Tandem devices

Figure 23 Photograph of a working tandem device based on a thin film of polycrystalline tungsten trioxide...

In order to understand the performance of the tandem device, low temperature transport studies are a valuable tool. Diodes made from pristine MDMO-PPV and in composites with PTPTB are compared. ITO/PEDOT and Au electrodes are chosen to guarantee hole-only devices. This special choice of the electrodes is a successful technique for improving our understanding of transport failures. The proper choice of contacts allows us to produce p-type or n-type diodes from the same semiconductor, depending on the selectivity of the contact. For instance, Au is a hole-injection contact for most of the polymeric semiconductors, while Ca is an electron-injection... 

A tandem device that achieves the direct cleavage of water into hydrogen and oxygen by visible light was developed in collaboration with two partner groups from the Universities of Geneva and Bern [123]. This is based on the in-series connection of two photosystems. A thin transparent film of nanocrystalline tungsten trioxide or ferric oxide absorbs the blue part of the solar spectrum (Eq. (63)). 

The experimental f-V characteristics of this tandem cell in the dark and under various intensities of simulated AM1.5 illumination are shown in Figure 12.25 (Xue et al., 2004b). Similar to single cells, the dark current of the two-cell tandem device exhibits diode characteristics with a high rectification ratio of lO -lO at 1.5 V. By fitting the forward-bias characteristics... 

Tandem OLED Devices 14.3.6.1 Tandem Device Architecture... 

Recently, a tandem (or stacked) OLED structure was developed in order to improve luminous efficiency and to increase lifetime. This was accomplished by vertically stacking several individual electroluminescent (EL) units, each of which comprise a HTL EML ETL, and driving the entire device with a single power source. The schematic structure of the tandem device is shown in Figure 14.19. 

Emission spectra of the white tandem devices with various EL units. (Reproduced from Spindler, J.P. and Hatwar, T.K., SID Int. Symp. Dig. Tech. Papers, 38,89,2007. With permission.)... 

Although the luminance efficiency for the tandem device structure shown in Figure 14.21 was sufficiently high, the higher voltage resulted in lower... 

Figure 2.14 Schematic arrangement for a monolithic tandem device for iight-induced water splitting W, space charge region (extended under the cataiyst particies that...

One of the most serious drawbacks that has been observed in the ionisation process with TSP, APCI, ESI interfaces, and also with FAB, is the soft ionisation of the analytes which mostly leads to molecular ions or molecular adduct ions. Though molecular mass information is provided, there is little or no structural information at all observable with PBI or electron impact (El) MS. This soft ionisation is clearly disadvantageous for any identification of environmental contaminants, since it generates either considerably less or no fragments at all, and hence is unable to confirm the presence of such compounds of environmental concern. With the commercial availability of tandem devices, tandem mass spectrometry (MS/MS) helped to overcome these identification obstacles via coUision-induced dissociation (CID) in MS/MS mode or via ion trap in MS mode. Today, even bench-top machines provide the possibility of MS . However, when TSP began to become the method of choice in environmental analysis and became commercially available, MS/MS technology was still quite expensive. Users of TSP ionisation with spectrometers not amenable for MS/MS had the possibility to record... 

Only a few materials were studied concerning their applicability to dye-sensitized hole injection processes. Among those are different copper(I) compounds (e.g. Cu(I)SCN, Cu(I)I, Cu2(I)0 [33-35]) and nickel(II) oxide [36]. Photovoltaic performances of such devices are orders of magnitudes poorer than those of classical dye-sensitized photoelectrochemical solar cells based on n-type materials. Substantial advantages could arise if an efficient photo-hole injection process would be available. The formation of solid-state tandem solar cells would become feasible, and a quantum step in device efficiency of dye-sensitized solar cells could be at reach. However, because of the poor performance of all known photocathodes, a combination of available photoanodes and photocathodes to a tandem device always results in a device that is photovoltaically less efficient than the photoanode on its own. The concept for electrolyte-based tandem cells exists. However, it contains strong potential to improve the photovoltaic performance in both electrolytic and in solid-state, dye-sensitized solar cells. 

As with the single-junction PV or PEC devices, efficiency bounds can be placed on multijunction configurations based on optical absorption limits. Figure 7.23 is a two-dimensi(Mial extension of Fig. 7.15 for tandem devices, where maximum photocurrent, and the corresponding STH levels for PEC devices, are calculated as a function of both top- and bottom-junction bandgaps. The assumptions included in the derivation of this graph are as follows ... 

Photoelectrode-based PEC-PEC tandem devices incorporating a coupled photoanode and photocathode. 

Hybrid photoelectrode PV-PEC tandem devices comprising a PEC photoanode or photocathode integrated with a buried single-junction PV cell. 

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