Current drift

When the temperature of a solar cell rises, cell conversion efficiency decreases because the additional thermal energy increases the thermally generated minority (dark-drift) current. This increase in dark-drift current is balanced in the cell by lowering the built-in barrier potential, lU, to boost the majority diffusion current. The drop in F causes a decrease in and F. Therefore, a cell s output, ie, the product of F and decreases with increasing cell temperature. is less sensitive to temperature changes than F and actually increases with temperature. 

If we place an ionic conductor between parallel-plate blocking electrodes that produce an electric field E parallel to the x-axis, the electrostatic potential varies as — xE on passing from one electrode at x = 0 to the other. At equilibrium, the mobile-ion concentration Cj(x) is proportional to exp(qEx/kT), and the ionic drift-current density (7(E in the field is balanced by a diffusion current due to the concentration gradient (Fick s law) ... 

In an exact calculation of the distribution of the electrostatic potential, the carrier densities and their currents, (4.81)-(4.87) are solved simultaneously, bearing in mind that only the sum of the diffusion and drift currents has physical significance. Due to the complexity of the above relations and in particular due to the coupling of electron and hole concentrations by Poisson s equation, analytical solutions exist only for a few, very specific conditions. Generally, the determination of local carrier concentrations, current densities, recombination rates, etc., requires extensive numerical procedures. This is especially true if they vary with time, but even in the steady state context. 

First, the drift current is calculated in the case of a constant electrical field, as one would expect for very thin bulk heterojunction solar cells. If the width W of the active layer is similar to the drift length of the carrier, the device will behave as a MIM junction, where the intrinsic semiconductor is fully depleted. The current is then determined by integrating the generation rate G = —dP/dx over the active layer, where P is the photon flux ... 

Assuming loss-free sweep-out of the carriers due to the driving force of the electrical field, the drift current is given by... 

Fig. 5.26. Schematic drawing of the absorption profile, energy bands and diffusion and drift current contributions, together with the minority carrier concentration of an n-type semiconductor under illumination...

Figure 5.26 sumarizes the findings. Bulk heterojunction solar cells, especially in the thin film limit, are expected to be dominated by the drift current. 

Note that, in the forward bias mode for an np rectifier, electrons from the n region, which carry the so-called drift current" of majority carriers, rush... 

The total current in the emitter IE, not too far from the emitter-to-base junction, consists of two contributions the electron current (drift current) 1 e, which proceeds under forward bias toward the emitter-to-base junction, and the much smaller hole current (a diffusion current), which originates in the base and proceeds in the opposite direction, but decays exponentially, as the distance from the junction increases. The total current in the collector, close to the base-to-collector junction, consists of electrons l c (a large fraction of 1 e) that have somehow evaded capture within the base and proceed against reverse bias in the collector region. The rest of the electron current in the collector is what in pn diodes is called reverse saturation current frs, and here it is called collector current with zero emitter current Iqo = hs- l c is the "useful" electron flow in the transistor. 

At low voltage the diffusion currents due to the majority carriers will increase while the drift currents will remain unchanged. Further increase in voltage will eventually reduce the potential barrier to zero, and the electrons and holes will move across the junction and recombine with the majority carriers of the other side of the junction. Thus a forward current consisting of majority carriers now flows across the junction and increases rapidly with only a small increase in voltage. 

The internal electric field F that drives this drift current under photovoltaic operation generally originates from the difference in the electrode work fimc-... 

Fig. 17 Simple relationship of open circuit voltage Vqc for drift-current dominated bulk heterojunction polymer solar cells. The first limitation arises from the molecular energy levels (Voci) secondly, improper match with the contact work function may further reduce the achievable voltage to 002- (Reprinted with permission from [105], 2003, American Institute of Physics)...

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