Colloidal quantum dots and metal halide perovskite hybridization for solar cell stability and performance enhancement

Metal halide perovskites and colloidal quantum dots (QDs) are two emerging classes of photoactive materials that have attracted considerable attention for next-generation high-performance solution-processed solar cells. In particular, the hybridization of these two types of materials has recently demonstrated remarkable performance enhancement due to the complementary nature of the two constituents. In this review, we will highlight the recent progress of QDs and perovskite hybridization in solar cell applications. More speci ﬁ cally, the unique properties of monophase perovskite QDs will be summarised, and are demonstrated by homogeneously hybridizing perovskite QDs into the perovskite lattice. We also discuss the recent progress in heterogeneously hybridizing discrete colloidal QDs into perovskite layers which results in signi ﬁ cant enhancement in perovskite ﬁ lm stability as well as corresponding solar cell performance improvement. PbS QDs, other chalcogenide QDs, and emerging two-dimensional QDs are further accounted through multiple methods, such as constructing bilayer architectures and core – shell structures or blending multiple QDs into perovskite layers. In the end, an outlook perspective of this ﬁ eld has been proposed to point out several challenges and possible solutions.


Introduction
Organic/inorganic halide perovskites have shown their potential to compete with the existing photovoltaic technologies, with a certied efficiency higher than 26%. 1 The high efficiency and performance of perovskite solar cells (PSCs) are supported by material properties such as a high optical absorption coefficient (10 5  suppressed recombination (defect tolerance), well-balanced charge transfer and so on. [2][3][4][5][6][7] However, the most challenging issue for the further commercialization of PSCs is the poor chemical stability of their performance under severe environments (e.g. humidity, light soaking, etc.). [8][9][10][11] In recent years, several routes 12 have been explored to enhance the stability of PSCs, such as compositional engineering, 13 additive and passivation strategies, [14][15][16][17] shape engineering, 18 introducing other robust functional layers, encapsulation techniques, and device structure engineering 19 by using two-dimensional (2D) and 2D/3D perovskites. 11 Among these, the integration of perovskites and colloidal quantum dots (QDs) into a single device appears to be an efficient approach for stability and device performance enhancement. QDs have received enormous attention because of their excellent properties emerging from quantum connement. They offer a high versatility with an easily tunable bandgap, high quantum efficiency, high colour purity, and generally better stability than organic chromophores. [20][21][22][23][24][25][26][27][28] One of the main elds of research and development is photovoltaics, which has demonstrated certied photovoltaic conversion efficiencies (lead sulde, PbS QDs) as high as 13.8%. [29][30][31] The integration combines the advantageous properties of both materials, leading to increased short circuit current due to the extended light absorption from both QDs and perovskites, enhanced stability due to the passivation of the QD surface, and a high open-circuit voltage and ll factor in the case of perovskite solar cells. In addition, low-cost solution processability at low temperatures also allows easy integration of QDs and perovskites.
It has been reported that multiple methods, such as constructing bilayer architectures 32 and core-shell structures 33,34 or blending one or several kinds of QDs into perovskite layers, 35 were utilized for the hybridization of QDs and perovskites. Hybridization devices such as solar cells, light-emitting diodes 36,37 and photodetectors [38][39][40] have been made with improved stability and performances so far. In this review, we summarise the intensive research efforts from the aspect of QDs and metal halide perovskite hybridization in solar cells, including (I) homogeneous hybridization: perovskite QD (PQD) hybrids into perovskite lms and (II) heterogeneous hybridization: PbS or other QD hybrids into perovskite lms.

Advantages of homogeneous hybridization
PQDs have been extensively explored due to their excellent photophysical properties and broad application prospects. [41][42][43][44] PQDs have a great potential for interface engineering in PSCs due to their tunable energy bands, high photoluminescence quantum yields and low excitation energy. [45][46][47][48] In terms of their same crystal structure (cubic ABX 3 ), and similar lattice parameters, chemical composition and processing requirements, the integration of PQDs with perovskite thin lms has unique advantages over other additives like polymers and molecular species. 15,49 Dispersing PQDs into perovskite bulk thin lms is considered as a simple and efficient strategy to passivate the grain boundaries and enhance the crystallinity of the perovskite absorber layer, passivate defects in the bulk or at the surface, as well as tune the interface structure and energetics. [50][51][52][53] Furthermore, the unique optical properties, especially the photoluminescence (PL) of PQDs, can be used to increase the absorption range of PSCs. In this section, we will focus on how different hybridizations have improved bulk thin lm-based PSC stability and performances through its unique optical properties and interface engineering. 54 56,58,59 One of the earliest examples was reported by Cha et al., using a MAPbBr 0.9 I 2.1 QD layer sandwiched between the MAPbI 3 bulk thin lm and Spiro-OMeTAD holetransporting material (HTM) layer (Fig. 1a). 56 As shown in Fig. 1b, by changing the Br/I ratio, the MAPbBr 3Àx I x QDs showed tunable energy levels. Interface engineering with the MAPbBr 0.9 I 2.1 QDs provided appropriate band energy alignment and facilitated the hole transfer, resulting in signicant improvements of ll factor, short-circuit photocurrent and power conversion efficiency (PCE) (13.32%), which shows 29% increase compared with the PQD-free case. 56 This suggested a promising means for interface engineering towards efficient charge carrier extraction and hence high photovoltaic performance. However, the stability of PSCs with and without MAPbBr 0.9 I 2.1 QDs didn't show much difference.
