Optimizing Organic Photovoltaic Efficiency Through Controlled Doping of ZnS/Co Nanoparticles

Abstract

Thin-film organic solar cells (TFOSCs) are gaining momentum as next-generation photovoltaic technologies due to their lightweight nature, mechanical flexibility, and low cost-effective fabrication. In this pioneering study, we report for the first time the incorporation of cobalt-doped zinc sulfide $(\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o})$ nanoparticles (NPs) into a poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) bulk-heterojunction photoactive layer. $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs were successfully synthesized via a wet chemical method and integrated at varying concentrations (1%wt, 3%wt, and 5%wt) to systematically investigate their influence on device performance. The optimal doping concentration of 3%wt yielded a remarkable power conversion efficiency (PCE) of 4.76%, representing a 102% enhancement over the pristine reference device (2.35%) under ambient laboratory conditions. The observed positive trend is attributed to the localized surface plasmon resonance (LSPR) effect and near-field optical enhancement induced by the presence of ZnS/Co NPs in the active layer, thereby increasing light-harvesting capability and exciton dissociation. Comprehensive morphological and optical characterizations using high-resolution scanning electron microscopy (HRSEM), high-resolution transmission electron microscopy (HRTEM), and spectroscopic techniques confirmed uniform nanoparticle dispersion, nanoscale crystallinity, and effective light absorption. These findings highlight the functional role of $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs as dopants in enhancing TFOSC performance, providing valuable insights into optimizing nanoparticle concentration. This work offers a scalable and impactful strategy for advancing high-efficiency, flexible, and wearable organic photovoltaic devices.

1. Introduction

With growing concerns over climate change and energy security, solar energy has emerged as a leading candidate among renewable energy sources due to its abundance, cleanliness, and sustainability. As global demand for environmentally friendly energy alternatives continues to rise, the development of cost-effective solar technologies that can efficiently convert sunlight into electricity becomes increasingly critical. Among various options, organic solar cells (OSCs), particularly polymer solar cells (PSCs), have gained significant attention owing to their low fabrication costs, lightweight nature, and mechanical flexibility. These features enable vacuum-free processing and compatibility with thin-film deposition techniques, making OSCs ideal for scalable, portable, and flexible solar energy applications [1,2,3,4,5,6,7]. Recent advancements in material design, device architecture, and characterization have led to substantial improvements in OSC performance. Notably, power conversion efficiencies (PCEs) exceeding 20% have been achieved in devices employing non-fullerene acceptors, positioning OSCs as competitive alternatives to conventional silicon-based solar cells [8,9]. Most high-performance OSCs are based on the bulk-heterojunction (BHJ) architecture, where a conjugated donor polymer and a soluble acceptor, typically a fullerene derivative, are blended to form a nanoscale interpenetrating network that facilitates efficient exciton dissociation and charge transport [10]. Despite these achievements, OSCs still face intrinsic limitations. The photoactive layer exhibits relatively low absorption in the near-infrared (NIR) region, resulting in incomplete light harvesting [10]. Moreover, the inherently low charge carrier mobilities of organic materials restrict the thickness of the active layer. While thinner layers suffer from insufficient light absorption, thicker layers are prone to increased bulk recombination losses, both of which compromise device efficiency [11]. To address these challenges, various light-trapping strategies have been explored to enhance photon absorption within thin active layers. These include the use of optical spacer layers [12], photonic crystals [12], diffraction gratings [12], and, more recently, metallic nanoparticles (NPs) [13,14,15,16,17,18]. Metallic NPs offer a promising route to improve light harvesting through localized surface plasmon resonance (LSPR), which amplifies the local electromagnetic field and enhances light absorption.

Table 1. Provides an overview of prior studies on metallic nanoparticles (NPs) and highlights their corresponding efficiency improvements in TFOSCs.

