Developing Monolithically Integrated CdTe Devices Deposited by AP-MOCVD
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Developing Monolithically Integrated CdTe Devices Deposited by AP-MOCVD S.L. Rugen-Hankey1*, V. Barrioz1, A. J. Clayton1, G. Kartopu1, S.J.C. Irvine1, C. White2, G. Rutterford2, G. Foster-Turner2 1
Centre for Solar Energy Research, Glyndwr University, OpTIC Technium, St Asaph, North Wales, UK OpTek Systems, Unit 14 Blacklands Way, Abingdon Business Park, Abingdon, Oxford, OX14 1DY * [email protected] (01745 535 213)
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ABSTRACT Thin film deposition process and integrated scribing technologies are key to forming large area Cadmium Telluride (CdTe) modules. In this paper, baseline Cd1-xZnxS/CdTe solar cells were deposited by atmospheric-pressure metal organic chemical vapor deposition (APMOCVD) onto commercially available ITO coated boro-aluminosilicate glass substrates. Thermally evaporated gold contacts were compared with a screen printed stack of carbon/silver back contacts in order to move towards large area modules. P2 laser scribing parameters have been reported along with a comparison of mechanical and laser scribing process for the scribe lines, using a UV Nd:YAG laser at 355 nm and 532 nm fiber laser. INTRODUCTION A key advantage in the large scale production of thin film photovoltaics (PV) is that an inline process does not require the assembly of smaller cells into modules, as in the case of crystalline or polycrystalline silicon wafer based systems, but instead uses monolithically integrated cells. This well-known approach reduces cost and allows for continuous inline processes to be used, such as roll-to-roll production lines [1]. CdTe solar cells, deposited by atmospheric pressure metal organic chemical vapour deposition (AP-MOCVD), have achieved > 15 % [2] using evaporated gold back contacts. This back contacting process is convenient at the research scale but when moving to large scale, evaporated gold is expensive compared to alternative contacting materials and is a fast diffuser in CdTe. Moving towards thin film PV modules means that alternative back contacts need to be assessed without resulting in excessive losses in device performance. Furthermore, the alternating scribing and deposition of different layers to form monolithic integration of cells, connected in series with each other, can be challenging as electrical characteristics of thin film devices are influenced by the scribing parameters [3,4]. The resolution and repeatability of the scribed lines, precision in substratepattern alignment, cell interconnects having low series resistance and high shunt resistance, are key to module performance [5]. It is therefore essential to optimise the laser parameters to create scribes with minimal heat affected zone (HAZ), smooth edges and no recast debris. This is further complicated as the area between the transparent conducting oxide (TCO) and back contact scribes (P1 and P3, respectively) is not active, as seen in Figure 1. With a sub-cell width generally limited to 10 mm or less to reduce the lateral conduction losses through the TCO and scribe lines, in the order of several tens of mic
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