The surge in IoT devices has spurred innovation in indoor light harvesting technologies to sustain wireless sensor networks (WSNs). High-efficiency flexible amorphous silicon (a-Si:H) thin-film photovoltaic modules based on polyimide (PI) substrates achieve a breakthrough aperture efficiency of 9.1% under low-light indoor conditions. The Millennial Steady-State Light Degradation Aging Test Chamber provides corresponding environmental simulation and accelerated testing for thin-film PV module quality control. After 1000 hours of aging under high light intensity, maximum power degradation remains below 10%, offering a novel solution for self-powered systems in electronic devices.

PECVD Process

a-Si:H layers were deposited at 190°C using a standard PECVD system. The critical parameter hydrogen dilution ratio (R = H₂/SiH₄) varied between 2 and 40. Research indicates that high-performance amorphous silicon material is obtained when the R value is slightly below the threshold for microcrystalline silicon phase formation. Variations in R significantly affect film stress: at R=2, tensile stress of +1.8 GPa is observed, while at R=5, it shifts to compressive stress of -4.33 GPa, correlating with SiH₂/SiH bond concentration.

(a) Flexible silicon module design; (b) Fabricated structure on polyimide foil

Cell Structure

The p-i-n cell employs in-situ doping with trimethylborane (TMB) and PH₃. The back contact material was optimized with a focus on comparing SnO₂:F (TCO) and molybdenum (Mo). The study revealed that Mo as the back contact exhibits superior Schottky contact performance with p-type a-Si:H compared to SnO₂:F, yielding an approximately 20 mV higher built-in voltage (Vbi).

Module Structure

(a) Component layout; (b) Photograph of flexible polyimide substrate-based component

The final optimized module structure consists of sequentially deposited layers on a polyimide substrate: a Mo back contact layer, an a-Si:H p-i-n layer, and a ZnO:Al (AZO) transparent front contact layer. Multiple cells are monolithic integrated on the substrate to form a 6×5 cm² series-connected module.

Indoor photovoltaic module performance

Flexible Photovoltaic Module I-V Curve and Electrical Parameters Table (300 lx, F12 Spectrum)

Tested under simulated indoor conditions (F12 fluorescent spectrum, 300 lx), the 5 cm x 6 cm flexible module achieved an overall area efficiency of 8.7% and a high effective area efficiency of 9.1%. This represents a significant improvement over previous results (6%), attributable to optimizations in the a-Si:H deposition process (particularly the hydrogen dilution ratio R).

Efficiency of flexible photovoltaic modules with irradiance: (a) 100–1000 lx, (b) 1000–5000 lx

The modules exhibit stable efficiency across a broad irradiance range from 100 lx to 5000 lx, indicating no significant performance degradation under low-light conditions.

Reliability and mechanical properties

Maximum Power Decay of Components with Different Hydrogen Silane Ratios (R) After 1000 Hours at 3000 lx

Photodegradation: Optimizing the hydrogen dilution ratio R effectively suppresses the Staebler-Wronski effect. After 1000 hours of aging under high light intensity (3000 lx, F12 spectrum including UV), the maximum power decay remains below 10%.

(a) Flexure test setup and (b) Performance changes after 800 flexures at a 1.9 cm radius

Mechanical Flexibility: The module exhibits outstanding mechanical flexibility. After over 800 repeated flexures at a bending radius of approximately 2 cm—significantly smaller than the industry standard (5 cm)—its photovoltaic performance showed no noticeable degradation.

A polyimide-based flexible a-Si:H photovoltaic module was successfully developed. Through optimized PECVD low-temperature deposition and Mo contact, it achieved a 9.1% aperture efficiency under 300 lx indoor illumination. The module combines broad irradiance adaptability, bending resistance, and low light degradation, providing an efficient energy solution for wireless sensor networks.

Millennial Steady-State Light Degradation Aging Test Chamber

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The Millennial Steady-State Light Degradation Test Chamber employs metal halide lamps capable of simulating full-spectrum light sources to reproduce destructive light waves present in various environments. It provides corresponding environmental simulation and accelerated testing for photovoltaic module product development and quality control.

✔ Light exposure testing complies with stability testing requirements specified in clauses MQT08 and MQT09 of IEC 61215

✔ Irradiation area up to 6040×2560mm

✔ Illuminance levels compliant with BBA standards

The Millennial Steady-State Photodegradation Test Chamber evaluates the long-term stability of photovoltaic modules under sustained illumination. By measuring performance degradation during high-intensity exposure, it validates the effectiveness of process optimization.