Characterization Techniques for Perovskite Solar Cell Materials

Meysam Pazoki, Anders Hagfeldt, Tomas Edvinsson

• Publisher: Elsevier
• Year: 2019
• Print ISBN: 9780128147276
• eBook ISBN: 9780128147283
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Characterization Techniques for Perovskite Solar Cell Materials: Characterization of Recently Emerged Perovskite Solar Cell Materials to Provide an Understanding of the Fundamental Physics on the Nano Scale and Optimize the Operation of the Device Towards Stable and Low-Cost Photovoltaic Technology explores the characterization of nanocrystals of the perovskite film, related interfaces, and the overall impacts of these properties on device efficiency. Included is a collection of both main and research techniques for perovskite solar cells. For the first time, readers will have a complete reference of different characterization techniques, all housed in a work written by highly experienced experts.- Explores various characterization techniques for perovskite solar cells and discusses both their strengths and weaknesses- Discusses material synthesis and device fabrication of perovskite solar cells- Includes a comparison throughout the work on how to distinguish one perovskite solar cell from another

Chapter 1

Bandgap tuning and compositional exchange for lead halide perovskite materials

Somayeh Gholipour^1,2 and Michael Saliba^3,4, ¹Department of Physics, Nanophysics Research Laboratory, University of Tehran, Tehran, Iran, ²Department of Physics, Faculty of Basic Sciences, University of Mazandaran, Babolsar, Iran, ³Technical University of Darmstadt, Department of Materials Science - Optoelectronics, Darmstadt, Germany, ⁴IEK-5 Photovoltaik, Forschungszentrum Jülich GmbH, Jülich, Germany

Abstract

Perovskite solar cells (PSCs) have rapidly emerged as a potential competitive photovoltaic technology reaching high power conversion efficiencies (PCEs) from single digits to a certified 23.7% in just a few years. At this stage, the key issues are the further improvement of the PCE and long-term device stability. In this chapter, we summarize the recent developments in the quest to phase stabilize the perovskite material. Since the organic cations are volatile and sensitive to moisture, heat, light and thus a long-term risk factor for phase stability, our focus is on incorporation of inorganic cations such as cesium and rubidium to enhance the photo-active black phases by means of modifications in optoelectronic properties of perovskites. The challenges associated with PSC industrialization include the role of cation/anion in phase stability, phase segregation of the halides, toxicity, organic/inorganic ion mixing and determination of the best tuned-band gap composition for tandem applications.

Keywords

Multi-cation perovskites; Tandem solar cell; Band gap engineering; Phase stability; Photovoltaic devices

Goldschmidt tolerance factor determines the suitability of cation to be compatible with the 3D structure of ABX₃ perovskite, which empirically was found to be between 0.8 and 1 for photoactive “black phase” perovskites.

1.1 Introduction

PSCs consisted of materials with ABX₃ structures (see Fig. 1.1B), where A=cesium (Cs), methylammonium (MA), or formamidinium (FA); B=tin (Sn) or lead (Pb); X=chlorine (Cl), bromine (Br), or iodine (I). They have improved considerably in recent years starting with PCEs from 3.8% [1] in 2009 to 24.2% [2] in 2019. Engineering of material properties and specifically the bandgap of perovskite materials for solar cell applications was conducted recently by changing the material composition. The aim of the current chapter is to present the recent efforts and available techniques for such a compositional engineering, by considering the impacts and consequences on solar cell performance.

Figure 1.1 Band gap tunability for ABX₃ structures ranging from 1.15 to 3.06 eV. (A) Colloidal library of CsPbX₃ (X=Cl, Br, I) solutions under UV light. Reproduced with permission [8]. Copyright 2019, American Chemical Society. (B) Crystal structure of a generic ABX₃ perovskite. (C) Representative PL spectra of CsPbX₃ extended towards MA(Sn/Pb)I₃ perovskites. The PL peaks for the colloidal CsPbCl₃ and CsPbI₃ are at 405 nm (3.06 eV) and 700 nm (1.77 eV), respectively [8]. For the Sn/Pb metal, the peaks for MAPbI₃ and MASnPbI₃ are at 780 nm (1.59 eV) and 960 nm (1.29 eV), respectively [11]. Due to the band gap anomaly of these compounds, the most red-shifted peak can be found for MASn_0.8Pb_0.2I₃ at 1080 nm (1.15 eV) [11]. Reproduced with permission L. Protesescu, S. Yakunin, M.I. Bodnarchuk, F. Krieg, R. Caputo, C.H. Hendon, R.X. Yang, A. Walsh, M.V. Kovalenko, Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut, Nano Lett. 15 (2015) 3692–3696. Available from: https://doi.org/10.1021/nl5048779. Copyright 2019, American Chemical Society. For the purpose of this chapter, the shape of these PL spectra is simulated using Gaussian functions to resemble the colloidal PL data illustrating the versatile shifting of the PL position from CsPbCl₃ (3.06 eV) to MASn_0.8Pb_0.2I₃ (1.15 eV).

