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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).

The thermodynamic limit for the efficiency of solar cells is predominantly defined by the energy bandgap of the used semiconductor. In case of organic solar cells both energetics and kinetics of three different species play role: excitons, charge transfer states and charge separated states. In this work, we clarify the effect of the relative energetics and kinetics of these species on the recombination and generation dynamics. Making use of detailed balance, we develop an analytical framework describing how the intricate interplay between the different species influence the photocurrent generation, the recombination, and the open-circuit voltage in organic solar cells. Furthermore, we clarify the essential requirements for equilibrium between excitons, CT states and charge carriers to occur. Finally, we find that the photovoltaic parameters are not only determined by the relative energy level between the different states but also by the kinetic rate constants. These findings provide vital insights into the operation of state-of-art non-fullerene organic solar cells with low offsets.

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.

The subtle link between photogenerated charge generation yield (CGY) and bimolecular recombination in organic semiconductor-based photovoltaics is relatively well established as a concept but has proven extremely challenging to demonstrate and probe especially under operational conditions. Received wisdom also teaches that charge generation in excitonic systems will always be lower than non-excitonic semiconductors such as GaAs – but this view is being challenged with the advent of organic semiconductor blends based upon non-fullerene acceptors (NFAs) with power conversion efficiencies exceeding 18%. Using a newly developed approach based upon temperature dependent ultra-sensitive external quantum efficiency measurements, we observe near unity CGY in several model NFA-based systems measured with unprecedented accuracy. We find that a relatively small increase in yield from 0.984 to 0.993 leads to a reduction in bimolecular recombination from 400 times to 1000 times relative to the Langevin limit. In turn, this dramatic reduction delivers the best thick junction performance to date in any binary organic solar cell – notably 16.2% at 300 nm. The combination of high efficiency and thick junction is the key for industrial fabrication of these devices via high-throughput deposition processing such as roll-to-roll, and thus central to a viable solar cell technology. These results also clearly reveal and elucidate the relationship between photo-generation and recombination in excitonic semiconductor photovoltaics thus providing an important bridge between basic device physics and practical cell engineering.

Significantly reduced bimolecular recombination relative to the Langevin recombination rate has been observed in a limited number of donor-acceptor organic semiconductor blends. The strongly reduced recombination has been previously attributed to a high probability for the interfacial charge-transfer (CT) states (formed upon charge encounter) to dissociate back to free charges. However, whether the reduced recombination is due to a suppressed CT-state decay rate or an improved dissociation rate has remained a matter of conjecture. Here we investigate a donor-acceptor material system that exhibits significantly reduced recombination upon solvent annealing. On the basis of detailed balance analysis and the accurate characterization of CT-state parameters, we provide experimental evidence that an increase in the dissociation rate of CT states upon solvent annealing is responsible for the reduced recombination. We attribute this to the presence of purer and more percolated domains in the solvent-annealed system, which may, therefore, have a stronger entropic driving force for CT dissociation.

Organic photovoltaic (OPV) efficiencies continue to rise, raising their prospects for solar energy conversion. However, researchers have long considered how to suppress the loss of free carriers by recombination-poor diffusion and significant Coulombic attraction can cause electrons and holes to encounter each other at interfaces close to where they were photogenerated. Using femtosecond transient spectroscopies, we report the nanosecond grow-in of a large transient Stark effect, caused by nanoscale electric fields of ∼487 kV/cm between photogenerated free carriers in the device active layer. We find that particular morphologies of the active layer lead to an energetic cascade for charge carriers, suppressing pathways to recombination, which is ∼2000 times less than predicted by Langevin theory. This in turn leads to the buildup of electric charge in donor and acceptor domains-away from the interface-resistant to bimolecular recombination. Interestingly, this signal is only experimentally obvious in thick films due to the different scaling of electroabsorption and photoinduced absorption signals in transient absorption spectroscopy. Rather than inhibiting device performance, we show that devices up to 600 nm thick maintain efficiencies of >8% because domains can afford much higher carrier densities. These observations suggest that with particular nanoscale morphologies the bulk heterojunction can go beyond its established role in charge photogeneration and can act as a capacitor, where adjacent free charges are held away from the interface and can be protected from bimolecular recombination.

Despite general agreement that the generation of free charges in organic solar cells is driven by an energetic offset, power conversion efficiencies have been improved using low-offset blends. In this work, we explore the interconnected roles that exciton diffusion and lifetime play in the charge generation process under various energetic offsets. A detailed balance approach is used to develop an analytic framework for exciton dissociation and free-charge generation accounting for exciton diffusion to and dissociation at the donor-acceptor interface. For low-offset systems, we find the exciton lifetime to be a pivotal component in the charge generation process, as it influences both the exciton and CT state dissociation. These findings suggest that any novel low-offset material combination must have long diffusion lengths with long exciton lifetimes to achieve optimum charge generation yields.

