Insight into the current output of polymer solar cells: A comparison between the standard and inverted architecture

The performances and the stability of a polymer solar cell (PSC) mainly depend on the architecture of the device and the materials used for the fabrication (photoactive and interface layers and contacts). In this work we made a comparative study between standard and inverted PSCs having an identical pair of hole and electron transport layers: molybdenum trioxide (MoO₃) and poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9dioctylfluorene)] (PFN), respectively. We realized devices using a blend film of poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4,5-b']dithiophene)-2,6-diyl-alt-(4-(2ethylhexanoyl)-thieno[3,4-b]thiopene)-2,6-diyl]/[6,6]-phenyl C₇₁ butyric acid methyl ester (PBDTTT-C:[70]PCBM). The standard and inverted cells sequences were ITO/MoO₃/PBDTTT-C:[70]PCBM/PFN/Ag and ITO/PFN/PBDTTT-C:[70]PCBM/MoO₃/Ag, respectively. We studied the performances of both kinds of devices in order to investigate the influence of the architecture (standard vs inverted) on the performance of the solar cells. All the devices were characterized by IV light, IV dark and quantum efficiency measurements. The best device reached a power conversion efficiency of 6%. The inverted device has an improved current output compared to the standard one. In order to elucidate the absorption of photons inside the blend, we performed the optical modeling of the devices using the transfer matrix formalism and we simulated the effective absorption in the photoactive layer and the optical electric field inside the devices.