Charge carrier mobility and lifetime versus composition of conjugated polymer/fullerene bulk-heterojunction solar cells
Introduction
During the last five years, important research efforts have been devoted to acquire a better understanding of the working principle of conjugated polymer:fullerene based bulk-heterojunction solar cells [1]. This knowledge sounds mandatory to allow further optimization and potential increase of the efficiency of these devices [2]. It appeared quickly that the ratio donor/acceptor does dictate not only the number of charge carriers created per incoming photons [3], but as well the ability of the device to collect the photo-induced charge carriers, that is, the transport properties of the active blend [4].
Several previous works focused on the investigation of solar cells characteristics versus conjugated polymer/fullerene ratio [4], [5], [6], [7], [8], [9]. In the case of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-phenylenevinylene] (MDMO-PPV):1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)-C61(PCBM) blends, short circuit current (Isc), fill factor (FF) and overall efficiency (η) were reported to show optimum values for PCBM concentration about 80% [7], [9]. These results have been interpreted in terms of competing effects between charge generation, taking essentially place in the MDMO-PPV molecules, and hole and electron respective mobilities. These latter have both been reported to increase up to two orders of magnitude upon increasing the PCBM concentration [9], [10]. Among others, ordering effects have been invoked to explain this unexpected phenomenon. This hypothesis has been substantiated by the numerous morphology studies performed on MDMO-PPV:PCBM, which show that PCBM tends to form nano-clusters due to its quite high diffusion coefficient in amorphous MDMO-PPV [7], [8]. This nano-clusters have been proposed to enhance the organization of the long MDMO-PPV chains, and hence the hole mobility [11].
In several models proposed to describe the working principle of conjugated polymer:fullerene solar cells, the charge collection is considered to be mostly ensured by field driven drift, yet diffusion might play a non-negligible role, especially close to the electrodes [9], [12], [13]. In this perspective, the distance performed by the charge carriers is given bywhere μ is the mobility of the charge carriers, τ their lifetime, and E the electric field in the device. Thus, the mobility indeed plays a major role in the collection of charge carriers. But so does as well the charge carrier lifetime. To the best of our knowledge, no one did yet report the evolution of τ versus the concentration of the MDMO-PPV:PCBM blend. Mihailetchi et al. expressed the necessity of their model to suppose increasing τ with decreasing PCBM to fit properly the experiment data, especially in the range below 50% of PCBM [9]. But no direct evaluation of τ was performed. Nevertheless, it has to be mentioned that Montanari et al. studied the recombination kinetic of charge carriers by transient absorption spectroscopy (TAS) [14]. However, the authors reported a PCBM concentration independent recombination kinetics as detected with this optical method.
Thus, we have used photo-induced charge carrier extraction by linearly increasing voltage technique (photo-CELIV) to investigate the charge carrier mobility, recombination kinetic and lifetime.
Section snippets
Experimental
As described in details elsewhere [15], [16], [17], photo-CELIV is a powerful method that allows the determination of charge carrier transport properties in the μs to ms range: a short laser pulse (3 ns, 532 nm, 0.5 mJ/pulse, Nd-YAG laser in our case) is absorbed by the device to be characterized; the charge carriers created are forced to recombine in the device thanks to an offset bias applied to compensate the Voc of the solar cell, what ensures flat-band condition; after a certain delay time τ
Results and discussion
Fig. 1a shows the photo-CELIV curves collected in the case of a 30% MDMO-PPV:70% PCBM active layer for various τdel. One can observe a capacitance induced displacement current to which is superimposed an extraction current [15], [16], [17]. This latter disappears after 8 μs, indicating a complete extraction of the charge carrier photogenerated. The mobility of the carriers can be calculated according to Eq. (2):where d is the thickness of the device, A is the
Conclusion
In conclusion, we have studied the transient mobility, charge carrier recombination kinetic (in the μs range) and charge carrier lifetime in MDMO-PPV:PCBM blends by photo-CELIV. We have observed that the charge carrier mobility increases by two orders of magnitude with increasing PCBM concentration, while the “effective” bimolecular lifetime of charge carriers decreases drastically, as theoretically proposed by Mihailetchi et al. Hence, the product mobility × lifetime of long lived charge
Acknowledgements
The authors gratefully acknowledge the financial support of the Austrian Foundation for the Advancement of Science (FWF NANORAC Contract No: FWF-N00103000). They thank H. Neugebauer and C. Lungenschmied for fruitful discussions.
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