Historia de las representaciones culturales de la maternidad

The use of multi-junction graded bandgap (MJGB) device structures for solar cell application is important in harvesting photons from various parts of the solar spectrum.

Some of the experimental works reported on solar cell device structures in Chapter 9 of

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this thesis are based on this type of device structure. These device structures are made up of different semiconductors having various energy bandgaps and arranged in a way that photons from different parts of the spectrum can be effectively absorbed to enhance the output short-circuit current density. The full MJGB solar cell structure was proposed by Dharmadasa et al. [30] in 2005 and it was experimentally tested with GaAs/AlGaAs using MOVPE growth technique and the obtained results showed a working model [31].

One of the focuses of this present research work is to develop the MJGB structures using low-cost electroplated semiconductors most especially from II-VI family.

Figures 2.20 (a) and 2.20 (b) demonstrate the two possible ways of designing the MJGB device structures as proposed by Dharmadasa et al. [30,31] . In Figure 2.20 (a), the starting window layer is a wide bandgap p-type semiconductor material while in Figure 2.20 (b), the starting window material is a wide bandgap n-type layer. In both cases, the last layer is a narrow bandgap layer. Starting from the front contact towards the back contact, both device structures show a gradual reduction in bandgap from E_g1 to E_gn. The device structure in Figure 2.20 (a) has the advantage of a higher potential barrier height over the one in Figure 2.20 (b). With higher ϕb, it is possible to obtain higher V_oc value. However, the device structure in Figure 2.20 (b) has been used in this work for solar cell fabrication due to the availability of electroplated materials.

Eg1

As light moves through the device structure, absorption of photons takes place starting from the ultra-violet (UV) region through the visible (Vis) and then finally to the infrared (IR) end of the solar spectrum. MJGB solar cells are designed in such a way that the high energy photons (UV and Vis) are absorbed by the wide bandgap semiconductor deposited first on the conducting substrate. The narrow bandgap thin

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films deposited afterwards absorb the low energy photons at the infrared end of the spectrum. This absorption process is essential so as to reduce thermalisation effect [32].

Thermalisation effects do occur when high energy photons are absorbed by low energy bandgap semiconductors; when this occurs, heat is produced within the device structure and this eventually lowers the solar cell parameters and the life span. Thermalisation effect in PV solar system is like an overload effect in induction motors of an electrical system.

The MJGB also make use of impact ionisation and impurity PV effect to reduce R&G process, increase photo-generated charge carriers, separate and transport the generated carriers to external circuit before they recombine in the device structure [5].

Considering Figure 2.20 (a), when high energy photons are absorbed at the front of the solar cell, they break bonds between the atoms and excite electrons from the VB to the CB. When this happens, they create electron and hole pairs which are separated to the back and front contacts respectively. The quick separation is initiated by the presence of strong internal electric field in the solar cell structure. The strong built-in electric field is represented by the steep slope produced as a result of the device structure design. High kinetic energy (KE) is gained by the electrons accelerating towards the back contact.

The electrons moving with high KE transfers its momentum to the atoms located at the rear end of the device and break the bonds between these atoms. This makes a photon which produce electrons with initial high KE to create two electron-hole pairs. This mechanism is referred to as band-to-band impact ionisation.

As explained by Dharmadasa et al. [32], the transmission of infrared photons towards the rear of the solar cell take place as a result of their low photon energy. These photons can equally break bonds between atoms and excite electrons from CB to VB due to low bandgap materials towards the back. In the same vein, heat from the surroundings or infrared radiation can break bonds between atoms and create electron-hole pairs.

However, most of the far infrared photons do not possess sufficient energy to directly promote the excited electrons to the CB but the energy is adequate to promote the excited electrons to one of the defect levels located below the CB. As earlier explained, there is presence of defects at the M/S interface and these defects can be very useful if well controlled. Some of these defects may be introduced during post deposition treatment process like etching or during growth while others may be introduced during growth. The ones introduced during growth are mainly native defects.

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The holes created are quickly transported to the front ohmic contact and this does not therefore permit the trapped electrons at the defect levels to fall back and recombine with the hole. Further absorption of infrared radiation can create another electron-hole pair (EHP) and the newly excited electron can push the initial excited electrons at one of the defect levels to the CB. Aside these, the high KE electrons accelerating down the slope can also promote the trapped electrons in defect levels to the CB. This mechanism of operation is referred to as impurity PV effect. The combination of impact ionisation and impurity PV effect can improve the device parameters of the fabricated solar cell showing avalanche of electrons created during this process.

2.6 Summary

In this chapter, solid materials and the different categories in which they exist have been briefly presented. Semiconductor materials have been classified into two major groups based on dopants addition and elemental composition. Brief discussions have also been made on the various interfaces which exist in an electronic device. Some of the current transport mechanisms taking place in metal/semiconductor interfaces were discussed, and highlights of some types of solar cells were also given. Since the thesis work was focused on development of graded bandgap devices, the new designs were presented to show their advantages over two-layer solar cell device structures.

