Ferroelectric materials with bulk photovoltaic effects sheds light on the next generation portable, sustainable and cost-effective optoelectronic devices. However, due to severe polarization deterioration caused by leakage current of photoexcited carriers, most of ferroelectrics are merely capable of absorbing 8-20% of visible-light spectra. Ferroelectrics with the narrow bandgap (<2.0 eV) are still scarce, hindering their practical applications. 

In a study published in Nature Communications, the research group led by Prof. LUO Junhua and Prof. SUN Zhihua from Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences reported a visible-light-absorbing biaxial ferroelectric in the family of two-dimensional hybrid perovskites, (isopentylammonium)₂(ethylammonium)₂Pb₃I₁₀ (PEPI).  

The researchers synthesized PEPI from the concentrated aqueous HI solutions, and grew the large-size crystals by the temperature cooling method with the temperature lowing rate of 1 K/day. The polarity switching of PEPI was solidly convinced by polarization-electric field hysteresis loop. 

They found that PEPI exhibits conclusive ferroelectric polarization (~5.2 μC/cm²) and a quite narrow optical bandgap of ~1.8 eV. This is the narrowest direct bandgap for the molecular ferroelectrics, transcending the constraints between bulk ferroelectricity and narrow optical bandgap. 

Unprecedented in-plane bulk photovoltaic effects are achieved inside its bc-plane. Deep analysis revealed that such in-plane bulk photovoltaic effects stem from the almost isotropy structural and charge-density along the crystallographic (100) plane. This exceptional in-plane bulk photovoltaic effect was reported for the first time.

Besides, the researchers developed PEPI as broadband self-driven photoelectric detector in the wide range of 365-670 nm by combining its narrow bandgap and exceptional in-plane bulk photovoltaic effects.  

Specifically, PEPI exhibits excellent photoelectric detection performance, including large switching ratio (>10⁵), high-density photocurrent (~1.5 μA/cm²) and ultrafast transient responding time (~2.7 ns).

This study provides a deep understanding of the origin of ferroelectric self-powered photodetection effects, and will promote the optoelectronic application potentials.

Broadband self-driven photoelectric detection of the visible-light-absorbing biaxial ferroelectric. (Image by Prof. LUO's group)