Î±-CsPbI3 Colloidal Quantum Dots: Synthesis, Photodynamics and

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#-CsPbI3 Colloidal Quantum Dots: Synthesis, Photodynamics and Photovoltaic Applications Jiantuo Gan, Jingxuan He, Robert L. Z. Hoye, Abdurashid Mavlonov, Fazal Raziq, Judith L. MacManusDriscoll, XiaoQiang Wu, Sean Li, Xiaotao Zu, Yiqiang Zhan, Xiaoyong Zhang, and Liang Qiao ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00634 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019

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ACS Energy Letters

α-CsPbI3 Colloidal Quantum Dots: Synthesis, Photodynamics and Photovoltaic Applications Jiantuo Gan,*a Jingxuan He, a Robert L. Z. Hoye,b Abdurashid Mavlonov,a Fazal Raziq,a Judith L. MacManus-Driscoll,b Xiaoqiang Wu,a Sean Li,c Xiaotao Zu,a Yiqiang Zhan,d Xiaoyong Zhang,e and Liang Qiao*a,c a. School of Physics, University of Electronic Science and Technology of China, Chengdu 610054, PR China. b. Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge, CB3 0FS, UK. c. School of Materials Science and Engineering, University of New South Wales, Sydney 2052, NSW AU. d. Center of Micro-Nano System, SIST, Fudan University, Shanghai 200433, PR China. e. School of Materials Science and Engineering, Xi’an Shiyou University, Xi’an 710065, PR China. ABSTRACT: Owing to their defect tolerance and phase-stability, α-CsPbI3 colloidal quantum dots (CQDs) with high mobility and 80–95% photoluminescence quantum yield (PLQY) are promising candidates for next-generation photovoltaics (PVs). Recently, α-CsPbI3 CQD PVs have begun to show promising power conversion efficiencies of 13.4%, with the open circuit voltage approaching the Shockley-Queisser limit. These devices are stable in ambient conditions for several months. However, the short circuit current density (JSC) of ~12 mA/cm2 is low and the limiting mechanisms are unclear. In this work, we review the strategies for improving the JSC, and the effect of interfaces and mobility of the charge transport layers on carrier extraction. We also evaluate strategies to enhance the stability of CsPbI3 CQDs under illumination, as well as methods to elucidate the recombination losses in the CQD PVs and methods to reduce these losses. This work provides routes to achieve efficient and stable α-CsPbI3 CQD PVs.

Solar photovoltaics (PVs) using colloidal quantum dots (CQDs) are attracting attention for the fabrication of efficient next-generation solar cells.1 Their theoretical efficiency has potential to break the Shockley-Queisser (S-Q) limit of 33.7 % through multiple exciton generation (MEG).2, 3 The nanocrystals can also be prepared by a low CO2 footprint method: LaMer synthesis, during which the nanocrystals begins to nucleate immediately after precursor injection at low temperatures of 200 ℃ or below.4-9 So far, solar cells based on PbS nanocrystals (with a Bohr radius of ~18 nm10) represent the highest experimental efficiencies of over 11.2 % at laboratory scale among the chalcogenides.11-13 Their limitation, however, is the low open circuit voltage VOC (~0.5–0.6 V).14 By contrast, their short circuit current densities (JSC) have exceeded 30 mA/cm–2 through the use of nanostructured electrodes and optimizing the band structure. These strategies have not had similar effects on the VOC.15-18 Defect analyses of PbS CQDs have also revealed the presence of traps 0.3–0.4 eV below the conduction band minimum(CBM) with a high density of ~1017 cm–3,14, 19 which can limit the open circuit voltage.20 The VOC deficit is also strongly influenced by carrier recombination at the interfaces.21 More importantly, it is challenging to achieve efficient MEG in PbS CQD PVs, owing to the charge transport layer’s low transmittance for the high energy photons needed in

MEG.22 On the other hand, new types of quantum dots, such as, α-CsPbI3 nanocrystals that can form shallower defects and have good mobilities (0.2 cm2·V–1·s–1) are promising alternatives for PVs. But research into this field is still at the nascent stages. We therefore review the prospects of α-CsPbI3 CQDs for solar PVs. This review begins by discussing the advantages of α-CsPbI3 CQDs for PVs as well as the current factors limiting their performance. Afterwards, we address the possibility of engineering both the quantum dot and charge transport layers for efficient charge extraction, with reduced defect recombination. Finally, we identify effective strategies for enhancing charge transport and solar efficiency in addition to photostability. Emerging colloidal quantum dots with high photoluminescence quantum yields and mobilities are urgently needed for highly efficient and stable PVs. The features of α-CsPbI3 CQDs for PVs. As an emerging solar absorber, CQDs of α-CsPbI3 hold several advantages over conventional lead chalcogenides and hybrid leadhalide perovskites (LHP). First, α-CsPbI3 shows an electronic structure that is conducive to forming shallow defects. This property, which enables defect tolerance, is typical to lead halide perovskites but rarely observed in

