Determining the optimum morphology in high-performance polymer-fullerene organic photovoltaic cells

This work was supported by the Engineering and Physical Sciences Research Council (grant number EP/I013288/1) and from the European Union Seventh Framework Programme under grant agreement 321305.The morphology of bulk heterojunction organic photovoltaic cells defines many of the device performance characteristics. Measuring the morphology is challenging due to the small length scales and low contrast between organic materials. Here we have utilised nanoscale photocurrent mapping, ultrafast fluorescence and exciton diffusion to observe the detailed morphology of a high performance blend. We show that optimised blends consist of elongated fullerene-rich and polymer-rich fibre-like domains which are 10-50 nm wide and 200-400 nm long. These elongated domains provide a concentration gradient for directional charge diffusion which helps extraction of charge pairs with 80% efficiency. In contrast, blends with agglomerated fullerene spheres show a much lower efficiency of charge extraction of ~45% which is attributed to poor electron and hole transport. Our results show that formation of narrow and elongated domains are desirable in bulk heterojunction solar cells.Publisher PDFPeer reviewe

Additive-assisted supramolecular manipulation of polymer : fullerene blend phase morphologies and its influence on photophysical processes

It is well known that even small variations in the solid-state microstructure of polymer:fullerene bulk heterojunctions can drastically change their organic solar cell device performance. We employ pBTTT:PC₆₁BM as a model system and manipulate co-crystal formation of 1 : 1 (by weight) blends with the assistance of fatty acid methyl esters as additives. This allows us to evaluate the role of the intermixed phase in such binary blends through manipulation of their phase morphology—from fully intercalated to partially and predominantly non-intercalated systems—and its effect on the exciton- and carrier- dynamics and the efficiency of charge collection, with relevance for future device design and manufacturing.10 page(s

Materials Design Considerations for Charge Generation in Organic Solar Cells

This article reviews some of our recent progress on materials design guidelines for photoinduced charge generation in bulk-heterojunction organic solar cells. Over the last 7 years, our group has employed transient absorption measurement to determine the relative quantum yields of long-lived polaron pairs for over 300 different organic Donor/Acceptor blend films. We have shown that this optical assay of charge separation can be a strong indicator of photocurrent generation efficiency in complete devices. In this review, we consider the lessons that can be drawn from these studies concerning the parameters that determine efficiency of this photoinduced charge separation in such solar cells. We consistently find, from studies of several materials series, that the energy offset driving charge separation is a key determinant of the efficiency of this charge generation, and thereby photocurrent generation. Moreover, we find that the magnitude of the energy offset required to drive charge separation, and the strength of this energetic dependence, varies substantially between materials classes. In particular, copolymers such as diketopyrrolopyrrole- and thiazolothiazole-based polymers are found to be capable of driving charge separation in blends with PCBM at much lower energy offsets than polythiophenes, such as P3HT, while replacement of PCBM with more crystalline perylene diimide acceptors is also observed to reduce the energy offset requirement for charge separation. We go on to discuss the role of film microstructure in also determining the efficiency of charge separation, including the role of mixed and pure domains, PCBM exciton diffusion limitations and the role of material crystallinity in modulating material energetics, thereby providing additional energy offsets that can stabilize the spatial separation of charges. Other factors considered include the role of Coulombically bound polaron pair or charge transfer states, device electric fields, charge carrier mobilities, triplet excitons, and photon energy. We discuss briefly a model for charge separation consistent with these and other observations. We conclude by summarizing the materials design guidelines for efficient charge photogeneration that can be drawn from these studies