PEDOT:PSS in Solar Cells
PEDOT:PSS layers are often used in third generation photovoltaics like organic or perovskite solar cells. It is an attractive material for these applications due to its:
- Adaptable conductivity
- Relative ease of processing,
- Low cost
- High availability of the material.
PEDOT:PSS can be used as an electrode or a hole transport layer in solar cells. There are clear advantages to using PEDOT:PSS in solar cells, but there are also some limitations. However, there are many ways to adapt and tune the properties of PEDOT:PSS films.
PEDOT:PSS as Hole Transport Layers
PEDOT:PSS is one of the most commonly used HTLs in organic solar cells. Additionally, PEDOT:PSS can also be used as a hole transport layer for inverted perovskite solar cells.
The most important properties of a hole transport layer are
- High conductivity to transport electrons efficiently. Reduced conductivity can limit charge movement, lowering shunt resistance leading to low fill factor.
- Good energy match with the active layer to enable efficient charge movement through the material.
- Good material compatibility with other device layers
Benefits of PEDOT:PSS HTLs
One of the main benefits of PEDOT:PSS is its high transparency in the visible light region. This is important for HTLs, especially in inverted devices.
PEDOT:PSS can have reasonably high electrical conductivity for a polymer. The conductivity and work function of PEDOT:PSS can be easily tuned. It is relatively stable, and its low temperature processing techniques make it appropriate for use in flexible and stretchable solar cells.
PEDOT:PSS is water soluble. Most organic solar cell active layers are not water soluble. You can therefore deposit aqueous PEDOT:PSS solutions onto these, without redissolving the previous layers. PEDOT:PSS suspensions have a lower environmental impact than other HTL materials which use polar organic solvents.
Limitations of PEDOT:PSS HTLs
However, PEDOT:PSS is both hydroscopic and acidic. The acidic nature of PEDOT:PSS means it can corrode ITO electrodes. As PEDOT:PSS is hygroscopic, it will encourage moisture absorption and degradation - especially when you are dealing with organic and perovskite solar cells. Both of these qualities can lead to reduced PV performance over time.
Additionally, PEDOT:PSS is not as conductive as other hole transport layers. This is due to its structural inhomogeneity and the fact that PSS is not conductive. This low conductivity can be overcome by having thin HTL layers.
You can improve the conductivity of PEDOT:PSS through other methods such as:
- Controlling film morphology of the PEDOT:PSS film
- Reducing the relative amount of PSS to PEDOT
- Using additives or dopants
- Introducing interlayers
PEDOT:PSS in Electrodes
Electrodes play a pivotal role in the efficiency and functionality of solar cells. Among various materials, PEDOT:PSS electrodes have emerged as a notable choice due to its unique properties. Here’s a closer look at the benefits and limitations of using PEDOT:PSS as an electrode in solar cell applications.
Benefits of PEDOT:PSS Electrodes
PEDOT:PSS is highly valued in the solar cell industry for its mechanical flexibility, which makes it suitable for flexible photovoltaic devices. It also offers relatively low sheet resistance (for a polymer), high stability, and optical transparency. These features make it an choice for both bottom and top electrodes, enhancing device efficiency. Photovoltaic devices with PEDOT:PSS as the bottom electrode have achieved efficiencies over 10% on both rigid and flexible substrates.
Limitations of PEDOT:PSS Electrodes
Despite its advantages, PEDOT:PSS suffers from lower conductivity compared to traditional materials like ITO. This can impact the overall stability and efficiency of the device. Additionally, issues with the hydrophilic nature of PEDOT:PSS can lead to moisture penetration, which poses a risk to the longevity and reliability of the solar cells. Addressing these challenges is crucial for maximizing the potential of PEDOT:PSS in solar technology applications.
PEDOT:PSS Properties in Solar Cells
PEDOT to PSS Ratio
The ratio of PEDOT to PSS can change the conductivity of the resulting film significantly. Increasing the conductivity of your electrode or hole transport layer will improve your solar cell performance.
PEDOT:PSS complexes that are used as hole transport layers tend to have a higher ratio of PEDOT compared to PSS (e.g. AI 4083 with PEDOT:PSS ratio 1:6). This ratio is further reduced for PEDOT:PSS used for electrodes (e.g. PH 1000 PEDOT:PSS with ratio 1:2.5).
