Perovskites and Perovskite Solar Cells: An Introduction

The rapid improvement of perovskite solar cells has made them the rising star of the photovoltaics world and of huge interest to the academic community. Since their operational methods are still relatively new, there is great opportunity for further research into the basic physics and chemistry around perovskites. Furthermore, as has been shown over the past two years - the engineering improvements of perovskite formulations and fabrication routines has led to significant increases in power conversion efficiency (with recent devices reaching over 22%, as of April 2017).

Why are perovskite solar cells so significant?

There are two key graphs which demonstrate why perovskite solar cells have attracted such prominent attention in the short time since their breakthrough paper of 2012 [1].

The first of these graphs (which uses data taken from NREL solar cell efficiency tables) demonstrates the power conversion efficiencies of the perovskite-based devices over recent years, in comparison to emergent photovoltaic research technology, and also traditional thin-film photovoltaics.

The graph shows a meteoric rise compared to most other technologies over a relatively short period of time. In the space of three years, perovskite solar cells have managed to achieve power conversion efficiencies comparable to Cadmium Telluride, which has been around for nearly 40 years. Although it could be argued that more resources and better infrastructure for solar cell research have been available in the last few years, the dramatic rise in perovskite solar cell efficiency is still incredibly significant and impressive.

Solar Cell Efficiency
Perovskite solar cells have increased in power conversion efficiency at a phenomenal rate compared to other types of photovoltaics. Although this figure only represents lab-based "hero cells", it heralds great promise.

 

The second key graph below is the open-circuit voltage compared to the band gap for a range of technologies that compete against perovskites.

This graph demonstrates how much of a photon’s energy is lost in the conversion process from light to electricity. For standard excitonic-based, organic-based solar cells, this loss can be as high as 50% of the absorbed energy. However, for perovskite-based solar cells, the loss is far less. Perovskite-based solar cells are fast approaching the same level of photon energy utilisation as the current leading monolithic crystalline technologies, such as silicon and GaAs. Furthermore, they also have the potential for much lower processing costs.

Photon energy use for perovskite solar cells compared to other photovoltaic types
The maximum photon energy utilisation (defined as the open circuit voltage Voc divided by the optical bandgap Eg) for common single junction solar cells material systems.

 

Currently, the only major unknown in the field of perovskite research is the stability of devices over their operational lifetime. Although lifetime studies of actual devices are limited, research into the stability of these films has shown that there are several reaction pathways leading to degradation that involve water, oxygen, and even the diffusion of electrode materials. Current leading research is focused upon reproducing the high power conversion efficiencies, but with the addition of stabilising agents such as Caesium and Rubidium.

Another issue yet to be fully addressed is the use of lead in perovskite compounds. Though it is used in much smaller quantities than that which is currently present in either lead- or cadmium-based batteries, the presence of lead in products for commercial use is problematic. There is potential for a lead alternative to be used in perovskite solar cells (such as tin-based perovskites), but the power conversion efficiency of such devices is still significantly behind lead-based devices. Finally, there has also been little discussion of the optical density of these materials - which although is higher than silicon, is still lower than other active materials. As a result, the perovskite devices require thicker light-harvesting layers which may cause some fabrication limitations. These limitations apply particularly to solution processed devices where creating such thick layers with high uniformity can be difficult.

Over the past two years, the improvements in precursor material blends for the fabrication of perovskite solar cells have led to a significant increase in power conversion efficiency. A key development has been the improvement in processing techniques used. Previously, vacuum-based techniques offered the highest efficiency devices but lately, improvements in solution-based deposition through the use of solvent quenching techniques has shifted the record-breaking devices to solution-based processing.

To enable a truly low cost-per-watt will require perovskite solar cells to have the much heralded trio of high efficiency, long lifetimes, and low manufacturing costs. This has not yet been achieved for other thin-film technologies but perovskite-based devices so far demonstrate enormous potential for achieving this.

What are perovskites?

The terms "perovskite" and "perovskite structure" are often used interchangeably. Technically, a perovskite is a type of mineral that was first found in the Ural Mountains and named after Lev Perovski who was the founder of the Russian Geographical Society. A perovskite structure is any compound that has the same structure as the perovskite mineral.

True perovskite (the mineral) is composed of calcium, titanium and oxygen in the form CaTiO3. Meanwhile, a perovskite structure is anything that has the generic form ABX3 and the same crystallographic structure as perovskite (the mineral). However, since most people in the solar cell world aren’t involved with minerals and geology, perovskite and perovskite structure are used interchangeably.

