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Recent advances in interfacial engineering of perovskite solar cells

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2017 J. Phys. D: Appl. Phys. 50 373002

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Journal of Physics D: Applied Physics

1. Introduction

Photovoltaic cells have been widely recognized as the most effective way for directly converting solar energy into elec-tricity, a promising renewable energy technology support-ing the sustainable development of humanity. Recently,

photovoltaic cells based on organometallic perovskites ABX3 (figure 1; typically, A = CH3NH3 (MA), NH2CHNH2 (FA), or Cs; B = Pb or Sn; X = Cl, Br or I) have quickly gar-nered attention, because of their immense potential for high- performance all-solid-state solar cells, i.e. perovskite solar cells (PSCs) [1–9]. Over the past six years, PSCs have

Recent advances in interfacial engineering of perovskite solar cells

Meidan Ye1, Chunfeng He1, James Iocozzia2, Xueqin Liu3, Xun Cui2, Xiangtong Meng2, Matthew Rager2, Xiaodan Hong1, Xiangyang Liu1,4 and Zhiqun Lin2

1 Department of Physics, School of Physics and Mechanical and Electrical Engineering, Research Institute for Soft Matter and Biomimetics, Xiamen University, Xiamen 361005, People’s Republic of China2 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, United States of America3 Engineering Research Center of Nano-Geo Materials of Ministry of Education, China University of Geosciences, Wuhan, Hubei 430074, People’s Republic of China4 Department of Physics, Faculty of Science, National University of Singapore, 117542 Singapore

E-mail: mdye@xmu.edu.cn and zhiqun.lin@mse.gatech.edu

Received 12 September 2016, revised 19 June 2017Accepted for publication 30 June 2017Published 22 August 2017

AbstractDue to recent developments, organometallic halide perovskite solar cells (PSCs) have attracted even greater interest owing to their impressive photovoltaic properties and simple device manufacturing processes with the potential for commercial applications. The power conversion efficiencies (PCEs) of PSCs have surged from 3.8% for methyl ammonium lead halide-sensitized liquid solar cells, CH3NH3PbX3 (X = Cl, Br, I), in 2009, to more than 22% for all-solid-state solar cells in 2016. Over the past few years, significant effort has been dedicated to realizing PSCs with even higher performance. In this review, recent advances in the interfacial engineering of PSCs are addressed. The specific strategies for the interfacial engineering of PSCs fall into two categories: (1) solvent treatment and additives to improve the light-harvesting capabilities of perovskite films, and (2) the incorporation of various functional materials at the interfaces between the active layers (e.g. electron transporting layer, perovskite layer, and hole transporting layer). This review aims to provide a comprehensive overview of strategies for the interfacial engineering of PSCs with potential benefits including enhanced light harvesting, improved charge separation and transport, improved device stability, and elimination of photocurrent hysteresis.

Keywords: interfacial engineering, perovskite solar cells, charge separation, light harvesting, device stability

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experienced unprecedented development in power conver-sion efficiency (PCE) with performance soaring to over 22% [10]. As the heart of PSCs, hybrid ABX3 organic–inorganic halide perovskites possess ideal photovoltaic properties such as optimal band gaps (~1.5 eV), high absorption coefficients (1.5 × 104 cm−1 at 550 nm), long carrier diffusion lengths (100–1000 nm, with reported values larger than 175 µm in MAPbI3 single crystals [11]), good charge carrier mobility (~20 cm2 V−1 s−1), small exciton binding energy (~30 meV), high defect tolerance, and solution processability [12–17]. In addition to solar cells, organometallic perovskites have also been introduced into other optoelectronic devices such as light-emitting diodes [18], lasers, and photodiodes [19, 20].

In recent years, device structures and active materials for PSCs have undergone multiple significant optimization and transformation iterations. To date, the cell structures of PSCs have been generally divided into mesoscopic and planar heter-ojunction (PHJ) configurations [21]. As shown in figure 1(a), a typical mesoscopic PSC device generally starts with the deposition of a semiconductor metal oxide compact layer (e.g. c-TiO2 or c-ZnO) on a transparent conductive substrate (e.g. fluorine/indium-doped tin oxide (FTO/ITO) glass). This is followed by the deposition of a mesostructured semicon-ductor (m-TiO2, m-SnO2, and m-ZnO) or insulating (Al2O3, and ZrO2) metal oxide as the electron transporting material (ETM) and/or scaffold layer with subsequent deposition of a perovskite light-absorbing layer. A hole transporting mat-erial (HTM) layer (commonly 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene, spiro-OMeTAD) is then coated, followed by the deposition of a metal anode (e.g. Au, and Ag) [22–25]. The highest PCE reported for mes-oscopic PSCs is 22.1% [10]. However, the porous structure

of mesoscopic PSCs commonly requires a high-temperature sintering process at T > 450 °C to improve the film quality of the ETM/scaffold layer. In this regard, PHJ PSCs with a n-i-p junction (e.g. FTO/c-TiO2 (n)/MAPbI3−xClx (i)/spiro-OMeTAD (p)/Ag) without the m-TiO2 scaffold have been developed, which donot require high-temperature sintering [15]. The inverse of n-i-p (figure 1(b)), PHJ p-i-n PSCs (fig-ure 1(c)) have also been fabricated by employing poly(3,4- ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as the HTM layer and fullerene-based molecules as the ETM layer [26]. The low-temperature processability of PHJ PSCs enables device fabrication on flexible substrates (e.g. poly-ethylene terephthalate (PET) and polyethylene naphthalate (PEN)), enabling PSCs to be potentially incorporated into lightweight wearable electronics. More impressively, HTM-free PSCs (figure 1(d)) have also attracted considerable attention due to their optimized PCE over 13% [27–30]. The removal of the organic HTM layer not only simplifies the preparation procedure but can greatly reduce the production cost as spiro-OMeTAD is expensive.

Concurrently, the active materials for PSCs have been exten-sively researched. Much effort has been devoted to investigat-ing perovskite morphology and crystal optimization [12–14, 16, 17, 31–34], the rational design of blocking and ETM layers [35–39], and the optimal selection of HTM layers. Figure 2 lists some representative perovskites and electron/hole trans-porting materials as well as their energy levels. A series of mixed organometallic halide perovskites have been developed including MAPbI3, MAPbBr3, MAPbCl3, MAPbI3−xClx, and FAPbI3 [40–43]. In addition to these Pb-based perovskites, Sn-based perovskites have recently been introduced to par-tially or completely replace Pb and reduce the toxicity of PSCs

Figure 1. (a)–(d) Diagrams of various PHJ device structures employed in PSCs. The central image shows the generic unit cell for an organometallic halide perovskite crystal.

