Roderick C. I. MacKenzie

Visualizing charge separation in bulk heterojunction organic solar cells

Solar cells based on conjugated polymer and fullerene blends have been developed as a low-cost alternative to silicon. For efficient solar cells, electron-hole pairs must separate into free mobile charges that can be extracted in high yield. We still lack good understanding of how, why and when carriers separate against the Coulomb attraction. Here we visualize the charge separation process in bulk heterojunction solar cells by directly measuring charge carrier drift in a polymer:fullerene blend with ultrafast time resolution. We show that initially only closely separated (<1 nm) charge pairs are created and they separate by several nanometres during the first several picoseconds. Charge pairs overcome Coulomb attraction and form free carriers on a subnanosecond time scale. Numerical simulations complementing the experimental data show that fast three-dimensional charge diffusion within an energetically disordered medium, increasing the entropy of the system, is sufficient to drive the charge separation process.

A numerical study of mobility in thin films of fullerene derivatives

The effect of functional group size on the electron mobility in films of fullerene derivatives is investigated numerically. A series of four C(60) derivatives are formed by attaching saturated hydrocarbon chains to the C(60) cage via a methano bridge. For each of the derivatives investigated, molecular dynamics is used to generate a realistic material morphology. Quantum chemical methods are then used to calculate intermolecular charge transfer rates. Finally, Monte Carlo methods are used to simulate time-of-flight experiments and thus calculate the electron mobility. It is found that as the length of the aliphatic side chain increases, the configurational disorder increases and thus the mobility decreases.

Fullerene-free organic solar cells with an efficiency of 3.7% based on a low-cost geometrically planar perylene diimide monomer

The aggregate-induced limitation for high power-conversion efficiencies (PCEs) of perylene-diimide (PDI):polymer solar cells can be circumvented when two simple rules are respected; the aggregate size of PDI remains short enough and the omnipresent PDI aggregates are electronically interconnected. Following these guidelines, a PCE of 3.7% is delivered by using the solution-processable, planar PDI monomer of N,N′-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide as the electron acceptor mixed with the low-energy gap polymeric donor poly[(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b;4,5-b′]dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene))-2,6-diyl] (PBDTTT-CT). The PBDTTT-CT:PDI composite absorbs strongly the light in the region of 400 nm–800 nm and after adding a small amount of 1,8-diiodooctane (DIO) efficient photocurrent generation is achieved. Space-charge limited dark current and transient photovoltage measurements suggest that the use of the DIO component optimizes the electron/hole carrier mobility ratio, suppresses the non-geminate recombination losses and improves the charge extraction efficiency.

Design and Simulation of Next-Generation High-Power, High-Brightness Laser Diodes

High-brightness laser diode technology is progressing rapidly in response to competitive and evolving markets. The large volume resonators required for high-power, high-brightness operation makes their beam parameters and brightness sensitive to thermal- and carrier-induced lensing and also to multimode operation. Power and beam quality are no longer the only concerns for the design of high-brightness lasers. The increased demand for these technologies is accompanied by new performance requirements, including a wider range of wavelengths, direct electrical modulation, spectral purity and stability, and phase-locking techniques for coherent beam combining. This paper explores some of the next-generation technologies being pursued, while illustrating the growing importance of simulation and design tools. The paper begins by investigating the brightness limitations of broad-area laser diodes, including the use of asymmetric feedback to improve the modal discrimination. Next, tapered lasers are considered, with an emphasis on emerging device technologies for applications requiring electrical modulation and high spectral brightness.

The physical meaning of charge extraction by linearly increasing voltage transients from organic solar cells

Carrier mobility in organic solar cells is almost exclusively determined with the Charge Extraction by Linearly Increasing Voltage (CELIV) technique; indeed much of our understanding of the recombination and charge transport mechanisms in organic solar cells is based on CELIV measurements. However, since the conception of the CELIV method, our understanding of organic semiconductors has significantly advanced. In this work, we critically examine the CELIV methods ability to provide accurate material data in the light of recent advances in our understanding of trap states and their influence on mobility in organic semiconductors. We then apply this knowledge to understand the mechanisms responsible for degradation in organic solar cells.

