Ultrathin Solar Cell With Magnesium-Based Optical Switching for Window Applications

Abstract

Photovoltaic windows that can be switched between 4 transparent and energy harvesting mode can be realized by using 5 ultrathin solar absorbers embedded in an optical nanocavity. In the 6 present work, we use a 5 nm thick amorphous germanium absorber 7 integrated in a magnesium-based thin film optical cavity, which 8 switches from an absorptive to a transparent state due to hydrogen 9 absorption. We analyze the influence of the mirror layer thickness 10 on the light absorption, photocurrent generation, and transmission 11 as well as color neutrality of the device. The optical properties are 12 studied by 1-D transfer-matrix method by changing Mg thickness 13 between 0 and 100 nm, then compared to the experimental results of 14 fabricated devices. When the thickness of Mg increases, the switch-15 able average transparency varies between 25% and 0%, while the 16 power conversion efficiency rises up to 2.3%. The applicability of 17 the device is tested by modeling the annual power generation in 18 realistic scenarios. The influence of the cardinal orientation and 19 the seasons on the switchable photovoltaic window implemented in 20 a building facade with the abovementioned parameters is analyzed 21 for different switching scenarios. 22 Index Terms-Magnesium optical switching, smart window, 23 switchable solar cell, ultrathin solar cell. 24 I. INTRODUCTION 25 I N MODERN buildings, huge glass facades often separate the 26 interior from the exterior environment. They are supposed to 27 supply thermal and visual comfort to the inner of the building. 28 To improve this comfort, shading systems have to be imple-29 mented, which provide effective light and heat management 30 [1]-[3]. Dynamic daylight control can be provided by smart 31 windows, which attracted great interest in recent years [4]-[8]. 32 These windows are able to reduce the amount of energy needed 33 for cooling in the building [9]. The emergence of switchable Q1 34 solar cells added a new energy harvesting capability to these 35 smart windows. While still being able to dynamically block 36 sunlight from entering the interior of a building, these smart 37 solar windows generate electricity at the same time. For the 38 realization of a smart photovoltaic window, a technical solution 39 enabling reversible switching from a transparent state to an ab-40 sorbing photovoltaic state is required. Different approaches are 41 523

Tunable Photovoltaics: Adapting Solar Cell Technologies to Versatile Applications

“…Gasochromic Switching: Gasochromic switching involves the interaction of oxidizing or reducing gases, like oxygen and hydrogen, with transition metal oxides such as tungsten oxide (WO 3 ), or metals like magnesium and titanium. [122,123] Due to the absorption of gaseous molecules, the responsive materials undergo a phase change such as a transition from metallic to a metal hydride dielectric state upon hydrogenation. [124] This leads to a dramatic change in the optical response.…”

“…[125] Depending on the thickness and the material type of the gasochromic structure, the phase change can induce a transition from a reflective/absorptive to a transparent state. [122,123,126] Since the gasochromic mechanism requires a direct interaction with a gas, the responsive material is commonly exposed to the surrounding environment and therefore placed in the external electrode of the switchable SC. [122,123,126] A careful consideration for the thickness of the gasochromic layer is required to allow sufficient transmittance of visible light.…”

“…[122,123,126] Since the gasochromic mechanism requires a direct interaction with a gas, the responsive material is commonly exposed to the surrounding environment and therefore placed in the external electrode of the switchable SC. [122,123,126] A careful consideration for the thickness of the gasochromic layer is required to allow sufficient transmittance of visible light. For instance, the thickness can be above 100 nm for the case of WO 3 but, it is restricted to way below 100 nm for Mg due to the high opacity level.…”

“…For instance, the thickness can be above 100 nm for the case of WO 3 but, it is restricted to way below 100 nm for Mg due to the high opacity level. [123] Photochromic Switching: As for the photochromic mechanism, it typically consists of a reversible transition between a colorless state in the dark and colored state during illumination. [127] In PV, this switching dynamic mechanism has been exploited mainly in dye-sensitized solar cells (DSSCs) that allow a change in color and self-adjustment of the visible light transmission when irradiated upon.…”

“…[128] The stimulus-responsive element is then a part of the functional materials composing the SC and can be inserted in different regions from the inner absorber to the external electrode. [123,126,133,139] Hence, similar to smart window technologies, switchable PV can allow a reversible and dynamic optical modulation with a drastic change from transparent to opaque or colored states due to either chemical composition or structural changes.…”