A new flexible solar cell approach with 2D transition metal dichalcogenides

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Solar cells, and renewables in general, have been a hot topic, and we’re now at the stage where a number of solar cells are showing good efficiencies. In the solar cell space, inorganic and larger solar cells (such as those that use silicon as the semiconductor material for the photovoltaic junction) are the dominant cell type due to their low manufacturing cost and relatively good power conversion efficiency (PCE). However, even with many solar cells on the commercial market, scientists and engineers frequently seek to create different solar cells that are better suited to different situations and usage environments.

One of the areas where there has been a lot of interest is in creating flexible solar cells, because they can be attached to surfaces with complex geometries, so they are much more versatile in where they are. can be used. Over the years, there has been a lot of interest in creating flexible solar cells. Yet, despite silicon solar cells which currently dominate around 95% of the market, silicon is a fragile material with a low optical absorption coefficient. Therefore, it is not best suited for thin film and flexible solar cells, especially for demanding applications that require a high power-to-weight ratio.

This means the focus has shifted from the status quo for flexible solar cells to becoming a commercially viable option. Several flexible solar cells have been created using different organic molecules, but many have not yet reached high enough efficiencies to be commercially viable, and the theoretical limits of some organic molecules are low. Graphene, a technically organic cell that functions more like an inorganic substance, has shown promise. Graphene has been used in a number of solar cells, from flexible thin-film solar cells to integrating alongside silicon and other semiconductors in bulk solar cells, and there’s a lot of promise in this domain.

Graphene has demonstrated potential for a wide range of solar cells and encouraged researchers to investigate other 2D materials in the family. A 2D material of interest is transition metal dichalcogenides (TMDCs) which contain over 100 substances that exhibit semiconductor properties that could be useful in solar cell junctions.

TMDCs are attracting interest for many applications, some of which are related to the applications where graphene is entering the market, while others are entirely separate. Because there are many different types of TMDCs, you get a range of materials with a range of properties suitable for various applications. Some TMDCs are insulators, some are conductors, while some are semiconductor materials.

A TMDC is a 2D material with three atomic planes, where one plane of transition metal atoms is sandwiched between two planes of chalcogen (sulfur, tellurium, or selenium) atoms. Some people are led to believe that the ‘2D’ comes from the fact that it means only one layer (i.e. 2D geometrically). While this may be true, the 2D nature of TMDCs and other 2D materials stems from the quantum confinement of electrons at such small scales. Thus, for 2D materials, electrons are confined in 1 dimension and are free to move in 2 dimensions.

There has been much interest in semiconductor TMDCs for solar cells since TMDC materials with excellent optical absorption coefficients, narrow band gaps, and self-passivated surfaces are also good semiconductors. When used to create thin-film surfaces, TMDCs can also have nearly perfect broadband and omnidirectional absorption in the visible light spectrum. The range of semiconductor TMDCs available also means that there is a range of band gaps to choose from for different types of solar cells, including single junction, double junction and tandem solar cells.

It is therefore possible to use TMDCs for a wide range of solar cell types, including flexible solar cells, as the inherent thinness of 2D TMDC layers means that they are reasonably flexible in nature despite being of an entirely inorganic material. Additionally, theoretical models have shown that TMDC solar cells could have up to 27% PCE. Although this is lower than larger silicon solar cells, it is much higher than most other materials tested for making flexible solar cells.

Like many challenges presenting new technologies, the problems with TMDCs focus on what is possible now compared to theoretical possibilities. So while TMDC solar cells could theoretically have high PCEs, the current gold standard is still very low. There have been challenges in reaching these theoretical limits and integrating the active materials onto flexible substrates.

Overall, the PCE of many flexible TMDC solar cells did not exceed 2%, which is very low, and this low value was attributed to two main issues. The first is a strong Fermi-level pinning at the metallic contact, which causes the energy bands to bend, resulting in a higher energy barrier that the charge carriers (electrons and holes) must overcome to get together. meet at the photovoltaic junction. The second key issue is that many traditional doping approaches, such as diffusion and ion implementation, are not viable because they can easily damage TMDCs. Additionally, transfer processes to flexible substrates can often damage the TMDC-substrate interface or contaminate it with organic residues left behind, resulting in lower performance.

Several theorized paths could help solve some of these problems, including soft metal transfer methods, introducing an ultra-thin interlayer at the TMDC-metal interface, or different use of TMDCs in the form of a van der Waals (vdW) heterostructure (mainly with graphene). It has also been shown that certain doping methods could be more efficient with TMDCs, such as surface charge transfer, fixed charge doping with metal oxides, plasma doping or electrostatic doping.

Researchers have now tried to overcome some of these problems by adopting some of the above methods. The researchers used tungsten diselenide (WSe2) TMDCs, but also introduced transparent graphene contacts to mitigate Fermi-level pinning and used MoOx capping as the most suitable doping approach. For the transfer aspects, the team used a combination of exfoliation, spin-coating, and lithography methods to create a clean, non-damaging direct transfer method of the active materials onto an ultra-thin flexible polyimide substrate (5 nm).

Implementing these different approaches has enabled researchers to create a flexible WSe2 TMDC solar cell that outperforms other devices of this type. The PCE of the fabricated device was found to be 5.1% with a specific power of 4.4 Wg-1, which is also better than its predecessors and more than 100 times better than similar TMDC solar cells. This performance also puts TMDC on par with average PCEs of a range of other thin-film solar cell technologies, including cadmium telluride (CdTe), copper-indium-gallium selenide (CIGS), amorphous silicon and III-V semiconductors.

While the theoretical limits of these TMDCs have been set at around 27%, it is believed that the specific power of TMDC flexible solar cells could eventually reach 46 W g-1. Values ​​close to the theoretical limits could start to offer many opportunities for flexible, wearable and implantable electronics in different industrial sectors, as this is an area that currently attracts a lot of interest. While there are still a lot of design aspects to consider before you start seeing these values, there are many possibilities, and it is also believed that the MoOx doping strategy could play a key role. In effect, the MoOx layer acts as an effective anti-reflective coating for the solar cell. Increasing the thickness of this layer could help improve photon absorption in the active layer of WSe2, leading to higher PCEs.

Even though the PCEs achieved are not numerically high, nor do they approach the levels of larger commercial solar cells, you need to look at the gains made over the current gold standard and what the cell Average TMDC solar product in terms of efficiency. If you look at that instead of the PCE numbers directly, the latest developments are about 2 to 2.5 times what most flexible TMDC solar cells are capable of. Thus, by taking this factor into account, many gains have been made to improve the efficiency of TMDC solar cells while providing a way to transfer the active TMDC material to the flexible substrate in a non-destructive manner. 2D material solar cells have not yet reached their theoretical capacity, but if significant progress is made, TMDC solar cells could reach these heights in the future, as it is expected that all types of cells sunglasses inspired by the 2D material improve considerably over time. .

Reference:
Sarawat KS et al., High specific power flexible transition metal dichalcogenide solar cells, Nature Communications, 12, (2021), 7034.

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