As one of the most promising photovoltaic materials, the organic–inorganic metal halide perovskites have gained great attention worldwide during the past decade due to their outstanding optoelectronic properties. In particular, the perovskite solar cells (PSCs) have undergone great success and progress, achieving a certified cell efficiency of 25.7%. In this chapter, we comprehensively introduced the hybrid perovskite materials as well as perovskite-based optoelectronic devices. Fundamentals of the perovskite materials, like crystal structures, composition/bandgap tuning, optoelectronic properties, and film deposition methods, are summarized in the first part. The second part focuses on the revolution history, device architectures, state-of-the-art technologies, stability, and upscaling issues of PSCs. Some other important perovskite-based optoelectronic devices are briefly discussed in the following part. Finally, perspectives about future development of perovskite-based devices are proposed.

11.1 Organic–Inorganic Hybrid Perovskite Materials

As one of the emerging photovoltaic technologies, organic–inorganic hybrid metal halide perovskite solar cells (PSCs) have undergone great success and progress during the past decade. The hybrid perovskite materials have risen to stardom due to their extraordinary optoelectronic properties, such as high optical absorption coefficient, high carrier mobility, long free carrier diffusion length, low exciton binding energy, and easy solution processability. According to the best research-cell efficiency chart obtained from the Web site of National Renewable Energy Laboratory (NREL), [1] a certified champion power conversion efficiencies (PCE) of 25.7% have been obtained for PSCs. The high PCEs and low fabrication costs of the PSCs make it a promising candidate to replace silicon cells. In this chapter, we first summarize briefly the basic properties and fabrication approaches of the organic–inorganic hybrid perovskite materials. The second part mainly introduces the state-of-the-art organic–inorganic hybrid PSCs and some key challenges for commercialization. Some other important perovskite-based optoelectronic devices beyond PSCs are briefly discussed in the following part. Last, perspectives about future development of perovskite-based devices are proposed.

11.1.1 Crystal Structures

The organic–inorganic halide perovskite crystals share the same crystal structure with calcium titanium oxide (CaTiO3), which possesses the typical perovskite crystal structure, named after the Russian mineralogist Perovski (1792–1856). The general chemical formula of the hybrid perovskites is ABX3, where A represents monovalent inorganic or organic cations like cesium (Cs+), methylammonium (MA+), formamidinium (FA+), MA = CH3NH3+, and FA = CH(NH2)2+; B represents divalent metal cations like Pb2+ or Sn2+; and X represents halide anions like Cl−, Br−, or I−. In an ideal perovskite crystal structure (cubic), the body-centered B-site cation is coordinated with 6 nearest X-site anions, forming BX6 octahedra, while the A-site cations located at vertex positions filling the voids formed by BX6 octahedrons, as shown in Fig. 11.1. The crystal structure can be changed to lower symmetric phases (tetragonal, orthorhombic, hexagonal, etc.) through tuning the ionic radii or rotation/distortion of the BX6 octahedrons [2, 3]. The crystal structure formation and stability of perovskites can be speculated by the well-known Goldschmidt tolerance factor (t) [3].

$$ t = \frac{{R_{A} + R_{X} }}{{\sqrt 2 \left( {R_{B} + R_{X} } \right)}} $$
Fig. 11.1
A cubic crystal structure depicts perovskites with a chemical formula of A B X subscript 3, where A is located in the center of the crystal.

The ideal cubic crystal structure of typical organic–inorganic metal halide perovskites with chemical formula of ABX3

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where RA, RB, and RX represent the effective ionic radii for the mentioned ions at A, B, and X sites, respectively. Generally, a t value between 0.81 and 1.11 is found favorable for the formation of stable 3D organometal semiconducting perovskites [4]. The cubic structure in Fig. 11.1 is likely when t value lies between 0.89 and 1.0, while lower t values will give lower symmetric crystal structures [4]. Besides, phase transition happens with temperature increasing, and the high-temperature phase of the hybrid halide perovskites generally has cubic crystal structure. For the most frequently used perovskites, the room-temperature crystal symmetries of MAPbI3 and FAPbI3 are tetragonal and cubic, respectively.

11.1.2 Compositional Engineering and Bandgap Tuning

In general, organometal halide perovskites have direct-bandgap structure. The bandgaps of halide perovskites can be easily tuned by replacing A/B-site cations and X-site anions in the composition, covering the entire visible spectral range and part of the near-infrared region. At the early stage, the most widely used halide perovskite material for PSCs was MAPbI3, which has low crystallization temperature (~100 °C) and high efficiencies (>22%). However, its high-temperature (e.g., 85 °C) operating stability is not good enough, which is related with the low crystal formation energy [5]. Currently, most of the reported high efficiencies of PSCs are based on FAPbI3-dominated mixed perovskites, with incorporation like Cs, K, MA, and MDA cations and Br anion for partial substitution of FA and I in FAPbI3. Use of larger A-site cations has been proved to be useful for enhancing light absorption due to associated higher symmetry and smaller bandgap. Therefore, the bandgap of FAPbI3 perovskite (~1.45 eV) is slightly smaller than that of MAPbI3 (~1.55 eV), because of the larger ionic radius of FA cation than that of MA. The bandgap of FAPbI3 perovskite is closer to the “ideal” single-junction solar cell bandgap (1.1~1.4 eV), [6] demonstrating higher light current. As widely reported in the literature, by simultaneously incorporating serval cations (like FA, MA, MDA, Cs, etc.), the perovskite lattice strain can be relaxed and the resulting mixed-cation perovskites are generally thermally more stable and show higher PCEs with better reproducibility.

Regarding B-site metal cations, the two most widely studied ions are Pb2+ and Sn2+. Although Sn-based lead-free perovskites possess more ideal (smaller) bandgap than that of Pb-based perovskites, the performance of Sn-based devices lags far behind that of Pb-based PSCs, due to the inherent chemical instability of Sn2+ ions, which are prone to be oxidized into Sn4+ upon exposure in the air [7,8,9,10,11,12]. Recently, Sn–Pb mixed perovskites with narrow bandgaps (tunable from 1.2 eV to 1.3 eV) have enabled the construction of all-perovskite tandem solar cells, which are promising to exceed the PCEs of single-junction PSCs [13,14,15,16]. However, it is still challenging to prepare highly efficient and stable Sn–Pb mixed PSCs due to the easy oxidation of Sn2+ to Sn4+.

