Photocurrent losses

Photocurrent losses have been recorded for electrolytes dosed with electron acceptors such as O2 and iodine [341]. Nanocrystalline Ti02 electrodes with thick-... 

Photocurrent losses have been recorded for electrolytes dosed with electron acceptors such as O2 and iodine [226]. Nanocrystalline Ti02 electrodes with thicknesses ranging from 2 pm to 38 pm were included in this study. In the presence of these electron-capture agents, electron collection (i.e. photocurrent) at the rear contact was seriously compromised. On the other hand, as high as 10% of the photons were converted to current for a 38 pm thick film in a N2-purged solution [226]. 

As pointed out in the previous section, free carrier generation in P3HT PCBM blends proceeds at the picosecond time scale, unassisted by the applied electric field. It must, therefore, be the free carrier recombination (non-geminate recombination) in competition with charge extraction that accounts for most of the photocurrent loss within the working regime of P3HT PCBM blends. 

The cell efficiency was 3.65%. After illuminating for 3192 h, the short-circuit photocurrent density of the monolithic solid-state DSSC decreased from 7.44 to 6.83 mA cm. The major loss in short-circuit photocurrent occurred within the first 1272 h of the stability test (5.9%). Photocurrent losses over the following 1920 h were less than 2.4%, indicating that the initial photocurrent losses had plateaued. However, the open-circuit voltage increased steadily from 524 to 665 mV for the same period. As a result, the overall energy conversion efficiency of the monolithic device increased from 2.73 to 3.09% after 3192 h. 

It was observed in other works that in sulfide electrolyte, decomposition of ZnSe was still obtained stable PECs could be constructed though from singlecrystal, n-type, Al-doped ZnSe electrodes and aqueous diselenide or ditelluride electrolytes [124]. Long-term experiments in these electrolytes were accompanied by little electrode weight loss, while relatively constant photocurrents and lack of surface damage were obtained, as well as competitive electrolyte oxidation. Photoluminescence and electroluminescence from the n-ZnSe Al electrodes were investigated. 

Gartner s equation can be derived by calculating that part of the photocurrent which comes from the bulk. The concentration p(x) of holes obeys the following equation, which combines the familiar diffusion equation with a source and a loss term ... 

A p-type electrode is in depletion if a cathodic bias is applied. Illumination generates one electron per absorbed photon, which is collected by the SCR and transferred to the electrolyte. It requires two electrons to form one hydrogen molecule. If the photocurrent at this electrode is compared to that obtained by a silicon photodiode of the same size the quantum efficiencies are observed to be the same for the solid-state contact and the electrolyte contact, as shown in Fig. 4.13. If losses by reflection or recombination in the bulk are neglected the quantum efficiency of the electrode is 1. 

Oxidized MWCNTs have also been tested in conjunction with solid-state electrolytes [107]. Compared to pristine MWCNTs, the oxidized MWCNTs have a better miscibility with the ionic liquids used in the electrolyte. Overall, a much improved gelforming ability resulted. The latter was clearly reflected in the device performance. In particular, devices with oxidized MWCNTs outperformed those with pristine MWCNTs and the reference devices in terms of photocurrents, Vocs, and efficiencies. Importantly, the device stability was also greatly enhanced when oxidized MWCNTs were implemented - 100 days with a loss of overall efficiency by less than 10 °/o. The authors ascribed the drop in efficiency to phase separation and subsequent leakage of ionic liquids. 

Ip( i) is the photocurrent density at wavelength X. IPCE becomes 100% when all photons generate electron-hole pairs. However, in practical situations IPCE is always less than 100% due to the losses corresponding to the reflection of incident photons, their imperfect absorption by the semiconductor and recombination of charge carriers within the semiconductor, etc. 

This is clearly illustrated in the case of p-GaP as shown in Figure 5. In a 0.1 M NaOH solution decomposition is by Ga loss from the surface as the hydroxide, and photocurrent decay is rapid as a phosphate layer builds on the surface. In 0.1 M HgPO solution decay is again rapid but in this case the Ga is not solubilized but deposits on the surface as the metal. In the more oxidizing acids such as H2SO4 both Ga and P are removed from the surface and the photocurrent remains high as the surface is essentially photoetched. 

A lack of stabilization of the silicon means that when holes reach the surface the oxide continually grows. In our measurements on silicon, the loss of stability is thus observed by the decrease in photocurrent at a given voltage as the oxide grows. 

Unprimed, solid-line curves are photocurrent (left-hand scale) and primed, dotted-line curves are emission intensity (right-hand scale) monitored at Kmax 600 nm. Curves A and A result from excitation at 501.7-nm, 23°C Curves B and B from 514.5-nm, 23< C Curves C and C , 49°C and 501.7-nm excitation Curves D and D 86°C, 514.5-nm irradiation. Note that the ordinate of Curve D has been expanded by a factor of 10. Equivalent numbers of 501.7- and 514.5-nm photons were used to excite the photoelectrode in identical geometric configurations. The exposed electrode area is 0.41 cm2, corresponding to an estimated x for 501.7-nm excitation at 23°C and +0.7 V vs. Ag (PRE) of 0.50, uncorrected for solution absorbance and reflectance losses (9). 

Recombination in the depletion layer can become important when the concentration of minority carriers at the interface exceeds the majority carrier concentration. Under illumination minority carrier buildup at the semiconductor-electrolyte interface can occur due to slow charge transfer. Thus surface inversion may occur and recombination in the depletion region can become the dominant mechanism accounting for loss in photocurrent. 

Mechanisms 1 and 2 are included in the model that is used here for comparison with experimental data. Interface recombination and dark current effects are not included however, the experimental data have been adjusted to exclude the effects of dark current. To include the additional bulk and depletion layer recombination losses, the diffusion equation for minority carriers is solved using boundary conditions relevant to the S-E junction (i.e., the photocurrent is linearly related to the concentration of minority carriers at the interface). Using this boundary condition and assuming quasi-equilibrium conditions (flat quasi-Fermi levels) ( 4 ) in the depletion region, the following current-voltage relationship is obtained. 

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