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In high-grade serous ovarian carcinoma (HGSC), deleterious mutations in DNA repair gene RAD51C are established drivers of defective homologous recombination and are emerging biomarkers of PARP inhibitor (PARPi) sensitivity. RAD51C promoter methylation (meRAD51C) is detected at similar frequencies to mutations, yet its effects on PARPi responses remain unresolved. In this study, three HGSC patient-derived xenograft (PDX) models with methylation at most or all examined CpG sites in the RAD51C promoter show responses to PARPi. Both complete and heterogeneous methylation patterns were associated with RAD51C gene silencing and homologous recombination deficiency (HRD). PDX models lost meRAD51C following treatment with PARPi rucaparib or niraparib, where a single unmethylated copy of RAD51C was sufficient to drive PARPi resistance. Genomic copy number profiling of one of the PDX models using SNP arrays revealed that this resistance was acquired independently in two genetically distinct lineages. In a cohort of 12 patients with RAD51C-methylated HGSC, various patterns of meRAD51C were associated with genomic “scarring,” indicative of HRD history, but exhibited no clear correlations with clinical outcome. Differences in methylation stability under treatment pressure were also observed between patients, where one HGSC was found to maintain meRAD51C after six lines of therapy (four platinum-based), whereas another HGSC sample was found to have heterozygous meRAD51C and elevated RAD51C gene expression (relative to homozygous meRAD51C controls) after only neoadjuvant chemotherapy. As meRAD51C loss in a single gene copy was sufficient to cause PARPi resistance in PDX, methylation zygosity should be carefully assessed in previously treated patients when considering PARPi therapy. Significance: Homozygous RAD51C methylation is a positive predictive biomarker for sensitivity to PARP inhibitors, whereas a single unmethylated gene copy is sufficient to confer resistance.

Dynamic windows with adjustable tint give users control over the flow of light and heat to decrease the carbon footprint of buildings and improve the occupants’ comfort. Despite the benefits of dynamic windows, they are rarely deployed in buildings because the existing technology cannot achieve fast and colour-neutral tinting at an agreeable cost. Reversible metal electrodeposition is a promising approach to solve these problems. Here, we demonstrate the use of polymer inhibitors to reversibly deposit metal films with controlled morphology in dynamic windows. The windows that employ the polymer inhibitor can readily tint to below 0.001% visible transmittance in less than 3 min and exhibit high infrared reflectance (>70%), colour-neutral transmittance (C* 900 cm2 dynamic windows with fast response and excellent uniformity. Dynamic windows can reduce a building’s energy demand, yet the control of light and heat transmission remains challenging. Strand et al. show that polymer inhibitors improve the morphology of metal films allowing a wide modulation of light and heat flow in windows based on reversible metal electrodeposition.

Although inhibitors of the kinases CHK1, ATR, and WEE1 are undergoing clinical testing, it remains unclear how these three classes of agents kill susceptible cells and whether they utilize the same cytotoxic mechanism. Here we observed that CHK1 inhibition induces apoptosis in a subset of acute leukemia cell lines in vitro, including TP53-null acute myeloid leukemia (AML) and BCR/ABL–positive acute lymphoid leukemia (ALL), and inhibits leukemic colony formation in clinical AML samples ex vivo. In further studies, downregulation or inhibition of CHK1 triggered signaling in sensitive human acute leukemia cell lines that involved CDK2 activation followed by AP1-dependent TNF transactivation, TNFα production, and engagement of a TNFR1- and BID-dependent apoptotic pathway. AML lines that were intrinsically resistant to CHK1 inhibition exhibited high CHK1 expression and were sensitized by CHK1 downregulation. Signaling through this same CDK2–AP1–TNF cytotoxic pathway was also initiated by ATR or WEE1 inhibitors in vitro and during CHK1 inhibitor treatment of AML xenografts in vivo. Collectively, these observations not only identify new contributors to the antileukemic cell action of CHK1, ATR, and WEE1 inhibitors, but also delineate a previously undescribed pathway leading from aberrant CDK2 activation to death ligand–induced killing that can potentially be exploited for acute leukemia treatment. Significance: This study demonstrates that replication checkpoint inhibitors can kill AML cells through a pathway involving AP1-mediated TNF gene activation and subsequent TP53-independent, TNFα-induced apoptosis, which can potentially be exploited clinically.

Summary Nickel oxide (NiOx) hole transport layers (HTLs) are desirable contacts for perovskite photovoltaics because they are low cost, stable, and readily scalable; however, they deliver lower open-circuit voltages (VOCs) compared to organic HTLs. Here, we characterize and mitigate electron transfer-proton transfer reactions between NiOx HTLs and perovskite precursors. Using XPS and UPS characterization, we identify that Ni≥3+ metal cation sites in NiOx thin films act both as Bronsted proton acceptors and Lewis electron acceptors, deprotonating cationic amines and oxidizing iodide species, forming PbI2−xBrx-rich hole extraction barriers at the perovskite-NiOx interface. Titrating reactive Ni≥3+ surface states with excess A-site cation salts during perovskite active layer deposition yielded an increase in VOC values to 1.15 V and power conversion efficiencies of ∼20%. This may be a general finding for metal oxide contacts that act as Bronsted and Lewis acid-base reactants toward perovskite precursors, an observation that has also been made recently for TiO2 and SnO2 contacts.

Summary Electrochromic, “smart” windows allow users to control light and heat flow into and out of buildings or vehicles without sacrificing their view. Despite the appeal of dynamic windows, they have yet to achieve extensive commercialization due to their inability to simultaneously achieve fast, color-neutral tinting over a wide optical dynamic range at a low cost. Reversible metal electrodeposition (RME) is a promising alternative that addresses the drawbacks of existing technologies. Here, we study several possible RME electrolytes at various pHs with different supporting anions. We identify that acidic perchlorate electrolytes permit fully reversible metal electrodeposition without harming the substrate or introducing irreversible side reactions, which was not seen among all other electrolytic compositions. This perchlorate electrolyte shows promising long-term durability, demonstrating 10,000 stable cycles with no evidence of electrode etching. In addition, we construct 15 × 15 cm2 dynamic windows that tint uniformly with fast, color-neutral switching (>60% contrast in

Low-bandgap Sn/Pb ABX3 perovskites have reached photovoltaic power conversion efficiencies >20%, but they usually have poor stability due to the common use of acidic poly(3,4-ethylenedioxythiophene...