Besides, the bandgap of the hybrid perovskites can also be continuously tuned by using mixed halides (I, Br, Cl). Previous studies have shown that the bandgap of MAPbIxBr3-x perovskites can be tuned between 1.55 eV (MAPbI3) and 2.30 eV (MAPbBr3) by varying the halide composition between iodide (I) and bromide (Br), while FAPbIxBr3-x perovskites have tunable bandgap between 1.45 eV (FAPbI3) and 2.23 eV (FAPbBr3), thus enabling perovskite films with a variety of colors [17, 18]. The introduction of Br element also enhances the crystal stability of the perovskite films in ambient conditions. Similarly, the bandgap of MAPbClxBr3−x perovskites can be tuned between 2.30 eV (MAPbBr3) and 3.16 eV (MAPbCl3) by adjusting the composition ratio of Br and Cl [19].

In addition to the three-dimensional (3D) perovskites discussed above, the so-called two-dimensional (2D) metal halide perovskites (or layered perovskites), as an important category of perovskite materials, have attracted much attention of researchers for their desirable optoelectronic properties and improved stability compared to their 3D counterparts. The 2D layered perovskites have a general chemical formula of L2An−1BnX3n+1, where L is a long-chain organic cation (spacer), A is a small organic cation, B is a metal cation, and X is a halide anion [20,21,22]. The 2D perovskites possess a very unique architecture, with n layers of inorganic BX6 octahedral sheets sandwiched between two layers of organic chains. And their corresponding optoelectronic properties depend greatly on the value of n. Compared with their 3D counterparts, a wide variety of large organic cations (L) are suitable for the 2D crystal structure. Generally, the 2D perovskites are not a good choice as solar cell absorbers directly, because of their wide bandgaps and limited charge transport abilities of the 2D layers. Nevertheless, 2D perovskites are recently proved to be an excellent choice for 3D perovskite surface passivation.

11.1.3 Optoelectronic Properties

The outstanding optical and electronic properties of perovskite light absorbers are important fundamentals for their success and superior performance in photovoltaic and other optoelectronic applications. Being direct-bandgap semiconductors, the organic–inorganic hybrid perovskites have high light-absorption coefficients (~105 cm−1), [23, 24] long carrier diffusion lengths (micrometer scale),[25,26,27] long carrier lifetimes (microsecond scale),[28] low trap density (~1010 cm−3) [25, 29], and modest carrier mobilities (102~103 cm2V−1 s−1) [30] Owing to their high optical absorption coefficients, a perovskite film less than 1 μm-thick is generally sufficient to absorb almost all the sunlight, [23] which can greatly reduce the material costs. Moreover, the exciton binding energy (Eb) for the iodide-based perovskite materials commonly used in PSCs was measured to be only a few millielectronvolts at room temperature, [31,32,33] which means that no high-electric fields across the absorber are required for the generation of free charges upon light illumination. Therefore, these superior optoelectronic properties ensure that charge carriers can be freely transported across the entire perovskite film (<1 μm) before recombination. However, for the polycrystalline perovskite films, it is noteworthy that the surface recombination (non-radiative, trap-assisted recombination at grain boundaries and film surface) is a major mechanism for losses in PSCs in compassion with bulk recombination inside perovskite crystals or grains [34, 35].

11.1.4 Deposition Methods of Perovskite Thin Films

In most cases, polycrystalline organic–inorganic metal halide perovskite thin films work as the core part of the perovskite-based optoelectronic devices. The film surface coverage, morphology, uniformity, crystallinity, etc., are closely related to the optoelectronic properties of the perovskite films. Therefore, depositing high-quality perovskite thin films is crucial for fabricating high-performance perovskite-based electronic devices. The most widely used lab-scale deposition methods of perovskite thin films generally include one-step route, two-step route, and vapor deposition route, as shown in Fig. 11.2. Among them, both one-step route and two-step route are solution-based processes (e.g., spin-coating), which are simple and less costly compared to the vapor deposition route.

Fig. 11.2
Three illustrations depict the following methods, one-step route, two-step route, and vapor deposition.

Schematic deposition methods of perovskite thin films

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  1. (1)

    One-step Route

In the one-step route, stoichiometric metal halides (PbI2, PbBr2, SnI2, etc.) and organic halides (MAI, FAI, etc.) are firstly dissolved in organic solvents like dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), γ-butyrolactone (GBL), and N-methyl pyrrolidone (NMP), and then, the precursor solution is spin-coated on substrates directly, followed by thermal annealing on hotplates for complete crystallization of the perovskite films (see Fig. 11.2a). However, since all these solvents have high viscosities and high boiling points (150 ~ 200 ºC), the slow evaporation of the solvents limits the perovskite nucleation rate, which consequently leads to low coverage (with pinholes) and high surface roughness of the prepared perovskite films [36]. In order to address this problem, an anti-solvent-assisted one-step route has been widely adopted by researchers, i.e., dripping volatile anti-solvents (such as chlorobenzene, toluene, diethyl ether, ethyl acetate) on the spinning substrate during the one-step perovskite spin-coating process. These anti-solvents cannot dissolve the solute but are miscible with the perovskite solvents (DMF, DMSO, etc.). Therefore, the dripped anti-solvents can extract perovskite solvents quickly, reduce the solute solubility, and create supersaturation conditions, leading to rapid precipitation or crystallization of perovskite films. Additionally, the anti-solvents can also wash away part of the perovskite solvents since they are miscible with each other, which can further promote the rapid crystallization process. Consequently, the anti-solvent-assisted method increases the nucleus density during the film formation process, so that smooth, uniform, and pinhole-free perovskite films can be produced [37, 38]. Notably, the anti-solvent volume, dripping time, duration, flow speed, etc., are important influence factors of perovskite film quality.

