porous silicon heterojunctions

porous silicon heterojunctions

Solar Energy Materials and Solar Cells ELSEVIER Solar Energy Materials and Solar Cells 46 (1997) 261-269 Electrical and optical characterization of...

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Solar Energy Materials and Solar Cells

ELSEVIER

Solar Energy Materials and Solar Cells 46 (1997) 261-269

Electrical and optical characterization of crystalline silicon/porous silicon heterojunctions C. Palsule a'*, S. Liu a, S. Gangopadhyay a, M. Holtz a, D. Lamp a, M. Kristiansen b "Department of Physics, Texas Tech University, Lubbock, TX 79409, USA b Department of Electrical Engineering, Texas Tech University, Lubbock, TX 79409 USA

Received 31 May 1996; received in revised form 17 October 1996

Abstract We have investigated the photovoltage and photocurrent spectra of crystalline silicon/porous silicon heterojunctions. The porous silicon layers were prepared using anodic etching of p-type crystalline silicon at a current density of 25 mA/cm 2. From the spectral dependence of the photovoltage and photocurrent, we suggest that the photovoltaic properties of the j unction are dominated by absorption in crystalline silicon only. We have also studied the effect of increase in the thickness of porous silicon layers on these spectra. We find that the open-circuit voltage of the devices increases, but the short-circuit current decreases with an increase in the thickness of the porous silicon layers. We propose a qualitative explanation for this trend, based on the increase in the series and the shunt resistance of these devices. The effect of hydrogen passivation on the junction properties by exposing the devices to hydrogen plasma is also reported. Keywords: Photovoltage spectra; Photocurrent spectra; Crystalline silicon/porous silicon het-

erojunctions

1. Introduction and background The visible light emission from p o r o u s silicon (PS) [1] has generated a great deal of interest due to its potential technological applications. This has stimulated

* Correspondence address. Hewlett-Packard, Integrated Circuits Business Division, Corvallis, OR 97330, USA. 0927-0248/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved Pll S 0 9 2 7 - 0 2 4 8 ( 9 7 ) 0 0 0 0 4 - 4

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a significant amount of basic research in this material to understand its structure, composition and the origin of its highly efficient visible photoluminescence (PL). In addition to PL, it has also been shown that PS exhibits electroluminescence [2] and a photovoltaic effect [3]. A PS film on top of crystalline silicon (c-Si) can serve as a window layer to absorb high-energy photons efficiently, the surface texturing of PS can enhance light trapping, thus, decreasing the optical losses, and front or back surface fields due to its wider bandgap can decrease the surface recombination to improve the efficiency of c-Si solar cells. However, in order to prepare devices, it is essential to understand the physics of PS/c-Si junctions completely. The visible photoluminescence band (1.6-1.8 eV) and the absorption spectra of self-supporting PS films suggest that the bandgap of PS is larger than 2 eV [4], which is larger than c-Si. Several models have been proposed to explain the increase in the bandgap [-1, 4-6]. The original and most prevalent of these has been the quantum confinement model [1, 7]. According to this model, the increase in the bandgap is due to confinement of electrons and holes in 2-3 nm diameter wire-like pillars of crystalline material present in PS. To a first-order approximation, the increase in valence and conduction band energies is given by [7] AE~,~

h2 4m~,eD2 ,

(1)

where mh.e * are the hole and electron effective masses, respectively, and D is the wire diameter. Since PS is prepared by anodically etching c-Si, there is always a good electrical contact between PS and the underlying c-Si. It has been proposed [7] that during the etching there is a depletion of holes in the PS and as a result, the Fermi level in PS moves towards the middle of the bandgap making the material intrinsic/n-type compared to the substrate. A thermal equilibrium at the junction between these two materials requires the alignment of the two Fermi energies and hence, a built-in electric field. The effective doping density and hence, the position of the initial Fermi level in PS can be varied by changing the etching conditions during the preparation of PS. In this work, we have studied the photovoltaic properties of Au/c-Si/PS/Au structures as a first step towards a complete characterization of PS/c-Si junctions. We report on the changes in the photovoltage (PV) and photocurrent (PC) spectra of these devices upon changing the thickness of the PS layer and propose an explanation based on the series and the shunt resistance of the devices. We also compare the photovoltaic properties of the devices before and after exposure to hydrogen plasma.

