Finding the optimal morphology of novel organic photovoltaic (OPV) polymer blends is a major obstacle slowing the development of more efficient OPV devices. With a focus on accelerating the systematic morphology optimisation process, we demonstrate a technique offering rapid high-resolution, 3-dimensional blend morphology analysis in the scanning electron microscope. This backscattered electron imaging technique is used to investigate the morphological features and lengthscales defining the promising PffBT4T-2OD:PC70BM blend system and show how its photovoltaic performance is related to the nature of its phase separation. Low-voltage backscattered electron imaging can be used to probe for structure and domain stacking through the thickness of the film, as well as imaging surface morphology with highly competitive spatial resolution. For reference, we compare our results with equivalent images of the widely studied P3HT:PC60BM blend system. Our results also demonstrate that backscattered electron imaging offers significant advantages over conventional cross-sectional imaging techniques, and show that it enables a fast, systematic approach to control 3-dimensional active layer morphology in polymer:fullerene blends

Detection of signal electrons belongs amongst the key parameters of the Scanning Electron Microscope (SEM). The traditional approach is the ETD detector for secondary electrons (SE) and a below-the-lens detector for backscattered electrons (BSE). State-of-the-art SEMs can be equipped with up to three inlens detectors capable of collecting both the SE and BSE signal. Moreover, the possibility to sort electrons according to their energies and/or emission angles is becoming common. Such a selective detection is usually done by influencing the SE or BSE trajectories by electric or magnetic fields. This paper introduces compound-lens-controlled energy selective detection of BSE on a new FEI SEM.The compound final lens combines the magnetic final lens inside the pole piece (ML1), the immersion magnetic lens (ML2) and the electrostatic lens (EL1) formed by the potential at the T1 detector (Fig. 1a). Main function of the compound final lens is to focus the primary electron beam to the sample. However, independent control of these three lenses enables us to use ML1 and EL1 to focus the primary beam and the ML2 to affect the trajectories of signal electrons. The ML2 behaves as a chromatic sensitive lens which focuses the high-loss BSE into the aperture of the annular T1 detector. The low-loss BSE are less sensitive to the ML2 magnetic field and reach the T1 detector (Fig. 1b). SE also pass through the aperture in the T1 and are collected further in the column by the T2 and T3 detectors.The main goal of the energy selective BSE detection is the enhancement of the compositional contrast. The BSE signal typically contains both the high-and the low-loss fractions. The BSE coefficient (η) is little dependent on the atomic number (Z) for the high-loss BSE, while it is strongly dependent in the case of the low-loss BSE. Therefore, in order to enhance the compositional contrast, low-loss BSE need to be selectively detected. This is exactly the mechanism behind the contrast enhancement with the compound lens. Besides the BSE filtration, the magnetic field of the ML2 decreases the diameter of the primary beam and thus improves the resolution. This enables us to acquire compositional contrast images in high resolution, such as the Pd nano-particles on the CeO2 matrix in Fig. 2a [2]. Another important improvement is the collection efficiency of the T1 BSE detector. Thanks to its design and position, the T1 detector covers a large BSE emission angle and provides high signal to noise ratio even at low probe currents. Naturally, BSE filtering cuts off a significant part of the T1 signal. However, the T1 detector keeps providing low noise images at probe currents down to 25 pA and less. The possibility to work at low probe currents together with the energy selective BSE detection enables charge-free imaging of non-conductive samples. The high-loss BSE carrying the charge information are filtered out so they don't contribute to the T1 detector image. Collection of low-loss BSE at probe current of 25pA produces charge free images of insul...

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The quality of an image is determined by both resolution and contrast: the size of the features we can see and the type of information that the image gives about them. Recent advances in scanning electron microscopy have improved both, for example using magnetic immersion or electrostatic lenses for improved resolution, or by using in-lens, angle-sensitive detection for tunable contrast. Here, we introduce a new FEI SEM that presents a further improvement both in resolution and in contrast using a new compound electrostatic-magnetic final lens.The SEM makes use of a combination of a magnetic final lens in the pole piece, a magnetic immersion lens and an electrostatic lens formed by the potential in the bottom of the column. The combination of these lenses focuses the primary electron beam to a very tight spot. The resolution of the system is specified as 1.0 nm at 1 kV. Figure 1 demonstrates the performance at 500 V landing energy.The contrast performance of the system benefits from the in-lens backscatter detector (T1) located at a position close to the sample. The T1 detector receives a high signal intensity due to its position. This enables ultra-low beam current BSE imaging, as is shown in Figure 2. This is important for beam sensitive samples, such as polymers, porous materials or other fragile samples, which require the maximum amount of signal to be acquired in the shortest amount of time with the smallest dose possible. Furthermore, with the T1 detector BSE imaging is possible even during TV-rate navigation, so that materials contrast is always available, even when working at short working distance or with a tilted sample. Combined with the new compound final lens, the T1 detector is capable of acquiring an energyfiltered BSE image. Tuning of the lens strength allows for selective detection of low-loss BSEs. This enables precise materials contrast on the smallest particles, as also evidenced in Figure 2. Moreover, this energy selection works as an effective charge filter, allowing the acquisition of charge-free images on insulating samples. A low vacuum mode with chamber pressure up to 500 Pa completes the capabilities on charging samples, essential for analytical measurements on uncoated insulators.The information that can be acquired from the sample is further increased by the use of detector segmentation. This is true for the T1 detector, which has two segments, but also for the retractable below-the-lens backscatter detector DBS, which has both 4 angular or 4 annular segments, which can be read out in any combination by selecting the desired mode in the software.In conclusion, we present a new versatile SEM that brings a range of technologies including a compound final lens, BSE filtering, and segmented detection all into one tool. The system delivers the resolution and contrast that allows materials researchers to capture the maximum amount of information from their sample, with the right detail, with the least amount of work.