Reduction of recombination at the interface of perovskite and electron transport layer with graded pt quantum dot doping in ambient air-processed perovskite solar cell | Scientific Reports

Scientific Reports volume 14, Article number: 24254 (2024) Cite this article

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The study of charge transfer in thin film solar cells made of several layers is of high importance since they may lose their energy via the recombination process at the interfaces, specifically at the interface of the electron transport layer (ETL) and perovskite. Titanium dioxide (TiO2) is mostly used as an ETL in perovskite solar cells due to its many advantages. However, TiO2 has some disadvantages, such as low electron mobility compared to the perovskite layer and electron trap states on its top at the interface. These effects cause the accumulation of carriers at the ETL/perovskite interface then the non-radiative recombination will be enhanced, which is considered as one of the significant losses in the Perovskite Solar Cells (PSCs). In this work, a new technique is taken for more optimal ETL doping. We fabricated the ETL layers with graded doping of platinum quantum dots (Pt QDs), in which Pt QDs concentration is high at the ETL/Perovskite interface and zero at the FTO/ETL interface. This strategy not only suppresses the recombination at the ETL/perovskite interface and subsequently enhances the device efficiency from 12.92 to 14.36% but also improves the stability of the PSCs.

The need for dependable clean energy resources is increasing day by day. Photovoltaics, as one of many ways of solar energy capturing systems, are the most important devices among those that convert solar energy to electricity and have become one of the hottest topics in the past decades1,2,3,4. Among the new kinds of solar cells, PSCs are of particular interest. Because they have certain favorable features like the high mobility of charge carriers, tunable bandgap, long charge carrier diffusion length, and low cost of production. For these reasons, many studies have been carried out to enhance and optimize their performance. From the beginning of the emergence of this type of solar cells until now, its efficiency has improved consistently and continuously from about 3% to over 26%5,6,7,8,9,10,11.

Solar cells are composed of different layers, electron transport layer (ETL), active light absorbing layer, hole transport layer (HTL), and metallic electrodes (cathode and anode), each of which has its own specific functionality in the cell. The ETL/HTL layer which transfers the generated electrons/holes from the active absorbing perovskite layer to the electron-collecting/hole-collecting electrode, also might prevent the permeation of electrons/holes to the anode/cathode. Much research carried out to optimize the performance of the ETL in PSCs through the structure engineering of the ETL. One of the most important losses in PSCs is the non-radiative recombination at the interface of the ETL/perovskite, which mainly comes from the presence of traps on the surface of the ETL. Moreover, due to the low mobility of the carriers in the oxide ETL compared to the perovskite layer, accumulation of the free carriers will occur at the interface of the perovskite and the ETL layer12,13,14. Therefore, free electrons get trapped and subsequently recombine with free holes which is detrimental to the solar cell efficiency. To reduce the recombination at the interface, the mobility of the ETL might be enhanced which will allow easy transferring of electrons and thus lowering the non-radiative recombination rate. Many researches carried out to study and overcome the problem of the recombination of the free carriers at the interface and enhance the electron extraction15,16,17. In the PSCs, mostly TiO2 is used as an ETL and hole-blocking layer. Its conduction band aligns properly with the perovskite layer, and besides this, it has advantages like high transparency, stability, and low cost. These advantages make it suitable to be used as ETL in PSCs. Nevertheless, TiO2 ETL has some drawbacks like lower electron mobility compared to the perovskite and also defects that can act as electron trapping sites at the layer surface. Therefore, due to the lower mobility of ETL, carriers will accumulate at the interface in which the trapping sites are available. So, a high non-radiative recombination rate at the interface of ETL/perovskite is expected, which is considered as one of the most important losses in the PSCs18,19,20,21,22,23.

Nowadays, many efforts have been made to address this issue and optimize the TiO2 film24,25. An effective strategy to enhance TiO2 mobility is doping with electron-donating agents26,27. Doping not only increases the electron mobility of the TiO2 layer but also deactivates the traps at the layer surface28,29. Using metal quantum dots (QDs) as a doping agent offers an applicable and effective procedure to treat the TiO2 ETL layer. In 2021, Qureshi reported doping of TiO2 ETL by zirconium. They reported a better extraction rate for charge carriers in the PSC24. In 2020, Ishag investigated the modification of TiO2 ETL by doping Zinc nanoparticles25. In spite of the fact that the doping procedure looks easy since PSCs normally are processed from solution, its processing could be problematic. The most important problem could be the uniform distribution of the QDs in the ETL. The contact of these QDs with the FTO electrode is not favorable and can facilitate the penetration of holes into the cathode, but TiO2 ETL might block the holes30,31,32,33. Whereas, the presence of nanoparticles on the upper surface of the TiO2 film is required to facilitate electron transfer from the perovskite layer to the ETL, their presence at the interface of ETL/FTO is detrimental as mentioned above. Uniform doping deactivates traps and reduces non-radiative recombination at the ETL perovskite interface but makes hole permeation to the cathode easier. Thus, a new technique for optimal ETL doping is needed. This means an optimal electrode might not only facilitate electron extraction (subsequently decrease the interface recombination) but also might block the hole permeation into the cathode.

