The relentless pursuit of sustainable and cost-effective energy solutions has positioned thin film solar panels at the forefront of photovoltaic research. Unlike their bulky crystalline silicon counterparts, thin film solar panels offer advantages such as flexibility, lightweight design, reduced material consumption, and the potential for lower manufacturing costs. The heart of any thin film solar panel is the light-absorbing semiconductor layer, whose properties ultimately dictate the device’s power conversion efficiency (PCE). For decades, materials like cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) have dominated this space, achieving impressive laboratory efficiencies. However, concerns regarding the toxicity of cadmium, the scarcity and cost of indium and tellurium, and the complex vacuum-based fabrication processes have spurred the search for alternative, earth-abundant absorbers.
This quest led to the development of quaternary compounds like Cu2ZnSn(S,Se)4 (CZTSSe). While promising, CZTSSe-based thin film solar panels suffer from a significant voltage deficit, limiting their maximum practical efficiency far below the theoretical Shockley-Queisser limit. This deficit is largely attributed to a high density of detrimental cation disorder defects that act as recombination centers. Another candidate, the ternary compound CuSbS2, while composed of abundant elements, exhibits an anisotropic, layered crystal structure leading to an indirect bandgap—properties not ideal for high-efficiency photovoltaic conversion.
Recent theoretical and experimental studies have highlighted a new, promising material: bournonite CuPbSbS3. By conceptually incorporating PbS into the CuSbS2 matrix, the crystal structure transforms from a two-dimensional layered arrangement to a three-dimensional “rock-salt” derivative. This three-dimensional electronic dimensionality is a crucial prerequisite for efficient charge carrier transport and collection in a thin film solar panel. First-principles calculations predict that CuPbSbS3 possesses a nearly direct bandgap of approximately 1.3 eV, which is close to the ideal value for single-junction solar cells under the AM1.5G spectrum. Furthermore, it exhibits a high optical absorption coefficient (>105 cm-1), intrinsic p-type conductivity, and a notable degree of defect tolerance. These attributes make CuPbSbS3 a compelling candidate for the next generation of thin film solar panels.
In our research, we focused on developing a simple, solution-processable method to fabricate high-quality CuPbSbS3 thin films and integrate them into functional solar cell devices. We employed a butyldithiocarbamic acid (BDCA) solution-based approach, which allows for the homogeneous mixing of metal precursors at a molecular level before deposition. Our initial devices, fabricated using a single spin-coating step, showed modest performance. We hypothesized that the film thickness might be a limiting factor, as excessively thin absorber layers cannot fully harvest the incident sunlight, while overly thick layers may increase series resistance and carrier recombination. To systematically investigate this and optimize the device, we adopted a multiple spin-coating strategy. This report details our study on the photovoltaic performance of CuPbSbS3 thin film solar cells based on varying spin-coating times, demonstrating that a twice-coated absorber layer significantly enhances the device’s efficiency.
Experimental Methodology: Fabrication of CuPbSbS3 Thin Film Solar Panels
Our device architecture follows a standard n-i-p (substrate) configuration: Fluorine-doped tin oxide (FTO) glass acts as the transparent conducting substrate, followed by a compact SnO2 electron transport layer (ETL), the CuPbSbS3 light absorber, a Spiro-OMeTAD hole transport layer (HTL), and a thermally evaporated silver (Ag) top electrode. The focal point of our process is the deposition and optimization of the CuPbSbS3 absorber layer.
1. Precursor Solution Preparation:
The BDCA precursor solutions for Cu, Pb, and Sb were prepared separately. For the Cu precursor, copper(I) oxide (Cu2O) powder was dissolved in a mixture of carbon disulfide (CS2) and ethanol. Butylamine was then added dropwise under vigorous stirring to form the stable copper butyldithiocarbamate complex. Similar procedures were followed using lead(II) oxide (PbO) and antimony(III) oxide (Sb2O3) to obtain the Pb and Sb precursors, respectively. These three clear precursor solutions were then mixed in a specific molar ratio (Cu:Pb:Sb ≈ 0.9:1:1.25) and stirred to obtain the final homogeneous CuPbSbS3 precursor ink. This solution-based method ensures excellent stoichiometric control and uniformity, which is vital for the performance of the resulting thin film solar panel.
2. Device Fabrication Process:
The FTO glasses were meticulously cleaned. A compact layer of SnO2 was deposited by spin-coating a commercial colloidal dispersion and annealing at 150°C to form the ETL.
The key step involves depositing the CuPbSbS3 absorber:
- Single Coating (1C): The precursor ink was spin-coated onto the SnO2/FTO substrate at 2000 rpm for 60 seconds. The film was then dried on a hotplate at 100°C for 10 minutes to remove solvents.
