Hall effect and transient surface photovoltage (SPV) study of Cu₃BiS₃ thin films

Efecto Hall y estudio de fotovoltaje superficial transiente (SPV) en películas delgadas de Cu₃BiS₃

Efeito Hall e estudo de foto voltagem superficial transiente (SPV) em películas delgadas de Cu₃BiS₃

F. Mesa¹, A. Dussan², B. A. Paez-Sierra³, H. Rodriguez-Hernandez³

Edited by Beynor Paez & Alberto Acosta

¹Unidad de Estudios Universitarios, Universidad del Rosario, Bogotá, Colombia.
²Departamento de Física, Grupo de Materiales Nanoestructurados, Universidad Nacional de Colombia - Bogotá, Colombia.
³Thin films and Nanophotonics Group, Pontificia Universidad Javeriana, Bogotá, Colombia.

Funding: Pontificia Universidad Javeriana; Banco de la República; Fundación para la Promoción de la Investigación y la Tecnología (FPIT).
Electronic supplementary material: N/A

Received: 22-11-2013 Accepted: 25-03-2014 Published on line: 10-04-2014

Para citar este artículo / To cite this article

Mesa F, Dussan A, Paez-Sierra BA, Rodriguez-Hernandez H (2014) Hall effect and transient surface photovoltage (SPV) study of Cu₃BiS₃ thin films. Universitas Scientiarum 19(2): 99-105 doi: 10.11144/Javeriana.SC19-2.ehef

Abstract

Here, we present the electrical properties of the compound Cu₃BiS₃ deposited by co-evaporation. This new compound may have the properties necessary to be used as an absorbent layer in solar cells. The samples were characterized by Hall effect and transient surface photovoltage (SPV) measurements. Using Hall effect measurements, we found that the concentration of n charge carriers is in the order of 10¹⁶ cm³ irrespective of the Cu/Bi mass ratio. We also found that the mobility of this compound (μ in the order of 4 cm² V^-1s^-1) varies according to the transport mechanisms that govern it and are dependent on temperature. Based on the SPV, we found a high density of surface defects, which can be passivated by superimposing a buffer layer over the Cu₃BiS₃ compound.

Keywords: Electric transport; Cu₃BiS₃; transient surface photovoltage; defects.

Resumen

Se presentan las propiedades eléctricas del compuesto Cu₃BiS₃ depositado por co-evaporación. Este es un nuevo compuesto que puede tener propiedades adecuadas para ser utilizado como capa absorbente en celdas solares. Las muestras fueron caracterizadas a través de medidas de efecto Hall y fotovoltaje superficial transiente (SPV). A través de medidas de efecto Hall se encontró que la concentración de portadores de carga n es del orden de 10¹⁶ cm^-3 independiente de la relación de masas de Cu/Bi. También se encontró que la movilidad de este compuesto (μ del orden de 4 cm²V ^-1s^-1) varía de acuerdo con los mecanismos de transporte que la gobiernan en dependencia con la temperatura. A partir de las medidas de SPV se encontró alta densidad de defectos superficiales, defectos que son pasivados al superponer una capa buffer sobre el compuesto Cu₃BiS₃.

Palabras clave: Transporte eléctrico; Cu₃BiS₃; fotovoltaje superficial transiente; defectos.

Resumo

Apresentam-se as propriedades elétricas do composto Cu₃BiS₃ depositado por co-evaporação. É um composto novo que pode ter as propriedades adequadas para ser utilizado como capa absorvente em células solares. As amostras foram caracterizadas através de medidas do efeito Hall e foto voltagem superficial transiente (SPV). Através de medidas do efeito Hall se encontro que a concentração de portadores de carga n é da ordem de 10¹⁶ cm³ independentemente da relação de massas de Cu/Bi. Também se encontrou que a mobilidade des composto (μ da ordem de 4 cm²V ^-1s^-1) varia de acordo com os mecanismos de transporte que a governam em dependência com a temperatura. Partindo das medidas de SPV se encontrou uma alta densidade de defeitos superficiais, defeitos que são passivados a sobrepor uma capa buffer sobre o composto Cu₃BiS₃.

Palavras-chave: Transporte elétrico; Cu₃BiS₃; foto voltagem superficial transiente; defeitos.

Introduction

I-III-VI₂ chalcopyrite compounds are direct gap semiconductors and have adequate properties for use in the fabrication of single-junction cells or tandem cells (Repins et al. 2008, Kayes et al. 2011, Choi et al. 2012, Seo et al. 2012). Nevertheless, the maximum conversion efficiency reported up to date for single-junction thin film technology solar cells was obtained by fabricating the device using Cu-III-VI2 group compounds as the absorbing layer (Contreras et al. 2009, Green et al. 2012). These solar modules which are based on a thin film of Cu (InGa)Se₂ (CIGS) offer greater stability and price competitiveness for production processes. By contrast, Cu-V-VI2 based compounds are characterized by having a forbidden bandgap that varies between values from 1.15 to 2.73 eV, which makes this kind of compounds potentially useful and attractive candidates for photovoltaic applications (Bar et al. 2008, Kawamura et al. 2009, Meiss et al. 2011, Mõmg et al. 2011, Tadjarodi et al. 2012).

