PEROVSKITE SOLAR CELL

A perovskite solar cell (PSC) 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 and all-inorganic cesium lead halide, are cheap to produce and simple to manufacture.

                          Solar cell efficiencies of laboratory-scale devices using these materials have increased from 3.8% in 2009 to 25.5% in 2020 in single-junction architectures, and, in silicon-based tandem cells, to 29.15%, exceeding the maximum efficiency achieved in single-junction silicon solar cells. Perovskite solar cells have therefore been the fastest-advancing solar technology as of 2016. With the potential of achieving even higher efficiencies and very low production costs, perovskite solar cells have become commercially attractive. Core problems and research subjects include their short- and long-term stability.

Advantages

Metal halide perovskites possess unique features that make them useful for solar cell applications. The raw materials used, and the possible fabrication methods (such as various printing techniques) are both low cost. Their high absorption coefficient enables ultrathin films of around 500 nm to absorb the complete visible solar spectrum.These features combined result in the possibility to create low cost, high efficiency, thin, lightweight and flexible solar modules. Perovskite solar cells have found use in powering low-power wireless electronics for the ambient powered internet of things applications, and may help mitigate climate change.

Materials

Crystal structure of CH₃NH₃PbX₃ perovskites (X=I, Br and/or Cl). The methylammonium cation (CH₃NH₃⁺) is surrounded by PbX₆ octahedra.^[10]

The name 'perovskite solar cell' is derived from the ABX₃ crystal structure of the absorber materials, which is referred to as perovskite structure and where A and B are cations and X is an anion. A cations with radii between 1.60 Å and 2.50 Å were found to form perovskite structures.  The most commonly studied perovskite absorber is methylammonium lead trihalide (CH₃NH₃PbX₃, where X is a halogen ion such as iodide, bromide or chloride), with an optical bandgap between ~1.55 and 2.3 eV depending on halide content. Formamidinium lead trihalide (H₂NCHNH₂PbX₃) has also shown promise, with bandgaps between 1.48 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. The first use of perovskite in a solid state solar cell was in a dye-sensitized cell using CsSnI₃ as a p-type hole transport layer and absorber. A common concern is the inclusion of lead as a 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.

Shockley-Queisser limit

Solar cell efficiency is limited by the Shockley-Queisser limit. This calculated limit sets the maximum theoretical efficiency of a solar cell using a single junction with no other loss aside from radiative recombination in the solar cell. Based on the AM1.5G global solar spectra, the maximum power conversion efficiency is correlated to a respective bandgap, forming a parabolic relationship.

This limit is described by the equation

${\displaystyle \eta =t_{s}\times u(x_{g})\times v(f,x_{g},x_{c})\times m(vx_{g}/x_{c})}$

Where

${\displaystyle x_{g}=V_{g}/V_{s}\ ;\ x_{c}=V_{c}/V_{s}}$

and u is the ultimate efficiency factor, and v is the ratio of open circuit voltage V_op to band-gap voltage V_g, and m is the impedance matching factor, and V_c is the thermal voltage, and V_s is the voltage equivalent of the temperature of the Sun.

The most efficient bandgap is found to be at 1.34 eV, with a maximum power conversion efficiency (PCE) of 33.7%. Reaching this ideal bandgap energy can be difficult, but utilizing tunable perovskite solar cells allows for the flexibility to match this value. Further experimenting with multijunction solar cells allow for the Shockley-Queisser limit to be surpassed, expanding to allow photons of a broader wavelength range to be absorbed and converted, without increasing thermalisation loss.

