The real illumination intensity was monitored during the measurement using a Si photodiode and the exact illumination intensity was used for efficiency calculations. An elegant and already well‐established approach to determine the nid is to measure the VOC as a function of the light intensity (I). All the obtained values are reported in Table 1. It is common to neglect the thermal generation current (the term -1, multiplied by ), which is a good approximation for voltages some larger than 0. Often less extreme overestimation, but just the same: do not do it;-). acknowledges the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—Project No. ∝ 03SF0540), and the German Federal Ministry for Economic Affairs and Energy (BMWi) through the “PersiST” project (Grant No. This means that if you measure () pairs for a (wide) range of different illumination intensities (thus varying ), the points should overlap with the dark curve! However, despite the continuous advance of the scientific community in increasing the power conversion efficiencies (PCEs), perovskite solar cells are still limited by the open‐circuit voltage (VOC). In contrast, reducing the quality of the perovskite/TL interface decreases the value of nid (along with a decrease of the VOC), irrespectively of whether Emaj, S or both of them are increased. q n , Moreover, we demonstrated that increased interfacial recombination reduces the ideality factor towards 1 in the case of cells with a PEDOT:PSS and P3HT HTL. k Through the years, several studies spotlighted the perovskite surface[7-9] and the grain boundaries[9, 10] as main recombination centers in the perovskite absorber. The intensity of the laser was adjusted to a 1 sun equivalent intensity by illuminating a 1 cm2 size perovskite solar cell under short‐circuit and matching the current density to the JSC under the sun simulator (22.0 mA cm−2 at 100 mW cm−2, or 1.375 × 1021 photons m−2 s−1). 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. A simplified expression for the current density, as a function of the applied voltage, has been systematically derived from a charge transport model, based on drift-diffusion theory, that includes ion migration in the perovskite layer [4,5]. If the ideality factor was equal to one, one could call this the ideal Shockley equation. In contrast, if we consider only bulk recombination (device with ideal interfaces), then the ideality factor is considerably higher (≈1.8). In a last step, three fluorescent test samples with high specified PLQY (≈70%) supplied from Hamamatsu Photonics were measured where the specified value could be accurately reproduced within a small relative error of less than 5%. (Note, although pretty evident I think: all figures in this post show calculated data, not measurements!) How can one determine the ideality factor and the dark saturation current (at least in principle, see below for a better way on real devices)? Moreover, fast interface recombination at this interface induces a slower increase of ne in the ETL layer compared to the perovskite bulk. [30] By corroborating our results by drift diffusion simulations, we clarify that a single non‐radiative recombination process at the interface can cause such mixed (between 1 and 2) nid values. The active area was 6 mm2 defined as the overlap of ITO and the top electrode. We succeeded in modeling a range of different nid values, from 1 to 2, considering only first‐order SRH recombination and the carrier densities (nh and ne) in the proximity of the dominant recombination channel. ) In case of only one dominant interface this QFLS is then equal to the VOC (see Figure 3 and Figure S8A, Supporting Information). I’d say a good rule of thumb is: if the slope of the current–voltage characteristics at short circuit is (close to) zero (i.e., ), then is a good assumption. Note that for solar cells with good fill factor, can be approximated by the short circuit current . Consequently, and to some extent counterintuitively, a higher nid may actually correspond to a better perovskite device. ( Log Out /  P.S. Excitation for the PL measurements was performed with a 445 nm continuous wave laser (Insaneware) through an optical fibre into an integrating sphere. the explanation that crossing point is due to the field dependent separation of polaron pairs is not correct. On the other hand, despite an overall higher QFLS, a passivated neat perovskite film presents a higher nid value due to reduced surface recombination. Learn about our remote access options, Institute of Physics and Astronomy, University of Potsdam, Potsdam, 14476 Germany, Young Investigator Group Perovskite Tandem Solar Cells, Helmholtz‐Zentrum Berlin für Materialien und Energie GmbH, Berlin, 12489 Germany, E‐mail: capriogl@uni-potsdam.de; neher@uni-potsdam.de; stolterf@uni-potsdam.de, Department of Physics, Swansea University, Singleton Park, Swansea, Wales, SA2 8PP UK, Institute for Silicon Photovoltaics, Helmholtz‐Zentrum Berlin für Materialien und Energie GmbH, Berlin, 12489 Germany, Faculty IV – Electrical Engineering and Computer Science Technical, University Berlin, Berlin, 10587 Germany. Under illuminated conditions. The current flowing out of the diode is defined to be negative. Fill in your details below or click an icon to log in: You are commenting using your WordPress.com account. These HTLs include undoped poly(3‐hexylthiophene) (P3HT) (Emaj ≈ 0.2 eV) and doped poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) (Emaj ≈ 0.4 eV). Essentially, the charge carriers which can flow out are the generated ones (e.g. The current flowing out of the diode is defined to be negative. The corresponding VOC was monitored with a Keithley 2400 system in a two‐wire configuration. A spectral correction factor was established to match the spectral output of the detector to the calibrated spectral irradiance of the lamp. In this work, the effects of bulk and interface recombination on the nid are investigated experimentally and theoretically. 