Tuning band gaps with three halides Tandem solar cells can boost solar cell efficiency by using two active layers to absorb the solar spectrum more completely, provided that the two cells are current-matched. Inorganic-organic perovskites tuned to the appropriate wide band gap (∼1.7 electron volts) as top cells that contained iodine and bromine or bromine and chlorine have short carrier diffusion lengths and undergo photo-induced phase segregation. Xu et al. now report a method for incorporating chloride that allows for fabrication of stable triple-halide perovskites with a band gap of 1.67 electron volts. Two-terminal tandem silicon solar cells made with this material had a power conversion efficiency of 27%. Science , this issue p. 1097

Summary Multijunction all-perovskite solar cells offer a route toward efficiencies of III-V materials at low cost by combining the advantages of low thermalization loss in multijunction architectures with the beneficial properties of perovskites—namely, low processing cost, high-throughput fabrication, and compatibility with flexible substrates. However, there are two main challenges for enabling high-efficiency tandems: (1) design of a recombination layer to efficiently combine two perovskite subcells while also preventing bottom cell damage during top cell processing and (2) achieving high open-circuit voltage of the wide-gap subcell. Herein, we overcome both of these challenges. First, we demonstrate a nucleation layer consisting of an ultra-thin polymer with nucleophilic hydroxyl and amine functional groups for nucleating a conformal, low-conductivity aluminum zinc oxide layer by atomic layer deposition (ALD). This method enables ALD-grown recombination layers that reduce shunting as well as solvent degradation from solution processing on top of existing perovskite active layers. Next, we demonstrate a band-gap tuning strategy based on A-site cations of mismatched size (dimethylammonium and cesium) to enable a 1.7 eV perovskite with high, stable voltages. By combining these advances, we fabricate two-terminal all-perovskite tandem solar cells with 23.1% power conversion efficiency on rigid substrates and 21.3% on flexible plastic substrates.

Acquired PARP inhibitor (PARPi) resistance in BRCA1- or BRCA2-mutant ovarian cancer often results from secondary mutations that restore expression of functional protein. RAD51C is a less commonly studied ovarian cancer susceptibility gene whose promoter is sometimes methylated, leading to homologous recombination (HR) deficiency and PARPi sensitivity. For this study, the PARPi-sensitive patient-derived ovarian cancer xenograft PH039, which lacks HR gene mutations but harbors RAD51C promoter methylation, was selected for PARPi resistance by cyclical niraparib treatment in vivo. PH039 acquired PARPi resistance by the third treatment cycle and grew through subsequent treatment with either niraparib or rucaparib. Transcriptional profiling throughout the course of resistance development showed widespread pathway level changes along with a marked increase in RAD51C mRNA, which reflected loss of RAD51C promoter methylation. Analysis of ovarian cancer samples from the ARIEL2 Part 1 clinical trial of rucaparib monotherapy likewise indicated an association between loss of RAD51C methylation prior to on-study biopsy and limited response. Interestingly, the PARPi resistant PH039 model remained platinum sensitive. Collectively, these results not only indicate that PARPi treatment pressure can reverse RAD51C methylation and restore RAD51C expression, but also provide a model for studying the clinical observation that PARPi and platinum sensitivity are sometimes dissociated.

Acquired PARP inhibitor (PARPi) resistance in BRCA1- or BRCA2-mutant ovarian cancer often results from secondary mutations that restore expression of functional protein. RAD51C is a less commonly studied ovarian cancer susceptibility gene whose promoter is sometimes methylated in the tumor, leading to homologous recombination deficiency and PARPi sensitivity. For this study, the PARPi-sensitive patient-derived xenograft PH039, which lacks demonstrable repair gene mutations but harbors RAD51C promoter methylation, was selected for PARPi resistance by repeated 21-day niraparib treatments in vivo. PH039 acquired PARPi resistance by the third cycle of treatment and demonstrated unimpeded growth during subsequent exposure to either niraparib or rucaparib. Transcriptional profiling throughout the time course of resistance development showed widespread pathway level changes along with a marked increase in RAD51C mRNA, which reflected loss of RAD51C promoter methylation. Analysis of RAD51C methylation in patient tumor samples from the ARIEL2 Part 1 clinical trial of rucaparib monotherapy likewise indicated that loss of RAD51C methylation prior to on-study biopsy was associated with limited response. Interestingly, the PARPi resistant PH039 model remained platinum sensitive. Collectively, these results not only indicate that PARPi treatment pressure can reverse RAD51C methylation and restore RAD51C expression, but also provide an important model for studying the clinical observation that PARPi and platinum sensitivity are sometimes dissociated.

While loss of BRCA1 promoter methylation has been shown to cause PARP inhibitor (PARPi) resistance in high-grade serous ovarian carcinoma (HGSC), the impacts of RAD51C methylation (meRAD51C) remain unresolved. In this study, three PARPi-responsive HGSC patient-derived xenografts (PDX) with RAD51C gene silencing and homologous recombination deficiency were found to have either homogeneous or heterogeneous patterns of meRAD51C. PDX could lose meRAD51C following PARPi treatment (rucaparib/niraparib), where a single unmethylated RAD51C copy was sufficient to drive PARPi-resistance. Genomic profiling revealed this resistance was acquired independently in two distinct PDX lineages. Furthermore, we describe a patient sample where 1/3 RAD51C gene copies lost methylation following neoadjuvant chemotherapy. We show meRAD51C is a positive predictive biomarker for PARPi response and should be screened for routinely in patients. However, methylation loss in a single gene copy is sufficient to cause PARPi resistance and should be carefully assessed in previously treated patients considering PARPi therapy.

Triple halide perovskites have been used to make perovskite-Si tandems with 27% efficiency and excellent stability. Reactions between NiO and perovskites have been studied.

