Elsevier

Solar Energy

Volume 199, 15 March 2020, Pages 772-781
Solar Energy

Analysis of different annealing conditions on physical properties of Bi doped CdTe thin films for potential absorber layer in solar cells

https://doi.org/10.1016/j.solener.2020.02.066Get rights and content

Highlights

  • Analysis of different annealing conditions on CdTe:Bi films is reported.

  • Structural properties reveal to cubic phase with preferred (1 1 1) plane.

  • Optical band gap is found 1.42 eV for 300 °C air annealed films.

  • Surface morphology reveals to homogenous deposition of films.

  • Air annealed films at 300 °C are found more suitable for potential absorber.

Abstract

The mainstream technology used for production of high-efficiency CdTe photovoltaics involves Cu doping to CdTe absorber and CdTe/CdS junction activation by cadmium chloride (CdCl2), however, both procedures restrict their use for long term operation. With purpose of seek alternatives, herein, air and MgCl2 annealing along with CdCl2 is performed over e-beam evaporated CdTe:Bi films for their role in absorber photovoltaics. Structural properties indicate appearance of cubic (1 1 1) preferred peak where grain size is found to be affected with nature and temperature of annealing. Optical properties divulge variation in absorbance with annealing, also direct energy band gap is estimated in 1.42–1.69 eV range. All investigated films demonstrated ohmic behavior where conductivity is affected by Bi dopant. Surface morphological properties demonstrated uniform deposition with spherical-shaped grains for pristine as well as 150 °C and 300 °C air annealed films which changed to polyhedral at further annealing. Appeared Cd and Te peaks in EDS patterns validated CdTe films deposition. Surface topographical properties display ditch and dike structures for pristine and 150 °C annealing and hillock structures for higher annealing. Thus, present study imparts critical insights to air, CdCl2 and MgCl2 annealing effect on properties to CdTe:Bi films and designate that chloride treatment is less-effective during Bi doping to CdTe.

Introduction

In the cutting edge technology, an increase in energy demand of society has drawn a significant concern towards energy shortage of the world. To fulfill the energy requirement, researchers have shown their focus on renewable energy sources like solar, tidal, wind energy as non-renewable energy sources like fossil fuels are limited in stock. Among the available renewable energy sources, the solar energy is foremost as it is available freely in abundant amount. It can be converted into electrical energy through solar photovoltaics (PVs) for utility purposes. In order to accelerate the use of PVs, it is essential to enhance the performance and trim down the cost. Thin film PV technologies can be assumed apotheosis for these purposes which share nearly 5% solar market where polycrystalline CdTe-based PVs hold half of this market share (Artegiani et al., 2019) and CdTe PVs are probably second highly utilized solar cells where first one is Silicon based PVs. Presently CdTe-based PVs are significantly contributing towards energy need of the world due to their distinct development process and cost effectiveness. Thin film CdTe PVs provide electricity at cheaper cost and directly contending with conventional fuels as well as Si-technology (Kranz et al., 2013, Duenow and Metzger, 2019).

The dramatic achievement in record efficiency of more than 22% for lab scale and 18% for modules of CdTe PVs has drawn a significant attention of academia and industrials for further breakthrough in performance where the ‘First Solar’ holds the record for maximum achieved efficiency. The achieved higher efficiency of CdTe PVs is due to outstanding material characteristics like near optimum band gap (1.45 eV), larger optical absorption coefficient of greater than 103 per meter, p-type conductivity etc (Orellana et al., 2019, Chavez et al., 2018, Huang et al., 2019). In order to achieve further advancement in efficiency, it is imperative to increase the open circuit voltage (Voc) as there is no significant scope remained for fill factor. The stagnant Voc is mainly limited due to two critical issues viz. intricacies in formation of stable ohmic contact and grain boundaries (GBs) problem which are frequently observed for polycrystalline CdTe PVs (Kranz et al., 2013, Metzger et al., 2019).

