Sabina Abdul Hadi

Thin Film a-Si/c-Si1-xGex/c-Si Heterojunction Solar Cells: Design and Material Quality Requirements

a-Si:H/crystalline-Si 1-x Ge x /c-Si heterojunction solar cells (HIT cells) are simulated and fabricated for the first time. Cells with junction layers consisting of Si, Si 0.75 Ge 0.25 , and Si 0.59 Ge 0.41 are compared to study the effect of increasing Ge concentration. The results show a V oc drop from 0.6V for Si cells to 0.4V for Si 0.59 Ge 0.41 , consistent with the reduction in bandgap. The measured J sc increases from ~18.5 mA/cm 2 for Si cells to 20.3 mA/cm 2 for the Si 0.59 Ge 0.41 cells, for one light pass. Simulations suggest that the measured J sc for the Si 0.59 Ge 0.41 based solar cells is limited by a low lifetime. In order for Si 1-x Ge x based cells to exceed the efficiency of Si, simulations indicate that Ge percentages larger than 40% and lifetimes above 1 ∝s are required.

Thin film a-Si/c-Si 1−x Ge x /c-Si heterojunction solar cells with Ge content up to 56%

Thin film a-Si(n⁺)/c-Si_1-xGe_x(p)/c-Si(p⁺) heterojunction solar cells are fabricated with Ge content up to 56 atomic percent. Solar cells with junction layers consisting of Si, Si_0.75Ge_0.25, Si_0.59Ge_0.41, and Si_0.44Ge_0.56 are compared to study the effect of increasing Ge concentration. The measured short-circuit current (J_sc) increases from ~14 mA/cm² for Si cells to 21 mA/cm² for the Si_0.44Ge_0.56 cells, for one light pass and a 2 μm-thick SiGe layer. The results show an open-circuit voltage (V_oc) of 0.61 V for Si cells, dropping to 0.32 V for Si_0.44Ge_0.56, consistent with the reduction in band-gap. Quantum efficiency measurements highlight the improved spectral response for higher Ge percentages. Physics based TCAD simulations combined with the experimental results are used to extract lifetime and interface velocity.

Effect of germanium fraction on the effective minority carrier lifetime in thin film amorphous-Si/ crystalline-Si1xGex/crystalline-Si heterojunction solar cells

The effect of germanium fraction on the effective minority carrier lifetime (τeff) for epitaxial Si1-xGex layers is extracted using measurements on amorphous(a) Si(n+)/crystalline(c)-Si1-xGex(p)/crystalline(c)-Si(p+) heterojunction solar cells with x = 0.25, 0.41 and 0.56. The τeff extracted for Si0.75Ge0.25 is ∼1 μs, decreasing to ∼ 40 ns for Si0.44Ge0.56. In addition, the band-gap voltage offset (Woc) increases from 0.5 eV for Si to 0.65 eV for 56% Ge indicating an increase in non-radiative recombination consistent with the reduction in effective lifetime.

a-Si/c-Si 1−x Ge x /c-Si heterojunction solar cells

The performance and material quality requirements of thin film a-Si/c-Si_1-xGe_x/Si heterojunction solar cells are investigated by modeling and simulation. The effects of Ge content, Si_1-xGe_x thickness, Si_1-xGe_x lifetime and a-Si/c-Si_1-xGe_x interfacial quality have been studied. The simulations predict that Si_1-xGe_x based thin film solar cells provide a significant increase in solar cell output current for Ge fractions larger than 30%, due to the narrower band-gap and increased absorption. In addition, the efficiency of thin (2μm) Si_1-xGe_x solar cells surpasses that of Si for minority carrier lifetimes larger than 0.5μs. For these 2μm thin layers, simulations predict reduced material quality requirements for Si_1-xGe_x cells, with a clear performance advantage relative to Si based solar cells.

Theoretical efficiency limit for a two-terminal multi-junction “step-cell” using detailed balance method

Here we present detailed balance efficiency limit for a novel two-terminal dual and triple junction “step-cell” under AM 1.5G and AM 0 incident spectrums. The step-cell is a multi-junction (MJ) solar cell in which part of the top cell is removed, exposing some of the bottom cell area to unfiltered incident light, thus increasing bottom cells photogenerated current. Optical generation of the bottom cell is modeled in two parts: step part, limited by the bottom cell bandgap, and conventional part, additionally limited by the top cell absorption. Our results show that conventionally designed MJ cell with optimized bandgap combination of 1.64 eV/0.96 eV for dual junction and 1.91 eV/1.37 eV/0.93 eV for triple junction has the highest theoretical efficiency limit. However, the step-cell design provides significant efficiency improvement for cells with non-optimum bandgap values. For example, for 1.41 eV ( ∼GaAs)/Si dual junction under AM 1.5G, efficiency limit increases from ∼21% in a conventional design to 38.7% for optimized step-cell. Similar benefits are observed for three-junction step-cell and for AM 0 spectrum studied here. Step-cell relaxes bandgap requirements for efficient MJ solar cells, providing an opportunity for a wider selection of materials and cost reduction.

