Evolution of the Perovskite Solar Cell

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1		Tapas Mallick	--	5839	2023-05-18 15:16:22	\|
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This entry is adapted from the peer-reviewed paper 10.3390/en16104031

Perovskite solar cells (PSCs) have seen a rapid increase in power conversion efficiencies (PCEs) over just a few years and are already competing against other photovoltaic (PV) technologies. The PCE of hybrid PSCs exhibiting distinct properties has increased from 3.8% in 2009 to ≈30% in 2023, making it a strong contender for the next generation of PV devices.

photovoltaic perovskite solar cell efficiency stability

1. Introduction

The perovskite solar cell consists of perovskite, classed as an inorganic–organic hybrid compound that harnesses solar energy and works as a charge carrier conductor. A large revolution in perovskite materials in solar cell technology in terms of providing extraordinarily effective and phenomenally improved power conversion efficiency has been witnessed since 2009. The reports of authors dating back to the early 1990s show the inception of studying perovskite as a solar absorber.

In 2006, Japanese researchers achieved an exceptional PCE of 2.2%, which is regarded as a landmark. Dye-sensitised solar cells with CH₃NH₃PbBr₃ as a sanitiser was used in the experiment ^[1]. The first organic halide compound reported in 2009 was fabricated using CH₃NH₃PbBr₃ as a sanitiser. They recorded a PCE of 3.81% for the CH₃NH₃PbBr₃ device and 3.13% for CH₃NH₃PbI₃. Later, the same group again measured a PCE of 6.5% using the CH₃NH₃PbI₃ compound with iodine liquid electrolyte contact and a better fabrication condition.

However, this type of PSC was unstable due to the decomposition of perovskite nanocrystals in the iodine liquid electrolyte. Afterwards, numerous researchers entered this field in the following years to improve the PCE of perovskite solar cells. The fundamental photophysical properties, functioning, and unique nature offer many advances in optoelectronic applications that triggered this technology ^[2]. Perovskite materials also have distinct electronic properties such as piezoelectric properties, thermoelectric properties, superconductivity, and semi-conductivity, which are dependent on the selected materials. Due to these favourable properties, exciting experimental results, the mechanism to explain the higher PCE of these materials, various fabrication methods, and a considerable increase in efficiency beyond 25% were reported only in a few years. The PCE of 9.7% was achieved using Spiro-OMeTAD as the hole transport material (HTM) and CH₃NH₃PbI₃ as the perovskite solar absorber to fabricate solar state DSSCs ^[3]. This DSSC has shown dramatically improved device stability compared to liquid electrolytes, yet the device stability is a significant challenge for researchers in terms of commercialisation and mass production.

2. Characteristics of Perovskite Solar Cells

The optical properties of PSC, including bandgap, polarisation, absorption coefficient, and photoluminescence, have been discussed widely in the literature. Regarding the optical properties of perovskite materials, e.g., CH₃NH₃PbI₃ and CHNH₂PbI₃, they manifest relatively higher absorption coefficients than Sn-based perovskite materials. Replacement of methylammonium (MA) with other organic cations also allows for the tuning of the bandgap materials without affecting valance band maximum (VBM) by modifying the Pb-I-Pb bond length and angle ^[4].

The optical behaviour was observed to investigate the variation in energy levels and bandgap of the perovskite materials. The reduction was detected in the valence band maximum (VBM) and conduction band minimum (CBM) levels to 110 meV and 77 meV, respectively, by the increment of temperature to 25–85 °C by employing absorbance and PL spectroscopy and the further increase of 33 meV in the bandgap level of MAPbI₃ with the rise of temperature ^[5]. The researchers used the density functional theory (DFT) to present a more in-depth perception of thermal expansion that was dependent on a shift in VBM level, providing significant results for further designing MAPbI₃ at different applied temperatures. Another group observed an 100 °C temperature causing a shift in the band edge of MAPbI₃. The decomposition of MAPbI₃ at a lower temperature cannot be used commercially ^[6].

Single-crystal BaZrO₃ showed an indirect and high optical band gap energy of ≈4.8 eV. The Raman study exploited the second-order spectrum and opened up the possibility for fundamental studies due to the availability of single-crystal BaZrO₃ ^[7]. The plot of spectra at ambient temperature and low temperature did not indicate any phase transition. Slight changes could have been due to thermal effects.

