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

Abstract

Charge transfer (CT) states at a donor-acceptor heterojunction have a key role in the charge photogeneration process of organic solar cells, however, the mechanism by which these states dissociate efficiently into free carriers remains unclear. Here we explore the nature of these states in small molecule-fullerene bulk heterojunction photovoltaics with varying fullerene fraction and find that the CT energy scales with dielectric constant at high fullerene loading but that there is a threshold C 60 crystallite size of B4 nm below which the spatial extent of these states is reduced. Electroabsorption measurements indicate an increase in CT polarizability when C 60 crystallite size exceeds this threshold, and that this change is correlated with increased charge separation yield supported by CT photoluminescence transients. These results support a model of charge separation via delocalized CT states independent of excess heterojunction offset driving energy and indicate that local fullerene crystallinity is critical to the charge separation process.

Predicting thermal stability of organic solar cells through an easy and fast capacitance measurement

“…Cg can be expressed by the equation Cg= ɛɛ 0 A/L where A is the area of the device, L is the thickness of the active layer, ɛ 0 the dielectric constant of vacuum and ɛ the static permittivity of the active layer. When the temperature is increased two phenomena can influence Cg due to significant morphological modification of the active layer: i) variation of the active layer thickness (L), and ii) change in the dielectric properties of the blend (ɛ) [58]. To define the conditions at which the Cg should be monitored, we measured the capacitance-voltage characteristics in the dark for each system, from -1 V to 1,5 V at different frequencies.…”

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

“…[4][5][6][7][8][9] Such complex, but more realistic, structural models are motivating studies of the correlations between film morphology and the processes of charge generation and device function in OSC. 6,8,[10][11][12][13][14][15][16][17][18][19] In this study, we address this issue for blend films and devices employing an amorphous donor polymer silaindacenodithiophene donor (SiIDT-DTBT) previously shown to exhibit high miscibility with PC60BM. 20 Our study employs a range of blend ratios to modulate the blend morphology and both rigorous structural and spectroscopic characterization allowing us to quantitatively analyse the correlations between blend structure and device performance.…”

Modulating the Exciton Dissociation Rate by up to More than Two Orders of Magnitude by Controlling the Alignment of LUMO + 1 in Organic Photovoltaics

“…[1][2][3][4][5][6][7][8] With the rapidly growing interests in OPVs in both scientific and industrial communities, theoretical modeling of the electronic structure of excitons and their accompanying dynamics has been widely implemented and now plays an important role in interpreting the elementary processes or designing new materials. [5][6][7][8][9][10][11][12][13][14][15][16][17] Due to the large size of the system, the theoretical investigations are mainly based on simplified models with the common assumption that only the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of each molecule participate in the exciton dynamics within the donor (D)/acceptor (A) heterojunction, i.e.…”