Dual‑Functional a‑Si:H Solar Cells: Optimized Photonic Design for High‑Efficiency Power Generation and Vivid RGB Display

Abstract

Colored solar cells (SCs) are rapidly emerging as key components for building‑integrated photovoltaics (BIPVs), yet most designs emphasize aesthetics at the expense of electrical performance. In this work we present a planar amorphous silicon (a‑Si:H) SC that delivers both a high‑purity red, green, or blue display and a respectable power‑conversion efficiency. The color quality is achieved through a thin‑film photonic stack that combines a distributed Bragg reflector (DBR) with dual‑layer antireflection coatings (ARC) and a TiO₂ buffer. Comprehensive optoelectronic simulations—solving Maxwell’s equations via RCWA and COMSOL, coupled with Poisson and carrier‑transport equations—predict efficiencies of 4.88%, 5.58%, and 6.54% for the red, green, and blue cells, respectively. Using the SCs to render the Soochow University logo demonstrates robust wide‑angle pattern display, paving the way for aesthetically appealing, zero‑energy facades.

Background

Urban energy demand has accelerated the development of BIPVs that can simultaneously generate electricity and enhance façade aesthetics. Conventional SCs, however, appear dull or black, limiting their architectural appeal. Recent progress in color‑controlled SCs shows promise, yet many approaches sacrifice significant efficiency to achieve vivid color or produce a limited color gamut. Photonic strategies—such as Fabry‑Perot filtering, selective transparent conducting photonic crystals (STCPC), and DBRs—can tailor reflected spectra, but often introduce side‑band reflections that degrade color purity and reduce the usable solar spectrum. Moreover, most studies focus solely on optical response without addressing how the engineered layers influence carrier generation, transport, and collection within the semiconductor junctions. A holistic design that simultaneously optimizes both optics and electronics is essential for practical, high‑performance colored SCs.

Methods

The optical response of each multilayer stack is calculated by solving Maxwell’s equations with rigorous coupled‑wave analysis (RCWA) and COMSOL Multiphysics, yielding layer‑specific reflection and absorption spectra. Carrier dynamics are modeled by coupling the Poisson equation with drift–diffusion equations for electrons and holes, incorporating surface recombination and realistic contact boundary conditions. The reflection spectrum is mapped onto the CIE 1931 chromaticity diagram to determine perceived color. All material optical constants are taken from Palik’s database; a 5‑nm mesh and perfectly matched layers are used in the optical simulations. Electrical simulations are performed using the same carrier‑transport framework described in our prior work.

Results and Discussion

Figure 1 shows the device stack: a 500‑nm a‑Si:H active layer (30‑nm n‑type, 50‑nm p‑type), 100‑nm ZnO electron‑transport layer, 20‑nm ITO hole‑transport layer, 55‑nm TiO₂ buffer, and a 6‑pair ZnS/ZnO DBR. The DBR’s reflectivity (R) and bandwidth (Δλ₀) are governed by R = [(n₀n₂²ᴺ – nₛn₁²ᴺ)/(n₀n₂²ᴺ + nₛn₁²ᴺ)]² and Δλ₀ = (4λ₀/π) arcsin[(n₂ – n₁)/(n₂ + n₁)]. A modest refractive‑index contrast and a larger number of pairs yield high color saturation while keeping the bandwidth narrow.

Figure 2a displays the DBR reflection spectra for the target RGB wavelengths (λ₀ = 625, 520, 445 nm) with peak reflectivities of 74.82%, 72.1%, and 76.31%. The subsequent integration of the SC shifts the peaks slightly (R ≈ 87–82%) but introduces side‑band reflections that can dilute color purity.

Inserting dual‑layer ARCs (MgF₂ and SnO₂) atop the DBR and a TiO₂ buffer suppresses side‑band reflections and narrows the resonant bandwidth, as shown in Figure 2e. The resulting CIE coordinates (Figure 2f) lie within 52.7% of the sRGB gamut, with ΔE values of 16.8 (red), 47.6 (green), and 41.7 (blue). This demonstrates that the photonic design achieves a color space comparable to commercial displays while maintaining high reflectivity for brightness.

Optoelectronic simulations reveal that the absorption (A) and external quantum efficiency (EQE) of each RGB cell exhibit dips at the reflected wavelengths, as expected for color‑display operation. The EQE remains above 80% across most of the solar spectrum, with a slight reduction due to recombination in the thin active layer. Figure 3d shows the J–V characteristics: the red, green, and blue cells achieve efficiencies of 4.88%, 5.58%, and 6.54% respectively, compared to 7.59% for a conventional a‑Si:H cell with a 100‑nm SiO₂ ARC. The open‑circuit voltage and fill factor are largely unaffected by the photonic layers, confirming that the primary trade‑off is the intentional absorption loss for color.

Tolerance studies indicate that random thickness variations of ±5 % in the DBR and ARC layers cause color shifts (ΔE) of 1.9–11.2 % for red, 1.3–15.7 % for green, and 0.5–2.9 % for blue, while the efficiency variation remains within ±0.4 %. Incident‑angle analysis (Figure 5) shows that blue and green colors retain their hue up to 70°, whereas red shifts toward green beyond 70°. Despite some color drift, the Soochow University logo remains legible across a wide range of angles, indicating robust pattern display performance.

Fabrication of the planar RGB SCs follows established commercial processes: PECVD deposition of the a‑Si:H layers on a TCO substrate, sputtering of the ZnO/ITO contacts, magnetron sputtering of the TiO₂ buffer, DBR, and dual‑layer ARC. This mature workflow ensures scalability and cost‑effectiveness for large‑area BIPV applications.

Conclusions

We have engineered planar a‑Si:H solar cells that simultaneously deliver high‑purity RGB display and respectable power conversion. By integrating a DBR, dual‑layer ARC, and TiO₂ buffer, the cells achieve color coordinates within the sRGB gamut and efficiencies up to 6.54% (blue). The design is robust against realistic fabrication tolerances and maintains clear pattern display under oblique illumination. These results demonstrate a viable pathway to aesthetically pleasing, energy‑producing façades, and the underlying design principles can be extended to other photovoltaic technologies.

Abbreviations

A:

Absorption

ARCs:

Antireflection coatings

BIPVs:

Building‑integrated photovoltaics

CAL:

Color‑adjusting layer

CIE:

Commission Internationale de L’Eclairage

DBR:

Distributed Bragg reflector

Eff:

Photoconversion efficiency

EQE:

External‑quantum efficiency

FF:

Fill factor

F-P:

Fabry‑Perot

Jsc:

Short‑circuit current density

J-V:

Current‑voltage

NTSC:

National television system commission

PECVD:

Plasma‑enhanced chemical vapor deposition

R:

Reflectivity

RCWA:

Rigorous coupled‑wave analysis

RGB:

Red‑green‑blue

SCs:

Solar cells

sRGB:

Standard red‑green‑blue

STCPC:

Selectively transparent and conducting photonic crystal

TCO:

Transparent conducting oxide

Voc:

Open‑circuit voltage