1 Introduction

The increasing demand for energy all over the world and the environmental pollution caused by excessive usage of conventional fuels necessitates the requirement for eco-friendly energy alternatives. Gearing up to deal with these issues involves a quest for sustainable energy sources along with their efficient harnessing and storage techniques [1, 2]. Among the most sought-after resources, solar energy is a promising, reliable, secure, and renewable energy outlet, provided we are equipped with coherent harvesting and storage devices. According to the International Energy Agency (IEA), the demand for solar power generation has increased many-fold in the past decade, owing to feasible, environment-friendly, and cost-effective energy production [3].

Progress in technology and science over the years has shouldered the growth of solar cell advancement, presently reaching the third generation of photovoltaics, which are smoothly making way for the upcoming fourth generation [4]. A comparative study of the different generations of photovoltaics is given in Table 1.

Table 1 A comparative study of the different generations of photovoltaics, and their types and characteristics. The maximum reported efficiencies are taken from the NREL database
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The first-generation, silicon-based solar cells, comprising single-crystalline and polycrystalline cells, are technologically mature and are widely commercialized [11, 12]. However, high cost and complex manufacturing demands for first-generation photovoltaics paved the way for second-generation solar cells based on thin-film technology involving materials like amorphous silicon, cadmium-telluride (CdTe) [11, 12], copper indium gallium di-selenide (CIGS) [11,12,13], copper indium di-selenide (CuInSe2) [14], and copper-zinc-tin-sulfide (CZTS) [8, 15, 16].

Thin-film solar cells are cost-effective as they ensure effective material utilization compared to silicon solar cells [7]. The main bottleneck faced by thin-film solar cells is the scarcity of raw materials, which are often toxic [7, 8].

Figure 1 depicts the highest confirmed laboratory photoconversion efficiency for various photovoltaic cells from 1975 to 2023. Here, emerging photovoltaics have shown a tremendous increase in conversion efficiency from the 1980s, with the least recorded efficiency reaching 13% (inorganic cells and dye-sensitized cells) and the highest recorded efficiency at 33.7% (perovskite/Si tandem monolithic) by 2023 [17].

Fig. 1
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(Source: NREL efficiency report) [17]

Recorded efficiencies of all generations of solar cells (1975 to 2023)

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Presently, the new generation of solar cells—the third-generation photovoltaics based on nanocrystals, polymers, dyes, perovskites, and organic materials—is a highly flourishing field in solar energy research [18]. Even though the achieved power conversion efficiency and stability are low in most cases, third-generation solar cells are renowned due to their numerous practical applications [21].

Organic photovoltaics are flexible and lightweight compared to rigid crystalline silicon solar cells. These properties, along with the factor of being low cost, become significant in applications like the Internet of Things (IoT) [22] and remote area power distribution, where long-term usage of batteries has become impracticable. Even though thin-film solar cells also have the inherent advantage of flexibility, the requirement of easily bendable substrate metal foil that withstands high fabrication temperatures (CIGS (> 550 °C) and CdTe (> 600 °C)) increases the complexity of cost and technology [23]. Hence, organic photovoltaics have become significant despite their limitations, like immature technology and lack of long-term operational stability. Table 2 summarizes the mechanical hardness of different types of solar cells.

Table 2 Mechanical hardness of various types of solar cells
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While conventional thin-film solar cells bend at a curvature radius of a few millimeters, organic cells outperform them in terms of flexibility. Kaltenbrunner et al. [26] have reported that glueing the flexible device to a pre-stretched elastomer allows the devices to resist quasi-linear compression to below 70% of their original area. Additionally, they discovered that the device performance suffered only minimal loss after 20 complete cycles of cyclic compression and stretching to 50% without any discernible damages on the peripheral contact points of the device. The extraordinary flexibility and specific weight of organic photovoltaics make them an ideal ultrathin-film power source for Internet of Things applications [28]. Replacing indium-tin-oxide (ITO) electrodes with flexible transparent conductive electrodes like conductive polymers or conductive carbon nanomaterials or their hybrid composites will result in an increment in the pliability of these solar cells.

