Some transport properties of P3HT:PCBM thin films

A. FATH ALLAH; LONGEAUD, C.; J.A. SCHMIDT; M. EL YAAKOUBI; S. BERSON; N. LEMAITRE

Aachen

Conferencia; 26th International Conference on Amorphous and Nanocrystalline Semiconductors; 2015

ICANS

During the last decade organic polymers have received a considerable attention for their potential application in solar energy conversion. Conversion efficiencies are now of the order of 10% and researches are still going on to increase this figure. One of the important parameter controlling the thin active layer quality is the ambipolar diffusion length Ld that determines the ability of the carriers to be separated and eventually collected after their generation. Is it possible to apply the well known technique of Steady State Photocarrier Grating (SSPG) to organic blends? How is the Ld parameter reflected in the solar device behavior?In this paper we present some transport measurements performed on different blends of P3HT:PCBM. In particular, we have focused on the estimate of Ld using the SSPG technique that proves to be very efficient for semi-insulating inorganic thin films as hydrogenated amorphous silicon. Different blends of P3HT:PCBM (weight ratio 1:2, 1:1, 1:0.6, 1:0.3) with a thickness of the order of 200 nm have been deposited on glass at INES, kept in nitrogen atmosphere during transportation, fitted with two parallelelectrodes 1 mm apart and subsequently maintained under vacuum to avoid air contamination. Dark conductivity, photoconductivity and diffusion length were measured at room temperature. The 1:2 sample presented a high resistivity and was not photoconductive. For the other films, we have observed an increase of the conductivity and photoconductivity accompanied by a decrease of Ld from 170 nm to 90 nm with the decrease of the proportion of PCBM in the film. Transport properties of the 1:1 blend were also studied as function of temperature. Besides, we have investigated on the influence of the contacts on the transport properties and found that the response time to a perturbation of the samples fitted with aluminum contacts could depend on the blend proportion. Finally, in this communication we compare the transport properties we have found with the electrical properties (Jsc, FF) of the devices achieved with the same active blends.The two promising blends for device application are the 1:0.6 and 1:1. However, though the 1:0.6 presents a lower Ld than the 1:1 (140 nm instead of 170 nm) its higher photoconductivity results in better device performances.