Charge carrier mobility and lifetime of organic bulk heterojunctions analyzed by impedance spectroscopy
Introduction
Organic conducting materials are at the heart of bulk heterojunction (BHJ) solar cells, a promising alternative to silicon-based solar cells due to their optical, electronic and mechanical properties [1]. Bulk heterojunctions are formed by an interpenetrating blend of an optically active polymer and electron accepting molecules. At the optically active polymer excitons are created by light absorption. This exciton dissociates at the polymer/molecule interface, thus spatially separating the electrons and holes which therefore can be transported to the electrodes by hopping.
When using derivatives of poly(phenylene vinylene) (PPV) as hole transporting material (HTM), some aspects of the BHJ device operation (particularly the internal electric field distribution [2] and the transport mechanisms [3], [4]) have been properly interpreted. Blends formed of PPV derivatives and fullerenes are considered to form undoped systems. Therefore, the basic device model resembles that proposed for a-Si thin film p–i–n structures [3]. Transport is mainly determined by charge carrier drift upon the electrical field driving force. Recently, BHJ organic solar cells using P3HT as HTM have shown high performance [5]. P3HT exhibits relatively high hole mobility (∼10−4–10−3 cm2 V−1 s−1) [6] and can easily undergo p-doping when exposed to the air or moisture [7], [8], with the consequent formation of Schottky contacts. Models which regard polymer semiconductors as undoped materials should be then revised to include the effect of band bending (depletion zones) and minority carrier injection and storage in the diode bulk.
The central magnitude that informs about carrier accumulation is the capacitance, which can be readily determined by impedance spectroscopy. This is because in all solar cells the generation of positive and negative carriers creates a splitting of Fermi levels that is ultimately responsible for the photovoltage [9]. In silicon solar cells, for instance, the capacitance shows two main components as a function of the bias [10], [11]: a Mott–Schottky characteristic, due to the modulation of the Schottky barrier, at reverse and moderate forward bias, and a chemical capacitance [9], that increases exponentially for intense forward bias. The first characteristic indicates the presence of doping whereby the solar cell device is able to accumulate substantial minorities. Such carrier storage is manifest in the second characteristic, the chemical capacitance, which directly reflects the carrier statistics [9]. In standard dye-sensitized solar cells (DSC) based on nanostructured TiO2, the shape of the voltage dependence of the capacitance is somewhat different, since the “hole conductor” is a liquid electrolyte with high ionic concentration, so that the Schottky barrier in the active semiconductor layer is not found. However, the chemical capacitance is very clearly observed and shows the density of states of electrons accumulated in the electron-transporting material (TiO2) [12], [13]. The identification of the voltage-dependent capacitance has become then a major tool for assessing the energetics in a DSC [14] and for interpreting the recombination lifetime [15]. The strong accumulation associated with the unconstrained rise of the Fermi level with bias appears in high performance solar cells [13], while in many other cases, the charge storage is inhibited by additional mechanisms and the solar cell capacitance may become negative at strong forward bias [10].
Given the general significance of the capacitance for interpretation of the fundamental transport and recombination mechanism governing the operation of new classes of solar cell devices, in this paper we present direct measurements and interpretation of the capacitance under reverse and forward voltages of a BHJ structure of the type ITO/PEDOT:PSS/P3HT:PCBM/Al. Reverse bias capacitance exhibits Mott–Schottky-like behavior signaling the formation of a Schottky junction (band bending) at the P3HT:PCBM-Al contact. Impedance modeling allows to extract both the recombination time and mobility of the minority carriers (electrons) at forward bias in the dark.
Section snippets
Experimental
We have built diodes with structure ITO/PEDOT:PSS/P3HT:PCBM/Al following the next procedure: RR-P3HT from Rieke Materials and PCBM from Nano-c were dissolved in xylene in a weight ratio of (1:0.75). The solution was heat up to 65 °C and continuously stirred for 3 h. In parallel, ITO substrates from Diamond Coatings with a sheet resistance of 40 Ω/□ were cleaned in 5 min subsequent ultrasonic baths of acetone, methanol and isopropanol. They were later introduced in oxygen plasma for 5 min. Baytron P
Results and discussion
It is well-known that P3HT is a conjugated polymer that in exposure to oxygen and/or moisture results p-doped [7], [8]. Under such conditions, it has been suggested that the P3HT-Aluminum contact shows a Schottky diode behavior. Band bending with a corresponding depletion zone is formed at the contact as indicated in Fig. 1a [16], [17]. For a Schottky diode [18] the junction capacitance, which appears as a consequence of the modulation of the depletion layer, exhibits a bias dependence
Conclusion
In summary, it has been shown that the P3HT:PCBM-Al contacts show a behavior consistent with a Schottky diode in the dark and under reverse bias. Furthermore, the capacitance values at forward bias are viewed as a strong indication that minority charge carriers play a significant role. With such assumption and measuring ac electrical impedance spectra we propose a model to interprete the diffusion and recombination processes which allows us to determine both the diffusion and recombination time
Acknowledgment
We thank financial support from Ministerio de Educacion y Ciencia under project HOPE CSD2007-00007 (Consolider-Ingenio 2010).
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