Future prospects of InGaN/GaN Multiple Quantum Well Solar Cells Research for High Quantum Efficiency and Cost effectiveness Arvind Trivedi1, Yogendra K Yadav2, K Takhar2, T.D Das2, Servin Rathi2, and D Biswas1 1 Electronics and Electrical Comm. Engg, Indian Institute of Technology Kharagpur, Kharagpur-721302, West Bengal, India 2 Advanced Technology Development Center, Indian Institute of Technology Kharagpur, Kharagpur-721302, West Bengal, India E-mail:
[email protected] Abstract. A review is presented on the efforts and accomplishments is made using various approaches for achieving high efficiency InGaN/GaN solar cells, specifically with multi-junction tandem, Multiple Quantum Wells (MQW) and Quantum Dots (QD), using molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) techniques on sapphire substrate. The InGaN/GaN QW system is a useful technique for enhancement of photon absorption in ultra thin cells designed for optimum photovoltaic response. The Quantum well thickness and period of the MQW active region need to be optimized for maximizing light absorption. Generally, GaN based photovoltaic’s have been fabricated on sapphire (Al2O3) substrates because it have been much appreciated due to their superior performance, but their widespread applications of these devices have been hindered due to the higher cost of the substrate, low thermal conductivity and unavailability in large diameter. Silicon can be used as an alternative substrate considering the benefits of low cost, availability in large diameter, high thermal conductivity (1.5W/cm-K) and have well characterized electrical and thermal properties. Thus, using Si as a substrate, InGaN photovoltaic can be a novel with respect to its high efficiency and cost effectiveness. The high performance and low cost solar cells can be realized by appropriate polarization/band engineering, careful device design, simulation prior to fabrication.
1. Introduction The III-nitrides semiconductor material have been proved as an excellent candidate for making electronics and optoelectronic devices like LED, Solar cell, Photodetector and Laser diode etc. Indium gallium nitride (InGaN) based solar cells with a direct energy bandgap covering nearly the entire solar spectrum 0.7–3.4 eV [1-2] have been of particular interest to researchers. The properties of III-nitrides include large carrier mobility, high drift velocity, strong optical absorption, and resistance to radiation, making them ideal for the development of photovoltaic [3]. The current world’s record efficiency of triple-junction tandem solar cells of 41.6% have already been achieved [4]. Despite the tremendous advantages and potential applications provided by InGaN-based solar cells, growing an InGaN layer above 100 nm with high crystal quality remains a challenge and has severely limited the number of
studies on p-GaN/i-InGaN/n-GaN heterojunction solar cells. Increasing the spectral response of InGaN based solar cells requires a high proportion of indium, (exceeding 20%), in the light absorbing layer [5]. Solar cell materials are characterized on the basis of two properties: how strongly they absorb light, and the created charge carriers reach the electrical contacts, successfully. Thus, III-V nitrides are promising extensive materials for third-generation ultrahigh efficiency photovoltaic’s to achieve external quantum efficiencies over 50%. 2. Way to maximize solar cell efficiency There are several ways to enhance the solar cell efficiency. 1. Maximize absorption of light and conduction of photo carriers. There is a trade-off between top layer thickness and absorption of light. The thinner layer increases high electric field at semiconductor junction, so that electron and hole go successfully towards n and p contact, but it reduces the light absorption, hence reduces efficiency. 2. High electron diffusion length L
( n KT n / q ) , maximize carrier life time, short diffusion
length increases the recombination. 3. By reducing the material thickness of emitter increase the light absorption properties of active region enhance the photocurrent and external quantum efficiency of photovoltaic device. Development of solar cells with quantum-confinement. 4. Antireflection coating Prevents incident light from reflecting off of the device. 8. The silver nanoparticles increase light scattering, light trapping, and carrier collection in the III-N semiconductor layers leading to enhancement of the external quantum efficiency. 9. The quantum efficiency can be enhanced by optimizing grid contacts for low Ohmic contact resistance. 10. The effective life time of charge carrier is derived from three recombination process which is mainly depends on the nature and quality of the semiconductor, the doping level, and cell irradiance [6]. The effective life time of charge carrier can be expressed as
1 1 1 1 eff SRH rad Auger where SRH , Auger and rad are the carrier lifetimes associated with Shockley–Read–Hall (SRH) processes recombination through traps in the forbidden gap, Auger recombination and radiative recombination, respectively. 3. Problems and key Challenges for high conversion efficiency There are several challenges need to be overcome in the III-nitrides to take full advantage of its potential applications in high frequency RF and optoelectronics devices . 1. The lattice mismatch between InN and GaN and low temperature growth of InGaN induce impurity incorporation and morphological defects (v-defects) generating nonradioactive recombination centers (NRCs). These NRCs reduce carrier lifetimes and reduce the solar cell short circuit current. An efficient conversion of lower energy photons requires high In content InGaN, which limits the maximum thickness of this layer, reducing its light absorption [7]. 2. As InGaN solar cells progress toward higher Indium contents (above 35%) to expand the spectral response to longer wavelengths, management of polarization will become increasingly challenging, which tends to the polarization-induced electric fields. Although InGaN material exhibit high threading dislocation (TD) densities in the range of (108-1010 cm-2). 3. When the thickness and/or mole fraction of InxGa1−xN materials increase, Indium (In) rich clusters in InGaN films easily induce phase separation. This leads to lower open-circuit voltages VOC compared with theoretical values, low fill factor (FF), and large recombination rate by defect states that degrade the short-circuit current density (JSC) of InGaN-based PV devices [8].
