Drift-Diffusion Studies of Compositional Morphology in Bulk Heterojunctions: The Role of the Mixed Phase in Photovoltaic Performance

Abstract

The active layers of most OPV devices are constructed from a blend of two organic compounds. The two materials spontaneously segregate into pure component phases during device fabrication, creating a bicontinuous network of conduction pathways that are selective for electron or hole charge carriers. The morphological distribution of these materials within the active layer has long been known to influence charge transport and resulting device performance. In addition to the two purecomponent phases present in these devices, a third, mixed-composition phase exists at the interface between the two pure phases. The exact effects of this mixed-composition phase on OPV device performance are not well understood, although it has been argued that the presence of a mixed phase is necessary for optimal device operation. In this work, we probe the effects of having a mixed-composition, interfacial phase on the performance and charge transport characteristics of organic photovoltaic (OPV) devices through a series of drift-diffusion model simulations. We start with set of model morphologies with only pure component phases and then introduce an interfacial, mixed-phase in a controllable fashion. Our simulations show that a modest amount of mixing initially improves device efficiency by reducing the tortuosity of the device's conduction pathways and easing morphological traps. However, an excessive amount of mixing can actually degrade highconductivity pathways, reducing photovoltaic performance. The point at which mixing switches from being beneficial to instead detrimental to OPV performance differs depending on the average domain size of a device's morphology. Devices with smaller feature sizes are more susceptible to the debilitating effects of overmixing, so that the presence of a mixed-phase may either raise power conversion efficiency by as much as 100% or lower it by as much as 50%, depending on the average domain size and the extent of mixing. This suggests that variations in the amount of mixed-composition phase with different processing conditions is one of the key factors that makes optimizing bulk heterojunction OPV devices difficult.

Intercalated vs Nonintercalated Morphologies in Donor–Acceptor Bulk Heterojunction Solar Cells: PBTTT:Fullerene Charge Generation and Recombination Revisited

“…13,[21][22][23][24][25] In addition, the nature of the morphological distribution in the donor-acceptor interface, whether blurred with a continuous composition gradient or presenting an abrupt transition between the donor and acceptor phase, was predicted to affect the efficiency of exciton harvesting and charge extraction, thus impacting upon the overall device performance. 23,26,27 A key uncertainty in such analyses is the nature of the interfacial bound charges in such bulk heterojunction (BHJ) devices and their impact upon charge generation and recombination.…”

“…These observations are consistent with a recent semi-empirical theoretical study of the effect of intermixed regions upon charge recombination. 27 In contrast, in the non-intercalated 1:1 PBTTT:ICTA system, there is a shallow energetic barrier to separate the interfacial CT state, and yet most of the photogenerated charge carriers are either morphologically bound or energetically trapped charges in a more disordered blend. 37 As a consequence, polaron recombination as obtained by TAS is first order, and the EGE remains at a low level independently of the applied voltage.…”

Flexible ITO-Free Roll-Processed Large-Area Nonfullerene Organic Solar Cells Based on P3HT:O-IDTBR

“…In general, the changes observed in surface roughness may be a consequence of bulk morphology changes induced by the coating-temperature variations that influence the drying kinetics [41]. The hot drum and hot slot-die head induce highly ordered molecular packing [24], which improves carrier transport inside the bulk heterojunction [24,25,42], improving J SC and FF (Table I).…”

Device Physics in Organic Solar Cells and Drift-Diffusion Simulations

“…[80,[120][121][122][123][124] There are two types of disorder, which can affect OSC device performance in different ways: energetic disorder, which is usually a consequence of the conformational freedom of the molecules or polymer chains, and structural disorder, results from the BHJ phase separation that can lead to "dead-ends" and other morphological conditions that are not optimal for carrier transport. [125] Several studies have examined the effects of energetic disorder on OSC performance, usually assuming a Gaussian distribution of energetic states or an exponential tail of trap states in 1D drift-diffusion simulations. [123,124] Martin et al [130] suggested an ideal ordered BHJ for the 2D drift-diffusion simulation.…”

“…Finck and Schwartz also simulated the effects of DA mixed-composition by utilizing morphologies generated by Cahn-Hilliard (CH) modeling [131] of binary fluid phase separation. [125] With the CH model, the component domain sizes can be tuned with a single parameter. Then, a mixed interfacial region by smoothing the binary morphology in a continuously controllable fashion can be introduced.…”