Mechanistic Investigation into the Light-Soaking Effect Observed in Inverted Polymer Solar Cells Using Indium Tin Oxide Electrodes Modified By Piperazine Derivatives

Abstract

The well-controlled electrical double layer in polymer solar cells, which is formed between active layer and electrode, promotes both increase in built-in electric field and decrease in carrier injection barrier, resulting in effective carrier collection on the electrode. Previously, in ITO/P3HT:PCBM/PEDOT:PSS/Au inverted solar cells, we observed the improvement of power conversion efficiency according to decrease in ionization potential of ITO by modifying with amine compounds. However, the light-soaking effect which the open-circuit voltage (Voc) gradually increased with white light irradiation containing UV component was observed. In this work, we have investigated the mechanism of light-soaking effect by combining time-dependences of photovoltaic properties and electric capacity of the inverted polymer solar cells. To prepare the amine-modified ITO electrodes, we used ethanol as a solvent for piperazine, N’(2-aminoethyl)piperazine (NAP) and 1,4-bis(3-aminopropyl)piperazine (BAP). The dilute solutions containing the amines were spin-coated on the ITO substrates. The substrate was subsequently dried at 120 °C for 10 min on a hotplate. Additionally, to remove any extra amines from the ITO, the same operation was performed using a solvent instead of the amine solution. Their ionization potentials (I P) were measured by a photoelectron spectroscopy in air. To fabricate the inverted polymer solar cells, a P3HT:PCBM blend solution and a PEDOT:PSS dispersion solution were spin-coated on the modified ITO substrates sequentially. The Au rear electrode was vacuum-deposited on the PEDOT:PSS layer. Finally, the device was mechanically protected by laminating it with a film at 150°C for 5 min under a reduced pressure. The time-dependence of the current-voltage (I-V) curves of these solar cells was measured under AM 1.5G-100 mW cm-2 simulated sunlight condition. Using ac impedance spectroscopic measurement, both time-dependences of resistance and electric capacity (CPE) of these solar cells were obtained in the dark and under simulated sunlight irradiation. The cell fabrications and the measurements were carried out under an ambient atmosphere. The IP values of BAP-modified ITO electrodes decreased with increasing the concentration of the BAP solutions. The minimum IP was 4.39 eV, decreasing by ca.0.3 eV compared to that of bare ITO. In addition, the power conversion efficiency of the inverted cells increased by using the BAP-modified ITOs. This improvement is perhaps because of an efficient drift migration of photo-produced charge carriers by increase in the built-in potential. However, when white light was irradiated to the cells, the light-soaking effect was observed. The time taken to arrive at a constant Voc decreased as the IP of the amine-modified ITO decreased. That is, the light-soaking effect was small when the amine-modified ITO which had smaller IP was used. In addition, the CPE in an electric double layer formed in the ITO/active layer interface was estimated by ac impedance spectroscopy. This CPE gradually decreased by continuous white light irradiation. Furthermore, the degree of change of the CPE was small when the amine-modified ITO with smaller IP was used. The good correlation between both time-dependences of VOC and CPE was observed. The decrease in CPE and the increase in VOC were caused by decrease in IP . That is, the CPE corresponded to an electron injection barrier in the ITO/active layer interface, and the electron injection barrier became small according to decrease in IP . In summary, the improvement of VOC was caused by decreasing the electron injection barrier because of decrease in IP .