Highly near-infrared transparent GeO ₂-doped In ₂O ₃ electrodes for bulk heterojunction organic solar cells

We report on the characteristics of GeO ₂-doped In ₂O ₃ (IGO) films for use as transparent electrodes in organic solar cells (OSCs). The electrical, optical, and structural properties of IGO electrodes were investigated as a function of RF power and post-annealing conditions. At optimized conditions, the IGO electrode exhibited a low sheet resistance of 14.0 /square, a high optical transmittance of 86.9%, a root mean square roughness of 1.27 nm and a work function of 5.2 eV. In particular, the IGO film showed higher optical transmittance in the near-infrared region due to a lower free carrier concentration and higher mobility than conventional ITO electrodes. The higher Lewis acid strength of the Ge ⁴ (3.06) dopant, compared to that of a Sn ⁴ (1.62) dopant, led to higher mobility of the IGO films. In addition, we observed that the strongly oriented (222) grains in the IGO films enhanced carrier mobility and relaxation time. Furthermore, a bulk heterojunction OSC with the optimized IGO anode exhibited a good cell performance with a fill factor of 67.38%, a short circuit current of 8.438 mA/cm ², an open circuit voltage of 0.606 V, and a power conversion efficiency of 3.443%, which are comparable to OSCs with ITO anodes.