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Figure 1. (A) Schematic of the band structure of α-CsPbI3 showing the tendency to form traps resonant with the bands or shallow within the bandgap.23 (B) TRPL spectra for different types of CQDs, including: MAPbI3, FAI-CsPbI3, CsPbI3, PbSe and PbS CQDs. (C) Bar charts of hole mobility and lifetime for these CQDs.6 (D) Absorption spectra for α-CsPbI3 CQDs under different synthesis temperature (60–185 ℃ of CQD dispersing solution in the inset). (E) TEM image of cubic phase α-CsPbI3 CQD array. (F) XRD pattern for α-CsPbI3 CQDs processed at 60–185 ℃.5 Reprinted with permission from refs 5,6,23.

traditional semiconductors.23, 24 As illustrated in Fig. 1A, both the conduction band (CB) and valence band (VB) are dominated by anti-bonding orbitals, which favor either shallow trap states or defects resonant within the CB or VB, leading to lower rates of Shockley-Read-Hall recombination.25 By contrast, bandgaps for traditional metal chalcogenide semiconductors typically form bonding and anti-bonding orbitals, which tends to localize deep traps in the band gap.23 Defect calculations on α-CsPbI3, show that the lowest formation energy defects (e.g., lead or iodide vacancies) have transitions levels resonant within the bands, which is consistent with Fig. 1A and suggests that these are not recombination active. The defects forming deep transition levels (namely IPb anti-site defects) have high formation energies, and therefore are not expected to be present.26 Both indicate α-CsPbI3 to be defect tolerant, and α-CsPbI3 has been found to exhibit an order of magnitude longer carrier lifetime (~2 ns) than PbS, despite also having high defect densities (~1016 cm-3) (see Figs.1B&C).6 Similar effects are observed in hybrid LHPs, in which long lifetimes are achieved despite high defect densities.27 In addition, α-CsPbI3 CQDs also shows higher hole mobility (μ~0.2–0.5 cm2/V·s) than that of PbS (Fig.1C).6 As a result, the power conversion efficiency (PCE) of α-CsPbI3 CQDPVs has recently been demon-

strated to reach a record value of ~13.4% with a high VOC (~1.2 V). Although the PLQY was not given in this work, other authors have achieved PLQYs near unity with αCsPbI3 CQDs, and this is consistent with the presence of few non-radiative losses, which is needed to achieve such high Vocs.5, 6, 28 Further, α-CsPbI3 shows size-dependent phase-stability.6, 29 Although bulk α-CsPbI3 thin films have exhibited longer carrier lifetimes and have achieved higher device efficiencies (14–15 %),30, 31 it is not easy to maintain the desired cubic structure in the bulk film. From thermodynamic considerations, cubic α-CsPbI3 transforms to an orthorhombic (δ-phase) structure at 320 ℃,32 leading to significant degradation in the device performance. To solve this problem, the nanocrystals with reduced dimensions are synthesized. These CsPbI3 CQDs (with sizedependent bandgap Eg of 1.73–2.3 eV, in Fig.1D&E) exhibited better phase stability. This was attributed to the larger surface energy of the nanocrystals compare to the bulk grainsm,6, 29, 33 but further research is needed to clarify the precise mechanism. In addition, the excess unreacted precursors can be removed without inducing aggregation of the α-CsPbI3 nanocrystals using methyl acetate (MeOAc), thus avoiding the phase transitions of the cubic phase to the undesired orthorhombic phase (Fig.1F). Thirdly, α-CsPbI3 nanocrystals are normally reported to


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ACS Energy Letters

Table 1. PCE, related J-V characteristics and stability of α-CsPbI3 CQD solar cells. η

Eg

(%)

VOC (V)

JSC (mA/cm2)

FF (%)

(eV)

FTO/TiO2/α-CsPbI3/spiro-OMeTAD/MoO3/Al

10.77

1.23

13.47

65

FTO/TiO2/FAI coated α-CsPbI3/spiroOMeTAD/MoO3/Al

13.4

1.2

14.37

FTO/TiO2/μ-graphene α-CsPbI3/PTAA/Au

11.64

1.1

FTO/TiO2/varied thickness α-CsPbI3/spiroOMeTAD/MoO3/Ag

9.78

FTO/TiO2/α-CsPbI3/PTB7/MoO3/Ag FTO/TiO2/GeI2 additive α-CsPbI3/spiro-OMeTAD/Au

Device Structure

Stability

Year

~1.73

60 days

20165

78

~1.73

-

20176

12.18

76

1.82

1 month in N2

201834

1.06

12.24

73

~1.7

within 60 hours

201835

12.55

1.27

12.39

80

-

-

201836

12.15

1.11

14.80

74

-

3 months

201937

Figure 2. (A) Schematics of the layer structure in a α-CsPbI3 CQD solar cell and (B) corresponding SEM cross-sectional view.5 (C) XRD patterns for α-CsPbI3 CQDs after one day and two months. (D) J-V characteristics. (E) EQE spectrum and integrated JSC.5 (F)VOC as a relation with absorber bandgap for different types of CsPbX3.36 Reprinted with permission from refs 5,36.

have high photo-luminescence quantum yields (PLQYs) between 80–95 % in the green-red wavelength region.23 The values have recently been demonstrated to reach near-unity with a stability of up to a month using tri-noctylphosphine (TOP) ligands instead of oleylamine.38 In PbS CQDs, it is possible to generate more-than-one pair of electrons and holes, if the energy of the incident photons is higher than the MEG threshold (typically ~3Eg).39 In reality, the charge transport layer in the devices tend to have low transmittance for photons with such high energy, such that most are lost before they can be absorbed by the PbS CQDs.23 Thus, so far it has been challenging to harvest these potential multiple carriers. Fortunately, this is no such threshold energy requirement for reaching unity PLQY and thus the bandgap of the charge transport layer is not the main challenge for α-CsPbI3