The interdiffusion of PQDs into perovskite lms can reduce ionic defects at the surface and grain boundaries. 59 Multi-cation hybrid halide perovskite QDs, namely Cs x (MA 0.17 FA 0.83 ) 1Àx -PbBr 3 (abbreviated as QDs-Cs5), were dispersed in anhydrous hexane and integrated into (FAPbI 3 ) x (MAPbBr 3 ) 1Àx lms. 59 In this work, the composition of Cs-incorporated multi-cation PQDs was found to inuence the effectiveness of ionic defect passivation. The solid-state interdiffusion process leads to the passivation of imperfections and the enhancement of photovoltaic performance characteristics observed upon treating the solar cells with QDs-Cs5. This boosted the PCE of organicinorganic halide PSCs exceeding 21%, which retained more than 90% of their initial PCE despite exposure to continuous illumination for more than 550 h. 59 PQDs are also used as nucleation sites to facilitate the growth of perovskite lms. MAPbBr 3 QDs with an average diameter of 4.9 nm were successfully prepared using a short alkyl ligand of n-butyl amine and dispersed in an antisolvent as nucleation centers for the growth of MAPbI 3 lms, as shown in Fig. 1c. 60 The heterogeneous nucleation based on high lattice matching (5.5% at (110) crystalline plane) and the low free-energy barrier signicantly improve the crystallinity of MAPbI 3 lms with decreased grain sizes (Fig. 1d), resulting in a longer carrier lifetime and lower trap-state density in the lms. Meanwhile, the derived "perovskite light-emitting solar cells" with the p-i-n structure exhibit improved electroluminescence and current efficiency when operating as LEDs, and enhanced photovoltaic characteristics and stability as solar cells. The current efficiency is enhanced by an order of magnitude as LEDs, and meanwhile the PCE increases from 14.49% to 17.10% as SCs, compared to the reference device without QDs. 60 Besides the above-mentioned blending approaches, constructing bilayer or multilayer structures is also a homogeneous hybridization strategy for improved efficiency and stability in PQD solar cells. Que et al. demonstrated a method that entails solution-deposition of high-Cs-content Cs 1Àx FA x PbI 3 alloy QDs onto a bulk Cs-lean FAPbI 3 -based thin lm (Fig. 1d), which shows a PCE (reverse scan) of 20.82% with signicantly reduced hysteresis. 61 The QD modication approach not only improves the charge dynamics in the devices but also improves ambient stability enhancement effects for both the FAPbI 3 -based thin lms and the associated PSCs. As shown in Fig. 1f, when increasing the RH from 20% to 90%, within only 24 h, the PCE of the QD-free device decreased by $90% of its initial value, while only an $20% decrease was observed for the QD-modied device.
For a quick overview of organic-inorganic PQDs in perovskite bulk lms, the homogeneous hybridization strategy is summarised in the upper part of Table 1. PQDs are utilized as interface modiers, nucleation sites or spectral absorption broadening components to facilitate the growth of perovskite lms through blending and fabricating bilayer or multilayer structures. In most cases, the photovoltaic efficiency of PSCs improved remarkably, however, their stability didn't see much enhancement, especially for pure MAPbI 3 QDs. When the inorganic part, Cs + , is incorporated into organic-inorganic PQDs, the hybridizing strategy achieves both high efficiency and long-term stability in QD-Cs modied perovskite solar cells due to enhanced defect passivation, and more efficient charge extraction or photocarrier harvesting.

All-inorganic perovskite QDs in perovskite bulk thin lms
Compared with organic-inorganic perovskites, all-inorganic perovskite with Cs + cations was thought to exhibit excellent atmospheric stability due to their intrinsically higher thermaldecomposition temperature. 18,71 Besides, the narrower energy band of cubic phase CsPbX 3 (X ¼ I, Br, or Cl) perovskites contributes to higher PCE as they can absorb long-wavelength light. However, owing to the low Goldschmidt tolerance factor, the desired a-CsPbX 3 perovskite can only be stable at high temperatures (>200 C), 72 limiting its applications. When decreasing the size into nanoscale QDs or nanocrystals (NCs), the phase stability of all-inorganic perovskites would be enhanced because of reduced surface strain. Furthermore, the optoelectronic properties and bandgap energy of PQDs can be ne-tuned by designing the composition, dimensional size and shape of PQDs. 73 Similarly, interfacial engineering has been widely employed with all-inorganic PQDs leading to both stability and photovoltaic efficiency enhancements. 51 Based on the fact that FA 0.85 MA 0.15 Pb(I 0.85 Br 0.15 ) 3 and CsPbBr 3 possess the same crystal structure and similar lattice parameters (only 7.6% lattice mismatch), Zai et al. developed an interfacial engineering method to construct hybrid perovskite heterojunction devices with CsPbBr 3 QDs penetrating the perovskite precursor mixture during lm growth. 63 The best performance of 20.56% was achieved thanks to the more favourable energy alignment, enhanced light harvesting, and reduced carrier recombination ( Fig. 2a and b). 63 In another study, CsPbBr 3 QDs were introduced between the perovskite/ HTL interface to improve the morphology and crystallinity of MAPbI 3 lms. 64 CsPbBr 3 QDs led to the crystallization of a passivation layer of Cs 1Ày MA y PbI 3Àx Br x on top of the perovskite layer with fewer grain boundaries and lower defect density. Therefore, the authors obtained environmentally stable perovskite solar cells with 20.46% efficiency, with reduced charge recombination and facilitated charge transfer/extraction at the interfaces ( Fig. 2c and d).
