Recently, there has been a dramatic increase in the global demand for renewable and green energy sources due to the exhaustion and environmental issues associated with conventional energy sources, such as fossil fuels and nuclear energy (Rego de Vasconcelos and Lavoie, 2019; Ashok et al., 2020; Lin et al., 2020; Subhan et al., 2020; Tiwari et al., 2020). In this regard, solar energy, a low-cost, renewable, naturally abundant and clean energy source, has attracted enormous research effort as a promising alternative to traditional energy sources (Subramanyam et al., 2019; Zhong et al., 2020; Rabaia et al., 2021). Among the new-generation photovoltaic devices that convert solar energy into electricity, organic solar cells (OSCs) are being widely investigated owing to their low production cost, facile fabrication procedure, abundance of raw materials, easy scalability, lightweight, excellent flexibility and environmentally friendly nature (Gusain et al., 2019; Lee, 2019; Li et al., 2020a; Shah et al., 2020; Shoyiga et al., 2020; Liu et al., 2021).

A typical OSC is composed of the electrodes (cathode and anode), charge transport layers (electron transport layer (ETL) and hole transport layer (HTL)), and the photoactive layer, assembled in the conventional or inverted configurations, as presented elsewhere (Muchuweni et al., 2020a). In an OSC, photons from incident solar radiation are transmitted through the substrate, bottom electrode and charge transport layer, so that they reach the photoactive layer, where they are absorbed by the donor material, in which excitons, i.e., strongly bound electron-hole pairs, are generated (Jeon et al., 2017; Ramasamy et al., 2019; Wang et al., 2020; Kang et al., 2021) and localized owing to the large exciton binding energy in the polymer matrix (Hatton et al., 2008). The photogenerated excitons subsequently diffuse within their limited diffusion distance to the interface between materials with dissimilar electron affinities and ionization potentials, i.e., between the donor-acceptor interface, where they are absolutely separated into free charge carriers after overcoming the binding energies (Tessema Mola et al., 2018). However, since the exciton diffusion length in OSCs is small, a bulk heterojunction (BHJ) should exist within the short diffusion distance so that excitons can always reach the donor-acceptor interface for charge separation to occur, and a continuous interpenetrating channel should exist for transporting charge carriers to the electrodes (Jeon Y.-J. et al., 2018; Ramasamy et al., 2019). Actually, the charge carriers are separated into electrons and holes in the acceptor’s lowest unoccupied molecular orbital (LUMO) and donor’s highest occupied molecular orbital (HOMO) levels, respectively, as illustrated in Figure 1. Hence, for the efficient transportation of electrons from the LUMO via the ETL to the cathode, the LUMO level of the acceptor should match well with the ETL’s work function (Huang et al., 2019). Similarly, for the efficient transportation of holes through the HTL to the anode, the donor HOMO level should be well-matched with the HTL’s work function (Huang et al., 2019). Thus, free electrons are transported to the cathode via the ETL, whereas holes are transported in the opposite direction to the anode via the HTL under internal electric fields, which lead to photocurrent generation (Amollo et al., 2018). Finally, the electrodes allow the flow of the photogenerated current to and from the external circuit so that the cell can power a given load.

Despite having a lower projected cost of < $0.07/Wp relative to < $0.35/Wp for commercially available silicon solar cells (Riede et al., 2021), the power conversion efficiency (PCE) of state-of-the-art OSCs (18–25%) (Cho et al., 2020; PV-Magazine, 2020; Salim et al., 2020) is still lower than that of commercially available silicon-based solar cells (above 26%) (Andreani et al., 2018). In addition, when compared with silicon solar cells, OSCs suffer from poor long-term environmental stability, which limits their commercialization (Chen, 2019; Burlingame et al., 2020; Duan and Uddin, 2020; Wang et al., 2021). Hence, this has prompted significant research interest in developing highly efficient and sustainable devices through approaches, such as incorporating novel materials into the different components of OSCs, to overcome the limitations of the commonly used traditional materials.

