Functionalization of ETL/Perovskite Interface: Comparison
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Please note this is a comparison between Version 2 by Alfred Zheng and Version 1 by Elias Stathatos.

Perovskite solar cells (PSCs) have revolutionized the field of photovoltaics, achieving certified power conversion efficiencies reaching 26% at the laboratory scale. High performance, enhanced stability, and long lifetime are prerequisites for the industrialization and commercialization of this class of third-generation photovoltaic technology. The electron transport layer (ETL) plays a pivotal role in obtaining stable perovskite solar cells with a high power conversion efficiency (PCE). It must be characterized by high transparency to visible light, photostability, and compatibility with the perovskite used. Therefore, a thorough comprehension and optimization of the interaction between perovskite materials and TiO2 ETL underlayers, as well as a special focus on the behavior of the corresponding devices, are necessary.

  • perovskite solar cells
  • perovskite interface
  • electron transport layer

1. Introduction

In recent years, intensive research activity has been conducted in the field of third-generation photovoltaics, especially in the development of metal halide perovskite materials and perovskite-based PV devices, where the certified power conversion efficiency (PCE) now reaches 26% [1][2][3][4][5].
Perovskites employed in solar cells are usually described by the general formula ABX3, where A stands for organic (MA or FA) and/or inorganic (Cs or Ru) cations, B stands for metal cations (Pb or Sn), and X refers to halide anions (I, Br, or X) [6]. A PSC device, independently of its normal or inversed architecture, has a multilayered structure (Figure 1), where the perovskite active layer, which absorbs light and creates photogenerated charge carriers (electrons and holes), is placed between two charge extraction/transport layers (electron transport layer (ETL) and hole transport material (HTM)). A conducting glass substrate (FTO or ITO) below the ETL (HTM) and a metal evaporated film (Au, Ag, or Al) on top of the HTM (or ETL) ensure the charge collection [7]. Despite PCEs outperforming those of silicon counterparts, the poor stability of their absorber when humidity, oxygen, and/or light is present is the main issue impeding the long-term operation of PSCs and affecting their industrialization and commercialization. To address the stability issues and obtain significantly efficient and robust PSCs, a number of advanced strategies have been proposed in the literature, including optimization in terms of composition (cation–anion mixing, perovskite doping, and lead-free perovskite) and band-gap, additive, solvent/antisolvent, film deposition, and interface engineering [8][9]. In the last case, interface functionalization is realized through perovskite dimension (3D/0D, 3D/1D, and 3D/2D), molecular (dyes, polymers, etc.), ETL (SnO2, PCBM, fullerene derivatives, sulfides, and metal doped-oxides), and HTM (spiro-OMeTAD replacement, NiOx, C-based PSCs, and other p-type materials) innovative engineering approaches [10][11][12]. This leads to appropriate energy-level alignment, minimal defects, and the development of highly hydrophobic interfaces with a high resistance to humidity attack.
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Figure 1. Schematic representation of conventional n-i-p (left) and inverted p-i-n (right) PSC architectures, depicting the functionalization of ETL/perovskite and perovskite /HTM interfaces (ETL: electron transport layer; HTM: hole transport material; M: Ag, Au, or Al metal contacts/charge collectors).