CQDPVs, regardless of their short Bohr radius (~6–12 nm).40-42 α-CsPbI3 CQDs are very promising for future PV as well other optoelectronic applications. Even so, until 2016 α-CsPbI3 CQD n-i-p solar cells22 (Figs.2A&B) began to show PCE of 10.77 % with the cubic phase remaining stable for two months under ambient conditions, as shown in Fig.2C. The obtained EQE varies between 40–70 % in the wavelength range of 350–700 nm, while it is higher in the short wavelength range (350–500 nm) than that in the long wavelength range (500–700 nm) (Figs2.D&E). The efficiency rapidly reached 13.4% as a result of more efficient charge transport through nanocrystal modification, such as FAI-coating, micrometer-



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Figure 3. (A)-(C) Photographs of α-CsPbI3 nanocrystals made via LaMer synthesis, with halide exchange of Br– ions in different proportions.35 (D) Schematic of μ-modified CQDs for efficient charge transport and resistance to moisture.34 (E) Schematic of CsPbBr3 CQDs with dynamic binding using conventional capping ligands in the ionized form (OA–, Br– and OLAH–). (F) Tight binding with Zwitterionic capping ligand.43 (G) Schematics of MAPbI3 CQDs modified by PPA functional conjugated ligands and (H) by n-octylamine.44 Reprinted with permission from refs. 34,35,43,44.


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ACS Energy Letters

sized graphene (μ-graphene) cross-linking, GeI2 additives etc, as summarized in Table 1.5, 6, 34-37 A hole transport layer of PTB7 is also helpful for charge transport through improved band alignment with the CsPbI3.36 So far, in contrast to PbS, the VOC of α-CsPbI3 has reached close to the SQ limit of ~1.2 V (see blue triangles in Fig.2F), whereas the JSC still has plenty of room for improvement from its current low values. To achieve a high JSC, the device should be studied from the following two aspects: 1) efficiency of carrier extraction and 2) reducing recombination loss at the interfaces. In this work, we therefore focus on strategies for improving charge transport within the CQDs and carrier extraction at the interfaces with the charge transport layers. For device applications, the photo-stability is another important factor which should be considered. We summarize the currently used strategies to improve the optoelectronic properties of the CQD layer. Furthermore, engineering of the charge transport layer is also discussed for improvement of charge injection rate at the hetero-interface. The performance of α-CsPbI3 CQDPVs is mainly limited by a low JSC, which critically depends on charge carrier transport in the CQD layer and injection into the charge transport layers. Improvement in charge transport in α-CsPbI3 CQDPVs. By analogy to the doping strategy (i.e., extrinsic ion replacement with lattice sites in ABX3 perovskite) used in traditional semiconductors, cation or anion exchange in LHPs has also been considered as a useful method to improve the efficiency. However, it has been shown that anion exchange (Figs. 3A-C) for perovskite α-CsPbI3 is not ideal for application in PVs.6 This is because exchanging I with Br or Cl will increase the bandgap, leading to a smaller fraction of the solar spectrum being reduced (in Fig.2F).45 On the other hand, A-site exchange with formamidinium (FA+) has recently been demonstrated to alter the surface chemistry of α-CsPbI3 CQDs and has doubled their mobility to 0.5 cm2/V·s. As a result, both JSC and fill factor (FF) were increased, leading to the record efficiency of α-CsPbI3 CQDs.6 Apart from lattice-site substitution, incorporation of suitable additives has also been shown to improve the efficiency. For example, Liu et al. introduced GeI2 into αCsPbI3 CQDs and demonstrated that an excess in iodide ions is effective for passivating CQDs leading to a stabilized near-unity PLQY, in comparison of a drop of 10 % in the control sample after 30 days. The PV devices increased in efficiency from approximately 10% to 12% after ~2 months, before dropping back to approximately 11 % after 3 months.37 Wang et al. hybridized μ-graphene with α-CsPbI3 CQDs (Fig. 3D) and found that μ-graphene with high mobility (800 cm2·V−1·s−1) facilitated cross-linking of the CQDs, ultimately leading to an increase in the JSC.34 The μ-graphene added created extra channels for efficient transport of the dissociated excitons. Carrier dynamics investigations by time-resolved photo-luminescence (TRPL) also confirmed that cross-linking promoted

