Investigating Differences in Performances of Top and Bottom Illuminated Organic Solar Cells

Organic solar cell (OSC) devices built on opaque substrates need to be illuminated from the top, unlike those on transparent substrates which may be illuminated from the bottom (substrate side). Top illuminated devices are, so far, reported with inferior solar cell parameters and power conversion efficiencies compared to bottom illuminated devices. Top and bottom illuminated OSCs, along with semi-transparent devices with transparent electrodes of the earlier devices, are built on glass substrates with various thicknesses of poly 3-hexylthiophene (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend layer as the photoactive layer (PAL). The devices are characterised (with the semitransparent device characterised with top, as well as bottom illumination) and the corresponding solar cell parameters are compiled and analysed. Theoretical estimation of light coupled in different devices for different thicknesses are compared. Simulated OSC characteristics are then fitted on measured data by tuning device parameters. Higher trap concentrations at the transparent electrode stack and PAL interface closer to illumination in top illuminated devices. The higher trap values are confirmed by analysis of device dark current and EQE data. The overall analysis indicates that minimising traps at the device illumination side interfaces in top illuminated OSCs and optimal choice of PAL thickness should help these devices perform at least as well as traditional bottom illuminated devices.