Organic spin-valves: from unipolar to bipolar devices

We review the recent advances in both unipolar and bipolar organic spin valves. We discuss mechanisms that dominate the spin relaxation of injected spin aligned carriers and limit their spin diffusion length. A space charge limited current model with ferromagnetic electrodes is used to describe the bipolar organic spin valve operation. For unipolar spin valves we take into account finite spin diffusion length and obtain a general expression that is valid for devices with thick organic interlayers. For bipolar spin valves where the injected spin aligned electrons and holes selectively form polaron pairs having singlet or triplet spin configurations, we obtain modified expressions for the magneto-resistance and magneto-electroluminescence and show that they are proportional to the difference in singlet-triplet generation rates and the spin injection polarization degree of the ferromagnetic electrodes.