Here we show that perovskite-based solar cells have two universal features: an ideality factor close to two and a space-charge-limited current regime. [16, 17] This allows us to study the impact of a particular interface on the nid with the aim to ultimately understand which recombination mechanism controls its value in the full cell. k Thus, generation = recombination — or more specifically, thermal generation current = recombination current — which essentially implies that 0V correspond to the open circuit voltage in the dark. 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! PHYSICAL REVIEW APPLIED 11, 044005 (2019) Identifying Dominant Recombination Mechanisms in Perovskite Solar Cells by Measuring the Transient Ideality Factor Phil Calado,1,* Dan Burkitt, 2Jizhong Yao,1 Joel Troughton,2 Trystan M. Watson,2 Matt J. Carnie, Andrew M. Telford,1 Brian C. O’Regan,3 Jenny Nelson,1 and Piers R.F. The corresponding VOC was monitored with a Keithley 2400 system in a two‐wire configuration. (Note, although pretty evident I think: all figures in this post show calculated data, not measurements!) ) Change ). This is shown for perovskite solar cells with various HTLs characterized by different majority carrier energetic offsets and interface recombination at the p‐interface. so that the ideality factor can be determined from the inverse slope of the ln(current) at forward bias, and the dark saturation current from the current-axis offset. As shown in the figure, the fill factor for a measured device (which happens always with the applied voltage, of course;-) is clearly lower as compared to the one plotted against the internal voltage. The photogenerated current was measured using a lock‐in‐amplifier (EG&G Princeton Applied Research Model 5302, integration times 300 ms) and evaluated after calibrating the lamp spectrum with an UV‐enhanced Si photodetector (calibrated at Newport). Note that interface recombination may cause a significant bending of the majority quasi‐Fermi levels in the perovskite bulk (EF,e at the ETL and EF,h at the HTL), which has its origin in the depletion of the majority carrier density in the perovskite near the TL due to a large energy offset in combination with fast surface recombination. I Therefore, the measured VOC will not necessarily be equal to the QFLS at the dominant recombination side; however, this is considered in the model. a lumped circuit model is commonly used to simulate solar cell operation. [23, 24, 38] On the other hand, when increasing S with an ideal band alignment (Emaj = 0 eV), the decrease of nid is less sudden and it remains above one. , The resulting JV‐curve and the voltage dependent recombination losses (in the bulk, interface, contacts, etc.) n Here, the electron (, a) Numerically simulated intensity‐dependent, orcid.org/https://orcid.org/0000-0002-3465-2475, I have read and accept the Wiley Online Library Terms and Conditions of Use. Lastly, we note that the non‐passivated perovskite lies in between with nid = 1.45 (Figure S4, Supporting Information). The AM1.5G short‐circuit current of devices matched the integrated product of the external quantum efficiency (EQE) spectrum within 5–10% error.  We, therefore, performed measurement of the PLQY and VOC as function of illumination intensity with different exposure times (see Figure S2, Supporting Information). The derivation of the simple diode equation uses certain assumption about the cell. The single diode model, as shown in fig. Note that from here on we will discuss the impact of these parameters on the external nid. 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). It derivation can be found in semiconductor text books, but it can also be derived based on thermodynamic arguments (see Peter Würfel’s excellent book on the physics of solar cells). q In the case of polymer:fullerene solar cells, the ideality factors derived by the two methods usually differ substantially.  Overall, the simulations can well reproduce the intensity dependence of the VOC of our cells as shown in Figure 1b. ) Figure 2 illustrates the operation of the solar cell. Measuring Ideality Factor. Lastly, it is worth to note that the above analysis does not give the correct ideality factor if the electron/hole densities are considered at the “wrong spot” in the device, i.e., at a location where the recombination rate is comparatively small and not limiting the VOC. All the obtained values are reported in Table 1. Related terms: Solar Cells; Photovoltaics; Open Circuit Voltage; Shunt Resistance; Barrier Height; Heterojunctions [15-20] One of the most popular approaches to assess the dominant recombination mechanism is the measurement of the ideality factor (nid). e However, the shunt resistance still does! Although the simulation tool used here does not include ion motion in the absorber layer, given the excellent match of the simulations with a large number of different experiments and the absence hysteresis in our device, we believe that for the particular systems studied here, using fullerenes as ETL, the ion movement is not a decisive parameter, consistent with previous reports. In this work, we analyze perovskite solar cells with different architectures (planar, mesoporous, HTL-free), employing temperature dependent measurements (current–voltage, light intensity, electroluminescence) of the ideality factor to identify dominating recombination processes that limit the … ( Log Out / Patterned indium tin oxide (ITO) (Lumtec, 15 Ω sqr.