Reducing optical and resistive losses in graded silicon-germanium buffer layers for silicon based tandem cells using step-cell design

Si solar cells with a SiGe graded buffer on top are fabricated as the initial step in GaAsP/Si tandem cell fabrication. Using this structure, the impact of the SiGe buffer layer on the Si solar cells is characterized. To mitigate the impact of the narrow-bandgap SiGe on the electrical and optical characteristics of the Si sub-cell, a portion of the underlying Si is exposed using a step-cell design. The step-cell design is demonstrated to increase the Jsc of the SiGe/Si stack from 5 to 20 mA/cm2. The layout of the top mesa is shown to have an impact on the device characteristics with the finger design giving better results than the rectangular mesa with respect to fill factor and series resistance. In addition, utilizing the step-cell design increases overall spectral response of the bottom cell, with significant improvements in the short wavelength range.

Growth and characterization of GaAsP top cells for high efficiency III–V/Si tandem PV

We investigate the growth, microstructure and device characteristics of 1.71eV bandgap GaAs_0.76P_0.24 solar cells grown on Si substrates using Si_yGe_1-y graded buffers. Our optimized growth conditions suppress defect nucleation at the GaAsP/SiGe heterointerface and enable the demonstration of single junction solar cells with a threading dislocation density of 3.4×10⁶ cm^-2, a 3× reduction compared to reported GaAs_xP_1-x cells grown on Si. The solar cells have high open-circuit voltages (V_oc) of 1.22 V, bandgap-voltage offset (W_oc) of 0.48 V (representing a 45 mV reduction over prior art). The short-circuit current density (j_SC) is 11 mA/cm² and the fill factor (FF) 82%, under AM1.5G irradiance without an anti-reflection coating (ARC). Integration of an ARC would push these GaAs_0.77P_0.23 single junction solar cells to >15% efficiency, making them well suited to cascade with a Si solar cell for high efficiency tandem solar cells.

Theoretical efficiency limits of a 2 terminal dual junction step cell

In this paper we present theoretical analysis for upper efficiency limit of a novel 2 terminal dual junction stepcell. Results show that step cell design relaxes bandgap requirements for efficient tandem cell. While conventional tandem cell with optimized bandgap combination (1.64 / 0.96 eV) has the highest efficiency (45.78 %), the step-cell design provides significant efficiency improvement for cells with non-optimized bandgap values. Efficiency upper limit for Si based step-cell with top cell bandgap equal to 1.41 eV (~ GaAs), efficiency upper limit increases from ~21% in conventional tandem cell to 38.7% for optimized step-cell design. Step-cell design provides opportunity for wider selection of materials used in tandem solar cell applications.

Design Optimization of Single-Layer Antireflective Coating for GaAs

Single-layer antireflective coating (SLARC) materials and design for GaAs1_xPx/Si tandem cells were analyzed by TCAD simulation. We have shown that optimum SLARC thickness is a function of bandgap, thickness, and material quality of top GaAs_1-xP_x/Sisubcell. Cells are analyzed for P fractions x = 0, 0.17, 0.29, and 0.37, and ARC materials: Si₃N₄, SiO₂ , ITO, 11fO₂, and Al₂O₃. Optimum ARC thickness ranges from 65-75 nm for Si₃N₄ and ITO to ~100-110 nm for SiO2. Optimum ARC thickness increases with increasing GaAs1_xPx absorber layer thickness and with decreasing P fraction x. Simulations show that optimum GaAs_1-xP_x/Siabsorber layer thickness is not a strong function of ARC material, but it increases from 250 nm for x = 0 to1 μm for x = 0.29 and 0.37. For all P fractions, Si₃N₄, 11fO₂, and Al₂O₃ performed almost equally, while SiO2 and ITO resulted in ~1% and ~2% lower efficiency, respectively. Optimum SLARC thickness increases as the material quality of the top cell increases. The effect of ARC material decreases with decreasing GaAs1_xPx material quality. The maximum efficiencies are achieved for cells with ~1-μm GaAs0.71P0.29 absorber (r = 10 ns): ~26.57% for 75-nm Si₃N₄ SLARC and 27.62% for 75-nm SiO2/60-nm Si₃N₄ double-layer ARC.

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A novel GaAs0.71P0.29/Si tandem cell is proposed and simulated. In order to grow GaAs0.71P0.29 layers on Si, Si1-yGey (SiGe) buffer layers can be used but optical losses are expected. To reduce large optical losses a wafer bonded/layer transferred structure can be used that eliminates the SiGe buffer layer. In this work we propose a novel tandem step-cell design that partially exposes the underlying Si cell for both wafer bonded and SiGe based cells. We demonstrate by experiment and simulation mitigation of the optical losses associated with SiGe buffer layers. For an optimized GaAs0.71P0.29/Si tandem cell without the step cell design, simulations estimate ~20% efficiency for the bonded structure and ~3% for the as grown structure with a SiGe buffer. With the proposed novel step-cell design, optimum efficiency of bonded structure increases to ~32% while for structures with SiGe the simulated efficiency reaches ~23%. Optimum exposure of bottom cell area increases with increasing thickness and lifetime of layers above the bottom Si cell.