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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.

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.

Charge generation and recombination processes at interfaces between electron donating (donor, D) and accepting molecules (acceptor, A) are mediated by intermolecular charge-transfer (CT) states. Si...

Quantifying energetic disorder in organic semiconductors continues to attract attention because of its significant impact on the transport physics of these technologically important materials. Here, we show that the energetic disorder of organic semiconductors can be determined from the relationship between the internal quantum efficiency of charge generation and the frequency of the incident light. Our results for a number of materials suggest that energetic disorder in organic semiconductors could be greater than previously reported, and we advance ideas as to why this may be the case.

Power conversion efficiencies of donor/acceptor organic solar cells utilizing nonfullerene acceptors have now increased beyond the record of their fullerene-based counterparts. There remain many fu...

Trap-assisted recombination caused by localised sub-gap states is one of the most important first-order loss mechanism limiting the power-conversion efficiency of all solar cells. The presence and relevance of trap-assisted recombination in organic photovoltaic devices is still a matter of some considerable ambiguity and debate, hindering the field as it seeks to deliver ever higher efficiencies and ultimately a viable new solar photovoltaic technology. In this work, we show that trap-assisted recombination loss of photocurrent is universally present under operational conditions in a wide variety of organic solar cell materials including the new non-fullerene electron acceptor systems currently breaking all efficiency records. The trap-assisted recombination is found to be induced by states lying 0.35-0.6 eV below the transport edge, acting as deep trap states at light intensities equivalent to 1 sun. Apart from limiting the photocurrent, we show that the associated trap-assisted recombination via these comparatively deep traps is also responsible for ideality factors between 1 and 2, shedding further light on another open and important question as to the fundamental working principles of organic solar cells. Our results also provide insights for avoiding trap-induced losses in related indoor photovoltaic and photodetector applications., Trap-assisted recombination caused by localised sub-gap states is one of the factors limiting power-conversion efficiency in solar cells, yet the presence and relevance is still under debate in organic solar cells. Here, the authors reveal that this recombination loss is universally present under operational conditions in these devices.

In crystalline semiconductors, absorption onset sharpness is characterized by temperature-dependent Urbach energies. These energies quantify the static, structural disorder causing localized exponential-tail states, and dynamic disorder from electron-phonon scattering. Applicability of this exponential-tail model to disordered solids has been long debated. Nonetheless, exponential fittings are routinely applied to sub-gap absorption analysis of organic semiconductors. Herein, we elucidate the sub-gap spectral line-shapes of organic semiconductors and their blends by temperature-dependent quantum efficiency measurements. We find that sub-gap absorption due to singlet excitons is universally dominated by thermal broadening at low photon energies and the associated Urbach energy equals the thermal energy, regardless of static disorder. This is consistent with absorptions obtained from a convolution of Gaussian density of excitonic states weighted by Boltzmann-like thermally activated optical transitions. A simple model is presented that explains absorption line-shapes of disordered systems, and we also provide a strategy to determine the excitonic disorder energy. Our findings elaborate the meaning of the Urbach energy in molecular solids and relate the photo-physics to static disorder, crucial for optimizing organic solar cells for which we present a revisited radiative open-circuit voltage limit., The sub-gap absorption coefficient in organic semiconductors is often characterized by Urbach energies, which quantify both structural and dynamic disorders, yet the fundamental is not well-understood. Here, the authors provide a strategy to determine excitonic disorder energy, and reveal that absorption at energies well below the gap is universally dominated by thermal broadening.

Nonradiative losses to the open-circuit voltage are a primary factor in limiting the power-conversion efficiency of organic photovoltaic devices. The dominate nonradiative loss in the bulk is intrinsic to the active layer and can be determined from the quasi-Fermi-level splitting (QFLS) and the radiative thermodynamic limit of the photovoltage. Quantification of the QFLS in thin-film devices with low mobility is challenging due to the excitonic nature of photoexcitation and additional sources of nonradiative loss associated with the device structure. This work outlines an experimental approach based on electromodulated photoluminescence, which can be used to directly measure the intrinsic nonradiative loss to the open-circuit voltage, thereby quantifying the QFLS. Drift-diffusion simulations are carried out to show that this method accurately predicts the QFLS in the bulk of the device regardless of device-related nonradiative losses. State-of-the-art PM6:Y6-based organic solar cells are used as a model to test the experimental approach and the QFLS is quantified and shown to be independent of device architecture. This work provides a method to quantify the QFLS of organic solar cells under operational conditions, fully characterizing the different contributions to the nonradiative losses of the open-circuit voltage. The reported method will be useful not only in characterizing and understanding losses in organic solar cells but also in other device platforms such as light-emitting diodes and photodetectors.

