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Self-Powered Systems Chemie low temperatures could potentially facilitate the wider application of ZnO in DSSCs.[22e,38] Novel strategies, such as the integration of ZnO or TiO2 nanowires with 3D optical fibers or planar waveguides, have been shown to address issues such as insufficient internal surface area and deficient dye loading, which are shortcomings of DSSCs based on 1D nanostructures. Such strategies offer the potential for the manufacture of DSSCs with high flexibility, adaptability, and enhanced efficiency for concealed applications.[37b,39] Advances have also been made in the development of photosensitizers and electrolytes to enhance the efficiency of energy conversion in DSSCs. The combination of a donor-p- bridge-acceptor Zn–porphyrin dye and a tris(bipyridyl)cobalt- (II/III)-based redox electrolyte was recently demonstrated to boost the power-conversion efficiency of a DSSC to 12.3%, which is comparable to the performance of thin-film PV devices.[40] The liquid redox electrolyte conventionally used in DSSCs is associated with sealing and long-term-stability issues. It can be replaced by a solid p-type semiconductor to enhance the stability of DSSCs.[41] Currently, DSSCs suffer from stability issues caused by both intrinsic and extrinsic factors, such as degradation of electrolytes and dye molecules by elevated and varied temperatures, humidity, prolonged illumination, and sealant leakage. If DSSCs are to be deployed practically at a level comparable to the use of standard silicon PV technology, substantial effort has to be devoted to the improvement of their stability through scientific development and engineering. It is also technically feasible to engineer DSSCs into flexible formats with mechanical robustness and adaptability for novel applica- tions.[42] DSSCs could be suitable for powering distributed MNSs for self-sufficient, maintenance-free applications, since ideally DSSCs convert sunlight into electricity in a self-sustainable manner, without suffering any permanent chemical trans- formation or consumption of the materials in the system.[43] Although their conversion efficiency is still less than that of the best thin-film cells, DSSCs could still possibly compete with conventional options for electricity generation if their price/performance ratio was improved to meet grid parity by taking advantage of emerging fabrication techniques, such as a roll-to-roll printing process for the large-scale manufacture of DSSCs at low cost with reasonable efficiency. 3.1.2. Organic Solar Cells As a result of the increasing requirement for inexpensive renewable energy sources, organic-polymer-based PV devices (OPV) have been introduced as one option for the production of energy from light at very low cost.[44] The ability to chemically manipulate electrical conductivity through the molecular/electrochemical doping of polymers, together with the development of facile and cheap processing techniques, has significantly boosted the application of polymer-based solar cells. In organic solar cells, excitons are separated into free electron–hole pairs by the effective field created across the heterojunction between two dissimilar organic materials, known as the donor and acceptor molecules.[45] Typical device architectures for organic solar cells include the single layer, the bilayer heterojunction, the bulk heterojunction, and the diffusive bilayer heterojunction.[45b] The conversion efficien- cies for single-layer and bilayer organic solar cells have been below or around 1%, and most recent attention has been focused on solution-processed polymer bulk-heterojunction (BHJ) solar cells, for which conversion efficiencies of around 6–8% have been attained by systematically engineering the polymer properties and morphology and the device struc- ture.[46] The optical absorption coefficient is high for organic molecules, and the cost of fabricating organic solar cells with decent conversion efficiencies could be much lower than the cost of fabricating the current thin-film solar cells. In analogy with DSSCs, emerging fabrication techniques, such as roll-to- roll processing, enables the manufacturing of organic solar cells to be scaled up to a level at which OPV devices are viable and competitive for powering flexible MNSs and even consumer electronics.[47] However, there are still problems associated with OPV cells, such as their relatively low conversion efficiency, their vulnerability to variations in humidity/temperature, and instability in device performance. Organic materials in OPV cells are susceptible to chemical degradation caused by oxygen, moisture, and even reactions with electrode materials, as well as physical degradation resulting from morphological changes and the spatial diffu- sion of materials inside the cells at the microscale.[48] In addition to the conversion efficiency, improvement of the stability, cost, and processing of OPV cells deserves consid- erable attention for the deployment of OPV techniques to become practical. In-depth investigations need to be carried out to gain an understanding of and, more importantly, alleviate degradation (such as the oxidation of organic material upon illumination) and low efficiency due to micro- phase separation during annealing and operation. 3.1.3. Quantum-Dot and Plasmonic Solar Cells Semiconductor quantum dots (QDs), within which the charge carriers are confined spatially by potential barriers, have attracted widespread attention as a result of their novel optoelectronic properties,[49] which can be tailored by simply varying the dimensions of the QDs.[50] QD-based solar cells have also drawn a lot of attention during the past few years owing to the possibility of boosting energy-conversion efficiency beyond the traditional Shockley–Queisser limit for silicon-based solar cells.[51] Significant progress has been made in photoelectrodes based on 3D arrays of QDs of various materials.[52] The relatively delocalized while still quantized states found in QD arrays can facilitate multiple- exciton generation (MEG), which involves the generation of multiple electron–hole pairs upon the absorption of a single photon and can hence considerably increase the conversion efficiency of solar cells. However, measurement of the exact efficiency of MEG is experimentally challenging, and appar- ent inconsistencies between the reported values of quantum yield have led to much controversy in this field.[53] A detailed discussion on the controversy, status, and prospects of research on MEG in nanocrystal QDs can be found else- where.[54] Angew. Chem. Int. Ed. 2012, 51, 2 – 24 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org &&&& These are not the final page numbers! Angewandte Ü ÜPDF Image | Self-Powered Nanosystems
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