Research Article
Vol. 55, No. 13 / May 1 2016 / Applied Optics
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Theory of plasmonic quantum-dot-based intermediate band solar cells SINA FOROUTAN
AND
HAMED BAGHBAN*
School of Engineering-Emerging Technologies, University of Tabriz, Tabriz 5166614761, Iran *Corresponding author: h‑
[email protected] Received 16 December 2015; revised 4 March 2016; accepted 22 March 2016; posted 22 March 2016 (Doc. ID 255975); published 22 April 2016
High scattering cross section of plasmonic nanoparticles in intermediate band solar cells (IBSCs) based on quantum dots (QDs) can obviate the low photon absorption in QD layers. In this report, we present a modeling procedure to extract the optical and electrical characteristics of a GaAs-based plasmonic intermediate band solar cell (PIBSC). It is shown that metal nanoparticles (MNPs) that are responsible for scattering of incident photons in the absorber layer can lead to photocurrent enhancement, provided that an optimum size and density is calculated. Proper design of QD layers that control the intermediate energy band location, as well as the lossscattering trade-off of MNPs, can result in an efficiency increase of ∼4.2% in the PIBSC compared to a similar IBSC, and an increase of ∼5.9% compared to a reference GaAs PIN cell. A comprehensive discussion on the effect of intermediate band region width and current-voltage characteristics of the designed cell is presented. © 2016 Optical Society of America OCIS codes: (350.6050) Solar energy; (230.5590) Quantum-well, -wire and -dot devices; (240.6680) Surface plasmons. http://dx.doi.org/10.1364/AO.55.003405
1. INTRODUCTION Intermediate band solar cells have been asserted as promising photovoltaic cells for realizing solar power conversion efficiencies >60% [1], compared to the Shockley–Queisser limit of 40.7% for a single gap cell under maximum concentration [2]. Due to the existence of an intermediate band (IB) within the bandgap of a semiconductor, intermediate band solar cells (IBSCs) can generate photocurrent from the subbandgap photons, without reducing the voltage [3]. Such an energy band can be created utilizing confined states in a three-dimensional quantum dot (QD) where periodic arrays of QDs, located in a flat-band potential region, form an intermediate band due to the overlap of electron states in QDs [4]. Even though IB-associated multiband transitions have been theoretically demonstrated to attain significant enhancement in power conversion efficiency, usually the measured absorption of the QDs related with such transitions are quite small and one or more orders of magnitude lower than that of their bulk materials [5–7]. One way of boosting this absorption is to grow more QD layers; however, this method causes strain-induced dislocations that reduce the device throughput. Processes are being developed to reduce strain, but still this is a challenging issue. Improved photocurrent in QD solar cells with straincompensated layers has been discussed in [8,9]. Despite these reported successes, the generated photocurrent from subbands is about 1% of the current generated from the interband process [10]. It has also been demonstrated that placing InAs 1559-128X/16/133405-08 Journal © 2016 Optical Society of America
QDs in the intrinsic (I) layer of a P-I-N solar cell broadens the photoresponse spectra and provides higher short circuit current (∼53% increase) [11]. Comparing the experimental results obtained from an InAs/ GaAs-based QD IBSC with theoretical predictions it has been confirmed that the operational characteristics of IBSCs are originated from the carrier dynamics via the IB at room temperature [12]. Based on these observations, it seems critical to achieve a functional IBSC in which larger photocurrent is generated through the subband energy levels and can be significantly enhanced while the output voltage is preserved. So far numerous efforts have been allocated to surface plasmon-enhanced solar cells by utilizing metal nanoparticles (MNPs) [13–16]. In particular, MNPs have been proposed to enhance absorption in QDs of intermediate band solar cells [17–19]. An enhancement of about 2 orders of magnitude has been reported due to the intense photon scattering by the near field in metallic nanoparticles by placing MNPs in close adjacency of quantum dots [17]. However, besides the optical analysis performed, there is no report on the electrical properties of the IBSC. It has also reported that MNPs located on the surface of InAs/GaAs quantum dot solar cell result in an enhancement in the photocurrent and spectral response, which leads to a power conversion efficiency enhancement from 8.0% to 8.9% for Ag nanoparticles, and to 9.5% for Au nanoparticles [18]. Also, a colloidal deposition technique has been utilized in [19]
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Research Article
Vol. 55, No. 13 / May 1 2016 / Applied Optics
to fabricate ordered hybrid arrays of self-assembled MNPs and QDs. The effect of MNPs on the absorption of the quantum dot layer has been analyzed in this study by measuring the optical loss within the active layer before and after placing the MNPs on the surface and inside the QD layer. However, no report on the electrical properties of the developed IBSC has presented. Besides the experimental reports on the improved photocurrent induced by MNPs, there still exists a lack of modeling procedure to fully deploy the optical and electrical properties of plasmonic intermediate band solar cells (PIBSCs), which will be covered in the current study. Moreover, metallic loss and high absorption cross section of MNPs especially for nanoparticles with small dimensions (diameters
