Christopher J.M. Emmott

Reducing the efficiency-stability-cost gap of organic photovoltaics with highly efficient and stable small molecule acceptor ternary solar cells

Technological deployment of organic photovoltaic modules requires improvements in device light-conversion efficiency and stability while keeping material costs low. Here we demonstrate highly efficient and stable solar cells using a ternary approach, wherein two non-fullerene acceptors are combined with both a scalable and affordable donor polymer, poly(3-hexylthiophene) (P3HT), and a high-efficiency, low-bandgap polymer in a single-layer bulk-heterojunction device. The addition of a strongly absorbing small molecule acceptor into a P3HT-based non-fullerene blend increases the device efficiency up to 7.7 ± 0.1% without any solvent additives. The improvement is assigned to changes in microstructure that reduce charge recombination and increase the photovoltage, and to improved light harvesting across the visible region. The stability of P3HT-based devices in ambient conditions is also significantly improved relative to polymer:fullerene devices. Combined with a low-bandgap donor polymer (PBDTTT-EFT, also known as PCE10), the two mixed acceptors also lead to solar cells with 11.0 ± 0.4% efficiency and a high open-circuit voltage of 1.03 ± 0.01 V.

Organic photovoltaic greenhouses: a unique application for semi-transparent PV?

Organic photovoltaics are an emerging solar power technology which embody properties such as transparency, flexibility, and rapid, roll to roll manufacture, opening the potential for unique niche applications. We report a detailed techno-economic analysis of one such application, namely the photovoltaic greenhouse, and discuss whether the unique properties of the technology can provide advantages over conventional photovoltaics. The potential for spectral selectivity through the choice of OPV materials is evaluated for the case of a photovoltaic greenhouse. The action spectrum of typical greenhouse crops is used to determine the impact on crop growth of blocking different spectral ranges from the crops. Transfer matrix optical modelling is used to assess the efficiency and spectrally resolved transparency of a variety of commercially available semi-conducting polymer materials, in addition to a non-commercial low-band-gap material with absorption outside that required for crop growth. Economic analysis suggests there could be a huge potential for OPV greenhouses if aggressive cost targets can be met. Technical analysis shows that semi-transparent OPV devices may struggle to perform better than opaque crystalline silicon with partial coverage, however, OPV devices using the low-band-gap material PMDPP3T, as well as a high efficiency mid-band-gap polymer PCDTBT, can demonstrate improved performance in comparison to opaque, flexible thin-film modules such as CIGS. These results stress the importance of developing new, highly transparent electrode and interlayer materials, along with high efficiency active layers, if the full potential of this application is going to be realised.

Hydrogen or batteries for grid storage? A net energy analysis

Energy storage is a promising approach to address the challenge of intermittent generation from renewables on the electric grid. In this work, we evaluate energy storage with a regenerative hydrogen fuel cell (RHFC) using net energy analysis. We examine the most widely installed RHFC configuration, containing an alkaline water electrolyzer and a PEM fuel cell. To compare RHFCs to other storage technologies, we use two energy return ratios: the electrical energy stored on invested (ESOIe) ratio (the ratio of electrical energy returned by the device over its lifetime to the electrical-equivalent energy required to build the device) and the overall energy efficiency (the ratio of electrical energy returned by the device over its lifetime to total lifetime electrical-equivalent energy input into the system). In our reference scenario, the RHFC system has an ESOIe ratio of 59, more favorable than the best battery technology available today (Li-ion, ESOIe = 35). (In the reference scenario RHFC, the alkaline electrolyzer is 70% efficient and has a stack lifetime of 100 000 h; the PEM fuel cell is 47% efficient and has a stack lifetime of 10 000 h; and the round-trip efficiency is 30%.) The ESOIe ratio of storage in hydrogen exceeds that of batteries because of the low energy cost of the materials required to store compressed hydrogen, and the high energy cost of the materials required to store electric charge in a battery. However, the low round-trip efficiency of a RHFC energy storage system results in very high energy costs during operation, and a much lower overall energy efficiency than lithium ion batteries (0.30 for RHFC, vs. 0.83 for lithium ion batteries). RHFCs represent an attractive investment of manufacturing energy to provide storage. On the other hand, their round-trip efficiency must improve dramatically before they can offer the same overall energy efficiency as batteries, which have round-trip efficiencies of 75–90%. One application of energy storage that illustrates the tradeoff between these different aspects of energy performance is capturing overgeneration (spilled power) for later use during times of peak output from renewables. We quantify the relative energetic benefit of adding different types of energy storage to a renewable generating facility using [EROI]grid. Even with 30% round-trip efficiency, RHFC storage achieves the same [EROI]grid as batteries when storing overgeneration from wind turbines, because its high ESOIe ratio and the high EROI of wind generation offset the low round-trip efficiency.

Dynamic carbon mitigation analysis: the role of thin-film photovoltaics

The introduction of substantial levels of renewable energy technologies will incur greenhouse gas (GHG) emissions during product manufacture. We have developed a model to assess the impact on GHG emissions of a growth of solar photovoltaic (PV) capacity. The model is applied to PV growth scenarios in India and Germany, locations which differ in their insolation and the carbon intensity of the local grid. The impact of growth is to delay net GHG emission reductions by around 4 and 9 years in the Indian and German scenarios receptively. This dynamic approach quantifies the benefit of technologies with a lower GHG footprint in achieving rapid GHG emission reductions. In addition, short lifetime PV technologies, with a low GHG footprint, such as organic PV, can show greater emission reductions despite a higher levelised global warming potential (gCO2eq per kWh). Finally, a measure of the dynamic cost of GHG emission reductions is proposed to assess the cost, over the short term, of emission reductions from renewable energy technologies.

Can solar power deliver

Solar power represents a vast resource which could, in principle, meet the worlds needs for clean power generation. Recent growth in the use of photovoltaic (PV) technology has demonstrated the potential of solar power to deliver on a large scale. Whilst the dominant PV technology is based on crystalline silicon, a wide variety of alternative PV materials and device concepts have been explored in an attempt to decrease the cost of the photovoltaic electricity. This article explores the potential for such emerging technologies to deliver cost reductions, scalability of manufacture, rapid carbon mitigation and new science in order to accelerate the uptake of solar power technologies.

Life cycle analysis of an off-grid solar charging kiosk

Solar power has a huge potential in electrifying rural communities, particularly in the developing world. It offers the possibility of a clean, affordable energy source which may reduce the environmental impact of existing, fossil fuel based sources. The life-cycle carbon emissions resulting from off-grid solar powered lighting solutions are an important factor influencing the environmental impact of implementing such solutions. This issue is particularly relevant when assessing the case for carbon financing for such a project. However, few studies have addressed the carbon saving potential of such off grid systems. Here, we analyse a distribution model known as a Solar Charging Kiosk which enables access to photovoltaic electricity for rural, off-grid communities. Using a kiosk which has been established in the Bugesera region of Rwanda as a model system, the carbon savings avoided from reduced use of kerosene based lighting are calculated based on real system performance and usage data of customers of the kiosk. Strategies to further increase the emissions mitigation potential of the system are proposed.