Multiple Core–Shell Silicon Nanowire-Based Heterojunction Solar Cells

Article pubs.acs.org/JPCC

Multiple Core−Shell Silicon Nanowire-Based Heterojunction Solar Cells Guobin Jia,*,† Björn Eisenhawer,† Jan Dellith,† Fritz Falk,† Annett Thøgersen,‡ and Alexander Ulyashin‡ †

Institute of Photonic Technology, Albert-Einstein-Str. 9, 07745 Jena, Germany SINTEF Materials and Chemistry, Forskningsveien 1, 0314 Oslo, Norway

‡

ABSTRACT: Silicon nanowire-based solar cells received increasing attention due to their enhanced light harvesting properties and the potential to use lowcost materials to produce solar cells comparable with those on costly monocrystalline counterparts. It is essential to improve the performance of nanowire solar cells by suppressing surface recombination. A multiple core (crystalline silicon nanowires)−shell silicon nanowire-based heterojunction solar cell has been fabricated to deal with this problem. To this end, an ultrathin passivating Al2O3 tunnel layer was deposited on the highly doped ptype a-Si:H emitter prior to a transparent conducting oxide by atomic layer deposition (ALD). Both open circuit voltage and current density increase significantly due to the insertion of the ultrathin Al2O3 layer. An efficiency of 10.0% has been reached by using this multiple core−shell structure.

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INTRODUCTION Silicon nanowires (SiNWs) have been proposed to be used as sensors1−3 and field effect transistors.4−6 Moreover, SiNWs find applications more and more in the energy chain, either for energy storage as efficient batteries7 or as energy harvesting devices like thermal electrical generators,8,9 electrochemical cells for generation of hydrogen by water splitting,10 and most importantly as solar cells.11,12 Nanowire-based solar cells are very promising for the third generation low-cost highly efficient light harvesting devices. Two types of nanowire-based solar cells, i.e., with axial or radial p−n junction, are under intensive investigation. An issue of the axial p−n junction configuration is that the surface is greatly enlarged. To keep surface recombination acceptable, one should passivate the p- as well as the n-type region, which is difficult in practice. The radial p−n junction configuration relies on a core−shell structure. Because of the small diameters of the nanowires, most of the core region is depleted from majority carriers so that the electron−hole pairs generated can be immediately separated by the electrical field by drift so that the recombination in the crystalline silicon (c-Si) cores is very low. As a consequence, materials with rather low minority carrier lifetimes could be tolerated to fabricate solar cells with efficiencies comparable with those of costly monocrystalline counterparts.11 Furthermore, a densely packed nanowire array provides antireflection properties at the surface and light trapping within the arrays. Light absorption is greatly enhanced within a nanowire carpet only several micrometers thick, so that highly efficient thin film solar cells could be realized by using silicon nanowires. However, even if light absorption is greatly enhanced, efficiencies of SiNW-based solar cells are still very low.13,14 Only one group reported an efficiency above 10%.15 However, highly sophisticated technologies have been used in their © 2013 American Chemical Society

experiment. In the present work, we present a low-cost process for SiNW solar cell preparation resulting in an efficiency slightly above 10%. A very promising approach of nanowire based core−shell structure is a heterojunction with intrinsic thin layer (HIT) configuration, in which crystalline silicon (c-Si) nanowires are used as a core and hydrogenated amorphous silicon (a-Si:H) as a shell. Owing to the excellent passivation of the silicon surface by the intrinsic a-Si:H layer, wafer-based HIT solar cells deliver higher open circuit voltages,16 and high efficiency solar cells can be easily realized. The a-Si:H shell can be simply deposited by plasma enhanced chemical vapor deposition (PECVD)17 in which the doped and the intrinsic a-Si:H layers can be made with and without adding a doping gas during the deposition. An efficiency of 7.29% has been already realized in our group.17 To further improve the performance of the nanowire-based HIT core−shell solar cells, one has to understand the hurdles in the preparation as well as in the operation. A close comparison of the HIT cells on planar wafers and those on SiNW arrays is needed. Conventional HIT solar cells consist of an n-type monocrystalline silicon wafer with a-Si:H (i + p+)-emitter on the front side covered by a transparent conducting oxide (TCO) contacting the cell. The most critical point is the thickness of the a-Si:H emitter. Since the minority carrier diffusion length in the a-Si layer is very low and the surface recombination at the TCO/a-Si:H interface could be high, light absorbed within the a-Si:H emitter is usually considered as a loss. Therefore, the total a-Si:H layer is kept as thin as possible (in the range of ∼10 nm) to minimize the loss by the absorption in the a-Si:H layer, so that the surface recombinaReceived: November 8, 2012 Revised: December 21, 2012 Published: January 7, 2013 1091