By tuning the ratio of halide anions (I, Br, Cl and their mixtures), inorganic PQD hybrids in PSCs show a further improvement of the PCE and stability. Akin et al. incorporated an interfacial layer of inorganic CsPbBr 1.85 I 1. 15 QDs between triple-cation based perovskite and Spiro-OMeTAD layers. 65 CsPbBr 1.85 I 1. 15 QDs increased the ability of hole extraction and suppressed the charge recombination by preventing the back-ow of electrons, resulting in a hysteresis-free best efficiency as high as 21.14%. Moreover, PSCs with PQDs benet from the high moisture resistivity as well as suppressed ion migration, and thus better operational stability, and retained 94% of their initial performance under continuous light irradiation over 400 h.
Besides the functional constituents for interfacial engineering layers and gradient heterojunctions in PSCs, inorganic PQDs with dopant and surface-ligand carrying vehicles are attractive for both elemental passivation and molecular surface modication. Zheng et al. utilized CsPbBrCl 2 QDs as elemental dopants and molecular surface modiers for MAPbI 3 solar cells. 51 When the QDs are distributed across the MAPbI 3 precursor, they will decompose and leave elemental dopants inside the perovskite lm and ligands on the surface of the perovskite lm. The long hydrophobic alkane tails from the CsPbBrCl 2 QD ligands were anchored to Pb sites on the MAPbI 3 surface and retarded the moisture permeation by blocking the escape of MA cations (Fig. 2e). The PQDs passivated the defect trap states from both the bulk of active layers as well as the surface structures. Thus, this QD passivation strategy mitigated the energy disorder of MAPbI 3 , narrowed band-tail electronic states, and reduced mid-gap states. As a result, a CsPbBrCl 2 QDtreated device yielded PCEs up to 21.5% (Fig. 2f). Under one sun of continuous illumination for 500 h, the device with 0.25 wt% QDs maintained $80% of the initial PCE; in contrast, the pristine device without QDs dramatically degraded to 27% of its initial PCE. 51 Although organic-inorganic PSC hybrids with PQDs have achieved dramatic improvement in device efficiency, their longterm stability remains a major concern. To address this issue, extensive research efforts were dedicated to exploiting all- Table 1 Selective studies of PQD hybrids with perovskite films Cell type Year Ref. inorganic PSCs hybrids with all-inorganic PQDs. Bian et al. utilized a graded bandgap with the CsPbBrI 2 (E g ¼ 1.91 eV) lm under CsPbI 3 (E g ¼ 1.77 eV and 300 nm thick) QDs as component cells, to achieve a favourable energy-level alignment for carrier collection ( Fig. 3a and b). 66 Optimizations including Mn 2+ substitution, thiocyanate (SCN-) capping, and [(NH 2 ) 2 CH] + treatment resulted in an extended photoresponse, high carrier mobility, and well-matched energy levels, which taken together contributed to a PCE of 14.45%. 66 Notably, the PCE showed very little degradation aer 3 weeks without encapsulation in dry nitrogen and oxygen atmospheres. However, the black-phase CsPbI 3 (a-and g-phases) was not stable at room temperature, and it tended to convert to the nonperovskite d-CsPbI 3 phase. In another study by Bai et al., an efficient approach was described to prepare stable black-phase CsPbI 3 via the formation of a heterostructure consisting of 0D Cs 4 PbI 6 and 3D CsPbI 3 through tuning the stoichiometry of the precursors between CsI and PbI. 68 The 0D Cs 4 PbI 6 was suggested to surround the black-phase CsPbI 3 grains to simultaneously passivate the defects and stabilize the phase stability. The corresponding PSCs with the Cs 1.2 PbI 3.2 lm can yield an impressive PCE of 16.39% with improved stability, and retained their initial PCE under continuous illumination (AM 1.5G, 100 mW cm À2 , room temperature) in a nitrogen-lled glovebox for 500 h ( Fig. 3c and d). All-inorganic 0D/3D Cs 4 Pb(IBr) 6 /CsPbI 3Àx Br x mixeddimensional perovskite solar cells were also fabricated, by spontaneously distributing the 0D Cs 4 Pb(IBr) 6 phase in the 3D CsPbI 3Àx Br x perovskite phase (Fig. 3e). 70 Due to the good energy level alignment and lattice match, a 0D-3D heterojunction structure was formed. The defect passivation and non-radiative recombination suppression within the lms effectively promoted carrier transport in the PSCs, boosting the PCE to 14.77%. The derived devices showed an obviously enhanced stability, retaining 93.9% of the initial efficiency aer 60 days in an N 2 atmosphere. Selected studies of all-inorganic PQDs homogeneous hybridization in perovskite bulk lms are summarised in the lower part of Table 1. With the in-depth study and continuous optimization of the hybridization methods, as well as interfacial engineering of all-inorganic PQDs and perovskite lms, the PCE and stability of PSCs have seen considerable improvement by increasing the ability of charge transfer/extraction, passivating the defect trap states, depressing ion migration or enhancing the crystallinity of the perovskite absorber layer.