Another valuable optical property of PV material is photoluminance (PL), which helps to provide a wide range of information about bandgap, charge separation, and chemical purity. The phenomena behind photoluminance are that when light is directed onto semiconductor material, protons absorption occurs on the material surface, and photo excitement occurs, followed by relaxing excitation to the ground state. This phenomenon was investigated by using organometal halide. An integrating sphere is suggested to reduce angular distribution for a better PL effect on PV cells. Some substituting methods are also recommended, e.g., substitution of methylammonium (λ = 776 nm) and formamidinium (λ = 776 nm) in lead-iodide-based perovskite ^[8]. The formation of broader PL peaks was reported by an increased formamidinium ratio, indicating the evolution of the reliable solution of MA and FA in the perovskite lattice. The investigation of the simultaneous intercalation of MA and FA cations with different compositions by XRD has been reported.

The same phenomena were observed for substituting iodide with bromide in lead-halide-based perovskite ^[9]. The property of PL in perovskite material is highly stable and showed promising results when exposed to various environments ^[10]. The charge separation mechanism can be figured out via PL quenching measurements. The PL behaviour of CH₃NH₃PbI₃/SpiroMeTAD and CH₃NH₃PbI_3-x Cl_x /PCBM was studied ^[11]^[12]. A significant decrease was noticed in PL intensity when both materials were in contact with the electron transport layer (ETL) and the hole transport layer (HTL) caused by the injection of electrons and holes. Due to the high conduction band, electrons are injected into the mesoporous layer but not the Al₂O₃ layer. Some studies have reported that inorganic HTL, e.g., CuSCN and NiO, act as charge separators. However, NiO showed less efficiency in terms of charge separation than CuSCN ^[13].

The impedance measurement is a crucial parameter in perovskite solar cells in terms of providing information about carrier transport, diffusion length, and recombination phenomena. According to impedance studies, the carrier conductivity for two different mesoporous and planar devices was similar, but the planar-based cell had higher recombination resistance than mesoporous-based cells ^[13]. Impedance studies also provide information about the occurrence of polarisation in PSC devices. However, perovskite material polarisation phenomena are likely to cause I-V hysteresis ^[14].

Electronegativity as a fundamental chemical property of PSC may explain the competence of an ion to attract electron density in the chemical bond. In halide ions, an electronegativity rise from I⁻ to F⁻ originates from the cutback of ionic size. However, F⁻ exhibits excellent robust bonding with the H atom of A site and B site compared to other halogens. The formation of better charge transport leads to an enhanced electron mobility rate and also limits lead leaching. The B-X bond can lead the way to a lower bandgap, which is requestioned for Vis and NIR range of optoelectronics ^[15].

3. Improving Efficiency through Structure Modulation

3.1. A-Site Cation

In the crystal structure of perovskite, it is generally understood that the A-site cation has no direct impact on electronic properties, but the bandgap structure can be tuned to adjust, which can affect the electronic characteristics of the perovskite material. A cation can expand or contract the lattice if it is of a smaller or larger size that can affect the bandgap by changing the B-X bond length ^[16]. The first mixed A-cation MA_xFA_1-xPbI₃ was reported to tune the bandgap by varying ratios of MA⁺ to FA⁺ and attained an efficiency of 14.9% ^[8]. The enhanced PCE of this perovskite material compared to the MA⁺ engaged to the higher absorption rate due to better film quality, fewer pin-holes, higher crystal quality, larger grain sizes, and smaller roughness within the grains.

Another stacked structure of FAPbI₃/MAPbI₃ films by ion exchanging was prepared, which increased the absorption rate and higher efficiency of 16.01% with a current density of 20.22 mA cm⁻² ^[17]. The three-dimensional symmetrical structure was also reported by employing the vacancy at the A site by a tiny monovalent cation, e.g., rubidium (Rb), caesium (Cs), farmamidium (FA), and methylammonium (MA). The most used cation in mixed A-site perovskite is methylammonium (MA). For CH₃NH₃PbI₃ devices, power conversion efficiency was reported beyond 15%. The structure of CH₃NH₃PbI₃ is tetragonal symmetry rather than cubic due to the tiny dimensions of the CH₃NH₃⁺ cation. As a result, it exhibits an absorption rate of 1.51–1.55 ev, which is significantly higher than the optimised limit of Shockley–Queisser for mono junction devices ^[18]^[19]. It is well known that inorganic-material-based perovskite devices show better efficiency, performance, and stability as compared to organic material. The idea of replacing organic cations by inorganic cations to design PSC devices was initiated by Choi et al. Cs_x(MA)_1-xPbI₃ PSC was devised, and it obtained 7.68% efficiency ^[20].

