Perovskite-silicon tandem solar cells enhanced by nano-optics

Tandem perovskite-on-silicon photovoltaic cells have the potential to outperform standard silicon (Si)-based photovoltaic cells in terms of power conversion efficiency (PCE).

Study: Nano-optical designs for high-efficiency monolithic perovskite-silicon tandem solar cells. Image Credit: Dave Weaver/Shutterstock.com

An article published in the journal Nature’s nanotechnology presented perovskite-on-silicon tandem photovoltaic cells exhibiting periodic nanotextures that provide multiple benefits while retaining the material qualities of solution-treated perovskite sheets.

The rise of tandem solar cells with perovskite

It is essential to improve the power conversion efficiencies of photovoltaic cells to extend their applications. Each solar cell is ultimately limited by the substance it is made of, which impacts the efficiency it can achieve.

Figure 1. Nanotextured PSTSC design. a–vsSEM cross-sectional micrographs of the front and back side of the planaria (a), nanotextured (b) and nanotextured + RDBL (vs) PSTSC. c-Si, crystalline silicon. DAFM image of the front face of the lower cell in nanostructured silicon before the deposition of the contact layers. ePhotographs of the final PSTSC with a blue active area between the front silver ring of approximately 1 cm² (left) and the RDBL in the back (right). Image credit: Stannowski, B., Becker, C. et al., Nature Nanotechnology

Monolithic dual-terminal perovskite-on-silicon tandem (PSTSC) solar cells have recently successfully overcome the physiological limitation of traditional single-junction crystalline silicon photovoltaic cells by achieving a PCE greater than 31%.

Improvements in the electronic and optical characteristics of perovskite-on-silicon tandem solar cells have enabled PSTSCs to achieve such high power conversion efficiencies.

Cell polarity can be changed for better transmission per top contact as well as adjustment of the different layers for improved optical characteristics.

The electronic characteristics can be improved via an optimization of the contact layers by using additives and by modifying the perovskite deposit.

Figure 2. Formation and morphology of the perovskite film. a,bRepresentative perovskite films coated by planar spin (a) and nanotextured (b) lower silicon cells coated with Me-4PACz (approximate area, 25 × 25 mm²). vsManufacturing yield of PCTSCs processed on nanotextured planar silicon bottom cells. D,eLateral photographs of perovskite solution droplets on planar (D) and nanotextured (e) silicon substrate with roll-off angle (RA) and static contact angle (CA; inset) shown. FProjected area of ​​domain sizes of perovskite layers deposited on planar and nanotextured substrates determined from top view SEM images (Supplementary Fig. 2). g, Azimuthal (χ) intensity profiles of the (100) reflection of perovskite averaged over 39 angles of incidence from 0.1° to 2°, determined from GIWAXS measurements. Image credit: Stannowski, B., Becker, C. et al., Nature Nanotechnology

How can texturing improve performance?

Efficient light control plays an important role in providing nanostructured device interfaces for better power conversion efficiencies, as evidenced by numerical analyzes of perovskite-on-silicon tandem solar cells.

For light control in Si-based photovoltaic cells, KOH-etched random pyramid textures with microscale sizes are commonly used.

Unfortunately, these textures are incompatible with solution-treated perovskite absorbers which have shown the largest PCEs to date among all perovskite single-junction photovoltaic cells.

Different techniques for implementing light control textures in nanostructured PSTSCs have been investigated so far, either by modifying the perovskite deposition method while keeping the pyramid texture unchanged, or by modifying the pyramid textures of a way that would make perovskite solution processing possible.

Developing a suitable texture capable of balancing the constant trade-off between optical and electronic capabilities of nanostructured PSTSCs with solution-treated perovskite top cells continues to prove challenging.

