As the record single-junction efficiencies of perovskite solar cells now rival those of CIGS, CdTe, and multicrystalline silicon, they are becoming increasingly attractive for use in tandem solar cells, due to their wide, tunable bandgap and solution processability. Previously, perovskite/silicon tandems were limited by significant parasitic absorption and poor environmental stability. Here, we improve the efficiency of monolithic, two-terminal, 1 cm 2 perovskite/silicon tandems to 23.6% by combining an infrared-tuned silicon heterojunction bottom cell with the recently developed cesium formamidinium lead halide perovskite. This more stable perovskite tolerates deposition of a tin oxide buffer layer via atomic layer deposition that prevents shunts, has negligible parasitic absorption, and allows for the sputter deposition of a transparent top electrode. Furthermore, the window layer doubles as a diffusion barrier, increasing the thermal and environmental stability to enable perovskite devices that withstand a 1000-hour damp heat test at 85 °C and 85% relative humidity.

Quantum chemistry is an important area of application for quantum computation. In particular, quantum algorithms applied to the electronic structure problem promise exact, efficient methods for determination of the electronic energy of atoms and molecules. The Bravyi-Kitaev transformation is a method of mapping the occupation state of a fermionic system onto qubits. This transformation maps the Hamiltonian of n interacting fermions to an Oðlog nÞ-local Hamiltonian of n qubits. This is an improvement in locality over the JordanWigner transformation, which results in an O(n)-local qubit Hamiltonian. We present the Bravyi-Kitaev transformation in detail, introducing the sets of qubits which must be acted on to change occupancy and parity of states in the occupation number basis. We give recursive definitions of these sets and of the transformation and inverse transformation matrices, which relate the occupation number basis and the BravyiKitaev basis. We then compare the use of the Jordan-Wigner and Bravyi-Kitaev Hamiltonians for the quantum simulation of methane using the STO-6G basis.

Crystalline silicon comprises 90% of the global photovoltaics (PV) market and has sustained a nearly 30% cumulative annual growth rate, yet comprises less than 2% of electricity capacity. To sustain this growth trajectory, continued cost and capital expenditure (capex) reductions are needed. Thinning the silicon wafer well below the industry-standard 160 µm, in principle reduces both manufacturing cost and capex, and accelerates economicallysustainable expansion of PV manufacturing. In this Analysis piece, we explore two questions surrounding adoption of thin silicon wafers: (a) what are the market benefits of thin wafers? (b) what are the technological challenges to adopt thin wafers? In this Analysis, we re-evaluate the benefits and challenges of thin Si for current and future PV modules using a comprehensive technoeconomic framework that couples device simulation, bottom-up cost modeling, and a sustainable cash-flow growth model. When adopting an advanced technology concept that features sufficiently good surface passivation, the same high efficiencies are achievable for both 50-µm wafers and 160-µm ones. We then quantify the economic benefits for thin Si wafers in terms of poly-Si-to-module manufacturing capex, module cost, and levelized cost of electricity (LCOE) for utility PV systems. Particularly, LCOE favors thinner wafers for all investigated device architectures, and can potentially be reduced by more than 5% from the value of 160-µm wafers. With further improvements in module efficiency, an advanced device concept with 50-µm wafers could potentially reduce manufacturing capex by 48%, module cost by 28%, and LCOE by 24%. Furthermore, we apply a sustainable growth model to investigate PV deployment scenarios in 2030. It is found that the state-of-theart industry concept could not achieve the climate targets even with very aggressive financial scenarios, therefore the capex reduction benefit of thin wafers is advantageous to facilitate faster PV adoption. Lastly, we discuss the remaining technological challenges and areas for innovation to enable high-yield manufacturing of high-efficiency PV modules with thin Si wafers. Broader ContextClimate change is among the greatest challenges facing humankind today. Given the urgency of transitioning to a carbon-neutral energy system, we need to accelerate the deployment of existing renewable technology in the near term. With rapid technological progress and cost decline, silicon photovoltaics (PV) modules is a proven technology to be deployed to a multi-terawatt scale by 2030. Despite the high growth rate in the past decade, the capital-intense nature of silicon PV manufacturing hinders the sustainable growth of the industry. Today, the most significant contribution to capital expenditure (capex) of PV module fabrication still comes from silicon wafer itself. Reducing wafer thickness would have a proportionate effect on wafer and poly capex; however, wafer thickness reduction has been much slower than anticipated. This study revisits the concept of wafer thinning...