For these reasons, Cu₃BiS₃ compound has taken great importance in optoelectronic applications. This compound grows in a "wittichenite" type orthorhombic crystalline lattice, where this phase is obtained at 300°C temperatures (Naiding 1994, Razmara et al. 1997). Several techniques have been used for the preparation of Cu₃BiS₃ samples, such as evaporation (Mesa et al. 2009, Colombara et al. 2012), via PEG (polyethylene glycol) (Yan et al. 2012), reactive sputtering (Gerein et al. 2006), and CBD (Chemical Bath Deposition) (Estrella et al. 2003). Nevertheless, it is reported that with the use of CBD there is growth of a metallic Bi thin film which is deposited by thermal evaporation on CuS obtained by CBD. As a result of this study the layers presented p-type conductivity (∼0.03 Ω^-1cm^-1) and an energy gap E_g- of 1.28 eV These are photoconductive, and the product of the mobility by the life time of the photo-generated carriers is ∼10^-6 cm²V^-1. In work carried out by N. Gerein and J. Haber (Gerein et al. 2006 a,b) it is reported that layers deposited by reactive sputtering in one or two phases, and then annealed in an H2S atmosphere present electrical resistivities in the range between 3-200 Ωcm, a forbidden band gap (E_g) of 1.4 eV and an absorption coefficient of 1x10⁵ cm^-1.

In the present work, Cu₃BiS₃ samples were deposited by evaporation processes, varying the mass ratios m_Cu/(m_Cu+m_Bi). Emphasis was made on studying the effect of the Cu/Bi ratio on the concentration of mobile charge carriers. From transient surface photovoltage (SPV) measurements, the effect of the addition of a buffer layer on the compound was also observed.

Materials and Methods

Samples of Cu₃BiS₃ were deposited by the method of evaporation of precursors varying the mass ratios m_Cu/(m_Cu+m_Bi) between 0.43 to 0.49. Additionally, Hall voltage measurements were carried out on Cu₃BiS₃ samples, using Al as electrical contact. The Hall voltage measurement system is composed of:

Chamber with prevacuum system, sample heating and cooling system, and Hall voltage measurement system, which in turn is composed of model 7065 Hall effect card and a Keithley model 7001 Switch System. The Hall voltage measurement system is described in detail in the reference (Gutiérrez et al. 2007). The concentration of mobile charge carriers is calculated from the Hall voltage measurements by following an experimental procedure developed by the Keithley firm, which is based on the Van der Pauw method and the ASTM (American Society for Testing and Materials 1991) F 76 standard.

Surface photovoltage (SPV) measurements were taken using a capacitor composed of a Cr semitransparent electrode (transparency around 70% and diameter d=5mm) evaporated on a quartz crystal; a mica (width d=50μm) as dielectric and the sample deposited over a conducting substrate (reference electrode). The photovoltage signal is recorded through a lock-in amplifier taking the drop in voltage over the charge resistance R. A pulsated laser NT343/1/UVE (EKSPLA) was used, where the pulse duration was around 5 ns and a maximum intensity of 0.5 mJ/cm² (Fu et al. 2014).

Results

HRTEM images of medium and high magnification measured in a specific area of Cu₃BiS₃ thin films, show that this compound grows in a nanocrystalline lattice, with grains of different size, whose average value is in the vicinity of 15 nm (Mesa et al. 2012). On the other hand, due to the nanocrystalline feature of the samples, it is hard to perform the corresponding indexation. Nevertheless, the diffraction pattern of the selected area is shown in the image (Figure 1).

With the aim of deepening in the study of electrical transport properties of Cu₃BiS₃ thin films, the p concentration and the μ mobility of charge carriers was determined from measurements of electrical conductivity and Hall coefficient as a function of temperature. The p vs T and μ vs T curves for Cu₃BiS₃ thin films deposited with varying m_Cu/(m_Cu+m_Bi) ratio are analyzed (Figure 2).

Additionally, transient SPV curves were obtained with Cu₃BiS₃ thin films and with Cu₃BiS₃/In₂S₃ system, using photoexcitation at different wavelengths (Figure 3). From these results it is observed that the transient SPV signal, obtained with Cu₃BiS₃ films excited with wavelengths greater than the gap (<890 nm) is positive, with a decay time constant of ∼0.1 μs. Nevertheless, when the sample is excited with wavelengths smaller than the energy gap (<890 nm), the SPV signal decay has negative values.

Discussion

The results show that a significant increase in the p concentration of carriers is observed at temperatures greater than 250 °K, apparently caused by thermal excitation of electrons from extended states at the valence band toward accepting states within the gap, thus generating an increase in the density of negatively ionized accepting impurities (Figure 2). The results also show that the sharp increase in the conductivity of Cu₃BiS₃ thin films observed at temperatures greater than 250 °K (Mesa et al. 2009), is basically caused by an increase in the concentration of carriers at this range of temperatures (Kehoe et al. 2013).

