Variations of cell performance in ITO-free organic solar cells with increasing cell areas

This study examined the effects of a cell area on the cell performances in ITO-free organic solar cells (OSCs) based on poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C₆₁ (PCBM). Highly conductive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films with two different sheet resistances (R_sh) were used as polymeric transparent anodes for cost-effective ITO-free OSCs. Changes in the power conversion efficiency (PCE), the fill factor (FF), the short-circuit current (J_sc), and the open-circuit voltage (V_oc) that resulted from changing the cell area or sheet resistance of transparent electrodes were systematically investigated. With increasing cell area from 4.5 to 49.5 mm², the device performance of ITO-free OSCs was continuously decreased mainly due to the decrease in the FF and the series resistance (R_s). In addition, the performance of OSCs was critically dependent on R_sh of the PEDOTPSS electrode. Upon reducing R_sh of the polymer anode from ∼200 to ∼90 Ω/□, the FF and PCE showed better values at an identical large cell area and exhibited a relieved cell performance degradation with increasing cell area, suggesting that the sheet resistance of transparent electrodes is a dominant factor to limit cell efficiencies in practical large-area solar cells.