Perovskite solar cells combine high carrier mobilities with long carrier lifetimes and high radiative efficiencies. Despite this, full devices suffer from significant nonradiative recombination ...losses, limiting their VOC to values well below the Shockley–Queisser limit. Here, recent advances in understanding nonradiative recombination in perovskite solar cells from picoseconds to steady state are presented, with an emphasis on the interfaces between the perovskite absorber and the charge transport layers. Quantification of the quasi‐Fermi level splitting in perovskite films with and without attached transport layers allows to identify the origin of nonradiative recombination, and to explain the VOC of operational devices. These measurements prove that in state‐of‐the‐art solar cells, nonradiative recombination at the interfaces between the perovskite and the transport layers is more important than processes in the bulk or at grain boundaries. Optical pump‐probe techniques give complementary access to the interfacial recombination pathways and provide quantitative information on transfer rates and recombination velocities. Promising optimization strategies are also highlighted, in particular in view of the role of energy level alignment and the importance of surface passivation. Recent record perovskite solar cells with low nonradiative losses are presented where interfacial recombination is effectively overcome—paving the way to the thermodynamic efficiency limit.
Perovskite solar cells have reached certified efficiencies of 25.2% within just ten years due to their excellent optoelectronic properties. Nonradiative recombination at the interface between the perovskite absorber and charge‐transporting layers is identified as the major source of open‐circuit‐voltage losses in state‐of‐the‐art devices, requiring advanced strategies to study and to control efficiency‐limiting interfacial processes.
Here we report a new and simple synthetic pathway to form ordered, hollow carbon nitride structures, using a cyanuric acid–melamine (CM) complex in ethanol as a starting product. A detailed analysis ...of the optical and photocatalytic properties shows that optimum hollow carbon nitride structures are formed after 8 h of condensation. For this condensation time, we find a significantly reduced fluorescence intensity and lifetime, indicating the formation of new, nonradiative deactivation pathways, probably involving charge-transfer processes. Enhanced charge transfer is seen as well from a drastic increase of the photocatalytic activity in the degradation of rhodamine B dye, which is shown to proceed via photoinduced hole transfer. Moreover, we show that various CM morphologies can be obtained using different solvents, which leads to diverse ordered carbon nitride architectures. In all cases, the CM-C3N4 structures exhibited superior photocatalytic activity compared to the bulk material. The utilization of CM hydrogen-bonded complexes opens new opportunities for the significant improvement of carbon nitride synthesis, structure, and optical properties toward an efficient photoactive material for catalysis.
This work elucidates the impact of charge transport on the photovoltaic properties of organic solar cells. Here we show that the analysis of current-voltage curves of organic solar cells under ...illumination with the Shockley equation results in values for ideality factor, photocurrent and parallel resistance, which lack physical meaning. Drift-diffusion simulations for a wide range of charge-carrier mobilities and illumination intensities reveal significant carrier accumulation caused by poor transport properties, which is not included in the Shockley equation. As a consequence, the separation of the quasi Fermi levels in the organic photoactive layer (internal voltage) differs substantially from the external voltage for almost all conditions. We present a new analytical model, which considers carrier transport explicitly. The model shows excellent agreement with full drift-diffusion simulations over a wide range of mobilities and illumination intensities, making it suitable for realistic efficiency predictions for organic solar cells.
Today's perovskite solar cells (PSCs) are limited mainly by their open‐circuit voltage (VOC) due to nonradiative recombination. Therefore, a comprehensive understanding of the relevant recombination ...pathways is needed. Here, intensity‐dependent measurements of the quasi‐Fermi level splitting (QFLS) and of the VOC on the very same devices, including pin‐type PSCs with efficiencies above 20%, are performed. It is found that the QFLS in the perovskite lies significantly below its radiative limit for all intensities but also that the VOC is generally lower than the QFLS, violating one main assumption of the Shockley‐Queisser theory. This has far‐reaching implications for the applicability of some well‐established techniques, which use the VOC as a measure of the carrier densities in the absorber. By performing drift‐diffusion simulations, the intensity dependence of the QFLS, the QFLS‐VOC offset and the ideality factor are consistently explained by trap‐assisted recombination and energetic misalignment at the interfaces. Additionally, it is found that the saturation of the VOC at high intensities is caused by insufficient contact selectivity while heating effects are of minor importance. It is concluded that the analysis of the VOC does not provide reliable conclusions of the recombination pathways and that the knowledge of the QFLS‐VOC relation is of great importance.
The lack of selectivity and energy alignment of the charge transport layers in perovskite solar cells induce a mismatch between the external open‐circuit voltage and the internal quasi‐Fermi level splitting due to enhanced interface recombination. This limits the maximum open‐circuit voltage potentially achievable and results in its saturation at high illumination intensities.
