Rear textured p-type high temperature passivating contacts and their implementation in perovskite/silicon tandem cells.

Silicon solar cells based on high temperature passivating contacts are becoming mainstream in the photovoltaic industry. Here, we developed a high-quality boron-doped poly-silicon hole contact. When combined with a co-processed phosphorus-doped poly-silicon electron contact, high-voltage silicon bottom cells could be demonstrated and included in 28.25%-efficient perovskite/Si tandems. The active area was 4 cm2 active area and the front electrode was screen-printed.

Understanding and Mitigating the Degradation of Perovskite Solar Cells Based on a Nickel Oxide Hole Transport Material during Damp Heat Testing.

The development of stable materials, processable on a large area, is a prerequisite for perovskite industrialization. Beyond the perovskite absorber itself, this should also guide the development of all other layers in the solar cell. In this regard, the use of NiOx as a hole transport material (HTM) offers several advantages, as it can be deposited with high throughput on large areas and on flat or textured surfaces via sputtering, a well-established industrial method. However, NiOx may trigger the degradation of perovskite solar cells (PSCs) when exposed to environmental stressors. Already after 100 h of damp heat stressing, a strong fill factor (FF) loss appears in conjunction with a characteristic S-shaped J-V curve. By performing a wide range of analysis on cells and materials, completed by device simulation, the cause of the degradation is pinpointed and mitigation strategies are proposed. When NiOx is heated in an air-tight environment, its free charge carrier density drops, resulting in a band misalignment at the NiOx/perovskite interface and in the formation of a barrier impeding hole extraction. Adding an organic layer between the NiOx and the perovskite enables higher performances but not long-term thermal stability, for which reducing the NiOx thickness is necessary.

Fully textured monolithic perovskite/silicon tandem solar cells with 25.2% power conversion efficiency.

Tandem devices combining perovskite and silicon solar cells are promising candidates to achieve power conversion efficiencies above 30% at reasonable costs. State-of-the-art monolithic two-terminal perovskite/silicon tandem devices have so far featured silicon bottom cells that are polished on their front side to be compatible with the perovskite fabrication process. This concession leads to higher potential production costs, higher reflection losses and non-ideal light trapping. To tackle this issue, we developed a top cell deposition process that achieves the conformal growth of multiple compounds with controlled optoelectronic properties directly on the micrometre-sized pyramids of textured monocrystalline silicon. Tandem devices featuring a silicon heterojunction cell and a nanocrystalline silicon recombination junction demonstrate a certified steady-state efficiency of 25.2%. Our optical design yields a current density of 19.5 mA cm-2 thanks to the silicon pyramidal texture and suggests a path for the realization of 30% monolithic perovskite/silicon tandem devices.

Hydrophobic Organic Hole Transporters for Improved Moisture Resistance in Metal Halide Perovskite Solar Cells.

Solar cells based on organic-inorganic perovskite semiconductor materials have recently made rapid improvements in performance, with the best cells performing at over 20% efficiency. With such rapid progress, questions such as cost and solar cell stability are becoming increasingly important to address if this new technology is to reach commercial deployment. The moisture sensitivity of commonly used organic-inorganic metal halide perovskites has especially raised concerns. Here, we demonstrate that the hygroscopic lithium salt commonly used as a dopant for the hole transport material in perovskite solar cells makes the top layer of the devices hydrophilic and causes the solar cells to rapidly degrade in the presence of moisture. By using novel, low cost, and hydrophobic hole transporters in conjunction with a doping method incorporating a preoxidized salt of the respective hole transporters, we are able to prepare efficient perovskite solar cells with greatly enhanced water resistance.

Solution Synthesis Approach to Colloidal Cesium Lead Halide Perovskite Nanoplatelets with Monolayer-Level Thickness Control.

We report a colloidal synthesis approach to CsPbBr3 nanoplatelets (NPLs). The nucleation and growth of the platelets, which takes place at room temperature, is triggered by the injection of acetone in a mixture of precursors that would remain unreactive otherwise. The low growth temperature enables the control of the plate thickness, which can be precisely tuned from 3 to 5 monolayers. The strong two-dimensional confinement of the carriers at such small vertical sizes is responsible for a narrow PL, strong excitonic absorption, and a blue shift of the optical band gap by more than 0.47 eV compared to that of bulk CsPbBr3. We also show that the composition of the NPLs can be varied all the way to CsPbBr3 or CsPbI3 by anion exchange, with preservation of the size and shape of the starting particles. The blue fluorescent CsPbCl3 NPLs represent a new member of the scarcely populated group of blue-emitting colloidal nanocrystals. The exciton dynamics were found to be independent of the extent of 2D confinement in these platelets, and this was supported by band structure calculations.

Process for the synthesis of symmetric and unsymmetric oxygen bridged dimers of boron subphthalocyanines (µ-oxo-(BsubPc)2s).