Device Architecture	NPs	NPs Location	PCE (%) Without NPs	PCE (%) with NPs	Ref.
ITO/PEDOT:PSS/P3HT:PCBM:NPs/LiF/Al	Ag/Mg	P3HT/PCBM	2.29	4.11	[18]
ITO/PEDOT:PSS:CuS/P3HT:PC61BM:NPs/LiF/Al	CuS	PEDOT:PSS	2.01	4.51	[19]
ITO/PEDOT:PSS/P3HT:PCBMNP/LiF/Al	ZnS	P3HT:PCBM	1.90	4.00	[20]
ITO/ZnO/PTB7:PCBM/MoO3/Al	Ag	ZnO	6.53	7.25	[21]
ITO/PEDOT:PSS/PTB7:PC71BM/Ca/Al	Au@Ag@SiO2	PTB7:PC71BM	7.72	9.56	[22]
ITO/ZnS/Y2O3/PTB7:PC71BM/MoO3/Ag	ZnO/Y2O3	ZnO	5.77	6.22	[23]
ITO/PEDOT:PSS:Au/PTB7:PC71BM/Ca	Au	PEDOT:PSS	7.50	8.10	[24]
ITO/PEDOT:PSS/PTB7:PC71BM/Ca/Al	Au@Ag@SiO2	PEDOT:PSS	7.72	9.04	[22]

provides an overview of representative studies that highlight the effects of incorporating metallic NPs on the performance metrics of OSCs in various device architectures.

Additionally, their scattering properties extend the optical path length, increasing exciton generation and improving charge transport. Beyond noble metals, various metal and semiconductor sulfide NPs, including copper sulfide (CuS) [25], zinc oxide (ZnO) [26], and zinc sulfide (ZnS) [20], have shown promise in enhancing OSC performance. More recently, cobalt (Co) doping has emerged as a strategy to further improve the optoelectronic properties of metal NPs, thereby increasing their effectiveness in photovoltaic applications [27]. To the best of our knowledge, this study is the first to report the incorporation of cobalt-doped zinc sulfide $(\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o})$ NPs into a P3HT/PC61BM polymer blend as the active layer in OSCs. The introduction of $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs significantly enhanced the optical absorption of the photoactive film, leading to improved exciton generation and charge transport. These enhancements led to a notable improvement in overall device performance, as discussed in the following sections.

2. Materials and Methods Section

2.1. Materials

All chemical reagents used for the synthesis of cobalt-doped zinc sulfide $(\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o})$ NPs were obtained from commercial suppliers and used without further purification. The precursors included sodium borohydride $(\mathrm{N}\mathrm{a}{\mathrm{B}\mathrm{H}}_4,>99.98\%)$, cobalt nitrate $(\mathrm{C}\mathrm{o}{(\mathrm{N}\mathrm{O}}_3{)}_2,>98\%)$, zinc nitrate $(\mathrm{Z}\mathrm{n}{(\mathrm{N}\mathrm{O}}_3{)}_2,>99.98\%)$, and thiourea $\left({\mathrm{C}\mathrm{H}}_4{\mathrm{N}}_2\mathrm{S}\right)$, all purchased from Sigma-Aldrich (St. Louis, MO, USA). Polymer materials employed in device fabrication, including poly-(3,4-ethylene dioxythiophene): poly-(styrene-sulfonate) (PEDOT: PSS), Poly-(3-hexylthiophene) (P3HT), and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), each with a purity of 95%, were sourced from Ossila Co. Ltd., Sheffield (UK). Indium tin oxide (ITO)-coated glass substrates, with a sheet resistance of 8–12 Ω/sq, were also procured from Ossila.

