Page - 9 - in Photovoltaic Materials and Electronic Devices

In order to further investigate the effects of additive on the charge carrier transport, the hole-only devices have been fabricated based on pristine PTB7 films and blend PTB7:PC71BM films with/without additives, respectively. High-work-function material gold (Au) is used as the cathode to block the back injection of electrons. The dark J-V curves of the hole-only devices with the configuration of ITO/PEDOT:PSS/PTB7/Au and ITO/PEDOT:PSS/PTB7:PC71BM/Au are measured and shown in Figure 4. The hole transport through the polymer film is limited due to the accumulation of space chargewhenasufficientvoltage isappliedto thishole-onlydevice. Thespacecharge limitedcurrent(SCLC) isdescribedbytheequation[29,30]: J“ 9 8 εµ V2 d3 “ 9 8 ε0εrµ V2 d3 (1) where ε0 is the permittivity of free space, εr is the dielectric constant of the blend material, µ is the hole mobility, V is the voltage drop across the device and d is the active layer thickness. The parameter εr is assumed to be 3, which is a typical value for conjugated polymers. The hole will be collected by the ITO electrode, which is very similar to hole transport process in the OPVs. The J-V characteristics of neat PTB7 and PTB7:PC71BM with/without additives of 3 v% DIO and 1 wt % PS fully agree with the SCLC model. Accordingto the J-Vcurves, theholecurrentdensityof thehole-onlydeviceswith 1 wt % PS is larger than that of the hole-only devices without any additive, the hole current density of the hole-only devices with 3 v% DIO is larger than that of thehole-onlydeviceswith1wt%PSandtheholecurrentdensityof thehole-only devices with additives of both 3 v% DIO and 1 wt % PS is larger than that of the hole-only devices with only 3 v% DIO. This means that hole carrier transport in the hole-onlydeviceswith3v%DIOand/or1wt%PShasbeenimprovedcompared with that of the hole-only devices without any additive. The result indicates that eventhoughthe thicknessof thefilmwithPSisgreater thanthatwithoutPS in the samefabricatecondition, theholemobilityof thedevicewithPSisbetter thanthat without PS, which shows PS can be good for hole carrier transport. Moreover it further demonstrates that the improved hole carrier transport could be one of the reasonfor the increased JscofOPVs[30,31]. 9