Non-fullerene acceptors (NFAs) are far more emissive than their fullerene-based counterparts. Here, we study the spectral properties of photocurrent generation and recombination of the blend of the...

The impact of intermixed donor-acceptor domains in organic bulk heterojunction (BHJ) solar cells, using low-donor-content devices as model systems, is clarified. At low donor contents, the devices are found to exhibit anomalously high open-circuit voltages independent of the donor-acceptor energetics. These observations can be consistently explained by a theoretical model based on optical release of trapped holes, assuming the donors behave as trap sites in the gap of the acceptor. Our findings provide guidelines for reducing the large open-circuit voltage losses in organic solar cells and avoiding morphology-induced losses in state-of-the-art BHJ solar cells and photodetectors.

The mixed caesium and formamidinium lead triiodide perovskite system (Cs1−xFAxPbI3) in the form of quantum dots (QDs) offers a pathway towards stable perovskite-based photovoltaics and optoelectronics. However, it remains challenging to synthesize such multinary QDs with desirable properties for high-performance QD solar cells (QDSCs). Here we report an effective oleic acid (OA) ligand-assisted cation-exchange strategy that allows controllable synthesis of Cs1−xFAxPbI3 QDs across the whole composition range (x = 0–1), which is inaccessible in large-grain polycrystalline thin films. In an OA-rich environment, the cross-exchange of cations is facilitated, enabling rapid formation of Cs1−xFAxPbI3 QDs with reduced defect density. The hero Cs0.5FA0.5PbI3 QDSC achieves a certified record power conversion efficiency (PCE) of 16.6% with negligible hysteresis. We further demonstrate that the QD devices exhibit substantially enhanced photostability compared with their thin-film counterparts because of suppressed phase segregation, and they retain 94% of the original PCE under continuous 1-sun illumination for 600 h.

Organic photodetectors (OPDs) with a performance comparable to that of conventional inorganic ones have recently been demonstrated for the visible regime. However, near-infrared photodetection has proven to be challenging and, to date, the true potential of organic semiconductors in this spectral range (800-2500 nm) remains largely unexplored. In this work, it is shown that the main factor limiting the specific detectivity (D*) is non-radiative recombination, which is also known to be the main contributor to open-circuit voltage losses. The relation between open-circuit voltage, dark current, and noise current is demonstrated using four bulk-heterojunction devices based on narrow-gap donor polymers. Their maximum achievableD* is calculated alongside a large set of devices to demonstrate an intrinsic upper limit ofD* as a function of the optical gap. It is concluded that OPDs have the potential to be a useful technology up to 2000 nm, given that high external quantum efficiencies can be maintained at these low photon energies. Vandewal, K (corresponding author), UHasselt Hasselt Univ, Inst Mat Res IMO, Agoralaan 1,Bldg D, B-3590 Diepenbeek, Belgium ; IMEC, Associated Lab IMOMEC, Wetenschapspk 1, B-3590 Diepenbeek, Belgium. Armin, A (corresponding author), Swansea Univ, Dept Phys, Singleton Pk, Swansea SA2 8PP, W Glam, Wales. ardalan.armin@swansea.ac.uk; koen.vandewal@uhasselt.be

The measurement of the external quantum efficiency (EQE) for photocurrent generation at photon energies below the bandgap of semiconductors has always been an important tool for understanding pheno...

Despite the myriad of organic donor:acceptor materials, only few systems have emerged in the life of organic solar cells to exhibit considerable reduced bimolecular recombination, with respect to the random encounter rate given by the Langevin equation. Monte Carlo simulations have revealed that the rate constant of the formation of electron–hole bound states depends on the random encounter of opposite charges and is nearly given by the Langevin equation for the domain sizes relevant to efficient bulk heterojunction systems. Recently, three studies suggested that charge transfer states dissociating much faster than their decay rate to the ground state, can result in reduced bimolecular recombination by lowering the recombination rate to the ground state as a loss pathway. A separate study identified another loss pathway and suggested that forbidden back electron transfer from triplet charge transfer states to triplet excitons is a key to achieving reduced recombination. Herein we further explain the reduc...

