Thin film solar panels have garnered significant attention in recent decades due to their potential for high efficiency, low cost, and ease of fabrication. As a key component, the light-absorbing semiconductor layer dictates the overall performance of these panels. Traditional materials such as copper indium gallium selenide (CIGS), cadmium telluride (CdTe), and halide perovskites have achieved remarkable photoelectron conversion efficiencies (PCEs), yet they face challenges including material scarcity, toxicity, and instability. This has spurred research into alternative absorber materials that are abundant, environmentally benign, and stable. Among these, CuPbSbS3 has emerged as a promising candidate due to its three-dimensional electronic structure, near-direct bandgap, high optical absorption coefficient, moderate p-type doping, and defect-tolerant characteristics. In this study, we explore the photovoltaic performance of CuPbSbS3 thin film solar panels fabricated via a solution-based process, with a focus on optimizing the film thickness through multiple spin-coating cycles. The aim is to enhance device efficiency by improving film quality and interfacial properties, thereby contributing to the advancement of next-generation thin film solar panels.
The fabrication of thin film solar panels often involves scalable and cost-effective techniques. For CuPbSbS3, we employed a butyldithiocarbamic acid (BDCA) solution method to deposit the absorber layer. This approach allows for precise control over composition and morphology. The process begins with the preparation of precursor solutions containing copper, lead, and antimony complexes, which are mixed in stoichiometric ratios to form a homogeneous CuPbSbS3 precursor. The solution is then spin-coated onto substrates, followed by thermal annealing to crystallize the film. To increase film thickness—a critical factor for light absorption—we implemented a multiple spin-coating strategy. Specifically, we compared single, double, and triple spin-coating cycles, each involving coating and annealing steps, to investigate the impact on film properties and device performance. The resulting thin film solar panels were configured in a superstrate structure: fluorine-doped tin oxide (FTO) as the transparent conductive oxide, a tin dioxide (SnO2) electron transport layer, the CuPbSbS3 absorber, a spiro-OMeTAD hole transport layer, and a silver (Ag) back contact. This design facilitates efficient charge extraction and minimizes recombination losses, which are essential for high-performance thin film solar panels.
Characterization of the CuPbSbS3 films revealed significant insights into their structural and electronic properties. Scanning electron microscopy (SEM) images showed that single-coat films were relatively smooth and dense, but with inadequate thickness for optimal light harvesting. Double-coat films exhibited increased thickness with minimal pinhole formation, whereas triple-coat films developed numerous pinholes, leading to potential shunt paths and reduced device performance. X-ray diffraction (XRD) patterns confirmed the formation of pure-phase CuPbSbS3, with characteristic peaks corresponding to the (002), (112), and (122) crystal planes, indicating a well-crystallized material suitable for thin film solar panels. X-ray photoelectron spectroscopy (XPS) analysis further verified the chemical states, with Cu⁺, Pb²⁺, Sb³⁺, and S²⁻ present, confirming the desired stoichiometry and phase purity. These results underscore the effectiveness of the BDCA method in producing high-quality CuPbSbS3 absorber layers for thin film solar panels.