A general mechanism for controlling thin film structures in all-conjugated block copolymer:fullerene blends

Block copolymers have the potential to self-assemble into thermodynamically stable nanostructures that are desirable for plastic electronic materials with prolonged lifetimes. Fulfillment of this potential requires the simultaneous optimisation of the spatial organisation and phase behaviour of heterogeneous thin films at the nanoscale. We demonstrate the controlled assembly of an all-conjugated diblock copolymer blended with fullerene. The crystallinity, nanophase separated morphology, and microscopic features are characterised for blends of poly(3-hexylthiophene-block-3-(2-ethylhexyl) thiophene) (P3HT-b-P3EHT) and phenyl-C61-butyric acid methyl ester (PCBM), with PCBM fractions varying from 0–65 wt%. We find that PCBM induces the P3HT block to crystallise, causing nanophase separation of the block copolymer. Resulting nanostructures range from ordered (lamellae) to disordered, depending on the amount of PCBM. We identify the key design parameters and propose a general mechanism for controlling thin film structure and crystallinity during the processing of semicrystalline block copolymers.

Increasing organic solar cell efficiency with polymer interlayers

We demonstrate how organic solar cell efficiency can be increased by introducing a pure polymer interlayer between the PEDOT:PSS layer and the polymer:fullerene blend. We observe an increase in device efficiency with three different material systems over a number of devices. Using both electrical characterization and numerical modeling we show that the increase in efficiency is caused by optical absorption in the pure polymer layer and hence efficient charge separation at the polymer bulkheterojunction interface.

The impact of hot-phonons on the performance of 1.3µm dilute nitride edge-emitting quantum well lasers

A robust opto-electronic device simulation tool is extended to model the phonon bottleneck in edge-emitting 1.3µm InGaAsN double quantum well (QW) laser diodes. Both the steady state operation and the transient response of the phonon bottleneck are examined as a function of injection current and heatsink temperature. It is found that the hot phonon population can raise the electron and hole temperatures in the QW active region by up to 7K above the equilibrium lattice temperature at moderate injection currents. At high injection currents, it is found that the phonon bottleneck can significantly decrease the optical power.

Combining plasmonic trap filling and optical backscattering for highly efficient third generation solar cells

Metal oxide contact layers such as ZnO and TiOx are commonly used in third generation solar cells as they can be solution processed and have a relatively high conductivity. It is well known that by ultraviolet (UV) light-soaking such devices, their overall device efficiency can be boosted. This improvement in efficiency is due to high energy UV light exciting hot carriers which then fill the trap states in the metal oxide film. Unfortunately, UV causes degradation of the active layer and thus must be filtered out if long lifetimes are to be achieved. In this work, we use plasmonically excited metal nano-structures embedded in a ZnO metal oxide layer to generate hot charge carriers from visible light alone, thus removing the need for UV light soaking. Using this approach, the solar cells also exhibit better charge transport/recombination properties as well as enhanced light trapping behavior. We demonstrate that the power conversion efficiency of a low-bandgap thieno[3,4-b]thiophene/benzodithiophene (PTB7) based solar cell can be increased from 7.91% to 9.36%.

Organic semiconductors with a charge carrier life time of over 2 hours at room temperature

Recently, Gao et al. reported being able to measure significant quantities of photogenerated charge up to one hour after it had been generated in an organic semiconductor device. The aim of this paper is twofold; (a) to provide conclusive experimental evidence to support the picture of device operation; and (b) to understand and demonstrate how changes to the device structure and materials can be used to tune the charge carrier lifetime. By tuning both the materials used, and the device structure we are able to observe a charge carrier life time of over 2 hours and still extract significant amounts of charge from the device after 5 hours. This is achieved by engineering the band structure of the device to control the spatial overlap of the stored photoexcited electron and hole populations and thus the recombination rate. By performing lifetime measurements as a function of charge carrier density and applied voltage we find the recombination rate has a 0th order dependence on carrier density, and elucidate the mechanisms responsible for these long charge carrier life times. This work is of technological significance for the development of organic electronic high sensitivity photodetectors and memory elements.