We report the synthesis and characterization of a novel thiocarbonyl iso-DPP derivative, namely 1,3,4,6-tetraphenylpyrrolo[3,2-b]pyrrole-2,5(1H,4H)-dithione. Even without solubilising alkyl chains, the small molecule could be processed from organic solvents such as dichloromethane, chloroform or dichlorobenzene, and it was found that the optical properties of neat thin films were strongly dependent on the solvent used. Field effect hole mobilities were of the order 10−4 cm2 V−1 s−1, with mobilities measured in a diode configuration solvent dependent and at least an order of magnitude lower. Importantly, blends of the iso-DPP derivative with PC70BM, a typically used electron acceptor in bulk heterojunction solar cells, were found to possess hole mobilities of up to 10−3 cm2 V−1 s−1 in a diode configuration, which was an order of magnitude larger than the electron mobility. Finally, simple bulk heterojunction solar cells were fabricated with maximum power conversion efficiencies of 2.3%.

Energy states below the bandgap of a semiconductor, such as trap states or charge transfer states in organic donor acceptor blends, can contribute to light absorption. Due to their low number density or ultrasmall absorption cross-section, the absorption coefficient of these states is challenging to measure using conventional transmission reflection spectrophotometry. As an alternative, the external quantum efficiency (EQE) of photovoltaic devices is often used as a representative of the absorption coefficient, where the spectral line shape of the EQE is considered to follow the absorption coefficient of the active layer material. In this work, it is shown that the subbandgap EQE is subject to thickness dependent low finesse cavity interference effects within the device, making this assumption questionable. A better estimate for the absorption coefficient is obtained when EQE spectra corresponding to different active layer thicknesses are fitted simultaneously for one attenuation coefficient using an iterative transfer matrix method. The principle is demonstrated for two model acceptor-donor systems (PCE12ITIC and PBTTTPC71BM) and accurate subgap absorption coefficients are determined. This approach has particular relevance for both understanding sub-gap states and their utilization in organic optoelectronic devices.

Transient photovoltage (TPV) is a technique frequently used to determine charge carrier lifetimes in thin-film solar cells such as organic, dye sensitized and perovskite solar cells. As this lifetime is often incident light intensity dependent, its relevance to understanding the intrinsic properties of a photoactive material system as a material or device figure of merit has been questioned. To extract complete information on recombination dynamics, the TPV measurements are often performed in conjunction with charge extraction (CE) measurements, employed to determine the photo-generated charge carrier density and thereby the recombination rate constant and its order. In this communication, the underlying theory of TPV and CE is reviewed and expanded. Our theoretical findings are further solidified by numerical simulations and experiments on organic solar cells. We identify regimes of the open-circuit voltage within which accurate lifetimes and carrier densities can be determined with TPV and CE experiments. A wide range of steady-state light intensities is required in performing these experiments in order to identify their 'working dynamic range' from which the recombination kinetics in thin-film solar cells can be determined.

Radiative and non-radiative charge carrier recombination in thin-film diodes plays a key role in determining the efficiency of electronic devices made of next generation semiconductors such as organic, perovskite and nanocrystals. In this work, we show that lowering the bulk recombination does not necessarily result in enhanced performance metrics of electronic devices. From the perspective of charge carrier extraction and injection, the radiative limit of the open-circuit voltage of solar cells, noise current of photodetectors and lasing threshold of injection lasers cannot be improved if the contacts are not perfectly selective. A numerical drift-diffusion model is used to investigate the interplay between bulk recombination and surface recombination of minority carriers at the contacts in bipolar thin diode devices based on low-mobility semiconductors. The surface recombination becomes prominent in case of reduced bulk recombination strengths when non-selective contacts, i.e., contacts that are either metallic or have imperfect charge-selective interlayers, are employed. Finally, we derive analytical approximations for the case when diffusion-limited surface recombination of minority carriers at Ohmic contacts dominates the dark current. These results indicate that having perfectly selective contacts becomes crucial in systems with suppressed bulk recombination – a challenging requirement for the future state-of-the-art thin-film solar cells, light-emitting devices and photodetectors made of next generation semiconductors.

