Perovskite solar cell

A perovskite solar cell is a type of solar cell which includes a perovskite structured compound, most commonly a hybrid organic-inorganic lead or tin halide-based material, as the light-harvesting active layer. Perovskite materials such as methylammonium lead halides are cheap to produce and simple to manufacture.

Solar cell efficiencies of devices using these materials have increased from 3.8% in 2009 to 22.1% in early 2016, making this the fastest-advancing solar technology to date. With the potential of achieving even higher efficiencies and the very low production costs, perovskite solar cells have become commercially attractive, with start-up companies already promising modules on the market by 2017.

Features

See also: Building-integrated photovoltaics

Varun Sivaram, Samuel Stranks, and Henry Snaith in an article for Scientific American about perovskite solar cells said that the solar panels of tomorrow will be transparent, lightweight, flexible, and highly efficient. We will be able to coat shingles or skylights or windows with them, and it will all be as cheap as putting up wallpaper. In July 2015 major hurdles were that the largest perovskite solar cell was only the size of a fingernail and that they degraded quickly in moist environments.

Materials

Crystal structure of CH₃NH₃PbX₃ perovskites (X=I, Br and/or Cl). The methylammonium cation (CH₃NH₃⁺) is surrounded by PbX₆ octahedra.

The name 'perovskite solar cell' is derived from the ABX₃ crystal structure of the absorber materials, which is referred to as perovskite structure. The most commonly studied perovskite absorber is methylammonium lead trihalide (CH₃NH₃PbX₃, where X is a halogen atom such as iodine, bromine or chlorine), with an optical bandgap between 1.5 and 2.3 eV depending on halide content. Formamidinum lead trihalide (H₂NCHNH₂PbX₃) has also shown promise, with bandgaps between 1.5 and 2.2 eV. The minimum bandgap is closer to the optimal for a single-junction cell than methylammonium lead trihalide, so it should be capable of higher efficiencies. A common concern is the inclusion of lead as component of the perovskite materials; solar cells based on tin-based perovskite absorbers such as CH₃NH₃SnI₃ have also been reported with lower power-conversion efficiencies.

In another, recent development solar cells based on transition metal oxide perovskites and heterostructures thereof such as LaVO₃/SrTiO₃ are studied.

Processing

Perovskite solar cells hold an advantage over traditional silicon solar cells in the simplicity of their processing. Traditional silicon cells require expensive, multistep processes, conducted at high temperatures (>1000 °C) in a high vacuum in special clean room facilities. Meanwhile the organic-inorganic perovskite material can be manufactured with simpler wet chemistry techniques in a traditional lab environment. Most notably, methylammonium and formamidinium lead trihalides have been created using a variety of solvent techniques and vapor deposition techniques, both of which have the potential to be scaled up with relative feasibility.

In one-step solution processing, a lead halide and a methylammonium halide can be dissolved in a solvent and spin coated onto a substrate. Subsequent evaporation and convective self-assembly during spinning results in dense layers of well crystallized perovskite material, due to the strong ionic interactions within the material (The organic component also contributes to a lower crystallization temperature). However, simple spin-coating does not yield homogenous layers, instead requiring the addition of other chemicals such as GBL, DMSO, and toluene drips. Simple solution processing results in the presence of voids, platelets, and other defects in the layer, which would hinder the efficiency of a solar cell. Recently, a new approach for forming the PbI₂ nanostructure and the use of high CH₃NH₃I concentration which are adopted to form high quality (large crystal size and smooth) perovskite film with better photovoltaic performances. On one hand, self-assembled porous PbI₂ is formed by incorporating small amount of rationally chosen additives into the PbI₂ precursor solutions, which significantly facilitate the conversion of perovskite without any PbI₂ residue. On the other hand, through employing a relatively high CH₃NH₃I concentration, a firmly crystallized and uniform CH₃NH₃PbI₃ film is formed. Another technique using room temperature solvent-solvent extraction produces high-quality crystalline films with precise control over thickness down to 20 nanometers across areas several centimeters square without generating pinholes. In this method "perovskite precursors are dissolved in a solvent called NMP and coated onto a substrate. Then, instead of heating, the substrate is bathed in diethyl ether, a second solvent that selectively grabs the NMP solvent and whisks it away. What's left is an ultra-smooth film of perovskite crystals." In another solution processed method, the mixture of lead iodide and methylammonium halide dissolved in DMF is preheated. Then the mixture is spin coated on a substrate maintained at higher temperature. This method produces uniform films of up to 1 mm grain size.