Two critical issues associated with semi-transparent, n–i–p perovskite solar cells for 2-terminal tandem devices are parasitic absorption and long-term instability associated with the widely used spiro-OMeTAD and MoOx hole transport and buffer layers, respectively. In this work, we present an alternative hole contact bilayer that consists of a 30 nm undoped layer of spiro-TTB in conjunction with 9 nm of air-stable vanadium oxide (VOx) deposited via atomic layer deposition. The low absorption of UV and visible light in this bilayer results in the fabrication of a semi-transparent perovskite cell with 18.9 mA cm−2 of photocurrent, a 14% increase compared to the 16.6 mA cm−2 generated in a control device with 150 nm of doped spiro-OMeTAD. The ALD VOx buffer layer shows promise as a stable alternative to MoOx; an unencapsulated Cs0.17FA0.83Pb(Br0.17I0.83)3 device with ALD VOx and ITO as the top contact maintains its efficiency following 1000 hours at 85 °C in a N2 environment. Lastly, we use transfer matrix modeling of the optimized perovskite stack to predict its optical performance in a monolithic tandem cell with heterojunction silicon.

This review article examines the current state of understanding in how metal halide perovskite solar cells can degrade when exposed to moisture, oxygen, heat, light, mechanical stress, and reverse bias. It also highlights strategies for improving stability, such as tuning the composition of the perovskite, introducing hydrophobic coatings, replacing metal electrodes with carbon or transparent conducting oxides, and packaging. The article concludes with recommendations on how accelerated testing should be performed to rapidly develop solar cells that are both extraordinarily efficient and stable.

Photoinduced halide segregation currently limits the perovskite chemistries available for use in high-bandgap semiconductors needed for tandem solar cells. Here, we study the impact of post-deposition surface modifications on photoinduced halide segregation in methylammonium lead mixed-halide perovskites. By coating a perovskite surface with the electron-donating ligand trioctylphosphine oxide (TOPO), we are able to both reduce nonradiative recombination and dramatically slow the onset of halide segregation in CH3NH3PbI2Br films. This result, coupled with the observation that the rate of halide segregation can be tuned by varying the selective contact, provides a direct link between surface modifications and photoinduced trap formation. On the basis of these observations, we discuss possible mechanisms for photoinduced halide segregation supported by drift-diffusion simulations. This work suggests that improved understanding and control of perovskite surfaces provides a pathway toward stable and high-perf...

We apply a series of transient measurements to operational perovskite solar cells of the architecture ITO/PTAA/FA0.83Cs0.17Pb(I0.83Br0.17)3/C60/BCP/Ag, and similar cells with FA0.83MA0.17. The cells show no detectable JV hysteresis. Using photocurrent transients at applied bias we find a ∼1 ms time scale for the electric field screening by mobile ions in these cells. We confirm our interpretation of the transient measurements using a drift-diffusion model. Using Coulometry during field screening relaxation at short circuit, we determine the mobile ion concentration to be ∼1 × 1018/cm3. Using a model with one mobile ion species, the concentration and the screening time require an ion mobility of ∼3 × 10–7 cm2/(V s). As far as we know, this article gives the first direct measurement of the ion mobility and concentration in a fully functional perovskite solar cell. The measured ion mobility is 2 orders of magnitude higher than the highest estimates previously determined using perovskite solar cells and perov...

The rapid rise in efficiency and tunable bandgap of metal-halide perovskites makes them highly attractive for use in tandems on silicon. Recently we demonstrated a perovskite–silicon monolithic two...

Metal halide perovskite semiconductors possess excellent optoelectronic properties, allowing them to reach high solar cell performances. They have tunable bandgaps and can be rapidly and cheaply deposited from low-cost precursors, making them ideal candidate materials for tandem solar cells, either by using perovskites as the wide-bandgap top cell paired with low-bandgap silicon or copper indium diselenide bottom cells or by using both wide- and small-bandgap perovskite semiconductors to make all-perovskite tandem solar cells. This Review highlights the unique potential of perovskite tandem solar cells to reach solar-to-electricity conversion efficiencies far above those of single-junction solar cells at low costs. We discuss the recent developments in perovskite-based tandem fabrication, and detail directions for future research to take this technology beyond the proof-of-concept stage. Perovskites, with their wide bandgap range, are good partners for both commercial and novel photovoltaic technologies in multijunction solar cells. Here, McGehee and co-workers review recent material and device developments and highlight future challenges and opportunities for perovskite-based tandems.

Although two-terminal perovskite-silicon tandem solar cells have rapidly increased in efficiency, they have only been demonstrated with n-type silicon, which currently constitutes less than 5% of the global photovoltaics market. In this paper, we realize the first two-terminal perovskite tandem with p-type silicon by developing a recombination contact that enables voltage addition without damaging either subcell. We find that silicon interband recombination contacts are limited by a SiO x charge-extraction barrier, which forms during oxidative top-cell fabrication. A sputtered 30-nm indium tin oxide layer is found to protect the silicon cell surface from oxidation, while forming a recombination contact with the p-type nickel oxide hole transport layer for the perovskite top cell. Using this recombination contact we achieve voltage addition between the perovskite top cell and aluminum back-surface field p-type silicon bottom cell. We also find that minimizing moisture on the nickel oxide surface is important for achieving a stable open-circuit voltage under illumination. The recombination contact developed herein could play an important role in near-future developments.

The performance of perovskite solar cells has rapidly increased above 22%, and their environmental stability is also progressing. However, the mismatch in thermal expansion coefficients and low fracture energy of layers in perovskite solar cells raise a concern as to whether devices can withstand mechanical stresses from temperature fluctuations. We measured the fracture energy of a perovskite film stack, which was shown to produce 23.6% efficiency when incorporated in a monolithic perovskite-silicon tandem. We found that the fracture energy increased by a factor of two after 250 standardized temperature cycles between −40 °C and 85 °C and a factor of four after laminating an encapsulant on top of the stack. In order to observe how the increased mechanical stability translated from film stacks to device performance and reliability, we carried out a comparative study of perovskite solar cells packaged between glass and two commonly used encapsulants with different elastic moduli. We demonstrated that solar cells encapsulated with a stiffer ionomer, Surlyn, severely decreased in performance with temperature cycling and delaminated. However, the solar cells encapsulated in softer ethylene vinyl acetate withstood temperature cycling and retained over 90% of their initial performance after 200 temperature cycles. This work demonstrates a need for an encapsulant with a low elastic modulus to enable mechanical stability and progress toward 25 year operating lifetime.