charge transport. An additional advantage of μ-graphene addition is that it acts as an agglomeration spacer and moisture barrier, so that it stabilizes the α-phase under different temperatures in N2 for one month.34 Ligand modification is another potential strategy to increase JSC. Krieg et al. reported that capping the CQDs with zwitterionic molecular groups led to improved charge transport as well as retention of the PLQY in CsPbX3 CQDs compared to single head group capping.43 The conventional single head groups are oleic acid ions (OA–), Br– or oleylammonium ions (OLAH+), and are dynamically bound to the nanocrystals (see Fig.3E). The new types of zwitterionic molecules include sulfabetaine, phosphocholine and γ-aminoacid (see Fig.3F for molecular structures). In each molecule, both the deprotonated acid group and the quaternary ammonium are tightly bound to the nanocrystals by the chelate effect. More importantly, the cationic and anionic groups in the zwitterionic molecules prevent the external neutralization, so that the nanocrystals are not surrounded by the insulating molecules. Consequently, charge transport is improved, as reflected in a high photo-conductivity and current density in the light emitting diodes. Very recently, Dai et al. demonstrated that the 3-Phenyl-2-propen-1amine (PPA) functional conjugated ligand is more suitable for charge transport in comparison to the conventional n-octylamine ligand (Fig. 3G-H).44 Due to the insulating nature of the long alkyl-chain in n-octylamine, the device efficiency is greatly limited. In PPA, on the other hand, intrinsic delocalization of the molecular orbitals and π-π stacking of its aromatic ring ensures sufficient orbital overlap of neighboring ligand groups. As a result, the PPA modified CQDs are found to exhibit a 22 times enhancement in carrier mobility compared to CQDs using noctylamine, as evidenced by Hall effect measurements.44 Although PPA-enhanced efficiency is reported in hybrid LHPs, we envision that similar effects could also be found in α-CsPbI3 CQDs. Apart from above-mentioned treatment of the CQD layer, interface engineering is another approach to promote charge transport. In particular, the hole transport layer (HTL) PTB7 was observed to have a more favorable band alignment with the CQD layer than Spiro-OMeTAD, rr-P3HT or PTB7-Th did, leading to higher JSC, using the device structure shown in Fig.4A.36 Four different molecules were investigated as the HTL and the energy-level electronic band alignments of these layers with α-CsPbI3 were determined by ultraviolet photoelectron spectroscopy (see Fig. 4B&C). In this case, the CQDs are first treated with a mixed solution of Pb(NO3)2 and methyl acetate (MeOAc) with MeOAc helping to remove the insulating oleic acid/oleylamine (OA/m) ligands. In order to study the charge transfer mechanism, the carrier dynamics were evaluated by TRPL, where the carrier lifetime in four types of heterojunctions were determined (Fig.4D&E). The shortest carrier lifetime (~3 ns) is observed in PTB7, consistent with its relatively smaller VB offset with αCsPbI3 under type-I band alignment geometry. Given the



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Figure 4. (A) Schematics of CQDPV with hole transport layer. (B) Chemical structures of hole conductor materials of SpiroOMeTAD, rr-P3HT, PTB7 and PTB7-Th and (C) Band alignment of α-CsPbI3 CQDs and different HTLs. (D) The lifetime histogram of the sample extracted from 2D lifetime images. (E) The lifetime imaging results of α-CsPbI3 QD film.36 The Reprinted with permission from ref 36.

important role of HTL in charge transport in traditional PVs, more detailed studies of the influence of HTL on the structure-property correlation in α-CsPbI3 CQDPVs is greatly needed. It is advantageous to have further surface modification in α-CsPbI3 to enhance charge carrier transport. Photo-stability of α-CsPbI3 CQD solar cells. As with other LHPs, stability is a critical issue for α-CsPbI3. The stability of α-CsPbI3 includes the phase stability, photostability, thermal stability and stability against moisture. In this section, we will focus on the photo-stability against phase transitions under continuous light illumination. Recent works indicate that the nanocrystals exhibit superior cubic-phase stability in comparison with bulk materials, making them potentially more suitable for PVs. However, degradation occurs when they are continuously illuminated (white light with intensity of 100 mW/cm2) (see Figs.5A-D) and the cubic phase transformed to other phases within a few hours.46 Although Cs is almost the largest alkaline element, it is still not large enough to hold the even bigger corner-sharing [PbX6]4- octahedron and

CsPbI3 has a small Goldschmidt tolerance factor.47-49 Thus a phase transition to the more thermodynamically stable orthorhombic structure (δ-phase) is unavoidable (Figs.5E&F), as evidenced by XRD. In the study by An et al., the heat introduced during photoexcitation led to thermally-induced structural changes in addition to mechanical stress within the CQDs,46, 50, 51 leading to photodegradation. The degradation is independent of wavelength and intensity of the light source. To make it worse, detachment of the capping agents occurred upon long light illumination, lead to aggregation of surface lead interstitials (Pb0), which together with dangling bonds on the CQD surface, acted as trap states, increasing the carrier recombination.46 i) In order to stabilize the cubic phase, B-site doping has been demonstrated to be a successful strategy. Fig.5G compares the change in formation energy (ΔE) per divalent Pb2+ ions in undoped and doped nanocrystals. The higher ΔE values indicate greater stability (i.e.: more negative formation energy). From the chart, Mn2+, Zn2+, Sr2+, and Sn2+ are potential dopants, as reflected by their more negative formation energy.52 Akkerman et al. showed that


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Figure 5. (A)-(D) TEM images α-CsPbI3 CQDs at different degradation times.46 (E) Density of states for Mn2+ ions doped αCsPbI3 CQDs.47 (F) Evolution of XRD patterns for α-CsPbI3 CQDs.47 (G) Bar chart of formation energy required for B-site dopants in α-CsPbI3 CQDs.53 (H) Increased photo-stability with core/shell structure.54 (I) Different shapes of α-CsPbI3 under reaction temperature and length of acid chains during LaMer hot injection.55 Reprinted with permission from refs 46,47,53–55.