The perovskite lattice arrangement is demonstrated below. As with many structures in crystallography, it can be represented in multiple ways. The simplest way to think about a perovskite is as a large atomic or molecular cation (positively-charged) of type A in the centre of a cube. The corners of the cube are then occupied by atoms B (also positively-charged cations) and the faces of the cube are occupied by a smaller atom X with negative charge (anion).

Perovskite crystal structure in the form ABX3
A generic perovskite crystal structure of the form ABX3. Note however that the two structures are equivalent – the left hand structure is drawn so that atom B is at the <0,0,0> position while the right hand structure is drawn so that atom (or molecule) A is at the <0,0,0> position. Also note that the lines are a guide to represent crystal orientation rather than bonding patterns.

 

Depending on which atoms/molecules are used in the structure, perovskites can have an impressive array of interesting properties including superconductivity, giant magnetoresistance, spin-dependent transport (spintronics) and catalytic properties. Perovskites therefore represent an exciting playground for physicists, chemists and material scientists.

In the case of perovskite solar cells, the most efficient devices so far have been produced with the following combination of materials in the usual perovskite form ABX3:

  • A = An organic cation - methylammonium (CH3NH3)+
  • B = A big inorganic cation - usually lead(II) (Pb2+)
  • X3= A slightly smaller halogen anion – usually chloride (Cl-) or iodide (I-)

Since this is a relatively general structure, these perovskite-based devices can also be given a number of different names, which can either refer to a more general class of materials or a specific combination. As an example of this, we’ve created the below table to highlight how many names can be formed from one basic structure.

A B X3
Organo Metal Trihalide (or trihalide)
Methylammonium Lead Iodide (or triiodide)
Plumbate Chloride (or trichloride)

The perovskite name picking table: pick any one item from columns A, B or X3 to come up with a valid name. Examples include: Organo-lead-chlorides, Methylammonium-metal-trihalides, organo-plumbate-iodides etc.

 

The table demonstrates how vast the parameter space is for potential material/structure combinations, as there are many other atoms/molecules that could be substituted for each column. The choice of material combinations will be crucial for determining both the optical and electronic properties (e.g. bandgap and commensurate absorption spectra, mobility, diffusion lengths, etc). A simple brute-force optimisation by combinatorial screening in the lab is likely to be very inefficient at finding good perovskite structures. As such, while the field of perovskite solar cells has progressed rapidly so far with only a basic know-how of the photo physics and structural chemistry, far more in-depth knowledge than currently available is required to fully optimise devices.

 

Fabrication and Measurement of Perovskite Solar Cells

Although perovskites come from a seemingly different world of crystallography, they can be incorporated very easily into a standard OPV (or other thin film) architecture. While the best perovskite structures have been vacuum deposited to give better, more uniform film qualities, this process requires the co-evaporation of the organic (methylammonium) component at the same time as the inorganic (lead halide) components. The accurate co-evaporation of these materials to form the perovskite therefore requires specialist evaporation chambers that are not available to many researchers. This may also cause the practical issues of calibration and cross-contamination between organic and non-organic sources which would be difficult to clean.

However, the development of low-temperature solution deposition routes offer a much simpler method to incorporate perovskites, and can even be used with existing materials sets. Although perovskite solar cells originally came out of DSSC research, the fact that they no longer require an oxide scaffold means the field is branching out, and that many device architectures now look very similar to thin-film photovoltaics (except with the active layers substituted with the perovskite). The key to enabling this is by ensuring that the perovskite precursor materials use relatively polar solvents for deposition. Therefore, orthogonal solvent systems for the different layers can be easily developed.

The structure below represents a standard (non-inverted) perovskite solar cell based upon a standard glass/ITO substrates with metal back contact. All that is required to form a working device from the perovskite are two charge-selective interface layers (for the electrons and holes respectively).

Many of the standard interface layers from the world of organic photovoltaics work relatively well. For example, PEDOT:PSS and the PTAA-class of polymers work well as hole interface layers, while PCBM, C60, ZnO and TiO2 makes effective electron interfaces. However, the field is so new that there is a vast archive of possible interface materials to be explored. Understanding and optimising the energy levels and interactions of different materials at these interfaces offers a very exciting area of research.

Perovskite solar cell generic architecture 
Generic structure of a standard (non-inverted) perovskite solar cell.

 

The main issues for practical device fabrication of perovskite solar cells are film quality and thickness. The light-harvesting (active) perovskite layer needs to be several hundred nanometres thick – several times more than for standard organic photovoltaics. Unless the deposition conditions and annealing temperature are optimised, rough surfaces with incomplete coverage will form. Even with good optimisation, there will still be a significant surface roughness remaining. Therefore, thicker interface layers than might normally be used are also required. However, the fact that efficiencies of over 11% have already been achieved for spin coated devices [2] is highly encouraging.