Perovskite solar cells now compete with their inorganic counterparts in terms of power conversion efficiency, not least because of their small open-circuit voltage (VOC) losses. A key to surpass ...traditional thin-film solar cells is the fill factor (FF). Therefore, more insights into the physical mechanisms that define the bias dependence of the photocurrent are urgently required. In this work, we studied charge extraction and recombination in efficient triple cation perovskite solar cells with undoped organic electron/hole transport layers (ETL/HTL). Using integral time of flight we identify the transit time through the HTL as the key figure of merit for maximizing the fill factor (FF) and efficiency. Complementarily, intensity dependent photocurrent and VOC measurements elucidate the role of the HTL on the bias dependence of non-radiative and transport-related loss channels. We show that charge transport losses can be completely avoided under certain conditions, yielding devices with FFs of up to 84%. Optimized cells exhibit power conversion efficiencies of above 20% for 6 mm2 sized pixels and 18.9% for a device area of 1 cm2. These are record efficiencies for hybrid perovskite devices with dopant-free transport layers, highlighting the potential of this device technology to avoid charge-transport limitations and to approach the Shockley-Queisser limit.
Organic solar cells are composed of electron donating and accepting organic semiconductors. Whilst a significant palette of donors has been developed over three decades, until recently only a small ...number of acceptors have proven capable of delivering high power conversion efficiencies. In particular the fullerenes have dominated the landscape. In this perspective, the emergence of a family of materials–the non‐fullerene acceptors (NFAs) is described. These have delivered a discontinuous advance in cell efficiencies, with the significant milestone of 20% now in sight. Intensive international efforts in synthetic chemistry have established clear design rules for molecular engineering enabling an ever‐expanding number of high efficiency candidates. However, these materials challenge the accepted wisdom of how organic solar cells work and force new thinking in areas such as morphology, charge generation and recombination. This perspective provides a historical context for the development of NFAs, and also addresses current thinking in these areas plus considers important manufacturability criteria. There is no doubt that the NFAs have propelled organic solar cell technology to the efficiencies necessary for a viable commercial technology–but how far can they be pushed, and will they also deliver on equally important metrics such as stability?
Organic photovoltaics have long promised low embodied energy, low cost solar power but have yet to make the commercial transition. Recent advances in efficiencies are potentially about to change this status‐quo, driven by a new class of semiconductors called the non‐fullerene electron acceptors. The emergence of these materials is reviewed, and perspectives provided as to future challenges and performance.
Perovskite solar cells with all‐organic transport layers exhibit efficiencies rivaling their counterparts that employ inorganic transport layers, while avoiding high‐temperature processing. Herein, ...it is investigated how the choice of the fullerene derivative employed in the electron‐transporting layer of inverted perovskite cells affects the open‐circuit voltage (VOC). It is shown that nonradiative recombination mediated by the electron‐transporting layer is the limiting factor for the VOC in the cells. By inserting an ultrathin layer of an insulating polymer between the active CH3NH3PbI3 perovskite and the fullerene, an external radiative efficiency of up to 0.3%, a VOC as high as 1.16 V, and a power conversion efficiency of 19.4% are realized. The results show that the reduction of nonradiative recombination due to charge‐blocking at the perovskite/organic interface is more important than proper level alignment in the search for ideal selective contacts toward high VOC and efficiency.
Perovskite solar cells have emerged as one of the most promising solar‐cell technologies for the next generation of photovoltaics. Despite simple and cheap processing, the solar cells exhibit comparably little loss in open‐circuit voltage. An approach to further reduce this loss by optimizing the charge selectivity of the fullerene‐based electron contact in efficient devices is shown.
The functionality of organic semiconductor devices crucially depends on molecular energies, namely the ionisation energy and the electron affinity. Ionisation energy and electron affinity values of ...thin films are, however, sensitive to film morphology and composition, making their prediction challenging. In a combined experimental and simulation study on zinc-phthalocyanine and its fluorinated derivatives, we show that changes in ionisation energy as a function of molecular orientation in neat films or mixing ratio in blends are proportional to the molecular quadrupole component along the π-π-stacking direction. We apply these findings to organic solar cells and demonstrate how the electrostatic interactions can be tuned to optimise the energy of the charge-transfer state at the donor-acceptor interface and the dissociation barrier for free charge carrier generation. The confirmation of the correlation between interfacial energies and quadrupole moments for other materials indicates its relevance for small molecules and polymers.
Organic solar cells are currently experiencing a second golden age thanks to the development of novel non‐fullerene acceptors (NFAs). Surprisingly, some of these blends exhibit high efficiencies ...despite a low energy offset at the heterojunction. Herein, free charge generation in the high‐performance blend of the donor polymer PM6 with the NFA Y6 is thoroughly investigated as a function of internal field, temperature and excitation energy. Results show that photocurrent generation is essentially barrierless with near‐unity efficiency, regardless of excitation energy. Efficient charge separation is maintained over a wide temperature range, down to 100 K, despite the small driving force for charge generation. Studies on a blend with a low concentration of the NFA, measurements of the energetic disorder, and theoretical modeling suggest that CT state dissociation is assisted by the electrostatic interfacial field which for Y6 is large enough to compensate the Coulomb dissociation barrier.
The efficiency of photocurrent generation is studied in the high‐efficiency nonfullerene PM6:Y6 blend, using a combination of field‐ and temperature‐dependent optoelectronic measurements. These experiments reveal barrierless free charge generation, despite a small driving force. Theoretical modeling suggests the existence of a large electrostatic interfacial field, which pushes charges away from the donor–acceptor interface.
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