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propanol subsequently, and then, it was put into a H2SO4 (97%)/H2O2 (30%) solution (1:1 by volume) at 80 °C for about 10 min to remove any organic and inorganic contaminations at the surface. The cleaned wafer was mounted into an etching cell, which allows the polished side only to be in contact with the etching solution. A mixture of 10 mL of AgNO3 (0.02 M) and 10 mL of HF (5 M) was added to the etching cell. Etching was stopped after 30 min, and the sample was rinsed thoroughly with deionized water. The resulting silicon nanowires are densely packed, irregularly shaped, and have a length of ∼600−800 nm. To remove silver, we used a cleaning procedure according to ref 17 with just slight modifications. After 10 min of treatment in concentrated HNO3 (65%) and rinsing with deionized water, subsequently, a small amount of H2O2 (30%) was added to a NH4OH (∼5%)/2-propanol (3:1 by volume) solution for ionizing the Ag. The soluble Ag complex [Ag(NH3)2]+ will be released from the surface and removed by rinsing with ultrapure deionized water (resistivity of 18.2 MΩ·cm) afterward. The Na2S2O3 treatment was omitted in this work. The formed oxide layer was removed by dipping into an HF (2%)/2-propanol solution for 8 min. The 2-propanol serves as a surfactant to reduce the surface tension so that the etchant can penetrate into the nanowire array to completely remove the oxide around the NWs. Subsequently, the samples were rinsed by ultrapure deionized (DI) water and blown dry with N2. The samples were immediately placed into a PECVD chamber to deposit the a-Si:H layer. The deposition was performed at 225 °C. A thin intrinsic a-Si:H layer (20 s at a silane flow rate of 2 sccm; chamber pressure of 1 mbar) was deposited prior to a highly doped p-type a-Si:H (2 min at a silane flow rate of 2 sccm and a diborane flow rate of 1.45 sccm (2% diluted in He); chamber pressure 1 mbar). After the a-Si:H deposition, the sample was cleaved into two parts. One part was covered by 100 nm aluminum-doped ZnO (AZO) as the TCO contact by atomic layer deposition (ALD) on top of the a-Si:H at a temperature of 225 °C. The other was covered by 1.2 nm Al2O3 by ALD and subsequently by 100 nm AZO. The thin Al2O3 layer acts as a tunnel oxide layer for the transport of the carriers as well as a passivating layer for the highly doped p-type a-Si:H layer. The samples were mesa-etched to remove the AZO, Al2O3, and nanowires outside the solar cells as well as the parasitic AZO layer at the back surface. The cell areas were determined by a microscope. Finally, an ohmic back side contact was made by rubbing InGa liquid alloy onto the rear side of the wafer.

tion at the a-Si:H emitter does not play an important role for HIT solar cells made on planar wafers.18 However, for a SiNW core−shell (c-Si/a-Si:H) configuration, the surface is greatly enlarged, the thickness of the a-Si:H layer is getting comparable with the diameters of the nanowires, and light can also be scattered between the densely packed core−shell nanowire structures and therefore gets several times the chance to be absorbed by the a-Si:H layer. As a result, a large portion of light is absorbed in the a-Si:H emitter, and the loss due to surface recombination at TCO/a-Si:H interface cannot be ignored anymore. The passivation of the a-Si:H surface becomes a critical issue to further improve the performance of the device. Furthermore, also the loss due to light absorption in the TCO layer is enhanced because of light scattering between the nanowires. In this work, we fabricated a novel multiple core−shell TCO/Al2O3/a-Si:H (p+ + i)/c-Si nanowire structure as sketched in Figure 1. In comparison with our former work,17

Figure 1. Sketch of the multiple core−shell nanowire based solar cell on SiNW arrays.

a 1.2 nm thin Al2O3 layer is introduced between the TCO and the a-Si:H emitter. This ultrathin Al2O3 layer serves as a passivating layer to suppress the surface recombination as well as a tunnel barrier for the front contact. It is demonstrated that both VOC and JSC are increased significantly by inserting the ultrathin Al2O3 layer, and an initial efficiency of 10.04% has been reached.

