Visualizing charge separation in bulk heterojunction organic solar cells

Abstract

Solar cells based on conjugated polymer and fullerene blends have been developed as a low-cost alternative to silicon. For efficient solar cells, electron-hole pairs must separate into free mobile charges that can be extracted in high yield. We still lack good understanding of how, why and when carriers separate against the Coulomb attraction. Here we visualize the charge separation process in bulk heterojunction solar cells by directly measuring charge carrier drift in a polymer:fullerene blend with ultrafast time resolution. We show that initially only closely separated (o1 nm) charge pairs are created and they separate by several nanometres during the first several picoseconds. Charge pairs overcome Coulomb attraction and form free carriers on a subnanosecond time scale. Numerical simulations complementing the experimental data show that fast three-dimensional charge diffusion within an energetically disordered medium, increasing the entropy of the system, is sufficient to drive the charge separation process.

Delocalization and dielectric screening of charge transfer states in organic photovoltaic cells

“…Moreover, in the presence of sufficiently sized C 60 crystallites, CT delocalization is not dependent on excess driving energy from the initial CT reaction as it is observed here under resonant EA excitation 28,29 . The impact of fullerene nanocrystallinity on charge separation has been noted previously, where it was attributed to high local electron mobility within the traditional Onsager framework 11,30 as well as to shifts in electron affinity of the fullerene crystallites 31 . In contrast, our results here ARTICLE point to the role of delocalization, analogous to recent findings for organic-inorganic HJs 32 , and are consistent with a growing number of recent reports in which charge separation is driven by accessing band-like CT states that subsequently decay on a sub-50 ps timescale (an upper limit based on the resolution of our PL transient data) into separated, free charge carriers 3,4,29,33 .…”

A Close Look at Charge Generation in Polymer:Fullerene Blends with Microstructure Control

“…If a Stark effect is caused by those charges, an EA contribution is seen in the TA data. In contrast to the case where a uniform electric field is externally applied across the bulk of the film using electrodes, 43,65,66 the Stark effect in the TA data depends on the local radial electric fields around free charges and the local electric dipoles around electron−hole pairs. It can be observed even if the overall macroscopic field cancels due to a random orientation of the electron−hole pairs in the BHJ (in particular for a quadratic dependence on the local field magnitude).…”

Morphology, Temperature, and Field Dependence of Charge Separation in High-Efficiency Solar Cells Based on Alternating Polyquinoxaline Copolymer

“…Initially, these models were semi-classical based on (i) the thermodynamic balance between the lowest CT state and 'free' separated charges (SC) energy levels, usually referred to as 'cold' dissociation 9,23 and (ii) a kinetic model of the competition between thermalisation and dissociation [17][18][19]38 referred to as 'hot' dissociation. An enchancement of the local mobility [39][40][41] or dissociation during cooling (due to energetic disorder) has also been proposed to explain the efficient separation of charges. 42 In parallel, a number of quantum mechanical models have been proposed in which the electronic dynamics is determined by delocalization 15,24,43 and vibronic [44][45][46][47] effects.…”