The performance of photodetectors fabricated from emerging semiconductors such as perovskites, quantum dots, two-dimensional materials or organics, for example, can be prone to misinterpretation. This Comment exposes the problems and proposes some guidelines for accurate characterization.

Why, despite considerable R&D efforts and significant translational investment over the past 20 years, has the technology of solution-processed thin film solar cells not become a commercial reality? The manufacturing cost-to-power conversion efficiency ratio seems persuasive, as do the energy payback and embodied energy metrics. As new perovskite-based semiconductors achieve impressive efficiencies and organic semiconductors enjoy a resurgence, the lab-to-manufacturing translation and scaling questions require urgent attention. This comment addresses the challenges in solution processable photovoltaic technologies faced by scientists and engineers in addressing these questions, and highlights the concept of thick junctions as a promising solution.

Organohalide perovskite optoelectronics based upon large (mm-sized) single crystals present exciting opportunities for new device platforms and fundamental studies. Herein, we report CHNHPbBr and CHNHPbI single crystals prepared via an inverse temperature crystallization method with strong near infrared photoresponses significantly below the optical gap. Light intensity dependent photocurrent measurements reveal the photoresponse is not a two-photon phenomenon, but rather is derived from a linear mechanism. The effect (including responsivity and speed) is enhanced in a photoresistor architecture, indicating that the photoresponse is due to absorption into surface trap states in the crystal. Without any optimisation, respectable NIR responsivities at room temperature of ∼10 A W at a low 1V bias operating voltage are achieved. These results again demonstrate the remarkable potential of organohalide perovskites as light sensing materials, and the possibilities for engineering a new class of single crystal-based optoelectronics. (Figure presented.).

Photocurrent generation in organic bulk heterojunction (BHJ) solar cells is most commonly understood as a process which predominantly involves photoexcitation of the lower ionization potential species (donor) followed by electron transfer to the higher electron affinity material (acceptor) [i.e., photoinduced electron transfer (PET), which we term Channel I]. A mirror process also occurs in which photocurrent is generated through photoexcitation of the acceptor followed by hole transfer to the nonexcited donor or photoinduced hole transfer (PHT), which we term Channel II. The role of Channel II photocurrent generation has often been neglected due to overlap of the individual absorption spectra of the donor and acceptor materials that are commonly used. More recently Channel II charge generation has been explored for several reasons. First, many of the new high-efficiency polymeric donors are used as the minority component in bulk heterojunction blends, and therefore, the acceptor absorption is a significant fraction of the total; second, nonfullerene acceptors have been prepared, which through careful design, allow for spectral separation from the donor material, facilitating fundamental studies on charge generation. In this article, we review the methodologies for investigating the two charge generation channels. We also discuss the factors that affect charge generation via Channel I and II pathways, including energy levels of the materials involved, exciton diffusion, and other considerations. Finally, we take a comprehensive look at the nonfullerene acceptor literature and discuss what information about Channel I and Channel II can be obtained from the experiments conducted and what other experiments could be undertaken to provide further information about the operational efficiencies of Channels I and II.

Engineering the interface between the perovskite absorber and the charge-transporting layers has become an important method for improving the charge extraction and open-circuit voltage (V-OC) of hybrid perovskite solar cells. Conjugated polymers are particularly suited to form the hole-transporting layer, but their hydrophobicity renders it difficult to solution-process the perovskite absorber on top. Herein, oxygen plasma treatment is introduced as a simple means to change the surface energy and work function of hydrophobic polymer interlayers for use as p-contacts in perovskite solar cells. We find that upon oxygen plasma treatment, the hydrophobic surfaces of different prototypical p-type polymers became sufficiently hydrophilic to enable subsequent perovskite junction processing. In addition, the oxygen plasma treatment also increased the ionization potential of the polymer such that it became closer to the valance band energy of the perovskite. It was also found that the oxygen plasma treatment could increase the electrical conductivity of the p-type polymers, facilitating more efficient charge extraction. On the basis of this concept, inverted MAPbI(3) perovskite devices with different oxygen plasma-treated polymers such as P3HT, P3OT, polyTPD, or PTAA were fabricated with power conversion efficiencies of up to 19%.

