Etapa de determinación de carencias en localidades

Though the cells with Cd/Te ratio 0.5 ratio exhibited higher VOC’s the cells frequently flaked during subsequent processing steps; especially during the CdCl2 rinse and back contact etch (performed using Br-Methanol solution). As a result, the performance was not consistent from cell to cell on the same substrate. The variation in VOC for the cells on the same substrate with Cd/Te ratio 0.5 and 2.0 is shown in Figure 37.

The huge error bar on the 0.5 ratio cell is not seen on the cell with ratio 2.0. SEM cross section images revealed the presence of voids at the CdTe/CdS interface (shown in Figure 38). No voids were observed for the Cd/Te ratio 2.0 sample. It has been reported in an earlier study that faster growth rate causes voids and discontinuities in grains due to rapid growth of adjacent faces [131]. In this study, films which exhibited voids were deposited at higher growth rate. Therefore, it is possible that the higher growth rate for Te-rich films is the reason for the presence of voids. The variation in VOC could be a result of pinholes

Figure 37: Variation in VOC for cells on the same substrate for ratios 0.5 and 2.0 0

formed during processing steps as a result of poor adhesion/void presence. In order to improve adhesion and reduce inconsistencies, two approaches were used:

1. Reduced Ratio window 2. 2-layer deposition 6.6.2 Reduced Ratio Window

Since films deposited at Cd/Te ratios below 0.5 flaked, solar cells were fabricated using CdTe films with Cd/Te ratios 0.7, 1. aand 1.4. The SEM images showed larger grain sizes for films with Cd/Te ratio 0.7. The cross sections did not reveal any voids and no flaking was observed for the 0.7 ratio films (Figure 39). The JV and Q.E for these cells are shown in Figure 40. Solar cells made with Cd/Te ratio 0.7 (Te-rich)

Figure 38: SEM cross sections for Cd/Te ratios 0.5, 2.0. Voids are present for the 0.5 ratio film.

Figure 39: SEM cross section for ratio 0.7 film

exhibited the highest VOC of 800 mV. The roll-over was present due to a non-ohmic back contact present in all cells.

Figure 41 (left) shows the measured carrier concentrations from Capacitance – Voltage measurements for CdCl2 annealing temperatures 375 and 390° C as a function of Cd/Te ratio. The doping for Cd/Te ratio 0.7 was higher than ratio 1.0 and 1.4. Also, the doping for 390° C CdCl2 HT was higher. The error bars indicate the highest and lower carrier concentrations obtained from the doping vs depletion width profile shown in Figure 41 (right). An order of magnitude higher doping was observed for Te-rich samples

Figure 41: Doping concentrations Vs Cd/Te ratios (left) for different CdCl2 annealing temperatures 1E+13

(3×10¹⁴ cm^-3), compared to Cd-rich (1×10¹³ cm^-3) deposited films. CdCl2 HT improves the carrier concentration by forming complex defects called A-centers. However the formation of A-centers is limited by the availability of VCd’s. In case of Cd-rich conditions, the availability of VCd’s is limited and therefore Cl forms compensating donor defects ClTe. The Voc’s showed good correlation to the carrier concentrations.

6.6.3 2 Layered Depositions

Alternatively, films were deposited in two layers. The initial layer was Cd/Te ratio ≥1 (which showed better adhesion to CdS layer) followed by a second layer of Cd/Te ratio <1. The initial layer was timed to achieve an estimated thickness of 0.5 µm while the overall film thickness was aimed to be 5 µm. Since the thickness of the initial layer was only 10% of the total thickness, the second layer was also expected to influence the performance of the solar cell on par or slightly more than the initial layer. Two sets of samples were deposited using this approach: First set had Cd/Te ratios 0.3, 0.5 and 0.7 with initial layer of Cd/Te ratio 1.0 and second set with an initial layer of ratio 2.0. The flaking observed in single layer depositions

Figure 42: SEM cross section images for Cd/Te ratios 0.7, 0.5 and 0.3 with a starting layer of ratio 1.0

was not observed in two-layered depositions. Figure 42 shows SEM cross section images for films deposited with Cd/Te ratios 0.3, 0.5 and 0.7, with an initial layer of Cd/Te ratio1.0. All films showed grains extending along the cross-section with no signs of voids. Similar growth was observed for the films with initial layer of Cd/Te ratio 2.0. Solar cells showed consistency and repeatability in performance for Cd/Te ratios as low as 0.3. Figure 43 shows the JV characteristics for solar cell with Cd/Te ratio 0.7 with 1.0 and 2.0 as the initial layers. The cells with initial Cd/Te ratio 1.0, performed better with a VOC of 790 mV compared to 740 mV for the cell with initial layer of ratio 2.0. Though the variations in VOC on different cells on the same substrate was eliminated, the cell performance did not show variation as a function of Cd/Te

ratio. This shows that the initial layer has a larger impact on the performance than anticipated, shadowing the effect of Cd/Te ratio. Figure 44 shows the variations in VOC for ratio 0.5, 0.7 and 0.7 with initial layer of 1.0.

6.6.4 Summary of Key Findings from Adhesion Issues

● Cells fabricated with Cd/Te ratios < 0.5 flaked, possibly due to fast growth rate; two approaches were used to improve CdTe adhesion to CdS: a) Reduced ratio window b) 2-layered method

Figure 43: JV curve for Cd/Te ratio 0.7 with starting layer of ratio 1.0 and 2.0 -0.03

-0.02 -0.01 0.00 0.01 0.02 0.03 0.04

-0.50 0.00 0.50 1.00 1.50 Current Density, [A/cm2]

Voltage, [Volts]

0.7/1.0 0.7/2.0

● Both the approaches helped in eliminating the flaking observed for CdTe films grown under Te-rich conditions. The variations in VOC’s on the same substrate were eliminated with both the approaches.

● The cells made with Cd/Te ratio of 0.7 showed the best VOC of 800 mV. Te-rich growth conditions appear to yield higher VOC’s.

● The carrier concentrations for Te-rich samples was higher than Cd-rich samples and correlates with observed VOC trends

6.7 Cu Effects