- Twice Coating (2C): The aforementioned spin-coating and drying steps were repeated exactly once on the same substrate, effectively building up the film thickness in a second layer.
- Thrice Coating (3C): The spin-coating and drying steps were repeated twice on the same substrate.
Following the coating step(s), all samples were simultaneously annealed in a tube furnace at 320°C for 2 minutes under a nitrogen atmosphere. This pyrolysis step converts the metal-organic complexes into crystalline CuPbSbS3 sulfide.
Subsequently, the Spiro-OMeTAD HTL was spin-coated onto the absorber layer, and the devices were left in a dry environment for oxidation. Finally, a 70 nm Ag electrode was thermally evaporated through a shadow mask to complete the thin film solar panel structure: FTO/SnO2/CuPbSbS3/Spiro-OMeTAD/Ag.
3. Materials and Device Characterization:
The surface morphology of the CuPbSbS3 films was examined using scanning electron microscopy (SEM). The crystal structure and phase purity were confirmed by X-ray diffraction (XRD). The chemical states of the elements within the film were verified by X-ray photoelectron spectroscopy (XPS). The current density-voltage (J-V) characteristics of the complete thin film solar panels were measured under simulated AM 1.5G illumination (100 mW/cm²) to determine photovoltaic parameters: open-circuit voltage ($V_{oc}$), short-circuit current density ($J_{sc}$), fill factor (FF), and power conversion efficiency (PCE). Electrochemical impedance spectroscopy (EIS), steady-state power output (SPO), and dark J-V measurements were conducted to analyze charge transfer and recombination dynamics. Space-charge-limited current (SCLC) measurements were performed on electron-only devices to estimate the trap-state density in the CuPbSbS3 films.
Results and Discussion: From Film Properties to Device Physics
1. Characterization of the CuPbSbS3 Absorber Films:
The SEM images reveal the profound impact of the coating strategy on film morphology. The single-coated (1C) film appears relatively smooth and dense, providing good coverage of the substrate. The twice-coated (2C) film retains a generally compact morphology but exhibits a slightly rougher texture and some minor pinholes, which is a common trade-off when increasing film thickness via solution processing. In stark contrast, the thrice-coated (3C) film shows a significant increase in pinholes and a porous, irregular surface. These morphological defects in the 3C film can act as shunting paths, promoting charge recombination and leakage current, which is detrimental to the performance of a thin film solar panel.
The XRD patterns for films deposited on glass substrates confirm the successful formation of crystalline CuPbSbS3. All major diffraction peaks correspond to the standard pattern for bournonite, with no detectable secondary phases such as CuSbS2 or PbS. This indicates that our BDCA solution process and annealing conditions yield phase-pure material, a critical foundation for building an efficient thin film solar panel.
XPS analysis provides deeper insight into the chemical composition. The high-resolution spectra for Cu 2p, Pb 4f, Sb 3d, and S 2p are all consistent with the expected oxidation states in CuPbSbS3: Cu(I), Pb(II), Sb(III), and S(-II). The absence of peaks corresponding to Cu(II) or metallic Pb(0) confirms the phase purity and proper formation of the compound. This chemical uniformity is essential for achieving predictable and stable semiconductor behavior in the absorber layer of a thin film solar panel.
2. Photovoltaic Performance and Optimization:
The J-V characteristics under simulated sunlight clearly demonstrate the effect of the coating strategy. The photovoltaic parameters extracted from the best-performing devices for each category are summarized in Table 1 below.
| Device | $V_{oc}$ (mV) | $J_{sc}$ (mA/cm²) | Fill Factor (FF) | PCE (%) |
|---|---|---|---|---|
| Single Coating (1C) | 155 | 6.87 | 0.312 | 0.332 |
| Twice Coating (2C) | 215 | 9.26 | 0.335 | 0.667 |
| Thrice Coating (3C) | 177 | 8.42 | 0.301 | 0.448 |
The twice-coated device delivers the highest performance, with a PCE double that of the single-coated device. This improvement stems from concurrent increases in both $V_{oc}$ (from 155 mV to 215 mV) and $J_{sc}$ (from 6.87 mA/cm² to 9.26 mA/cm²). The enhancement in $J_{sc}$ can be directly attributed to the increased optical absorption due to the greater absorber thickness, allowing the thin film solar panel to capture more photons. The significant boost in $V_{oc}$ is more nuanced and indicates improved electronic quality of the absorber and/or better interface properties.
The steady-state power output, measured at the maximum power point voltage, further validates the performance. The 2C device maintained a stable photocurrent density of 5.17 mA/cm², corresponding to a stabilized PCE of 0.667%, confirming the reliability of the J-V measurement. In contrast, the 1C and 3C devices showed lower stabilized outputs of 3.28 mA/cm² and 3.98 mA/cm², respectively.
