DBR effect

After then, we measured the transmittance of the cell with QDs. And we found out the transmittance of the cell with QDs is still high(figure 4.3.4-1). That means many photons loss, we didn’t get use of it. So we think how to solve this situation, and comes the DBR material in.

From figure 4.3.4-1, we observe the transmittance of cell with QDs is slightly higher than reference. The reason is the anti-reflectivity characteristic of QDs. QDs let more light in the cell, but it can’t absorb all of them. So some light come out the cell.

360 380 400 420 440

0 10 20 30 40 50

EQE(% )

Wavelength(nm)

QDs

Reference

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Figure 4.3.4-1 The measured transmittance of reference cell and cell with QDs.

DBR can reflect the light of specific wavelength. We design a DBR that reflect the light effectively from 370nm to 430 nm. The reflectance spectrum is shown in figure 4.3.4-2.

Figure 4.3.4-2 The measured reflectance of DBR.

360 380 400 420 440

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From figure 4.3.4-2, we observe the reflectance of DBR is very high (R>95%). We think that may reduce the problem mentioned before. So we add the DBR to the back of the cell and want to reflect the light into cell to be used again. We measured the absorption spectrum of reference cell, cell with QDs, cell with DBR and cell with both QDS and DBR(figure4.3.4-3).

Figure 4.3.4-3The absorption spectrum of reference, cell with QDs, cell with DBR and cell with both QDS and DBR.

We can see the cell with QDs is higher than reference before 400nm, it means the QDs do convert the light into the cell to be used. And the cell with DBR is higher after 370nm because it reflect the light back to the cell to be used again. As we expect, the best absorption is the cell with QDs and DBR. It shows the broadband enhancement in absorption from 350nm to 460nm.

So we measured the J-V curve to see if the efficiency is improved. The results is shown in figure 4.3.4-4. We compare the reference cell, cell with QDs and cell with both QDs and DBR. The device characteristics were summarized in Table 4.3-3.

The cell with both QDs and DBR can effectively enhance the short-circuit current density from 1.01 to 1.15 mA/cm² and the power conversion efficiency from 1.019 to 1.162 %,

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corresponding to a 14 % enhancement compared to cell with QDs.

From the J-V curve result, we can observe the J_sc of cell with DBR is higher than the reference ones and cell with QDs. It means the more photon is absorbed, and generates more electron-hole pairs. And more photons are absorbed because of the reflection of the DBR. It reflect the light into the cell to be use again, and reduce the loss in photon.

Figure 4.3.4-4 The measured current-voltage characteristics of reference cell, cell with QDs and cell with both QDs and DBR.

.

Voc (V) Jsc(mA/cm2) F.F. Efficiency(%)

Reference 1.62 0.95 62.82 0.97

QDs 1.61 1.01 62.28 1.019

QDs+DBR 1.63 1.15 62.23 1.162

Table 4.3-3 The details of measured current-voltage characteristics of reference cell, cell with QDs and cell with both QDs and DBR

After all, we measured the external quantum efficiency of the reference, cell with QDs

0.0 0.5 1.0 1.5 2.0

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and cell with QDs and DBR.

Figure 4.3.4-5 shows the EQE of the reference cell, cell with QDs and cell with QDs and DBR. The cell with DBR and QDs layers shows an enhanced EQE in the range from 350nm to 440nm. Especially in the range 350nm to 420nm, it shows the higher enhancement, just matching to the high reflectance of the DBR.

Figure 4.3.4-5 The measured external quantum efficiency of reference cell, cell with QDs and cell with both DBR and QDs.

360 380 400 420 440

0 10 20 30 40 50

EQ E (%)

Wavelength (nm)

Reference QDs

QDs+DBR

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Chaper 5. Conclusion

In summary, we successfully demonstrate the textured PDMS film serve as anti-reflectance coating layer. The advantages are the low-cost, non-vacuum system, large area and simple process. PDMS film provides a refractive index gradient to reduce the reflectance. From the reflectance spectroscopy and angle-resolved reflective spectra, we observe that both flat and rough PDMS show the the better omnidirectional and broadband anti-reflective characteristic (ARC) compared to reference cell.

Then we demonstrated the PDMS film which is useful in harvesting solar photon and enhancing the power conversion efficiency of InGaN/GaN multiple quantum well solar cells.

Compared cell with flat PDMS and cell with rough PDMS to reference one, the power conversion efficiency achieved 4.8% and 7.1% enhancement, and the Jsc achieved 4.1% and 7.2% enhancement respectively.

Second part, we successfully demonstrate the quantum dots(QDs) serve as photon down conversion centers. From the reflectance spectroscopy and angle-resolved reflective spectrum , the result indicates the anti-reflective characteristic of cells with QDs is better than reference cell over a broadband wavelength range. We prove that the cell with QDs has the anti-reflectance coating effect.

From the J-V curve, the GaN solar cell with QDs layer can effectively enhance the short-circuit current density from 0.95 to 1.01 mA/cm² and the power conversion efficiency from 0.97 to 1.03 %, corresponding to a 7.2 % enhancement compared to reference ones. The overall shape of EQE enhancement resembles the absorption curves of QDs, and we believe this is a strong indication of QD absorption and down-conversion process. The enhancement is higher before 400nm, just mapping to the absorption spectrum from quantum dots.

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