While the performance of laboratory-scale organic solar cells (OSCs) continues to grow, development of high efficiency large area OSCs remains a big challenge. Although a few attempts to produce large area organic solar cells (OSCs) have been reported, there are still challenges on the way to realizing efficient module devices, such as the low compatibility of the thickness-sensitive active layer with large area coating techniques, the frequent need for toxic solvents and tedious optimization processes used during device fabrication. In this work, highly efficient thickness-insensitive OSCs based on PTB7-Th:PC71BM that processed with single-component green solvent 2-methylanisole are presented, in which both junction thickness limitation and solvent toxicity issues are simultaneously addressed. Careful investigation reveals that this green solvent prevents the evolution of PC71BM into large area clusters resulting in reduced charge carrier recombination, and largely eliminates trapping centers, and thus improves the thickness tolerance of the films. These findings enable us to address the scalability and solvent toxicity issues and to fabricate a 16 cm(2) OSC with doctor-blade coating with a state-of-the-art power conversion efficiency of 7.5% using green solvent.

Nature Reviews Materials 2, 16100 (2017) In the last row of Table 2 the reference was wrongly cited as 87. The correct citation is reference 82. This has been corrected in both the HTML and PDF versions.

The origin of photocurrent losses in the power-generating regime of organic solar cells (OSCs) remains a controversial topic, although recent literature suggests that the competition between bimolecular recombination and charge extraction determines the bias dependence of the photocurrent. Here the steady-state recombination dynamics is studied in bulk-heterojunction OSCs with different hole mobilities from short-circuit to maximum power point. It is shown that in this regime, in contrast to previous transient extracted charge and absorption spectroscopy studies, first-order recombination outweighs bimolecular recombination of photogenerated charge carriers. This study demonstrates that the first-order losses increase with decreasing slower carrier mobility, and attributes them to either mobilization of charges trapped at the donor:acceptor interface through the Poole–Frenkel effect, and/or recombination of photogenerated and injected charges. The dependence of both first-order and higher-order losses on the slower carrier mobility explains why the field dependence of OSC efficiencies has historically been attributed to charge-extraction losses.

Efficient light detection is central to modern science and technology. Current photodetectors mainly use photodiodes based on crystalline inorganic elemental semiconductors, such as silicon, or compounds such as III–V semiconductors. Photodetectors made of solution-processed semiconductors — which include organic materials, metal-halide perovskites and quantum dots — have recently emerged as candidates for next-generation light sensing. They combine ease of processing, tailorable optoelectronic properties, facile integration with complementary metal–oxide–semiconductors, compatibility with flexible substrates and good performance. Here, we review the recent advances and the open challenges in the field of solution-processed photodetectors, examining the topic from both the materials and the device perspective and highlighting the potential of the synergistic combination of materials and device engineering. We explore hybrid phototransistors and their potential to overcome trade-offs in noise, gain and speed, as well as the rapid advances in metal-halide perovskite photodiodes and their recent application in narrowband filterless photodetection. Conventional photodetectors, made of crystalline inorganic semiconductors, are limited in terms of the compactness and sensitivity they can reach. Photodetectors based on solution-processed semiconductors combine ease of processing, tailorable optoelectronic properties and good performance, and thus hold potential for next-generation light sensing.

The dielectric properties of three pairs of organic semiconductors that contain increasing numbers of cyclopentadithiophene-co-benzothiadiazole moieties (monomer, dimer and polymer) were studied and compared. The materials in each pair differed in the nature of the ‘solubilizing groups’, which are either alkyl- or glycol-based. At low frequencies (

Fullerenes are electron transporting organic semiconductors with a wide range of applications. In particular, methanofullerenes have been the preferred choice for solution-processed solar cells and photodiodes. The wide applicability of fullerenes as both ‘n-type’ transport materials and electron acceptors is clear. However, what is still a matter of debate is whether the fullerenes can also support efficient transport of holes, particularly in diode geometries. In this letter, we utilize a number of recently developed experimental methods for selective electron and hole mobility measurements. We show for the two most widely used solution processable fullerenes, PC70- and-PC60BM, that whilst both exhibit electron mobilities as high as 10−3 cm2/Vs, their hole mobilities are

