phd thesis on perovskite

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Developing highly efficient lead halide perovskite solar cells

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The Upscaled Fabrication and Encapsulation of Perovskite Solar Cells

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Perovskite materials for multi-junction solar cell applications

The work presented in this thesis will describe studies on organic-inorganic perovskite materials when being used in tandem and multi-junction solar cell architectures. Perovskite solar cells (PSC) have recently emerged as a viable absorber material for photovoltaic applications, owing to its remarkable optoelectronic properties along with its ease of fabrication. These attributes have led to a rapid progress in device performance, where over the last few years, perovskite solar cells have...

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Carbon-based electrodes for perovskite solar cells

First published on 16th July 2021

The cost-effective processability and high stability of carbon-based perovskite solar cells (C-PSCs) have shown great potential to positively devote to the development of large-scale production processes. However, there are certain critical issues such as inferior performance and poor interface contact between perovskites and carbon electrodes, which have to be resolved first. The review shows that three main carbon materials, namely, carbon black, graphenes and carbon nanotubes display high photoelectric conversion efficiencies when being mixedly used as rigid electrodes and show excellent robustness in mechanical deformation as flexible carbon electrodes in carbon-based perovskite solar cells. Moreover, the specific development of and the comparison among three primary types of C-PSCs, namely, meso C-PSCs, embedment C-PSCs and paintable PSCs are emphasized. Furthermore, this work discusses the latest progress of C-PSC interface engineering from four aspects, namely, energy alignment, hysteresis effect, interface passivation and built-in electric field, and the differences among them are explained. Finally, further challenges and perspectives of C-PSCs are outlined. This work will be a profound influence and guidance on the significance of C-PSCs in commercialization.

1. Introduction

The metal electrode here is usually gold (Au), silver (Ag) or other highly conductive metals, but the metal layer is formed in a vacuum environment for thermal evaporation coating with high energy consumption, which severely hinders large-scale production and commercialization of PSCs. 20–24 From the perspective of work-function ( W f ), carbon materials ( W f ∼ −5.0 eV) have the potential to replace Au ( W f ∼ −5.1 eV) as the black electrode of the device. Carbon materials have the advantages of abundant sources, high electrochemical stability, and hole extraction, and these advantages are unavailable for metal electrodes. Therefore, the manufacturing process of carbon-based structures is simplified because of the lack of the hole transport layer. In addition, the specific hydrophobicity of the carbon material structure can also significantly enhance the stability of the solar cells. As illustrated in Fig. 1(a) , the traditional M-PSCs consist of five layers (Au, hole transport layer (HTL), light-absorbing, electron transport layer (ETL), fluorine-doped tin oxide (FTO)). 19 Then, the HTL was removed and the Au electrode was replaced with the carbon electrode, as shown in Fig. 1(b) . 19 In M-PSCs, the interaction between the perovskite film and the ETL determines the electron quasi-Fermi level ( E fn ), while the interaction between the perovskite film and the HTL determines the hole quasi-Fermi level ( E fp ) ( Fig. 1(c) ). 19 When removing the HTL, the interaction between the perovskite/carbon electrode predominantly determines the E fp value in carbon-based perovskite solar cells (C-PSCs), which will uplift the E fp position thanks to the higher Femi level of carbon materials than the highest occupied molecular orbital (HOMO) of the HTL ( Fig. 1(d) ). 19 Consequently, it is the premise that improving the V OC and PCE of C-PSCs is to reduce the Femi level of carbon materials.