Advantages of heterogeneous hybridization
Unlike monophase PQDs, co-sensitization of perovskites and other QDs has also aroused considerable interest regarding the combination of the two material families. They show more freedom in tunability or optimization of their properties. Chalcogenide QDs are promising photovoltaic materials due to their extraordinary physical properties, such as tunable bandgaps by size control, intrinsic high extinction coefficients, and high dipole moments. 74 Owing to the small lattice mismatch between the two types of materials, the integration of lead halide perovskites and lead chalcogenide QDs with various microstructures and compositions has also been used in solar cells. Particularly, PbS QDs have been likely the most widely used because of their broad light-harvesting capability to the infrared (IR) region, similar cubic crystal structures, and the relatively low lattice mismatch with perovskites. 75 This section describes heterogeneous hybridization, mainly focusing on the hybridization of PbS QDs with perovskite matrixes. Some other promising chalcogenide QDs and emerging 2D QDs are also introduced.

PbS QD hybrids with organic-inorganic perovskites
PbS QDs have been integrated into planar PSCs to form epitaxial interfaces such as an effective HTM with appropriate band  (Fig. 4a). Having an appropriate band energy alignment is a crucial parameter to achieve a good performance of solar cells. By tuning the size of PbS colloidal QDs, as shown in Fig. 4b, Hu et al. engineered the energy alignment between PbS QDs acting as the HTM and the perovskite active layer, allowing the harvesting of photons with up to 1000 nm wavelength and increased signicantly the performance. 74 Dang et al. reported a synergistic effect of employing PbS QDs into PSCs, as they induced hydrophobic modication of the perovskite surface, leading to an improvement of the device stability in air ( Fig. 4c and d). 77 One of the critical issues on the efficiency improvement for both depleted and sensitized colloidal QD solar cells has been the passivation of surface trap states. 80 The long-chain surface ligands can stabilize and control the size of colloidal QDs, however, they are also electronic barriers which compromise photogenerated carrier transport of QDs, and ultimately affect the overall performance of solar cells. 81 The ligand exchange procedure opens the possibility to exchange to short ligands, improving the interdot electronic communication, and moreover passivating the trap states.
The bulky organic capping layer of colloidal QDs was exchanged by the perovskite or perovskite precursor shell. In 2014, Dirin et al. presented a general methodology for the surface functionalization of colloidal NCs with perovskites and other metal halide complexes as inorganic capping ligands, showing that MAPbI 3 was the hybrid capping of PbS NCs able to retain highly efficient IR photoluminescence quantum yield. 82 Seo et al. carried out perovskite passivation of PbS QDs. 83 In the same year, Ning et al. produced highly concentrated quantum dot inks, using butylamine as the solvent, that can be directly deposited into thick, uniform, QD lms. 84 In the following year, they reported for the rst time the preparation of thin lms with the perovskite matrix and embedded colloidal QDs. 33 Structurally, the PbS QD lattice matches well with the MAPbI 3 structure both three-dimensionally (Fig. 5a) and two-dimensionally (Fig. 5b), showing less than 4.6% lattice mismatch. 33 The density functional theory (DFT) calculation of an interface formation energy between PbS (100) and MAPbI 3 (110) planes showed an interfacial energy less than 10 meVÅ À2 , suggesting that growth of MAPbI 3 on PbS at room temperature is nearly homoepitaxy. 33 DFT calculations further reveal that the epitaxial three-dimensional embedding of PbS QDs inside a perovskite matrix is achieved without the formation of interfacial defects. By combining the electrical transport properties of the perovskite matrix with the high radiative efficiency of the QDs, strong PL emission is achieved from PbS QDs as photogenerated electrons transfer from the perovskite to embedded QDs with an efficiency exceeding 80%. Since these early demonstrations, perovskite lms with embedded QDs were applied as an active layer in different optoelectronic devices, such as solar cells, [85][86][87] LEDs, 88 and photodetectors. 89 The introduction of PbS QDs into the perovskite matrix has increased the photoconversion performance of QD solar cells by the effect of QDs in the crystallization process of the perovskite layer. Recently, Gaulding et al. systematically investigated how the QD size, surface chemistry, and metal halide perovskite (MHP) lm formation methods affect the resulting optoelectronic properties of QD/MHP "dot-in-matrix" systems, as shown in Fig. 4e. 79 PbS acted as effective seeding sites to promote the growth of perovskite crystals, delivering substantial morphological improvements in terms of grain size, surface coverage, and uniformity. Through analyses of time-resolved crystal formation kinetics obtained from synchrotron X-rays with a fast subsecond probing time resolution, an important "catalytic" role of the seed-like PbS NCs is clearly elucidated. 90 The PSC performance observed by the appropriate incorporation of PbS ( Fig. 5c and d), with concentration 1.0 wt%, reached 17.4%, approximately a 25% improvement with respect to the reference devices. 87 Han et al. attained a performance of 18.6% with the QD in the perovskite strategy. 85 A similar effect in the grain size by QD addition has also been observed in other studies. 86,[91][92][93] The main mechanism in the ligand exchange from organic QD capping to the halide or perovskite shell is based on the binding of iodine anions to lead atoms on the QD surface. 94 The sturdy and inert shell insulate the QDs from oxygen or water molecules and protect them from photocorrosion. However, agglomeration could be produced. With proper optimization of the ligand exchange process, Lan et al. introduced more iodine on the QD surface without the detrimental effect of fusion, enabling increased passivation, and enhancing the performance of QD solar cells to 10.6%. 