For nearly a decade the Kurt J Lesker Company has been shipping Physical Vapor Deposition (PVD) tools into the field of perovskite solar cells around the world and continues to support world class research into this exciting technology. A key topic in the fabrication of vapor deposited perovskite photovoltaics is how much the crystal grain boundaries affect the solar cell performance (see Figure 1). Recent work, published in ACS Energy Letters, titled "Control over crystal size in vapor deposited metal-halide perovskite films' by Killian Lohmann, Jay Patel, Mathias Uller Rothmann, Chelsea Xia, Robert Oliver, Laura Herz, Henry Snaith and Michael Johnston from the University of Oxford, in the UK, has not only identified key parameters that effect the grain growth in perovskite thin films but the team has also developed a novel method to control the deposition of the organic precursor, resulting in highly efficient solar cells.
Solar cells work by using materials that absorb photons from sunlight in a broad spectral range and in turn effectively convert this captured light into free charges that produce electricity. Modern commercial solar cells are mainly based on crystalline silicon, which is a cheap and abundant semiconductor. In recent decades thin film technologies such as CIGS, CdTe, amorphous silicon and organic photovoltaic (OPV) materials have all strived to create solar cells that have high efficiencies coupled with good cell stability and low manufacturing costs but come with the advantages of being a thin film by being lightweight and flexible. However, there is a new type of material that has the potential to revolutionize the field of photovoltaics: perovskite-structure based materials. A perovskite structure is any material with the formula ABX3, and it is the recent work centred around metal halide perovskites (MHPs) that has recently garnered a lot of interest in the solar energy world. These easy to fabricate, versatile semiconductors have shown rapid growth in solar to exhibit electrical power conversion efficiencies (PCEs) from 3.8% to 25.5% over the past few years - which is unprecedented for a solar cell technology and why so many research groups around the world are interested in them.
MHP semiconductors are typically composed of a variety of chemical components comprised of organic and/or inorganic cations, e.g. A = methylammonium (MA+), formamidinium (FA+); divalent metals e.g. B = lead (Pb2+), tin (Sn2+) and halide anions, e.g. iodide (I-), bromide (Br-), etc.. With a wide range of band-gaps they are well suited for all-perovskite tandem solar cell configurations or being combined with existing silicon technologies.
Whilst solution processed methods are common to create a thin film, vacuum vapor deposition has many advantages versus solution processing, the main being controllability, reproducibility, scalability and much higher device yield. However, the deposition process and growth mechanism is not fully understood, particularly with co-deposition, where the precursors are sublimed at the same time in a vacuum process chamber and once the MHP films are deposited there is further uncertainty on how the crystal grain boundaries affect the solar cell performance.
The Oxford team fabricated a device structure of: Fluorene-doped tin oxide (FTO)/C60/MAPbI3/Spiro-OMeTAD/Au. By developing a novel substrate rate control deposition methodology and by measuring how the temperature of the substrate, along with the type of substrate material affected the film grain size a novel insight of the growth mechanism of vacuum co-evaporated MHPs was established which allows the creation of more efficient perovskite solar cells in the future[9, 10].
The n-i-p device structure was partly synthesized in two separate Kurt J Lesker deposition systems at the University of Oxford:
- A 10nm thick undoped C60 fullerene, N-type contact later, was thermally evaporated in a custom glovebox-integrated Kurt J Lesker Mini SPECTROS tool (see Figure 2) at a deposition rate of 0.1 Å/s at a pressure below 5x10-6 mbar.
- Using Low Temperature Evaporation (LTE) sources, PbI2 was co-evaporated in the same custom Kurt J Lesker thermal evaporation tool at a rate of 0.30 Å/s along with CH3NH3I (MAI) which was evaporated by controlling the temperature of the LTE source, between 175.1°C and 183°C, such that the deposition rate as measured by the quartz crystal sensor located at the substrate was 0.45 Å/s. The chamber walls were maintained at 17°C during the MAI deposition.
- An 80nm thick Au top contact layer was evaporated using a separate Kurt J Lesker Nano 36 (see Figure 3) evaporation tool at a deposition rate of 0.8 - 2.5 Å/s. Using a mask, devices of 0.0919cm2 were fabricated.
The current-voltage characteristics of the devices were measured using a calibrated solar simulator. Scanning Electron Microscopy (SEM) was used to analyze the morphology of the fabricated device whilst X-ray diffraction (XRD) was used to probe the crystallinity. External quantum efficiency (EQE) measurements were performed using Fourier-transform infrared (FT-IR) spectroscopy to optically characterize the devices along with optical-pump THz-probe spectroscopy (OPTPS) and photoluminescence measurements to determine the optoelectronic properties of the thin films.
The analysis of the CH3NH3PbI3 thin films fabricated using vacuum co-deposition at different substrate temperatures revealed that when the films were deposited onto cold substrates they were made up of large, micrometer-sized grains whilst those deposited at room temperature were comprised of much smaller grains in the hundreds of nm size. When these films were measured under simulated sunlight the large grain films exhibited the worst PCE performance owing to a much lower current density. Figure 4 shows the SEM images of the co-deposited MHP films at different substrate temperatures along with current-voltage (J-V) and steady-state PCE curves.
This understanding of the mechanism of how grain boundaries are created and how they affect solar cell performance is a key point with vacuum deposited MHPs. Subsequently the isolation of the temperature effects of the substrate on the growth of the thin films and the development of a novel substrate rate control deposition method led to CH3NH3PbI3 solar cells being fabricated with a PCE of 18.2%. The device performance was found to be directly linked to the crystal growth which in turn was found to be affected by the CH3NH3I precursor adsorption rate which was strongly influenced by the substrate temperature. Not only this but further experiments looking at the effects of the substrate on the crystal growth identified that the type of the substrate material plays an important role too. This work has been published in ACS Energy Letters. The group from Oxford University will use the findings to continue to fabricate highly efficient solar cells in the future.
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