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Graphene-Based Nanocomposites for Improved Performance and Long-Term Stability in Perovskite Solar Cells

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Tabitha A. Amollo and Qi Hua Fan

Submitted: 23 January 2024 Reviewed: 08 April 2024 Published: 02 May 2024

DOI: 10.5772/intechopen.114965

Nanocomposites - Properties, Preparations and Applications IntechOpen
Nanocomposites - Properties, Preparations and Applications Edited by Viorica Parvulescu

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Nanocomposites - Properties, Preparations and Applications [Working Title]

Dr. Viorica Parvulescu and Dr. Elena Maria Maria Anghel

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Abstract

Rapid progress and advancement in the development of perovskite solar cells (PSCs) have been witnessed in the recent past. PSCs are being fronted as the next-generation devices for cost-effective and high-efficiency solar energy conversion. They are characterized by high absorption coefficients and superior photovoltaic performance. Nonetheless, PSCs suffer from poor device stability and charge transport. Graphene, because of its unique material properties such as high carrier mobility, and material strength, has the potential to circumvent the challenges of PSCs. Further, graphene-based nanocomposites extend the functionality of graphene for solution-based device processing. The graphene-based nanocomposites improve charge transport via the creation of charge percolation pathways and enhance charge extraction by providing favorable energy level alignment. The nanocomposites employed in the interfacial and as an interlayer promote the formation of smooth perovskite film morphology. Furthermore, the nanocomposites form an effective moisture barrier and effectively passivate the perovskite film’s surface defects, thus ensuring long-term stability. Graphene in the nanocomposites plays a crucial role in effecting PSCs’ long-term stability. Hence, the use of graphene-based nanocomposites in the interfacial layers and as an interlayer of PSCs is a potent route to attaining effective solar energy conversion and long-term stability in these devices.

Keywords

  • graphene
  • nanocomposites
  • perovskite solar cells
  • stability
  • photovoltaic performance

1. Introduction

The development and deployment of renewable and clean energy technologies have been recognized as a sustainable approach to meeting the ever-increasing global energy demands and addressing the climate crisis. Renewable and clean energy resources include solar, wind, geothermal and hydro energy. The renewables are not only environmentally friendly but also sustainable and affordable. Of the renewables, solar energy is a “good candidate” for energy reliability, sustainability and security. The energy produced by the sun is sufficient to meet the global energy demands. Nonetheless, solar energy supplies approximately 3.6% [1] of the worldwide energy consumption. On the same breadth, tremendous efforts are being put in place to develop materials and technologies for the efficient exploitation of solar energy.

Perovskite solar cells (PSCs) are one of the most promising solar energy conversion technologies. PSCs are compatible with solution-based processing, which implies simple and low-cost production. Certified power conversion efficiency (PCE) as high as 26.1% has been achieved in PSC [2]. The perovskite material is characterized by high optical absorption, high charge carrier mobility, low exciton binding energy and long charge carrier diffusion lengths [3, 4]. Nevertheless, the poor device stability of PSCs remains a major challenge to their commercialization. Materials engineering is one route to improve the photovoltaic performance and the long-term device stability of PSCs. In this regard, 2D nanomaterials, including graphene, transitional metal dichalcogenides (TMDCs) and transition metal carbide and nitrides (MXenes), have been and are being employed in PSCs. This chapter focuses on the application of graphene-based nanocomposites in PSCs.

Graphene, the first true 2D nanomaterial, is characterized by outstanding properties such as high carrier mobility, electrical conductivity, optical transparency, material strength and specific surface area [4]. These properties suit graphene for application in PSCs. Graphene oxide (GO), an oxygen-functionalized form of graphene, enables surface modification and further functionalization, such as heteroatom doping via its oxygen functional groups [5]. The functionalization makes graphene hydrophilic and thus dispersible and processable in solvents such as in PSCs fabrication. Besides, these functional groups passivate the defects in the perovskite films by strengthening the bonds and blocking the ion diffusion channels to affect long-term stability in the devices [6, 7]. The high specific surface area of graphene adds impetus to the passivation effects. Similarly, the tunable electronic structure and high charge carrier mobility mitigate charge recombination in PSCs for enhanced performance. Graphene nanocomposites capitalize on the synergy between the constituents’ properties to yield suitable materials properties for solar energy conversion.

