In recent years, the rapid growth in global energy demand has intensified interest in renewable energy sources, with solar energy standing out as a clean and sustainable option. Among various photovoltaic technologies, perovskite solar cells have garnered significant attention due to their high efficiency, low cost, simple fabrication processes, and material versatility. These devices are broadly categorized into conventional (n-i-p) and inverted (p-i-n) structures. The inverted perovskite solar cell configuration offers advantages such as simpler processing and better suitability for large-scale industrial production. In inverted perovskite solar cells, the hole transport layer (HTL) serves as a foundational substrate for the perovskite active layer, directly influencing both the morphology of the perovskite film and the charge carrier transport dynamics, thereby impacting overall device performance.
Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is widely adopted as the HTL in inverted perovskite solar cells owing to its straightforward processing, good electrical conductivity, uniform film formation, and cost-effectiveness. However, PEDOT:PSS suffers from inherent drawbacks, including acidity and hygroscopicity, primarily due to the sulfonic acid groups in the PSS chains. These issues can lead to corrosion of transparent electrodes, degradation of the perovskite layer, and reduced ion mobility, ultimately compromising the photovoltaic performance and long-term stability of perovskite solar cells. To address these challenges, various doping strategies have been explored to modulate the properties of PEDOT:PSS.
In this study, we introduce a novel approach by incorporating lysine, an alkaline amino acid, into the PEDOT:PSS solution. The amino groups in lysine neutralize the sulfonic acid groups in PSS, effectively reducing the acidity of PEDOT:PSS. This modification not only improves the interfacial properties but also enhances the quality of the perovskite film deposited atop the HTL. We systematically investigate the effects of lysine doping concentration on the photovoltaic parameters and stability of inverted perovskite solar cells. Our findings demonstrate that optimized lysine doping significantly boosts device efficiency and durability, offering a promising pathway for advancing perovskite solar cell technology.
The reaction between lysine and PEDOT:PSS involves acid-base neutralization, where the amino groups (R-NH2) of lysine interact with the sulfonic acid groups (R-SO3H) of PSS. This interaction can be represented by the following equation:
$$ \text{R-NH}_2 + \text{R-SO}_3\text{H} \rightarrow \text{R-NH}_3^+ + \text{R-SO}_3^- $$
This reaction mitigates the aggregation of sulfonic acid groups, thereby reducing the acidity and hygroscopicity of PEDOT:PSS. To quantify the acidity adjustment, we measured the pH of PEDOT:PSS solutions with varying lysine concentrations. The pH increased from 3.5 for pristine PEDOT:PSS to approximately 7.0 for the 20 wt% lysine-doped PEDOT:PSS solution, indicating effective neutralization.
We fabricated inverted perovskite solar cells with the structure ITO/L(x wt%)-PEDOT:PSS/MAPbI3−xClx/PCBM/BCP/Ag, where x represents the lysine doping concentration (0, 1, 5, 10, 20, and 30 wt%). The photovoltaic performance of these devices was evaluated under AM 1.5G illumination. The key parameters, including open-circuit voltage (VOC), short-circuit current density (JSC), fill factor (FF), and power conversion efficiency (PCE), are summarized in Table 1.
| Device | VOC (V) | JSC (mA cm−2) | FF (%) | PCE (%) |
|---|---|---|---|---|
| P-PEDOT:PSS | 0.94 | 20.81 | 80.31 | 15.71 |
| L(1 wt%)-PEDOT:PSS | 0.96 | 20.94 | 81.92 | 16.47 |
| L(5 wt%)-PEDOT:PSS | 1.00 | 21.28 | 81.05 | 17.25 |
| L(10 wt%)-PEDOT:PSS | 1.02 | 21.21 | 80.68 | 17.46 |
| L(20 wt%)-PEDOT:PSS | 1.04 | 21.35 | 79.47 | 17.65 |
| L(30 wt%)-PEDOT:PSS | 1.06 | 20.22 | 79.01 | 16.94 |
The device with 20 wt% lysine-doped PEDOT:PSS (L(20 wt%)-PEDOT:PSS) exhibited the highest PCE of 17.65%, with a VOC of 1.04 V and JSC of 21.35 mA cm−2. This represents a significant improvement over the reference device (P-PEDOT:PSS), which had a PCE of 15.71%, VOC of 0.94 V, and JSC of 20.81 mA cm−2. The enhancement in VOC is attributed to better energy-level alignment between the HTL and the perovskite layer, while the increased JSC suggests improved charge carrier extraction and transport. The external quantum efficiency (EQE) spectra and integrated current densities further corroborate these findings, with the L(20 wt%)-PEDOT:PSS device showing higher EQE values in the 350–500 nm range and an integrated JSC of 20.61 mA cm−2, consistent with the J-V measurements.