Another emerging technology, using conjugated polymers as donor materials in solar cells, has also been reported to enhance the device performance. According to reported studies, employing a low bandgap polymer as a donor and fullerene derivative as an acceptor effectively pushes efficiency beyond 10%. Adaptable and tunable physical and chemical properties [29] of polymers, including electrical conductivity, have been used in various applications [30,31,32]. Owing to their three-dimensional network structure, polymer templates can be employed to create mesoporous materials or polymeric matrices in solid electrolytes [33,34,35]. In addition, certain polymers have strong carrier mobility, which makes it possible for them to serve as electron and hole transfer materials. Many functional groups in polymers aid in controlling the bulk morphology and interfacial layers to passivate defects, enhancing the performance of devices [36, 37]. Polymers can also be employed as photoactive layers or buffer layers in organic photovoltaics owing to their varied structures and modified functional groups, which tune the optical absorption and electron mobility [38, 39]. Easy processing methods of polymers make the fabrication of organic solar cells more viable and cost-effective.

1.1 Device structure and working of polymer solar cells

Polymer solar cells work based on photoexcitation, leading to exciton generation followed by charge separation by proper energy level alignment of different layers (Fig. 2a). Well-defined energy level arrangement of different layers in the photovoltaic device, as shown in Fig. 2b, enables a highly directional flow of charges and efficient charge separation. The electrons from the donor polymer are excited to the LUMOD (Lower Unoccupied Molecular Orbit) levels upon light irradiation, forming excitons. At the donor–acceptor interface, the electrons from the generated excitons are transferred to the LUMO level of electron acceptor (LUMOA) and then to the electrode. Likewise, the generated holes move from the Highest Occupied Molecular Orbit of the Donor material (HOMOD) to the other electrode through intermediate layers like hole transport layer (HTL). The generated photocurrent and the cell parameters like photoconversion efficiency (PCE), fill factor (FF), short circuit current (JSC), and open circuit voltage (VOC) depend on a number of factors like incident light intensity, carrier diffusion length, and surface recombinations [40,41,42,43,44].

Fig. 2
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Schematic representation of a working processes and b energy level alignment of a polymer solar cell. Structural arrangement of c single layer, d bilayer, and e bulk heterojunction polymer solar cells

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The structure of polymer solar cells varies depending on the materials and combinations used; these can basically be classified into three types. A single-layer polymer photovoltaic (Fig. 2c) comprises a photoactive layer sandwiched in between the charge transport layers, which are followed by the electrodes. In case of a bilayer polymer solar cell, the photoactive layer comprises an electron donor and an acceptor material stacked one above the other, as shown in Fig. 2d, whereas, in a bulk heterojunction, a donor–acceptor blend, as given in Fig. 2e, is used as the active layer. Apart from these structures, many reported studies have included additional layers or have excluded a few layers to enhance the operational efficiency and viability of the fabricated device [45,46,47,48,49,50,51,52]. We shall be coming across several such device structures and the roles of materials used in the upcoming sections.

2 Polymer-nanocarbon composites—significance

Each layer in a polymer photovoltaic plays a significant role in the device functioning, and thereby the selection of materials for each layer is crucial in enhancing the performance. Even though conducting polymers have many advantages over conventional materials, issues like low hole mobility, narrow light absorption range, lower efficiency, and stability call for further exploration to overcome the limitations. As an effective technique of remediation, the conducting polymers can be blended with various nanocarbon materials. The high surface-to-volume ratio of nanocarbon materials helps in tuning properties like absorption range, whereas the size of these materials aids in the miniaturization of devices [53]. The other noteworthy properties of carbon nanomaterials include high conductivity, mechanical strength, and good interaction with external agents. Hence, the blending of polymers and carbon nanomaterials will complement each other to make a positive impact on the device structure and performance [54,55,56,57]. Table 3 gives a brief outlook on the advantages and disadvantages of polymer-nanocarbon composites.

Table 3 A brief outlook on the advantages and disadvantages of polymer-nanocarbon composites
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3 Polymer-nanocarbon composites—synthesis

Preparation of polymer-nanocarbon composites often involves less complex, and comparatively scalable techniques, which can either be making a composite out of separately synthesized polymer and nanocarbon or be co-synthesis for obtaining the composite. Zhang et al. [58] have discussed special strategies like homogeneous, laminate, alignment, and network architectures to enhance the desired properties of polymer-nanocarbon composites. To attain the homogeneous architecture of nanocarbon in a polymer matrix, different methods like powder-blending [59], melt blending [60], solution blending [61], and in situ polymerization [62] can be employed. Methods like evaporation [63], vacuum filtration [64], scaffold infiltration [94], and this blending boosted JSC by about 30% and PCE by about 40% without altering the VOC. The incorporation of SWCNTs made charge transfer, enhancing the JSC from 3.75 to 4.95 mA/cm2. Bhatia et al. [95] functionalized MWCNTs with two different aryl azides, which are nitrene generation precursors, and these functionalized MWCNTs were incorporated into the P3HT:PCBM active layer. The reported performance enhancement is attributed to the shortening of exciton dissociation pathway brought about by the functionalized MWCNTs. In 2021, Khan et al. [96] incorporated varying concentrations of MWCNTs into the P3HT:PCBM photoactive layer of organic photovoltaics with device structure ITO/PEDOT:PSS/P3HT:PCBM:MWCNTs/Ca/Al. The device with a 4% concentration of MWCNTs in the active layer gave the best performance with a PCE of 1.88%.

