  In organic bulk heterojunction solar cells, the open circuit voltage
($V_\mathrm{oc}$) suffers from an ultra-high loss at low temperatures. In this
work we investigate the origin of the loss through calculating the
$V_\mathrm{oc}-T$ plots with the device model method systematically and
comparing it with experimentally observed ones. When the energetic disorder is
incorporated into the model by considering the disorder-suppressed and
temperature-dependent charge carrier mobilities, it is found that for
nonselective contacts the $V_\mathrm{oc}$ reduces drastically under the low
temperature regime, while for selective contacts the $V_\mathrm{oc}$ keeps
increasing with the decreasing temperature. The main reason is revealed that as
the temperature decreases, the reduced mobilities give rise to low charge
extraction efficiency and small bimolecular recombination rate for the
photogenerated charge carriers, so that in the former case they can be
extracted from the wrong electrode to form a leakage current which counteracts
the photocurrent and increases quickly with voltage, leading to the anomalous
reduction of $V_\mathrm{oc}$. In addition, it is revealed that the charge
generation rate is slow-varying with temperature and does not induce
significant $V_\mathrm{oc}$ loss. This work also provides a comprehensive
picture for the $V_\mathrm{oc}$ behavior under varying device working
conditions.
