Materials Letters 112 (2013) 62–65
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Comparison of pillar-array and hole-array patterned Si solar cells Seung-Hyouk Hong a, Hyunyub Kim b, Hyunki Kim c, Joondong Kim a,n a b c
Department of Electrical Engineering, Incheon National University, Incheon 406772, Republic of Korea School of Information and Communication Engineering, Sungkyunkwan University, Suwon 440746, Republic of Korea Department of Electrical Engineering, Kunsan National University, Kunsan 753701, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 18 July 2013 Accepted 31 August 2013 Available online 7 September 2013
Periodically patterned pillar-arrays and hole-arrays were applied to solar cells. A pillar has a width of 1.81 μm and a hole has a width of 2.28 μm in a similar periodic length of about 4 μm at a fixed depth of 2 μm. The three-dimensional patterns provide an optical benefit of reduction in reflection and thus, drive more photons into a Si absorber. From an electrical aspect, a patterned structure has a short carriercollection length that results in an improved open-circuit voltage, yielding a value of 609 mV, which is better than the value of from 592 mV for a textured Si device. & 2013 Published by Elsevier B.V.
Keywords: Pillar-array Hole-array Patterned structures Silicon solar cells Collection length
1. Introduction Si photovoltaics dominate the industry; however, a significant improvement is highly required for large-scale deployment. High cost, which is a main burden of Si solar cells, which can be relieved by achieving highly efficient performance. The enhancement of light absorption in a thin light-absorber [1,2] is crucial to relieve the cost burden. A suitable front surface, the point at which the incident light comes in, is crucial in the design of effective light trapping strategies [3–6]. For an ideally rough substrate, Lambertian light trapping can be achieved by the extension of the optical path length by 4n2, where n is the refractive index [7]. Recently, a periodic structure has been found to possibly provide a paramount extension of the optical path length by 14.5n2. A recent theoretical report has showed that a hole-array structure can satisfy the equivalent efficiency of wafer Si solar cells with only one-twelfth the mass and one-sixth the thickness [1]. Hole-arrays are superior to rod arrays both in terms of their mechanical robustness [8] and in their efficiency enhancement; they also allow an easier modulation of the Si fill fraction [6]. A radial pillar structure is an effective design to reduce the carrier collection length; such a reduction will result in improved current performance [5]. Moreover, pillar structures spontaneously work as anti-reflection layers and provide an optical coupling effect that improves the light utilization in a Si absorber [9].
n
Corresponding author. Tel.: þ 82 32 8358770; fax: þ 82 32 835 0773. E-mail address:
[email protected] (J. Kim).
0167-577X/$ - see front matter & 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.matlet.2013.08.132
Although much attention has been paid to the periodic structures for solar cells, little has been devoted to the comparison of hole and pillar structures in terms of their electrical and optical aspects. Herein, we present pillar-array and hole-array solar cells. Periodic three-dimensional (3D) Si absorbers were patterned for a pillar-array absorber and a hole-array absorber with similar geometric features. The 3D structures efficiently drive more photons into the Si and effectively collect more photogenerated carriers. The pillar-array, having the shortest collection length provides the highest conversion efficiency with a significantly enhanced solar cell voltage.
2. Experimental procedure Hole-arrays and pillar-arrays were formed on a p-type (100) 4 in.-Si wafer having a resistivity of 1–10 Ω cm. The photoresist (PR) process was performed to shape the PR mask patterns, protecting the Si substrate during the reactive ion etching. For the hole-array patterns, positive PR processes were performed. UV exposed PR patterns were removed during a developing process to leave inversely replicated shapes from the glass photomask. Meanwhile, negative PR processes were applied for the pillar-array patterns. UV exposed PR patterns were remained on the Si substrate; these patterns had the same features as those of the patterns of the glass photomask. Reactive ion etching was performed with an initial flow of C4F8 gas to form a polymer coating layer. SF6-plasma was used to etch the residual polymer layer and the PR-maskless Si substrate. After the PR mask patterns were removed, periodic-arrays were achieved. In comparison, textured Si was also prepared by a conventional alkaline-based texturing method.
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Fig. 1. SEM images of: (a) hole-array Si and (b) pillar-array Si. SEM images of SiNx-film thickness of (c) planar Si substrate and (d) patterned-Si. (e) Reflectance profiles of textured Si, hole-array, and pillar-array.
Fig. 2. I–V characteristics: (a) under dark condition and (b) under one-sun illumination.
For p–n junction formation, n-type doping was performed both for the patterned Si wafer and for the flat Si wafer. Phosphorous oxychloride (POCl3) as an n-type doping source was driven into a furnace for 40 min at 800 1C.
3. Results and discussion Fig. 1 shows hole-arrays and pillar-arrays patterned on a Si substrate with similar depths of 2 μm. Two-different shapes were formed from an identical photomask. The size of the PR-region was designed to be 2 μm 2 μm with a period of 4 μm. A positive PR process was used to open the UV-exposed PR-region. Meanwhile a negative PR process remained the UV-exposed PR-region. After the RIE process, the hole had a width of 2.28 μm with a period of 3.95 μm; the pillar had a width of 1.81 μm, with a period of 4.04 μm. One can imagine the formation of a planar Si substrate by overlapping a pillar-array pattern on a hole-array pattern. Light in the red wavelength ( 600 nm) is the most significant for Si materials [10]. A thin SiNx was designed as an anti-reflection
Table 1 Solar cell performances. Device
Jsc [mA/cm2]
Voc [mV]
Efficiency [%]
Fill factor
Textured Si Hole-arrays Pillar-arrays
34.26 35.5 35.4
592 598 609
15.0 15.6 16.2
74.3 72.9 74.6
coating layer for the Si and the air systems. The objective wavelength of 600 nm was targeted to have a minimum light-reflection. The SiNx has a refractive index (n) of 1.9; the optimum thickness (d) of 80 nm-SiNx was calculated in order to satisfy the quarter wavelength (λ) design scheme (d¼λ/4n). A SiNx-coating was processed for the hole-arrays and the pillar-arrays. For comparison, a SiNx-coating was also applied for planar Si, under the same conditions. A SiNx-film of 79 nm was achieved from a planar Si substrate, as shown in Fig. 1(c). A similar thickness of 76 nm was formed along the patterned-shape, as can be seen in Fig. 1(d).