2D nanocarbon materials like graphene (G) and its derivatives have also been reported as additives to the polymer-based active layer materials of organic solar cells. Liu and colleagues [97] fabricated a polymer solar cell with solution-processable functionalized graphene (SPF-G) as an electron acceptor additive to the P3OT (poly(3-octyltheophene)) donor in a bulk heterojunction structure, achieving an efficiency of 1.4% for SPF-graphene concentration of 5 wt%. Later, the same group [54] replaced P3OT with P3HT, attaining a PCE of 1.1% with the incorporation of 10% weight of SPF-graphene. In another reported study by Wang et al. [98], with a similar device structure and an active layer comprising SPF-graphene, P3OT, and PCBM composite, a PCE of 1.14% was achieved.

All the above-analyzed literature elucidates the effectiveness of nanocarbon materials as additives to polymers in the photoactive layer of polymer solar cells. Table 4 gives a comparison between different device structures, materials, and solar cell parameters of different reported organic photovoltaics with nanocarbon-polymer composites as active layers. The reported polymer-nanocarbon blends have shown improved conductive properties, microstructure, temperature stability, and visible light absorption compared to the bare polymer materials. Most of the studies have shown that incorporation of nanocarbon materials has improved the fill factor and short circuit current, thereby increasing the light-conversion efficiency, owing to better charge separation and faster charge transportation.

Table 4 Comparison between different device structures, materials, and solar cell parameters of different reported organic photovoltaics with nanocarbon-polymer composites as active layers
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4.2 Polymer-nanocarbon composites as charge transport layers in polymer solar cells

Effective separation of excitons generated by the photovoltaic effect in the active layer, followed by the transportation of charges to desired directions with a minimal number of undesired recombinations, makes up a crucial and complicated process in the operation of an organic solar cell. The charge transport layers, be it HTL (hole transport layer) or ETL (hole transport layer), play important roles in the extraction of charges from the active layer and their movement in specific directions.

Even though there are reported works based on materials like WS2 [99] and metal oxides [100], the majority of efficient materials reported for HTL application in organic solar cells fall under the category of polymers, like the well-known PEDOT:PSS [101, 102], and their composites [103,104,105]. Moon et al. [106] used nitrogen-doped GQDs (n-GQDs) along with PEDOT:PSS as the HTL in PTB7:PC71BM-based organic solar cells to enhance the energy downshift and, thereby, hole extraction. Nitrogen-doped GQDs, whose optical characteristics can be tuned by varying treatment temperatures, were synthesized by solvothermal cutting of poly-acrylonitrile carbon frameworks. As a result of GQD incorporation, there was a notable increase of PCE up to 14.5%. Dang et al. [107] functionalized multiwalled carbon nanotubes (MWCNTs) using boronic acid and blended them with PEDOT:PSS to form the HTL of PCDTBT:PC71BM-based organic solar cells. A 28% hike in PCE is reported upon the addition of 0.4% weight of br-MWCNTs which is attributed to the increase in short-circuit current caused by enhanced conductivity and hole mobility. Zhang et al. [108] hybridized unzipped single-walled carbon nanotubes (u-SWCNTs) with PEDOT:PSS, forming the HTL in PBDB-T-2F:IT-4F-based polymer solar cells, which yielded a PCE of 14.6%. The notable enhancement in fill factor and short circuit current is ascribed to the improved conductivity and carrier mobility induced in the PEDOT:PSS as a consequence of u-SWCNT incorporation.

