The value and temperature dependence of the ideality factor provides essential information about the dominant recombination route in solar cells. This study presents experimental results of accurate ...ideality factor determination for representative organic photovoltaic cells (OPV) evaluated at different temperatures over a large current density regime. It is noted that standard dark I–V curves strongly deviate from those obtained by evaluations based on short circuit current density (J
SC)–open circuit voltage (V
OC) pairs. This is attributed to the applied external voltage in a dark I–V measurement not being representative of internal chemical potential, particularly at lower temperatures. Complementary electroluminescence measurements attest that the current density dependence of the ability of the solar cell to emit light is better correlated to the series resistance free ideality factor. For the studied set of OPV devices it is observed that the ideality factors are quite low, and with very weak temperature dependence. The J
SC–V
OC method to determine ideality factors further allows good estimates of activation energies as well as recombination current prefactors J
00. The findings imply that the principal OPV non‐radiative recombination mechanism is not recombination of free carriers with trapped carriers in an exponential density of tail states as previously reported.
Improved determination of ideality factors is performed by evaluating open circuit voltage–short circuit current pairs as opposed to traditional dark I–V curves. The true temperature dependence of the ideality factor can then be obtained for organic solar cells otherwise substantially limited by series resistance effects. The relation to radiative efficiency, activation energy, and dark saturation current is clarified.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Charge transfer complexes are interfacial charge pairs residing at the donor–acceptor heterointerface in organic solar cell. Experimental evidence shows that it is crucial for the photovoltaic ...performance, as both photocurrent and open circuit voltage directly depend on it. For charge photogeneration, charge transfer complexes represent the intermediate but essential step between exciton dissotiation and charge extraction. Recombination of free charges to the ground state is via the bound charge transfer state before being lost to the ground state. In terms of the open circuit voltage, its maximum achievable value is determined by the energy of the charge transfer state. An important question is whether or not maximum photocurrent and maximum open circuit voltage can be achieved simultaneously. The impact of increasing the CT energy—in order to raise the open circuit voltage, but lowering the kinetic excess energy of the CT complexes at the same time—on the charge photogeneration will accordingly be discussed. Clearly, the fundamental understanding of the processes involving the charge transfer state is essential for an optimisation of the performance of organic solar cells.
Charge transfer complexes are interfacial charge pairs residing at the donor‐acceptor heterointerface in organic solar cell. Experimental evidence shows them to be crucial for the photovoltaic performance: both photocurrent and open circuit voltage depends on them. For charge generation and recombination, charge transfer complexes represent essential intermediate steps. Clearly, understanding the role of the charge transfer state is essential for an optimisation of organic solar cells.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Bright and efficient blue emission is key to further development of metal halide perovskite light-emitting diodes. Although modifying bromide/chloride composition is straightforward to achieve blue ...emission, practical implementation of this strategy has been challenging due to poor colour stability and severe photoluminescence quenching. Both detrimental effects become increasingly prominent in perovskites with the high chloride content needed to produce blue emission. Here, we solve these critical challenges in mixed halide perovskites and demonstrate spectrally stable blue perovskite light-emitting diodes over a wide range of emission wavelengths from 490 to 451 nanometres. The emission colour is directly tuned by modifying the halide composition. Particularly, our blue and deep-blue light-emitting diodes based on three-dimensional perovskites show high EQE values of 11.0% and 5.5% with emission peaks at 477 and 467 nm, respectively. These achievements are enabled by a vapour-assisted crystallization technique, which largely mitigates local compositional heterogeneity and ion migration.
Abstract
Point defects in metal halide perovskites play a critical role in determining their properties and optoelectronic performance; however, many open questions remain unanswered. In this work, ...we apply impedance spectroscopy and deep-level transient spectroscopy to characterize the ionic defect landscape in methylammonium lead triiodide (MAPbI
3
) perovskites in which defects were purposely introduced by fractionally changing the precursor stoichiometry. Our results highlight the profound influence of defects on the electronic landscape, exemplified by their impact on the device built-in potential, and consequently, the open-circuit voltage. Even low ion densities can have an impact on the electronic landscape when both cations and anions are considered as mobile. Moreover, we find that all measured ionic defects fulfil the Meyer–Neldel rule with a characteristic energy connected to the underlying ion hopping process. These findings support a general categorization of defects in halide perovskite compounds.
