Fan-Lei Wu

Thin Film Solar Cells Fabricated Using Cross-Shaped Pattern Epilayer Lift-Off Technology for Substrate Recycling Applications

This study reports the use of cross-shaped pattern epitaxial lift-off (ELO) technology to release crack-free single crystal epilayers with a solar cell structure from a gallium arsenide (GaAs) substrate. A cross-shaped pattern array was used to define cell size and provide the etch path for the etchant solution. AlAs was used as a sacrificial layer and etched using a hydrofluoric acid etchant through the cross-shaped hole. Results indicate that the entire wafer can be etched simultaneously. The desired carrier, i.e., the electroplate nickel substrate, can directly contact the epilayer without wax or low-viscosity epoxy, and can also be applied to an external force through magnetic attraction to decrease the release time. After the cross-shaped pattern ELO process, the separated GaAs substrate can be recycled through chemical cleaning. The performance of solar cells grown on new and recycled GaAs substrates remained above 90% of the initial performance when the substrate was recycled less than three times.

Performance of GaAs/Mirror/Cu-Substrate Thin-Film Solar Cells

This paper presents the thermal effect on the performance of a GaAs thin-film solar cell with a 50-200- μm-thick copper (Cu) substrate. The GaAs thin-film solar cell is fabricated by transferring a GaAs solar cell on a Cu substrate with a AuGe/Au mirror. The thin-film solar cells with sufficiently thick Cu substrates exhibit significant performance stability (e.g., Voc, Jsc, and FF) as the temperature increases. This stability can be attributed to the following two factors: 1) The highly reflective AuGe/Au mirror enhances the light absorption of the thin-film cell with a thin base layer, and 2) the good thermal dissipation of the Cu substrate reduces thermal degradation.

Thin-film vertical-type AlGaInP LEDs fabricated by epitaxial lift-off process via the patterned design of Cu substrate.

In this study, the thin-film vertical-type AlGaInP LEDs on Cu substrates were fabricated. By performing the epitaxial lift-off (ELO) process, the LED device can be transferred from GaAs to Cu substrate. Then the GaAs substrate was separated and the ELO-LED was completed. To overcome the drawback of crack formation in the epilayer during the ELO process, various patterned Cu substrates were designed. Moreover, the finite element method was used to simulate the stress distribution in the LED sample during the ELO process. From the simulation results, an optimum structure of patterned Cu substrate was obtained since its maximum stress can be confined to the chip edges and the stress was decreased significantly during the ELO process, resulting in an apparent reduction of crack generation after separating the GaAs substrate. This optimum patterned Cu substrate was employed for the fabrication of ELO-LED. In addition, the chemical etching process was also used to etch the GaAs substrate, and this device transferred to Cu substrate was denoted as CE-LED. Based on the measurements of device performances, the forward voltages (@350 mA) of the CE-LED and ELO-LED were measured to be 2.20 and 2.29 V, while the output powers (@350 mA) of these two devices were 49.9 and 48.2 mW, respectively. Furthermore, the surface temperatures (@350 mA) of these two samples were 46.9-48.3 and 45.2-47.0 °C, respectively. Obviously, the device characteristics of the ELO-LED are very similar to those of the CE-LED. It confirms that the design of patterned Cu substrate is very helpful to obtain the thin-film vertical-type AlGaInP LEDs. Additionally, via the ELO process, the separated GaAs substrate can be reused for production cost down.

High separation rate of epitaxial lift-off using hydrophilic solvent for III–V solar cell and reusable applications

Through the epitaxial lift-off (ELO) process using the HF solutions mixed with hydrophilic substances consisting of acetone (ACE), isopropanol (IPA), and methanol (MA), the separation rate for the GaAs substrate and the III-V solar cell can be improved significantly. Especially for the use of HF:ACE etchant, a extremely high lateral etching rate (14.3 μm/min) of the AlAs sacrificial layer can be achieved, as compared with that employing the pure HF etchant (3.6 μm/min). In addition, by conducting the ELO technique, the GaAs substrate was reused for four times, indicating its high potential for the reusable applications.

Optoelectric Properties of GaInP p-i-n Solar Cells with Different i-Layer Thicknesses

The optoelectric properties of GaInP p-i-n solar cells with different intrinsic layer (i-layer) thicknesses from 0.25 to 1 μm were studied. Both emission intensity and full width at half maximum features of the photoluminescence spectrum indicate that the optimum i-layer thickness would be between 0.5 and 0.75 μm. The integrated current results of photocurrent experiment also point out that the samples with 0.5 to 0.75 μm i-layer thicknesses have optimum value around 156 nA. Electroreflectance measurements reveal that the built-in electric field strength of the sample gradually deviates from the theoretical value larger when i-layer thickness of the sample is thicker than 0.75 μm. I-V measurements also confirm crystal quality for whole samples by obtaining the information about short currents of photovoltaic performances. A series of experiments reflect that thicker i-layer structure would induce more defects generation lowering crystal quality.

