Page - 81 - in Photovoltaic Materials and Electronic Devices

In both n-i-p substrate and the p-i-n superstrate PV device configurations, most incidentphotonsareabsorbedin the intrinsic layerwithphoto-generatedelectrons andholes transportedto thecontacts. Hence,optimizationof i-layer iscriticaland theoptical responseandphasecompositionof these intrinsic layers tremendously impactsolarcellperformance. The intrinsicSi:HlayersarepreparedatT =200 0C, p = 0.8 Torr, and P = 0.04 W/cm2 as a function of R varied from 10 to 50. The growth evolution diagram for intrinsic Si:H as a function of variable hydrogen dilution, 10ďRď50,onton-layercoatedBRshasbeendevelopedandisshowninFigure6. Thehydrogendilutionandthicknessofn-layerwasfixedatR=50and~200Å,based on protocrystallinity observed in the n-layer growth evolution diagram. For the intrinsic layer, R = 15 is the lowest hydrogen dilution ratio at which the aÑ(a+nc) transitionisobservedwithin~3000Åoflayergrowth. Thedecreaseinthe(a+nc)Ñnc thickness with R may indicate higher nucleation density of crystallites for higher hydrogendilution. Hence,R=10 is identifiedhereasoptimizedforn-i-pa-Si:Hsolar cells incorporatinga~3000Åthickprotocrystallineabsorber [2]. Thethicknessof thep-layershouldbethinenoughtomaximize transparency but thick enough to generate an electric field in the intrinsic layer. Typical p-layer thicknesses are ~100–150 Å, and a large optical band gap assists in minimizing parasitic absorption of incident light within this layer. Within the amorphous and protocrystallinephasethebandgapof thep-layergenerally increaseswith increasing R. The intrinsic layer, p-layer, and their interface are most directly responsible for open circuit voltage optimization, which can be guided using growth evolution diagrams [63,64]. The p-type Si:H layers are prepared at T = 100˝C, p = 1.5 Torr, P = 0.066 W/cm2, and D = 0.0125 as a function of R varied from 50 to 200 on borosilicate glass initially coated with ~3000 Å thick intrinsic a-Si:H prepared at R = 10. From the growth evolution diagram, it can be observed that the p-layer depositions with R > 150 nucleate crystallites within the typical p-layer thickness used in a-Si:H based PV. The R =110film grows initially as a-Si:H and the aÑ(a+nc) transition occurs after a bulk layer thickness of 545 Å. Depositions at 50ďRď 100 indicate that this transition occurs for thicknesses greater than the deposited 650 Å, which is outside the range of interest for solar cells. At R = 200, the aÑ(a+nc) transition occurs at a bulk thickness of 40 Å, and the (a+nc)Ñnc transition occurs within 200 Å. The p-layer should be deposited at the maximum R that can be sustainedwithoutcrossingtheaÑ(a+nc) transitionboundarythroughoutthedesired thickness of 100–150 Å here. This p-layer growth evolution diagram is comparable to previouslypublisheddiagrams[11,12,63,64]. The slope of db, r(t) = d(db(t))/dt, was used to determine the deposition rate of each film even though ε for films containing nanocrystallites are not accurate duetophaseevolutionwith thickness. Figure7showsvariations ingrowthrateas functions of R for n-, i-, and p-layers. The deposition rate shows a familiar trend 81