ABX3 Perovskites for Tandem Solar Cells

Abstract

Worldwide energy consumption is experiencing a drastic increase due to our current way of life. Solar energy is proposed to be the main clean source to replace the burning of fuels. The transformation of solar energy in electricity is made through compounds that absorb solar radiation and spatially support the separation of electrical charge. In recent years a new semiconducting material based on ABX3 perovskites has emerged on the photovoltaic scene, recasting the field of new-generation solar cells since these materials promise superior efficiency at low cost. The research in perovskites pushed their photoconversion efficiency near to the thermodynamic limit. Now this barrier can be overcome by tandem architectures whereby two or more solar cells are stacked together, making more effective use of the incident solar radiation. In this review, we cover the state of the art of perovskite-based tandem solar cells from the description of ABX3 optoelectronic properties to their integration into multi-junction systems. Perovskite solar cells carry the banner for emerging photovoltaics since they have demonstrated power conversion efficiency values well above 20%, which were traditionally only accessible for fairly established technologies such as silicon. Indeed, ABX3 perovskite materials have revolutionized solar cells due to their ease of processing and outstanding electronic and optical properties, which make them ideal candidates for the development of multi-junction devices aiming to surpass limits associated to stand-alone technologies. In this review we discuss the latest regarding this matter. First, we introduce standard materials and processing techniques involved in the preparation of state-of-the-art perovskite solar cells. We then discuss the development of perovskite-based tandem devices in which ABX3 perovskite acts as the active material in the top subcell and Si, CIGS, polymer, or ABX3 act as bottom subcells. Finally, we provide the reader with a discussion on the different lines of research that this rapidly developing field may follow. Perovskite solar cells carry the banner for emerging photovoltaics since they have demonstrated power conversion efficiency values well above 20%, which were traditionally only accessible for fairly established technologies such as silicon. Indeed, ABX3 perovskite materials have revolutionized solar cells due to their ease of processing and outstanding electronic and optical properties, which make them ideal candidates for the development of multi-junction devices aiming to surpass limits associated to stand-alone technologies. In this review we discuss the latest regarding this matter. First, we introduce standard materials and processing techniques involved in the preparation of state-of-the-art perovskite solar cells. We then discuss the development of perovskite-based tandem devices in which ABX3 perovskite acts as the active material in the top subcell and Si, CIGS, polymer, or ABX3 act as bottom subcells. Finally, we provide the reader with a discussion on the different lines of research that this rapidly developing field may follow. In a world where the energy demand is still governed for more than 80% by fossil fuels, any realistic path to reach the objective of reducing CO2 emissions and prevent a further increase in the global average temperature must have renewable energy as its core.1European Commission (2015). Paris Agreement, 21st Conference of the Parties.Google Scholar, 2International Energy Agency (2016). World Energy Outlook 2016. http://www.worldenergyoutlook.org/publications/weo-2016/.Google Scholar In this scenario, solar energy surpasses by orders of magnitude the potential of all other renewable alternatives combined, as displayed in Figure 1A.3Perez R. Perez M. A fundamental look at supply side energy reserves for the planet.IEA/SHC Solar Update. 2015; 62: 4-6Google Scholar Consequently, solar photovoltaic technologies (PV) are attracting tremendous social interest: markets are rapidly increasing their investments in PV, and academia is devoting tireless efforts to study the mechanisms of solar-to-electrical energy conversion to push efficiency limits. Figure 1B displays the evolution of the number of scientific articles related to PV published since 1980, which reflects a continuously growing interest in the development of solar technologies.4Result of the search ‘PV and solar cell’ in the Web of Science. Accessed on 06/01/2017. https://webofknowledge.com/.Google Scholar In this context, solar cells based on metal-halide (ABX3) perovskites emerged in the last decade as a new technology that demonstrates both high performance and low cost. This type of cell is driving a revolution in PV, holding great promise for the generation of green energy on a large scale. Indeed, perovskite solar cells (PSCs) already represent 10% of all papers published in the field of PV and have been recently highlighted as one of the top ten emerging technologies according to the World Economic Forum.5World Economic Forum (2016). Top 10 emerging technologies of 2016. https://www.weforum.org/agenda/2016/06/top-10-emerging-technologies-2016/.Google Scholar This is due to the natural abundance of the precursors employed to synthesize perovskite absorbers, their weightlessness, and, above all, the swift rise in power conversion efficiency (PCE) demonstrated for this technology. All of these facts make perovskite devices potential competitors of well-established technologies such as those based on silicon. Not without success, PSCs evolved from featuring a modest 3.81% in 20096Kojima A. Teshima K. Shirai Y. Miyasaka T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells.J. Am. Chem. Soc. 2009; 131: 6050-6051Crossref PubMed Scopus (5418) Google Scholar to over 20% in several works nowadays,7Correa-Baena J.