Microcrystalline Single-Junction and Micromorph Tandem Thin Film Silicon Solar Cells
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[4]) and for another tandem of 12 % (not yet confirmed; measurement based on outdoor conditions [5]). In these tandems gc-Si:H bottom cells with approximative 450 mV were incorporated. Obviously, higher efficiencies for micromorph tandem cells could be obtained, if the Voc of the gc-Si:H bottom cells is increased. In the first part of this paper, new results on pgc-Si:H single cells, with increased values of Voc, are presented. In the second part we focus on the temperature performance of ýlc-Si:H and micromorph devices and compare them with a-Si:H and c-Si cells. Note that the temperature behaviour is indeed an important aspect of solar cells working under real outdoor conditions, especially if the total yearly energy yield per installed Wp is considered. In the last part of this paper, the dark I-V characteristics of gc-Si:H, crystalline and amorphous single junction cells are compared. EXPERIMENTAL The deposition of microcrystalline p-i-n solar cells has been described in earlier studies [1-7]. A careful reoptimisation of the p-doped layers [9] and a better control of impurity contaminations in the i-layer were carried out leading to a further improvement in cell performance. The silane gas purifier technique introduced by our group [3,6] was used for the deposition of all Rc-Si:H i-layers. The cells were characterised under AM 1.5 conditions at 100 mW/cm 2 by a two-source solar simulator (Wacom WXS-140S-10). The short-circuit current 139 Mat. Res. Soc. Symp. Proc. Vol. 507 © 1998 Materials Research Society
densities were calibrated by integrating the product of spectral response (SR) data in the range of 350 to 1000 nm times the AM1.5 sun spectrum. For the temperature-dependent measurements a Pt 100 sensor (100 Q platinum thermometer) was glued onto the back side of the cells. The cells were kept in an isothermal environment by a "heating box" equipped with a glass window in order to illuminate the devices. The temperature variation during an I-V scan was less than I 'C for all measurements. The temperature was varied from 10 'C to 90 'C, both for the illuminated as well as for the dark I-V characteristics. The semi-logarithmic I-V dark characteristics in the linear region was adapted by least square fit. RESULTS AND DISCUSSION
1. State of the art of microcrystalline p-i-n cells The challenge for a further improvement of the micromorph tandem cell efficiencies lies in obtaining open circuit voltages higher than 450 mV for the Rtc-Si:H bottom cell. Therefore special efforts were invested for the preparation of jic-Si:H single-junction cells, with the aim to improve the Voc. The results of these new i.c-Si:H cells are given in Tab. I. Table I: Recent new results of gc-Si:H single p-i-n solar cells. r' [%] Jsc [mA/cm 2 l FF [%]
7.7
8.3
7.7
8.1
8.5
3.2
4.4
25.3 67.9
25.2 68.2
21.5 71.1
23.2 68
22.9 69.8
18.4 30.5
17.9 41.8
Voc [mV]
448
483
503
512
531
568
592
Table I shows clearly that Voc-values over 500 mV combined with good fill factors are obtainable for [tc-Si:H single-junction cells.
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