2. Experimental details The PS films studied in this work were prepared by anodic etching of (111)-oriented boron-doped c-Si (resistivity ~ 10 - 30 ~cm) in a solution of H F (48~0) : ethanol (99.5%) = 3 : 7 at a constant current density of 25 mA/cm 2. A 1000 A gold film deposited on the back side of the wafer by thermal evaporation served as the anode, while a platinum wire was used as the cathode for the etching. The etching was performed in a specially prepared Teflon cell under tungsten lamp illumination. After

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Table 1 Etching parameters for the PS films C o m m o n parameters 25 mA/cm 2, H F : Ethanol 3 : 7 Sample

Etching time (min)

PS film thickness (lain)

A B C D E

2 4 8 16 32

5.6 9.1 16.3 24.7 30.0

the etching, the samples were rinsed using deionized water and blown dry with nitrogen. Semi-transparent gold films were deposited onto the PS layers to serve as the top electrode. The samples were then stored in light-tight boxes to avoid degradation due to simultaneous exposure to light and air. A series of samples were prepared using different etching times (see Table 1). The film thicknesses were estimated using an optical microscope and a solid-state camera. Under each etching condition, three samples were prepared to check the reproducibility. To characterize the PS/c-Si junctions, photovoltage (PV) and photocurrent (PC) spectra were used. In both cases, the raw spectra were corrected for the combined spectral response of the halogen lamp and the monochromator, which were used during these experiments. The photocurrent spectra in this work were recorded under short-circuit conditions, while the photovoltage spectra were recorded under opencircuit conditions. The samples were then exposed to hydrogen plasma in an electron-cyclotron resonance (ECR) system [-8] at 200 W microwave power for 2 h.

3. Results and discussion

The dark I - V characteristics of the c-Si substrate (with gold contacts on either side) without a PS layer do not show any rectification. In contrast, all samples with PS layer on top show rectification in dark I - V characteristics, indicating the presence of a built-in electric field. All the samples with PS layer on top also exhibit photovoltaic properties. A representative dark I - V characteristics for a sample etched for 8 min at a current density of 25 mA/cm 2 is shown in Fig. 1. Also shown in the figure is the I - V characteristics of the same sample under 100 mW/cm 2 illumination from a solar simulator. It has been already shown that, of the two possible junctions, gold/PS and PS/c-Si, the gold/PS junction has very little contribution towards the total photovoltage [9]. As a result, all the photovoltaic properties are dominated by the PS/c-Si junction and a study of these properties yields information about PS/c-Si junction. From the PV measurements, we find that the potential of the c-Si substrate side is positive in comparison with the PS, which is consistent with previous literature [3]. Figs. 2 and 3 show a comparison of the PV and the PC spectra for all the samples used in this work. Qualitatively, both the PV and the PC spectra exhibit similar spectral

264

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-

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Fig. 1. Dark and light (100 mW/cm 2) I - V characteristics for Au/PS/c-Si/Au structure.

dependence. In order to understand this spectral dependence, we have to compare the absorption spectra of crystalline silicon and PS. From such a comparison of absorption spectra [10], it is quite clear that for energies below 1.7-1.8 eV, the absorption coefficient of crystalline silicon is at least an order of magnitude larger than PS. The absorption length of PS is 30 ~tm at 1.7-1.8 eV. Thus, for all the devices in this study, the PS film is transparent to light below this energy. Since the PV and the PC spectra peak in this energy range, it is quite clear that most of the useful absorption takes place in the crystalline silicon part of the sample. Hence, the sharp cut-offat 1.1-1.2 eV range corresponds to the room temperature bandgap of the underlying c-Si. For photon energies above 1.7-1.8 eV, since the absorption length of PS is smaller than 30 ~tm, the absorption in PS will become signficant depending on the thickness of the PS film. But in all the cases, the PV and the PC spectra peak in the energy range of 1.1-1.5 eV and drop off in the high-energy range. This suggests that in high-energy range, even though most of the electron-hole pairs are generated in PS, the carriers either cannot reach the built-in electric field or cannot diffuse towards the respective collecting electrode. In addition, due to absorption in PS, light reaching the crystalline silicon at these energies will be considerably attenuated and hence, the density of photogenerated electron-hole pairs will be low - which results in a drop-off at these energies. This explains the shape of an individual PV or PC spectrum. Now, we attempt to explain the complementary behavior of the PV and the PC spectra with an increase in the thickness of the PS film. In homojunction solar cells it has been shown [11] that an increase in the series resistance does not affect the open-circuit voltage much, but results in a decrease in the fill-factor as well as the short-circuit current of the device. Similarly, a decrease in the shunt resistance results

