Commonly, Spiro-OMeTAD (Spiro-MeOTAD) is one of the most studied and suitable hole transport material used in Perovskite solar cells. What does make this material, the best hole transfer material?
HOMO energy level, good hole transport mobility and stability of Spiro-OMeTAD determine its excellent hole injection/transport for perovskite solar cells and other electronic devices.
Thank you Xuhua Wang but how HOMO level determine superiority of Spiro-OMeTAD? What is the best HOMO value for a good hole transfer material?
The role of HTL is of great significance in a heterojunction PSC without which the perovskite layer comes in contact with the metal electrode resulting in recombination and poor Voc. Organic polymeric HTLs such as spiro-OMeTAD, PTAA have been the go-to material so far in many PSC efficiency records. The favorable match between the valence band of the perovskite and the highest occupied molecular orbital of the HTL along with good hole conductivity made spiro-OMeTAD a popular choice. However, researchers are on the lookout for new HTLs alternatives recently to replace spiro-OMeTAD. Variations in conductivity upon exposure to oxygen or light, multi-step synthesis, and expensive sublimation steps for purification are a few drawbacks of spiro-OMeTAD that is prompting this search.
Ideally, the perfect HTL would be environmentally stable having conductivity high enough that is compatible with the perovskites and be thin yet stable enough to support a fast charge transit that boosts the FF and Voc. Syntheses of such HTLs have to be facile and inexpensive as well to facilitate the large-scale commercialization of PSCs. Inorganic HTLs such as transition metal chalcogenides have shown some promises in this regard, however, their performance in heterojunction PSCs requires further evaluation.
HOMO level of Spiro-OMeTAD is determined by its molecular structure. The measurements of energy level can be done via various techniques. Normally cyclic voltammetry combined with UV-VIS is very often used to measure HOMO and LUMO energy levels of a polymer. When you design electric devices such as OLEDs, organic solar cells or perovskite solar cells, the energy level of hole injection or transport layer must match the energy level of HOMO of organic semiconductors or valance band of perovskite materials. It is also called energy alignments between layer to layer.
Whether it is good hole transporter or not? The charge carrier mobility of a semiconductor has to be determined by different techniques, too such as time of flight, JV curves.
Dear Abdolazim Hasseli ,
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Ramadan Kareem!
As the colleagues hinted, the HTL must have the following properties:
- high p-type doping concentration
- High conductivity which means also high mobility.
- Its homo level must lie slightly lower than that of the absorber to easily extract the holes from the absorber layer.
- Its lumo level must be appreciably higher than that of the absorber to reflect the electrons back to the absorber.
- Its energy gap must be wide to be transparent for the photons of the incident solar radiation. So, basically it must be transparent to the incident solar radiation.
- It must be highly stable against the environmental effects such as oxygen and moisture.
- It must be compatible in the fabrication processes.
May be the there are two defects with the SPIRO-OMeTAD its low mobility as an organic material and also it less stability against environmental conditions.
Metal oxide p-type HTL are advantageous such that NiO and Cu2O.
For more information about the function of the transport layers please follow the paper in the link:Conference Paper Generic Analytical Models for Organic and Perovskite Solar Cells
Best wishes
Thank you Abdelhalim abdelnaby Zekry and God bless you in this holy month of Ramadan.
Hole transport layers should have high charge carrier mobility (hole extraction capability), better stability, and better hydrophobicity. Their (fabrication) cost should be low as much as possible. Furthermore, they should have better bandgap alignment. In this context, inorganic HTLs could be the good options.
Conjugated polyelectrolyte consists of conducting polymer embedded with ionizable side group which is refered as electrolyte active group... They have multiple oxygenic functionalities which passivates the charge trap state on pervoskite layer. Functionalization improves the electrolyte activity or hole transport in pervoskite sensitized solar cell. Donor-pi-acceptor moity regulates the energy level in polyelectrolyte.
Hole transport material should exhibits high ptype conductivity, stability and interaction with photoactive layer. Several HTMs that has been employed in PSC are SpiroMeOTAD, PEDOT:PSS, CuI, Cu2O and currently GO is utlizing in PSC to enhance its performance.
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