Strategy to mitigate the dipole interfacial states in (i)a‐Si:H/MoO_x passivating contacts solar cells

Abstract

Molybdenum oxide (MoOx) is attractive for applications as hole‐selective contact in silicon heterojunction solar cells for its transparency and relatively high work function. However, the integration of MoOx stacked on intrinsic amorphous silicon (i)a‐Si:H layer usually exhibits some issues that are still not fully solved resulting in degradation of electrical properties. Here, we propose a novel approach to enhance the electrical properties of (i)a‐Si:H/MoOx contact. We manipulate the (i)a‐Si:H interface via plasma treatment (PT) before MoOx deposition minimizing the electrical degradation without harming the optical response. Furthermore, by applying the optimized PT, we can reduce the MoOx thickness down to 3.5 nm with both open‐circuit voltage and fill factor improvements. Our findings suggest that the PT mitigates the decrease of the effective work function of the MoOx (WFMoOx) thin layer when deposited on (i)a‐Si:H. To support our hypothesis, we carry out electrical simulations inserting a dipole at the (i)a‐Si:H/MoOx interface accounting the attenuation of WFMoOx caused by both MoOx thickness and dipole. Our calculations confirm the experimental trends and thus provide deep insight in critical transport issues. Temperature‐dependent J‐V measurements demonstrate that the use of PT improves the energy alignment for an efficient hole transport.

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

“…In order to control the growth of high quality MoO x thin films by achieving an optimal oxygen content, different ( i )a‐Si:H/MoO x interface treatments were employed to investigate their effect on MoO x properties. Accordingly, two different conditions were adopted after depositing ( i )a‐Si:H: using a highly hydrogen diluted (~170 sccm) gas mixture of (1) SiH 4 , H 2 and CO 2 , namely, PT (Plasma Treatment), or (2) SiH 4 , H 2 , CO 2 and B 2 H 6 , namely, PTB (Plasma Treatment with Boron radicals) 19 . The inclusion of boron is optimal for deposition MoO x as successfully demonstrated in our previous work 19 .…”

“…Accordingly, two different conditions were adopted after depositing ( i )a‐Si:H: using a highly hydrogen diluted (~170 sccm) gas mixture of (1) SiH 4 , H 2 and CO 2 , namely, PT (Plasma Treatment), or (2) SiH 4 , H 2 , CO 2 and B 2 H 6 , namely, PTB (Plasma Treatment with Boron radicals) 19 . The inclusion of boron is optimal for deposition MoO x as successfully demonstrated in our previous work 19 . As it will be shown in Section 3.3, these plasma treatments introduce two layers on top of porous ( i )a‐Si:H consisting of a‐SiO x :H/a‐Si:H. PT and PTB processes used the same pressure (2.2 mbar) but different power density (76 and 90 mW/cm 2 , respectively).…”

“…The interest in TMO thin films stems mainly from their wide‐range tunable work function (WF), electronic properties, and good transparency, all of which are important for application in SHJ solar cells 13,14 . Molybdenum oxide (MoO x ) as one of the most promising hole transport layers (HTL) has been well examined 15–19 . In this respect, different processing methods have been investigated to exploit the potential of MoO x layers.…”

“…For instance, MoO x films have been produced by a solution process using organic precursors followed by thermal treatment, 20 atomic layer deposition, 21–23 and thermal evaporation 23,24 . Furthermore, SHJ solar cells using MoO x have achieved above 22% efficiency 16,19 and, so far, a record performance of 23.5% with the potential for further improvements 25 …”

“…In particular, several works attempted to limit the reaction between the MoO x and the intrinsic hydrogenated amorphous silicon, ( i )a‐Si:H, underneath by annealing 25 and by pre‐growth of a SiO x layer 32,33 . Specifically, a surface treatment on the ( i )a‐Si:H layer before MoO x deposition improves solar cell performance 19 . Nevertheless, the fundamental knowledge of the phenomena occurring at the ( i )a‐Si:H/MoO x interface and its application for achieving efficient solar cells devices is not well understood yet.…”