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[44–46]. [6,6]-phenyl C61 butyric acid methyl ester (PCBM) is the most common ETM in PHJ p-i-n PSCs. There are now more ETMs available to be used in mesoscopic and PHJ n-i-p PSCs such as TiO2 [47], ZnO [48], WO3 [49], SnO2 [50], and ZnSnO4 [51] with a reduced cost compared to PCBM. Spiro-OMeTAD is the most popular HTM. However, poly-3-hex-ylthiophene (P3HT) and poly-triarylamine (PTAA) are also advantageous for hole extraction in mesoscopic and PHJ n-i-p PSCs [52]. In addition to PEDOT:PSS, various inorganic com-pounds, such as NiO [53], CuI [54], CuSCN [55], CuO [56], MoOx [57], graphene oxide (GO), and reduced graphene oxide (RGO) [58, 59], have also been reported as efficient HTMs for PHJ p-i-n PSCs. FTO and ITO substrates are typical cath-odes when combined with thin anodic Au or Ag metal layers in mesoscopic and PHJ n-i-p PSCs. Conversely, FTO and ITO can be used as anodes by integrating with cathodic Al, Ag, Au, Ni, or Ca metal layers in PHJ p-i-n PSCs [15, 24]. In HTM-free PSCs, carbon black is typically the anode layer with FTO or ITO glass as the cathode [30].

Interfacial engineering is a recognized strategy for maxi-mizing the performance of organic solar cells [60–62]. Although modification of the perovskite, charge transport and extraction layers is typically a hot research area in PSCs, more attention has been devoted to the interfacial engineer-ing of PSCs over the past three years [63–65]. In the follow-ing sections, an overview of recent interfacial engineering approaches in PSCs are reviewed (e.g. perovskite/ETM (P/E), perovskite/HTM (P/H), ETM/metal layer (E/M), and FTO or ITO substrate/ETM (S/E) interfaces). General observations on their roles in device performance (e.g. energy-level align-ment, electrical conductivity, trap passivation, improvement

in device stability, and elimination of photocurrent hysteresis) are provided.

2. Interfacial engineering in PSCs

Under illumination, charge carriers are generated in the perovskite layer after absorbing incident photons. The charge carriers are then immediately moved through the transport channels including the ETM/HTM layers, each interface in between (i.e. P/E, P/H, H/M and S/E interfaces), and finally extracted to the anode and cathode in PSCs (figure 3) [66, 67]. The clearly tortuous path taken by electrons and holes requires a well-constructed layered structure to prevent the carriers from being quenched during their journey. During the transport process, the free charge carriers can be captured or recombined by traps and defects in the perovskite films, and ETM/HTM layers due to their internal imperfections. The interfacial defects significantly contribute to charge loss, which leads to a reduction in fill factor (FF), open-circuit voltage (VOC), short-circuit current (JSC), and ultimately poor PCEs [68]. In addition to controlling the crystallization and morphology formation processes of perovskite films, modifi-cation of the PSC interfaces is an effective approach to maxi-mize the electron–hole separation/collection and minimize charge recombination.

Interfacial engineering can also address other problems in PSCs such as instability concerns and anomalous hysteresis behavior. The poor long-term stability of PSCs is mainly due to the inherent vulnerability of perovskite to moisture, light, and heat. The proper design of interfacial layers in PSCs can

Figure 2. The energy level diagram summarizes representative examples of organometallic perovskites and charge-extraction materials (electrons and holes). Dotted lines (far left and far right) represent the work function of the anode and cathode materials.

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effectively isolate perovskite films from exposure to these undesirable environments and improve device stability [10, 69–71]. Hysteresis behavior in photocurrent density–voltage (J–V) characteristics is frequently encountered in PSCs, par-ticularly PHJ PSCs. This typically leads to differences in the PCEs calculated from the forward scan (from short circuit to open circuit) and the backward scan (from open circuit to short circuit) [72, 73]. The hysteretic behavior is problematic for achieving stable and reliable PSC performance. Furthermore, the origin of hysteresis in PSCs is still not completely under-stood. Charge trapping, ion migration, and capacitive and fer-roelectric polarization are potential reasons underlying this phenomenon [72–74]. It has been reported that increasing the grain size and improving the crystal quality in perovskite films can reduce the hysteresis of PSCs [52, 75]. Interface pas-sivation can also alleviate this problem [76, 77].

2.1. Engineering perovskite films

To date, several routes for forming perovskite crystalline films with various degrees of surface coverage and crystal quality have been developed. Variations in these parameters directly affect PSC performance. Some of these routes include one-step solution processing [25], two-step sequential deposition [78], and vapor-assisted solution processing [15]. It is noteworthy that many secondary steps have also been developed to modify the morphology and crystallization of perovskites. As shown in table 1, solvent treatment (typically with DMSO [52], DMF [79], and isopropanol [80]), and additive incorporation [81–98] are the two main approaches for controlling perovskite crystal growth. In addition, controlling precursor and pro-cess conditions such as temperature are also used to modify the crystallization of perovskite films [12, 40, 99–102]. These modifications may also passivate the perovskite surface traps to suppress charge recombination, reduce photocurrent hyster-esis, and enhance the air/thermal stability of PSC devices.

For example, it was found that introducing a small amount of Pb(SCN)2 into the perovskite precursor when forming the MAPbI3 film via one-step solution processing can increase

the grain size and improve the crystallization of perovskite films. The resulting PHJ n-i-p PSC device exhibited a reduced hysteresis and increased FF. Furthermore, the introduction of PCBM into the SnO2 ETM layer contributed to a PCE enhance-ment from 15.57% to 18.42% for this device [90]. The addi-tion of a small amount of PbI2 to the perovskite precursor led to its selective formation at the perovskite grain boundaries (GBs) as well as at the TiO2/perovskite (P/E) interface during annealing. This had a passivation effect that improved carrier extraction with reduced a recombination at the GBs and the P/E interface (figure 4) [83]. Zhou et  al also indicated that the synergistic effect of PbI2 passivation and chlorine incor-poration improved the perovskite growth and reduced non-radiative recombination leading to the enhanced VOC and PCE [82]. Although perovskites are sensitive to moisture, the addi-tion of water to the precursor solution was found to effectively enhance the crystallization, surface coverage, and stability of perovskite thin films [95, 96], as well as reduce the photo-current hysteresis [96]. NH4Cl is also a beneficial additive to slow the crystallization process of perovskites and enable the formation of smooth perovskite films with maximum surface coverage and enhanced PSC performance [86–88]. Perovskite films with large multi-crystalline grains with smooth surfaces can be achieved through a combination of H2O addition and DMF vapor treatment. The corresponding PSCs showed a best PCE of 20.1%, no current hysteresis, and high device stability [103].

2.2. Engineering the interface between the perovskite and electron transport layers

In general, dealing with electron capture and transport is a greater challenge in photovoltaic devices for several reasons. In PSCs, the electron diffusion length is relatively shorter than that of holes leading to a higher chance of quenching [104]. This is considered as a major stumbling block for further improvement in device PCEs [104]. In addition, electrical con-ductivity of the TiOx ETM in mesoscopic or PHJ n-i-p PSCs, and the PCBM ETM in PHJ p-i-n PSCs is several orders of magnitude lower than HTM conductivity. Consequently, there is generally poorer electron collection efficiency with corre-spondingly lower JSC and FF [105, 106].

