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Abstract Enhancing the device electroluminescence quantum efficiency (EQE EL ) is a critical factor in mitigating non-radiative voltage losses (V NR ) and further improving the performance of organic solar cells (OSCs). While the common understanding attributes EQE EL in OSCs to the dynamics of charge transfer (CT) states, persistent efforts to manipulate these decay dynamics have yielded limited results, with the EQE EL of high-efficiency OSCs typically remaining below 10−2%. This value is considerably lower than that observed in high efficiency inorganic photovoltaic devices. Here, we report that EQE EL is also influenced by the dissociation rate constant of singlet states (k DS ). Importantly, in contrast to the traditional belief that advocates maximizing k DS for superior photovoltaic quantum efficiency (EQE PV ), a controlled reduction in k DS is shown to enhance EQE EL without compromising EQE PV . Consequently, a promising experimental approach to address the V NR challenge is proposed, resulting in a significant improvement in the performance of OSCs.

Abstract With power conversion efficiencies approaching 20%, organic solar cells can no longer be considered the ugly duckling of photovoltaics. Successes notwithstanding, there is still a need for further improvement of organic solar cells, both regarding energy and current management in these devices. At present, there are different and mutually exclusive interpretation schemes for the associated losses of energy and charge, hampering the rational design of next generations of organic solar cells. One critical factor that affects voltage, current, and fill factor losses is whether or not photogenerated charges are effectively near or far away from thermodynamic equilibrium. While it is commonly agreed that both the vibronic and (disordered) energetic structure of organic semiconductors affect the solar cell characteristics, the degree to which deviations from near‐equilibrium population of the associated energy level distributions matter for the photovoltaic performance is unclear: near‐equilibrium as well as kinetic descriptions have provided seemingly convincing descriptions of a wide range of experiments. Here, the most important concepts in relation to experimental results are reviewed, open questions are addressed and implications for device performance and improvement are highlighted.

Organic solar cells, despite their high power conversion efficiencies, suffer from open circuit voltage losses making them less appealing in terms of applications. Here, the authors, supported with experimental data on small molecule photovoltaic cells, relate open circuit voltage to photovoltaic gap, charge-transfer state energy, and donor-acceptor interfacial morphology.

Fabrication of efficient narrowband near-infrared photodetectors remains a challenge. Here, Liu et al. show a facile and general approach for achieving a sub-50 nm response by intentionally n-doping an optically-thick nonfullerene photodiode using an electron-donating amine-interlayer.

Abstract Modern lighting is expected to be light weight, flexible, efficient, non‐expensive and environmentally friendly fabricated. Organic light emitting diodes (OLEDs) meet all these requirements and can be manufactured using inexpensive and roll‐to‐roll compatible printing techniques. They, however, often use low work function, highly reactive metals, such as barium and calcium to facilitate electron injection, deposited using expensive and non‐continuous vacuum techniques. Efficient and stable alternatives can be found in the aliphatic amines, polyethylenimine (PEI) and polyethylenimine(ethoxylated) (PEIE), that shift the work function of aluminum favorably for electron injection. This work demonstrates ultrasonic spray coating of PEI(E) as electron injection and transport layer for OLEDs, reducing the work function of the aluminum cathode by 0.355 eV allowing a luminous efficacy comparable to that of the OLEDs using calcium/aluminum electrodes. Slightly higher luminous results are noted for the OLEDs with spin coated PEI(E), indicating that the surface morphology and thickness of the PEI(E) layer are crucial factors: ultrasonic spray coated PEI(E) layers have an increased overall thickness and surface roughness. This study shows the potential of ultrasonic spray coating and the suitability of PEI(E) as excellent electron injection and transport layer for OLEDs and paves the way towards fully spray coated OLEDs.

Complex morphology in donor/acceptor organic heterostructures hampers the understanding of device performance. Here, the origin of the high voltage loss in organic solar cells is ascribed to the too small spacing between the donor/acceptor molecules in the active layer, and the voltage loss can be reduced by increasing the donor/acceptor spacing.

Photomutiplication-type organic photodetectors (PM-OPDs) are attractive for various next-generation technologies due to their lower cost, higher sensitivity and technological utility. Here, the authors report vacuum-processed narrowband PM-OPDs with enhanced sub-bandgap external quantum efficiency.

The commercialisation of organic photovoltaic technology calls for research on material degradation mechanisms. Ramirez et al. show that triplet excitons produced by back charge transfer can significantly impact the photo-stability of fullerene-based devices even in the absence of water and oxygen.

The suppression of dark current in organic photodetectors (OPDs) is important for maximizing the performance of the devices. Here, the authors report the relationship between the high dark saturation current and the presence of mid-gap trap states in OPDs with a donor–acceptor structure.

