In the pursuit of sustainable energy solutions, the development of efficient and cost-effective photovoltaic technologies has become a global priority. Among these, thin film solar panels have garnered significant attention due to their potential for low-temperature processing, flexibility, and reduced material usage compared to traditional silicon-based solar cells. In this study, we focus on advancing p-type dye-sensitized solar cells (p-DSSCs) as a promising component for tandem solar cells, which can enhance light absorption and overall efficiency. Specifically, we investigate the performance of p-type solar cells utilizing nickel oxide (NiO) thin films, a wide-bandgap p-type semiconductor, modified through lithium (Li) doping and sensitized with cadmium selenide (CdSe) quantum dots. Our aim is to contribute to the optimization of thin film solar panels by improving the photocathode materials, ultimately enabling higher-performance p-n tandem devices.
The evolution of solar energy conversion technologies has seen the emergence of various thin film solar panels, including dye-sensitized solar cells (DSSCs), perovskite solar cells, and organic photovoltaics. DSSCs, in particular, have been widely studied since their groundbreaking demonstration by O’Regan and Grätzel in 1991, where a nanoporous TiO2 electrode sensitized with ruthenium complexes achieved a power conversion efficiency (PCE) of 7.1%. This innovation highlighted the potential of thin film solar panels based on sensitized semiconductors, offering advantages such as simple fabrication, low cost, and tunable optical properties. Over the years, DSSCs have evolved, with certified efficiencies reaching up to 13.0% as of 2021, yet further improvements are limited by inherent material constraints and device mechanisms. To overcome these limitations, researchers have explored p-type DSSCs, which use p-type semiconductors like NiO as photocathodes, complementing the traditional n-type photoanodes in tandem configurations. This approach can broaden the absorption spectrum and increase the open-circuit voltage, making thin film solar panels more competitive in the renewable energy market.
Nickel oxide (NiO) is a key material for p-DSSCs due to its wide bandgap (3.6–4.0 eV), chemical stability, and p-type conductivity stemming from nickel vacancies or dopant-induced holes. However, undoped NiO often exhibits low electrical conductivity, which hampers device performance. Doping with elements like lithium (Li) has been shown to enhance the electrical and optical properties of NiO thin films, making them more suitable for thin film solar panels. Li doping can increase hole concentration, reduce resistivity, and modify the film morphology, thereby improving charge transport and sensitizer adsorption. In this work, we employ a sol-gel method to prepare Li-doped NiO thin films on fluorine-doped tin oxide (FTO) glass substrates, followed by sensitization with CdSe quantum dots synthesized via a hot-injection method. We systematically characterize the structural, morphological, and optical properties of these films and evaluate their performance in p-DSSCs. Our findings demonstrate that optimal Li doping significantly boosts the photocurrent density and overall efficiency, underscoring the potential of engineered NiO thin films in advancing thin film solar panels.
To provide a comprehensive understanding, we first delve into the theoretical foundations of thin film solar panels, particularly p-DSSCs. The operation of a p-DSSC involves light absorption by a sensitizer (e.g., quantum dots) attached to a p-type semiconductor, generating holes that are injected into the valence band of the semiconductor and transported to the counter electrode via an electrolyte. The overall power conversion efficiency (η) is given by:
$$ \eta = \frac{J_{sc} \times V_{oc} \times FF}{P_{in}} $$
where \( J_{sc} \) is the short-circuit current density, \( V_{oc} \) is the open-circuit voltage, \( FF \) is the fill factor, and \( P_{in} \) is the incident light power density (typically 100 mW/cm² for standard testing). For thin film solar panels, enhancing \( J_{sc} \) and \( V_{oc} \) through material engineering is crucial. In NiO-based systems, the doping concentration can influence these parameters by altering the band structure and charge carrier dynamics. The relationship between doping and film properties can be described using models such as the Drude theory for electrical conductivity, where the conductivity (σ) is proportional to the carrier concentration (n) and mobility (μ):
$$ \sigma = n e \mu $$
Here, \( e \) is the elementary charge. For Li-doped NiO, Li+ ions act as acceptors, increasing hole concentration and thus conductivity, which is beneficial for thin film solar panels. Additionally, the film morphology affects the surface area available for sensitizer loading and electrolyte penetration, both critical for efficient device operation. We explore these aspects in detail through experimental investigations.
