Self-Assembled Monolayers vs PEDOT:PSS
Two commonly used materials for hole selective layer (HSL) or hole transport layer (HTL) are PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) and self-assembled monolayers (SAMs). Each offers distinct advantages and limitations that impact their effectiveness in solar cells. In the realm of electronics devices including light-emitting diodes (LEDs), field effect transistors (OFETs), and photovoltaics (OPVs), the choice of interfaces between electrodes and photoactive materials is crucial for optimizing device performance.
Advantages and Disadvantages: PEDOT:PSS vs SAMs
The advantages and disadvantages of using PEDOT:PSS and SAMs as hole selective layers are shown below:
| PEDOT:PSS | Self-Assembled Monolayers | |
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| Advantages |
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| Disadvantages |
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| Examples |
SAMs offer several advantages over traditional HTLs like PEDOT. Their ability to precisely control the interface at the molecular level can lead to improved energy level alignment, reduced recombination losses, and enhanced device stability.
Key properties to consider
Stability
SAMs generally offer better stability compared to PEDOT due to their non-hygroscopic and non-acidic nature. Compatibility of the HTL with the perovskite layer are critical. SAMs can form robust, non-acidic, and hydrophobic interfaces that protect the underlying layers from degradation. PEDOT:PSS can degrade over time due to its hygroscopic properties and acidity.
In perovskite solar cells with an active layer of Cs0.25FA0.75Sn0.5Pb0.5I3, cells with the SAM Br-2PACz as the HTL retained 80% of their initial efficiency after 230 hours under continuous working conditions. PEDOT:PSS-based devices dropped under 80% after only 72 hours.
Processing
The fabrication of SAMs can be more complex and less scalable compared to solution processed PEDOT.
Interface characteristics
SAMs offer superior control over the work function and energy level alignment at the interface, allowing for optimized charge transport and reduced recombination losses. While PEDOT also provides good energy level alignment, its fixed work function limits its tunability compared to SAMs.
Record high device efficiency of 19.51% for mixed inverted tin/lead (Sn/Pb) perovskite solar cells was achieved when Br-2PACz was used as the self-assembled monolayer contact interface. An efficiency (PCE) of 16.33% was achieved using PEDOT:PSS as the hole transport layer.
Thickness
PEDOT films are typically thicker than SAMs, which can be advantageous for forming a smooth and continuous layer. SAMs, being only a few nanometres thick, offer minimal disruption to the device's optical properties and ensure a more intimate contact with the active layer.
PEDOT:PSS as Hole Selective Layer
PEDOT:PSS is a widely used conductive polymer that has garnered significant attention due to its favourable properties for organic electronics. It is composed of two components:
- PEDOT: the conductive polymer
- PSS: a charge-balancing counterion
How PEDOT:PSS is applied, along with its electrical and optical properties, directly impact its performances as a hole selective layer.
Deposition
PEDOT:PSS is typically deposited from an aqueous solution, using various techniques such as spin coating, dip coating, and printing. The resulting film thickness can be easily controlled, and it usually ranges from tens to hundreds of nanometres. PEDOT:PSS is widely used in organic and perovskite solar cells due to its ability to provide a smooth, uniform, and conductive interface. It reduces surface roughness and improves the wettability of subsequent layers, which is essential for high-quality active layer deposition. PEDOT films are amorphous with some degree of phase separation between PEDOT-rich and PSS-rich regions, which influences their conductivity and transparency.
Electrical Properties
One of the standout features of PEDOT is its high electrical conductivity, which can be further enhanced through post-treatment processes like the addition of secondary dopants (e.g., DMSO or ethylene glycol). This conductivity is crucial for efficient hole transport and extraction in solar cells.
PEDOT also has a suitable work function (around 5.0 eV) that aligns well with the highest occupied molecular orbital (HOMO) levels of many organic semiconductors, facilitating efficient hole injection.
Optical Properties
PEDOT is highly transparent in the visible spectrum, a critical property for HTLs in solar cells since it ensures that a maximum amount of light reaches the active layer. Its transparency, combined with its conductive properties, makes it an excellent choice for solar cell applications.
Stability
One of the main drawbacks of PEDOT is its hygroscopic nature and acidic nature, which can lead to long-term stability issues and degradation of the underlying materials, especially in perovskite solar cells.
SAMs as Hole Selective Layer
Self-assembled monolayers are another type of hole selective layer that have gained attention for their unique properties and ability to fine-tune interface characteristics at the molecular level. SAMs consist of a single layer of molecules that spontaneously organize themselves on a substrate.
Deposition
SAMs are formed by the adsorption of molecules with a specific head group (e.g., thiols, silanes, phosphonates) on a substrate, creating a densely packed monolayer that is typically only a few nanometres thick. As mentioned above, the fabrication of SAMs can be more complex and less scalable compared to solution processed PEDOT.
Electrical Properties
The electrical properties of SAMs are highly dependent on the choice of molecules used. The choice of the head group, tail group, and functional groups in the molecules can be tailored to achieve desired properties.
SAMs offer excellent control over the interface chemistry, enabling precise tuning of the work function and surface energy. While SAMs themselves are not inherently conductive, they can modify the work function of the underlying substrate to optimize hole injection or extraction. By selecting appropriate molecules, SAMs can create favourable energy level alignment with the active layer, facilitating efficient hole transport.
Optical properties
Due to their molecularly thin nature, SAMs do not significantly affect the optical properties of the solar cell. They are essentially transparent, ensuring that they do not interfere with light absorption by the active layer.
Self-Assembled PEDOT:PSS Monolayers
Perovskite solar cells with PEDOT:PSS forming a self-assembled monolayer as the HTL can improve power conversion efficiency (PCE). An increase from 13.4% to 18.0% was observed with the monolayer of PEDOT:PSS cells compared with the solution as-cast PEDOT:PSS cells. It was suggested that ultra-thin layer of PEDOT:PSS can attach strongly onto ITO via In–O–S chemical bonds between the PSS chain and ITO. A bilayer structure can therefore form due to Coulomb interaction, inducing an oriented electric field from positively charged PEDOT to negatively charged PSS. This can accelerate the process of hole extraction.
Summary
Both PEDOT and self-assembled monolayers offer unique advantages as hole transport interface in solar cells. SAMs, with their excellent stability, precise interface control, and minimal impact on optical properties, present a compelling alternative for PEDOT:PSS, especially for applications where long-term stability and optimized energy level alignment are crucial. The choice between PEDOT:PSS and SAMs ultimately depends on the specific requirements of the electronic devices, including factors like stability, processability, and cost. As research in this field progresses, further advancements in both materials are expected to enhance the performance and durability of next-generation solar cells.
Self-Assembled Monolayers (SAMs)
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Self-Assembled Monolayers (SAMs) in Perovskite Solar Cells (PSCs)
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- A carbazole-based self-assembled monolayer as the hole transport..., Pitaro, M., J. Mater. Chem. A (2023)
- PEDOT:PSS monolayers to enhance the hole extraction and..., Hu, L., J. Mater. Chem. A (2018)
Further Reading
- A Comprehensive Review of Organic Hole-Transporting Materials for Highly Efficient and Stable Inverted Perovskite Solar Cells,Y. Duan et al. dv. Funct. Mater. (2024)
- Advantages and challenges of self-assembled monolayer as a hole-selective contact for perovskite solar cells; S. Wang et al. Mater. Futures 2, (2023)