Cathode interfacial material (CIM) is critical to improving the power conversion efficiency (PCE) and long-term stability of an organic photovoltaic cell that utilizes a high work function cathode. In this contribution, a novel CIM is reported through an effective and yet simple combination of triarylphosphine oxide with a 1,10-phenanthrolinyl unit. The resulting CIM possesses easy synthesis and purification, a high T g of 116 °C and attractive electron-transport properties. The characterization of photovoltaic devices involving Ag or Al cathodes shows that this thermally deposited interlayer can considerably improve the PCE, due largely to a simultaneous increase in V oc and FF relative to the reference devices without a CIM. Notably, a PCE of 7.51% is obtained for the CIM/Ag device utilizing the active layer PTB7:PC71BM, which far exceeds that of the reference Ag device and compares well to that of the Ca/Al device. The PCE is further increased to 8.56% for the CIM/Al device (with J sc = 16.81 mA cm−2, V oc = 0.75 V, FF = 0.68). Ultraviolet photoemission spectroscopy studies reveal that this promising CIM can significantly lower the work function of the Ag metal as well as ITO and HOPG, and facilitate electron extraction in OPV devices.

Lending Triarylphosphine Oxide to Phenanthroline: a Facile Approach to High-Performance Organic Small-Molecule Cathode Interfacial Material for Organic Photovoltaics utilizing Air-Stable Cathodes

A novel polymer-solar-cell architecture using the conjugated polymer PFS as both the anode and cathode interlayers is constructed, and a high power conversion efficiency of 9.48% is achieved using the corresponding photovoltaic device.

A Bifunctional Interlayer Material for Modifying Both the Anode and Cathode in Highly Efficient Polymer Solar Cells

A series of star-shaped small molecular cathode interlayer materials are synthesized for PTB7:PC71BM based polymer solar cells (PSCs), comprising neutral amino groups, quaternary ammonium ions, amino N-oxides, and sulfobetaine ions as pendant polar functionalities, respectively. For the first time, the effect of these different pendant functional groups with or without mobile counterions on the cells' photovoltaic properties is investigated in detail. A large improvement in device performance is observed by inserting these cathode interfacial layers (CILs) between the PTB7:PC71BM active layer and the Al electrode. The CILs could effectively lower the work function of the Al cathode, increase the built-in potential, and decrease the series resistance of the related PSCs. poly(9,9-dioctylfluorene-co-N-[4-(3-methyl-propyl)]-diphenylamine) with pendant quaternary ammonium ions shows the best cathode modification ability, giving rise to the highest power conversion efficiency of 10.1%, even better than that of the typical poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] based device. The design strategy and structure–property relationships concluded in this work will be helpful to develop more efficient cathode interface materials for high-performance PSCs in the future.

Polymer Solar Cells Exceeding 10% Efficiency Enabled via a Facile Star-Shaped Molecular Cathode Interlayer with Variable Counterions

Small Molecule Interlayers in Organic Solar Cells

In-depth considerations for better polyelectrolytes as interfacial materials in polymer solar cells

We investigate solution-processed tetra-n-alkyl ammonium bromides (TAABs) as electron extraction layers (EELs) in bulk heterojunction (BHJ) solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). The EELs of TAABs lead to simultaneous increases in open-circuit voltage (VOC), short-circuit current density, and fill factor, enhancing the power conversion efficiency of BHJ solar cells from 2.38% to 4.02–4.19%. It is interesting that the same increase in VOC of about 0.14 V is obtained for devices with Al, Ag, and Au cathodes by using the EEL of TOAB. The self-assembly of TOAB into the lamellar structure stacked upright atop P3HT:PCBM is corroborated by synchrotron X-ray diffraction. The surface morphologies of TAABs atop P3HT:PCBM, explored by atomic force microscopy, show that there are ultrathin layers of TAABs in the regions beside large island clusters. The underlying mechanism is inferred that the TAAB molecules introduce anisotropic dipoles toward P3HT:PCBM which significantly elevate the vacuum level of the metal cathode for the efficient electron extraction. Our results provide a simple method to fabricate high performance BHJ solar cells by solution processing.

Enhanced performance of polymer solar cells using solution-processed tetra-n-alkyl ammonium bromides as electron extraction layers

We demonstrate the significance of ions with an ordered arrangement for enhancing the electron injection in polymer light-emitting diodes (PLEDs) and the electron extraction in polymer solar cells (PSCs) by using tetraoctylammonium bromide (TOAB) as a model compound. The spin-coated films of TOAB are prepared on the active layers of both PLEDs and PSCs without any annealing (TOAB-No) as well as with annealing at 60 °C (TOAB-60) and 100 °C (TOAB-100). Subsequently, they are investigated by atomic force microscopy (AFM) and synchrotron X-ray diffraction (XRD) as well as simultaneously employed as the interfacial layers in PLEDs and PSCs. The AFM results show that the active layers are well covered with TOAB-No and TOAB-100, but poorly covered with TOAB-60 due to the formation of large islands. The XRD results show that TOAB-No and TOAB-60 are polycrystalline with the (002) lattice planes parallel to the surface of the active layers while TOAB-100 is amorphous. Of note, the electron injection in PLEDs and the electron extraction in PSCs are significantly enhanced using TOAB-No and TOAB-60, but both are worsened using TOAB-100. Evidently the enhanced electron injection/extraction is mainly attributed to the ordered arrangement of ions in the TOAB crystals. This study indicates that with an ordered arrangement of ions, all ionic compounds should be excellent for enhancing the electron injection/extraction in polymer optoelectronic devices.

Chih Yun Chin

Papers

Enhanced performance of polymer solar cells using solution-processed tetra-n-alkyl ammonium bromides as electron extraction layers

Significance of ions with an ordered arrangement for enhancing the electron injection/extraction in polymer optoelectronic devices