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2. Perovskite solar cells

PSCs can be fabricated in the planar heterojunction (PHJ) or mesoporous structures. The planar device structure is fabricated as transparent conductive oxide (TCO)/electron transport layer (ETL)/perovskite/hole transport layer (HTL)/metal as shown in Figure 1a. In contrast, the mesoporous structure has mesoscopic metal oxide like TiO2 included in the active layer. However, the evolution of the PHJ structure ruled out the need for mesoscopic metal oxide. Perovskites, based on organic-inorganic materials, form the photoactive medium of PSCs. These have the general formula of ABX3 where A stands for either an organic or inorganic cation, B stands for a metal cation and X stands for a halide anion. The most commonly used perovskites in PSCs are those based on organo-lead or tin-halide because of their favorable properties like long charge carrier diffusion length, good optical absorption, good charge separation and transfer [4, 8]. Both organic materials such as 2, 2′,7,7′-tetrakis(N,N-bis(p-methoxyphenyl) amino)-9,9′-spirobifluorene (Spiro-OMeTAD) and inorganic materials such as Cu2O and NiOx are used in the HTL while TiO2 and SnO2 are some of the commonly used ETL materials for the PHJ structure. The TCO is usually made of indium tin oxide (ITO) and fluorine tin oxide (FTO) while the metal contact is made of Au or Ag. The PHJ structure is susceptible to hysteric effects [9] caused by the ionic current associated with the perovskite material [10]. Such ionic currents arise from unpassivated trap states at the perovskite grain boundaries [10].

Figure 1.

PSCs of (a) conventional and (b) inverted device structure.

PSCs can also be fabricated in the conventional (n-i-p) or inverted structure (p-i-n), depending on the positioning of the ETL and HTL. In the n-i-p structure, ETL is deposited on top of the TCO, while for the p-i-n structure, the HTL is deposited on top of the TCO, as shown in Figure 1(a) and (b). The inverted PSCs commonly use PEDOT:PSS and the fullerenes as the HTL and ETL materials, respectively. The inverted structure offers the advantage of low-temperature (ca. 150°C) processing over the mesoscopic structure which requires high temperatures (ca. 450–500°C) for the deposition of titania. Nonetheless, PSCs suffer from poor stability originating from the intrinsic thermodynamic instability of perovskite materials [11]. Especially, the organometallic halides have high sensitivity to moisture, so their chemical stability is highly jeopardized by exposure to water and oxygen. Other factors that influence the performance of PSCs are the morphology and the grain size of the perovskite material. Typically, better photovoltaic performance is obtained in devices with thick active layers of large grain sizes, as this promotes light harvesting. The thickness of the perovskite absorber layer also influences the performance of PSCs. Thicker films result in better optical absorbance, though at a trade-off with efficient charge collection as the latter depends on the charge carrier diffusion length. Light trapping is a strategic route to utilizing optimum perovskite layer thickness for high optical absorbance and charge collection. Thus, the production of high-efficiency PSCs requires optimization of the active layer composition and quality as well as charge extraction and collection. Moreover, effective encapsulation technologies are needed to implement PSC device stability.

2.1 Application of graphene-based nanocomposite in perovskite solar cells

As aforementioned in section one, the oxygen functional groups in GO enable the functionalization of its carbon backbone. As such, GO would be the preferred starting point in solution-phase preparation of graphene-based nanocomposites. Furthermore, the reduction of GO to reduced graphene oxide (rGO) can be achieved during the functionalization process using a suitable synthesis method. Solution-phase synthesis methods such as hydrothermal, solvothermal, sol-gel and chemical reduction are suitable for the preparation of graphene-based nanocomposites. Moreover, the oxygen functional groups promote molecular-level interaction of the nanocomposites in the PSC layers. The graphene derivatives GO and rGO and their nanocomposite are applicable in PSCs’ interfacial and active layers. The following sections elucidate the effect of these 2D nanomaterials on the photovoltaic performance and device stability of PSCs. Cases of metallic nanoparticles/graphene, polymer/graphene and quantum dots/graphene nanocomposites are discussed. Also, a few cases of GO and rGO are considered to decipher the influence of the synergistic effect of the materials’ properties of nanocomposite constituents.

2.1.1 Graphene-based nanocomposites in the interfacial layers of perovskite solar cells

2.1.1.1 Hole transport layer

Silver nanoparticles/graphene oxide (Ag NPs@GO) nanocomposite was employed in inverted Cs0.15(CH3NH3)0.85PbI3 PSCs [12]. Slight improvement in photovoltaic performances was realized when the nanocomposite was employed in the PEDOT:PSS HTL with Voc, Jsc, FF and PCE of 0.96 V, 17.20 mA cm−2, 75 and 12.31% in comparison to the reference device with Voc, Jsc, FF and PCE of 0.99 V, 17.05 mA cm−2, 72 and 12.17%. The slightly improved performance was attributed to the near-field effect of the NPs. When the nanocomposite was employed as an interlayer between the HTL and the perovskite layer, local surface plasmon resonance (LSPR) effects of the Ag NPs were maximized. These devices realized better improvement in performance with Voc, Jsc, FF and PCE of 1.02 V, 18.56 mA cm−2, 74 and 14.00%, respectively. The enhanced Jsc and FF were attributed to improved optical absorption resulting from LSPR effects. On the other hand, the device with GO in the interlayer yielded Voc, Jsc, FF and PCE of 0.99 V, 17.20 mA cm−2, 73 and 12.73%, respectively, which was still a slightly better performance in comparison to the reference device. This was attributed to better perovskite film morphology and charge extraction at the interface. Thus, the nanocomposites improved light harvesting and charge extraction to yield enhanced PCE. Incorporation of the nanocomposite in the active layer produced poor perovskite film morphology, which resulted in device failure. This study shows not only the synergistic effect of the nanocomposite but also the fact that the LSPR effects can be maximized by placing the NPs near the perovskite layer. It is important to point out that the nanocomposite enhances the device performance when used as an interlayer but causes device failure when embedded in the active layer. Thus, it highlights that device engineering should be done judiciously to realize the benefits of materials engineering in solar cells.