In addition to property limitations, there are also structural complications associated with the ETMs. There is commonly a weak contact between the perovskite layer and the ETM layer in solution-processed perovskite-based PSCs. This is due primarily to the rough perovskite surface, which lowers the electron-extraction efficiency, and increases charge-carrier recombination leading to large current leakage [77]. Finally, optimal energy-level matching at the junction between the ETM and the perovskite layers is required for PSCs to per-form well [107]. To address the problems noted above, inter-facial modification at the contact between the perovskite and ETM layers has been widely investigated. As shown in table 2, interfacial engineering is performed in all four types of PSCs by employing a variety of materials. Materials used include: (1) inorganic materials [39, 108–120], (2) fullerene and its derivatives (C60) [105, 106, 121–127], (3) self-assembled

Figure 3. Schematic diagram of charge transport across the various interfaces in PSCs with each potentially contributing to losses and thereby reduced performance.

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Table 1. Summary of engineering strategies for perovskite films in PSCs.

Device structure Device type Methoda Materialb PCE (%) Engineering effect Reference

FTO/c-TiO2/m-TiO2/MAPbI3−xBrx/PTAA/Au

Mesoscopic One-step DMSO 16.46 Forms a highly uniform and dense perovskite surface, and eliminates photocurrent hysteresis

[52]

FTO/c-TiO2/m-TiO2/FAPbI3/spiro-OMeTAD/Ag

Mesoscopic One-step FACl 13.7 Suppresses the formation of non-perovskite phases

[81]

FTO/c-TiO2/m-TiO2/MAPbI3(Cl)/spiro-OMeTAD/hc-PEDOT:PSS

Mesoscopic One-step PbI2/PbCl2 18.4 Improves film growth and suppresses nonradiative recombination

[82]

FTO/c-TiO2/MAPbI3 /spiro-OMeTAD/Au

n-i-p Two-step PbI2 12.00 Passivates perovskite and reduces charge recombination

[83]

FTO/c-TiO2/MAPbI3−xClx/spiro-OMeTAD/Ag

n-i-p One-step MOF-525 14.5 Improves morphology and crystallinity of perovskite films

[84]

FTO/c-TiO2/MAPbI3 (MAPbI3−xClx)/spiro-OMeTAD/Ag

n-i-p Two (one)-step TBP 10.62 (15.01)

Improves perovskite crystallization

[85]

FTO/c-TiO2/MAPbI3−xBrx/spiro-OMeTAD/Al

n-i-p One-step NH4Cl 12.10 Slows down film crystallization and improves perovskite quality

[86]

FTO/c-TiO2/MAPbI3/Spiro-OMeTAD/Au

n-i-p One-step HONH3Cl 11.12 Improves perovskite crystallinity and morphology, reduces charge recombination

[89]

FTO/SnO2/PCBM/MAPbI3/spiro-OMeTAD/Au

n-i-p One-step Pb(SCN)2 18.42 Increases grain size and improves crystalline quality of perovskite films, and reduces the degree of hysteresis

[90]

FTO/c-TiO2/FA0.85MA0.15Pb(I0.85Br0.15)3/spiro-OMeTAD/Au

n-i-p One-step N-RGO 18.7 Reduces the grain boundaries and improves light-harvesting properties, passivates surface and reduces charge recombina-tion

[91]

ITO/PEDOT:PSS/MAPbI3 /PCBM/C60/BCP/Al

p-i-n Two-step DMF 15.6 Increases grain size and crystallinity, and improves effective charge diffusion length in perovskite films

[79]

ITO/PEDOT:PSS/MAPbI3−xClx/PCBM/C60/Ag

p-i-n One-step DIO 11.8 Improves crystallization and enhances charge transport

[92]

ITO/PEDOT:PSS/MAPbI3/PCBM/Al

p-i-n One-step NH4Cl 9.93 Improves perovskite crystallinity and morphology

[87]

ITO/PEDOT:PSS/MAPbI3−xClx/PCBM/C60/Al

p-i-n One-step Isopropanol 13.11 Increases purity and coverage of perovskite crystals, reduces leakage current, and interfacial charge recombination

[80]

ITO/PEDOT:PSS/MAPbI3−xClx/PCBM/Al

p-i-n One-step EAI 10.2 Improves perovskite crystallization and coverage, and enhances thermal stability

[93]

ITO/PEDOT:PSS/MAPbI3/PCBM/Ca/Al

p-i-n One-step NH4Cl 9.93 Improves perovskite crystallization and coverage without thermal treatment

[88]

ITO/PEDOT:PSS/MAPbI3/PCBM/Ca/Al

p-i-n One-step CaCl2 3.08 Improves perovskite crystallization and coverage

[94]

ITO/PEDOT:PSS/MAPbI3−xClx/PCBM/Bphen/Al

p-i-n One-step H2O 16.06 Enhances perovskite crystallization and ambient stability

[95]

ITO/PEDOT:PSS/MAPbI3/PCBM/Ca/Al

p-i-n Two-step H2O 18 Improves perovskite quality, prevents current hysteresis, and enhances light stability

[96]

(Continued )

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monolayers (SAMs) [128–132], (4) functional polymers [77, 107, 133, 134], and (5) other organics [71, 135–139]. These immense variety of interfacial layer materials play a number of important roles in PSCs including increasing the cover-age and crystallinity of perovskite films [136], passivating the charge trap states in the active layers [138], improving the electrical properties and charge extraction of ETM layers [140], eliminating photocurrent hysteresis [77], and improv-ing the long-term device stability [71, 116].

For example, graphene quantum dots (GQDs), single- or few-layer graphene with sizes of several nanometers, possess quantum-confinement and edge effects, and thus several novel optoelectronic properties such as long hot-electron lifetime (more than hundreds of picoseconds) and effective hot-electron extraction (time constant <15 fs) at the GQD/TiO2 interface [108]. When an ultrathin layer of GQDs was inserted at the perovskite/TiO2 interface (figure 5(a)), it acted as a superfast electron tunnel to improve the electron extraction and increase the PCE of the resulting mesoscopic PSCs (figure 5(b)) [108]. Carbon quantum dots (CQDs) were also found to be able to increase the electron

extraction efficiency when they were incorporated into TiO2 films for PSCs [141].

In another example, CsBr was used as an interfacial mat-erial between the perovskite layer and TiO2 ETM layer [119]. This improved the smoothness of the perovskite film, reduced the trap density of pinholes, and led to a negative shift in the work function of the c-TiO2 layer (figure 5(c)). These improve-ments led to improved electron transfer rates from the perovs-kite layer to the c-TiO2 layer and increased PCE. The CsBr interfacial layer also reduced the chemical reactivity of the TiO2 to UV light exposure, thereby reducing the defect density at the perovskite/TiO2 interface and enhancing the UV-light stability of PSCs (figure 5(d)) [119]. Similarly, electrically conductive LiSPS can fill pinholes on solution-processed per-ovskite to form a smoother morphology. An ultrathin LiSPS ionomer layer can be applied to engineer the perovskite/PCBM interface in PHJ p-i-n PSCs to reduce charge-carrier recom-bination, improve charge carrier collection, enhance device reproducibility, and lower photocurrent hysteresis [77].

