development of sustainable energy sources. [1] Among these, solar energy has the greatest potential, with more solar energy irradiating the surface of the Earth in one hour than human society consumes in one year. [2] However, the intermittency of solar energy limits its utility. In order for solar energy to provide power on a scale commensurate with that currently generated from fossil fuels, it must be stored and supplied to users on demand. [3] On short timescales (seconds to days) this is possible to achieve using batteries, which can store electricity generated from solar photovoltaics. [4,5] However, the expensive materials required and their relatively high rates of self-discharge make batteries unsuitable for seasonal energy storage. [6,7] Storing solar energy in the chemical bonds of a fuel, which can be stored indefinitely at low cost, transported, and converted to electrical or heat energy on demand is therefore highly desirable. Solar fuels such as H 2 , CH 3 OH, and CH 4 can be generated from abundant and renewable feedstocks such as water and CO 2 using semiconductor photocatalysts. The vast majority of research to date has focused on photocatalysts fabricated from wide bandgap semiconductors such as TiO 2 , [8] SrTiO 3 , [9] and carbon nitride (CN). [10-14] Photocatalysts based on some of these semiconductors have achieved operational stabilities exceeding 1000 h and maximum external quantum efficiencies (EQEs) of over 50% for overall water splitting. [9,15-18] However, their wide and difficult to tune bandgaps mean that photocatalysts fabricated from these semiconductors are almost exclusively active at UV wavelengths, which carry <5% of solar energy. [19] This fundamentally limits their efficiency below what is required for many practical solar fuels applications. For example, an estimated solar-to-hydrogen efficiency (η STH) of 5-10% would be required to photocatalytically produce H 2 at a cost that meets the U.S. Department of Energy's target of $2-4 kg −1 , which is difficult or impossible to achieve with the aforementioned wide bandgap semiconductors. [20] This has stimulated interest in developing novel photocatalysts based on semiconductors with narrower bandgaps that are able to absorb a greater proportion of the solar spectrum and can therefore achieve higher maximum theoretical solar energy conversion efficiencies. [18] Among these, non-CN organic semiconductors have recently gained prominence due to the Earth abundance of their constituent elements, and their high extinction coefficients and The photocatalytic synthesis of solar fuels such as hydrogen and methane from water and carbon dioxide is a promising strategy to store abundant solar energy in order to overcome its intermittency. Although this approach has been studied for decades using inorganic semiconductor photocatalysts, organic semiconductors have only recently gained notable attention. The tunable energy levels of organic semiconductors can enable the design of photocatalysts with optimized solar light utilization. However,...

Organic semiconductor nanoparticles (NPs) composed of an electron donor/acceptor (D/A) semiconductor blend have recently emerged as an efficient class of hydrogen‐evolution photocatalysts. It is demonstrated that using conjugated polymers functionalized with (oligo)ethylene glycol side chains in NP photocatalysts can greatly enhance their H2‐evolution efficiency compared to their nonglycolated analogues. The strategy is broadly applicable to a range of structurally diverse conjugated polymers. Transient spectroscopic studies show that glycolation facilitates charge generation even in the absence of a D/A heterojunction, and further suppresses both geminate and nongeminate charge recombination in D/A NPs. This results in a high yield of photogenerated charges with lifetimes long enough to efficiently drive ascorbic acid oxidation, which is correlated with greatly enhanced H2‐evolution rates in the glycolated NPs. Glycolation increases the relative permittivity of the semiconductors and facilitates water uptake. Together, these effects may increase the high‐frequency relative permittivity inside the NPs sufficiently, to cause the observed suppression of exciton and charge recombination responsible for the high photocatalytic activities of the glycolated NPs.

Four solution-processable, linear conjugated polymers of intrinsic porosity are synthesised and tested for gas phase carbon dioxide photoreduction. The polymers’ photoreduction efficiency is investigated as a function of their porosity, optical properties, energy levels and photoluminescence. All polymers successfully form carbon monoxide as the main product, without the addition of metal co-catalysts. The best performing single component polymer yields a rate of 66 μmol h−1 m−2, which we attribute to the polymer exhibiting macroporosity and the longest exciton lifetimes. The addition of copper iodide, as a source of a copper co-catalyst in the polymers shows an increase in rate, with the best performing polymer achieving a rate of 175 μmol h−1 m−2. The polymers are active for over 100 h under operating conditions. This work shows the potential of processable polymers of intrinsic porosity for use in the gas phase photoreduction of carbon dioxide towards solar fuels.