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Fig. 3. (a) IQE profiles and (b) relative IQE values.
To investigate the light transport at the surface, we measured the reflectance from the hole-array, the pillar-array, and the planar Si substrate, as shown in Fig. 1(e). An average reflectance of 9.56% was obtained from the textured Si in the range of 300–1100 nm. A minimum reflectance of 0.018% was achieved at a wavelength of 607 nm, which agrees well with the design of the SiNx-film thickness. More effective reduction in reflection was achieved, with values of 8.60% and 8.84% for the hole-array and the pillararray structures, respectively. To characterize the junction performances, we measured the I–V characteristics under dark conditions, as shown in Fig. 2(a). The textured Si device showed a stable junction quality, having an ideality factor of 1.35. The hole-array device and the pillar-array device have slightly higher ideality factor values of 1.42 and 1.76, respectively. This clearly shows that a p–n junction is fairly formed inside the 3D structures. Surface enlargement is a favorable advantage of these patterned structures; however, this simultaneously increases the surface recombination problem [11]. Under one-sun illumination (AM 1.5, 100 mW/cm2), a textured Si solar cell exhibits a short current density (Jsc) of 34.26 mA/cm2 and an open-circuit voltage (Voc) of 592 mV, giving a conversion efficiency of 15.0%, as presented in Fig. 2(b) and in Table 1. These values are little lower than those of a commercial solar cell. It is attributed to the thicker Si wafer thickness of 500 μm than 200 μm of a commercial solar cell wafer. A substantially improved efficiency was achieved from both the hole-array and the pillar-array structure. Hole-array and pillar-array devices provided similar current values. 3D structures (hole-arrays and pillar-arrays) were designed to have a maximum current by having a hole or a pillar width as a half of a period [12]. The pillar-array device is effective to achieve the highest Voc value (609 mV) among the devices. Conventionally, the value of Voc usually decreases as the surface area increases due to recombination loss [13]. Different from the typical tendency, pillar-arrays boost the Voc due to the short collection length. The pillar has a width of 1.81 μm. This means that half of the width (0.905 μm) is the collection length of the photogenerated carriers inside the pillar center. Meanwhile, the hole-array has a shape that is the reverse of that of the pillar-array. Each hole is interconnected to neighboring hole structures. Photogenerated carriers are produced in the interconnect region. Considering a hole width of 2.28 μm, the distance from the interconnect center is 1.18 μm to the closest p–n junction. For further investigation, we obtained internal quantum efficiencies (IQEs) for the hole-array and pillar-array solar cells, and for the textured device, with results as shown in Fig. 3(a). The collection efficiency is a function of the effective utilization of the incident photons to control the photogenerated current in a solar
cell. At a wavelength of 600 nm, the pillar-array solar cell showed the highest QE of 97.14%, which is higher than the value of 95.9% of the textured Si. This is a distinctive result from the nanoscale structure solar cell [11]. The relative IQE values shown in Fig. 3(b) clearly demonstrate the collection performances of the hole and the pillar structured solar cells compared to that of the planar device. For the visible range, it is clearly observed that the pillar-array and the hole-array fairly improve the QE performances. The wavelength of 1100 nm is the longest wavelength to have a higher energy than that of the bandgap of Si (1.1 eV). At that wavelength, substantial enhancements were achieved by 161.1% and 155.7% from the pillar-array and the hole-array devices, respectively. Meanwhile, a reduced QE performance was obtained from the pillar-array and the holearray devices at short wavelengths shorter than 400 nm. This clearly shows the existence of recombination loss in 3D structures. The patterning process always brings surface defects causing a recombination problem [11], which significantly deteriorates the carrier collection at short wavelengths. The relative IQE profiles certainly indicate a disadvantage of the structured light absorber at short wavelengths. However, the general tendency is an improvement of carrier collection with the use of 3D structures.
4. Conclusions Periodic 3D structured-Si absorbers were fabricated for solar cells. Hole-arrays and pillar-arrays were patterned with a similar period of 4 μm on p-type Si wafers. The hole pattern has a width of 2.28 μm and the pillar pattern has a width of 1.81 μm at a depth of 2 μm. Optically, both hole-arrays and pillar-arrays are effective, at reducing the reflection at the surface and therefore, drive more photons into the Si light-absorber. Electrically, the pillar-array has a shorter carrier collection length than those of the hole-array or the planar substrate; the pillar-array device provided the highest conversion efficiency of 16.2%, with a highly-improved voltage. An efficient 3D solar cell enables an enlargement of the light-reactive surface with efficiently reduced reflection, allowing the effective driving of the incident photons.
Acknowledgment The authors acknowledge the financial support of the Korea Institute of Energy Technology Evaluation and Planning (KETEP20113030010110). Seung-Hyouk Hong and Hyunyub Kim contributed equally to this work.
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