In vapor assisted techniques, spin coated or exfoliated lead halide is annealed in the presence of methylammonium iodide vapor at a temperature of around 150 °C. This technique holds an advantage over solution processing, as it opens up the possibility for multi-stacked thin films over larger areas. This could be applicable for the production of multi-junction cells. Additionally, vapor deposited techniques result in less thickness variation than simple solution processed layers. However, both techniques can result in planar thin film layers or for use in mesoscopic designs, such as coatings on a metal oxide scaffold. Such a design is common for current perovskite or dye-sensitized solar cells.

Both processes hold promise in terms of scalability. Process cost and complexity is significantly less than that of silicon solar cells. Vapor deposition or vapor assisted techniques reduce the need for use of further solvents, which reduces the risk of solvent remnants. Solution processing is cheaper. Current issues with perovskite solar cells revolve around stability, as the material is observed to degrade in standard environmental conditions, suffering drops in efficiency (See also Stability).

Physics

An important characteristic of the most commonly used perovskite system, the methylammonium lead halides, is a bandgap controllable by the halide content. The materials also display a diffusion length for both holes and electrons of over one micron. The long diffusion length means that these materials can function effectively in a thin-film architecture, and that charges can be transported in the perovskite itself over long distances. It has recently been reported that charges in the perovskite material are predominantly present as free electrons and holes, rather than as bound excitons, since the exciton binding energy is low enough to enable charge separation at room temperature.

Estimating the efficiency limit

Since perovskite bandgaps are tunable and can be optimised for the solar spectrum, these cells are able to attain the Shockley–Queisser limit radiative efficiency limit, also known as the detailed balance limit, which is about 31% under an AM1.5G solar spectrum at 1000W/m², for a Perovskite bandgap of 1.55 eV. This is smaller than the radiative limit of gallium arsenide of bandgap 1.42 eV which can reach a radiative efficiency of 33%.

According to the detailed balance model by Shockley and Queisser, the maximal output power of a solar cell can be achieved if the following set of hypotheses are fulfilled:

(1) Carrier populations obey Maxwell–Boltzmann statistics. Particularly, the quasi-Fermi levels of electrons and holes are uniformly split through the cell and the split equals the applied voltage. The assumption is reasonable if mobility of photocarriers (electrons and holes) are sufficiently large. Regarding perovskite materials, charge carrier mobility as high as 10 cm² V⁻¹ s⁻¹ has been observed.

(2) Radiative band-to-band (monomolecular) recombination mechanism is the only one existing. Nonradiative recombination, such as Auger recombination, trap (defect) assisted recombination, etc, are ignorable. Different from silicon with an indirect bandgap, perovskite material has a direct band gap. Therefore, Auger recombination is sufficiently suppressed, which has been verified in recent experimental results. Moreover, light emission from perovskite solar cells is dominated by a sharp band-to-band transition that has a radiative efficiency much higher than that of organic solar cells.

(3) Internal conversion efficiency reaches 100%. When one photon is absorbed, it produces one electron-hole pair; and when one electron-hole pair recombines, it produces one photon. For perovskite solar cells, the internal quantum efficiency approaches 100%.

(4) Photon recycling effect occurs in the cell. Although a photon will be created by one electron-hole pair recombination during the radiative recombination process, the photon can be reabsorbed at a different spatial location in the cell, which creates a new electron-hole pair. Designs of light trapping and angular restriction can improve the photon reabsorption process and thus maximize solar cell efficiency. For perovskite solar cells, the existence of a photon recycling effect is still unclear.

To analyze the efficiency limit of CH₃NH₃PbI₃ solar cells, the detailed balance model is given by

$J(V) = {J_t}(V) + {J_e}(V) - {J_p}$

where $V$ is the applied voltage of the solar cell system. $J_p$ is the photocurrent (photogenerated current) due to absorption of incident sunlight. $J_e$ and $J_t$ describe current density losses due to the radiative emission and trap-assisted nonradiative recombination, respectively.