Though single-material organic solar cells are attractive for next-generation photovoltaic technologies, designing new materials with ideal properties remains a challenge. Here, the authors report the use of orientation-dependent molecular electrostatics to realise efficient homojunction devices.

A steep absorption edge is preferred for high performance solar cells, but is less common for organic solar cells (OSCs). Here Panhans et al. find that the absorption tails are dominated by zero point vibrations and are responsible for the lowering of the open-circuit voltage and the performance of OSCs.

It has been commonly believed that the driving force at the donor-acceptor heterojunction is vital to efficient charge separation in organic solar cells. Here Zhong et al. show that the driving force can be as small as 0.05 eV without compromising the charge transfer rate and efficiency.

Strong light-matter coupling can tune exciton properties but its effect in photovoltaics remains unexplored. Here Nikolis et al. show that the photon energy loss from optical gap to open-circuit voltage can be reduced to unprecedented values by embedding organic solar cells in optical microcavities.

The performance of organic semiconductor devices depends heavily on molecular parameters. Here, Schwarze et al. point out that the molecular quadrupole moment largely influences device energy levels and show how quadrupole moments can reduce the energy barrier for charge generation in solar cells.

Molecular doping is routinely used in organic semiconductor devices nowadays, but the physics at play remains unclarified. Tietze et al. describe it as a two-step process and show it costs little, energetically, to dissociate charge transfer complexes due to energetic disorder of organic semiconductors.

Abstract A method to determine the doping induced charge carrier profiles in lightly and moderately doped organic semiconductor thin films is presented. The theory of the method of Charge Extraction by a Linearly Increasing Voltage technique in the doping-induced capacitive regime (doping-CELIV) is extended to the case with non-uniform doping profiles and the analytical description is verified with drift-diffusion simulations. The method is demonstrated experimentally on evaporated organic small-molecule thin films with a controlled doping profile, and solution-processed thin films where the non-uniform doping profile is unintentional, probably induced during the deposition process, and a priori unknown. Furthermore, the method offers a possibility of directly probing charge-density distributions at interfaces between highly doped and lightly doped or undoped layers.

Interfaces of organic donor-acceptor blends provide intermolecular charge-transfer states with red-shifted but weak absorption. By introducing an optical micro-cavity; Siegmundet al., enhance their photoresponse to achieve narrowband NIR photodetection with broad spectral tunability.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

The original version of this Article contained an error in Equation 1. A factor of ‘c’ was included in the right-hand term. This has been corrected in the PDF and HTML versions of the Article.

Three furan-fused BODIPYs were synthesized with perfluorinated methyl, ethyl and n-propyl groups on the meso-carbon. They were obtained with high yields by reacting the furan-fused 2-carboxylpyrrole in corresponding perfluorinated acid and anhydride. With the increase in perfluorinated alkyl chain length, the molecular packing in the single crystal is influenced, showing increasing stacking distance and decreasing slope angle. All the BODIPYs were characterized as intense absorbers in near infrared region in solid state, peaking at ~800 nm with absorption coefficient of over 280 000 cm-1. Facilitated by high thermal stability, the furan-fused BODIPYs were employed in vacuum-deposited organic solar cells as electron donors. All devices exhibit PCE over 6.0% with the EQE maximum reaching 70% at ~790 nm. The chemical modification of the BODIPY donors have certain influence on the active layer morphology, and the highest PCE of 6.4% was obtained with a notably high jsc of 13.6 mA cm-2. Sensitive EQE and electroluminance studies indicated that the energy losses generated by the formation of a charge transfer state and the radiative recombination at the donor-acceptor interface were comparable in the range of 0.14-0.19 V, while non-radiative recombination energy loss of 0.38 V was the main energy loss route resulting in the moderate V∞ of 0.76 V. [ABSTRACT FROM AUTHOR]

Global efforts and synergetic interdisciplinary collaborations on solution-processed bulk-heterojunction polymer solar cells (PSCs or OPVs) made power conversion efficiencies over 10% possible. The rapid progress of the field is credited to the synthesis of a large number of novel polymers with specially tunable optoelectronic properties, a better control over the nano-morphology of photoactive blend layers, the introduction of various effective interfacial layers, new device architectures and a deeper understanding of device physics. We will review the pioneering materials for polymer-fullerene solar cells and trace the progress of concepts driving their development. We discuss the evolution ofmorphology control, interfacial layers and device structures fully exploring the potential of photoactive materials. In order to guide a further increase in power conversion efficiency of OPV, the current understanding of the process of free charge carrier generation and the origin of the photovoltage is summarized followed by a perspective on how to overcome the limitations for industrializing PSCs. [ABSTRACT FROM AUTHOR]