III-nitrides are ideally suited to the fabrication of optoelectronics devices such as lasers and light-emitting diodes, due to their high efficiency and wide applicability. 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 photovoltaics.
1.2.1 The characteristics of InGaN
GaN-based photovoltaics with an InxGa1-xN absorption layer are promising these years.
InGaN alloys have been widely exploited as active materials for light-emitting diodes and laser diodes with emission wavelengths covering from near UV to green spectral regions.
InGaN alloys recently emerge as a new solar cell materials system due to their tunable energy band gaps (varying from 0.7 eV for InN to 3.4 eV for GaN), covering almost the whole solar spectrum and superior photovoltaic characteristics such as direct energy band gap in the entire alloy range and high carrier mobility, drift velocity, radiation resistance, and optical absorption of ~105 cm−1 near the band edge.
InGaN alloys have been shown to have superior high energy radiation resistance for space based PV applications. The band gap of InN was recently discovered to be 0.7 eV as opposed to the previously believed 1.3 eV. The importance of this discovery is that the band gap of the InGaN material system spans nearly the entire solar spectrum (0.7~3.4 eV), thus enabling design of multijunction solar cell structures with near ideal band gaps for maximum efficiency.
The figure shows the bang gap of several materials. For InxGa1-xN material, the ratio of InN is x, and the ratio of GaN is 1-x( x = 0~1 ). Since it’s a direct bandgap material, the bandgap of In InxGa1-xN obey the following formula (1-1):
Eg
InxGa1-xN= 0.7 * x + 3.4 * (1-x)-1.43*x*(1-x)
(1-1)6
Figure 1.2-1 The bandgap and lattice constant of several materials.
1.2.2 The advantages and the challenge of GaN-Based solar cells
III-nitrides are ideally suited to the fabrication of optoelectronics devices such as lasers and light-emitting diodes, due to their high efficiency and wide applicability. 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 photovoltaics.
Nowadays the p-GaN/i-InGaN/n-GaN heterojunction solar cells gets lots of attention because of some main advantages such as :
1. Direct band gap : Unlike Si-based solar cells, the band gap of GaN/InGaN is direct band gap. It means during the energy conversion, it won’t loss some energy like heat. This point enhances the use of sunlight.
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2. Tunable band gap : By mixing the different ratio of GaN/InGaN, we can have the band gap different from 0.7eV~3.4eV. The range is just overlap the solar spectrum.
It means we can make the band gap just what we need.
This direct and wide band gap range makes the InGaN material system useful for photovoltaic applications due to the possibility of fabricating not only high-efficiency multijunction solar cells but also third-generation devices such as intermediate-band solar cells based solely on the nitride material system.
Despite the tremendous advantages and potential applications provided by GaN/InGaN solar cells, there still some challenges such as :
1. Growth of InGaN : The most important challenging task for high efficiency III-nitride solar cell is to grow high quality InxGa1-xN materials with high In content.
GaN is the most extensively studied material and comparatively has matured among the III-nitrides, while the lower band gap InGaN, which is more useful for photovoltaic application, is still a topic of fundamental research. High quality InxGa1-xN layers with an In content from around 25 to 100% is required to obtain high-efficiency multijunction solar cells. Presently this becomes difficult with more than 20% content of Indium. Increasing the In content in InGaN during growth poses many challenges in controlling phase separation and defect density which results poor performances in the InGaN solar cells [7], [8], [9]. Growing an InGaN layer (>100 nm) with high crystal quality remains a challenge (Figure 1.2-2) and has severely limited the number of studies on p-GaN/i-InGaN/n-GaN heterojunction solar cells[10]-[13].
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Figure 1.2-2 The relationship between critical thickness and indium content ratio.
2. Lattice mismatch and defects : The III-nitride solar cells face important challenges in lattice mismatch issue. The large difference in interatomic spacing between InN and GaN results poor quality growth of InGaN, especially in the intermediate range of In content . In general, a large lattice mismatch between InN and GaN and the low temperature growth of InGaN detracts from the crystalline quality in epitaxial layers and induces defects due to threading dislocation (TD), thereby generating nonradiative recombination centers (NRCs) in the light absorbing layer.[14]
Furthermore, these defect centers trap and interact with photogenerated carriers, thereby reducing the carrier lifetime and short-circuit current carrier of the solar cells.[15]
3. Phase separation and In fluctuation and carrierdynamics : There are many roadblocks to achieving efficient III-nitride PV devices. Foremost, growing high quality InGaN layers with high indium compositions and the required thickness is difficult, which is already discussed. Phase separation in InGaN layer is also among the key challenges to obtain high performances solar cells. Phase separation is the
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formation of microscopic or macroscopic domains of variable constituent composition in a material. Phase separation incorporates lower band gap domains within the material that absorbs corresponding low energy light, its effect is opposite to that of intermediate bands or quantum dots. As the size and distribution of the lower bandgap phase-separated domains are not optimal, they act as recombination centers decreasing the short circuit current of the solar cell. It is also evident from the theory of quantum well solar cells [16] that the lower bandgap material tends to dominate by pinning down the open circuit voltage of the device.
When the thickness and/or mole fraction of InxGa1−xN materials increase, In-rich clusters in InGaN films easily induce phase separation. This leads to lower Voc compared with theoretical values, low FFs, and large recombination rate by defect states that degrade the Jsc of InGaN-based PV devices.
Although InGaN based solar cells offer tremendous potential for terrestrial as well as space photovoltaic applications, there are only a few reports on InGaN based solar cells.
Furthermore, most reported InGaN solar cells have In contents lower than 15% and band gaps near 3 eV, or larger, and therefore deliver diminishing quantum efficiency at wavelengths longer than 420 nm.[17]-[21] An earlier theoretical material system to obtain solar cells having a solar energy conversion efficiency greater than 50% can be fulfilled by InGaN alloys with In content of about 40%.[22] Additionally, III-nitride multijunction solar cells with near ideal band gaps for maximum solar energy conversion efficiency must incorporate InGaN layers with higher In contents or lower energy band gaps. However, the realization of high crystalline quality In- GaN films in the entire composition range is highly challenging. One of the biggest problems is attributed to the large lattice mismatch between InN and GaN, resulting in low solubility and phase separation.[23]-[24]
InGaN/GaN multiple quantum well solar cells is one way to improve lattice mismatch and critical thickness. In this way, we can grow higher In content layer with short thickness in
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order to enhance the absorption of the incident light.