Abstract
Mechanical stacking of a thin film perovskite-based solar cell on top of crystalline Si (cSi) solar cell has recently attracted a lot of attention as it is considered a viable route to overcome the limitations of cSi single junction power conversion efficiency. Effective light management is however crucial to minimize reflection or parasitic absorption losses in either the top cell or in the light in-coupling of the transmitted light to the bottom sub-cell. The study here is focused on calculating an optimum performance of a four-terminal mechanically stacked tandem structure by varying the optical property and thickness of the spacer between top and bottom sub-cells. The impact of the nature of the spacer material, with its refractive index and absorption coefficient, as well as the thickness of that layer is used as variables in the optical simulation. The optical simulation is done by using the transfer matrix-method (TMM) on a stack of a semi-transparent perovskite solar cell (top cell) mounted on top of a cSi interdigitated back contact (IBC) solar cell (bottom cell). Two types of perovskite absorber material are considered, with very similar optical properties. The total internal and external short circuit current (Jsc) losses for the semitransparent perovskite top cell as a function of the different optical spacers (material and thickness) are calculated. While selecting the optical spacer materials, Jsc for both silicon (bottom cell) and perovskite (top cell) were considered with the aim to optimize the stack for maximum overall short circuit current. From these simulations, it was found that this optimum in our four-terminal tandem occurred at a thickness of the optical spacer of 160 nm for a material with refractive index n = 1.25. At this optimum, with a combination of selected semi-transparent perovskite top cell, the simulated maximum overall short circuit current (Jsc-combined, max) equals to 34.31 mA/cm2. As a result, the four-terminal perovskite/cSi multi-junction solar cell exhibits a power conversion efficiency (PCE) of 25.26%, as the sum of the perovskite top cell PCE = 16.50% and the bottom IBC cSi cell PCE = 8.75%. This accounts for an improvement of more than 2% absolute when compared to the stand-alone IBC cSi solar cell with 23.2% efficiency.
Highlights
In the last several years, the photovoltaic industry based on silicon (Si), cadmium telluride (CdTe), and copper indium gallium diselenide (CIGS) has grown rapidly with power conversion efficiency
In order to calibrate and underline the accuracy of our optical simulations, Reflectance (R), Transmittance (T), and Absorptance (A) of a semi-transparent perovskite-based sub cell with MAPbI3 as absorber material in an architecture of the perovskite cell, as depicted in Figure 1, were measured where q is the elementary charge, h is Planck’s constant, c is the speed of the light, λ is the wavelength, 4 ofthe solar spectrum, Tcell is the transmission of the cell, and R is the reflection of cell
The sum (Jsc-combined ) of the short circuit currents for both the perovskite and the interdigitated back contact (IBC)-Si are calculated and the optimum optical spacer was chosen according to the maximum overall short circuit current
Summary
In the last several years, the photovoltaic industry based on silicon (Si), cadmium telluride (CdTe), and copper indium gallium diselenide (CIGS) has grown rapidly with power conversion efficiency. Materials 2018, 11, 2570 between 14–21% at a cost less than $1/W [1,2]. The industry still produces less than one percent of the world electricity with the desire to boost the power conversion efficiency to above. Perovskite has emerged as a promising absorber material for thin-film solar cell technology [4,5]. Solar cells that are based on perovskite have reached a certified 22.1% power conversion efficiency (PCE) in 2016 [6,7]. The perovskite material is based on a small organic cation (A), a cationic group 14 metal (B), and a halide anion (X).
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