Materials Design Considerations for Charge Generation in Organic Solar Cells

Abstract

This perspective 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 perspective we consider the lessons which can be drawn from these studies concerning the parameters which determine efficiency of this photo-induced 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, co-polymers 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, whilst 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 which can stabilise 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 summarising the materials design guidelines for efficient charge photogeneration which can be drawn from these studies.

Charge generation in polymer–fullerene bulk-heterojunction solar cells

“…67 This hot CT state theory was further supported by their subsequent work on other polymer:fullerene blends as well as polymer:perylene diimide blends ( Figure 6). [69][70][71][72][73][74] In addition to the above work, there are some other reports supporting the hot CT state theory. For example, the sub-bandgap CT excitation was reported to generate polarons which are more localized compared with those from the above-gap excitation, implying the importance of the excess thermal energy.…”

Charge Separation in Intermixed Polymer:PC70BM Photovoltaic Blends: Correlating Structural and Photophysical Length Scales as a Function of Blend Composition

“…25 Given that in most (but not all) blend films, polymer light absorption is responsible for most photocurrent generation, such intermixed domains will play a key role in photocurrent generation, as has been suggested in recent studies. 5,10,26 It is important to note that such short lengthscales start to approach the diameter of individual PC70BM molecules (~ 1 nm) and the monomer repeat unit length (and exciton wavefunction delocalisation length) of many donor polymers.…”

Cyclic Voltammetric, Chronocoulometric, and Spectroelectrochemical Studies of Electropolymerized Films Based on (3,4-Ethylenedioxythiophene)-Substituted 3,6-Dithiophen-2-yl-2,5-dihydropyrrole[3,4-c]pyrrole-1,4-dione

“…The thiophene derivative of DPP, DTDPP compound has also been used in the electronics industry to prepare new devices, including solar cells. [9][10][11][12] Because DTDPP cannot be electropolymerized, EDOT-DTDPP was designed and synthesized with EDOT covalently attached to the dye core. This design allows electropolymerization to occur via the pendant EDOT functionality while maintaining the highly conjugated DTDPP dye core.…”