Photon-to-current

With the D S SCs containing Hibiscus surattensis extract, the best performances were probably obtained because of the large amount of dye adsorbed on Ti02 (lxl cm2 active surface) in comparison with the other extracts from tropical flowers. Incident photon to current conversion efficiency (IPCE) values of 76% were calculated (2 — 590 nm). Jsc was 5.45 mAcm-2, Voc = 392mV, FF — 54%, and efficiency = 1.14%. Also, the stability of the photovoltaic devices was the best in the case of Hibiscus surattensis, even though it needs to be improved to achieve real long-term stability, especially as far as the sealing quality is concerned. 

The incident monochromatic photon-to-current conversion efficiency (IPCE), also called external quantum efficiency, is defined as the number of electrons generated by light in the external circuit divided by the number of incident photons as a function of excitation wavelength. It is expressed in Equation (7).29 In most cases, the photoaction spectrum overlaps with the absorption spectrum of the sensitizer adsorbed on the semiconductor surface. A high IPCE is a prerequisite for high-power photovoltaic applications, which depends on the sensitizer photon absorption, excited state electron injection, and electron transport to the terminals ... 

Figure 12 Photocurrent action spectra of nanocrystalline Ti02 films sensitized by fe(3,4-dicarboxypyridine) Ru11 (1,4,8,11,15,18,22,25-octamethyl-phthalocyanin) (54). The incident photon to current conversion...

Figure 17 Photocurrent action spectra of bare nanocrystalline Ti02 film, and the sensitizers (1), (22), (24), and (57) adsorbed on Ti02 films. The incident photon to current conversion efficiency is plotted as a...

Figure 20 Incident photon to current conversion efficiency of a nanocrystalline Ti02 cell sensitized by dye...

The overall process performance, as measured by photon efficiency (number of incident photon per molecule reacted, like the incident photon to current conversion efficiency, or IPCE, for PV cells), depends on the chain from the light absorption to acceptor/donor reduction/oxidation, and results from the relative kinetic of the recombination processes and interfacial electron transfer [23, 28]. Essentially, control over the rate of carrier crossing the interface, relative to the rates at which carriers recombine, is fundamental in obtaining the control over the efficiency of a photocatalyst. To suppress bulk- and surface-mediated recombination processes an efficient separation mechanism of the photogenerated carrier should be active. 

Quite differently, Pleux et al. tested a series of three different organic dyads comprising a perylene monoimide (PMI) dye linked to a naphthalene diimide (NDI) or C60 for application in NiO-based DSSCs (Fig. 18.7) [117]. They corroborated a cascade electron flow from the valance band of NiO to PMI and, finally, to C60. Transient absorption measurements in the nanosecond time regime revealed that the presence of C60 extends the charge-separated state lifetime compared to just PMI. This fact enhanced the device efficiencies up to values of 0.04 and 0.06% when CoII/m and P/Ij electrolytes were utilized, respectively. More striking than the efficiencies is the remarkable incident photon-to-current efficiency spectrum, which features values of around 57% associated to photocurrent densities of 1.88 mA/cm2. 

A very useful parameter for evaluating the performance of a photoelectrolysis cell is the incident photon to current conversion efficiency (IPCE). This is a measure of the effectiveness in converting photons incident on the cell to photocurrent flowing between the working and counter electrodes. IPCE is also called the external quantum efficiency. 

Fig. 3.24 Incident photon to current efficiency (IPCE) spectrum of a titania nanotube array photoelectrode.

Steady-state wavelength-specific photocurrents were measured for the Ti-Fe-0 films in a two-electrode arrangement at different applied voltages. Incident photon-to-current efficiencies (IPCE) are calculated using the following equation ... 

Fig. 5.60 Absorbed-photon-to-current-efficiency APCE) of Ti-Fe-0 nanotube array samples at (a) 0.5 V bias, and (b) 0.7 V bias.

The performance of the cell can be quantified on a macroscopic level with parameters such as incident photon to current efficiency (IPCE), open-circuit photo voltage (Voc), and the overall efficiency of the photovoltaic cell (i7ceM). 

An analogous behavior extends to other species having small reorganization energies and appropriate potentials such as the iron(II) complexes Fe(DMB)32 + and Fe(DTB)32 + (Ey2 0.95 V versus SCE). When used in the presence of an excess of Co(DTB)32 + and in conjunction with suitable sensitizers like the heteroleptic dye Ru(dnbpy)(H2DCB)22+ (Em = 1.25 V versus SCE) (Fig. 17.28), the iron(II) comediators clearly enhance the performance of the Co(DTB)32+ and outperform the I /I3 redox couple, at least in terms of monochromatic photon to current conversion efficiency, with maximum values close to 85%. 

Figure 7 shows the schematic energy diagram of a DSSC. The following steps contribute the photon to current conversion ... 

In contrast to a conventional p-n-junction-type solar cell, the mechanism of the DSSC does not involve a charge-recombination process between electrons and holes because electrons are injected from the dye photosensitizers into the semiconductor, and holes are not formed in the valence band of the semiconductor. In addition, electron transport takes place in the Ti02 film, which is separated from the photon absorption sites (i.e., the photosensitizers) thus, effective charge separation is expected. This photon-to-current conversion mechanism of the DSSC is similar to that for photosynthesis in nature, where chlorophyll functions as the photosensitizer and electron transport occurs in the membrane. 

Photovoltaic performance of the DSSC is described as follows Figure 8 shows the external spectral response curve of the photocurrent for nanocrystalline Ti02 solar cells sensitized by N3 and black dyes with the I /If redox mediator, where the incident photon-to-current conversion efficiency (IPCE) is represented as a function of wavelength. IPCE is obtained by the following equation ... 

It has been considered that the high stability of the dye in a DSSC system could be obtained by the presence of I - ions as the electron donor to dye cauons. Degradation of the NCS ligand to the CN ligand by a intramolecular electron-transfer reaction, which reduces consequently the Ru(III) state to the Ru(II) state, occurs within 0.1-1 sec [153], whereas the rate for the reduction of Ru(in) to Ru(II) by the direct electron transfer from I ions into the dye cations is on the order of nanoseconds [30]. This indicates that one molecule of N3 dye can contribute to the photon-to-current conversion process with a turnover number of at least 107—10s without any degradation [153]. Taking this into consideration, N3 dye is considered to be sufficiently stable in the redox electrolyte under irradiation. 

D. Incident Photon to Current Efficiency and Open-Circuit Photovoltage... 

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