Heterojunction Silicon Wafer Solar Cells using Amorphous Silicon Suboxides for Interface Passivation

Available online at www.sciencedirect.com

EnergyProcedia Procedia1500 (2011) Energy (2012) 97000–000 – 106

Energy Procedia www.elsevier.com/locate/procedia

International Conference on Materials for Advanced Technologies 2011, Symposium O

Heterojunction Silicon Wafer Solar Cells using Amorphous Silicon Suboxides for Interface Passivation Thomas Muellera,*, Johnson Wonga and Armin G. Aberlea,b a

Solar Energy Research Institute of Singapore, National University of Singapore, 7 Engineering Drive 1, Block E3A, Singapore 117574, Singapore b Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Block E4, Singapore 117576, Singapore

Abstract Current industrial monocrystalline silicon wafer solar cells based on screen-printing technology for contact formation and selective emitter have an efficiency potential of around 20%. Heterojunction silicon wafer solar cells combine plasma-deposited amorphous silicon thin films and n-type crystalline silicon wafers to industrially viable highefficiency solar cell devices. The excellent surface passivation provided by an intrinsic amorphous silicon interlayer leads to a very high open-circuit voltage, enabling industrial heterojunction cells with efficiencies of over 23%, as demonstrated by the Japanese company Sanyo. The key point of these structures is the removal of the highly recombination-active solar cell contacts from the crystalline surface, by insertion of a thin film with a wide bandgap. To reach the full device potential, the defect density at the hetero-interface must be minimised. Commonly used hydrogenated amorphous silicon films of only a few nanometer thickness are appealing candidates for this. Their bandgap (~1.7 eV) is much larger than that of c-Si and, when intrinsic, such films can reduce the c-Si surface state density by hydrogenation. Additional oxygen built into the amorphous network results in amorphous silicon suboxides, which in turn leads to an improved surface passivation. With this approach we are able to achieve implied voltages of 740 mV and an optical bandgap of about 2 eV, which improves the cell’s blue response. In addition, microcrystalline silicon emitter and back-surface-field layers provide enhanced doping efficiency, enabling the fabrication of contacts with low saturation current density values. This new approach has already demonstrated promising cell efficiencies of about 21%. This paper summarises the recent progress in heterojunction silicon wafer solar cell research using amorphous silicon suboxides. © by by Elsevier Ltd.Ltd. Selection and/orand/or peer-review under responsibility of the organizing committee © 2011 2011Published Published Elsevier Selection peer-review under responsibility of Solar Energyof International Conference on Materials for Advanced Technologies. Research Institute of Singapore (SERIS) – National University of Singapore (NUS). Keywords: Silicon wafer solar cells; amorphous silicon; heterojunction; a-SiOx; PECVD; surface passivation

* Corresponding author. Tel.: +65 6601 1292; fax: +65 6775 1943 E-mail address: [email protected]

1876-6102 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the organizing committee of International Conference on Materials for Advanced Technologies. doi:10.1016/j.egypro.2012.02.012