2. Functionalization of ETL/Perovskite Interface

The electron transport layer (ETL) plays a pivotal role in obtaining stable perovskite solar cells with a high power conversion efficiency (PCE) [13][14]. It must be characterized by high transparency to visible light, photostability, and compatibility with the perovskite used. Therefore, a thorough comprehension and optimization of the interaction between perovskite materials and TiO2 ETL underlayers, as well as a special focus on the behavior of the corresponding devices, are necessary. Working toward this target, novel approaches based upon interface engineering in terms of functionalization of the electron transport layer (ETL) of perovskite solar cells (PSC) have recently been reported [15][16].
Transition metal semiconducting oxides (TMSO, e.g., TiO2, SnO2, ZnO, Zn2SnO4, CeO2, Cr2O3, Fe2O3, and Nb2O5) [17] and fullerene derivatives (e.g., 6,6-phenyl-C61-butyric acid methyl ester/PCBM) [18][19][20] have been commonly employed as selective electron transport layers in PSCs. H.S. Kim et al. explored the addition of MXene/TMSO nanocomposites (MXenes: transition metal carbides, nitrides, or carbonitrides having a two-dimensional layered structure) to modify the PCBM ETL and further boosted the performance (PCE and long-term stability) of inverted perovskite solar cells (p-i-n PSCs) [21].
Inverted PSCs usually employ PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) fullerene derivative as the ETL. However, the efficiency and lifespan of the corresponding devices are limited by a high degree of disorder and severe self-aggregation of the PCBM ETL. Working on inverted planar PSCs, Y. Jiang et al. used the chelation effect as a very useful tool to reduce the ETL disorder and, thus, enhance the efficiency and stability of the devices. The authors designed a series of functional dyads FP-Cn (n = 4, 8, 12), where fullerene and terpyridine chelating groups are linked via a flexible alkyl chain spacer. Using the FP-C8/C60 ETL dyad as the electron transport layer and Cs0.05FA0.90MA0.05PbI2.85Br0.15 as the light absorber, PSCs with a PCE of 21.69%, minor hysteresis, good reproducibility, and high stability were obtained. By replacing perovskite with FAPbI3, PSCs with the FP-C8/C60 ETL gave an optimal PCE of 23.08%, which is one of the highest efficiency values ever obtained with solution-processed fullerene derivatives [22].
In addition, following an interlayer strategy involving the in situ generation of polyethylenimine-based two-dimensional (2D) perovskite, C. Wang et al. boosted the efficiency, stability, and reproducibility of inverted planar perovskite solar cells by effectively reducing the lattice match between the NiOx HTL and the MAPbI3 absorber, thereby suppressing the interfacial defect formation and developing perovskite layers with a high crystalline quality [23]. Moreover, working on FTO/NiOx/MAPbI3/PC61BM/BCP/Ag PSCs, I-H. Ho et al. modified the HTL/absorber interface with quaternary ammonium halide-containing cellulose derivatives and succeeded in producing perovskite films of high crystalline quality with large grains, low surface roughness, enhanced light absorption, and increased hole mobility. The coating with cellulose polymeric materials smoothens the NiOx HTL surface, tunes its wettability, improves the compatibility with the perovskite absorber, and passivates uncoordinated Pb2+ species. As a result, the MAPbI3-based inverted PSCs modified by cellulose polymers showed improved photovoltaic performance and high stability after storage under ambient conditions [24].
Taking the advantage that perovskite solar cells are considered the evolution of dye-sensitized solar cell technology, the idea of dye sensitization for optimizing the ETL/perovskite interface was investigated by N. Balis et al. [25]. The use of the solution-processable D35 [triphenylamine-based metal-free (E)-3-(5-(4-(bis(2’,4’-dibutoxy-[1,1’-biphenyl]-4-yl) amino) phenyl) thiophen-2-yl)-2-cyanoacrylic acid] D-π-A organic chromophore to sensitize the TiO2 compact layer (Figure 2) led to planar PSCs based on MAPbI3 achieving a power conversion efficiency of 17% (against 15% of those with a non-sensitized layer), which was accompanied by further improved stability. The obtained results suggest that this performance improvement can be attributed to enhanced recombination resistance, increased electron transport, better crystallization of the deposited perovskite, defect passivation, roughness reduction, dipole moment effects, and the humidity sealing character of the hydrophobic dye monolayer. Thus, for the first time in the literature, it was demonstrated that dye sensitization could be effectively applied to interface engineering in PSCs.