In addition to inorganic materials, soft polymeric mat-erials possess several advantages for interfacial modification

Figure 4. Proposed mechanism for PbI2 passivation in MAPbI3 perovskite films. (Reproduced with permission from [83]. Copyright 2014, American Chemical Society.)

ITO/PEDOT:PSS/MAPbI3−xClx/PCBM/C60/Ag

p-i-n One-step 1,4-DIB 13.09 Improves perovskite crystallization and coverage

[97]

ITO/PEDOT:PSS/MAPbI3−xClx/PCBM/PFN-P2/Ag

p-i-n One-step PFN-P1 13.2 Helps promote uniform crystal-lization and enhances stability

[98]

ITO/PEDOT:PSS/MAPbI3/PCBM/Ca/Al

p-i-n Two-step H2O/DMF 20.1 Improves perovskite quality, reduces current hysteresis, and enhances ambient stability

[103]

aone-step = one-step solution processing, two-step = two-step sequential deposition.bMOF-525: Zr-based porphyrinmetal-organic frameworks, TBP: 4-tert-Butylpyridine, DIO: 1,8-diiodooctane, EAI: ethylammonium iodide, N-RGO: N-doped reduced graphene oxide, 1,4-DIB: 1,4-diiodobutane, PFN-P1: poly[9,9-bis(3′-(N,N-dimethylamino)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)].

Table 1. (Continued )

Device structure Device type Methoda Materialb PCE (%) Engineering effect Reference

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Table 2. Summary of strategies for interfacial engineering of the PSC P/E interface.

Device structure Device type MaterialaPCE (%) Engineering effect Reference

FTO/c-TiO2/m-TiO2/GQDs/MAPbI3/spiro-OMeTAD/Au

Mesoscopic GQDs 10.15 Increases electron extraction efficiency [108]

ITO/c-TiO2/m-TiO2/CQDs/MAPbI3−xCl3/spiro-OMeTAD/Au

Mesoscopic CQDs 18.89 Improves electron mobility, matches energy levels

[141]

FTO/c-TiO2/m-TiO2/CsCO3/MAPbI3/spiro-OMeTAD/Au

Mesoscopic CsCO3 14.2 Improves electron transport and reduces recombination

[109]

FTO/c-TiO2/m-TiO2/HOCO-R-NH+3 I−/

MAPbI3/spiro-OMeTAD/Ag-AuMesoscopic HOCO-

R-NH+3 I−

12 Passivates surface traps and reduces charge recombination

[138]

FTO/c-TiO2/m-TiO2/MgO/MAPbI3/spiro-OMeTAD/Au

Mesoscopic MgO 13.9 Reduces recombination and enhances H2O/UV stability

[111]

FTO/c-TiOx/PEO/m-Al2O3/MAPbI3−xClx/spiro-OMeTAD/Au

Mesoscopic PEO 13.8 Reduces work function and facilitates electron extraction

[107]

FTO/c-TiO2/m-TiO2/glycine/MAPbI3/spiro-OMeTAD/Au

Mesoscopic glycine 12.02 Reduces perovskite defects and enhances charge transfer

[135]

FTO/c-TiO2/m-TiO2/PABA/MAPbI3/spiro-OMeTAD/Au

Mesoscopic PABA 10.58 Reduces traps and defects in the crystalline perovskite film

[128]

FTO/c-TiO2/m-TiO2/SrTiO3/MAPbI3/spiro-OMeTAD/Au

Mesoscopic SrTiO3 11.4 Blocks reverse electron transfer and promotes charge transfer

[113]

FTO/c-TiO2/ALD-Al2O3/mp-TiO2/FAPbI3-MAPbBr3/PTAA/Au

Mesoscopic Al2O3 17.05 Passivates surface and improves charge separation and transport

[115]

FTO/c-TiO2/m-TiO2/R-PhCOOH/FAxMA1−xPbI3-yBry/spiro-OMeTAD/Au

Mesoscopic R-PhCOOH 18.43 Passivates trap states and enhances charge extraction

[137]

FTO/c-TiO2/NaYF4:Yb/Er/MAPbI3/spiro-OMeTAD/Ag

Mesoscopic NaYF4:Yb/Er

18.1 Facilitates the growth of high-quality perovskite crystals and extends the spectral range to include NIR light

[118]

FTO/c-TiO2/m-TiO2/HOOC-R-SH/MAPbI3/HS-PhF5/Spiro-OMeTAD/Au

Mesoscopic thiols 14.1 Enhances electron transfer and improves air/light stability

[71]

ITO/ZnO/PCBM/MAPbI3 /PTB7-Th/MoO3/Ag

n-i-p PCBM 12.2 Suppresses trap-assisted charge recombination at the interface

[125]

FTO/c-TiO2/C60/MAPbI3−xClx /spiro-OMeTAD/Au

n-i-p C60 15.7 Passivates trap states and reduces charge recombination at the interface

[121]

ITO/ZnO/C3-SAM/MAPbI3 /spiro-OMeTAD/MoO3/Ag

n-i-p C3-SAM 15.67 Improves the crystallinity of perovskite, lowers work function, and strengthens electronic coupling

[129]

FTO/c-TiO2/PCBB-2CN-2C8/MAPbI3−xClx /spiro-OMeTAD/Au

n-i-p PCBB-2CN-2C8

17.35 Passivates trap states and reduces charge recombination at the interface

[105]

FTO/PFN-OX:ZnO/MAPbI3/spiro-OMeTAD/Au

n-i-p PFN-OX 15.5 Achieves appropriate energy level alignment, and optimizes ohmic contact with minimum resistance and high charge selectivity

[133]

FTO/c-TiO2/CsBr/MAPbI3/spiro-OMeTAD/Au

n-i-p CsBr 16.3 Enhances UV-light stability [119]

FTO/c-TiO2/PCBA/MAPbI3/spiro-OMeTAD/Au

n-i-p PCBA 13.78 Reduces contact resistance and promotes change transfer at the interface

[123]

FTO/c-TiO2/PCBA/MAPbI3/spiro-OMeTAD/Ag

n-i-p PCBA 17.76 Reduces charge recombination and facilitates electron extraction

[124]

ITO/c-TiO2/Fullerenol/MAPbI3−xClx /P3HT/MoO3/Ag

n-i-p Fullerenol 14.69 Facilitates the charge transportation and decreases the interfacial resistance

[106]

ITO/c-TiO2-Cl/FA0.85MA0.15PbI2.55Br0.45 /sipro-OMeTAD/Au

n-i-p Cl additive 20.1 Strengthens the interface binding, suppresses interfacial recombination, and improves stability

[142]

