Solar photovoltaics have great promise for a low-carbon future but remain expensive relative to other technologies. Greatly increased penetration of photovoltaics into global energy markets requires an expansion in attention from designs of high-performance to those that can deliver significantly lower cost per kilowatt-hour. To evaluate a new set of technical and economic performance targets, we examine material extraction costs and supply constraints for 23 promising semiconducting materials. Twelve composite materials systems were found to have the capacity to meet or exceed the annual worldwide electricity consumption of 17,000 TWh, of which nine have the potential for a significant cost reduction over crystalline silicon. We identify a large material extraction cost (cents/watt) gap between leading thin film materials and a number of unconventional solar cell candidates including FeS2, CuO, and Zn3P2. We find that devices performing below 10% power conversion efficiencies deliverthe same lifetime energy output as those above 20% when a 3/4 material reduction is achieved. Here, we develop a roadmap emphasizing low-cost alternatives that could become a dominant new approach for photovoltaics research and deployment.

We present the rational synthesis of colloidal copper(I) sulfide nanocrystals and demonstrate their application as an active light absorbing component in combination with CdS nanorods to make a solution-processed solar cell with 1.6% power conversion efficiency on both conventional glass substrates and flexible plastic substrates with stability over a 4 month testing period.

Iron pyrite (FeS 2 ) has long been a material of interest for photovoltaic devices. 1 With an indirect energy transition at 0.95eV, a direct transition at 1.03eV, 1b and an integrated absorption coefficient of 3.3×10 5 cm -1 for the energy spectrum of wavelength values (λ) between 300 nm and 750 nm, it is ideally suited for photovoltaic applications. This coupled with low procurement costs and vast abundance gives pyrite the potential to be a disruptive photovoltaic material when compared to many other candidate. 2 (1) (a) Ellmer, K.; Tributsch H. In Iron Disulfide (Pyrite) Numerous iron sulfides exist in nature, each with unique magnetic and electrical properties that are strongly related to the stoichiometric ratio between Fe and S as well as (2) crystalline structure. Pyrite has previously been prepared using several high temperature approaches including: MOCVD, sulfurization of iron films, sulfurization of iron oxide films, reactive sputtering and spray pyrolysis, 3 Semiconductor nanocrystals have been used as building blocks to assemble a range of electronic and photonic structures, including light emitting diodes, lasers, and photovoltaics.,1b Yet at elevated temperatures, segregation of iron and sulfur species is unavoidable, which could change the stoichiometry and material phase of the deposited film. In fact, the best demonstrated pyrite photovoltaic device by these techniques shows a modest 2.8% power conversion efficiency. 1a This low performance was partially explained by a high density of surface defects, but the unusually low open circuit voltage of 200 mV suggests that phase purity may also play a role. 1,3 Orthorhombic marcasite FeS 2 and hexagonal troilite FeS are both common iron sulfur phases but because they have much smaller band gaps (0.34 eV for marcasite and 0.04 eV for troilite), even trace amounts would explain the low open circuit voltage observed in this previous work. 4 Critical to the functionality of these types of devices are the purity, crystallinity, stoichiometry, and size of the nanocrystal building blocks. While some low temperature solution phase colloidal nanocrystal synthesis approaches to pyrite have been explored, 5Single source molecular precursors with precisely defined composition can provide a high degree of control of nanocrystal synthesis, as demonstrated with the growth of CdS, ZnS, CdSe, ZnSe, Sb 2 Te 3 , In 2 S 3 nanocrystals these efforts are early and unlike their thin film predecessors, there are no reports on photovoltaics made from these synthetic materials.

Successful materials innovations can transform society. However, materials research often involves long timelines and low success probabilities, dissuading investors who have expectations of shorter times from bench to business. A combination of emergent technologies could accelerate the pace of novel materials development by 10x or more, aligning the timelines of stakeholders (investors and researchers), markets, and the environment, while increasing return-on-investment. First, tool automation enables rapid experimental testing of candidate materials. Second, high-throughput computing (HPC) concentrates experimental bandwidth on promising compounds by predicting and inferring bulk, interface, and defect-related properties. Third, machine learning connects the former two, where experimental outputs automatically refine theory and help define next experiments. We describe state-of-the-art attempts to realize this vision and identify resource gaps. We posit that over the coming decade, this combination of tools will transform the way we perform materials research. There are considerable first-mover advantages at stake, especially for grand challenges in energy and related fields, including computing, healthcare, urbanization, water, food, and the environment. The development of novel materials has long been stymied by a mismatch of time constants (Figure 1). Materials development typically occurs over a 15-25-year time horizon, sometimes requiring synthesis and characterization of millions of samples. However, corporate and government funders desire tangible results within the residency time of their leadership, typically 2-5 years. The residency time for postdocs and students in a research laboratory is usually 2-5 years; when a project outlasts the residency of a single individual, seamless continuity of motivation and intellectual property is often the exception, not the rule. Market drivers of novel materials development, informed by business competition and environmental considerations, often demand solutions within a shorter time horizon. This mismatch in time constants results in a historically poor return-on-investment of energy-materials (cleantech) research relative to comparable investments in medical or software development. 1

Batteries have great promise for facilitating the grid integration of renewable energy and powering electric vehicles. One critical concern for the scale-up of battery production is the availability of the elements used in battery couples. We provide the first systematic comparison of supply limits and extraction costs of the elements in battery couples against short-and long-term scaling goals. Several couples can scale well beyond short-and long-term grid-storage goals, including: Na/S, Zn/Cl 2 , and FeCl 2 /CrCl 3. Li-based couples currently have the performance characteristics most suitable for electric vehicles, yet scaling beyond 10 MM vehicles per year will demand significant increases in Li production. We also provide a framework to evaluate new couples, such as those based on Mg, which may be an alternative to Li-based couples. While the extraction costs of the elements used in current battery couples are, in many cases, below 10 $ kWh −1 , the cost of finished battery cells is in the range of 150-1000 $ kWh −1 , well above cost targets of 100 $ kWh −1 for both grid and transportation applications. Currently high costs remain a critical barrier to the widespread scale-up of battery energy storage.

Rare earths, sometimes called the vitamins of modern materials, captured public attention when their prices increased more than tenfold in 2010 and 2011. As prices fell between 2011 and 2016, rare earths receded from public view, but less visibly, they became a major focus of innovative activity in companies, government laboratories, and universities. Geoscientists worked to better understand the resource base and improve our knowledge about mineral deposits that can be mines in the future. Process engineers carried out research that is making primary production and recycling more efficient. Materials scientists and engineers searched for substitutes that require fewer or no rare earths while providing properties comparable or superior to those of existing materials. As a result, even though global supply chains are not significantly different now than they were before the market disruption, the innovative activity motivated by the disruption will likely have far-reaching, if unpredictable, consequences for supply chains of rare earths in the future.

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