Jessica A. Alexander

Measurement of optical properties in organic photovoltaic materials using monochromated electron energy-loss spectroscopy

The optical and electronic properties of organic materials influence the functionality of all organic electronics. These properties can be measured in bulk materials easily, but determining the opto-electronic properties in thin films and at interfaces is challenging. In this report we describe how these properties can be measured with high spatial resolution using an optimized electron energy-loss spectroscopy (EELS) method in the scanning transmission electron microscope (STEM). EELS spectra were collected for poly(3-hexylthiophene) (P3HT), [6,6]phenyl-C61 butyric acid methyl ester (PCBM), copper phthalocyanine (CuPc), and C60. These organic materials are both susceptible to electron beam damage and commonly utilized in organic photovoltaics (OPVs). In order to prove that these spectra are representative of the pure materials and that the samples have not undergone any significant beam damage, the real and imaginary parts of the complex dielectric function obtained from these spectra have been compared to the same functions obtained using variable angle spectroscopic ellipsometry (VASE), a technique that should not induce any beam damage to the samples. Comparisons of these two data sets reveal good agreement in both measured peak intensities and their corresponding peak energies, thus validating this low-damage EELS acquisition method. EELS spectrum images were acquired from a CuPc/C60 bilayer structure to demonstrate that it is possible to collect spatially resolved EELS data from device-related structures comprised of these beam-sensitive materials.

Determining Optical Absorption Coefficients in Beam Sensitive Materials using Monochromated Electron Energy-Loss Spectroscopy

Using electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM), it is possible to measure optoelectronic properties of materials with high spatial and spectral resolutions. In particular, low-loss spectra (energy-losses of less than 50 eV) can be used to extract the real (ε1) and imaginary (ε2) parts of the complex dielectric function [1] over this entire range, which encompasses ultraviolet, visible, and infrared wavelengths. Since ε1 and ε2 are related to the refractive index (n) and the extinction coefficient (κ), both n and κ can also be derived over this entire range with high spatial and high energy resolutions. This makes STEMEELS a very powerful technique for probing the optoelectronic properties of materials.

Monochromated Electron Energy-Loss Spectroscopy of Lead-Free Halide Perovskite Semiconductors

Over the last few years, the performance of perovskite solar cells has improved considerably with efficiencies greater than 22% reported [1]. While this rapid rise in efficiency make them an extremely promising alternative to traditional silicon solar cells, perovskite solar cells suffer from stability issues in light and air. There are also ecological and toxicity concerns as the perovskites used in these devices contain lead. To address the latter toxicity concern, lead-free halide perovskite semiconductors have been developed recently [2]. These lead-free halide perovskites have similar band gaps and reflectance behavior as their lead-containing counterparts and could, ultimately, provide a safer alternative to the perovskite materials currently utilized in solar cell devices [2]. Knowledge of their optoelectronic properties is critical to their further development and utilization in actual solar cell devices.

Optimized Damage-Reduction 60 keV Monochromated Electron Energy-Loss Spectroscopy Measurements of Optical Properties at the Donor/Acceptor Interface in Organic Photovoltaic Devices

The key to improving the performance of OPVs is to understand the donor/acceptor interface within the device. The two primary areas of understanding that correlate to this interface that can affect the performance of the device are the morphology of the interface and the local electronic structure at the interface [1]. Multiple groups using different characterization techniques have extensively studied the morphology of the interface [2, 3, 4]. However, the local electronic structure at the interface has largely been ignored as these measurements require the use of electron energy-loss spectroscopy (EELS) to measure these optoelectronic properties as the transmission electron microscope is the only instrument that results in the high spatial resolution necessary to probe the interface [5].

Monochromated Electron Energy-Loss Spectroscopy of Organic Photovoltaics

Monochromated electron energy-loss spectroscopy (EELS) is opening up new opportunities for the study of the electronic structure in complex materials. The prospect of mapping band structure with high spatial and energy resolution is an exciting prospect. A particularly challenging task is to realise this in organic materials such as polymer matrix composites, biomaterials and organic electronic materials. For example, the processes that generate current in organic photovoltaics (OPVs) are highly dependent on the microand nano-structure of the devices, especially at the donor-acceptor (D-A) interface. Hence, the structure of this interface is vital to understanding the efficiency of devices, as this knowledge will provide a foundation for the engineering of new OPV devices with improved power conversion efficiency. Scanning transmission electron microscopy (STEM) EELS can be used to probe the nature and structure of interfaces in OPV devices because it is possible to obtain high energy resolution measurements over large ranges of energy-loss (ΔE).

Variable Angle Spectroscopic Ellipsometry and Electron Energy-Loss Spectroscopy

Electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) has considerable potential for investigation of interfaces in organic photovoltaic (OPV) devices. In particular, the low-loss region of the EEL spectrum can be used to obtain the complex dielectric function of the material. The complex dielectric function in turn allows us to distinguish between single electron transitions and collective excitations. Spatial mapping of the single electron transitions can then be used to learn about the chemistry and bonding in the vicinity of interfaces between the acceptor and donor interfaces in the OPV samples [1].

Electron Energy-Loss Spectroscopy of Organic Photovoltaics

Advances in organic photovoltaic (OPV) based solar cell device technology have increased power conversion efficiencies (PCE) beyond 11% [1], pushing flexible architecture OPV devices closer to being a viable low-cost, environmentally friendly alternative to contemporary inorganic based solar cells [2, 3]. Extending OPV performance beyond this limit is a critical challenge that requires better understanding of the PCE limiting processes. Since only photo-generated excitons that diffuse to the interface between electron donor and acceptor materials can dissociate into holes and electrons, understanding the chemistry and molecular structure of this interface is critical to identifying and mitigating these limitations. Other factors such as the amount of light absorption, efficiency of photogeneration of electrons and holes, and their collection efficiency at the respective electrodes must also be optimized in order to improve the device PCE. Electron energy-loss spectroscopy (EELS) is an extremely useful tool that can be used to probe the nature and structure of these interfaces and further the understanding of processes that occur there.

Investigation of the Use of Stereo-Pair Data Sets in Electron Tomography Characterization of Organic-Based Solar Cells

The processes that generate current in organic photovoltaics (OPVs) are highly dependent on the micro-and nano-structure of the devices, especially at the donor-acceptor (D-A) interface. Light trapping strategies have been proposed to tailor absorption of incident sunlight and generate more photocurrent at the D-A interface. Recent studies have reported the use of a range of plasmonic nanostructures, such as nanoparticles, slit arrays and nanohole arrays to improve the power conversion efficiency (PCE) in OPVs [1, 2]. While incorporation of plasmonic nanostructures for light trapping in thin-film PV cells is an attractive solution for enhancement of the optical absorption and current density in an OPV without increasing the thickness of its active layers, little is known about the detailed structure, chemistry and bonding between the active layer and the plasmonic nanostructures. The understanding of this interface is vital to understanding why these nanoparticles improve the efficiency of such devices. This knowledge will provide a foundation for the engineering of new OPV devices with improved PCE.