ITO/PEDOT:PSS/MAPbI3 /PCBM/C60/BCP/Al

p-i-n PCBM 14.9 Passivates charge trap states and eliminates photocurrent hysteresis

[126]

(Continued )

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including low cost, low toxicity, high transparency, high polarity, and easy film formability. The introduction of a thin layer of PEO at the perovskite/TiOx interface greatly reduced the work function of the TiOx ETM due to the formation of an interfacial dipole without impacting the morphology, transparency, or hydrophilicity of the TiOx layer. This led to a reduction in recombination and an enhanced electron collec-tion efficiency [107]. Fullerene C60 and its derivatives are also popular materials for interfacial engineering in PSCs due to their high electron transfer efficiency. A triblock-functional-ized fullerene derivative (PCBB-2CN-2C8) was designed for modifying the perovskite/TiO2 ETM interface in PHJ n-i-p PSCs (figure 5(e)) [105]. Investigation into the electrical prop-erties of the interfacial layer revealed that PCBB-2CN-2C8 can facilitate the formation of perovskite films with high crys-tallinity, uniform crystal size distribution, improved deep trap

state passivation on the TiO2 surface, reduced charge recom-bination, enhanced device stability under ambient conditions (figure 5(f)), and reduced hysteresis [105]. Recently, a chlo-rine-capped TiO2 colloidal nanocrystal film prepared via low-temperature processing was employed as an electron-selective layer in planar PSCs with a certified efficiency of 20.1%. The TiO2-Cl/perovskite interface showed improved binding strength between the TiO2/perovskite interface, reduced inter-facial recombination, and improved PSC stability [142].

2.3. Engineering the interface between the electron transporting and metal layers

PHJ p-i-n PSCs with a typical inverted structure (ITO/PEDOT:PSS/perovskite/PCBM/metal) are promising for future flexible electronic devices due to their low-temperature

Figure 5. (a) SEM cross-section image of a GQD-modified perovskite layer. (b) Schematic illustration of electron generation and extraction at the P/E interface with and without GQDs. (Reproduced with permission from [108]. Copyright 2014, American Chemical Society.) (c) Ultraviolet photoelectron spectroscopy spectra of c-TiO2 with/without CsBr modification. (d) The normalized PCE decay of devices under UV irradiation. (Reproduced with permission from [119]. Copyright 2016, The Royal Society of Chemistry.) (e) Schematic illustration of the PSC architecture with TiO2/PCBB-2CN-2C8 as the ETM. (f) Efficiency of polymer-containing and polymer-free devices stored on a windowsill in ambient air with 45%–50% humidity over several days. (Reproduced with permission from [105]. Copyright 2015, American Chemical Society.)

ITO/PEDOT:PSS/MAPbI3 /LiSPS/PCBM/Al p-i-n LiSPS 13.83 Reduces charge-carrier recombination and eliminates photocurrent hysteresis

[77]

ITO/PEDOT:PSS/MAPbI3−xClx/TPPI/PCBM/TPPI/Al

p-i-n TPPI 13 Improves coverage and crystallinity of the perovskite, and reduces contact resist-ance

[136]

FTO/NiO/MAPbI3−xClx/PCBM:PS/Al p-i-n PS 10.68 Prevents electron–hole recombination [133]FTO/c-TiO2/m-TiO2/Silane/ZrO2/MAPbI3/Carbon

HTM-free Silane 12.7 Tunes interface electronic structure and passivates the recombination process

[131]

FTO/c-TiO2/m-TiO2/BAA/MAPbBr3/Au HTM-free Bromoacetic acid

5.57 Passivates interface and suppresses charge recombination

[130]

FTO/c-TiO2/m-TiO2/NiO/MAPbI3/Carbon HTM-free NiO 11.4 Extends electron lifetime and improves hole extraction

[120]

aGQDs: graphene quantum dots, CQDs: carbon quantum dots, PCBA: [6,6]-phenyl-C61-butyric acid, PCBM: [6,6]-phenyl C61 butyric acid methyl ester, PCBB-2CN-2C8: [6,6]-phenyl-C61-butyric acid-dioctyl-3,3′-(5-hydroxy-1,3-phenylene)-bis(2-cyanoacrylate) ester, PABA: 4-aminobenzoic acid, C3: 3-aminopropanioc acid, PEO: polyoxyethylene, PFN-OX: poly[9,9-bis(6′-(N,N-diethylamino)propyl)-fluorene-alt-9,9-bis-(3-ethyl(oxetane-3-ethyloxy)-hexyl)-fluorene], LiSPS: 4-lithium styrenesulfonic acid/styrene copolymer, PS: polystyrene, TPPI: phosphoniumhalide salts, PTS: phenyltrichlorosilane.

Table 2. (Continued )

Device structure Device type MaterialaPCE (%) Engineering effect Reference

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Table 3. Summary of the strategies for interfacial engineering at the E/M interface in PSCs.

Device structure Device type Materiala PCE (%) Engineering effect Reference

ITO/PEDOT:PSS/MAPbI3−xClx /PCBM/ZnO/Al

p-i-n ZnO 15.9 Enhances electron extraction and im-proves ambient stability

[149]

ITO/PEDOT:PSS/MAPbI3 /PCBM/LiF/Al

p-i-n LiF 14.1 Reduces energy barrier, improves elec-tron extraction, and eliminates photo-current hysteresis

[152]

ITO/PEDOT:PSS/MAPbI3−xClx /PCBM/PN4N/Al

p-i-n PN4N 15.0 Reduces contact resistance and suppresses interfacial charge recom-bination

[160]

ITO/PEDOT:PSS/MAPbI3−xBrx /PCBM/BCP/Ag

p-i-n BCP 13.06 Reduces leakage current and decreases charge transfer resistance

[155]

ITO/PEDOT:PSS/MAPbI3−xClx/PCBM/Rhodamine 101/LiF/Al

p-i-n Rhodamine 101/LiF

13.2 Lowers work function and improves electron collection

[154]

ITO/PEDOT:PSS/MAPbI3−xClx /PCBM/C60/LiF/Al

p-i-n C60/LiF 14.24 Facilitates charge collection process and improves air stability

[163]

ITO/PEDOT:PSS/MAPbI3/PTCDI/Cr2O3/Cr/Au

p-i-n Cr2O3/Cr 12.5 Enhances air stability [151]

ITO/PEDOT:PSS/MAPbI3−xClx /PCBM/PDINO/Ag

p-i-n PDINO 14.0 Reduces series resistance and enhances air stability

[156]

ITO/PEDOT:PSS/MAPbI3 /PCBM/MUTAB/Ag

p-i-n MUTAB 16.5 Decreases contact resistance and improves ambient/thermal stability

[147]

ITO/PEDOT:PSS/MAPbI3−xClx /PCBM/DMAPA-C60/Al

p-i-n DMAPA-C60 13.4 Reduces work function, facilitates energy-level alignment, and inhibits carrier recombination

[164]

ITO/PEDOT:PSS-Ag NPs/MAPbI3−xClx /PCBM/Bphen/Ag

p-i-n Bphen 15.75 Improves electrical properties and charge extraction

[140]