95 This strategy has been further improved using lead halide ligands, reaching a solar cell efficiency of 11.28%. 96 Moreover, the passivation of lead halide also improved the stability of devices: 90% of initial efficiency was maintained aer 1000 h of storage under ambient conditions. 96 Yang et al. incorporated the core-shell structure of PbS/ MAPbI 3 QDs into a photovoltaic device with a graded band structure, achieving a better balance of open-circuit voltage (V oc ) and J sc , yielding a record solar cell performance (reaching 8.95% of PCE) using the depleted heterojunction QD solar cell conguration; however, QD solar cells strongly suffer from the hysteresis in photocurrent-voltage curves. 78 More recently, Ma et al. established a multifunctional interface layer of perovskite ligand modied PbS QDs to improve the performance and stability of PSCs, reaching a high V oc of 1.13 V and FF of 80% respectively, achieving a high PCE of 20.64% (Fig. 5e). 97 Moreover, the device with PbS-perovskite QDs was reported to stay nearly three times longer than the control device before its PCE decaying to 90% of its initial value under ambient conditions and continuous light soaking. They attributed the long-term stability to inhibited iodide ion mobilization, reduced defects, and increased moisture resistance ability by a more hydrophobic surface with the multifunctional interface layer. In another study reported by Masi et al., PbS QDs were used as a stabilizing agent for the FAPbI 3 perovskite black phase. They developed strong bonds with the black phase FAPbI 3 , setting   74 15.6 In an N 2 atmosphere at room temperature, the bilayer device maintains 84% of the initial PCE aer 350 h of aging, whereas the controls maintain 61% of the initial values a record of concomitantly fast formation for FAPbI 3 at temperatures as low as 85 C in just 10 minutes. FAPbI 3 thin lms obtained through this procedure preserve the original low band gap of 1.5 eV, reach a record open-circuit voltage of 1.105 V (91% of the maximum theoretical V oc ) and preserve high efficiency for more than 700 hours. 93 For a quick overview of the heterogeneous hybridization of PbS QDs and perovskite bulk lms, some typical reports are listed in Table 2. With an appropriate band energy alignment, PbS QDs have been integrated into planar PSCs as an effective HTM. The combination of perovskite and PbS QDs overcame the absorption limitation, resulting in a higher short-circuit current density (J sc ), compared with that for single perovskite devices with a contribution to the photocurrent of photons harvested by the two co-absorbers. PbS QDs acted as effective seeding sites to promote the growth of perovskite crystals, delivering substantial morphological improvements in terms of grain size, surface coverage, and uniformity. On the other hand, in order to reduce surface trap states, the perovskite or perovskite precursor shell acted as passivating agents and exchanged the capping layer of PbS QDs, thus increasing the photoconversion performance of QD solar cells. For comparison, in the lower part of Table 2, selected studies with heterogenous hybridization of all-inorganic perovskites with PbS QDs are shown, which will be explained in detail in Part 3.2. With the strategies of CsPbI 3 shell passivated PbS QDs, multi-cation perovskites, or nano-heterojunctions, sub-bandgap trap-state assisted recombination was diminished, and the charge collection was promoted. They exhibit not only improved PCE performance but also remarkable stability.

PbS QD hybrids with all-inorganic perovskite lms
An emerging alternative to organic-inorganic hybrid halide perovskites is the use of all-inorganic perovskites, which exhibit higher stability. 71,89,[98][99][100][101] It was reported that the CsPbI 3 shell passivated PbS QDs diminished the sub-bandgap trap-state assisted recombination, hence improving the charge collection and allowing performance as high as 10.5%. 98 In addition, colloidal QD solar cells based on PbS/CsPbI 3 QDs present extremely low hysteresis, not only in different scanning directions but also in different scan sweep steps 98 (Fig. 6a and b). Moreover, these devices also exhibit remarkable stability not only under continuous 1-sun illumination for 46 h, but also under ambient conditions for 42 days. 98 On the other hand, Jung et al. studied theoretically the bond formation and carrier connement at the PbS-CsPbBr 3 interface. They elucidated that PbS and CsPbBr 3 present just $0.5% lattice mismatch, much lower than the lattice mismatch between PbS and MAPbI 3 ($5%), which ensures high-quality epitaxy with low strain between PbS QDs and CsPbBr 3 . 88 Lattice mismatch can be further reduced by the use of mixed halide I-Br or Br-Cl perovskites. By tuning the ratio of Br to I in the CsPbBr x I 3Àx matrix composition, Liu et al. achieved nearzero lattice mismatch (3) for PbS QDs at a Br content of approximately 66% (3 < 0.2%), enabling the strain-free epitaxial growth of perovskite (Fig. 6c). The lattice-anchored QDs:perovskite solid exhibits a photoluminescence quantum efficiency of 30 percent for a QD solid emitting at infrared wavelengths, and a doubling in charge carrier mobility as a result of a reduced energy barrier for carrier hopping compared with the pure QD solid. The lm stability is improved from three days to more than six months when 13 vol% QDs are incorporated (Fig. 6d). 34 Furthermore, the devices with the 15 vol% CsPbBr 2 I matrix exhibited the highest PCE of 12.6%, which is a reproducibly enhanced performance relative to controls. They also showed improved photostability, retaining 95% of their initial PCE under continuous AM1.5G illumination for 2 h, unencapsulated, while the PCE of controls degraded to 70% of their initial value within an hour. This is attributed to the latticematching perovskite matrix which provides surface passivation and lowers the energy barrier for carrier hopping.
