Controlled Hydrothermal Synthesis of ZnO Nanocrystals for Enhanced Inverted Polymer Solar Cells

Background

Organic bulk‑heterojunction (BHJ) solar cells that employ n‑type inorganic metal‑oxide nanostructures for electron transport have gained traction due to their enhanced stability, low‑cost fabrication, and compatibility with solution processing [1–4]. ZnO nanocrystals stand out for their high electron mobility, optical transparency across the visible spectrum, and facile morphology control, making them ideal electron‑transport materials [5–7]. While prior studies have shown improved device metrics through nanorods, nanowalls, and nanotetrapods, the precise influence of morphology on photovoltaic performance remains debated.

In this work, we synthesize ZnO nanocrystals of distinct shapes using a straightforward hydrothermal process. By varying precursor concentration, temperature, and surfactant, we achieve nanorods, nanotetrapods, nanoflowers, and nanocubes. These nanostructures are then integrated into the active layer of inverted BHJ solar cells, and the resulting J–V characteristics reveal a clear morphology‑performance relationship. Our findings highlight that large surface area, optimal inter‑nanocrystal spacing for active‑layer infiltration, and continuous electron‑transport pathways are essential for maximizing device efficiency.

Methods

Deposition of ZnO Seed Layer

To nucleate ZnO growth on mismatched substrates, we deposit a seed layer via dip‑coating, following the protocol detailed in our previous publication [8].

Hydrothermal Growth of ZnO Nanocrystals

The ITO substrate, coated with the ZnO seed layer, is positioned upside‑down in a reaction vessel containing 40 ml of an aqueous mixture of zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) and hexamethylenetetramine (HMTA) at equal molarity. A chosen surfactant—either polyethylenimine (PEI) or sodium citrate—is added to tailor growth. The sealed vessel is then maintained at a constant temperature for a set duration. After growth, the ZnO nanocrystals are harvested, rinsed with deionized water, and dried at ambient conditions. For a detailed procedure, see reference [8].

Fabrication of the Solar Cells [9]

We first spin‑coat a thin PCBM layer (20 mg ml⁻¹ in dichloromethane, 1000 rpm, 30 s) onto the ZnO nanocrystals. This intermediate layer enhances infiltration of the active polymer into the nanocrystal matrix. Next, a P3HT:PCBM blend (10 mg ml⁻¹ P3HT, 16 mg ml⁻¹ PCBM in chlorobenzene) is spin‑coated (1000 rpm, 30 s) and baked at 225 °C for 1 min to remove solvent and promote polymer penetration. A PEDOT:PSS hole‑transport layer (4000 rpm, 40 s) is then applied and annealed at 130 °C for 15 min, yielding ~35 nm thickness. Finally, a 100 nm Al cathode is thermally evaporated, and the device is annealed at 130 °C for 20 min under nitrogen. The complete architecture is illustrated in Fig. 1.

Device architecture of the organic bulk heterojunction solar cell

Characterization

Field‑emission scanning electron microscopy (FE‑SEM; Hitachi FE‑S4800) examines the ZnO nanocrystal morphology. Photovoltaic performance is evaluated using a Keithley 2400 source‑meter under AM 1.5G illumination (100 mW cm⁻²). All measurements are performed in triplicate to ensure reproducibility.

Results and Discussion

By tuning the hydrothermal parameters, we successfully produced ZnO nanorods, nanotetrapods, nanoflowers, and nanocubes. The most compelling evidence of morphological control comes from the patterned ZnO nanorod array fabricated on a TiO₂ ring template derived from a self‑assembled polystyrene monolayer [11]. Figures 2a and 2b display the top view and 45° tilt of this array, grown from a 0.05 M Zn(NO₃)₂·6H₂O/HMTA solution at 80 °C for 3 h. The rods exhibit a uniform 380 nm diameter and a 200 nm spacing, providing a short, continuous electron‑transport path and ample space for active‑layer infiltration.

The nanotetrapod array (Fig. 2c,d) was synthesized under similar conditions (0.025 M, 50 °C, 6 h) but with 0.1 ml PEI added, which promotes axial growth while suppressing radial expansion [12]. Each tetrapod, composed of 3–7 nanorods, offers a significantly larger surface area than the single‑rod array.

a Top view and b 45° tilt of the ZnO nanorod array. c Top view and d 45° tilt of the ZnO nanotetrapod array

Figure 3 presents SEM images of the nanoflower and nanocube morphologies, fabricated via a two‑step hydrothermal route. Initial nanorods (0.025 M, 85 °C, 3 h) serve as templates for secondary growth. Nanoflowers are formed in a 0.0075 M Zn(NO₃)₂·6H₂O / 0.0075 M sodium citrate solution at 95 °C for 12 h, producing unordered, densely packed “petal” structures with ~30 nm inter‑petal gaps. Nanocubes emerge in a 0.0075 M Zn(NO₃)₂·6H₂O / 0.015 M sodium citrate solution at 95 °C for 6 h, yielding ~150 nm cubic particles that are isolated from one another.

Top view of a the ZnO nanoflowers and b the ZnO nanocubes. The inset of Fig. 3a is a zoom‑in of a single nanoflower.

Four distinct ZnO nanocrystal morphologies were incorporated into identical inverted BHJ devices (Fig. 1). For each morphology, five devices were fabricated on the same ITO substrate; only the top three devices (within 3% PCE deviation) were considered for reporting. The resulting J–V curves are shown in Fig. 4, and key performance metrics are summarized in Table 1.

J–V characteristics of the organic bulk heterojunction solar cells with different ZnO nanostructures

The nanotetrapod device achieved the highest PCE of 3.96%, followed closely by the nanorod (3.71%) and nanoflower (3.69%) devices, while the nanocube configuration yielded 3.25%. The superior performance of the nanotetrapod is attributable to its extensive surface area (~300 nm inter‑crystal spacing) that facilitates both light absorption and efficient electron extraction. In contrast, the nanorod array, with lower surface area, exhibits reduced dye loading and consequently a lower short‑circuit current density (J_SC). The nanoflower structure, despite its high surface area, suffers from tight inter‑petal spacing (<50 nm), hindering active‑layer infiltration and resulting in a slightly lower J_SC. Finally, the isolated nanocube morphology interrupts electron transport pathways, further diminishing J_SC and overall efficiency.

Conclusions

We have demonstrated that a simple, scalable hydrothermal process can produce ZnO nanocrystals with controlled morphologies—nanorods, nanotetrapods, nanoflowers, and nanocubes—by tuning precursor concentration, temperature, and surfactant. When integrated as the electron‑transport layer in inverted BHJ solar cells, morphology critically influences device performance. Optimal results arise from a large, accessible surface area, sufficient inter‑nanocrystal spacing for active‑layer infiltration, and an uninterrupted electron‑transport network.

Abbreviations

HMTA:

Hexamethylenetetramine

ITO:

Indium‑tin‑oxide

J_SC:

Short‑circuit current density

J-V:

Current density–voltage

P3HT:

Poly(3‑hexylthiophene)

PCBM:

[6]-Phenyl‑C₆₁-butyric acid methyl ester

PCE:

Power‑conversion efficiency

PEDOT:PSS:

Poly(3,4‑ethylenedioxythiophene):poly(styrene sulfonate)

PEI:

Polyethylenimine

SEM:

Field emission scanning electron microscopy

V_OC:

Open‑circuit voltage