These perovskites can be processed by a myriad of techniques (including inexpensive solution processing). They have a tunable band gap from about 1.2 to 3.0 eV by interchanging the above mentioned cations [3,4], metals [5,6], or halides [7]. This is illustrated schematically in Fig. 1.1A, where colloidal CsPbX₃ (X=Cl, Br, I) solutions under UV light have a photoluminescence (PL) peak (Fig. 1.1C) ranging from 1.77 (CsPbI₃) to 3.06 eV (CsPbCl₃). The intermediate band gaps can be reached by mixing the respective halides [8]. In addition, by using Sn/Pb metal mixtures, band gaps from 1.59 (MAPbI₃) to 1.29 eV (MASnI₃) become available. Interestingly, the mixed metals do not follow a linear Vegard trend [5,6,9,10] and the most red-shifted band gap is at 1.15 eV for an intermediary MASn_0.8Pb_0.2I₃ [11]. Moreover, germanium is the second closest element to lead in the periodic table with oxidation state 2+, which has a lower cationic size, lower electronegativity and lower toxicity [12]. Bandgap tuning of germanium based perovskites is pursued by utilizing different monovalent cations [13], halides [14] and mixtures of halides and cations [15] leading to bandgaps of 1.3–3.7 eV. Non-cubic germanium perovskite compounds show nonlinear optical properties as reported in ref. [13] and [16]. As an alternative, Bi³⁺ has been the most investigated trivalent metal cation for lead exchange in halide perovskite solar cell applications so far. Antimony is a neighbor of Bi in the periodic table with oxidation state 3+ and a smaller cationic radius. In similarity to bismuth, the antimony cation tends to form layered perovskites with the formula A₂Sb₃X₉. Thus, trivalent double perovskites including Cs₂Bi₃I₉, MA₂Bi₃I₉ and Cs₃Sb₂I₉ lead to large band gap of 2.0 eV [17], 2.1 eV [17], 1.96 eV [18], respectively.

The large band gap range has made perovskites attractive for various applications outside of photovoltaics, ranging from lasing [19], light-emitting devices [20], sensing [21], photodetectors [22,23] as well as X-ray and particle-detection [24–27].

This also enables various options for photovoltaic applications. Some of the simplest and most accessible lead-based perovskites, e.g. MAPbI₃ [28,29], can already achieve band gaps between 1.55 and 1.62 eV, which are nearly ideal for single-junction photovoltaic (PV) applications [30]. Even targeting directly the Shockley-Queisser optimum for single-junctions at 1.42 eV is an option by using a small amount of Sn in Sn/Pb metal mixtures [30]. Increasing the amount of Sn further results in band gaps of ≈ 1.2 eV which is close to silicon. Larger band gaps of 1.7 eV can also be achieved by using halide Br/I mixtures, enabling tandem solar cells with silicon. Even perovskite on perovskite tandem solar cells become accessible [31–36]. According to the Shockley-Queisser limit and the design of the single-junction solar cell, the maximum achievable efficiency is inherently limited to 31% due to thermalization and transmission losses [30]. Thus, a tandem design that consists of multiple absorber cells stacked in a horizontal fashion can be realized to absorb a larger portion of the solar spectrum and consequentially yield higher PCEs (see Chapter 12).

The same band gap can be achieved through different compositions by varying the ratios of the cations, metals, or anions. For example, increasing the Br content blue-shifts the band gap towards 1.7 eV, which is highly desired for perovskite/silicon tandems. However, this imposes new challenges: for example, using an equimolar amount of mixed halides (Br and I) may lead to phase segregation with distinct Br and I domains occurs in single cation MA-perovskites under full sun illumination as reported by Hoke et al. [37]. Such dephasing is problematic for long-term stable materials, since results in the formation of more iodine rich phases acting as recombination centers [38].

This effect can then be counteracted through perovskites with ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of contributors
• Preface
• Chapter 1. Bandgap tuning and compositional exchange for lead halide perovskite materials
• Chapter 2. X-ray diffraction and Raman spectroscopy for lead halide perovskites
• Chapter 3. Optical absorption and photoluminescence spectroscopy
• Chapter 4. Current-voltage analysis: lessons learned from hysteresis
• Chapter 5. Photoelectron spectroscopy investigations of halide perovskite materials used in solar cells
• Chapter 6. Time resolved photo-induced optical spectroscopy
• Chapter 7. Photovoltage/photocurrent transient techniques
• Chapter 8. Temperature effects in lead halide perovskites
• Chapter 9. Stability of materials and complete devices
• Chapter 10. Characterizing MAPbI3 with the aid of first principles calculations
• Chapter 11. Organic-inorganic metal halide perovskite tandem devices
• Chapter 12. Concluding remarks
• Index