Guan-Jhong Lin

Effect of indium fluctuation on the photovoltaic characteristics of InGaN/GaN multiple quantum well solar cells

Severe In fluctuation was observed in In0.3Ga0.7N/GaN multiple quantum well solar cells using scanning transmission electron microscopy and energy dispersive x-ray spectroscopy. The high In content and fluctuation lead to low fill factor (FF) of 30% and energy conversion efficiency (η) of 0.48% under the illumination of AM 1.5G. As the temperature was increased from 250 to 300 K, FF and η were substantially enhanced. This strong temperature-dependent enhancement is attributed to the additional contribution to the photocurrents by the thermally activated carriers, which are originally trapped in the shallow quantum wells resulting from the inhomogeneous In distribution.Severe In fluctuation was observed in In0.3Ga0.7N/GaN multiple quantum well solar cells using scanning transmission electron microscopy and energy dispersive x-ray spectroscopy. The high In content and fluctuation lead to low fill factor (FF) of 30% and energy conversion efficiency (η) of 0.48% under the illumination of AM 1.5G. As the temperature was increased from 250 to 300 K, FF and η were substantially enhanced. This strong temperature-dependent enhancement is attributed to the additional contribution to the photocurrents by the thermally activated carriers, which are originally trapped in the shallow quantum wells resulting from the inhomogeneous In distribution.

Efficiency Enhancement of InGaN-Based Multiple Quantum Well Solar Cells Employing Antireflective ZnO Nanorod Arrays

Antireflective ZnO nanorod arrays (NRAs) by a scalable chemical method have been applied for InGaN-based multiple quantum well solar cells. The length of the NRAs plays an important role in photovoltaic characteristics. It was found that the 1.1-μm-long NRA results in enhanced conversion efficiency due to the suppressed surface reflection. However, the 2.5- μm-long NRAs, although exhibiting the lowest reflection, lead to slightly deteriorated performances, possibly due to the increased absorption of the NRAs. The results indicate that the absorption of lengthened NRAs should be considered when optimizing their antireflection performances. We demonstrated a viable efficiency-boosting way for photovoltaics.

Origin of Hot Carriers in InGaN-Based Quantum-Well Solar Cells

InxGa1-xN/GaN multiple quantum-well (QW) (MQW) solar cells with x = 0.30 and 0.15 were characterized. The MQWs with x = 0.30 show deteriorated performances due to the inferior crystal qualities. At the temperatures above 200 K, the conversion efficiency (η) for x = 0.30 exhibits an abrupt increase led by the thermally activated carriers. Two potential origins are proposed for the hot carriers: 1) the native shallow donors in the MQWs and 2) the shallow QWs due to the compositional fluctuations. According to the distinct behavior of the device with x = 0.15, it is believed that the shallow QWs lead to the abrupt increase in η.

Efficiency enhancement of InGaN multi-quantum-well solar cells via light-harvesting SiO2 nano-honeycombs

Periodic sub-wavelength SiO2 nano-honeycombs are fabricated on GaN-based multiple quantum well solar cells by self-assembly polystyrene nanosphere lithography and reactive ion etching. The nano-honeycombs are found to be effective in suppressing the undesired surface reflections over a wide range of wavelengths. Under the illumination of air mass 1.5G solar simulator, conversion efficiency of the solar cell is enhanced by 24.4%. Simulations based on finite-difference time-domain method indicate that the improved performances result from the enhanced optical absorption in the active region due to the reflection suppression and enhanced forward scattering.

Solar energy harvesting scheme using syringe-like ZnO nanorod arrays for InGaN/GaN multiple quantum well solar cells

Syringe-like ZnO nanorod arrays (NRAs) synthesized by a hydrothermal method were applied as the light-harvesting layer on InGaN-based multiple quantum well (MQW) solar cells. Theoretical calculations show that the NRAs with an abrupt shrinkage of tip diameter can further suppress surface reflectance in comparison with the flat NRAs. InGaN-based MQW solar cells with the syringe-like NRAs exhibit greatly improved conversion efficiencies by 36%. These results are attributed to the improved flatness of the refractive index profile at the air/device interface, which results in enhanced light trapping effect on the device surface.

