Improving Effciency

When Justin Hodgkiss accepted his Fulbright fellowship from Alan MacDiarmid in 2001, he didn’t expect to join a research institute named in honour of the late Nobel laureate within less than a decade. In fact, he didn’t even expect to return to New Zealand, at least not as a scientist. “Back then,” he says, “I felt I had better opportunities abroad.”

That all changed as he watched the growth and success of the MacDiarmid Institute and eventually decided to return in 2009. At the beginning of 2010, he joined the Institute as one of its youngest Principal Investigators, pursuing research that builds directly on MacDiarmid’s discovery of conducting polymers.

Back in 2001, Hodgkiss had graduated in chemistry from the University of Otago and was bound for MIT to join the team of Professor Daniel Nocera, whose research focus is on the chemistry of renewable energy. “During my PhD at MIT, I was developing ultrafast laser experiments to study electron transfer reactions in molecules designed to mimic molecules in photosynthesis. I was working at an abstract, fundamental level, looking specifically at proton-coupled electron transfer. It’s a very important process in biology, where the motion of electrons is dependent on the motion of protons.”

Hodgkiss remembers the five years he spent at MIT as very tough to begin with but ultimately greatly rewarding and formative for his career. Although his own research focused on a small piece of the energy puzzle, it was apparent that many pieces contributed to an overriding theme. During his time, Nocera’s team was working on chemical catalysts that use sunlight to make chemical fuels. The research culminated in the development of an artificial photosynthesis process which uses the Sun’s energy to split water into hydrogen and oxygen, exploiting proton-coupled electron transfer steps along the way. In a paper published in 2008, Nocera announced that a new catalyst, consisting of cobalt metal, phosphate and an electrode, produced oxygen from water, opening the way for cheap and efficient storage of solar energy.

Hodgkiss’ next destination was a post-doc position at Cambridge University, with Professor Sir Richard Friend, which brought him closer to the work of Alan MacDiarmid himself. “Richard Friend’s group had distinguished itself in the development and understanding of polymer-based electronic devices. They had been pioneering in this work and were amongst the first to pick up on MacDiarmid’s discoveries and to develop impactful technologies. They were the first to make a polymer light-emitting diode and one of two groups to develop polymer solar cells. Alan MacDiarmid was a strong supporter of Richard Friend.”

Hodgkiss says he was attracted to work on polymer solar cells because it is an expanding area of research with many challenging science questions as well as largescale opportunities for application. “I also became interested in solar cells because it was an important area where I could apply my skills in laser spectroscopy. Ultrafast lasers are a powerful tool to understand what happens when something absorbs light, which gets right to the crux of solar cells.”

After two years with the Cambridge group, Hodgkiss returned to New Zealand and has recently built a new ultrafast spectroscopy laboratory at Victoria University of Wellington. His current research builds on work he began in Cambridge – using ultrafast lasers to study photoactive materials, particularly organic solar cells where the active layer is based on carbon instead of silicon or another inorganic  semiconductor. One of the advantages of carbon-based polymers is that they are cheap to make and can be printed onto a piece of plastic to make a complete device. While the power-conversion efficiency doesn’t yet match that of conventional solar cells, Hodgkiss says it is improving fast. “If measured in cost-per-Watt, polymer solar cells are already reaching parity with conventional solar cells. That has come about through rapid increases in efficiency, but there is still a lot of room for improvement and that’s what I am focusing my research on.”

When polymer solar cells were first made in the 90s, their efficiency was a fraction of a percent, but by the start of the millennium it had risen to four percent and recently jumped up again to eight percent, largely thanks to the development of new materials. “A few percent more make a big difference in the cost per Watt,” says Hodgkiss. “But the design of new materials has not always been well informed by the physics. These things just don’t work the way you’d expect from standard solar cell models. The measurements I’m doing in the laser lab provide quite detailed information about exactly what happens when a material absorbs light. We want absorbed photons to create an electrical current, but there are many competing processes that often overwhelm photocurrent generation and they can happen extremely quickly. Ultrafast lasers allow me to trace out the dynamics after photon absorption and see exactly what goes right or wrong and when, how long it takes, and what the material basis is so that we can go about designing new materials that will be even better.”

Spitfire Pro seems an appropriate name for Hodgkiss’ ultrafast laser, set up in a window-less room amid a maze of lenses and mirrors. It emits pulses of light of about 100 femtoseconds – 10-13 seconds or a tenth of a billionth of a second – a time so short that it is perhaps most meaningfully described by comparison. A femtosecond is to the second what a second is to about 31.7 million years, or what five minutes are to the age of the universe. “You can still see the light beam because there’s a train of these ultrafast pulses coming out. When photons are compressed into such a small pulse the instantaneous power is enormous.”

A solar cell on a rooftop is hit by photons arriving at different times and in different colours, with each triggering a new cascade of processes that can take anything from femto- to milliseconds. Hodgkiss’ laser allows him to trigger all processes at the same time with a short pulse and take a series of measurements, or spectra, to capture snapshots of the transient intermediates resulting from the first pulse. “The beam is split into multiple branches and one will be used as a probe pulse with a broad spectrum of colours. Depending on which colours are absorbed we can identify different types of excitations in the solar cell, whether we have free electrons, bound electrons or any decomposition products.”

Hodgkiss sees the MacDiarmid Institute as an ideal environment to bring all necessary expertise together to make significant progress in the development of polymer-based solar cells. Already, he is discussing potential collaborations with other MacDiarmid investigators, including Ashton Partridge, Richard Blaikie, and his former Otago lecturer Keith Gordon. “There are also several MacDiarmid Institute investigators working on transparent electrodes. In solar cells, the active material must be sandwiched between two electrodes and at least one needs to be transparent so that the light can get into the active layer. The physics of making transparent electrodes is tricky because the properties that cause materials to be conductive imply a certain electronic structure which also means that it absorbs light. Now we’re trying to bring these different MacDiarmid Institute projects together in a more coherent and comprehensive approach to the development of organic solar cells for low-cost energy systems.”