Susan Spencer

Contribution of Aggregate States and Energetic Disorder to a Squaraine System Targeted for Organic Photovoltaic Devices

Squaraine dyes have significant potential for use in organic photovoltaic devices because their chemical and packing structure tunability leads to a broad solid state panchromaticity. Nevertheless, broadening of the spectrum does not always give rise to increasing power conversion efficiencies. Furthermore, the same processing strategy used to make devices from different squaraines does not lead to the same optimized performance. In this work, by varying the environmental conditions of a set of anilinic squaraines, we demonstrate that spin-cast thin films are made up of a complex set of states, with each state contributing differently to the overall device efficiency. We demonstrate crystallochromy in that small changes in the packing structure give rise to dramatically different absorption spectra. Through a remarkable comparison between squaraines in poly(methyl methacrylate) solid solution and squaraine:PC60BM blends, we also show long-range and orientational disorder broadening, which distorts the ability to correlate qualitative spectroscopic assessment with an understanding of the device mechanism. We conclude that a full quantitative assessment of the populations of each excited state must be carried out in order to make progress toward an improved understanding of each states contribution to charge transfer at the bulk heterojunction interface.

Influence of squaraine aggregation on short-circuit current and device efficiency

Linear absorption measurements, current-voltage characteristics, and time-resolved microwave conductivity measurements were utilized to investigate the impact of molecular aggregation of a novel donor material on photovoltaic device efficiency. We report efficiencies of 2.4+/-0.3% and explain the increase in short-circuit current and efficiency as a function of the increased aggregation.

Theory and assignment of intermolecular charge transfer states in squaraines and their impact on efficiency in bulk heterojunction solar cells (Presentation Recording)

Squaraines are targeted for organic photovoltaic devices because of their high extinction coefficients over a broad wavelength range from visible to near infra-red (NIR). Moreover, their side groups can be changed with profound effects upon their ability to crystallize, leading to improvements in charge mobility and exciton diffusion. The broadening in squaraine absorption is often qualitatively attributed to H- and J-aggregates based on the exciton model, proposed by Kasha. However, such assignment is misleading considering that spectral shifts can arise from sources other than excitonic coupling. Our group has shown that packing structure influences the rate of charge transfer; thus a complete and accurate reassessment of the excited states must be completed before the true charge transfer mechanism can be confirmed. In this work, we will show how squaraine H-aggregates can pack in complete vertical stacks or slipped vertical stacks depending upon sidegroups and processing conditions. Hence, we uncover the contribution of an intermolecular charge transfer (IMCT) state through essential states modeling validated by spectroscopic and X-Ray diffraction data. We further show external quantum efficiency data that describe the influence of the IMCT state on the efficiency of our devices. This comprehensive understanding of squaraine aggregates drives the development of more efficient organic photovoltaic devices, leading towards a prescription for derivatives that can be tailored for optimized exciton diffusion, charge transfer, higher mobilities and reduced recombination in small molecule OPV devices.

Explaining the molecular structure of the bulk heterojunction with simple electronic measurements and marcus-hush theory for squaraine: PCBM organic photovoltaic devices

A unique donor material for small molecule organic solar cells is shown to be opto-electrically tunable with thermal annealing. The system is characterized and shown to make working devices (PCE=1.1 +/- 0.3%). Examination of the spectral response of the devices before and after thermal annealing show an interesting decrease in spectral response that is explained by examining both the material properties and through application of Marcus-Hush theory. The proposed explanation could lay the groundwork for a predictive model to analyze potential device candidates.

A study on hybrid organic/silicon heterojunction solar cells

In this study, work is presented towards Sili-con/Polyaniline (PAni) hybrid solar cell development. An external quantum efficiency (EQE) integrated short-circuit current density (Jsc) of up to 22.96 mA/cm2 was measured, while AM1.5G IV measured Jsc peaked at 6.31 mA/cm2, an open circuit voltage (Voc) of 469.4 mV was measured, and an overall AM1.5G device efficiency of 0.725% despite high grid shadowing of > 50% and highly resistive contacts. Rectifying behavior was also seen in Silicon/Poly(3,4-ethylenedioxythiopene)-poly(styrenesulfonate) devices, achieving VOC values of up to 411 mV.

A study of squaraine small molecule donor materials for application in organic photovoltaics

A unique donor material for small molecule organic solar cells is shown to be opto-electrically tunable with thermal annealing. The system is characterized and shown to make working devices (PCE=1.1 +/- 0.3%). Examination of the spectral response of the devices before and after thermal annealing show an interesting decrease in spectral response that is explained by examining both the material properties and through application of Marcus-Hush theory. The proposed explanation could lay the groundwork for a predictive model to analyze potential device candidates.

New candidates for near-infra-red-absorbing active layers in multijunction organic photovoltaics: Characterization and performance

Dye molecules were modified through exchange of a central nucleofugal functional group. Profound changes in the bandgap spacing and HOMO-LUMO levels were observed. Concentration quenching was observed for all the dyes and increases in external quantum efficiency of devices were observed after dyes were diluted through distribution within a polymer host. The addition of the phenoxy functional group in one derivatized dye appeared to reduce the effect of fluorescence concentration quenching. All characterization herein allows for prediction of the effects of other nucleofugal syntheses on these dyes for more successful application in NIR bulk heterojunction photovoltaics.