Full Length ArticleStudies on interface between In2O3 and CuInTe2 thin films
Introduction
Thin film solar cells with I-III-VI2 film as absorber layer, Mo film deposited on glass substrate serves as back electrode, with the glass plate providing the necessary mechanical strength. An alternate approach [1], [2], [3] to the back metallic contact is to replace Mo film with a transparent conducting oxide (TCO) film. An important advantage [3] of using TCO electrodes is that, apart from providing electrical contact, they can transmit unabsorbed portions of the incident radiation, as TCO’s have an average transmittance of about 85% in the visible and near infrared regions. Solar cells with TCO back contact are sometimes termed as bifacial [3], as it is possible to illuminate from both sides and therefore forms the basic unit of multi-junction or tandem solar cells. The bifacial structures are commonly visualized in two different forms: a) the conventional form in which the cell is developed by first forming the back contact TCO on the glass plate and b) the “inverted” structure in which the back contact is deposited last. The latter structure is called “superstrate” type cell. Two major factors [3], [4] that affect the performance of the solar cells with TCO back contact are 1) changes in the structural and phase homogeneity of the absorber and the TCO films, 2) interface layers, if any, formed between TCO and absorber films due to chemical reaction or inter-diffusion across the interface. For instance, any change in the characteristics of the TCO increases the series resistance and reduce the transmitted light, which in turn will affect the other cells that are in tandem. Therefore, application of TCO as back contact necessitates a detailed prior study of interface between chalcopyrite and TCO thin films.
The present work deals with probing of the interface formed between stepwise flash evaporated CuInTe2 films and dc sputtered In2O3 films using Rutherford backscattering spectrometry (RBS), glancing angle X-ray diffraction (GXRD) and micro-Raman spectroscopy. CuInTe2 belongs to the chalcopyrite ternary compound semiconductor family of compounds and exhibits characteristics displayed by good solar radiation absorber materials. On the other hand, In2O3 is a wide band gap semiconductor (band gap ∼3.75 eV) with an average optical transmittance of ∼85% in the visible region [5]. Stoichiometric In2O3 shows semi-insulating characteristics [6], however, it becomes a degenerate n-type semiconductor with metallic characteristics in oxygen deficient form. It is apparent that the function of In2O3, when it is in contact with other materials variable from buffer to Ohmic contact by controlling the oxygen vacancies alone.
In order to probe the interfaces, two types of structures, viz., Si/In2O3/CuInTe2 and Si/CuInTe2/In2O3 have been formed. The interface formed between In2O3 and CuInTe2 in the former is analogous to the TCO-chalcopyrite interface in absorber based solar cells with TCO back contact [3] as CuInTe2 is deposited on In2O3 film. On the other hand, the interface between In2O3 and CuInTe2 thin films in the latter structure is similar to the chalcopyrite-TCO interface in “superstrate” type solar cells with TCO Ohmic contact [1], [3]. Since, the order in which In2O3 and CuInTe2 films are formed and the methods used for depositing the two are different, there is a possibility that interface characteristics differ.
Section snippets
Experiments
CuInTe2 and In2O3 films were grown by step wise flash evaporation method and dc reactive sputtering respectively. The details of growth of CuInTe2 and In2O3 thin films were presented elsewhere [7], [8]. Briefly, CuInTe2 films were grown onto substrates held at 573 K using step wise flash evaporation. In2O3 thin films were grown by direct current reactive sputtering technique using indium target of 5N purity in the presence of 2% of O2 and Ar mixture. Sputtering was done at a constant power of 100
GXRD studies
Typical GXRD patterns of Si/In2O3/CuInTe2 and Si/CuInTe2/In2O3 recorded with a glancing angle of 3° are displayed in Fig. 1a and b respectively. Analysis of the pattern shown in Fig. 1a revealed the reflections are from the different planes of CuInTe2 and In2O3 films indicating that both are homogeneous. Peaks corresponding to In2O3 are prefixed as INO. In Fig. 1a, reflections from CuInTe2 thin film dominate the pattern as it forms the top layer. On the other hand, the pattern shown in Fig. 1b
Conclusion
The interface between dc sputtered In2O3 and CuInTe2 thin films have been studied by analyzing Si/In2O3/CuInTe2 and Si/CuInTe2/In2O3 structures using GXRD, RBS and Raman spectroscopy. The results show In2O3 and CuInTe2 films have retained their homogeneity in the former structure but for the presence of a thin inhomogeneous interface region, whereas in the latter structure CuInTe2 film has been completely destroyed. The destruction of CuInTe2 film is attributed to the striking of the substrate
References (24)
- et al.
Advantageous light management in Cu(In,Ga)Se2 superstrate solar cells
Sol. Energy Mater. Sol. Cells
(2016) - et al.
Novel device structure for Cu(In,Ga)Se2 thin film solar cells using transparent conducting oxide back and front contacts
Sol. Energy
(2004) Microstructural and diffusion properties of CIGS thin film solar cells fabricated using transparent conducting oxide back contacts
Thin Solid Films
(2005)- et al.
Development of Cu(In,Ga)Se2 superstrate devices with alternative buffer layers
Sol. Energy Mater. Sol. Cells
(2016) - et al.
Growth and characterization of stepwise flash evaporated CuInTe2 thin films
Sol. Energy Mater. Sol. Cells
(2009) - et al.
Characterization of interface between CuInSe2 and In2O3
J. Phys. Chem. Solids
(2005) Algorithms for the rapid simulation of Rutherford backscattering spectra
Nucl. Instrum. Methods B
(1985)- et al.
Characterization of CuGaxSey/ZnO for superstrate solar cells
Thin Solid Films
(2000) - et al.
Growth and Rutherford backscattering spectrometry study of direct current sputtered indium oxide films
Thin Solid Films
(2005) - et al.
Evaporated Sn-doped In2O3 films: basic optical properties and applications to energy-efficient windows
J. Appl. Phys.
(2005)
Direct current magnetron sputtered In2O3 films as tunnel barriers
J. Appl. Phys.
Migration of constituent atoms and interface morphology in a heterojunction between CdS and CuInSe2 single crystals
J. Appl. Phys.
Cited by (2)
Electrochemically growth and characterization of CuInTe<inf>2</inf> chalcopyrite thin films
2020, Journal of Materials Science: Materials in ElectronicsCuInTe<inf>2</inf> thin films synthesis using one-step electrodeposition process: structural, optical, and electrical characterization
2018, Applied Physics A: Materials Science and Processing