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Self-Assembled Monolayers (SAMs) in Perovskite Solar Cells (PSCs)

Incorporating self-assembled monolayers (SAMs) within perovskite solar cells has improved device efficiency. SAMs exist as ultrathin layers that can be engineered to improve various aspects of the solar cell including charge transport and stability. SAMs have benefits including:

Self-Assembled Monolayer
Self-Assembled Monolayer
  • Distinct energy levels
  • Charge carriers can travel a considerable distance without recombining
  • Low-cost
  • Very resistant to contamination and defects
  • Improved stability

SAMs are incorporated as an alternative to the hole transport layer or as an interface between device layers.

Typically, self-assembled monolayer molecules consists of:

Anchor Group Adsorbs onto transparent conductive substrate
Terminal / 𝜋-conjugated Group Extracts and transports photo-generated holes from the perovskite layer
Spacer Group Connects the anchoring group and 𝜋-conjugated unit to determine the intermolecular geometries

Key Advantages of Self-Assembled Monolayers in PSCs


There are three key advantages to including self-assembled monolayers in your perovskite solar cell:

  • The self-assembled molecules have a dipole moment (distribution of charge) which can alter the work function of a coated layer. This can improve the charge extraction and transfer between layers.
  • SAMs can have a variety of functional groups. These chemical groups can passivate the buried surface of perovskite polycrystalline film and reduce the interfacial non-radiative recombination and current leakage.
  • SAMs can significantly enhance the stability of PSCs by providing a protective barrier against environmental degradation caused by oxygen or water. SAMs can also reduce instability by providing defect-free surfaces for crystallization of the perovskite layer with fewer defects and grain boundaries.

Self-Assembled Monolayers as Hole Transport Layers in PSCs


Self-Assembled Monolayers as HTLs diagram
Self-Assembled Monolayers as HTLs

Self-assembled monolayers are used as alternative hole transport layers to improve electronic device efficiency and stability. This is especially the case for inverted perovskite solar cells (IPSCs), known as p-i-n cells. SAMs in IPSC devices have seen >25% power conversion efficiencies and enhanced operational lifetimes. This value is comparable to conventional silicon solar cells. The SAM molecule used was 2-(4-(bis(4-methoxyphenyl)amino)phenyl)-1-cyanovinyl)phosphonic acid (MPA-CPA).

Compared to traditional hole transport layers like PEDOT:PSS and NiOx, SAMs offer advantages such as:

  • Increased stability
  • Enhanced surface wettability
  • Improved perovskite film quality
  • Reduction in defects

SAMs used as hole transport layers can by categorized based on the terminal unit within the SAM molecules used. These are usually carbazole-based (eg. MeO-2PACz) or triarylamine based (eg. MPA-CPA). The terminal group interacts with the perovskite layer. It is selected to match the energy level of the perovskite to ensure efficient hole transport. This reduces non-radiative loss and increases device performance.

The main carbazole or triarylamine unit can be functionalized with different chemical groups. This is done to improve interactions between the SAM molecules as well as with the perovskite layer. Functionalization also alters the HOMO and LUMO of the SAM layer. This means the layer can be engineered to make the most efficient device possible for the PSC system selected.

Examples of carbazole and triarylamine-based SAM Molecules

Self-Assembled Monolayers as Interfaces in PSCs


Self-assembled monolayers are used to improve the interaction between different layers within perovskite solar cells. They can be used either side of the perovskite layer to improve charge transport to the hole transport or electron transport layer. SAMs alter the work function of the layer they are coated on, provide a protective coating and can improve perovskite crystallization.

HTL / Perovskite Interface

SAM molecules are used to improve device efficiency and stability at the interface between the hole transport layer and perovskite layer. The molecules self-assemble on the HTL layer through adsorption interactions from anchor unit. This creates a surface that can enhance perovskite crystallization and coverage leading to improved device performance.

Anchor Unit Selection

The anchor units of SAM molecules, typically acids such as phosphonic acid, interact with the HTL. The anchor unit of the SAM molecule is selected based on the HTL and the desired interactions. The SAM molecules have their own dipole moments (separation of positive and negative charge within a molecule). The introduction of the SAM layer causes charge redistribution at the interface. This alters the work function of the HTL and as a result enhances the efficiency of hole transport. Some examples of anchor groups and their interactions with HTLs is shown in the table below.

Anchor Group HTL Material Bond Type (Anchor-HTL)
Phosphonic Acid PEDOT:PSS O- - S+ (ionic)
Carboxylic Acid NiOx

O: - Ni (coordination)

O- - Ni+ (ionic)

Terminal Group Selection

Terminal groups on SAM molecules interact with the perovskite layer. Once the SAM is anchored to the HTL the exposed terminal groups influence perovskite crystallization.

The functional groups of the terminal unit interact with the perovskite layer which can reduce surface defects and alter the surface energy levels of the perovskite. The distribution of electrons within the SAM molecule is usually also controlled by the terminal group. Properties such as the molecular energy level structure as well as the magnitude and direction of the dipole moment are imparted. This influences the electronic coupling with the perovskite layer. For example when the SAM molecule phenylphosphonic acid (PPA) is modified with different chemical groups different work function and dipole moments are observed for NiOx:

SAM Molecule Functional Group Dipole Moment [μ] Dipole Moment Direction Work Function of modified NiOx (eV)
PPA n/a 0.66 NiOx -> Perovskite (+ve) 5.23
MPPA Methoxy (-OCH3) 1.85 NiOx -> Perovskite (+ve) 5.17
CNPPA Cyano (-CN) 3.59 Perovskite -> NiOx (-ve) 5.39

The electron-donating methoxy group showed a lower value of work function (WF). This is due to the positive dipole directing away from the oxide layer to the perovskite layer. NiOx modified with CNPPA showed an increased the WF as a result of a negative dipole directing towards the oxide surface. This is as a result of the electron-withdrawing properties of the cyano group.

Self-Assembled Monolayer Interface
Self-Assembled Monolayer Interface

Changing SAM-dipoles alters the interfacial WF and directly influences the quasi-Fermi level splitting under excitation. This in turn modifies the device open-circuit voltage (VOC). Modification can improve charge extraction and reduce energy losses during the charge transfer process. This enhances device short-circuit current (JSC) and fill factor (FF), improving device performance overall.

Perovskite / ETL Interface

Self-assembled monolayers have been used to solve issues of low electron mobility and high interfacial defect density between perovskite and the electron transport layer in regular n-i-p PSCs.

Anchor Group Selection

Anchor groups are typically coordinating to metal oxide electron transport layers such as TiO2, SnO2 and ZnO. Typical anchor groups are:

  • Phosphates
  • Carboxylic Acids
  • Silanes

The anchor groups reduce the number of exposed -OH groups on the metal oxide surface by forming anchoring bonds. By replacing the -OH groups with conjugated terminal groups of the SAM molecules the ETLs become more hydrophobic. This improves the quality of the perovskite film formed on it.

Terminal Group Selection

Fullerene functionalized SAM molecules have been used to improve electron transport between perovskite layers and ETLs such as TiO2. The strong interaction between fullerene groups and the iodide of the perovskite passivates interfacial defects and improves charge transport. This passivation reduces the chance of electrons being trapped at the interface allowing them to flow freely and minimizing recombination losses.

Self-Assembled Monolayers (SAMs)

Self-Assembled Monolayers

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Further Reading


Contributors


Edited by

Dr. Amelia Wood

Application Scientist

Diagrams by

Sam Force

Graphic Designer

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