Electronic Properties of Cu-In-S Solar Cells on Cu-Tape Substrate
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Electronic Properties of Cu-In-S Solar Cells on Cu-Tape Substrate Igor Konovalov, Jürgen Penndorf, Michael Winkler, Olaf Tober Institut für Solartechnologien, Im Technologiepark 7, 15236 Frankfurt (Oder), Germany ABSTRACT Thin film solar cells obtained by the “CIS on copper tape” technique are investigated. This technique promises a high throughput capability, but the efficiency of the cells is still about 5 % only. The model of the band structure of the absorber has been introduced into the model of the whole cell. Parameters of the model were determined experimentally by use of quantitative EBIC profiling, C-V doping profiling, Hall measurements, and AFM. The structure has been simulated using SCAPS-1D software. Results of the simulation show a good correlation to the measured I-V and DSR data of the cell. The benefits and drawbacks of the cell structure as well as factors limiting its efficiency are discussed. INTRODUCTION The process for Cu-In-S solar cell fabrication on Cu tape (“CISCuT” process) was developed as an alternative to the conventional slow rate vacuum CIS co-evaporation process. The major advantages of the CISCuT process are high throughput with sulphurisation/heat treatment time of about 6-7 s, continuous roll-to-roll manufacturing, cheap flexible copper substrate, electrochemically deposited In precursor and, as a consequence, potential low fabrication costs [1,2]. It turned out, that the very short heat treatment time, not achievable with any other contemporary CIS technology, makes it possible to create a spontaneous layered structure (so-called “as grown” cell, in this paper referred to as “absorber”) during a single sulphurisation/heat treatment step, with properties suitable for light energy conversion. Winkler et al. [3] have shown that the structure of the absorber consists of one top and two bottom CuInS2 layers (CIS layers) and a CuIn5S8 middle layer. Further, a p-CuI buffer layer and ZnO window layers improve the light conversion efficiency up to 5 %. The structure of the cell is supposed to be known in this paper (Figure 1, [3-5]). Here, its electronic properties are of interest. It has been shown [4] that the electronic structure of the “as grown” absorber contains two pn junctions, of which the upper one is photoactive and the lower one is short-circuited. In the present study, the internal absorber structure is further investigated. The current conduction mechanisms of a complete cell are discussed.
absorber
1 µm
n-ZnO i-ZnO CuI CuInS2 CuIn5S8 CuInS2 CuInS2 CuXInY Cu
Figure 1. Schematic structure of an „as grown“ cell after [3,4,5]. H5.15.1
EXPERIMENTAL DETAILS 100 µm thick Cu- tape is used as a substrate. A layer of 0.7 µm In has been deposited on the surface of the substrate electrochemically. During sulphurisation/heat treatment, the tape moves at a constant velocity of about 4.4 cm/s through a 30 cm long reaction zone. During this time, the tape is first preheated in the reactor, the temperature of which is about 580°C. Then the reaction with sulphur vapor occurs. The fina
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