Contactless Carrier Lifetime Characterization of Silicon Heterojunction Structures at Elevated Temperatures

We realized the contactless transient photoconductance decay (PCD) lifetime measurement method at elevated temperatures. We utilized illumination-controlled heating to adjust the sample temperature instead of using a hotplate to simulate the conditions of realistic solar cell operation. Heating and lifetime probing laser pulses are alternating continuously to keep the samples at stable temperature. Pseudo current density – voltage (J-V) characteristics and the emitter saturation current density, J0e, were recorded in the temperature range between 28°C and 62°C from silicon heterojunction (SHJ) precursor samples with and without transparent conductive oxides (TCO). For the proper evaluation, state-of-the-art intrinsic recombination, band gap narrowing and mobility models are applied. In terms of efficiency and fill factor, similar trends are acquired as in the case of conventional I-V measurements of finished solar cells at higher temperatures. Beyond the well-known efficiency reducing effect of the increased intrinsic carrier concentration at higher temperatures, we concluded, that the carrier lifetime gets larger at the maximum power point (MPP) of the cells around 1-2⋅1015 1/cm3 injection level, which results in improved efficiency compared to the expectations. We also found that the slope of the Kane-Swanson fit decreases with increasing temperatures as well, so both bulk and surface recombination properties improve. This effect explains the low temperature coefficient (< 0.3 %/°C) of the SHJ type solar cells.