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in a decrease in the open-circuit voltage and the fill-factor, but does not affect the short-circuit current much. For the samples in this study, in case of current flow perpendicular to the PS film, the series resistance will be proportional to the PS thickness. Since the PS resistivity is very high - 106-10v~ cm [123, a change in thickness will make a significant change in short-circuit current. The shunt resistance represents all the alternative conduction paths from one electrode to the other, in which carriers do not encounter the PS/c-Si junction. In these samples, all the connected undepleted crystalline silicon channels will represent the shunt resistance. So, as the thickness of the PS layer increases, the probability of having such connected channels from one electrode to another will decrease rapidly and the shunt resistance will increase accordingly. As the thickness of the PS layer is increased, the series resistance would increase resulting in a decrease in the short-circuit current. At the same time, an increase in shunt resistance due to increase in thickness would result in an improvement in the open-circuit voltage. This is the exact behavior we see in the PV and the PC spectra for devices C, D and E, but devices A and B do not fit this

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pattern. In order to understand this discrepancy, we have to look at the PS preparation more carefully. All the PS films were prepared under illumination in white light. Since the typical absorption length of PS above 2.5 eV is 5 gm or shorter, the electron-hole pair generation due to illumination can supply the necessary holes for further dissolution of crystalline silicon and result in an increased porosity in the top layer of the PS films [13]. This will result in a more complete depletion of holes in this region and a higher bandgap widening. The resulting increase in the conduction band offset will result in a slightly higher open-circuit voltage, but a lower short-circuit current due to an increase in the barrier height for electrons going from c-Si to PS. Such behavior is expected in heterojunction solar cells [11]. If this effect dominates over the changes in PC and PV due to an increase in series and parallel resistances accompanying the increase in the thickness of PS, then sample A will have a slightly higher open circuit voltage but a lower short circuit current than sample B. Since the high-energy photons from the illumination used during the etching process are absorbed within the first few microns of the PS film, they do not affect the properties

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268

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of the deeper layers and as a result, the porosity of the thicker films is fairly uniform. So as the PS film thickness increases further, its only effect is an increase in the series and the shunt resistance in case of devices C, D and E. One possible reason for poor high-energy response in PS could be shorter carrier lifetimes due to an increase in surface recombination at the large available internal and external surfaces. It has been reported that the main defect responsible for surface recombination in PS is closely related to the Pbo center - which is a [111J-axially symmetric silicon dangling bond at the c-Si/SiO2 interface [14]. Hydrogenation of the PS films has been shown to be effective in passivating the dangling bonds and enhancing the PL [15]. In order to study the effect of the passivation on the junction properties, one sample from each group was exposed to a hydrogen plasma at 200 W microwave power for 2 h in an ECR plasma system. Comparisons of the PV and PC spectra for sample E, both before and after exposure to hydrogen plasma are shown in Fig. 4a and Fig. 4b, respectively. After hydrogenation, there is a substantial improvement in the PC spectrum and a small improvement in the PV spectrum. Similar trends were observed for the other four samples. In the PC spectra, the improvement is larger on the low-energy side than on the high-energy side. In the low-energy region, most of the photogenerated carriers are in c-Si. Under short-circuit conditions, the electrons will first diffuse to the junction and then will be swept to the top electrode due to the electric field in the depletion region through the PS. Hence, higher improvement in the low-energy region indicates improved transport of electrons through the PS layer. The improvement in the PV spectra was uniform throughout the entire spectral range. Overall these results suggest that hydrogen exposure results in better passivation of dangling bonds. Since the dangling bonds are the major recombination centers, a reduction in the concentration of dangling bonds enhances the lifetimes of both the carriers. This results in an increase in the photogenerated current at the same photon flux which in turn means an increase in the short-cicuit current and the open-circuit voltage.

4. Summary and conclusions We have studied photovoltage and photocurrent spectra of c-Si/PS heterojunctions as a function of the PS thickness. These spectra clearly indicate that in these junctions, PS serves principally as a window layer while bulk silicon serves as the main light absorber. We also find that the series and the parallel resistances of these devices increase with an increase in the thickness of the PS layer. This results in a decrease in the short-circuit current and an increase in the open-circuit voltage, respectively. Hydrogenation of these devices results in an improvement in the PV and the PC spectra due to better passivation of dangling bonds.

Acknowledgements We would like to thank the Center for Energy Research (CER) at Texas Tech. University for the financial support.

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