In order to further improve the film quality, a solvent-engineering strategy (using mixed perovskite solvents, e.g., DMF/DMSO or GBL/DMSO) has been successfully used. In terms of MAPbI3 perovskite, a MAI-PbI2-DMSO intermediate phase is formed during the anti-solvent-assisted spin-coating process due to the strong interaction between and DMSO [39]. The presence of the intermediate phase can retard the rapid reaction between MAI and PbI2 during the spin-coating process, which leads to the formation of the uniform and smooth perovskite layer with comparatively larger grain size than that without DMSO. Besides, for FAPbI3-based perovskites, using methylammonium chloride (MACl) as additive in perovskite solution has been demonstrated an effective strategy to dramatically increase the grain size and crystallinity of perovskite films prepared through the above-discussed anti-solvent-assisted one-step route [40,41,42]. It is noteworthy that the addition of MACl can also enlarge the time window for dripping anti-solvents, which ensures good device reproducibility. In addition to the anti-solvent-based method, some other approaches have also been studied for one-step fabrication of perovskite films, such as hot-casting, [43] vacuum pumping[44, 45], and gas quenching techniques, [46] which have their own pros and cons.

  1. (2)

    Two-step Route

For the two-step sequential deposition route in early stages, a lead halide (like PbI2) layer is first spin-coated on substrate, followed with immersing in organic halide salts (like MAI) solution (dissolved in 2-propanol) and subsequent thermal annealing for the conversion to perovskite phase [47, 48]. Later, an improved two-step route was developed, i.e., sequential spin-coating of lead halide and organic salt bilayer film, followed by a subsequent thermal interdiffusion-driven process for the reactions between the lead halide and organic salts and complete crystallization (see Fig. 11.2b). However, one of the most common problems for the two-step route is the incomplete conversion of PbI2, with some residual insulating PbI2 material at the substrate/perovskite interface, which will definitely hinder the charge transport and increase the device sheet resistance. Regarding to this puzzle, Sang Il Seok et al. exploited an intramolecular exchange process (IEP) approach, wherein the PbI2(DMSO) film (instead of PbI2 film) was first deposited, followed by the coating of FAI and subsequent thermal annealing [49]. During annealing, DMSO was exchanged with FAI, consequently forming highly oriented FAPbI3 films with large grains and flat surfaces without residual PbI2. The resultant cells get a certified efficiency of 20.2%. Furthermore, mixed organic salts dissolved in 2-propanol, e.g., FAI:MAI:MACl or FAI:MABr:MACl, are often used for the fabrication of mixed perovskite films through two-step route [50, 51].

  1. (3)

    Thermal Vapor Deposition Route

The thermal vapor deposition of perovskite films involves the simultaneous or alternate thermal evaporation of the solid metal halides (e.g., PbI2, PbBr2) and the organic halide salts powder (e.g., MAI, FAI, MABr) onto a rotating substrate by using a dual-source evaporation system (see Fig. 11.2c). In 2013, Henry J. Snaith et al. first adopted vapor-deposited CH3NH3PbI3 −xClx perovskite film as the absorbing layer, with which the first planar heterojunction PSC was prepared successfully and a PCE up to 15.4% was achieved [52]. The precise controlling of the deposition rates, substrate temperature, and the molar ratio of the two precursor materials are crucial for the preparation of high-quality vapor-deposited perovskite films. Compared with perovskite films prepared through solution processes, the vapor-deposited ones were extremely flat and uniform without pinholes or voids; thus, the vapor deposition route is applicable to the large-area perovskite deposition and perovskite-based tandem solar cell applications. However, this vapor deposition method is the least frequently used one these days, because the perovskite deposition conditions are complicated and expensive high-vacuum equipment is required [53].

11.2 Perovskite Solar Cells (PSCs)

11.2.1 Evolution of PSCs

Referring to the best research-cell efficiencies chart downloaded from the Web site of National Renewable Energy Laboratory (NREL), [1] it is noteworthy that the efficiency growth rate of PSCs is faster than that of other PV technologies. Some of the important landmark efficiencies of PSCs are summarized in Fig. 11.3.

Fig. 11.3
A line graph depicts the P C E in percentage versus years. It exhibits an increasing trend from 3.8 percent for Toin U to 25.7 percent for U N I S T.

The efficiency roadmap of perovskite solar cells

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In 2009, Tsutomu Miyasaka and coworkers reported the photovoltaic application of metal halide perovskites for the first time, by depositing MAPbBr3 and MAPbI3 nanocrystalline particles on the ~10 μm-thick mesoporous TiO2 film as visible-light sensitizers in the dye-sensitized solar cells (DSSCs) configuration with liquid hole-conducting electrolyte [54]. A champion efficiency of only 3.8% was obtained for MAPbI3-sensitized cells at that time, while the cells were quite unstable, owing to the usage of the liquid hole-conducting electrolytes. Two years later, Nam-Gyu Park and colleagues prepared DSSCs with similar device structures based on MAPbI3-quantum-dot-sensitized TiO2 film (3.6 μm-thick), and they realized a cell efficiency of 6.5% in 2011 [55]. Similarly, the perovskite quantum-dot-sensitized solar cells possess very short lifetime (~10 min) due to the easy dissolution of the perovskite nanocrystals in the liquid electrolyte. This stimulated the replacement of the liquid electrolyte with solid-state hole transport materials (HTMs). Later in 2012, Nam-Gyu Park and Michael Gratzel et al. introduced a solid-state spiro-OMeTAD (2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene) hole conductor and successfully fabricated the first all-solid-state perovskite sensitized mesoscopic solar cell with dramatically improved device efficiency (9.7%) and shelf stability [56]. Herein, MAPbI3 nanoparticles were deposited onto a submicron-thick mesoporous TiO2 films (600 nm-thick), whose pores were infiltrated with spiro-OMeTAD solution, leaving only solid spiro-OMeTAD after solvent evaporation. Almost simultaneously in the same year, Henry J. Snaith et al. also reported success with spiro-OMeTAD hole conductor [36]. They replaced the TiO2 mesoporous layer and MAPbI3 absorber with an insulating Al2O3 scaffold and an iodide-chloride mixed-halide perovskite (MAPbI2Cl), respectively. As a result, the newly developed hybrid solar cell, termed as “meso-superstructured solar cell” (MSSC), delivered a higher PCE of 10.9% with increased photovoltage. This mixed-halide perovskite material was reported to have remarkably better stability and carrier transport property than its pure iodide equivalent. Notably, only perovskite nanoparticles or quantum dots, instead of perovskite thin films or capping layer, were employed up to that time. Therefore, those cells, essentially, still belong to the category of sensitized solar cells.

