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Wide Bandgap Perovskites

Wide bandgap (WBG) perovskites are a subset of perovskite semiconductor materials. These perovskite materials are characterized by a bandgap energy (Eg) >1.7 eV.

WBG perovskites have several attractive properties for various optoelectronic applications. Some of these attractive features include:

  • Potential for low-cost fabrication
  • Tuneable bandgap energies
  • High absorption coefficients
  • Remarkable defect tolerance
  • Lightweight and flexible so are suitable for applications where weight and form factor are critical considerations.

These materials hold significant promise for next-generation photovoltaics, light-emitting diodes (LEDs), photodetectors (PDs), and other optoelectronic devices. In photovoltaics, WBG perovskites find three uses: tandem/multijunction PV, indoor PV and agrivoltaics.

Fundamentals of Wide Bandgap


Wide bandgap (WBG) perovskites have a larger bandgap compared to intermediate and narrow bandgap perovskites. They absorb higher-energy photons (shorter wavelengths). This characteristic allows them to efficiently convert high-energy photons into electricity in photovoltaic devices.

Band gaps of narrow, intermediate wide band gap perovskites (less than 1.4 eV, between 1.4-1.6 eV and over 1.7 eV)
Bandgaps of narrow, intermediate and wide bandgap perovskites.

Low energy photons are not absorbed. Instead, they pass through the WBG perovskite layer. This partial transmission limits the efficiency of wide bandgap perovskites solar cells compared to intermediate bandgap solar cells.

However, this increased transmission means that WBG perovskites can be semi-transparent, producing semi-transparent solar cells.

Even with the efficiency limits due to their high band gaps, wide band gap perovskite solar cells have the potential to achieve respectable power conversion efficiencies (PCE), typically in the range of 20% to 25%. (Rühle et al., 2016). This is especially impressive considering that the theoretical efficiency limit for any solar cell is PCE=33.7% (attainable for an optimum bandgap value of 1.34 eV).

Light transmission through a wide band gap perovskite- any photons with energy  over 1.7 eV will be absorbed WBG perovskite).
Light transmission through wide band gap perovskites.

Examples of Wide Bandgap Perovskites


  1. Mixed Halide Perovskites: By tuning the halide ratio from iodine (I) to bromine (Br) on the X site of the perovskite crystal lattice, a wide range of bandgaps from 1.5 to 2.3 eV can be achieved. Examples include hybrid organic-inorganic perovskites like MAPb(I1-xBrx)3, FAPb(I1-xBrx)3, and inorganic perovskites like CsPb(I1-xBrx)3.
  2. Double and Triple Perovskites: Double and triple perovskites, such as Cs2AgBiBr6 and Cs3Bi2I9, exhibit wide bandgaps ranging from 2.0 to 2.3 eV. These materials offer intriguing properties for optoelectronic applications and can serve as alternatives to lead-based perovskites.
  3. Chalcogenide Perovskites: Lead-free chalcogenide perovskites, such as BaZrS3, possess bandgaps in the range of 1.8 to 1.9 eV. These materials are of interest due to their non-toxic composition and potential for use in environmentally friendly optoelectronic devices.

These examples demonstrate the diversity of wide bandgap perovskite materials and their potential for applications in photovoltaics, LEDs, photodetectors, and other optoelectronic devices, offering opportunities for bandgap tuning, high absorption coefficients, and flexibility in device design.

Applications


Applications of wide band gap include photodiodes, LEDs, along with various photovoltaics
Applications of wide band gap include photodiodes, LEDs, along with various photovoltaics.
  1. Tandem and Multijunction PV

In tandem and multijunction solar cells, solar cells with different bandgaps are stacked to increase the amount of light absorbed by the cell as a whole. These tandem and multijunction devices use a perovskite solar cell as the top sub-cell due its wider bandgap (1.7 – 2.0 eV).

Any light that is transmitted through the WBG top cell, will then be absorbed by the bottom-sub cell. As these two solar cells have totally different band gaps, between them they absorb different parts of the solar spectrum. This increases the absorbance (due to a wider spectral coverage) and therefore efficiency of the solar cell device. Perovskite tandem and multijunction solar cells can reach above 40% efficiencies (Hörantner et al., 2017).

  1. Indoor PV

Wide bandgap perovskites are better suited to absorb higher-energy photons, such as those found in indoor lighting sources like LEDs and fluorescent bulbs. By matching the absorption spectrum of the perovskite material to the indoor lighting spectrum, it is possible to enhance the efficiency of indoor PV systems to a range of 50-60%. (Jagadamma et al., 2021)

IoT is a smart network of connected physical objects with embedded sensors and actuators. Sustainably, powering these sensors is a huge challenge. These sensors only require μW-mW of power for their efficient functioning - so WBG perovskite solar cells provide an attractive alternative to batteries to power these small devices. 

  1. Agrivoltaics

Agrivoltaics, also known as agro-photovoltaics or solar farming, involves the co-location of agriculture and solar energy production on the same land. Wide bandgap perovskite solar cells hold promise for agrivoltaics systems due to their semi-transparency and efficient absorption of high-energy photons present in sunlight. This is useful in agrivoltaics settings where shorter wavelengths dominate due to reflections from crops or greenhouse structures, allowing perovskite cells to enhance overall energy conversion efficiency.