Mn2+ doping in α-CsPbI3, for instance resulted in, prolonged photo-stability for one month, compared to a few days for the undoped CsPbI3, along with increased PLQY and PL lifetimes (from 14 ns to 17 ns) as shown in Fig.5E.47 The reason for enhanced stability is attributed to a contraction in the unit cell caused by B-site substitution with the smaller divalent metals.56, 57 Simultaneously, the halide anions from doping precursors could possibly passivate the under-coordinated Pb atoms, resulting in conversion of the non-radiative defects into radiative states and ultimately enhancing both the PLQY and slow lifetime components (from 49.1 ns to 124.1 ns).58 Shen et al. investigated Zn-substituted α-CsPbI3, and found that partial substitution with Zn2+ led to enhanced stability of over 70 days in open air due to lattice contraction. At the same

time, the non-radiative PL decay was reduced because of a suppression in non-radiative recombination, leading to an increased PLQY of 98.5%.59 Yao et al. proved that Sr2+ substitution along with iodide passivation strategy was also effective to reduce the structural distortion by significantly increasing the formation energies. This led to stabilization of the cubic phase with nanocrystals 5–15 nm in size. These Sr-doped CsPbI3 CQDs retained high PLQY in the densely-packed film.60 In addition, Wang et al. used Sn as a dopant and found that with a Sn2+:Pb2+ ratio of 0.6:0.4, the α-CsPbI3 thin film solar cell showed the best phase-stability of ~150 days.32 More importantly, the exciton Bohr radius (distance between the electron-hole pair within an exciton) in CsSnI3 was on the order of ~46 nm, far exceeding the typical size of CQDs, leading to the formation of stronger quantum confinement with a large



exciton binding energy than in α-CsPbI3.61, 62 Therefore, replacing Pb with Sn is expected to yield more electronhole pairs, further boosting the photo-current. Moreover, the replacement of Pb2+ ions offered the potential for lead-free halide perovskite PVs.52, 63 On the other hand, replacing Pb2+ with other divalent ions for phase stabilization may also introduce new defect states as recombination centers, thus possibly compromising the defect tolerant nature of the host material.52, 64 Therefore, a comprehensive study on defects introduction or passivation by Bsite doping is necessary. ii) As an alternative to B-site doping, core/shell structures have also recently been demonstrated as a viable route to increase the photo-stability. Recently, Wang et al reported a successful synthesis route of (Cs,Rb)-based bromide core/shell nanocrystals through 2-step solution processing by rubidium oleate (RbOA) treatment (see Fig.5H).54 Compared with CsPbBr3, the core/shell structure exhibited better phase stability under illumination, as well as enhanced PLQY. So far, the PLQY stability in αCsPbI3 nanocrystals has not been improved by core/shell structures as much as through B-site doping. However, it has been effective in improving the stability of other bromide perovskite nanocrystals. Therefore, we suggest that more investigations should be made to understand how the core/shell structures affect the stability of α-CsPbI3 nanocrystals. iii) The stability of CQDs can also be improved by adjusting reaction temperature and the chain length of different solvent molecules (carboxylic acids and amines see Fig.5I).55 Both the reaction temperature and chain length can influence the PLQY of the CQDs during synthesis by LaMer hot injection. In general, the nucleation tends to form cubic structures at elevated temperatures (170 ℃) regardless of the molecules present and their chain length. However, the size of cubes can be reduced with longer acid chains, whereas the structure can be transformed from nanoplatelets to cubes with chain amine becoming longer, for instance, using oleylammonium. On the other hand, nanoplatelets can be formed at low synthesis temperatures (140 ℃ or below65) irrespective of their solvents. In particular, the longer chain amine leads to thicker nanoplatelets at the low temperature. Further, both reaction temperature and chain length can also impact on the photo-physical properties. iv) Surface modification of the α-CsPbI3 CQDs with phenyletylammonium (PEA) have been shown to successfully improve photo-stability. Dayan et al. recently examined the effect of the aromatic molecule PEA on the photo-stability of the CQDs compared to the linear-chain molecule hexylammonium (HA).66 The results show that PEA-CsPbI3 nanocrystals displays better photo-stability of over 700 hours under 1 sun illumination. The stability is only 72 hours for the HA-CsPbI3 nanocrystals under the same illumination conditions. Further, computations show that the reason of the enhanced stability using PEA modification molecules comes from different charge distribution over the perovskite structure.

