Control of exciton spin statistics through spin polarization in organic optoelectronic devices

Abstract

Spintronics based on organic semiconductor materials is attractive because of its rich fundamental physics and potential for device applications. Manipulating spins is obviously important for spintronics, and is usually achieved by using magnetic electrodes. Here we show a new approach where spin populations can be controlled primarily by energetics rather than kinetics. We find that exciton spin statistics can be substantially controlled by spin-polarizing carriers after injection using high magnetic fields and low temperatures, where the Zeeman energy is comparable with the thermal energy. By using this method, we demonstrate that singlet exciton formation can be suppressed by up to 53% in organic light-emitting diodes, and the dark conductance of organic photovoltaic devices can be increased by up to 45% due to enhanced formation of triplet charge-transfer states, leading to less recombination to the ground state.

Figure 1. The spin polarization effect in an organic optoelectronic device.

Without applying an external magnetic field, electrons and holes have random spin orientations, and singlet and triplet excitons are formed in a ratio of 1:3. By applying a magnetic field at low temperature, the spins of electrons and holes will be aligned with the external magnetic field. The magnetic field direction is denoted by green arrows. For perfect spin alignment, only the ↓↓ triplet state can be formed when an electron and hole recombine.

Figure 2. Magnetic field effects in a TFB:F8BT LED.

(a) Fractional change in EL as a function of magnetic field under constant biases at various temperatures, as indicated in the figure. The bias voltages are chosen to give a current density of ~4 × 10⁻⁴ mA cm⁻² at zero magnetic field. The black curves are fits to the model described in the text. (b) Fractional change in EL as a function of magnetic field at 3 K under various bias voltages. The black curves are fits to the model described in the text. (c) MC at 3 K under various bias voltages. (d) Fractional change in EL as a function of current density at 8.7 T at various temperatures. Here the current density is the zero-field value, and the device is under constant bias voltage during the measurement.

Figure 3. Schematic of recombination pathways and timescales.

Spin polarization due to the magnetic field changes the ratio of singlet and triplet charge-transfer states (¹CT and ³CT, respectively) formed by charge recombination, as shown in Fig. 1. For the LED device (a), triplet CT states recombine rapidly to form lower-energy triplet excitons. For the photovoltaic device (b), the triplet exciton state is higher in energy than the triplet CT state. Triplet CT states therefore have a long lifetime and can be redissociated to form free charges. This leads to a net free-charge recombination rate that depends on the magnetic field. T and S denote triplet and singlet excitons, respectively.

Figure 4. MC in an P3HT:PCBM device.

(a) MC (on a logarithmic scale) under constant bias at various temperatures. The zero magnetic field current density is always ~0.5 mA cm⁻². (b) Device current as a function of temperature under constant 5.0 V bias at various magnetic fields. (c) MC at 2 K under various bias voltages.