In silicon heterojunction (SHJ) solar cells, a wide bandgap material with a high work function is widely used as the hole extraction pathway to attain high efficiency. We introduced a molybdenum oxide (MoOx) film as an effective hole-transfer layer in carrier selective contact (CSC) solar cells by virtue of its wide bandgap along with high work function. The passivation characteristics, and optical and electrical properties of MoOx films were investigated by differing thickness and work function. The combination of 6 nm hydrogenated intrinsic amorphous silicon (a-Si:H(i)) and 7 nm thermally evaporated MoOx passivation layers provides excellent passivation properties, reduces carrier recombination, and improves the cell performance. The synthesized CSC solar cells showed promising results, with an open-circuit voltage (Voc) of 708 mV, short-circuit current (Jsc) = 37.38 mA/cm2, fill factor (FF) = 74.59%, and efficiency (η) = 19.75%. To justify the obtained result, an AFORS HET simulation was conducted based on the experimental results. The high work function and wide bandgap MoOx/c-Si(n) interface developed a considerable built-in potential and suppressed the electron–hole pair recombination mechanism. The CSC solar cell's simulated performance was enhanced from 1.62 to 23.32% by varying MoOx work function (ΦMoOx) from 4.5 to 5.7 eV.
The screen-printing process for making good contact of electrodes with the top layer of solar cells is crucial for enhancing the electrical properties of a solar cell. This paper reports the experimental approach adopted for the process of electrode formation and the resulting shape of electrodes in silicon-based heterojunction (SHJ) solar cells. It was observed that good contact between electrodes and the top transparent conductive oxide (TCO) layer strongly depends on the squeegee pressure, curing temperature, and curing time. By optimizing the squeegee pressure at 0.350 MPa, snap-off distance of 1.4 mm, squeegee speed of 80 mm/sec, curing temperature of 180°C, and curing time of 30 minutes, the height to width ratio (aspect ratio) of the fabricated electrodes was achieved at ~0.66. The results have been verified through 3D laser profiler, field emission scanning electron microscopy, transfer length method, and light current-voltage measurements. The SHJ solar cells were fabricated using an optimized condition and successfully achieved splendid properties of short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF), and efficiency (η) up to 40.57 mA/cm2, 723 mV, 81.03%, and 23.79%, respectively.
Carrier selective contact (CSC) layers have been extensively studied to realize high passivation effect in solar cells. Excellent passivation properties of Al2O3 and a-Si:H(i) as passivating interlayers between the hole-selective contact (HSC) MoOx and p-type c-Si wafer surface are reported herein. MoOx single layer exhibits a high work function value (≥5.0 eV), which can cause sufficient band bending in the band structure for HSC. An Al2O3/MoOx contact exhibits a significantly higher transmittance and surface passivation compared with that of an a-Si:H(i)/MoOx contact. The passivation results for Al2O3/MoOx contact are a carrier lifetime (τeff) of 830 μs and implied open circuit voltage (iVOC) of 726 mV, whereas for conventional a-Si:H(i)/MoOx contact, the corresponding values are 770 μs and 716 mV. Delicate thickness optimization was performed using experimental and simulation results for Al2O3/MoOx and a-Si:H(i)/MoOx stacks to achieve high performance in p-type c-Si solar cells.
Numerical simulation and experimental techniques were used to investigate lithium fluoride (LiFx) films as an electron extraction layer for the application of silicon heterojunction (SHJ) solar cells, with a focus on the paths toward excellent surface passivation and superior efficiency. The presence of a 7 nm thick hydrogenated intrinsic amorphous silicon (a-Si:H(i)) passivation layer along with thermally evaporated 4 nm thick LiFx resulted in outstanding passivation properties and suppresses the recombination of carriers. As a result, minority carrier lifetime (τeff) as well as implied open-circuit voltage (iVoc) reached up 933 μs and iVoc of 734 mV, accordingly at 120°C annealing temperature. A detailed simulated study was performed for the complete LiFx based SHJ solar cells to achieve superior efficiency. Optimized performance of SHJ solar cells using a LiFx layer thickness of 4 nm with energy bandgap (Eg) of 10.9 eV and the work function of 3.9 eV was shown as: Voc=745.7 mV, Jsc=38.21 mA/cm2, FF=82.17%, and =23.41%. Generally, our work offers an improved understanding of the passivation layer, electron extraction layer, and their combined effects on SHJ solar cells via simulation.
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