The twisted SBF-based molecule, spiro-OMeTAD is the most widely investigated small organic molecule hole conductor and has been responsible for consistently good device performance since its inception in 1998 by Bach and Grӓtzel 37.
Figure 1.5 Molecular structure of (a) spiro-linked systems and (b) spiro-OMeTAD (adapted from reference 38).
From the crystal structure of spiro-OMeTAD it is observed that the phenylenes in the methoxyphenyl-amine substituents have a relatively anisotropic arrangement in the crystals. The amine nitrogen is planar in nature with the methoxy groups (-OCH3) on the
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side chains being oriented in a propeller configuration. Such an arrangement favours delocalization of charge and packing of spiro-OMeTAD occurs as a result of sharing of electrons between the p-orbitals of nitrogen and the electron-donating methoxy groups. The anisotropically arranged phenylene groups further contribute to the packing of the molecules. The crystal structure of spiro-OMeTAD reveals π-π stacking between the fluorene units and random alignments with respect to the fluorene backbone. Molecular dynamic calculations performed by Shi et al 39. and Yavuz et al 40. demonstrated several possible interfluorene packing configurations. The π-π stacking between molecular pairs may have contribution from interflourene spiral packing motifs as well as slipped π-π
stacking between neighbouring phenylene units. During intermolecular π-π stacking two fluorene rings are separated by a vertical plane-to-plane distance of ~3.8 Å. The amine nitrogen being the active site for reaction during Li-TFSI doping of spiro-OMeTAD, may become positively charged during the doping procedure. The methoxy groups were introduced into spiro-OMeTAD to control the oxidation potential and thus help in adjusting the electronic properties of the hole conductor. The methoxy groups being electron-donating in nature help in stabilising the positive charge on the nitrogen atom of the tertiary amine by resonance stabilization and the extension of π-electron delocalization over the whole molecule. The methoxy groups also play an important role in aligning the HOMO level of the molecule apart from anchoring the spiro-OMeTAD molecule to the perovskite absorber layer in n-i-p configuration.
This cross-linked structure results in improved morphological stability of low molar mass materials while retaining their electronic properties. They have several advantages such as
1. The orthogonally interconnected flourene rings leads to high steric hindrance which suppresses intermolecular interactions between the π systems.
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2. This arrangement leads to better solubility of the Spiro compounds compared to the non-Spiro-linked molecules.
3. Doubling of the molecular weight along with the crosslinking structure imparts certain rigidity to the Spiro compounds which causes entanglement in the amorphous state and hinders recrystallization. Glass transition temperature of spiro-OMeTAD is 120° C.
4. Spiro-OMeTAD is compatible with solution-processed techniques such as spin coating, dip coating, slot-die coating etc.
5. The methoxy groups in spiro-OMeTAD structure are not only responsible for adjusting the HOMO level of the material but also help in anchoring the film onto the underlying perovskite layer in n-i-p architecture.
Although spiro-OMeTAD has a lot of advantages but it suffers from low hole conductivity. The following sections will describe the methods to increase conductivity.
Objective 1: Doping and oxidation of spiro-OMeTAD
Though widely explored as HTM in PSCs, spiro-OMeTAD suffers from low hole mobility (~10−5𝑐𝑚2𝑉−1𝑠−1) 35,41 and conductivity (~10−8 𝑆 𝑐𝑚−1) in its pristine form 37,42,43. The low conductivity and hole mobility of spiro-OMeTAD stems from the sp3
hybridized N atom with pyramidal structures which leads to large intermolecular distances. Thus, charges have to move along longer paths and hence the conductivity is lowered 44. The lower hole mobility can also be explained in terms of the two types of
steric hindrances which exist in unit cells of spiro-OMeTAD and affect the π-π stacking of the molecules. The steric hindrance between two molecules in a single crystal unit cell prevents the formation of close π-π stacking upon crystallization whereas the
hindrance between the outer fragments of each cell prohibits the formation of continuous π-π stacking. The second mechanism is possibly due to the twisted nature of the central
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Spiro carbon 39. This discontinuity inhibits delocalization of charge carriers and charge
transport occurs primarily by hopping from one spiro-OMeTAD molecule to another 39.