Several developing solar cell technologies—including perovskite cells, thin-film cells, epitaxially grown cells, and many tandem cells on silicon—have fabrication constraints that require a planar front surface. However, flat front surfaces result in large reflection losses and poor light trapping within the cell. We investigate scattering layers made from polydimethylsiloxane (PDMS) polymer carrying a random-pyramid texture to reduce such losses. The layers are first tested on a model system consisting of silicon heterojunction solar cells that have zero, one, or two surfaces textured with the same random pyramids (the other surfaces being planar) in order to elucidate the potential and limitations of employing a textured transparent layer instead of a textured absorber. PDMS layers result in short-circuit current density enhancements of 3.0 mA/cm 2 and 1.7 mA/cm 2 when applied to the front of a cell with flat front and rear surfaces, and a cell with a flat front surface and textured rear surface, respectively. Optical simulations reveal that the majority of the gain is due to a reduction in front-surface reflection and that the layers contribute only marginally to trapping weakly absorbed infrared light; nevertheless, a cell with a textured rear surface and a PDMS layer at its flat front surface can come to within 0.7 mA/cm 2 of the performance of a double-side-textured cell. Finally, a PDMS scattering layer is implemented in a planar perovskite solar cell, boosting its short-circuit current density by 1.9 mA/cm 2 and thus its efficiency by 10.6% relative.

Tin-lead perovskites have low band gaps (1.2 eV – 1.4 eV) that offer complementary absorption spectra to mixed halide perovskites with band gaps in the range of 1.6 eV – 1.85 eV. In combination, tin-lead and mixed halide perovskites can be used to make efficient all-perovskite tandem solar cells. While these can achieve high efficiency, they are hampered by the unproven longterm stability of tin-containing perovskites. We make the first demonstration of tin-lead perovskite solar cells that pass benchmark 1000-hour tests of stability under stressors of heat, light, and atmospheric exposure. We identify that mixed tin-lead perovskites oxidise by a different chemical pathway, one that is much less favourable, than the oxidation pathway followed by pure tin perovskites, making them significantly more stable. Fortuitously, mixed tin-lead perovskites also have band gaps that are close to ideal for the rear cell of an all-perovskite monolithic two-junction tandem. Beyond selecting the composition of the perovskite to be one that has inherently suppressed tendency to oxidise, we also fabricate a novel heterojunction-based architecture that eliminates the commonly used acidic PEDOT:PSS layer, which leads to enhanced stability. By packaging the resulting solar cell in a glass-on-glass package with a pliable low-elastic-modulus polyolefin encapsulant and a butyl rubber edge seal, we also demonstrate that the low band gap perovskite solar cell that passes the IEC damp heat test for thin film solar cells – a milestone that demonstrates the removal of a significant impediment to the commercialization of high efficiency perovskite tandem solar cells.

We found Br/I mixed perovskite with high fraction of Cs can steadily incorporate Cl-based perovskite to tune the bandgap. The resulting films exhibit significant improvement in the mobility and lifetime of photocarriers. In 1.67eV wide-bandgap perovskite top cells optimized for perovskite/Si tandems, we achieved a great reducing of voltage-of-open-circuit (VOC) deficit from 0.57eV to 0.45eV, which is among the best of perovskite top cells in Si tandems reported so far. The stabilized PCE is improved from 17-18% to 20%, featured with high VOC (1.22V) and high FF (83%). This advance in wide-bandgap top cells holds promise for improving perovskite/Si tandems.

In Methyl Ammonium Lead Iodide (MAPI) perovskite solar cells, screening of the built in field by mobile ions has been proposed as part of the cause of the large hysteresis observed in the current/voltage scans in many cells. We show that photocurrent transients measured immediately (e.g. 100 μs) after a voltage step can provide direct evidence that this field screening exists. Just after a step to forward bias, the photocurrent transients are reversed in sign (i.e. inverted), and the magnitude of the inverted transients can be used to find an upper bound on the width of the space charge layers adjacent to the electrodes. This in turn provides a lower bound on the mobile charge concentration, which we find to be 1 x 10 17 /cm 3 . Using a new photocurrent transient experiment, we show that the space charge layer thickness remains approximately constant as a function of bias, as expected for mobile ions in a solid electrolyte. We also discuss additional characteristics of the inverted photocurrent transients that imply either an unusually stable deep trapping, or a photo effect on the mobile ion conductivity.

We developed transmissive-to-black electrochromic devices by assembling p-aminotriphenlyamine (pAT) on the surface of mesoporous tin-doped indium oxide (mITO) electrodes. pAT-modified electrodes possess an optical contrast ratio of up to 64%, switching speeds of 1–4 s, and enhanced chemical reversibility as compared to unbound pAT in solution. Gel permeation chromatography (GPC) and inductively coupled plasma mass spectrometry (ICP-MS) were used to analyze the electrochemical degradation products of pAT-modified electrodes and pAT in solution. The rare transparent-to-black electrochromism of pAT makes it a promising small-molecule, anodically coloring material for smart window applications.

