CUENTA CUSTODIA DE MULTAS DEL CORREDOR

The InAs/GaAsN QD-IBSCs fabricated at Prof. Okada’s Lab at the Research Center for Advance Science and Technology (RCAST) at the University of Tokyo were also subjected to low temperature concentrated light experiments. The layer structure of these cells consists of a top 50 nm layer corresponding to the contact layer, a 150 nm thick GaAs p-emitter, 50 stacked InAs QD layers separated by 20 nm thick GaAsN intrinsic spacers, a 1,000 nm thick GaAs n-emitter and a 250 nm thick n-GaAs buffer layer. The reference cell is similar except for hosting a 1,000 nm thick intrinsic GaAs layer instead of the QD region.

Concentration measurements in Fig. 2.6(a) show the QD and reference cell JL-VOC

and dark J -V characteristics at room temperature, where the two red curves (representing the concentration measurements) remain approximately parallel. This result confirms that the very high electronic thermalization and tunneling rates make the QD cell to behave as a single gap solar cell (of reduced bandgap) at this temperature. The latter implies that both the reference and QD cells recover their VOC with an equivalent trend (when

the concentration is increased), i.e. they both have same slopes, conversely to the case explained in the previous section, where low temperatures apply and the QD cell behaves as a real IBSC.

Fig. 2.6(b) shows the JL-VOC and dark curves of the QD cell measured at different

temperatures. The recombination characteristic of this cell decreases as the temperature decreases, as expected from the increase of the bandgap (which strongly affects the amount of radiative recombination) and the reduction of the non-radiative component due to the progressive extinction of the phononic modes that favor this type of recombination. All of the foregoing makes the VOC to increase when the temperature decreases, up to a value

slightly above 1.2 V (reached for the lowest temperature, T =20 K). Conversely to the previous InAs/GaAs solar cell fabricated at the University of Glasgow, this value does not correspond to the bandgap of the host material (GaAs, with which the emitters are made

(a) (b)

Figure 2.6: Concentrated light JL-VOC and dark characteristics of a 50 stacked QD layer IBSC from

RCAST, plotted together with their corresponding dark curves. (a) Room temperature comparison be- tween the p-i-n GaAs reference cell and the InAs/GaAsN QD-IBSC. (b) QD-IBSC measured at different temperatures from T =298 K down to T =20 K. The plot shows the voltage recovery not being completely achieved in this case.

of), but most likely to the bandgap of the GaAsN. We can, hence, infer that the voltage preservation principle is not fulfilled for this QD technology or at least that it is limited by the bandgap (divided by the electron charge) of the GaAsN, that is notably lower than that of the emitters (GaAs).

Nevertheless, even if the electronic thermal escape is almost completely suppressed at this low temperature, the thin spacers (of 20 nm) that separate each QD layer will not prevent the electronic tunneling. It would be so, especially because the spacer material is made of GaAsN instead of GaAs, and it has a lower energy CB due to the BAC effect produced by the dilute nitrogen, i.e. the smaller the energy between the QD ground state and the CB of the barrier material, the larger the tunneling component.

A remarkable measuring problem occurs in these cells at low temperatures. When the concentrated light factor surpasses approximately 100 suns (in these cells, this is equivalent to about 10-2 _mAcm-2_{), one of the problems that will be explained in section 6.3.3.1 and}

referred to as the unavoidable heating of the cell when exposed to very high irradiances takes place. It can be observed in the upper part of the JL-VOC curve at T =77 K. It is

probably caused by a defective processing of the device, which could hinder the evacuation of the heat generated during the exposure of the cell to a very high irradiance. As a consequence, the bending of the JL-VOC curve takes place toward lower voltages as the

concentration increases, which is counterintuitive. Conversely to the conventional heating that usually occurs in this type of concentrated light measurement, which affects the cell temperature constantly during the whole flash pulse (provoking a parallel displacement of

the curve), in this case, the heating (and the subsequent VOCdecrease) is more pronounced

in the very first part of the flash pulse and then progressively disappears. As a result, the VOC at the higher part of the T =77 K plot is not correctly measured. The higher part of

the curve at T =20 K has not been represented because this effect is even more pronounced.

(a) (b)

(c)

Figure 2.7: Concentrated light JL-VOC characteristics of the second batch of samples from RCAST. (a)

QD-IBSC with Si-direct doping including a two-diode fitting (with rS=0). (b) QD-IBSC without QD

doping (also with fitting). (c) p-i-n GaAs reference cell.

Regarding the second batch of samples provided by RCAST, Fig. 2.7 represents the room temperature JL-VOC and dark J -V plots, together with a fitting to a two-diode

recombination model, with rS=0 in order to reproduce the JL-VOC curve. The results of

the fitting: J01, J02 and the slopes corresponding to each diode, are also indicated. The

maximum VOC obtained in the experiments is also represented in the plots. The fact that

these QD cells operate as single gap solar cells at room temperature, make possible a quite accurate fitting of the recombination curves. Otherwise, it would not be possible to fit them, since there is not an analytical model of the IBSC recombination that is equivalent to the Shockley’s model for a conventional solar cell.

Figure 2.8: VOC obtained from the solar cells of the second batch of samples fabricated at RCAST and

processed at IES-UPM. The batch consists of two 30 QD stacked layer InAs/GaAsN QD-IBSCs, with and without Si-direct doping and a p-i-n GaAs reference cell. The measurements were performed at room temperature and for a wide range of concentrated lights.