Our experimental methodology begins with the preparation of NiO thin films via the sol-gel method, a versatile technique for fabricating uniform thin film solar panel components. We dissolved nickel precursors in appropriate solvents and added lithium chloride (LiCl) at varying atomic percentages (0%, 0.5%, 1%, and 5% relative to nickel content) to achieve Li doping. The sol was aged for 72 hours to ensure homogeneity, then deposited on FTO glass substrates using a doctor-blade technique to form precursor films. These films were annealed at 400°C for 30 minutes in air to crystallize the NiO and remove organic residues. This process yields porous NiO thin films with tailored properties for thin film solar panels. For sensitization, we synthesized CdSe quantum dots via a hot-injection method, which allows precise control over particle size and optical properties. The quantum dots were dispersed in toluene and then deposited onto the NiO films using electrophoretic deposition (EPD) at 48 V for 15 minutes, resulting in a deep-red colored sensitized photocathode. The EPD method ensures uniform coverage and strong adhesion, which is essential for high-performance thin film solar panels.
To assemble the p-DSSCs, we paired the sensitized NiO photocathodes with a counter electrode made of copper sulfide (Cu2S) on FTO glass, prepared according to a literature procedure. The electrolyte was a polysulfide solution (Na2S and S in water), which facilitates hole transport. The cells were configured in a sandwich structure with an active area of 0.25 cm². Characterization included X-ray diffraction (XRD) for phase analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for morphology assessment, and ultraviolet-visible (UV-Vis) spectroscopy for optical absorption measurements. Photovoltaic performance was evaluated under simulated sunlight (AM 1.5G, 100 mW/cm²) using a source meter to record current-voltage (I-V) curves. All measurements were conducted at room temperature to ensure consistency.
The structural properties of the Li-doped NiO thin films were first examined using XRD. Figure 1 (not shown here, but referenced in context) presents the XRD patterns for undoped and Li-doped NiO films. All samples exhibit diffraction peaks corresponding to the cubic phase of NiO (JCPDS No. 47-1049), with prominent peaks at (111), (200), and (220) planes. As the Li doping concentration increases, the intensity of the (200) peak rises, and the full width at half maximum (FWHM) decreases, indicating improved crystallinity and larger grain sizes. Moreover, a slight shift in the diffraction angles to higher values is observed, particularly for the (200) and (220) peaks. This shift can be attributed to lattice contraction due to the substitution of Ni2+ ions (ionic radius 0.069 nm) by smaller Li+ ions (ionic radius 0.06 nm), consistent with Bragg’s law:
$$ 2d \sin \theta = n\lambda $$
where \( d \) is the interplanar spacing, \( \theta \) is the diffraction angle, \( n \) is the order of reflection, and \( \lambda \) is the X-ray wavelength (0.15406 nm for Cu Kα radiation). The reduction in \( d \) with increasing Li content confirms successful doping. We calculated the crystallite size \( D \) using the Debye-Scherrer equation:
$$ D = \frac{k \lambda}{\beta \cos \theta} $$
where \( k \) is the shape factor (0.9), \( \lambda \) is the wavelength, \( \beta \) is the FWHM in radians, and \( \theta \) is the Bragg angle. The results are summarized in Table 1, showing that grain size increases with Li doping, from approximately 24 nm for undoped NiO to 33 nm for 5% Li-doped NiO. This growth in crystallite size can enhance charge transport in thin film solar panels by reducing grain boundaries, but it may also affect porosity.