In 2013, Sang Il Seok and Michael Gratzel et al. reported a PCE up to 12% by using a polymeric hole conductor (poly-triarylamine, denoted as PTAA) in the device structure of mp-TiO2(600 nm-thick)/MAPbI3/PTAA/Au [57]. Specifically, a solid perovskite capping layer was found on top of the mp-TiO2/MAPbI3 composites. By using similar device structures (with mp-TiO2 and PTAA) and MAPb(I1-xBrx)3 perovskites, Seok’s group further improved the device efficiency to 12.3% (when x = 0.2) with greatly improved stability. Later in 2013, Gratzel’s group reported a sequential deposition route (i.e., the two-step method), permitting much better control over the perovskite formation [47]. PbI2 solution is first infiltrated into a mp-TiO2 scaffold film and subsequently transformed into the MAPbI3 perovskite after exposing in MAI solution. The resultant mesoscopic solar cells got a PCE of 15.0% (certified efficiency of 14.1%). This two-step method has been widely adopted by researchers ever after for its better reproducibility. Subsequently, Snaith’s group reported similar efficiency (15.4%, without certification) by employing a compact vapor-deposited MAPbI3-xClx perovskite film and a simple planar heterojunction thin-film device architecture (FTO/compact-TiO2/perovskite/spiro-OMeTAD/Ag) [52]. It is noteworthy that the vapor-deposited perovskite films were extremely uniform and flat in comparison with the solution-processed ones. Moreover, the innovative planar device structure (without the mesoporous scaffold) developed by Snaith et al. exerted a profound effect on the subsequent evolution of PSC structures, although this device structure at that time tends to have significant I-V hysteresis phenomenon.

Soon afterward in 2014, Seok’s group reported a certified PCE of 16.2% (certified in 2013) without I-V hysteresis by using a bilayer solar cell architecture: FTO/compact-TiO2/mp-TiO2 + perovskite(200 nm-thick)/perovskite capping layer/PTAA/Au [39]. Extremely uniform and dense mixed-halide MAPbI3−xBrx perovskite films were prepared by using a solvent-engineering technique (GBL/DMSO mixed solvents) and anti-solvent quenching approach, which became very popular in the PSC community afterward. Subsequently in early 2015, a higher certified efficiency of 17.9% (certified in early 2014) was reported also by Seok’s group by using the (FAPbI3)1−x(MAPbBr3)x mixed perovskites [58]. The incorporation of MAPbBr3 was found capable of stabilizing the FAPbI3 perovskite phase for efficient and stable PSCs.

Later in 2015, the first confirmed efficiency of over 20% (20.1%, certified in late 2014) was reported by Seok’s group [49]. They prepared high-quality FAPbI3-based perovskite films through the IEP process (as discussed in 11.1.4). Then in early 2016, Gratzel’s group reported a certified efficiency ~21% (certified in late 2015), enabled by cesium (Cs)-containing triple cation perovskite compositions, i.e., Csx(FA0.83MA0.17)1−xPb(I0.83Br0.17)3 [59]. The device structure used is a stack of FTO/compact-TiO2/Li-doped mp-TiO2/perovskite/spiro-OMeTAD/Au. Subsequently in 2017, Seok et al. reported an iodide management strategy based on the previously adopted IEP process and achieved a certified PCE of 22.1% (certified in early 2016), which was further improved to 22.7% (certified in mid-2017) [60]. The introduction of additional iodide ions into the FAI (MABr) solution decreases the concentration of deep-level defects.

Further progress was reported by Jingbi You’s group from China in 2019 [51]. They used an organic salt phenethylammonium iodide (PEAI) for the surface passivation of FA1−xMAxPbI3 perovskites (prepared by two-step route) to suppress the surface defects. As a result, certified efficiencies of 23.3% and 23.7% (both certified in 2018) were obtained, respectively. Also in 2019, Seok’s team got the same certified efficiency of 23.7% by stabilizing α-FAPbI3 perovskite with the incorporation of methylenediammonium dichloride (MDACl2, 3.8 mol%) [41]. Later, an increased efficiency of 24.4% (certified in early 2019) was reported in 2020 by the same group through simultaneous substitution of Cs and MDA in FAPbI3 perovskite for lattice strain relaxation [42].

Strikingly, a few efficiencies exceeding the 25% mark were obtained independently by several different research groups from 2019 to 2021. Moungi G. Bawendi1 and Jangwon Seo et al. reported a certified PCE of 25.2% by using a chemical-bath-deposited SnO2 electron transport layer, [61] while Jin Young Kim and colleges achieved an equal certified efficiency by pseudo-halide anion engineering of FAPbI3 perovskite using formate (HCOO−), both of which were certified in 2019 [62]. Dong Suk Kim et al. adopted polyacrylic acid-stabilized SnO2 quantum dots on the compact-TiO2 (QD-SnO2@c-TiO2) as electron transport layer, which enabled a certified efficiency of 25.4% [63]. Soek’s group obtained a certified PCE of 25.5% in mid-2020 by coupling Cl-bonded SnO2 with a Cl-containing perovskite precursor [64]. Besides, the very recent new champion efficiency of 25.7% (certified in late 2021) [1] is likely still from Seok’s group. Therefore, the SnO2-based planar PSCs seem to have great potential for achieving new record efficiencies.

11.2.2 Device Architectures

PSCs are usually composed of perovskite light absorbers, charge transport layers, and electrodes. Currently, the most often adopted device architectures for PSCs can be categorized into three types: [65] mesoporous n-i-p structure, planar n-i-p structure, and planar p-i-n structure, as shown in Fig. 11.4. Among them, n-i-p and p-i-n architectures correspond to normal structure and inverted structure, respectively. Herein, n and p denote n-type (e.g., TiO2, ZnO, SnO2, PCBM) and p-type (e.g., spiro-OMeTAD, PTAA, PEDOT:PSS, NiOX, CuSCN) semiconducting material films, [66] working as electron transport layers (ETLs) and hole transport layers (HTLs), respectively, while I represent the perovskite light-absorbing layer. In principle, charge transport layers should have good energy level alignment with perovskite layer and electrodes, for efficient charge transport and collection.

Fig. 11.4
Three illustrations, a, b, and c, depict the layers of mesoporous normal, planar normal, and planar inverted, respectively.