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v) Additionally, the ligand TOP rather than OA/m can further maintain the stability CsPbI3 nanocrystals with PLQY close to 100 % within 1 month.38 Liu et al. explained that the high PLQY in the TOP-CsPbI3 nanocrystals comes from the enhanced PL lifetime. The chemical stability of TOP-CsPbI3 nanocrystals originates from the strong retention of the alkylammonium oleate ligands to the surface of the CQDs (the ligands remain even after washing with anti-solvent twice). The interface between the absorber and charge transport layer is important in influencing the charge extraction rate. Engineering of the electron transport layer. Apart from charge transport within the CQD layer, charge injection across the interface between the absorber and electron transport layer (see Fig.6A), is also very important. Unlike the situation in well-studied metal chalcogenides (PbS),67 the detailed structure, underlying mechanisms, and correlations with device efficiency of the charge transport layer in α-CsPbI3 remains unclear. Therefore, we will focus on it in this part, particular the interface with the n-type charge transport layer. So far, sol-gel TiO2 nanoparticles (NPs) is frequently used as the electron transport layer in α-CsPbI3 CQD PVs. Compared to ZnO, sol-gel TiO2 NP films have a higher density of states and degree of coupling as well as intermediate states, leading to a higher carrier injection rate k at the interface of ~1.3–2.1×1010 s–1,68, 69 (Figs.6B-C) calculated using Eq.1a:69

where the time constant τ is the lifetime for CQDs or different hetero-junction interfaces, determined by an exponential fit to the transient absorption decay. Altogether, the injection efficiency can also be calculated using Eq.1b:

Because of the wide bandgap, high electron mobility, transmittance and suitable band alignment with photoactive layers, ZnO, SnO2 and TiO2 are often used as charge transport layers for conventional PbS CQDPVs to transmit light, selectively extract electrons and blocking holes.11, 12, 70, 71 More importantly, interface recombination can be further suppressed through charge transport layer engineering by e.g., Mg doping in ZnO72, 73 or Cl doping in SnO2.70 Further, instead of commonly used sol-gel methods, Zang et al. reported an increase in JSC in conventional CQDPVs using a ZnO layer deposited by sputtering.74 The increase in JSC is attributed to an improvement in both charge separation and transport efficiency from the smooth urface of the sputtered ZnO. Therefore, engineer ing of charge transport layer is an effective way to both reduce hetero-interface recombination and charge separate tion/transport for the conventional CQDPVs. Howev-


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ACS Energy Letters

Figure 6. Schematics of photo-excitation, ground state recombination, transferring and diffusion process at the hetero-interface in the band structure (A) and in the NPs and CQDs (B), respectively. 69, 75 (C) Transient absorption spectra for α-CsPbI3 CQDs (in size of 15 nm) in hexane and adsorbed on TiO2 and Al2O3 films.69 (D) Schematics of the high bandwidth transient photocurrent setup. (E) Transient photocurrent signal obtained for perovskite nanocrystals. (E) Transient photocurrent signal obtained for perovskite nanocrystals. (F) Conductivity as a function of the applied electric field under dark condition. The inset is a SEM image of uncovered nano-structure electrodes. (G) Responsivity of the film of over μm spaced interdigitated and nano-trench electrodes.76 (H) Heterojuntion structures in porous structured electrode; (I) in ordered one-dimensional nanostructure electrode; (J) bulk heterojunction structure.77 Reprinted with permission from refs 69,75,76,77.

er, there is not much work reported in this area for the purpose of efficiency improvement in α-CsPbI3 CQDPVs. In this section, we will focus on charge transport layer engineering from the following aspects and expect enhancement of JSC in the α-CsPbI3 CQDPVs. i) Although the sol-gel method is widely used, it is not a flexible method for producing uniform TiO2 thin films at scale, even if it is less expensive in comparison to physical vapor deposition methods.78, 79 More importantly, sol-gel TiO2 in general exhibits lower conductance in comparison with other methods. Sol-gel TiO2 typically gives a resistivity of (103–1010 Ω·cm) depending on the illumination level,80-82 whereas pulsed laser deposited (PLD) TiO2 gives a resistivity on the scale of 10–2–106 Ω·cm, depending on post thermal treatment and O2 partial pressure during synthesis.64, 83, 84 From the device point of view, the PV perfor-

mance relies on an efficient charge transfer at the heterointerface with less recombination, where the electrical conductance, band alignment and defect property of the charge transport layer are concerned. Thus, we expect that PLD charge transport layers might be better for the particular case of low JSC in α-CsPbI3 CQDPVs condsidering their enhanced electrical, optical as well as better structural properties through modulation of processing parameters. In addition, the thickness can also be altered simply by parameters of, e.g., pulse number.78, 85 ii) Moreover, in a recent study, Mir et al. show that nano-structure engineering of the charge transport layer can also greatly impact on the device efficiency. They measured the photo-response using a transient photocurrent setup, see Fig.6D, to determine the lifetime of carriers (Fig.6E). In particular, they point out that the