ITO/PEDOT:PSS/MAPbI3−xClx /PCBM/Bphen/CsCO3-MoO3/Ag

p-i-n CsCO3-MoO3 15.59 Lowers work function and enhances electron collection efficiency

[153]

ITO/PEDOT:PSS/MAPbI3 /PCBM/CTAB-doped ZrOx/Ag

p-i-n CTAB-doped ZrOx

15.9 Reduces work function, facilitates electron extraction, and enhances ambient stability

[143]

ITO/PEDOT:PSS/MAPbI3 /CTAB-doped PCBM/Ag

p-i-n CTAB 17.11 Reduces work function, facilitates electron extraction, and enhances ambient stability

[148]

ITO/PEDOT:PSS/MAPbI3 /DMOAP-doped PCBM/Ag

p-i-n DMOAP 18.06 Reduces work function, facilitates electron extraction, and enhances ambient stability

[145]

ITO/NiO/MAPbI3 /PCBM/PCB-DAN/Ag

p-i-n PCBDAN 17.2 Reduces interface barrier, facilitates electron transport, and improves air stability

[144]

ITO/PEDOT:PSS/MAPbI3−xClx /PCBM/PEIE/Ag

p-i-n PEIE 12.36 Reduces work function and facilitates electron extraction

[146]

ITO/PEDOT:PSS/MAPbI3−xClx /PCBM/Bphen-Ir(MDQ)2(acac)/Ag

p-i-n Bphen-Ir(MDQ)2(acac)

15.87 Reduces current leakage and decreases carrier recombination

[162]

ITO/PEDOT:PSS/PSS-Na/MAPbI3−xClx /PCBM/ZnO:PFN/Al

p-i-n ZnO:PFN 12.76 Reduces interface charge recombination and improves long-term stability

[159]

ITO/PEDOT:PSS/PSS-Na/MAPbI3−xClx /PCBM/ZnO:PEI/Al

p-i-n ZnO:PEI 11.76 Reduces work function, facilitates electron extraction, and enhances thermal stability

[158]

ITO/PEDOT:PSS/MAPbI3 /PCBM/PEOz/Ag

p-i-n PEOz 14 Reduces work function and facilitates electron extraction

[161]

a PN4N: amino-functionalized polymer, BCP: bathocuproine, PDINO: perylene-diimide, MUTAB: thiol-functionalized cationic surfactant (11-mercaptoundecyl)trimethyl ammonium bromide, DMAPA-C60: an amine functionalized fullerene derivative, Bphen: 4,7-diphenyl-1,10-phenanthroline, CTAB: cetyltrimethyl ammonium bromide, DMOAP: N,N-dimethyl-N-octadecyl(3-aminopropyl)trimethoxysilyl chloride silane, PCBDAN: [6,6]-phenyl-C61-butyric acid 2-((2-(dimethylamino)ethyl)(methyl)amino) ethyl ester, PEIE: ethoxylatedpolyethylenimine, Ir(MDQ)2-(acac): iridium(III) bis(2-methyldibenzo [f,h]quinoxaline)-(acetylacetonate), PFN: poly[(9,9-bis(3′-(N,N-dimethylamion)propyl)-2,7-fluo-rene)-alt-2,7-(9,9-dioctyl)-fluorene], PEI: polyethylenimine, PEOz: poly(2-ethyl-2-oxazoline).

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processability [26]. It is important that the work functions of the cathode and anode match the quasi-Fermi levels of the active layers to ensure ohmic contact, maximum Voc, and minimal energy barrier for charge extraction to reduce interface carrier recombination [143, 144]. Accordingly, low work function metals such as Al (~4.1 eV) or Ca (~2.9 eV) are sometimes used as cathode electrodes in PHJ p-i-n PSCs for efficient electron extraction (tables 1–4). Unfortunately, these low work function metals are readily oxidized in ambi-ent air leading to device instability. Thus, stable and high work function metals such as Au (~5.1 eV) or Ag (~4.6 eV) are typically used instead. However, the mismatch between the work function of these metals and the lowest unoccu-pied molecular orbital of PCBM (~4.2 eV) is undesirable for high-performance devices [145–147]. To address this issue, recent efforts have employed interfacial engineering of the ETM layer/cathode interface of PHJ p-i-n PSCs in order to improve device efficiency and stability [145–147]. The most common approach is to incorporate an additional cathode buffer layer (CBL) between the PCBM ETM layer and the metal cathode. As shown in table 3, a number of materials are employed as the CBL, including (1) modified PCBM [145, 148], (2) inorganic materials [143, 149–153], (3) organic compounds [147, 154–157], (4) functional polymers [146, 158–161], (5) 4,7-diphenyl-1,10-phenanthroline (Bphen) [140] and iridium(III) bis(2-methyldibenzo[f,h]quinoxaline)-(acetylacetonate) (Ir(MDQ)2-(acac))-doped Bphen [162], and (6) fullerene C60 and its derivatives [62, 144, 163, 164]. These materials are incorporated into the PCBM/metal inter-face to lower the work function of the metal (e.g. Au or Ag) cathodes, decrease contact resistance, reduce the energy bar-rier, facilitate electron extraction, enhance ambient/thermal stability, and eliminate photocurrent hysteresis.

For example, LiF is an electrical insulator and its insertion at the PCBM/Al interface decreased the energy barrier due to the formation of a dipole moment at the interface for enhanced device performance from increased electron extraction, reduced hysteresis, and improved stability [152]. In addition, zwitterions, like rhodamine 101 derivatives, have distinct separate positive and negative charges on the same molecule that are able to lower the work function of electrodes through the formation of surface dipoles. When double interlayers composed of a rhodamine 101 layer and a LiF layer were introduced at the PCBM/Ag inter-face of PHJ p-i-n PSCs, they exhibited an improved electron-collection efficiency compared to a single interlayer composed of either LiF or rhodamine 101 [154]. The incorporation of a C60 layer between the PCBM and LiF layers can also protect the perovskite/PCBM interface by blocking water and oxygen intru-sion and thereby significantly enhancing device stability (figures 6(a) and (b)) [163]. Similarly, a ZnO nanocrystal layer prepared by an annealing-free process is often used as a selective electron-transporting interlayer (i.e. hole blocking) to improve the cath-ode interface and enable stable and reproducible PHJ p-i-n PSCs [149, 150]. The incorporation of ZnO:PEI or ZnO:PFN compos-ites has also been reported to further reduce the interface charge recombination for improved PCE performance and stabilize the interface for enhanced device stability [158, 159].

