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Three-Terminal Heterojunction Bipolar Transistor Solar Cells with Non-Ideal Effects Efficiency Limit and Parametric Optimum

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Three-Terminal Heterojunction Bipolar Transistor Solar Cells with Non-Ideal Effects Efficiency Limit and Parametric Optimum Energy Conversion and Management 188 (2019) 112–119 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman Three-terminal heterojunction bipolar transistor solar cells with non-ideal effects: Efficiency limit and parametric optimum selection T ⁎ ⁎ Xin Zhanga,b, Yee Sin Angb, Zhuolin Yea, Shanhe Sua, Jincan Chena, , Lay Kee Angb, a Engineering Research Center of Micro-nano Optoelectronic Materials and Devices, Ministry of Education, Fujian Key Laboratory of Semiconductors and Applications, Collaborative Innovation Center for Optoelectronic Semiconductors and Efficient Devices, and Department of Physics, Xiamen University, Xiamen 361005, People’s Republic of China b Singapore University of Technology and Design-Massachusetts Institute of Technology International Design Center & Science and Math Cluster, Singapore University of Technology and Design, 8 Somapah Road, Singapore 487372, Singapore ARTICLE INFO ABSTRACT Keywords: Without fabricating intermediate tunnel junctions or wafer bonding schemes for interconnecting the subcells, Three-terminal solar cell heterojunction bipolar transistor solar cells offer a promising new route in solar energy conversion. In this work, Heterojunction bipolar transistor an improved theory for the three-terminal heterojunction bipolar transistor solar cell is presented with inclusion Irreversible loss of non-ideal effects missing from the previous treatment, namely the non-radiative recombination and the Performance evaluation thermal conduction losses that are inevitably present in realistic devices. Following detailed balance theory, the Parametric optimization revised analytical formula for the cell conversion efficiency is derived, and the maximum efficiencies under different conditions are further calculated. Under the condition of 100 sun irradiance and 50% injection effi- ciency, a Gallium arsenide/Gallium antimonide-based solar cell operating at 465 K yields a maximum efficiency of 46.4%. Moreover, the effects of solar concentration, injection efficiency, and other key parameters on the cell performance are analyzed, and, consequently, optimal operating conditions and limiting factors on the con- version efficiency are determined. Simulation results show that such a solar cell operating with low injection efficiency under moderate concentration factor and low cell temperature can significantly boost its conversion performance. This work provides new physical insights for optimal designs, thus paving a route towards the development of low-cost high-performance solar cells. 1. Introduction Shockley-Queisser efficiency limit of 32.2% for silicon-based PV cells operated at room temperature under unconcentrated solar illumination Solar energy, a promising renewable energy source, has attracted [7]. widespread interests due to its abundant reserves and environmental For the purpose of improving light utilization and minimizing the friendliness [1]. It plays a significant role in reducing global climate thermalization losses, numerous methods, including multi junction [8], change and the next-generation energy resources [2]. The most heavily spectrum splitting [9], hot carrier collection [10], and intermediate- deployed technology for solar power conversion is single junction si- band [11] photovoltaics have been developed to minimize the ther- licon-based photovoltaics (PV) with a record efficiency of 28.3% [3]. modynamic losses by expanding the device wavelength responses to the Despite worldwide cumulative photovoltaic installed capacity exceeded ultraviolet and infrared regions. In 1958, Jackson first presented the 401 GW by 2018, the conversion efficiencies for most of this manu- model of multi-junction cells with different bandgap semiconductors in factured output still remain in the range of 10–18% [4]. In such PV a series connected architecture, in which each subcell acts as a filter cells, photons with energy larger than semiconductor bandgap excite absorbing the spectral range corresponding to the bandgap of each electrons into the conduction band, which diffuses to the electrodes to semiconductor layer, thus allowing the solar spectrum to be more ef- form an electrical current [5]. However, photons with energy lower fectively utilized [12]. One disadvantage of this design is reflected in than the bandgap cannot be utilized, while photons with energy above the epitaxial growth of the single-crystalline semiconductor layers and the bandgap will lose excess energy due to thermalization [6]. Together the intermediate tunnel barrier buffer layers, which requires complex with radiative recombination losses, these spectral