Christopher J. Fell

Organic Solar Cells: Understanding the Role of Förster Resonance Energy Transfer

Organic solar cells have the potential to become a low-cost sustainable energy source. Understanding the photoconversion mechanism is key to the design of efficient organic solar cells. In this review, we discuss the processes involved in the photo-electron conversion mechanism, which may be subdivided into exciton harvesting, exciton transport, exciton dissociation, charge transport and extraction stages. In particular, we focus on the role of energy transfer as described by Förster resonance energy transfer (FRET) theory in the photoconversion mechanism. FRET plays a major role in exciton transport, harvesting and dissociation. The spectral absorption range of organic solar cells may be extended using sensitizers that efficiently transfer absorbed energy to the photoactive materials. The limitations of Förster theory to accurately calculate energy transfer rates are discussed. Energy transfer is the first step of an efficient two-step exciton dissociation process and may also be used to preferentially transport excitons to the heterointerface, where efficient exciton dissociation may occur. However, FRET also competes with charge transfer at the heterointerface turning it in a potential loss mechanism. An energy cascade comprising both energy transfer and charge transfer may aid in separating charges and is briefly discussed. Considering the extent to which the photo-electron conversion efficiency is governed by energy transfer, optimisation of this process offers the prospect of improved organic photovoltaic performance and thus aids in realising the potential of organic solar cells.

Towards the development of a virtual organic solar cell: An experimental and dynamic Monte Carlo study of the role of charge blocking layers and active layer thickness

Monte Carlo (MC) simulations have been used to fully model organic solar cells. The quantum efficiency and short-circuit current of these virtual devices are in excellent agreement with experimental measurements. Simulations show that, contrary to expectation, indium tin oxide/poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)/poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methylester (PCBM)/aluminium devices lack effective charge blocking layers at the electrode interfaces. X-ray photoelectron spectroscopy depth profiling shows that despite a PCBM-rich region near the cathode, interface intermixing at the electrodes combined with incomplete PCBM coverage leads to significant interface recombination. This work highlights the effectiveness of MC simulations as a predictive tool and emphasizes the need to control electrode interface processes.

Efficiently-cooled plasmonic amorphous silicon solar cells integrated with a nano-coated heat-pipe plate

Solar photovoltaics (PV) are emerging as a major alternative energy source. The cost of PV electricity depends on the efficiency of conversion of light to electricity. Despite of steady growth in the efficiency for several decades, little has been achieved to reduce the impact of real-world operating temperatures on this efficiency. Here we demonstrate a highly efficient cooling solution to the recently emerging high performance plasmonic solar cell technology by integrating an advanced nano-coated heat-pipe plate. This thermal cooling technology, efficient for both summer and winter time, demonstrates the heat transportation capability up to ten times higher than those of the metal plate and the conventional wickless heat-pipe plates. The reduction in temperature rise of the plasmonic solar cells operating under one sun condition can be as high as 46%, leading to an approximate 56% recovery in efficiency, which dramatically increases the energy yield of the plasmonic solar cells. This newly-developed, thermally-managed plasmonic solar cell device significantly extends the application scope of PV for highly efficient solar energy conversion.

Modelling transport in nanoparticle organic solar cells using Monte Carlo methods

Water-based nanoparticle (NP) organic solar cells eliminate the need for harmful organic solvents during deposition. However, the core-shell NP structure should limit charge extraction resulting in poor performance. Here, we use dynamic Monte Carlo modelling to show that for optimised NP structures the core-shell character does not severely limit performance. Simulations further reveal that small NPs are more susceptible to extensive phase segregation, which diminishes charge carrier percolation pathways from the cores to the electrodes and thus inhibits charge extraction. Simulated performance behaviour was used to propose an explanation for the experimentally observed change in performance due to annealing.

The origin of fine structure in near-field scanning optical lithography of an electroactive polymer

Near-field scanning optical lithography (NSOL) has been used to produce arbitrary structures of the electroactive polymer polyphenylenevinylene at sizes comparable to optical wavelengths, which are of interest for integrated optical devices. The structures are characterized using AFM and SEM and exhibit interesting fine structure. The characteristic size and shape of the lithographic features and their associated fine structure have been examined in the context of the electric field distribution at the near-field scanning optical microscope tip. In particular, the Bethe–Bouwkamp model for electric field distribution at an aperture has been used in combination with a recently developed model for precursor solubility dependence on UV energy dose to predict the characteristics of lithographic features produced by NSOL. The fine structure in the lithographic features is also investigated and explained. Suggestions for the further improvement of the technique are made.