Among the reported nanocarbon materials used as additives in polymer HTLs, graphene and its derivatives take up a major share, owing to the exceptional properties of these two-dimensional nanomaterials. Dericiler et al. [109] used electrochemically exfoliated graphene, synthesized using sulfate salts, to modify the PEDOT:PSS HTL of organic photovoltaics based on P3HT:PCBM active layer. The addition of graphene enhanced the PCE by about 66%, which indicates improved carrier transportation efficiency of the HTL. Maity et al. [110] used an interlayer of HNO3-treated graphene between PEDOT:PSS and ITO layers in PTB:PC71BM-based organic photovoltaics, yielding an efficiency of 7% with a JSC of 14.3 mA/cm2. When this same formation was used, excluding the ITO layer, the device performance improved even higher, reaching a PCE of 8.4%, with a JSC of 16.1 mA/cm2, kee** the VOC constant at 0.75 V. Similarly, Hilal et al. [111] have also reported incorporating graphene additives to PEDOT:PSS to form the anode interfacial layer, where the chemical interactions between graphene and PEDOT:PSS enabled increased electron transfer from graphene to PEDOT:PSS, increasing the conductivity of graphene by giving it a net positive charge, consequently resulting in an enhanced PCE of 4.52%. Iakobson et al. [112] studied the performance of polyaniline-PAMPSA (poly(2-acrylamido-2-methyl-1-propane sulfonic acid)-graphene nanosheet composites as an HTL material in P3HT:PC70BM-based organic solar cells by varying the range of graphene oxidization. The unoxidized graphene nanosheets showed better performance owing to their increased roughness, which, in contact with the active layer, promotes better exciton separation and hole extraction. Pei et al. [113] have reported sulfonated graphene (SG)–modified PEDOT:PSS HTLs for PM6:Y6 and PM6:L8BO-based devices, attaining notably high PCEs of 17.48% and 18.56% respectively. The sulfonated graphene, on one hand, reduced the undesired energy barriers and on the other hand improved the surface morphology and conductivity enabling faster hole extraction.

Graphene oxide (GO) [114,115,116], which is a popular and potential derivative of graphene, is one of the most widely reported nanocarbon materials to be used as an additive to modify the polymer HTL of organic photovoltaics. Ozcan et al. [117] doped PEDOT:PSS with GO to form the HTL of P3HT:PC71BM-based inverted organic solar cell, attaining an efficiency of 2.75%, owing to the superior electronic properties brought about by GO do**. Raj et al. [118] have also reported a similar work, blending PEDOT:PSS and GO to form the HTL of PTB7:PCBM-based polymer photovoltaics, yielding a maximum efficiency of 7.68%. The energy level alignment, as shown in Fig. 5a, facilitates the desired directional flow of electrons and holes. The HTL incorporated with GO shows good conductivity, enabling efficient hole extraction and transfer, thereby improving the performance parameters of the device. Figure 5b shows the J-V characteristics for devices fabricated with varying (PEDOT:PSS):GO ratios in the HTL, where the ratio of 15:1 gave the best result with a JSC of 14.9 mA/cm2, and a VOC of 0.75 V. Another reported study by Iwan et al. [119] also used GO to modify the HTL layer of both PTB7:PC71BM and P3HT:PC61BM active layer-based solar cells. The energy level diagrams of different materials used in the study, including various ratios of [PEDOT:PSS]:GO composites, are given in Fig. 5c. A maximum PCE of 5.32% was attained using the P3HT:PC61BM active layer accompanied by a 1:1 ratio of [PEDOT:PSS]:GO composite HTL. The better HOMO–LUMO matching attained by addition of GO, together with enhanced hole mobility, contributed to the performance of the device. Rafique et al. [120] used [PEDOT:PSS]/GO double-decked HTL in PCDTBT:PC71BM (where PCDTBT is poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)])-based photovoltaics, where these devices outperformed both other devices with single material HTLs made of GO and PEDOT:PSS separately, as elucidated in Fig. 5d, e, f, and g. This superior performance is attributed to primarily the work function matching between PCDTBT and [PEDOT:PSS]/GO, and then to the reduction in series resistance, which enabled better carrier mobility and charge transportation. Also, the high shunt resistance caused by GO played significant role in suppressing undesired carrier recombinations. Rafique et al. [121], in another study, treated the [PEDOT:PSS]/GO bilayer with UV radiation in the presence of ozone molecules to improve the work function and charge extraction properties, resulting in a device with PCE of 5.24%. The improved performance is also attributed to the reduction of oxygen in GO by ozone molecules under UV irradiation, which in turn enhanced the conductivity of the HTL.