losses result in the and expensive ultrahigh-vacuum crystal-growth techniques [13]. ⁎ Corresponding authors. E-mail addresses: [email protected] (J. Chen), [email protected] (L.K. Ang). https://doi.org/10.1016/j.enconman.2019.03.034 Received 16 January 2019; Accepted 12 March 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved. X. Zhang, et al. Energy Conversion and Management 188 (2019) 112–119 −2 Nomenclature Psun incident solar energy (W cm ) q Elementary charge (C) C solar concentration TS sun temperature (K) Cm maximum theoretical solar concentration TC cell temperature (K) −1 c speed of light (cm s ) TA ambient temperature (K) −1 −2 Efe electrons quasi-Fermi level (eV) U heat leak coefficient (W K cm ) Efh holes quasi-Fermi level (eV) VE Voltage output of the emitter terminal (V) ET top bandgap (eV) VC Voltage output of the collector terminal (V) EB Bottom bandgap (eV) F solar concentration factor Greek symbols − h the Planck constant (eV s 1) e −2 ffi JBC current density crossing BC junction (A m ) γ Injection e ciency h −2 JB current density in the Base region (A m ) λ sensitivity for the non-radiative process e −2 ffi JEB current density crossing EB junction (A m ) η e ciency −2 JC (A m ) −2 JE (A m ) Abbreviations − k Boltzmann constant (eV K 1) n refraction index BC base–collector − P power output density (W cm 2) EB emitter–base −2 Pleak heat flux due to heat conduction (W cm ) HBTSC heterojunction bipolar transistor solar cell −2 Prad radiative flux from the HBTSC (W cm ) Additionally, due to the current-matching, the lowest current generated without the need of an intermediate tunnel junctions or wafer bonding by the subcell will ultimately limit the overall cell current. An alter- schemes for cell interconnection [26], HBTSC shares the same limiting native approach is the use of spectrum-splitting technology to overcome efficiency as the dual-junction solar cell and is free from the current the limitations of current matching and material choice for the different mismatch problem [27]. Nowadays, several theoretical investigations subcells, by splitting the solar spectrum into two bands of photons with regarding the HBTSC have been reported. The pioneering work by different-wavelength [14]. Here, photons with energy greater than the Luque and Martí put forward a concrete theoretical framework that bandgap are directed to the PV cell, while those with energy less than underlies the novel concept and calculated the efficiency limits of n/p/ the bandgap are absorbed by thermal receiver [15]. Ross and Nozik first n-type ideal HBTSCs [22]. The simulation results predicted that the reported that the conversion efficiency of the hot carrier solar cell HBTSC obtains a detailed-balance limit of 54.7% under the maximum under AM1.5 can reach a theoretical limit of 66%, which is 52% higher concentration, which is the same as that of a dual-junction solar cell. than that of traditional Si PV cell systems [16]. König et al. further Martí et al. further discussed the working principle of the HBTSC based proposed the principle, materials and design of hot carrier solar cells on a circuit model, revealing the transistor effect should be avoided with energy selective contacts [17]. The main challenges of this tech- [28]. Linares et al. presented three-terminal solar cells resembled by a nology are the lack of suitable materials with drastically reduced carrier heterojunction BJT, and then explored Si, a-Si: H, III-Vs, nanomaterials cooling rates and the fabrication of selective energy contacts to extract and perovskites for practical implementation [23]. the photogenerated carriers, which severely impedes its progress to- For practical HBTSCs, the cell temperature always increases dyna- wards commercial applications [18]. Luque and Martí theoretically mically above the ambient when exposed to concentrated sunlight. analyzed the efficiency improvement of ideal solar cells by introducing Moreover, there exist several irreversible losses in the HBTSC, which an intermediate band between the conduction and valence bands to may cause additional performance degradation. Such effects are, absorb low-energy photons [19]. Based on this concept, Wang et al. however, not considered in the proof-of-concept calculation presented fabricated bulk intermediate band solar cell using Zinc telluride doped in Ref. [22]. Importantly, for the realistic simulation of HBTSCs, it is with oxygen impurities to form the intermediate band [20]. The prac- necessary to include these effects, so to obtain a reliable theoretical tical implementation of this approach remains challenging due to the efficiency limit and the optimum design of HBTSC. expensive cost of nanostructured intermediate-band materials and the In this paper, an updated model of a HBTSC are proposed with in- fabrication complexity in achieving energy-level alignment [11]. The clusions of: (i) variable temperatures
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