2.2. Synthesis of ZnS/Co NPs

The $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs were synthesized using a wet chemical processing method, which was selected for its simplicity and ability to achieve controlled doping, a critical step in tuning the optical and electronic properties of the resulting NPs. In this procedure, aqueous solutions of zinc nitrate (40 mM), thiourea (40 mM), cobalt nitrate (20 mM), and sodium borohydride (0.5 M) were prepared by dissolving each compound in 50 mL of deionized (DI) water in separate clean flasks. The synthesis began by sequentially adding the zinc nitrate and thiourea solutions into a 500 mL beaker under continuous stirring. Following this, the cobalt nitrate solution was introduced dropwise to ensure uniform distribution of the dopant within the reaction medium. Subsequently, sodium borohydride was added as a reducing agent to initiate the formation of nanoparticles. The reaction mixture was stirred continuously on a magnetic hot plate for 4 h at an average temperature of 45 °C to promote homogeneity and facilitate the formation of $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs. After synthesis, the resulting suspension was repeatedly washed and filtered with DI water to remove residual sodium ions and other byproducts. The purified product was then dried in a vacuum oven at 80 °C for 2 h, yielding $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs suitable for incorporation into the active layer of OSCs.

2.3. Device Fabrication of ZnS/Co NPs

The patterned ITO-coated glass was first etched using an acid solution composed of HCl, H₂O, and HNO₃ in a volume ratio of 48:48:4. Following etching, the substrates were thoroughly cleaned by sequential sonication in DI water, acetone, and isopropanol for 15 min each, followed by drying at 100 °C in an oven. The conventional device architecture employed in this study was Glass/ITO/PEDOT:PSS/P3HT:PC61BM+ZnS/Co/LiF/Al, as illustrated in Figure 1.

A thin hole-extraction layer of PEDOT: PSS was deposited onto the ITO-coated glass by spin-coating at 3500 rpm for 1 min, followed by drying in an oven at 90 °C for 25 min under ambient conditions. The P3HT and PC61BM blended solution, with a concentration of 20 mg/mL, was prepared by dissolving the active layer in a mixed solvent of chloroform at a ratio of 1:1 by weight. However, $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs were incorporated into the mixture solution at concentrations of $1\%\mathrm{w}\mathrm{t}$, $3\%\mathrm{w}\mathrm{t}$, and $5\%$wt, with each concentration prepared in a separate solution. The mixtures were stirred on a hot plate at $45°\mathrm{C}$ for 4 h to ensure homogeneity. The photoactive layer with incorporated $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs at different concentrations were mainly spin-coated onto the PEDOT: PSS layer at 1200 rpm for 40 s and subsequently dried in a furnace at 90 °C for 6 min under a nitrogen atmosphere. Finally, a thin layer of lithium fluoride (LiF, 0.5 nm) and an aluminum (Al) electrode (70 nm) were deposited in a vacuum chamber at a pressure of 10⁻⁶ mbar.

2.4. Device Characterization Section

The current density–voltage (J–V) characteristics of the fabricated organic solar cells were measured using a Keithley HP2420 source meter in conjunction with a solar simulator (model SS50AAA) operating under standard AM1.5G illumination at an intensity of 100 mW/cm². These measurements were conducted to evaluate the photovoltaic performance of the devices. Space-charge-limited current (SCLC) is measured under dark conditions using a solar simulator to investigate the effect of photon-generated traps in polymer solar cells. The optical properties of the photoactive films, including absorption behavior, were characterized using ultraviolet–visible (UV–Vis) spectroscopy. This analysis provided insights into the light-harvesting capabilities of the active layers and the influence of $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs incorporation on the spectral response. High-resolution scanning electron microscopy (HRSEM) and high-resolution transmission electron microscopy (HRTEM) utilize advanced imaging strategies, signal-detection modes, and complementary analytical techniques to investigate materials or samples at the nanoscale, often achieving atomic-level resolution. Energy-dispersive X-ray spectroscopy (EDX) was employed to determine the percentage composition of the nanoparticles. Fourier transform infrared spectrophotometer (FTIR) is a notable technique used to determine additional aspects of the prepared nanostructures by identifying their characteristic peaks and confirming the sample’s functional groups and chemical structure.