The photogenerated current is given by

${J_p} = q\int_0^\infty {a(\lambda ,W)} \frac{{\Gamma (\lambda )\lambda }}{{h{c_0}}}d\lambda$

where $c_0$ is the speed of light in free space, $\Gamma$ is the global AM1.5G spectrum of Sun (W•m-2•nm-1), $\lambda$ is the wavelength and $q$ is the elementary charge. The absorptivity $a$ depends on light-trapping and angular-restriction designs.

According to Maxwell–Boltzmann statistics, the radiative current density is expressed as

${J_e}(V) = {J_0}\left[ {\exp \left( {\frac{{qV}}{{{k_B}T}}} \right) - 1} \right]$

where $k_B$ is the Boltzmann constant and $T$ is the Kelvin temperature. The above is the same as Shockley’s diode equation but the dark current $J_0$ should be represented with the aid of the black-body radiation law. Solar cells absorb photon energy and randomly generate electron-hole pairs, which are the physical original of dark current. Meanwhile, photons are emitted immediately after electrons and holes recombine with each other. Under thermal equilibrium condition, the quantity of absorbed photons should be balanced with that of emitted photons. Finally, the dark current is formulated as

${J_0} = q\int_0^\infty {a(\lambda ,W)} \frac{{{\Gamma _0}(\lambda )\lambda }}{{h{c_0}}}d\lambda$

The dark current shows a similar mathematical expression to the photocurrent except the sun spectrum $\Gamma$ should be replaced by the black-body (thermal) emission spectrum of solar cell $\Gamma _0$.

For perovskite solar cells, the dominant nonradiative recombination process is the bimolecular (or Shockley-Read-Hall) recombination. Therefore, the nonradiative current can be expressed as

${J_t}(V) = q\gamma {n_i}W\exp \left( {\frac{{0.5qV}}{{{k_B}T}}} \right)$

where $\gamma$ is the monomolecular (recombination) rate, $n_i$ is the intrinsic carrier density, and $W$ is the cell thickness.

Along with analytical calculations, there have been many first principle studies to find the characteristics of the perovskite material, numerically. These include but not limited to bandgap, effective mass, and defect levels for different perovskite materials. Also there have some efforts to cast light on the device mechanism based on simulations where Agrawal et al. suggests a modeling framework, presents analysis of near ideal efficiency, and talks about the importance of interface of perovskite and hole/electron transport layers. However, Sun et al. tries to come up with a compact model for perovskite different structures based on experimental transport data.

Architectures

Schematic of a sensitized perovskite solar cell in which the active layer consist of a layer of mesoporous TiO₂ which is coated with the perovskite absorber. The active layer is contacted with an n-type material for electron extraction and a p-type material for hole extraction. b) Schematic of a thin-film perovskite solar cell. In this architecture in which just a flat layer of perovskite is sandwiched between to selective contacts. c) Charge generation and extraction in the sensitized architecture. After light absorption in the perovskite absorber the photogenerated electron is injected into the mesoporous TiO₂ through which it is extracted. The concomitantly generated hole is transferred to the p-type material. d) Charge generation and extraction in the thin-film architecture. After light absorption both charge generation as well as charge extraction occurs in the perovskite layer.

Perovskite solar cells function efficiently in a number of somewhat different architectures depending either on the role of the perovskite material in the device, or the nature of the top and bottom electrode. Devices in which positive charges are extracted by the transparent bottom electrode (cathode), can predominantly be divided into 'sensitized', where the perovskite functions mainly as a light absorber, and charge transport occurs in other materials, or 'thin-film', where most electron or hole transport occurs in the bulk of the perovskite itself. Similar to the sensitization in dye-sensitized solar cells, the perovskite material is coated onto a charge-conducting mesoporous scaffold – most commonly TiO₂ – as light-absorber. The photogenerated electrons are transferred from the perovskite layer to the mesoporous sensitized layer through which they are transported to the electrode and extracted into the circuit. The thin film solar cell architecture is based on the finding that perovskite materials can also act as highly efficient, ambipolar charge-conductor. After light absorption and the subsequent charge-generation, both negative and positive charge carrier are transported through the perovskite to charge selective contacts. Perovskite solar cells emerged from the field of dye-sensitized solar cells, so the sensitized architecture was that initially used, but over time it has become apparent that they function well, if not ultimately better, in a thin-film architecture. More recently, some researchers also successfully demonstrated the possibility of fabricating flexible devices with perovskites, which makes it more promising for flexible energy demand. Certainly, the aspect of UV-induced degradation in the sensitized architecture may be detrimental for the important aspect of long-term stability.