Utilizing carbon black/graphite as an anode in PSCs, a PCE of 6.6% was first achieved. 25 After that, low-temperature cured carbon electrodes for PSCs were fabricated, which were developed as substitutes for noble metal electrodes in HTL-free PSCs. This carbon black electrode had achieved a PCE of 8.31%, which reached 9% by optimizing the conditions (doctor-blading technique). 25,26 In addition, a novel preparation of a low-temperature carbon black electrode for HTL-free PSCs had been developed under high relative humidity, and the black electrode had a small sheet resistance and a good interfacial contact with the substrate. 27 Subsequently, a room-temperature solvent-exchange method was developed to fabricate self-adhesive macroporous carbon electrodes. 28 A PCE up to 19.2% was achieved, which was the best efficiency for C-PSCs. The typical PCE improvement of C-PSCs along with years is shown in Fig. 2 . It can be seen that C-PSCs doped with other materials exhibit superior efficiency, on account that the dopant can change the conductivity and W f of the black electrode. 29–34

Carbon electrode materials were fabricated by two main deposition methods. The first method required high temperatures (400–500 °C). 19 Mesoporous carbon was deposited at the top of the insulating layer by screen printing or doctor-blading technology, and subsequently sintered. The insulating layer had the role of preventing the interface contact between the transparent and the rear electrode, and avoided photocurrent loss. Another strategy was layer-by-layer deposition, 19 thanks to the development of low-temperature carbon electrodes, which first formed perovskite films and then screen-printed or doctor-bladed carbon films on the perovskite layer or HTL. The current carbon electrode materials mainly include carbon black, graphenes and carbon nanotubes. Fig. 3(a) and (b) indicate that the PCE of M-PSCs was higher than that of C-PSCs, due to which the smooth surface of the Au electrode reflected incident light and caused the light-absorbing layer to absorb repeatedly. 35 In addition, the cross-sectional scanning electron microscopic (SEM) image of the obtained device is displayed in Fig. 3(c) and (d) . 36,37 Hence, the low-cost and low-temperature carbon black electrode had a great potential in massive flexible manufacturing of PSCs. Recently, three carbon materials have been explored to carbon electrodes such as carbon black, graphenes, and carbon nanotubes. 38–47 However, the carbon electrode still exhibits low conductivity, carrier recombination, poor interface contact, etc.

In this review, we focus on the major progress of C-PSCs in recent years, including the development of carbon-based materials, their preparation and new strategies to improve device structures. The following content was mainly composed of three parts: (I) the influences of the proportions of the three carbon materials (carbon black, graphenes, and carbon nanotubes) on the PCE are summarized; (II) the preparation methods of meso C-PSCs, embedment C-PSCs and paintable C-PSCs were described in detail, and their performance was emphasized and (III) starting from several aspects such as energy alignment, hysteresis effect, passivation of defects and built-in electric field, a broad overview of the interface engineering development of C-PSCs was carried out. Finally, the challenges and potentials of C-PSCs were summarized. We envision that this work will inspire researchers to employ the remarkable stability, low cost and hydrophobic properties of carbon materials to highly efficient and stable C-PSCs.

2. Carbon-based electrode materials in PSCs

2.1 carbon black.

Ku et al. used carbon black/graphite as the rear electrode of PSCs for the first time. 25 Then, Wei et al. utilized pure carbon black as the rear electrode for C-PSCs by collecting candle soot. 56 The structure of this carbon black was highly porous. In addition, the carbon electrode was printed on the pre-fabricated perovskite film during device preparation. Finally, the cell achieved a PCE of 2.6% because of the low conductivity of this carbon black. After that, they determined to employ commercial carbon black as the rear electrode, and the PCE of the device was greatly boosted which eventually reached 11.0%. 56 Huang et al. had also done related research. 57 It was necessary to first use methylammonium iodide (MAI) and carbon black to form carbon ink, resulting from the small size of carbon black, then they used inkjet printing technology to deposit carbon electrodes. When carbon ink was placed on the PbI 2 precursor layer, MAI reacted with PbI 2 in the ink and crystallized to a cubic perovskite phase, thus immobilizing and embedding carbon black to produce carbon electrons. It enhanced the back contact in the C-PSCs, resulting in the acceleration of carrier migration and reduced charge recombination. As a result, a PCE of 11.6% was achieved. 57