Schematic illustrations of the most commonly used device architectures of PSCs. Reprinted with permission from Ref. [78]

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The mesoporous n-i-p structure first evolved from the dye-sensitized solar cells (DSSCs) configuration with a thick mesoporous metal-oxide scaffold layer (like mp-TiO2) [54]. The mesoporous layer allows the infiltration of perovskite precursor solution and the formation of mp-TiO2/perovskite intermixed layer. In the very early stage, the mesoporous TiO2 layer (mp-TiO2) usually has a thickness of more than one micrometer for sufficient light absorption, which however impedes the cell efficiency because the perovskite grains are confined by the dimensions of the pores and cannot grow into larger ones. Currently, mesoporous layer with thickness less than 300 nm is often utilized for the mesoporous n-i-p devices, and a dense and uniform perovskite capping layer with large perovskite grains will form on top of the mesoporous layer (see Fig. 11.4a). PSCs with device structure of FTO/compact-TiO2/mp-TiO2/perovskite/spiro-OMeTAD/Au now can achieve certified efficiencies higher than 25% [62, 67]. However, the high-temperature sintering process of the mesoporous layer hampers the development of PSCs since it is unfavorable for scaling.

In planar structures, the mesoporous layer is omitted; therefore, there is no mesoporous/perovskite intermixed layer in the device structure, which not only simplifies the device preparation processes, but also allows for the low-temperature preparation. In the planar n-i-p architecture, the perovskite layer is directly deposited on the thin ETL (~30 nm), followed by the coating of HTL and top electrodes (see Fig. 11.4b). To date, the best n-i-p PSCs normally employ SnO2 as ETL, with device structure of FTO/SnO2/perovskite/spiro-OMeTAD/Au, and can attain similar or even slightly higher efficiencies (>25%) than that of mesoporous devices [61, 63, 64]. Notably, the I-V hysteresis is negligible for well-prepared planar PSCs based on SnO2 ETL. In contrast with the planar n-i-p structure, in the planar p-i-n structure, the perovskite layer is deposited on top of ultrathin HTL (mostly PTAA or NiOX), as shown in Fig. 11.4c. High-performance planar p-i-n PSCs generally possess device structure of ITO/PTAA/perovskite/PCBM/BCP/Ag, and the champion cell efficiencies are ~24% [68,69,70,71,72,73,74,75,76]. In addition, the simple planar device structure enables the easy preparation of semitransparent (using transparent electrodes like graphene) and flexible PSCs (on flexible substrates) through low-temperature solution processes [77,78,79].

11.2.3 State-Of-The-Art PSC Technologies

To date, the champion cell efficiencies of single-junction PSCs (25.7%) have crossed the 25% threshold, approaching the highest certified efficiencies of single-crystal silicon solar cells (26.1%) and silicon heterostructure cells (26.7%) [1]. Currently, the state-of-the-art PSCs are usually based on organic–inorganic lead halide PSCs with normal device architectures, either mesoporous n-i-p structure (with mp-TiO2 layer) or planar n-i-p structure (SnO2 as ETL).

In terms of the perovskite active layers for the state-of-the-art PSCs, FAPbI3-dominated mixed perovskites are generally prepared through the above-mentioned anti-solvent-assisted one-step route, with DMSO incorporation for Lewis acid–base adduct formation. Notably, methylammonium chloride (MACl) is commonly utilized as additive in perovskite precursor solution to help mediate the crystallization process of perovskites [41, 42, 61, 62, 64, 80]. The MACl additive is shown to induce the formation of intermediate phase with pure α-phase FAPbI3 perovskite films without annealing [40, 81,82,83]. Then, the intermediates disappear quickly and transform into perovskite phase during later thermal annealing process, because of the volatile property of MACl. Notably, almost no MACl exists in the annealed perovskite films. By tuning the incorporation amount of MACl (optimized value ~35 mol%), high-quality and stable perovskite film with enlarged grain size, enhanced crystallinity, and increased carrier lifetime can be obtained.

As discussed above, photoinactive δ-phase FAPbI3 (yellow phase) is prone to form during the film deposition process; meanwhile, the α-phase FAPbI3 (black phase) film is intrinsically unstable and can spontaneously transform into the undesirable δ-phase, which will both greatly deteriorate the performance of PSCs [84, 85]. In order to fabricate and also stabilize phase-pure α-phase FAPbI3 (black phase) films, a series of compositional engineering strategies have been developed, such as the incorporation of MAPbBr3 and Cs+, i.e., (FAMA)Pb(IBr)3 or (FAMACs)Pb(IBr)3,[58, 59] whereas the introduction of MA, bromine, and caesium components (normally < 15 mol%) will widen the mixed perovskite bandgap, which is unfavorable for the light absorption and photocurrent generation. To maximize the light-absorption region, Sang Il Seok et al. used the inherent bandgap of α-phase FAPbI3 (~1.45 eV) to increase the photocurrent of PSCs [41]. They stabilized the α-FAPbI3 phase by doping with only 3.8 mol% methylenediammonium dichloride (MDACl2) while inducing only a slight change in the perovskite bandgap; therefore, they achieved certified short-circuit current densities of over 26 mA/cm2 and a certified efficiency of 23.7%. Further optimization of the FAPbI3-MDA strategy by Cs-MDA codoping, as well as the replacement of compact-TiO2/mp-TiO2 double layer with atomically coherent SnO2 thin film, contributes to the much higher certified efficiencies of 24.43% and 25.5%, respectively [42, 64]. Except for the above cation-doping strategies, Jin Young Kim et al. also introduced an anion engineering concept using pseudo-halide anion formate (HCOO − , 2 mol% addition) to suppress anion-vacancy defects located at perovskite film surface and grain boundaries, with enhanced film crystallinity [62]. The resulting PSCs attain a certified PCE of 25.2% and maintain excellent operational stability.

