1. Introduction
Advanced energy materials play a critical role in the continued development of photovoltaic technologies, particularly crystalline silicon solar cells, which continue to dominate the global renewable energy market [1]. Despite their high degree of technological maturity, the performance of silicon photovoltaic devices remains fundamentally limited by non-radiative recombination losses occurring at surfaces, interfaces, and within defect-rich bulk regions. These recombination pathways directly reduce the achievable open-circuit voltage and, consequently, the overall power conversion efficiency.
Among the various surface engineering strategies, hydrogenated silicon nitride (SiNₓ:H) has emerged as a key passivation layer due to its dual functionality: chemical passivation of dangling bonds and field-effect passivation arising from fixed charges within the dielectric layer [2]. Nevertheless, further improvements in passivation quality require characterization techniques capable of resolving recombination dynamics with both temporal and spectral selectivity.
Spectrally selective open-circuit voltage decay (OCVD) analysis represents an advanced transient diagnostic method that is particularly well suited for probing charge carrier recombination in silicon photovoltaic energy materials [3,4]. By exploiting the wavelength-dependent absorption coefficient of silicon, this technique enables depth-resolved investigation of recombination processes, offering direct insight into the relative contributions of surface, emitter, and bulk regions.
2. Methods
A spectrally selective OCVD system was developed to investigate recombination dynamics in monocrystalline (c-Si) and multicrystalline (mc-Si) silicon photovoltaic energy materials. Industrial p-type Al-BSF solar cells fabricated using standard industrial processes were examined in both full-area devices (10 × 10 cm²) and mini-cell configurations (2 × 2 cm²) in order to capture global as well as localized recombination behavior.
Pulsed optical excitation was provided by light-emitting diodes operating at discrete wavelengths of 458, 524, 633, and 864 nm. These wavelengths were selected to enable depth-resolved carrier generation, ranging from near-surface excitation to bulk-dominated absorption, in accordance with the spectral absorption characteristics of silicon. Pulse generation, timing, and synchronization were controlled using an Arduino-based system interfaced with MATLAB/Simulink, providing microsecond-scale temporal resolution.
To assess the impact of passivation on recombination dynamics, a low-temperature thermal annealing treatment at 115 °C for 3 minutes was applied. The resulting OCVD transients were analyzed using exponential fitting procedures, allowing extraction of characteristic decay times corresponding to the effective minority carrier lifetime under different excitation conditions.
3. Results
The spectrally selective OCVD analysis reveals pronounced differences in recombination dynamics between monocrystalline and multicrystalline silicon solar cells. Under broad-area, high-injection excitation, full-size c-Si devices exhibited voltage decay times exceeding 15 ms, indicative of superior crystalline quality and reduced bulk defect density. In contrast, localized excitation in mini-cell configurations resulted in decay times in the microsecond regime, highlighting the strong influence of injection level and probing scale on recombination kinetics.
Short-wavelength excitation at 458 nm, which predominantly probes the near-surface and emitter regions, produced a marked increase in both open-circuit voltage and decay time following thermal annealing. This behavior confirms the effectiveness of hydrogen-related defect passivation in reducing surface recombination velocity. Conversely, near-infrared excitation at 864 nm, which is primarily sensitive to bulk recombination processes, showed minimal change after annealing, particularly in multicrystalline silicon. This observation indicates the persistence of bulk defect-related recombination associated with grain boundaries and extended defects.
Excitation at intermediate wavelengths exhibited coupled surface–bulk recombination behavior, further validating the depth-resolved capability of the spectrally selective OCVD approach.
4. Conclusions
This work demonstrates that spectrally selective OCVD analysis is a powerful, non-destructive characterization technique for investigating recombination dynamics in silicon photovoltaic energy materials. By enabling clear separation of surface and bulk recombination mechanisms, the method provides direct insight into the effectiveness of passivation treatments. The results confirm that low-temperature thermal annealing primarily enhances surface passivation, with a more pronounced impact on monocrystalline silicon compared to multicrystalline material. Through the integration of wavelength-resolved excitation and high temporal resolution, the proposed approach offers a robust pathway for guiding material optimization and passivation engineering in next-generation high-efficiency silicon photovoltaic technologies.
References
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[3] J. Vollbrecht et V. V. Brus, « Effects of Recombination Order on Open-Circuit Voltage Decay Measurements of Organic and Perovskite Solar Cells », Energies, vol. 14, no 16, p. 4800, août 2021, doi: 10.3390/en14164800.
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