The nano-heterojunction or contact heterojunction structure is different from the core-shell structure. Discrete nanoparticles each composed of QDs and hetero nanoparticles are combined by chemical bonding or physical contact. 35,47,92,102,103 The nano-heterojunction structure may incorporate new functions beyond those of each independent component. Yang et al. devised a strategy based on n-and p-type ligands that judiciously shied the QD band alignment. 35 The best performing devices and controls were achieved using a mixture of organohalide perovskite ligand based MAPbI 3 for electron acceptor (Atype) QDs, and thioglycerol for donor (D-type) dots. Interdot carrier transfer and exciton dissociation studies conrmed efficient charge separation at the nanoscale interfaces between the two classes of QDs. The rst fabricated mixed-QD solar cells achieved a power conversion of 10.4%, surpassing the performance of previously reported bulk heterojunction QD devices fully two-fold 35 (Fig. 7a and b). The unencapsulated device retains 80% PCE aer 10 days of storage in air. In another study ( Fig. 7c and d), heterostructured CsPbX 3 -PbS (X ¼ Cl, Br, I) QDs were successfully synthesized via a room temperature in situ growth of PbS method by using hexamethyldisilathiane ((TMS) 2 S) as the sulfur precursor. 103 The CsPbX 3 -PbS QDs exhibited dual PL peaks in visible and near infrared regions, corresponding to CsPbX 3 and PbS, respectively. The femtosecond TAS study conrmed that exciton energy effectively transferred from CsPbBr 3 to PbS in CsPbBr 3 -PbS QDs when CsPbBr 3 is excited. Nano-heterojunctions containing PQDs and hetero QDs may promote the development of highly efficient photovoltaic and optoelectronic devices.

Other chalcogenide QD hybrids with perovskite lms
Besides PbS QDs, other chalcogenide QDs such as CdSe, 102,104 CdTe, 105 ZnSe, 106 CdSe/ZnS or CdS/ZnS core/shell QDs, 53,104 SnS, 107 ZnO, 14 copper indium sulphide (CIS), 108 Cu 12 Sb 4 S 13 (CAS), 109 etc. are utilized to hybridize with perovskites. CdSe/ CsPbI 3 QDs were used as an interface layer between the MAPbI 3 perovskite lm and the hole transport layer to promote interfacial charge extraction and enhance light-harvesting ability simultaneously. 102 Compared with pristine PSCs, hybrid PSCs achieve 21% enhancement in PCE, which can be attributed to the ultrafast charge carrier dynamics and Förster resonance energy transfer effect ( Fig. 8a and b). In Fig. 8c, it is shown that the modied devices with QDs show better stability than those without QDs, which should be attributed to the weakening of the affinity of the perovskite lm to water by the QD layer. 102 Hanmandlu et al. perovskite to passivate the surface charge traps and grain boundaries. 104 Based on this passivation strategy, the target devices exhibited a PCE close to 20% with suppressed hysteresis in the J-V characteristics and a ll factor of 81.44%. Moreover, they achieved long-term stability under an N 2 atmosphere at 50 C, retaining 80 and 75% of their initial PCE aer 720 h and aer 2400 h, respectively; in contrast, the control device lost 70% aer 360 h.
Similarly, with the congeneric junction contact between the perovskite and CdTe QDs, the devices with the CdTe QDs in perovskite lms achieve a high efficiency ($19.3%, averaged) with a signicantly reduced hysteresis (Fig. 8d). 105 The reduced hysteresis is partially contributed from faster hole extraction at the interface thanks to the high hole mobility in CdTe.
Tavakoli et al. synthesized a quasi-core shell structure of ZnO/reduced graphene oxide (rGO) QDs as an electron transfer layer (Fig. 8e). 14 In this regard, rGO not only passivated the surface of ZnO nanoparticles to prevent the reaction with MAI, but also extracted the charge carriers quickly from the perovskite layer to reduce carrier recombination. The resulting PSC on the ZnO/rGO layer exhibited a stable PCE as high as 15.2% and 11.2% on uorine-doped tin oxide (FTO) glass and polyethylene terephthalate (PET) substrates, respectively, under AM1.5G illumination. Besides, the PCE dropped only 10% aer 30 days, which was much more stable than that of the pure ZnO one (90% drop) (Fig. 8f). In this case, the ZnO/rGO quasi-core shelled QDs were inserted into a layer-by-layer structure, which is considered as one type of hybrid structure in a broad sense. However, because of our review focus on direct hybridization of QDs with perovskites at the level of the materials, we will not expand more discussions for layer-by-layer hybridizations.
