Surface modification of TiO2 layer with phosphonic acid monolayer in perovskite solar cells: Effect of chain length and terminal functional group
Graphical abstract
Introduction
Perovskite solar cells (PSCs) attract enormous attention due to their excellent photovoltaic properties and incredible progress in its efficiency in recent years, with the latest power conversion efficiency (PCE) of 25.2% being reported [1]. The performance of PSCs depends on a number of factors, including the perovskite film quality, choice of contacts, and the interfaces between perovskite to electron transport layer (ETL) and perovskite to hole transport layer(HTL). The composition engineering of the ABX3 perovskite controls the band gap and the optoelectronic properties [2,3], whereas the film processing techniques determine the perovskite crystal and film quality [[4], [5], [6]]. Smooth, pinhole-free, and large grain-sized films are required to give highly efficient cells so as to minimize the energy loss due to defects, traps or resistance [7].
In a conventional PSC structure of [FTO/c-TiO2/mp-TiO2/Perovskite/Spiro-OMeTAD/Ag], the excitons are generated in the perovskite layer upon absorption of incident photons. Diffusion of excitons and then charge splitting at the perovskite/Spiro-OMeTAD interface result in hole and electron carriers in Spiro-OMeTAD and perovskite layer respectively. Holes and electrons drift to the anode and cathode separately and recombine in the external circuit to generate the current. To enhance the performance of a solar cell, it is necessary to maximize and even balancing the amount of carriers collected from the perovskite layer to both electrodes (FTO and Ag). In general, there is no Schottky barrier for charge transporting between mp-TiO2/perovskite, and perovskite/spiro-OMeTAD interfaces, but a working device usually does not give the theoretically predicted performance due to various losses within the channel resulting lower open circuit voltage (Voc), the current density (Jsc), and the fill factor (FF). These losses originate either from the perovskite crystal and film quality or from interfacial conditions.
Engineering the interface between the ETL and perovskite layer was demonstrated by additive engineering [8], doping [[9], [10], [11], [12], [13], [14]], and core/shell structure [15,16], multifunctional fullerene derivative [17], polyoxyethylene [18], and reduced-graphene scaffold [19]. However, balancing charges arriving the electrodes is seldom considered.
Self-assembled monolayers [[20], [21], [22], [23], [24], [25], [26], [27]] (SAMs) of organic molecules are widely used to modulate the surface property of a substrate through judicial choices of the head group, spacer group and tail group of the molecule. The head group is used to bind to the substrate through specific interactions; the spacer group between the head and tail may define the thickness of the layer and the nature of the linkage (insulating or conductive). The tail group, which will be in contact with outside, provides a platform to modulate the physical and chemical properties of the substrate: such as surface energy, wettability, reactivity, as well as the work function. In particular, the SAM modulates the work function (WF) through the dipoles collectively installed by the molecules at the interface. Upon formation of closely-packed and oriented assembly, a local electric field is built at the surface to influence the escape or acceptance of charges across the surface. The direction and magnitude of the dipole moment of adsorbate along the surface normal have been proposed to be responsible for the WF shift [28]. SAMs have also been demonstrated to modulate the interface between ETL/perovskite and perovskite/HTL for PSCs [[29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42]]. The modification by molecules carrying electron-withdrawing groups increases the work function (i.e., lowers the Fermi energy level) whereas those with electron-donating group decreases the work function through the abrupt shift of potential across the dipole layer [43]. SAMs can also tune the film morphology grown on top of them through its interaction with the film-forming material. Lijian Zuo et al. demonstrated the interfacial chemical interactions between the SAM molecules and perovskite are crucial to the optoelectronic properties of perovskite solar cells [40].
In the fabrication of PSCs, surface treatments such as O2-plasma or UV-Ozone of TiO2 layer are often employed to remove the residual organics from the surface. After these treatments, the surface wetting property improves as the surface energy is increased. Moreover, such treatments also bring down the Fermi energy level (increasing WF). However, these surface treatments do not last long [44]. A stable WF is highly desirable for maintaining the same performance of the solar cell over time.
