Organohalide Lead Perovskites for Photovoltaic Applications

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Organohalide Lead Perovskites for Photovoltaic Applications Abd. Rashid bin Mohd Yusoff, and Mohammad Khaja Nazeeruddin J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b02893 • Publication Date (Web): 17 Feb 2016 Downloaded from http://pubs.acs.org on February 18, 2016

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The Journal of Physical Chemistry Letters

Organohalide Lead Perovskites for Photovoltaic Applications Abd. Rashid bin Mohd Yusoff1,2, and Mohammad Khaja Nazeeruddin1* 1

Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and

Engineering, École Polytechnique Fédérale de Lausanne, Lausanne CH-1015, Switzerland. 2

Advanced Display Research Center, Department of Information Display, Kyung Hee

University, Dongdaemoon-gu, 130-701, Seoul, Korea. AUTHOR INFORMATION Corresponding Author *[email protected]

ABSTRACT. Perovskite solar cells have recently exhibited a significant leap in efficiency due to their broad absorption, high optical absorption coefficient, very low exciton binding energy, long carrier diffusion lengths, efficient charge collection, and very high open-circuit potential, similar to III-IV semiconductors. Unlike silicon solar cells, perovskite solar cells can be developed from a variety of low-temperature solutions processed from inexpensive raw materials. By optimizing the perovskite absorber film formation using solvent engineering, a power conversion efficiency of over 21% has been demonstrated, highlighting the unique photovoltaic properties of

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perovskite materials. Here, we review the current progress in perovskite solar cells and charge transport materials. We highlight crucial challenges and provide a summary and prospects.

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Perovskite solar cells have recently attracted significant attention due to various unique properties, including a high absorption coefficient, broad absorption range, outstanding charge carrier mobility, and long diffusion length (LD)1-4. Miyasaka et al., 2009 reported the potential of perovskite as a light absorber and possible material for solar cells, noting a power conversion efficiency (PCE) of 3.8%5. Nam-Gyu Park demonstrated a PCE of 6.5% with CH3NH3PbI3, perovskite materials6. The key breakthrough, which sparked broad excitement in the PV community, was solid-state perovskite solar cells, which were developed by the Grätzel, Park and Snaith groups7-8. In less than 3 years, the efficiency of perovskite solar cells has reached 20%9. Based on the rapid progress of this field, this review investigates multiple aspects of perovskite solar cells, such as strategies to rationally design promising perovskite materials via bandgap engineering, increase the LD, crystallinity and morphology control, and interface engineering of hole transport materials (HTMs). We also highlight the current challenges facing perovskite solar cells and new device configurations. Organohalide lead perovskites have the chemical structure ABX3, where A is the organic cation situated at the eight corners of the unit cell, B is the metal cation located at the body center, and X denotes the halide anion in the six face center (see Figure 1). Tolerance (t) and octahedral (µ) factors determine the possible structure and crystallographic stability, where in an ideal solid sphere model, t is defined as the ratio of the bond length A-X to that of B-X =

, ℎ rA, rB and rX are the ionic radii", and the ratio is defined as µ10. Ranges

of 0.81 < t < 1.11 and 0.44 < µ < 0.90 are typical for halide perovskites (X= F, Cl, Br, and I)10. A t ranging from 0.89 to 1.0 illustrates a cubic structure, whereas lower values indicate less symmetric tetragonal and orthorhombic structures. For example, in hybrid halide perovskite


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systems, A is typically a larger organic cation, namely, methylammonium (CH3NH3+), with rA = 0.18 nm11, even though better performance has been reported for ethylammonium (CH3NH2NH3+), with rA = 0.23 nm, and formamidinium (NH2CH=NH2+), with rA = 0.19 to 0.22 nm12-15. X is a halogen anion, generally iodine, bromide, or chloride, with rX = 0.22 nm, 0.196 nm, and 0.181 nm, respectively. Cation B usually consists of Pb and B with rB = 0.119 nm and 0.11 nm, respectively. The Sn cation has also been widely used for high-performance perovskite solar cells due to the lower theoretical bandgaps16. The lower stability related to Sn is due to the ease of oxidation of Sn to SnI4, whereas relativistic effects afford greater oxidation protection in Pb. A) Bandgap engineering Solar energy is mostly concentrated in the visible and near-infrared (NIR) regime, and in order to be effectively harvested, the absorption spectrum of perovskite must have a significant overlap with the solar spectrum. However, if the bandgap is too small, the device will be able to collect extra current but the open-circuit voltage (VOC) will be too small. If the bandgap is too wide (2 eV), only a small fraction of solar energy can be absorbed. Thus, a semiconductor with a bandgap of approximately 1.4 - 1.6 eV is preferred for solar cells developed from a single junction. The bandgap of the perovskite material decreases with i) an increase in the in-plane MI-M bond angle (via octahedral tilting distortions), ii) an increase in the dimensionality of the MI(X)6 network, and iii) a decrease in the electronegativity of the anions17. i)

Increasing the angle of the M-I-M bonds

The variation of the M-I-M bond angle has a remarkable impact on the bandgap. Using AMI3 (M = Ge2+, Sn2+, and Pb2+), the M–I–M angles in the MI6 octahedra are 166.27(8)°, 159.61(5)°, and


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155.19(6)° for Ge2+, Sn2+, and Pb2+, respectively18. Moreover, a decrease in the covalent character of the M-I bond was observed while descending the 14th group due to an increase in the difference between the electronegativity of the two atoms (Ge 4p < Sn 5p < Pb 6p). Accordingly, the bandgap of AMI3 with different M ion follows the trend of AGeI3 < ASnI3 < APbI3. Sn2+based perovskites are more suitable for photovoltaic applications than Pb2+-based perovskites due to their smaller bandgap red-shift (260 nm). In a recent study based on Sn2+ perovskite solar cells, a high short-circuit current density of approximately 20.04 mA cm-2 was attained19 (see Figure 2a,b). However, the efficiency was significantly lower than that of Pb2+ using a similar device structure, and the low device performance was attributed to the lower VOC and FF. ii)

Increasing the dimensionality of the BX6 network

A previous study reported that the A cation does not contribute to the band structure and electronic properties20. However, the size can potentially affect the symmetry of the BX64− octahedral network, depending on the differences of the tolerance factor, and can thus modify the bandgap. For three different A cations HN=CHNH3+ (FA+), CH3NH3+ (MA+) and Cs+, there is a symmetry lowering trend: FAPbI3 > MAPbI3 > CsPbI32. The Pb-I-Pb bridging angles gradually deviate from the ideal linear conformation from FAPbI3 to CsPbI3, yielding a reduction in the orbital overlap of Pb-I. Thus, a corresponding bandgap widening trend is observed: FAPbI3 < MAPbI3 < CsPbI3 (see Figure 2c). By the design of a mixture of FA+ and MA+ in the A site, the optical-absorption onset of MAxFA1−xPbI3 is red-shifted compared to that of MAPbI3; thus, enhanced solar-light harvesting properties are expected. Furthermore, MAxFA1−xPbI3 exhibits superior carrier-collection efficiency compared to the single-cation analogues due to its longer exciton lifetime20. Although FA+-based perovskites have narrower bandgaps, they suffer from instability to a greater extent than MA+-based perovskite. Very recently, by rational