As an absorber layer in PSCs, SnS QDs-MAPbI 3 hybrid lms were fabricated by a facile in situ crystallization method, enabling easy embedding of the QDs in the perovskite layer (Fig. 8g). 107 Compared with SCs based on pure MAPbI 3 , the champion SCs achieve a 25% enhancement in efficiency, giving rise to an efficiency of 16.8%. The improved performance can be attributed to the optimized crystallinity of the absorber, enhanced photo-induced carrier separation and transport within the absorber layer, and increased absorption intensity in the visible and NIR light region.
The incorporation of a CIS NC layer into the perovskite solar cell gave rise to enhanced hole transport and moisture stability. 108 Water contact angles of perovskite lms without and with CIS NCs show that CIS is more hydrophobic, and is likely to slow the rate at which moisture is able to inltrate into the perovskite. With the protection of CIS NCs, they prevented moisture from penetration into the perovskite and acted as a moisture resistant coating. 108 The derived devices exhibited superior resistance to humidity and improved photovoltaic stability aer 4 h under 90% humidity without device encapsulation (Fig. 8h). In contrast, the PCE of PSCs without a CIS layer decreased signicantly. Meanwhile, the CIS NC layer can modify the interface between the perovskite and hole transporting material, suppressing charge recombination pathways.
The surface oleylamine ligands of CAS QDs are exchanged with 3-mercaptopropionic acid, realizing enhanced electronic coupling and a reduced band gap. 109 The all-inorganic PQD SCs with CAS QDs exhibit a PCE of 10.02%, achieving a high J sc of 18.28 mA cm À2 because of the improved light absorption and hole extraction ability of CAS QDs. Moreover, CAS QD based PSCs exhibit enhanced long-term stability and retain 94% of their initial PCE aer storage in ambient air for 360 h. Table 3 summarises selective studies of chalcogenide QDs hybridized into PSCs. The heterogeneous hybridization of chalcogenide QDs in PSCs promoted interfacial charge transport and extraction, crystallinity of the absorber, and lightharvesting ability simultaneously, which gave rise to enhanced performance and stability. This is associated with high hole mobility, broadening absorption range, grain boundary passivation, or the weakening affinity to water by the chalcogenide QDs.

Emerging 2D QD hybrids with perovskite lms
In addition to chalcogenide QDs, QDs such as carbon QDs, 50,110,111 graphene QDs (GQDs), 112,113 black phosphorus QDs (BPQDs), 114 etc., have emerged as efficient perovskite absorbers to improve the PCE and the stability of PSC devices. Ma et al. added hydroxyl and carbonyl functional group-containing carbon QDs into the perovskite precursor solution to passivate the uncoordinated Pb 2+ ions in grain boundaries (Fig. 9a). 50 This method decreased nonradiative recombination, leading to higher PL intensities and longer carrier lifetimes in the perovskite lm. The champion device achieved a PCE of 18.24% and retained 73.4% of its initial PCE aer aging for 48 h under 80% humidity in the dark at room temperature (Fig. 9b).
GQDs have been another focus in the interfacial engineering of PSCs due to their high charge carrier mobility and high transmittance in the entire visible light spectrum, showing great promising for photovoltaic applications. Inserting an ultrathin layer of GQDs between the perovskite and the mesoporous titanium dioxide ETL facilitates electron transfer, leading to the boosting of the photocurrent and thus the efficiency of the corresponding solar cells. 112 The graphene QDs act as a superfast electron tunnel that strongly quenches the PL of the perovskite absorber and decreases the electron extraction time from (260-307 ps) to (90-106 ps) (Fig. 9c). Zhou et al. combined graphene QDs and SnO 2 in an effective ETL for highperformance exible PSCs. 113 The optimized graphene QDs/ SnO 2 ETL has higher electron mobility, better lm coverage and better energy level alignment matching compared to the pristine SnO 2 , resulting in promoted charge transfer and suppressed charge recombination (Fig. 9d). Moreover, the optimized device yielded PCEs of 19.6% for the rigid substrate and 17.7% for the exible substrate.
Recently, BPQDs are incorporated as effective nucleation sites to modulate the crystalline growth of CsPbI 2 Br perovskite thin layers, facilitating crystallization and lm morphology. 114 The lone-pair electrons of BPQDs can induce strong interaction of intermolecular combination with molecules of the CsPbI 2 Br precursor solution. The BPQDs@CsPbI 2 Br hybrid lms concomitantly reinforce a stable CsPbI 2 Br crystallite and suppress the oxidation of BPQDs. Consequently, an impressive PCE of 15.47% was achieved for BPQDs@CsPbI 2 Br (0.7 wt%) hybrid lm devices, with an enhanced cell stability, under ambient conditions. 2D QDs have emerged as additives for interfacial engineering to enhance both the PCE and stability of PSC devices (Table 4). Due to their unique optoelectronic properties like high charge carrier mobility and their excellent nonlinear optical properties, they result in enhanced-PL intensities, longer carrier lifetimes, larger grain boundaries, reduced non-radiative recombination and facilitated crystallization and lm morphology when hybridizing in the perovskite layer. Their interaction suggested a promising route toward the future realization of efficient and stable PSCs.