Recently, Gong et al. have introduced an intermediate layer of carbon black between the perovskite film and the carbon rear electrode. 48 The device was fabricated with the structure of the perovskite/carbon black/carbon electrode, where carbon black was employed to facilitate the hole extraction. They adopted a one-step spin-coating method with a preheating process for a high-quality perovskite film. 58–62 The configuration and band alignment of the device are illustrated in Fig. 4(c) and (d) . 48 Fig. 4(e) depicts the manufacture procedures of the perovskite layer, carbon black interlayer and carbon rear electrode. Obviously, the carbon black interlayer played a role of transition layer between perovskite and carbon rear the electrode, which accelerated hole extraction. It improved the extraction efficiency of holes because of the larger contact area and proper energy band arrangement in the interface perovskite/carbon electrode. As a result, the device achieved a recorded PCE of 13.13%. 48 Chu et al. designed a new type of carbon rear electrode with nanoscale carbon black and carbon fiber to improve the perovskite/carbon interface. 63 HTL-free PSCs were obtained by using double-layer carbon electrodes (coherent layer and conductive layer), thereby increasing the PCE to 14.1%. 63

2.2 Graphene

In 2015, You et al. applied graphene as a transparent rear electrode in the inverted PSCs for the first time. 73 The front electrode (FTO side) and the rear electrode (graphene side) were irradiated under 1 sun illumination, and the PCE were 12.02% and 11.6%, respectively. Graphene was widely used in inverted PSCs and replaced transparent conductive oxide (TCO) as the front electrode. 74 By using graphene transparent anodes and organic hole transport materials poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), high-efficiency TCO-free PSCs were prepared. By adding MoO 3 and optimizing the thickness of the MoO 3 layer between the graphene anode and PEDOT:PSS, the highest PCE of 17.1% was obtained. Simultaneously, the PCE of the device with ITO transparent electrode was 18.8%. 74

These studies found that PCE vitally depended on the number of graphene layers. 74–76 The performance of the two-layer graphene device was the best and achieved the PCE with 12.37% certified value, while the PCE of the single-layer assembled device was 9.18%. 77 Nevertheless, the increased graphene layers resulted in deterioration of PCE (PCE for three and four layers were 11.45% and 11.27%, respectively). 77 A large quantity of layers reduced the transmittance of electrodes, and resulted in a decrease in quantities of photons reaching the absorber layer. In 2019, by Zhang et al. showed that graphene-based PSCs (G-PSCs) exhibit a PCE of 18.65%. 66 The optimized unpackaged device kept 90% of its initial PCE after aging for 1000 hours at a high temperature of 85 °C. Fig. 5(g) shows the SEM image of the G-PSC structure.

2.3 Carbon nanotubes

Furthermore, the mechanical property of CNTs was proved to be extremely beneficial for the exploitation of flexible PSCs. 88,89 Compared with the high treatment temperature of the carbon black/graphite composite electrode (above 450 °C), CNT films could be integrated into PSCs without any sintering, since remarkable optical characteristic and flexibility CNTs were very prospective in flexible PSC electrode materials. Luo et al. reported a cross-stacked carbon nanotube (CSCNT) film, 78 and later used an SnO 2 -coated carbon nanotube (SnO 2 @CSCNT) film as the electrode in flexible inverted PSCs in their work. 89 Compared with devices without SnO 2 coating, flexible inverted PSCs with SnO 2 @CSCNT cathodes had significantly improved photovoltaic performance. The results indicated that SnO 2 @CSCNT was a promising cathode material for long-term PSC operation. The typical SEM image of the CSCNT film is shown in Fig. 6(b) .