−1) was washed with acetone, Hellmanex III, deionized‐water, and isopropanol. ( A review of techniques to determine the ideality factor of solar cell has been given by Bashahu and Nkundabakura . With that, we thoroughly explain, experimentally and theoretically, that a low ideality factor in many cases correlates to low VOCs and poor device performances. M.S. In this picture, the ideality factor of the cell depends essentially on the asymmetry of the electron and hole quasi‐Fermi levels at the dominant recombination site. On the contrary, in the interface limited region, no interplay between different recombination processes is observed. However, the true meaning of its values is often misinterpreted in complex multilayered devices such as PSC. So, what’s next. S.A. acknowledges funding from the German Federal Ministry of Education and Research (BMBF), within the project “Materialforschung für die Energiewende” (Grant No. If the ideality factor was equal to one, one could call this the ideal Shockley equation. The Shockley diode equation describes the current–voltage characteristics of a diode. The PLQY was measured by exciting the sample inside an integrating sphere with a 455 nm laser diode with varying intensity. the term becomes zero as the open circuit voltage is “measured” without current flow, so the series resistance does not apply. The current flowing out of the diode is defined to be negative. I plan to write two more posts on the ideality factor, one on its relation to the recombination rate, and one the transport resistance (see recent papers by [Würfel/Neher et al 2015] and [Neher/Koster et al 2016]. ideality n = 1 reverse saturation current. . e The spectral photon density was obtained from the corrected detector signal (spectral irradiance) by division through the photon energy (hf) and the photon numbers of the excitation and emission obtained from numerical integration using Matlab. On the other hand, despite an overall higher QFLS, a passivated neat perovskite film presents a higher nid value due to reduced surface recombination. J Moreover, the ideality factor of the device is identical (≈1.3) regardless whether recombination in perovskite bulk (both radiative and SRH) is implemented or not. Therefore, nid = 1 must not be misinterpreted as radiative bimolecular recombination of free carriers, as often wrongly assumed. Addressing confusion about physics of disordered materials, and adding to it… ;-). A couple of years ago, I wrote about some general properties of current-voltage characteristics of organic solar cells, but did not describe the ideality factor.1 I think the ideality factor was mentioned only once, and then without details. However, this often used approach to connect the value of the ideality factor to the order of recombination relies on several critical assumptions. Where does one start after so long an absence — meaning only the blog abstinence; I have been working and publishing since last time;-) One of the things which have been on my mind is the ideality factor, a figure of merit for the charge carrier recombination mechanism in a semiconductor diode. The latter is indeed considerably below the maximum theoretically achievable VOC due to the nonradiative recombination of charges. In other words, the plot shows that an nid of 1 is not necessarily representing an efficient cell as often believed (and suggested in other works). the ﬁll factor of a solar cell depends critically on the diode ideality factor (besides, of course, the resistances and the saturation current). I By coupling intensity‐dependent quasi‐Fermi level splitting measurements with drift diffusion simulations of complete devices and partial cell stacks, it is shown that interfacial recombination leads to a lower nid compared to Shockley–Read–Hall (SRH) recombination in the bulk. Theoretical models were proposed to clarify the much higher ideality factors. ϑ Revealing Energy Loss and Nonradiative Recombination Pathway in Mixed-Ion Perovskite Solar Cells. Enter your email address below and we will send you your username, If the address matches an existing account you will receive an email with instructions to retrieve your username, Our combined experimental/simulation study focusses on, a) Intensity dependent quasi‐Fermi level splitting, QFLS(, In order to provide further insights into the origin of these ideality factor values, we analyzed the hole (, Schemes of interfacial energy levels and quasi‐Fermi level splitting (QFLS) based on a simulated energy diagram. The real illumination intensity was monitored during the measurement using a Si photodiode and the exact illumination intensity was used for efficiency calculations. 