  Wetzelaer, Koster, and Blom [PRL 107, 066605] recently observed that the
classic Einstein relation $\frac{D}{\mu}=\frac{kT}{q}$ is still valid in
disordered semiconductors in thermal(quasi)equilibrium by studying the
diffusion-driven currents of single-carrier diodes. This is in contrast to the
previous theoretical prediction [2] and experiment observation from Leo et.al.
[3] where the Einstein relation increases with 1/T. We will discuss the reason
for this discrepancy here and suggest that one-carrier diodes experiment might
nor be suitable for verifying Einstein relation in organic semiconductors.