Jeon et al. developed a vapor doping method based on ex situ triflic acid (TFMS), which minimized the interaction with t-BP and shown the maximum doping effect. 79 In the case of traditional M-PSCs, an increase in the concentration of 2,2,7,7-tetrakis( N , N -di- p -methoxyphenylamine)-9,90-spirobifluorene (Spiro-MeOTAD) in HTL would decrease the PCE ( Fig. 6(c) ). It was in that the increase in HTL concentration resulted in a thicker Spiro-MeOTAD layer, causing a decrease in the hole transport capacity and vice versa . 79 However, it did not happen in the case of CNT lamination PSCs, as CNTs had already extracted holes in the interface before Spiro-MeOTAD restricted hole extraction ( Fig. 6(d) ). Combined the optimized carbon nanotubes and Spiro-MeOTAD ( Fig. 6(e) ), the best PCE of C-PSCs was higher than that of M-PSCs and reached 18.8% ( Fig. 6(f) ). 79 This was due to the superior hole selectivity of carbon nanotubes and the improvement of electrical conductivity by doping TFMS.

In conclusion, from a view of the application, the performance of graphene as a transparent electrode is the best due to good conductivity, high flexibility and light transmittance. 74 Carbon nanotubes are suitable to be counter electrodes because of their unique structure to promote hole extraction, 90 while carbon black can increase the pores of carbon paste, promote charge transfer and improve interface contact. 48 Therefore, it is reasonable to believe that if the carbon materials will be utilized as the basic electrode material, the PCE will possibly catch up with that of conventional M-PSCs.

3. Three types of C-PSC progress

3.1 meso c-pscs.

In the initial study of meso C-PSCs, the solution of PbI 2 and MAI was employed to the device via a one-step deposition technique, which achieved a PCE with 6.6%. 25 The inferior performance was due to the poor infiltration of the perovskite precursor solution in the device. To improve solution infiltration, Xu et al. applied an ordered porous carbon material as the carbon electrode, which boosted the solution infiltration and lightly increased the PCE to 7%. 113 As illustrated in Fig. 7(c) , the photocurrent density–voltage characteristic ( J – V ) curve of meso C-PSCs was plotted using OG-15, OG-16 and CG as counter electrodes, respectively. 113 Fig. 7(d) and (e) represent the assembled framework C-FDU-15 and C-FDU-16, respectively. 113 Fig. 7(f) shows the TEM image of C-FDU-15 seen in the (110) direction, and Fig. 7(g) shows the TEM image of C-FDU-16 viewed from the same direction. 113 A two-step technique was also explored for meso C-PSCs by Han's group, 111 which involved pre-deposition of PbI 2 , and then reached reaction in MAI solution to generate MAPbI 3 . By employing the TiO 2 nanosheet as the ETL, the meso C-PSCs exhibited the best PCE of 10.6%, 111 which enhanced to 11.6% after altering the graphite size and thickness of the carbon counter electrode. 112 Baranwal et al. evaluated the stability of the PCE of meso C-PSCs. 114 The results indicated that the efficiency of the encapsulated meso C-PSCs was not almost degraded at 100 °C, which indicated that the high thermal stability of the device had a chance to be achieved. Although the thick carbon layer could play a role in reducing the perovskite layer from being affected by moisture degradation, it must also avoid direct exposure of the rear electrode. There were two kinds of sealing positions of the device, as illustrated in Fig. 7(h) and (i) . These results indicated that meso C-PSCs had the opportunity to achieve high-efficiency PSCs. 114

3.2 Embedment C-PSCs

Carbon black was often used in the work of embedment of C-PSCs. However, the poor connectivity of carbon black would lead to an inferior interfacial contact. The addition of other adhesives would increase the connectivity, but prevent the infiltration of the solution and inhibit the conversion of PbI 2 into MAPbI 3 . In order to solve this issue, the bonding properties of other carbon materials (carbon black, graphite and MWCNTs) were compared. Finally, it was concluded that the FF values of the three carbon materials were very different, in the order of graphite (0.64) < carbon black (0.65) < MWCNTs (0.75). 64

In 2019, Yang et al. reported embedment C-PSCs based on carbon nanotubes. Carbon nanotubes were coated on PbI 2 and then immersed in CH 3 NH 3 I solution for 12 h to form a CH 3 NH 3 PbI 3 perovskite layer. Finally, the prepared CH 3 NH 3 PbI 3 perovskite layer was annealed on a hot plate at 100 °C for 10 min to obtain a complete device. On the basis of this device, they added a ferroelectric oxide (PbTiO 3 ) between the ETL and the perovskite layer. The PbTiO 3 was formed on TiO 2 by coating TiO 2 with (Pb(OAc) 2 ·3H 2 O), and then annealing at 450 °C for 1 h. The PbTiO 3 could provide a larger internal electric field and inhibit the non-radiative recombination of carriers, and the corresponding PCE of device reached 16.37%.

