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IEEE ELECTRON DEVICE LETTERS, VOL. 31, NO. 10, OCTOBER 2010
Improved Efficiency by Using Transparent Contact Layers in InGaN-Based p-i-n Solar Cells Jae-Phil Shim, Seong-Ran Jeon, Yon-Kil Jeong, and Dong-Seon Lee
Abstract—InGaN/GaN p-i-n solar cells were fabricated either without a current spreading layer or with ITO or Ni/Au spreading layers. A 10.8% indium composition was confirmed within an i-InGaN layer using X-ray diffraction. I–V characteristics were measured at AM1.5 conditions, with solar cell parameters being obtained based on I–V curves in all cases. Current spreading layers produced strong effects on efficiency. The solar cell with the ITO current spreading layer showed the best results, i.e., a short circuit current density of 0.644 mA/cm2 , an open circuit voltage of 2.0 V, a fill factor of 79.5%, a peak external quantum efficiency of 74.1%, and a conversion efficiency of 1.0%. Index Terms—Efficiency, Indium Gallium Nitride (InGaN) solar cell, spreading layer.
I. I NTRODUCTION
I
NDIUM gallium nitride (InGaN) has been widely used in various optoelectronic devices, including light-emitting diodes (LEDs) and laser diodes [1], [2]. Such a material has also been widely promoted in solar cell fabrication due to superior optical properties such as high absorption coefficients and good electrical properties, i.e., high mobility and saturation velocity, and strong radiation tolerance [3]–[7]. One of the biggest advantages of InGaN materials is the feasible engineering of bandgap energy from 0.7 eV, for InN, to 3.4 eV, for GaN, by varying indium composition, almost covering the entire solar spectrum from ultraviolet to infrared [8]. If GaNbased solar cells absorb most of the solar spectrum via bandgap engineering, then significantly improved efficiency will be possible. To increase absorption wavelength regions, research into InGaN layer growth with high In content continues. Most InGaN-based solar cells have relied on the identical electrode structure used in small LEDs, i.e., a single metal electrode Manuscript received May 10, 2010; revised June 28, 2010; accepted June 28, 2010. Date of publication September 13, 2010; date of current version September 24, 2010. This work was supported by the Ministry of Education, Science and Technology under Grant 2009-0065432. The review of this letter was arranged by Editor C. Jagadish. J.-P. Shim is with the School of Information and Communications, Gwangju Institute of Science and Technology, Gwangju 500-712, Korea (e-mail:
[email protected]). S.-R. Jeon is with the Korea Photonics Technology Institute, Gwangju 500779, Korea (e-mail:
[email protected]). Y.-K. Jeong is with the Research Institute for Solar and Sustainable Energies (RISE), Gwangju Institute of Science and Technology, Gwangju 500-712, Korea (e-mail:
[email protected]). D.-S. Lee is with the School of Information and Communications and the Reserch Institute for Solar and Sustainable Energies (RISE), Gwangju Institute of Science and Technology, Gwangju 500-712, Korea (e-mail:
[email protected]). 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.2010.2058087
for each n- and p-contact. Such a shape definitely presents drawbacks as device dimension increases. For such a reason, grid-type metal electrodes have been introduced, demonstrating higher external quantum efficiency (EQE) [4]. Enhanced sunlight transmission and generated carrier extraction are also critical factors to consider. In addressing such factors, various InGaN solar cells rely on Ni/Au semitransparent layers to achieve transparency as well as to establish Ohmic contact to p-GaN. However, the transmittance of Ni/Au layers represents less than 60% in 350–400-nm wavelength regions near GaN bandgap energy [9], [10]. In order to reduce absorption in conventional Ni/Au transparent layers, a new transparent material application is required. In this letter, the results of grid metal electrode fabrication and solar cell performance are reported. In addition, InGaN/GaN solar cell characteristics are compared and contrasted among current spreading layers such as Ni/Au or ITO, as well as in the absence of such layers. II. E XPERIMENT InGaN/GaN p-i-n structures were grown using metal organic chemical vapor deposition (MOCVD). Trimethylgallium, ammonia, and trimethylindium were used as Ga, N, and In sources, respectively. Si-doped 2.2-μm n-GaN was grown on an undoped GaN/Al2 O3 template. A 200-nm intrinsic InGaN layer, at 805 ◦ C, and a Mg-doped 100-nm p-GaN layer were grown sequentially. The composition of the InGaN layer was estimated by X-ray diffraction (XRD). An ω−2θ scan curve of (0002) InGaN peak revealed In composition at 10.8%, corresponding to bandgaps of 3.02 eV (not shown here). Devices of 2.5 mm × 2.5 mm were fabricated on an epitaxial structure with metal grid electrodes situated on top. The structure of the fabricated device and real image are shown in Fig. 1(a) and (b), respectively. Two sets of dissimilar current spreading layers, i.e., Ni/Au (5 nm/5 nm) and ITO (150 nm), were deposited on the p-GaN. Then, a grid-shaped Ni/Au (30 nm/500 nm) p-contact pad was formed on each current spreading layer, producing Au-to-PCB wire bonding. An additional set without a current spreading layer, including only a p-contact pad, was also fabricated. A Pd/Au (30 nm/500 nm) p-contact pad, previously reported as displaying lower contact resistance than a Ni/Au p-contact pad, was formed for the set [11]. Three different sets were annealed at 500 ◦ C for 1 min under ambient conditions by means of rapid thermal annealing. Furthermore, all three devices utilized the identical Ti/Al/Ti/Au (30 nm/80 nm/20 nm/500 nm) n-contact pad. Current–voltage (I–V ) characteristics were measured by an Oriel Class AAA
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SHIM et al.: IMPROVED EFFICIENCY BY USING CONTACT LAYERS IN InGaN-BASED p-i-n SOLAR CELLS
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Fig. 3. External quantum efficiency of fabricated InGaN/GaN p-i-n solar cells by varying current spreading layers (ITO, Ni/Au, no spreading layer). The solar cell with ITO showed 74.1% at 382 nm.