In addition to the stabilizing of perovskite absorbers, interfacial engineering at perovskite/HTL and perovskite/ETL interfaces is also of crucial importance for achieving highly efficient and stable PSCs. As is known, the interfaces between the perovskite and charge transport layers contain much higher concentrations of defects than that within the perovskite layer, specifically deep-level defects, which seriously hamper the device efficiencies [29, 86, 87]. Currently, the most popular perovskite surface (perovskite/HTL interface) passivation materials are alkylammonium halides bearing long alkyl chains, such as phenethylammonium (PEA+), ethylammonium (EA+), and n-butylammonium (BA+), which improve the device efficiencies and stabilities by forming 2D perovskite capping layers [28, 42, 51, 61, 62, 64, 88, 89]. The so-called 2D/3D passivation strategy nowadays has been widely used for perovskite surface passivation. However, it is difficult to effectively passivate the underlying perovskite/ETL interface, because the passivation materials deposited on ETL may be dissolved by perovskite solution during perovskite film spin-coating process. In view of this issue, Sang Il Seok et al. reported the formation of an ultrathin atomically coherent interlayer at the SnO2/perovskite interface, achieved by coupling Cl-bonded SnO2 with a Cl-containing perovskite precursor [64]. This interlayer leads to the decreased interfacial defects and enhanced charge extraction and transport from the perovskite layer, resulting in a certified high efficiency of 25.5% under standard light illumination. The Cl-bonded SnO2 films used here were deposited by using either spin-coating or chemical bath deposition methods, from either SnCl2⋅2H2O solution in ethanol or SnCl4 solution in deionized water.

11.2.4 Stability of PSCs

Although PSCs have lots of advantages in competition with silicon and other thin-film solar cells, poor long-term operating stability of PSCs remains a main issue that hinders their commercialization. Generally, there are intrinsic and extrinsic factors that influence the device degradation processes of PSCs. The intrinsic device instabilities of PSCs are closely correlated with the intrinsic crystallographic and chemical instability of the perovskite absorbers, as well as other negative effects like ion migration, electrode-perovskite reactions, residual strains, and the instable nature of organic charge transport layers [90,91,92,93,94]. On the other hand, various extrinsic environmental factors, like UV-light, moisture, oxygen, thermal stress, etc., will accelerate the decomposition of ABX3 crystal into its constituents and additional by-products, which tend to magnify the intrinsic instabilities of perovskites and further aggravate the perovskite and device degradation processes [8, 95, 96].

At present, the majority of the PSC stability studies are focusing on FAPbI3-based devices. While FAPbI3 has better thermal stability than MAPbI3 perovskite, the black FAPbI3 perovskite phase (α-phase) is not stable at room temperature, due to the easy formation of the undesirable photoinactive δ-phase (yellow phase). Therefore, many recent researches have been conducted with regard to the stabilizing of the FAPbI3 perovskite phase, and outstanding progress has been made. Interestingly, many above-mentioned strategies for achieving high efficiencies (>24%), e.g., compositional engineering, additive engineering, 2D/3D interfacial engineering, and charge transport layer design, in most cases, can also help achieve and maintain α-FAPbI3 phase in PSCs [40, 62, 64]. For example, based on the principle of tolerance factor adjustment, the incorporation of inorganic cations (Cs+, Rb+) with smaller ionic radius to A-site can suppress phase transition processes to yellow phase and improves crystal stability [59, 97]. Moreover, 2D perovskite surface passivation layer has been proved to be quite effective for stabilizing the underlying 3D perovskites, because 2D perovskites exhibit better humidity stability compared with 3D perovskites, and enhanced activation energy barrier for phase transition (black phase to yellow phase) [98]. The hydrophobic alkane chain in 2D perovskites can also help suppress the influence of humidity on 3D perovskites. Recently, Wei Huang and coworkers reported the synthesis of stable α-FAPbI3 in ambient air, irrespective of atmosphere humidity (20~90%) and temperature (25~100 °C), based on vertically aligned PbI2 thin films grown from an ionic liquid methylamine formate (MAFa), instead of using the commonly used solvents like DMF and DMSO [99]. The vertically grown PbI2 film has numerous nanometer-scale ion channels, which facilitate the diffusion of FAI into the PbI2 thin films for fast and robust transformation to α-FAPbI3. PCEs of over 24% were achieved for the PSCs prepared in ambient air. The unencapsulated cells can retain 80% and 90% of their initial efficiencies after operation at maximum power point for 500 h at 85 °C and continuous light stress, respectively.

11.2.5 Upscaling of PSCs

At present, the efficiencies of large-area PSCs (>1 cm2) still lag behind that of small-sized devices (mostly around 0.1 cm2). The development of large-area PSCs and modules is essential to their mass production and commercialization; thus, more and more researchers and companies have been devoted to the upscaling of PSCs over the past decade. The main challenge for the manufacture of high-performance large-scale PSCs is the scalable fabrication of high-quality large-area perovskite films with uniform and dense surfaces. Here for large-area films, the laboratory-scale spin-coating method is not applicable any more, due to the poor reproducibility and the non-uniformity of films from center to edge, while some other scalable deposition approaches are utilized instead by researchers, such as blade (bar) coating, slot-die coating, inkjet printing, spray coating, screen printing, vacuum deposition methods, and roll-to-roll printing (for flexible substrates) [100,101,102,103,104]. To date, many notable results on large-area PSCs have been achieved. For example, a certified stabilized efficiency of 18.6% with an aperture area of ~30 cm2 (corresponding to an active area efficiency of 20.2%) has been obtained for minimodules with blade-coated perovskite layer, [101] and a PCE of 17.9% has been achieved by inkjet printing for perovskite module with area ~800 cm2, according to the Champion Photovoltaic Module Efficiency Chart plotted by National Renewable Energy Laboratory (NREL) [105]. The increasing module efficiencies indicate large potential for practical use in the future. However, the perovskite module stability is also a main challenge in for large-scale manufacturing and applications [106].

When it comes to commercialized perovskite solar modules, it is necessary to pattern the ITO or FTO electrodes and divide the full-sized bottom electrodes into multiple subcells, because of the relatively low sheet resistances of ITO or FTO electrodes (7~15 Ω/sq). Two main architectures are normally employed, i.e., series-connected and parallel-connected architectures. To realize interconnections of such architectures, laser-scribing techniques (known as P1, P2, and P3 scribing processes) are generally used in industrial production [107]. The P1, P2, and P3 scribing steps are conducted for the patterning of bottom electrodes, charge transport layers/active layer, and top electrodes, respectively. Additionally, the deposition of metal grid on ITO or FTO electrodes is often employed as an strategy to increase the charge collection on the bottom electrodes [108].

