Design guidelines for perovskite/silicon 2-terminal tandem solar cells: an optical study

Abstract

Perovskite/silicon 2-terminal tandem cells have made significant advances towards >25% efficiency. Despite this, there is limited understanding of how the optical properties of the materials affect the optical losses within the tandem cell. Using an accurate optical model, we investigate, identify and propose solutions to the optical loss mechanisms inherent in a typical perovskite/silicon 2-terminal tandem cell. The results highlight, firstly, the requirement for low absorption in all layers above the perovskite film, and secondly, the importance of the proper choice of refractive index and thickness of charge transport layers of the perovskite cell, in order to minimize reflection at the interfaces formed by these layers. We demonstrate that the proper choice of these parameters is based on, and can be guided by, basic optics principles which serve as design guidelines. With careful selection of charge transport materials, optimization of the perovskite absorber thickness and the introduction of light trapping within the silicon cell, a matched current of over 20 mA/cm² can be realized, enabling efficiencies greater than 30% using currently available cell processing methods and materials.

Improved Optics in Monolithic Perovskite/Silicon Tandem Solar Cells with a Nanocrystalline Silicon Recombination Junction

“…[9][10][11]20,21] While in principle effective, this approach suffers from several drawbacks: parasitic absorption at wavelengths above 800 nm due to free-carrier absorption, [22] poor refractive index matching with silicon causing enhanced reflection losses at the TCO/silicon interface, [23] and high lateral conductivity, promoting shunt paths through the top cell. So far, mainly transparent conductive oxides (TCOs) have been used as recombination layers in perovskite-based monolithic tandems.…”

Electron‐Beam‐Evaporated Nickel Oxide Hole Transport Layers for Perovskite‐Based Photovoltaics

“…Indeed, the PV module certification standards of the International Electrotechnical Commission (e.g., IEC 61646) specify that manufacturers need to ensure operating temperatures as high as 85 °C for periods up to 1000 h. Additionally, diffusion of metal atoms or dopants of the spiro-MeOTAD is known to degrade the devices. [33,34] To tackle all these problems, novel materials for charge extracting layers-in particular, HTLs-are required. [31,32] In high-performing single-junction perovskite solar cells, spiro-MeOTAD is usually deposited on the backside of the device, thus influencing the optical losses in the device only marginally.…”

Recent Development of Organic-Inorganic Perovskite-Based Tandem Solar Cells

“…The simulation results have supported the idea that high refractive index carrier transport materials should be used between the transparent electrode and the perovskite active layer to lower the interface reflection. Grant et al have lead a further detailed investigation of the optical loss in the tandem structure based on a complete optical model for the monolithic stack. Their simulation work was based on an open‐source, simulation package called EMUstack, utilizing a combination of scattering matrices and a Finite Element Method (FEM) to calculate the light propagation through multilayered structures.…”

“…For example, Liu et al demonstrated the possibility of employing low‐temperature solution‐processed NiO x thin film as the hole transport layer in both inverted (p‐i‐n) planar and regular (n‐i‐p) mesoscopic perovskite solar cells, which could offer valuable references in the optimization of perovskite/Si tandem solar structure. When it comes to the electron transport layers (ETL), the calculation results indicated that a 20 nm thick ETL with a refractive index of n = 3.6 was optically optimized . However, ETL materials with such a high index have not yet been found.…”

“…(a) Light distribution for the p‐i‐n perovskite/silicon tandem structure, with layer composition MgF 2 /unannealed IZO/spiro‐OMeTAD/MAPbI 3 /compact (cp)‐TiO 2 /annealed ITO/c‐Si/Ag with layer thicknesses 105 nm/44 nm/160 nm/300 nm/44 nm/30 nm/200 µm/200 nm, respectively. (b) Light distribution for the perovskite/silicon tandem with 80 nm NiO x HTL instead of spiro‐OMeTAD . Copyright 2016, Optical Society of America.…”