It is also observed that the Cu₃BiS₃ samples present low mobility, probably due to the fact that the samples deposited by co-evaporation present a high density of native defects and ionized impurities affecting mobility (Figure 2). Also, it is observed that by increasing temperature, mobility increases, and the increase is sharp at temperatures greater than 250 °K (especially in samples with a mass ratio of 0.46 and 0.49). This behavior might be explained by assuming that when temperature increases a greater quantity of carriers are excited from states at band tails towards states in a valence band region with lower effective mass (they become lighter). Nevertheless, it may also be observed that when temperature increases, carriers in grain boundary states are excited into the valence band, inducing thus a decrease in the height of the grain boundary barrier Ob. On the other hand, the mobility of the sample with mass ratio of 0.43 presents a different behavior at the low temperature region (T<150 °K), which may be caused by a decrease in the probability that the carriers are displaced from a localized state into another via tunnel, in turn caused by some unknown mechanism.

Nevertheless, this disordered nanocrystalline feature of Cu₃BiS₃ films (Figure 1), causes the formation of a high density of associated states and point defects and incomplete bonds with high density grain boundaries, which are responsible for the low mobility values measured in this kind of samples, and for the formation of localized states allowing electrical transport via hopping.

From these results it is established that the generation of positive surface photovoltage when the Cu₃BiS₃ film is excited with photons of wavelength λ>890 nm (Figure 3), occurs because in this case the excitation of electrons is induced from surface states towards the conduction band, which causes a decrease in the deflection of bands in the surface region, which in turn causes an increase in the SPV value. When the excitation is done with photons of wavelength λ<890 nm, there is simultaneously an excitation of electrons from surface states to conduction bands, which causes positive SPV values, and fundamental excitation (transition of electrons from valence band to conduction band), which are diffused into surface states, giving rise to negative SPV values. As the negative effect of the fundamental electron transition is dominant, the net SPV value turns out to be negative (Mesa et al. 2010).

Moreover, it is observed that the transient SPV curves obtained with Cu₃BiS₃/In₂S₃ samples do not present change of sign and their values are generally negative, except in the case in which the excitation is done with very low energy photons (λ=1500 nm), where the SPV change is not present (Figure 3). In this case, wavelengths longer than the In₂S₃ gap were chosen (λ_G=688 nm), in order to avoid excitation of electrons through fundamental absorption.

From obtained results it is observed that negative transient SPV of Cu₃BiS₃/In₂S₃ system decays further (two orders of magnitude), in comparison with the Cu₃BiS₃ negative transient SPV, apparently due to the passivation of Cu₃BiS₃ surface states, which causes elimination of the positive contribution in the SPV net value (Figure 3). Also, it is observed that the SPV value decreases as the excitation wavelength increases; this behavior is due to the fact that the decrease in energy from photons with the increase in λ, causes a decrease in the density of electrons photogenerated in the Cu₃BiS₃, which move (by diffusion and drag) toward surface states of the In₂S₃, thus reducing the net measured value of transient SPV. Measurements carried out with 1500 nm excitation show that there is no change in the SPV signal, indicating that the energy of the photons is lower than the activation energy of Cu₃BiS₃ and In₂S₃ surface states.

The experimental results presented here lead to the following interpretations. The Cu₃BiS₃ thin film surface presents a high density of surface states, where the emitted electrons are conducted to the surface generating a positive SPV value. These states are distributed within the Eg, indicated by the start of the SPV at approximately 0.5 eV The best distribution range for this work function agrees with that.

The energy band diagrams for Cu₃BiS₃ and for Cu₃BiS₃/In₂S₃ system show both the effect of the surface states on the surface deflection of the bands (Figure 4), causing the surface photovoltage changes observed in Cu₃BiS₃ films, and the effect of Cu₃BiS₃ surface state passivation on Cu₃BiS₃/In₂S₃ system surface photovoltage, which takes place when the In₂S₃ layer is deposited on it.

Conclusion

The Cu₃BiS₃ samples were grown by evaporation. From Hall effect and transient surface photovoltage measurements, it was found that the disordered nanocrystalline feature of the samples causes the formation of a high density of states associated with point defects and incomplete bonds with a high density of grain boundaries, which are responsible for the low mobility values, causing the formation of localized states which allow electrical transport via hopping. Nevertheless, when a buffer layer of In₂S₃ is included on the Cu₃BiS₃ surface, there is a passivation of defect states, improving the quality of the material.

Acknowledgements

This work is supported by several projects from Universidad del Rosario. Special thanks also go to Universidad Nacional de Colombia, QUBITeXp International Trade SAS, Colombia. OFI at Pontificia Universidad Javeriana, projects 4513 and 4515, Banco de la República with Fundación para la Promoción de la Investigación y la Tecnología (FPIT), Project 2954 "Construcción de prototipo de pantalla plana tipo OLED con resolución 200X200".

Conflict of Interest

This work does not present any conflicts of interest.

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