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incorporation of MAPbBr3 into FAPbI3, a stable and efficient mixed-perovskite, light-harvesting layer was obtained, which improved the device PCE to 18%21. Some perovskite oxides, ABO3 (where A is an alkaline-earth metal=Ca, Sr or Ba, and B is a transition metal=Ti), are widely utilized in thin film electronic components and electrooptical materials. These perovskite oxides have wide bandgaps ranging from 3-5 eV due to the large difference in electronegativity between the oxygen and metal atoms22. For instance, a low bandgap BiFeO3 is obtained by reducing the difference in electronegativity between oxygen and the metal atom due to the higher electronegativity of Fe than Ti23. Moreover, due to the smaller difference in electronegativity between I− and Pb atoms (I−, 2.66 vs. Pb2+, 2.33), the bandgap of CH3NH3PbI3 is only 1.55 eV, which is much lower than that of the ATiO3 system. iii)

Decrease of the electronegativity of the anions

For CH3NH3PbX3, experimental studies have shown that absorption was shifted to the blue region by moving from I− to Br− to Cl−. For example, CH3NH3Pb(I1−xBrx)3 demonstrated a blueshifted absorption band-edge compared to that of CH3NH3PbI3 due to the lower electronegativity of the I− anion than that of the Br− anion24,25. iv)

Tetragonal to cubic transition

At high temperatures, the structure of CH3NH3PbBr3 is cubic phase, whereas it becomes tetragonal below 235 K26,27. This transition initiates from tilting of the PbBr6 octahedral and orientation ordering of the MA cation26,28. Mashiyama and coworkers classified the tetragonal to cubic transition as displacive29. A recent work by Quarti and coworkers demonstrated a gradual blue-shift of the absorption spectra in the MAPbI3 systems from 310 K to 400 K, where the optical bandgap increases from ~1.61 to ~1.69 eV29 (Figure 3a,b). The fitting of the data gives an


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estimation of the temperature dependence of the bandgap for MAPbI3. The blue-shift in the absorption spectra is in agreement with the previous work reported by Foley and coworkers, who demonstrated that the bandgap shifting is due to the down-shift of the valance band30 (Figure 3c,d,e). B) Photophysical mechanism in CH3NH3PbI3 i)

Diffusion length One of the vital parameters in determining the solar cell performance is the carrier

diffusion length (LD). A longer LD indicates a thicker absorber layer for better light harvesting. Several groups have reported long LD4,21,26,32. In a recent work conducted by Huang et al., they observed a decreased LD in solution-grown CH3NH3PbI3 single crystals with increased light intensities21 (see Figure 4a). The decrease is elucidated by an increased recombination rate when the excited carrier concentration is too high. The remarkable versatility of the perovskite also attracted Stranks and coworkers to conduct transient absorption and photoluminescencequenching measurements to extract the electron-hole LD, diffusion constants, and lifetimes in CH3NH3PbI3 and mixed halide CH3NH3PbI3−xClx perovskite thin films4 (see Figure 4b,c). Promising results were reported; the electron and hole LD values were greater than 1 µm for the mixed halide perovskite, approximately a factor of ~5 to 10 greater than the absorption depth because the recombination lifetime is much longer. Related phenomenon observed by other groups supports these findings26. Femtosecond transient optical spectroscopy was fully utilized, and by assuming that charge carrier quenching occurred at only the extraction layer interface with 100% efficiency, the extracted electron and hole LD values of 130 and 110 nm, respectively, were obtained (see Figure 4d).


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Bakr and colleagues observed a low trap-state density and long LD in organolead trihalide, MAPbX3 perovskite32. In addition, the mobility of Br− is significantly larger than that of I− (115 vs. 2.5 cm2 V-1 s-1). However, both the Br− and I− based perovskite crystals displayed a superposition of fast and slow dynamics τ = 22 and 357 ns (MAPbBr3) and τ = 22 and 1032 ns (MAPbI3). For the MAPbBr3, using the lower mobility and shorter lifetime, the estimated LD is ~ 3 µm, whereas using the longer lifetime gives an LD of ~ 17 µm. Following the same reasoning, the best LD is ~ 8 µm and the worst LD is ~ 2 µm in MAPbI3. In parallel, Wehrenfennig et al., determined the charge carrier dynamics in the organohalide CH3NH3PbI3-xClx by means of terahertz spectroscopy33. The obtained charge carrier mobility (33 cm2 V-1 s-1) was greater than the previous study by the same group due to the different morphology and crystalline domain sizes5,34,35. Under the normal solar-cell operating conditions, the charge carrier lifetimes were observed to be limited by only mono-molecular decay processes, such as trap-mediated recombination, which have previously been observed to be exceptionally slow. At charge-carrier densities below ~ 1017 cm-3, the intrinsic LD is approximately 3 mm. This observation shows the ability of dual-source evaporation as a fabrication route for organolead halide absorber layers in highly efficient, planar-heterojunction solar cells. Recent work on CH3NH3PbI3-xClx perovskite system sparked interest among the scientific communities36-40. In one of these works, the sequential deposition technique was superior to the conventional spin-coating, where the best device via sequential solution deposition exhibited 11.7% efficiency compared to 4.8% via spin-coating36. This work suggests that sequential deposition could be useful to control film morphology and crystallinity because it significantly improved light absorption. The role of Cl− as a dopant in MAPbI3-xClx was described in detail, and the incorporation of Cl− was only allowed at low concentration, less than approximately 3-


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4%, in iodide-based perovskite37. The incorporation of Cl− as a dopant considerably improves the charge transport within the perovskite layer. Recombination lifetimes up to 446 ns and relatively good stability have also been reported in mixed halide perovskites using CH3NH3PbBr3−xClx (x = 0.6–1.2)41 (see Figure 4e). The bandgap widening observed by the absorption is blue-shifted after replacing Br− with Cl− (see Figure 4f). CH3NH3PbBr3−xClx has a longer average recombination lifetime compared to that of the Br-based perovskite (100 ns), with the longest being extended to 446 ns. This result indicates an extraordinary low-bulk recombination rate compared to other methylammonium, lead halide perovskites. ii)