Effects of buffer layers and front electrode angles on the performance of In 0.16 Ga 0.84 As solar cells

Three buffer layers, that included 2-step, step-graded, and linear-graded layers, were designed to improve the performance of In_0.16Ga_0.84As solar cell. The In₀₁₆Ga₀₈₄As solar cells fabricated on these three buffer layers were denoted as 2S-cell, S-cell, and L-cell, respectively. The efficiencies of these three devices were 9.8%, 14.4%, and 16.3%, respectively. Obviously, the linear-graded buffer layer is most helpful to enhance the efficiency of In0.16Ga0.84As solar cell. Additionally, the front electrode angle of In_0.16Ga_0.84As solar cell was varied from 0° to 90°. When the angles were fixed at 0° and 90°, the devices possessed higher efficiencies of 16.3% and 16.6%, respectively.

Separation-rate improvement of epitaxial lift-off for III-V solar cells

Thin-film III-V semiconductor solar cells have a number of advantages compared with other types of solar cells. For example, tuning the bandgap of III-V compound materials to match the solar spectrum gives the resulting solar cells unsurpassed conversion efficiencies. The virtues of these devices notwithstanding, the semiconductor substrate used in fabricating them is expensive, which adds to their cost. A method known as epitaxial lift-off (ELO) enables substrate reuse, which enhances affordability.1 However, the technique relies on hydrofluoric acid (HF) solution, a popular chemical etchant, and long-term exposure to the etchant increases the surface roughness of either the epilayer (i.e., the thin film containing the device) or the substrate. This roughness in turn hinders both substrate reuse and the performance of the solar cells. To solve this problem, various chemical fluids have been proposed to clean the substrate and modify the surface structure.2 But chemical cleaning is difficult to control because it is isotropic (that is, it etches at the same rate in every direction). A gallium arsenide (GaAs) solar cell on a (100) GaAs substrate consists of a 0.2 m-thick GaAs buffer layer, a 0.2 m-thick indium gallium phosphide (InGaP) etching stop layer, a 3 mthick GaAs buffer layer, a 20nm-thick aluminum arsenide (AlAs) sacrificial layer, and a 2.6 m-thick GaAs device epilayer. For ELO to be practical, etching time needs to be fast. However, arsine (AsH3) bubbles formed during the ELO process are known to obstruct the etching slits and prevent the AlAs sacrificial layer from reacting with the HF solution. Previous research3 established that oxygen is required for chemical etching of AlAs in HF solution. Blocking of the etching slits by the AsH3 bubbles Figure 1. (a) Lateral etching rate for the aluminum arsenide (AlAs) sacrificial layer during solar cell fabrication using various hydrofluoric acid (HF) solution mixtures. (b) Photograph of the sample. H2O: Water. ACE: Acetone. IPA: Isopropanol. MA: Methanol.

Improvement in etching rate for epilayer lift-off with surfactant

In this study, the GaAs epilayer is quickly separated from GaAs substrate by epitaxial lift-off (ELO) process with mixture etchant solution. The HF solution mixes with surfactant as mixture etchant solution to etch AlAs sacrificial layer for the selective wet etching of AlAs sacrificial layer. Addiction surfactants etchant significantly enhance the etching rate in the hydrofluoric acid etching solution. It is because surfactant provides hydrophilicity to change the contact angle with enhances the fluid properties of the mixture etchant between GaAs epilayer and GaAs substrate. Arsine gas was released from the etchant solution because the critical reaction product in semiconductor etching is dissolved arsine gas. Arsine gas forms a bubble, which easily displaces the etchant solution, before the AlAs layer was undercut. The results showed that acetone and hydrofluoric acid ratio of about 1:1 for the fastest etching rate of 13.2 μm / min. The etching rate increases about 4 times compared with pure hydrofluoric acid, moreover can shorten the separation time about 70% of GaAs epilayer with GaAs substrate. The results indicate that etching ratio and stability are improved by mixture etchant solution. It is not only saving the epilayer and the etching solution exposure time, but also reducing the damage to the epilayer structure.

Investigation of InGaAs Epilayer Crystalline Quality and Photovoltaic Device Application

This paper reports the quality of InxGa1-xAs (0＜x＜0.2) layers grown on misoriented GaAs substrate (2°-and 15°-off) by metalorganic chemical vapor deposition. The crystalline quality of the InxGa1-xAs epilayers is determined by x-ray reciprocal space mapping (RSM). RSM results show that the crystalline quality of InxGa1-xAs epilayers grown on 2°-off GaAs substrate is better than that of 15°-off GaAs substrate. It could be due to small strain relaxation in the epilayer grown on 2°-off GaAs substrate. The photovoltaic performance of In0.16Ga0.84As solar cell grown on 2°-off GaAs substrate is better than that of In0.16Ga0.84As grown on a 15°-off GaAs substrate, because the InxGa1-xAs films grown on 15°-off GaAs substrate show a large strain relaxation in the active layer of the solar cell. A large strain relaxation causes the high dislocation density at the initial active layer/InxGa1-xAs graded layer interface for the solar cell grown on 15°-off GaAs substrate.