Recently the authors have demonstrated that compensated or “midgap” intrinsic hydrogenated microcrystalline silicon (μc-Si:H), as deposited by the Very High Frequency Glow Discharge (VHF-GD) technique, can be used as active layer in p-i-n solar cells. Compared to amorphous silicon (a-Si:H), μc-Si:H was found to have a significantly lower energy bandgap of around 1 eV. The combination of both materials (two absorbers with different gap energies) leads to a “real” tandem cell structure, which was called the “micromorph” cell. Micromorph cells can make better use of the sun's spectrum in contrast to conventional double-stacked a-Si:H / a-Si:H tandems.The present study will show that the compensation technique (involving boron “microdoping”) used sofar for obtaining midgap μc-Si:H can be replaced by the application of a gas purifier. The use of this gas purifier has a beneficial influence on the transport properties of undoped intrinsic μc-Si:H. By this procedure, increased cell efficiencies in both, single microcrystalline silicon p-i-n as well as micromorph cells could be obtained. In the first case 7.7 % stable, and in the second case 13.1% initial efficiency could be achieved under AM1.5 conditions. Preliminary light-soaking experiments performed on the tandem cells indicate that microcrystalline silicon could contribute to an enhancement of the stable efficiency performance. Micromorph cell manufacturing is fully compatible to a-Si:H technology; however, its deposition rate is still too low. With further increase of the rate, a similar cost reduction potential like in a-Si:H technology can be extrapolated.

bstractRecently, we have demonstrated that intrinsic hydrogenated microcrystalline silicon, as deposited by the very high frequency glow-discharge technique, can be used as the active layers of p-i-n solar cells. Our microcrystalline silicon Ž represents a new form of thin film crystalline silicon that can be deposited in contrast to any other approach found in . literature at substrate temperatures as low as 2008C. The combination of amorphous and microcrystalline material leads to a 'real' silicon-based tandem structure, which we label 'micromorph' cell. Meanwhile, stabilised efficiencies of 10.7% have been confirmed. In this paper, we present an improved micromorph tandem cell with 12% stabilised efficiency measured under outdoor conditions. Dark conductivity and combined SIMS measurements performed on intrinsic microcrystalline silicon layers reveal a post-oxidation of the film surface. However, a perfect chemical stability of entire microcrystalline cells as well as micromorph cells is presented. Variations of the pri interface treatment show that an increase of the open circuit voltages from 450 mV up to 568 mV are achievable for microcrystalline cells, but such devices have reduced fill factors.

Recent progress of solar cells based

Hydrogenated microcrystalline silicon prepared at low temperatures by the glow discharge technique is examined here with respect to its role as a new thin-"lm photovoltaic absorber material. XRD and TEM characterisations reveal that microcrystalline silicon is a semiconductor with a very complex morphology. Microcrystalline p}i}n cells with open-circuit voltages of up to 560}580 mV could be prepared.`Micromorpha tandem solar cells show under outdoor conditions higher short-circuit currents due to the enhanced blue spectra of real sun light and therefore higher e$ciencies than under AM1.5 solar simulator conditions. Furthermore, a weak air mass dependence of the short-circuit current density could be observed for such micromorph tandem solar cells. By applying the monolithic series connection based on laser patterning a "rst micromorph mini-module (total area of 23.6 cm) with 9% cell conversion e$ciency could be fabricated.

Intrinsic microcrystalline silicon opens up new ways for silicon thin-film multi-junction solar cells, the most promising being the ''micromorph'' tandem concept. The microstructure of entirely microcrystalline p-i-n solar cells is investigated by transmission electron microscopy. By applying low pressure chemical vapor deposition ZnO as front TCO in p-i-n configurated micromorph tandems, a remarkable reduction of the microcrystalline bottom cell thickness is achieved. Micromorph tandem cells with high open circuit voltages of 1.413 V could be accomplished. A stabilized efficiency of around 11% is estimated for micromorph tandems consisting of 2 mm thick bottom cells. Applying the monolithic series connection, a micromorph module (23.3 cm 2 ) of 9.1% stabilized efficiency could be obtained.

Compensated" microcrystalline silicon is obtained by adding 8 -20 ppm diborane in the plasma gas phase. p-in cells with such i-layers have increased infrared sensitivity, when compared to a-Si:H p-i-n cells. The preparation of the world's first "mixed stacked" a-Si:H/yc-Si:H tandem cell with an initial efficiency of 9.1 % is reported. The pc-Si:H cells showed no degradation of the cell performance under intense light-soaking. Typical properties of the pc-Si:H cell indicate that the electronic transport is dominated by the crystalline phase of the material.

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Amorphous silicon p-i-n solar cells were deposited on "in house" developed LP-CVD zinc oxide and compared with commercially available SnOa (Asahi type U2) substrates.While for both front TCO materials comparably high values of the open circuit voltage (860-900 mV) and of the fill factor (72 -74 %) were obtained, a remarkable enhancement of the short-circuit current density could be observed for LP-CVD ZnO substrates. Optical characterizations confirm for LP-CVD ZnO a more efficient light-trapping effect, as compared to SnOa. By applying this low-cost LP-CVD ZnO, a stabilized a-Si:H p-i-n solar cell efficiency of 9 % has been achieved.