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The performance of perovskite solar cells is predominantly limited by non-radiative recombination, either through trap-assisted recombination in the absorber layer or via minority carrier recombination at the perovskite/transport layer interfaces. Here, we use transient and absolute photoluminescence imaging to visualize all non-radiative recombination pathways in planar pintype perovskite solar cells with undoped organic charge transport layers. We find significant quasi-Fermi-level splitting losses (135 meV) in the perovskite bulk, whereas interfacial recombination results in an additional free energy loss of 80 meV at each individual interface, which limits the open-circuit voltage (V-oc) of the complete cell to similar to 1.12 V. Inserting ultrathin interlayers between the perovskite and transport layers leads to a substantial reduction of these interfacial losses at both the p and n contacts. Using this knowledge and approach, we demonstrate reproducible dopant-free 1 cm(2) perovskite solar cells surpassing 20% efficiency (19.83% certified) with stabilized power output, a high V-oc (1.17 V) and record fill factor (>81%).
Charge transport layers (CTLs) are key components of diffusion controlled perovskite solar cells, however, they can induce additional non-radiative recombination pathways which limit the open circuit voltage (V-OC) of the cell. In order to realize the full thermodynamic potential of the perovskite absorber, both the electron and hole transport layer (ETL/HTL) need to be as selective as possible. By measuring the photoluminescence yield of perovskite/CTL heterojunctions, we quantify the non-radiative interfacial recombination currents in pin- and nip-type cells including high efficiency devices (21.4%). Our study comprises a wide range of commonly used CTLs, including various hole-transporting polymers, spiro-OMeTAD, metal oxides and fullerenes. We find that all studied CTLs limit the V-OC by inducing an additional non-radiative recombination current that is in most cases substantially larger than the loss in the neat perovskite and that the least-selective interface sets the upper limit for the V-OC of the device. Importantly, the V-OC equals the internal quasi-Fermi level splitting (QFLS) in the absorber layer only in high efficiency cells, while in poor performing devices, the V-OC is substantially lower than the QFLS. Using ultraviolet photoelectron spectroscopy and differential charging capacitance experiments we show that this is due to an energy level mis-alignment at the p-interface. The findings are corroborated by rigorous device simulations which outline important considerations to maximize the V-OC. This work highlights that the challenge to suppress non-radiative recombination losses in perovskite cells on their way to the radiative limit lies in proper energy level alignment and in suppression of defect recombination at the interfaces.
Perovskite/silicon tandem photovoltaics (PVs) promise to accelerate the decarbonization of our energy systems. Here, we present a thorough subcell diagnosis methodology to reveal deep insights into the practical efficiency limitations of state-of-the-art perovskite/silicon tandem PVs. Our subcell selective intensity-dependent photoluminescence (PL) and injection-dependent electroluminescence (EL) measurements allow independent assessment of pseudo-V-OC and power conversion efficiencies (PCEs) for both subcells. We reveal identical metrics from PL and EL, which implies well-aligned energy levels throughout the entire cell. Relatively large ideality factors and insufficient charge extraction, however, cause each a fill factor penalty of about 6% (absolute). Using partial device stacks, we then identify significant losses in standard perovskite subcells due to bulk and interfacial recombination. Lastly, we present strategies to minimize these losses using triple halide (CsFAPb(IBrCI)(3)) based perovskites. Our results give helpful feedback for device development and lay the foundation toward advanced perovskite/silicon tandem PVs capable of exceeding 33% PCE.
We introduce a new and simple method to quantify the effective extraction mobility in organic solar cells at low electric fields and charge carrier densities comparable to operation conditions under one sun illumination. By comparing steady-state carrier densities at constant illumination intensity and under open-circuit conditions, the gradient of the quasi-Fermi potential driving the current is estimated as a function of external bias and charge density. These properties are then related to the respective steady-state current to determine the effective extraction mobility. The new technique is applied to different derivatives of the well-known low-band-gap polymer PCPDTBT blended with PC70BM. We show that the slower average extraction due to lower mobility accounts for the moderate fill factor when solar cells are fabricated with mono- or difluorinated PCPDTBT. This lower extraction competes with improved generation and reduced nongeminate recombination, rendering the monofluorinated derivative the most efficient donor polymer.
The fact that organic solar cells perform efficiently despite the low dielectric constant of most photoactive blends initiated a long-standing debate regarding the dominant pathways of free charge formation. Here, we address this issue through the accurate measurement of the activation energy for free charge photogeneration over a wide range of photon energy, using the method of time-delayed collection field. For our prototypical low bandgap polymer:fullerene blends, we find that neither the temperature nor the field dependence of free charge generation depend on the excitation energy, ruling out an appreciable contribution to free charge generation though hot carrier pathways. On the other hand, activation energies are on the order of the room temperature thermal energy for all studied blends. We conclude that charge generation in such devices proceeds through thermalized charge transfer states, and that thermal energy is sufficient to separate most of these states into free charges.
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.
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.
We have applied time-delayed collection field (TDCF) and charge extraction by linearly increasing voltage (CELIV) to investigate the photogeneration, transport, and recombination of charge carriers in blends composed of PCPDTBT/PC70BM processed with and without the solvent additive diiodooctane. The results suggest that the solvent additive has severe impacts on the elementary processes involved in the photon to collected electron conversion in these blends. First, a pronounced field dependence of the free carrier generation is found for both blends, where the field dependence is stronger without the additive. Second, the fate of charge carriers in both blends can be described with a rather high bimolecular recombination coefficients, which increase with decreasing internal field. Third, the mobility is three to four times higher with the additive. Both blends show a negative field dependence of mobility, which we suggest to cause bias-dependent recombination coefficients.