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EXPERIMENTAL SECTION SiNW arrays were prepared following the method described by Peng et al.,19 i.e., by etching in AgNO3/HF solution at room temperature. An n-type (100) Si wafer, thickness of 675 μm, with resistivity of 1.5 Ω·cm was cleaned by acetone and 2-

Figure 2. Top view SEM image (left) of the nanowire based solar cells (inset, high-resolution SEM image) and SEM cross-section image (right). 1092

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RESULTS AND DISCUSSION

The left image in Figure 2 shows the top view scanning electron microscopy (SEM) image of a large area homogeneous silicon nanowire array embedded in the AZO. Still single nanowire structures can be distinguished by high-resolution SEM image in the inset of the left image in Figure 2. Most of the nanowires are connected with each other by the AZO, which provides a very good ohmic contact at the front side. In spite of a high aspect ratio of the densely packed nanowires, a conformal coating of the nanowires20 by AZO was achieved thanks to the ALD technique (right image of Figure 2). The total a-Si:H thickness is 5−7 nm, as shown by high-resolution transmission electron microscopy (HRTEM) in the left image of Figure 3. With the help of plasmon energy loss filtered HRTEM, the thin Al2O3 layer can be distinguished (see the right image in Figure 3).

Figure 4. Comparison of the IV curves (upper) and the EQE (lower) of the two types of solar cells (red, cell without Al2O3; black, cell with ultrathin Al2O3 layer). The inset in the lower image is a magnification of the EQE at the range from 300 to 360 nm.

Figure 3. High-resolution TEM images (left and inset) of the nanowire-based solar cells. The right image is a plasmon energy loss filtered (23 eV) TEM image showing the thin Al2O3 layer.

Table 1. Parameters Determined by IV Curves and Suns-VOC (Second Values) of Two Different Types of Solar Cells: A, 100 nm AZO/1.2 nm Al2O3/a-Si:H (p+ + i)/c-Si Nanowires; B, 100 nm AZO/a-Si:H (p+ + i)/c-Si Nanowires

IV curves were taken under AM 1.5 cw illumination and in dark as demonstrated in the upper image of Figure 4. The parameters of the solar cell can be extracted from the IV curve measured under AM 1.5 illumination. Suns-VOC is another very useful method to determine the photovoltaic (PV) parameters21 because it is independent of the contact resistance so that it gives the performance of the solar cells under ideal contact conditions. Suns-VOC was measured using a Sinton Suns-VOC150 equipment. Table 1 lists the parameters of the two types of solar cells determined by IV and Suns-VOC. The deviation of VOC is mainly resulting from different shadowing areas by the contact tips and the heating of the cells during IV measurement. External quantum efficiency (EQE) measurements of both types reveal that the solar cell with ultrathin Al2O3 exhibits higher EQE almost over the whole range of the spectrum as shown in the lower image in Figure 4. Large improvement of the EQE is mainly obtained in the wavelength range from 450 to ∼950 nm, which is strongly absorbed in the a-Si:H layer. It is also observed that the cell with ultrathin Al2O3 layer has a slightly higher EQE in the spectral range from 300 to 350 nm (see the inset of the lower image in Figure 4). Upon optimization of the a-Si:H emitter, an efficiency of 10.04% has been reached on the multiple core−shell nanowires based solar cells. Figure 5 shows the IV curves measured in dark (black line) and under AM 1.5 illumination (red line). The multiple core−shell nanowire-based solar cell shows an open circuit voltage (VOC) as high as 517 mV, a current density (JSC) of 26.46 mA/cm2, a conversion efficiency of 10.04%, and a fill

A B

VOC (mV)

JSC (mA/cm2)

η (%)

FF (%)

512/532 494/509

23.13 20.59

8.20/9.93 7.28/8.33

66.5/80.7 71.5/79.6

Figure 5. IV curves measured in the dark (black line) and under AM 1.5 (red line) of the solar cell with 10% efficiency. The second values in the inset are determined by Suns-VOC measurements.

factor (FF) of 73.4%. The solar cell shows an even better performance by Suns-VOC measurements, where the series resistance is ignored and the heating of the cells during the measurement minimized (VOC, 532 mV; η, 11.0%; FF, 78.4%). Passivation Effect by Multiple Core−Shell Structure. The increase of JSC and VOC clearly shows the passivation effect 1093