3. Results and Discussion Section

3.1. Morphological and Structural Characterization of Cobalt-Doped ZnS NPs

To gain comprehensive insights into the morphology and internal structure of the synthesized $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs, both high-resolution scanning electron microscopy (HRSEM) and high-resolution transmission electron microscopy (HRTEM) analyses were conducted. HRSEM imaging revealed a relatively uniform distribution of nanoparticles, exhibiting a mixture of disc-like and spherical morphologies. This uniformity suggests a well-controlled synthesis process and is essential for ensuring homogeneous dispersion within the polymer matrix, minimizing aggregation-related losses, and promoting consistent device performance. Similar morphologies have been reported in ZnS nanocrystals synthesized via wet chemical methods, indicating that the observed shapes are typical of this synthesis route [20]. HRTEM analysis provided detailed structural information, revealing well-defined lattice fringes with interplanar spacings of 0.310 nm and 0.266 nm, as clearly observed in Figure 2a. These values correspond to the (111) and (200) planes of cubic ZnS, respectively, and are consistent with those reported in previous studies on ZnS and Co-doped ZnS nanocrystals [20,28,29]. Notably, the HRTEM images also revealed features indicative of core–shell-like structures, characterized by a darker central region surrounded by lighter shells, as shown in Figure 2a–c, with relative distribution in terms of size and shape.

This morphology may result from compositional gradients or differences in atomic density between the ZnS core and the Co-doped shell. The particle size distribution, estimated from HRTEM images, ranged between 5 and 10 nm. This nanoscale dimension is particularly advantageous for organic solar cell applications, as it facilitates intimate contact with the polymer matrix, enhances interfacial charge transfer, and supports efficient exciton dissociation. Nanoparticles within this size range have been shown to integrate well into polymer blends, maintaining phase stability and promoting effective charge transport [20]. Elemental analysis using HRSEM coupled with energy-dispersive X-ray spectroscopy (EDX), shown in the inset of Figure 2d, confirmed the presence and uniform distribution of zinc (Zn), sulfur (S), and cobalt (Co) throughout the sample. The distinct intensity peaks corresponding to these elements provide strong evidence for the successful formation of $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs. The detection of cobalt alongside Zn and S supports the effective incorporation of the dopant into the ZnS matrix, which is essential for tailoring the optical and electronic properties of the nanoparticles. A minor carbon (C) signal was also observed, likely due to partial surface oxidation or ambient contamination during sample handling, a common occurrence in nanoparticle characterization. The elemental uniformity observed in the EDX mapping further reinforces the structural homogeneity of the synthesized $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs, which are critical for consistent performance when integrated into the active layer of OSC devices.

3.2. Fourier Transform Infrared Spectrophotometer of ZnS/Co NPs

The chemical structures of the synthesized $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs were investigated using a Fourier transform infrared spectrophotometer (FTIR), with the results presented in Figure 3. The successful synthesis of $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs were confirmed through FTIR spectroscopy, which provided critical insights into both the structural framework and surface chemistry of the material. The vibrational signatures observed in the FTIR spectrum serve as compelling evidence for the formation of the ZnS lattice and the effective incorporation of cobalt dopants.

