Interface Treatments for High-Efficiency MoO_x Based Silicon Heterojunction Solar Cells

Silicon based solar cells dominate the market of photovoltaics, which hold the highest potential for green electricity production. A front/back-contacted architecture combined with the silicon-based heterojunction (SHJ) concept realized a world record efficiency of 26.3%¹. However, conventional silicon-based doped layers, which work as carrier selective transport layers, are not optically transparent.<br/>Transition metal oxides are promising candidates for SHJ solar cells due to their advantageous opto-electronic properties. Molybdenum oxide (MoO_x) with high work function (WF) has achieved highly efficient solar cells with 23.5% efficiency². However, depositing high quality MoO_x thin film on top of a-Si:H is challenging due to highly reactive surfaces³. This interaction may result in interfacial oxidation, lower Mo oxidation states and reduced WF^4,5. In turn, electrical performances of solar cells featuring MoO_x are poorer³ than standard SHJ devices.<br/>Several attempts have been made to prevent the reaction between MoO_x and (<i>i</i>)a-Si:H. Pre-annealing⁴of a-Si:H and pre-growth of SiO_x layer⁶ have been proposed as effective ways to prevent the degradation of MoO_x. Nevertheless, those approaches require extra process steps that make MoO_x integration not immediately viable in industrial environment. In our previous work, we proposed a plasma treatment (<i>PT</i>) at MoO_x/(i)a-Si:H interface that demonstrated to mitigate dipole effect and resulted in improved cell performance⁷. After optimizing the interface treatment, even thinner MoO_x layer could be applied.<br/>The solar cells were fabricated using 260±20 μm 4-inch double-side textured n-type &lt;100&gt; FZ wafers. (<i>i</i>)a-Si:H was deposited by plasma-enhanced chemical vapor deposition on both sides, while (<i>n</i>)a-Si:H only at the rear side. We carried out two different plasma treatments on the (<i>i</i>)a-Si:H interface prior MoO_x deposition: one called <i>PT</i> from a mixture of SiH₄, H₂and CO₂ and another called <i>PTB</i> from a mixture of SiH₄, H₂, CO₂ and B₂H₆. One sample without plasma treatment (<i>noPT</i>) was used as reference to evaluate the efficacy of the proposed method. A thickness series from 1 to 4 nm of MoO_x was thermally evaporated from MoO₃ powder at the pressure of 5×10^-6 mbar. After that, optimized 50-nm and 150-nm thick tungsten-doped indium oxide (IWO) layers were sputtered through a hard mask at front and rear side, respectively, defining six 2×2 cm² solar cells per wafer. As metal contact at the front side, we used room temperature Cu plating⁸ with a metal coverage fraction of 1.575%; at the rear side, 500-nm thick Ag layer was sputtered on the full device area.<br/>We studied the impact of different plasma treatment conditions on MoO_x film quality and cells’ performance. MoO_x film quality was assessed based on O vacancies inside the film⁴. The samples with interface treatments contain less O vacancies than the sample without treatment. It is noticeable that the cell precursors endowed with <i>PTB</i> yield higher fill factor (<i>FF</i>) than other types of precursors. Instead, <i>PT</i> and <i>noPT</i> samples benefitted from high short-circuit current density (<i>J</i>_SC). Ultimately, <i>PTB</i> and <i>PT</i> samples achieved similar conversion efficiency. For the variation of MoO_x thickness, we observed that even a slightly change in its thickness influences the solar cells’ electrical performance drastically. An ultra-thin, ~1.7-nm thick MoO_x layer could provide sufficient field passivation (open circuit voltage, <i>V</i>_OC = 721.4 mV) and good opto-electrical properties (<i>FF</i> = 82.18% and <i>J</i>_{SC-illuminated-area} = 40.20 mA/cm²). With our approach we demonstrate word record cell with ISFH-certified conversion efficiency of 23.83%.<br/>¹ LONGi Solar 2021.<br/>² J. Dréon, et al., Nano Energy, 2020.<br/>³ J. Geissbühler et al., Appl. Phys. Lett. 2015.<br/>⁴ M.T. Greiner, et al., Adv. Funct. Mater. 2013.<br/>⁵ M.T. Greiner, et al., Adv. Funct. Mater. 2012.<br/>⁶ J. Tong, et al. ACS Appl. Mater. Interfaces. 2021.<br/>⁷ L. Mazzarella, et al., Prog Photovolt Res Appl. 2020.<br/>⁸ C. Han, et al., Solar RRL 2022.