Towards an understanding of light activation processes in titanium oxide based inverted organic solar cells

Abstract

The light activation phenomenon in inverted P3HT:PCBM bulk heterojunction organic solar cells based on titanium oxide sublayer (TiOx) is characterized by fast acquisition of current-voltage (J-V) curves under light bias as function of time. TiOx layers were thermally treated under inert atmosphere at different temperatures prior active layer deposition and for every device an activation time was extracted. It is shown that the higher the TiOx annealing temperature, the faster the activation. The improvement of the overall device performances is also observed for devices with TiOx layers baked above 100 °C. The evolution of the characteristic of the organic semiconductors (OSC) device, from dielectric to diode, is attributed to the increase of TiOx conductivity by three orders of magnitude upon white light illumination. Additionally, devices based on baked TiOx present higher conductivity than those based on unbaked TiOx which would explain the gain in performances and the short activation time of the OSC. In order to understand the origin of the phenomenon, deactivation experiments were also performed under different conditions on OSC. The deactivation process was shown to be thermally dependent and fully reversible under inert atmosphere, which suggest that deep traps are responsible for the activation phenomenon. An optimal annealing temperature was found at 120 °C and gives a reasonable short activation time of approximately 1 min and photo conversion efficiency up to 4%.

Direct observation of UV-induced charge accumulation in inverted-type polymer solar cells with a TiOx layer: Microscopic elucidation of the light-soaking phenomenon

“…When the device was irradiated by white light, the cell performance greatly improved, showing a proper J-shaped J-V curve, and the curve then maintained the J-shape under irradiation of both white light and brief UV-cut light. [8][9][10][11][12][13]15 Direct evidence for the light-soaking-induced charge accumulation is demonstrated by ESR measurements. In these experiments, a continuous-wave method was used with a modulation frequency of 100 kHz for the external magnetic field H. 18 Thus, the observed light-induced ESR signals are due to accumulated (or deeply trapped) photogenerated charge carriers with a lifetime of >10 ls.…”

“…[3][4][5][6][7] One of the fundamental issues is the light-soaking phenomenon with respect to UV-light irradiation, which has been commonly observed in IOSCs with electron-collecting layers such as TiO x . 2,[8][9][10][11][12][13][14][15] Although high-temperature annealing treatments on the TiO x layer can prevent the phenomenon, such treatments are undesirable for low-temperature device fabrication using flexible plastic substrates. 15 Additionally, long-term UV-light irradiation on IOSCs can cause device degradation.…”

“…Although the light-soaking phenomenon has been explained on the basis of charge-trap fillings in the electron-collecting layers using phenomenological methods, the mechanism is not yet completely understood, and a microscopic investigation using a direct research method is greatly desired. [9][10][11][12][13][14]16 To fully elucidate the light-soaking phenomenon, electron spin resonance (ESR) spectroscopy is the most suitable technique because ESR is a highly sensitive and nondestructive technique for directly observing the charges in organic semiconductors and their devices at the molecular level. [17][18][19][20][21][22][23][24] ESR studies have demonstrated a clear correlation between performance deterioration and charge accumulation (or trappings) in a normal-type OSC, 18 and have clarified degradation mechanism due to charge formation during device fabrication.…”

Vertical phase separation and light-soaking effect improvements by photoactive layer spin coating initiation time control in air-processed inverted organic solar cells

“…The metal oxides were baked at 150°C before depositing the photoactive layer to dry and improve conductivity. This minimizes the light-soaking time of the OSC [27].…”

“…This can be overcome by exposing the cells to a combination of illumination and temperature [25]. A certain period of UV activation on ETL metal oxides restrains the 'S-curve' and increases PCE [26,27]. This is because electrons on valence band of the metal oxide get excited to fill up any trap sites created and increase the electron number on conduction band, which enhance the transfer rate.…”