The work function and surface property of ZnO can be simply tuned by the self-assembled monolayer (SAM) molecules derived from benzoic acid such as 4-methoxybenzoic acid (MBA), 4-tert-butylbenzoic acid (BBA), and 4-fluorobenzoic acid (FBA), which have different dipole orientation and magnitude. MBA, BBA, and FBA treated ZnO layers were used as an electron injection/transporting layer for inverted type polymer solar cells (PSCs) with a structure of ITO/SAM treated ZnO/active layer (P3HT:PC61BM)/MoO3/Ag. The power conversion efficiency (PCE) of PSCs based on MBA and BBA treated ZnO reaches 3.34 and 2.94%, respectively, while the PCE of the device based on untreated ZnO is 2.47%. In contrary, the PCE of the device with FBA treated ZnO is 1.81%. The open circuit voltage (V oc) of the device with MBA, BBA, and FBA treated ZnO is 0.63 and 0.62 V, respectively, while the V oc of PSC with untreated ZnO is 0.60 V. Contrarily, the V oc of the device with FBA treated ZnO is 0.53 V. The PCE and V oc of PSCs based on MBA and BBA treated ZnO are better than those of the other devices. This seems to be related with the direction of dipole moment of benzoic acid derivatives. Also, the morphology of the active layer seems to be affected by the substituent on the 4-position of benzoic acid. The active layer on MBA treated ZnO shows optimized morphology, and its device shows the best performances. We demonstrate that the work function and morphology of the active layer can be controlled by SAM treatment of the ZnO surface with different dipole orientation and a substituent on the 4-position of benzoic acid. These are very simple and effective methods for improving the performances of PSCs. The results provide an alternative strategy to improve the interface property between inorganic and organic materials in organic electronic devices.

A nonconjugated anionic polyelectrolyte, poly(sodium 4-styrenesulfonate) (PSS-Na), was applied to the optoelectronic devices as an interfacial layer (IFL) at the semiconducting layer/cathode interface. The ultraviolet photoelectron spectroscopy and the Kelvin probe microscopy studies support the formation of a favorable interface dipole at the organic/cathode interface. For polymer light-emitting diodes (PLEDs), the maximum luminance efficiency (LEmax) and the turn-on voltage (Von) of the device with a layer of PSS-Na spin-coated from the concentration of 0.5 mg/mL were 3.00 cd/A and 5.5 V, which are dramatically improved than those of the device without an IFL (LEmax = 0.316 cd/A, Von = 9.5 V). This suggests that the PSS-Na film at the emissive layer/cathode interface improves the electron injection ability. As for polymer solar cells (PSCs), the power conversion efficiency (PCE) of the device with a layer of PSS-Na spin-coated from the concentration of 0.5 mg/mL was 2.83%, which is a 16% increase compared to that of the PSC without PSS-Na. The PCE improvement is mainly due to the enhancement of the short-circuit current (12% increase). The results support that the electron collection and transporting increase by the introduction of the PSS-Na film at the photoactive layer/cathode interface. The improvement of the efficiency of the PLED and PSC is due to the reduction of the Schottky barrier by the formation of a favorable interface as well as the better Ohmic contact at the cathode interface.

Polyviologen (PV) derivatives are known materials used for adjusting the work function (WF) of cathodes by reducing the electron injection/collection barrier at the cathode interface. To tune and improve device performance, we introduce different types of counteranions (CAs), such as bromide, tetrafluoroborate, and tetraphenylborate, to a PV derivative. The effective WF of the Al cathode is shown to depend on the size of the CA, indicating that a Schottky barrier can be modulated by the size of the CA. Through the increased size of the CA from bromide to tetraphenylborate, the effective WF of the Al cathode is gradually decreased, indicating a decreased Schottky barrier at the cathode interface. In addition, the change of the power conversion efficiency (PCE) and the short circuit current (Jsc) value show good correlation with the change of the WF of the cathode, signifying the typical transition from a Schottky to an Ohmic contact. The turn-on electric field of the electron-only device without PV was 0.21 MV/cm, which is dramatically higher than those of devices with PV-X (0.07 MV/cm for PV-Br, 0.06 MV/cm for PV-BF4, and 0.05 MV/cm for PV-BPh4) This is also coincident with a decrease in the Schottky barrier at the cathode interface. The device ITO/PEDOT/P3HT:PCBM/PV/Al, with a thin layer of PV derivative and tetraphenylborate CA as the cathode buffer layer, has the highest PCE of 4.02%, an open circuit voltage of 0.64 V, a Jsc of 11.6 mA/cm2, and a fill factor of 53.0%. Our results show that it is possible to improve the performance of polymer solar cells by choosing different types of CAs in PV derivatives without complicated synthesis and to refine the electron injection/collection barrier height at the cathode interface.

A poly(vinyl alcohol) is applied to organic optoelectronic devices as a cathode buffer layer to improve the performances.

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