Microdome InGaN-based multiple quantum well solar cells

InGaN-based multiple quantum well (MQW) solar cells (SCs) employing the p-GaN microdome were demonstrated to significantly boost the conversion efficiency by 102%. The improvements in short-circuit current density (Jsc, from 0.43 to 0.54 mA/cm2) and fill factor (from 44% to 72%) using the p-GaN microdome are attributed to enhanced light absorption due to surface reflection suppression. The concept of microdome directly grown during SC epitaxial growth preserving mechanical robustness and wafer-scale uniformity proves a promising way in promoting the photovoltaic performances of SCs without any additional process.

Characterizations of low-temperature electroluminescence from ZnO nanowire light-emitting arrays on the p-GaN layer

Low-temperature electroluminescence from ZnO nanowire light-emitting arrays is reported. By inserting a thin MgO current blocking layer in between ZnO nanowire and p-GaN, high-purity UV light emission at wavelength 398 nm was obtained. As the temperature is decreased, contrary to the typical GaN-based light emitting diodes, our device shows a decrease of optical output intensity. The results are associated with various carrier tunneling processes and frozen MgO defects.

Efficiency dip observed with InGaN-based multiple quantum well solar cells

The dip of external quantum efficiency (EQE) is observed on In(0.15)Ga(0.85)N/GaN multiple quantum well (MQW) solar cells upon the increase of incident optical power density. With indium composition increased to 25%, the EQE dip becomes much less noticeable. The composition dependence of EQE dip is ascribed to the competition between radiative recombination and photocurrent generation in the active region, which are dictated by quantum-confined Stark effect (QCSE) and composition fluctuation in the MQWs.

Nanorod photon management in nitride-based devices

The unique properties of indium gallium nitride (InGaN) alloys, including their wide electronic band gap spanning 0.7–3.4eV, high absorption coefficient, high carrier mobility, and superior radiation resistance,1, 2 make them ideal candidates for optoelectronic devices. Intense research efforts since the mid-1990s have led to remarkable successes in LEDs. More recently, the promising photovoltaic characteristics of InGaN have also attracted increasing research interest.3–5 In the development of nitride-based optoelectronic devices, one of the main challenges is the difficulty of growing highcrystal-quality InGaN layers on GaN. GaN usually makes up pand n-contacts. The lattice mismatch between InGaN and GaN can adversely affect device performance once the InxGa1 xN layer grows beyond a critical thickness on the GaN substrate.6 To address the issue, multiple quantum wells (MQWs) are used to form an active layer, ensuring excellent radiative recombination efficiencies for LEDs while also avoiding the undesired tradeoff between crystal quality and absorption efficiency for solar cells. Furthermore, the quantized energy levels in MQWs offer an additional level of control of solar absorption through proper selection of well and barrier widths without changing the indium content.7 Although progress has been made, many challenges still remain. For LEDs, the internal electrical field in MQWs that results from spontaneous and piezoelectric polarization leads to charge separation and reduces internal quantum efficiency.8 The great difference between the refractive index of GaN and that of air also prevents a high proportion of photons from escaping out of the device.9 In addition to the strong surface reflection caused by the large change in refractive index, the thin InGaN active layer also limits the absorption of solar energy, yielding suboptimal photovoltaic operation. Figure 1. Process schematics for textured gallium nitride (GaN)-based LEDs. (a) SiO2/Ag (silica/silver) layers are deposited on indium tin oxide (ITO). (b) Material following thermal annealing at 270 for a few minutes and (c) following a reactive ion etching process. (d) The silver nanoparticles (NPs) are then removed by nitric acid.