Photoinduced Transient Stark Spectroscopy in Organic Semiconductors: A Method for Charge Mobility Determination in the Picosecond Regime

Abstract

Subpicosecond photoinduced Stark spectroscopy experiments are carried out for measuring charge carrier mobility in organic semiconductors. The technique is demonstrated in state-of-art devices based on methanofullerene. The transient mobility of photogenerated charge carriers is measured in the picosecond time domain. Electric field dependent mobility is observed from the earliest time scales. In addition, two distinct transport regimes are revealed: a short-lived state, approximately 10 ps, of high mobility and a transient towards the trap limited transport, associated with the mesoscopic structure of the medium. DOI: 10.1103/PhysRevLett.96.106601 PACS numbers: 72.80.Rj, 72.80.Ng, 78.47.+p, 78.66.Tr Charge carrier mobility in organic semiconductors is a critical material property that determines the performance characteristics of many electronic devices, including fieldeffect transistors [1], light-emitting diodes [2], and solar cells [3]. For this reason, many experimental techniques and theoretical modeling have been devoted to its measure and interpretation [4 -7]. It turns out that different experiments provide different values of carrier mobility in the same material, due to varying experimental conditions and different device architectures. For instance, it is found that the hole mobility of common polymer semiconductors is several orders of magnitude lower when tested in lightemitting diodes than in field-effect transistors [8,9]. Such variations make material evaluation difficult if not impossible.In amorphous organic materials charge carrier mobility is often the average of a number of slowing down processes where defects and traps dominate. Ideally one would like to measure the carrier mobility in a pure, defect free material, so meaningful comparison between different experiments and theory would be allowed. Close to this ideal case is the use of time resolved microwave conductivity technique, an electrodeless method that monitors charge dipoles absorption in the transient regime [7,10]. The time resolution of this technique is limited, however, to the nanosecond domain. Here we present an alternative approach, based on photoinduced transient Stark effect in which optical probing is exploited for extracting mobility values in the picosecond time regime. A related technique, exploiting the Franz-Keldysh effect in extended band states, was successfully employed by Shank et al. to measure nonequilibrium carrier transport in inorganic semiconductors [11][12][13]. The validity of our method relies on the fact that charge transport is monitored in situ on the compound without current extraction, avoiding pitfalls caused by carrier migration along macroscopic regions of the sample as well as by carrier extraction at semiconductor-metal electrode interfaces. Mobilities obtained in this way can be considered as upper limit values, unaffected by limiting processes such as trapping or recombination. This information is intrinsic and constitutes a direct feedback for the design of new materials with improved...

A Close Look at Charge Generation in Polymer:Fullerene Blends with Microstructure Control

“…If a Stark effect is caused by those charges, an EA contribution is seen in the TA data. In contrast to the case where a uniform electric field is externally applied across the bulk of the film using electrodes, 43,65,66 the Stark effect in the TA data depends on the local radial electric fields around free charges and the local electric dipoles around electron−hole pairs. It can be observed even if the overall macroscopic field cancels due to a random orientation of the electron−hole pairs in the BHJ (in particular for a quadratic dependence on the local field magnitude).…”

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

“…on how the electron donor (typically a conjugated polymer) and the electron acceptor (typically a fullerene derivative) arrange from the nano-to the microscopic scale [1][2][3]. 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.…”

“…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].…”

Subpicosecond photoinduced Stark spectroscopy in fullerene-based devices

“…Results and discussion are an extension of a previous short paper, which was focused on mobility determination from the early time dynamics. 11 Our results suggest that geminate recombination is responsible for a loss of 40% of the initially photogenerated charges in the first 450 ps.…”

“…In PCBM we reported previously a quadratic dependence with the applied electric field characteristic of excitonic Stark shift. 11 In the second case, the signal depends on 1 as well as 2. In addition to EA, signatures from charge induced absorption ͑bands c 1 and c 2 ͒, are present at 1, as expected from the high rectification ratio of the J-V diode curve and the large charge injection achieved at positive bias ͑inset Fig.…”