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In the last decade, perovskite semiconductors have triggered a revolution in solar cell research. However, critical issues remain concerning the stability of metal-halide perovskites, which need to be overcome to enable a large scale commercialisation of perovskite photovoltaics (PV). While the rather poor environmental stability of these perovskites is usually attributed to their ionic nature rendering them sensitive to moisture and oxygen, the actual contribution of mobile ions to the total degradation loss under different environmental conditions is poorly understood. In this work, we reveal that the initial degradation of perovskite semiconductors is largely the result of mobile ion-induced internal field screening - a phenomenon that has not been previously discussed in relation to the degradation of perovskite solar cells. The increased field screening leads to a decrease in the steady-state power conversion efficiency mainly due to a large reduction in current density, while the efficiency at high scan speeds (>1000 V/s) where the ions are immobilized is much less affected. We also show that interfacial recombination does not increase upon ageing, yet the open-circuit voltage (VOC) decreases as the result of an increase in the mobile ion density upon ageing. Furthermore, similar ionic losses appear under different external stressors, in particular when there are free charges present in the absorber layer. This work reveals a key degradation mechanism, providing new insights into initial device degradation before chemical or extrinsic mechanical device degradation effects manifest, and it highlights the critical role mobile ions play therein.

Researchers demonstrate that the dark saturation current in organic photodiodes is fundamentally limited by mid-gap trap states. This leads to an upper limit for specific detectivity. Photodiodes are ubiquitous in industry and consumer electronics. Constantly emerging new applications for photodiodes demand different mechanical and optoelectronic properties from those provided by conventional inorganic-based semiconductor devices. This has stimulated considerable interest in the use of organic semiconductors, which provide a vast palette of available optoelectronic properties, can be incorporated into flexible form factor geometries, and promise low-cost, low-embodied energy manufacturing from earth-abundant materials. The sensitivity of a photodiode depends critically on the dark current. Organic photodiodes (OPDs), however, are characterized by a much higher dark current than expected for thermally excited radiative transitions. Here we show that the dark saturation current in OPDs is fundamentally limited by mid-gap trap states. This new insight is generated by the universal trend observed for the dark saturation current of a large set of OPDs and further substantiated by sensitive external-quantum-efficiency- and temperature-dependent current measurements. Based on this insight, an upper limit for the specific detectivity is established. A detailed understanding of the origins of noise in any detector is fundamental to defining performance limitations and thus is critical to materials and device selection, and design and optimization for all applications. Our work establishes these important principles for OPDs. This work was funded through the Welsh Government’s Sêr Cymru II Program ‘Sustainable Advanced Materials’ (Welsh European Funding Ofice—European Regional Development Fund). C.K. is recipient of a UKRI EPSRC Doctoral Training Program studentship. P.M. is a Sêr Cymru II Research Chair and A.A. is a Rising Star Fellow also funded through the Welsh Government’s Sêr Cymru II ‘Sustainable Advanced Materials’ Program (European Regional Development Fund, Welsh European Funding Ofice and Swansea University Strategic Initiative). This work was also funded by UKRI through the EPSRC Programme grant EP/T028513/1 Application Targeted Integrated Photovoltaics. S.G. acknowledges the Research Foundation–Flanders (FWO Vlaanderen) for granting him a PhD fellowship. K.V. and W.M. are grateful for project funding by the FWO (G0D0118N, G0B2718N and GOH3816NAUHL).

In crystalline and amorphous semiconductors, the temperature-dependent Urbach energy can be determined from the inverse slope of the logarithm of the absorption spectrum and reflects the static and dynamic energetic disorder. Using recent advances in the sensitivity of photocurrent spectroscopy methods, we elucidate the temperature-dependent Urbach energy in lead halide perovskites containing different numbers of cation components. We find Urbach energies at room temperature to be 13.0 ± 1.0, 13.2 ± 1.0, and 13.5 ± 1.0 meV for single, double, and triple cation perovskite. Static, temperature-independent contributions to the Urbach energy are found to be as low as 5.1 ± 0.5, 4.7 ± 0.3, and 3.3 ± 0.9 meV for the same systems. Our results suggest that, at a low temperature, the dominant static disorder in perovskites is derived from zero-point phonon energy rather than structural disorder. This is unusual for solution-processed semiconductors but broadens the potential application of perovskites further to quantum electronics and devices.

Funder: Alexander von Humboldt Foundation; Id: http://dx.doi.org/10.13039/100005156, Perovskite semiconductors are an attractive option to overcome the limitations of established silicon based photovoltaic (PV) technologies due to their exceptional opto‐electronic properties and their successful integration into multijunction cells. However, the performance of single‐ and multijunction cells is largely limited by significant nonradiative recombination at the perovskite/organic electron transport layer junctions. In this work, the cause of interfacial recombination at the perovskite/C60 interface is revealed via a combination of photoluminescence, photoelectron spectroscopy, and first‐principle numerical simulations. It is found that the most significant contribution to the total C60‐induced recombination loss occurs within the first monolayer of C60, rather than in the bulk of C60 or at the perovskite surface. The experiments show that the C60 molecules act as deep trap states when in direct contact with the perovskite. It is further demonstrated that by reducing the surface coverage of C60, the radiative efficiency of the bare perovskite layer can be retained. The findings of this work pave the way toward overcoming one of the most critical remaining performance losses in perovskite solar cells.

Suppressing nonradiative recombination at the interface between the organometal halide perovskite (PVK) and the charge-transport layer (CTL) is crucial for improving the efficiency and stability of PVK-based solar cells (PSCs). Here, a new bathocuproine (BCP)-based nonconjugated polyelectrolyte (poly-BCP) is synthesized and this is introduced as a "dual-side passivation layer" between the tin oxide (SnO

Organic semiconductors are disordered molecular solids and as a result, their internal charge dynamics and ultimately, the performance of the optoelectronic devices they constitute, are governed by energetic disorder. To ascertain how energetic disorder impacts charge generation, exciton transport, charge transport, and the performance of organic semiconductor devices, an accurate approach is first required to measure this critical parameter. In this work, we show that the static disorder has no relation with the so-called Urbach energy in organic semiconductors. Instead, it can be obtained from photovoltaic external quantum efficiency spectra at wavelengths near the absorption onset. We then present a detailed methodology, alongside a computational framework, for quantifying the static energetic disorder associated with singlet excitons. Moreover, the role of optical interference in this analysis is considered to achieve a high-accuracy quantification. Finally, the excitonic static disorder was quantified in several technologically-relevant donor-acceptor blends, including high-efficiency PM6:Y6., 21 pages, 6 figures. 9 supplementary pages, 3 supplementary figures, and 4 supplementary tables. Submitted to Advanced Functional Materials