In order to verify whether the trend of the VOC of any of these QD cells (with and

without Si-direct doping) approaches the VOCof the reference cell as the concentration in-

creases, another type of representation can be performed consisting of plotting the VOC(X)

functions represented in semilogarithmic scale (X being the concentration). These func- tions, plotted in Fig. 2.8, show similar behavior both for the QDs with doping as well as for the QDs without doping. The difference in VOC between the GaAs p-i-n reference cell

and the QD cells is roughly constant (300 mV), verifying that the VOC of the QD cells

does not recover faster than the GaAs reference cell at room temperature. As it can be observed in Fig. 2.7(c), the reference cell suffers again from the same unavoidable heating and thus, it cannot be subjected to very high concentrations.

The third batch of QD solar cells, which layer structure is discussed in Fig. 5.23, is aimed to solve the problems encountered in the previous batches, where the GaAsN barrier material limits the performance of the device and the thin spacers do not block the carrier tunneling between QD layers. The results of the low temperature concentrated light measurements are plotted in Fig. 2.9. The p-i-n GaAs reference cell, represented in Fig. 2.9(a), apparently fulfills the superposition principle at the four different temperatures, even at T =20 K (of course, only in the low concentration range, where the effect of the series resistance is not very pronounced), which also occurred for the p-n GaAs reference cell from Glasgow. This cell almost reaches a VOC of 1.5 V, while the QD cell slightly

surpasses this value (but does not surpass the EG/q value), accurately reproducing the

voltage recovery result exposed in section 2.2.1 and published in Ref. [Linares et al., 2012b]. The confirmation of this result is important regarding the voltage preservation principle of InAs/GaAs QD-IBSCs at low-temperature.

(a) (b)

Figure 2.9: Concentrated light JL-VOCand J -V dark characteristics of the third batch samples (the one in

which GaAsN barriers are replaced by thick GaAs ones) fabricated at RCAST and processed, encapsulated and measured at IES-UPM. (a) p-i-n GaAs reference. (b) InAs/GaAs QD-IBSC. Both figures are measured at different temperatures from T =298 K down to T =20 K.

The concentrated light experiment carried out with the GaAs cell of this first batch of samples could not measure JL-VOC at the highest concentration (∼800 suns, inside the

cryostat), because the large rS of this cell required a reverse bias more negative than -6

V, which cannot be provided by our set-up. Therefore, the JL is slightly underestimated,

although this effect is almost unnoticeable. On the other hand, the QD sample experienced a deterioration of the rP only at T =20 K, which is apparent only throughout the low

concentration range (which crosses the curve at T =77 K).

2.2.3 InAs/GaAs QD-IBSCs manufactured at Rochester Institute of Technology

Another type of strain-compensation QD cells have also been characterized in this Thesis work. In this case, a batch of QD-IBSCs with 5 InAs/GaP stacked QD layers, similar to the ones used in Ref. [Hubbard et al., 2008], were fabricated at the Rochester Institute of Technology in collaboration with NASA Glenn Research Center. These samples were also characterized at low temperature and concentrated light. The GaP, as well as the GaAsN, also has a smaller lattice constant than the GaAs, exerting a tensile strain that compensates the compressive strain introduced by the InAs QDs. The main difference between the GaP strain-compensating technique used in this case compared to the GaAsN used from RCAST is that the GaP layer is sandwiched between a thicker GaAs barrier material, instead of being GaAsP alloy. The resulting band diagram is then different than the one generated by a single alloyed material barrier layer, although this, in principle,

will not significantly affect the results. Regarding the electronic properties, the GaP has a larger bandgap than GaAs, conversely to the GaAsN case. For this experiment, no reference cells were provided to IES-UPM as a benchmark.

(a) (b)

Figure 2.10: (a) Microscope picture of the encapsulated InAs/GaAsP QD-IBSC from Rochester. The solar cell is in this case a small portion of a 1x1 cm2 _{solar cell that was cut into smaller pieces in order to}

appropriately measure it under concentrated light. (b) Spectral response of the QD cell. A small response in the sub-bandgap region is observed.

Fig. 2.10(a) shows a microscope picture of the QD cell used in this experiment, taken in the wire-bonding system. The cell is a small portion of a larger (squared 1 cm2) cell, which was too large for our concentration system because the source-meter cannot handle large currents (it is limited to 6 A). The spectral response of the cell is represented in Fig. 2.10(b), where the GaAs bandgap signature can be observed at 870 nm as well as a sub-bandgap response as large as 2% of the integrated photocurrent. Nevertheless, this sub-bandgap photoresponse is very likely almost exclusively related to the WL.

The result of the low temperature concentration experiment is shown in Fig. 2.11. It is remarkable how the superposition principle is almost completely fulfilled for all the temperatures, even at T =20 K. In this case, the voltage recovery is produced up to 1.43 V at T =20 K, which is 86 mV below the EG/q value of 1.516 eV. Nevertheless, this can be

qualitatively considered as having fulfilled the voltage preservation principle (taking into account that the same concentration could not be achieved at T =20 K).

It can then be concluded that the GaP (or any related alloyed compound such as GaAsP) is a valid strain compensation material for the InAs/GaAs based QD technology, regarding its operation as an IBSC, conversely to the GaAsN material, which reduced bandgap dramatically limits the electrical performance.

Figure 2.11: Concentrated light JL-VOCcharacteristics of the InAs/GaAsP strain compensated QD-IBSCs

(with 5 stacked QD layers) fabricated at NanoPower Research Laboratories at Rochester Institute of Technology. Seven different measurements acquired from T =298 K down to T =20 K are shown, including their corresponding dark curves for comparison.