Significance Electrolysis of water to generate hydrogen fuel could be vital to the future renewable energy landscape. Electrodes that can sustain seawater splitting without chloride corrosion could address the issue of freshwater scarcity on Earth. Herein, a hierarchical anode consisting of a nickel–iron hydroxide electrocatalyst layer uniformly coated on a sulfide layer formed on Ni substrate was developed, affording superior catalytic activity and corrosion resistance in seawater electrolysis. In situ-generated polyanion-rich passivating layers formed in the anode are responsible for chloride repelling and high corrosion resistance, leading to new directions for designing and fabricating highly sustained seawater-splitting electrodes and providing an opportunity to use the vast seawater on Earth as an energy carrier., Electrolysis of water to generate hydrogen fuel is an attractive renewable energy storage technology. However, grid-scale freshwater electrolysis would put a heavy strain on vital water resources. Developing cheap electrocatalysts and electrodes that can sustain seawater splitting without chloride corrosion could address the water scarcity issue. Here we present a multilayer anode consisting of a nickel–iron hydroxide (NiFe) electrocatalyst layer uniformly coated on a nickel sulfide (NiSx) layer formed on porous Ni foam (NiFe/NiSx-Ni), affording superior catalytic activity and corrosion resistance in solar-driven alkaline seawater electrolysis operating at industrially required current densities (0.4 to 1 A/cm2) over 1,000 h. A continuous, highly oxygen evolution reaction-active NiFe electrocatalyst layer drawing anodic currents toward water oxidation and an in situ-generated polyatomic sulfate and carbonate-rich passivating layers formed in the anode are responsible for chloride repelling and superior corrosion resistance of the salty-water-splitting anode.

BACKGROUND Perovskite solar cells (PSCs) have attracted intensive attention because of their ever-increasing power conversion effi­ciency (PCE), low-cost materials constituents, and simple solution fabrication process. Initi­ated in 2009 with an efficiency of 3.8%, PSCs have now achieved a lab-scale power conversion efficiency of 23.3%, rivaling the performance of commercial multicrystalline silicon solar cells, as well as copper indium gallium selenide (CIGS) and cadmium telluride (CdTe) thin-film solar cells. Thousands of articles re­lated to PSCs have been published each year since 2015, highlighting PSCs as a topic of in­tense interest in photovoltaics (PV) research. With high efficiencies achieved in lab devices, stability and remaining challenges in upscal­ing the manufacture of PSCs are two critical concerns that must be addressed on the path to PSC commercialization. ADVANCES We review recent progress in PSCs and discuss the remaining challenges along the pathway to their commercialization. Device configurations of PSCs (see the figure) include mesoscopic formal (n-i-p) and inverted (p-i-n) structures, planar formal and inverted struc­tures, and the printable triple mesoscopic structures. PCEs of devices that use these structures have advanced rapidly in the case of small-area devices (~0.1 cm 2 ). PSCs are also attracting attention as top cells for the construction of tandem solar cells with existing mature PV technologies to increase efficiency beyond the Shockley-Queisser limit of single-junction devices. The stability of PSCs has attracted much well-deserved attention of late, and notable progress has been made in the past few years. PSCs have recently achieved exhibited life­times of 10,000 hours under 1 sun (1 kW/m 2 ) illumina­tion with an ultraviolet filter at a stabilized temperature of 55°C and at short-circuit conditions for a printable triple mesoscopic PSCs. This irradiation is equivalent to the total irradiation of 10 years of outdoor use in most of Europe. However, within the PSC community, standard testing protocols require further development. In addition, transpar­ency in reporting standards on stability tests needs to be improved; this can be achieved by providing both initial photovoltaic performance and normalization parameters. The upscaling of PSCs has also progressed steadily, leading to PSC mini-modules, standard-sized modules, and power systems. PV companies have set out to manufacture large-area PSC modules (see the figure), and a 110-m 2 perovskite PV system with screen-printed triple mesoscopic PSC modules was recently debuted. Studies of these increased-area modules and systems will promote the development of PSCs toward commercializa­tion. PSC research is expanding to cover fundamental topics on materials and lab-sized cells, as well as to address issues of in­dustrial-scale manufacturing and deployment. OUTLOOK The PV market has been continu­ously expanding in recent years, bringing op­portunities for new PV technologies of which PSCs are promising candidates. It is impera­tive to achieve a low cost per watt, which means that both efficiency and lifetime need improve­ment relative to current parameters. The efficiency gap between lab cells and industrial modules has seen impressive reduc­tions in crystalline silicon; PSCs must simi­larly enlarge module areas to the panel level and need to achieve lifetimes comparable to those of legacy PV technologies. Other improvements will need to include industry-scale electronic-grade films, recycling methods to address concerns regarding lead toxicity, and the adoption of standardized testing protocols to predict the operation lifetime of PSCs. Modules will need to endure light-induced degradation, potential-induced degradation, partial-shade stress, and mechanical shock. The field can benefit from lessons learned during the development of mature PV technologies as it strives to de­fine, and overcome, the hurdles to PSC com­mercial impact.

Understanding the formation chemistry of metal halide perovskites is key to optimizing processing conditions and realizing enhanced optoelectronic properties. Here, we reveal the structure of the crystalline precursor in the formation of methylammonium lead iodide (MAPbI3) from the single-step deposition of lead chloride and three equivalents of methylammonium iodide (PbCl2 + 3MAI) (MA = CH3NH3). The as-spun film consists of crystalline MA2PbI3Cl, which is composed of one-dimensional chains of lead halide octahedra, coexisting with disordered MACl. We show that the transformation of precursor into perovskite is not favored in the presence of MACl, and thus the gradual evaporation of MACl acts as a self-regulating mechanism to slow the conversion. We propose the stable precursor phase enables dense film coverage and the slow transformation may lead to improved crystal quality. This enhanced chemical understanding is paramount for the rational control of film deposition and the fabrication of superior optoelectronic devices., The existence of a crystalline precursor is key to perovskite film formation, but the precise chemistry of the precursor and its transformation into perovskite are poorly understood. Here, the authors identify the crystal structure and conversion chemistry of the precursor for PbCl2-derived methylammonium lead iodide perovskites.

Despite concerns about environmental and mechanical stability, perovskite solar cells (PSC) were made stable with careful solar cell design and encapsulation. Replacing methylammonium cations with a mixture of cesium and formamidinium and using an ITO protective contact enabled 140}}$^{circ}$ C encapsulation and enhanced moisture stability. Vacuum laminating mechanically fragile PSCs using glass and a low elastic modulus encapsulant with a butyl rubber edge seal around the perimeter enabled them to pass the IEC 61215 200 temperature cycling and the 1000-hour damp heat test at 85 C85 relative humidity. Removing TiO 2 from the PSC stacks also resulted in the encapsulated PSCs passing the UV exposure test.