It is instructive to highlight the mechanisms by which metallic NPs (MNPs) improve the performance of solar cells namely near-field effect, far-field scattering, plasmon-induced charge separation and creation of intensive optical absorption bands [13]. Far-field scattering increases the optical path length thus promoting light harvesting. The light trapping affected by MNPs in PSCs is influenced by their composition, size, shape and position in the devices. For example, cylindrical and hemispherical MNPs are more effective in light trapping than nanospheres [14]. Also, the MNPs only improve light trapping at an optimum size [15] which favors forward light scattering. The near-field effect constitutes the creation of a strong electromagnetic field near the MNPs when embedded in the photoactive medium. This enhances light harvesting through the phonon’s high density of states and charge carrier excitation by the MNPs [16]. The near-field effect is prominent in small-sized MNPs and is suited for absorber materials with short carrier diffusion lengths. Plasmon-induced charge separation occurs when interband and intraband transitions cause excitation of electron-hole pairs thus leading to energy transfer to primary hot charge carriers [17, 18]. More hot electrons are generated by electron-electron scattering which causes further energy redistribution [19, 20].

Zhang et al. crosslinked CsPbBr3 quantum dots (QDs) with GO (GO/(CsPbBr3 QD)) through the Pb-O bond and used it as an interlayer between the active layer and the Spiro-MeOTAD HTL of FAPbI3-based PSCs [21]. The device with the GO/QD interlayer performed better than the pristine (without interlayer) and the QD-only interlayer with Voc, Jsc, FF and PCE of 1.14 V, 24.05 mA cm−2, 78 and 21.42%, respectively. The pristine device yielded Voc, Jsc, FF and PCE of 1.09 V, 23.56 mA cm−2, 76 and 19.58%, respectively, while the QD interlayer device yielded Voc, Jsc, FF and PCE of 1.12 V, 23.82 mA cm−2, 77 and 20.59%. The better performance of the GO/QD interlayer device was attributed to various factors: (i) improved conductivity of the nanocomposite i.e. GO partially replaced organic ligands of the QD and crosslinked the isolated QDs to form charge percolation pathways, (ii) the perovskite film on the GO/QD interlayer showed a smoother morphology than the pristine film; this favored the formation of smooth and non-porous HTL which minimizes resistive losses, (iii) GO/QD interlayer provided the most favorable band alignment between the perovskite film and the HTL, (iv) the GO/QD interlayer devices exhibited the shortest charge extraction lifetime implying effective charge extraction and reduced charge recombination and (v) the GO/QD interlayer device had the lowest trap density. Using the same method, perovskite solar modules (PSMs) consisting of six sub-cells in series were fabricated to explore the scalability of the approach. The GO/QD interlayer PSMs yielded Voc, Jsc, FF and PCE of 6.50 V, 3.74 mA cm−2, 76 and 18.55%. Further, the devices were tested for thermal and illumination stability. The PSMs were encapsulated, and their long-term thermal stability was studied at 85°C with a relative humidity of 60%. After 1000 hours, the GO/QD interlayer module maintained 91% of its PCE implying excellent thermal stability. On the other hand, the QD interlayer device retained 60% of its PCE while the pristine module was nearly destroyed. Besides, the water contact angle was greater for the perovskite film with GO/QD interlayer (ca. 79.8°) compared to the pristine film (ca. 52.94°) which showed that the former film was more hydrophobic thus, more effective as a moisture barrier. Therefore, the excellent stability of the GO/QD interlayer was attributed to its high hydrophobicity on the one hand and effective passivation of the perovskite film surface defects which suppressed defect-induced film decomposition, on the other hand. With regards to illumination stability, the GO/QD interlayer device again showed superior stability maintaining 90% of its PCE after 1000 hours while the pristine and QD interlayer devices degraded. Usually, under illumination, PSCs degrade due to ion migration caused by light-induced potential. The GO/QD interlayer formed an effective barrier to ion migration, thus protecting the metal contact from iodide erosion and consequently ensuring device stability. It was inferred that the GO in the nanocomposite played a crucial role in ensuring long-term device stability. This study highlights the different dynamics that affect the performance of PSCs and the influence of a graphene-based nanocomposite on PSCs’ stability.

