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Internal Quantum Efficiency limitations in Organic Photovoltaics

Final Report Summary - IQEOPV (Internal Quantum Efficiency limitations in Organic Photovoltaics)

The EU funded research project IQEOPV, Internal Quantum Efficiency limitations in Organic Photovoltaics, has in large been devoted to raise the understanding of the main limiting processes in organic photovoltaics devices. The detailed mechanisms for photogeneration and non-geminate recombination of charge carriers in these solar cells strongly depend on the voltage across the device, either due to a field dependent geminate recombination rate or a charge carrier concentration dependent non-geminate recombination rate. Therefore, we have focused our investigation within this project on the voltage and temperature dependence of recombination mechanisms and their relation to charge transfer states as a mediator of both geminate and non-geminate recombination.
A detailed set of studies has provided us with deeper insights into the overall voltage dependence on recombination. The parameter characterizing the voltage dependence is the diode ideality factor. We have for the first time appropriately determined this ideality factor for a set of organic solar cells over a large range of charge carrier concentrations and temperatures. From our accurate determination of the real diode diffusion current, we observed that the open circuit voltage for many high bandgap materials is clearly deviating from the expected logarithmic behavior relative to current (or light intensity) when approaching high currents or light intensities. As surface recombination occurring at a non-selective electrode is likely accountable for this deviation, it should be considered a substantial culprit in limiting the voltage under solar illumination conditions. This confirmed limitation is certainly associated with a non-radiative transition at the electrode.
We have further been able to relate the ideality factor directly to the voltage dependency of radiative efficiency for OPVs. The soft character of the polymer and fullerene materials used in organic photovoltaics has been demonstrated to substantially affect the spectral energy tail of absorption as well as the spectral energy onset of radiative recombination. This considerably limits the performance of soft material solar cells in such a way that the achievable photovoltage is remaining far below the theoretical upper thermodynamic limit. We identified the relaxation energy of charge carriers within the charge transfer states as major factor for this voltage loss.
A significant improvement of the important figure of merit for potential of a material to achieve high photovoltaic power conversion efficiencies — the radiative efficiency — was however clearly identified with the new type of solar cells based on organic-inorganic methylammonium lead iodide perovskites. This device type does not suffer of the same relaxation energy loss we reported for the organic solar cells. As these hybrid solar cells can also be processed from solution, they will most likely open a new route to achieve easily manufactured large area photovoltaics with substantially lower parasitic recombination and accordingly a higher open circuit voltage and power conversion efficiency.
Pure organic photovoltaics, however, still have to overcome substantial obstacles before being able to contribute significantly to the production of renewable energy. Their soft structure may limit their potential to reach very high power conversion efficiencies. However, as we were able to identify this shortcoming, strategies to circumvent it can be investigated in the near future. This is important, as organic photovoltaics promises unparalleled ease of manufacturing and immediate large scale/large area advantage associated with solution processing, still strongly motivates further research in the field of organic photovoltaics. Several types of higher efficiency solar cells will never be implemented since they cannot be manufactured cheap over large areas, in contrast to organic photovoltaics. The fact that sunlight is a rather dilute form of energy, in the end always forces any technology, which intends to harvest large amounts of sunlight, to be compatible with very large area implementation. A continuation of funding through organizations on national and European level is therefore still of high importance to develop this market in view of renewable energy production. Therefore, the better understanding of OPV operational principles and the identification of major loss mechanisms is not only of academic significance, but will in the long run also contribute towards a more environmentally friendly society in Europe. The skills acquired by the applicant during the IEF program will contribute to benefiting European excellence, as the fellow has strong intentions to further his career in Europe and to continue pursuing his scientific and technical motivations in the field of organic photovoltaics. The results of the Marie Curie program are further projected to be of value for the coming European OPV industry. Independent of weather the OPV industry targets the smaller consumer electronics market or the huge general energy market; any such initiatives will inevitably depend on higher power conversion efficiency to, in the end, reach appropriate financial revenues and profits.
Collaborations between scientists from various groups in different countries is indeed a key aspects in research progress on a European level, as can be seen in many examples where a continuous improvement and development occurs in small incremental steps that unlikely takes place alone in just a sole research group. Close connection between researchers, bringing together different types of expertise, thus substantially accelerates the general development of the much needed cheaper photovoltaic alternatives.