Magnetic Field Effects in $\pi$-Conjugated Polymer-Fullerene Blends: Evidence for Multiple Components

We studied magnetoconductance (MC) and magnetoelectroluminescence (MEL) in organic diodes from blends of $\pi$-conjugated polymers and fullerenes at various concentrations, $c$. The MC response is composed of several components that depend on the applied bias voltage and $c$. A dominant positive low-field (LF) component, which also governs the MEL response, dramatically decreases and broadens in the blends, thus unraveling a positive high-field and negative LF components. The positive MC components are caused by electrostatically bound $e\mathrm{\text{−}}h$ polaron pairs in unblended devices, and charge transfer pairs in the blends, which are dominated by two different field-induced spin sublevel mixing mechanisms. In contrast, the negative LF response is due to $e\mathrm{\text{−}}e$ and $h\mathrm{\text{−}}h$ pairs; this is confirmed by studying MC in electron- and hole-unipolar devices, which lack positive MC response.

Figure 1

Magnetoconductance (MC) response, $\Delta I/I$ vs field, $B$ in unblended diodes $c=0$ (a) and $c=100\%$ (d), and OPV devices (b) and (c) based on blends of MEH-PPV/PCBM with various PCBM concentrations, $c$ (in weight) and biasing voltage, $V$, at room temperature. The red lines through the data points are fits using a two-mechanism model for the spin-mixing process (see text).

Figure 2

(a) The room temperature MC value at $B=160\mathrm{mT}$, namely $[I(160\mathrm{mT})-I(0)]/I(0)$ as a function of the biasing voltage, $V$ for MEH-PPV devices at four PCBM concentrations, $c$. (b) Same as in (a) but for unipolar MEH-PPV devices at 6 K; electron-only (black squares) and hole-only red circles for HF, and red squares for the LF components (multiplied by a factor of 10).

Figure 3

Comparison between MEL (blue circles; right axis) and MC (black squares; left axis) responses in MEH-PPV/PCBM OPV devices at two PCBM concentrations and biasing voltages; (a) $c=1\%$, $V=4$ volt; (b) $c=50\%$ $V=1.7$ volt. The red line through the data points are fits using the two-mechanism model as in Fig. 1 (see text).

Figure 4

MC response of unipolar MEH-PPV devices ($c=0$) at 6 K. (a) hole-only diode at $V=1.2$ volt, where the low-field and high-field components are defined; (b) electron-only diode at $V=3$ volt.