Functionalized rGO and poly(3-hexylthiophene) (P3HT), nanocomposites were employed as the HTL of mixed cation perovskite Cs0.15FA0.85PbI3 PSCs [22]. The rGO was functionalized by 4-(hexyloxy)phenyl (PhOHex) containing alkyl chains and 4-[(2-20-bithiophene)-5-yl]phenyl (PhBiTh) containing thienyl groups. PSCs with Spiro-MeOTAD, P3HT, rGO-PhOHex@P3HT and rGO-PhBiTh@P3HT HTLs yielded PCEs of 9.4, 8.7, 9.8 and 8.1%, respectively. The lower PCE in the rGO-PhBiTh@P3HT HTL device originated from low Jsc and FF caused by higher charge recombination. From photoluminescence measurements, charge extraction was efficient in the P3HT-based HTLs, even better than the Spiro-MeOTAD HTL, except for rGO-PhBiTh@P3HT HTL. rGO-PhOHex@P3HT HTL exhibited the smoothest morphology, while the rGO-PhBiTh@P3HT HTL film was the roughest. In the former HTL, the rGO-PhOHex nanofillers were uniformly dispersed in the polymer matrix, while the latter HTL was characterized by clusters of graphene, which acted as charge recombination sites. The uniform dispersion of the rGO-PhOHex nanofillers showed that the hexyl chains were effective in promoting homogenous film formation during deposition. On the other hand, the bithienyl groups of rGO-PhBiTh induced aggregation of graphene by promoting intramolecular π-stacking interactions. This study shows the influence of the chemical nature of the graphene functional group on molecular-level interaction and, consequently, on the film morphology. Moreover, it evinces that smooth or homogenous films are not necessarily achieved using such moieties.

In order to elucidate the effects of the graphene-based nanocomposites on PSCs performance, a case of graphene derivatives is considered. GO and rGO reduced by hydrazine (rGO-NH), sodium borohydride (rGO-BH) and 4-hydrazino benzenesulfonic acid (rGO-HBS) were utilized as the HTLs of inverted planar methylammonium lead iodide PSCs [23]. Table 1 shows the electrical parameters of the fabricated devices. As indicated in Table 1, PCE was highest in the rGO-HBS HTL device and lowest in the GO HTL device, even lower than the PEDOT:PSS HTL reference device. Generally, the rGO HTL devices outperformed the GO and PEDOT:PSS devices. This better performance resulted from better Jsc and Voc values. The lower performance of the rGO-BH compared to the other rGO HTL devices was attributed to a poor coverage of rGO on the ITO substrate, while rGO-NH and rGO-HBS formed uniform films on ITO. GO and rGO-HBS HTL devices exhibited better stability than the PEDOT:PSS HTL device. The PCE of these devices reduced only up to half the original values after 1000 hours of storage in ambient conditions without encapsulation. Although the GO HTL enabled rapid hole extraction compared to the rGO HTLs, it simultaneously allowed for rapid charge recombination, which resulted in lower charge collection efficiencies as indicated by the lower Jsc. With the reduced oxygen functional groups in rGO HTLs, lower hole injection would occur within the graphene structure, but the transfer of delocalized holes in the benzene rings would reduce charge recombination to yield better performance. This study shows that even though the presence of oxygen functional groups in a graphene sample would reduce its electrical conductivity, they are desirable for the formation of favorable film morphology. This is due to the associated hydrophilicity of such samples, which enables the formation of stable suspension during device processing.

HTL materialVoc (volts)Jsc (mA) cm−2)FF (%)PCE (%)
PEDOT:PSS0.8720.48314.8
GO0.9419.57513.8
rGO-NH0.9621.37916.0
rGO-BH0.9721.47415.3
rGO-HBS0.9622.17716.4

Table 1.

Electrical parameters of inverted PSCs with GO and various rGO materials in the HTL [23].

2.1.1.2 Electron transport layer

rGO/polyaniline (G-PANI) nanocomposite was employed in the mesoporous TiO2 (mp-TiO2) ETL of (CsMAFA)Pb(IBr)3 triple cation PSCs [24]. In the nanocomposite, PANI nanoparticles were well dispersed on the rGO sheets. The nanocomposite-modified devices yielded better performance with a Voc, Jsc, FF and PCE of 0.96 V, 26.96 mA cm−2, 64 and 16.48%, respectively. In contrast, the reference device yielded a Voc, Jsc, FF and PCE of 0.96 V, 20.48 mA cm−2, 63 and 12.32%, respectively. The improved performance originated from enhanced optical absorption and crystallinity of the perovskite layer as well as reduced charge recombination at the active layer/ETL interface. Both rGO and G-PANI improved the devices’ performances, albeit the improvement caused by the former ca. Voc, Jsc, FF and PCE of 0.99 V, 25.53 mA cm−2, 64 and 14.81%, respectively, was slightly lower. The aging test was carried out on unencapsulated devices kept inside a dry airbox with a relative humidity of 20% in the dark at a temperature of 20–30°C. The G-PANI modified device maintained 85% of its PCE after 1870 hours while the PCE of the reference device was reduced to 15% over the same period and conditions. The nanomaterials significantly improved the device’s stability, as shown in Figure 2(b). This was attributed to the effective passivation of grain boundaries, as shown in Figure 2(a).

