Benjamin J. Leever

Consequences of Anode Interfacial Layer Deletion. HCl-Treated ITO in P3HT:PCBM-Based Bulk-Heterojunction Organic Photovoltaic Devices

In studies to simplify the fabrication of bulk-heterojunction organic photovoltaic (OPV) devices, it was found that when glass/tin-doped indium oxide (ITO) substrates are treated with dilute aqueous HCl solutions, followed by UV ozone (UVO), and then used to fabricate devices of the structure glass/ITO/P3HT:PCBM/LiF/Al, device performance is greatly enhanced. Light-to-power conversion efficiency (Eff) increases from 2.4% for control devices in which the ITO surface is treated only with UVO to 3.8% with the HCl + UVO treatment--effectively matching the performance of an identical device having a PEDOT:PSS anode interfacial layer. The enhancement originates from increases in V(OC) from 463 to 554 mV and FF from 49% to 66%. The modified-ITO device also exhibits a 4x enhancement in thermal stability versus an identical device containing a PEDOT:PSS anode interfacial layer. To understand the origins of these effects, the ITO surface is analyzed as a function of treatment by ultraviolet photoelectron spectroscopy work function measurements, X-ray photoelectron spectroscopic composition analysis, and atomic force microscopic topography and conductivity imaging. Additionally, a diode-based device model is employed to further understand the effects of ITO surface treatment on device performance.

Narrow diameter distributions of metallic arc discharge single-walled carbon nanotubes via dual-iteration density gradient ultracentrifugation

The outstanding electrical and mechanical properties of single-walled carbon nanotube (SWCNT) thin fi lms [ 1 ] have brought them to the forefront of materials research for a variety of applications including transparent conductors. [ 2–4 ] While as-produced SWCNT samples inherently contain a variety of tube diameters and chiral angles, leading to a mixture of metallic and semiconducting tubes that presents challenges for practical electronic applications, density gradient ultra-centrifugation (DGU) has enabled the isolation of large quantities of electronically monodisperse material. [ 5 ] Transparent conducting fi lms fabricated using DGU-sorted SWCNTs have been well-characterized [ 6–8 ] and the advantages of using monodisperse metallic SWCNTs have been demonstrated in devices such as organic photovoltaics. [ 9 ] However, these metallic SWCNT samples are comprised of several species with varying diameters and thus exhibit broad optical transitions, which both hinder optoelectronic applications where sharp spectral features are required and impede empirical studies aimed at elucidating the dependence of SWCNT properties on nanotube diameter. Efforts to further refi ne electronically sorted SWCNTs by diameter have typically relied on using starting material from different fabrication techniques such as high pressure carbon monoxide conversion (HiPCO), [ 10 ] laser ablation, [ 11 ] and electric arc discharge [ 12 ] that, when sorted via DGU, provide electronically monodisperse SWCNTs centered about a different tube diameter according to the synthesis method [ 5 , 6 , 13 , 14 ] and/ or batch. [ 15 ] More recent separation techniques that are able to target single ( n , m ) semiconducting species have focused on small-diameter CVD-grown SWCNTs and have not yet distinguished between metallic species of different diameters. [ 16–18 ]

Spatially resolved photocurrent mapping of operating organic photovoltaic devices using atomic force photovoltaic microscopy

A conductive atomic force microscopy (cAFM) technique, atomic force photovoltaic microscopy (AFPM), has been developed to characterize spatially localized inhomogeneities in organic photovoltaic (OPV) devices. In AFPM, a biased cAFM probe is raster scanned over an array of illuminated solar cells, simultaneously generating topographic and photocurrent maps. As proof of principle, AFPM is used to characterize 7.5×7.5μm2 poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester OPVs, revealing substantial device to device and temporal variations in the short-circuit current. The flexibility of AFPM suggests applicability to nanoscale characterization of a wide range of optoelectronically active materials and devices.

Tandem Solar Cells from Accessible Low Band-Gap Polymers Using an Efficient Interconnecting Layer

Tandem solar cell architectures are designed to improve device photoresponse by enabling the capture of wider range of solar spectrum as compared to single-junction device. However, the practical realization of this concept in bulk-heterojunction polymer systems requires the judicious design of a transparent interconnecting layer compatible with both polymers. Moreover, the polymers selected should be readily synthesized at large scale (>1 kg) and high performance. In this work, we demonstrate a novel tandem polymer solar cell that combines low band gap poly isoindigo [P(T3-iI)-2], which is easily synthesized in kilogram quantities, with a novel Cr/MoO3 interconnecting layer. Cr/MoO3 is shown to be greater than 80% transparent above 375 nm and an efficient interconnecting layer for P(T3-iI)-2 and PCDTBT, leading to 6% power conversion efficiencies under AM 1.5G illumination. These results serve to extend the range of interconnecting layer materials for tandem cell fabrication by establishing, for the first time, that a thin, evaporated layer of Cr/MoO3 can work as an effective interconnecting layer in a tandem polymer solar cells made with scalable photoactive materials.

Prediction of the Wetting Behavior of Active and Hole-Transport Layers for Printed Flexible Electronic Devices Using Molecular Dynamics Simulations

Molecular dynamics (MD) simulations were used to predict the wetting behavior of materials typical of active and hole-transport layers in organic electronics by evaluating their contact angles and adhesion energies. The active layer (AL) here consists of a blend of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (P3HT:PCBM), whereas the hole-transport layer (HTL) consists of a blend of poly(3,4-ethylenedioxythiophene) and poly(styrenesulfonate) (PEDOT:PSS). Simulations of the wetting of these surfaces by multiple solvents show that formamide, glycerol, and water droplet contact angle trends correlate with experimental values. However, droplet simulations on surfaces are computationally expensive and would be impractical for routine use in printed electronics and other applications. As an alternative, contact angle measurements can be related to adhesion energy, which can be calculated more quickly and easily from simulations and has been shown to correlate with contact angles. Calculations of adhesion energy for 16 different solvents were used to rapidly predict the wetting behavior of solvents on the AL and HTL surfaces. Among the tested solvents, pentane and hexane exhibit low and similar adhesion energy on both of the surfaces considered. This result suggests that among the tested solvents, pentane and hexane exhibit strong potential as orthogonal solvent in printing electronic materials onto HTL and AL materials. The simulation results further show that MD can accelerate the evaluation of processing parameters for printed electronics.