Recent improvements to device processing have led to significant increases in the surface coverage while reducing the surface roughness. One method for improving the surface coverage and the roughness is to add small amounts of acids such as hydoriodic, hydrobromic, or hydrochloric acid. These materials are byproducts of the synthesis of methylammonium halides however the presence of these acids have an impact upon the solubility of the lead components. We previously discussed this in a post about the purity of MAI vs lead chloride solubility. Another method is by precise control of the timing of precipitation of the salts, this is done by solvent quenching methods with precise timings of the quench and volumes of the quenching solvents needed to give the optimal performance. To help with our understanding of this we decided to build the new Ossila syringe pump which has allowed us to use this quenching process to push in house power conversion efficiency values over 16%.

Even now there are further improvements being made in all areas of perovskite processing which are of great interest; some of these include mixed phase perovskites, two dimensional perovskite structures, inorganic perovskites, and 

For those just beginning their perovskite research, we have produced a video guide demonstrating the entire process of fabricating and measuring perovskite photovoltaics. In our own labs, we have reached efficiencies in excess of 11% using this particular fabrication routine.

 

Perovskite Fabrication Video Guide

 

Ossila Products for Perovskite Solar Cells

Ossila’s products are rapidly being adapted for use with perovskite solar cells and have already been used in a number of configurations. At the most basic level, most perovskite-based solar cells are based upon a transparent conductive oxide coated glass substrate with evaporated metal cathode and top encapsulation. As such, our existing substrate infrastructure and perovskite materials are already being used in high performance solution processed perovskite devices. Our standard encapsulation epoxy is also perfectly suited for laminating glass or other barrier layers – as used in Gratzel’s (2014) Nature paper[3].

The Ossila Spin Coater is also routinely used for the deposition of our interface and active layers with high accuracy and simple operation. In addition the Ossila Syringe Pump is being used for automatic the dispensing and quenching of our perovskite layers to obtain the highest quality films. Our academic colleagues have also made some exciting progress on solution-processed perovskite solar cells via spray coating onto our standard substrates.

Over recent months we have also worked with our academic collaborators to bring more perovskite-based products to market, including: High purity Methylammonium Iodide, Methylammonium Bromide, Formamidinium Iodide, and Formamidinium Bromide. We have also released our first set of perovskite inks, the first of these is I101 (MAI:PbCl2), is designed to be processed in air and has demonstrated efficiencies in our labs up to 11.7%. Our second ink, I201 (MAI:PbCl2:PbI2) is formulated to be processed in a nitrogen atmosphere, and so far we have seen efficiencies up to 11.8%. Both inks are designed to help our customers reach high efficiencies incredibly quickly when first starting out with their perovskites research. We include optimised processing routines with both inks to maximise results.

We will continue to develop our range of perovskite products in order to simplify fabrication and measurement, while also developing better characterisation tools and techniques. As always, please contact us should you have any questions.

Perovskite Ink

References (please note that Ossila has no formal connection to any of the authors or institutions in these references):

[1] Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Michael M. Lee et al., Science magazine, Vol 338, p643-647 (2012)
[2] Morphological Control for High Performance, Solution-Processed Planar Heterojunction Perovskite Solar Cells. Giles E. Eperon et al., Advanced Materials, Vol 24, p151-157 (2014)
[3] Perovskite solar cells employing organic charge-transport layers. Olga Malinkiewicz et al., Nature Photonics, Vol 8, p128-132 (2014)

Further Reading:

Perovskite solar cell overviews:

  • Hybrid solar cells- Perovskites under the Sun. Maria Antonietta Loi & Jan C. Hummelen, Nature Materials, V 12, p1087-1089 (2013)
  • Perovskite-Based Solar Cells. Gary Hodes, Science magazine, Vol 342, p317-318 (2013)

A general perovskite review by Henry Snaith:

  • Perovskites: The Emergence of a New Era for Low-Cost, High-Efficiency Solar Cells. Henry J. Snaith, The Journal of Physical Chemistry Letters, Vol 4, p3623-3630 (2013)

Key perovskite papers:

  • Efficient organometal trihalide perovskite planar-heterojunction solar cells on flexible polymer substrates. Pablo Docampo et al., Nature Communications, Vol 4, Article 2761 (2013)
  • Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber. Samula D. Stranks et al., Science magazine, Vol 342, p341-344 (2013)
  • High Charge Carrier Mobilities and Lifetimes in Organolead Trihalide Perovskites. Christian Wehrenfennig et al., Advanced Materials, Vol 26, p1584-1589 (2013)
  • Low-temperature processed meso-superstructured to thin-film perovskite solar cells. Energy Environmental Science, James M. Ball et al., Vol 6, p1739-1743 (2013)