There is another different class of architectures, in which the transparent electrode at the bottom acts as cathode by collecting the photogenerated p-type charge carriers.

History

These perovskite materials have been well known for many years, but the first incorporation into a solar cell was reported by Miyasaka et al. in 2009. This was based on a dye-sensitized solar cell architecture, and generated only 3.8% power conversion efficiency (PCE) with a thin layer of perovskite on mesoporous TiO₂ as electron-collector. Moreover, because a liquid corrosive electrolyte was used, the cell was only stable for a matter of minutes. Park et al. improved upon this in 2011, using the same dye-sensitized concept, achieving 6.5% PCE.

A breakthrough came in 2012, when Henry Snaith and Mike Lee from the University of Oxford realised that the perovskite was stable if contacted with a solid-state hole transporter such as spiro-OMeTAD and did not require the mesoporous TiO₂ layer in order to transport electrons. They showed that efficiencies of almost 10% were achievable using the 'sensitized' TiO₂ architecture with the solid-state hole transporter, but higher efficiencies, above 10%, were attained by replacing it with an inert scaffold. Further experiments in replacing the mesoporous TiO₂ with Al₂O₃ resulted in increased open-circuit voltage and a relative improvement in efficiency of 3–5% more than those with TiO₂ scaffolds. This led to the hypothesis that a scaffold is not needed for electron extraction, which was later proved correct. This realisation was then closely followed by a demonstration that the perovskite itself could also transport holes, as well as electrons. A thin-film perovskite solar cell, with no mesoporous scaffold, of > 10% efficiency was achieved.

In 2013 both the planar and sensitized architectures saw a number of developments. Burschka et al. demonstrated a deposition technique for the sensitized architecture exceeding 15% efficiency by a two-step solution processing, and at a similar time Liu et al. showed that it was possible to fabricate planar solar cells by thermal evaporation, also achieving more than 15% efficiency. Docampo et al. also showed that it was possible to fabricate perovskite solar cells in the typical 'organic solar cell' architecture, an 'inverted' configuration with the hole transporter below and the electron collector above the perovskite planar film.

A range of new deposition techniques and even higher efficiencies were reported in 2014. A reverse-scan efficiency of 19.3% was claimed by Yang Yang at UCLA using the planar thin-film architecture. In November 2014, a device by researchers from KRICT achieved a record with the certification of a non-stabilized efficiency of 20.1%.

In December 2015, a new record efficiency of 21.0% was achieved by researchers at EPFL.

As of March 2016, researchers from KRICT and UNIST hold the highest certified record for a single-junction perovskite solar cell with 22.1%.

Stability

One big challenge for perovskite solar cells is the aspect of short-term and long-term stability. The water-solubility of the organic constituent of the absorber material make devices highly prone to rapid degradation in moist environments. Encapsulating the perovskite absorber with a composite of carbon nanotubes and an inert polymer matrix has been demonstrated to successfully prevent the immediate degradation of the material when exposed to moist ambient air at elevated temperatures. However, no long term studies and comprehensive encapsulation techniques have yet been demonstrated for perovskite solar cells. Beside moisture instability, it has also been shown that the embodiment of devices in which a mesoporous TiO₂ layer is sensitized with the perovskite absorber exhibits UV light induced instability. The cause for the observed decline in device performance of those solar cells is linked to the interaction between photogenerated holes inside the TiO₂ and oxygen radicals on the surface of TiO₂. The measured ultra low thermal conductivity of 0.5 W/(Km) at room temperature in CH₃NH₃PbI₃ can prevent fast propagation of the light deposited heat, and keep the cell resistive on thermal stresses that can reduce its life time. The PbI₂ residue in perovskite film has been experimentally demonstrated to have a negative effect on the long-term stability of devices. The stabilization problem is claimed to be solved by replacing the organic transport layer with a metal oxide layer, allowing the cell to retain 90% capacity after 60 days.