11.3 Other Perovskite-Based Devices

The organic–inorganic hybrid metal halide perovskite materials, which have led to unprecedented success in photovoltaic devices, have also been proved to be promising candidates for many other optoelectronic applications, such as light-emitting diodes, lasers, photodetectors, sensors, photocatalytic devices, thermoelectric devices, and memory devices. These perovskite-based devices normally inherit the typical device architectures and working principles of the organic devices discussed in previous chapters, while demonstrating many special advantages due to the extraordinary optoelectronic features and low-cost fabrication of the metal halide perovskites. In this part, several representing types of perovskite-based devices will be introduced briefly.

11.3.1 Perovskite Light-Emitting Diodes and Lasers

Apart from the great advances of PSCs, the perovskite light-emitting diodes (PeLEDs) with perovskite emitting layers have become another hot topic and witnessed great progress in recent years. The effective light emission from the hybrid perovskites originates from their remarkable characteristics, including: (i) high photoluminescence quantum yield (PLQY), (ii) narrow emission linewidth, (iii) wide bandgap tunability through composition engineering, (iv) low defect density, (v) low non-radiative recombination, (vi) easy solution processability, and (vii) negligible Stokes shifts. Light emissions of various color (from visible to near-infrared region) with high brightness can be obtained due to the wide-range tunable bandgaps of the hybrid perovskites, which may help surmount some of the shortcomings of both traditional LEDs (suboptimal color quality) and OLEDs (low maximum brightness and color quality). The demonstration of effective perovskite electroluminescence offers scope for developing efficient and color-tunable light emitters for low-cost lighting, display, and other optical applications.

The electroluminescence performance of PeLEDs can be quantified via the external quantum efficiency (EQE), that is, the ratio between emitted photons to the electrons flowing in the circuit. In general, to maximize the EQE of the PeLEDs, important factors like leakage current, non-radiative recombination, charge balance, and photon recycling should be carefully considered [109, 110]. Highly efficient PeLEDs will result from the combination of an optimized perovskite emitter with suppressed non-radiative recombination, as well as the device architecture designs favoring the radiative charge recombination in the perovskite layer and the extraction of the emitted photons. Generally, halide perovskites with different dimensionalities, including 3D (bulk, nanocrystals, and quantum dots) and quasi-2D perovskites, are utilized as the perovskite emitting layer. In contrast to 3D perovskites with very low exciton binding energies (a few meV), the quasi-2D layered perovskites possess very large exciton binding energies (several hundreds of meV) due to enhanced electron–hole interactions originating from their special natural quantum-well structures, where the inorganic layers act as “wells” and the organic molecules act as “barriers.”

In 2014, Richard H. Friend et al. reported the first room-temperature operating PeLED based on solution-processed organometal halide perovskites [111]. They prepared infrared (MAPbI3−xClx) and green (MAPbBr3) light-emitting devices, with very low EQEs of 0.76% and 0.1%, respectively. Since then, the device performances have experienced rapid progressing. So far, the EQEs of PeLEDs have surpassed 20% for green, red, and near-infrared (NIR) emission and more than 10% for blue and white emission [112,113,114,115,116].

Along with the rapid performance enhancements for PeLEDs, extensive efforts are focusing on resolving their instable issue, which is mainly caused by chemical reactions and ion migration in the perovskite emitting layers and at interfaces [117]. The device stability of PeLEDs mainly suffers from moisture, temperature, photodegradation, and bias-induced degradation. To date, the champion operating half-life (T50) of PeLEDs is 2400 h, [118] while most of the reported T50 values are less than 100 h under operation, which still lag far behind that of the state-of-the-art inorganic LEDs and OLEDs (up to a million hours). More studies on device optimization strategies and degradation mechanisms are required before achieving industry-ready operational stability.

For luminescent applications beyond LEDs, the metal halide perovskites have also shown great potential as gain media for solution-processed laser diodes, in consideration of their excellent carrier mobilities, low non-radiative recombination, sharp absorption onsets, and large gain cross section at the emission wavelength [119]. In fact, optically pumped lasers with high-quality perovskite gain media have already been demonstrated successfully through Fabry–Perot cavities or whispering mode cavities [120,121,122]. However, to date, the realization of electrically driven perovskite lasers remains a great challenge, mainly because of the low stability of the metal halide perovskites, especially at the lasing working conditions with high temperatures induced at the lasing regime.

11.3.2 Perovskite Photodetectors

In terms of light detection, high-performance perovskite photodetectors (PePDs) could be obtained by using lead halide perovskites in device forms of photodiodes, photoconductors, and phototransistors. Recently, PePDs have gained increasing attention and research interests due to their fast response speeds, high specific detectivities, high responsivities, wide response wavelength range, low noise, and controllable wavelength-dependent response [123, 124]. Compared with traditional silicon-based photodetectors, the hybrid perovskite films with thickness of only a few-hundred nanometers in PePDs can absorb a much greater number of photogenerated charge carriers per unit of thickness, leading to improved EQEs and low response time. The detection wavelengths of PePDs cover a wide range of the electromagnetic spectrum from visible to near-infrared and even to X-ray band. Therefore, PeLEDs have many promising applications, including imaging, communication, medical sensors, automatic control, etc.

Recently, hybrid PePDs based on perovskite/2D material heterojunctions have attracted increasing attention [125]. Herein, the perovskite films with large absorption coefficients are mainly responsible for light harvesting, while high-mobility ultrathin 2D materials work as charge transport layer/channel. This heterojunction structure increases the device performance greatly, with ultrafast interfacial charge transfer (based on the photogating effect) and high responsivity along with fast response. A variety of 2D materials, such as graphene or its derivatives, transition-metal dichalcogenides, transition-metal nitrides, black phosphorous, and MXene, have also been successfully integrated with perovskites for PePDs applications.

