Visualizing the Vertical Energetic Landscape in Organic Photovoltaics

Abstract

Energy level diagrams in organic electronic devices play a crucial role in device performance and interpretation of device physics. In the case of organic solar cells, it has become routine to estimate the photovoltaic gap of the donor:acceptor blend using the energy values measured on the individual blend components, resulting in a poor agreement with the corresponding open-circuit voltage of the device. To address this issue, we developed a method that allows a direct visualisation of the vertical energetic landscape in the blend, obtained by combining ultraviolet photoemission spectroscopy and argon cluster etching. We investigate both model and high-performance photovoltaic systems and demonstrate that the resulting photovoltaic gaps are in close agreement with the measured CT energies and open-circuit voltages. Furthermore, we show that this method allows us to study the evolution of the energetic landscape upon environmental degradation, critically important for understanding degradation mechanisms and development of mitigation strategies.

Energy Level Alignment in Ternary Organic Solar Cells

“…In order to gain a more realistic picture of the vertical energetic landscape throughout ternary blend films, we applied high resolution UPS depth profiling, [44] which allows us to estimate the photovoltaic gap at each depth in both ternary systems for all mixing ratios. In short, this method combines UPS with gas cluster ion etching, allowing us to acquire a UPS spectrum for every depth of the active layer.…”

“…We note, that compared with other state-of-the-art systems, PBDB-T-2Cl:PC70BM:ITIC-2F-based ternaries exhibit very low recombination losses. [44] In addition to energetic depth profiles, UPS provides valuable information concerning the compositional distribution of materials in ternary blends. We compare the compositional profiles as probed by both XPS and UPS depth profiling in Fig.…”

“…UPS and XPS depth profiling was carried out with the MAGCIS dual mode ion source (ESCALAB 250Xi, Thermo Scientific) operated in the argon gas cluster ion beam mode, which has been shown to minimize etching induced material damage. [44,45] The cluster mode was used at an energy of 4 keV with "large" clusters (in the order of Ar2000) and an etch crater size of (2.5 x 2.5) mm 2 . To create a depth profile, XPS or UPS measurements were altered with cluster etch steps and a 10s break after etching to prevent charging.…”

“…As a result, experimental investigation of the correlation between the changes in the device open-circuit voltage and the evolution of energetics in ternary blends has not been performed to date. Herein, we apply our recently developed ultraviolet photoemission spectroscopy (UPS) depth profiling technique [44] to TOSC systems in order to probe their vertical energetic and compositional landscape. We al, [43] has been proposed to operate via electron cascade mechanism.…”

Tuning Spin Current Injection at Ferromagnet-Nonmagnet Interfaces by Molecular Design

“…In order to compare the electronic structures of the three derivatives of DNTT at the interface with Py, ultraviolet photoemission spectroscopy (UPS) depth profiling measurements were carried out on thin (∼40 nm) OSC films on Py following the methodology developed in Ref. [31] (see Sec. S5 of the Supplemental Material [17]).…”