A possible way to reduce absorber layer thickness in thin film CdTe solar cells V. Krishnakumar ⁎, A. Barati, H.-J. Schimper, A. Klein, W. Jaegermann Darmstadt University of Technology, Institute of Materials Science, Petersenstrasse 32, D-64287 Darmstadt, Germany abstract article info Available online 6 December 2012 Keywords: Thin film solar cell CdCl 2 -activation CdTe Reducing the thin film module production cost is a strong focus in the current thin film solar cell research. One possible way to reduce the cost is minimising material consumption by reducing film thickness. Ultra thin CdTe layer will also help to design p–i–n structure solar cells. The standard CdTe layer thickness is nor- mally in the range of 4–5 μm. Experimental trials were carried out in order to reduce the CdTe layer thickness below 1 μm using close spaced sublimation technique. Simply reducing the CdTe layer thickness induces pin- holes in the CdTe layer which results in poor device performance. The film thickness reduction below 1.5 μm was achieved by employing a double layer structure. In the process of making CdTe double layer the first layer was deposited at higher substrate temperature (~520 °C) and the second layer was deposited at low substrate temperature (~350 °C). The maximum cell efficiency of 12.5% was obtained for ~3 μm CdTe layer and comparable device performance was obtained for the 1.5 μm CdTe layer (11.2%). Further reducing the film thickness below 1.5 μm reduces the device performance. Solar cell efficiencies for ~ 0.8 μm and 0.5 μm CdTe layers were 9.5% and 5.2% respectively. The Capacitance–Voltage measurements show that the CdTe layer is fully depleted when the thickness is reduced below 1 μm. Further, the possible cause for the decrease in efficiency with decrease in CdTe film thickness is discussed in this article. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Polycrystalline thin film solar cells are promising in order to achieve low module production cost. Among the thin film solar cells, CdTe shows more potential for low cost, high efficiency and stable solar cells. One of the main goals in thin film solar cell (CdTe and Copper In- dium Gallium Selenide) research is achieving low film thickness in the range of ~1 μm without compromising the efficiency. Another focus is the minimisation of material needs of elements with limited resources as e.g. Te. High efficiency solar cells are prepared normally with CdTe film thickness in the range of 3 to 7 μm [1–3]. Although the CdTe layer has high absorption coefficient (10 5 cm -1 ) which requires film thick- ness only in the range of 1–1.5 μm [4], CdTe solar cells are normally prepared with higher film thickness in order to avoid shunting effects due to the formation of pinholes [1]. The thickness reduction in CdTe solar cell will help to reduce material consumption, thin film deposition time and post deposition treatment time. Thus, reducing the absorber layer thickness will clearly help to reduce the module production cost. In addition, reduced layer thickness may provide the applicability of alternative device structure as e.g. p–i–n cells [5]. Researchers demon- strated CdTe film thickness reduction by Metal Oxide Chemical Vapour deposition technique [6] and controlling deposition parameters by sputtering [7] and varying substrate temperature profile during growth by Close Spaced Sublimation (CSS) [8]. Plotnikov et al. [9] reported that the device with CdTe film thickness below 1 μm is possible by control- ling the sputter parameters together with non etching Cu based back contact procedure. In this work we demonstrate a simple and an alter- nate possible way of reducing CdTe layer below 1.5 μm. The aim of this work is to show that the better working solar cell can be produced at low film thickness (1–1.5 μm) comparable to thick CdTe layers (4–5 μm). 2. Experimental details The CdTe solar cells described in this work were prepared in superstrate structure using Pilkington TEC-15 (SnO 2 :F/SnO 2 ) glass plates as substrate. Prior to thin film deposition, the substrates were cleaned by standard cleaning procedure. The CdS and CdTe thin film depositions were made using the dedicated CSS deposition unit integrated with DAISY-SOL (Darmstadt Integrated System for SOLarcell research) [10]. The CdS layer was deposited in double layer structure [11]. The first layer was deposited at higher substrate temperature in the range of 520–540 °C and the second layer was deposited at 250 °C substrate tem- perature. Following the CdS layer, the CdTe layer was deposited without breaking the vacuum. No special gas was used in the chamber and the base pressure of the chamber during the film growth was of the order of 10 -7 mbar. Source to substrate distance is ~5 mm. During the CdTe layer growth the substrate temperature was varied from 490 to 525 °C. It is believed that the variation in substrate temperature during the film growth can help to minimise possible pinhole formation. The film growth time was varied to obtain the required film thickness. In order Thin Solid Films 535 (2013) 233–236 ⁎ Corresponding author at: CTF Solar GmbH, Industriestrasse 2, 65779 Kelkheim, Germany. Tel.: +49 61511669662; fax: +49 351 896705 19. E-mail address: krishnakumarphy@yahoo.co.in (V. Krishnakumar). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.11.085 Contents lists available at SciVerse ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf