Rahul Dewan - Jacobs University

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2. FUNDAMENTAL CONCEPTS (a) Current Dark V OC Voltage I L I mp V mp P mp Voltage Illuminated I SC Power (b) P mp V OC Figure 2.1: (a) I-V characteristic of a solar cell in dark and under illumination conditions. (b) Output power curve for the corresponding solar cell under illumination. The characteristic parameters of a solar cell are also highlighted in the figure. V mp factor, F F . The I-V curve of a solar cell, along with these parameters, is shown in Fig. 2.1(a). And the output power from the corresponding solar cell under illumination as a function of the bias voltage is also shown in Fig. 2.1(b). Short circuit current is generated due to the generation and collection of photon-generated carriers (when no voltage is produced between the anode and cathode). And ideally, this current I SC is equal to the illumination current I L , i.e. it is the largest current that can be drawn from a solar cell. V OC is the maximum possible voltage available from the solar cell. The voltage of the solar cell reaches the open circuit voltage for current flow of I = 0. From Eq. 2.1 it can be derived that for a given illumination current I L , the open circuit voltage increases logarithmically with decreasing saturation current I S . Even though I SC and V OC are the maximum possible current and voltage obtainable from the solar cell, the power obtained at these two points are zero. In Fig. 2.1 the operating point that generates the maximum power (P mp = V mp I mp ) that can be obtained from the solar cell is also highlighted. Therefore, the maximum thermodynamic efficiency η of the photovoltaic energy conversion process for a solar cell is: 6 η = V mpI mp P IN (2.2)
2.1. Characteristic Parameters of Solar Cells where P IN is the total incident power. We can also observe from Fig. 2.1 that for an ideally shaped I-V curve, it would look rectangular. Which means the solar cell would deliver a constant current density I SC until the open voltage V OC . The term called the fill factor (F F ) has been introduced to measure how close a given characteristic curve of a solar cell is to the ideal rectangular I-V shape. Together with I SC and V OC , the F F is defined by the following ratio F F = V mpI mp V OC I SC (2.3) By definition, F F ≤ 1. By inserting the relationship from Eq. 2.3 into Eq. 2.2, we obtain η = V OCI SC F F P IN (2.4) Along with the three output characteristic parameters of the solar cell, the previous equation defining the efficiency of a solar cell is the most commonly utilized quantity to measure the performance of solar cells. 2.1.1 Shockley-Queisser Efficiency Limit In the last five decades, several authors have studied the efficiency limits of solar cells using different hypothesis on the optical and electrical losses from the solar cell, and have published the efficiency limits for single junction or multi-junction solar cells [15–22]. But due to its simplicity in formalizing the problem, the paper on detailed balance efficiency limits published in 1961 by Shockley and Queisser still provides the benchmark for evaluating the performance of a single junction solar cell [15]. Shockley and Queisser calculated the maximum efficiency of solar cells by only taking the fundamental radiative recombination losses into account. In their work, they assumed the sun and the solar cell in consideration to be two thermodynamic bodies with temperatures of 6000 K and 300 K, respectively. Thus the spectral density of the incident power used in Shockley and Queisser’s work followed the blackbody radiation which is described by the Planck’s law. But due to scattering, absorption and reflection in the atmosphere, sunlight that reaches the Earth’s surface is reduced in power. Therefore in this section we consider the current standard spectral densities for sunlight, namely AM 0 and AM 1.5 1 . The solar radiation just outside the earth’s atmosphere is called AM 0, where the losses are primarily caused by the ultraviolet absorption in ozone and infrared absorption in water vapor. The incident power for AM 0 illumination is approximately 1.353 kW/m 2 . For solar cell performance 1 When the sun is at angle θ to overhead, the air mass (AM) is given by: AM = 1/cosθ. E.g. when the sun is 48.2 ◦ off overhead, the radiation is AM 1.5. 7
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BIBLIOGRAPHY [9] A. Lin and J. Phil
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BIBLIOGRAPHY Photovoltaic Solar Ene
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BIBLIOGRAPHY [57] J. I. Owen, J. H
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BIBLIOGRAPHY [79] K. Yee, “Numeri
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BIBLIOGRAPHY [102] C. Heine and R.
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BIBLIOGRAPHY [124] H. W. Deckman, C
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BIBLIOGRAPHY nanoparticles,” Jour
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LIST OF SYMBOLS AND ACRONYMS Ag Sil
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10. I. Vasilev, R. Dewan, M. Marink
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Acknowledgement • I would like to
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Rahul Dewan - Jacobs University

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