Noel Bristow

Outdoor performance of organic photovoltaics: Diurnal analysis, dependence on temperature, irradiance, and degradation

The outdoor dependence of temperature and diurnal irradiance on inverted organic photovoltaic (OPV) module performance has been analysed and benchmarked against monocrystalline-silicon (c-Si) photovoltaic technology. This is first such report and it is observed that OPVs exhibit poorer performance under low light conditions, such as overcast days, as a result of inflexion behaviour in the current-voltage curves, which limits the open-circuit voltage (VOC) and fill factor. These characteristics can be removed by photo-annealing at higher irradiance levels, which occur diurnally as irradiance increases after sunrise. We also report the first temperature coefficients for OPVs from outdoor data; the OPV modules showed a positive temperature coefficient, which compared to a negative coefficient from the c-Si modules. Overall, the cell degradation outdoors appears very severe for these modules and highlights the need for improved barrier.

Application of UV-absorbing silver(I) luminescent down shifter for PTB7 organic solar cells for enhanced efficiency and stability

Presented in this work is the application of a new silver(I) complex [Ag(POP)(Bphen)](BF4) (denoted AgPOP) to provide a means for down-shifting incident ultraviolet (UV) light into the visible range for PTB7-based organic solar cells. The synthesis, material, and optical characterisation of the AgPOP with PTB7 are reported. This material was applied to the OPV light-incident surface without the need for a host polymer. The luminescent down shifting layer serves two purposes. Firstly it improves the photocurrent of the OPV, and thus the efficiency through enhancement of the UV spectral response of the device. This is corroborated by external quantum efficiency data showing enhanced performance at UV wavelengths. Secondly, the AgPOP layer reduces the effect of degradation that originates from UV exposure, which leads to enhanced lifetime.

Three dimensional corrugated organic photovoltaics for building integration; improving the efficiency, oblique angle and diffuse performance of solar cells

The lamination of OPV modules to corrugated roof cladding has been undertaken. The 3-dimensional form of the cladding provides three advantages for outdoor OPV deployment; firstly the ‘footprint’ of the solar cell is reduced, which leads to ∼10% improved power conversion (PCE) efficiency per unit area. Secondly, the oblique angle performance is enhanced, leading to increased output in the early morning and evening. Indoor characterisation showed a 9-fold enhancement in efficiency was obtainable, when compared to a flat module. Thirdly, an improvement in performance under diffuse lighting conditions was measured, when compared to a flat module. The average daily yield of the 3D module was 17–29% higher than a flat module, with higher relative enhancements observed on cloudier days. Geographically, the 3D module appears to be well-suited to countries with a high latitude, due to the enhanced diffuse light levels and the fact that tilting the module in both ‘latitude’ and ‘longitude’ directions away from normal, leads to the best achievable enhancement in solar cell performance. The approach set out in this paper could yield a product that has profound advantages over existing BIPV products and is potentially applicable to other flexible inorganic solar cell technologies.

Development of multidye UV filters for OPVs using luminescent materials

Luminescence down-shifting (LDS) is used in several photovoltaic technologies aiming to improve the photon conversion efficiency (PCE) of the devices through the increase of the light harvesting in the regions of the electromagnetic spectrum where the EQE of the solar cells is poor. The aim of this work was to produce films of mixtures (blends) of two luminescent materials, dispersed in a poly-methyl methacrylate (PMMA) matrix, hoping to improve their properties both as LDS layer and as UV filter when applied on the clear, external surface of P3HT:PC61BM photovoltaic devices. The best results led to an increment of 7.4% in the PCE of the devices, and a six fold enhancement in their half-life (T 50%). This study indicates that multidye LDS layers with optimized optical properties can lead to an effective improvement in the performance and operational stability of OPVs.

Optimisation of the material properties of indium tin oxide layers for use in organic photovoltaics

The influence of indium tin oxide [(In2O3:Sn), ITO] material properties on the output performance of organic photovoltaic (OPV) devices has been modelled and investigated. In particular, the effect of altering carrier concentration (n), thickness (t), and mobility (μe) in ITO films and their impact on the optical performance, parasitic resistances and overall efficiency in OPVs was studied. This enables optimal values of these parameters to be calculated for solar cells made with P3HT:PC61BM and PCPDTBT:PC71BM active layers. The optimal values of n, t and μe are not constant between different OPV active layers and depend on the absorption spectrum of the underlying active layer material system. Consequently, design rules for these optimal values as a function of donor bandgap in bulk-heterojunction active layers have been formulated.