Time-independent, high electron mobility in thin PC 61 BM films: Relevance to organic photovoltaics

Abstract

a b s t r a c tUltrafast optical probing of electric field by means of electroabsorption combined with conventional photocurrent measurements was employed to investigate the drift and mobility dynamics of photo-generated charge carriers in the pristine PC 61 BM film and in the blend with a merocyanine dye. Electrons passed a 40 nm thick PC 61 BM film within a few picoseconds with time-independent and weakly dispersive mobility. The electron mobility is 1 cm 2 /(V s) at 1 MV/cm and an estimate of the zero-field mobility yields 5 Á 10 À2 cm 2 /(V s). The initial electron mobility in the blend is of the order of 10 À2 cm 2 / (V s) and decreases rapidly. We conclude that electron motion in PC 61 BM based organic bulk hetero-junction solar cells is limited by barriers between PC 61 BM domains rather than by intrinsic PC 61 BM properties.

Unveiling the Nature of Charge Carrier Interactions by Electroabsorption Spectroscopy: An Illustration with Lead-Halide Perovskites

“…EAS and TREAS have been successfully applied to scrutinize the dynamics of photocarriers in small molecule-and conjugated polymer-based organic photovoltaic (OPV) systems. [1][2][3] They are currently believed to constitute very powerful tools for the study of another moment and polarizability changes upon the optical transition of interest (0 → k).…”

Using the Stark effect to understand charge generation in organic solar cells

“…A particularly interesting model system is pBTTT (poly(2,5-bis(3-hexadecyl-thiophen-2-yl)thieno [3,2-b]thiophene) blended with PCBM ( [6,6]-phenyl C 60 butyric acid methyl ester), since the phase morphology can in this case be carefully controlled during solution processing [4,5]. We present here a relatively new technique, time-resolved electromodulated differential absorption (EDA) [6][7][8][9], applied to solar cell devices containing either neat pBTTT or a 1:1 (by weight) blend of pBTTT with PCBM. In neat pBTTT, the polymer chains arrange into ordered lamellae, allowing high transistor mobility [10].…”

Non-Equilibrium Charge Motion in Organic Solar Cells

“…Figure 11 shows typical carrier mobility values found in various OPV systems [12][13][14][15][16][17][18][19][20][51][52][53][54][55][56][57] , as estimated by the indicated experimental technique (each technique is described in the following chapters). The trend in the mobility data strongly resembles the gradual thermalization of photo-created charges, shown in Figure 10.…”

“…This assignment is based on the considerably lower electron mobility in neat PC61BM on a time scale of several picoseconds, as determined using Stark-shift spectroscopy (µe = 0.5×10 -2 cm 2 V -1 s -1 in the zero-field limit) 16,51 and TRTS measurements on APFO3:PC61BM blends (µe = 0.3×10 -2 cm 2 V -1 s -1 ) 52 . Measurements on APFO3:PC61BM blends show increasing TRTS mobility with increasing polymer chain length 52 , whereas TRTS measurements on P3HT:PC61BM blends show only a minor effect of side chain regioregularity on TRTS mobility 53 .…”

“…Therefore, at the 20 ns -1 µs time scale the photo-generated carriers in TQ1: PC71BM have not yet reached quasi-equilibrium. 51 To summarize, the combined TRMC and TA measurement allows to estimate the mean carrier mobility on the 20 ns -1 µs time scale following photoexcitation. In OPVs employing fullerene acceptors, TRMC mostly monitors electron transport, since the hole mobility is typically much lower.…”