3.3 Paintable C-PSCs

Zhang et al. applied a commercial carbon paste to paintable C-PSCs for the first time. 26 By systematically altering the thickness of the TiO 2 layer, the device obtained a PCE of 8.3%. In the beginning, the PCE of paintable C-PSCs was very low, but the simplest manufacturing process continued to inspire researchers to enhance the efficiency of the device. By optimizing the ratio of paint composition (carbon black and graphite), Yang et al. obtained a PCE of 10.2%. 128 In 2015, Wei et al. developed a flexible carbon paste printing on the LHP layer, which effectively avoided the negative influence of the solvent on the perovskite film, and finally obtained a PCE with 13.5%. 52

The research on the stability of paintable C-PSCs had been reported by literatures. 129,130 Wei et al. compared the effects of epoxy resin and Ag coating on the carbon electrode. 131 Epoxy resin was a kind of polymer in carbon paste and had superior water resistance. Compared with the pure carbon electrode, carbon paste as the electrode had a better hydrophobicity. The Ag coating could enhance the hydrophobicity. In addition, the Ag coating could also improve the conductivity of the carbon electrode. Because perovskite was easily degraded by moisture, hydrophobicity was very important. 131 This was proved by the environmental stability measurement. The device with epoxy resin had higher stability, and the Ag coating acted like the icing on the cake. 131

Zhang et al. exploited a self-adhesive macroporous carbon film by solvent exchange at room temperature. 28 In the previous process, the solvent in the carbon paste was removed by high-temperature volatilization, which seriously affected the flexibility and porosity of the carbon paste, labeled as C1. It was found that ethanol could effectively inhibit the curing of carbon paste during solvent exchange. Fig. 9(c) depicts the microscopic curing mechanism of carbon paste. 28 After the exchange process was completed, the carbon film fell off from the glass substrate and formed a self-supporting film, which was marked C2. This method can avoid high-temperature curing. Fig. 9(d) shows the SEM images of devices with C1 and C2 as black electrodes, respectively. 28 It was found that there was a large gap between C1 and the Spiro-MeOTAD layer, while there was almost no gap between C2 and the Spiro-MeOTAD layer, and hence, it had a better interfacial contact.

There were obvious differences among the three kinds of C-PSCs in the preparation process, especially in the preparation and deposition of carbon paste. The multi-layer mesoporous structure in meso C-PSCs limited the complete conversion of pre-deposited PbI 2 . 99–101,132 In the preparation of embedment C-PSCs, the TiO 2 layer and the carbon layer should be deposited separately by a two-step method when pre-depositing PbI 2 . 71,133–135 For paintable C-PSCs, it was deposited layer by layer. No matter it was the one-step method, the two-step method or other development methods, it could be applied. Due to its superior stability, low cost of production and large market potential, C-PSCs had become a critical part of the PSC field. 136–141 Table 3 lists the fabrication parameters of different techniques. It can be seen that the PCE of paintable C-PSCs exhibited the best.

Although C-PSC-related research had achieved excellent results, the inferior interface contact between the perovskite film and the carbon electrode remained to be solved.

4. Interface engineering of C-PSCs

4.1 energy alignment.