A process for the gram scale synthesis of the oxygen bridged dimer of boron subphthalocyanine, µ-oxo-(BsubPc)2, has been developed. During the development it was found that a wide range of reaction pathways under diverse conditions lead to µ-oxo-(BsubPc)2 formation. However, obtaining µ-oxo-(BsubPc)2 as the main reaction product in appreciable yields and its subsequent isolation were extremely challenging. The best balance of purity, yield and conversion was achieved with a time controlled reaction of an equimolar reaction of HO-BsubPc with Br-BsubPc in the presence of K3PO4. The purification involved sequentially Soxhlet extraction, Kauffman column chromatography and train sublimation. We have repeated the process and yields ranged from 27 to 30% of pure, doubly-sublimed µ-oxo-(BsubPc)2. This process also enabled the synthesis of unsymmetric µ-oxo-(BsubPc)2s by reaction of HO-BsubPc with Br-F12BsubPc, Cl-Cl6BsubPc and Cl-Cl12BsubPc. After synthesis the solution-state properties of the unsymmetric µ-oxo-(BsubPc)2s were investigated, and compared to the symmetric µ-oxo-(BsubPc)2 and more broadly to other BsubPcs. The electronic properties of µ-oxo-(BsubPc)2 were found to differ from its unsymmetric counterparts, but were found to be similar to halo-BsubPcs. Furthermore, the photophysical properties of µ-oxo-(BsubPc)2, both symmetric and unsymmetric, differed greatly from all other known BsubPcs.

Modified boron subphthalocyanines with stable electrochemistry and tuneable bandgaps.

The synthesis of boron subphthalocyanines (BsubPc) from modified phthalonitriles is reported. The BsubPcs have intense red-shifted absorption compared to normal BsubPcs and readily tuneable optoelectronic properties including enhanced electrochemical stability and the presence of up to two reversible electrochemical reductions.

The use of siloxanes, silsesquioxanes, and silicones in organic semiconducting materials.

Optimization of the physical and electronic properties of organic semiconductors is a key step in improving the performance of organic light emitting diodes, organic photovoltaics, organic field effect transistors, and other electronic devices. Separate tuning of the physical and electronic properties of these organic semiconductors can be achieved by the hybridization of organo-silicon structures (silicones, siloxanes, silsesquioxanes) with organic semiconductors. Common chemical means to achieve this hybridization are summarized while a large range of literature examples are covered to demonstrate the range and flexibility of this synthetic strategy.

Siliconized triarylamines as redox mediator in dye-sensitized solar cells.

A new class of triarylamine compound functionalized with bulky triisopropylsilyl ether (-OTIPS) groups is used as a hole transport material in dye-sensitized solar cells (DSSCs). Using both optical and photoelectrochemical techniques, we compared the performance of this compound with that of a parent compound containing methyl ethers as well as the conventional I3⁻/I⁻ redox couple. DSSCs fabricated with the triisopropylsilyl ether-substituted triarylamine exhibited high open circuit potentials (V(oc) > 0.9 V on average) and efficiencies of up to 1.9%. However, cells fabricated with triarylamine containing methyl ethers performed very poorly, pointing to the importance of -OTIPS in the overall performance of this material.

Liquid triarylamines: the scope and limitations of Piers-Rubinsztajn conditions for obtaining triarylamine-siloxane hybrid materials.

New liquid triarylamine-siloxane hybrid materials are produced using the Piers-Rubinsztajn reaction. Under mild conditions, liquid analogues of conventional and commonly crystalline triarylamines are easily synthesized from readily available or accessible intermediates. Using a diverse selection of triarylamines, we explored the effects of siloxane group and substitution pattern on the physical properties of these materials, and we have demonstrated that relatively large molecular liquids with desirable electrochemical properties can be produced. The interactions between the strongly Lewis acidic catalyst used for this transformation, tris(pentafluorophenyl)borane (BCF), and the Lewis basic triarylamine substrates were studied. Through UV-vis-NIR and (19)F NMR spectroscopy, we have proposed that the catalyst undergoes a reversible redox reaction with the substrates to produce a charge transfer complex. The formation of this charge transfer complex is sensitive to the oxidation potential of the triarylamine and can greatly affect the kinetics of the Piers-Rubinsztajn reaction.

Hole Mobility of a Liquid Organic Semiconductor.

The first detailed study of charge transport through a liquid organic semiconductor (LOS) is reported with the goal of elucidating the effects of molecular motion on charge transport through molecular liquids. Using a liquid, silyl ether-substituted triarylamine, hole transport mobilities were obtained over a wide range of temperatures above the glass transition temperature of the material. Analysis of this data reveals that molecular motion(s) have a negligible effect on macroscopic charge transport through a molecular liquid. The results strongly resemble transport behavior found in conventional, disordered solids and suggest that silyl ether-substituted LOSs may be good candidates for integration into electronic devices, by those who are familiar with the application of traditional triarylamines, where their unique physical state can or could be exploited.

Siloxane-triarylamine hybrids: discrete room temperature liquid triarylamines via the Piers-Rubinsztajn reaction.

A series of room temperature liquid siloxane-triarylamine hybrids were synthesized using the Piers-Rubinsztajn reaction. These materials displayed well behaved electrochemical oxidation and low T(g)'s and were free-flowing liquids. The interaction between the Lewis acidic tris(pentafluorophenyl)borane catalyst and the Lewis basic starting triarylamine substrate was also investigated by steady state UV-vis spectroscopy and (19)F NMR.