On the physics of dispersive electron transport characteristics in SnO₂ nanoparticle-based dye sensitized solar cells

The present study elucidates dispersive electron transport mediated by surface states in tin oxide (SnO₂) nanoparticle-based dye sensitized solar cells (DSSCs). Transmission electron microscopic studies on SnO₂ show a distribution of ∼10 nm particles exhibiting (111) crystal planes with inter-planar spacing of 0.28 nm. The dispersive transport, experienced by photo-generated charge carriers in the bulk of SnO₂, is observed to be imposed by trapping and de-trapping processes via SnO₂ surface states present close to the band edge. The DSSC exhibits 50% difference in performance observed between the forward (4%) and reverse (6%) scans due to the dispersive transport characteristics of the charge carriers in the bulk of the SnO₂. The photo-generated charge carriers are captured and released by the SnO₂ surface states that are close to the conduction band-edge resulting in a very significant variation; this is confirmed by the hysteresis observed in the forward and reverse scan current-voltage measurements under AM1.5 illumination. The hysteresis behavior assures that the charge carriers are accumulated in the bulk of electron acceptor due to the trapping, and released by de-trapping mediated by surface states observed during the forward and reverse scan measurements.