The magneto conductance responses in polymer photovoltaic devices

Abstract

This article studies the magneto conductance (MC) responses of regioregular poly(3-hexylthiophene) (P3HT)-based polymer photovoltaic devices. The net MC results of devices are the sum of the positive and negative MC effects. The positive MC effect is due to the increased population of the singlet polaron-pair (PP) states (of the high dissociation rate to charge carriers) under the magnetic field. The negative MC effect results from the decline on the triplet exciton–charge reactions to charge carriers (the mobility and concentration of triplet excitons are decreased by the magnetic field). The MC responses of P3HT-based photovoltaic devices can be manipulated by the applied magnetic field, the electrical bias, the built-in potential, and the interfacial dipole layer. An inversion in the MC response is observed at the electrical bias near the open-circuit voltage. In addition, blending of an electron acceptor material, [6,6]-phenyl C61-butyric acid methyl ester, in P3HT active layer quenches the photo-excited states at the donor–acceptor interface and results in distinct MC responses of photovoltaic cells, which probably are related to the features indicating the formation of inter-molecular charge–transfer complexes at donor–acceptor junction.

Introduction

Organic conjugated molecules/polymers have very unique properties [1], [2], [3], they can be used as organic semiconductors in fabricating novel electronic devices [4], [5], [6], [7], [8], [9]. In addition, the electronic devices incorporating organic conjugated molecules/polymers as the active layers, exhibit strong magneto responses [10], [11]. The output performance, such as the electroluminescence [10], [12], [13], [14], resistance or conductance [10], [11], [12], [13], [14], [15], [16], and photocurrent [11], [17], [18], [19], [20] in organic/polymer diodes and photovoltaic (PV) cells can be modulated by the applied magnetic field, although the basic device configurations are composed of intrinsically non-magnetic components. The magneto responses of organic semiconductors would have new applications in organic electronics and spintronic devices. [21], [22], [23], [24].

A large magneto resistance (MR) or magneto conductance (MC) response in organic/polymer light-emitting diodes (O/PLEDs) has been reported by many researchers, which is intimately associated with the threshold of light emission and the ambipolar injection of charge carriers [10], [25]. While the organic/polymer diodes are operated at PV regime with illumination, the magnitude of photocurrent is varied with the applied magnetic field [11], [17], [18], [19], [20]. These results suggest that the dissociations and charge-reaction processes of the photo-excited states to the generation of free charge carriers are susceptible to changes by the applied magnetic field. The excited states, induced either electrically (in O/PLEDs) or optically (in PV cells) in organic semiconductor layers, are essentially important to the MR or MC responses in organic/polymer diodes and PV cells [12], [26], [27], [28], [29].

The excited states (excitons) in the regioregular poly(3-hexylthiophene) (P3HT)-based polymer diodes are Coulombically bound electron–hole (e–h) pairs, commonly having a short range of the separation distance [27], [30], [31], [32], [33]. Those excitons may undergo a relaxation process to become the excited polaron-pairs (PP) states with a longer range of e–h separation distance than that of excitons. Fig. 1a illustrates the schemes for formation of intra- and inter-molecular excited PP states from excitons. Both the excitons and PP states are the excited states of conjugated organic molecules possessing singlet and triplet electronic configurations. The lifetime of the singlet and triplet PP states is shorter than that of the singlet and triplet exciton states [34], [35]. The magnitude of exchange energy for the singlet/triplet excited states is exponentially decayed with respect to the e–h separation distance and is related to the energy difference between singlet and triplet states [30], [31]. Accordingly, the status of the long-range (>1 nm) PP states having exchange energies of the order smaller than 10⁻³ meV is subjected to changes brought in by the applied magnetic field. Sakagughi et al. reported that the external magnetic field dependencies on the photodecomposition reaction in solution, which were attributed to the electronic Zeeman and hyperfine interactions in the intermediate radical pair [36]. The intersystem crossing process of singlet/triplet PP states as illustrated in Fig. 1b involves the spin orbital coupling (SOC) [37], [38] and the hyperfine interaction (HFI) [19], [27], [39], [40], [41]. The population of singlet and triplet PP states can be modified by the external magnetic field, such as the changes of the splitting and difference of the energy states by Zeeman effect [19], [27], [28]. Since the photocurrent of P3HT-based diodes is the sum of “current flows” generated by the photo-excited states (excitons and PP states), any variations induced by an applied magnetic field on the distribution, dissociation, and charge-reaction processes of the singlet/triplet excited states (excitons and PP states) would certainly modulate the magnitude of photocurrent and MC response in PV devices [18], [29]. In this manuscript, we studied how the change of the electrical bias and the device electrode influences the dissociation and charge-reaction processes of photo-excited states in P3HT-based polymer diodes. Increasing the applied magnetic field increases the distribution of singlet PP states and reduces the mobility of triplet excitons by Zeeman splitting [28], [29]. The dissociation rate of the singlet PP states is higher due to the stronger ionic feature than that of triplet PP states, [19], [42], [43] which is accountable for an increase of free charge carriers and a positive MC effect. However, the decline of triplet exciton–charge reactions to charge carriers (reduce in mobility and concentration of triplet excitons), caused by the applied magnetic field [44], [45], [46], results in the decrease of free charge carriers and contributes to a negative MC effect. The net MC responses of the photovoltaic cells are the sum of positive and negative MC effects, in which the processes are schematically illustrated in Fig. 1c. Additionally, the MC responses are influenced by built-in electrical field of the diodes, which is tunable by varying the bias voltages and the work function of electrodes. An inversion in MC response is observed at the electrical bias near the open-circuit voltage (V_oc). The magneto responses for inter-molecular charge–transfer complex states at the donor–acceptor interface are investigated by blending an electron acceptor material, [6,6]-phenyl C61-butyric acid methyl ester (PCBM), in P3HT as the active layer. The distinct MC features suggest the formation of inter-molecular charge–transfer complexes at donor–acceptor junction.