Multiple-carrier generation was observed in a MOS cell with the same structure we discussed in Section 10.3.3 but with different active layers, here consisting of a SRO layer with more silicon excess. A superlinear dependence of Isc on the incident light power was observed. This was explained by the presence of the subbandgap interface states and by the multiple-carrier generation process. Such a cell has the potential to be used as a high-efficiency solar cell.
Two SRO layers are used as the active layer of the MOS structure shown in Figure 10.11. The thickness and chemical composition of these two layers, named Г3 and T3N, are reported in Table 10.3. Both layers show PL emission with a peak wavelength larger than 900 nm, which means that the diameter of the Si-NCs is larger than 5nm. Both Г3 and T3N layers are more conductive than the Г10 one (see the Section 10.3.3), as shown in Figure 10.15. T3N is more conductive than Г3 due to the higher silicon and nitrogen content.
The I – V characteristics in the dark and under illumination (with a 633-nm laser) along with the photocurrent, Jph = JL-JD, of the MOS device with T3N as the active
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Table 10.3 Chemical composition, optical, and electrical characteristics of Г3 and r3N layers.
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Voltage /V Figure 10.16 Dark-current density, current density under illumination (633 nm, 700 mW/cm2), and the difference of the two (photocurrent density) as a function of the applied voltage. |
layer are shown in Figure 10.16 [110]. The Jph plot shows four distinct regions in the I – V characteristics. Under reverse bias, it shows a typical photodiode behavior (PD). Under forward bias, the curve enters into the fourth quadrant showing a photovoltaic (PV) region with a large spectral response. Beyond the open circuit voltage, the current under illumination, JL, has been found to be lower than dark current, JD, indicating a negative photoconducting region (-PC). Above a forward bias of about 1.7 V, JL becomes larger than JD and the device acts as a photoconductor (PC).
The superlinear dependence of Isc on the incident optical power shown in Figure 10.17 was explained by an internal gain mechanism [111]. Isc increases at low optical powers as a power function with a scaling exponent of about 2.3. Isc saturates at high optical powers.
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Figure 10.17 Isc as a function of incident power intensity of a 633-nm laser. |
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Voltage /V Figure 10.18 Iph vs. applied voltage under 633 nm, IR, and the illumination from both light sources. |
What is peculiar in this cell is the fact that the photocurrent /ph depends on the photon wavelength in an apparently strange way (Figure 10.18). Under IR illumination /ph increases only when visible photons are also absorbed. This is explained by an inverse Auger effect occurring on the interface subbandgap states [108].
A simple phenomenological model based on an internal gain mechanism or carrier multiplication due to impact excitation of electrons from the subbandgap interface states by photogenerated conduction-band electrons was developed to explain this phenomenon [111]. This is illustrated in Figure 10.19. The incident light is absorbed by the Si substrate generating e-h pairs, and then the photogenerated electrons are injected into the SRO layer, where they have an excess energy with respect to the fundamental Si-NC state. Thermalization to the fundamental Si-NC state occurs then by an inverse Auger process,
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Figure 10.19 Qualitative sketch ofthe mechanism of the internal carrier multiplication. |
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by which the excess energy is absorbed by an electron trapped into a subbandgap interface state that, in turn, is excited at the fundamental Si-NC state. In this way, a multiplication of the photoelectrons occurs, which explains the superlinear dependence of the Isc on the incident light power. The emptied interface states are eventually refilled by IR absorption, as shown in Figure 10.18.
The PV conversion efficiency of this device is about 14% under 1 Sun illumination. The Voc and Isc are 600 mV and 1.15 mA, respectively. Its fill factor is only 0.13, which is mostly due to the large parasitic resistances. An optimization of the device is needed to achieve higher efficiency.
As we mentioned before, the addition of nitrogen improves the conductivity of the SRO layer and, hence, affects its PV properties. As can be seen in Figure 10.20, the Isc is greatly increased by the addition of nitrogen. It could be argued as well that nitrogen reduces the size of Si-NCs and increases the number of interface states, which in turn increases the carrier multiplication.
