Amorphous/Crystalline Silicon Interface Passivation: Ambient-Temperature Dependence and Implications for Solar Cell Performance

Silicon heterojunction (SHJ) solar cells feature amorphous silicon passivation films, which enable very high voltages. We report how such passivation increases with operating temperature for amorphous silicon stacks involving doped layers and decreases for intrinsic-layer-only passivation. We discuss the implications of this phenomenon on the solar cell's temperature coefficient, which represents an important figure-of-merit for the energy yield of devices deployed in the field. We show evidence that both open-circuit voltage (Voc) and fill factor (FF) are affected by these variations in passivation and quantify these temperature-mediated effects, compared with those expected from standard diode equations. We confirm that devices with high Voc values at 25°C show better high-temperature performance. However, we also argue that the precise device architecture, such as the presence of charge-transport barriers, may affect the temperature-dependent device performance as well.

I. Introduction

Silicon heterojunction (SHJ) solar cells are increasingly attracting attention, thanks to their potential for stable high conversion efficiencies at competitive production costs [1]. Record efficiencies of 24.7% [2] and 25.6% [3] were recently reported by Panasonic, Japan, for standard two-side-contacted and interdigitated back-contacted device structures, respectively. These results refer to devices certified under standard testing conditions (STC, 1000 W·m⁻², 25 °C, AM1.5 g spectrum). Independently from these outstanding results, the values taken in STC and used for certification could be questioned, as encapsulated devices deployed in the field can reach operating-temperatures (T) as high as 90 °C [4]. In fact, photovoltaic devices typically show significant performance losses with increasing T. Hence, the temperature coefficient of the conversion efficiency () represents an important figure-of-merit for the energy yield of a given photovoltaic technology.

¹If not stated differently, we refer to TC as the relative temperature coefficient, i.e., for a solar cell parameter X (open-circuit voltage, etc.), TC = 1/X⁰ X/T (X⁰: value at the reference temperature, typically 25 °C) [5]. The absolute TC refers to the absolute value of X/T.

In this respect, compared to conventional crystalline silicon homojunction technologies ( for standard homojunction and −0.35%/°C for homojunctions with passivating contacts [6]), SHJ solar cells are less sensitive to increasing operation temperatures ( [7] to −0.1%/°C [8]

²This is comparable with thin-film solar cells, which exhibit high V_ocs as well [9].

). To underline the importance of a favorable , starting from a 20%-efficient cell (at 25 °C), a difference in of 0.12 or 0.35%/°C results in ∼5.4 or 15.8% relative difference in efficiency at an operating-temperature of 70 °C.