Film structure plays a critical role in defining the performance of all organic optoelectronic devices, with the importance clearly illustrated in the development of organic acceptor–donor bulk heterojunction (BHJ) photovoltaic (OPV) devices where solvent and/or thermal annealing of the deposited active layer affect solar cell output. Herein we report that the polymer–polymer interactions in solution, which are dependent on the thermal history of the solution, are a first order parameter in controlling the properties of the final active layer and hence device performance. We illustrate the key role played by organic semiconductor interactions in solution with the high efficiency donor–acceptor co-polymer, poly[N-9′′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), and its blends with [6,6]-phenyl C71-butyric acid methyl ester (PC71BM). Differences in the cooling rate of the casting solution after dissolution can lead to a 20% variation in the ultimate efficiency of cells with identical active layer thicknesses with slow-cooled solutions giving rise to poorer devices. The oft-ignored intermolecular (polymer–polymer) interactions that occur in solution are manifest by dramatic differences in viscosity and are a function of concentration and molecular weight. Hence solution thermal history represents a critical new dimension in the processing landscape for organic polymer semiconductors.

Organic solar cells (OSCs) are composed of one or more layers of order 100 nm thickness sandwiched between metallic and transparent electrodes. As such they are low finesse multilayer optical cavities where the optical field distribution is governed by the complex refractive indices and thicknesses of all layers in the “solar cell stack”. Optical interference and parasitic absorbance in nonactive layers can have a dramatic effect on the shape of the measured external quantum efficiency (EQE) the parameter often used to optimize device structure and derive critical insight regarding charge generation and extraction. In this communication we study a model high efficiency OSC system (PCDTBT/PC70BM) as a function of active layer thickness blend composition and processing. The spectral shapes of the measured EQEs show strong thickness and blend ratio dependence. However when correctly determined the internal quantum efficiencies (IQEs) are spectrally flat. The differences in EQE spectral shape predominantly originate from optical interference and parasitic absorptions rather than charge generation or transport phenomena. We also demonstrate similar results for a second model system (PCPDTBT/PC60BM) in which an energy dependent “IQE like” response has recently been used to justify the existence of hot excitons. Once again we show the origin of these spectral phenomena to be optical not electronic. These cases highlight the fact that thin film organic solar cells (even single junction) must be properly considered as low finesse electro optical cavities a point that is not universally appreciated.

Light-emitting diodes made from perovskite semiconductors have reached a milestone in the efficiency with which they emit light — potentially ushering in a new platform for lighting and display technology. Perovskite LEDs that efficiently convert electrons into light.

Color discrimination in photodetection is conventionally achieved using broadband-absorbing inorganic semiconductors with passive optical filters. Organic semiconductors show promise to deliver narrowband spectral responses due to their tunable optical properties. While achieving narrow-absorbing organic semiconductors is an ongoing endeavor in the synthetic chemistry community, charge collection narrowing is introduced as a “material-agnostic” technique to realize narrowband spectral responses using broadband absorbers such as blends of organic semiconductors, inorganic nanocrystals, and perovskites in a photodiode architecture. Charge collection narrowing in organic semiconductors demands photoactive junction thicknesses on the order of a few micrometers causing fabrication difficulties and limitations in device metrics such as frequency bandwidth. In this work it is shown that electrical inversion can result in charge collection narrowing in organic photodiodes with active layer thicknesses on the order of hundreds of nanometers and hence much easier to achieve via high throughput solution processing techniques. Additionally, it is shown that an indium tin oxide/gold electrode with modified work function acts as a cavity mirror, further narrowing the spectral response and at the same time delivering an extremely selective cathode, suppressing the dark current dramatically. Nearly voltage independent detectivities of 10 Jones are achieved with an active sensing area of 0.2 cm .

The competition between charge extraction and nongeminate recombination critically determines the current-voltage characteristics of organic solar cells (OSCs) and their fill factor. As a measure of this competition, several figures of merit (FOMs) have been put forward; however, the impact of space charge effects has been either neglected, or not specifically addressed. Here we revisit recently reported FOMs and discuss the role of space charge effects on the interplay between recombination and extraction. We find that space charge effects are the primary cause for the onset of recombination in so-called non-Langevin systems, which also depends on the slower carrier mobility and recombination coefficient. The conclusions are supported with numerical calculations and experimental results of 25 different donor/acceptor OSCs with different charge transport parameters, active layer thicknesses or composition ratios. The findings represent a conclusive understanding of bimolecular recombination for drift dominated photocurrents and allow one to minimize these losses for given device parameters.

