Heterogeneity in Polymer Solar Cells: Local Morphology and Performance in Organic Photovoltaics Studied with Scanning Probe Microscopy

Abstract

The use of organic photovoltaics (OPVs) could reduce production costs for solar cells because these materials are solution processable and can be manufactured by roll-to-roll printing. The nanoscale texture, or film morphology, of the donor/acceptor blends used in most OPVs is a critical variable that can dominate both the performance of new materials being optimized in the lab and efforts to move from laboratory-scale to factory-scale production. Although efficiencies of organic solar cells have improved significantly in recent years, progress in morphology optimization still occurs largely by trial and error, in part because much of our basic understanding of how nanoscale morphology affects the optoelectronic properties of these heterogeneous organic semiconductor films has to be inferred indirectly from macroscopic measurements. In this Account, we review the importance of nanoscale morphology in organic semiconductors and the use of electrical scanning probe microscopy techniques to directly probe the local optoelectronic properties of OPV devices. We have observed local heterogeneity of electronic properties and performance in a wide range of systems, including model polymer-fullerene blends such as poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM), newer polyfluorene copolymer-PCBM blends, and even all polymer donor-acceptor blends. The observed heterogeneity in local photocurrent poses important questions, chiefly what information is contained and what is lost when using average values obtained from conventional measurements on macroscopic devices and bulk samples? We show that in many cases OPVs are best thought of as a collection of nanoscopic photodiodes connected in parallel, each with their own morphological and therefore electronic and optical properties. This local heterogeneity forces us to carefully consider the adequacy of describing OPVs solely by "average" properties such as the bulk carrier mobility. Characterizing this local heterogeneity in the morphology of an OPV and the consequent variations in local performance is vital to understanding OPV operation.

Theoretical estimation of the rate of photoinduced charge transfer reactions in triphenylamine C60 donor–acceptor conjugate

“…It has been extensively studied experimentally and theoretically. [12,[15][16][17][18][19][20][21] For instance, rate constants (kct, subscript ct stands for charge transfer; it can be turned into subscripts cs and cr standing for charge separation and charge recombination, respectively) for photoinduced charge transfer (CT) reactions occurring in P3HT-PCBM reveal the potential of such materials in the construction of efficient DSSCs. Based on quantum chemistry methods, these kct have been demonstrated [22] to be tunable by a few parameters so that the power conversion efficiency may be increased; [23][24][25] which highlights, in fact, the importance of theoretical developments for the understanding and design of DSSCs with improved efficiency.…”

Structure‐Property Optimizations in Donor Polymers via Electronics, Substituents, and Side Chains Toward High Efficiency Solar Cells

“…These reviews have covered various aspects of this interdisciplinary research fi eld, such as design of polymers, [26][27][28] device physics, [ 29,30 ] physical chemistry, [ 31,32 ] morphology control, [33][34][35][36][37][38][39] and stability/economics. [ 40,41 ] Rather than contributing another comprehensive review, we attempt to direct the readers' attention to the latest advances in the design of new polymeric materials for BHJ solar cells.…”

Multi-Scale Modeling of Bulk Heterojunctions for Organic Photovoltaic Applications

“…Electron microscopy can image domain sizes, but suffers from poor signal-to-noise due to very little contrast between P3HT and PCBM (Andersson et al, 2009;Oosterhout et al, 2009;van Bavel et al, 2010;2008;Yang et al, 2005). Scanning probe techniques mostly give information about the top surface, though some charge transport information can be derived from specialized scanning probe techniques (Groves et al, 2010;Pingree et al, 2009;Reid et al, 2008). The vertical segregation of materials can be determined non-destructively using refraction techniques (including ellipsometry (Campoy-Quiles et al, 2008;Madsen et al, 2011), X-ray (Andersen, 2009;Germack et al, 2010), and neutron scattering (Lee et al, 2010;Mitchell et al, 2004)).…”