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1. Introduction Recently, heterojunction solar cells, in particular amorphous/crystalline silicon solar cells, have attracted attention as an excellent candidate for industrially viable high-efficiency silicon wafer solar cells. Typically, such cells feature very thin (nanometer scale) hydrogenated amorphous silicon films deposited on crystalline silicon (c-Si) substrates and provide a very high energy conversion efficiency, which is due to fact that higher open-circuit voltages can be obtained using heterojunction solar cells compared to homojunction cells. To fabricate a high-efficiency heterojunction solar cell, the abrupt discontinuity of the crystal network at the c-Si surface to the amorphous or microcrystalline emitter results usually in a large density of interface defects in the c-Si bandgap, due to a high density of dangling bonds. These defects at the hetero-interface often reduce the solar cell performance. Although an electrical field can reduce the recombination at or near the hetero-interface, the junction properties are still governed by the interface state density. Therefore, in order to obtain high efficiency, it is essential to reduce the interface state density of heterojunction solar cells. Hydrogenated amorphous silicon (a-Si:H) has been accepted as a suitable heterojunction material for a-Si:H/c-Si solar cells, forming both the emitter layer as well as the interfacial passivation layer. Nevertheless, the optical absorption in the intrinsic a-Si:H(i) and the doped a-Si:H(n, p) layers is rather strong. Thus the short-circuit current density of a-Si:H/c-Si solar cells rapidly decreases with increasing a-Si:H layer thicknesses, as few of the electron-hole pairs generated in the a-Si:H contribute to the cell’s current. Therefore, it is preferable to employ an a-Si:H-based alloy that has larger optical bandgap than standard a-Si:H. Fujiwara et al. [1] investigated a-Si:H/c-Si heterointerfaces, stating that they necessitate an immediate a-Si:H deposition (after the wafer cleaning step) and an abrupt interface to the c-Si substrate in order to ensure a good interface passivation. High-temperature (> 130 ºC) growth of a-Si:H, however, often leads to epitaxial layer formation on c-Si. A (partial) epitaxial growth leads to a high dark saturation current due to interfacial defect states D it, which in turn reduces the Voc. Thus, this epitaxial growth narrows the process window for the a-Si:H layer growth and makes the solar cell optimisation more difficult. Physical and structural properties of a-Si:H, however, strongly vary with the growth conditions. Also, due to the inherent strong blue light absorption, only ultrathin a-Si:H films can be allowed. A considerable number of studies were published describing a-Si:H/c-Si heterojunction solar cells (see, for example, Refs. [1-16]). Nevertheless, there is little consensus on the properties of the a-Si:H/c-Si heterointerface, and thus a clear insight into the a-Si:H/c-Si solar cell is still lacking. This is also reflected in the discrepancy of the solar cell efficiencies achieved in the various studies listed above [5]. A passivation scheme featuring highly-transparent plasma-deposited hydrogenated nanocomposite amorphous silicon suboxides (a-SiOx:H), grown at low temperature, represents a material system suitable for this application and is an attractive alternative to the standard a-Si:H incorporated in a-Si/c-Si heterojunction solar cells. The passivation scheme using a-SiOx:H was introduced in Ref. [17]. The work was continued in Refs. [18, 19] where applicability of the plasma-deposited a-SiOx:H as a high-quality surface passivation scheme for Si wafer based solar cells was proven. A novel approach for heterojunction solar cell fabrication, featuring nanocomposite plasma-deposited amorphous silicon suboxides (a-SiOx:H) for high-quality surface passivation combined with overlaying plasma-deposited doped microcrystalline silicon (μc-Si:H(p+)/μc-Si:H(n+)) for use as emitter and backsurface-field layers, is used in this work to form the heterojunction. A schematic of the cell structure is shown in Fig. 1. This cell architecture was introduced in Ref. [4]. The present work focuses on the recent developments using plasma-deposited wide-gap amorphous silicon suboxides to form interface

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passivation layers, and plasma-deposited wide-gap highly-conductive μc-Si:H (or a-Si:H that is grown close to the transition to μc-Si:H silicon) to form the emitter and BSF layers. The µc-Si:H layers are grown at very high frequency using PECVD, and were discussed in detail in Refs. [4, 5]. TCO

µc-Si(p+)

a-SiOx:H(i)

n-Si base

µc-Si(n+)

Fig. 1. Cross-sectional view of the heterojunction Si solar cell design used in this work. The structure was proposed in Refs. [4, 5].

2. Results and discussion 2.1. Nanocomposite amorphous silicon suboxides used for surface passivation As for heterojunction solar cells, a-Si:H(i) is the standard material applicable as buffer layer between c-Si and doped a-Si:H, serving as surface passivation layer of the Si wafer. Therefore, the surface passivation quality of a-SiOx:H is compared to the surface passivation quality of standard a-Si:H(i) in this experiment. The passivation quality is characterised by the effective lifetime on 1-Ωcm n-type Fz Si wafers. Both a-SiOx:H and a-Si:H(i) are deposited on front and rear, using optimised process conditions. An excellent effective lifetime of more than 5 ms after post-annealing of a-SiOx:H was achieved with these structures, as shown in Fig. 2. This is the highest effective lifetime value ever measured for a plasma-deposited amorphous silicon-passivated 1-Ωcm n-type Si wafer. The used film thickness was larger than 50 nm, to ensure a saturation of the effective lifetime. For comparison, the effective lifetime values for our best a-Si:H(i) films are 2 ms after post-annealing on the same type of Si wafer, which is comparable to the results published by other groups. Our effective lifetime is also higher than that obtained with thermal silicon dioxide passivation or plasma silicon nitride passivation (see, for example, Refs. [20, 21]). Another benefit is that the a-SiOx:H films feature a high optical bandgap (Eg = 1.9 eV), which suppresses optical absorption within this layer. 2.2. Optical confinement of a-SiOx:H layers From spectroscopic ellipsometry (SE) measurements of the a-SiOx:H films, the refractive index n and the extinction coefficient k can be deduced. For this purpose, a-SiOx:H films were deposited on a c-Si/ SiO2 substrate, whereby the 100 nm SiO2 layer serves as an optical separation layer. For the SE analysis, a simple three-layer model c-Si/SiO2/a-SiOx:H was used. Figure 3 shows the absorption coefficient as a function of the photon energy for the a-SiOx:H compared to that of standard a-Si:H and c-Si. The