In the low-wavenumber region, the spectrum displayed distinct absorption peaks at 597 cm⁻¹ and 453 cm⁻¹, which are characteristic of Zn–S stretching vibrations. These peaks fall within the well-established range of 400–706 cm⁻¹, commonly associated with Zn–S bond formation. Notably, the peak at 597 cm⁻¹ aligns with previously reported values for Co-doped ZnS systems, typically observed between 500 and 650 cm⁻¹ [30,31,32,33]. Such vibrational modes have been documented in the literature [33], including peaks at 662 cm⁻¹ and 467 cm⁻¹, further reinforcing the conclusion that the ZnS lattice was successfully formed and structurally stabilized by the incorporation of Co. Complementing the FTIR findings, HRTEM analysis provided detailed structural insights, revealing well-defined lattice fringes with interplanar spacings of 0.310 nm and 0.266 nm. These spacings correspond to the (111) and (200) planes of cubic ZnS, respectively. The consistency between the vibrational signatures observed in FTIR and the crystallographic features revealed by HRTEM confirms the successful synthesis and structural integrity of the Co-doped ZnS lattice. Beyond structural confirmation, the mid- to high-frequency regions of the FTIR spectrum provided valuable information regarding surface-bound functional groups and residual species. A broad absorption band at 3376 cm⁻¹ was attributed to O–H stretching vibrations, indicative of surface-adsorbed water molecules or hydroxyl groups. These functional groups are known to play a pivotal role in photocatalytic processes, particularly in the generation of reactive hydroxyl radicals (•OH) under irradiation. The peak at 1619 cm⁻¹ was assigned to H–O–H bending vibrations of molecular water or possibly C=O stretching, likely resulting from atmospheric CO₂ absorption. This peak, along with those at 977 cm⁻¹ and 597 cm⁻¹, may also reflect Co(H–O–H) bending interactions, suggesting a degree of coordination between cobalt ions and water molecules on the nanoparticle surface. Additional absorption bands observed at 3816 cm⁻¹, 2675 cm⁻¹, 2328 cm⁻¹, 2100 cm⁻¹, 1982 cm⁻¹, 1340 cm⁻¹, and 977 cm⁻¹ are consistent with the presence of residual water and carbon dioxide molecules, commonly introduced during synthesis or through environmental exposure. Notably, bands in the 2849–2930 cm⁻¹ range correspond to C–H stretching vibrations, which may be attributed to organic residues from precursor materials or stabilizing agents used during the co-precipitation process.

3.3. Optical Absorption of the Photoactive Films

A comprehensive investigation of $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs and their composite films were conducted using UV–Vis spectroscopy, a pivotal technique for elucidating the optical and electronic properties of metallic materials. Figure 4 presents the extensive absorber films, both with and without the incorporation of $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs. Figure 4a shows the optical absorbance spectrum of $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs powder suspended in DI water. A broad absorption peak centered around 410 nm is observed, along with continuous absorbance across the entire measured spectrum. This broad and extended absorption is attributed to the size distribution of the $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs, which enhances photon absorption within the medium. Interestingly, Tauc’s plot is one of the most widely used methods to estimate the optical bandgap energy, as demonstrated below:

$$\alpha hv={\beta (hv-E_g)}^n$$

(1)

here, $h$ is Planck’s constant, $v$ is the photon frequency, $\alpha$ is the absorption coefficient, $\beta$ is the material-dependent constant, $E_g$ is the optical energy bandgap, and $n$ is an exponent, which depends on the nature of the transition [30]. The optical $E_g$ was investigated from the intercept of the tangent to the $\left(\alpha hv\right)$ versus $hv$ plot, extrapolated to ${\left(\alpha hv\right)}^2$ = 0. From this analysis, the direct bandgap of $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs were observed to be 1.94 eV (see Figure 4b).

In composite films consisting of a P3HT/PCBM blend with varying concentrations of $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs loading from (pristine, 1%wt, 3%wt, and 5%wt), the UV−Vis absorption spectra are predominantly influenced by the characteristic absorbance of the polymer blend, with a typical absorption peak ranging from 504 to 510 nm (see Figure 4c). However, the incorporation of $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs resulted in notable spectral changes, including the emergence of additional absorption bands in the UV region, with maximum absorption occurring between 390 and 401 nm. There were also increased peak intensities and broadening of the absorption envelope across the visible and near-infrared (NIR) regions. These alterations suggest enhanced light-harvesting capabilities and possible changes in the electronic structure of the composite films resulting from the presence of the metal nanoparticles. The optical absorption intensity varied with the concentration of $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs in the active layer. A maximum absorption peak was observed at a concentration of 3 wt%. However, increasing the concentration to 5%wt led to a noticeable decrease in absorption intensity. This reduction is attributed to the formation of a higher density of defects within the active layer, which facilitates charge carrier recombination and subsequently reduces the optical absorption. However, Figure 4d and

Table 2. The J-V performance of TFOSCs with different concentrations of $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs in the P3HT/PCBM active layer.