The higher electron affinity of CdTe requires high work function metal to form an ohmic contact otherwise a schottky barrier will be generated resulting in increment in contact resistance and decrement in performance. This issue can be solved by reducing the barrier by improving the conductivity of CdTe films (Li et al., 2015, Lisco et al., 2016). For this purpose, CdTe is generally doped with Cu which lower the barrier and improve the carrier concentration but Cu doping has demerit too as Cu can generate Cui interstitials as well as diffuse to CdS/CdTe junction and create recombination centers and instability issues respectively which assume to deny its operation for long terms (Deng et al., 2016, Amarasinghe et al., 2019). Group-V elements like P, As, Sb, Bi can be used as a substitution to Cu doping which also introduce accepter state in CdTe and improve the film properties and performance of device concerned (Metzger et al., 2019, McCandless et al., 2019, Himanshu et al., 2019). Therefore, in the present work, CdTe is intentionally doped with Bi to prepare CdTe: Bi 5% alloy followed by deposition of films.

The grain boundaries in polycrystalline CdTe thin films also limit the Voc where these boundaries are operated/ performed as recombination centers and trap states for charge carriers and reduced carrier life time followed by trim down in performance. These grain surfaces are usually deactivated/passivated by chloride treatment where Cl-atoms are segregated along these interfaces followed by their passivation. The chloride treatment also facilitates the grain growth, recrystallization and improvement in conductivity (Metzger et al., 2019, Li et al., 2014, Zhang et al., 2008, Paulauskas et al., 2014). The development of high efficiency CdTe PVs often prefers use of CdCl2 compound for chloride treatment but its venomous nature is lethal for user as well as environment. Alternative to CdCl2, different non-toxic Cl-compounds viz. MgCl2, NH4Cl, ZnCl2, SrCl2 etc. may be used where MgCl2 compound yields similar performance to that of CdCl2 (Patel et al., 2019, Ordonez et al., 2017, Hashmi et al., 2019, Patel et al., 2019, Kumarage et al., 2018, Major et al., 2014).

Hence both the Bi doping and chloride treatment improve the carrier life time and then Voc and efficiency of the device concerned. Therefore, in this report, Bi doped CdTe films of thickness 550 nm are deposited and subjected to three different post growth processes viz. air, CdCl2 and MgCl2 annealing treatment at 150 °C, 300 °C and 450 °C for one hour. Since there is scarcity of Te on earth, therefore, thickness of CdTe:Bi films is kept lower (550 nm) vis-à-vis to typical thickness of 3–5 μm to reduce the material consumption and lower the cost. The structural, optical, electrical, surface topographical and morphological analyses have been carried out to optimize the properties and seek their aptness for potential absorber layer in photovoltaic applications.

Section snippets

Materials synthesis

Preparation of CdTe:Bi 5% alloy was carried out using melt growth technique where high purity Bi (Alfa Aeser) and CdTe (Sigma Aldrich) were weighed in respect of their atomic weight percentage to develop 1.5 gm Bi doped CdTe material. The weighed material was transferred into quartz ampoule followed by its vacuum sealing. In order to get the proper and homogenous melt of materials, the vacuum sealed ampoule was placed in a furnace (NASKAR) and initially temperature of furnace was raised to

Structural analysis

The typical X-ray diffractograms of pristine, air, CdCl2 and MgCl2 annealed Bi doped CdTe thin films are depicted in Fig. 1 which have been analyzed for investigation of structural (crystallographic) properties. All the films are strongly textured along (1 1 1) orientation in sphalerite (cubic phase with zinc blende crystal) structure for annealing up to 300 °C which signify that growth of crystallites (grains) is taken place along (1 1 1) reflection.

In case of air annealed samples, the

Conclusion

In this contribution, an endeavor has been made to investigate the effect of air, CdCl2 and MgCl2 annealing on properties of Bi doped CdTe thin films for absorber layer photovoltaic applications. The fabrication of thin films of thickness 550 nm was accomplished employing e-beam evaporation technique where melt growth prepared CdTe:Bi 5% alloy was utilized as source material for films deposition. The XRD analysis showed that preferential growth of films is obtained along (1 1 1) orientation of

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The Science and Engineering Research Board (SERB), New Delhi is greatly appreciated by all the authors for financial support through Extramural Research Project vide EMR/2017/003330 to undertake the present work. The Malaviya National Institute of Technology Jaipur, Mohanlal Sukhadia University Udaipur and PSG College of Technology Coimbatore are acknowledged for providing deposition and characterizations facilities. The first author, Himanshu sincerely shows his gratitude to the Department of

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