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. Ideality factors are derived from either the slope of the dark current/voltage curve or the light intensity dependence of the open-circuit voltage in solar cells and are often a valuable method to characterize the type of recombination. QFLS , Numerical simulations and VOC versus I experiments of systems with different nid are exemplified in Figure 4a. ) Halide perovskite solar cells (PSC) have the potential to trigger a revolution in the photovoltaic sector due to their low‐cost production and outstanding efficiencies. (Please note that under realistic conditions, is not only pretty small and difficult to measure in principle, it is also hidden behind shunt currents in the device. ) 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. However, we emphasize that we cannot exclude that other parameters may affect this trend in other devices. V Learn more. I 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). The sun simulator was calibrated with a KG5 filtered silicon solar cell (certified by Fraunhofer ISE). T ) JV‐curves were measured under N2 with a Keithley 2400 system in a two‐wire configuration with a scan speed of 0.1 V s−1 and voltage step of 0.02 V. One sun illumination at ≈100 mW cm−2 of AM1.5G irradiation was provided by a Oriel class ABA sun simulator. At zero volt, . In order to verify the Voc-Isc method, a serie… From these results, the QFLS in the perovskite absorber was calculated at each intensity, following the approach as outlined in our previous works (see also Figure S3, Supporting Information, for further details). In case of only one dominant interface this QFLS is then equal to the VOC (see Figure 3 and Figure S8A, Supporting Information). q Only then, the ideality factor is related to the recombination order via the well‐known relation nid = ϑ/α. Ideality factors reported in relatively high efficiency dye-sensitized solar cells were around 2.0 ; From: Nanostructured Materials for Solar Energy Conversion, 2006. E.g. Saturation current (I 0) and ideality factor (n)ofap-n junction solar cell are an indication of the quality of the cell. [15, 16] We have recently measured the intensity dependence of the QFLS and the VOC of complete perovskite solar cells for two different polymer‐based hole transporting materials. The ideality factor could only be determined from the dark characteristics using the “remaining” part of the exponential current–voltage regime. 0 ∝ Here, we extend our previous studies by utilizing intensity dependent PL measurements on perovskite films with and without transport layers in order to obtain the internal nid (from QFLS) of the individual junctions of the cell and the neat material and to rationalize the origin of the nid values previously observed. It is only in the case of optimized interfaces and highly suppressed interface recombination that an nid of 1 would be again desirable, being representative of predominant free carrier recombination and reduced SRH in the bulk. The PL of the samples was readily recorded after mounting the sample and after an exposure of 1 s at each laser intensity subsequently, the incident laser was blocked by a shutter and the filter wheel position adjusted while the sample was kept in dark conditions avoiding any effects induced by constant illumination. J The temperature dependence of the solar cell ideality factor can give valuable information about the main recombination mechanism in … The current flowing out of the diode is defined to be negative. 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%. As pointed out above, the recombination under a 1 sun equivalent illumination intensity in p‐i‐n‐type perovskite solar cells is mainly a first‐order non‐radiative trap‐assisted process at the perovskite/TL interfaces. [39, 40]. 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. Interestingly, also in a hypothetical solar cell with a strongly misaligned (but undoped) PTAA layer (Figure S8B, Supporting Information), the situation is almost identical to PEDOT:PSS, suggesting a stronger influence of the energetic offset on the nid rather than doping. The measurement of the ideality factor (nid) is a popular tool to infer the dominant recombination type in perovskite solar cells (PSC). The latter was recorded using a home‐built setup utilizing a Philips Projection Lamp (Type7724 12 V 100 W) in front of a monochromator (Oriel Cornerstone 74100) and the light was mechanically chopped at 70 Hz. The ideality factor has been derived from the slope of the semi-logarithmic dark J-V curve and represented by equation 13 given below. 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European Funding Office been derived from the bracket or equal to one, one could call this ideal... Is measured by monitoring the evolution of Vas a function of the factor... Recombination in the bulk, interface, contacts, etc. Generally from! Cells with various HTLs characterized by different carrier reservoirs Figure 3 visually the... Layer interface low nonradiative recombination of charges be negative transient ideality factor along the entire curve. Qfls yields nid, int ≈ 1.3 causing this deviation at high intensities and parasitic losses is! Matched the integrated product of the simple diode equation uses certain assumption about the.!, however ideality factor solar cell the recombination rate is completely governed by ne and nh depend the... Be obtained from the perovskite QFLS device designs I experiments of systems with different nid investigated...
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