Revealing Underlying Processes Involved in Light Soaking Effects and Hysteresis Phenomena in Perovskite Solar Cells

Abstract

continuously increase with light illumination while the shortcircuit current ( J SC ) experiences a quick increase and then a decrease upon light exposure. The C -V measurements fi nd that light soaking can decrease the charge accumulation at the electrode interfaces. Essentially, the light soaking-decreased charge accumulation at electrode interface can be attributed to following two possible processes. First, the photogenerated carriers can neutralize the interfacial defects at electrode interface upon light illumination. Second, the migration of ions can change the built-in electric fi eld and then affects the charge accumulation at electrode interfaces. In particular, these two processes can largely increase the V OC by increasing interfacial potential barrier at electrode interfaces during light illumination. The time-dependent PL and frequency-dependent capacitance ( Cf ) fi nd that the bulk defects within perovskite fi lm are mainly positively charged and can be neutralized by photogenerated electrons upon light illumination. In particular, our frequency-dependent capacitance provides the fi rst direct evidence that light soaking can decrease bulk-electrical polarization within organo-metal halide perovskites. Especially, decreasing the bulk-electrical polarization causes a decrease on J SC in light soaking. However, neutralizing the defects at electrode interfaces and bulk perovskite fi lm can enhance the transport of the dissociated charge carriers to respective electrodes, increasing the FF during light illumination. Clearly, our experimental studies provide an in-depth understanding on internal coupling between electrode and bulk parameters in light soaking and hysteresis phenomena in perovskite solar cells under deviceoperating condition. Figure 1 shows the light soaking effects on device performance for perovskite solar cells. On initial light exposure the device shows a lower photovoltaic performance with V OC = 0.51 V, J SC = 18.34 mA cm −2 , FF = 53.1%, and power conversion effi ciency (PCE) = 4.97%. With continuous light exposure, device performance is signifi cantly improved over time. After ≈20 min of light soaking, the enhanced V OC = 0.83 V, J SC = 18.18 mA cm −2 , FF = 69.5% are obtained, resulting in a PCE = 10.49%. As we know, a continuous light illumination can cause heating and charge trapping in the development of light soaking and hysteresis effects. Here, our studies indicate that the light soaking and hysteresis effects come from charge trapping rather than heating. Specifi cally, we observe that the surface temperature of device can be increased from 26 to 42 °C when the cells are continuously exposed to light illumination. However, the device performance only slightly decreases, according to the J -V characteristics (inset in Figure 2 ), when the temperature increased to 42 ºC equivalent to the temperature produced by continuous light illumination. After removing the heating and cooling the device to room temperature, we can see that the device performance is again signifi cantly

Impact of Structural Dynamics on the Optical Properties of Methylammonium Lead Iodide Perovskites

“…[88] This rising behavior or PL enhancement (Figure 4e), is also obtained in inorganic semiconductor QDs, e.g., CdSe QDs, [76] ZnS-capped CdSe QDs, [78] etc. This PL rising behavior has been attributed to the migration of charge carriers, followed by a trap-filling process at energetically shallow defects within the film.…”

Is Excess PbI2 Beneficial for Perovskite Solar Cell Performance?

“…It can be observed that both S1 and S3 devices exhibit good stability, and for both devices with alternating light/dark periods we can observe some recovery of the effi ciency upon illumination on the next day. Improvement in the device performance after light soaking has been reported in perovskite devices with different architecture, [35][36][37] which was attributed to the reduced trap density under illumination. [ 35 ] We observe some initial improvement in the effi ciency without heating, followed by decrease and then recovery upon illumination next day, and this effi ciency increase is more pronunced in aged cells.…”

“…Improvement in the device performance after light soaking has been reported in perovskite devices with different architecture, [35][36][37] which was attributed to the reduced trap density under illumination. [ 35 ] We observe some initial improvement in the effi ciency without heating, followed by decrease and then recovery upon illumination next day, and this effi ciency increase is more pronunced in aged cells. It is expected that aged cells would have higher density of defects and traps and thus may exhibit improvement in performance upon illumination after storage in the dark.…”

Rational design of SnO2-based electron transport layer in mesoscopic perovskite solar cells: more kinetically favorable than traditional double-layer architecture

“…As aforementioned, the photocatalytic activity of SnO 2 is less pronounced due to a higher band gap of~3.8 eV, which is favorable to the device's stability when exposed to UV light. In planar or mesoporous PSCs, hysteresis effect in current-voltage characterizations is a common phenomenon observed under different conditions including scan directions and rates, as well as applying bias voltage and light soaking prior to measurement [42,43]. To date, many reasons have been indicated to be the causes of hysteresis behavior, such as ion migration and transient ferroelectric polarization [44].…”