Light‐modulated TiO_x Interlayer Dipole and Contact Activation in Organic Solar Cell Cathodes

Abstract

Understanding working mechanisms of selective interfacial layers and the underlying energetics of the organic semiconductor/electrode interface is an issue of primary concern for improving organic solar cell technologies. TiO x interlayers are used here to tune the selectivity of the cathode contact to electrons by controlled UV‐light activation. The S‐shaped kink observed for deactivated titania interlayers completely disappears after 2 min of UV‐light exposure yielding high fill factor (≈60%) and adequate efficiencies. UV‐light activation of complete cells alters the work function of the oxide that decreases about 650 mV as observed by Kelvin probe measurements. Capacitive techniques reveals a light‐intensity dependent shift in flat‐band voltage of up to 1.2 V under 1 sun illumination (without UV) in the case of deactivated TiO x interlayers. An increase in the magnitude of the light‐modulated dipole present at the oxide layer accounts for that voltage shift. Although the sign of the interface dipole would favor the extraction of electrons, the concomitant modification of the band bending in the organic semiconductor hinders an efficient extraction of carriers at positive voltages and originates the S‐shaped characteristics. After contact activation, the dipole strength does not change with the light intensity.

Toward High‐Temperature Stability of PTB7‐Based Bulk Heterojunction Solar Cells: Impact of Fullerene Size and Solvent Additive

“…[37,57,60] The quality of an interface is influenced by potential charge accumulation and formation of leakage current, which depends on the concentration of donor and acceptor at the contacts. [61,62] Here, we observe that leakage current is reduced after PAS-140 accompanied by an increase in V oc . These electronic modifications were studied in details with impedance spectroscopy.…”

Mechanistic Investigation into the Light Soaking Effect Observed in Inverted Polymer Solar Cells Containing Chemical Bath Deposited Titanium Oxide

“…On the contrary, Trost et al 15 insisted that the origin of the light-soaking effect was the TiO x /organic interface which had an energy barrier for the electron extraction. Further it was reported in recent work 16,27-29 that the light-soaking effect was observed because the energy barrier at such an interface reduced with an increase in light irradiation time. In addition, we think that the existence of electron trap sites in TiO x near the TiO x /P3HT:PCBM interface, which was suggested by ESR measurements, can become one of the reasons for the light-soaking effect.…”

“…Therefore, the interface charge transport may become the main factor that influences cell performance. In inverted solar cells that contain sol-gel titanium oxide as an electron collection layer, a light soaking effect that cell performance improves gradually with an increase in light irradiation time has often been observed [10][11][12][13][14][15][16] . Although this mysterious phenomenon is now well known, there is not necessarily satisfactory explanation for the mechanism.…”

Nanoscale mapping by electron energy-loss spectroscopy reveals evolution of organic solar cell contact selectivity

“…All Nyquist plots show two arcs at high applied voltages, and a contribution at still lower frequencies is observed only for the fresh sample, previously connected to a contact resistance (R CO ). 40 Values of 30-75 Ω cm 2 are observed for R CO being this a significant contribution to the overall series resistance of the device. Interestingly, this resistance is not present in aged devices indicating that in the initial state the fullerene is not able to transfer electrons to the ZnO increasing as a consequence the contact resistance.…”