When photoluminescence, electroluminescence, and open-circuit voltage diverge : light soaking and halide segregation in perovskite solar cells

Perovskite solar cells suffer from various instabilities on all time scales. Some of them are driven by light, in particular when employing compounds with mixed halides. Such light soaking effects have been observed in performance changes of solar-cell devices. They have also been spectroscopically investigated in detail on films, where the formation of a low-gap iodine rich phase, seen in a red shift of the PL has been made responsible for a reduced open-circuit voltage. However, studies synchronously examining device performance and its relation to spectroscopy data, are scarce. Here, we perform an in-operandum study, where we investigate changes of open-circuit voltage (Voc) and photocurrent during light soaking and complement it with photo- (PL) and electroluminescence (EL) data on devices, which allow analysis of the Voc-limiting processes using optical and optoelectronic reciprocity relations. We find that changes in the Voc for stable single halide compositions are quantitatively correlated with changes in the PL intensity, showing that the Voc follows changes in the quasi-Fermi level splitting. In contrast, changes in Voc for the mixed halide composition are not correlated to the emergence of the low-gap phase, confirming that this phase is not the sole culprit for a low and instable Voc. Instead, non-radiative voltage losses influenced by mobile ions are dominant in devices containing compositions with high Br content. Interestingly, the low-gap phase contributes less to photocurrent, as seen by a wavelength-dependent PL quenching at short circuit. This observation might be explained by the formation of emissive but partially insulated iodine-rich regions in the film. Such an effect is also possible for single halide systems, when the perovskite composition is not stable, seen in an increase of PL at short circuit during light soaking. This indicates that ion migration in general causes photovoltaically inactive regions, without enhancing non-radiative recombination. EL measurements confirm that Rau’s reciprocity relation between external EL quantum efficiency and Voc cannot readily be applied to absorbers with such different phases.Perovskite solar cells suffer from various instabilities on all time scales. Some of them are driven by light, in particular when employing compounds with mixed halides. Such light soaking effects have been observed in performance changes of solar-cell devices. They have also been spectroscopically investigated in detail on films, where the formation of a low-gap iodine rich phase, seen in a red shift of the PL has been made responsible for a reduced open-circuit voltage. However, studies synchronously examining device performance and its relation to spectroscopy data, are scarce. Here, we perform an in-operandum study, where we investigate changes of open-circuit voltage (Voc) and photocurrent during light soaking and complement it with photo- (PL) and electroluminescence (EL) data on devices, which allow analysis of the Voc-limiting processes using optical and optoelectronic reciprocity relations. We find that changes in the Voc for stable single halide compositions are quantitatively correlated with changes in the PL intensity, showing that the Voc follows changes in the quasi-Fermi level splitting. In contrast, changes in Voc for the mixed halide composition are not correlated to the emergence of the low-gap phase, confirming that this phase is not the sole culprit for a low and instable Voc. Instead, non-radiative voltage losses influenced by mobile ions are dominant in devices containing compositions with high Br content. Interestingly, the low-gap phase contributes less to photocurrent, as seen by a wavelength-dependent PL quenching at short circuit. This observation might be explained by the formation of emissive but partially insulated iodine-rich regions in the film. Such an effect is also possible for single halide systems, when the perovskite composition is not stable, seen in an increase of PL at short circuit during light soaking. This indicates that ion migration in general causes photovoltaically inactive regions, without enhancing non-radiative recombination. EL measurements confirm that Rau’s reciprocity relation between external EL quantum efficiency and Voc cannot readily be applied to absorbers with such different phases

The bottlenecks of Cs2AgBiBr6 solar cells : how contacts and slow transients limit the performance

Cs2AgBiBr6 has attracted much interest as a potential lead-free alternative for perovskite solar cells. Although this material offers encouraging optoelectronic features, severe bottlenecks limit the performance of the resulting solar cells to a power conversion efficiency of below 3%. Here, the performance-limiting factors of this material are investigated in full solar cells featuring various architectures. It is found that the photovoltaic parameters of Cs2AgBiBr6-based solar cells strongly depend on the scan speed of the J/V measurements, suggesting a strong impact of ionic conductivity in the material. Moreover, a sign change of the photocurrent for bias voltages above 0.9 V during the measurement of the external quantum efficiency (EQE) is revealed, which can be explained by non-selective contacts. The radiative loss of the VOC from sensitive subgap-EQE measurements is calculated and it is revealed that the loss is caused by a low external luminescence yield and therefore a high non-radiative recombination, supported by the first report of a strongly red shifted electroluminescence signal between 800 and 1000 nm. Altogether, these results point to a poor selectivity of the contacts and charge transport layers, caused by poor energy level alignment that can be overcome by optimizing the architecture of the solar cell

2D/3D hybrid Cs2AgBiBr6 double perovskite solar cells : improved energy level alignment for higher contact‐selectivity and large open circuit voltage

