Elucidating the role of current injection on the influence of open-circuit voltage in small-molecule organic photovoltaic devices: From the aspects of charge transfer and electroluminescent spectrum

•The change in VOC is investigated using temperature dependent electrical measurement and EL spectra.•The device performance of organic photovoltaic devices can be dominated by charge transfer states.•The property of charge transfer state extracted from the EQE and EL reduced spectra.•The calculation of barrier height at the rubrene/C60 interface yields the prediction of Voc.•The nonradiative recombination loss is highly correlated with the injected current of device.

We demonstrate that the open-circuit voltage (VOC) of organic photovoltaic (OPV) devices composed of rubrene and C60 can be considerably different when the anode and active layer are changed. Two types of anodes and active layers were compared. In plasma-treated indium-tin-oxide (ITO) OPV devices, the parameter VOC exhibits an improvement from 0.68 V to 0.76 V when the device structure is varied from a bilayer to a mixed structure. However, in the OPV devices that use ITO/MoO3 as the anode, a similar VOC is observed regardless of the device structure. A series of temperature-dependent measurements are conducted to investigate these results. The calculation of barrier height at the rubrene/C60 (or rubrene:C60) interface yields the prediction of VOC, suggesting that an excess energetic loss occurs in the mixed structures. The electroluminescent (EL) spectra of these devices show that the mixed structure can completely quench the EL of rubrene single layer. A broad band of the charge transfer (CT) emission is observed clearly. A temperature-dependent measurement for the extracting injection barrier is conducted and shows that the mixed structure is favorable for the hole current injection. The CT properties are obtained using the external quantum efficiency and EL spectra of the OPV devices. We find that the nonradiative recombination loss is highly correlated with the injected current; the lower the injection barrier induced the less the nonradiative recombination loss. Therefore, the parameter VOC can be improved when the injected current is increased.

Graphical abstractWe relate the nonradiative recombination loss and open-circuit voltage to the rubrene/C60 device's injected current using the charge transfer states and electroluminescent spectra analyses.Figure optionsDownload full-size imageDownload as PowerPoint slide