Perovskite Photovoltaic Devices with Carbon‐Based Electrodes Withstanding Reverse‐Bias Voltages up to –9 V and Surpassing IEC 61215:2016 International Standard

Abstract

One of the key challenges of perovskite photovoltaics (PV) is the long‐term stability. Although efforts are made to improve the lifetime of perovskite PV devices, their degradation under reverse‐bias conditions is barely addressed. Herein, perovskite solar cells with carbon‐based electrodes are presented which demonstrate superior resilience against reverse‐bias‐induced degradation. Although their breakdown voltage is identified to be at approximately −3.6 V, cells do not degrade until the applied reverse‐bias exceeds −9 V. Two main degradation mechanisms are identified: 1) iodine loss due to hole tunneling into perovskite, which takes place even at low reverse‐bias but decomposes the perovskite only after long time durations; and 2) rapid heating at large reverse‐bias leading to formation of PbI2, which starts at shunts and then follows the path of the least resistance for the cell current, which is primarily influenced by the electrode sheet resistances. Finally, perovskite solar modules with carbon‐based electrodes are demonstrated, which are subjected to a “hotspot” test described in the IEC 61215:2016 international standard at an accredited module testing laboratory. Passing this accelerated test for the first time confirms the superior stability of perovskite PV devices with carbon‐based electrodes and highlights their large industrialization potential.

Reverse-Bias and Temperature Behaviors of Perovskite Solar Cells at Extended Voltage Range

“…Contrary to PSC-A, both PSC-B and PSC-C started to exhibit relevant reverse current densities (on the order of tens of mA cm –2 ) at reverse biases equal to or higher than 7.5 V. The drop of the specific resistance of the cells, visualized in logarithmic scale in the inset of Figure 2 b, can be attributed to the formation of local shunts, 12 which can therefore occur easily in the cells without cTiO 2 and may be also associated with Au ion migration effects. 12 According to Ohm’s law, the power dissipated in the form of heat by the reverse-biased device ( P ) is given by the product of the current ( I ) and the voltage ( V ); i.e., P = V × I . Figure S2 reports the mean power density dissipated by each device as a function of the reverse bias, revealing maximum values as high as 9.41 W cm –2 for a PSC-A at 10 V reverse bias.…”

“…Such radical deterioration was caused by a pronounced temperature gradient over time caused by hot-spots, which are attributed to local shunts. 12 This disruptive mechanical deterioration was not observed in PSC-B and PSC-C, whose temperatures progressively increased (in some cases up to more than 160 °C; see replica 2 for PSC-B) with increasing the reverse bias until severe damages caused the electrical disconnection between cathode and anode through cracking and interface loosening of the ETL/perovskite/HTL structure. Such effects typically occurred at reverse biases between 15 and 25 V. Therefore, as shown in the photographs reported in Figure 2 d, the entire active area of PSC-B and PSC-A was deteriorated after applying a reverse bias of 30 V, evidencing pronounced cracking/stripping/dissolution of the Au electrodes.…”

“… 5 These results suggest that very high (absolute) current densities may be locally reached over subsecond time scale. Another recent study on the reverse-bias behavior of PSCs with carbon-based electrodes extended the analysis at reverse biases exceeding 9 V. 12 In addition, it was shown that the reverse-bias stability of such cells was superior compared to those based on metallic electrodes because of the lack of metal-induced degradation mechanisms (metal ion migration and even electrode melting at elevated temperatures). 12 Nevertheless, it is still crucial to provide an in-depth understanding of the reverse-bias behavior of PSC with metallic electrodes, which are currently the most efficient PSC architecture, extending the analysis above a few volts (up to more than 10 V).…”

“…Another recent study on the reverse-bias behavior of PSCs with carbon-based electrodes extended the analysis at reverse biases exceeding 9 V. 12 In addition, it was shown that the reverse-bias stability of such cells was superior compared to those based on metallic electrodes because of the lack of metal-induced degradation mechanisms (metal ion migration and even electrode melting at elevated temperatures). 12 Nevertheless, it is still crucial to provide an in-depth understanding of the reverse-bias behavior of PSC with metallic electrodes, which are currently the most efficient PSC architecture, extending the analysis above a few volts (up to more than 10 V). Moreover, the analysis of PSC behavior at such reverse biases can serve as an accelerating aging test, which provides an approximate evaluation of the reliability of the devices in the presence of electric biases (that trigger ion migration/charge accumulation leading to thermally activated traps 15 or detrimental parasitic electrochemical reactions 13 ) 14 or local defects/malfunctions, such as microstructural failures (e.g., pinholes in large-area perovskite films 16 and ion-migration-induced conductive channels, 17 which both cause a spatially confined reduction of the shunt resistance).…”

“…Therefore, it is reasonable that the temperature reached in the active materials of the devices is significantly higher than those measured by our IR thermal imaging system at the front side of the cell, as also proved by rear–front analyses ( Figure 2 e). According to a recent study, 12 hot-spots generated by the passage of high current in filamentary shunt can cause the thermal decomposition of the perovskite into PbI 2 . Since the latter is less conductive, the surrounding areas of perovskite can become the path of least resistance, which in turn will be degraded by the heating induced by the increase of the current.…”