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absorption in the a-SiOx:H film in the blue light region around 3.0 eV is significant lower than that of aSi:H. Consistently, the fraction of light transmitted to the wafer which is available for solar conversion increases drastically. As the passivation quality depends strongly on the thickness of an applied a-Si(O):H layer, it is evident that with decreasing absorption in the passivation layer, the thickness could be increased to gain a better passivation quality.

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Fig. 2. Surface passivation quality of a-SiOx:H compared to a-Si:H. A passivation layer thickness of more than 50 nm was used. From Ref. [4].

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Energy (eV) Fig. 3. Absorption coefficient of a-SiOx:H deduced from SE data fit compared to published absorption coefficients of crystalline silicon. From Ref. [19].

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2.3. Impact of a-SiOx:H thickness on surface passivation To validate the applicability of the a-SiOx:H surface passivation for use as buffer layer in heterojunction solar cells, it is important to investigate the surface passivation quality of a-SiOx:H films as a function of the film thickness. Usually a film thickness of 2-5 nm is adequate to effectively passivate the c-Si surface and yet thin enough to let carriers pass. For better comparability, 1-Ωcm n-type Fz wafers were used for all depositions. The thicknesses of the deposited films vary from 250 nm down to 2 nm, which is the lowest thickness ensuring a homogeneous, shunt-free film. The results of the effective lifetime τeff and corresponding Seff values for an a-SiOx:H passivated 1-Ωcm n-type wafer are shown in Fig. 4. As can be seen, a film thickness as low as 10 nm provides an almost as good surface passivation quality as a 250 nm thick film. Further reduction of the thickness down to 3 nm results in a slight decrease of the effective lifetime to 3.1 ms. For a film thickness of below 2 nm, the a-SiOx:H films could not be deposited homogeneously across the whole wafer surface with the used PECVD system. Therefore, the effective lifetime decreases drastically for these thicknesses (not shown here). To apply the optimal a-SiOx:H film as passivating buffer layer sandwiched between the c-Si wafer and the doped emitter/BSF layer in the heterojunction solar cells, the film thickness is determined by a trade-off between passivation quality, current losses due to low conductivity of the a-SiOx:H films (resulting in high series resistance), light absorption, and deposition homogeneity. Further details on the composition of the a-SiOx:H films and their passivation level can be found in Ref. [18].

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Fig. 4. Values for τeff and Seff as a function of applied a-SiOx:H film thickness, before and after annealing. The results were obtained from quasi-steady-state photoconductance (QSSPC) transient photoconductance (TPC) measurements. The thickness of the deposited films varies from 250 nm down to 2 nm; 2 nm is the lowest thickness ensuring a homogeneous, shunt-free film. The lines are a guide to the eye. From Ref. [4].

2.4. Surface passivation quality of stacked a-SiOx:Hand doped µc-Si:H films

To determine the passivation quality of the a-SiOx:H films in stacks with the doped µc-Si:H(p+) and µc-Si:H(n+) films described in the previous chapter, the effective lifetime of 1-Ωcm n-type wafers passivated symmetrically with a-SiOx:H, doped µc-Si:H(p+) emitter on the top and doped µc-Si:H(n+) BSF on the rear is measured. Earlier works on the passivation quality of a-Si:H (for example Ref. [22])