Solution-processed polyelectrolyte interlayers, such as PEIE and P3TMAHT have also been applied to modify the PCBM/Ag interface. Forming a surface dipole with a negative charge toward the Ag layer and a positive charge toward the PCBM layer enables the reduction of the metal vacuum level (figure 6(c)), thereby lowering the electron injection barrier to PCBM (figure 6(d)) [146]. The deposition of a thin layer of Cr/Cr2O3 at the interface between the PCBM or N, N′-dimethyl-3,4,9,10-tetracarboxylicperylenediimide (PTCDI)

Figure 6. (a) SEM cross-section and (b) stability tests of p-i-n PSCs with a PCBM/C60/LiF interlayer at the E/M interface. (Reproduced with permission from [163]. Copyright 2015, American Chemical Society.) (c) Schematic energy-level diagram of devices with and without the polyelectrolyte interlayer under flat-band conditions, and (d) J–V characteristics of ITO/ZnO/PCBM devices with and without a polyelectrolyte /Ag interlayer. (Reproduced with permission from [146]. Copyright 2014, American Chemical Society.) (e) TEM image of PSCs prepared on foil, and (f) stability tests of devices in air under continuous illumination with and without a Cr2O3/Cr interlayer. (Reproduced with permission from Macmillan Publishers Ltd: Nature Materials [151]. Copyright 2015.

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ETM layer and the metal cathode can improve the device sta-bility and enable the use of metal electrodes such as Au, Cu, or Al under ambient conditions (figure 6(e) and (f)) [151]. This is because the Cr/Cr2O3 interlayer can greatly reduce the for-mation of oxidized and halide-containing species at the metal surface [151].

2.4. Engineering the interface between the perovskite and hole transporting layers

Although perovskite is able to function as both an efficient light harvester and hole transporter in HTM-free PSCs, the participation of a HTM can further promote hole transport from perovskites to anodes for higher-performance devices

Table 4. Summary of the strategies for interfacial engineering of the P/H and other interfaces of PSCs.

Device structure Device type Interface MaterialaPCE (%) Engineering effect Reference

FTO/c-TiO2/m-TiO2/MAPbI3/Al2O3/spiro-OMeTAD/Au

Mesoscopic P/H Al2O3 4.69 Enhances moisture/sunlight stability [116]

FTO/c-TiO2/m-TiO2/MAPbI3/TPB/spiro-OMeTAD/Au

Mesoscopic P/H TPB 13.10 Tunes energy levels and reduces charge recombination

[175]

FTO/c-TiO2/m-Al2O3/MAPbI3−xClx/IPFB/spiro-OMeTAD/Ag

Mesoscopic P/H IPFB 15.7 Passivates surface trapping states and improves charge extraction

[172]

FTO/c-TiO2/m-TiO2/MAPbI3/GO/spiro-OMeTAD/Au

Mesoscopic P/H GO 14.5 Improves interfacial contact and reduces charge recombination

[70]

FTO/c-TiO2/m-TiO2/MAPbI3/PS/spiro-OMeTAD/Au

Mesoscopic P/H PS 17.80 Suppresses charge recombination and enhances humidity stability

[177]

FTO/c-TiO2/m-TiO2/MAPbI3/graphene/spiro-OMeTAD/Au

Mesoscopic P/H Graphene 14.6 Reduces surface traps and enhances hole extraction

[176]

FTO/c-TiO2/m-TiO2/HOOC-R-SH/MAPbI3/HS-PhF5/spiro-OMeTAD/Au

Mesoscopic P/H Thiols 14.1 Enhances electron transfer and improves air/light stability

[71]

FTO/c-TiO2/m-TiO2/(FAPbI3)0

.85(MAPbBr3)0.15/FAPbBr3−xIx/spiro-OMeTAD/Au

Mesoscopic P/H FAPbBr3−xIx 21.3 Matches energy levels and reduces charge carrier recombination

[179]

FTO/c-TiO2/m-Al2O3/MAPbI3 /Al2O3/spiro-OMeTAD/Au

n-i-p P/H Al2O3 13.07 Lowers device series resistance, reduces shunting degradation, and improves stability

[174]

FTO/c-TiO2/MAPbI3/C12-silane/spiro-OMeTAD/Ag

n-i-p P/H C12-silane 13.74 Blocks electron recombination and improves moisture stability

[178]

FTO/NiO/meso-Al2O3/MAPbI3 /PCBM/BCP/Ag

p-i-n P/H Al2O3 13.5 Minimizes interfacial recombination loss

[120]

ITO/PEDOT:PSS/X-QUPD/MAPbI3−xClx /PCBM/Al

p-i-n P/H X-QUPD 13.06 Reduces leakage current and suppress-es electron–hole recombination

[165]

ITO/PEDOT:PSS/AgOTF-doped GO/MAPbI3−xClx /PCBM/Au

p-i-n P/H AgOTF-doped GO

11.90 Tunes electronic structure and improves charge collection

[181]

ITO/PEDOT:PSS/Au NPs-PN4N/MAPbI3−xClx /PCBM/Al

p-i-n P/H Au NPs-PN4N

13.7 Improves electrical contact and generates plasmonic scattering effect

[180]

ITO/PEDOT:PSS-Ag NPs/MAPbI3−xClx /PCBM/Bphen/Ag

p-i-n P/H Ag NPs 15.75 Improves electrical properties and charge extraction

[141]

FTO/graphene/c-TiO2/Al2O3MAPbI3−xClx/spiro-OMeTAD/Au

Mesoscopic S/E Graphene 15.6 Provides superior charge collection ability

[187]

ITO/PEIE/c-Y:TiOx/MAPbI3−xClx/spiro-OMeTAD/Au

n-i-p S/E PEIE 19.3 Reduces work function and facilitates electron extraction

[190]

ITO/c-TiO2/MAPbI3/spiro-OMeTAD/MoO3/Al

n-i-p H/M MoO3 10.2 Enhances air stability [57]

FTO/c-TiO2/MAPbI3/spiro-OMeTAD/Al2O3/Ag

n-i-p H/M Al2O3 15.2 Enhances moisture stability [69]

ITO/ZnO/MAPbI3/graphite/car-bon black

HTM-free P/M graphite 10.2 Improves electrical contact and charge collection

[190]

FTO/c-TiO2/m-TiO2/MAPbI3/AlOx/Au

HTM-free P/M AlOx 11.10 Acts as an effective electron blocking layer to enhance charge collection

[189]

a TPB: N,N,N′,N′-tetraphenyl-benzidine, IPFB: iodopentafluorobenzene, PS: polystyrene, C12-silane: dodecyltrimethoxysilane, X-QUPD: (N,N′-bis(4-(6-((3-ethyloxetan-3-yl)-methoxy)-hexyloxy)phenyl)-N,N′-bis(4-methoxy-phenyl)biphenyl-4,4′-diamine, AgOTf: silver trifluoromethanesulfonate, PN4N: amine-containing polymer, PEIE: ethoxylatedpolyethylenimine.

