Recombination losses

Mohapatra, S.K., Raja, K.S., Mahajan, V.K., and Misra, M. (2008) Efficient photoelectrolysis of water using Ti02 nanotube arrays by minimizing recombination losses with organic additives. Journal of Physical Chemistry C, 112 (29), 11007-11012. 

Fig. 13 Recombination losses occurring under forward bias in a typical OPV device. Holes injected from the anode Fermi level into the HOMO level ( ) of the donor and electrons injected from the cathode Fermi level ( f,c) into the LUMO level ( J of the acceptor are transported to the D/A interface. Coulombic attraction between holes and electrons yields the (D A ) CT state with energy Ect- Charge recombination reaction (D A ) D + A occurs with rate constant...

Fig. 17 Detailed balance approach for determining open circuit voltage based on the energy of the D/A charge-transfer state and its coupling to recombination loss modes. Reproduced with permission from [195]. Copyright 2009 Macmillan Publishers Limited...

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. 

In nanosized particle film electrodes, photogenerated holes can be rapidly transferred to the semiconductor/electrolyte interface and there be captured by the redox species in the electrolyte. In this way, the recombination losses can be diminished. This is of great importance for semiconductors like hematite with a very short hole diffusion length (2-4 nm). Another advantage is the large internal surface area, which characterize nanostructured semiconductor film electrodes. The latter decreases the current density per unit area of semiconductor / electrolyte interface. 

A fascinating aspect of the sensitized colloidal semiconductor films is that injected electrons created throughout the semiconductor network are collected in the external circuit with high efficiency. This implies that carrier transport through the 10 pm thick film occurs with no measurable recombination loss. The mechanisms of carrier transport have been studied in some detail. Carrier transport in a semiconductor film can be described by the continuity equation [155] ... 

This equation is obtained by using eqs. 22 and 23 from Bolton et al. (1980). In the range where eq. 2.78 is valid, the quasi-Fermi level difference fi should be as small as possible in order to avoid recombination losses, i.e., p. = -i- qrj +. ... 

Ross R. T. and Colhns J. M. (1980), Efficiency of quantum-utilizing solar energy converters in the presence of recombination losses , J. Appl. Phys. 51,4504-4507. 

Figure 12.23 Photon energy input and heat losses for a photoelectrode/Uquid junction. (1) Photonic energy input (2) relaxation loss (3) drift loss (4) recombination loss (5) liberation of Peltier heat at the back contact (6) relaxation loss and liberation of Peltier heat at the electrolyte contact.

This high photovoltage is remarkable because it is close to the thermodynamically possible value of around 0.7 V (see also Section 11.1.1.3). Such a high value has not even been achieved in simple p-n homojunctions, for which the lower photovoltage has been interpreted as recombination losses in the depletion layer [23]. None of these losses seem to occur in the n-Si/CH OH liquid junction. Residual losses in the short-circuit arise from optical reflectivity and absorption processes and losses in the fill factor arise from concentration overpotentials and uncompensated series resistance losses from the potentiostat [20]. Neglecting the latter losses, this cell is the first system with which the same efficiency was obtained as that found with Si homojunctions. 

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