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state that whereas even a very thin layer of a-Si:H(i) of around 3 nm may still yield a very low surface recombination velocity on low-resistivity wafers (~1 Ωcm), the surface passivation properties are lost when the intrinsic film is subsequently covered by a plasma-deposited doped a-Si:H emitter or BSF layer. Figure 5 illustrates quasi-steady-state photoconductance (QSSPC) curves measured with a standard lifetime tester (Sinton Consulting), obtained for a 1-Ωcm n-type c-Si wafer passivated on one side by a 10 nm a-SiOx:H film capped by a 20 nm p-doped µc-Si:H(p+) emitter film and by a 20 nm n-doped µcSi:H(n+) BSF film on the other side. An implied open-circuit voltage iVoc of 735 mV was obtained for this structure at 1 sun intensity. For comparison, the corresponding QSSPC curve for a simple 10 nm thick a-SiOx:H passivation without additional capping layers is also shown. field effect passivation increase

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Fig. 5. Effective lifetime as a function of excess carrier density before and after the deposition of the heavily doped µc-Si:H layers. The corresponding implied Voc of the structure is 735 mV at 1 sun intensity.

From these experiments it is clearly seen that the effective lifetime of a 1-Ωcm n-type Si wafer covered by symmetrical 10 nm a-SiOx:H films is maintained at around 4 ms when the capping layers are added, see Fig. 5. The additional capping of the a-SiOx:H layers with a µc-Si:H(p+) emitter layer on the front and a µc-Si:H(n+) BSF layer on the rear, as shown in Fig. 5, even leads to a small increase of the effective lifetime (from 4.0 to 4.6 ms) at an excess carrier density of 10 15 cm-3. This stems from an increased fieldeffect passivation induced by the µc-Si:H(n+) and µc-Si:H(p+) layers due to their high doping efficiency, while the expected increase in interface state density after capping layer addition can be observed for higher excess carrier densities, i.e. > 1016 cm-3. The increase in surface recombination is thus prevented by an increase in field-effect passivation. This is a clear indication that the precursors are well suited for high-efficiency heterojunction Si solar cells. 2.5. Heterojunction silicon wafer solar cell results Heterojunction solar cells, using textured substrates, were fabricated by incorporating a-SiOx:H layers for surface passivation and doped µc-Si:H layers for emitter and BSF formation. Both the a-SiOx:H and µc-Si:H are the precursors for a heterojunction solar cell.

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A full description of the device’s electronic properties would require a precise knowledge of the band positions and band discontinuities, which have been shown to be critical [22]. However, the injection level dependence of the effective lifetime can be transformed into an illumination vs. implied V oc curve and then into a predicted I-V curve. This allows a meaningful determination of the limits on V oc and FF solely imposed by recombination, thus neglecting series resistance imposed effects. This approach can be applied to the precursor structure, passivated by a-SiOx:H and capped with the corresponding µc-Si:H emitter and BSF layers, but also for a precursor structure additionally contacted with a transparent conductive oxide (TCO) layer on top of the emitter and BSF layers. Comparing these two pseudo I-V curves (also called lifetime I-V curves) and additionally comparing them to the Suns-Voc curve and to the real I-V curve measured on the completely contacted device permits to diagnose any effect of the TCO deposition on the µc-Si:H/a-SiOx:H/c-Si precursors. The Suns-Voc data of complete solar cells comprises further losses due to shunting, whereas the real I-V curve measured under 1-sun illumination additionally includes the series resistance effects [6]. Figure 6 shows the lifetime I-V curve (without TCO, but with emitter and BSF capping layers) and the Suns-Voc I-V curve of a silicon wafer heterojunction solar cell made on a 1-Ωcm n-type c-Si wafer after TCO sputtering. Here, indium tin oxide (ITO) was used. The implied V oc of the lifetime I-V measurements predicts 735 mV, whereas the 1-sun Voc of the completed solar cell is 705 mV.

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Fig. 6. Lifetime I-V (without ITO) and 1-sun I-V curves of a heterojunction silicon wafer solar cell formed on a 1-Ωcm n-type wafer, after ITO sputtering. The implied Voc of the lifetime I-V measurements predicts 735 mV, whereas the 1-sun Voc measured with the Suns-Voc method is around 705 mV.