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a light concentrator will surely improve the performance of the nanowire solar cells. To further clarify the second point, an estimate was made to compare the solar cells made on planar wafers and that on nanowires. The enlargement of the surface due to the nanowires can be calculated as follows. Assuming the average radius r and the length l of the nanowires as illustrated in Figure 6, from simple geometric consideration, a factor F = 2αl/r + 1

by the ultrathin Al2O3 layer. Such passivation effect of the highly doped p-type c-Si is attributed to the field effect by the negatively charged Al2O3.22 This mechanism should be valid for the highly doped p-type a-Si:H emitter as well, where the negatively charged Al2O3 will repulse the minority carriers away from the surface, which effectively reduces the recombination at the surface of the a-Si:H emitter. The slight decrease of the FF of the cell with an ultrathin Al2O3 layer may be a result of increased series resistance due to the Al2O3. The EQE measurements on the two types of samples confirm the passivating effect of the p+ a-Si:H surface by the thin Al2O3 layer. The sample with the ultrathin Al2O3 layer exhibits higher EQE almost over the whole spectral range of interest. However, the very low EQE at short wavelengths around 500 nm indicates that there is a substantial loss due to the absorption within the AZO and a-Si:H layer, whereas the conventional HIT solar cells23 show very high EQE (80%) in this range. Because of the scattering of the light within the nanowire array, the effective layer thickness of AZO and a-Si becomes larger, and the absorption loss in the AZO layer as well as in the a-Si is greatly enhanced. From the measured data of JSC and VOC of the two types of solar cells, we conclude according to eq 1 that the cell with ultrathin Al2O3 has a much lower saturation current (JS),24 almost half of (∼56%) the one without the Al2O3 layer.

VOC ≈

kT ⎛ JSC ⎞ ln⎜⎜ ⎟⎟ q ⎝ JS ⎠

Figure 6. Nanowires with average radius r and length l on the silicon wafer.

results, describing the relative increase of the surface, where α is the ratio of projective area occupied by the nanowires. On the basis of this equation, an estimation can be made on the nanowire-based solar cells taking l ≈ 800 nm and r ≈ 50 nm and assuming that half of the original area is removed by the etching, i.e., α = 1/2. In this case, F = 17 results, i.e., the nanowire sample has 17 times larger surface than the planar one, and the nanowire-based solar cell has just 1/17 of the current density of the planar ones, i.e., (JSC)P = 17(JSC)N where (JSC)P and (JSC)N denote the current densities for planar and nanowire solar cells, respectively. Provided that the saturation current JS is the same for both kinds of cells, the difference of the VOC can be calculated according to eq 1 as

(1)

k, T, and q denote Boltzmann’s constant, temperature, and elementary charge, respectively, and kT/q ≈ 26 mV at room temperature. The reduction of JS originates from the increase of the effective minority carrier diffusion length in the a-Si:H emitter, which can be attributed to the surface passivation at the a-Si:H. This result should be very helpful to improve the HIT solar cells made on planar wafers as well. Up to now, the passivation of the a-Si:H emitter surface is omitted, and very thin a-Si:H is used to minimize the absorption loss. Generally, the multiple core−shell structure can be considered as a stack of a semiconductor−insulator−semiconductor (SIS)25,26 junction and an a-Si:H/c-Si (HIT) heterojunction solar cell based on silicon nanowires. The current contribution of the SIS junction is small but can be observed at the EQE in the wavelength range from 300 to 350 nm as shown in the inset of the lower image in Figure 4. In this spectral range, the photon energy is above the band gap of the AZO, and absorption occurs already in the AZO layer. The sample without Al2O3 shows low EQE in this range. Such SIS junctions were established recently by Pradhan et al.,27 where it was stated that the carriers generated within the AZO layer penetrate through the oxide barrier and contribute to the photocurrent. Operation of the Nanowire-Based Solar Cells. In comparison with an HIT solar cell made on planar wafers (possibly with textured surface), the performance of the nanowire based core−shell HIT solar cell is still lower. This is to the one hand due to absorption losses in AZO and a-Si layer as discussed above. However, the area of the p−n junction is much larger in the nanowire cells so that the local current density is much smaller than that determined by the projected area, leading to a lower open circuit voltage. This behavior is similar to a solar cell operating at weak light conditions,28 under which it shows lower efficiency. Increasing the light intensity by