Figure 3. Implementation of a backside RDBL for high-performance nanotextured PSTSCs. a,bSchematic of the PSTSC layer stack with standard reflector (a) and with an RDBL on the back (b). (i), (n) and (p) refer to intrinsic, n-doped and p-doped layers. The numbered black frames mark the standard rear face (1) and the SiO₂ (2) and the gate finger regions (3) of the RDBL. vs2D map of J_{pH, Si} for different SiO₂ (y-axis) and TCO thicknesses (x-axis). The configurations corresponding to the stacks of experimental layers are marked by arrows and the corresponding J_{pH, Si} is indicated. D, 2D map of power losses in the lower silicon cell for different gate finger pitch (y-axis) with 40 µm finger width and TCO thickness (x-axis). Power dissipated in mW cm⁻² is indicated for the RDBL configuration and for the standard reflector. e, EQE spectra (dotted lines, perovskite EQE; solid lines, silicon EQE) of a planar PSTSC with a standard backside (black) and a nanotextured PSTSC (nt) with an RDBL (red). The integrated photogenerated current densities of the perovskite (J_ph,pero) and silicon (J_{pH, Si}) are indicated. F, Current density/voltage (J–V) characteristics of a nanotextured PSTSC with a CalLab-certified RDBL at Fraunhofer ISE. Image credit: Stannowski, B., Becker, C. et al., Nature Nanotechnology

Research Highlights

Nanotexturing dramatically reduces reflection losses, dramatically increases manufacturing yield with remarkable film-forming capabilities, and improves open-circuit voltage by approximately 15 mV.

To mitigate spurious absorption losses at near-infrared wavelengths, a reflector containing a dielectric buffer layer (RDBL) was added to the rear of the nanostructured silicon bottom cell.

The team, led by Dr. Bernd Stannowski, developed a perovskite-on-silicon tandem solar cell having a smooth sinusoidal nanotexture that added the benefits of the nanostructured silicon surface while retaining the material qualities of solution-treated perovskite.

The group designed a nanostructured perovskite-on-silicon tandem solar cell with a slightly grooved texture on the surface of the lower silicon cell.

Commenting on the effect of the nanostructure, one of the authors, Dr. Christiane Becker said: “More surprisingly, this texture brings several advantages at the same time: it reduces the losses by reflection and ensures a more regular formation of the perovskite film.”

Conclusions of the study

In this work, the research team added soft nanotexturing and improved retro-reflector arrangement in monolithic nanostructured PSTSCs.

The nanoscale textures enabled a significant increase in the process yield of perovskite top cells from about 50% for planar cells to nearly 95% for nanostructured Si bottom cells.

A marked decrease in reflection losses was observed, equivalent to a current density equivalent to approximately 0.5 mA cm^-2. Total short circuit current densities up to 40 mA cm^-2 were detected in silicon and perovskite cells.

Sensitivity studies revealed that nanoscale textures significantly increased performance durability when there was a divergence from the ideal nanocrystalline SiO.₂ layer thickness.

A 15 mV increase in open circuit voltage was a major factor in the improved efficiency of the PSTSC. The increase in open-circuit voltage was primarily the result of improved perovskite cell performance, as evidenced by selective light-emitting characterizations of the subcells.

In addition to the nanoscale textures on the front of the lower Si cell, the team also integrated a reflector containing a dielectric buffer layer on the back.

By decreasing parasitic absorption losses, the perovskite-on-silicon tandem solar cell design improved the current density in the Si cell by about 0.3 mA cm^-2.

Dr. Stannowski’s team achieved a certified power conversion efficiency of 29.8% by integrating RDBL and nanotextures into a single PSTSC. They argued that tailored nanotextures could help improve perovskite semiconductor materials on several levels.

The results of this work are crucial for perovskite-on-silicon tandem solar cells, as well as optoelectronic systems based on metal halide perovskites.

Reference

Stannowski, B., Becker, C. et al. (2022). Nano-optical designs for high-efficiency monolithic perovskite-silicon tandem solar cells. Nature’s nanotechnology. Available at: https://doi.org/10.1038/s41565-022-01228-8

Source: https://www.helmholtz.de/en/

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