Optimizations of high-efficiency silicon heterojunction solar cells for tandem applications

Abstract

Silicon heterojunction (SHJ) solar cells have exhibited efficiencies well above 25%. To further boost the efficiencies of c-Si-based solar cells, high-bandgap perovskite cells are stacked on top achieving a record efficiency of 29.52%. However, as most of the high-quality perovskite films are solution-processed, the front surface of the bottom device should be flat. Therefore, in this work SHJ bottom c-Si cells featuring front-side-flat and rear-side-textured morphology, which delivers high VOC together with excellent near-infrared response, have been optimized as bottom cells for tandem configurations.

Firstly, RF-PECVD deposition conditions of a (i)a-Si: H monolayer for symmetric <100> flat c-Si surfaces were optimized. The optimized (i)a-Si:H monolayer ( 10-nm-thick) was obtained using pure SiH4, which results in rather moderate passivation performances (teff = 1.2ms, i-VOC = 701 mV).
To improve further the passivation quality of monolayer (i)a-Si:H on flat <100> surface, other passivation approaches aiming at incorporating more H without promoting detrimental epitaxial growth have been investigated.
With a bilayer deposition approach, which features firstly a less H-containing (i)a-Si:H to prevent epitaxial growth and then a second H-rich (i)a-Si:H layer, the passivation properties were slightly enhanced to τeff=1.4 ms and i-VOC=704 mV. Subsequently, by combining the bilayer approach with a post HPT, τeff of 2.0
ms and an i-VOC of 714 mV were achieved. Finally, by combining the bilayer approach with an intermediate HPT, the optimal passivation sample was deposited, with τeff of 2.4 ms and an i-VOC of 720 mV on the flat <100> surface.
To gain a better understanding of the correlation between passivation qualities and the microstructure properties of (i)a-Si:H on flat <100> surface, the layers have been characterized mainly via Fourier-transform infrared spectroscopy (FTIR). From the analysis, it can be concluded that the passivation layer that contains
sufficient H and a higher fraction of monohydrides is beneficial for achieving a better passivation quality.

For the two-terminal tandem solar cells, bottom cells with (n)-contact on top are preferred due to the optical advantage of the perovskite top cells with the p-i-n configuration. Therefore, a first tandem cell with (n)a-Si:H has been fabricated in collaboration with TU Eindhoven resulting in 22.2% efficiency. Starting from
this first fabricated tandem cell, its main optical limitations have been identified by performing advanced optical simulations using GenPro4, and the main strategies to overcome these optical drawbacks have been defined. By optimizing the front anti-reflection layers (MgF2 and ITO) thicknesses (at 100 nm and 20 nm, respectively), and reducing C60 thickness from 20 to 10 nm, front reflections, and parasitic absorption can be minimized. Thus a gain of implied photocurrent density of 1.8 mA/cm2 for the tandem cell was obtained.
Further, by implementing (n)nc-SiOx:H doped layer in the SHJ bottom cell, instead of standard (n)a-Si:H layer the reflection between the top and bottom cell is also reduced, and enhanced light incorporation into the bottom cell is obtained. By adopting all the above optimizations and also adjusting the perovskite
layer from 473 nm to 530 nm, a total improvement of 2.7 mA/cm2 in implied photocurrent density with respect to the initial 22.2% tandem cell can be achieved.
After having identified different optically optimized SHJ bottom cells for tandem applications, both rear junction and front junction single-side-textured SHJ solar cells were fabricated. Firstly, the passivation quality of (i)a-Si:H/(n)-layer and (i)a-Si:H/(p)-layer on different (i)a-Si:H were investigated. Then RJ solar cells
with three different (n)-type layers [(n)nc-SiOx:H;(n)nc-Si:H;(n)a-Si:H)] have been fabricated with optimal thicknesses individuated from the tandem optical simulations. Furthermore, a tunnel recombination junction SHJ solar cell with a layer stack of (n)nc-Si:H/(p)nc-SiOx:H/(p)nc-Si:H has been fabricated and measured as well.

In conclusion, various doped contacts (both n- and p-type) were successfully implemented into SHJ solar cells, which delivered VOCs range from 700 to 714 mV and FFs range from 77.8% to 80.9%. Therefore, different well-functioning SHJ solar cells have been developed and are ready to be implemented as bottom cells for high-efficiency tandem devices.