گسیلندگی حرارتی سلولهای خورشیدی ناهمگون سیلیکون
Abstract
1- Introduction
2- Materials and methods
3- Modelling
4- Results
5- Discussion
6- Conclusions
References
Abstract
The aim of this work is to evaluate whether silicon heterojunction solar cells, lacking highly emissive, heavily doped silicon layers, could be better candidates for hybrid photovoltaic thermal collectors than standard aluminium-diffused back contact solar cells. To this end, the near and mid infrared emissivity of full silicon heterojunction solar cells, as well as of its constituent materials – crystalline silicon wafer, indium tin oxide, n-, i- and p-type amorphous silicon – have been assessed by means of ellipsometry and FTIR. The experimental results show that the thermal emissivity of these cells is actually as high as in the more traditional structures, ~80% at 8 μm. Detailed optical modelling combining raytracing and transfer matrix formalism shows that the emissivity in these cells originates in the transparent conductive oxide layers themselves, where the doping is not high enough to result in a reflection that exceeds the increased free carrier absorption. Further modelling suggests that it is possible to obtain lower emissivity solar cells, but that a careful optimization of the transparent conductive layer needs to be done to avoid hindering the photovoltaic performance.
Introduction
Thermal emissivity of solar cells is gaining interest both due to its effect on the normal cell operating temperature, and therefore efficiency in the field [1], and due to its effect on the thermal performance of hybrid photovoltaic-thermal (PV-T) collectors [2,3]. We have shown in recent experiments that radiative emissivity of aluminium-diffused back contact solar cells is extremely high, and have deduced from modelling that this originates from the highly doped emitter and back surface field layers [4]. Silicon heterojunctions (SHJ) solar cells offer an excellent opportunity to boost the performance of hybrid PV-T collectors for two reasons. First, their low thermal coefficient of power will help to keep high efficiency at elevated temperatures. The impact of this property in PV-T systems has been analysed in detailed by Mellor et al. [2]. Second, SHJ cells are expected to have lower mid-infrared (MIR) emissivity than the traditional silicon solar cell design as they lack highly doped silicon layers and instead have a front indium tin oxide layer, which is a material often used as a low emissivity coating. In this work we assess the latter hypothesis by measuring the MIR emissivity of a state-of-the-art SHJ solar cell, together with their individual components. Integrated raytracing and transfer-matrix method simulations are used to get further insight into the role and impact of each of the layers that make the structure.