Free carriers or excitons After more than five years of continuous experimenting, one of the unanswered questions

is whether the excited carriers are excitons or free carriers. In brief, excitons are tightly bound, whereas free carriers are collected at the bottom and top electrodes. The binding energy between holes and electrons decreases when the dimensionality is increased. Mathews and coworkers reported that the exciton binding energy is in the range of 19-50 meV42, which is comparable to the room temperature thermal energy of approximately 25 meV. From various series of measurements, including optical absorption, magneto-absorption and temperature-dependent PL, the calculated exciton binding energy for CH3NH3PbI3 is comparable to the thermal energy of ~25 meV at room temperature. Even for a 19 meV binding energy, the fraction of excitons is estimated to result in ~57% of the generated excitons dissociating spontaneously and 43% remaining as excitons. Exciton binding energies also depend on the morphology and crystal size, where for large crystal size, exciton absorption peaks become apparent at low temperature. In contrast, in the case of small crystal size, discrete, free carrier absorption behavior at low temperatures is observed. Despite the swift progress in perovskite solar cells reported recently,


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the fundamental photophysical process is not fully understood, and further research is required to move forward. iii)

Defects

Yin and coworkers used density functional theory to investigate the unusual defect physics of perovskite solar cells43. In a hybrid perovskite system, the defects with the lowest energy formation are the positively charged lead (Pb2+) and methylammonium (MA+). According to Figure 5, both positively charged ions create shallow charge-carrier trap states. In addition, Pb2+ and MA+ defects are strongly dependent on the preparation method. Wang and coworkers found that CH3NH3PbI3 can either be n- or p-type self-doped by varying the ratio of the two precursors during perovskite formation44. The authors showed that the film composition can be tuned via film formation methods, precursor composition, and process conditions. Duan, Samiee and coworkers demonstrated that these shallow charge-carrier trap states do not function as nonradiative recombination sites45,46. Moreover, Buin and coworkers demonstrated from their calculation work that the large charge diffusion lengths found in the iodide-free precursors are associated with the amount of iodine deposited because it tends to have higher density recombination center47. C) Crystallinity and morphology control Despite the rapid improvement of PCE in the perovskite solar cells, another vital issue for perovskite solar cells is the ability to control the crystallinity and morphology of the perovskite film. Several groups have adopted a “non-tricky” approach to improve the device performance by increasing the perovskite thickness to optimize light harvesting. However, the most challenging part is that both the shunt pathways and the amount of the light absorption due to the thicker film have to be addressed. Therefore, achieving excellent uniformity and quality films


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with 100% surface coverage, fewer pinholes, and that are highly crystalline is important to obtain highly efficient perovskite solar cells. Many photophysical properties, namely, light harvesting, charge carrier transport and LD, are significantly affected by the morphology and crystallinity of the perovskite film48-52. The defect states and grain boundaries of the perovskite crystallites act as the traps of charge carriers, which in turn aggravate the charge recombination. In principle, mesoporous perovskite solar cells provide a physical limitation on the crystal dimensions, which affects the quality of perovskite films53. Incorporating a perovskite overlayer can be a wise strategy to enhance device performance, increasing the LD to the same range as in the planar structure solar cell54. A significant reduction in the crystallite size (from >500 to < 100 nm) has been reported in the mesoporous Al2O3 system (see Figure 6)55. Therefore, the charge trapping and recombination at the grain boundaries are the main factors for the drop in JSC in the mesostructured perovskite solar cells. Different sintering and annealing temperatures and the difference in the surface energy of Al2O3 scaffolds are responsible for the contradictory observations from the same group, who previously reported the optimum mesosuperstructured organometal halide perovskite solar cell with a perovskite overlayer. Early reports on α-Al2O3 crystals showed that heating at 500 °C decreases the surface density of the hydroxyl groups, whereas heating above 500 °C eventually affects the surface morphology and surface chemistry, which in turn makes the surface less hydrophilic56. Thus, in the case of a mesoporous Al2O3 scaffold, the high-quality perovskite thin film can be obtained by ensuring the scaffold has a hydrophilic surface. In Volmer-Weber growth, non-ideal surface energies are responsible for the discontinuous film with large grain size accompanied with holes due to the fast growth rate of the perovskite thin film. From these observations, the ability to control the crystallization of the perovskite film is relatively important in fabricating high-


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performance devices, and finding effective routes to manipulate the nucleation and growth of perovskite crystals, particularly in planar architecture is important to achieve high-quality morphology and crystallinity. D) Hole transport materials (HTM) Despite the swift progress in PCE associated with the evolution of different types of perovskites and device fabrication techniques, the interfacial layers between the electrodes and the perovskite absorber layer are introduced to assist this process, including both HTM and electron transport material (ETM) in single-junction perovskite solar cells. In general, these interfacial layers serve to i) tune the work function of the electrode to promote Ohmic contact at the absorber layer and electrode interface; ii) determine the polarity of the device; iii) improve the selectivity toward holes or electrons while blocking the other and minimizing charge carrier recombination at the interface; iv) enhance light harvesting; and v) improve device stability. Due to space limitations, here we mainly discuss recently reported HTMs, which have proven to be the most useful in perovskite solar cells. In selecting appropriate HTMs for perovskite solar cells, there are several aspects that must be given serious attention: i) good hole mobility; ii) a compatible HOMO energy level; iii) good solubility and film forming properties, and iv) cost-effectiveness. Thus far, two types of HTMs have been employed in perovskite solar cells: organic and inorganic HTMs. i)

Organic HTMs

In the race toward achieving highly efficient devices, various small molecules and conducting polymers for HTMs have been adopted, and some have comparable photovoltaic performance to spiro-OMeTAD. High mobility polymers, such as poly(triaryl amine) (PTAA), have been tested by Heo and coworkers, who demonstrated 12% efficiency using a CH3NH3PbI3 perovskite light


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harvester57. In 2014, Seok and coworkers recorded the highest certified PCE (16.2%) employing PTAA as the HTM53; an even higher certified PCE of 17.9% has also been demonstrated58. The exploitation of poly(3-hexylthiophene-2,5-diyl) (P3HT) as the HTM was demonstrated by Abrusci38 and Heo57 and coworkers, where PCEs of 3.8% and 6.7% were achieved, respectively. A remarkable improvement from 9.2% to 12.4% in the P3HT-based perovskite solar cells was observed after doping P3HT with bis(trifluoromethane)sulfonamide lithium salt (Li-TFSI) and 2,6-di-tert-butylpyridine (D-TBP)59. The use of organic HTMs provides freedom in tuning their oxidation potentials and surface properties. The prospect of higher VOC is possible with the combination of CH3NH3PbBr3 and HTMs with deeper HOMO levels. The first demonstration was realized by Cai and colleagues by deploying poly[N-9-heptadecanyl-2,7-carbazole-alt-3,6bis(thiophen-5-yl)-2,5-dioctyl-2,5-dihydropyrrolo-[3,4]pyrrole-1,4-dione) (PCBTDPP)60. The developed perovskite solar cells exhibited high VOC of 1.16 V. In addition, an extremely high VOC of 1.5 V was reported by Edri and colleagues61 and was supported by the work reported by Seok’s group, who demonstrated a high VOC of 1.4 V using a combination of a triarylaminebased HTM and CH3NH3PbBr3 absorber62. Further progress was made by Park’s group, who introduced a hydrophobic DPP-based polymer (PDPPDBTE)63. Due to excellent hole mobility of 10-3 cm2 V-1 s-1 and deep HOMO level of -5.4 eV, the fabricated perovskite solar cells exhibited a PCE of 9.2%, significantly higher than that of devices employing a spiro-OMeTAD HTM. Our group used a pyrene-core arylamine and achieved a PCE of 12.4% under 1 Sun. Further optimization could enhance the VOC because the HOMO level of Py-C is -5.11 eV, slightly higher than spiro-OMeTAD (-5.22 eV)64. Other promising results have been reported by Choi and colleagues using a fused quinolizino acridine (OMeTPA-FA) HTM combined with a CH3NH3PbI3 absorber layer65.