Lead-based organohalide perovskites have recently emerged as arguably the most promising of all next generation thin film solar cell technologies. Power conversion efficiencies have reached 20% in less than 5 years, and their application to other optoelectronic device platforms such as photodetectors and light emitting diodes is being increasingly reported. Organohalide perovskites can be solution processed or evaporated at low temperatures to form simple thin film photojunctions, thus delivering the potential for the holy grail of high efficiency, low embedded energy, and low cost photovoltaics. The initial device-driven "perovskite fever" has more recently given way to efforts to better understand how these materials work in solar cells, and deeper elucidation of their structure-property relationships. In this Account, we focus on this element of organohalide perovskite chemistry and physics in particular examining critical electro-optical, morphological, and architectural phenomena. We first examine basic crystal and chemical structure, and how this impacts important solar-cell related properties such as the optical gap. We then turn to deeper electronic phenomena such as carrier mobilities, trap densities, and recombination dynamics, as well as examining ionic and dielectric properties and how these two types of physics impact each other. The issue of whether organohalide perovskites are predominantly nonexcitonic at room temperature is currently a matter of some debate, and we summarize the evidence for what appears to be the emerging field consensus: an exciton binding energy of order 10 meV. Having discussed the important basic chemistry and physics we turn to more device-related considerations including processing, morphology, architecture, thin film electro-optics and interfacial energetics. These phenomena directly impact solar cell performance parameters such as open circuit voltage, short circuit current density, internal and external quantum efficiency, fill factor, and ultimately the all-important power conversion efficiency. Finally, we address the key challenges pertinent to actually delivering a new and viable solar cell technology. These include long-term cell stability, scaling to the module level, and the toxicity associated with lead. Organohalide perovskites not only offer exciting possibilities for next generation optoelectronics and photovoltaics, but are an intriguing class of material crossing the boundaries of molecular solids and banded inorganic semiconductors. This is a potential area of rich new chemistry, materials science, and physics.

Blends of electron-donating and -accepting organic semiconductors are widely used as photoactive materials in next-generation solar cells and photodetectors. The yield of free charges in these systems is often determined by the separation of interfacial electron–hole pairs, which is expected to depend on the ability of the faster carrier to escape the Coulomb potential. Here we show, by measuring geminate and non-geminate losses and key transport parameters in a series of bulk-heterojunction solar cells, that the charge-generation yield increases with increasing slower carrier mobility. This is in direct contrast with the well-established Braun model where the dissociation rate is proportional to the mobility sum, and recent models that underscore the importance of fullerene aggregation for coherent electron propagation. The behaviour is attributed to the restriction of opposite charges to different phases, and to an entropic contribution that favours the joint separation of both charge carriers., In organic solar cells, the photogeneration of free charge carriers is limited by the dissociation of interfacial charge transfer states. Here, the authors study the impact of charge carrier mobilities in operational devices and show that the slowest charge carriers limit the dissociation of charge transfer states.

Bimolecular recombination in bulk heterojunction organic solar cells is the process by which nongeminate photogenerated free carriers encounter each other, and combine to form a charge transfer (CT) state which subsequently relaxes to the ground state. It is governed by the diffusion of the slower and faster carriers toward the electron donor–acceptor interface. In an increasing number of systems, the recombination rate constant is measured to be lower than that predicted by Langevin's model for relative Brownian motion and the capture of opposite charges. This study investigates the dynamics of charge generation, transport, and recombination in a nematic liquid crystalline donor:fullerene acceptor system that gives solar cells with initial power conversion efficiencies of >9.5%. Unusually, and advantageously from a manufacturing perspective, these efficiencies are maintained in junctions thicker than 300 nm. Despite finding imbalanced and moderate carrier mobilities in this blend, strongly suppressed bimolecular recombination is observed, which is ≈150 times less than predicted by Langevin theory, or indeed, more recent and advanced models that take into account the domain size and the spatial separation of electrons and holes. The suppressed bimolecular recombination arises from the fact that ground-state decay of the CT state is significantly slower than dissociation.

Partially converted poly(1,4-phenylenevinylene) (PPV) is used as a thin p-type interlayer in planar organohalide pervoskite solar cells resulting in a high V, 1.06 V, a power conversion efficiency of ≈15%, and minimized charge carrier recombination.