In this respect, carbon-based materials, such as graphitic carbon nitride, carbon quantum dots, carbon nanotubes (CNTs) and graphene (Jeon et al., 2018b; Nguyen et al., 2019; Ouyang, 2019; Hu et al., 2020; Pan et al., 2020; Qin et al., 2020; Subramanyam et al., 2020; Shin et al., 2021; Vercelli, 2021), have attracted considerable research attention due to their unique physicochemical properties, low-cost, natural abundance of carbon, non-toxicity and compatibility with large-scale solution synthesis (Delacou et al., 2017). Among these, CNTs are more appealing owing to their large specific surface area, tunable band gap, high optical transmittance in the visible region, competitive electrical conductivity, high charge carrier mobility, excellent flexibility and superior mechanical, thermal and chemical stability (Khan et al., 2018; Oseni et al., 2018).

CNTs, one of the stiffest and strongest materials ever discovered, consist of a cylindrical nanostructure of hexagonally oriented carbon atoms as shown in Figure 2, and can be classified as either semiconducting or metallic depending on their length, diameter and arrangement of hexagonal rings (Alturaif et al., 2014). Also, how graphene layers are wrapped to form a nanotubular morphology, i.e., the chirality of the tubes, significantly determines the electrical properties of the CNTs (Alturaif et al., 2014). CNTs that consist of a single round roll of graphene with a typical diameter of around 0.4–10 nm are referred to as single-walled CNTs (SWCNTs), whereas those consisting of two or more rolled layers of graphene sheets with a typical diameter of 1.4–100 nm are called double-walled CNTs (DWCNTs) and multiwalled CNTs (MWCNTs), respectively (Hatton et al., 2008; Agbolaghi, 2019b; Nguyen et al., 2019). Currently, SWCNTs have been relatively more studied than MWCNTs owing to the novel properties of SWCNTs, such as their band gap energy that can be tuned from 0 to ∼2 eV, thereby varying their electrical conductivity. In addition, the diameter of SWCNTs falls within the preferred range of up to 20 nm for OSC applications since the thickness of the organic layer in OSCs is typically up to 200 nm (Hatton et al., 2008). Hence, SWCNTs exhibit unusual properties of either semiconducting or metallic materials, whereas MWCNTs are zero band gap materials with metallic properties (Obaidullah et al., 2018; Ouyang, 2019).

CNTs have been prepared by means of several techniques, such as chemical vapour deposition (CVD), laser ablation and arc discharge (Jeon et al., 2018c; Khan et al., 2021; Shen, 2021). Among these, CVD is more appealing owing to its potential for large-scale synthesis. Also, CNTs can be deposited onto various components of OSCs by using different techniques, such as spray coating, dip coating, spin coating, sputtering and CVD (Khan et al., 2018). However, the insolubility in organic solvents, entanglement and poor alignment of CNTs, in addition to the presence of metal impurities, are the main limitations for the incorporation of CNTs into various layers of OSCs. These limitations cause unfavourable short-circuits, surface charge carrier trapping and reduction in charge carrier mobility, thereby increasing leakage current and recombination pathways (Oseni et al., 2018). Although sonication has been employed to disperse CNTs, it often causes them to break and alter their properties; hence, covalent and non-covalent functionalization have been developed in an attempt to improve the dispersibility of CNTs (Tasis et al., 2006; Hadi et al., 2020; Khan et al., 2021). Among these functionalization techniques, covalent functionalization involves the use of chemical reactions to introduce various functional groups onto the surfaces of CNTs (Nan et al., 2016). Nonetheless, the harsh chemical reactions impair the π-bonds of CNTs and degrade the CNTs’ intrinsic electrical, mechanical and thermal properties (Nan et al., 2016).

On the other hand, non-covalent functionalization involves the adsorption of dispersants onto the surface of CNTs through non-covalent interactions, such as interactions between the π-electronic systems of CNTs and dispersants, which break the van der Waals forces between the individual CNTs, without any disruptions to the CNTs’ π-electron conjugated structure (Keru et al., 2014). This, in turn, improves the interaction of CNTs with other constituent materials that make up the different components of OSCs, thereby increasing charge carrier mobility (An et al., 2017; Hadi et al., 2020). This also addresses the challenge of leakage current due to short-circuits that usually originate from the bundling of poorly dispersed CNTs. In addition, magnetic or electric fields may be applied, or the CNTs may be grown along a single direction to improve their alignment (Khan et al., 2021), thereby minimizing undesirable short-circuits that usually emanate from poorly aligned CNTs, and facilitating the efficient transportation of charge carriers, which subsequently leads to a substantial improvement in device performance.