Hysteretic current-voltage behavior

Another major challenge for perovskite solar cells is the observation that current-voltage scans yield ambiguous efficiency values. The power-conversion efficiency of a solar cell is usually determined by characterizing its current-voltage (IV) behavior under simulated solar illumination. In contrast to other solar cells, however, it has been observed that the IV-curves of perovskite solar cells show a hysteretic behavior: depending on scanning conditions – such as scan direction, scan speed, light soaking, biasing – there is a discrepancy between the scan from forward-bias to short-circuit (FB-SC) and the scan from short-circuit to forward bias (SC-FB). Various causes have been proposed such as ion movement, polarization, ferroelectric effects, filling of trap states, however, the exact origin for the hysteretic behavior is yet to be determined. But it appears that determining the solar cell efficiency from IV-curves risks producing inflated values if the scanning parameters exceed the time-scale which the perovskite system requires in order to reach an electronic steady-state. Two possible solutions have been proposed: Unger et al. show that extremely slow voltage-scans allow the system to settle into steady-state conditions at every measurement point which thus eliminates any discrepancy between the FB-SC and the SC-FB scan. Henry Snaith et al. have proposed 'stabilized power output' as a metric for the efficiency of a solar cell. This value is determined by holding the tested device at a constant voltage around the maximum power-point (where the product of voltage and photocurrent reaches its maximum value) and track the power-output until it reaches a constant value. Both methods have been demonstrated to yield lower efficiency values when compared to efficiencies determined by fast IV-scans. However, initial studies have been published that show that surface passivation of the perovskite absorber is an avenue with which efficiency values can be stabilized very close to fast-scan efficiencies. Initial reports suggest that in the 'inverted architecture', which has a transparent cathode, little to no hysteresis is observed. This suggests that the interfaces might play a crucial role with regards to the hysteretic IV behavior since the major difference of the inverted architecture to the regular architectures is that an organic n-type contact is used instead of a metal oxide.

The observation of hysteretic current-voltage characteristics has thus far been largely underreported. Only a small fraction of publications acknowledge the hysteretic behavior of the described devices, even fewer articles show slow non-hysteretic IV curves or stabilized power outputs. Reported efficiencies, based on rapid IV-scans, have to be considered fairly unreliable and make it currently difficult to genuinely assess the progress of the field.

The ambiguity in determining the solar cell efficiency from current-voltage characteristics due to the observed hysteresis has also affected the certification process done by accredited laboratories such as NREL. The record efficiency of 20.1% for perovskite solar cells accepted as certified value by NREL in November 2014, has been classified as 'not stabilized'.

Perovskite in tandem cells

A perovskite cell combined with bottom cell such as Si or copper indium gallium selenide (CIGS) as a tandem design can suppress individual cell bottlenecks and take advantage of the complementary characteristics to enhance the efficiency. For example, Using a four terminal configuration in which the two sub-cells are electrically isolated, Bailie et al. obtained a 17% and 18.6% efficient tandem cell with mc-Si (η ~ 11%) and copper indium gallium selenide (CIGS, η ~ 17%) bottom cells, respectively. A 13.4% efficient tandem cell with a highly efficient a-Si:H/c-Si heterojunction bottom cell using the same configuration was obtained. Mailoa et al. used a c-Si bottom cell in a two terminal tandem design to demonstrate a 13.7% cell.

There have been some efforts to predict the theoretical limits for these traditional tandem designs using perovskite as top cell of c-Si or a-Si/c-Si heterojunction bottom cell. Also to show that even further output power enhancement is possible; a bifacial structure has been studied. It was concluded that by a practical albido reflection 8% extra output power can be extracted from the bifacial structure.

In May 2016, IMEC and its partner Solliance announced a tandem structure with a semi-transparent perovskite cell stacked on top of a back-contacted silicon cell. A combined power conversion efficiency of 20.2% was claimed, with the potential to exceed 30%.

See also

• Dye-sensitized solar cell
• Emerging photovoltaics
• Hybrid solar cell
• List of types of solar cells
• Nanocrystal solar cell
• Perovskite (mineral)
• Polymer solar cell
• Thin film solar cell
• Third generation photovoltaic cell