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Most organic HTMs require solvent additives to enhance their properties. Considering the morphology compatibility, a few chemists have proposed excellent HTMs that do not require an additive to achieve a PCE over 11%, including superior ambient condition stability66. Ko and Nazeeruddin synthesized a donor-acceptor type quinolizino acridine HTM (fused-F), which possessed a well-matched HOMO level with CH3NH3PbI3 (-5.23 eV vs. 5.43 eV), and a higher PCE (12.8%) was achieved compared to spiro-OMeTAD-based perovskite solar cells (see Figure 7a,b)67. The various distinctive features, such as long LD and the ambipolar nature of organolead halide perovskite materials4,26, have sparked considerable interest in constructing HTM-free perovskite solar cells. This concept significantly reduced the processing and manufacturing costs, which is advantageous for practical, cost-effective, large-scale commercialization. Eliminating HTM in the perovskite solar cells also eliminates oxidation and offers better lifetime and more consistent data. Our group demonstrated HTM-free perovskite solar cells using TiO2 nanosheets as the mesoporous layer68. FTO was employed as an electrode and was coated successively by compact TiO2, TiO2 nanosheets, and CH3NH3PbI3. Gold was thermally deposited as the counter electrode. Under 1 Sun, this first attempt achieved a JSC of 16.1 mA cm2

, an FF of 0.57, a VOC of 0.631 V, corresponding to a light to electric PCE of 5.5%. A depletion

layer forms at the TiO2/CH3NH3PbI3 interfaces, where it is has been suggested that the total depletion width facilitates the charge separation and suppresses the back reaction, leading to the improvement in the PCE (10.85%) in HTM-free perovskite solar cells69 (see Figure 7c). Therefore, CH3NH3PbI3 perovskites can serve both as effective light harvesting materials and as hole transporters. Further optimization via the layer’s crystallinity has resulted in significant enhancement in PCEs ranges from 8-11%69-74.


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The lack of an HTM means there is direct contact between the perovskites system and the counter electrode, which has a great influence on the device performance. By taking this into consideration, Zhang and colleagues demonstrated a doctor-blading carbon paste as the top electrode in their HTM-free perovskite solar cells75. With low-temperature processing conditions, a PCE of 8.31% has been reported. Moreover, the reported unencapsulated devices also demonstrated excellent long-term stability (800 h) without significant deterioration in all photovoltaic parameters (see Figure 7d). A slight improvement in PCE was demonstrated with the use of a low-cost carbon top electrode76. The low-cost carbon was deposited at low temperature onto the MAPbI3 perovskite layer and exhibited 9% efficiency with over 2000 h of darkness stability. Following methods introduced in Refs. 75 and 76, Han and coworkers reported highly efficient, fully printed, mesoscopic, HTM-free perovskite solar cells using carbon top electrode and FAPbI3 absorber layer77. The highest PCE (11.9%) was achieved in the case of FAPbI3, which was slightly higher than that of the MAPbI3 printed devices (11.4%). Carbon materials have also inspired several groups to develop perovskite solar cells as low-cost HTM78-87. Han and coworkers demonstrated a post-deposited perovskite absorber layer on the mesoporous TiO2/ZrO2/carbon monolithic structure of HTM-free perovskite solar cells79. In their proposed structure, a high PCE of 12.8% was obtained, with notable stability of more than 1000 hours. Despite the long lifetime, the carbon layer had to be sintered at 400 °C, which limited their potential fabrication on a plastic substrate. These observations indicate that the use of carbon electrode reduces the processing cost, simplifies the fabrication and could potentially be used in large-scale manufacturing. In separate studies, Yang and coworkers used candle soot and graphene as the HTM for perovskite solar cells and achieved a PCE of 11%83,84. Another interesting idea was demonstrated by Wong and coworkers, where semi-transparent devices


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using carbon nanotubes as HTM achieved a 9.9% PCE85. The main drawback in their study is the high sheet resistance of CNT films resulting in a lower FF. Commercial conductive carbon pastes have been utilized in the perovskite absorber layers of HTM-free cells, achieving an 8-9% PCE86,87. However, the presence of solvents in the pastes might influence the device performance. The above-mentioned observations indicate that the use of a carbon electrode reduces the processing cost and simplifies the fabrication, and carbon electrodes could potentially be used in large-scale manufacturing. ii)

Inorganic HTMs

Although the progress in inorganic HTMs is slow due to the limited selection of materials, their low-cost and stable under ambient conditions verify that they are compatible with organic HTMs. CuI-based HTM provided higher FF and better JSC stability compared with devices based on spiro-OMeTAD due to the higher electrical conductivity88. However, the high recombination in the CuI layer limited the VOC. Another copper-based, inorganic p-type, hole-conductor, copper thiocyanate (CuSCN), has also been actively studied, and the highest PCE (12.4%) reported was achieved after optimization of the perovskite morphology66. In another report by Yan’s group with NiO, a PCE of 9.11% was reported, and a slight improvement was achieved by Wang and coworkers, with a PCE of 9.51% using NiO as a hole-collecting electrode89,90. Although the performance of p-type semiconductors is less promising, the highest VOC of approximately 1.05 V reported recently for NiO HTM is important91. A solution to this problem could make NiO a viable replacement for spiro-OMeTAD88. Research on new HTM in highly efficient perovskite solar cells has shown that for each perovskite system, the proper selection of interfacial layers leads to the formation of the desired Ohmic contact, suppressed bimolecular recombination, and maximized VOC, JSC, and FF. Despite


