Blend ratio and applied voltage effects on the charge recombination in bulk heterojunction polymer solar cells based on anthracene-containing poly(arylene-ethynylene)-alt-poly(arylene-vinylene) studied using magnetoconductance
Introduction
Polymer solar cells have great potential for use in lightweight, flexible solar energy conversion devices. In an effort to realize, low-cost and high-efficiency organic photovoltaic (OPV) cells, the application of several new concepts in electron spin engineering to device development has recently been reviewed [[1], [2], [3], [4](d), [4], [4](a), [4](b), [4](c), [4], [5]]. Power conversion efficiencies (PCEs) of >6% have already been achieved for this type of organic solar cell [[6], [7], [8]], together with the prospect of high production rates and low-cost processing [9,10]. Consequently, they are generating more and more interest in the academic and industrial community. Therefore, extensive research has been performed on the physical mechanisms governing these types of blends [11]. Qualitatively, the working principles of OPV cells include the following steps [12]: The absorption of light in an organic material creates singlet excitons, which they can usually be dissociated at the contact electrodes [11]. When an exciton is generated in the donor material (D), it can reach the interface with the acceptor material (A) if it is located within its diffusion length. At the interface, it is energetically favorable to transfer the electron to the electronegative acceptor. After the dissociation of the exciton, the resulting electron and hole are spatially separated on two different materials. However, they are Coulombically bound pairs, often known as a charge transfer complex [11] or charge transfer state [13]. The final limiting step in OPV cells is the transport of the charge carriers to their respective electrodes. Electrons will generally be transported through the acceptor material and holes through the donor material.
One of the major problems in organic semiconductors (OSCs) is magnetoconductance (MC), which is related to the current shift observed under an external magnetic field. MC has been observed in a number of OSC devices, such as organic light emitting diodes (OLEDs) [[14], [15], [16], [17]], OSCs [18,19], and organic field effect transistors (OFETs) [20,21]. The effects observed at room temperature are as large as 10% [22], but their underlying mechanism is still under discussion. The exact origin of MC on organic materials is still widely discussed in the scientific community and the literature suggests many models, such as the bipolaron model [23], the electron-hole model [24] and the exciton-charge model [25]. The explanations of the MC effect observed in organic solar cells are in conflict with various investigators. In fact, Shakya et al. [26] reported that the increase in MC on P3HT:PCBM was caused by an increase in the inter-system crossing rate between the singlet and triplet excitonic states under an applied magnetic field, resulting in an increase in the triplet exciton population, which in turn increases the efficiency of photocurrent generation via exciton dissociation. However, Lei et al. [27] explained the positive MC in the photocurrent on the same system by mixing the magnetic field-dependent hyperfine field between the singlet and triplet polaron pairs, leading to increased dissociation and photocurrent of singlet polaron-pair dominated charge. Finally, Zang et al. [28] stated that the combination of the electrical drifting force by an applied bias and attraction of Coulombs in the polaron-pair states represents the probabilities of polaron pair formation and dissociation at the donor-acceptor (D/A) interface and consequently affects the MC in the photocurrent. In this work, we studied the magnetic field effects (MFE), namely, magnetoconductance (MC) in organic bulk-heterojunction solar cells devices prepared using blends of anthracene-containing poly(arylene-ethynylylene)-alt-poly(arylene-vinylene) (AnE-PVstat), which was synthesized according to the procedures described previously [29,30], and fullerene molecules at various ratios (1:1, 1:2, 1:3 and 1:4) and under different applied voltages ranging from 0.7 to 0.95 V in steps of 0.05 V. The MC is positive and monotonic over the full range of the blend ratios and applied voltages studied. We analyzed the MC effect using an MC model controlled by the interconversion of the singlet and triplet e-h pairs states based on density matrix formalism using the stochastic Liouville equation, considering that the singlet and triplet e-h pairs have different recombination and dissociation rates, to theoretically reproduce the line shape of the MC. For all the applied voltages studied, the recombination and dissociation rates of the e-h pairs on the singlet and triplet states decreased upon increasing the fullerene content until it reached a maximum with a PCBM content of 75 wt%. After this concentration, the latter rates start to increase. However, for each blend ratio, the recombination and dissociation rates of the singlet and triplet e-h pairs increase when the voltage increases.
Section snippets
Experimental section
We used an ambipolar polymer (AnE-PVstat), which exhibits very good ambipolar behavior, i.e., both electrons and holes can be injected and transported, and can form (e-h) pairs or excitons [29,30]. The material used as the hole transport layer, poly(3,4-ethylene dioxythio-phene) (PEDOT) doped with poly(styrene sulfonic acid) (PSS) (PEDOT:PSS Clevios P500), was purchased from Heraeus and n-type semiconductor phenyl C60 butyric acid methyl ester (PC60BM) was obtained from Sigma-Aldrich. PCBM was
Photovoltaic characterization of the ITO/PEDOT:PSS/AnE PVstat:PCBM/LiF/Al solar cells
Fig. 2a shows the current versus applied voltage characteristics of the ITO/PEDOT:PSS/AnE-PVstat:PCBM/LiF/Al structures prepared using different blend ratios (1:1, 1:2, 1:3 and 1:4) for both polarities under dark and illuminated conditions. As inferred from the plot, the structures possess diode behavior under dark conditions. Under illumination, the devices show solar cell behavior. The photovoltaic parameters of the various cells are given in Table 1, which include the open circuit voltage (V
Conclusions
We have studied the MC effect on organic solar cells devices prepared using different blend ratios at different applied voltages. We observed a positive MC effect for all the devices, which increased with an increase in the fullerene content until reaching a maximum at a PCBM content of 75 wt%. After this concentration, the MC effect started to decrease. We used a stochastic Liouville Equation in the framework of the MIST model to reproduce the experimental MC effect. It has been found that the
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to thank the Tunisian Ministry of Higher Education and Scientific Research and the Austrian Science Foundation FWF for their financial support inside the Wittgenste Prize project for Prof. Niyazi Serdar Sariciftci.
Moufid RADAOUI would like to thank Dr. Markus Clark Scharber (Institute of Physical Chemistry and Linz Institute of Organic Solar Cells, Johannes Kepler University Altenberger Straβe, 69 4040 Linz, Austria) for his many helpful discussions.
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