(VOC)P − (VOC)N = =

(J )N ⎞ kT ⎛ (JSC )P ⎜⎜ln − ln SC ⎟⎟ q ⎝ JS JS ⎠ kT (JSC )P ln q (JSC )N

≈ 26 mV × ln F ≈ 74 mV

(2)

where (VOC)P and (VOC)N denote the open circuit voltages of the planar and nanowire-based solar cells. The result shows that, at the same global current densities, the VOC of the nanowire-based solar cell is smaller than that of a planar one and so is the efficiency. However, if the actual local current density can be increased by concentrating the sunlight to approximately a factor of F, VOC will reach the same level of the planar solar cell. The efficiency of the nanowire-based solar cells at such conditions is becoming comparable with the planar one. Usually, HIT solar cells are not operated in concentrated light because an increase of the carrier density causes Auger recombination to dominate, and so, the cell efficiency is reduced. In the case of the nanowire-based solar cells, the minority carriers (holes) are quickly extracted from the core region by the electrical field of the space charge region, and the absence of holes will dramatically reduce the probability of Auger recombination. Consequently, concentrated sunlight could be tolerated. This result is consistent with our result acquired on planar wafers. 1094

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The value F is an important parameter, which determines the electronic parameter of the nanowire-based solar cells. Furthermore, it can be readily used as a parameter during the heterojunction a-Si:H (i + p+) deposition. In comparison with that on planar wafers, the average deposition rate on the nanowire arrays is ∼1/F, i.e., in order to get the same a-Si:H thickness, the time needed is factor F longer than that on planar wafers. Further Possibilities to Improve the Nanowire-Based Solar Cells. To make a high quality intrinsic a-Si:H layer, a cleaned c-Si surface is needed. This could be the biggest issue for a densely packed nanowire array because a trace of the metal catalyst used during the etching procedure is difficult to completely remove; so, the p−n junction properties are in jeopardy. It has been observed that the cleaning procedure plays an important role for the performance of the finished solar cells. After using the cleaning procedure detailed in this work, no Ag remnants can be detected by EDX or SIMS. However, one should be aware that contaminations may still be present on the surface of the nanowires, which are under the detection limit. The presence of such contaminations at the junction interface may influence the performance dramatically due to recombination. Light absorption in the UV to blue spectral region in the AZO layer is another issue, which could probably be solved by tuning the band gap of the AZO layer to obtain maximum transparency or by using deep UV transparent contact materials like graphene,29 Ga-doped ZnO,30 or β-Ga2O331 layers as transparent contact. The properties of the a-Si layer should be optimized to obtain the highest VOC and JSC. It is noted that high efficiency HIT solar cells prepared on wafers include a back surface field for passivation at the rear side,18,32 so that a higher VOC as well as JSC can be reached. Such add-ons can be included in the nanowire-based solar cells to further improve its performance. The advantages of the nanowire-based core−shell solar cells are that the properties are mostly determined by the interface or the surface of the device, and they can tolerate rather low quality of the substrate material. The transfer of this technology onto low-cost substrates, such as metallurgical grade silicon wafers, thin crystalline silicon film,33 and so on is on the way. Such technology will dramatically reduce the cost of the solar cells and make PV comparable with conventional energy sources.

cells and guiding the real deposition rate on the nanowires during the PECVD. Further improvement of the nanowire-based core−shell solar cells may be reached by using deep-UV transparent TCO for the front contact, introducing a back surface field, and optimizing the solar cells by using a more sophisticated multiple core−shell tandem junction. It is noted that this work has been done on nanowires etched into monocrystalline Cz-Si wafers. A transfer of the technology onto low-cost silicon materials like wafers made from metallurgical grade silicon and crystalline silicon thin films deposited on low-cost substrates is in progress.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +49 3641 206421. Fax: +49 3641 206499. E-mail: guobin. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support by the European Commission in Projects SiNAPS and NanoPV under the grant numbers 257856 and 246331, respectively. We would like to thank Sven Schönherr at IFK of the Friedrich Schiller University of Jena for the EQE measurements.

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REFERENCES

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CONCLUSIONS Multiple core−shell nanowire-based solar cells with the structure AZO/Al2O3/a-Si:H (p+ + i)/c-Si nanowires have been fabricated. The insertion of an ultrathin (1.2 nm) Al2O3 layer deposited by ALD shows an excellent passivation effect on the highly doped p-type a-Si:H emitter of the heterojunction HIT solar cells. Both, VOC and JSC increase by the insertion of the ultrathin Al2O3 layer. Upon optimization of the a-Si:H emitter, the multiple core−shell nanowire-based solar cell has reached an efficiency slightly over 10%. The limiting issue for the performance of the nanowire-based solar cells is their large surface. Therefore, the relevant local current density at the p−n junction is much lower than in flat solar cells resulting in a lower VOC. The performance of the solar cells may be improved by using concentrated photovoltaics. A parameter F determining the enlargement of the total surface area was found to be very useful in governing the electronic parameters of the solar 1095

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