Solar cells based on organohalide perovskites (PSCs) have made rapid progress in recent years and are a promising emerging technology. An important next evolutionary step for PSCs is their up-scaling to commercially relevant dimensions. The main challenges in scaling PSCs to be compatible with current c-Si cells are related to the limited conductivity of the transparent electrode, and the processing of a uniform and defect-free organohalide perovskite layer over large areas. In this work we present a generic and simple approach to realizing efficient solution-processed, monolithic solar cells based on methylammonium lead iodide (CH3NH3PbI3). Our devices have an aperture area of 25 cm2 without relying on an interconnected strip design, therefore reducing the complexity of the fabrication process and enhancing compatibility with the c-Si cell geometry. We utilize simple aluminum grid lines to increase the conductivity of the transparent electrode. These grid lines were exposed to an UV-ozone plasma to grow a thin aluminum oxide layer. This dramatically improves the wetting and film forming of the organohalide perovskite junction on top of the lines, reducing the probability of short circuits between the grid and the top electrode. The best devices employing these modified grids achieved power conversion efficiencies of up to 6.8%.

Major growth in the image sensor market is largely as a result of the expansion of digital imaging into cameras, whether stand-alone or integrated within smart cellular phones or automotive vehicles. Applications in biomedicine, education, environmental monitoring, optical communications, pharmaceutics and machine vision are also driving the development of imaging technologies. Organic photodiodes (OPDs) are now being investigated for existing imaging technologies, as their properties make them interesting candidates for these applications. OPDs offer cheaper processing methods, devices that are light, flexible and compatible with large (or small) areas, and the ability to tune the photophysical and optoelectronic properties − both at a material and device level. Although the concept of OPDs has been around for some time, it is only relatively recently that significant progress has been made, with their performance now reaching the point that they are beginning to rival their inorganic counterparts in a number of performance criteria including the linear dynamic range, detectivity, and color selectivity. This review covers the progress made in the OPD field, describing their development as well as the challenges and opportunities.

The high power conversion efficiencies (PCEs) of laboratory-scale polymer-based organic solar cells are yet to translate to large area modules because of a number of factors including the relatively large sheet resistance of available transparent conducting electrodes (TCEs), and the high defect densities associated with thin organic semiconductor junctions. The TCE problem limits device architectures to narrow connected strips (< 1 cm) causing serious fabrication difficulties and extra costs. Thin junctions are required because of poor charge transport (imbalanced mobilities) in the constituent organic semiconductors. These issues are addressed using a combination of approaches to create thick junctions conformally coated on low sheet resistance metal grid TCEs. An essential feature of these thick junctions is balanced carrier mobilities, which affords high fill factors and efficient carrier extraction. Conformal coating is achieved by promoting enhanced intermolecular interactions in the coating solution using a high molecular weight polymeric semiconductor and appropriate solvent system. This combination of balanced mobilities, conformal coating and metallic grid TCEs is a simple and generic approach to the fabrication of defect-free large area organic solar cells (OSCs). The approach is demonstrated with 25 cm 2 monolithic devices possessing aperture-corrected power conversion efficiencies of 5% and fill factors exceeding 0.5.

Correction for ‘Dielectric constant enhancement of non-fullerene acceptors via side-chain modification’ by Jenny E. Donaghey et al., Chem. Commun., 2015, 51, 14115–14118.

Charge extraction rate in solar cells made of blends of electron donating/accepting organic semiconductors is typically slow due to their low charge carrier mobility. This sets a limit on the active layer thickness and has hindered the industrialization of organic solar cells (OSCs). Herein, charge transport and recombination properties of an efficient polymer (NT812):fullerene blend are investigated. This system delivers power conversion efficiency of >9% even when the junction thickness is as large as 800 nm. Experimental results indicate that this material system exhibits exceptionally low bimolecular recombination constant, 800 times smaller than the diffusion-controlled electron and hole encounter rate. Comparing theoretical results based on a recently introduced modified Shockley model for fill factor, and experiments, clarifies that charge collection is nearly ideal in these solar cells even when the thickness is several hundreds of nanometer. This is the first realization of high-efficiency Shockley-type organic solar cells with junction thicknesses suitable for scaling up.