The actual band gap for formamidinium (FA) lead trihalide can be tuned as low as 1.48 eV, which is closer to the ideal bandgap energy of 1.34 eV for maximum power-conversion efficiency single junction solar cells, predicted by the Shockley Queisser Limit. More recently, the 1.3 eV bandgap energy has been successfully achieved with the (FAPbI
3)
1−x(CsSnI
3)
x hybrid cell, which has a tunable bandgap energy (E_g) from 1.24 – 1.41 eV

Multi-junction solar cells

Multi-junction solar cells are capable of a higher power conversion efficiency (PCE), increasing the threshold beyond the thermodynamic maximum set by the Shockley–Queissier limit for single junction cells. By having multiple bandgaps in a single cell, it prevents the loss of photons above or below the band gap energy of a single junction solar cell. In tandem (double) junction solar cells, PCE of 31.1% has been recorded, increasing to 37.9% for triple junction and 38.8% for quadruple junction solar cells. However, the metal organic chemical vapor deposition (MOCVD) process needed to synthesize lattice-matched and crystalline solar cells with more than one junction is very expensive, making it a less than ideal candidate for widespread use.

Perovskite semiconductors offer an option that has the potential to rival the efficiency of multijunction solar cells, but can be synthesized under more common conditions at a greatly reduced cost. Rivaling the double, triple, and quadruple junction solar cells mentioned above, are all-perovskite tandem cells with a max PCE of 31.9%, all-perovskite triple-junction cell reaching 33.1%, and the perovskite-Si triple-junction cell, reaching an efficiency of 35.3%. These multijunction perovskite solar cells, in addition to being available for cost-effective synthesis, also maintain high PCE under varying weather extremes – making them utilizable worldwide.

Chiral ligands

Utilizing organic chiral ligands shows promise for increasing the maximum power conversion efficiency for halide perovskite solar cells, when utilized correctly. Chirality can be produced in inorganic semiconductors by enantiomeric distortions near the surface of the lattice, electronic coupling between the substrate and a chiral ligand, assembly into a chiral secondary structure, or chiral surface defects. By attaching a chiral phenylethylamine ligand to an achiral lead bromide perovskite nanoplatelet, a chiral inorganic-organic perovskite is formed. Inspection of the inorganic-organic perovskite via Circular Dichroism (CD) spectroscopy, reveals two regions. One represents the charge transfer between the ligand and the nanoplatelet (300-350 nm), and the other represents the excitonic absorption maximum of the perovskite. Evidence of charge transfer in these systems shows promise for increasing power conversion efficiency in perovskite solar cells.

Inorganic Perovskites

Improvement in the power conversion efficiency of Inorganic Perovskites over the past decade of development, basic structure

The highest performing perovskites solar cells utilize Organic-Inorganic Hybrid Perovskites (OIHPs), which have a power conversion efficiency (PCE) of ~25%. These cells, however, come with significant drawbacks due to their long term chemical instability. The organic components  - such as methylammonium or formamidinium - are inherently volatile and prone to destructive reaction with water, air, heat, or even simple illumination. OIHPs thus require encapsulation, an expensive and energy intensive process to prevent their interaction with air and water. Fully inorganic perovskites are designed to avoid this volatility, using only chemically stable inorganic compounds. The first fully inorganic perovskites, developed in 2012, were tin-based CsSnI₃ cells with a PCE of just 0.88%. Significant development has occurred throughout the past decade, and modern inorganic perovskites have PCE over 17%.  These modern, high performing fully inorganic perovskite cells are created using CsPbI₃, which has a band gap similar to that of high performing OIHPs (~1.7 eV), as well as excellent optoelectrical properties. Although chemically stable, these perovskite materials face significant issues with phase stability that prevent its broad industrial application. In high efficiency CsPbI₃, for example, the photoactive black α-phase is prone to transforming into the inactive yellow δ-phase seriously inhibiting the performance, especially when exposed to moisture.  The challenge of stabilizing the photoactive black α-phase of inorganic perovskite materials has been tackled in a variety of strategies, including octahedral anchoring and secondary crystal growth.

Other research and developments

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

Rice University scientists have discovered a novel phenomenon of light-induced lattice expansion in perovskite materials.

Research is being conducted to develop a new 3rd generation perovskite quantum dot solar cell technology, which could solve the long-term reliability issues that have been the roadblock to the success of other perovskite solar cells despite their high conversion efficiencies. 

In order to overcome the instability issues with lead-based organic perovskite materials in ambient air and reduce the use of lead, perovskite derivatives, such as Cs₂SnI₆ double perovskite, have also been investigated.