conductivity of nano-structured TiO2 shows an abrupt increase at high electrical fields (as a shown in Fig.6F). Whereas, the μm-spaced electrode is not field-dependent and its conductivity remains at a low value. In addition, it exhibits orders of magnitude enhancement of photoresponsivity in comparison with μm-spaced electrode under different solar irradiances (Fig.6G). From this perspective, the structures, e.g., hetero-junction with a porous electrode, nanowire electrode, bulk heterjunction (seeFig.6H-J) are probably better than typical planar structure for charge transfer in solar cells. Therefore, it is reasonable to expect that the JSC can be enhanced with increased carrier injection rate and reduced defect recombination loss by the nano-structure engineering of the charge transport layer.76 iii) In addition, it is also demonstrated that modification of the charge transport layer can also improve the band alignment with the absorber and reduce the activities of defects, leading to lower charge recombination rate and higher charge extraction efficiency. For example, Yan et al. show that a SnO2/ZnO bilayer can modify the electron affinity of the charge transport layer, so that the desirable band alignment is realized for efficient charge transport in CsPbI2Br solar cells. More importantly, the interface recombination is also suppressed, resulting in enhanced VOC, FF and PCE in the device.86 Further, Liu et al. reported that C60 modified ZnO is also efficient in extracting charge carriers while reducing leakage, so that the efficiency can reach 13.3 %.87 Therefore, we expect that modification of charge transport layer might indeed lead to the same achievement in the α-phase CsPbI3 nanocrystal PVs. Defect Analysis in CQD PVs. In PVs, carrier recombination is in general considered a major factor for limiting efficiencies, where electrons and holes are captured by defects through several different channels on varied range of timescales.16 Apart from sub-bandgap defects on the surface of CQDs, photo-generated carriers might also recombine via interface defects.21 Before defect treatment through passivation, it is important to understand the change in defect activities with the strategies discussed above (e.g., doping). In addition, it is helpful to eliminate the defect activities from the TRPL study for charge transport layer engineering together with PLQY measurement, where carrier relaxation is a competing process with recombination. Hence, we will briefly cover defect characterization based on impedance spectroscopy, along with strategies for enhancement of JSC and associated efficiency in α-CsPbI3 CQD PVs. In order to understand the underlying mechanisms of the trapping process, the equivalent electrical circuit model of the PV hetero-junction is usually setup and theoretical models are established to determine the trap density with electrical element. In particular, the defect density can be determined from the following equations based on impedance spectroscopy measurements, where the following two equations 2a&b can be used for calculating the trap density in the bulk and at the heterointerface, respectively:88, 89

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where the traps are classified into bulk and interface defects according to their formation, location and influence on the carriers, respectively. As can be noticed, for the bulk trap, see Eq.2a, the density is merely dependent on differential capacitance, ω·dC/dω. Whereas for interface defects, it is both dependent on the differential capacitance as well as probing energy Eω (which is related to external bias) and temperature T. Due to the difference in the origin of the defects, the most effective strategy to mitigate the effects of defects will be different according to individual devices. Again, the defect study will be able to exclude the influence of defect recombination on interface charge transfer by TRPL. Because these are the two major competing processes after photo-excitation. So far, impedance spectroscopy has been employed for investigation of defects in PVs and it is helpful in determining the trap density and their energy level. In particular, Bozyigit el al. has revealed that trap density is 1.8×1017 cm–3 with an activation energy of 0.36 eV below the CB for PbS CQDPVs using impedance spectroscopy to measure the capacitance of the space charge region and its relation with the measurement frequency.90 Yet the defect analysis has rarely been implemented for the αCsPbI3 CQDPVs. Therefore, we expect that it will provide useful guidance for future development of PVs with effective defect passivation strategies. Current Challenges and Future Prospects. Owing to their excellent performance, phase-stability and high PLQY, α-CsPbI3 CQDs are promising for future application in solar PVs. However, in order for them to achieve high efficiency, the JSC should be improved through improved mobility or carrier extraction, both within the nanocrystal array as well as at the interfaces between the absorber and charge transport layers. Photo-stability should also be enhanced for future commercial applications. From our discussion, we have arrived at the following suggestions for future development in α-CsPbI3 CQD PVs: •

Efficient charge transport in α-CsPbI3 CQDs can be enabled by A-site exchange with FA+ to achieve high carrier mobility. Enhanced charge transport can also be realized by incorporating suitable additives such as GeI2 and μ-graphene, which can passivate surface defects and facilitate crosslinking of the CQDs, respectively. Charge transport can be improved in other CQDs by molecular capping, e.g., zwitterionic molecular groups and PPA, where they can prevent external neutralization and delocalize the molecular orbitals, respectively. It is expected that these strategies could enhance charge transport in α-CsPbI3 CQDs as well. Furthermore, charge extraction can be improved using an interfacial layer of PTB7, reflected by a carrier lifetime (~3 ns) in the CQD PV using PTB7 .


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•

•

•

ACS Energy Letters For photo-stability, B-site doping with ionic dopants of e.g., Mn2+, Zn2+, Sr2+, and Sn2+ to substitute Pb2+ is an effective method, so that the unit cell of α-CsPbI3 is contracted with a larger tolerance factor and subsequently structural degradation is avoided during continous light illumination. Also, (Cs,Rb)-based bromide core/shell nanocrystals through a 2-step solution processing by rubidium oleate (RbOA) treatment is shown to prolong the stability in PLQY. We expect that it will also show similar effect in α-CsPbI3. In addition, the cubic structure tends to form at higher reaction temperature by using solvents with longer chain length during synthesis. Furthermore, α-CsPbI3 nanocrystals modified with aromatic molecule PEA demonstrates longer photo-stability (of up to 700 hours) than α-CsPbI3 modified with the linearchain molecule HA. This is a result of charge redistribution. TOP treated CsPbI3 nanocrystals show better chemical stability in comparison with OA/m, which arises from the alkylammonium oleate ligands strongly binding to the nanocrystals. Charge transfer and injection across the heterointerface is also very important to increase JSC. In particular, we have discussed the possibility of suppressing interface recombination in conventional CQDPVs by doping the charge transport layers. At the same time, the JSC in the device can be increased by using sputter deposited ZnO as a result of smooth hetero-junction interface formed. We therefore expect such an effect will take place in the α-CsPbI3 CQD based PVs. Further, in order to elucidate the underlying mechanism of low JSC in the α-CsPbI3 CQDs, Mir et al, pointed out that nano-structuring the electrodes might lead to superior performance, through improved carrier extraction. Last but not least, defect analysis (e.g., through impedance spectroscopy) is also necessary to monitor defect recombination in the device. Consequently, it helps to identify more effective treatment methods for defect passivation. On the other hand, defect study is also beneficial for study of charge transport layer engineering, so that the loss caused by the competing process can be eliminated during analysis of charge transfer.