Figure 2.

(a) FESEM image of the perovskite films on the G-PANI modified ETL and (b) stability test of G-PANI modified ETL PSCs [24].

Bagha et al. [25] used Ni-doped ZnO/rGO and Ag-doped ZnO/rGO bilayers as the ETLs of planar (Cs0.05(MA0.17FA0.83)0.94Pb (I0.83Br0.17)3) PSCs. The devices’ performance shown in Table 2 illustrates the systematic influence of the ETL constituent materials. Ni-doping of ZnO leads to a decrease in all the photovoltaic parameters. The reduction in Jsc was attributed to increased charge carrier recombination caused by trap states, while the decrease in Voc and FF was ascribed to a reduction of the energy gap between the ETLs lowest unoccupied molecular orbital (LUMO) and the minimum of the perovskite’s conduction band by the Burstein-Moss effect. On the other hand, Ag doping slightly enhanced all the photovoltaic parameters. The enhancement was attributed to passivated defect states and improved charge separation, which in turn reduces charge carrier recombination. Further enhancement in the photovoltaic performance of all the devices was achieved by rGO incorporation. Ag-doped ZnO/rGO exhibited superior electrical parameters, showing the effect of both the Ag NPs and rGO in performance enhancement. The rGO served to passivate the defects further, thereby increasing charge extraction. The significant improvement in the Jsc of the Ag-doped ZnO/rGO ETL device originated from increased optical absorption. Stability tests were carried out for ZnO, ZnO:Ag and ZnO:Ag /rGO ETL devices under free-encapsulation ambient conditions at 30°C and relative humidity of 30–35% for up to 150 hours. As shown in Figure 3, the champion device was the most stable, with a negligible change in its PCE during the entire period. This was attributed to the defect passivation effects of rGO.

ETL materialVoc (volts)Jsc (mA cm−2)FF (%)PCE (%)
ZnO0.7314.70616.59
ZnO:Ni0.6813.90555.27
ZnO:Ag0.7714.98627.25
ZnO:rGO0.7814.89617.01
ZnO:Ni /rGO0.7214.70566.03
ZnO:Ag /rGO0.8816.826610.03

Table 2.

Electrical parameters of planar PSCs with Ni-doped ZnO/rGO and Ag-doped ZnO/rGO bilayers in the ETL [25].

Figure 3.

Stability test of PSCs with Ag and rGO-modified ZnO ETLs [25].

N-doped graphene/ZnO nanorod nanocomposite (NG-ZnO NR NCs) was used as the ETL in CH3NH3PbI3 PSCs of planar device structure [26]. The concentration of NG was varied from 0 to 1 wt.%. As shown in Figure 4(a), the Jsc of the devices increased with the concentration of NG from 0 to 0.8% then dropped slightly at 1%. A similar trend was observed for the other photovoltaic parameters. At the optimum NG concentration, i.e. 0.8%, the device yielded a Voc, Jsc, FF and PCE of 1.02 V, 21.98 mA cm−2, 75 and 16.8%, respectively. On the other hand, the reference device yielded a Voc, Jsc, FF and PCE of 1.04 V, 17.38 mA cm−2, 71 and 12.9%, respectively. The enhanced performance was attributed to increased perovskite loading on the nanocomposite ETL, leading to a dense and crystalline perovskite film. The NG-ZnO NR NCs ETL was more effective in promoting the growth of a highly uniform perovskite film with better surface area coverage than the ZnO NRs. Consequently, the optical absorption of the perovskite film (Figure 4b) improved, leading to the enhanced Jsc. Similarly, a higher photoluminescence quenching was observed for the perovskite film deposited on the nanocomposite than on the ZnO NRs ETL. The stronger photoluminescence quenching was attributed to a higher density of states (DOS) for electron acceptors, facilitating effective charge carrier extraction at the ETL/perovskite layer interface. The effect of this is a reduced charge carrier recombination. A slight decrease in the photovoltaic performance was observed at 1 wt.%. At this higher concentration, the NGs formed aggregates, which led to increased series resistance and, thus, lower charge collection efficiency. Similar devices were also fabricated with pristine graphene-ZnO NRs NCs as the ETL. This device’s performance was slightly lower than the NG-ZnO NR NCs ETL device with a Voc, Jsc, FF and PCE of 1.02 V, 18.57 mA cm−2, 72 and 13.57%, respectively. The better performance of the NG-ZnO NR NCs ETL compared to the pristine graphene-ZnO NRs NCs ETL was attributed to its better electrical conductivity. N-doped graphene, being an n-type conductor, provided better charge selectivity for electrons. This study shows the influence of the perovskite film quality and the electronic transport characteristics of the constituent graphene sample on the PSC’s performance. Also, the nanocomposites concentration should be optimized to realize its benefits on photovoltaic performance.