Additionally, these cells perform well under partial shading conditions commonly encountered in agrivoltaics systems. Here WBG perovskite devices outperform traditional silicon solar cells. There are the benefits of combining solar energy production with agriculture, including improved crop yields and land use efficiency. 

  1. LEDs

Perovskites, as direct bandgap materials, typically have better radiative recombination rates compared to indirect bandgap semiconductors like silicon. Wide bandgap perovskites solar cells often have high photoluminescence quantum yields (PLQY), signifying their efficiency in converting absorbed energy into emitted light. This is fundamental for attaining both high brightness and efficiency in perovskite-based light-emitting diodes (LEDs). CsPbBr3 is known for its green and blue light emission in LEDs and leads ongoing LED device research.

  1. PDs

Perovskite photodetectors have fast response times, allowing them to quickly detect changes in incident light intensity. This property is important for applications requiring rapid detection, such as in imaging systems or communication devices.

Wide bandgap perovskite photodetectors can be integrated with other optoelectronic devices, such as light-emitting diodes (LEDs) or solar cells, to create multifunctional systems. EQE of 84.9% (Yun et al., 2022), the highest reported EQE in blue PDs, uses triple cation mixed halide wide bandgap perovskites.

Challenges for Wide Bandgap Perovskites


Apart from the stability issues under moisture, oxygen, and heat, wide bandgap perovskites suffer from the following issues and challenges:

  1. Mixed Halide Perovskites: Compared to their lower bandgap counterparts, these perovskites often exhibit relatively poor photovoltaic performance. Mixed halide wide band gap perovskites need improvement in both efficiency and stability for commercial viability.
    • Non-Radiative Losses: Wide bandgap mixed halide perovskites face challenges due to increased levels of non-radiative losses, both within the bulk material and in device configurations.
    • Understanding Poor Performance: The mechanisms underlying the inferior performance of wide bandgap materials remain yet to be understood.
    • Halide Segregation: What is known to us is that achieving wide bandgaps requires a bromine (Br) content of 20% or higher. However, in mixed halide perovskites this leads to light-induced halide segregation (Hoke effect). Halide segregation leads to the formation of iodide-rich lower bandgap phases and bromide rich higher bandgap phases. The iodide-rich lower bandgap phases limit the open circuit voltage to around 1.2 V. Ideally, wide bandgap perovskites can reach VOC > 1.5 V for maximum performance. (Ramadan et al., 2023)
    • Organic A-site vulnerabilities: Halide segregation is particularly severe in perovskites containing methylammonium (MA) on the A-site; this effect also affects other organic A-site perovskites. Inorganic perovskites have better stability under light illumination than the organic-inorganic hybrid perovskites. Compositional engineering, additives engineering as well as interface engineering aspects are being looked at to mitigate the halide segregation issues.
  2. Double and Triple Perovskites: Double and triple perovskites often require intricate synthesis methods compared to conventional single-component perovskites.
    • Synthesizing these materials can lead to issues achieving high-quality and uniform crystals, affecting device performance. Variations in composition and crystal structure can lead to unpredictable changes in bandgap energy, affecting device performance and efficiency.
    • Efficient charge carrier transport is crucial for optoelectronic devices, but double and triple perovskites may suffer from limited charge mobility or carrier recombination rates. Poor carrier transport properties can lead to decreased device performance and lower overall efficiency.
  3. Chalcogenide Perovskites: Chalcogenide perovskites require very high crystallization temperatures (e.g. for BaZrS3, crystallisation takes place at 900°C) (Karthick et al., 2022), making device fabrication difficult and expensive. The bottom layers degrade under such conditions. One potential solution for this is to use a patterned back contact architecture to mitigate these challenges.

Conclusion


In conclusion, wide bandgap perovskites represent a promising subset of perovskites with significant potential for revolutionizing various optoelectronic technologies such as photovoltaics, light-emitting diodes (LEDs), and photodetectors. While challenges such as stability, efficiency, fabrication complexity, and carrier transport limitations exist, ongoing research and development efforts are steadily advancing wide bandgap perovskite technologies. Addressing these challenges and harnessing the full potential of wide bandgap perovskites could lead to the development of highly efficient, low-cost, and environmentally friendly optoelectronic devices, contributing to the transition towards a sustainable energy future.

Further Reading and References


Ramadan, A. J., Oliver, R. D., Johnston, M. B., & Snaith, H. J. (2023). Methylammonium-free wide-bandgap metal halide perovskites for tandem photovoltaics. Nature Reviews Materials, 8(12), 822–838. doi:10.1038/s41578-023-00610-9

Rühle, S. (2016). Tabulated values of the Shockley–Queisser limit for single junction solar cells. Solar Energy, 130, 139–147. doi:10.1016/j.solener.2016.02.015

Contributors


Written by

Dinesh Behera

Scientific Writer

Diagram by

Sam Force

Graphic Designer

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