Christopher M. Proctor

A High‐Performing Solution‐Processed Small Molecule:Perylene Diimide Bulk Heterojunction Solar Cell

By combining the molecular donor p-DTS(FBTTh2 )2 with a readily produced perylene diimide acceptor we are able to achieve a power conversion efficiency of 3.0%, making this one of the most efficient non-fullerene organic solar cells to date. The reduced power conversion efficiency of the present system compared to the use of phenyl-C71 -butyric acid methyl ester as an electron acceptor is shown to primarily be related to a significant reduction in the internal quantum efficiency. These results indicate the potential of small-molecule:non-fullerene bulk-heterojunction organic photovoltaics.

Mobility Guidelines for High Fill Factor Solution‐Processed Small Molecule Solar Cells

Analysis of measured charge-carrier mobilities and fill factors in solution-processable small-molecule bulk-heterojunction solar cells reveals that in order to achieve a high FF, the hole and electron mobilities must be >10(-4) cm 2 V(-1) s(-1) . Neat-film mobility measurements are also found to be a useful predictor of the maximum blend film mobility and FF obtained in blend film solar cells.

Tri‐Diketopyrrolopyrrole Molecular Donor Materials for High‐Performance Solution‐Processed Bulk Heterojunction Solar Cells

Two new high-performance DPP-containing donor molecules employing a molecular architecture with three DPP chromorphores (tri-DPP) in conjugated backbones are synthesized and characterized. The two tri-DPP molecules with only a structural difference on alkyl substitutions, when blended with PC71 BM, lead to power conversion efficiencies up to 4.8 and 5.5%, respectively.

Optimization of energy levels by molecular design: evaluation of bis-diketopyrrolopyrrole molecular donor materials for bulk heterojunction solar cells

We report a series of solution-processable, small-molecule, donor materials based on an architecture consisting of two diketopyrrolopyrrole (DPP) cores with different aromatic π-bridges between the DPP units and different end-capping groups. In general, this architecture leads to desirable light absorption and electronic levels for donor materials. Out of the compounds investigated, a material with a hydrolyzed dithieno(3,2-b;2′,3′-d)silole (SDT) core and 2-benzofuran (BFu) end capping groups leads to the most favorable properties for solar cells, capable of generating photocurrent up to 800 nm while producing an open-circuit voltage of over 850 mV, indicating a small loss in electrical potential compared to other bulk heterojunction systems. Device properties can be greatly improved through the use of solvent additives such as 2-chloronaphthalene and initial attempts to optimize device fabrication have resulted in power conversion efficiencies upwards of 4%.

Importance of Domain Purity and Molecular Packing in Efficient Solution-Processed Small-Molecule Solar Cells

Connections are delineated between solar-cell performance, charge-carrier mobilities, and morphology in a highperformance molecular solar cell. The observations show that maximizing the relative phase purity and structural order while simultaneously limiting the domain size may be essential for achieving optimal solar-cell performances in solution-processed small-molecule solar cells .

Effect of leakage current and shunt resistance on the light intensity dependence of organic solar cells

In this report, we demonstrate that parasitic leakage currents dominate the current voltage characteristics of organic solar cells measured under illumination intensities less than one sun when the device shunt resistance is too low (<106 Ω cm2). The implications of such effects on common interpretations of the light intensity dependence of the solar cell open circuit voltage, fill factor, short circuit current, and power conversion efficiency are discussed in detail.

Understanding the Charge‐Transfer State and Singlet Exciton Emission from Solution‐Processed Small‐Molecule Organic Solar Cells

Electroluminescence (EL) from the charge-transfer state and singlet excitons is observed at low applied voltages from high-performing small-molecule bulk-heterojunction solar cells. Singlet emission from the blends emerges upon altering the processing conditions, such as thermal annealing and processing with a solvent additive, and correlates with improved photovoltaic performance. Low-temperature EL measurements are utilized to access the physics behind the singlet emission.

Enhancement of the Photoresponse in Organic Field-Effect Transistors by Incorporating Thin DNA Layers**

A mechanistic study of the DNA interfacial layer that enhances the photoresponse in n-type field-effect transistors (FET) and lateral photoconductors using a solution-processed fullerene derivative embedded with disperse-red dye, namely PCBDR, is reported. Incorporation of the thin DNA layer simultaneously leads to increasing the electron injection from non-Ohmic contacts into the PCBDR active layer in dark and to increasing the photocurrent under irradiation. Such features lead to the observation of the enhancement of the photoresponsivity in PCBDR FETs up to 10(3) . Kelvin probe microscopy displays that in the presence of the DNA layer, the surface potential of PCBDR has a greater change in response to irradiation, which is rationalized by a larger number of photoinduced surface carriers. Transient absorption spectroscopy confirms that the increase in photoinduced carriers in PCBDR under irradiation is primarily ascribed to the increase in exciton dissociation rates through the PCBDR/DNA interface and this process can be assisted by the interfacial dipole interaction.

Effect of structural variation on photovoltaic characteristics of phenyl substituted diketopyrrolopyrroles

A comprehensive study has been performed on a series of solution processable phenyl substituted diketopyrrolopyrroles blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) in order to investigate how systematic chemical modifications such as solubilizing groups and conjugation length impact solar cell performance. We find that replacement of linear alkyl chains with bulky ethyl-hexyl groups or the removal of linear alkyl chains on the terminal thiophene units leads to micron scale phase segregation at high donor:acceptor blend ratios. It is found that the conjugation length can be used to simultaneously tune energy levels, solubility, and molecular ordering. We show that over-extending the conjugation length can reduce solubility making film fabrication difficult while decreasing the conjugation length past a critical limit can significantly enhance molecular ordering thereby inducing micron scale phase segregation in blend films. This work shows that a materials potential device performance can be limited by slight chemical modifications which prevent device optimization at high donor:acceptor blend ratios and elevated annealing temperatures where charge mobility is balanced and charge collection is enhanced in the donor and acceptor phase.

Effect of copper metalation of tetrabenzoporphyrin donor material on organic solar cell performance

The effects of copper metalation of tetrabenzoporphyrin on the properties and performance of organic solar cells are studied. Tetrabenzoporphyrin (BP) and copper tetrabenzoporphyrin (CuBP) are both solution processed from soluble precursor materials and thermally converted in the thin film. Despite high field-effect hole mobility above 1 cm2 V−1 s−1, the power conversion efficiency (PCE) of solar cell devices with CuBP is severely diminished compared to those with BP. Conducting atomic force microscopy (c-AFM) is used to show that CuBP films are highly conductive in the direction perpendicular to the substrate, relative to those comprising BP. By analyzing the donor absorption characteristics as well as the external quantum efficiency and short-circuit current density of bilayer OPV devices as a function of donor layer thickness, it is determined that the differences in performance are likely due to a prohibitively short effective exciton diffusion length (LD) in the metalated derivative. By modeling the external quantum efficiency of bilayer OPV devices, we are able to approximate this difference in effective LD to be 15 nm for BP and 2 nm for CuBP.