A New Flexible Solar Cell Approach with 2D Transition Metal Dichalcogenides
19-01-2022 | By Liam Critchley
Solar cells, and renewable energy in general, has 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 bulkier solar cells (such as those which use silicon as the semiconducting material for the photovoltaic junction) are the dominating type of cell because of 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 look at creating different solar cells 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 these can be attached to surfaces with complex geometries, so they are much more versatile in where they can be used. Over the years, there has been a lot of interest in creating flexible solar cells. Still, despite silicon solar cells currently dominating around 95% of the market, silicon is a brittle material with a low optical absorption coefficient. Therefore, it is not the best suited for thin-film and flexible solar cells, especially for demanding applications that require a high power per weight ratio.
This meant that the focus has shifted from the status quo for flexible solar cells to become a commercially viable option. There have been several flexible solar cells created using different organic molecules, but many of these 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 thin-film flexible solar cells to being integrated alongside silicon and other semiconductors in bulk solar cells, and there is a lot of promise in this area.
Graphene has demonstrated potential for a wide range of solar cells and has encouraged researchers to investigate other 2D materials in the family. One 2D material of interest is transition metal dichalcogenides (TMDCs) which contain more than 100 substances that exhibit semiconducting properties that could be useful in solar cell junctions.
The Interest in TMDC Solar Cells
TMDCs are gathering interest in many applications, some of which are related to the applications where graphene is entering the market, while others are entirely separate. Because there are so many different types of TMDC, you get a range of materials with a range of properties suitable for various applications. Some TMDCs are insulators, others are conductors, while others are semiconductor materials.
A TMDC is a 2D material with three atomic planes, where a plane of transition metal atoms is sandwiched between two planes of chalcogen atoms (sulphur, tellurium, or selenium). Some people are led to believe that the ‘2D’ comes from meaning only one layer (i.e., 2D geometrically). While this can be true, the 2D nature of TMDCs and other 2D materials arises from the quantum confinement of electrons are such small scales. So, for 2D materials, the electrons are confined in 1 dimension and are free to move in 2 dimensions.
There has been a lot of interest in semiconducting TMDCs for solar cells since the 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 an almost perfect broadband and omnidirectional absorption in the visible light spectrum. The range of semiconducting 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.
So there is the potential to utilise TMDCs for a wide range of solar cell types, including flexible solar cells, because the inherent thinness of 2D TMDC layers means that they are reasonably flexible in nature despite being a wholly inorganic material. Moreover, theoretical models have shown that TMDC solar cells could have up to 27% PCE. While this is lower than bulkier silicon solar cells, it is much higher than many of the other materials being trialled to make flexible solar cells.
Current Challenges with Flexible TMDC Solar Cells
Like many challenges presenting new technologies, the issues with TMDCs are focused on what is possible now compared to the theoretical possibilities. So, while TMDC solar cells could theoretically have high PCEs, the current gold standard is still very low. There have been challenges with reaching these theoretical limits and integrating the active materials onto flexible substrates.
Overall, the PCE of many flexible TMDC solar cells has not exceeded 2%, which is very low, and this low value has been attributed to two main issues. The first is a strong Fermi-level pinning at the metal contact, which causes the bending of the energy bands, resulting in a higher energy barrier that the charge carriers (electrons and holes) need to overcome to 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 the TMDCs. Moreover, the transfer processes to the flexible substrates can often cause damage at the TMDC-substrate interface or contaminate it with left-behind organic residues, leading to lower performance.
Several routes theorised could help with some of these issues, including gentle metal transfer methods, introducing an ultrathin interlayer at the TMDC-metal interface or utilising the TMDCs differently in the form of a van der Waals (vdW) heterostructure (mostly with graphene). It’s also been shown that certain doping methods could be more effective with TMDCs, such as surface charge transfer, fixed charge doping with metal oxides, plasma doping or electrostatic doping.
An Approach to Make Flexible TMDC Solar Cells more Efficient
Researchers have now tried to overcome some of these issues by adopting some of the methods above. The researchers used tungsten diselenide (WSe2) TMDCs, but they also introduced transparent graphene contacts to mitigate the Fermi-level pinning and used MoOx capping as the more suitable doping approach. For the transfer aspects, the team used a combination of exfoliation, spin coating and lithographic methods to create a clean, non-damaging direct transfer method of the active materials onto an ultrathin (5 nm) flexible polyimide substrate.
The implantation of these different approaches enabled the researchers to create a WSe2 TMDC flexible solar cell that outperforms other devices of its 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 100x greater than similar TMDC solar cells. This performance also puts the TMDC on par with the average PCEs from a range of other thin-film solar cell technologies, including cadmium telluride (CdTe), copper indium gallium selenide (CIGS), amorphous silicon and III-V semiconductor devices.
While the theoretical limits of these TMDCs have been set to around 27%, it’s thought that the specific power of flexible TMDC 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 across different industry sectors—as this is an area where there is a lot of interest currently. While there still needs to be a lot of design aspects to be considered before we start to see these values, there are many avenues of opportunity, and it’s also thought that the MoOx doping strategy could play a key role. This is because the MoOx layer acts as an effective anti-reflection coating for the solar cell. Increasing the thickness of this layer could help improve the absorption of photons within the active WSe2 layer, leading to higher PCEs.
Overall Outlook
Even though the PCEs obtained aren’t high from a numerical standpoint, nor are they anywhere near the levels of bulkier commercial solar cells, you need to look at the gains made over the current gold standard and what the average TMDC solar cell produces in terms of efficiency. If you look at this instead of the PCE numbers directly, the latest developments are around 2 to 2.5 times greater than what most flexible TMDC solar cells are capable of. So, looking at that factor, a lot of gains have been made towards improving the efficiency of TMDC solar cells while offering a way of transferring the active TMDC material to the flexible substrate in a non-destructive way. Solar cells made of 2D material have not yet reached their theoretical capacity, but if significant progress is made, TMDC solar cells could reach those heights in the future, as it is expected that all types of 2D material inspired solar cells will vastly enhance over time.
