP/Ag/MoOx leads to negligible voltage and series-resistance losses. Under 1-sun illumination the multijunction solar cells exhibit a power conversion efficiency of 5.5 ± 0.2% with an FF of 0.685 ± 0.002 and a V(OC) of 1.65 ± 0.02 V, corresponding to the sum of the V(OC) of the component subcells. These devices exhibit a broad spectral response (in the wavelength range of 350 nm to 850 nm) but are limited by subcell external quantum efficiencies between 20% and 30% over the photoactive spectrum.