High-quality textured SnSe thin films for self-powered, rapid-response photothermoelectric application

Highlights

• High-quality (400)-textured SnSe polycrystalline thin films are prepared by chemical vapor deposition.

• The SnSe films possess a dimensionless ZT of 0.15 and power factor of 0.322 mW m⁻¹ K⁻² at 550 K.

• Self-powered SnSe-based photothermoelectric detectors are demonstrated.

• The detectors exhibit higher responsivity and speed comparing with single crystal.

Abstract

Tin selenide (SnSe) has attracted great attention in thermoelectrics (TE) thanks to its record high figure of merit (ZT) discovered in single crystal (SC). However, the practical application of SnSe SC is limited due to its extreme growth condition. With the development of flexible integrated electronics, using flexible polycrystalline TE films to substitute rigid SC becomes a main trend. Nevertheless, large resistances of TE films restrict their application as power devices. Here, self-powered SnSe-based photothermoelectric (PTE) detectors are demonstrated. The PTE detectors are sensitive to wide-spectrum and exhibit higher responsivity and respnse speed comparing with SC, due to the small heating volume, small conductive heat flux and large dissipation content of thin films. High-quality (400)-textured SnSe polycrystalline thin films are one-step synthesized by chemical vapor deposition. In-situ in-plane TE measurement shows that the obtained SnSe films possess a dimensionless ZT of 0.15 and a power factor of 0.322 mW m⁻¹ K⁻² at 550 K, showing SC level TE performance at low temperatures.

Introduction

Thermoelectric (TE) materials enable direct energy conversion between heat and electricity [1], offering diverse applications such as power generation [2] and TE refrigeration [3]. The performance of TE materials is evaluated by the dimensionless figure of merit: ZT = (S²σ/κ)T, where S, σ, κ, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively [4]. The recorded ZT = 2.6 observed in undoped SnSe single crystal (SC) has attracted great attention. SnSe is a two-dimensional p-type semiconductor with earth abundant elements, which is economy efficient and non-toxic. In addition, Sn–Se compounds only have three phases, α-SnSe, β-SnSe and SnSe₂, thus it is easy to acquire pure SnSe phase [5]. However, the practical use of SnSe SC is limited by high cost and extreme growth condition. Polycrystallizaton and nanocrystallization are two of the most effective ZT tuning strategies [[6], [7], [8], [9], [10], [11]], as the increased boundaries and surface in polycrystalline films give rise to phonons scattering interfaces that reduce the thermal conductivity. Nevertheless, SnSe polycrystals cannot achieve the SC level TE performance. The compromise between high ZT and low cost significantly impedes the practical applications of SnSe.

With the development of flexible integrated electronics, using polycrystalline films to substitute expensive rigid SC becomes a main trend. SnSe thin films are potential to realize both high-performance and practicability [11,12]. Devices based on nanoscale SnSe show significant potentials in the fields including photodetector [12], capacitor anode [13], sodium-ion battery [14] and TE device [15] due to their unique physical and chemical properties.

Synthesis of textured SnSe thin films is of great importance to acquire high TE performance in SnSe polycrystals, due to the strong anisotropic ZT of SnSe SC [1]. For example, SnSe film with (111) orientation prepared by thermal evaporation [16] showed an ultra-low in-plane thermal conductivity, while its maximum ZT was merely 0.042 at 550 K. Because of the excellent ZT of SnSe SC in b-c plane, its (400) textured film possessed high TE performance [17]. Typically, high (400) textured SnSe film with high power factor (defined as PFS²σ) was synthesized through solution method and post-annealing. However, the solution method needed repeated spin coating and heating processes which restricted the preparation efficiency for scalable applications. Recently, SnSe nanosheets have been synthesized by chemical vapor deposition (CVD) [[18], [19], [20]]. However, the scalable growth of high quality SnSe is still a major obstacle to develop polycrystalline SnSe thin films.

Because resistances of thin films are larger than those of bulks, the power of TE generators based on thin films is lower. It is required to develop other applications which can take full advantage of TE thin films. The photothermoelectric (PTE) effect enables the conversion of temperature difference induced by non-uniform illumination to electric voltage. PTE detector based on SC (SrTiO₃) [21], carbon nanotubes [22], organic film (PEDOT: PSS) [23] and nanophotonics [24] have been reported. It is reported that the localized heating and cooling in TE thin films are 10⁵ orders of magnitude faster than in bulks [15]. Consequently, the response and recovery of TE thin films to thermal excitation are faster. A simplified simulation of one side heating demonstrated that a larger temperature gradient would be constructed in thinner samples, resulting in larger TE voltages. The required length for acquiring the same temperature difference is shorter in thin films, indicating reduced space requirement of thin films. With the above-mentioned three advantages (rapid response, high TE voltage and reduced space requirement), TE thin films are expected to be superior in rapid response PTE detectors [22].

Herein, we report self-powered SnSe-based PTE detectors, which are sensitive to wide spectrum and exhibit higher responsivity and speed comparing with single crystal SnSe, due to the small heating volume, small conductive heat flux and large dissipation content of thin films. SnSe polycrystalline thin films with strong (400) texture prepared by one-step CVD. The obtained SnSe films possess a dimensionless ZT of 0.15 and a power factor of 0.322 mW m⁻¹ K⁻² at 550 K.