Interface Modification for Energy Level Alignment and Charge Extraction in CsPbI3 Perovskite Solar Cells.

In perovskite solar cells (PSCs) energy level alignment and charge extraction at the interfaces are the essential factors directly affecting the device performance. In this work, we present a modified interface between all-inorganic CsPbI3 perovskite and its hole-selective contact (spiro-OMeTAD), realized by the dipole molecule trioctylphosphine oxide (TOPO), to align the energy levels. On a passivated perovskite film, with n-octylammonium iodide (OAI), we created an upward surface band-bending at the interface by TOPO treatment. This improved interface by the dipole molecule induces a better energy level alignment and enhances the charge extraction of holes from the perovskite layer to the hole transport material. Consequently, a Voc of 1.2 V and a high-power conversion efficiency (PCE) of over 19% were achieved for inorganic CsPbI3 perovskite solar cells. Further, to demonstrate the effect of the TOPO dipole molecule, we present a layer-by-layer charge extraction study by a transient surface photovoltage (trSPV) technique accomplished by a charge transport simulation.

Highly efficient p-i-n perovskite solar cells that endure temperature variations.

Daily temperature variations induce phase transitions and lattice strains in halide perovskites, challenging their stability in solar cells. We stabilized the perovskite black phase and improved solar cell performance using the ordered dipolar structure of ß-poly(1,1-difluoroethylene) to control perovskite film crystallization and energy alignment. We demonstrated p-i-n perovskite solar cells with a record power conversion efficiency of 24.6% over 18 square millimeters and 23.1% over 1 square centimeter, which retained 96 and 88% of the efficiency after 1000 hours of 1-sun maximum power point tracking at 25° and 75°C, respectively. Devices under rapid thermal cycling between -60° and +80°C showed no sign of fatigue, demonstrating the impact of the ordered dipolar structure on the operational stability of perovskite solar cells.

Halogen Bonding in Perovskite Solar Cells: A New Tool for Improving Solar Energy Conversion.

Hybrid organic-inorganic halide perovskites (HOIHPs) have recently emerged as a flourishing area of research. Their easy and low-cost production and their unique optoelectronic properties make them promising materials for many applications. In particular, HOIHPs hold great potential for next-generation solar cells. However, their practical implementation is still hindered by their poor stability in air and moisture, which is responsible for their short lifetime. Optimizing the chemical composition of materials and exploiting non-covalent interactions for interfacial and defects engineering, as well as defect passivation, are efficient routes towards enhancing the overall efficiency and stability of perovskite solar cells (PSCs). Due to the rich halogen chemistry of HOIHPs, exploiting halogen bonding, in particular, may pave the way towards the development of highly stable PSCs. Improved crystallization and stability, reduction of the surface trap states, and the possibility of forming ordered structures have already been preliminarily demonstrated.

Perfluorinated Self-Assembled Monolayers Enhance the Stability and Efficiency of Inverted Perovskite Solar Cells.

Perovskite solar cells are among the most exciting photovoltaic systems as they combine low recombination losses, ease of fabrication, and high spectral tunability. The Achilles heel of this technology is the device stability due to the ionic nature of the perovskite crystal, rendering it highly hygroscopic, and the extensive diffusion of ions especially at increased temperatures. Herein, we demonstrate the application of a simple solution-processed perfluorinated self-assembled monolayer (p-SAM) that not only enhances the solar cell efficiency, but also improves the stability of the perovskite absorber and, in turn, the solar cell under increased temperature or humid conditions. The p-i-n-type perovskite devices employing these SAMs exhibited power conversion efficiencies surpassing 21%. Notably, the best performing devices are stable under standardized maximum power point operation at 85 °C in inert atmosphere (ISOS-L-2) for more than 250 h and exhibit superior humidity resilience, maintaining ∼95% device performance even if stored in humid air in ambient conditions over months (∼3000 h, ISOS-D-1). Our work, therefore, demonstrates a strategy towards efficient and stable perovskite solar cells with easily deposited functional interlayers.

Monitoring Charge Carrier Diffusion across a Perovskite Film with Transient Absorption Spectroscopy.

We have developed a new noninvasive optical method for monitoring charge carrier diffusion and mobility in semiconductor thin films in the direction perpendicular to the surface which is most relevant for devices. The method is based on standard transient absorption measurements carried out in reflectance and transmittance modes at wavelengths below the band gap where the transient response is mainly determined by the change in refractive index, which in turn depends on the distribution of photogenerated carriers across the film. This distribution is initially inhomogeneous because of absorption at the excitation wavelength and becomes uniform over time via diffusion. By modeling these phenomena we can determine the diffusion constant and respective mobility. Applying the method to a 500 nm thick triple cation FAMACs perovskite film revealed that homogeneous carrier distribution is established in few hundred picoseconds, which is consistent with mobility of 66 cm2 (V s)-1.

Refractive index change dominates the transient absorption response of metal halide perovskite thin films in the near infrared.

Perovskites have lately attracted a lot of attention as promising materials for the next-generation of efficient, low-cost, and solution processable optoelectronics. Their complex transient photophysics, in time scales ranging from femtoseconds to seconds, have been widely investigated. However, in most of the reported works the spectral window of ultrafast transient absorption (TA) spectroscopy of perovskite films is limited to the visible region, hence missing crucial information coming from the near-infrared (NIR). Furthermore, the measured TA responses are affected by light interference in a thin perovskite layer making data interpretation a challenge even in the visible part of the spectrum. Here, we demonstrate a method that allows us to separately obtain the changes in absorption and refractive index from conventional transmission and reflection pump-probe measurements. We show that the contribution of the absorption change to the response of metal halide perovskite thin films in the NIR is much smaller than that of the refractive index change. Furthermore, the spectral shape of TA responses in the NIR range is predominantly determined by perovskite layer thickness and its refractive index. However, the time profile of the responses bears important information on the carrier dynamics and makes the NIR a useful range to study perovskite photophysics.

Hypogammaglobulinaemia secondary to cow-milk allergy in children under 2 years of age.

Symptomatic hypogammaglobulinaemia in children younger than 2 years of age was studied to rule out a primary immunodeficiency. Thirty-four patients were referred to the Immunology Service to study the hypogammaglobulinaemia-associated clinical picture. Food allergy was documented in 10 patients by personal and familial history, presence of specific immunoglobulin E (IgE) and elevated total serum IgE levels. Coeliac disease and human immunodeficiency virus infection were also ruled out. Protein loss through stools was assessed by clearance of alpha1-antitrypsin (AAT). Serum immunoglobulin levels were determined by nephelometry and functional antibodies were studied by enzyme-linked immunosorbent assay. The cellular immune response was assessed by in vitro lymphocyte proliferation in response to mitogens and cell subsets were analysed by flow cytometry. In five patients of the 10 patients we suspected a protein loss through the mucosa. Four of these five patients showed an increased AAT and the other showed an extensive cutaneous lesion. Immunological studies revealed normal antibody function, in vitro lymphoproliferative responses and cell numbers in four of the 5 patients. One patient showed abnormally low numbers of CD4(+) T cells as well as a defective proliferative response to mitogens. After diagnosis of cow milk allergy, milk was replaced with infant milk formula containing hydrolysed proteins. Recovery of immunoglobulin values and clinical resolution were achieved. Hypogammaglobulinaemia during early childhood in some children may be secondary to cow milk allergy, and immunoglobulins and cells may leak through the inflamed mucosa. Resolution of symptoms as well as normalization of immunoglobulin values may be easily achieved by avoidance of the offending allergen.