Colocalized Nanoscale Electrical and Compositional Mapping of Organic Solar Cells

Abstract

Organic photovoltaics based on the bulk heterojunction is an emergent technology with the potential to enable low-cost, lightweight, and mechanically flexible energy conversion applications. Organic photovoltaic performance is intimately linked to the heterogeneous nanoscale structuring of the active layer. Means of directly assessing the interplay between local nanoscale structure and local nanoscale function are lacking, however. This work combines the complementary strengths of energy-filtered transmission electron microscopy and conductive atomic force microscopy to perform colocalized nanoscale measurements of bulk heterojunction chemical composition and charge carrier mobility in a high-performance small molecule organic photovoltaic system. We find that the nanoscale donor concentration and hole mobility maps are uncorrelated, unlike device-scale measurements that show a strong dependence of hole mobility on donor concentration. These results challenge standard interpretations of nanoscale structural and electrical maps, e.g., that a high local donor concentration implies a high local hole mobility and vice versa. Our results demonstrate instead that factors such as local phase continuity have a greater impact on charge transport than the local amount of each phase. These results also support an emerging picture for small molecule bulk heterojunctions in which electrical connectivity within finely mixed domains plays a decisive role in charge migration. The devised colocalized approach can be generalized to a broad range of transmission electron microscope and atomic force microscope modes, opening vast opportunities for nanoscale structure−function mapping of materials.

Probing the Contribution of Lateral Pathways to Out‐of‐Plane Charge Transport in Organic Bulk Heterojunctions

“…Therefore, in the current images, bright regions correspond to high hole current in well‐connected donor domains. [ 42 ] The hole current shows a strong dependence on the degree of phase separation. At 0 vol% DIO (Figure 2a), the average hole current is 51.6 pA, and small conductive spots can be seen, which span from tens to hundreds of nanometers in size.…”

Elucidating the Role of Ion Migration and Band Bending in Perovskite Solar Cell Function at Grain Boundaries via Multimodal Nanoscale Mapping

“…With this approach, current-voltage (I − V) curves are recorded at an array of pixel locations. [23][24][25] At each pixel, the probe approaches the sample surface, measures an I − V curve under a constant probe-sample force, then retracts from the surface before moving to another pixel location. In this way, PPIV mapping produces a rich dataset that can be mined to simultaneously map a wide range of photovoltaic metrics at the nanoscale, including short-circuit current (I sc ), open-circuit voltage (V oc ), series resistance (R s ), shunt resistance (R sh ), and maximum power (P max ).Although not yet applied to perovskite systems, this method is promising for capturing a comprehensive account of local photovoltaic function in perovskite active layers.Owing to the soft lattice structure of methylammonium lead halide perovskites, ion mobility has been observed to impact perovskite solar cell function and is likely to play a role in the local photovoltaic properties at GBs.…”

“…With this approach, current-voltage (I − V) curves are recorded at an array of pixel locations. [23][24][25] At each pixel, the probe approaches the sample surface, measures an I − V curve under a constant probe-sample force, then retracts from the surface before moving to another pixel location. In this way, PPIV mapping produces a rich dataset that can be mined to simultaneously map a wide range of photovoltaic metrics at the nanoscale, including short-circuit current (I sc ), open-circuit voltage (V oc ), series resistance (R s ), shunt resistance (R sh ), and maximum power (P max ).…”

“…This work shows the potential of PPIV and ionic strain mapping for elucidating nanoscale function in perovskite thin films. In combination with complementary nanoscale measurement tools, [22,24,43] these approaches hold promise for revealing the nanoscale origins of localized photovoltaic performance in emerging perovskite materials.…”

Correlating Nanoscale Optical Coherence Length and Microscale Topography in Organic Materials by Coherent Two-Dimensional Microspectroscopy

“…In addition, whereas the above-mentioned methods provide information on the nanoscale topography, photocurrent, lattice fringes, chemical composition, and 3D tomography in organic photovoltaics, − they normally cannot measure the optical coherence length. In general, the optical coherence length refers to the number of stacked molecules within a crystalline aggregate across which the excited-state wave function is delocalized, that is, coherently distributed with a fixed phase relation between adjacent entities .…”