Charge selective interlayers are of critical importance in order for solar cells based on low mobility materials, such as polymer‐fullerene blends, to perform well. Commonly used anode interlayers consist of high work function transition metal oxides, with molybdenum trioxide (MoO 3 ) being arguably the most used. Here, it is shown that a thin interlayer of MoO 3 causes unintentional bulk doping in solar cells based on polymers and polymer‐fullerene blends. The doping concentrations determined from capacitance–voltage measurements are larger than 10 16 cm −3 and are seen to increase closer to the anode, reference devices without MoO 3 are undoped. Using time of flight secondary ion mass spectroscopy it is furthermore shown that molybdenum is present on the surface of all films with an interfacial layer of MoO 3 beneath the active layer. Doping concentrations of this magnitude are detrimental for device performance, especially for active layers >100 nm.
The charge-carrier mobility and the built-in potential are important device parameters for optimizing the performance of thin-film solar cells. Charge extraction by a linearly increasing voltage (CELIV) is a commonly used technique for mobility measurements, but the original CELIV method applies only to devices with blocking contact---a severe limitation, since most operating devices have Ohmic contacts. Here the CELIV method is extended to determine these parameters in thin-film metal-insulator-metal ($m\ensuremath{-}i\ensuremath{-}m$) or $p\ensuremath{-}i\ensuremath{-}n$ diodes, rendering it a powerful tool for such devices.
The field of organic solar cells has recently gained broad research interest due to the introduction of non-fullerene small-molecule acceptors. The rapid improvement in solar cell efficiency put increased demand on moving toward scalable device architectures. An essential step toward this is enabling thicker active layers for which the hole and electron mobilities and their ratio become increasingly important. In this work, we demonstrate selective charge-carrier mobility determination using the charge extraction by a linearly increasing voltage (CELIV) method. By tuning the contact properties of the solar cell diodes, the hole and electron mobilities are determined separately using the recently developed metal–intrinsic semiconductor–metal-CELIV (MIM-CELIV) technique. Balanced mobility is measured both in non-fullerene and in ternary blends with the recently published PBBF11 polymer. The mobility results are confirmed using the well-established metal–insulator–semiconductor (MIS) and photo-CELIV techniques.
In this work, we measure the hole mobility in the model polymer system poly(3-hexylthiophene-2,5-diyl) by using different measurement techniques. Our main purpose is to determine how the recently developed charge extraction by a linearly increasing voltage technique for metal–insulator–metal devices (MIM-CELIV) compares to other commonly used methods, such as space charge limited currents and time-of-flight. Our findings suggest that the MIM-CELIV technique gives a slightly lower mobility than the other techniques, which is understandable given that the method measures the mobility of relaxed charge carriers in the dark unlike, for example, time-of-flight where charge carriers are photogenerated. In addition, we scrutinize the accuracy and reliability of the techniques used, showing that differences in mobility smaller than a factor of two are not detectable unless statistics from many devices are available.
The kinetics at contacts plays a crucial role in sandwich-type thin-film devices based on organic semiconductors. This is of particular importance in organic and perovskite solar cells where selective contacts that are able to efficiently collect majority carriers, simultaneously blocking minority carriers, are desired. Despite the vast progress made, a comprehensive understanding, needed for developing new electrode materials to improve and optimize device performance is still lacking. A key parameter for obtaining information about processes taking place at the contacts is the effective surface recombination velocity.[1] However, means to quantitatively measure surface recombination velocities at contact interfaces in sandwich-type thin-film devices based on organic semiconductors are lacking. The Charge Extraction by a Linearly Increasing Voltage (CELIV) technique is one of the most common methods to measure the charge transport properties in organic semiconductor devices. In this work, we show how CELIV can be used to determine surface recombination velocities at selective and/or blocking contacts in thin-film devices. The analytical framework behind the method is presented, and confirmed by numerical drift-diffusion simulations. We furthermore demonstrate the method on organic semiconductor devices, employing TiO2 and SiO2 as cathode buffer layers. The method allows for an increased understanding of contact properties in sandwich-type thin-film devices based on organic semiconductors. [1] O. J. Sandberg, A. Sundqvist, M. Nyman, and R. Österbacka, Phys. Rev. Appl. 5, 044005 (2016).
The recently introduced perovskite solar cell (PSC) technology is a promising candidate for providing low-cost energy for future demands. However, one major concern with the technology can be traced back to morphological defects in the electron selective layer (ESL), which deteriorates the solar cell performance. Pinholes in the ESL may lead to an increased surface recombination rate for holes, if the perovskite absorber layer is in contact with the fluorine-doped tin oxide (FTO) substrate via the pinholes. In this work, we used sol-gel-derived mesoporous TiO
Titanium dioxide (TiO2) is a commonly used electron selective layer in thin-film solar cells. The energy levels of TiO2 align well with those of most light-absorbing materials and facilitate extracting electrons while blocking the extraction of holes. In a device, this separates charge carriers and reduces recombination. In this study, we have evaluated the hole-blocking behavior of TiO2 compact layers using charge extraction by linearly increasing voltage in a metal–insulator–semiconductor structure (MIS-CELIV). This hole-blocking property was characterized as surface recombination velocity (SR) for holes at the interface between a semiconducting polymer and TiO2 layer. TiO2 layers of different thicknesses were prepared by sol–gel dip coating on two transparent conductive oxide substrates with different roughnesses. Surface coverage and film quality on both substrates were characterized using X-ray photoelectron spectroscopy and atomic force microscopy, along with its conductive imaging mode. Thicker TiO2 coatings provided better surface coverage, leading to reduced SR, unless the layers were otherwise defective. We found SR to be a more sensitive indicator of the overall film quality, as varying SR values were still observed among the films that looked similar in their characteristics via other methods.
The charge extraction (of injected carriers) by linearly increasing voltage in metal-insulator-semiconductor structures, or MIS-CELIV, is based on the theory of space-charge-limited currents. In this work, the validity of MIS-CELIV for mobility determination in organic thin-film devices has been critically examined and clarified by means of drift-diffusion simulations. It is found that depending on the applied transient voltage, the mobility might be overestimated by several orders of magnitude in the case of an ohmic injecting contact. The shortcomings of the MIS-CELIV theory can be traced back to the underlying assumption of a drift-dominated transport. However, the effect of diffusion can be taken into account by introducing a correction factor. In the case of non-ohmic injecting contacts, the extracted mobility becomes strongly dependent on device parameters, possibly leading to large deviations from the actual mobility.
Unintentional doping of the active layer is detrimental to an organic solar cell's performance, but is often overlooked. This work clarifies the impact of doping on charge-collection efficiency in low-mobility solar cells. By analytical derivation and numerical simulation, the authors find the collection efficiency of photogenerated charge carriers in a doped active layer to be independent of light intensity, but distinctly dependent on voltage, resulting in an electric field dependence of the photocurrent. The results will help to overcome the effect of unwanted doping, and to distinguish between different recombination loss mechanisms in these important photovoltaics.
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