Perovskites (PP2.3)

PP2.3 Perovskites

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Aims and objectives

 

This work package will build on ACAP’s existing leadership in perovskite solar cell research to enhance their performance both in terms of power conversion efficiency and stability. Efforts will focus on i) improving the external radiative efficiency by one order of magnitude above that of the current state of the art and ii) demonstrating single junction perovskite cells with bandgap close to the ideal and power conversion efficiencies exceeding 27%.

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For the more thermally-stable, inorganic-perovskite cells, research will focus on improving carrier lifetimes towards 10 µs, reducing surface recombination velocities (towards 103 cm/s) for demonstrating 22% 1.72eV bandgap cells, 19.0% 1.90eV cells and 16.6%, 2.05eV cells.

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In terms of stability our goal is to develop flat-plate encapsulated cells that pass the full IEC 61215 standard tests and stable flexible encapsulated cells with power-to-weight ratios towards 30 W/g.

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These advances in perovskites will be harnessed within tandem cells (PP3) and various emerging applications (PP6).

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Background

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Since the emergence of perovskite solar cells more than a decade ago, there has not been any slowdown in their research and development activities. Indeed, recent progress has been fuelled by increasing public and private sector funding

 

[1]. Perovskites exhibit many alluring photovoltaic attributes, including i) high power output with respective to weight ii) flexibility, iii) ease of fabrication by solution processes, and iv) tunable bandgaps, creating significant interest in the potential for low-cost and high performing next generation photovoltaic technologies.

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To achieve a respectable, levelized cost of energy however, sufficient for manufacturers to invest in perovskite cell technology, both high efficiency and long operational lifetime are critically important. For that reason, research activities in ACAP will be focussed on achieving single junction durable and high-efficiency perovskite flat plate and flexible solar cells (which will be covered in this section, as well as their application in perovskite tandems, which will be covered in PP3.

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Analyses of various cell technologies reveal that the external radiative efficiencies (ERE), a measure of a solar cell’s effectiveness in absorption and radiation without non-radiative losses, is one order of magnitude lower than those of silicon and GaAs, the state of the art for terrestrial and space photovoltaics. Therefore, there is scope for efficiency improvement even for single junction perovskite cells which will one of the research themes in this work package

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Research Activities and Plans

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PP2.3.1 High Efficiency Hybrid and Inorganic Perovskite Solar Cells

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The aim of the research is to demonstrate single junction perovskite cells with ERE approaching 100%. For cells with bandgaps close to the ideal, i.e., <1.6eV, the power conversion efficiency is expected to reach or exceed 27%.

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Key strategies include composition engineering for reducing the bandgap of the state-of-the-art from 1.574eV while maintaining the same level of ERE.

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Strategies for increasing ERE include reduction or elimination of intrinsic defects by demonstrating mono-crystalline or close to “mono” perovskites and developing defect-free heterojunctions.

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The high-performance, single-junction, perovskite-cells research program will also include inorganic, lead-halide perovskite (e.g., CsPbIXBr3-X) solar cells which have emerged in the last 5 years with an added advantage of thermal stability.

 

The higher bandgaps of CsPbIXBr3-X also make them desirable for tandem solar cells. It is apparent that these cells are performing well optically with some reaching 90% of their theoretical current output limits. However, low carrier lifetime and high surface recombination are limiting the voltages and fill factors of these cells.

 

Research will be conducted: i) to improve CsPbIXBr3-X film quality with minimum defects with carrier lifetimes towards 10µs, enabling thick absorbers for high optical absorption without jeopardizing voltage output, ii) best transport layer and interlayer designs producing positive band-offsets for carrier transports and reduced surface recombination velocities thereby improving fill factors and output voltages.

 

The aim is to improve cell performance reaching 80% of their theoretical limits. This translates to 21.7%, 19.0% and 16.6%, respectively for CsPbI3 (1.72eV), CsPbI2Br (1.90eV), and CsPbIBr2 (2.05eV)

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PP 2.3.2 Stable Perovskite Cells and Modules

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This activity will include research for:

1.  developing a hierarchy of basic stability test (elevated temperature or/and light exposure) procedures to allow rapid assessment of perovskite cells before subjecting them to more sophisticated and more time and effort involved tests (e.g., IEC 61215 standards),

2. developing new characterisation and modelling methods to distinguish sources and reasons of instability, e.g., perovskite bulk vs interfaces and which particular interface - important for tailoring perovskite materials (compositions and additives),

3. designing new cell designs (new interfaces and flipping polarities) and modifying fabrication techniques for improved stability, and

4. developing low cost, low temperature glass-glass encapsulation techniques with hermetic electric feedthroughs. The anticipated outcome of these activities is perovskite cells and modules that pass the full IEC 61215 standard tests while maintaining very high efficiencies.

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PP 2.3.3 Low-Weight Low-Cost Flexible Perovskite Modules

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Perovskite devices have the highest power-per-weight ratios, as high as 29.4 W/g, amongst other photovoltaic technologies making them suitable for lightweight such as mobile applications. However, such a high specific power has been demonstrated with only small cells and no respectable output power has been achieved to date. Upscaling such low-weight PV devices poses extra challenges to the already challenging upscaling of perovskite PV.

 

The key challenges would be achieving conductivity of transparent conducting electrodes (TCEs) on ultra-thin substrates and producing patterned thin films of functional materials with the right registration on such thin flexible films. Therefore, the first step of the activity would be developing capability of producing patterned TCEs on thin flexible films.

 

TCEs have been identified as one of the biggest cost components of roll-to-roll manufactured perovskite PV. [7] Developing the capability would make it possible to produce low-weight and low-cost perovskite modules. The TCEs will then be used with upscaling technologies developed in PP2.9.

 

Another barrier to the achievement of ultra-low weight perovskite cells is the current use of thick polymer encapsulations that defeat the unique advantage, the high specific power, of perovskite cells.  Therefore, this project will develop new encapsulation-by-coating methods thereby reducing the thickness required by the front and rear barrier films.

 

The aim is to demonstrate ultra-light large-area encapsulated modules.

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Investigators: Anita Ho-Baillie (USYD), Klaus Weber (ANU), Thomas White (ANU), Doojin Vak (CSIRO), Jacek Jasieniak (Monash), Udo Bach (Monash), Prof. Paul Burn (UQ), Dr Paul Shaw (UQ), Xiaojing Hao (UNSW) and Martin Green (UNSW)

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