Metal halide perovskites are promising candidates for the wide band gap top cell in tandem solar cells. By introducing a tin oxide buffer layer via atomic layer deposition, we have improved the efficiency of monolithic, two-terminal, perovskite/silicon tandems to 23.6% by combining an infraredtuned silicon heterojunction bottom cell with the more stable cesium formamidinium lead halide perovskite. We show that further improvements to current and voltage can be made by introducing a PDMS scattering layer to reduce front surface reflection and using compositional engineering to simultaneously increase bandgap and Voc, while mitigating voltage losses arising from photo-induced halide segregation.

Tin-lead halide perovskites show promise as low- band gap absorbers to enable efficient all-perovskite tandem solar cells. While they have reached high efficiencies in single junction and in tandem solar cells, tin-containing perovskites face unique challenges- such as a tendency to degrade by oxidation of tin and sometimes very short carrier lifetimes that limit diffusion lengths. We map oxidation stability and important optoelectronic properties- band gap, carrier lifetime, and solar cell performance - across a wide range of compositions varying the A-site cation and the tin:lead ratio at the B-site. We identify mechanisms by which composition tunes the band gap in ways distinct from what has been reported for pure lead-based perovskites. We show that alloying tin with lead changes the mechanism by which tin is oxidized to a pathway that is relatively unfavourable. This significantly stabilizes the perovskite toward oxidation. Using a low band gap perovskite absorber, we demonstrate a 17.8% efficient single junction solar cell. Further, by employing a sputtered ITO top electrode, we demonstrate continuous operation of a solar cell in air with only a small drop in efficiency that is reversible upon storage in the dark This work creates a set of guiding principles to intelligently choose the best compositions for making efficient and stable solar cells based on tin-containing low band gap perovskites.

The fill factor of perovskite solar cells is considerably lower than that of GaAs and silicon, though they have similar open-circuit voltage (V oc ) deficits. To probe the fill factor loss, we applied the J sc -V oc characterization technique on three different types of perovskite solar cells. Based on the J sc -V oc measurements, we calculated the series resistance at maximum power point under different illumination—and thus injection— levels. We found that the series resistances of all three perovskite solar cells are illumination dependent, unlike silicon solar cells. Further analysis is underway to determine the origin of this phenomenon.

Perovskite/silicon tandems have the potential to surpass the efficiencies possible with single-junction silicon solar cells. Recently, perovskites with wide bandgaps have been demonstrated that can reduce the thermalization losses and make it feasible to boost the efficiency. However, perovskite/silicon tandems on planar silicon cells are multi-layered devices that exhibit strong interference which makes current matching difficult to achieve in two-terminal devices. We perform comprehensive optical modeling of perovskite/silicon tandems with a range of different bandgap perovskites and textures to determine the top-cell thicknesses necessary to achieve current matching for varying top-cell bandgap. Our results show it is possible to achieve 27% efficiency with 1.67 eV bandgap perovskite, and even higher efficiency is possible by minimizing parasitic losses.

Energy levels in the mixed polymer:fullerene phase of three-phase bulk heterojunction solar cells are significantly shifted from their values in the pure materials. These shifts provide an important driving force for separating charge carriers. Through cyclic voltammetry, we measure a gradual shift in the polymer HOMO and fullerene LUMO as a function of blend composition in a model bulk heterojunction (BHJ) system that only contains amorphous polymer. The effective band gap of the polymer:fullerene blend varies by up to 300 meV with varying blend composition. The shifts in polymer HOMO and fullerene LUMO can be quantitatively accounted for by the electrostatic potential generated by induced dipoles at the polymer:fullerene interfaces. Remarkably, however, the measured charge transfer state energy and open-circuit voltage shift far less.

Polymer:fullerene solar cells depend heavily on the electronic coupling of the polymer and fullerene molecular species from which they are composed. The intermolecular interaction between the polymer and fullerene tends to be strong in efficient photovoltaic systems, as evidenced by efficient charge transfer processes and by large changes in the energetics of the polymer and fullerene when they are molecularly mixed. Despite the clear presence of these strong intermolecular interactions between the polymer and fullerene, there is not a consensus on the nature of these interactions. In this work, we use a combination of Raman spectroscopy, charge transfer state absorption, and density functional theory calculations to show that the intermolecular interactions do not appear to be caused by ground state charge transfer between the polymer and fullerene. We conclude that these intermolecular interactions are primarily van der Waals in nature.

Fullerene dimerization has been linked to short circuit current (Jsc) losses in organic solar cells comprised of certain polymer–fullerene systems. We investigate several polymer–fullerene systems, which present Jsc loss to varying degrees, in order to determine under which conditions dimerization occurs. By reintroducing dimers into fresh devices, we confirm that the photo-induced dimers are indeed the origin of the Jsc loss. We find that both film morphology and electrical bias affect the photodimerization process and thus the associated loss of Jsc. In plain fullerene films, a higher degree of crystallinity can inhibit the dimerization reaction, as observed by high performance liquid chromatography (HPLC) measurements. In blend films, the amount of dimerization depends on the degree of mixing between polymer and fullerene. For highly mixed systems with very amorphous polymers, no dimerization is observed. In solar cells with pure polymer and fullerene domains, we tune the fullerene morphology from amorphous to crystalline by thermal annealing. Similar to neat fullerene films, we observe improved light stability for devices with crystalline fullerene domains. Changing the operating conditions of the investigated solar cells from Voc to Jsc also significantly reduces the amount of dimerization-related Jsc loss; HPLC analysis of the active layer shows that more dimers are formed if the cell is held at Voc instead of Jsc. The effect of bias on dimerization, as well as a clear correlation between PL quenching and reduced dimerization upon addition of small amounts of an amorphous polymer into PC60BM films, suggests a reaction mechanism via excitons.

