A Novel Silicon Photovoltaic Cell Using a Low ... - IEEE Xplore

IEEE ELECTRON DEVICE LETTERS, VOL. 28, NO. 7, JULY 2007

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A Novel Silicon Photovoltaic Cell Using a Low-Temperature Quasi-Epitaxial Silicon Emitter Mahdi Farrokh-Baroughi and Siva Sivoththaman, Senior Member, IEEE

Index Terms—Chemical vapor deposition (CVD), epitaxy, silicon, solar cell.

have been studied in detail [6], [7]. In this letter, we present a novel LT (250 ◦ C) solar cell based on a quasi-epitaxial (qEpi-Si) n+ emitter developed on a (p) multicrystalline silicon (mc-Si) substrate without using any TCO layer or intrinsic buffer layer. The high conductivity, mobity, and optical properties of the newly developed (n+ )qEpi-Si films on (p)mc-Si substrates are advantageous over a-Si emitters and simplify the device design and process. The novel p-n junction diode was characterized using different electrical techniques. Ultimately, simple solar cells without any surface texture or back-surface field features were fabricated with > 10% conversion efficiency (η).

I. I NTRODUCTION

II. E XPERIMENTAL P ROCESSES

Abstract—A new silicon solar cell fabricated using a lowtemperature process is demonstrated with a highly conductive (n+ ) quasi-epitaxial (qEpi-Si) silicon emitter deposited on silicon substrates, without using transparent conductive oxides. The emitter was formed by a plasma-enhanced chemical vapor deposition process on granular multicrystalline silicon (mc-Si) substrates at a substrate temperature of 250 ◦ C. The new (n+ )qEpi-Si/(p)mc-Si junction was found to be of good quality for photovoltaic applications. Solar cells of 1-cm2 area and conversion efficiencies exceeding 10% have been fabricated in a simple fabrication process and device structure.

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HILE crystalline silicon wafer-based solar cells dominate nearly 90% of the current solar cell market [1], the cost of the base Si material and the device manufacturing accounts for almost half of the cost of photovoltaic (PV) systems [2]. Deposition of Si thin films on non-Si substrates is being researched as a way to reduce Si usage [3]. Si-wafer-based lowcost approaches focus on the use of Si materials of different quality and different technologies [4]. The use of low-cost defective Si substrates requires defect passivation techniques such as hydrogenation. Developing simple device structures and compatible fabrication technologies taking into account the material-imposed restrictions such as process temperature are also important. Low-temperature (LT) processes therefore are attractive. Hydrogenated amorphous Si/crystalline Si (a-Si:H/c-Si) heterojunction (HJ) solar cells have shown a great potential in providing LT substitute for conventional homojunction solar cells [5]. Typical a-Si:H/c-Si HJ solar cells utilize a thin doped a-Si emitter (7–20 nm) along with a transparent conductive oxide (TCO) as conduction film [6]. It has also been shown that the interface quality at the HJ plays a critical role. The interface is normally passivated by an ultrathin intrinsic a-Si:H layer (“buffer” layer) [5]. Implementation of ultrathin a-Si:H films requires some process control and also critical process optimization. Using thicker a-Si:H emitter could also result in significant photocarrier loss due to the emitter absorption. Recombination mechanisms in a-Si/c-Si hetero interfaces Manuscript received November 14, 2006; revised April 16, 2007. The review of this letter was arranged by Editor P. K.-L. Yu. The authors are with the Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, ON N2L3G1, Canada. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2007.897873

The process for growing (n+ )qEpi-Si emitters at 250 ◦ C on mc-Si substrates was developed in a plasma-enhanced chemical vapor deposition (PECVD) system using SiH4 and PH3 precursor gases diluted in H2 . The film properties were characterized by high-resolution transmission electron microscopy (HRTEM) and electrical probing techniques in order to establish suitability for device application. In the optimized process, the pressure, RF power, and PH3 /SiH4 /H2 flow rates were 900 mtorr, 70 mW/cm2 , and 0.04/4/500 sccm, respectively. Then, 16-mm2 test diodes and 1-cm2 solar cells were fabricated on (p)mcSi substrates with 50-nm thick (n+ )qEpi-Si films deposited as emitter using the optimized conditions. The front and rear Al contacts were formed by RF sputtering. For solar cells, the front contact had a 10% metal coverage. A 75-nm PECVD silicon nitride antireflection coating was deposited on the front surface at 125 ◦ C. The process conditions for the nitride deposition were as follows: pressure of 400 mtorr, RF power density of 65 mW/cm2 , and the SiH4 /NH3 /H2 flow of 4/80/200 sccm. III. M EASUREMENT R ESULTS AND D ISCUSSION High active doping density and carrier mobility are essential for high σ in Si films. A very high crystallinity obtained from epitaxial growth can substantially increase both doping efficiency and carrier mobility. HRTEM analysis of crystallinity was carried out on 90-nm films. The HRTEM image of the (n+ )qEpi-Si/(p)mc-Si interface in Fig. 1(a) clearly shows an epitaxial growth; the atomic arrangement in the film is very similar to that in the substrate. The inset in Fig. 1(b) shows the film on a grain boundary (GB) of the mc-Si substrate, indicating that the growth rate of the film is independent of the crystal orientation of substrate grains. Fig. 1(b) also shows the bulk and interface regions on the GB in more detail. Epitaxial

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IEEE ELECTRON DEVICE LETTERS, VOL. 28, NO. 7, JULY 2007

Fig. 2. (a) I–V –T characteristics of a 4-mm2 (n+ )qEpi-Si/(p)mc-Si diode. The temperature was varied from 200 K to 360 K. (b) Extracted saturation currents of the first and the second diodes from the I–V –T characteristics of Fig. 2(a).