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[25]. Spiro-OMeTAD is a small molecule commonly used as an HTM in solid-state dye-sensitized solar cells and meso-scopic/PHJ n-i-p PSCs. However, pristine spiro-OMeTAD has poor hole transport properties due to its low conductiv-ity (4 × 10−7 S cm−1), thereby leading to high series resist-ance and low device performance [165]. Hence, lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) was doped into spiro-OMeTAD to increase its conductivity up to 2 × 10−4 S cm−1 and 4-tert-butylpyridine (t-BP) was used as an addi-tive to promote the dissolution of the lithium salt in the spiro-OMeTAD matrix [165, 166]. Recently, Ni nanobelts were dispersed in spiro-OMeTAD to enhance the stability of PSCs [167]. In addition, modifying the structure of spiro-OMeTAD by moving the methoxyl groups from para to ortho positions improved the hole transporting properties [168]. Despite improvements, spiro-OMeTAD still suffers from stability problems, because of the oxidative doping, chemical degrada-tion, and poor shielding in moist conditions [76]. In order to make high-performance PSCs with long-term stability, it is imperative to develop novel HTMs with decent hydrophobic-ity, stable morphology, suitable energy level alignment with perovskites, fast hole transport rate, high optical transparency, and low cost [169–171]. In parallel, appropriate engineer-ing of the perovskite/HTM interface can also improve hole extraction and enhance PSC device stability [166, 172, 173]. Recently, a series of materials have been investigated as buf-fer layers for the perovskite/HTM interface (table 4) both in

mesoscopic/PHJ n-i-p PSCs [70, 71, 116, 172, 174–179], and PHJ p-i-n PSCs [140, 165, 180, 181].

Owing to their unique structure and properties, graphene-based materials (i.e. functionalized graphene, GO and RGO) play multi-faceted roles in PSCs. For example, they can serve as an effective HTM [58, 59, 150, 182], replace the metal layer as a cathode [183–185], serve as an additive to mod-ify the crystallization and morphology of perovskite [91], and function as a conducting interlayer to tune the interfaces between each active layer [70, 181, 186, 187]. Amino-rich graphene was introduced at the perovskite/spiro-OMeTAD interface (P/H) to serve as an effective hole extraction and transfer pathway and passivate surface traps in perovskite via coordination between amines on the graphene surface and the under-coordinated Pb2+ on the perovskite surface (figure 7(a)) [176]. GO was used as an amphiphilic material to enhance the interfacial contact between perovskite and spiro-OMeTAD leading to improved PSC performance with increased JSC due to the improved charge extraction and enhanced VOC and FF attributed to reduced charge recombination [70]. In addition, silver trifluoromethanesulfonate (AgOTf) was employed to adjust the properties (e.g. structure, work function, and mobil-ity) of single-layer GO films. AgOTf-doped GO was incorpo-rated into PEDOT:PSS to provide desirable charge collection for the PHJ p-i-n PSCs [181].

Insulating Al2O3 is another versatile material for PSC. It can substitute mesoporous TiO2 to function as a simple

Figure 7. (a) Schematic of a PSC with an amino-rich graphene interlayer at the perovskite/HTM interface. (Reproduced with permission from [176]. Copyright 2016, The Royal Society of Chemistry.) (b) Energy-level diagram (versus vacuum), and (c) fitted interfacial recombination resistance (Rct) from electrochemical impedance spectroscopy spectra of an Al2O3-modified PSC in the dark at different applied potentials. (Reproduced with permission from [120]. Copyright 2015, The Royal Society of Chemistry.)

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scaffold for perovskite in PSCs [188]. Al2O3 has become an attractive interlayer material for use at nearly all the interfaces in PSCs, including the perovskite/spiro-OMeTAD [116], spiro-OMeTAD/Ag [69], perovskite/TiO2 [174, 189], per-ovskite/Au [190], FTO/m-TiO2 [114], and NiO/perovskite interfaces [120]. A solution-processed Al2O3 layer can pro-tect the perovskite from degradation by moisture and suppress charge recombination between TiO2 and spiro-OMeTAD if pinholes are present [115]. A hybrid interfacial layer com-posed of a NiO compact layer and a mesoporous Al2O3 scaf-fold applied in mesoscopic PSCs can strongly passivate the FTO/perovskite interface and block the shunt paths between NiO and PCBM due to the infiltration of perovskite inside the mesoporous Al2O3 layer (figures 7(b) and (c)) [120].

2.5. Engineering other interfaces

Apart from the abovementioned interfacial engineering approaches, modification can be extended to other interfaces (table 4; ITO or FTO/TiO2 [187, 191], spiro-OMeTAD/Ag or Al [57, 69], and perovskite/Au or carbon [190, 192] inter-faces in HTM-free PSCs) for improved PSC performance. Ethoxylated polyethyleneimine (PEIE), a polymer contain-ing simple aliphatic amine groups, was used to modify the ITO electrode to reduce its work function to 4.0 eV to enhance electron transport between the ETM and ITO layers [191]. Nanocomposites of high-quality graphene flakes and TiO2 nanoparticles were exploited as the electron collection layers in mesoscopic PSCs in which the graphene nanoflakes pro-vide superior charge collection in the nanocomposites and enabled device fabrication at low temperatures [187].

3. Conclusions and outlook

In this review, we summarize the latest advances in interfacial engineering strategies for PSCs with the most typical device configurations. Research has discovered that an immense vari-ety of materials can serve as functional interlayers at virtually all interfaces in PSCs, with this review covering all of the rep-resentative material types. Solvent treatment and addition are promising for preparing high-quality perovskite films for PSCs with improved light-harvesting efficiency, improved stability, and low current hysteresis. The introduction of a passivation layer between the perovskite and electron transport layers is an effective way to reduce charge recombination and improve electron extraction efficiency in PSCs. Similarly, the use of special materials as interfacial layers between the perovskite and hole transporting materials can enhance hole extraction efficiency and PSC device stability. Finally, the inclusion of the chosen inorganic and organic materials as interfacial lay-ers between the electron transporting and metal layers enables well-aligned device energy levels and improved PSC sta-bility. The attractive properties of organic–inorganic halide perovskites have quickly made it a promising material for use in solar cells. However, we are still a long way off see-ing PSCs in the marketplace. This is largely due to device instability, Pb toxicity, and large-scale production limitations. Therefore, greater effort should be devoted to exploiting more

favorable materials and cell configurations for satisfactory light- harvesting, developing more robust deposition methods for high-quality films, and engineering optimal interfaces for effec-tive charge extraction in order to further optimize device per-formance and accelerate future commercialization. Interfaces in PSCs play an essential role in charge carrier extraction and transport. At their best, interfaces serve as smooth bridges delivering photo-excited free carriers. However, they can also become traps leading to charge recombination at interfacial defects. Thus, employing interlayers to engineer interfaces in PSCs has been and will continue to be a vital research direction for optimizing perovskite properties, enhancing charge collec-tion, and improving environmental stability to ultimately real-ize industrially viable perovskite solar cells.

Acknowledgments

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21503177), the Air Force Office of Scientific Research (FA9550-14-1-0037 and FA9550-16-1-0187), the Natural Sci-ence Foundation of Fujian Province of China (No. 21171075), the Fundamental Research Funds for the Central Universities of China (No. 20720150031), and the ‘111’ Project (B16029).

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