Formamidinium (FA) has been extensively probed due to its favourable characteristics of better symmetry than CH₃NH₃⁺. A bandgap of ≈1.43–1.48 eV with an absorption rate of 840 nm, close to the optimum value of 1.4 ev that was reported by the FAPbI₃ crystal process ^[18]. A non-perovskite polymorph of FAPbI₃ was devised, and it was anticipated to limit PV efficiency due to unpropitious bond collaboration with the TiO₂ layer ^[21]. In future, the complete elimination of non-perovskite is expected to lead to the efficiency of FA-based devices to exceed that of MA.

3.2. B-Site Cation

The position of the B-site cation in the structure of hybrid perovskite material is usually taken by metals of the IVA group in a +2-oxidation state. The most widely used metals are P²⁺, Ge²⁺, and Sn²⁺. Sn and Pb both belong to the same group in the periodic table, as mentioned above. Pb has a toxicity problem with it. Different alternatives of Pb have been reported, such as Sn in the fabrication of PSC devices ^[22]. Lead is the most studied material with higher performance, but lead has a toxic nature, which results in the instability of the devices ^[23]. Sn has less of an inert electron pair effect, and the toxic character of this metal also improves results in a reduction in the bandgap. Generally, Sn²⁺-metal-based perovskite devices present a lower bandgap as compared to Pb²⁺-based devices influencing the stability of the device and reduced PCE. Thus, the idea to use mixed Sn and Pb to prepare perovskite material in the B site was proposed to obtain an absorption near the infrared region. It is believed hypothetically that MASnX₃ manifests extended and higher bandgap values than MAPbX₃ ^[24]. Zuo et al. reported an inverted planar structure device using a combination of double Pb-Sn perovskite and obtained 10.1% of PCE ^[25]. The same structure was used to fabricate PSC with modified C₆₀ Sn-Pb perovskite films and showed a PCE of 13.9%.

Moreover, when the compound of Sn-Pb perovskite with a C₆₀ additive was exposed to an ambient environment without any encapsulating, it showed excellent stability and superb efficiency ^[26]. Stoumpos et al. observed that both MASnI₃ and MAPbI₃ follow tetragonal arrangements in the ambient environment. Pb was replaced by Sn to produce a more uniform perovskite absorbing layer with enhanced co-ordination involving complexes ^[27]^[28].

Marshell et al. used inorganic CsSnI₃ perovskite material to elucidate the significant effect of adding SnCl₂ in the perovskite light-absorbing layer to stabilise the device without compromising on PCE ^[29]. Furthermore, a wide range of studies on replacing lead (Pb) by Ge or Bi has been reported to indicate the reason for PCE in electron–hole carrier recombination and solubility in perovskite material ^[30].

3.3. X-Site Anion

A handful of studies have been reported to explain mixed halide ions on the X site incorporated in perovskite material ^[31]^[32]. The electronic structure of perovskite ABX₃ is generally associated with the p orbit of X and B. The bandgap of perovskite material can be swayed by modifying the p orbit of the mixed X-site anion and for the absorption of visible light when exposed to sun radiation ^[33]. Noh et al. first demonstrated the PV properties of a mixed X halide anion, and this achieved 12.3% efficiency ^[34]. They examined both lower (<10%) and higher (>20%) Br content. They yielded high initial efficiency for lower content due to a narrow bandgap, but for higher content, they obtained excellent stability against humidity (RH 55% for 20 days).

Bromide is a favourable metal used to attain a high bandgap of perovskite, as has been reported earlier. Moscon et al. showed that assimilating Br metal in iodine-based perovskite can cause structural deformity, resulting in a higher bandgap ^[35]. MAPbI_xBr_3-x was reported by chemical amalgamation to improve stability by tuning the bandgap, as well as improved PCE ^[34]. Chlorine has also been extensively studied in the literature to enhance the efficiency of perovskites. Cl presented excellent performance, mainly in planar heterojunction configuration, in order to improve carrier lifetime and diffusion length, and thus a perovskite device showed improved PCE. Cl subliming transformation into pure MAPbI₃ was suggested by Unger and co-workers. The electronegative of I⁻ (2.66 EN) is close to Pb(2.33 En), so its bond character is neither covalent nor ionic but somewhat mixed. Thus, it showed the most stable behaviour when incorporated with perovskites. However, a downside of iodine-based hybrid halide perovskite is its instability towards humidity.

