Strain effect for different phosphorus content of InGaAs/GaAsP super-lattice in GaAs p–i–n single junction solar cell Kentaroh Watanabe a,n , Yunpeng Wang a , Hassanet Sodabanlu a , Masakazu Sugiyama b , Yoshiaki Nakano a,b a Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan b Department of Electrocal Engineering & Information System, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 133-8656, Japan article info Keywords: A1. Superlattices A2. Metalorganic vapor phase epitaxy A3. Quantum wells B2. Semiconducting III–V materials B3. Solar cells abstract The GaAs p–i–n single junction solar cell with InGaAs/GaAsP super-lattice (SL) in the i-region was fabricated by metal organic vapor phase epitaxy (MOVPE). Using the in situ wafer curvature monitoring, a series of SL solar cell samples with different phosphorus composition in the barrier GaAsP layer was evaluated the accumulated strain during MOVPE growth. The sample with larger phosphorus content in GaAsP barrier layer reduced total strain accumulation, resulted in improved solar cell performance regardless to the higher potential barrier. This result indicated the about 3-nm thick barrier is sufficiently thin for carrier extraction by assisting the tunneling effect. Furthermore, the accumulated strain during MOVPE growth of SL deteriorate solar cell. & 2014 Elsevier B.V. All rights reserved. 1. Introduction In recent years, the conversion efficiency of the photovoltaic (PV) solar cell basis on III–V semiconductors has been rapidly increasing with well developed epitaxial crystal growth technol- ogies [1–3]. A multiple quantum wells (MQWs) solar cell has an advantage of higher open-circuit voltage (V OC ) compared with a single junction solar cell composed of host barrier material alone [4]. Because the material with around 1.2 eV effective band gap as on optical absorber can be realized with low degradation of PV performance, the strain-compensated InGaAs/GaAsP MQWs is one of the promised candidates as a middle cell of current-matched triple-junction solar cell [5]. By compensating a strain force on InGaAs well and GaAsP barrier layer each other, totally minimum accumulated strain provided the maximum PV performance with MQWs cell [6]. The simple problem of the MQWs cell is the limitation of the optical thickness of the MQWs. The applied electric field to MQWs located in the i-region of p–i–n structure is defined by the thickness of the i-region. Because more than 3 μm of total thickness of MQWs are required to achieve sufficient absorption, so such a large i-region thickness reduced carrier extraction due to the small built in potential applying to MQWs which easily trap the photo carrier in the wells [7]. A super-lattice (SL) consisting of InGaAs well and extremely thin GaAsP barrier (  3 nm) is expected to improve the carrier extraction efficiency by assisting with tunneling effect through thin GaAsP barriers. We previously demonstrated the SL solar cell with minimal degrada- tion of open-circuit voltage at 1 SUN illumination compared to relatively thick (  10 nm) barrier thickness [8]. Furthermore, the InGaAs/GaAsP SL absorber in the i-region of GaAs p–i–n structure was clarified to provide better PV performance in concentrated sun irradiation by enhanced V OC maintaining the short-circuit current density (J SC ) increment [9]. In order to elucidate the detail mechanism of carrier extraction from SL cells, we focused on both the crystal quality of SL and the carrier dynamics with a tunnel effect thorough the barrier layer for the SL solar cells. In this study, we prepared various phosphorus contents of InGaAs/GaAsP SL in GaAs p–i–n solar cells and reported the evaluation result of the effect of difference of accumulated strain for PV performances. 2. Experiment 2.1. MOVPE growth of GaAs p–i–n structure with SL Fig. 1 shows the schematic structure of GaAs p–i–n single junction solar cell with 60 periods of InGaAs/GaAsP super-lattice in the i-region. All of our samples were grown by the planetary metal-organic vapor phase epitaxy (MOVPE) system (AIXTRON, AIX2000HT) on n-type Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth http://dx.doi.org/10.1016/j.jcrysgro.2014.02.053 0022-0248/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: +81 3 5452 5367. E-mail addresses: kentaroh@hotaka.t.u-tokyo.ac.jp, kentapon1230@gmail.com (K. Watanabe). Please cite this article as: K. Watanabe, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2014.02.053i Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