However, the PCE of CNT-based OSCs is still relatively lower than that of OSCs based on traditional materials; hence, the optoelectronic properties of CNTs need further optimization for them to produce devices that can compete with those based on their traditional counterparts. Also, recent applications of carbonaceous nanostructures, especially CNTs, to improve the efficiency and sustainability of BHJ-OSCs for possible commercialization are yet to be satisfactorily reported. Therefore, this review focuses on the recent applications of CNTs, mainly over 2018–2021, in the electrodes, active layers and charge transport layers of OSCs, to improve not only the PCE, but also to enhance the long-term operational stability, thereby paving the way for the commercial application of OSCs. In addition, the merits, limitations and outlook for the future fabrication of high performance and sustainable OSCs are discussed.

The most widely used transparent conducting electrode is indium tin oxide (ITO) due to its high optical transparency and electrical conductivity (Zhang Y. et al., 2020; Du et al., 2021), which facilitate the entrance of more light into the cell, and the efficient collection and transportation of photogenerated charge carriers to the external circuit. However, indium is an expensive and toxic rare earth metal, which limits the sustainability of ITO (Muchuweni et al., 2016a; Muchuweni et al., 2016b; Muchuweni et al., 2017a; Muchuweni et al., 2017b). Also, ITO is brittle; hence, not compatible with flexible substrates, and has instability issues in acidic and basic environments, which lead to device degradation (Jeon et al., 2015; Patil et al., 2021). In addition, good quality ITO is usually prepared using complicated, expensive and high-temperature vacuum-based processes (Lu et al., 2017; Muchuweni et al., 2020b).

Therefore, considerable effort has been exerted on developing alternative transparent conducting electrodes by using novel materials, such as metal grids (Mo et al., 2016; Jeong et al., 2018), silver nanowires (AgNWs) (Sun et al., 2019; Arulkumar et al., 2021), CNTs (Yu et al., 2016; Jeon et al., 2017) and graphene (Jeon et al., 2017; Keyvani-Someh et al., 2017), owing to their competitive optoelectronic properties. Among these, CNTs are more appealing due to their low-cost, easy availability of raw materials, low-temperature solution processability, large specific area, flexibility, stability, and capability of offering a good balance between high optical transmittance in the visible region and low sheet resistance (Lu et al., 2017; Matsuo, 2021; Shen, 2021). In addition, the work function of CNTs is comparable to that of ITO, which reduces the energy barrier; hence, facilitating the efficient collection of photogenerated charge carriers at the electrode.

Although CNTs have been recently employed to replace or modify the commonly used ITO anode in conventional OSCs, relatively few or no detailed studies have reported their application as the cathode, mainly due to the relatively higher work function of CNTs, as compared with the low work function of the widely used ETLs, which increases the potential barrier, thereby limiting the collection of electrons at the cathode. Thus, for the cathode to efficiently collect and transport electrons, its work function should be relatively lower than that of the ETL (Muchuweni et al., 2020a); hence, making the high work function of CNTs not suitable for cathode applications. Therefore, this section only reviews the recent application of CNTs in the anode of BHJ-OSCs.