Summary and outlook
Both QDs and perovskites are highly promising in the photovoltaic eld and have achieved signicant progress in a short period of time. The homogeneous and heterogeneous combination of QDs and perovskites leads to high-performance photovoltaics. On the basis of their crystal structures, the relatively low lattice mismatch promotes the crystallization of the perovskite lattice with large grains. The complimentary lightharvesting extends the absorption from visible to the NIR region with QD-enhanced light collection. Besides, the band alignment between the perovskite and QDs can also be tailored to favour efficient carrier transport and rapid extraction of charge carriers. By surface and interface engineering, the perovskite can increase the passivation of the QD surface, thus enhancing the stability.
Although impressive progress has been achieved in the performance and stability of PSCs with perovskite and QD hybridization, the improvement mechanism is not clear until now. The PCE values are relatively lower than those of pure PSCs with the highest efficiency. This is associated with the material and device properties of both QDs and perovskite solar cells, as well as the lattice mismatch and energy alignment between the two material families. On the other hand, the design of the device structure is signicant for performance improvement. Future studies call for a systematic exploration of interface engineering, band-structure of materials, and optimized congurations of the hybridization of QDs and perovskite. We compare and analyze the above-mentioned hybrid structures, and then we summarise the prospects of the future directions of this hybrid QDs/perovskite SC as follows in Fig. 10.

Summary of hybrid structures
To date, there are four types of perovskite and QD heterostructures reported: (1) QD blending in the continuous perovskite matrix; (2) multilayer architectures composed of perovskites and QDs; (3) capping traditional QDs with a perovskite layer in PSCs, or a reverse structure to grow an entire shell on perovskite QDs; (4) nano-heterojunctions of perovskites and other QDs. One primary goal of introducing QDs into or on top of perovskite lms was device operation related, namely to improve the device performance by enhancing energy level alignment, and efficient charge extraction and charge transport rate. In other cases, interface engineering has also been applied between the perovskite bulk lm and charge transport layers for surface defect passivation, QD coupling and device performance improvements.
Annealing time is a crucial parameter to determine whether the hybridizing type is blending or formation of a multilayer structure. When the QD solution was added before lm annealing, it is suggested that QDs would be incorporating into the perovskite lm via diffusion reaction. Otherwise, solar cells with a multilayer structure would be fabricated via layer-by-layer deposition with a multi-junction stacking technique. However, the multilayer or bilayer structures do not mean they are interaction-free at the interface. A component-graded heterojunction is formed throughout the perovskite/QD bilayer interface via interface engineering. Specically, some of the ions in the perovskite layer are exchanged with ions from QDs, and vice versa.
The resultant inuences of the interaction between QDs and the perovskite lm include ligand exchange and nano heterojunctions or contact heterojunctions. The ligand exchange procedure is adopted in a core-shell structure, which can exchange to a short ligand, improve the interdot electronic communication, and thus passivate the trap states. Nanoheterojunction structures combine the two materials by chemical bonding or physical contact, and terminate the dangling bond. When fabricating solar cells, one or more techniques could be applied at the same time. For example, a core-shell structured QD could be blended in perovskite bulk lms, and also used as an interface layer to fabricate multiple layers and quantum junctions for optoelectronic applications. In Table 5, we have summarized the best PCE enhancement with homogeneous hybridization and heterogeneous hybridization in PSCs. It is worth mentioning that we didn't compare the stability here as there isn't a uniform standard for stability.

Materials innovation
The tunable optical and electronic properties of QDs and perovskites provide a great possibility for optimizing the hybridizations. Besides the above-mentioned hybridizations, the selection of stoichiometry in QDs, multi-cation strategies by substituting A, B or X sites in the perovskite crystal structure (ABX 3 ), as well as optimization of surface ligands are promising to further facilitate the near-perfect interfacial lattice match for QD/perovskite heterostructures. For example, other octahedral metal hexa-halide cluster ligands, such as MA 4 PbI 6 , MA 3 BiI 6 , MA 4 -InCl 6 , and MA 4 MnCl 6 , also allow the quasi-epitaxial growth of the ligand shell. 115 Moreover, the processing methods of both material families and their interfacial physics and chemistry require further in-depth study, which may offer more insights into exploring effective strategies to achieve defect-free interfaces.

Device structure innovation
The homogeneous and heterogeneous hybridization of QDs and perovskites has been proved to improve the performance of optoelectronic devices based on strategies like fabricating multilayer structures and core-shell structures, blending or constructing nano-heterojunction structures. With the emergence of tandem and/or cascade junction solar cells with boosting efficiency, the solution processability of both QDs and perovskite provides promising opportunities towards applications for hybrid devices. Besides, the interaction between the perovskite and QDs also presents new properties that do not exist in single-phase materials, such as the formation of the exciplex state at lower energies than both the perovskite and QD band gap, demonstrating the potentiality for the development of advanced optoelectronic devices, such as tunable color LEDs 116

Conflicts of interest
There are no conicts to declare.