4. The current reports on PV response of InGaN are limited to low indium contents. The absorption spectra of InGaN materials, covering near UV to far-infrared regions, provide a promising potential for PV applications. At present, growing InGaN epitaxial layers on GaN with a band gap energy lower than 2 eV and with a thickness larger than hundreds of manometers has remained a challenge. The material quality degrades rapidly when either the indium contents or the thickness of InGaN epitaxial layers are increased [9-10]. 5. When InGaN-based materials are used for making solar cell, the dense defects, most likely the TDs, can become carrier killers. This results in a reduction in conversion efficiency. The highdensity defects lead to a short diffusion length of the carriers, resulting in poor efficiency because the photogenerated carriers recombine with the defect states before they reach to electrodes [11]. 4. Multiple quantum well (MQW) The Multiple Quantum Well (MQW) is a practicable approach to design high efficiency solar cell based on InGaN semiconductor alloy with relatively high Indium content for optimal solar conversion efficiency. It means that the Quantum well (QW) or quantum dot (QD) structures can also enable photons of lower energy than the bandgap of the semiconductor material. The carriers generated in the QWs/QDs can thermally escape onto the conduction band for electrons or valence band for holes to contribute to the total enhancement of photocurrent [12]. Hence, it increases the quantum efficiency of solar cell. Fig. 1 (a) Mini-bands will assists the carriers must not thermalize from one band to another, it must uses quantum dot approaches. Fig. 1 (b) Loacalized energy levels are illustrating the transport requires that collection or escape time less than recombination time.
Fig. 1 (a) Mini-bands
(b) Multiple Quantum Wells
(c) Structure of InGaN/GaN MQW solar cell
Table 1. Performance of III-Nitride solar cell on sapphire substrate Solar performance
cell
(InGaN/GaN) p-i-n Solar cell In0.03Ga0.97N/GaN MQW Solar cell InGaN based MQW Solar cell In0.28 Ga0.72N/GaN MQW Solar cell In0.12Ga0.88N/GaN MQW Solar cell
Open circuit voltage (VOC) Volt
Short circuit current (JSC) mA/cm2
Fill factor (FF) %
External Quantum Efficiency %
Illumination
References
1.75
1.11
73
50
1 Sun AM0
[1]
1.90
1.32
71
60
[3]
2.05
1.09
51
50
2.04
2.00
63
69
1.89
1.06
79
72
1.5Sun AM1.5 1Sun AM1.5G 1Sun AM1.5G 1Sun AM1.5G
[5] [6] [7]
5. Conclusion In this paper, recent developments in the field of III-nitride semiconductor based solar cells have been have been discussed. The III-nitride semiconductor are ideal candidate, used in many different electronic and optoelectronics applications for making high efficiency solar cell, high brightness LED, and laser diode due to their direct band gap and high reliability. The choice of the substrate is crucial and determines a wide variety of essential material properties, which in turn impact resulting device performance and reliability. InGaN is a novel material that offers substantial potential for high-efficiency photovoltaics, through which solar energy costs can be driven lower. The aim of this work is to develop a high-performance InGaN solar cell in the 1.6–3.4 eV range on sapphire substrate using buffer layer approach. The development process of high efficiency solar cell requires an iterative approach for design, simulation, growth, fabrication and characterization. The InGaN photovoltaic on Si substrate can be a novel with respect to its high efficiency and cost effectiveness, using buffer layer appproaches. Thus, the III-V nitrides are promising extensive materials for third-generation ultrahigh efficiency photovoltaics to achieve efficiencies over 50%.
ACKNOWLEDGEMENT This work has been supported by ‘MEP’ project, Department of Information Technology, DIT, Government of India. References [1] S. Nakamura and G. Fasol, The Blue Laser Diode (Springer, Berlin, 1997). [2] S. Nakamura, S. Pearton, and G. Fasol, The Blue Laser Diode, 2nd ed. (Springer, Berlin, 2000). [3] Y. Nanishi, Y. Saito, and T. Yamaguchi, Jpn. J. Appl. Phys., Part 1 42, 2549 (2003). [4] D. J. Friedman, Curr. Opin. Solid State Mater. Sci. 14, 131 (2010). [5] Ya-Ju Lee, Min-Hung Lee, Chun-Mao Cheng, and Chia-Hao Yang. Appl. Phys. Lett. 98, 263504 (2011). [6] Carl J. Neufeld, Samantha C. Cruz, Robert M. Farrell, Michael Iza, Stacia Keller, Shuji Nakamura, Steven P. DenBaars, James S. Speck and Umesh K. Mishra Appl. Phys. Lett. 99, 071104 (2007). [7] Elison Matioli, Carl Neufeld, Michael Iza, Samantha C. Cruz, Ali A. Al-Heji, Xu Chen, Stacia Keller, Umesh Mishra, Shuji Nakamura, James Speck and Claude Weisbuch Appl. Phys. Lett. 98, 021102 (2011). [8] X. Wu, C. Elsass, A. Abare, M. Mack, S. Keller, P. Petroff, S. DenBaars, J. Speck, and S. Rosner, Appl. Phys. Lett. 72, 692 (1998). [9] D. Cherns, S. Henley, and F. Ponce, Appl. Phys. Lett. 78, 2691 (2001). [10] J. Wierer, A. Fischer, and D. Koleske, Appl. Phys. Lett. 96, 051107 (2010). [11] R. Dahal, B. Pantha, J. Li, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 94, 063505 (2009). [12] Paxman, M., Nelson, J.; Braun, B., Connolly, J., Barnham, K.W.J., Foxon, C.T., and Roberts, J.S. J. Appl. Phys. 1993, 74, 614–621.
