Interface Stability of Polymer and Small-Molecule Organic Photovoltaics - Organic Solar Cells: Materials and Device Physics - page 161

Fig. 6.17 (left) Device structure of an inverted OPV; (right) I-V characteristics of the devices
with CaO and Ca as the electron-transporting layers under 100 mW/cm 2 illumination [ 92 ].
(Reprinted from [ 92 ], with permission from Elsevier) ( http://www.sciencedirect.com/science/
journal/09270248 )
can still work properly. The CaO layer was obtained by purposely oxidizing the Ca
deposited (expose the Ca to dry air). All the other layers are exactly the same.
Figure 6.17 (right) shows the comparison of the I-V characteristics of two
devices with the structures of ITO/Ca/P3HT:PCBM/MoO 3 /Ag and ITO/CaO/
P3HT:PCBM/MoO 3 /Ag under 100 mW/cm 2 . It is obvious that the device with
CaO exhibits rather poor performance compared to that with Ca. In this way, we
can indirectly confirm that Ca is not oxidized (at least not fully oxidized).
Therefore, it can be concluded that the Ca still primarily remains in its metallic
state.
Moreover, we have compared the effect of various metals (Ca, Mg, Al, and Ag)
on electron collection. The results indicate that Ca and Mg give comparable
photovoltaic effect. By comparison of inverted and normal devices, the
improvement of the efficiency for inverted devices is explained by the possible
origin from the favored vertical composition gradient of the P3HT:PCBM layer for
the inverted device. It was reported that the active layer of P3HT:PCBM fabricated
by spin-coating and post-annealing process usually has a composition with a
higher PCBM (electron acceptor) content close to the substrate and a higher P3HT
(electron donor) content close to the free (air) surface [ 73 , 95 ]. This vertical
composition gradient has an adverse effect on the photovoltaic performance of a
normal forward device using glass/ITO/PEDOT:PSS as the bottom anode. How-
ever, this composition gradient is favorable to our inverted device because a
P3HT-rich donor phase on the hole collecting side (MoO 3 /Au) and a PCBM-rich
acceptor phase on the electron collecting side (ITO/Ca) would reduce the charge
recombination at the electrodes [ 96 ]. In addition, the possible metal penetration or
damage to the active layer, caused by the deposition of the top metal contact [ 97 ],
can be prevented in the inverted structure due to the protection of MoO 3 layer.
It is worth mentioning that all devices were not encapsulated for the
measurement of air stability. The air stability of the inverted device is significantly