Perovskite-Based Solar Cells

From Fundamentals to Tandem Devices

Saida Laalioui

• Publisher: De Gruyter
• Year: 2022
• ISBN:
  9783110760651
• Edition: 1
• Topic: Tecnología e ingeniería
• Subtopic: Recursos energéticos

"Perovskite-Based Solar Cells: From Fundamentals to Tandem Devices" gives fundamental understanding of perovskite solar cells from the chemical composition of each thin layer composing the different stacks to the whole device. Special attention has been given to the development of the materials forming the perovskite solar cell and their effect on the device performance, in addition to the recent progress of this emerging technology. Moreover, light has been shed on the perovskite elaboration techniques, in addition to the several techniques proposed to improve both the efficiency and the stability of perovskite solar cells. Furthermore, special emphasis was given to the three types of tandem solar cells and their recent advances starting from Perovskite/perovskite tandem solar cells to Perovskite/ CIGS tandem cells to perovskite/ heterojunction silicon tandem solar cells. The latter constitute a promising solution to improve photovoltaic solar cells performance.

Chapter 1 Perovskite-based solar cells

1 Introduction

Several materials are under development in order to further make photovoltaic technologies competitive with conventional energies. This chapter is dedicated to perovskite-based solar cells, with a focus on their component materials, characteristics, structures, and principle of operation, as well as their scientific progress.

2 Perovskites

Since 2009, hybrid perovskites, consisting of both organic and inorganic groups, have become attractive absorbers for third-generation solar cells with high efficiencies exceeding 25% [4, 5].

Perovskites refer to a family of materials of formula ABX₃ with a crystal structure similar to that of calcium titanium oxide (CaTiO₃) called perovskite. Hybrid perovskites are a subfamily of the ABX₃ perovskites. They are the result of the combination of the organic cations A of type (R-NH₃), a divalent metal B of group IV smaller than A, and a halide X or combination of halides, forming a three-dimensional (3D) inorganic structure that contains the organic cation [6].

3 Crystal structure of the hybrid perovskite

As shown in Figure 1, in the perovskite structure, B is a metal cation forming BX₆ octahedra with X anions. These octahedra touch at the vertices forming a 3D network. Cations A are located in the interstices between the BX₆ octahedra [7]. The crystalline structure of the perovskite depends strongly on the elements used [7, 8].

Figure 1: Crystalline structure of the hybrid perovskite. Source: Reprinted with permission from Dian Wang, Matthew Wright, Naveen Kumar Elumalai, Ashraf Uddin, Stability of perovskite solar cells, Solar Energy Materials and Solar Cells, Vol. 147, p. 255–275, Copyright (2016), with permission from Elsevier.

Indeed, the atoms and molecules used to form the hybrid perovskites are those who obey to the Goldschmidt tolerance factor, according to the equation:

$t=\frac{\left(R_{\mathrm{A}}+R_{\mathrm{X}}\right)}{\sqrt{2(}{R}_{\mathrm{B}}+R_{\mathrm{X}})}$

where R_A, R_B, and R_X are the ionic radii of A, B, and X, respectively.

Indeed, the cubic structure is the most stable perovskite structure with a tolerance factor between 0.9 and 1, which is well suited for solar applications. Furthermore, for a tolerance between 0.8 and 0.9, the perovskite structure becomes distorted, giving less symmetric structures. However, when the tolerance factor becomes less than 0.8 or higher than 1, a nonperovskite structure is formed [9, 10].

Indeed, the atoms and compounds that meet the requirements of their crystalline structure and offer optical bandgaps suitable for semiconductors are listed below [4, 6, 9]:

A: Methylammonium CH₃NH₃⁺ (MA), formamidinium HC(NH₂)₂⁺ (FA), and cesium (Cs)

B: Lead (Pb) and tin (Sn)

X: I, Br, Cl, or mixture of two halides

Figure 2: The absorption spectrum of the perovskite CH₃NH₃Pb_(1–X)Sn_XI_3.Source: Reprinted with permission from Reprinted with permission from Q. Chen, N. De Marco, Y. Michael, T. Song, C. Chen, and H. Zhao, “Under the spotlight: The organic — inorganic hybrid halide perovskite for optoelectronic applications,” Nano Today, vol. 10, no. 3, p. 355–396, 2015.

Figure 3: Optical bandgap for the perovskite CH₃NH₃SnI_3–xBr_x.Source: Reprinted with permission from Springer Nature Customer Service Centre GmbH: Nature, Nature Photonics, Lead-free solid-state organic–inorganic halide perovskite solar cells, HAO, Feng, STOUMPOS, Constantinos C., CAO, Duyen Hanh, et al., 2014, vol. 8, no 6, p. 489–494.15. Copyright © 2014, Nature Publishing Group.

4 Characteristics of perovskites

The properties of perovskite depend on the materials used and their fabrication history. On the other hand, the family of hybrid perovskites has some striking characteristics, namely:

• Absorption of wavelengths up to 800 nm as shown in Figure 2 [7, 11, 12]
• A direct bandgap between 1.30 and 2.23 eV [4, 7, 11, 13]
• Possibility of adjusting their optical bandgaps by an appropriate selection of mixed cations and halides as shown in Figure 3 [7, 12]
• Good separation of electrical charges, which explains their high open-circuit voltages (V_oc) up to 1.31 V [14]. Indeed, thanks to the spontaneous electric polarization of the perovskites, internal electric fields are induced, which makes it possible to separate the photoexcited charge carriers, while reducing their recombination [7].
• High charge-carrier mobilities, of the order of 7.5 cm²/Vs for the holes and 12.5 cm²/Vs for electrons, measured at terahertz TH_Z frequencies [7]
• Good light absorption, which explains the high short-circuit current density (J_sc) values, up to 23 mA/cm² [7]
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