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

Commercialization and scale-up of organic solar cells (OSCs) using industrial solution printing require maintaining maximum performance at active-layer thicknesses400 nm─a characteristic still not generally achieved in non-fullerene acceptor OSCs. NT812/PC71BM is a rare system, whose performance increases up to these thicknesses due to highly suppressed charge recombination relative to the classic Langevin model. The suppression in this system, however, uniquely depends on device processing, pointing toward the role of nanomorphology. We investigate the morphological origins of this suppressed recombination by combining results from a suite of X-ray techniques. We are surprised to find that while all investigated devices are composed of pure, similarly aggregated nanodomains, Langevin reduction factors can still be tuned from ∼2 to1000. This indicates that pure aggregated phases are insufficient for non-Langevin (reduced) recombination. Instead, we find that large well-ordered conduits and, in particular, sharp interfaces between domains appear to help to keep opposite charges separated and percolation pathways clear for enhanced charge collection in thick active layers. To our knowledge, this is the first quantitative study to isolate the donor/acceptor interfacial width correlated with non-Langevin charge recombination. This new structure-property relationship will be key to successful commercialization of printed OSCs at scale.

The photogeneration of free charges in light-harvesting devices is a multistep process, which can be challenging to probe due to the complexity of contributing energetic states and the competitive character of different driving mechanisms. In this contribution, we advance a technique, integral-mode transient charge extraction (ITCE), to probe these processes in thin-film solar cells. ITCE combines capacitance measurements with the integral-mode time-of-flight method in the low intensity regime of sandwich-type thin-film devices and allows for the sensitive determination of photogenerated charge-carrier densities. We verify the theoretical framework of our method by drift-diffusion simulations and demonstrate the applicability of ITCE to organic and perovskite semiconductor-based thin-film solar cells. Furthermore, we examine the field dependence of charge generation efficiency and find our ITCE results to be in excellent agreement with those obtained via time-delayed collection field measurements conducted on the same devices.

Photodiodes are ubiquitous in industry and consumer electronics. New applications for photodiodes are constantly emerging, such as the internet of things and wearable electronics that demand different mechanical and optoelectronic properties from those provided by conventional inorganic devices. This has stimulated considerable interest in the use of next generation semiconductors, particularly the organics, which provide a vast palette of available optoelectronic properties, can be incorporated into flexible form factor geometries, and promise extremely low cost, low embodied energy manufacturing from earth abundant materials. The sensitivity of a photodiode to low light intensities (typically important in these new applications) depends critically on the dark current. Organic photodiodes, however, are characterized by a much higher dark current than expected for thermally excited band-to-band transitions. Here, we show that the lower limit of the dark current is given by recombination via mid-gap trap states. This new insight is generated from temperature dependent dark current measurements of narrow-gap photodiodes for the near-infrared. Based on Shockley-Read-Hall statistics, a diode equation is derived which can be used to determine an upper limit for the specific detectivity and to explain the general trend observed for the light to dark current ratio as a function of the experimental open-circuit voltage for a series of organic photodiodes. 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, design and optimisation for all applications. Our work establishes these important principles for organic semiconductor photodiodes for the near-infrared.

The photovoltaic performance of inorganic perovskite solar cells (PSCs) still lags behind the organic-inorganic hybrid PSCs due to limited light absorption of wide bandgap CsPbI3-xBrx under solar illumination. Constructing tandem devices with organic solar cells can effectively extend light absorption toward the long-wavelength region and reduce radiative photovoltage loss. Herein, we utilize wide-bandgap CsPbI2Br semiconductor and narrow-bandgap PM6:Y6-BO blend to fabricate perovskite/organic tandem solar cells with an efficiency of 21.1% and a very small tandem open-circuit voltage loss of 0.06 V. We demonstrate that the hole transport material of the interconnecting layers plays a critical role in determining efficiency, with polyTPD being superior to PBDB-T-Si and D18 due to its low parasitic absorption, sufficient hole mobility and quasi-Ohmic contact to suppress charge accumulation and voltage loss within the tandem device. These perovskite/organic tandem devices also display superior storage, thermal and ultraviolet stabilities.

Single-crystal halide perovskites exhibit photogenerated-carriers of high mobility and long lifetime, making them excellent candidates for applications demanding thick semiconductors, such as ionizing radiation detectors, nuclear batteries, and concentrated photovoltaics. However, charge collection depreciates with increasing thickness; therefore, tens to hundreds of volts of external bias is required to extract charges from a thick perovskite layer, leading to a considerable amount of dark current and fast degradation of perovskite absorbers. However, extending the carrier-diffusion length can mitigate many of the anticipated issues preventing the practical utilization of perovskites in the abovementioned applications. Here, single-crystal perovskite solar cells that are up to 400 times thicker than state-of-the-art perovskite polycrystalline films are fabricated, yet retain high charge-collection efficiency in the absence of an external bias. Cells with thicknesses of 110, 214, and 290 µm display power conversion efficiencies (PCEs) of 20.0, 18.4, and 14.7%, respectively. The remarkable persistence of high PCEs, despite the increase in thickness, is a result of a long electron-diffusion length in those cells, which was estimated, from the thickness-dependent short-circuit current, to be ≈0.45 mm under 1 sun illumination. These results pave the way for adapting perovskite devices to optoelectronic applications in which a thick active layer is essential.

The spectral shape near the energy gap determines the radiative limit of the open-circuit voltage in organic photovoltaic devices. In this work, we employ ultrasensitive photocurrent measurements and detect sub-gap states with energies far below gap in a large number of different donor-acceptor blends. We provide evidence that these low-energy sub-gap states are associated with radiative mid-gap trap states, generating photocurrent via an optical release process. To account for the radiative mid-gap states, we implement a two-diode model which accurately describes both the dark current and the open-circuit voltage in organic solar cells. These findings provide important insights for our current understanding of organic photovoltaic devices.