Consequently, further research needs to find the substitution of iodine or other mixed halide perovskites. The fluorine ion (F) as a worthy candidate for PV solar device application is emerging as a perovskite material due to its superior characteristics of electron withdrawing. Moreover, it has the potential of forming a hydrolytic bond (N-H-F) in comparison with other halides ^[36]. However, fluorine has the drawback of being tinier in size than iodine, which leads to an unfavourable tolerance factor and a considerable amount of strain lattice that can halt the capacity of light absorptivity and conductivity. Even though PSCs have improved their output using different techniques and materials, stability is still a concern for researchers.

4. Stability Studies of PSC

4.1. Crystal Structural Stability

The perovskite material has the general crystal structure of ABX₃, as described earlier.

The predicted steady-state performance and stability of standard PV modules is 20 to 25 years; however, depending on various characteristics and properties of the components of PV devices, such stability has not been demonstrated in the last few years.

The structural stability of PSC contemplates the capability of the crystal structure to remain stable against a wide range of factors. The crystal symmetry of halite perovskite can be obtained by maintaining a feasible tolerance factor. That tolerance factor of the crystal structure can determine the rough estimation of the stability and the deformity of the crystal structure. Moreover, it also provides an idea of the phase structure in terms of whether it is cubic or whether it deviates into other shapes such as orthorhombic or tetragonal phase ^[37].

Methyl-ammonium lead iodide is the most studied and efficient material for the perovskite light-absorbing layer. However, the toxic nature of lead can lead to the production of degradation of PSC devices. Some researchers emphasised replacing lead with other metal ions for further large-scale manufacturing ^[38]. Several other inorganic cations, organic cations, and halide anions have been integrated into PSC structure engineering to improve the stability and efficiency of the PSC devices ^[39].

It is also noteworthy that MAPbI₃ has reported a tetragonal phase to exist, even after heating at a temperature of 373 K ^[40]. This indicates the stable nature of the tetragonal phase in thin films for a specific temperature, but there is an ambiguous phase transition between the tetragonal and cubic phases. Perovskite materials possess different phases depending on the variation of temperature. Perovskites display a stable orthorhombic phase when the temperature is below 100 K. With an increased temperature to 160 K, it displays the tetragonal phase and replaces the original orthorhombic phase ^[41]. When the temperature increases to 330 K, the cubic phase replaces the tetragonal phase ^[42].

The phase transition from a tetragonal to a cubic structure can be due to nucleation of lead iodide at the interface due to the intrinsic degradation process. The activation energy related to the dynamic exchange of proton-originating volatile species was found to be ≈1.54 eV during inert condition without the involvement of water and decreased to ≈0.96 eV with water molecules ^[43].

The incorporation of organic molecules (FA) and inorganic molecules (PbI₂) is another strategy to improve ^[44] the stability of perovskite against air, the ambient environment, and vacuum. Formamidinium has been reported to obtain phase transition at high temperatures, indicating more stability than MAPbI₂. Moreover, another report suggested light soaking as a cause of the reversible phase transition of perovskite materials ^[45]. However, more research attention needs to reveal the reason behind this behaviour.

Integration of CsPbI₂ has been reported for better stability of the perovskites phase of FAPbI₂ ^[46]. CsPbI₂ and FAPbI₂ are capable of mixing in the perovskite phase. However, because of the similarity of volume and structure of FAPbI₂ and CsPbI₂ and the dissolubility of the latter, they show instability and can lead to further massive energy loss ^[47]. The amalgam of CsPbI₂ and FAPbI₂ tuned the Goldschmidt effective tolerance factor and showed improved stability against humidity, resulting in better PSC device performance ^[46]. CsPbI₂Br was studied and presented stabilised PCE of 5.6% with J-V scanning efficiency of up to 9.8% for PCS devices ^[48].

MAPbI₃ showed a lattice phase change near 55 °C. Temperature is for the real solar cell working temperature. If this type of cell is encapsulated from moisture, it can perform at up to 80 °C. If the temperature is limited below that value, heating will not be a problem, even in concentrated photovoltaic systems. This idea has been experimentally performed to show a retained efficiency of 92% of PCE after exposure of 63 h with a 0.2% loss of photocurrent per hour ^[49].