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this remarkable progress achieved for HTMs toward enhancing both device performance and stability, there is still room for future improvement by carefully designing materials with wellcontrolled electronic, optic, and chemical properties. E) Hysteresis The current hysteresis issue in perovskite solar cells has sparked another theme in thin film photovoltaics. Hysteresis has been detected in both mesoscopic and thin-film perovskite solar cells, which indicates a mismatch of the J–V curves scanned in the forward and reverse directions. This anomalous phenomenon has also been detected in DSSCs, CIGS, CdTe, and amorphous silicon92-95. It has been proposed that the hysteresis originated from the chemical capacitance, which is discharged under the forward and reverse scans, yielding a reduced and increased JSC, respectivey96. However, hysteresis in these types of photovoltaic devices can be suppressed by using a low scan rate. Extreme hysteresis has also been observed at a significantly reduced scan rate of 0.011 V s−1. Various possible mechanisms have been proposed to explain the hysteresis in perovskite solar cells: i) photoinduced traps for charge carriers in the MAPbI3, ii) ferroelectric effect caused by dipole alignment in the material induced by a combination of illumination and the bias applied across the MAPbX3, iii) ion migration throughout the film under illumination and electric fields, and iv) imbalanced charge extraction95,96. The photoinduced traps explanation is unreasonable because it cannot explain the case of increased photoconductivity over time97. Thus, the most probable hysteresis associated with the transient effects is the ferroelectric properties of the perovskite thin films. The underestimated PCE attained from the forward scan arises from the excess polarization caused by the nonrelaxed ferroelectric domain. The overestimated PCE observed from the reverse scan is caused by the opposite effect98. Recent work demonstrated that illumination, which weakens the


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hydrogen bonds between the MA cation and the inorganic scaffold, improved the rotational freedom of the MA cations. These dipole cations can align along an electric field immediately, if present, or generate aligned domains by dipole-dipole interactions. However, adjustment of the inorganic scaffold might take a much longer time99. These observations are in agreement with the capacitance and time characteristics at low frequency, which explains the long timescale of the transient effect100-103. The hysteresis is also related to the slow dielectric relaxation process because the realignment of the perovskite system cannot occur at a higher scan rate. Therefore, extreme hysteresis occurs when the scan rate decreases. Multiple studies have suggested that ion migration is responsible for the anomalous hysteresis observed in perovskite solar cells. A recent theoretical work led by De Angelis suggested that ion migration and consequent accumulation close to the electrode lead to significant band bending and thus influence the extraction of the generated excitons104. In addition, the dynamics and kinetics of ions migration were recently investigated by Eames and colleagues, who suggested a desirable vacancy-assisted diffusion of I− ions with the lowest activation energy compared to that of Pb2+ or MA+ (0.58 vs. 2.31 vs. 0.84 eV)105. This report also suggests that the diffusion of CH3NH3+ is negligible due to the significantly lower diffusion coefficient of CH3NH3+ compared to that of I− (10-16 cm2 s-1 vs. 10-12 cm2 s-1). The significantly higher activation energy of Pb2+ suggests that this migration is improbable. In addition, our latest work suggests that hysteresis occurs on the same timescale predicted for ionic transport106. Hysteresis behavior will be observed if the timescale of the slow process is similar to the J-V scan. Moreover, the voltage-dependent ionic redistribution leads to a build-up of ionic space charge close to the contact, resulting in varying built-in voltage of the perovskite. Consequently,


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based on the bias history, changes in the built-in voltage might lead to various J-V characteristics. In addition to the above-mentioned probable mechanism, charge trapping, which provides competitive recombination channels, is also detrimental to solar cell performance107. A low charge carrier trap density is associated with a high performance device, and vice versa. Nevertheless, these traps can be formed at interfaces with the HTM or ETM and can be enhanced with time due to the chemical/structural changes in the material synthesis and processing conditions. A significantly low scan rate (~0.01 V s-1) eliminates the hysteresis108,109. One probable explanation why mesoporous TiO2 perovskite solar cells demonstrate unnoticeable/negligible hysteresis compared to planar type perovskite solar cells is the reduced crystal size in the mesoporous TiO2 devices. The small size confinement reduces the number of ferroelectric domains. The less pronounced hysteresis also originates from the capability of the mesoporous TiO2 layer to release the polarization effect via electron injection. The growth condition and the type of organic cations and halide ions are also responsible of the polarization properties of the perovskite film110. Previous works found a strong dependence of the shape of the J-V characteristics on the light and voltage bias prior to actual measurements. Highly efficient and less pronounced hysteresis perovskite solar cells with high FF have been reported with pre-conditioning by lightsoaking109,111. In this pre-conditioning case, there are contradictory explanations for the less obvious hysteresis, which results from ferroelectric polarization and ion migration111. The energy barrier decrement for the polarization alignment and a significant increase in the dielectric constant can also be seen with light illumination112,113. Accordingly, high device performance is


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expected due to pre-conditioning and forward bias, which assists the charge transport and separation processes. Opposite behavior was found under dark-soaking and reverse bias109,111. In conclusion, the apparent PCE of the perovskite solar cells is significantly affected by various factors, such as the device structure, crystal size of the perovskite, selective contacts, measurement details, and pre-conditions. F) Stability Despite recent advancements towards high PCEs, device stability is another important factor that has to be taken into account before commercial application of perovskite solar cells can become widespread. To date, the degradation process is not fully understood, although several groups have proposed potential mechanisms. For instance, organometal halide perovskites are intrinsically susceptible to moisture and heat. Niu and coworkers found a decreased absorption between 530 and 800 nm in the lead halide film subjected to humidity114. Color changes from dark brown to yellow in lead halide films were also observed by Habisreutinger and coworkers upon degradation in air, which could be attributed to the bond dissociation among the crystal units115. One of the first degradation studies on perovskite solar cells was conducted by Kim and coworkers and reported a long lifetime over 500 h when cells were kept in air at room temperature7. Later, Grätzel reported long-lifetime (> 1000 h), light-soaking, encapsulated solar cells and damp heat stability measurements (1000 h at 85% relative humidity and 85 °C)116. In addition, we previously demonstrated that the negligible JSC evolution for encapsulated CH3NH3PbI3 based solar cells indicates that encapsulation is a plausible route to improve device stability117. In this study, the light-soaked devices degraded only 20% after 500 h of testing.


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Another promising finding was reported by Law and coworkers, where the perovskite cells degraded 8% after 60 h of stability measurements118. It has been speculated that during light excitation, the generated holes in the valance band could potentially recombine with the electrons at the TiO2 surface, yielding free electrons in the conduction band and subsequently forming null oxygen vacancy sites. These unfilled sites may attract generated electrons, and they could recombine with holes in the HTM, leading to instability of the device. To overcome this situation, especially in TiO2-based devices, Leitjens and coworkers used an insulating Al2O3 layer instead of TiO2 and found that the constructed devices had better stability, decreased PCE by approximately 50% of its original performance and remarkable performance, when the devices underwent light soaking testing over 1000 h45. Introducing an additional protection layer also helps to restrain the exposure of oxygen and moisture towards the perovskite layer. Similar improvement has been reported by substituting organic HTMs with a carbon nanotube-PMMA composite layer, resulting in improved stability up to 100 h at 80 °C115. The hydrophobic nature of PMMA helps to prevent the penetration of water molecules. A thin Al2O3 layer was enough to improve the moisture stability of the CH3NH3PbI3-based perovskite solar cells, and the PCE was degraded by approximately 50% from its initial value upon exposure to high humidity (60%) compared to a decrease approximately 80% in the free Al2O3 protection layer devices114. Employing HTMs with higher hydrophobicity also improved moisture stability because it prevents water penetration into the absorber layer. Kwon and coworkers utilized hydrophobic polymers in their perovskite solar cells and achieve only 20% efficiency decrement over 1000 h of stability measurements, which is slightly better compared to the devices with spiro-OMeTAD as the HTM63.