Understanding how intermixed domains and pure domains in bulk heterojunction (BHJ) thin films affect charge generation and recombination in polymer solar cells (PSCs) is important to further improve PSC device performance. We control the proportion of intermixed domains and pure domains in an amorphous polymer and fullerene BHJ thin films by varying the fullerene concentration. By studying these thin films with ultrafast spectroscopy, we found that the coherent ultrafast charge generation process is nonexistent in this BHJ system due to the lack of pure polymer domains. We also found that the BHJ thin film with 100% intermixed domains begins charge recombination immediately after the initial charge generation process, while the BHJ thin film with pure fullerene domains showed an additional slow excitonic charge generation process before a slower charge recombination takes place. These results clearly demonstrate that pure domains are important for optimal BHJ solar cell performance.

For long-term performance, chemically robust materials are desired for organic solar cells (OSCs). Illuminating neat films of OSC materials in air and tracking the rate of absorption loss, or photobleaching, can quickly screen a material’s photochemical stability. In this report, we photobleach neat films of OSC materials including polymers, solution-processed oligomers, solution-processed small molecules, and vacuum-deposited small molecules. Across the materials we test, we observe photobleaching rates that span 7 orders of magnitude. Furthermore, we find that the film morphology of any particular material impacts the observed photobleaching rate and that amorphous films photobleach faster than crystalline ones. In an extreme case, films of amorphous rubrene photobleach at a rate 2500 times faster than polycrystalline films. When we compare density to photobleaching rate, we find that stability increases with density. We also investigate the relationship between backbone planarity and chemical reactivit...

We report the fabrication of monolithic all-perovskite tandem solar cells with a stabilized power conversion efficiency of 19.1% and demonstrate improved thermal, atmospheric, and operational stability of the tin–lead perovskite (FA0.75Cs0.25Sn0.5Pb0.5I3) used as the low gap absorber. To achieve a high matched current density in the two-terminal tandem, we develop a route to fabricate uniform and thick tin–lead perovskites that enable the two-terminal tandem to attain external quantum efficiencies >80% in the near infrared. By post-processing the as-deposited tin–lead perovskite films with methylammonium chloride vapor, we increase grain sizes to over a micron and boost solar cell open circuit voltage and fill factor. Tin–lead perovskite solar cells made by this method exhibit the most stable operation at maximum power under simulated sunlight of any reported small bandgap perovskite solar cell to date. We show that an unencapsulated tin–lead perovskite solar cell maintains its full performance after 150 hours at 85C in air, which is substantially better than has been observed previously. This work identifies strategies for attaining highly efficient all-perovskite tandem solar cells while maintaining thermal and operational stability.

We report on reversible, light-induced transformations in (CH3NH3)Pb(BrxI1−x)3. Photoluminescence (PL) spectra of these perovskites develop a new, red-shifted peak at 1.68 eV that grows in intensity under constant, 1-sun illumination in less than a minute. This is accompanied by an increase in sub-bandgap absorption at ∼1.7 eV, indicating the formation of luminescent trap states. Light soaking causes a splitting of X-ray diffraction (XRD) peaks, suggesting segregation into two crystalline phases. Surprisingly, these photo-induced changes are fully reversible; the XRD patterns and the PL and absorption spectra revert to their initial states after the materials are left for a few minutes in the dark. We speculate that photoexcitation may cause halide segregation into iodide-rich minority and bromide-enriched majority domains, the former acting as a recombination center trap. This instability may limit achievable voltages from some mixed-halide perovskite solar cells and could have implications for the photostability of halide perovskites used in optoelectronics.

We have deposited perovskite solar cells with a bandgap of 1.68 eV onto heterojunction silicon solar cells that by themselves have an efficiency of 21% to create a 1 square centimeter monolithic tandem solar cell with an efficiency of 25.3%. We have also made all-perovskite tandems using a new ABX3 perovskite composition containing a mixture of tin and lead on the B site that have greater than 20% efficiency. With solar cells packaged between two sheets of glass with rubber edge seals, we have passed the industry standard 1000 hour 85°C 85% humidity damp heat test as well as 200 cycles between 85°C and -40°C. One of the keys to obtaining high efficiency and stability was optimizing the composition of cesium and formamidinium on the A site and iodine and bromine on the X site. We will show how light -induced phase separation occurs when there is too much bromine in the films. Another crucial step towards improving stability is the use of atomic layer deposition to deposit a tin oxide buffer layer on the perovskite that enables the sputter deposition of an indium tin oxide transparent electrode. ITO is less reactive with perovskites than the metals that are typically used in perovskite solar cells. The ALD layer has minimal parasitic absorption and prevents shunting. Our progress towards achieving 30% power conversion efficiency and passing even more aggressive stability tests will be presented.

Tin and lead iodide perovskite semiconductors of the composition AMX3, where M is a metal and X is a halide, are leading candidates for high efficiency low cost tandem photovoltaics, in part because they have band gaps that can be tuned over a wide range by compositional substitution. We experimentally identify two competing mechanisms through which the A-site cation influences the band gap of 3D metal halide perovskites. Using a smaller A-site cation can distort the perovskite lattice in two distinct ways: by tilting the MX6 octahedra, or by simply contracting the lattice isotropically. The former effect tends to raise the band gap, while the latter tends to decrease it. Lead iodide perovskites show an increase in band gap upon partial substitution of the larger formamidinium with the smaller cesium, due to octahedral tilting. Perovskites based on tin, which is slightly smaller than lead, show the opposite trend – they show no octahedral tilting upon Cs-substitution, but only a contraction of the lattice, leading to progressive reduction of the band gap. We outline a strategy to systematically tune the band gap and valence and conduction band positions of metal halide perovskites through control of the cation composition. Using this strategy, we demonstrate solar cells that harvest light in the infrared up to 1040 nm, reaching a stabilized power conversion efficiency of 17.8%, showing promise for improvements of the bottom cell of all-perovskite tandem solar cells. The mechanisms of cation-based band gap tuning we describe are broadly applicable to 3D metal halide perovskites, and will be useful in further development of perovskite semiconductors for optoelectronic applications. We thank Kevin A. Bush and Kyle Frohna for productive and stimulating conversations, Adam H. Slavney and David A. Hanifi for insightful comments on the manuscript, and Caleb C. Boyd for assistance with solar cell fabrication. This research was supported by the Office of Naval Research, U.S. Department of Defense. Use of the Stanford Synchrotron Radiation Light-source, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. T.L. is funded by a Marie Sklodowska Curie International Fellowship under grant agreement H2O2IF-GA-2015-659225. A.G. is supported by NSF GRFP (DGE-1147470). B.C. is a postdoctoral research fellow of the Research Fund Flanders (FWO). A.B. is a Ph.D. fellow of FWO.