Electronic doping is a suitable technique to tune the type and density of charge carriers as well as bring about controllability and reproducibility for efficient device performance 41,45–50. Doping of organic semiconductors relies on the charge transfer mechanism between the organic host and the dopant. The latter maybe either a donor (n- type doping) or acceptor (p-type doping) moiety that leaves part of the host molecules in a reduced or oxidized state. The ionization potential or electron affinity of the dopant with respect to the energy level of the host plays a significant role in understanding the feasibility of the doping procedure. Controlled doping with molecular-dopants are expected to be applicable for the controllability and reproducibility of PV performance. Addition of dopants not only generates additional charge carriers but also tunes the electronic properties and different dopants such as Li-TFSI 45, Na-TFSI 51, Ag-TFSI 52,
Co (III) complexes 42,53,54, WO3 55, Cu(II) salts 56,57 have been utilised to dope spiro-
OMeTAD 45,55,58–62.
(a) Lithium bis (trifluoromethyl-sulfonyl)imide (Li-TFSI) for doping spiro-OMeTAD:
Spiro-OMeTAD was originally synthesized for hole transport application in solid-state DSSCs and in pristine form achieved a PCE of 0.74% which increased to 2.56% with additives 4-tert-butylpyridine (t-BP) and lithium bis(trifluoromethane-sulfonyl)imide (Li-TFSI) 63. Li-TFSI improves the conductivity of spiro-OMeTAD by shifting the Fermi level towards the HOMO, which implies p-type doping 45,59. Addition of Li-TFSI leads to improvement of the solar cell efficiency, and reduces recombination losses at interfaces which might be due to alteration of the TiO2 band edge position for higher
potentials 37,64. Lithium and antimony-based salts and N(PhBr)3SbCl6, were the first
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salt was proposed to act as dopant by introducing free charge carriers by the oxidation of spiro-OMeTAD. The lithium salt was used to control the surface electrochemical properties of TiO2 by adsorption and intercalation of Li+ ions 37,64. Addition of 12% Li-
TFSI was reported to have increased hole mobility of spiro-OMeTAD by two orders of magnitude. This increase in mobility was explained on the basis of increased disorder and broadening of the density of states as well as screening of deep Coulomb traps to reduce the potential barrier for charge transfer 41.
(b) Role of tert-butyl pyridine (tBP) additive in doping spiro-OMeTAD: The concept
of 4-tert-butyl pyridine as additive has stemmed from the usage of tBP-dipped TiO2
electrode in DSSCs which was reported to substantially enhance the VOC and fill factor
(FF). Defects such as oxygen vacancies originating from TiO2 upon heating has a
negative impact on the solar cell performance 65. tBP is used to passivate the under- coordinated titanium atoms within the lattice, thus minimising charge recombination. Kruger et al. proposed the improvement in photovoltaic performance when tBP was blended with spiro-OMeTAD and Li-TFSI solution due to reduction in interfacial charge recombination 63. More recently, Juarez-Perez et al. 66 and Wang et al. 67 reported that tBP improves miscibility of spiro-OMeTAD with Li-TFSI and impedes phase segregation in solution, resulting in uniform distribution of Li-TFSI during film deposition. Molecular structure of Li-TFSI and tBP additives used for doping spiro- OMeTAD are shown in Figure 1.6. In this thesis both LiTFSI and tBP have been used for doping hole conductors and studied using synchrotron spectroscopic techniques to understand the influence of the dopants on the materials properties.
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Figure 1.6 Li-TFI and tBP as p-type additives used for doping spiro-OMeTAD hole conductor for perovskite solar cells.