Min R.,Huazhong University of Science and Technology |
Zhang Q.,IMRA Europe |
Tong Q.,Huazhong University of Science and Technology |
Zou X.,Huazhong University of Science and Technology |
And 2 more authors.
IEEE Transactions on Power Electronics | Year: 2017
Exploring high-performance controller for buck converter is challenging since it can be easily affected by converter model accuracy. In this paper, a novel nonaveraged current discrete-time (NCD) model is proposed, in which inductor current is expressed as time-varying equations during switch-on and switch-off states. It achieves higher accuracy than the conventional averaged model at high-frequency range, thus can be used to optimize high-speed controller design. Based on the NCD model, a multiloop minimum switching cycle (MMSC) control strategy, composed of output feedback (OF), line feed forward (LFF), and reference feed forward (RFF) loops, is proposed and tuned for buck converter operating in continuous conduction mode. Mutual influences among three loops are considered and eliminated by specifically designed LFF and RFF compensations, which adapt the OF compensation. With consideration of sampling and calculation delays, relationship between transient switching cycles and geometric center of controller poles is discovered from a calculated output voltage error series. Furthermore, theoretical minimum switching cycles are calculated by moving the center inside the unit cycle of complex plane, which ensures system stability. Moreover, load/line transient response and reference tracking time are simultaneously optimized to the minimum switching cycles. Effectiveness of the controller is proved by converter closed-loop pole/zero plots, transient response simulations, and experiments. © 1986-2012 IEEE.
Clarke T.M.,University of Wollongong |
Peet J.,Konarka Technologies |
Nattestad A.,University of Wollongong |
Drolet N.,Konarka Technologies |
And 4 more authors.
Organic Electronics: physics, materials, applications | Year: 2012
Organic photovoltaic devices based on the donor:acceptor blend of poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4′, 7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C 61 butyric acid methyl ester (PCBM) have received considerable attention in recent years due to their high power conversion efficiencies and the ability to achieve close to 100% internal quantum efficiency. However, the highest efficiencies were all attained using active layers of less than 100 nm, which is not ideal for either maximised potential performance or commercial viability. Furthermore, more recent reports have documented significant charge carrier trapping in these devices. In this paper two charge extraction techniques (photo-CELIV and time-of-flight) have been used to investigate the mobility and recombination behaviour in a series of PCDTBT:PCBM devices. The results not only confirm significant charge carrier trapping in this system, but also reveal close to Langevin-type bimolecular recombination. The Langevin recombination causes a short charge carrier lifetime that results in a short drift length. The combination of these two characteristics (trapping and fast bimolecular recombination) has a detrimental effect on the charge extraction efficiency when active layers greater than ∼100 nm are used. This accounts for the pronounced decrease in fill factor with increasing active layer thickness that is typically observed in PCDTBT:PCBM devices. © 2012 Elsevier B.V. All rights reserved.
Bi Y.,Technical University of Delft |
Felter K.,Technical University of Delft |
Hoogland S.,King's College |
Grozema F.C.,Technical University of Delft |
And 3 more authors.
Journal of Physical Chemistry C | Year: 2016
The low photovoltaic efficiency of iron pyrite-based solar cells is often related to the presence of sulfur deficiencies. In this paper surfur-rich iron pyrite nanocrystals (FeS2 NCs) are synthesized by the hot injection method and deposited using layer by layer deposition. Optical absorption measurements show substantial sub-bandgap absorption, which is attributed to a sulfur-rich, thin surface layer. Microwave photoconductance measurements show very little signal of films with the original long ligands, while an approximately 100-fold higher signal is observed for films treated with FeCl2 and 1,2-ethanedithiol (EDT) solutions. In mesoporous hybrid systems of FeS2/SnO2 both sub-band-gap and above-band-gap photons lead to electron injection from FeS2 into the SnO2 conduction band. We explain these findings by proposing that pinning of the Fermi level by the surface layer leads to a downward band bending in the direction of the surface within the FeS2 NC. Hence, photoexcited electrons will first move toward the shell where they relax into empty surface states. As the holes remain behind in the core of the nanocrystals, this results in a charge-separated state with a long lifetime. Interestingly, these surface electrons are able to migrate in the FeS2 NP layer by interparticle tunneling and can still decay by injection into SnO2. Hence, our results indicate that SnO2 is a suitable electron acceptor for FeS2. The long-lived charge-separated electrons and holes could be exploited efficiently in photodetectors. © 2016 American Chemical Society.
Cowan S.R.,University of California at Santa Barbara |
Leong W.L.,University of California at Santa Barbara |
Banerji N.,University of California at Santa Barbara |
Dennler G.,IMRA Europe |
Heeger A.J.,University of California at Santa Barbara
Advanced Functional Materials | Year: 2011
Small amounts of impurity, even one part in one thousand, in polymer bulk heterojunction solar cells can alter the electronic properties of the device, including reducing the open circuit voltage, the short circuit current and the fill factor. Steady state studies show a dramatic increase in the trap-assisted recombination rate when [6,6]-phenyl C 84 butyric acid methyl ester (PC 84BM) is introduced as a trap site in polymer bulk heterojunction solar cells made of a blend of the copolymer poly[N-9″-hepta-decanyl-2,7- carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′, 3′-benzothiadiazole) (PCDTBT) and the fullerene derivative [6,6]-phenyl C 61 butyric acid methyl ester (PC 60BM). The trap density dependent recombination studied here can be described as a combination of bimolecular and Shockley-Read-Hall recombination; the latter is dramatically enhanced by the addition of the PC 84BM traps. This study reveals the importance of impurities in limiting the efficiency of organic solar cell devices and gives insight into the mechanism of the trap-induced recombination loss. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Boix P.P.,Jaume I University |
Larramona G.,IMRA Europe |
Jacob A.,IMRA Europe |
Delatouche B.,IMRA Europe |
And 2 more authors.
Journal of Physical Chemistry C | Year: 2012
All-solid semiconductor-sensitized solar cells lack models allowing their characterization in terms of the fundamental processes of charge transport and recombination. Nanostructured TiO 2/Sb 2S 3/CuSCN solar cells were characterized by impedance spectroscopy, and a model was proposed for this type of cells. One important feature resulting from this analysis was the hole transport diffusion, which could be assimilated to a series resistance affecting the cell fill factor. The other important feature was the recombination rate, which could be described in a similar manner as other cells using nanostructured TiO 2 electrodes and which had an important impact on the open circuit. A simulation of the current-voltage curves using such model allowed us to get an approximate quantification of the losses caused by each process and to evaluate the possible improvements on the performance of this kind of cell. © 2011 American Chemical Society.