| Li Doping Concentration (atomic %) | 2θ for (200) Peak (°) | FWHM (°) | Calculated Crystallite Size (nm) |
|---|---|---|---|
| 0 | 43.157 | 0.459 | 23.74 |
| 0.5 | 43.191 | 0.354 | 30.79 |
| 1.0 | 43.229 | 0.345 | 31.51 |
| 5.0 | 43.231 | 0.328 | 33.20 |
Morphological analysis via SEM reveals significant changes in the NiO thin films with Li doping. Undoped NiO films appear relatively smooth and dense with smaller particles, whereas Li-doped films become rougher and more porous, with particle sizes ranging from 20 to 40 nm. This increased roughness and porosity are advantageous for thin film solar panels, as they provide a larger surface area for CdSe quantum dot adsorption and facilitate electrolyte infiltration, thereby improving charge collection efficiency. TEM images of the 0.5% Li-doped NiO film after CdSe sensitization show some aggregation of NiO particles, with clusters around 40 μm in size, but the porous structure remains intact, supporting effective sensitizer loading. The enhanced morphology due to Li doping is a key factor in optimizing thin film solar panels for better performance.
Optical properties were assessed using UV-Vis absorption spectroscopy. The absorption spectra of CdSe-sensitized NiO films with different Li doping concentrations exhibit similar profiles, with an absorption edge around 610 nm corresponding to the CdSe quantum dots. However, a blue shift in the absorption peak to approximately 550 nm is observed, likely due to quantum dot aggregation on the NiO surface, which alters the effective particle size. This shift can be described by the quantum confinement effect, where the bandgap energy \( E_g \) of semiconductor quantum dots is size-dependent:
$$ E_g = E_{g,bulk} + \frac{h^2}{8R^2} \left( \frac{1}{m_e^*} + \frac{1}{m_h^*} \right) $$
Here, \( E_{g,bulk} \) is the bulk bandgap, \( h \) is Planck’s constant, \( R \) is the particle radius, and \( m_e^* \) and \( m_h^* \) are the effective masses of electrons and holes, respectively. For thin film solar panels, controlling quantum dot size and distribution is crucial to maximize light harvesting. Our results indicate that Li doping does not significantly alter the optical absorption of the sensitized films, but the morphological changes enhance the overall light absorption capacity.
The photovoltaic performance of the assembled p-DSSCs was evaluated through I-V measurements under standard conditions. Figure 2 (not shown) displays the current-density-voltage (J-V) curves for cells with undoped and Li-doped NiO photocathodes. The key parameters extracted from these curves are listed in Table 2. Notably, the cell with 0.5% Li-doped NiO achieves the highest short-circuit current density (\( J_{sc} \)) of 4.19 mA/cm², compared to 3.08 mA/cm² for the undoped cell. This represents a 36% improvement, attributed to enhanced electrical conductivity and better quantum dot loading due to the porous structure. The open-circuit voltage (\( V_{oc} \)) shows a slight decrease with increasing Li doping, from 0.21 V for undoped NiO to 0.194 V for 5% Li-doped NiO, possibly due to changes in the band alignment or increased recombination. The fill factor (FF) remains around 36% for optimal doping, leading to a peak power conversion efficiency (η) of 0.31% for the 0.5% Li-doped cell, a 63% enhancement over the undoped cell (η = 0.19%). These results underscore the importance of precise doping control in thin film solar panels to balance conductivity and morphological properties.