Processing

Perovskite solar cells hold an advantage over traditional silicon solar cells in the simplicity of their processing and their tolerance to internal defects.  Traditional silicon cells require expensive, multi-step processes, conducted at high temperatures (>1000 °C) under high vacuum in special cleanroom facilities.Meanwhile, the hybrid organic-inorganic perovskite material can be manufactured with simpler wet chemistry techniques in a traditional lab environment. Most notably, methylammonium and formamidinium lead trihalides, also known as hybrid perovskites, have been created using a variety of solution deposition techniques, such as spin coating, slot-die coating, blade coating, spray coating, inkjet printing, screen printing, electrodeposition, and vapor deposition techniques, all of which have the potential to be scaled up with relative ease except spin coating.

Deposition methods

The solution-based processing method can be classified into one-step solution deposition and two-step solution deposition. In one-step deposition, a perovskite precursor solution that is prepared by mixing lead halide and organic halide together, is directly deposited through various coating methods, such as spin coating, spraying, blade coating, and slot-die coating, to form perovskite film. One-step deposition is simple, fast, and inexpensive but it’s also more challenging to control the perovskite film uniformity and quality. In the two-step deposition, the lead halide film is first deposited then reacts with organic halide to form perovskite film. The reaction takes time to complete but it can be facilitated by adding Lewis-bases or partial organic halide into lead halide precursors. In two-step deposition method, the volume expansion during the conversion of lead halide to perovskite can fill any pinholes to realize a better film quality. The vapor phase deposition processes can be categorized into physical vapor deposition (PVD) and chemical vapor deposition (CVD). PVD refers to the evaporation of a perovskite or its precursor to form a thin perovskite film on the substrate, which is free of solvent. While CVD involves the reaction of organic halide vapor with the lead halide thin film to convert it into the perovskite film. A solution-based CVD, aerosol-assisted CVD (AACVD) was also introduced to fabricate halide perovskite films, such as CH₃NH₃PbI_3, CH₃NH₃PbBr₃, and Cs₂SnI₆.

One-step solution deposition vs two-step solution deposition

One-step solution deposition

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.

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.

Pb halide perovskites can be fabricated from a PbI₂ precursor, or non-PbI₂ precursors, such as PbCl₂, Pb(Ac)₂, and Pb(SCN)₂, giving films different properties.

Two-step solution deposition

In 2015, a new approach for forming the PbI₂ nanostructure and the use of high CH₃NH₃I concentration have been 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 amounts 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. Furthermore, this is an inexpensive approach.

Vapor deposition

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.

Scalability

Scalability includes not only scaling up the perovskite absorber layer, but also scaling up charge-transport layers and electrode. Both solution and vapor 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).

In 2014, Olga Malinkiewicz presented her inkjet printing manufacturing process for perovskite sheets in Boston (US) during the MRS fall meeting – for which she received MIT Technology review's innovators under 35 award. The University of Toronto also claims to have developed a low-cost Inkjet solar cell in which the perovskite raw materials are blended into a Nanosolar ‘ink’ which can be applied by an inkjet printer onto glass, plastic or other substrate materials.

Scaling up the absorber layer

In order to scale up the perovskite layer while maintaining high efficiency, various techniques have been developed to coat the perovskite film more uniformly. For example, some physical approaches are developed to promote supersaturation through rapid solvent removal, thus getting more nucleations and reducing grain growth time and solute migration. Heating, gas flow, vacuum, and anti-solvent can all assist solvent removal. And chemical additives, such as chloride additives, Lewis base additives, surfactant additive,^] and surface modification, can influence the crystal growth to control the film morphology. For example, a recent report of surfactant additive, such as L-α-phosphatidylcholine (LP), demonstrated the suppression of solution flow by surfactants to eliminate gaps between islands and meanwhile the surface wetting improvement of perovskite ink on the hydrophobic substrate to ensure a full coverage. Besides, LP can also passivate charge traps to further enhance the device performance, which can be used in blade coating to get a high-throughput of PSCs with minimal efficiency loss.