ZnS/Co (%wt)	${\mathit{E}}_{\mathit{g}}$ (eV)	${\mathit{E}}_{\mathit{l}\mathit{o}\mathit{s}\mathit{s}}$ (eV)	${\mathit{V}}_{\mathit{o}\mathit{c}}$ (V)	${\mathit{J}}_{\mathit{s}\mathit{c}}$ (mAcm−2)	FF (%)	PCE (%)	Rs (Ωcm2)
0%wt	1.62	1.08	0.54	10.23	44.40	2.35	850
1%wt	1.54	0.98	0.56	14.85	49.78	3.91	443
3%wt	1.50	0.94	0.56	15.71	57.41	4.76	359
5%wt	1.57	1.01	0.56	13.36	47.52	3.55	571

reveal slight variations in the energy bandgap of the absorber films. The differences in bandgap values are due to reduced charge recombination, caused by the interaction between $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs and the polymer cells. Moreover, the effective suppression of charge recombination was observed at $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs doping levels of 1%wt, 3%wt, and 5%wt by weight. Consequently, photon energy loss was evaluated by analyzing the variation in the energy bandgap of the absorber films. Mathematically, it is expressed as $E_{loss}=$ $E_g$ − q$V_{oc}$, where q$V_{oc}$ is the energy extracted from the cell. An increase in $E_{loss}$ from 0.94 eV to 1.08 eV was observed across the tested solar cells, indicating a dependence on the $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs’ doping concentrations. However, the highest energy loss was recorded in the undoped reference device, confirming that $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs help reduce energy loss by limiting charge recombination. Finally, the presence of $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs promotes exciton dissociation and charge transport, thereby accounting for the observed improvement in device performance.

3.4. Electrical Properties of the Photoactive Films

It is worth noting that organic solar cells (OSCs) were fabricated with a photoactive layer composed of P3HT/PCBM as the reference device, and P3HT/PCBM doped with $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs at various concentrations. The J−V (current density–voltage) characteristics recorded for the newly fabricated solar cells are provided in Figure 5. The reference device, fabricated without the inclusion of $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs, produced an efficiency of 2.35%, with a $J_{sc}$ (short-circuit current density) of 10.23 ${\mathrm{m}\mathrm{A}\mathrm{c}\mathrm{m}}^{-2}$. However, a significant enhancement in device performance was observed at the doping levels of 1%wt, 3%wt, and 5%wt concentrations of $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs in the photoactive layer (see Table 2). The maximum PCE value recorded in this experimental work was 4.76% at an optimal doping concentration of 3%wt. The observed enhancement is attributed to a notable increase in $J_{sc}$ to 15.71 ${\mathrm{m}\mathrm{A}\mathrm{c}\mathrm{m}}^{-2}$, thereby providing a 53.57% improvement over the reference solar cell.

The increase in device performance is generally credited to the improved photocurrent collection, facilitated by the incorporation of $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs. The observed positive trend is attributed to the LSPR effect and near-field optical enhancement induced by the presence of $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs in the active layer, thereby increasing light-harvesting capability. This is supported by UV-Vis spectral data, which show the highest intensity peak absorption at a 3%wt NP loading (see Figure 4c), promoting exciton dissociation and enhancing charge carrier collection. As shown in Table 2, the fill factor (FF) increased from 44.40% in the reference device to 57.41% at a 3%wt doping, indicating reduced internal power losses. This improvement in FF is correlated with a significant reduction in current leakage, facilitated by near-field effects that promote more efficient exciton dissociation [34,35], as well as a lower series resistance ($R_s)$, which decreased from 850 ${\Omega \mathrm{c}\mathrm{m}}^2$ to 359 ${\Omega \mathrm{c}\mathrm{m}}^2$. Additionally, the photon energy loss (E_loss) decreased from 1.08 eV in the pristine device to 0.94 eV at optimal doping, suggesting more efficient charge separation and suppressed recombination (see Table 2). Finally, the high conductivity and electron mobility of $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs are instrumental in this reduction, as they integrate seamlessly into the polymer matrix, facilitating charge extraction. These findings clearly demonstrate that incorporating ZnS/Co NPs into the P3HT/PC61BM photoactive layer enhances its optical absorption and establishes favorable conditions for exciton dissociation and charge transport, resulting in improved photon harvesting in polymer solar cells.