The photovoltaic performance of the fabricated thin film solar panels was evaluated under standard AM 1.5G illumination. Current density-voltage (J-V) measurements demonstrated that devices with double-spin-coated CuPbSbS3 absorbers achieved the highest PCE of 0.667%, with an open-circuit voltage (VOC) of 215 mV, a short-circuit current density (JSC) of 9.26 mA/cm², and a fill factor (FF) of 0.335. In comparison, single-coat devices yielded a PCE of 0.332% (VOC = 155 mV, JSC = 6.87 mA/cm², FF = 0.312), and triple-coat devices showed a PCE of 0.450% (VOC = 177 mV, JSC = 8.42 mA/cm², FF = 0.301). The improvement in double-coat devices primarily stemmed from enhanced VOC and JSC, attributed to better film thickness and reduced recombination. To quantify these effects, we analyzed key parameters using formulas common in thin film solar panel research. The photoelectron conversion efficiency is given by:
$$ \text{PCE} = \frac{V_{\text{OC}} \times J_{\text{SC}} \times \text{FF}}{P_{\text{in}}} \times 100\% $$
where \( P_{\text{in}} \) is the incident power density (100 mW/cm² for one sun). The fill factor represents the squareness of the J-V curve and is defined as:
$$ \text{FF} = \frac{V_{\text{mp}} \times J_{\text{mp}}}{V_{\text{OC}} \times J_{\text{SC}}} $$
with \( V_{\text{mp}} \) and \( J_{\text{mp}} \) being the voltage and current density at the maximum power point. For thin film solar panels, the ideal diode equation helps understand VOC limitations:
$$ J = J_0 \left( \exp\left(\frac{qV}{nkT}\right) – 1 \right) – J_{\text{ph}} $$
where \( J_0 \) is the reverse saturation current density, \( n \) is the ideality factor, \( q \) is the elementary charge, \( k \) is Boltzmann’s constant, \( T \) is temperature, and \( J_{\text{ph}} \) is the photocurrent density. Lower \( J_0 \) values, as observed in double-coat devices, correlate with higher VOC, indicating suppressed non-radiative recombination. This is crucial for advancing thin film solar panels toward their theoretical efficiency limits.
Further analysis involved electrochemical impedance spectroscopy (EIS) and dark J-V measurements to probe charge transport and recombination dynamics. Nyquist plots revealed smaller charge-transfer resistances for double-coat devices, suggesting improved interfacial charge extraction—a vital aspect for efficient thin film solar panels. Dark current analysis showed lower reverse saturation currents, confirming reduced recombination losses. Additionally, space-charge-limited current (SCLC) measurements on symmetric devices provided insights into trap density. The trap-filled limit voltage (\( V_{\text{TFL}} \)) is related to trap density (\( N_t \)) by:
$$ V_{\text{TFL}} = \frac{e N_t L^2}{2 \varepsilon \varepsilon_0} $$
where \( e \) is the electron charge, \( L \) is the film thickness, \( \varepsilon \) is the relative permittivity, and \( \varepsilon_0 \) is the vacuum permittivity. Double-coat devices exhibited a lower \( V_{\text{TFL}} \) (0.221 V) compared to single-coat (0.432 V) and triple-coat (0.365 V) devices, indicating fewer trap states and better electronic quality, which are desirable for high-performance thin film solar panels.
To summarize the photovoltaic parameters, Table 1 presents a comparison of devices based on spin-coating cycles. This data highlights the optimization achieved through double spin-coating, reinforcing the importance of film thickness and morphology in thin film solar panels.
| Spin-Coating Cycles | VOC (mV) | JSC (mA/cm²) | Fill Factor | PCE (%) | Steady-State PCE (%) |
|---|---|---|---|---|---|
| Single | 155 | 6.87 | 0.312 | 0.332 | 0.285 |
| Double | 215 | 9.26 | 0.335 | 0.667 | 0.667 |
| Triple | 177 | 8.42 | 0.301 | 0.450 | 0.406 |
The enhancement in double-coat devices can be attributed to several factors. First, increased film thickness improves light absorption, as described by the Beer-Lambert law:
$$ I(z) = I_0 e^{-\alpha z} $$
where \( I(z) \) is the light intensity at depth \( z \), \( I_0 \) is the incident intensity, and \( \alpha \) is the absorption coefficient. For CuPbSbS3, \( \alpha \) is high (on the order of 10⁴ cm⁻¹ for photons above the bandgap), so a thicker film captures more photons, boosting \( J_{\text{SC}} \). Second, better crystallinity and reduced pinhole density minimize shunt paths, enhancing FF and VOC. Third, improved interfacial energy alignment between the absorber and transport layers facilitates charge separation and collection. These aspects are critical for the development of efficient thin film solar panels, as they directly impact power output and stability.
