Henri Vahlman

Carbon-Double-Bond-Free Printed Solar Cells from TiO2/CH3NH3PbI3/CuSCN/Au: Structural Control and Photoaging Effects

Carbon double bond-free printed solar cells have been fabricated with the structure <F-doped SnO2 (FTO)/dense TiO2/nanocrystalline TiO2/CH3NH3PbI3/Au> and <FTO/dense TiO2/nanocrystalline TiO2/CH3NH3PbI3/CuSCN/Au>, in which CuSCN acts as a hole conductor. The thickness of the CH3NH3PbI3 layer is controlled by a hot air flow during spin coating. The best conversion efficiency (4.86%) is obtained with <FTO/dense TiO2/nanocrystalline TiO2/thin CH3NH3PbI3 (hot-air dried)/CuSCN/Au>. However, a thick CH3NH3PbI3 layer on CuSCN is better for light-exposure stability (100 mW cm(-2) AM 1.5) when not encapsulated. Without the CuSCN coverage, the black CH3NH3PbI3 crystal changes to yellow during the light-exposure stability test, which is due to the transformation of the CH3NH3PbI3 perovskite crystal into hexagonal PbI2.

Recombination activity of light-activated copper defects in p-type silicon studied by injection- and temperature-dependent lifetime spectroscopy

The presence of copper contamination is known to cause strong light-induced degradation (Cu-LID) in silicon. In this paper, we parametrize the recombination activity of light-activated copper defects in terms of Shockley—Read—Hall recombination statistics through injection- and temperature dependent lifetime spectroscopy (TDLS) performed on deliberately contaminated float zone silicon wafers. We obtain an accurate fit of the experimental data via two non-interacting energy levels, i.e., a deep recombination center featuring an energy level at Ec−Et=0.48−0.62 eV with a moderate donor-like capture asymmetry ( k=1.7−2.6)  and an additional shallow energy state located at Ec−Et=0.1−0.2 eV, which mostly affects the carrier lifetime only at high-injection conditions. Besides confirming these defect parameters, TDLS measurements also indicate a power-law temperature dependence of the capture cross sections associated with the deep energy state. Eventually, we compare these results with the available literature d...

Modeling of light-induced degradation due to Cu precipitation in p-type silicon. II. Comparison of simulations and experiments

The presence of copper impurities is known to deteriorate the bulk minority carrier lifetime of silicon. In p-type silicon, the degradation occurs only under carrier injection (e.g., illumination), but the reason for this phenomenon called copper-related light-induced degradation (Cu-LID) has long remained uncertain. To clarify the physics of this problem, a mathematical model of Cu-LID was introduced in Paper I of this article. Within the model, kinetic precipitation simulations are interlinked with a Schottky junction model for electric behavior of metallic precipitates. As this approach enables simulating precipitation directly at the minority carrier lifetime level, the model is verified in this second part with a direct comparison to the corresponding degradation experiments and literature data. Convincing agreement is found with different doping and Cu concentrations as well as at increased temperature, and in the dark, both simulated degradation and measured degradation are very slow. In addition, ...

Modeling of light-induced degradation due to Cu precipitation in p-type silicon. I. General theory of precipitation under carrier injection

Copper contamination causes minority carrier lifetime degradation in p-type silicon bulk under illumination, leading to considerable efficiency losses in affected solar cells. Although the existence of this phenomenon has been known for almost two decades, ambiguity prevails about the underlying defect mechanism. In Paper I of this two-part contribution, we propose the first comprehensive mathematical model for Cu-related light-induced degradation in p-type silicon (Cu-LID). The model is based on the precipitation of interstitial Cu ions, which is assumed to be kinetically limited by electrostatic repulsion from the growing Cu precipitates. Hence, growth and dissolution rates of individual Cu precipitates are derived from the drift-diffusion equation of interstitial Cu and used in a kinetic precipitation model that is based on chemical rate equations. The kinetic model is interlinked to a Schottky junction model of metallic precipitates in silicon, enabling accurate calculation of the injection-dependent ...

Vertically integrated modeling of light-induced defects: Process modeling, degradation kinetics and device impact

As photovoltaic (PV) device architectures advance, they turn more sensitive to bulk minority charge carrier lifetime. The conflicting needs to develop ever advancing cell architectures on ever cheapening silicon substrates ensure that various impurity-related light-induced degradation (LID) mechanisms will remain an active research area in the silicon PV community. Here, we propose vertically integrated defect modeling as a framework to accelerate the identification and mitigation of different light induced defects. More specifically, we propose using modeled LID-kinetics to identify the dominant LID mechanism or mechanisms within complete PV devices. Coupling the LID-kinetics model into a process model allows development of process guidelines to mitigate the identified LID-mechanism within the same vertically integrated simulation tool. We use copper as an example of a well-characterized light-induced defect: we model the evolution of copper during solar cell processing and light soaking, and then map the deleterious lifetime effect of Cu-LID onto device performance. We validate our model using intentionally Cu-contaminated material processed on an industrial PERC-line and find that our model reproduces the LID-behavior of the manufactured solar cells. We further show via simulations that Cu-LID can be mitigated by reducing the contact co-firing peak temperature, or the cooling rate after the firing process.

Rapid thermal anneal activates light induced degradation due to copper redistribution

While it is well known that copper impurities can be relatively easily gettered from the silicon bulk to the phosphorus or boron–doped surface layers, it has remained unclear how thermally stable the gettering actually is. In this work, we show experimentally that a typical rapid thermal anneal (RTA, a few seconds at 800 °C) used commonly in the semiconductor and photovoltaic industries is sufficient to release a significant amount of Cu species from the phosphorus-doped layer to the wafer bulk. This is enough to activate the so-called copper-related light-induced degradation (Cu-LID) which results in significant minority carrier lifetime degradation. We also show that the occurrence of Cu-LID in the wafer bulk can be eliminated both by reducing the RTA peak temperature from 800 °C to 550 °C and by slowing the following cooling rate from 40–60 °C/s to 4 °C/min. The behavior is similar to what is reported for Light and Elevated Temperature degradation, indicating that the role of Cu cannot be ignored when studying other LID phenomena. Numeric simulations describing the phosphorus diffusion and the gettering process reproduce the experimental trends and elucidate the underlying physical mechanisms.While it is well known that copper impurities can be relatively easily gettered from the silicon bulk to the phosphorus or boron–doped surface layers, it has remained unclear how thermally stable the gettering actually is. In this work, we show experimentally that a typical rapid thermal anneal (RTA, a few seconds at 800 °C) used commonly in the semiconductor and photovoltaic industries is sufficient to release a significant amount of Cu species from the phosphorus-doped layer to the wafer bulk. This is enough to activate the so-called copper-related light-induced degradation (Cu-LID) which results in significant minority carrier lifetime degradation. We also show that the occurrence of Cu-LID in the wafer bulk can be eliminated both by reducing the RTA peak temperature from 800 °C to 550 °C and by slowing the following cooling rate from 40–60 °C/s to 4 °C/min. The behavior is similar to what is reported for Light and Elevated Temperature degradation, indicating that the role of Cu cannot be ignored when ...