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A 21.02% efficiency has been achieved although less attention is given to enhancing device lifetime. A better understanding of the degradation mechanisms will assist the scientific community in proposing a route to enhance device stability via interfacial engineering and physical encapsulation approaches. In addition, standard degradation measurement conditions must be standardized. G) Novel configurations As the field progresses, several groups have proposed novel device configurations, including the introduction of organic materials to serve as the HTM/ETM in perovskite solar cells62,107,119-124. Because of their compatibility in continuous roll-to-roll devices, flexible perovskite solar cells received remarkable attention with the best PCE of 12.20%125. ITO/PEN was employed as an electrode and was coated successively by compact TiO2, perovskite and spiro-OMeTAD. Silver served as the top electrode. A superior bending performance, i.e., its efficiency does not change after 1000 bending cycles under conditions of bending radii of 10 mm, has also been reported. The use of titanium foil as a substrate and transparent conductive adhesive on a PET film embedded with a nickel (Ni) grid as a counter electrode have also been demonstrated126. The device exhibited a PCE of 10.3% with a dramatic increase of both JSC and FF due to the implementation of PEDOT:PSS as the HTM on a spiro-OMeTAD layer. Following this work, another interesting and appealing approach is to optically engineer the perovskite solar cells to maximize light coupling in the absorber layer127,128. In one of the earliest incorporations of core-shell metal nanoparticles into perovskite solar cells, a significantly improved JSC helped to enhance the PCE of perovskite solar cells from 10.7% to 11.4% (see Figure 8a). The authors found that reduced exciton binding energy, which led to enhanced generation of free charge carriers is responsible for the improved JSC compared to pristine perovskite solar cells127.


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Another plasmonic study demonstrated that the presence of popcorn-shaped nanoparticles led to faster charge transfer at the TiO2/perovskite interface128. On the other hand, the enhancement of VOC has been attributed to the suppression of charge recombination due to the strong PL quenching. The plasmonic devices exhibited a PCE of 10.3%, an approximately 15.7% enhancement compared to the initial value. Semiconductor quantum dots exhibit size-tunable absorption, a high optical extinction coefficient, and the ability to generate multiple excitons, and it is particularly interesting to incorporate these quantum dots into perovskite solar cells129-131. The first demonstration of quantum dot perovskite solar cells exhibited a PCE of 6.20% on a relatively large active area (0.309 cm2)6 (see Figure 8b). The considerable JSC enhancement was due to the enhanced charge carrier extraction from the quantum dots6,108. Recent incorporation of 3 nm quantum dots in CH3NH3PbBr3 perovskite systems had brought improvement in all key photovoltaic parameters112. The best quantum dot perovskite solar cells exhibited a PCE of 11.40%, a considerably high VOC of 1.11 V, and an FF greater than 70%. To further improve device performance, multi-junction perovskite cells in a horizontal structure can be implemented to harvest a larger portion of the solar energy. The best approach is first to have the sunlight project onto the wide bandgap subcell and then continue progressively onto the low bandgap subcell. Although promising performance can be obtained from this type of device, few studies have been reported. One of the first reported multi-junction devices utilizing perovskite and crystalline silicon as the top and bottom cell, respectively, exhibited a PCE of 13.4% compared to single-junction perovskite solar cells with only 11.6%113 (see Figure 8c). With a similar concept and a 4-terminal tandem structure, a PCE of 17% was obtained employing silicon and perovskite in the tandem devices110. The PCE was improved with further


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combination optimization comprising CIGS and perovskite along with the PCE of 18.6% compared to 12.7% in single-junction perovskite solar cells. In comparison with 4-terminal tandem devices, 2-terminal tandem devices do not require extra wiring but require a suitable and robust interconnecting layer to physically and electrically connect the top and bottom cells. The first 2-terminal demonstration PCE was lower than the single-junction perovskite solar cell109 because the authors employed a semi-transparent top electrode, which blocked the majority of incoming light. Promising 2-terminal demonstration has reached a PCE of 10.23%, higher than the respective single-junction perovskite and polymer devices132. The latest improvement of the 2-terminal monolithic tandem devices was accomplished using perovskite and silicon solar cells, which produced a VOC of 1.65 V along with the best PCE of 13.7%133 (see Figure 8d). Despite encouraging efficiency, it is low compared to the best recorded efficiency for perovskite or silicon devices. Possible improvements can be obtained by substituting the HTM with wider bandgap materials, improving the quality of the perovskite thin film, and utilizing better surface passivation on the front and bottom devices. In addition to proper bandgap engineering for the top and bottom devices, a suitable interconnecting layer is important to avoid voltage losses. H) Commercialization and potential applications i)

Commercialization barrier and potential solutions Perovskite solar cell technology has a crucial role in providing much needed power in the

context of growing global concerns about sustainable energy supplies and protecting our environment from the adverse effects of fossil fuel utilization. However, there are several barriers that hinder the adoption of perovskite solar cell technology, including i) financial barriers, ii) lack of awareness, and iii) technical barriers. a) Financial awareness


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The initial costs of solar cell systems are high; therefore, the acceptance of solar cell systems depends strongly on the financial viability of the investments in the solar cell systems. Solar cell systems are characterized by high initial capital requirements. Hence, well-adapted financial schemes and support are required for the dissemination of these technologies. Financial institutions are not always interested in providing credit for private solar cell investors. Thus, there is urgency for some type of financing support along with the traditional banking channels. Perhaps, the government could help with a financing mechanism and provide financing and access to affordable credit plans to encourage potential users to become actual users. The government could also help in promoting the development of the local solar cell market by encouraging local manufacturers, especially by temporarily removing taxes for purchasing any related solar cell equipment. Moreover, providing duty-free import of solar equipment and all related accessories would help in lowering the initial costs of solar cell systems. It is generally accepted that the high initial costs of solar cell systems slows the growth of the solar cell market. Therefore, any measure taken by the government to lower the initial cost of solar cell systems is the next logical step. b) Lack of awareness and inadequate promotion support The majority of the people constituting the potential market are unaware of the capabilities of solar cell technologies. The government should assume the responsibility for creating awareness among the industry and public about the benefits of solar cell systems. This awareness can be achieved via various approaches, such as advertising campaigns and solar cell documentation programs through television, radio, newspaper, and highway billboards. Awareness creation can also be achieved through exhibitions, conferences, seminars, workshops, study tours and