In 2019, Wang et al. employed a kind of high conductivity and low-temperature carbon paint to paintable C-PSCs. 108 The carbon paint had good perovskite compatibility and conductivity, and the final PCE was 11.7%. 108 The interfacial contact was improved and hole extraction was increased by introducing hole transport layer (3,4-ethylenedioxythiophene:polystyrene sulfonate (PEDOT:PSS)). Fig. 10(c) shows the fabrication process of this device. The cross-sectional scanning electron microscopic image of cryogenic C-PSCs is shown in Fig. 10 (d). It was worth noting that it was difficult to match PEDOT:PSS ( W f ∼ 4.8 eV) and carbon ( W f ∼ 4.8–5.0 eV), as shown in Fig. 10(e) . Therefore, the close contact between perovskite and carbon electrode accelerated the hole collection and achieved 14.55% PCE. 108

4.2 Hysteresis effect

The HTL-free C-PSCs not only reduced the cost but also increased the stability. However, the performance of the TiO 2 -based ETL was unstable due to the hysteresis phenomenon. 173,174 Therefore, the non-hysteresis phenomenon C 60 was chosen as the ETL, to form the All-C-PSCs. Meng et al. fabricated a FTO/C 60 /MAPbI 3 /carbon device structure, in which a PCE of C 60 -based device was 15.38%, while that of TiO 2 -based device was only 12.06%. 30 The cross-sectional SEM image of the All-C-PSCs is shown in Fig. 11(a) . The C 60 value effectively improved the electron extraction, suppressed the charge recombination and reduced the sub-band gap state at the interface with the perovskite. Furthermore, the All-C-PSCs prevented moisture from entering the perovskite layer and had superior operation stability in a humid environment. Graphene was further used as a transparent conductive electrode to make it real All-C-PSCs and finally achieved 13.93% PCE. The high performance of the All-C-PSCs came from the bonding flexibility and electronic versatility of carbon materials. 175–179 The SEM image of the All-C-PSCs is shown in Fig. 11(b) , where the thickness of the carbon layer was about 20 μm, which prevented moisture from infiltration into the perovskite. 30 Fig. 11(c) shows the J – V curve of the All-C-PSCs based on graphene transparent electrodes. The reverse scan was 0.35% higher than the PCE of the forward scan. 30 Fig. 11(d) shows the reverse and forward scan J – V curves of C 60 and TiO 2 ETL, under the condition of AM 1.5G 100 mW cm −2 . 30 It can be seen that the TiO 2 -based C-PSCs showed an obvious hysteresis. When replaced by C 60 , the efficiency of electron extraction was enhanced and the J – V hysteresis was eliminated. 30

As shown in Fig. 11(h)–(j) , there was almost no difference between the forward and reverse directions of the J – V curves of carbon black and MWCNTs at the same scanning rate, indicating that the hysteresis of the two materials was very small. However, there was an obvious difference in the J – V curve of graphite, which mean that there was a large hysteresis phenomenon in graphite. Because the three kinds of carbon materials had similar cell configuration and device structure, it was considered that the effect of hole transfer on the hysteresis effect was different if the contact between carbon electrode and perovskite interface was different. 127 Through the study of the hysteresis effect of three kinds of carbon materials, MWCNT was expected to prepare high-efficiency C-PSCs without hysteresis. 127

4.3 Passivation defect

Yang et al. applied a polyethyleneimine-functionalized carbon nanotube (PEI/CNT) to the interface between the perovskite layer and the carbon electrode to improve the interface contact. 181 PEI molecules also had the effect of passivating interface defects. 181 It was found that PEI molecules were anchored on the perovskite framework by amino groups via the coordination interaction between –NH 2 (–NH–) and Pb 2+ or the hydrogen bond between H atoms in –NH 2 (–NH–) and I − , thereupon passivating the surface trap state in CsPbI 3 . 181 Due to the improvement of the contact between the perovskite/carbon interface, the charge transfer was accelerated. C-PSCs finally obtained 10.55% PCE and 0.71 FF. 181 Fig. 12(g) shows the specific structure of the device, and the existence of the PEI/CNT bridge could be observed. 181 During the preparation of C-PSCs, the PEI/CNT bridge was deposited on the perovskite/carbon electrode interface and partially infiltrated in the perovskite layer, as shown in Fig. 12(h) . 181 CNTs were first oxidized by mixed acids (sulfuric acid and nitric acid), and then carboxyl groups were introduced into the surface of CNTs. Finally, PEI molecules were connected to CNTs by the dehydration reaction between carboxyl and amino groups. 185 Hence, the PEI/CNT interface bridging method was expected to prepare efficient and stable C-PSCs.