Scaling up the charge-transport layer

Scaling up the charge-transport layer is also necessary for the scalability of PSCs. Common electron transport layer (ETL) in n-i-p PSCs are TiO₂, SnO₂ and ZnO. Currently, to make TiO₂ layer deposition be compatible with flexible polymer substrate, low-temperature techniques, such as atomic layer deposition, molecular layer deposition, hydrothermal reaction, and electrodeposition,are developed to deposit compact TiO₂ layer in large area. Same methods also apply to SnO₂ deposition. As for hole transport layer (HTL), instead of commonly used PEDOT:PSS, NiO_x is used as an alternative due to the water absorption of PEDOT, which can be deposited through room-temperature solution processingCuSCN and NiO are alternative HTL materials which can be deposited by spray coating, blade coating, and electrodeposition, which are potentially scalable. Researchers also report a molecular doping method for scalable blading to make HTL-free PSCs.

Scaling up the back electrode

Evaporation deposition of back electrode is mature and scalable but it requires vacuum. Vacuum-free deposition of back electrode is important for full solution processibility of PSCs. Silver electrodes can be screen-printed, and silver nanowire network can be spray-coated as back electrode. Carbon is also a potential candidate as scalable PSCs electrode, such as graphite, carbon nanotubes, and graphene.

Toxicity

Toxicity issues associated with the Pb content in perovskite solar cells strains the public perception and acceptance of the technology.The health and environmental impact of toxic heavy metals has been much debated in the case of CdTe solar cells, whose efficiency became industrially relevant in the 1990s. Although, CdTe is a thermally and chemically very stable compound with a low solubility product, K_sp, of 10⁻³⁴ and, accordingly, its toxicity was revealed to be extremely low, rigorous industrial hygiene programmes  and recycling commitment programmes have been implemented. In contrast to CdTe, hybrid perovskites are very unstable and easily degrade to rather soluble compounds of Pb or Sn with K_SP=4.4×10^−9, which significantly increases their potential bioavailability and hazard for human health, as confirmed by recent toxicological studies. Although the 50 % lethal dose of lead [LD₅₀(Pb)] is less than 5 mg per kg of body weight, health issues arise at much lower exposure levels. Young children absorb 4–5 times as much lead as adults and are most susceptible to the adverse effects of lead.In 2003, a maximum blood Pb level (BLL) of 5 μg/dL was imposed by the World Health Organization,  which corresponds to the amount of Pb contained in only 5x5 mm² of the perovskite solar module. Furthermore, the BLL of 5 μg/dL was revoked in 2010 after the discovery of decreased intelligence and behavioral difficulties in children exposed to even lower values.

Efforts in Reducing Lead Toxicity

Replacing Lead in Perovskites

Various studies have been performed to analyze promising alternatives to lead perovskite for use in PSCs. Good candidates for replacement, which ideally have low toxicity, narrow direct bandgaps, high optical absorption coefficients, high carrier mobility, and good charge transport properties, include Tin/Germanium-halide perovskites, double perovskites, and Bismuth/Antimony-halides with perovskite-like structures.

Research done on Tin halide-based PSCs show that they have a lower power conversion efficiency (PCE), with those fabricated experimentally achieving a PCE of 9.6%. This relatively low PCE is in part due to the oxidation of Sn²⁺ to Sn⁴⁺, which will act as a p-type dopant in the structure and result in higher dark carrier concentration and increased carrier recombination rates. Gemanium halide perovskites have proven similarly unsuccessful due to low efficiencies and issues with oxidising tendencies, with one experimental solar cells displaying a PCE of only 0.11%  Higher PCEs have been reported from some Germanium Tin alloy-based Perovskites, however, with an all-inorganic CsSn_0.5Ge_0.5I₃ film having a reported PCE of 7.11%. In addition to this higher efficiency, the Germanium Tin alloy Perovskites have also been found to have high photostability.