Since their introduction in 2017, the efficiency of lead-free halide perovskite solar cells based on Cs2AgBiBr6 has not exceeded 3%. The limiting bottlenecks are attributed to a low electron diffusion length, self-trapping events and poor selectivity of the contacts, leading to large non-radiative VOC losses. Here, 2D/3D hybrid double perovskites are introduced for the first time, using phenethyl ammonium as the constituting cation. The resulting solar cells show an increased efficiency of up to 2.5% for the champion cells and 2.03% on average, marking an improvement by 10% compared to the 3D reference on mesoporous TiO2. The effect is mainly due to a VOC improvement by up to 70 mV on average, yielding a maximum VOC of 1.18 V using different concentrations of phenethylammonium bromide. While these are among the highest reported VOC values for Cs2AgBiBr6 solar cells, the effect is attributed to a change in recombination behavior within the full device and a better selectivity at the interface toward the hole transporting material (HTM). This explanation is supported by voltage-dependent external quantum efficiency, as well as photoelectron spectroscopy, revealing a better energy level alignment and thus a better hole-extraction and improved electron blocking at the HTM interface

Roadmap on commercialization of metal halide perovskite photovoltaics

Perovskite solar cells (PSCs) represent one of the most promising emerging photovoltaic technologies due to their high power conversion efficiency. However, despite the huge progress made not only in terms of the efficiency achieved, but also fundamental understanding of the relevant physics of the devices and issues which affect their efficiency and stability, there are still unresolved problems and obstacles on the path toward commercialization of this promising technology. In this roadmap, we aim to provide a concise and up to date summary of outstanding issues and challenges, and the progress made toward addressing these issues. While the format of this article is not meant to be a comprehensive review of the topic, it provides a collection of the viewpoints of the experts in the field, which covers a broad range of topics related to PSC commercialization, including those relevant for manufacturing (scaling up, different types of devices), operation and stability (various factors), and environmental issues (in particular the use of lead). We hope that the article will provide a useful resource for researchers in the field and that it will facilitate discussions and move forward toward addressing the outstanding challenges in this fast-developing field

Nutritional Status Of Preterm Infants At Discharge: A Study At Tehran Valie-E-Asr Hospital

Background: Nutrients meeting recommended dietary intakes take time to establish and once established are rarely maintained throughout hospital stay in preterm infants. Methods: Our purpose is to define the nutritional status and growth parameters of hospitalized patients at discharge in our hospital. Data were obtained concerning all 50 infants born weighing <1800 g and <34 weeks of gestation who survived until at least 21 days of age. At weekly intervals, intakes of fluid, energy, and protein from all sources were determined and body weight, head circumference and length were recorded. Results: 50 premature neonates with gestational age of 30.5 ± 2.35 weeks and birth weight of 1337 ±232 g comprising 26 (52%) females and 24 (48%) males were entered into this study from April to July 2004. The mean time of caloric intake of 120Kcal /kg/d was 18.41 ± 6.7 days of age. 37.8% of mothers provide breast milk, 51% preterm formula and the rest of the patients consumed both. Biochemical markers of nutritional status showed that 64% of neonates had blood urea nitrogen <5 mg/dL, 70% had al-kaline phosphatase >450 IU/L, 73% had serum phosphorus <4.5 mg/dL, and 49% had serum albumin <3 g/dL at time of discharge. Except for serum albumin, comparisons of biochemical parameters between small for gestational age & appropriate for gestational age infants were not significant at discharge time. Comparisons of weight and head circumference at birth and at discharge time showed that a significant number of neonates became small for gestational age at discharge (p<0.05). Conclusion: Availability of appropriate amino acid solutions for neonates, parenteral phosphorus, trace elements and human fortifier could reduce these problems

Nutritional Status Of Preterm Infants At Discharge: A Study At Tehran Valie-E-Asr Hospital

Background: Nutrients meeting recommended dietary intakes take time to establish and once established are rarely maintained throughout hospital stay in preterm infants. Methods: Our purpose is to define the nutritional status and growth parameters of hospitalized patients at discharge in our hospital.Data were obtained concerning all 50 infants born weighing 450 IU/L, 73% had serum phosphorus <4.5 mg/dL, and 49% had serum albumin <3 g/dL at time of discharge. Except for serum albumin, comparisons of biochemical parameters between small for gestational age & appropriate for gestational age infants were not significant at discharge time. Comparisons of weight and head circumference at birth and at discharge time showed that a significant number of neonates became small for gestational age at discharge (p<0.05).Conclusion:Availability of appropriate amino acid solutions for neonates, parenteral phosphorus, trace elements and human fortifier could reduce these problems

Negative capacitance in perovskite solar cells

So-called negative capacitance seems to remain an obscure feature in the analysis of the frequency-dependent impedance of perovskite solar cells. It belongs to one of the puzzling peculiarities arising from the mixed ionic-electronic conductivity of this class of semi- conductor. Here we show that apparently high capacitances in general (positive and nega- tive) are not related to any capacitive feature in the sense of a corresponding charge accumulation. Instead, they are a natural consequence of slow transients mainly in forward current of the diode upon ion displacement when changing voltage. The transient current leads to a positive or negative ‘capacitance’ dependent on the sign of its gradient. The ‘capacitance’ appears so large because the associated resistance, when thinking of a resistor- capacitor element, results from another physical process, namely modified electronic charge injection and transport. Observable for a variety of devices, it is a rather universal phe- nomenon related to the hysteresis in the current–voltage curve