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The role of amorphous silicon and tunneling in heterojunction with intrinsic thin layer (HIT) solar


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The role of amorphous silicon and tunneling in heterojunction with intrinsic thin layer(HIT) solar cells

仲玉泉

Compansolar

20

10.05.06

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I. INTRODUCTION

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pA schematic of a typical HIT cell with a p-type wafer is shown in Fig. 1(a). layers: pThe p-wafer solar cell is constructed with five layers: 1、highly n-doped indium tin oxide [ITO(n+)], na[a2、n-doped a-Si :H layer [a-Si :H(n)], alayer[a3、intrinsic a-Si :H layer[a-Si :H(i)], c[c4、p-doped c-Si wafer [c-Si(p)], a5、 a-Si :H(i) and a-Si :H(p).

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The amorphous layers are typically just several na nometers thick and the ITO layer is generally tens of nanometers across. These layers are very thin r layer, elative to the crystalline silicon layer, which is of t microns. he order of hundreds of microns. Because the intrinsic amorphous layers are so thin and not doped, their contribution to the electric small, devicefield is small,Hence, from a device-modeling pperspective, the p-wafer cells can be conceptualized( conceptualized(概 ) as an n+ /n/ p structure. pIn this case, just one dominant p-n junction is present。 present。
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n1( The n-wafer solar cell is shown in Fig. 1(b). It also consists layers, dimensions( pof ?ve layers,with similar dimensions( ) to the p-wafer cell. paHere, the ITO and the p-doped a-Si :H layer form a n+ / p acdiode, while the a-Si :H and c-Si form a p/ i /n diode. Again, because the intrinsic amorphous layers are so thin, they do not contribute signi?cantly to the electric ?eld. So, the doped layers essentially form a n+ / p/n structure

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The p-type region is just 5 nm across and centered between two much thicker n-type layers. This is not a traditional p-n junction solar cell; instead, there are two opposing diodes. Our device simulations show that this structure generally produces irregular current-density-volta ge (J-V) behavior and poor performance, which have been observed experimentally at times. The question then arises: how can Sanyo achieve 22% efficiency with n-wafer HIT cells?
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Previous HIT models circumvented this question by eliminating( )the transparent conducting oxide (TCO) layer or representing the TCO layer as a metallic contact.This conve rts the n+ / p/n structure into a p/n structure. Clearly,this simplification makes it much easie r to attain standard J-V curves and convergen ce(会 )in typical device simulations. However In2O3 :Sn(ITO), the standard TCO used in HIT cells, is a semiconductor with a measured band gap of 3.7eV,and it should not be represented as a metallic contact.
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The issue highlights the fact that the device physics of HITsolar cells is not well understood. Hypotheses for the dominant transport mechanisms(传输机 ) and the material parameters that impact device performance the most vary significantly among differ ent authors(不同的作者对于主要的传输机制的假设和最影响器件性能的材料参 数的假设都相去甚远)Multistep tunneling across the junction i nterfaces has been suggested as the dominant trans port mechanism for low bias by several authors; but others have suggested recombination in the bulk or in the space-charge region(一些作者认为越过结面的多级隧道效应是 低偏压的主要传输机制,其他人则认为空间电荷区的复合才是主要的) In general, tunneling across interfaces has rarely been included i n device simulations www.art-com.co.kr
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In this work, we have incorporated (合并)ITO into the cell structure as a n-type semiconductor, and included tunneling as a transport mechani sm. Our analysis highlights the different roles of a-Si :H layers and tunneling in p- and nwafer HIT cells, and it demonstrates the carrier transport required for cells with n-wafers to achieve ef?ciencies beyond 20%.

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II. MODEL

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The basic material parameters and thickness for each layer are given in Table I.

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A schematic of the defects in a-Si :H layers is shown in Fig. 2. The amorphous layers have two Gaussian defects and exponential(指数)band tails from the band edges. The energy difference between the donor-typ e and acceptor-type defect is 0.3 eV, and they are symmetrically positioned 0.15 eV from the mid-gap. The conduction-band tail is 20 meV wide, and the valence-band tail 50 meV

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III. RESULTS AND DISCUSSION

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pA. p-wafer cells arole of the a-Si :H 1、By inserting a-Si :H between the c-Si and ITO lay ers, the a-Si :H layer forms a buffer that is importa nt for surface passivation but otherwise has little in ?uence on device performance. 2、Another role of the a-Si :H is to prevent tunneling. 3、At the back contact, the role of the a-Si :H layer is to lower minority-carrier recombination
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pA. p-wafer cells role of the tunneling
tunneling at the back contact can play an important r ole in device performance. It is possible that other tra nsports such as defect-level hopping (从缺 跃迁) across the a-Si :H could also be bene?cial

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B. n-wafer cell

In this structure, the thin a-Si :H emitter is the only p-type layer.
The light-generated current can be represented as a current source connected to the a-Si :H/c-Si diode because the carrier generation is mainly in the c-Si. This model results in an irregular J-V behavior

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When the ITO layer is present, a larger p-type regi on between two n-type regions creates stronger b and bending and more robust (强健 )diode behavi or. As the thickness is reduced, the diode beha vior becomes weaker and Voc decreases. On the other hand, if the ITO layer is excluded,the device is a classical p-n diode, and less p-type m aterial is required to sustain a large Voc. Fujiwara and Kondo observed a large reduction in Voc when the thickness of the p-doped a-Si :H lay er was lowered. This is dif?cult to explain without the inclusion of the ITO layer
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IV. CONCLUSIONS

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We have incorporated the ITO layer and tunnelin g into HIT solar cell models. In p-wafer devices, the a-Si :H layers between ITO and c-Si provide interface passivation and a spacer to decrease tunneling. At the back con tact, tunneling through the a-Si :H layers is im portant to avoid deleterious back contact barrier effects. In n-wafer devices, the a-Si :H layer is the only p-type region, and hence, its doping and thickn ess are critical to device performance. To attain good solar performance from the n/ p/n struct ure, tunneling across heterointerfaces at the jun ction is critical.
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