Figure 4.

(a) J-V characteristics and (b) optical absorption of PSCs with NG-ZnO NR NCs as ETL [26].

rGO/mesoporous TiO2 (mp-TiO2) was used in the ETL of mesostructured PSCs [27]. The study also investigated the effect of the concentration of rGO on photovoltaic performance. The rGO-modified ETL devices performed better than the reference mp-TiO2 ETL device, as shown in Figure 5. At the optimum concentration of rGO (0.4 vol.%), the device yielded a Voc, Jsc, FF and PCE of 0.91 V, 21.0 mA cm−2, 71 and 13.5%, respectively. On the other hand, the reference device yielded a Voc, Jsc, FF and PCE of 0.86 V, 19.6 mA cm−2 67 and 11.5%, respectively. At a lower (0.2 vol.%) and higher concentration (1.0 vol.%) of rGO in the ETL, the devices yielded a PCE of 13.5 and 11.7%, respectively. The optimum concentration of rGO in the ETL improved the electron collection efficiency of these devices, which is shown by the improved Jsc in Figure 5. However, a higher concentration of rGO caused a decrease in optical transmittance by up to 9%. This decreased light harvesting by the perovskite layer resulted in a lower Jsc in this device. The measured incident photon to current conversion efficiency (IPCE) was better for the rGO-modified ETL device (0.4 vol.%) ca. 88.02% compared to the reference device ca. 82.99%. The work function measured for rGO (−4.4 eV) was lower than that for TiO2 (−4.0 eV). This would promote the transfer of electrons from the perovskite and TiO2 to the rGO. Thus, the incorporation of rGO at optimum concentration in the ETL would improve charge transfer and collection efficiency thereby, mitigating charge carrier recombination in the device. Eletrospun GO/TiO2 nanofibers were used as electron transport material in mixed cation (FAPbI30.8MAPbBr30.2) PSCs [28]. The perovskite layer deposited on the GO/TiO2 ETL (at an optimum concentration) was characterized by a dense film with uniform grain size. The nanocomposite facilitated better extraction of electrons from the conduction band of the perovskite to the electrodes. This resulted in an improved performance ca. Voc, Jsc, FF and PCE of 1.18 V, 23.89 mA cm−2, 71 and 20.14%, respectively, in comparison to the TiO2 ETL device with Voc, Jsc, FF and PCE of 1.132 V, 23.73 mA cm−2, 69 and 18.42%, respectively.

Figure 5.

Current-voltage characteristics of rGO/mp-TiO2 ETL PSCs at various concentrations of rGO [27].

GO-ZnO and GO-Ag-ZnO nanocomposites prepared by the hydrothermal route were utilized as electron transport material in triple cation Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 PSCs [29]. The perovskite film on the GO-Ag-ZnO ETL consisted of larger crystal grains (ca. 250 nm) in comparison to the film deposited on ZnO film (ca. 230 nm). Also, GO enabled the formation of a homogenous film with greater surface coverage. Table 3 shows the photovoltaic performance of the devices. GO-Ag-ZnO effected a favorouble energy level alignment which resulted in the increased Voc, Jsc and PCE. The better performance was also attributed to better charge transport and collection.

ETL materialVoc (volts)Jsc (mA cm−2)FF (%)PCE (%)
ZnO0.9714.79517.27
GO-ZnO0.8312.41424.36
Ag-ZnO0.9013.22495.84
GO-Ag-ZnO0.9914.98598.72

Table 3.

Electrical parameters of PSCs with GO-Ag-ZnO nanocomposites in the ETL [29].

