Random Pyramidal Textures for Commercial Solar Cells

By Surbhi JainReviewed by Susha Cheriyedath, M.Sc.May 30 2022

In an article recently published in the journal ACS Applied Energy Materials, researchers discussed efficient light-harvesting in the thick perovskite solar cells (PSCs) with industry-applicable random pyramidal textures.

Study: Efficient Light Harvesting in Thick Perovskite Solar Cells Processed on Industry-Applicable Random Pyramidal Textures. Image Credit: Alberto Masnovo/Shutterstock.com

Background

Perovskite/silicon (Si) tandem solar cells, especially the monolithic two-terminal (2T) architecture, have become one of the potential candidates for high-efficiency, low-cost next-generation photovoltaics (PVs) with the capability to surpass the theoretical limit of single-junction solar cells in recent years.

To maximize and match the current generation in both sub-cells, the 2T architecture requires effective light management algorithms. However, front-side polished Si wafers are used in most of the reported highly efficient 2T perovskite/Si tandem solar cells. While this method is suitable for laboratory-scale manufacturing, it is not the most cost-effective alternative for industrial use.

For textured perovskite/Si tandem solar cells, developing a thick perovskite absorber layer that completely covers such intermediate-sized pyramidal patterns has been identified as crucial. Various ways for fabricating perovskite films have been proposed to resolve this challenge. However, chemicals in the perovskite precursor solution are the most common method for increasing the diffusion length.

The ability to investigate the optical gains associated with processing thick PSCs on intermediate-sized textures is enabled by having an efficient, thick, and stable perovskite absorber over a flat surface.

About the Study

In this study, the authors reported high-efficiency PSCs manufactured on replicated industry-applicable random pyramidal textures having a decreased pyramid size of about 1-2 μm. In the first step, planar PSCs with perovskite absorber layers that were only a few micrometers thick maintained efficient charge carrier extraction and had a power conversion efficiency of up to 18% were created by using a Lewis base addition.

The team described the preparation of highly efficient thick PSCs using replicated industry-applicable textured Si wafers that displayed very low reflectance, similar to standard industrial textured Si wafers. Firstly, the Lewis base urea (CH₄N₂O) was used to improve the crystal development of the micrometer thick double-cation perovskite absorber layer, which resulted in thick planar reference PSCs with effective charge extraction and power conversion efficiencies (PCEs) up to 18%.

The researchers evaluated the optical advantages of processing the produced thick perovskite absorber over intermediate-sized textures by using highly scalable nanoimprint lithography to replicate the Si texture on the glass substrates. A spin-coating formulation based on the anti-solvent quenching approach was created, which used urea as a Lewis base addition in a double cation perovskite precursor solution, to obtain suitably thick perovskite layers for entirely covering such intermediate-sized textures.

Observations

When the prepared thick films are used in textured PSCs with inverted pyramids, the AM 1.5G weighted reflectance was lowered from 9.9% to 5.2%, which was better than the planar reference. Reduced broadband reflectance combined with improved light trapping boosted current generation by 87.3% of the maximum possible short-circuit current density. Despite the increased surface area of the roughness, a high open-circuit voltage (V_OC) and fill factor (FF) equivalent to the planar reference PSC was maintained. As a result, the proposed textured PSC had a stable power output of 18.7% for five minutes at maximum power point tracking. The textured PSCs showed better current generation for all the angles of incidence, which highlighted their benefits in bifacial applications and realistic irradiation settings.

When compared to the references, prototype thick PSCs with urea additive comprised of the layer stack ITO/2PACz/perovskite/C60/BCP/Ag showed an average increase in V_OC of 50 mV, along with an average increase in short-circuit current density (J_SC) of about 1 mA/cm² and FF from 67 to 77%.

Over the broadband spectrum, the textured PSCs had lower reflection losses than the planar reference, both in normal incidence and for an angle of incidence (AOI) up to 70°, which resulted in dramatically increased light harvesting. The optical gains yielded a satisfactory J_SC of 87.3%, which was 4.7% higher than the planar reference in absolute terms.

Conclusions

In conclusion, this study elucidated the development of close to a millimeter thick PSCs for efficient light harvesting. The proposed texture had a 1-2 μm pyramid dimension and provided effective broadband light in-coupling along with excellent light trapping near the perovskite band-gap.

Urea was used as a Lewis base addition to increase the crystallization and, as a result, the film quality and grain size of the prepared thick perovskite absorber layers were allowed to achieve sufficiently long diffusion length and efficient charge carrier extraction. Thick PSCs processed over the industry-applicable random pyramid textures were demonstrated after optimizing the thick absorber layer over a planar stack. Using highly efficient nanoimprint lithography, the textures were recreated on glass substrates.

The influence of light control utilizing the proposed approach on the PCE of the textured PSCs with a stabilized power output of up to 18.7% was reflected on the PCE of the textured PSCs with V_OC and FF equivalent to the planar reference. The authors believe that the proposed technique could be useful for mono- or bifacial single-junction PSCs in real-world applications, in addition to the possible application for textured 3T and 2T perovskite/Si tandem solar cells.

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