The active layer of BHJ-OSCs is composed of an intimate blend of electron donor and acceptor materials, which does not only absorb light, generate excitons and provide multiple sites with large donor-acceptor interfacial area for effective dissociation of charge carriers, but also removes the requirement for long exciton diffusion lengths, and provides percolation pathways for efficient charge carrier transport (Oseni and Mola, 2017; Hou et al., 2018; Rafique et al., 2018; Gusain et al., 2019; Wu et al., 2019; Chen et al., 2020; Song et al., 2020; Xu et al., 2021). However, the photoactive layer often has issues, such as low optical absorption in the visible range, the need for more energy to dissociate the strongly bound photogenerated excitons, the presence of defects and charge carrier traps, short lifetime of charge carriers owing to recombination, discontinuous pathways for charge carrier transport, poor charge carrier mobility due to the hopping transport mechanism, and long-term instability due to degradation of the active layer materials (Paul et al., 2017; Subramanyam et al., 2020). Hence, traditional active layer materials need to be replaced or modified; to broaden the absorption spectrum for effective photon absorption and exciton generation; to increase the donor-acceptor interfacial area for significant exciton dissociation; to provide additional conductive networks for efficient charge carrier transport; and to prevent air and moisture penetration for improving long-term stability.

The most commonly used donor and acceptor materials are the p-type semiconducting polymer, P3HT, and fullerene, PCBM, respectively (Lim et al., 2018; Rathore et al., 2018; Dlamini et al., 2020; Gao et al., 2020; Milanovich et al., 2020; Ghosekar and Patil, 2021). This is due to the merits of P3HT, such as outstanding solubility in organic solvents, high absorption in the visible region, enhanced crystallinity, excellent charge carrier mobility and high stability (Khanh et al., 2020). In addition, PCBM has an excellent electron accepting capability and forms suitable nanoscale morphological networks with P3HT (Sivakumar et al., 2020). However, PCBM suffers from drawbacks, such as high-cost, low electrical conductivity, low charge carrier mobility, limited energy level engineering, weak optical absorption in the visible region, inferior mechanical flexibility, poor air and thermal stability, and complicated synthesis procedures (Amollo et al., 2017; Aïssa et al., 2019; Liang et al., 2019; Sivakumar et al., 2020; Zhao et al., 2020). Therefore, to address the aforementioned limitations, recent studies have focussed on replacing or modifying PCBM with carbon-based materials, particularly CNTs, owing to their excellent electron-accepting capability, large specific surface area, broad absorption spectrum, low reflectance, high charger carrier mobility, facile tunability of band gap, superior stability, and low-cost (Bhatia and Kumar, 2017; Fraga Domínguez et al., 2017). Furthermore, the band offset of CNTs and the donor polymer, and high built-in electric field at the polymer-CNT interface, have the potential to enhance the dissociation of excitons at the donor-acceptor interface, thereby facilitating the efficient transfer of electrons from the polymer to the CNTs (Fraga Domínguez et al., 2017; Agbolaghi, 2019b).

Being motivated by this, ZnO:SWCNTs have been incorporated into the polythieno [3,4-b]thiophene-co-benzodithiophene (PTB7):PCBM and P3HT:PCBM active layer blends of conventional BHJ-OSCs (Oseni et al., 2018), as illustrated in Figure 5A. Increasing the ZnO:SWCNT concentration to 6 wt% improved the generation of excitons and enhanced the interpenetrating networks for both hole and electron transport, which increased the J_sc and FF, resulting in relatively higher PCEs of 4.66 and 3.10% for the devices based on PTB7:PCBM:ZnO:SWCNTs and P3HT:PCBM:ZnO:SWCNTs, respectively, when compared with 2.76 and 1.92% for the pristine PTB7:PCBM and P3HT:PCBM devices, respectively. However, the introduction of excessive concentrations of ZnO:SWCNTs into the active layer led to a decrease in the J_sc and FF, probably due to the high series resistance and low shunt resistance, originating from the agglomeration and bundling of nanotubes, which produces charge carrier traps that prevent the smooth flow of charge carriers, resulting in high charge carrier recombination, as well as causing high leakage current due to short-circuits, thereby impairing device performance.