3. Analysis of Charge Carrier Dynamics:
To understand the origin of the improved $V_{oc}$, we employed several electrical characterization techniques. The dark J-V curves show that the 2C device has the lowest reverse saturation current density, suggesting suppressed non-radiative recombination compared to the 1C and 3C devices. This directly correlates with its higher $V_{oc}$, as expressed by the diode equation:
$$ V_{oc} = \frac{n k T}{q} \ln\left(\frac{J_{sc}}{J_0} + 1\right) $$
where $J_0$ is the reverse saturation current density, $n$ is the ideality factor, $k$ is Boltzmann’s constant, $T$ is temperature, and $q$ is the elementary charge. A lower $J_0$ leads to a higher $V_{oc}$.
Electrochemical impedance spectroscopy (EIS) performed in the dark at the $V_{oc}$ bias provides information on charge transfer resistance ($R_{ct}$) at the interfaces. The Nyquist plot for the 2C device exhibits a smaller semicircle radius in the high-frequency region compared to the 1C and 3C devices. This indicates a lower $R_{ct}$, meaning more efficient charge extraction at the interface between the CuPbSbS3 absorber and the charge transport layers. Efficient extraction minimizes carrier accumulation at interfaces, reducing interfacial recombination losses—a common bottleneck in thin film solar panels.
The most revealing analysis comes from space-charge-limited current (SCLC) measurements on electron-only devices. The trap-filled limit voltage ($V_{TFL}$) is related to the density of trap states ($N_t$) in the semiconductor by:
$$ N_t = \frac{2 \epsilon \epsilon_0 V_{TFL}}{e L^2} $$
where $\epsilon$ is the relative permittivity, $\epsilon_0$ is the vacuum permittivity, $e$ is the electron charge, and $L$ is the film thickness. The $V_{TFL}$ values followed the order: 1C (0.432 V) > 3C (0.365 V) > 2C (0.221 V). This clearly indicates that the twice-coating process resulted in an absorber layer with the lowest trap-state density. Fewer trap states mean reduced Shockley-Read-Hall recombination, which is a primary non-radiative loss mechanism. This reduction in bulk recombination centers is a key reason for the superior $V_{oc}$ and overall performance of the 2C thin film solar panel.
The degradation in performance for the 3C device, despite its greater thickness, can be explained by the observed poor morphology. The numerous pinholes and porous structure likely create direct shunting paths between the ETL and HTL, increasing leakage current (lower FF). Furthermore, the highly defective and possibly poorly crystallized bulk material in the 3C film (as suggested by the higher $V_{TFL}$ compared to 2C) enhances bulk recombination, pulling down both $V_{oc}$ and $J_{sc}$. This underscores that simply increasing thickness is not sufficient; the optoelectronic quality of the added material is paramount for building an efficient thin film solar panel.
Conclusion and Perspective
In summary, we have successfully demonstrated a simple yet effective twice-spin-coating strategy to enhance the performance of solution-processed CuPbSbS3 thin film solar panels. This optimization addresses a common challenge in solution processing: achieving an optically thick yet electronically high-quality absorber layer. The twice-coated film offers an optimal balance, providing sufficient thickness for light absorption while maintaining good crystallinity and low defect density. This leads to improved charge extraction, suppressed bulk and interfacial recombination, and consequently, significantly better photovoltaic parameters.
Our champion device achieved a power conversion efficiency of 0.667% with an open-circuit voltage of 215 mV. While this efficiency is preliminary, it represents a crucial step in the development of CuPbSbS3 as a novel absorber material. The performance gain from the coating optimization validates the potential of this material system. The key formula governing the efficiency of a thin film solar panel is:
$$ PCE = \frac{J_{sc} \times V_{oc} \times FF}{P_{in}} $$
Our work effectively improved all three parameters in the numerator ($J_{sc}$, $V_{oc}$, FF) for the 2C device, leading to the doubling of the PCE.
Future work to advance CuPbSbS3 thin film solar panels will focus on several fronts: (1) Further refining the solution chemistry and annealing process to improve the morphology of thicker films and eliminate pinholes. (2) Exploring interface engineering, such as the use of different buffer layers or passivation strategies, to reduce interfacial recombination losses further. (3) Doping and alloying, for instance, with Se to form CuPbSb(S,Se)3, to fine-tune the bandgap and electrical properties. (4) Scaling up the deposition process towards larger-area modules. The insights gained from this study on the relationship between fabrication process, film quality, and device performance provide a solid foundation for these future endeavors in developing efficient, stable, and earth-abundant thin film solar panels based on the promising CuPbSbS3 semiconductor.