11.3.3 Perovskite X-ray Detectors

As a special branch of photodetectors, X-ray detectors and imagers are widely applied in our daily life, such as being used for medical diagnostics, security check, and non-destructive product inspection. Generally, highly sensitive and low-dose imaging is desired to reduce the side effects and risks caused by X-ray irradiation. The metal halide perovskites naturally with high Z atoms (e.g., Cs, Pb) could attenuate X-ray effectively. Perovskite X-ray detectors with lower fabrication cost have showed much higher sensitivity and lower detectable X-ray dose than the commercial α-Se X-ray imagers in recent reports [126,127,128,129]. Moreover, the low-temperature solution growth of large perovskite single crystals offers the opportunity of fabricating large-area single-crystal X-ray imagers, which could suppress the ghosting or image lags with the absence of grain boundaries. These superior properties establish the potential of perovskite X-ray detectors to be the next-generation X-ray imagers. However, there is still lots of work that needs to be done before the commercialization of perovskite X-ray imagers. Except for the stability issue, some commercially concerned parameters, such as detective quantum efficiency (DQE), image lag, and ghosting, should be further optimized for better comparison with the commercial detectors.

In addition to the above-discussed applications, the organometal halide perovskites have also been employed for a variety of other optoelectronic applications, such as photocatalytic, thermoelectric, sensing, and memory devices. When it comes to the photocatalysis research and applications, the metal halide perovskites have been employed for solar water splitting, hydrogen generation, organic contaminants degradation, etc. For sensor applications, the hybrid perovskite materials have been used for a variety of analytes, such as gases, solvents, metal ions, organic compounds, humidity, and temperature. However, the perovskite stability issue needs to be further addressed to achieve high-performance optoelectronic devices.

11.4 Summary and Perspectives

As one of the most promising photovoltaic materials, the organic–inorganic metal halide perovskites have gained great attention worldwide during the past decade, showing promising potential in many kinds of optoelectronic devices. In particular, the perovskite photovoltaic devices (PSCs) have undergone great success and progress. The hybrid perovskite materials have risen to stardom due to their extraordinary optoelectronic properties, such as high optical absorption coefficient, high carrier mobility, long free carrier diffusion length, low exciton binding energy, and easy solution processability. To date, the highest certified cell efficiency (25.7%) of PSCs is already very close to 26%. And the reported PCEs of perovskite/silicon tandem solar cells have reached 29.8% [1]. The excellent device performance and low fabrication costs of the PSC technology make it a promising candidate to compete with silicon-based solar panels. However, some issues like long-term stability, lead toxicity, and upscaling fabrication are critical challenges to face for PSCs. Due to the excellent defect tolerance of perovskite, its applications expand not only to photoelectric conversion but also to LEDs, photodetectors, etc.

In order to achieve highly efficient PSCs with long-term operational stability, it is essential to have high-quality perovskite films with smooth surface, full coverage (pinhole-free), high crystallinity (large grain size), stable photoactive crystal phase, low defect density on surface and at grain boundaries, and good heterojunction contacts. Therefore, improving the crystal quality of perovskite films, defect passivation and suppression of non-radiative recombination at interfaces (interface engineering), and rational selection of charge transport layers are expected to further improve the device efficiency. Notably, for the widely adopted FAPbI3-based mixed perovskites, the formation of photoactive yellow phase, as well as phase segregation phenomenon, often occurs during the film fabrication process, which has not been well solved to date.

Currently, progress in crystal growth, compositional engineering, solvent engineering, intermediate phase engineering, and film deposition approaches have made it possible to prepare organometal halide perovskite thin films with minimized bulk trap density, such that defects are predominantly located at the interfaces and grain boundaries. This has motivated the development of interfacial defect passivation. The perovskite top and bottom interface engineering through surface treatment is often adopted in the literature for defect passivation. For example, some organic halide salts are widely used to modify the perovskite surface and form an interfacial 2D perovskite layer (2D/3D heterostructure), which can dramatically stabilize the perovskite surface. However, a recent study shows that surface treatments may induce a negative work function shift (i.e., more n-type), which activates halide migration to aggravate PSC instability [130]. Therefore, despite the beneficial effects of surface passivation, this detrimental side effect may limit the maximum device stability attainable for PSCs. In view of this phenomenon, some new interfacial modification materials that will not change perovskite work function are desired. One important choice is 2D materials. As reported, high-mobility 2D flakes like black phosphorous are promising for perovskite surface modification, which can greatly enhance charge transport and device performance [96]. Therefore, the heterointerfaces in PSCs, as well perovskite grain boundaries, should be optimized carefully to achieve better device performance.

In addition to device efficiency and stability, recently, research topics on reducing lead toxicity and lead leakage have entered people's vision. Since the heavy metal Pb (Pb2+) is soluble in water, lead leakage from the cracked perovskite solar panels will show serious potential threat to the environment and human health. Moreover, due to the ionic nature of the organometal halide perovskites, they tend to decompose quickly in the presence of water, which accelerates the leakage of Pb. In some recent studies, perovskite surface layers, like metal–organic framework, 2D perovskites, and polymers, are employed to help slow down the perovskite film dissolution speed and reduce lead leakage, while the long-term operational stability can also be substantially improved [131,132,133,134]. Besides, the development of more reliable cell encapsulation techniques is also very important for improving device stability and preventing lead leakage.

Further development of PSCs also calls for solutions for the upscaling fabrication issue. For PSCs, the slight increase in area (e.g., from 0.1cm2 to 1cm2) will lead to a dramatic drop of the cell efficiency (e.g., from 25% to less than 22%). In literature, PSCs with active area of 1cm2 are often described as large-area device. However, such small area (1cm2) is still far from the requirements of practical solar panel products. Perovskite modules with larger area show even lower efficiencies. For example, the highest certified module efficiency to date is as low as 17.9% (802 cm2, Panasonic) [105]. Therefore, minimizing the efficiency loss between small-area cells and large-area modules still remains a great challenge for PSCs. The wide gap between small-area cells and large-area modules comes from several reasons. Firstly, the quality of perovskite films fabricated by using scalable approaches (e.g., blade coating, inject printing, roll-to-roll printing) is poorer with more defects. Secondly, the relatively high resistance of the transparent conducting electrode (ITO or FTO) used in PSCs is another key aspect that accused for the loss of the module efficiency. Normally, this effect can be relieved by dividing a large panel into many small subcells, which are serial or parallel-connected through interconnections. Therefore, the contact resistance of these interconnections will also contribute to the series resistance increase and efficiency loss of the perovskite module. Although the development of PSC is facing a variety of great challenges, it has already shown great promise toward large-scale production and applications in the near future.