Phosphonic acids are known to form covalent bonding on metal oxide layers, such as AlOx/Al, ITO, and TiO2 substrates [[45], [46], [47]]. In this work, we used terminally functionalized alkylphosphonic acid SAMs (Fig. 1) to modify the surface of mp-TiO2 layer, with a systematic change of the alkyl chain length while keeping the same terminal functional group, or change of the terminal function group while keeping the same alkyl chain length. We are particularly interested in the effect of the tunnel barrier, imposed by the alkyl chain length, and the WF, resulted from different functional groups, on the charge injection from perovskite layer to mp-TiO2 layer and its impact on the performance and long term stability of the device. As we have previously shown, with the same terminal functional group but different alkyl chain length, the WF and thus the energy gap between LUMO of perovskite and TiO2 is expected to be similar [48]. Studying the effect of tunnelling barrier on charge collection from perovskite may provide information on the charge balance situation in the device, which is a useful information for designing ideal interface modifier next. With the same alkyl chain length but different functional group, we expect the energy level alignment and possibly the surface energy to be different and their influence on charge collection will be analysed and compared.
Section snippets
Results and discussion
Two series of phosphonic acids were chosen to study the interfacial effect on charge extraction in perovskite solar cell. Firstly, iodo-terminated alkylphosphonic acids of different chain length, iodomethylphosphonic acid (IMPA), 4-iodobutylphosphonic acid (IBPA), and 6-iodohexylphosphonic acid (IHPA) (Fig. 1a) were used to study particularly the influence of chain length on charge injection from the perovskite to the mp-TiO2. In these cases similar energy level alignment is expected
Conclusions
Modification of mp-TiO2 layer with phosphonic acid monolayer provides an opportunity to examine the charge extraction at the interface of perovskite solar cells. By changing the chain length and/or functional group of the monolayer molecule, the work function and the tunnelling distance/barrier can be systematically modulated. By modifying with iodo-terminated phosphonic acids of different chain length, a correlation between PCE and the chain length was found: longer spacer group led to
Chemicals and materials
All the chemicals used in this study were received from the suppliers without further purification. FTO (7 Ω) was purchased from UNI-ONWARD Corp. Titanium diisopropoxide bis(acetylacetonate) solution (75% in 2-propanol) was obtained from Sigma-Aldrich. Perovskite precursors PbI2 (99.9985%) and CH3NH3I (98%) were obtained from Alfa Aesar and Lumtec respectively. Spiro-OMeTAD was purchased from Lumtec. Li-TFSI was obtained from ACROS. 4-tert-Butylpyridine (TBP) was obtained from Ak Scientific
Declaration of competing interest
The authors declare no competing financial interest.
Acknowledgments
We are grateful to the support from Academia Sinica and Ministry of Science and Technology, Taiwan (Grant No. 108-2113-M-001 -022 -).
References (55)
- et al.
Under the spotlight: the organic–inorganic hybrid halide perovskite for optoelectronic applications
Nano Today
(2015) - et al.
Enhanced short-circuit current density of perovskite solar cells using Zn-doped TiO 2 as electron transport layer
Sol. Energy Mater. Sol. Cells
(2016) - et al.
Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance
Science
(2016) - et al.
The rapid evolution of highly efficient perovskite solar cells
Energy Environ. Sci.
(2017) - et al.
Growth of CH3NH3PbI3 cuboids with controlled size for high-efficiency perovskite solar cells
Nat. Nanotechnol.
(2014) - et al.
Solvent engineering for high-performance inorganic-organic hybrid perovskite solar cells
Nat. Mater.
(2014) - et al.
High-efficiency solution-processed perovskite solar cells with millimeter-scale grains
Science
(2015) - et al.
Improving the TiO2 electron transport layer in perovskite solar cells using acetylacetonate-based additives
J. Mater. Chem.
(2015) - et al.
Interface engineering of highly efficient perovskite solar cells
Science
(2014)