TÍTULO DE PAGO POR DISPONIBILIDAD A FAVOR DE [●]

Manyof the IBSCs presented in this Thesis are based on III-V materials, which show a number of advantages regarding the high ratio of radiative to non-radiative recombination, the relatively high absorption coefficients, the possibility of tunning alloyed compounds, etc. However, crystalline Si (Si-c) technology is very important in PV because of the avail- ability of the material, the reduced cost, the maturity of the technology, etc. Regarding the IBSC, Si does not have an adequate bandgap for implementing an efficient IBSC, because this is too small and far from the optimum. Besides, any IB created within such a small semiconductor bandgap energy (∼1.12 eV) will very likely have a high thermal escape rate. Nevertheless, Si, as InAs QDs is an excellent workhorse for studying the implementation of the IBSC concept.

The incorporation of large amounts of Ti in c-Si has been proposed as a possible candidate material for IBSCs [Olea et al., 2008]. As it has been demonstrated in Ref. [Antol´ın et al., 2009], the initially degraded carrier lifetime of a semiconductor doped with a DL impurity (Ti on Si, in this example), can be recovered if the impurity is incorporated in very high concentrations. This experiment verified the theory presented in Ref. [Luque et al., 2006a], where it was stated that at sufficiently high concentration, the delocalization of the electronic wave-function in DLs in meant to inhibit non-radiative recombination.

In our case, a Si-based IBSC with Ti has been implemented thanks to the collaboration of the Universidad Complutense de Madrid (UCM) and the Universidad Polit`ecnica de Catalu˜na (UPC). This IB material is based on crystalline Si where a highly doped Ti- layer is incorporated by ion implantation and subsequent recrystallization by pulsed-laser melting (PLM). The IB region is grown on an n-type Si layer and the isolation from the

contacts is completed by adding a hydrogenated amorphous Si (a-Si:H) p-type layer as in a heterojunction with intrinsic thin layer (HIT) [Taguchi et al., 1990, Tanaka et al., 1992]. The HIT technology is a promising and relatively cheap Si-based technology with a reported efficiency of 22% [Tsunomura et al., 2009].

Figure 3.10: Absolute external QE measurements of the HIT Si:Ti solar cell and its reference cell (without Ti). On the left part of the figure, the measurement in linear scale is shown, then the low energy range of the external QE represented in logarithmic scale and on the right, the values of the integrated JSCof both

cells. Courtesy of Mrs. Esther L´opez.

Fig. 3.10 shows the absolute external QE plot of the HIT Si:Ti IBSC implemented in this work, together with the results of an equivalent cell (except for the incorporation of Ti) used as benchmark. The QE experiment shows that the reference cell (solid black line) has a higher response than the Si:Ti cell (solid red line) in the supra-bandgap range, although the Si:Ti has a weak but noticeable response in the sub-bandgap region, until approximately 1.8 µm.

Characterization under concentrated light has also been performed on these cells. Low temperature concentrated light experiments are only shown in the JSC-VOCformat for the

HIT Si reference cell (see Fig. 3.11(a)), because equivalent results from the HIT Si:Ti cell present some anomalies and its interpretation can be confusing. Actually, in this case, the more simple J -V curves under concentration are preferred. The experiment is strongly affected by a very large rP which again prevents the measurement at reverse bias, thus

obtaining JSC and not JL. An electronic noise typically associated to the source-meter

biasing of relatively bad quality solar cells in the low photocurrent range, prevents the measurement of the concentrated light JSC-VOC pairs below 10-2 mAcm-2. On the other

hand, the desynchronization of both the ISC(t) and the VOC(t) signals with respect to the

received irradiance (from the flash light) can be observed in Fig. 3.11(b). The saturation of the VOC(t), severely hinders the appropriate acquisition of this measurement. It is specially

(a) (b)

Figure 3.11: (a) Concentrated light JSC-VOC characteristics of the HIT Si reference cell measured at room

and low temperatures. A very strong noise and a pronounced desynchronization of the VOC signal affects

the measurement at low temperatures. (b) VOC, ISCand irradiance signals represented in accordance with

time. The uncorrelated maximums of these signals are represented with dashed lines.

pronounced at low temperatures. The superposition principle is fulfilled in three out of the four curves. Only at T =20 K this superposition cannot be verified. Nevertheless, this may be due to the impossibility of acquiring sufficient data in the low current range because of the high electronic noise. The behavior of the JSC-VOC at T =150 K and T =77 K in the

high concentration range is very similar. Surprisingly, the T =77 K curve is even displaced towards the left, which is counterintuitive regarding the conventional VOC enhancement

that takes when the temperature is lowered. Actually, the maximum VOC (841 mV) is

obtained for the measurement at T =150 K.

Fig. 3.11(b) presents an example (at room temperature) of the desynchronization be- tween the ISC and the VOC signals with respect to the evolution of the flash irradiance over

time. As deduced from the previous low temperature curves under concentration, besides being desynchronized, the VOC saturates. Even the current response is desynchronized,

which is not very common in this type of measurement.

Fig. 3.12 shows the concentrated light J -V curves of the HIT Si:Ti IBSC measured at a maximum concentration (approximately 800 suns). Two main results can be extracted from these plots. On the one hand, the VOC of the IBSC recovers from 387 mV at room

temperature up to 540 mV at T =20 K, which is a remarkable increase, but still very far from the absorption threshold energy: 0.73 eV at room temperature (deduced from an absolute external QE measurement, not shown in this work). On the other hand, such maximum VOC is obviously still far from EG/q of the cell, which in this case would

correspond to the c-Si emitter bandgap (which is more restrictive than the a-Si:H): 1.12 V at room temperature.

(a) (b)

Figure 3.12: Concentrated light J -V curves of the HIT Si:Ti IBSC measured at a maximum concentration of approximately 800 suns. (a) At room temperature. (b) At very low temperature (T =20 K). This experiment demonstrates that the voltage preservation principle is not fulfilled in these cells.