SWCNTs have also been incorporated into the P3HT:PCBM active layer blend of BHJ-OSCs (Kadem et al., 2018; Aïssa et al., 2019) as illustrated in Figure 5B. This resulted in devices with optimum PCEs of 2.20 and 3.54%, which outperformed their pristine P3HT:PCBM-based counterparts that had PCEs of 1.52 and 1.17%, respectively. This was attributed to the synergy between the electron-accepting nature of the fullerene and the rapid transportation of electrons through the additional percolation pathways created by the SWCNTs, which promoted the effective separation and transportation of charge carriers, thereby increasing the photogenerated current. In addition, the low R_rms of the SWCNT-based active layers of less than 5 nm, demonstrated that the hybrid active layer films were smooth and homogeneous; hence, prone to fewer short-circuits and few charge carrier traps, resulting in low leakage current and less recombination, which improves electron transport and subsequently increases the devices’ PCE. A further increase in PCE from 2.20 to 2.32% was observed after incorporating acid-treated SWCNTs into the P3HT:PCBM blend (Kadem et al., 2018), which was ascribed to the increase in electrical conductivity from 130 mS cm⁻¹ for the P3HT:PCBM blend with non-treated SWCNTs to 230 mS cm⁻¹ for the P3HT:PCBM blend with acid-treated SWCNTs.

A further improvement was made by introducing MWCNTs:rGO into the P3HT:PCBM active layer blend of BHJ-OSCs, which resulted in more stable devices with a relatively higher PCE of 4.13% when compared with 2.91% for the pristine P3HT:PCBM devices (Mahakul et al., 2019). This was ascribed to the introduction of MWCNTs:rGO as additives to P3HT:PCBM, which not only increased the optical absorbance and donor-acceptor interfacial area for effective photon harvesting, exciton generation and exciton dissociation, but also provided smooth conductive pathways for the efficient transportation of photogenerated charge carriers and acted as protective layers against air and moisture penetration; hence, improving device performance and stability.

MWCNTs have also been integrated with the P3HT:PCBM active layer blend of OSCs (Khanh et al., 2020; Subramanyam et al., 2020; Khan et al., 2021), which significantly enhanced the optical absorption and created large donor-acceptor interfaces for improving the generation and dissociation of excitons. Also, the relatively higher work function of the MWCNTs of ∼5.1 eV provided proper energy level alignment for the fast extraction and transportation of holes from the polymer blend to the MWCNTs, in addition to the faster electron transport by PCBM as illustrated in Figure 6, which played a substantial role in suppressing charge carrier recombination, thereby increasing the FF and J_sc, resulting in devices with relatively higher PCEs of 2.35 (Khanh et al., 2020), 1.88 (Khan et al., 2021), and 4.86% (Subramanyam et al., 2020), when compared with the pristine P3HT:PCBM-based devices. Nevertheless, extremely high concentrations of MWCNTs reduced the J_sc and FF, which in turn impaired the device performance due to the agglomeration of MWCNTs as their quantity increased, giving rise to leakage current through short-circuits. Furthermore, the agglomeration of CNTs often reduces the surface area available for the formation of heterojunctions, thereby reducing exciton generation; hence, significantly reducing the photogenerated current, and subsequently giving rise to devices with low PCE (Mahakul et al., 2019).

Recently, MWCNTs grafted with poly (3-dodecylthiophene) (MWCNTs-graft-PDDT) were crystallized with P3HT (Agbolaghi, 2019b; Hadi et al., 2020) and poly[bis(triiso-propylsilylethynyl) benzodithiophene-bis(decyltetradecylthien) naphthobisthiadiazole] (PBDT-TIPS-DTNT-DT) (Agbolaghi, 2019a), and the resulting nanohybrids were integrated with P3HT:PC₇₁BM (Agbolaghi, 2019b; Hadi et al., 2020) and PBDT-TIPS-DTNT-DT:PC₇₁BM (Agbolaghi, 2019a) active layer blends of BHJ-OSCs. Interestingly, the surface modification of MWCNTs helped to increase their dispersion in organic solvents, which enhanced the interaction between the P3HT and MWCNT phases. This, in turn, reduced the agglomeration of MWCNTs, and hence reduced short-circuits, surface charge trapping and recombination, resulting in devices with relatively higher PCEs of 5.40 (Agbolaghi, 2019b) and 4.18% (Hadi et al., 2020), respectively, when compared with 2.13 and 1.14% for the corresponding unmodified MWCNT-based devices. Also, the larger specific surface area and larger crystallite sizes of MWCNTs-graft-PDDT/P3HT with fewer grain boundaries and better crystallinity, respectively, increased the donor-acceptor interfacial area and formed conductive pathways, which increased the exciton dissociation rate and charge carrier mobility, resulting in high J_sc, thereby improving the device performance. However, the grafting of MWCNTs to PDDT weakened the assembling and crystallization of the PBDT-TIPS-DTNT-DT chains onto the MWCNTs, which reduced the PCE from 4.07 to 3.69% for the unmodified MWCNT- and the grafted MWCNT-based devices, respectively (Agbolaghi, 2019a).