The excellent optoelectronic properties demonstrated by hybrid organic/inorganic metal halide perovskites are all predicated on precisely controlling the exact nucleation and crystallization dynamics that occur during film formation. In general, high‐performance thin films are obtained by a method commonly called solvent engineering (or antisolvent quench) processing. The solvent engineering method removes excess solvent, but importantly leaves behind solvent that forms chemical adducts with the lead‐halide precursor salts. These adduct‐based precursor phases control nucleation and the growth of the polycrystalline domains. There has not yet been a comprehensive study comparing the various antisolvents used in different perovskite compositions containing cesium. In addition, there have been no reports of solvent engineering for high efficiency in all‐inorganic perovskites such as CsPbI3. In this work, inorganic perovskite composition CsPbI3 is specifically targeted and unique adducts formed between CsI and precursor solvents and antisolvents are found that have not been observed for other A‐site cation salts. These CsI adducts control nucleation more so than the PbI2–dimethyl sulfoxide (DMSO) adduct and demonstrate how the A‐site plays a significant role in crystallization. The use of methyl acetate (MeOAc) in this solvent engineering approach dictates crystallization through the formation of a CsI–MeOAc adduct and results in solar cells with a power conversion efficiency of 14.4%.

Organo-metal halide perovskites are an intriguing class of materials that have recently been explored for their potential in solar energy conversion. Within a very short period of intensive research, highly efficient solar cell devices have been demonstrated. One of the heavily debated questions in this new field of research concerns the role of chlorine in solution-processed samples utilizing lead chloride and 3 equiv of methylammonium iodide to prepare the perovskite samples. We utilized a combination of X-ray photoelectron spectroscopy, X-ray fluorescence, and X-ray diffraction to probe the amount of chlorine in samples before and during annealing. As-deposited samples, before annealing, consist of a crystalline precursor phase containing excess methylammonium and halide. We used in situ techniques to study the crystallization of MAPbI3 from this crystalline precursor phase. Excess methylammonium and chloride evaporate during annealing, forming highly crystalline MAPbI3. However, even after prolonged a...

2,2',7,7'-Tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD), the prevalent organic hole transport material used in solid-state dye-sensitized solar cells and perovskite-absorber solar cells, relies on an uncontrolled oxidative process to reach appreciable conductivity. This work presents the use of a dicationic salt of spiro-OMeTAD, named spiro(TFSI)2, as a facile means of controllably increasing the conductivity of spiro-OMeTAD up to 10(-3) S cm(-1) without relying on oxidation in air. Spiro(TFSI)2 enables the first demonstration of solid-state dye-sensitized solar cells fabricated and operated with the complete exclusion of oxygen after deposition of the sensitizer with higher and more reproducible device performance. Perovskite-absorber solar cells fabricated with spiro(TFSI)2 show improved operating stability in an inert atmosphere. Gaining control of the conductivity of the HTM in both dye-sensitized and perovskite-absorber solar cells in an inert atmosphere using spiro(TFSI)2 is an important step toward the commercialization of these technologies.

International audience; Among π-conjugated polymer donors for efficient bulk-heterojunction (BHJ) solar cell applications, poly(benzo[1,2-b:4,5-b′]dithiophene–thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) polymers yield some of the highest open-circuit voltages (VOC, ca. 0.9 V) and fill-factors (FF, ca. 70%) in conventional (single-cell) BHJ devices with PCBM acceptors. In PBDTTPD, side chains of varying size and branching affect polymer self-assembly, nanostructural order, and impact material performance. However, the role of the polymer side-chain pattern in the intimate mixing between polymer donors and PCBM acceptors, and on the development of the BHJ morphology is in general less understood. In this contribution, we show that ring substituents such as furan (F), thiophene (T) and selenophene (S)—incorporated into the side chains of PBDTTPD polymers—can induce significant and, of importance, very different morphological effects in BHJs with PCBM. A combination of experimental and theoretical (via density functional theory) characterizations sheds light on how varying the heteroatom of the ring substituents impacts (i) the preferred side-chain configurations and (ii) the ionization, electronic, and optical properties of the PBDTTPD polymers. In parallel, we find that the PBDT(X)TPD analogs (with X = F, T, or S) span a broad range of power conversion efficiencies (PCEs, 3–6.5%) in optimized devices with improved thin-film morphologies via the use of 1,8-diiodooctane (DIO), and discuss that persistent morphological impediments at the nanoscale can be at the origin of the spread in PCE across optimized PBDT(X)TPD-based devices. With their high VOC ∼1 V, PBDT(X)TPD polymers are promising candidates for use in the high-band gap cell of tandem solar cells.

Long-term stability of polymer solar cells is determined by many factors, one of which is thermal stability. Although many thermal stability studies occur far beyond the operating temperature of a solar cell which is almost always less than 65 °C, thermal degradation is studied at temperatures that the solar cell would encounter in real-world operating conditions. At these temperatures, movement of the polymer and fullerenes, along with adhesion of the polymer to the back contact, creates a barrier for electron extraction. The polymer barrier can be removed and the performance can be restored by peeling off the electrode and depositing a new one. X-ray photoelectron spectroscopy measurements reveal a larger amount of polymer adhered to electrodes peeled from aged devices than electrodes peeled from fresh devices. The degradation caused by hole-transporting polymer adhering to the electrode can be suppressed by using an inverted device where instead of electrons, holes are extracted at the back metal electrode. The problem can be ultimately eliminated by choosing a polymer with a high glass transition temperature.