Film Morphology
PEDOT:PSS is a two phase system made up of conductive, insoluble PEDOT and insulating PSS. The way that these two phases organize in a thin film changes film conductivity and morphology - affecting PV performance.
For example, the rougher the PEDOT:PSS film is, the more trap states form at the interface between the active layer and the PEDOT:PSS layer. This can lead to unwanted charge carrier recombination.
Work Function and Energy Alignment
In order to achieve optimal charge transport throughout a device, you need to tune the work function and corresponding energy levels of every layer. By increasing energy level alignment, you can aid efficient charge carrier movement throughout your solar cell.
PEDOT:PSS has a high work function of 5.0-5.2 eV. The work function of PEDOT:PSS is well matched with the HOMO of many donor materials in organic solar cells, facilitating efficient hole transport.
However, this is not a direct alignment for perovskite solar cells (such as MAPbI3). Any gap between the work functions of these layers can limit charge movement through the device or encourage unwanted charge recombination.
Improving PEDOT:PSS in Perovskite Solar Cells and Organic Solar Cells
Additives and Dopants
You can use additives and dopants to improve the conductivity, work function, acidity and hydrophobicity of your PEDOT:PSS film. Some examples of dopants you can use include:
- Dopants to increase conductivity such as PEO, metal chlorides, 2-propanol or silver nanoparticles.
- Solvent additives which also increase film conductivity by changing the ratio of PSS to PEDOT, and how they arrange in the film. These include DMF, DMSO, methanol,2-methoxyethanol. This also includes glycerol and ethylene glycerol (EG).
- Dopants that improve film stability. These can include cross-linking agents, strong bases to neutralize the acidic nature of PEDOT:PSS (KOH, NaOH, etc) and molecules to reduce absorption of moisture by the film (dopamine, F4-TCNQ).
- Dopants that change the work function of PEDOT:PSS. Some examples of these include PSS-Na or PFI. Another method for this is using surfactants or polar organic solvents like DMSO.
Post Treatment Methods
Post treatment techniques mainly affect the surface of the PEDOT:PSS layer. These methods can improve surface morphology of the film, smoothing the interface between the PEDOT:PSS and subsequent device layers. These treatments can also reduce the amount of PSS in the film, improving conductivity.
Post-treatment materials for PEDOT:PSS include polar organic compounds, acids, or cosolvents. Solvents used as additives, such as EG and graphene-oxide, can also be used as post-treatment to change the surface morphology.
Interface Layers
Thin interface layers can help modify the work function of HTLs improving charge transport through the device. Examples of these layers include DPP-DTT, PCDTBT, P3HT, PCPDTBT, poly-TPD. Other examples also include small molecules films or metal oxides.
Additionally, these interlayers can provide a more uniform layer for subsequent materials to be deposited onto. They also provide some protection for materials that are sensitive to water, such as many perovskites.
References
Reza, K. M., Mabrouk, S., & Qiao, Q. (2018). A review on tailoring PEDOT:PSS Layer for improved performance of Perovskite Solar Cells. Proceedings of the Nature Research Society, 2. https://doi.org/10.11605/j.pnrs.201802004
Hu, L., Song, J., Yin, X., Su, Z., & Li, Z. (2020). Research progress on polymer solar cells based on PEDOT:PSS Electrodes. Polymers, 12(1), 145. https://doi.org/10.3390/polym12010145
Xia, Y., Yan, G., & Lin, J. (2021). Review on tailoring PEDOT:PSS layer for improved device stability of perovskite solar cells. Nanomaterials, 11(11), 3119. https://doi.org/10.3390/nano11113119
Fan, X., Nie, W., Tsai, H., Wang, N., Huang, H., Cheng, Y., Wen, R., Ma, L., Yan, F., & Xia, Y. (2019). PEDOT:PSS for flexible and Stretchable Electronics: Modifications, strategies, and applications. Advanced Science, 6(19).
Xu, H., Yuan, F., Zhou, D., Liao, X., Chen, L., & Chen, Y. (2020). Hole transport layers for organic solar cells: Recent progress and prospects. Journal of Materials Chemistry A, 8(23), 11478–11492. https://doi.org/10.1039/d0ta03511
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