3.5. Charge Transport Properties of the Photoactive Films

Understanding the electrical properties of polymer solar cells critically depends on analyzing charge transport processes. In thin-film organic solar cells (TFOSCs), the space-charge-limited current (SCLC) regime provides a reliable method for evaluating charge transport characteristics within the photoactive layer. To reduce the effect of photon-generated traps, the SCLC measurements were conducted under dark conditions (see Figure 6a). This approach ensures that the extracted charge transport parameters more appropriately reflect the intrinsic properties of the samples. Figure 6b shows the SCLC characteristics derived from the forward bias region of the dark J−V curve, beginning at the injection-limited regime around 1.54 V and extending to the point where current saturation occurs.

The SCLC data were subsequently fitted using the field-dependent mobility model, as described by Mott–Gurney’s law (Equation (1)) [36,37].

$$J_{SCLC}={\displaystyle \frac{9}{8}}\epsilon {\epsilon }_0{\mu }_0{\displaystyle \frac{V^2}{L^3}}exp\left(0.89\gamma \sqrt{{\displaystyle \frac{V}{L}}}\right)$$

(2)

here, V is the applied voltage corrected for built-in voltage ($V_{bi}$), while L refers to the photoactive layer’s thickness, which is 100 nm in the device. ε is the dielectric constant of the photoactive layer, and ε₀ = ${8.85\times 10}^{-12} \mathrm{F}/\mathrm{c}\mathrm{m}$, which is the permittivity of free space, while µ₀ and γ are the zero-field mobility and the field activation factor, respectively. It is worth noting that the mobility of polymer solar cells decreases at high applied electric fields due to the negative values observed in the field activation factor, as reported in the literature [38]. The results are presented in

Table 3. The charge transport parameters of TFOSCs fabricated at various concentration levels of $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs.

ZnS/Co (%wt)	${\mathit{\mu }}_0$ (${\mathbf{c}\mathbf{m}}^2{\mathbf{S}}^{-1}{\mathbf{V}}^{-1}$)	γ ($\mathbf{c}\mathbf{m}{\mathbf{V}}^{-1}$)
0%wt (Pristine)	$2.6785\times {10}^{-4}$	$-1.9081\times {10}^{-4}$
1%wt	$1.8124\times {10}^{-3}$	$-1.3114\times {10}^{-4}$
3%wt	$1.7933\times {10}^{-3}$	$-1.1340\times {10}^{-4}$
5%wt	$1.9961\times {10}^{-3}$	$-1.4072\times {10}^{-4}$

. The results in Table 3 further indicate that the zero-field mobility is in one order of magnitude higher than that of the pristine reference solar cell, which is a clear indication that the $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs embedded with an active layer at various concentrations play a significant role in enhancing charge transport and charge dissociation, due to the presence of the LSPR effect in polymer solar cells. These findings validate the efficacy of incorporating $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ nanoparticles into the active layer as a transformative strategy for enhancing the performance of organic solar cells.

4. Conclusions

In summary, we investigated the photovoltaic performance of organic solar cells (OSCs) by incorporating $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs into the P3HT/PCBM active layer. The doped photoactive layer exhibited enhanced light absorption compared to the pristine reference cell, leading to improvements in $J_{sc}$ (short-circuit current density), (FF) fill factor, and overall power conversion efficiency (PCE). As a result, the highest PCE of 4.76% was achieved at a 3%wt concentration of $\mathrm{Z}\mathrm{n}\mathrm{S}/\mathrm{C}\mathrm{o}$ NPs, compared to the pristine (0%wt) device. This improvement is attributed to the LSPR effect introduced by the nanoparticles, which augments exciton dissociation and charge transport, while also reducing series resistance and increasing the device’s electrical conductivity. We believe this approach offers a promising pathway for augmenting the efficiency of OSC devices and could inspire future research on solar cells utilizing non-fullerene acceptors.