Beyond empirical results, theoretical models provide context for the potential of CuPbSbS3 in thin film solar panels. The Shockley-Queisser limit defines the maximum PCE for a single-junction solar cell based on its bandgap (\( E_g \)). For CuPbSbS3 with \( E_g \approx 1.3 \) eV, the theoretical PCE is around 33%. However, practical efficiencies are lower due to non-ideal factors such as recombination, resistive losses, and optical losses. The deficit in VOC, often expressed as \( E_g/q – V_{\text{OC}} \), is a key metric for thin film solar panels. In our study, double-coat devices showed a VOC deficit of approximately 1.085 V (assuming \( E_g = 1.3 \) eV), which, while still significant, represents an improvement over single-coat devices (1.145 V deficit). This suggests that double spin-coating mitigates some recombination pathways, moving closer to the ideal performance of thin film solar panels.
To delve deeper into material properties, we consider the defect tolerance of CuPbSbS3. Defect formation energies (\( E_f \)) influence carrier concentration and recombination rates. For a defect species X in charge state q, \( E_f \) is given by:
$$ E_f[X^q] = E_{\text{total}}[X^q] – E_{\text{total}}[\text{bulk}] – \sum_i n_i \mu_i + q(E_{\text{VBM}} + E_F) + \Delta^q $$
where \( E_{\text{total}} \) is the total energy from density functional theory (DFT) calculations, \( n_i \) and \( \mu_i \) are the number and chemical potential of species i, \( E_{\text{VBM}} \) is the valence band maximum energy, \( E_F \) is the Fermi level, and \( \Delta^q \) is a correction term. CuPbSbS3 exhibits high defect tolerance, meaning that dominant defects are benign and do not create deep traps that promote recombination. This property, combined with its three-dimensional electronic structure, makes it an attractive absorber for thin film solar panels, as it can maintain good performance even with non-stoichiometric or imperfect films.
The optical absorption characteristics of CuPbSbS3 further support its suitability for thin film solar panels. The absorption coefficient \( \alpha(\hbar\omega) \) near the band edge can be modeled using the Tauc relation for direct bandgap materials:
$$ (\alpha \hbar\omega)^2 = A (\hbar\omega – E_g) $$
where \( \hbar\omega \) is the photon energy, and \( A \) is a constant. Experimental data from spectroscopic ellipsometry or UV-Vis spectroscopy typically confirm a high \( \alpha \) value, enabling thin films (e.g., 1-2 μm) to absorb most above-bandgap light. This allows for reduced material usage and cost, a key advantage for thin film solar panels over traditional silicon-based panels. Moreover, the bandgap can be tuned via alloying, such as substituting S with Se to form CuPbSb(S,Se)3, which may lower \( E_g \) and enhance infrared absorption, potentially increasing \( J_{\text{SC}} \) in thin film solar panels.
In terms of device architecture, the superstrate configuration used here offers benefits for thin film solar panels. The transparent conductive oxide (TCO) front contact allows light entry, while the absorber is deposited atop, followed by hole and back contacts. This structure protects the absorber from environmental exposure, improving stability—a common concern for thin film solar panels, especially those with moisture-sensitive materials. However, alternative substrates like flexible polymers or metal foils could be explored for lightweight and portable thin film solar panels, expanding application scenarios such as building-integrated photovoltaics or wearable electronics.
To quantify the impact of spin-coating cycles on film thickness and optical properties, we can use simple models. The film thickness \( d \) after multiple spin-coatings can be approximated as:
$$ d = N \times d_0 $$
where \( N \) is the number of coats and \( d_0 \) is the thickness per coat, which depends on solution viscosity, spin speed, and annealing conditions. For our BDCA process, \( d_0 \) was estimated at around 200 nm based on cross-sectional SEM. Thus, double-coat films reached ~400 nm, while triple-coat films approached ~600 nm. The optical generation rate \( G(x) \) in the film is given by:
$$ G(x) = \int \Phi(\lambda) \alpha(\lambda) e^{-\alpha(\lambda) x} d\lambda $$
where \( \Phi(\lambda) \) is the photon flux at wavelength \( \lambda \), and \( x \) is the position from the front surface. Integrating \( G(x) \) over the film thickness yields the total photogenerated current, which increases with \( d \) until the diffusion length limit is reached. For CuPbSbS3, the minority carrier diffusion length (\( L_d \)) is a critical parameter; if \( d > L_d \), carriers generated near the back contact may recombine before collection. Optimization thus involves balancing thickness with \( L_d \), which can be enhanced via passivation strategies in thin film solar panels.