Jiantuo Gan is a postdoctor/Assistant Professor at the University of Electronic Science and Technology of China (UESTC). His current research interest includes synthesis of colloidal quantum dots, band structure engineering and defect study for future PV applications. He received his Ph.D. from the University of Oslo, Norway (2017). Home Page: https://scholar.google.de/citations?hl=en&pli=1&user=AwS_ CgsAAAAJ Jingxuan He joined Prof. L. Qiao’s group while pursuing his M.Sc. at UESTC, after completing his B.Sc. from the same University in 2017. His current research topic is engineering of low dimensional photoelectric materials using pulsed laser deposition for applications in PVs and photo-catalysis, financially supported by the University through scholarship schemes. Dr. Robert Hoye is a Royal Academy of Engineering Research Fellow leading a group at the University of Cambridge working on the development of optoelectronic materials. He completed his PhD in the University of Cambridge (20122014), worked at MIT as a postdoctoral researcher (2015-2016), before performing research in the Optoelectronics Group at Cambridge as a Nevile Research Fellow (Magdalene College, Cambridge; 2016-2018). Abdurashid Mavlonov received his Ph.D. degree in Semiconductor Device Physics & Materials Science from the University of Leipzig in 2016. He currently works as a postdoctoral researcher in School of Physics, UESTC. His research interests mainly focus on development of new materials and devices for solar cells. Fazal Raziq received his Ph.D. degree in Chemistry & Materials Science in 2017. He received distinguished Ph.D. scholar award from the Chinese Government (2016) and outstanding Ph.D. scholar award from Heilongjiang University (2017). Currently, he is working as a postdoctor at UESTC, where he studies visible-light responsive materials for selective CO2 conversion and water splitting. Professor Judith Mac-Manus Driscoll is Professor of Materials Science at University of Cambridge. She is a Fellow of the Materials Research Society, American Physical Society, IOM3, IOP and Royal Academy of Engineering. Xiaoqiang Wu received his Ph.D. from Northwestern Polytechnical University, China in 2017. Since then, he is working as a postdoctor/Assistant Professor at UESTC. His current research interest includes synthesis of perovskite thin films, band structure engineering and ethanol electrooxidation/oxygen evolution reaction/oxygen oxidation reaction for fuel cells applications.

AUTHOR INFORMATION Professsor Yiqiang Zhan is the Director, a Professor and the Principle Investigator in the Micro-Nano System Center, at Fudan University. He obtained his Ph.D. in physics at Fudan University in 2005 before performing his postdoctoral research at ISMN-CNR bologna, Italy. From 2007, he continued his research at Linkoping University, Sweden as an Assistant Professor.

Corresponding Author *E-mail: [email protected] (J. Gan) *E-mail: [email protected] (L. Qiao)

ORCID Jiantuo Gan: 0000-0001-7486-6382

Notes The authors declare no competing financial interest

Professor Xiaotao Zu has been a Professor at University of Electronic Science and Technology of China since 2003. In 2006-2009, he served as deputy head of the School of Physics.

Biographies



His research interests include development of photoelectric nanocomposites and their physical properties. Professor Sean Li is the director of UNSW Materials and Manufacturing Futures Institute at The University of New South Wales, Sydney, Australia. His research interests focus on energy and multifunctional materials with financial supports from Australian Research Council and industries. Professor Xiaoyong Zhang is a Professor at Xi’an Shiyou University and also the head of School of Materials Science and Engineering. His research work is financially supported by Chinese government funding foundation and oil coporation funding bodies (NSFC, NSF Shaanxi, CNPC, China Sinopec, CNOOC, etc). Professor Liang Qiao is a Professor at UESTC. He obtained his Ph.D. in Materials Science from Beihang University in 2009. Afterwards, he worked as a postdoctoral researcher in PNNL, ORNL and a Lecturer at The University of Manchester. His research interest is focused on low dimensional functional materials for electronics, optoelectronics, and energy applications.

ACKNOWLEDGMENT J.G. and J.H. contributed equally to this work. The research funding from the China Postdoctoral Science Foundation through the project (No.:2018M640906) is acknowledged by the authors. RLZH acknowledges funding from the Royal Academy of Engineering through the Research Fellowships scheme (No.: RF\201718\17101), as well as from the Isaac Newton Trust (Minue 19.07(d)). L.Q. was supported by the National Natural Science Foundation of China (Grant No.: 11774044).

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Î±-CsPbI3 Colloidal Quantum Dots: Synthesis, Photodynamics and

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