Characteristics of InGaNsapphire-based photovoltaic devices with different superlattice absorption layers and buffer layers

Characteristics of InGaN/sapphire-based Photovoltaic Devices

with Different Superlattice Absorption Layers and Buffer Layers

Chih-Ciao Yang,* Jinn-Kong Sheu, Min-Shun Huang, Shang-Ju Tu, Feng-Wen Huang, Kuo-Hua Chang and Wei-Chih Lai

Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan City, Taiwan 70101

Ming-Lun Lee

Department of Electro-Optical Engineering, Southern Taiwan University, Tainan, Taiwan 71005

E-mail: [email protected]

ABSTRACT

In this study, hetero-structure p-i-n type epitaxy wafers were deposited on the GaN/sapphire templates with different buffer layers by the MOVPE system. The absorption layers sandwiched in top p-GaN and bottom n+-GaN layers were designed into different short-period InGaN/GaN superlattice structures with specific pair numbers to maintain a total absorption thickness of 200 nm. As the buffer layer was properly adjusted, the V_OC and J_SC were enhanced by 35% and 95%, respectively. In addition to material qualities, the thickness of GaN buffer layers and piezoelectric-induced stain in the InGaN film itself also influenced the PV device performance.

INTRODUCTION

Bandgap engineering of group III-nitride compound semiconductors between indium nitride (InN, ~ 0.7eV) and gallium nitride (GaN, ~3.4 eV) by the In_XGa_1-XN alloy system makes it possible for the full-solar-spectrum photovoltaic (PV) applications since the bandgap energy of InN was confirmed in 2002 [1-2]. Recently, because of the potential multi-junction solar cells for ultra-high conversion efficiency over 50% by optimum bandgaps, aiming at terrestrial power plants (high concentrated PV) and extraterrestrial satellites, many research groups have reported their efforts in the research of the InGaN/sapphire-based PV devices, mainly including multiple quantum well (MQW) [3-5], p-i-n junction [6-9] and short-period superlattice (SPS) designs [10-11].

EXPERIMENTS

The InGaN/GaN epitaxy wafers in this study were deposited on c-face sapphire (Al₂O₃) substrates by a metalorganic vapor-phase epitaxy (MOVPE) reactor (EMCORE D-180). Figure 1 shows the schematic structures of the PV devices. First, two templates consisting of different thickness of 2- and 4-μm-thick undoped GaN (u-GaN) buffer layers were prepared, respectively. Then, InGaN/GaN p-i-n structure was simultaneously deposited on both the templates with 2- and 4-μm-thick u-GaN, where the samples were labeled as A-2 and A-4, respectively. Samples A-2 and A-4 consisted of a 1-μm Si-doped n+-GaN bottom layer (n~ 5×1018 cm-3), an undoped InGaN/GaN (2 nm/4 nm for 33 pairs) SPS structure, and a 100-nm-thick Mg-doped p-GaN (p~ 5×1017 cm-3) top layer in series. Meanwhile, another main p-i-n structure with a different InGaN/GaN (3 nm/4 nm for 28 pairs) SPS structure was also deposited on both u-GaN templates, and the samples were labeled as B-2 and B-4, respectively. Both the A- and B-series samples have the same active-layer thickness (200 nm) of the InGaN/GaN SPS layers.

RESULTS & DISCUSSION

CONCLUSION

The underlying u-GaN buffer layer thickness was increased to grow main InGaN-based PV devices. The J_SC of devices A-4 and B-4 were enhanced by 22% and 94%, as compared with that in devices A-2 and B-2, respectively. This is because the probability of scattering is decreased in the active regions with lower TD density, and hence photogenerated carriers could transport more easily to reach the external circuits. Device B-4 with a better coupled wavefunction in the superlattice active-region yielded the AM1.5G power conversion efficiency of 0.82%, mainly limited by the epitaxy technology so far. It is necessary to improve the material qualities to lower down the TD density and reduce the R_S of InGaN materials before the full-solar-spectrum InGaN-based PV devices are realized. When the indium content of In_XGa_1-XN is increased to absorb more sunlight, the polarization effects of InGaN layers in SPS structures must take into consideration. Although the opposite trend of built-in electric fields caused by the polarization of InGaN layers could be decreased by increasing the thickness of GaN barrier layers or aluminum-containing barrier layers (e.g. AlGaN), the localization of photogenerated carriers in the well layers could be a main problem for SPS or MQW InGaN PV designs.

REFERENCES

Fig. 2. The schematic structures of the PV devices of two buffer layer thickness and two compositions of short-period superlattice (SPS) absorption layers.

Fig. 3. The (002) XRD patterns of different p-i-n PV epitaxial wafers. The enlargement indicated a little blue-shift when the main structures were grown on thicker u-GaN templates, which could be attributed that lattice constants of epitaxial layers were more relaxed on the thicker u-GaN template layers [12].

Fig. 1. The solar spectral irradiance at air mass 0 (AM0) and global air mass 1.5 (AM1.5G) and the cutoff wavelength of semiconductor materials for common PV applications. The In_xGa_1-xN alloys have excellent potential for full-solar-spectrum absorption and tandem solar cells.

Table 1. Measured PV parameters of each fabricated device by AM1.5G illumination.

Fig. 5. Simulated energy-band diagrams of InGaN/GaN (2 nm/4 nm) under forward bias at 0, 1 and 2 V.

As the thickness of underlying u-GaN buffer layers was increased, the PV devices showed enhanced open-circuit voltage (V_OC) in devices A-4 and B-4, as compared with devices A-2 and B-2, respectively. The increased V_OC could be attributed to the fewer leakage paths in the devices with 4-μm-thick u-GaN buffer layers that the TD density in GaN/sapphire epitaxial layers becomes lower when the thickness of the buffer layer is increased [13]. The decreased TD density could potentially reduce the saturation current and therefore increase V_OC values [17]. The less TD density in sample A-4 and B-4 could potentially reduce the saturation current and therefore increase V_OC values [14]. Besides, the solar response of each SPS PV device shows non-typical J-V curves that the fill factor (FF) is quite low. This could be attributed to the recombination in the space charge region of InGaN/GaN SPS layers with densely charge-related defects [15]. As shown in figure 4, PV devices exhibited large series resistance (R_S), which degrades the PCE, mainly resulting from the 200-nm-thick i-layer of InGaN/GaN SPS structures. This phenomenon leads to a low FF and reduced photocurrent values of the PV devices.

Fig. 4. Typical J-V characteristics of PV devices illuminated under AM1.5G standard testing conditions. The inset shows the recombination current biased at low forward voltage.

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Optoelectronic Materials, Devices and Applications

Gallium Nitride Materials and Devices: 7939-54

Fig. 6. Simulated energy-band diagrams of PV devices with different thickness of In_0.24Ga_0.76N/GaN SPS structures at zero bias.