Recombination mechanisms in thin film solar panels include radiative, Shockley-Read-Hall (SRH), and surface recombination. The SRH recombination rate \( U_{\text{SRH}} \) via trap states at energy \( E_t \) is:
$$ U_{\text{SRH}} = \frac{np – n_i^2}{\tau_p (n + n_1) + \tau_n (p + p_1)} $$
with \( n \) and \( p \) as electron and hole concentrations, \( n_i \) as intrinsic concentration, \( \tau_n \) and \( \tau_p \) as carrier lifetimes, and \( n_1, p_1 \) as constants dependent on \( E_t \). Reduced trap density in double-coat films, as inferred from SCLC, likely decreases \( U_{\text{SRH}} \), contributing to higher \( V_{\text{OC}} \). Surface recombination velocity \( S \) also affects performance; it can be mitigated by interface engineering, such as using suitable buffer layers in thin film solar panels.
Looking beyond efficiency, stability is a paramount concern for thin film solar panels. Accelerated aging tests under damp heat (85°C/85% RH) or light soaking can assess long-term performance. While not covered in this study, CuPbSbS3’s inorganic nature suggests good stability compared to organic-inorganic perovskites, making it a robust candidate for durable thin film solar panels. Future work could involve encapsulation and outdoor testing to validate this hypothesis.
From an economic perspective, thin film solar panels based on abundant materials like CuPbSbS3 could reduce manufacturing costs. The BDCA solution process is scalable via roll-to-roll or spray coating, aligning with low-cost production methods. A cost model might consider material expenses, processing energy, and device yield. For instance, the levelized cost of electricity (LCOE) for thin film solar panels is given by:
$$ \text{LCOE} = \frac{\text{Total Cost}}{\text{Total Energy Output}} = \frac{C_{\text{cap}} + \sum_{t=1}^{T} \frac{C_{\text{O&M}}}{(1+r)^t}}{\sum_{t=1}^{T} \frac{E_{\text{out}}}{(1+r)^t}} $$
where \( C_{\text{cap}} \) is capital cost, \( C_{\text{O&M}} \) is operation and maintenance cost, \( E_{\text{out}} \) is annual energy output, \( r \) is discount rate, and \( T \) is lifetime. Higher efficiency and stability lower LCOE, emphasizing the importance of optimizing CuPbSbS3 thin film solar panels.
In conclusion, this study demonstrates that multiple spin-coating cycles, particularly double coating, significantly enhance the photovoltaic performance of CuPbSbS3 thin film solar panels. Through improved film thickness, reduced trap density, and better interfacial properties, we achieved a PCE of 0.667% with a \( V_{\text{OC}} \) of 215 mV. These findings underscore the potential of CuPbSbS3 as a promising absorber material for thin film solar panels, offering a combination of desirable electronic properties and solution-processability. Future research should focus on further optimizing film quality, exploring alternative device architectures, and assessing long-term stability to advance the commercialization of thin film solar panels. As the demand for renewable energy grows, such innovations in thin film solar panels will play a crucial role in achieving sustainable and affordable solar power.
To further elaborate, the role of thin film solar panels in the global energy landscape cannot be overstated. With increasing emphasis on decarbonization, thin film solar panels offer a pathway to widespread solar adoption due to their flexibility, lightweight nature, and potential for integration into various surfaces. CuPbSbS3, with its earth-abundant constituents, aligns with this vision by avoiding reliance on rare or toxic elements. The double spin-coating method presented here provides a simple yet effective route to improve device performance, which could be adapted to other solution-processed thin film solar panels. Moreover, the insights gained from characterization techniques like SEM, XRD, XPS, and J-V analysis contribute to a deeper understanding of material-behavior relationships in thin film solar panels.
In summary, the optimization of CuPbSbS3 thin film solar panels through spin-coating cycles represents a step forward in the development of efficient and cost-effective photovoltaic technologies. By continuing to refine fabrication processes and material properties, we can unlock higher efficiencies and drive the adoption of thin film solar panels in diverse applications, from residential rooftops to large-scale solar farms. The journey toward next-generation thin film solar panels is fueled by such incremental advancements, each bringing us closer to a sustainable energy future.