The 3D finite element simulation method was used to determine the photovoltaic parameters for inverted CH3NH3PbI3 PCSs with graphene/TiO2 nanocomposite (Gr/TiO2 NC) as the ETL [30]. The nanocomposite ETL device exhibited better performance with Voc, Jsc, FF and PCE of 1.14 V, 17.69 mA cm−2, 84 and 17.01%, respectively, in comparison to the TiO2 ETL device with Voc, Jsc, FF and PCE of 0.99 V, 19.07 mA cm−2, 76 and 14.42%, respectively. Gr ETL device exhibited Voc, Jsc, FF and PCE of 0.89 V, 21.73 mA cm−2, 83 and 16.03%, respectively. Optical absorption of the active layer was higher with Gr than with TiO2 ETL as the Gr optical absorption was low at 2.3%. The high optical absorption of the active layer on Gr ETL together with the high charge carrier mobility exhibited by the Gr led to a significant increase in Jsc. On the other hand, the Voc decreased in the Gr ETL device as compared to the TiO2 due to current leakage. The Gr work function ca. 4.4 eV provided good energy level alignment between the perovskite and the FTO, promoting electron transfer. At the same time, given that the Gr work function was higher than the perovskite valence band, some holes could flow from the perovskite valence band to Gr ETL and cause charge recombination, which gave rise to the current leakage. Superior performance was obtained in the Gr/TiO2 NC ETL device because of reduced charge recombination. The valence band in the NC ETL was lower than the perovskite’s creating a potential barrier to holes transfer from the active layer to the ETL. Also, the high carrier mobility of the Gr increases the electrical conductivity of the ETL for effective charge transport at the ETL/active layer interface. This leads to increased Voc in the device. Overall, electron transport was better in Gr/TiO2 NC ETL than in either Gr or TiO2 ETLs. Optical absorption in the Gr/TiO2 NC ETL was higher than in TiO2 at wavelengths greater than 500 nm, implying lower absorption of the active layer and subsequently reduced charge photogeneration. This caused the reduced Jsc in these devices. This study highlights that optical absorption and charge transport are the critical parameters of the photovoltaic performance of PSCs.

2.1.2 Graphene derivative in the active layer of perovskite solar cells

rGO at different reduction levels by HIAcOH-assisted reduction method was employed as an additive in the perovskite absorber layer and Spiro-OMeTAD HTL [31]. The rGO samples were obtained by tuning the reduction process time from 12 to 48 hrs to produce rGO12, rGO24 and rGO48, with carbon/oxygen ratios of 3.86, 8.18 and 11.49, respectively, while that for GO was 0.94. The devices with rGO12, rGO24 and rGO48 in the perovskite layer exhibited PCEs of 20.0, 19.34 and 19.04%, respectively, while the reference device (without rGO in the active layer) exhibited a PCE of 19.56%. The rGO, which was of small flakes, led to the formation of comparable perovskite layer morphologies to those without rGO additives. Thus, the small flaked rGO did not cause crystallization of the perovskite film. Also, the devices with rGO in the active layer showed better shelf-life stability than those without rGO. Devices with rGO24 simultaneously employed in the perovskite active layer and the Spiro-OMeTAD HTL showed a slower device degradation with a PCE drop of 23% after 20 hours when put on a hot plate at 85°C in ambient air. In comparison, the reference device’s PCE dropped by 37% under the same conditions. Hence, using rGO in the active layer and HTL improves thermal and moisture stability.

Reduced graphene oxide-cysteine nanogold (rGO-CysAu) nanocomposite was incorporated in the active layer of inverted CH3NH3PbI3 PSCs [32]. At an optimum concentration of the nanocomposite, the perovskite:rGO-CysAu films were characterized by smooth pin-hole free morphology of dense and well-crystallized grains. The device yielded a Voc, Jsc, FF and PCE of 1.09 V, 23.89 mA cm−2, 79% and 20.59%, respectively. This was an improved performance compared to the reference device with Voc, Jsc, FF and PCE of 1.06 V, 21.69 mA cm−2, 80% and 18.32%, respectively. The rGO-CysAu in the active layer increased the optical absorption and charge carrier collection efficiency which resulted in a 12% increase in the PCE. Beyond the optimum nanocomposite concentration, the perovskite film exhibited poor crystallization and surface defects which increased series resistance resulting in a lower PCE of 16.22%. Further, the nanocomposite-modified device exhibited better stability retaining its original PCE after 120 hrs, when stored in a glove box, whilst the reference device retained 98%.

2.2 Encapsulation of perovskite solar cells

Following the discussions in the above sections on the influence of graphene-based nanocomposites on PSCs’ long-term stability, it is instructive to highlight the contemporary encapsulation techniques which mitigate the degradation of these devices. Encapsulation is necessary to avoid PSCs degradation because real-time operation involves solar irradiation in the presence of air and moisture. Thus, encapsulation protects the devices from degradation by external factors such as moisture, temperature, oxygen and UV illumination. Materials for encapsulation should be characterized by high optical transparency [33], good moisture and oxygen barrier property [34], good insulating property [35], chemical inertness [36] and good mechanical properties (i.e low elastic modulus, high tensile strength and high creep resistance) [37]. The commonly used encapsulants in PSCs are thermoplastics and thermosets [38]. Encapsulation technologies for PSCs include single-layer, multi-layer, UV-curable adhesive and glass-glass vacuum laminated encapsulation [39]. The single-layer encapsulation is preferred because of its simplicity in production and integration in devices [38]. Nonetheless, both the single- and multi-layer encapsulation provide a limited barrier to moisture and oxygen intrusion [39]. UV-curable adhesive and glass-glass vacuum laminated encapsulation are effective in solving the moisture and oxygen penetration in PSCs. Both internal encapsulation (of grain boundaries, surfaces and interfaces) and external encapsulation (against environmental factors) should be systematically performed to ensure PSCs’ long-term stability.