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Figure 2. Functionalization of the TiO2/MAPbI3 interface following sensitization of the TiO2 compact layer (CL) by the hydrophobic D35 dye.
PSCs’ instability remains the most significant issue preventing them from industrial scaling up. In this context, the dye-sensitization approach was expanded by investigating its effect on the stability of planar PSCs against thermal and light stresses [26]. The stability investigation showed an improved endurance of devices after the insertion of D35 under shelf-shield conditions and especially after accelerated thermal treatment (retaining almost 80% of their initial efficiency after 60 min at 100 °C) and prolonged light saturation exposure (low degradation following continuous illumination for 7 h at 76.5 mWcm−2 incident irradiance in the 300–800 nm spectral range). This study confirmed the plethoric role of the dye-sensitization approach and the advantages it confers to interfacial engineering via organic chromophores for achieving efficient and stable PSCs. Further developments are expected as the dye-sensitization methodology can further employ a large number of molecular hydrophobic dyes, disposing exceptional structural and optoelectronic properties.
The effect of dye modification on TiO2 and ZnO electron transport layers in planar PSCs was also reported by R. Chouk et al. [27]. As a sensitizer, the authors employed a Schiff base–cobalt complex derived from ninhydrin and glycine ligands and succeeded in improving the photoinduced electron transfer and the resulting device efficiency and stability. The authors confirmed the existence of strong interactions between the Cobalt (II) dye and the ETLs and obtained a significant efficiency increase in the performance of the corresponding FTO/TiO2/Co-NG/MAPbl3/Spiro-OMeTAD/Ag and FTO/ZnO/Co-NG/MAPbl3/Spiro-OMeTAD/Ag solar cells (equal to 18.94% and 16.32%, respectively).
Noh et al. [28] selected an electron-accepting n-type organic semiconductor [3,9-bis(2-metylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno [2,3-d:2′,3′-d’]-s-indaceno [1,2-b:5,6-b’] dithiophene/ITIC—Scheme 1a] to passivate the surface of SnO2 and, thus, developed an organic/inorganic double ETL. The glass/ITO//PEIE-SnO2-ITIC//(FAPbI3)0.95(MAPbBr3)0.05//Spiro-OMeTAD/Au planar architecture, incorporating the optimized ETL, presents improved energy band alignment, low contact resistance, reduced trap-state density, and reached PCE values exceeding 16% (with marginal hysteresis), which remained practically unchanged for 200 h.
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Scheme 1.
ITIC (
a
) and QA (
b
) molecular structures.
Molecular dyes can also be used to functionalize inverted architectures. Y. Qi et al. [29] designed novel organic cationic cyanine dye molecules and were able to efficiently passivate the interface between the PC61BM and Ag electrode in ITO/Glass//PTAA//perovskite//PCBM/Dye/Ag devices, improving the PCE from a value of 14.24% (control) to 19.14% (functionalized). The efficiency increase was attributed to reduced interface charge recombination and improved charge transport. The addition of the dye interlayer offered additional protection from moisture, and the corresponding devices maintained 90% of their initial PCE for 120 h (under ambient conditions).
In order to address the long-term stability issues of PSCs, Q. He et al. introduced a novel perovskite (MAPbI3 and triple cation) surface passivation strategy involving quinacridone (QA—Scheme 1b) hydrophobic coating. The addition of such an insoluble, low-cost industrial organic pigment results in passivated glass/ITO//SnO2//MAPbI3//Spiro-OMeTAD//Au PSCs with considerably improved performance (PCE of 21.1% with low hysteresis) and notable stability (maintaining 85.7% of their initial PCE after 240 h of storage at 85 °C) [30].
