Maximizing the short-circuit current density via novel silicon heterojunction solar cell designs

Y. Zhao, P. Procel, L. Cao, S. Smits, K. Kovačević, L. Mazzarella, O. Isabella

Photovoltaic Materials and Device group, Delft University of Technology, the Netherlands

E-mail: O.Isabella@tudelft.nl

Abstract

Recent  advancements  in  crystalline  silicon  (c-Si)  solar  cell  technologies  have  demonstrated certified power conversion efficiencies of over 26.8% for front/back-contacted (FBC) and 27.3% for interdigitated/back-contacted (IBC) solar cells [1], [2]. Those efficiencies, achieved by silicon heterojunction (SHJ) technology together with high-quality wafers, have almost reached their maximum potential [3]. Therefore, it is increasingly relevant to explore alternative SHJ solar cell designs to further enhance the device performance with reduced material usage and simplified processing.

Conventional  FBC-SHJ  solar  cells,  with  a  full-area  silicon  thin-film  stack  and  transparent conductive oxide (TCO) layers on the illuminated side, suffer from parasitic absorption losses that limit the attainable efficiency. To minimize those losses, various alternative SHJ solar cell designs have been proposed [4]. Among them, one promising approach is to substitute doped silicon thin- film layers with transition-metal-oxide (TMO) layers, which are significantly more transparent and can further improve the short-circuit current density (JSC) of solar cells. With this, we demonstrated a Cu-plated front junction FBC-SHJ solar cell that exhibited a high JSC value of 40.2 mA/cm²  and a JSC,EQE  value (integrated from external quantum efficiency (EQE) measurement without metal shading) of 41.63 mA/cm²  by using a  1.7-nm-thick Molybdenum  oxide  (MoOx) layer on the illuminated sides [5]. The potential of such designs has been evaluated via consistently coupled advanced optical and electrical simulations, which suggest an attainable JSC of  42.30 mA/cm² with a conversion efficiency of 27.40% for TMO-based FBC solar cells [6].

A more promising FBC-SHJ design involves localizing both the silicon thin-film stack and the TCO layer beneath the metal contact, while the unshaded area is covered with transparent anti- reflection layers. We demonstrate the potential of this architecture through Cu-plated rear junction SHJ solar cells featuring the front localized contacts, achieving a high JSC  of 40.5 mA/cm² and enabling an efficiency exceeding 23%, representing a 2%abs. Efficiency improvement over the original conventional architecture [7]. Additionally, according to our simulations, this design has the potential to reach a JSC of 43.38 mA/cm² and an efficiency of 28.00% [6].

Moreover, IBC designs can mitigate the metal shading on the illuminated side, further boosting the  conversion efficiency. Our simulations indicate an achievable efficiency of 28.61% for IBC-SHJ  solar cells when implementing optimized pitch design and c-Si wafers (e.g. resistivity and thickness) [6]. However, atypical IBC fabrication process requires complex patterning of both n-type and p- type contacts on the rear side. This can be simplified by using a blanket MoOx layer over a patterned  n-type silicon thin-film layer, where electrons are collected via a novel n-layer/MoOx contact stack.  IBC cells with this design have achieved a JSC of 40.6 mA/cm²  and an  efficiency above 23%,  enabling  a  simpler  fabrication process and with significant  potential for further efficiency enhancement [8].

References：

1. H. Lin etal., “Silicon heterojunction solar cells with up to 26.81% efficiency achieved by  electrically optimized nanocrystalline-silicon hole contact layers,” Nat. Energy, vol. 8,no. 8,pp. 789–799, 2023.
2. H. Wu et al., “Silicon heterojunction back contact solar cells by laser patterning,” Nature, 2024.
3. A. Richter, M. Hermle, and S. W. Glunz, “Reassessment of the Limiting Efficiency for Crystalline Silicon Solar Cells,” IEEE J. Photovoltaics, vol. 3,no. 4,pp. 1184–1191, 2013.
4. Y. Zhao et al., “Strategies for realizing high-efficiency silicon heterojunction solar cells,” Sol. Energy Mater. Sol. Cells, vol. 258,p. 112413, 2023.
5. L. Cao et al., “Achieving 23.83% conversion efficiency in silicon heterojunction solar cell with ultra‐thin MoOx hole collector layer via tailoring (i)a‐Si: H/MoOx interface,” Prog.   Photovoltaics Res. Appl., 2022.
6. P. Procel, Y. Zhao, and O. Isabella, “Novel c-Si solar cell architectures exceeding conversion efficiency well above 27%,” To be submitted.
7. S. Smits, Y. Zhao, P. Procel, and O. Isabella, “Silicon heterojunction solar cells featuring localized front contacts,” To be submitted.
8. K. Kovačević, Y. Zhao, P. Procel, L. Cao, L. Mazzarella, and O. Isabella, “Interdigitated‐ back‐contacted silicon heterojunction solar cells featuring novel MoOx‐based contact stacks,” Prog. Photovoltaics Res. Appl., 2024.