Fig. 1. (a) A InGaN/GaN p-i-n solar cell device structure. (b) Real image of fabricated device.
Fig. 2. I–V characteristics of InGaN/GaN p-i-n solar cells with various current spreading layers (ITO, Ni/Au, no spreading layer) under AM1.5 conditions.
solar simulator under AM1.5 conditions, with EQE being analyzed by the incident photon current efficiency measurement system. III. R ESULT AND D ISCUSSION As shown in Fig. 2, obtained fill factors (F.F.) under AM1.5 conditions represented 79.5%, 73.9%, and 79.2%, corresponding to ITO, Ni/Au, and spreading layer absence, respectively.
Open-circuit voltage (Voc ) stood at approximately 2.0 V for all samples. A large Voc value was characteristic of InGaN solar cells, with the present value revealing a similar or larger value than those of previous publications [12]–[14]. Although a relatively high F.F. and Voc were obtained for all samples, short circuit current density (Jsc ) varied for each sample. The Jsc of the device with the ITO spreading layer stood at 0.644 mA/cm2 , while the Jsc without a spreading layer stood at 0.599 mA/cm2 . However, the device using the Ni/Au semitransparent layer, widely used as spreading layers in InGaN solar cells, showed the lowest value, i.e., 0.507 mA/cm2 . Using either a variety of spreading layers, or none at all, clearly impacted Jsc of the devices. Short circuit current density is affected by both incident light absorption and carrier extraction rate. By minimizing light absorption within the top layer of solar cells, incident light absorption is maximized in i-InGaN active layers [4]. Simultaneously, generated electrons within i-InGaN active layers move to metal pads with no loss. Fig. 3 shows current density differences more clearly by analyzing EQE according to different wavelengths. Major absorption of light occurred within the 360–400-nm range for all solar cells. All samples displayed zero efficiency at over 400 nm, corresponding to the estimated bandgap energy (3.02 eV) of the In0.11 Ga0.89 N layer, formerly measured by XRD. In addition, EQE decreased to below ∼370 nm, approximating the bandgap energy of GaN (3.42 eV) because of absorption on the p-GaN layer. Current density differences were explained by an EQE variance of 360–400 nm. The solar cell with the ITO current spreading layer showed the highest EQE among the three samples, 74.1% at 382 nm. Furthermore, ITO transmittance, which increased up to 85% near the 400-nm region, enhanced EQE more than the Ni/Au sample [9], [10]. Relatively low transmittance of the Ni/Au current spreading layer (∼50%) at a short wavelength (∼400 nm) resulted in absorption reduction of incident light probability from 360 to 400 nm. Interestingly enough, the solar cell with the Ni/Au spreading layer showed lower EQE than such a cell without a spreading layer. Total sunlight absorption was estimated on layer surfaces due to the absence of absorption on spreading layers. An approximate 100% transparency rate in devices
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TABLE I S OLAR C ELL PARAMETERS
ACKNOWLEDGMENT The authors would like to thank D. Lee in the Organic Solar Cell Laboratory, Seoul National University, for the EQE measurements and helpful discussions. R EFERENCES
without spreading layers, as opposed to devices with Ni/Au spreading layers, resulted in higher EQE; however, compared to ITO spreading layers, EQE lowered as wavelengths passed over the ∼340-nm region. Although no spreading layer had a high enough transparency rate to stand out among the three samples, less efficient electron extraction during spreading layer absence, compared to the ITO spreading layer sample, lowered EQE. Solar cell parameters are summarized and compared in Table I. The solar cell with the ITO current spreading layer showed the best results, a 79.5% F.F. and 1% conversion efficiency, which is better than the previously published value of 0.8% measured under 1.5 AM [13]. EQE of the solar cell with the ITO current spreading layer was lower than such a cell without a spreading layer in the region below ∼340 nm. The present result occurred due to ITO absorption below the corresponding bandgap (∼3.6 eV). Nevertheless, the solar cell with the ITO current spreading layer represented the highest conversion efficiency among all samples. In order to significantly improve the overall conversion efficiency, pouring much effort into improving InGaN layer quality, particularly thick In-rich InGaN layers, would be clearly justified. Then, ITO is getting more and more important since it revealed its excellent performance in the visible range through LEDs. IV. C ONCLUSION The p-i-n InGaN/GaN structure with an InGaN layer containing 10.8% of In was grown by means of MOCVD. On the epitaxial layer, three sets of different devices with dissimilar current spreading layers were fabricated. The solar cell with the ITO spreading layer under AM 1.5 conditions showed the following superior results: 2.0 V of open circuit voltage, 79.5% F.F., 74.1% peak EQE, and 1% conversion efficiency. InGaN-/GaN-based solar cells should be more improved not only by means of fabrication optimization, including the use of ITO and antireflection layers, but also by improving InGaN layers, particularly thick In-rich InGaN.
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