4.2. Effect of Humidity

During assembling and testing the device, oxygen, moisture, humidity, and high-energy photons from UV can directly affect the stability of the perovskite layer. Most organometal halides are highly sensitive to moisture, and the crystallinity of perovskites can be damaged due to the presence of excess water. Oxygen degradation is one of the greater concerns among all factors, as mentioned earlier in a dry atmosphere. Oxygen can bond with perovskite ions such as MA⁻, Pb⁺, and I⁻ and form electronic traps and charge barriers. Severe oxygen degradation can occur on the perovskite film surface due to Pb–O bonds ^[50]. However, in PSC, moisture degradation can be protected with an encapsulated device and in a controlled operating environment.

However, some authors suggest that the presence of humidity or moisture is beneficial for forming perovskites ^[51]. Liduo et al. proposed the decomposition process of CH₃NH₃PbI₃ to investigate the humidity factor. They stored films at 35 °C with 60% relative humidity (RH) for 18 h. X-ray diffraction and UV-visible spectrum were used for characterisation before and after storage.

Here, perovskite hydrolysed directly into PbI₂ in the presence of moisture, and then methylammonium iodide was decomposed to produce HI and later departed in the presence of oxygen, which was due to exposure to UV light. Kelly et al. demonstrated a positive correlation between humidity and PCE using in situ absorption spectroscopy and in situ grazing incident X-ray diffraction. Their work showed improved PCE by carefully controlling the relative humidity by adjusting the constant flow rate of saturated water vapours and dilatant carrier gas particles. Adding the HTM layer was also able to reduce the moisture rate ^[52]. The absorption of perovskite at room temperature, stored for 14 days in the dark at 50% relative humidity (RH), was reduced in all the visible range of the spectrum ^[53]. The XRD pattern displayed some diffraction peaks after storage in the dark, which could be indexed as (CH₃NH₃)₄PbI₆ 2H₂O_, but no peak was seen for PbI₂.

Therefore, reversible degradation of CH₃NH₃PbI₃ can occur when stored in the dark or processed in a vacuum. The absorbance spectra of CH₃NH₃PbI₃ and PbI₂ were very close, indicating the transformation of CH₃NH₃PbI₃ into PbI₂ under light.

Heat treatment was another solution to cure the degradation of MAPbI₂ perovskite in the presence of humidity ^[54]. Moreover, some materials showed almost zero PCE of the device under the high temperature of 55 °C in air and for the internal device at 85 °C with a RH of 80%. Due to such severe sensitivity to moisture, the fabrication process must be processed in the glove box filled with inert gas ^[55]^[56]. Kwon et al. ^[57] developed poly [2,5-bis(2-decyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-€-1,2-di(2,2′-bithiophen-5-yl) ethene](PDPPDBTE), which possesses excellent hydrophobic properties to pervade water to the perovskite layer. The stability test was conducted for Spiro-MeTAD and PDPPDBTE; Spiro-MeTAD showed that 27.6% of PCE decreased after 1000 h of the initial value, whereas the PDPPDBTE-based device showed better performance in terms of stability than Spiro-MeTAD. This experiment was performed at room temperature without encapsulation and with ≈20% RH.

The degradation mechanism of CH₃NH₃PbI₃ exhibiting smooth morphology was studied by spectroscopic ellipsometry characterisation. The result showed that degradation might occur due to the elution of CH₃NH₃I and PbI₂ formation, as well as the hydration of CH₃NH₃PbI₃ by H₂O integration^[58].

Pristine CH₃NH₃PbI₃ layers were investigated by Martin Ledinsky and co-workers. They studied the Raman spectrum and showed the presence of the PbI₂ layer as the final degradation product ^[59]. Moreover, in situ electrical resist measurement and X-ray diffraction analysis were performed to study the interaction between moisture and perovskite film. The results showed that exposure to moisture chemisorption phenomena occurred for a short time, and a decrease in resistance was reversible. However, by extending time, this absorption occurred due to chemical reaction transformation by hydration and consequently the decomposition of perovskite film ^[60]. An ab initio molecular dynamics simulation was used to investigate the interaction phenomena between the MAPbI₃ layer and the present water environment. The results showed that the terminated MAI surfaces went through salvation due to water and Pb interaction, which caused the I atom to release the surface. An intermediate hydrate phase was able to occur due to the incorporation of one water molecule into the terminated PbI₂ ^[61]. A planar perovskite/perovskite tandem solar cell was fabricated using the ALD technique. The quick charge transportation between the interfaces due to ALD AZO’s tunnelling effect was considered. A comprehensive study was carried for the current matching condition. They achieved PCE of ≈31% with a realistic V_OC of 2.16 V, an FF of ≈82.2%, and a J_SC of 17.65 mA/cm². ALD AZO also played a fencing role to intercept oxygen and water from entering the device, resulting in the significantly improved stability of the tandem device ^[62].

