Investigation of Well-Defined Pinholes in TiO2 Electron Selective Layers Used in Planar Heterojunction Perovskite Solar Cells

The recently introduced perovskite solar cell (PSC) technology is a promising candidate for providing low-cost energy for future demands. However, one major concern with the technology can be traced back to morphological defects in the electron selective layer (ESL), which deteriorates the solar cell performance. Pinholes in the ESL may lead to an increased surface recombination rate for holes, if the perovskite absorber layer is in contact with the fluorine-doped tin oxide (FTO) substrate via the pinholes. In this work, we used sol-gel-derived mesoporous TiO2 thin films prepared by block co-polymer templating in combination with dip coating as a model system for investigating the effect of ESL pinholes on the photovoltaic performance of planar heterojunction PSCs. We studied TiO2 films with different porosities and film thicknesses, and observed that the induced pinholes only had a minor impact on the device performance. This suggests that having narrow pinholes with a diameter of about 10 nm in the ESL is in fact not detrimental for the device performance and can even, to some extent improve their performance. A probable reason for this is that the narrow pores in the ordered structure do not allow the perovskite crystals to form interconnected pathways to the underlying FTO substrate. However, for ultrathin (~20 nm) porous layers, an incomplete ESL surface coverage of the FTO layer will further deteriorate the device performance.

. X-ray reflection (XRR) interference patterns of various TiO2 thin films deposited on glass.
Nanomaterials 2020, 10, 181; doi:10.3390/nano10010181 3 of 12 3 Figure S2. Film thickness and density dependence derived from the XRR data for mesoporous TiO2 thin films with the highest block co-polymer content when varying the solvent amount.

Description of MIS-CELIV measurements and sample preparation
Surface recombination velocity SR of holes at the FTO/TiO2 contact was measured in hole only diodes with device structure FTO/TiO2/P3HT/Au using the MIS-CELIV technique, which stands for charge extraction by linearly increasing voltage in a metal-insulator-semiconductor structure [1]. In these model devices, holes are injected from the Au top contact through the semiconducting P3HT layer by a steady-state offset voltage Voff applied in forward bias. The injected holes are blocked at the hole blocking TiO2 layer and a charge reservoir is formed at the TiO2 contact. Subsequently, a linearly increasing voltage pulse is applied in reverse bias of the diode in order to extract the injected hole reservoir. The measured current transient j(t) is given by the sum of the displacement current from the geometric capacitance of the device j0 and a time dependent linearly increasing extraction current Δj(t). The extracted charge Qextr can be determined by integrating the extraction current where textr is the time when all injected carriers have been extracted and JD is the injected dark current density. The surface recombination velocity of holes SR can be determined from the extracted charge Qextr and the measured steady state dark current JD(Voff) at the applied Voff according to where is the relative dielectric constant of P3HT (≈ 3), is the vacuum permittivity, k is the Boltzmann constant, T is the temperature and q is the elementary charge [1].
A lower SR value means that the TiO2 layer is blocking holes more efficiently. Here, the SR value can therefore be interpreted as a measure of how well the TiO2 layer prevents the P3HT layer from getting in contact with the FTO substrate, as FTO is not hole blocking. In other words, more intentional pinholes is expected to result in a higher surface recombination velocity. The measured CELIV current transients are shown in Figure S3 and calculated surface recombination velocities are plotted in Figure  S4. The SR values saturate to a constant value in the correct measurement regime at higher Voff, clearly larger than Vbi, when a large reservoir is accumulated at the TiO2 layer.
Samples for SR measurements were made on FTO glass substrates dipcoated with the dip coating solutions listed in Table 1. Spin coating was performed in a nitrogen-filled glove box, using a 40 mg/mL P3HT solution in chlorobenzene, at 1000 rpm for 1 minute and 30 seconds to yield a film thickness of around 300 nm. After spin coating the samples were annealed at 120 °C for around 10 minutes. Finally, 60 nm gold was evaporated on top. Measurements were conducted using a pulse generator (SRS model DG 535) and a function generator (SRS model DS345) for applying the offset voltage and the linearly increasing extraction voltage pulse. An oscilloscope (Tektronix TDS 680B) was used for recording the transient current response. The setup was controlled from a computer using a LabVIEW program.
For the measurements, a voltage rise speed of A = 3 V/ 10 ms was used. Samples were measured at varying applied offset voltage from 0 V to -1.4 V.