Ultrathin and compact electron transport layer made from novel water-dispersed nanoparticles to accomplish UV-stable perovskite solar cells

UV induced decomposition of perovskite material is one of main factors to severely destroy perovskite solar cells for instability. Here we report a UV stable perovskite solar cell with a Fe 2 O 3 electron transport layer (ETL) made by spin-coating water dispersed Fe 3 O 4 nanoparticles. Devices with the Fe 2 O 3 ETL prepared from 10 nm Fe 3 O 4 nanoparticles have nearly no decrease of photoelectric conversion efficiency (PCE) under continuous exposure of very high UV light intensity (300W Xe lamp) for 10 hours in contrast to the TiO 2 ETL based samples with more than 30% reduction of PCE, and its PCE (14.33) is also much superior to that of devices with the Fe 2 O 3 ETL made conventionally from FeCl 3 solution (7.7%). Through the study of Fe 2 O 3 thin film prepared perovskite solar cell, it is found that compactness, high transmittance, low leakage and low transmission impedance devices can be obtained by using an appropriate size of Fe 3 O 4 nanoparticles. Our major findings are expected to provide a guide to design the UV-protection compact electron transport layers for UV-stable perovskite solar cells. Perovskite solar cells with Fe 2 O 3 film could keep less than 5 % decreased under 300W Xe lamp continuous exposure for 10 hours which the TiO 2 used samples show 30 % reduction in PCE. The mechanism for excellent performance has been studied by investigating the cell parameters including the size and concentration of Fe 3 O 4 nanoparticles, thickness and annealing temperature of layer, transmittance and absorbance of light usage, leakage current and charge transfer and recombination processes of solar cells. Our work provides an easy, promising and environmentally friendly way to prepare UV-stable Fe 2 O 3 layer to increase the performance of perovskite solar cells.


Introduction
photocatalytic reaction between the electron transport layer materials and the perovskite layer, which destroys the perovskite layer and reduces the stability of the device. Therefore, Fe 2 O 3 has been widely used as a UV-stable ETL in perovskite solar cells. Wang  preparing high-quality Fe 2 O 3 thin films, but it is difficult due to the low electronic conductivity and crystallinity of Fe 2 O 3 . It has been reported an effective approach to prepare SnO 2 and NiO x thin film by firstly preparing nanoparticles and then assembling them for a thin film. [30][31][32][33] At the same time, nanocomposites also have unique properties and wide applications. [34,35] Here, we made UV-stable perovskite solar cells with

Characterization and measurement
The morphologies of the samples were characterized using transmission electron microscopy (JEOL, 2100F) and field-emission scanning electron microscopy (Hitachi, SU8010). The films were also investigated by X-ray diffractometer (Bruker, D8 Advance), X-ray photoelectron spectroscopy (Thermo, Escalab 250Xi) and Raman (Horiba, Labram Hr Evolution). The Photoluminescence and Time-resolved photoluminescence were tested with 530 nm laser (Edinburgh Instruments, LP320).
The absorbance measurement was tested by UV-2600 (Shimadzu). The photovoltaic parameters of solar cells were measured under Newport solar simulator AM 1.5G irradiation (100 mW cm -2 ) with a Keithley 2400 Source Meter, and IPCE curves were characterized by Zolix system. Electrochemical Impedance Spectroscopy (EIS and M-S plots was measured under an AM 1.5G light condition with an alternative signal

Results and Discussion
A high-quality nanoparticle film requires very good size uniformity, and thereby we use a magnetic field control method to prepare Fe 3 O 4 nanoparticles to effectively avoid agglomerations of crystal nuclei in the growth process by adjusting the magnetic field, while tuning the reaction time for differently sized nanoparticles. Besides, growing in aqueous solution is also a very important condition. Considering we will apply the nanoparticles in green energy solar cells, we hope to minimize the use of environmentally unfriendly solution.  Figure S1 shows the images of solution with same 6 mg/ml concentration (measured by Fe). The different size nanoparticles in same concentration have different colors which may due to the size effect in light transfer. Figure 1d shows the HRTEM of Fe 3 O 4 nanoparticles, from which we can see the high-quality crystalline with 2.52 Å as (311) plane and the surface of nanoparticles is clean which is good for carrier transport. [37,38] Scanning electron microscopy (SEM) was used in here to show the morphology of as-prepared films on In-doped tin oxide (ITO) structure. Figure 2a shows the schematic of different Fe 2 O 3 films which were prepared by nanoparticles and FeCl 3 solution. Due to the excellent hydrophilic property of water dispersed nanoparticles, the films can be self-assembled on the ITO surface, and the high-quality iron oxide films can be obtained because of the little change of crystal structure in the annealing process.
For the films prepared by FeCl 3 solution, the spin coated FeCl 3 film has a good density, but in the post annealing process, the crystallization process of iron oxide will cause a large number of holes in the original compact film. Figure 2b shows the morphology of ITO glass without any treatment, which has clear surface and acicular grains. Figure   2c shows the morphology of ITO glass with Fe 2 O 3 film which is prepared by spin-  Figure 2d shows the morphology of Fe 2 O 3 film which is prepared by spin-coating FeCl 3 solution and annealed, from which we can see the film is more like a network structure than a compact film. These holes in the mesh may lead to the direct contact between the perovskite layer and ITO glass, which will damage the carriers' transport process of the device. Generally speaking, the local network structure has better hydrophilicity, which is conducive to the diffusion of liquid on the surface,

Conflicts of interest
There are no conflicts to declare.

Materials Advances Accepted Manuscript
Open