Table 2 summarizes the photovoltaic parameters of OSCs with CNT-based active layers. From the reviewed reports, the highest PCE of 5.40% was observed in P3HT:PCBM:MWCNTs-graft-PDDT/P3HT active layer-based devices (Agbolaghi, 2019b). This demonstrates the excellent potential of CNTs as future additives of choice to the P3HT:PCBM active layer blend, rather than as substitutes to PCBM, capable of increasing not only the donor-acceptor interfacial area for the effective dissociation and extraction of excitons, but also providing smooth conductive pathways for the efficient transportation of photogenerated charge carriers, and as protective layers against air and moisture penetration; hence, improving device performance and stability.

The extraction and transportation of charge carriers from the active layer to the electrodes of OSCs have been significantly improved by introducing charge transport layers that reduce the energy barrier and prevent the direct contact between the active layer and electrodes for the effective extraction and transportation of charge carriers and suppression of recombination (Amollo et al., 2018; Ramasamy et al., 2019; Tian et al., 2021). From this viewpoint, HTLs and ETLs extract and transport holes and electrons from the photoactive layer to the anode and cathode, respectively, while selectively blocking electrons (for HTLs) and holes (for ETLs), thereby suppressing charge carrier recombination. In addition, the charge transport layers are usually transparent to allow the incoming photons to reach the active layer for exciton generation. Also, the charge transport layers improve the stability of devices by forming protective layers that seal the active layer materials from the diffusion of moisture and air (Rafique et al., 2018; Amusan et al., 2019; Liu et al., 2019). Therefore, the charge transport layers play a substantial role in determining the overall efficiency and stability of OSCs.

This review has presented the recent trends in the application of CNTs in electrodes, active layers and charge transport layers of OSCs, in response to the increasing demand to develop alternative materials that can replace or modify the traditional materials, which are currently producing devices with relatively low PCE and poor long-term stability. Among the possible alternatives, carbon-based materials, especially CNTs, are more appealing due to their solution processability, non-toxicity, the natural abundance of carbon, low-cost, competitive optoelectronic properties and excellent stability. However, the efficiency of CNT-based OSCs is still relatively lower than that of the state-of-the-art OSCs fabricated with traditional materials due to drawbacks, such as the relatively low visible region optical transparency of CNT-based electrodes and charge transport layers, which limit the passage of incoming photons to the active layer, thereby reducing the exciton generation rate, and hence lowering the photogeneration of current. In addition, the sheet resistance of CNT-based electrodes is relatively higher than that of commonly used ITO electrodes. This limits the efficient collection and transfer of photogenerated current from the charge transport layers to the external circuit, thereby impairing device performance. Nonetheless, the tunable band gap of CNTs, in addition to selective charge carrier transportation and blocking capabilities of CNT-based charge transport layers, enable the provision of an Ohmic contact at the photoactive layer-electrode interface, thereby improving charge carrier extraction and mobility, which increases the efficiency of devices. Also, the hydrophobic nature of CNTs helps to prevent the diffusion of moisture and air into the photoactive layer, which protects the photoactive layer materials from degradation, thereby enhancing the stability of OSCs. Furthermore, the large specific surface area of CNTs provides a large interfacial area between the donor and acceptor, which enhances exciton dissociation, in addition to the highly conductive CNT interpenetrating network, which improves the transfer of electrons and holes to their respective electrodes before recombination, thereby improving device performance. Therefore, as a future research direction, optimization of the optoelectronic properties of CNTs via approaches, such as chemical doping and formation of nanocomposites, is envisaged to pave the way for the commercialization of CNT-based OSCs through the fabrication of highly efficient, sustainable and cost-effective devices.