Fig. 1. (a) HRTEM image of a (n+ )qEpi-Si/(p) mc-Si interface. (b) TEM (inset) and HRTEM images of the (n+ )qEpi-Si film on a GB.

growth is observed at least up to 50-nm thickness. Our HRTEM analyses also indicated that the density of crystal defects starts to increase gradually at regions far from the interface, which is why we call the films quasi-epitaxial. But, a high crystallinity is still observed over thicknesses that are sufficient to form device emitters. The σ of the (n+ )qEpi-Si films was measured using specific electrical test structure [8], and σ values of about 150 Ω−1 · cm−1 were obtained. These values are about an order of magnitude higher than those of high-quality (n+ )nanoor microcrystalline Si films of about the same thickness [9], [10]. The fabricated diodes were used for the evaluation of the (n+ )qEpi-Si/(p)mc-Si interface. Fig. 2(a) shows the diode dark I−V −T where three different regimes can be identified in the following: 1) medium (“first” diode), 2) low (“second” diode), and high, forward bias regions. Fig. 2(b) shows the extracted saturation currents of the first and second diodes and the corresponding activation energies (EA ). The EA of 1.16 eV (very close to c-Si bandgap) indicates that the diffusion in quasineutral region (QNR) is the major carrier transport mechanism

TABLE I MEASURED PERFORMANCE PARAMETERS OF SOME OF THE FABRICATED (n+ )qEpi-Si/(p)mc-Si SOLAR CELLS

in medium forward bias. Also, the EA of 0.59 eV (about half c-Si bandgap) indicates that the recombination in the space charge region and the interface region is the major carrier transport mechanism in the low forward bias regime. This clearly shows that the transport mechanism in the (n+ )qEpiSi/(p)mc-Si junction is quite similar to that of a single sided c-Si homojunction. Further, the dominance of the diffusion in QNR in medium forward bias (the operation regime of solar cells) indicates that the interface of the (n+ )qEpi-Si/(p)mc-Si junction can be considered clean for PV application. The high crystallinity of the film and the junction quality indicate that

FARROKH-BAROUGHI AND SIVOTHTHAMAN: SILICON PV CELL USING LT

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IV. C ONCLUSION Highly conductive (n+ )qEpi-Si emitters were epitaxially grown on granular Si substrates using a standard PECVD system at 250 ◦ C. The HRTEM analysis confirmed the epitaxial growth and that the emitter benefits from high crystallinity. The dark I–V –T characteristic of the (n+ )qEpi-Si/(p)mc-Si diodes is very similar to the dark I–V –T characteristic of homojunctions. The interface is found to be of good quality for PV applications. Solar cells with η > 10% were fabricated, without using a TCO layer, using a simple device structure in an LT technology. ACKNOWLEDGMENT The authors would like to thank T. Moriarty of NREL for cell measurements. R EFERENCES Fig. 3. Illuminated I–V characteristic of the (n+ )qEpi-Si/(p)mc-Si solar cell measured at the National Renewable Energy Laboratory (device schematic and table have been inserted for clarity).

the new deivce can potentially be as good as crystalline Si p-n junction solar cells. High-frequency capacitance–voltage measurements performed on the diodes gave a built-in potential of 720 mV, which is comparable to the values of Si homojunctions, e.g., 0.8 V. This indicates that the charged interface states that affect the built-in potential [11] are not significantly present at our (n+ )qEpi-Si/(p)mc-Si interface. Table I lists the measurement results on the fabricated (n+ )qEpi-Si/(p)mc-Si solar cells. As listed in the table, the fill factor (FF) of 74%–76% obtained without using any TCO layer confirms that the (n+ )qEpi-Si emitter does not introduce noticeable resistive loss. The high FF also shows that the junction and the interface are of device quality. Fig. 3 shows an current–voltage (I–V ) of a 1.02-cm2 (n+ )qEpi-Si/(p)mc-Si solar cell independently confirmed at 25 ◦ C under AM1.5 illumination at the National Renewable Energy Laboratory (NREL, Colorado). It should be noted that these devices had a very simple structure. Features such as surface texturing and back surface field will further improve the efficiency. The LT nature and simplicity of the process make it ideally suited for low-quality defective Si substrates that can be improved by preprocess hydrogen passivation treatments.

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