To summarise, the CH₃NH₃PbI₃ compound with a certain amount of water can form an intermediate compound, leading to a reversible process in an inert atmosphere. However, CH₃NH₃PbI₃ can directly decrease to PbI₂ in the presence of water and is entirely irreversible. Nevertheless, to prevent voids and holes, the role of perovskite is crucial. A multi-layer encapsulated PSC scheme with the Al2O3 layer and hygroscopic layer deposition was demonstrated.

4.3. UV Light Stability

Apart from the moisture present in the air, UV light exposure was demonstrated, which strongly affected the performance of PSC devices. A multidimensional PSC system is the best strategy to cope with light behaviour ^[63]. The degradation mechanism of perovskite by UV light could occur due to integration of TiO₂ mesoporous layer in the fabrication of PSC. TiO₂, having a 3.2 eV bandgap, is an excellent catalyst for oxidising water to produce oxygen radicals and an oxidising organic compound. Upon UV light exposure, TiO₂ can extract an electron from I⁻ (iodide) when used as a photoanode, such as in typical DSSC, leading to the destruction of the crystal structure of perovskite and building up the strong ionic reaction of organic cations. In this process, extracted electrons trapped by vacant sites will recombine with excess oxygen molecules and generate O⁻₂ ^[64]. Upon illumination of UV light, oxygen is removed from the surface where it is absorbed, leaving vacant sites behind, which serve as a trap site for electrons. These trapped electrons are not mobile and may combine with holes in HTM, resulting in a low photo-generated current. Hitoshi et al. analysed CH₃NH₃PbI₃ transformation to PbI₂ by decreasing the UV-VIS absorption spectrum and XRD results. They proposed that extracted electrons from I⁻ by the TiO₂ layer degrade the structure stability of the perovskite by originating I₂.

Followed by the continued elimination of H⁺ and the evaporation process of CH₃NH₃PbI₃, the reduction of I by the interaction of the TiO₂ layer and CH₃NH₃PbI₃ on the interface occurs. The researchers integrated Sb₂S₃ between TiO₂ and CH₃NH₃PbI₃ at the interface to improve the stability of the perovskite upon UV light and noticed a significant enhancement in the stability by using this blocking layer. This worked as a deactivator for the I⁻/I₂ reaction at the TiO₂ surface, which enhanced stability.

Inserting of the CsBr layer between the ETL- and CH₃NH₃PbI_3-xCl_x-based absorbing layer was proposed to lead to UV light stability with a planar-based structure. They acquired 70% of initial PCE after UV light exposure for 20 min, and the control device under the same operating conditions degraded to zero in the air ^[65]. Dong et al. reported amino acid as an avert candidate to prevent decomposition of the perovskite layer under illumination by hydroxyl radicals and superoxide anions generated by O₂ and H₂O present at the interface of the TiO₂ layer ^[66]. Another way to overcome the instability originated by UV light absorption by TiO₂ is by applying a U-V filter in front of the TCO substrate before the deposition of the TiO₂ layer. However, as compared to other approaches, the U-V filter might lead to unavoidable fabrication costs due to extra material costs, loss in photogenerating a current, and a further decrease in the efficiency rate.

Al₂O₃ has shown a promising scaffold as a substitution for the TiO₂ layer. The stability of PSC was examined for over 1000 h at 40 °C, encapsulating a window glass lid and epoxy resin in a Ni-filled glove box, whereas 3 h exposure of TiO₂-based cells caused degradation to almost zero. Therefore, 5% maintenance of PCE was observed within the first 200 h due to a decrease in F.F and Voc ^[67]. A comparison of the TiO₂ layer Al₂O₃ scaffold stability was performed by Flannan T. F. O’Mahony et al. in an ambient environment.

