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Lead-free halide perovskites for the highest efficient solar energy conversion

Periodic Reporting for period 2 - FREENERGY (Lead-free halide perovskites for the highest efficient solar energy conversion)

Reporting period: 2020-08-01 to 2022-01-31

The overarching goal of FREENERGY is to demonstrate tin-based halide perovskites for solar energy conversion with the highest efficiency. The transition from fossil fuels towards sustainable energy sources is an enormous challenge that society must address as soon as possible. Solar energy is one of the most promising sustainable energy sources that can meet our needs: it is the most abundant source, and it is available every day in all areas of the world. Photovoltaic solar cells can directly convert the energy of sunlight into electric power by making use of the photovoltaic effect in semiconducting materials. For decades, silicon has dominated the market of photovoltaic materials. The price of silicon was too high for large-scale energy production when Bell Laboratories reported the first solar cell in 1954—after that, exploiting new materials and fabrication procedures systematically decreased the price of photovoltaic. The novel, innovative material and device concepts that draw from and stimulate several disciplines may open up new, presently unforeseen prospects for future photovoltaics. The design and fabrication of such materials and their integration in working devices is the very guiding principle of the FREENERGY project.
The ultimate challenge for photovoltaics is to achieve the highest solar energy conversion efficiency, i.e. the 28.8% value reached by GaAs solar cells, using inexpensive and environmentally friendly materials. The PI deems tin-based halide perovskites as the core material to address the ultimate photovoltaic challenge. Tin-based halide perovskites have all it takes to reach this ambitious goal except for the oxidative instability of tin. Therefore, FREENERGY proposes a ground-breaking approach to overcome this issue, developing new stable tin-based halide perovskites and proving them in the highest efficiency solar cells. The PI designed the research strategy twofold:

(i) the synthesis of new materials, focusing on inorganic mixed ion compositions to engineer the perovskite lattice towards intrinsically more stable tin-based perovskites;

(ii) the application of new specific supramolecular interactions to create inert self-assembly monolayers onto the perovskite surface that passivate defects and prevent the degradation at the interface with other device components.

FREENERGY will reveal the disruptive potential of tin halide perovskites as photovoltaic materials by crossing borders of materials, chemistry and physics science, ultimately aiming at the highest solar energy conversion with inexpensive and environmentally friendly solar cells.
The oxidative stability of Sn2+ in halide perovskites under solar cells operation is the fundamental scientific challenge addressed in FREENERGY. State-of-the-art approaches to this challenge aim to identify the best antioxidants to preserve the perovskite from external sources of degradation, such as oxygen and water, or reduce the interaction with them. While these approaches enabled longer material lifetimes, they did not directly face the intrinsic oxidative instability of Sn2+. The result is that the photovoltaic efficiency is still far from the real potential of this material. The PI proposes three fundamental strategies that differ from the approaches so far explored:

1. Going inorganic. The presence of organics enhances oxidative instability during device manufacturing and functioning. The PI expects that working with inorganics, including the perovskite, the contact materials and solvent-free processing, will enable an intrinsically more stable system.

2. Tuning the lattice parameters. The oxidative stability of Sn2+ within the bulk of the perovskite crystals hinges on the formation energy of lattice defects. The PI aims to control tuning the lattice parameters with mixed compositions of inorganic ions.

3. Fluorinating the surface. The degradation of the perovskite starts at the surface and then propagates within the bulk of the crystals. The PI will make use of specific supramolecular interactions to passivate the perovskite surface with fluorinated self-assembly monolayers.

Since the beginning of the project, we advanced more rapidly in the last strategic approach - Fluorinating the surface. The interface between the perovskite and the charge-selective contacts composing the device is paramount to control the stability. Undercoordinated ions at the surface are particularly reactive, and they cause rapid oxidation of Sn2+. We demonstrated that the self-assembly of Lewis acids or bases on specific sites at the surface of the perovskite enabled to stabilise the surface of the perovskite. We are now ready to implement this material in devices to measure the practical lifetime in working conditions. More recently, we made significant advances for point 2 in understanding the oxidative stability of Sn2+. We identify and eliminate an oxidative channel involving the use of solvent. Thus we managed to identify alternative solvents to prevent the oxidation of Sn2+. We are currently focusing on understanding the influence of the lattice parameter on the stability of the material. Some recent results showed that tin perovskite could crystallise in different polymorphs within the same film. Different polymorphs are highly detrimental for the charge extraction since it results in a topological distribution of bandgap within the same film, which prevents an effective charge extraction from the solar cells.
We will start working on point 1 in the coming months, exploring different inorganic tin perovskite compositions.
Perovskite solar cells (PSCs) approached the power conversion efficiency of established technologies, such as silicon, cadmium telluride (CdTe) and copper indium gallium selenide (CIGS). PSCs' next big challenge is to make stable and efficient lead-free devices, i.e. with more than 20% power conversion efficiency and stability for 25 years of outdoor usage.
Several scientific works have proposed alternative lead-free compositions since the first demonstrations of lead halide perovskite as photovoltaic materials. Sn-based halide perovskites are among the most promising lead-free options. However, the oxidative stability of Sn2+ under solar cells operation need to be addressed. State-of-the-art approaches to this challenge aim to identify the best antioxidants to preserve the perovskite from external sources of degradation, such as oxygen and water, or reduce the interaction with them. While these approaches enabled longer material lifetimes, they did not directly face the intrinsic oxidative instability of Sn2+. The result is that the photovoltaic efficiency is still far from the real potential of this material. Implementing the strategies described in the previous section, we expected the following results until the end of the project:

1. Lead-free perovskite solar cells with power conversion efficiency over 20%: state-of-the-art PSCs contain lead despite the restrictions of the European Union on hazardous substances in electronics.
We achieved the highest ever reported power conversion efficiency for a lead-free perovskite solar cell (more than 9%). We expect to have a further improvement in the next coming months.

2. Solvent-free perovskite crystallisation: manufacturing without solvents reduces the environmental impact.
We identified an alternative strategy making use of a green solvent that can prevent the oxidation of Sn2+. The use of solvent-free processing is no longer a contain, yet we will explore this possibility.

3. Stable devices for 25 years outdoor usage: long-term energy production defines the energy yield and thus the return on investment.
We are focusing on achieving the highest power conversion efficiency ( around 15%, currently our record is just over 9%) before starting a systematic investigation of the operational stability
Shockley-Queisser limit and effective record efficiency for different photovoltaic materials