A number of transition metal oxides and related compounds (including TiO2 and perovskite oxides) have been successfully used as ETLs [31][32][33][34][35][36][37][38][39][40][41]. However, despite its extensive use as both a compact and a mesoporous layer, titanium dioxide (TiO2) is characterized by low electron mobility and poor conductivity, and may act as a photocatalyst of chemical reactions, leading to the degradation of perovskites and permanent polarization of the film [42][43][44][45]. Innovative engineering strategies focusing on ETL/perovskite interface optimization are necessary to address the above issues. Metal (Y, Co, Li, Ag, Sn, Fe, Ru, Nb, Zn, Ta, or Mg) and non-metal (F, Cl, or S) doping have been proposed as effective ETL modification strategies that can lead to enhanced electrical conductivity, increased charge transport, and reduced charge recombination [46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65][66]. Furthermore, S.-H. Chen et al. synthesized mesoscopic Ag-doped TiO2 (meso-Ag:TiO2) to address the serious hysteresis problems encountered in planar structures. Thus, perovskite devices [FTO glass/dense TiO2/meso-Ag:TiO2/CH3NH3Pbl3/spiro-OMeTAD/Ag] incorporating a meso-Ag:TiO2 ETL present low hysteresis, and their optimization results in a PCE as high as 17.7% [67]. On the other hand, the efficiency and stability of PSCs are sensitive to UV light, heat, and humidity, and strongly depend on the properties of ETLs. Transition metal oxides such as TiO2 can trigger light instability due to photocatalysis [68]. Indeed, it has been recently demonstrated that the presence of a hygrophobic copper-modified TiO2 ETL (Figure 3) primarily mitigates the photodegradation action of the substrate, boosts the perovskite nanomorphology, passivates the surface trap states of the perovskite absorber, and facilitates electron transport to the ITO charge collector [69]. The addition of Cu monovalent cations downshifts the Fermi level of TiO2 and gives rise to a significant improvement in the performance of perovskite nanohybrids in terms of efficient energy conversion to electricity.
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Figure 3.
Architecture of a planar PSC device based on Cu-modified TiO
2
ETL.
Indeed, perovskite devices using a Cu-modified compact TiO2 ETL present a PCE exceeding 18%, outperforming by more than 1% the corresponding efficiency of the reference device [69]. This work confirms the advantages of interface engineering via metal ion doping as a totally aspiring and novel strategy with multiple consequences in the field of PSCs. The above strategy can be employed to many photosensitive metal oxide ETL materials possessing suitable optoelectronic and structural characteristics with TiO2, thus enabling the development of highly efficient and more robust energy systems (solar cells, LEDs, and FETs) against environmental stresses.
According to the literature, mesostructured devices are among the most efficient PSCs where titanium dioxide pastes are commonly employed to deposit compact and mesoporous ETLs. Graphitic carbon nitride (g-C3N4) is a very promising two-dimensional (2D) polymeric material for photovoltaic applications due to its good stability and suitable electronic properties (heat-resistant n-type semiconductor).
Z. Liu et al. employed n-type g-C3N4 ultrathin films to modify the ETL/perovskite and perovskite/HTL interfaces in planar PSCs, and despite a challenging band alignment, they achieved a PCE as high as 19.67% and long-term stability, which was attributed to the dramatically reduced trap density at the ETL/perovskite and perovskite/HTL interfaces [70]. In a recent contribution, it was demonstrated that the presence of a 2D g-C3N4 material on the surface of a mesoporous TiO2 ETL (Figure 4 on the left) primarily results in the exact conduction band alignment of mp-TiO2 and CH3NH3PbI3 perovskite, thus enhancing the charge carrier transfer and minimizing the energy losses of the photogenerated excitons [71]. Furthermore, the conduction band alignment is extended until the FTO by substituting the widely used TiO2 compact layer with an ultrathin ALD ZnO layer (Figure 4 on the right), and thus, the electron transport to the charge collector is further facilitated, achieving PCE values as high as 20.53%. Another significant aspect of this interface modification is the long-term stability of these devices, which can be attributed to the hydrophobic environment that g-C3N4 creates before the perovskite layer. As such, this gives rise to a substantial improvement in the performance of perovskite nanohybrids in terms of efficient energy conversion to electricity. This work confirms the advantages of interface engineering via 2D carbon-based materials as a totally aspiring and novel approach with multiple consequences in the field of PSCs. This strategy can be further exploited by investigating the impact of other 2D structures and nanocarbon composites.
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Figure 4.
Functional incorporation of graphitic carbon nitride (g-C
3
N
4
) at the TiO
2
/perovskite interface for planar (
left
) and mesostructured (
right
) PSCs.

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