The photogenerated electrons might migrate to combine oxygen molecules, leading to the formation of superoxides. Then, these superoxides cause the degradation of perovskite by the deprotonating of methylammonium cation ^[68]. However, avoiding parasitic reactions between photogenerated electrons and oxygen present in the environment can improve the stability of PSC. The fast electron extraction from the perovskite layer with meso-TiO₂ as an acceptor can help to circumvent parasitic reactions. ETM as a substitution for TiO₂ can be helpful in improving the stability of PSCs ^[69]. SnO₂-based PSC performed well to remain stable for 700 h of storage as compared to TiO₂-based PSC ^[70]. A highly stable SnO₂ nanocrystal layer was used to show 90% retained PCE of its initial value of 18.8% after storage of 30 days with <70% RH at an ambient environment ^[71]. PSC performance under prolonged light exposure is not only due to high-intensity light or optoelectronic effects. It can behave worse on the mechanical integrity of the device by inducing high temperatures ^[72]. A highly efficient Mg_xZn_1-xO-based (MZO-based) PSC was reported. MZO has a high conduction band and electron mobility when compared with traditional electrode TiO₂. These properties can reduce charge accumulation at the MZO/perovskite interface and increase the charge transfer between the interfaces. The encapsulated device retained 76% of the initial Jsc after one year of aging and 8 h UV irradiation, while for TiO₂, only 12% of its initial Jsc was retained. The work outcomes show great potential for MZO ETL in perovskite application ^[62].

4.4. Thermal Stability

Like moisture, the temperature can cause the degradation of perovskite material, and other main defective components can be a hole transition material. According to international standards, 85 °C is the minimum temperature required to be the best competitor to other solar cell technologies ^[73]. However, organometal halide perovskite materials have been reported to maintain the stability above 300 °C reported earlier. The degradation of the perovskite material was reported to operate below a 140 °C temperature in the literature ^[74]. Typically, perovskite material fabricated by solution process requires an annealing process, where 80 °C is the minimum temperature for the complete decomposition of PbI₂ and CH₃NH₃I in CH₃NH₃PbI₃. CH₃NH₃PbI₃(MAPbI₃) has been widely discussed as generating thermodynamic degradation due to the origin of volatile molecular defects ^[75]. Perovskite material using CH₃NH₃PbI_3-xCl_x was fabricated in a N’₂ atmosphere at up to 100 °C temperature, and a 3D perovskite structure was formed at 90 °C, whereas the degradation mechanism was seen at 100 °C ^[76].

Fan et al. studied the crystal structure of the MAPbI₃ microplate using a high in situ resolution transmitted electron microscope. They showed that after 100 s of heating at 85 °C, almost 75% of the transformation of the initial tetragonal phase to trigonal PBI₂ occurred ^[77]. The degradation mechanism occurred due to breaking the weak Pb-I-Pb bond along the (001) direction.

Han et al. investigated the thermal stability of the PSC device using a temperature-controlled environmental chamber operating at −20 °C to 200 °C ^[78]. The environmental temperature was lower than the actual cell temperature, e.g., 55 °C (85 °C). Cross-sectional focused ion beam scanning electron microscope (FIB-SEM) analysis was performed to study the degradation mechanism of CH₃NH₃PbI₃ for 500 h. Different components of the encapsulated device were observed to be defective due to direct exposure to one sun illumination. The degradation of silver layer formation of voids in HTM and the light-absorbing perovskite layer, the separation of the perovskite layer and TiO₂ layer, and the main formation of PbI₂ were observed. The degradation possibly occurred due to an encapsulated device’s reaction between Ag and HI gas. Therefore, the replacement of the Ag layer by appropriate metal and highly heat-repellent material and encapsulation is recommended by the authors. Herz et al. observed MAPbI₃ at an operating temperature of 100 °C and showed a gradual shift in the bandgap using PL and transmittance measurements. Their results suggested that inherent thermal degradation of the PSC devices operating at low temperatures can limit its use at the commercial level ^[6].

Wu et al. reported improvement in crystallisation and oxidation in Spiro-MeTAD-based perovskite devices after annealing. This was better for hole transport and transfer. Consequently, higher I_sc was achieved. Therefore, due to Li-TFSI transfer to the TiO₂ layer and evaporation occurrence of 4-tert-butylpyridine, the fermi level of TiO₂ shifted down. Consequently, low FF, reduced PCE, and V_oc were acquired. However, the absence of HTM is a suitable choice for a capable PSC device to be stable. A PCE of 10.5% was obtained without HTM layer integration in the perovskite structure ^[19]. Li et al. tested PSC devices fabricated with solution processing with meso/TiO₂/ZrO₂ scaffold coating and back contact of carbon black, being encapsulated to avoid moisture.