To elucidate the underlying mechanisms, we performed a series of material characterizations. X-ray photoelectron spectroscopy (XPS) analysis of the S 2p and O 1s orbitals revealed changes in the chemical states of PEDOT:PSS upon lysine doping. For the S 2p spectrum, the peaks corresponding to R-SO3H and R-SO3− groups shifted to higher binding energies in the L(20 wt%)-PEDOT:PSS film, indicating interactions between lysine and these functional groups. Similarly, the O 1s peaks associated with PEDOT and PSS components exhibited binding energy shifts, suggesting altered electronic structures due to lysine incorporation. These changes are consistent with the reorganization of sulfonic acid and sulfonate groups, reducing acidity and improving interfacial properties.
Ultraviolet photoelectron spectroscopy (UPS) measurements determined the work functions of the PEDOT:PSS films. The work function of P-PEDOT:PSS was –5.02 eV, while that of L(20 wt%)-PEDOT:PSS was –5.22 eV, which aligns more closely with the perovskite layer’s work function (–5.4 eV). This improved energy-level matching facilitates hole extraction and reduces energy losses, contributing to the higher VOC observed in lysine-doped devices.
Atomic force microscopy (AFM) showed that the L(20 wt%)-PEDOT:PSS film had a lower root-mean-square (RMS) roughness (1.61 nm) compared to P-PEDOT:PSS (2.81 nm). The smoother surface promotes the formation of a high-quality perovskite film with larger and more uniform grains. Scanning electron microscopy (SEM) images confirmed that the perovskite film deposited on L(20 wt%)-PEDOT:PSS had an average grain size of 0.49 μm, whereas that on P-PEDOT:PSS was 0.46 μm. The enhanced crystallinity was further verified by X-ray diffraction (XRD), which showed stronger diffraction peaks at 14.16° and 28.56° for the perovskite on lysine-doped PEDOT:PSS, indicating improved crystal quality.
Optical characterization through UV-Vis transmittance spectra revealed that both P-PEDOT:PSS and L(20 wt%)-PEDOT:PSS films exhibit high transparency (>80% across visible wavelengths). However, the L(20 wt%)-PEDOT:PSS film showed slightly higher transmittance in the 400–500 nm range, explaining the improved EQE in this region. Photoluminescence (PL) spectroscopy demonstrated a lower PL intensity for the perovskite film on L(20 wt%)-PEDOT:PSS, signifying more efficient charge extraction and reduced radiative recombination at the HTL/perovskite interface.
Electrical properties were investigated through conductivity measurements, dark J-V curves, electrochemical impedance spectroscopy (EIS), transient photocurrent (TPC), and transient photovoltage (TPV). The conductivity of ITO/L(20 wt%)-PEDOT:PSS/Ag devices was higher than that of ITO/P-PEDOT:PSS/Ag, indicating improved charge transport. The trap-filled limit voltage (VTFL) from hole-only devices was lower for L(20 wt%)-PEDOT:PSS (0.88 V) than for P-PEDOT:PSS (1.17 V). The trap density (Nt) was calculated using the formula:
$$ N_t = \frac{2 \varepsilon \varepsilon_0 V_{\text{TFL}}}{q L^2} $$
where ε is the dielectric constant of the perovskite, ε0 is the vacuum permittivity, q is the elementary charge, and L is the perovskite film thickness. The Nt for L(20 wt%)-PEDOT:PSS was 1.92 × 1016 cm−3, lower than that for P-PEDOT:PSS (2.55 × 1016 cm−3), confirming reduced trap-assisted recombination. EIS analysis yielded lower series resistance (Rs = 20 Ω) and transport resistance (Rtra = 43 kΩ) for L(20 wt%)-PEDOT:PSS devices compared to P-PEDOT:PSS (Rs = 33 Ω, Rtra = 57 kΩ), indicating enhanced charge transport. TPV and TPC measurements showed longer carrier lifetimes (13.60 μs) and shorter extraction times (0.47 μs) for L(20 wt%)-PEDOT:PSS devices, reflecting suppressed recombination and improved charge collection.
The stability of unencapsulated devices was assessed under both nitrogen and air environments. In N2 atmosphere (25°C), the L(20 wt%)-PEDOT:PSS device retained 86.54% of its initial PCE after 2160 hours, while the P-PEDOT:PSS device degraded to 77.74%. In air (25°C, 15% relative humidity), the L(20 wt%)-PEDOT:PSS device maintained 85.88% of its initial PCE after 360 hours, compared to 68.30% for the reference. The enhanced stability is attributed to the reduced acidity of PEDOT:PSS and the improved quality of the perovskite film, which minimizes moisture ingress and degradation.
In conclusion, lysine doping effectively optimizes PEDOT:PSS for use as an HTL in inverted perovskite solar cells. By neutralizing acidity and improving film properties, lysine enhances both photovoltaic performance and long-term stability. This work provides a simple and efficient strategy for advancing perovskite solar cell technology, highlighting the potential of amino acid additives in material engineering for sustainable energy applications.
The development of high-performance and stable perovskite solar cells is crucial for the widespread adoption of solar energy. Our approach using lysine-doped PEDOT:PSS addresses key challenges in inverted perovskite solar cells, paving the way for more efficient and durable photovoltaic devices. Future work could explore other biocompatible additives and their effects on various perovskite compositions to further push the boundaries of perovskite solar cell performance.