2 Importantly, for this type of devices, the internal QFLS and external VOC match within the light intensity regime studied here. Revisiting these old posts makes me acutely aware of what I did not know then and do know now a bit more about. ( An ideal diode has an ideality factor of 1, indicating the structure of the p-n device is perfect with no defects, while an ideal diode is impossible to produce. Scientists aim to fabricate a diode which diode characteristics curve could approaching the ideal diode the most. The diode ideality factor in organic solar cells: basics. From these results, the QFLS in the perovskite absorber was calculated at each intensity, following the approach as outlined in our previous works[16] (see also Figure S3, Supporting Information, for further details). [18] We, therefore, performed measurement of the PLQY and VOC as function of illumination intensity with different exposure times (see Figure S2, Supporting Information). The ideality factor is derived from the slope of the dark-IV, Suns-Voc and occasionally the Light-IV curve. The reason is that qVOC is the difference between the Fermi levels at the two contacts, which in this special case, is identical to the QFLS at the dominant recombination region. J The respective JV‐characteristic of all devices are presented in Figure S11 in the Supporting Information, while the nid of the LiF passivated cell with a PCE of ≈21% is shown in Figure S12 in the Supporting Information. It is noted that standard dark Consequently, analyzing the total recombination current as function of VOC may lead to wrong conclusions about mechanism of the recombination in the absorber and at its interfaces to the TLs. However, this often used approach to connect the value of the ideality factor to the order of recombination relies on several critical assumptions. . In this work, we demonstrated the application of intensity dependent QFLS measurements on perovskite/transport layer junctions to gain a comprehensive understanding of the processes determining the ideality factor in perovskite solar cells. After spin coating samples were annealed at 100 °C for 1 h. Afterwards, the samples were transferred to an evaporation chamber and C60 (30 nm), bathocuproine (8 nm) and copper (100 nm) were deposited under vacuum (p = 10−7 mbar). ) These effects can be approximated by considering a series resistance and a parallel (shunt) resistance . from the Perovskite/Hole Transport Layer Interface That means, the internal voltage at the solar cell is reduced by a voltage drop across the series resistance, and the diode current is essentially superpositioned on a shunt current. This can also be seen when comparing the dark current-voltage characteristics for an internal voltage with the same current plotted at the external voltage , which is reduced compared to the internal one by the (series) resistance. Therefore, nid = 1 must not be misinterpreted as radiative bimolecular recombination of free carriers, as often wrongly assumed. Surprisingly, this value is nearly identical to the value of nid,ext ≈ 1.3 as deduced from the intensity dependence of the VOC, provided that leakage through the thin PTAA layer can be avoided. The basic cell equation in the dark is: , where I is the current through the diode, V is the voltage across the diode, I 0 is the dark saturation current, n is the ideality factor and T is the temperature in kelvin. Figure 3 visually depicts the scenarios of the two cases described above. Moreover, we rationalized that nid = 1 does not always originate from predominant bimolecular recombination, but it can correspond to solar cells limited by interface recombination or recombination at the metal contacts in the case of a selectivity failure. T However, in case of predominant recombination at the perovskite/TL interface, the QLFS in the perovskite is irrelevant for the interfacial recombination rate as the recombination rate is determined by the difference of the electron and hole quasi‐Fermi levels at the HTL interface. [Update 2016-05-15] added “-” everywhere, terribly sorry! In other words, the value of nid is given by the share of the QFLS that EF,min gets when the QFLS increases as function of light intensity. [15, 16] Consistent with earlier studies, both types of devices show ideality factors approaching 1 and low VOCs. Ideality factors are used to identify the dominant form of recombination in many types of solar cells and guide future development. 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