Open-circuit voltage

Summary

Open-circuit voltage (abbreviated as OCV or VOC) is the difference of electrical potential between two terminals of an electronic device when disconnected from any circuit. There is no external load connected. No external electric current flows between the terminals. Alternatively, the open-circuit voltage may be thought of as the voltage that must be applied to a solar cell or a battery to stop the current. It is sometimes given the symbol Voc. In network analysis this voltage is also known as the Thévenin voltage. The open-circuit voltages of batteries and solar cells are often quoted under particular conditions (state-of-charge, illumination, temperature, etc.). The potential difference mentioned for batteries and cells is usually the open-circuit voltage. The value of the open-circuit voltage of a transducer equals its electromotive force (emf), which is the maximum potential difference it can produce when not providing current. Example Consider the circuit: If w

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Development of high band gap protocrystalline material for p-i-n thin film silicon solar cells

Fabio Maurizio

High band gap thin film silicon solar cells are of high interest for the use as top cell in triple junctions solar cells. Protocrystalline silicon, in the transition phase between amorphous and microcrystalline silicon, is such a high band gap material. During this project, protocrystalline intrinsic layers have been developed and optimized in p-i-n single junction solar cells by plasma-enhanced chemical vapour deposition (PECVD) in a new multi-chamber deposition system. In the first series, cells and layers were deposited with different dilutions going from silane to hydrogen flux ratios of 1:1 to 1:64. At the dilutions 1:16 and 1:32, protocrystalline ma-terial was obtained which showed amorphous characteristics only up to a certain thickness, where crystallites started to evolve. With dilution 1:16, a second cell series has been deposited with variation of the intrinsic layer thick-ness. The best cells in terms of open-circuit voltage and fill factor product were obtained with an i-layer thickness of 100 nm. These cells have open-circuit voltages and fill factors up to 0.975 V and 75.8 % (0.932 V and 69.9 %) for the initial (degraded) state. These are promising values for the use of this material in top cells. However, the low current density means that the optimum i-layer thickness is probably higher for triple cell application. Further cells have been deposited at different temperatures. These results imply that there is still room for further optimization, when current limitations of the heating system will be overcome.

2012

Electronic transport properties of vertical and lateral graphene-based heterostructures

Roberto Urcuyo Solorzano

Graphene has some drawbacks, like the absence of an electronic band gap which leads to only small gate switching ratios, and its fast carrier recombination that limits its use in optoelectronics. In order to overcome these issues, the aim of this thesis was to tune the properties of graphene by two different modification approaches. The first approach involved chemical functionalization to create lateral graphene heterostructures, whereas the second approach was directed toward vertical heterostructures that combine graphene with another 2D material. For the former case, two novel chemical functionalization methods were investigated. The first method relied upon hyperthermal molecular ion collisions with 4,4¿-azobis(pyridine). This enabled the covalent functionalization of graphene with a functionalization degree of up to 3%, as demonstrated by Raman in combination with XPS. AFM studies revealed that the functionalized sheets retain their topographic integrity. Thus obtained stripe-like patterns of covalently graphene within extended graphene sheets enabled enhanced on/off ratios upon gate switching. In the second functionalization, OsO4 was used to selectively introduce hydroxyl groups to graphene. Best results were obtained using UV light activation, which yielded graphene of high covalent functionalization degree, as concluded from its optical transparency and gate-induced on/off switching ratios of up to 500. Temperature dependent electrical measurements revealed 2D hopping as the dominant transport mechanism. The first type of vertical heterostructure, gr-TiOx-Ti diodes were fabricated and their electrical properties in the dark and under visible light illumination studied. In contrast to conventional MIM diodes, the performance of the graphene-based diodes was found to increase with decreasing thickness of the oxide insulator. Bias-induced modulation of the work function of graphene was identified as the key to the operation mechanism of the graphene diodes, enabling them to reach a very high asymmetry and nonlinearity (9000 and 8). The diodes compete well with state-of-the-art MIM diodes. Furthermore, the graphene-TiOx-Ti diodes could be operated as photovoltaic cell, with a maximum open-circuit voltage of 0.3 V and a short-circuit photocurrent of 14 nA under global illuminationt. This finding constitutes the first proof-of-principle of hot carrier extraction from graphene, based on the photovoltaic effect. The second type of vertical heterostructures was based on black phosphorus (BP). As a first step, thin BP sheets were combined with n-GaAs into novel pn devices. They displayed pronounced rectification behavior that, in the low bias regime, approaches that of ideal diodes. Moreover, when operated as photodiodes they reached external quantum efficiencies (EQE) above 30% under higher reverse bias. On this basis, the bP was then combined with gr into a p+/p heterojunction. Thus obtained devices achieved a maximum internal quantum efficiency (IQE) and responsivity of 13% and 10.7 mA/W, respectively. This IQE is the highest thus far reported for diodes comprising BP and another 2D material. Moreover, with the aid of a top gate it was possible to modulate both, the photo-conversion efficiency and photocurrent generation mechanism of the black phosphorous-graphene diodes. Together with the possibility to tune the photocurrent response by the thickness of the black phosphorous, these diodes emerge as promising photodetectors.

