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 the methylammonium or formamidinium 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 [1] to a certified 20.1% in 2014, making this the fastest-advancing solar technology to date. Their high efficiencies and low production costs make perovskite solar cells a commercially attractive option, with start-up companies already promising modules on the market by 2017.[2][3]

Materials

Unit cell of the most commonly employed perovskite absorber in solar cells: methylammonium lead trihalide (CH3NH3PbX3, where X is on of the following halogen ions I−, Br−, Cl−)

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 ion such as I−, Br−, Cl−), with an optical bandgap between 2.3 eV and 1.6 eV depending on halide content. Formamidinum lead trihalide (H2NCHNH2PbX3) has also shown promise, with bandgaps between 2.2 eV and 1.5 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.[4] 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.[5][6][7]

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 requiring high temperatures (upwards of 1000 °C) and vacuums in special clean room facilities to produce high purity silicon wafers.[8] These techniques are harder to scale up, while the organic-inorganic perovskite material can be manufactured with simpler wet chemistry and processing techniques in a traditional lab environment.[9] 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.[10]

In solution processing, lead halide and methylammonium iodide can be dissolved in 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.[11] 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. However, 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 (DEE), a second solvent that selectively grabs the NMP solvent and whisks it away. What's left is an ultra-smooth film of perovskite crystals."[12]

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.[13] This technique holds an advantage over solution processing, as it open up the possibility for multi-stacked thin films over larger areas.[14] 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.[4][15] The materials also display a diffusion length for both holes and electrons of over one micron.[16][17] 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.[18]

Architectures

a) 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 (anode), 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.[16] 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.[19] 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.[20]

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.[1] 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.[21]

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.[22][23] 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.[24] 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.[14] 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.[25] A thin-film perovskite solar cell, with no mesoporous scaffold, of > 10% efficiency was achieved.[19][26][27]

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,[28] 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.[29][30] 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.[31]

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.[32] In November 2014, a device by researchers from KRICT achieved a new record with the certification of a non-stabilized efficiency of 20.1%.[33] In November 2014, at the 6th World Conference on Photovoltaic Energy Conversion in Kyoto, Japan, the achievement of a single-junction perovskite solar cell with a power-conversion efficiency of 24% was mentioned without further details.[34]

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.[35] 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.[35][36] 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.[37] 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.[37]

Hysteretic current-voltage behavior

Another major challenge for perovskite solar cells is the observation that current-voltage scans yield ambiguous efficiency values.[38][39] The power-conversion efficiency of a solar cell is usually determined by characterizing its current-voltage (JV) behavior under simulated solar illumination. In contrast to other solar cells, however, it has been observed that the JV-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).[38] Various causes have been proposed such as ion movement, polarization, ferroelectric effects, filling of trap states,[39] however, the exact origin for the hysteretic behavior is yet to be determined. But it appears that determining the solar cell efficiency from JV-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.[39] 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 JV-scans.[38][39] 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.[5][40] Initial reports suggest that in the 'inverted architecture', which has a transparent cathode, little to no hysteresis is observed.[20] This suggests that the interfaces might play a crucial role with regards to the hysteretic JV 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 JV curves or stabilized power outputs. Reported efficiencies, based on rapid JV-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'.[33]

See also

References

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  39. ↑ 39.0 39.1 39.2 39.3 Unger, Eva L.; Hoke, Eric T.; Bailie, Colin D.; Nguyen, William H.; Bowring, Andrea R.; Heumuller, Thomas; Christoforo, Mark G.; McGehee, Michael D. (2014). "Hysteresis and transient behavior in current-voltage measurements of hybrid-perovskite absorber solar cells". Energy & Environmental Science x (x): xxx. doi:10.1039/C4EE02465F.
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