Page - 79 - in Photovoltaic Materials and Electronic Devices

ResultsofVIAshowanincrease insurfaceroughness followedbyadecrease within the first ~300 Å of material accumulation, indicating crystallite nucleation on the substrate followed by coalesce of the clusters. The nanocrystallite fraction increases with bulk layer thickness, then converges to 1.0 as expected for a nanocrystalline film. Voids initially appearwith the nucleation of crystallites, which thensubsequentlydecreaseandstabilizenear fvoid =0.04 throughout thegrowthof this layer. Depending on the source of reference ε for nc-Si:H, this behavior could indicate that the grains under these conditions were not well passivated with a-Si:H as is desirable in nc-Si:H PV [22,59]. Optimized nanocrystalline/microcrystalline PV devices often incorporate layers prepared at lowest hydrogen dilution where crystallitegrowthcanoccur,andnc-Si:Hlayersareoftenfabricatedusinghydrogen dilution grading approaches to manipulate the degree of crystallinity. For very high values of hydrogen dilution, such as R = 50 in this example, the material is likely not optimized for solar cells, because cracks related to voids can promote shunts in the cells and channels by which contamination (e.g., oxygen) can enter into the layer [14,21,60]. Comparison of the structural behavior of the aÑ(a+nc) and (a+nc)Ñnc transitionsasa functionofsingledepositionparametershasbeenusedtoproduce so-called deposition phase diagrams or growth evolution diagrams which have helped guide the development of optimization principles in Si:H based PV. For example, the structural evolution can be controlled by the dilution of reactive silicon carrying gases with hydrogen during the deposition process. Films prepared at lowR remainamorphous throughout their total thickness,while thosepreparedat higher R nucleate crystallites. The thickness at which the aÑ(a+nc) transition occurs decreaseswith increasingR.Optimuma-Si:HbasedPVdevices incorporate layers prepared at the highest R that will remain amorphous throughout the full thickness of the absorber layer while optimum nc-Si:H PV incorporates layers prepared at the lowest R where crystallite growth can occur [7,59–62]. For the case of a-Si:H, the additional hydrogen dilution improves ordering in the a-Si:H network, while for nc-Si:H low hydrogen dilution ensures that hydrogen etching does not occur and the grainboundaries remainwell-passivated. The growth evolution diagrams of n-type, intrinsic, and p-type Si:H layers in the n-i-p/BR/glass configuration are depicted in Figure 6. The n-type Si:H layers are prepared at T = 200˝C, p = 1.5 Torr, P = 0.032 W/cm2, and D = 0.0125 as a function of R varied from 20 to 80. For R < 50 the n-layer remains amorphous at least to a thickness of 500 Å. At R = 50 nanocrystallites nucleate in the n-type Si:H at about 450 Å of bulk layer thickness. The amorphous material prior to the aÑ(a+nc) transition of these depositions is protocrystalline [2]. A ~200 Å thick n-layer is typical for n-i-p configuration devices, and the best R for optimized n-i-p a-Si:H solar cells with a protocrystalline n-layer is identified here as near 79