Comparison of methods for estimating the impact of spectrum on PV output

Spectral correction factors are often used to characterize the impact of spectral variations on PV module output. We compare three different methods to predict a spectral correction factor and validate these against outdoor spectroradiometer measurements. It is found that commonly used models lead to a constant error due to assumptions made when determining coefficients describing geometric air mass dependence. We term this error the site spectral offset. This offset directly impacts estimations of the available solar resource and therefore affects the accuracy of energy yield predictions. We determine its value for the temperate coastal climate of Newcastle, Australia to be -2%, +1% and -3% for c-Si, CdTe and CIGS respectively. Errors in the daily resource estimation compared to pyranometer values of up to 15% are found for one module type while absolute variation of up to 60 Wm-2 in hourly AM1.5 equivalent insolation is seen for all examples studied. Applying a modified version of the CREST estimation method results in a significant reduction in estimation errors over all timescales

Comparing standard translation methods for predicting photovoltaic energy production

Translation equations underpin all predictive models for the energy output of photovoltaics in the outdoor environment. These equations translate the performance of a PV device to an arbitrary temperature and irradiance, based on measurements taken under reference conditions. Little work has been done to compare and contrast the three translation methods recommended under IEC 60891. This is partly due to a lack of comprehensive test data, and partly due to the flexibility in the way these methods can be applied. Based on comprehensive outdoor test data, we have used software scripts to evaluate the performance of these three standard translation methods, where the choice of reference conditions has been optimized to produce the best result in different ranges of irradiance and temperature. This allows a fair comparison of their performance that is independent of the test site location. We map the performance of the three methods over a wide range of irradiance and temperature.

Investigation of the photochemistry of the poly{p-phenylenevinylene} precursor system: implications for nanolithography.

The photochemistry of poly{p-phenylene[1-(tetrahydrothiophen-1-io)ethylene chloride]} (PPTEC), a water soluble precursor of the semiconducting polymer, poly{p-phenylenevinylene} (PPV), has been studied both under atmospheric conditions and in environments devoid of oxygen. UV-visible spectroscopy and photoluminescence data has been used to provide a picture of the mechanistic pathways involved in UV irradiation of the PPTEC material. A new quantitative model for the effect of UV irradiation upon film morphology is presented, which leads to insights for the improved control of the characteristics of PPV nanostructures produced via near-field scanning optical lithography.

Energy yield potential of perovskite-silicon tandem devices

Metal-halide perovskite photovoltaic devices have recently become of great interest to the research community with efficiencies of the thin film devices already reported to exceed a single junction cell efficiency of 20%. Of particular interest with this type of device is its potential to be integrated with the well established silicon photovoltaic technology into a monolithic tandem device with the potential to deliver efficiencies of greater than 30% In such devices the most attractive prospect is to monolithically integrate the perovskite and silicon materials into a planar single device for ease of deposition and device construction. Such a series connected device comes with inherent current matching restrictions on the operating performance of the individual junctions when working in tandem. These restrictions significantly complicate the calculation of potential energy yield for such a tandem device due to the impact of spectrum, angle of incidence and temperature. We model the performance of a tandem device using experimentally determined performance characteristics combined with representative resource and environment datasets to evaluate the energy yield potential of perovskite-silicon tandem under outdoor conditions. We observe that the largest impact is caused by the spectral irradiance distribution (7.6%) followed by angle of incidence (2.7%) and temperature response differential (0.7%). Our modelling indicates that these combine to alter the energy yield for a tandem device by 4.5%.

Determining uncertainty for I–V translation equations

Photovoltaic energy yield predictions rely on methods that estimate outdoor performance under arbitrary conditions based on a set of reference measurements. The uncertainty in these predictions has direct economic impacts on both the perceived feasibility of new systems and the energy market value. A common approach uses a set of translation equations such as those proposed in IEC 60891. Previous work examining the performance of the standard translation methods has not addressed the important issue of uncertainty. This is due to both a lack of comprehensive test data, and the scope of the problem resulting from flexibility in the way these methods can be applied. We used synthetic data to characterize the behavior of these standard translation methods in the presence of input uncertainty. The inputs and their uncertainty can then be mapped to the prediction uncertainty over a wide range of conditions.