2.3 Perspectives and challenges

As discussed in the preceding sections, the incorporation of graphene-based nanocomposites in PSCs comes with benefits to the devices’ performance. Nonetheless, many factors influence the extent and nature of these beneficial effects. The constituent materials of the nanocomposite and their spot in the device are at the core of the performance level. For example, graphene/metallic nanoparticles nanocomposites improve the performance level by both the plasmonic effects of the NPs and the good charge transport capabilities of graphene. Ag NPs@GO yielded better performance when used as an interlayer between HTL and the active layer than when used in the HTL because of maximized LSPR effects. Conversely, devices made with the same nanocomposite in the active layer failed. On the other hand, the graphene/metallic nanoparticles nanocomposites in the ETL enhance the device’s performance. ZnO:Ag/rGO ETL device yielded improved photovoltaic performance through enhanced optical absorption of the perovskite film and charge extraction at the interface. The nanocomposites as an interlayer or interfacial layer provide favorable energy level alignment between the electrodes and active layer, thus promoting charge extraction. Also, used in a similar fashion, the nanocomposites promote the growth of smooth perovskite film morphology with charge transport network which suppresses charge carrier recombination. The nanocomposites used either as an interlayer or in the interfacial layers affect long-term device stability by serving as an effective moisture barrier and effectively passivating the surface defects of the perovskite film. The passivation of surface defects limits ion migration thus protecting the metal contact from iodide erosion. For example, as discussed above a PSC with G-PANI nanocomposite as ETL maintained a remarkable 85% of its PCE after 1870 hours. Graphene in the nanocomposites is the key constituent to device long-term stability. The morphology induced by the nanocomposites which depend on molecular-level interaction is important. The type of functional group on the graphene-based nanocomposite influences the intermolecular interactions to produce smooth or rough perovskite films. Also, it affects charge transport in the devices. For example, N-doped graphene being an n-type conductor offers charge selectivity at the interface thus suppressing charge recombination. On the other hand, rGO, being a better electrical conductor than GO, provides better charge transport in PSCs. Graphene-based nanocomposite, when used in the perovskite layer, causes device failure by the formation of poor film morphology, i.e. crystallization of the perovskite film. Nonetheless, small flaked rGO employed in the perovskite blend formed comparable film morphologies to pristine perovskite, slightly enhanced the photovoltaic performance and induced moisture and thermal stability in the devices. Similarly, rGO-CysAu formed a smooth morphology owing to the cysteine ligand. The graphene-based nanocomposites are more effective in enhancing PSC performance when used in the interfacial layers or as an interlayer than in the active layer. Nonetheless, the nanocomposites only produce enhanced photovoltaic performance at optimum concentration. Therefore, the application of graphene-based nanocomposites should be done judiciously to maximize the beneficial effects of the material’s properties to achieve enhanced PSCs’ performance. Furthermore, a combination of the use of graphene-based nanocomposites, which effectively passivates the surface defects and offers a moisture barrier, with external encapsulation is an effective strategy towards realizing long-term stability in PSCs.

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3. Conclusion

This chapter has examined the application of graphene-based nanocomposites in the interfacial and active layers as well as an interlayer in PSCs. In the interfacial layer and as an interlayer, these nanocomposites enhance device performance and long-term stability. The nanocomposites in the interfacial layers and as interlayers provide favorable energy level alignment and thus promote charge extraction, favor the growth of smooth perovskite film morphology and thus suppress resistive losses and enhance the active layer optical absorption. Given graphene’s high charge carrier mobility, the nanocomposites promote effective charge transport, which suppresses charge recombination. Contrariwise, the nanocomposites employed in the active layer cause device failure through the formation of poor film morphology caused by the crystallization of the perovskite. Nonetheless, graphene derivatives like rGO when employed in the active layer produce comparable film morphology and benefit the devices’ performance. A substantial benefit of using graphene-based nanocomposites in the PSCs is the long-term stability primarily caused by the graphene constituent. These nanocomposites form a moisture barrier and effectively passivate the surface defects of perovskite film thus, limits ion migration and consequently effects devices’ moisture, illumination and thermal stability. Therefore, the use of graphene-based nanocomposites in PSCs is an effort worth advancing to realize improved solar energy conversion and long-term device stability.

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Acknowledgments

The authors are grateful to the Alliance for African Partnership (AAP) and National Science Foundation (Award #2243110) for funding this work.

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Conflict of interest

The authors declare no conflict of interest.

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Written By

Tabitha A. Amollo and Qi Hua Fan

Submitted: 23 January 2024 Reviewed: 08 April 2024 Published: 02 May 2024

© 2024 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.