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publication of solar cell technologies information, such as technical reports, guides/manuals, directories, and books. c) Technical barriers Despite the work conducted by certain countries, especially in Europe and America, solar cell technology is still new to most developing countries, such as India, Malaysia, Indonesia, and African countries. There is a greater thrust that is needed to promote the use of solar cell systems as an alternative source of power. The industry is relatively new; therefore, the existing market infrastructure, which includes marketing networks and support systems needed to provide the required after sales service and ongoing maintenance, needs to be strengthened. ii)

Perovskite applications

As energy demands around the globe increase, the need for a renewable energy source that will not harm the environment has never been greater. Although more conventional sources of energy, such as fossil fuels, are still satisfying the majority of the world’s energy demand, solar cell systems are used in a variety of applications. The potential applications of perovskite technology can be classified into two groups: i) utility interactive systems and ii) stand-alone systems. a) Utility interactive systems Utility interactive systems are often used in homes or commercial buildings to offset electricity costs when the perovskite technology system is not large enough to satisfy all of the energy demands. These systems are especially attractive in displacing power bought from the utility during peak demand hours, which usually coincide with peak sunlight hours. Because most


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power companies incorporate a peak demand surcharge in their billing process for commercial customers, solar cell systems can significantly reduce electrical bills. A properly designed perovskite technology system with battery storage can be used to provide power during peak load periods, potentially leading to greater savings. Because the peak demand for electricity normally occurs during sunny periods, an increasing number of utility companies use large perovskite technology systems to supplement other systems of electrical generation. The unit costs of larger utility systems are lower because of the economies of scale that occur with larger systems. b) Stand-alone systems The simplest stand-alone systems are direct systems, which use electricity as it is produced. Often, however, demand for electricity exists when the sun is not shining. In that case, a battery storage system is used. In these systems, the perovskite technology array charges the system’s batteries during sunlight hours, and the batteries supply electricity at night and during cloudy periods. If necessary, stand-alone systems may also use conventional generators as backup systems. Both direct and battery storage systems can provide DC and AC power. Some examples of the numerous potential applications of stand-alone perovskite technology systems include: i) Lighting The availability of low power DC lighting, such as low pressure sodium and fluorescent lights, makes perovskite technology an ideal source for meeting remote or mobile lighting needs. Lighting demands are greatest at night, making battery storage essential. Perovskite technology systems can also be used to provide lighting for billboards, highway information signs, public-


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use facilities, parking lots, marinas, homes, vacation cabins, piers, and caboose lighting for trains. ii) Communications Radio, television, and phone signals over long distances need to be amplified. Relay towers, often called repeater stations, perform this function. The best sites for repeater stations are usually at the highest possible elevation, where power lines are not commonly found and transport of conventional generator fuels would be difficult and costly. In addition, as the use of fiber optic cable spreads, photovoltaic repeater stations will be required. Coaxial cable can carry power to amplify the signal carried, but fiber optic cable does not have this capability. Perovskite technology can also be used on travelers’ information transmitters, portable computer systems, cellular telephones, mobile radio systems, and emergency call boxes. iii) Remote Site Electrification Many residences and other structures are simply too far from the utility distribution network to establish a grid connection. Additionally, power is needed at construction sites before the connection has been installed. Perovskite technology systems are an attractive way to provide electricity in these areas. Conventional generators or other renewable energy sources, such as wind or micro hydroelectric generators, may be used in conjunction with the perovskite technology system to ensure uninterrupted power availability. Some examples of remote site electrification are for rural homes, visitor centers in parks, park ranger sites, vacation cabins, hunting lodges, remote farm workshops, village/island electrification, clinics and remote research facilities, highway rest stops, public beach facilities, campgrounds, and military test areas.


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iv) Remote Monitoring Often monitoring for scientific research or other purposes must take place at temporary sites far from conventional power sources. Perovskite technology systems can be effectively used as a power source to monitor meteorological information, highway/traffic conditions, structural conditions, seismic recording, irrigation control, and scientific research in remote locations. v) Signs and Signals Devices, such as navigational beacons, and audible warning signs, such as sirens, highway warning signs, railroad signals, aircraft warning beacons, buoys and lighthouses, are generally remote or even impossible to connect to the utility grid. Perovskite technology systems can provide reliable power for these critical applications. vi) Water Pumping and Control Perovskite technology is an ideal candidate for water pumping applications. Many water pumping needs, such as livestock watering, are greatest during the sunniest hours of the day. These systems may be either direct systems, operating the pump only when the sunlight is sufficient, or they may pump water to an elevated storage tower during sunny hours to provide available water at any time. Either system avoids the use of batteries, resulting in a decrease in initial cost and reduced maintenance needs. Perovskite technology powered water pumping can be used to provide water for campgrounds, irrigation, remote village water supplies, and livestock watering. vii) Charging Vehicle Batteries


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Perovskite technology systems may be used to directly charge vehicle batteries or to provide a “trickle charge” for maintaining a high battery state of charge on little-used vehicles, such as fire-fighting and snow removal equipment and agricultural machines, such as tractors or harvesters. Direct charging is useful for boats and recreational vehicles. Solar stations may be dedicated to charging electric vehicles. viii)

Disaster Relief Applications

Natural disasters, such as hurricanes, floods, tornadoes, and earthquakes, often destroy electric generating facilities and distribution systems. In situations where the power will be out for a long period of time and the affected area is relatively large, portable perovskite technology systems are useful for providing street and personal lighting and power for communications equipment, warning and message signs, water purification, refrigeration of medical supplies and food, and pumping water. When makeshift shelters or medical clinics are necessary, perovskite technology supplied electricity can be seen as a better choice than conventional fuel generators. ix) Cathodic Protection Metallic structures exposed to soils and water naturally experience corrosion due to electrolytic action. This corrosion occurs because the metals lose ions when exposed to an electrolyte. A voltage may be applied that will prevent the ion loss from the metal, preventing corrosion. This method of protection is called cathodic protection. Only a small DC voltage is necessary to protect the metals. If utility power is used, the voltage must be lowered, and the power must be converted from AC to DC. Perovskite technology systems are capable of producing the low voltage DC power directly, resulting in a much more efficient use of energy. Cathodic protection is used on pipes, tanks, wellheads, wharves, bridges, and buildings.