Han et al. also observed HTL-free PSC devices with carbon as the back contact without any ceiling, and after exposure to the sun for 1008 h in the ambient environment, they acquired a greater level of PCE than the initial amount ^[79].

The performance of PSC devices at a prolonged light period was theoretically studied. In a temperature range of 300–360 K, different PV parameters such as Voc, PCE, and Jsc were analysed.

5. Hysteresis

Another challenging characteristic of PV solar cells to restrict further development is the presence of I-V hysteresis when measured by applying different voltage sweep rates in both forward and reverse directions ^[80]. Usually, the enhanced performance is acquired with a forwarding bias, followed by a backward bias condition. The two significant categories of hysteresis are typical and inert hysteresis, which can be found together or separately, depending on the applied pre-pole bias ^[81]. The possible factors causing the mechanism of the hysteresis can be the capacitive effect, trapping of charge carriers at the interface of the perovskite, ionic replacement, and ferroelectric polarisation ^[82]^[83]^[84]^[85]. Several reviews have proposed different hypotheses about these factors ^[86]. Moreover, the origin of these factors, intrinsic or extrinsic, is still unrevealed ^[87]^[88]. The solution to the hysteresis issue is essential for successfully developing PSC device characteristics, improving stability, and enhancing breakthrough advancement in associated applications ^[89]^[90]. Recently, some reports claimed to acquire both PCE and device stability with low I-V hysteresis during the fabrication process of the PSC device. However, I-V hysteresis is still under consideration to identify the origin and other related mechanisms governing the hysteresis mechanism during PSC characterisation ^[91]^[92]. Van Reenen et al. proposed theoretical studies to report the combined effect of ion replacement and trapping of charge carriers causing hysteresis in PSC devices. They observed that for the reduction in the hysteresis effect and enhanced performance of PSC, the ionic migration and grabbed site recombination should be limited ^[93].

Their work also suggested that high-density charges offer unfavourable biasing by allowing non-radiative trapped charge recombination, which results in less photocurrent and low PCE of the device. On the contrary, charge with low-density offers favourable biasing by allowing the amalgam of trapped free holes and electrons, leading to better performance of the PSC device. Kutes et al. studied the first observation of the presence of a ferroelectric domain (≈100 nm) in terms of the improved quality of β-CH₃NH₃PbI₃ thin film of perovskite. They observed that poly with DC bias reverse switching of the ferroelectric domain could occur ^[94].

This phenomenon was observed by Dualeh et al. and presented the occurrence of electron conduction occupying ionic migration ^[95]. The production of an acceptor or shallow donor for PbI²⁺, I⁻, and CH₃NH₃⁺(MA⁺) vacancies was also suggested ^[96]^[97]. Moreover, ionic disorder over electronic disorder in CH₃NH₃Pb₃ is analogous to cation and anion vacancies. Ion migration in PSC might cause a stoichiometry change at the vacant contacts leading to the more complex behaviour of the device. However, incorporating methylammonium and iodide ions can help provide doping regions at contacts to influence contact selection, photocurrent properties, and I-V hysteresis. Sainth et al. proposed that cell architecture is likely to influence the I-V characteristics of the cell dependence on selective contacts ^[14]. They also observed that increased hysteresis by slowing down the steady state was obtained at any applied bias condition without depending on history. They obtained better results of reasonable charge-selected contacts at the interface with forwarding bias conditions. However, the short circuit condition device showed poor performance because of the empty trap site caused by direct charge transfer to adjacent contacts. This state remained until the trap was occupied again.

The capacity analysis is another crucial parameter to detect the cause of underlying I-V hysteresis. These analyses address the kinetic of the charging process and the nature of the charge distribution and solar cell current distribution phenomena. The capacity characteristics of PSC devices have been reported in some reports ^[82]^[98]^[99]. Further, a CH₃NH₃PbI₃-based system was proposed to exhibit a significant dielectric constant and a high polarisation factor raised by illumination and extended voltage. Moreover, low-frequency capacitance slows the dynamic process in PSC-originating I-V hysteresis ^[100]. Various reports revealed that excess capacitance in PSC could be caused by electronic traps occupied in methylammonium iodide films following a given state density ^[101]^[102]. There are several other parameters on which I-V hysteresis depends, e.g., the scan rate, interface of material, illumination condition, biasing history, device structure, reversible light and voltage, and bias pre-conditioning.

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