EPFL2016

Due to their high efficiencies and their potentially low production costs, dye-sensitized solar cells (DSSC) have attracted much attention during the last few years. The technology is based on a layer made of mesoscopic TiO2 film which significantly increases the optical path for light harvesting by the surface-anchored sensitizer molecules, whilst keeping an efficient contact with the electrolytic solution. These sensitizer molecules are often based on ruthenium polypyridyl complexes because of their high absorption coefficients in the visible, but any dye could be used to sensitize the semiconducting TiO2. The standard DSSC usually contain organic electrolytes with dissolved triiodide/iodide as the redox couple. Cells based on alternative electrolytes have been developed and were used in early DSSC designs, but showed somehow deceiving photovoltaic performances. Until now, endeavors to increase the photovoltage of dye-sensitized nanocrystalline cells by replacing iodide by a one-electron, outer-sphere redox couple with a more positive oxidation potential have not succeeded. This has been largely due to limitations imposed by the dark current and the slow diffusion of the alternate mediator. The work presented here focused on finding efficient alternative redox couples for the DSSC, able to rival the classical triiodide/iodide couple. The tested compounds were triarylamines, platinum complexes and cobalt complexes. Triarylamines and platinum complexes proved considerably less effective than the classical couple in term of photoelectrochemical efficiencies. Cobalt complexes however displayed far more interesting behavior. The experiments carried out demonstrated that the [Co(bip)2]3+/2+ complexes were the most suitable candidates to be used as redox mediators as their photoelectrochemical and kinetic behavior rivaled those of the triiodide/iodide redox couple. The system however had to be optimized to reach high efficiencies. Heteroleptic dyes incorporating long alkyl chains were found to be the most appropriate category of dye, boosting further the DSSC efficiency by reducing the dark current within the cells. Co-grafting the sensitizer and co-adsorbent led to an improved photovoltaic efficiency through the increased photocurrent. Silver addition in the dye solution also leads to a global improvement of the photovoltaic parameters. To avoid diffusion problems with the large molecules in the electrolyte, very thin nanocrystalline TiO2 layers had to be employed. A 3 μm thick nanoporous layer with a 3 μm scattering layer was found to be optimal. As cobalt complexes may undergo electrochemical processes on the SnO2 layer of the photoelectrode, a blocking layer of compact TiO2 was used to reduce the dark current. The electrochemical regeneration of the cobalt complexes with the SnO2 is not fast enough to support cell function and the counter electrode must be covered either with platinum or gold layer to ensure a good fill factor. The optimization of the electrolyte itself mainly concerned the choice of the solvent, the redox mediator concentration, the best degree of oxidation, the additives that can be used to improve the photovoltaic parameters and finally how to increase the redox couple solubility. The best efficiencies were achieved with an acetonitrile/ethylene carbonate (2:3) solvent mixture, with a total redox mediator concentration between 0.10 to 0.15 M. If 10% of this total concentration is oxidized, the current density reaches its maximum whilst having a negligible effect on the photovoltage. 4-tert-butylpyridine (TBP) and lithium perchlorate were tried as additives in the electrolyte, as TBP increases the photovoltage and Li+ the photocurrent. The optimal concentrations of these two additives was found to be around 0.05 M TBP and 0.1 M LiClO4 for an electrolyte having a total cobalt complex concentration of 0.1 M. Mass-transport within the cell is the limiting factor for high currents when using cobalt complexes as redox mediators. Attempts to reduce the solvent viscosity by using alternative solvents to the acetonitrile/ethylene carbonate mixture were not successful. One way to solve the diffusion problem was to irradiate the cell from the counter electrode side. This method works well at high irradiation intensities. Finally, reduction of the distance between the electrodes also enhances diffusion. The use of [Co(bip)2]3+/2+ complexes, with appropriate dyes and taking into account the optimizations already described above, allowed efficiencies of 8% under an irradiance of 95 W · m-2, as well as IPCEs over 80% at 540 nm to be obtained. At full sun, efficiencies of over 4% were reached, with short circuit current densities of 9.5 mA · cm-2 and open circuit voltages of the order of 860 mV.

EPFL2004