4.4 Build-in electric field

Lee et al. revealed that increasing the difference in W f between the carbon electrode and the perovskite layer could facilitate the driving force of carrier migration, which indicated that the built-in electric field was driven by the difference of W f . 194 The enhancement of the built-in electric field could not only promote the carrier migration, but also improve the hole collection of the carbon electrode and reduce the charge recombination. 26,195,196

In 2014, Yan et al. found that multi-layer graphene (MG) as a counter electrode would produce a Schottky barrier at the interface with the perovskite layer, which greatly improved the efficiency of the device. 186 Fig. 13(a) and (b) exhibit the energy band diagrams of SG/perovskite and MG/perovskite, respectively. It can be found that there was a large Fermi level shift at the interface between multi-layer graphene and perovskite, forming a Schottky barrier. The MG had some conservative sandwich graphite layers in the oxidation process, so it had a good electrical conductivity. In addition, the mutual accumulation of MG could also fill the defects between each other ( Fig. 13(c) and (d) ). Most importantly, thanks to the built-in field of the Schottky junction, the hole extraction speed was enhanced and the interface charge recombination was greatly suppressed. They achieved a PCE of 11.5%. In 2019, He et al. used copper phthalocyanine (CuPc) to enhance the built-in electric field between the carbon electrode and the perovskite layer, and the efficiency increased to 14.8%. 187 From the SEM images of Fig. 13(f) and (g) , it could be seen that the morphology of the carbon electrode was not affected by CuPc. The device structure with CuPc carbon electrodes is shown in Fig. 13(e) . Compared with the bare carbon electrode, the W f value of the CuPc-doped carbon electrode increased from 4.03 eV to 4.22 eV ( Fig. 13(h) ). The increase in W f difference led to the enhancement of built-in electric field at the interface between carbon electrode and perovskite layer, which improved J SC , V OC and FF values.

In summary, through the latest progress of the four interface engineering of C-PSCs shown above, it can be seen that the effect of eliminating hysteresis is the best, and the other three are achieved by mixing other materials with carbon electrodes. 64,197–202 Obviously, eliminating the hysteresis phenomenon is the most likely to update the highest efficiency of the carbon electrode.

5. Conclusion and outlooks

To further improve the efficiency, the main challenge facing C-PSCs in the future is still the problem of interface contact. This problem stems from the inherent physical properties of carbon materials, e.g. crystal structure, size, electrical conductivity. Simultaneously, the morphology of the perovskite layer also has a great influence on the device. This can be improved through the following strategies. (i) The other materials ( e.g. copper phthalocyanine, MXene) doped with carbon electrodes are used to accelerate charge transfer, enhance charge collection and reduce charge recombination. (ii) The high-quality perovskite films are prepared by a room temperature solvent exchange method, melt-assisted growth method and top seed method to reduce surface defects and improve interfacial contact. (iii) The interlayer ( e.g. copper phthalocyanine and polyethyleneimine-functionalized) is added to the carbon electrode/perovskite interface with improving the work function, reducing the energy level mismatch and increasing the contact area.

The low-temperature carbon electrode discussed in the literature has good interfacial contact and a series of studies can be carried out. These are good research directions. Furthermore, based on the light absorption properties of carbon materials, the development of new characterization methods of C-PSCs is also a very important direction. This can increase the light absorption capacity of carbon materials, resulting in more electron–hole pairs and increased efficiency. Overall, there is much room to enhance their stability and efficiency for future practical applications.

Author contributions

Conflicts of interest, acknowledgements.

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