Reduction of Collection Efficiency of Charge Carriers with Increasing Cell Size in Polymer Bulk Heterojunction Solar Cells

Changes in solar cell performance related to active area size were investigated using polymer bulk heterojunction devices. Cell geometry was defined by introduction of a sub-electrode. The cells were uniform up to 16 cm(2). The solar cells showed little change in performance up to a cell area of 1 cm(2). As cell area increased above 4 cm(2) the power conversion efficiency dropped significantly, mostly because of fill factor (FF) drop and short circuit current density (J(sc)) suppression. The changes in FF and J(sc) could not be described solely by a Shockley diode equation based on an equivalent circuit model unless photocurrent collection was also considered. As cell area increased, collection efficiency deviated from unity, which further reduced device performance. That deviation is attributed to acceleration of recombination loss at low built-in junction potentials.