Methylammonium lead halide perovskites are attracting intense interest as promising materials for next-generation solar cells, but serious issues related to long-term stability need to be addressed. Perovskite films based on CH3NH3PbI3 undergo rapid degradation when exposed to oxygen and light. Here, we report mechanistic insights into this oxygen-induced photodegradation from a range of experimental and computational techniques. We find fast oxygen diffusion into CH3NH3PbI3 films is accompanied by photo-induced formation of highly reactive superoxide species. Perovskite films composed of small crystallites show higher yields of superoxide and lower stability. Ab initio simulations indicate that iodide vacancies are the preferred sites in mediating the photo-induced formation of superoxide species from oxygen. Thin-film passivation with iodide salts is shown to enhance film and device stability. The understanding of degradation phenomena gained from this study is important for the future design and optimization of stable perovskite solar cells.

Photocatalysts formed from a single organic semiconductor typically suffer from inefficient intrinsic charge generation, which leads to low photocatalytic activities. We demonstrate that incorporating a heterojunction between a donor polymer (PTB7-Th) and non-fullerene acceptor (EH-IDTBR) in organic nanoparticles (NP) can result in hydrogen evolution photocatalysts with greatly enhanced photocatalytic activity. Control of the nanomorphology of these NPs was achieved by varying the

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for this application. [1][2][3][4][5][6] Recently, however, new classes of materials are being investigated for hydrogen evolution from water, such as nitrogen doped graphene oxide, [7] covalent organic frameworks, [8][9][10] conjugated polymer networks, [11][12][13][14] and linear conjugated polymers. [15][16][17][18][19][20] With the notable exception of graphitic carbon nitride, [21] most organic semiconductors have required an additional metal co-catalyst to produce an appreciable H 2 evolution rate, often introduced by Pt photo deposition, [22] or the addition of a molecular catalyst. [8,23,24] However, recent reports of photocatalysis employing both linear and cross-linked conjugated polymers suggest that these systems are able to produce H 2 under visible light irradiation without any added co-catalyst, at a much faster rate than commercial graphitic carbon nitride, even when the latter is subjected to Pt photodeposition. [11] Activity without the need for co-catalysts, which are often precious metals or molecular complexes with limited stability, [25] could be a very attractive feature of such polymer photocatalysts with the potential to vastly reduce the cost and complexity of current photocatalytic systems.A feature of these linear and cross-linked conjugated polymers that has not yet been systematically studied is the presence of significant quantities of residual Pd originating from their synthesis via Pd catalyzed polycondensation reactions. [11,12,15,26] It has previously been shown that the molecular Pd catalysts used in these reactions can decompose via deligation and subsequent Ostwald ripening to form metallic Pd 0 particles, which are strongly retained within the polymer matrix and are difficult to remove via classical purification techniques. [27][28][29][30][31] Similar to Pt 0 , Pd 0 is a highly active proton reduction electrocatalyst. [32][33][34] Therefore, it is possible that the residual Pd, dispersed within these polymer structures, facilitates H 2 evolution and removes the need for additional co-catalysts. Much uncertainty remains about the nature of the active site in conjugated polymer systems, and there are conflicting views in the literature on whether or not the residual amounts of Pd play a significant role. No correlation was seen between Pd concentration and hydrogen evolution reaction (HER) rate in a range of conjugated micro porous polymers, [11] and in a series of linear polymersThe effect of residual Pd on hydrogen evolution activity in conjugated polymer photocatalytic systems is systematically investigated using colloidal poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) nanoparticles as a model system. Residual Pd, originating from the synthesis of F8BT via Pd catalyzed polycondensation polymerization, is observed in the form of homogeneously distributed Pd nanoparticles within the polymer. Residual Pd is essential for any hydrogen evolution to be observed from this polymer, and very low Pd concentrations (<40 ppm) are sufficient to have a significant effect on the h...

Semiconducting polymers are versatile materials for solar energy conversion and have gained popularity as photocatalysts for sunlight-driven hydrogen production. Organic polymers often contain residual metal impurities such as palladium (Pd) clusters that are formed during the polymerization reaction, and there is increasing evidence for a catalytic role of such metal clusters in polymer photocatalysts. Using transient and operando optical spectroscopy on nanoparticles of F8BT, P3HT, and the dibenzo[ b,d ]thiophene sulfone homopolymer P10, we demonstrate how differences in the time scale of electron transfer to Pd clusters translate into hydrogen evolution activity optima at different residual Pd concentrations. For F8BT nanoparticles with common Pd concentrations of >1000 ppm (>0.1 wt %), we find that residual Pd clusters quench photogenerated excitons via energy and electron transfer on the femto-nanosecond time scale, thus outcompeting reductive quenching. We spectroscopically identify reduced Pd clusters in our F8BT nanoparticles from the microsecond time scale onward and show that the predominant location of long-lived electrons gradually shifts to the F8BT polymer when the Pd content is lowered. While a low yield of long-lived electrons limits the hydrogen evolution activity of F8BT, P10 exhibits a substantially higher hydrogen evolution activity, which we demonstrate results from higher yields of long-lived electrons due to more efficient reductive quenching. Surprisingly, and despite the higher performance of P10, long-lived electrons reside on the P10 polymer rather than on the Pd clusters in P10 particles, even at very high Pd concentrations of 27000 ppm (2.7 wt %). In contrast, long-lived electrons in F8BT already reside on Pd clusters before the typical time scale of hydrogen evolution. This comparison shows that P10 exhibits efficient reductive quenching but slow electron transfer to residual Pd clusters, whereas the opposite is the case for F8BT. These findings suggest that the development of even more efficient polymer photocatalysts must target materials that combine both rapid reductive quenching and rapid charge transfer to a metal-based cocatalyst.