-P. Abate A. Saliba M. Tress W. Jacobsson T.J. Grätzel M. Hagfeldt A. The rapid evolution of highly efficient perovskite solar cells.Energy Environ. Sci. 2017; 10: 710-727Crossref Google Scholar which reveals PSCs as the most efficient emerging PV devices.8N.R.EL chart. https://www.nrel.gov/pv/assets/images/efficiency-chart.png.Google Scholar, 9Green M.A. Emery K. Hishikawa Y. Warta W. Dunlop E.D. Levi D.H. Hohl-Ebinger J. Ho-Baillie A.W.Y. Solar cell efficiency tables (version 50).Prog. Photovolt. Res. Appl. 2017; 25: 668-676Crossref Scopus (212) Google Scholar Along with the efficiency, scientists are intensively working to develop perovskite materials and PSCs that are environmentally friendly and stable over time. Photoconversion record efficiencies of PSCs are approaching the thermodynamic limit established by the Shockley-Queisser (SQ) theory, which sets the maximum PCE achievable for single-junction cells depending on the bandgap energy (Ebg) of the absorbing material.10Shockley W. Queisser H.J. Detailed balance limit of efficiency of p-n junction solar cells.J. Appl. Phys. 1961; 32: 510-519Crossref Scopus (5341) Google Scholar Different strategies can be employed to surpass such a theoretical ceiling, but only a few have been studied for perovskite devices. Hot carriers have been demonstrated to travel up to 600 nm in CH3NH3PbI3 (MAPbI3) perovskite films but, in order to take advantage of this phenomenon, it will be necessary to develop energy-selective contacts.11Li M. Bhaumik S. Goh T.W. Kumar M.S. Yantara N. Grätzel M. Mhaisalkar S. Mathews N. Sum T.C. Slow cooling and highly efficient extraction of hot carriers in colloidal perovskite nanocrystals.Nat. Commun. 2017; 8: 14350Crossref PubMed Scopus (11) Google Scholar, 12Guo Z. Wan Y. Yang M. Snaider J. Zhu K. Huang L. Long-range hot-carrier transport in hybrid perovskites visualized by ultrafast microscopy.Science. 2017; 356: 59-62Crossref PubMed Scopus (12) Google Scholar Also, recent simulations suggest that multi-exciton generation might be possible in perovskites that show quantum confinement, although experimental demonstration remains a major challenge.13Vogel D.J. Kryjevski A. Inerbaev T.M. Kilin D.S. Photoinduced single- and multiple-electron dynamics processes enhanced by quantum confinement in lead halide perovskite quantum dots.J. Phys. Chem. Lett. 2017; 8: 3032-3039Crossref PubMed Scopus (3) Google Scholar Although such approaches may open new avenues for the development of devices that take advantage of photon energy excess to surpass SQ limits, their demonstration in actual devices seems difficult. On the other hand, multi-junction or tandem configurations have already been proved to improve PCEs over the limits imposed by SQ theory. The working principle of tandem solar cells is based on the combination of different subcells, each of them able to absorb a different part of the electromagnetic spectrum, which allows minimizing losses and reaching higher PCEs. In a standard single-junction solar cell, photons with energies lower than the Ebg of the active material cannot be absorbed while those with higher energies lead to carriers that eventually thermalize to band edges from where they are extracted. These undesirable effects result in current and potential losses, respectively. In contrast, a double-junction configuration combines top (front) and bottom (rear) subcells based on materials with absorption onsets at relatively shorter and longer wavelengths (see Figure 2A). Figure 2B presents canonical energy diagrams for a double-junction tandem cell. Photons with higher energies are harvested in the front cell while those of lower energies penetrate deeper into the device and are absorbed by the rear cell. As happens for single-junction devices, there also exists a thermodynamic limit imposed by the SQ theory in the case of double-junction (tandem) solar cells. This leads to an optimal combination of Ebg for the front and rear cells. It is possible to classify double-junction cells as two- or four-terminal devices based on the electric connection among subcell electrodes. In a two-terminal device, subcells are monolithically stacked, requiring a recombination layer or a tunnel junction to achieve charge neutrality (see Figure 2C). In this series connection scheme, currents through top and bottom subcells must be the same, being thus limited by the subcell that produces a lower photocurrent. A four-terminal tandem solar cell is composed of two self-working cells that are externally connected in series or in parallel according to convenience. As displayed in Figures 2D and 2E, they can be either mechanically stacked or coupled by optical filters. From a fabrication point of view, four-terminal devices are more convenient since the preparation techniques of the different junctions do not need to be compatible. Nevertheless, two-terminal configurations are more attractive from an industrial point of view since they present lower parasitic absorption and costs are reduced due to the lesser amount of transparent conductive material required. Tandem architectures have been widely employed in several PV technologies such as Si, III-V semiconductor, or organic solar cells in order to boost their performance.14Yu Z. 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Vacío. 2004; 17: 21-25Google Scholar In this context, ABX3 perovskite materials, which are easy to process and feature a tunable bandgap, offer a cost-effective alternative for producing highly efficient tandem solar cells. Thus interest in this option has been growing over the last years (see inset of Figure 1B). Herein, we present a review on the use of ABX3 perovskites for tandem solar cell applications. Firstly we introduce this family of materials, paying special attention to their fabrication methods and optoelectronic properties. We then discuss perovskite-based single-junction solar cells in the context of SQ limit. Afterward, we provide a thorough revision of the different demonstrations of perovskite-based tandem devices. PSCs can be combined as front subcells with well-established technologies that operate as rear subcells such as silicon, copper indium gallium selenide (CIGS), or organic PV devices, reaching outstanding efficiencies. Also, perovskite materials with different Ebg can be combined in tandem solar cells. In fact, we show that all-perovskite tandem devices represent a unique opportunity to develop third-generation PV for widespread use. We finish with an outlook whereby we share our view on the paths the community may follow in order to push the efficiency of ABX3 perovskite-based tandems toward new horizons. In the most simplified description, ABX3 perovskites consist on a hexa-coordinated metal (B) cation occupying the centers of octahedra which share their halide-type (X) corners; a cation (A) fills the voids left by every eight of those octahedra (see Figure 3A), located at the corners of a classical cubic cell. Among perovskite materials developed for PV, methyl-ammonium lead triiodide (MAPbI3) is the most widely employed compound. 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