Fig. 5
figure 5figure 5

a Energy level alignment of ITO/PEDOT:PSS:GO/PTB7:PCBM/PFN/Al device structure (PFN is poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorine)-alt-27-(9,9-dioctylfluorene)]). b Comparison of J-V characteristics for devices fabricated with varying (PEDOT:PSS):GO ratios in the HTL. Reprinted with permission from [118]. c Energy level diagrams of the different active layer and charge transport materials, including various ratios of [PEDOT:PSS]:GO composites, used in the study by Iwan et al. Reprinted with permission from [119]. The superior performance of devices with [PEDOT:PSS]/GO double-decked HTL compared to devices with single-material HTLs-PEDOT:PSS and GO separately, in terms of d current density. e Open-circuit voltage; f fill factor; and g PCE. Reprinted with permission from [120]

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Graphene oxide can be further reduced to form reduced-graphene oxide (r-GO), which, through the reduction of oxygen species, possesses enhanced conductivity and electronic properties. Amallo et al. [122] used germanium-doped r-GO to modify the PEDOT:PSS HTL in P3HT:PCBM-based organic solar cells, attaining a PCE of 2.4%. Goumri et al. [123] enhanced the PCE of inverted solar cells based on P3HT:PCBM active layer from 2.14 to 3.06% with the addition of 3% weight of r-GO in the PEDOT:PSS HTL. Graphitic carbon nitride [124] has also been used for do** the PEDOT:PSS layer to enhance solar cell performance owing to its exceptional electronic and morphological properties. Figure 6a and b give a comparative study of reported PCEs and fill factors of different organic solar cells with nanocarbon composites as HTL materials.

Fig. 6
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Comparative study of a PCE and b fill factor of various reported organic solar cells with nanocarbon composite materials as HTL

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Carbon nanomaterials, especially graphene and its derivatives, have proven to be effective additives to enhance the performance of polymer HTL materials like PEDOT:PSS. All the reported studies showed significant improvement in carrier mobility and charge transportation of the polymer material upon incorporation of nanocarbon materials. Most carbon nanomaterials improve the energy level alignment and reduce the number of undesired recombinations in the fabricated devices. Table 5 gives a comparison between different device structures, materials, and Solar cell parameters of different reported organic photovoltaics with nanocarbon-polymer composites as hole transport layers. Coming to the electron transport layer (ETL) of organic solar cells, the field is still dominated by materials like LiF, Ca, and PFN. The scope of polymer-nanocarbon composites is yet to be widely explored.

Table 5 Comparison between different device structures, materials, and solar cell parameters of different reported organic photovoltaics with nanocarbon-polymer composites as hole transport layers
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5 Conclusion and future prospects

Owing to cost-effectiveness, simple processing technology, mechanical flexibility, lightweightness, and extensive roll-to-roll production (R2R), the development of polymer solar cells has gained significant attention in the modern world. The tunability of optoelectronic properties in conjugated polymers by altering their molecular design opens up a wide range of scope for future energy technology. Also, nanocarbon structures like GQDs, CNTs, and graphene have a great future scope, as they are easily processable materials with exceptional properties that can be of great use for powering future technology. The next generation of photovoltaic materials will be dominated by hybrids where polymer-nanocarbon composites can play a very crucial and effective role. Incorporating nanocarbon structures into polymers has been a proven method to enhance the performance of the active layer as well as the charge transport layer, especially the HTL of organic solar cells. These polymer-nanocarbon composites have shown superior electronic and optical properties compared to pristine polymers and nanocarbons, thus enabling enhanced performances of the fabricated devices compared to their pristine ones. Probing the exemplary properties of polymer-nanocarbon composites towards the exploration of new hybrid technologies and materials for performance enhancement in organic solar cells has a promising future outlook. As we are on the verge of transition towards the fourth generation of photovoltaics, polymer-nanocarbon composites could enable improved efficiency, mechanical strength, and cost-effectiveness. The mechanical robustness and potential for scalability and low-cost manufacturing make these hybrids appealing for broader applications. Leveraging the flexibility of polymers and the conductivity of nanocarbons for optimized charge transport could be a research area of high scope in the near future. Additionally, the composites offer avenues for environmental sustainability through green synthesis and recyclability. The polymer-nanocarbon hybrids present a multidimensional approach to advancing photovoltaic technology.

Polymer solar cells have improved quite quickly, yet compared to their inorganic counterparts, they still have lower power conversion efficiencies and longevity. When compared to studies on application of nanocarbon materials in perovskite and dye-sensitized solar cells, the potential of using carbon nanomaterials in organic solar cells is still a less explored field. Even though nanocarbon-polymer composites show significant improvement in device parameters, there are many limiting factors that hinder the hike in efficiency and stability of organic solar cells to a level comparable to that of their co-runners—perovskite solar cells. Several nanocarbon materials have been reported as suitable ETL and electrode materials for organic photovoltaics, but the scope of utilizing polymer-nanocarbon composites in these layers is almost left untouched.