Apart from the Tin and Germanium based perovskites, there has also been research on the viability of double-perovskites with the formula of A₂M⁺M³⁺X₆. While these double-perovskites have a favorable bandgap of approximately 2 eV and exhibit good stability, several issues including high electron/hole effective masses and the presence of indirect bandgaps result in lowered carrier mobility and charge transport.^[82] Research exploring the viability of Bismuth/Antimony halides in replacing lead perovskites has also been done, particularly with Cs₃Sb₂I₉ and Cs₃Bi₂I₉, which also have bandgaps of approximately 2 eV.^[83] Experimental results have also shown that, while Antimony and Bismuth halide-based PSCs have good stability, their low carrier mobilities and poor charge transport properties restrict their viability in replacing lead-based perovskites.

Encapsulation to Reduce Lead Leakage

Recent research into the usage of encapsulation as a method for reducing lead leakage has been conducted, particularly focusing on the utilization of self-healing polymers. Research has been done on two promising polymers, Surlyn and a thermal crosslinking epoxy-resin, diglycidyl ether bisphenol A:n-octylamine:m-xylylenediamine = 4:2:1. Experiments showed a substantial reduction in lead leakage from PSCs using these self-healing polymers under simulated sunny weather conditions and after simulated hail damage had cracked the outer glass encapsulation. Notably, the epoxy-resin encapsulation was able to reduce lead leakage by a factor of 375 times when heated by simulated sunlight.

Coatings to Adsorb Lead Leakage

Chemically lead-binding coatings have also been employed experimentally to reduce lead leakage from PSCs. In particular, Cation Exchange Resins (CERs) and P,P′-di(2-ethylhexyl)methanediphosphonic acid (DMDP) have been employed experimentally in this effort. Both coatings work similarly, chemically sequestering lead that might leak from a PSC module after weather damage occurs. Research into CERs has shown that, through diffusion-controlled processes, Pb²⁺ lead is effectively adsorbed and bonded onto the surface of CERs, even in the presence of competing divalent ions such as Mg²⁺ and Ca²⁺ that might also occupy binding sites on the CER surface.

To test the efficacy of CER-based coatings in adsorbing lead in practical conditions, researchers dripped slightly acidic water, meant to simulate rainwater, onto a PSC module cracked by simulated hail damage. Researchers found that by applying a CER coating onto the copper electrodes of damaged PSC modules, lead leakage was reduced by 84%. When the CER was integrated into a carbon-based electrode paste applied to PSC and on the top of the encapsulating glass, the lead leakage decreased by 98%. A similar test was also performed on a PSC module with DMDP coated on both the top and bottom of the module to study the efficacy of DMDP in reducing lead leakage. In this test, the module was cracked by simulated hail damage, and placed in a solution of acidic water containing aqueous Ca²⁺ ions, meant to simulate acidic rain with low levels of aqueous Calcium present. The lead concentration of acidic water was tracked, and researchers found that the lead sequestration efficiency of the DMDP coating at room temperature 96.1%.

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.

Efficiency limits

Perovskite solar cell bandgaps are tunable and can be optimised for the solar spectrum by altering the halide content in the film (i.e., by mixing I and Br). The Shockley–Queisser limit radiative efficiency limit, also known as the detailed balance limit.is about 31% under an AM1.5G solar spectrum at 1000 W/m², for a Perovskite bandgap of 1.55 eV.  This is slightly smaller than the radiative limit of gallium arsenide of bandgap 1.42 eV which can reach a radiative efficiency of 33%.

Values of the detailed balance limit are available in tabulated form  and a MATLAB program for implementing the detailed balance model has been written.

In the meantime, the drift-diffusion model has found to successfully predict the efficiency limit of perovskite solar cells, which enable us to understand the device physics in-depth, especially the radiative recombination limit and selective contact on device performance.  There are two prerequisites for predicting and approaching the perovskite efficiency limit. First, the intrinsic radiative recombination needs to be corrected after adopting optical designs which will significantly affect the open-circuit voltage at its Shockley–Queisser limit. Second, the contact characteristics of the electrodes need to be carefully engineered to eliminate the charge accumulation and surface recombination at the electrodes. With the two procedures, the accurate prediction of efficiency limit and precise evaluation of efficiency degradation for perovskite solar cells are attainable by the drift-diffusion model.

Along with analytical calculations, there have been many first principle studies to find the characteristics of the perovskite material numerically. These include but are 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[edit]

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.