To investigate the reason of the comparatively low real Voc of around 705 mV, compared to the high implied Voc of 735 mV predicted by lifetime measurements, the c-Si surface passivation by a-SiOx:H, using the same layer thickness as in the solar cell, was checked first. A wafer passivated only by two 5 nm thin intrinsic a-SiOx:H passivation layers was investigated. As Fig. 7 shows, its passivation leads to an effective lifetime of over 3 ms. Suspecting then that the µc-Si:H(p) emitter deposition deteriorates the effective lifetime, the passivation performance of the corresponding symmetrical p/i/n/i/p stack was tested, using the same layer thicknesses as in the solar cell. As shown in Fig. 7, this stack shows an increased field-effect passivation with no increase in the interface density. Therefore it can be concluded

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from Fig. 5 and Fig. 7 that the µc-Si:H(n) BSF layer deposition results in an increased interface state density while being able to establish a field-effect passivation. The lifetime I-V curve of the passivated wafer additionally capped with an emitter and a BSF layer predicts (based on Fig. 5) a pseudo iVoc exceeding 735 mV. However, after the TCO deposition there is already a significant decrease in implied Voc (not shown). This can also be seen from the Suns-Voc curve. The ITO deposition and/or the metallisation is detrimental, as it causes increased recombination and consequently diminishes the V oc from 735 mV (lifetime I-V) to 705 mV (Suns-Voc). The slight difference is due to slight temperature fluctuations of the Suns-Voc measurement chuck. The real Voc of the solar cell finally amounts to 702 mV. The final comparison of the Suns-Voc and the 1-sun I-V curves indicates appreciable "series resistance" losses, which can be contributed to the transport of excess carriers within the solar cell, thereby basically reducing the fill factor, as shown in Fig. 8.

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Fig. 7. Measured effective lifetime of a symmetrically passivated n-type 1-Ωcm c-Si wafer with a-SiOx:H passivation (10 nm), before and after the deposition of a heavily doped µc-Si:H(p+) layer on both sides. The i/p stack passivation sample - having the layer thickness used in the cell devices - indicates a field effect passivation with no interface state density increase.

Nevertheless, the final heterojunction device had a promising efficiency of 21.1% on a small area of 11 cm2, by incorporating wide-gap intrinsic a-SiOx:H as well as wide-gap doped µc-Si:H, see Fig. 8. Excellent surface passivation by introducing the a-SiOx:H is evident in the high solar cell efficiency. There are still improvements possible, especially with regards to the TCO deposition, which so far reduces the implied Voc from 735 to 700 mV.

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Fig. 8. Comparison of the different I-V curves of the finalised heterojunction solar cell, using a 1-cm n-type wafer, 5 nm a-SiOx:H passivation layers, 20 nm µc-Si:H(p+) emitter layer, 20 nm µc-Si:H(n+) BSF layer and 80 nm ITO contact layer. (a) Lifetime I-V (passivated wafer capped with emitter and BSF layer but without ITO), I-V resulting from Suns-Voc (passivated wafer capped with emitter and BSF layers and additionally capped with ITO), and real 1-sun I-V curve (same structure as in (b) with an additional metal grid deposited onto the ITO). Cell area is 1.0 cm2.

3. Conclusions Intrinsic amorphous silicon suboxide thin films (a-SiOx:H) have demonstrated excellent surface passivation on n-type c-Si wafers. A very low interface recombination defect density with record effective lifetimes of more than 5 ms on 1-Ωcm n-type Fz Si wafers proves an outstanding surface passivation quality. The layers were grown at low temperature in a VHF-PECVD tool. The silicon-hydrogen-oxygen bonds lead to high optical bandgap of around 1.9 eV, leading to a reduced absorption within this layer. Low-defect-density and highly conductive microcrystalline silicon thin films (µc-Si:H) used as emitter and BSF layers were also grown via VHF-PECVD, and resulted in wide-gap highly transparent (1.95 eV) films. If deposited on top of intrinsic amorphous silicon suboxides, the magnitude of the field effect passivation can be tuned. Such stacks were used to form the emitter and BSF layer of microcrystalline/ amorphous/crystalline silicon heterojunction solar cells. High implied and real open-circuit voltages were reached (735 and 705 mV, respectively). The interpretation of lifetime measurements on heterojunction test structures allowed a rapid development by a fast device diagnostic procedure. In particular, using VHF-PECVD, full heterojunction devices with open-circuit voltages of above 700 mV and efficiency of up to 21.1% were fabricated. The full potential of this novel heterojunction structure is not yet realised, due to the deterioration of the Voc by the ITO sputtering process. Acknowledgements Parts of this work were carried out by T.M. at his previous affiliation, the University of Hagen, Germany. SERIS is sponsored by the National University of Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB).

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et al. / Procedia Energy Procedia 15 000–000 (2012) 97 – 106 T.Thomas MuellerMueller et al. / Energy 00 (2011)

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