| Li Doping Concentration (atomic %) | Open-Circuit Voltage, \( V_{oc} \) (V) | Short-Circuit Current Density, \( J_{sc} \) (mA/cm²) | Fill Factor, FF (%) | Power Conversion Efficiency, η (%) |
|---|---|---|---|---|
| 0 | 0.21 | 3.08 | 29 | 0.19 |
| 0.5 | 0.208 | 4.19 | 36 | 0.31 |
| 1.0 | 0.198 | 3.62 | 36 | 0.26 |
| 5.0 | 0.194 | 3.41 | 32 | 0.21 |
To further analyze the performance, we consider the diode equation for solar cells, which relates the current density \( J \) to the voltage \( V \):
$$ J = J_{ph} – J_0 \left[ \exp\left( \frac{qV}{nkT} \right) – 1 \right] $$
where \( J_{ph} \) is the photocurrent density, \( J_0 \) is the reverse saturation current density, \( q \) is the elementary charge, \( n \) is the ideality factor, \( k \) is Boltzmann’s constant, and \( T \) is the temperature. For thin film solar panels, a lower \( J_0 \) and higher \( J_{ph} \) are desirable. Li doping likely reduces \( J_0 \) by improving the NiO film quality, but excessive doping may introduce defects that increase recombination, explaining the efficiency drop at 5% doping. We also evaluated the external quantum efficiency (EQE), though not shown here, which correlates with the absorption spectra and confirms the contribution of CdSe quantum dots to photocurrent generation. The integration of EQE over the solar spectrum yields a \( J_{sc} \) consistent with our measurements, validating the performance of these thin film solar panels.
In discussing the broader implications, we note that thin film solar panels based on sensitized NiO offer a pathway to tandem devices with n-type photoanodes like TiO2. The tandem configuration can theoretically achieve higher efficiencies by utilizing a wider range of the solar spectrum. The efficiency of a tandem cell can be approximated by:
$$ \eta_{tandem} = \eta_p + \eta_n – \eta_p \eta_n $$
where \( \eta_p \) and \( \eta_n \) are the efficiencies of the p-type and n-type subcells, respectively. For instance, if a p-DSSC with η = 0.31% is combined with an n-DSSC with η = 10%, the tandem efficiency could reach around 10.3%, demonstrating the additive potential. However, challenges such as current matching and interfacial engineering must be addressed. Our study on Li-doped NiO thin films provides insights into optimizing the p-type component, which is crucial for advancing thin film solar panels toward commercial viability.
We also explored the stability of these thin film solar panels under continuous illumination. Preliminary tests indicate that the CdSe-sensitized Li-doped NiO cells maintain over 80% of their initial efficiency after 100 hours of light soaking, suggesting reasonable stability for p-DSSCs. Degradation mechanisms may include quantum dot desorption or electrolyte decomposition, which are common issues in thin film solar panels and require further encapsulation or electrolyte formulation improvements.
In conclusion, we have successfully fabricated and characterized Li-doped NiO thin films for p-type dye-sensitized solar cells. Using a sol-gel method, we achieved cubic-phase NiO films with tunable morphology and electrical properties through Li doping. The optimal doping concentration of 0.5% atomic Li resulted in a porous, spherical structure with an average grain size of 30 nm, enhancing the short-circuit current density to 4.19 mA/cm² and the overall efficiency to 0.31%. These improvements are attributed to increased hole concentration, better film conductivity, and enhanced surface area for quantum dot sensitization. Our findings highlight the significance of doping strategies in refining thin film solar panels, particularly for p-type applications. Future work will focus on further optimizing the doping levels, exploring alternative dopants, and integrating these photocathodes into tandem devices with n-type materials. By continuing to innovate in thin film solar panels, we can contribute to the development of efficient, low-cost solar energy solutions that meet global energy demands sustainably.
Throughout this study, we have emphasized the role of thin film solar panels in the renewable energy landscape. The versatility of materials like NiO and CdSe quantum dots allows for customization based on application needs, whether for flexible, lightweight panels or high-efficiency tandem cells. As research progresses, we anticipate that thin film solar panels will play an increasingly important role in decentralized power generation and integrated photovoltaic systems. Our work adds to the growing body of knowledge on p-type semiconductors, offering a practical approach to enhance performance through simple doping techniques. We encourage further investigations into charge transport mechanisms, interface engineering, and scalability to propel thin film solar panels toward widespread adoption.