49 References

[1] D. A. Neamen, Semiconductor physics and devices:basic principles, third ed., McGraw-Hill, New York (2003).

[2] S. M. Sze, K. K. Ng, Physics of Semiconductor Devices, third ed., John Wiley &

Sons, New Jersey (2007).

[3] A. K. Sharma, Semiconductor Electronics, second ed., New Age International Publishers Ltd., New Delhi, India (2001).

[4] J. Koryta, J. Dvorak, L. Kavan, Priciples of Electrochemistry, Second Edi, John Wiley & Sons, Ltd., Chichester, West Sussex, England (1993).

[5] I. M. Dharmadasa, Advances in Thin-Film Solar Cells, first ed., Pan Stanford Publishing Pte. Ltd., Boulevard, Singapore (2013).

[6] B. M. Basol, J. Appl. Phys., 55 (1984) 601–603.

[7] B. E. Mccandless, J. R. Sites, Cadmium Telluride Solar Cells, in: Handb.

Photovolt. Sci. Eng., first ed., John Wiley & Sons, Ltd, Chichester, West Sussex, England (2003): pp. 617–662.

[8] D. M. Chapin, C. S. Fuller, G. L. Pearson, J. Appl. Phys., 25 (1954) 676–677.

[9] J. L. Gray, The Physics of the Solar Cell, in: Handb. Photovolt. Sci. Eng., first ed., John Wiley & Sons, Ltd, Chichester, West Sussex, England (2003): pp. 61– 112.

[10] V. Kabra, L. Aamir, M. M. Malik, Beilstein J. Nanotechnol., 5 (2014) 2216– 2221.

[11] T. Soga, Nanostructured Materials for Solar Energy Conversion, in:

Nanostructured Mater. Sol. Energy Convers., first ed., Elsevier, (2006).

[12] H. Gómez, R. Henríquez, R. Schrebler, R. Córdova, D. Ramírez, G. Riveros, E.

A. Dalchiele, Electrochim. Acta, 50 (2005) 1299–1305.

[13] P. V. Meyers, Sol. Cells, 23 (1988) 59–67.

[14] W. G. Oldham, A. G. Milnes, Solid. State. Electron., 7 (1964) 153–165.

50

[15] B. G. Streetman, S. K. Banerjee, Solid State Electronic Devices, Sixth Edit, Pearson Prentice Hall, New Jersey (2010).

[16] J. Singh, Semiconductor Devices: Basic Principles, first ed., John Wiley & Sons, Inc., United States of America (2001).

[17] I. M. Dharmadasa, J. D. Bunning, A. P. Samantilleke, T. Shen, Sol. Energy Mater. Sol. Cells, 86 (2005) 373–384.

[18] E. H. Rhoderick, IEE Proc. I Solid State Electron Devices, 129 (1982) 1–14.

[19] M. F. Mabrook, D. Kolb, C. Pearson, D. A. Zeze, M. C. Petty, Adv. Sci.

Technol., 54 (2008) 474–479.

[20] J. R. Pugh, D. Mao, J. G. Zhang, M. J. Heben, A. J. Nelson, A. J. Frank, J. Appl.

Phys., 74 (1993) 2619–2625.

[21] H. Card, Solid. State. Electron., 16 (1973) 365–374.

[22] D. Kearns, M. Calvin, J. Chem. Phys., 29 (1958) 950–951.

[23] H. Spanggaard, F. C. Krebs, Sol. Energy Mater. Sol. Cells, 83 (2004) 125–146.

[24] P. N. Rivers, Leading Edge Research in Solar Energy, Nova Science Publishers, (2007).

[25] N. G. Park, Mater. Today, 18 (2014) 65–72.

[26] NREL, Best Res. Cell Effic., (2016).

http://www.nrel.gov/ncpv/images/efficiency_chart.jpg (accessed April 22, 2016).

[27] P. P. Boix, K. Nonomura, N. Mathews, S. G. Mhaisalkar, Mater. Today, 17 (2014) 16–23.

[28] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc., 131 (2009) 6050–6051.

[29] Y. Zhou, K. Zhu, ACS Energy Lett., (2016) 64–67.

[30] I. M. Dharmadasa, Sol. Energy Mater. Sol. Cells, 85 (2005) 293–300.

[31] I. M. Dharmadasa, J. S. Roberts, G. Hill, Sol. Energy Mater. Sol. Cells, 88 (2005) 413–422.

51

[32] I. M. Dharmadasa, N. D. P. S. R. Kalyanaratne, R. Dharmadasa, J. Natl. Sci.

Found. Sri Lanka, 41 (2013) 73–80.

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Chapter 3 - Techniques for materials growth, materials and device