In polymer:fullerene solar cells, the origin of the losses in the field‐dependent photocurrent is still controversially debated. We contribute to the ongoing discussion by performing photo‐induced ...charge extraction measurements on poly(3‐hexylthiophene‐2,5‐diyl):6,6‐phenyl‐C61 butyric acid methyl ester solar cells in order to investigate the processes ruling charge carrier decay. Calculating the drift length of photogenerated charges, we find that polaron recombination is not limiting the photocurrent for annealed devices. Additionally, we applied Monte Carlo simulations on blends of conjugated polymer chain donors with acceptor molecules in order to gain insight into the polaron pair dissociation. The dissociation yield turns out to be rather high, with only a weak field dependence. With this complementary view on dissociation and recombination, we stress the importance of accounting for polaron pair dissociation, polaron recombination as well as charge extraction when considering the loss mechanisms in organic solar cells.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The separation of photogenerated polaron pairs in organic bulk heterojunction solar cells is the intermediate but crucial step between exciton dissociation and charge transport to the electrodes. In ...state-of-the-art devices, above 80% of all polaron pairs are separated at fields of below 10(7) V/m. In contrast, considering just the Coulomb binding of the polaron pair, electric fields above 10(8) V/m would be needed to reach similar yields. In order to resolve this discrepancy, we performed kinetic Monte Carlo simulations of polaron-pair dissociation in donor-acceptor blends, considering delocalized charge carriers along conjugated polymer chain segments. We show that the resulting fast local charge carrier transport can indeed explain the high experimental quantum yields in polymer solar cells.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UM
The maximum efficiency of any solar cell can be evaluated in terms of its corresponding ability to emit light. We herein determine the important figure of merit of radiative efficiency for ...Methylammonium Lead Iodide perovskite solar cells and, to put in context, relate it to an organic photovoltaic (OPV) model device. We evaluate the reciprocity relation between electroluminescence and photovoltaic quantum efficiency and conclude that the emission from the perovskite devices is dominated by a sharp band-to-band transition that has a radiative efficiency much higher than that of an average OPV device. As a consequence, the perovskite have the benefit of retaining an open circuit voltage ~0.14 V closer to its radiative limit than the OPV cell. Additionally, and in contrast to OPVs, we show that the photoluminescence of the perovskite solar cell is substantially quenched under short circuit conditions in accordance with how an ideal photovoltaic cell should operate.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Solution‐processed organic–inorganic hybrid perovskites are promising emitters for next‐generation optoelectronic devices. Multiple‐colored, bright light emission is achieved by tuning their ...composition and structures. However, there is very little research on exploring optically active organic cations for hybrid perovskites. Here, unique room‐temperature phosphorescence from hybrid perovskites is reported by employing novel organic cations. Efficient room‐temperature phosphorescence is activated by designing a mixed‐cation perovskite system to suppress nonradiative recombination. Multiple‐colored phosphorescence is achieved by molecular design. Moreover, the emission lifetime can be tuned by varying the perovskite composition to achieve persistent luminescence. Efficient room‐temperature phosphorescence is demonstrated in hybrid perovskites that originates from the triplet states of the organic cations, opening a new dimension to the further development of perovskite emitters with novel functional organic cations for versatile display applications.
Room‐temperature phosphorescence is activated in hybrid perovskites through energy transfer between inorganic and organic layers. By designing a mixed‐cation perovskite system, nonradiative recombination is suppressed for achieving a high phosphorescence efficiency up to 11%. Multiple‐colored emission with millisecond lifetime is achieved. These results emphasize the importance of organic chemistry in further development of perovskite materials for future optoelectronics.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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