A new transient charge extraction technique is presented, which facilitates simultaneous measurements of mobility and lifetime of photogenerated charge carriers in organic solar cells under real operating conditions. An adaptive field control is implemented keeping the solar cell at open circuit conditions during recombination. The practical benefit of the new technique is demonstrated by determining the mobility-lifetime parameter of solar cells based on PCDTBT:PC(71) BM and P3HT:PC(61) BM.

Up to now the basic theoretical description of charge extraction by linearly increasing voltage ͑CELIV͒ is solved for a low conductivity approximation only. Here we present the full analytical solution, thus generalize the theoretical framework for this method. We compare the analytical solution and the approximated theory, showing that especially for typical organic solar cell materials the latter approach has a very limited validity. Photo-CELIV measurements on poly͑3-hexyl thiophene-2,5-diyl͒:͓6,6͔-phenyl-C 61 butyric acid methyl ester based solar cells were then evaluated by fitting the current transients to the analytical solution. We found that the fit results are in a very good agreement with the experimental observations, if ambipolar transport is taken into account, the origin of which we will discuss. Furthermore we present parametric equations for the mobility and the charge carrier density, which can be applied over the entire experimental range of parameters.

We investigated the charge carrier mobility in pristine poly(3-hexyl thiophene-2,5-diyl) (P3HT): [6,6]-phenyl-C 61 butyric acid methyl ester (PCBM) blend devices by applying the time resolved photoconductivity experiment in dependence on the donor:acceptor ratio. We observe a bipolar transport in all studied samples ranging from pure polymer to polymer:fullerene with 90% PCBM content. For the ratios P3HT:PCBM 1:4 and 1:1 we observe two transit times in the electron current transients, as well as hole double transients for P3HT:PCBM 1:2. We find high hole and electron mobilities in the order of 10 −3 -10 −2 cm 2 /Vs for a concentration of 90% PCBM in the blend.Due to its promising potential of low cost fabrication, organic photovoltaics has attracted a lot of attention during the last decade. Bulk heterojunction (BHJ) solar cells based on blends of poly(3-hexyl thiophene) P3HT and phenyl-C 61 butyric acid methyl ester PCBM belong to the most promising candidates for this application, as efficiencies in the range of 5% have been achieved recently [1].The mobility of photogenerated charges is one of the crucial factors determining the performance of organic solar cells [2], its impact being more complex than following any "faster equals better" stereotype. Nevertheless, one of the prerequisites for a well-performing BHJ solar cell is that the respective mobilities for transporting electrons on the fullerene and holes on the conjugated polymer are rather balanced, in order to avoid a space charge building up. However, as the blend of two different material types cannot be described by a simple superposition of the single material's properties, a prediction of the bipolar charge transport in a solar cell is not straightforward. Tuladhar et al. [3] investigated the dependence of the charge carrier mobility in poly[2methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene (MDMO-PPV) with varying PCBM content using time-offlight transient photoconductivity (TOF). They found an increase of the mobility for both carrier types with an increasing fullerene fraction. Additionally, ambipolar transport was found in PCBM embedded in an insulating polystyrene matrix. For the nowadays more relevant combination of P3HT:PCBM, field effect transistor (FET) experiments were reported in literature, observing an ambipolar transport in a limited range of blend ratios [4,5]. However, field effect measurements consider neither photogenerated charges nor low carrier concentrations. There is only one publication studying P3HT:PCBM blends with varying donor:acceptor ratio using the relevant TOF technique. Huang et al. [6] found an anomalous transition from dispersive to nondispersive and back to dispersive transport for an increasing PCBM fraction. The authors use films around one micrometer thickness, and * Electronic address: abaumann@physik.uni-wuerzburg.de † Electronic address: deibel@physik.uni-wuerzburg.de do not comment on the observed dispersivity which might be influenced by the layer thickness [7]. In this context, the ratio depende...

The slow decay of charge carriers in polymer-fullerene blends measured in transient studies has raised a number of questions about the mechanisms of nongeminate recombination in these systems. In an attempt to understand this behavior, we have applied a combination of steady-state and transient photoinduced absorption measurements to compare nongeminate recombination behavior in films of neat poly(3-hexyl thiophene) (P3HT) and P3HT blended with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). Transient measurements show that carrier recombination in the neat P3HT film exhibits second-order decay with a recombination rate coefficient that is similar to that predicted by Langevin theory. In addition, temperature dependent measurements indicate that neat films exhibit recombination behavior consistent with the Gaussian disorder model. In contrast, the P3HT:PCBM blend films are characterized by a strongly reduced recombination rate and an apparent recombination order greater than two. We then assess a number of previously proposed explanations for this behavior, including phase separation, carrier concentration dependent mobility, non-encounter limited recombination, and interfacial states. In the end, we propose a model in which pure domains with a Gaussian density of states are separated by a mixed phase with an exponential density of states. We find that such a model can explain both the reduced magnitude of the recombination rate and the high order recombination kinetics and, based on the current state of knowledge, is the most consistent with experimental observations.Comment: 9 pages, 4 figures; corrected a few minor typos and grammatical error

The charge carrier drift mobility in disordered semiconductors is commonly graphically extracted from timeof-flight (TOF) photocurrent transients yielding a single transit time. However, the term transit time is ambiguously defined and fails to deliver a mobility in terms of a statistical average. Here, we introduce an advanced computational procedure to evaluate TOF transients, which allows to extract the whole distribution of transit times and mobilities from the photocurrent transient, instead of a single value. This method, extending the work of Scott et al. (Phys. Rev. B 46, 8603), is applicable to disordered systems with a Gaussian density of states (DOS) and its accuracy is validated using one-dimensional Monte Carlo simulations. We demonstrate the superiority of this new approach by comparing it to the common geometrical analysis of hole TOF transients measured on poly(3-hexyl thiophene-2,5-diyl) (P3HT). The extracted distributions provide access to a very detailed and accurate analysis of the charge carrier transport. For instance, not only the mobility given by the mean transit time, but also the mean mobility can be calculated. Whereas the latter determines the macroscopic photocurrent, the former is relevant for an accurate determination of the energetic disorder parameter σ within the Gaussian disorder model (GDM). σ derived by using the common geometrical method is, as we show, underestimated instead.