Uncovering Buried Structure and Interfaces in Molecular Photovoltaics

Abstract

The processes that generate current in organic photovoltaics are highly dependent on the micro‐ and nano‐structure in the semiconductor layers, especially at the donor‐acceptor interface. Elucidating film properties throughout the thickness of the devices is therefore key to their further development. Here, a methodology is developed to gain unprecedented insights into the structure and composition of the molecular layers within the depth of device structure using high resolution transmission electron microscopy (HRTEM). The technique was applied to three archetypical solar cell configurations consisting of copper phthalocyanine (CuPc) and C60, which have been cross‐sectioned using a focused ion beam method optimized to minimize sample damage. The HRTEM images exhibit lattice fringes in both CuPc and C60, confirming the crystallinity and texture of both materials, and offering novel insight into the growth of C60 onto molecular materials. The donor‐acceptor interface morphology is further studied using scanning transmission electron microscopy (STEM) in combination with energy dispersive X‐ray (EDX) spectroscopy, extending the scope of our methodology to amorphous heterostructures.

Molecular Order in Buried Layers of TbPc2 Single‐Molecule Magnets Detected by Torque Magnetometry

“…[4b] Efforts have been made to develop lab-based techniques to access the orientation and aggregation of Pc using electron paramagnetic resonance, [11a] but again spatial selectivity is lacking, while local probes such as transmission electron microscopy provide detailed information but require extensive and aggressive sample preparation. [16] If the investigated molecule, or one isostructural analogue, possesses magnetic anisotropy, the orientation can be detected by measuring the torque exerted by an external magnetic field.…”

Why Inverted Small Molecule Solar Cells Outperform Their Noninverted Counterparts

“…This result is in agreement with published TEM measurements on related material systems, which do not indicate phase separation when deposited at room temperature, e.g., ZnPc:C 60 blends, [ 23 ] as well as cross-sections of CuPc:C 60 blends deposited on pristine CuPc. [ 24 ] However, the ZL images change dramatically when the BHJ is deposited on F 4 ZnPc at an elevated substrate temperature (Figure 2 b). The blend exhibits pronounced round, dark structures already in the ZL image.…”

Analytical transmission electron microscopy at organic interfaces

“…Mapping of different domains by core-loss EELS or EDX does not necessarily require the presence of an element in one phase but not the other. This was demonstrated in a study of copper phthalocyanine (CuPc)/C 60 bilayers by Gilchrist et al [47]. As C 60 contains a greater concentration of carbon compared to CuPc, differing carbon mass percentages were successfully used to distinguish phases through STEM-EDX spectrum imaging, and also to determine the location and roughness of the interface [47].…”

“…It is evident that these imaging conditions produce completely dissimilar contrast. The analytical TEM techniques which have been employed to enhance contrast in OPV systems include elemental mapping by EDX [47,48] or core-loss EELS [48,49], mapping variations in the plasmon peak position [42,44,45,[50][51][52][53][54] as well as single electron excitations in the low-loss region of the EEL spectrum [45]. …”