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x) Refrigeration Perovskite technology systems are excellent for remote or mobile storage of medicines and vaccines. xi) Consumer Products Perovskite technology can also be used on a variety of commercially available and successful consumer products. For example, perovskite technology can be used to power small DC appliances for recreational vehicles. Other examples include watches, lanterns, calculators, radios, televisions, flashlights, outdoor lights, security systems, gate openers, golf carts, and fans. As the technology improves and is applied in even more innovative ways, perovskite technology promises to cleanly provide a significant portion of the world’s electricity. The cost of perovskite technology systems will steadily fall. Their use will dramatically increase as they become more cost competitive with conventional forms of electrical generation. Perovskite technology systems are the best choice in hundreds of important applications. Summary and Future Outlook Perovskite solar cells may play a major role in our future photovoltaic technologies. The ability to tune the bandgap, increase the LD, and control the morphology and crystallinity properties might enable a range of device configurations. The perovskite solar cell is still in its early stage, so there is no prevailing technology that dominates the market. Thus, to be commercially viable, perovskite solar cells must substantially surpass the performance of other existing photovoltaics at a much lower manufacturing cost.


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Notable achievements and progress have been reported in terms of device configuration, working mechanism, film deposition, interfacial engineering and current problems related with perovskite solar cells. Perovskite film formation is important because it determines the crystal structure, composition, uniformity, texture and defects, which influence the final device performance. It has been generally accepted that interfacial engineering is a powerful tool to optimize device performance via charge generation, transportation, and collection. With the best performance realized through p-i-n architecture, perovskite films that have been reported to have a 1.55 eV bandgap are well-suited as the top cell in tandem structures. The ability to create multi-junction devices might enable harvesting more solar energy. Due to the uniqueness of perovskite thin films, we envision additional applications, such as wearable devices. The materials and device stability issues need to be understood in parallel with the progress in device improvement. Even though perovskites have created a tsunami phenomenon in several research fields, various challenges remain to be addressed, such as the toxicity and degradation of fabricated cells. Although a few groups have demonstrated the capability to fabricate lead-free devices, the rate of success is still low compared to the devices incorporating lead. On the other hand, the degradation and hysteresis mechanisms that appeared in most of our devices must be properly addressed before we can move to large-scale production. The durability of terrestrial-use for solar cells has already exceeded 20 years. Research must be devoted to improving the intrinsic properties of hybrid perovskites and to making the perovskites market-viable. An encouraging conclusion to be drawn from this review is that the general trend of development in perovskite solar cells is exciting and the future appears bright; however, there is


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significant research to be completed before these perovskite solar cell devices can have a significant impact on our renewable energy landscape and our society. AUTHOR BIOGRAPHIES

Prof. Dr. Mohammad Khaja Nazeeruddin (Male), Head of “Group for Molecular Engineering of Functional Materials” at EPFL. He authored over 500 papers, six book chapters, and inventor/coinventor of 60 patents. His total numbers of citations are over 49’064, with an h-index of 105. He appeared in Thomson Reuters 2015 Highly Cited Researchers, and Worlds Most Influential Scientific Minds. He is a PI of several industrial, national, European Union, and international projects. He has been invited to give over 100 keynote and plenary talks in various international conferences. He is directing several post-docs and PhD students.

Prof. Abd Rashid Bin Mohd Yusoff, is a visiting Professor from Kyung Hee University, Korea with expertise in organic devices including OPVs, and OLEDs. He authored over 60 papers, two books, four book chapters, and also served as Editor-In-Chief of Nanoscale for Special Issue (Graphene-based Energy Devices). His research mainly concerns the interface and device


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Figure 1. Perovskite structure. a, Crystal structure of organometal trihalide with chemical structure of ABX3, where A is the organic cation (green), B is the metal cation (blue) and X is the halide anion (red). b, An alternative view illustrating B cation assembled around X anions to form BX6 octahedra. c, Tilting of BX6 octahedra occurring from non-ideal size effects and other factors.


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Figure 2. Bandgap engineering. Widely tunable perovskite materials. Figures reproduced with permission from: a, ref. 18, ASC; b, ref. 18, ACS; c, ref. 2, RSC.


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Figure 3. (a) UV-vis absorption spectrum of MAPbI3 from 310 K to 400 K; (b) evolution of the band gap. Temperature dependent (c) absorbance; (d) photoluminescence spectra of MAPbI3; and (e) measured bandgap as a function of temperature. Figures reproduced with permission from: a, ref. 30, RSC; b, ref. 30, RCS; c, ref. 31, APS, d, ref. 31, APS, e, ref. 31, APS.


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Figure 4. Carrier recombination lifetime. a, Transient photovoltaic curves of the perovskite devices under 1 Sun and 0.1 Sun illumination. Time-resolved PL decay transients of: b, Mixed halide perovskite; c, Triiodide perovskite. d, Triiodide perovskite. e, MAPbBr3-xClx with different Cl− substitution ratios. f, Normalized blue-shifted absorption spectra of MAPbBr3-xClx. Figure reproduced with permission from: a, ref. 20, Science; b,c, ref. 4, Science; d, ref. 25, Science; e,f, ref. 41, RSC.


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Figure 5. Energy levels of MAPbI3 perovskite depicting defect states calculated by DFT: (a) acceptor and (b) donor states resulting from intrinsic defects. Figures reproduced with permission from: a, ref. 43, APS; b, ref. 43, APS.


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Figure 6. Morphology control. Cross-section SEM images of CH3NH3PbI3-xClx perovskite solar cells. a, ~400 nm thick scaffold without capping layer. b, ~80 nm thick scaffold with capping layer. c, without scaffold. d, schematic device structure. e, The respective photovoltaic parameters with different scaffold thickness. Figure reproduced with permission from ref. 55, RSC.


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Figure 7. Hole transport materials. a, J-V characteristics of CH3NH3PbI3 perovskite solar cells with different HTMs. b, IPCE spectra of the respective solar cells. c, Schematic structure of HTM-free TiO2/CH3NH3PbI3 perovskite solar cells and energy-level diagram of the respective solar cells. The vertical arrow shows the depletion region observed at the TiO2/CH3NH3PbI3 interface. d, Long-term lifetime of unencapsulated HTM-free perovskite solar cells over 800 h measured under 1 Sun. Figure reproduced with permission from: a,b, ref. 72, RSC; c, ref. 74, ACS; d, ref. 90, ACS.


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Figure 8. New device structures. a, Schematic structure of plasmonic perovskite solar cell. b, TEM images of i) CH3NH3PbI3 deposited on TiO2, ii) the incorporation of quantum dot in perovskite layer, iii) pure CH3NH3PbI3, iv) bare TiO2. c, 4-terminal tandem device structure with perovskite cell as the top cell and Si or CIGS as the bottom cell. d, Cross sectional SEM of monolithic tandem CZTSSE/perovskite solar cell. Figures reproduced with permission from: a, ref. 127, ACS; b, ref. 6, RSC; c, ref. 110, RSC. d, ref. 133, APS.


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QUOTES i.

ii.

Hybrid organic–inorganic methylammonium lead halide perovskites, are recognized for their excellent semiconducting properties and has been utilized as the intrinsic layer in perovskite solar cells. The major appeal of perovskite solar cells is that they can be developed at room temperature using solution processable methods, therefore much cheaper compared to its counterpart silicon solar cells.


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Organohalide Lead Perovskites for Photovoltaic Applications

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