Fig. 1.

Effects of strong coupling (SC) on the performance of SubNc/Cl₆-PhOSubPc organic solar cells. a Device structure of a normal SubNc/Cl₆-PhOSubPc solar cell employing ITO as bottom contact used as reference, and general device structure of solar cells exhibiting SC effects. For the SC-devices, the thickness d of the n-doped electron transport layer and p-doped hole transport layer are kept the same. b Normalized EQE spectra (upper panel) of the reference and an exemplary SC-device (with d = 55 nm) demonstrating the splitting of the absorption peaks of SubNc and Cl₆-PhOSubPc (lower panel). The blue solid arrows indicate splitting of the absorption peaks of both materials into upper-polariton, middle-polariton, and lower-polariton (UP, MP, and LP, respectively). As a result, the EQE spectrum of the SC-device is redshifted and its low-energy edge steepened. Inset pictures show the molecular structures of SubNc and Cl₆-PhOSubPc. c Current–voltage characteristic curves of the reference and SC-device show that the V_OC remains rather constant under SC. d Excited-state diagram illustrating the splitting of the first singlet excited state (S₁) into two polariton states. Electronic transitions (red arrows) can occur directly from the ground state (S₀) to the high-energy upper polariton (UP) and the low-energy lower polariton (LP). The energetically lower LP defines the optical gap (E_opt) of the polariton based solar cell