Built-in Potential and Open Circuit Voltage of Heterojunction CuIn 1-x GaxSe 2 Solar Cells
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Built-in Potential and Open Circuit Voltage of Heterojunction CuIn1-xGaxSe2 Solar Cells Akimasa Yamada, Koji Matsubara, Keiichiro Sakurai, Shogo Ishizuka, Hitoshi Tampo, Hajime Shibata, Tomoyuki Baba1, Yasuyuki Kimura1, Satoshi Nakamura1, Hisayuki Nakanishi1 and Shigeru Niki National Institute of Advanced Industrial Science and Technology AIST Central 2, Umezono, Tsukuba 305-8568 Japan 1 Tokyo University of Science 2641 Yamazaki, Noda, Chiba 278-8510 Japan ABSTRACT The reasons why the open circuit voltage (Voc) of high-x CuIn1-xGaxSe2 (CIGS)/ZnO solar cells remain low are discussed. Here it is shown that the Voc ceiling can be interpreted simply on the basis of a model that the valence-band energy (Ev) of CIGS is almost immovable irrespective of x. When the conduction-band energy (Ec) of ZnO is lower than that of high-x CIGS (DEc0 gives rise to a photoelectrons barrier in the conduction-band that partially cancels Vfb, thus the Voc of a low-x CIGS cell is governed by the Ec of CIGS. Based upon this concept, a material selection guideline is given for the windows and transparent electrodes appropriate for high-x CIGS absorbers-based solar cells. INTRODUCTION The bandgap energy (Eg) optimum for the absorber material in a single-junction solar cell is believed to be 1.4-1.5 eV for terrestrial applications. Figure 1 shows the parameters of an ideal solar cell as a function of Eg. These parameters include the conversion efficiency (h), open circuit voltage (Voc), short circuit current density (Jsc), voltage (Vm) and current density (Jm) at maximum output, and the fill factor (FF). The calculation was done using following equations: q N Jsc = hc / P]n gm (n) N = 1 c 2rhc - m ^ 1hm Dm E g n=1 (1) J sc V oc = kT q log ;1 + J 0 E (2) kT J qV m Vm = q cW ;b1 + Jsc l e E - 1m J m =J sc -J 0 dexp ; kT E -1n 0 , (3)
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where q, h, c, k, T and Jo are electron charge, Planck's constant, velocity of light, Boltzmann constant, temperature and the reverse saturation current density, respectively. T was fixed at 300 K and Jo was estimated using material parameters for Si [1] except for Eg. P(n) is the solar power spectrum per unit area sampled at wavelength l(n) within a band Dl; actual sunlight data [2] was used after modifying it to correspond to 100 mW/cm2 input power density (Pin). The Lambert W function, the inverse function of xex, was calculated using Mathematica. As a matter of course, h=JmVm/Pin and FF=JmVm/JscVoc. CuIn1-xGaxSe2 (CIGS) is a promising material for thin film solar cells because its Eg can be adjusted to that predicted theoretically giving the highest h by changing the value of x. In part due to the large absorption coefficient of CIGS, CIGS-based solar cells have achieved an h of over 19% [3]. Although the h of CIGS cells has been found to increase with increasing x until 0.3-0.4 (Eg~1.2 eV), further increasing x was found to lead to a fall in h well before the expected optimum value of Eg was reached [4]. The Voc of CIGS solar cells has been found to increase with x until 0.3-0.4, which corres
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