In this paper, to have better electron extraction and also prevention of hole permeation into the cathode, we investigated graded doping of the TiO2 layer with Pt QDs in which the doping is the highest at the perovskite side and the lowest at the cathode (FTO) side. Pt QD is chosen because it is one of the least reactive metals to oxygen and moisture. Our doping method of the TiO2 layer occur in the sintering process and during the process the QDs penetrate/diffuse into the TiO2 film and results in a reduction of non-radiative recombination and an improvement of ETL conductivity in an ambient air-processed PSC. Moreover, with this method, metallic nanoparticles are not present at the side of the ETL/electrode to allow hole transfer to the cathode anymore. In other words, the ETL blocks the holes effectively. A direct consequence of the mentioned effects is reflected in the efficiency and the stability of PSCs that is discussed in the following.

Perovskite solar cells were fabricated on Fluorine-doped Tin Oxide (FTO) substrates. To make it ready for TiO2 deposition, they are washed in a soap solution, cleaned in an ultrasonic bath in acetone and ethanol for 12 min, consecutively, dried in an oven (10 min), and treated in a UV-ozone photoreactor for 20 min.

TiO2 layer was spin-casted from the precursor including 350mL titanium [IV] isopropoxide and 25 mL HCl in 5mL of isopropanol onto FTO substrates at 3200 rpm for 30 s and then sintered at 500 °C for 35 min that results TiO2 thin layer about 60–70 nm. To prepare the Pt-QDs doped TiO2 layer, a layer of platinum (Pt) paste (with a concentration of 20 mg/ml with a particle size average of 5–8 nm) was deposited on the dried TiO2 film by doctor blade coating method and then sintered at 500 °C for 35 min. Sintering causes the diffusion of QDs into the TiO2 layer, thus resulting a non-uniform doping of the TiO2 layer in which the concentration of Pt QDs is higher at the perovskite side and lower at the other side. In other words, graded doping of Pt QDs occurs. To compare this non-uniform doping with the uniform doing of the TiO2 layer, uniformly doped TiO2 layers were also fabricated. To do this, we mixed the diluted Pt paste solution (8 mg/ml) with the solution containing TiO2 and spin-coated at 3000 rpm for 30 s followed by sintering at 500 °C for 35 min.

The perovskite layer is processed at normal ambient. The precursor solution was prepared by dissolving 36 mg of MAI: PbI2 = 1:1 in 1 ml N, N -dimethylformamide (DMF) solvent and stirred for 12 h at 60 °C. The prepared solution was spin-coated on the TiO2 film at 2000 rpm for 30 s and dried at 100 °C for 12 min. For the hole transport layer (HTL), two solutions were prepared. The first one was prepared by dissolving 90 mg spiro-MeOTAD powder in 1 mL of chlorobenzene, then 40 µL tert-Butylpyridine (TBP) was added. And the second solution was prepared by dissolving 510 mg Lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) in 1 mL Acetonitrile. Finally, all solutions were mixed together and spin-coated on perovskite layer at 3200 rpm for 30 s. To finalize the PSC structure, a silver electrode was coated on the HTL by doctor blade method using silver paste and ultimately the whole device dried at 90 °C for 30 min.

Low Angle X-ray diffraction (XRD) patterns were performed by PW1730-PHILIPS. For X-ray photoelectron spectroscopy (XPS) analysis, Thermo Scientific K-Alpha + was used. Surface morphology was investigated by atomic force microscopy (AFM, Multi-Mod ARA-AFM). Cross-section SEM image was measured by a scanning electron microscope (mira3-TESCAN). The absorption spectra were measured by an ultraviolet-visible (UV-vis) spectrophotometer (PerkinElmer Lambda 750). Steady-state photoluminescence (PL) spectra of the perovskite films on different ETL were measured by VARIAN CARY ECLIPSE. Current density-voltage (J-V) characteristics of PSCs were obtained using an IV-28 IRASOL source meter under AM 1.5G solar irradiation (1000 Wm− 2).

To test how electron extraction can be affected by doping, we fabricated three kinds of devices with different ETLs. The first one is our control device with TiO2 ETL which has no doping. The two other devices doped with Pt QDs but with different techniques; one of the techniques gives a uniform doping and the other results in a graded doping as shown schematically in Fig. 1a. This Figure beside the sequence of the layers, show energy alignment of the layers that will be discussed in the next paragraph. Figure 1b shows the cross-sectional SEM image of the device and according to the figure, the thickness of the perovskite layer is about 250 nm. Furthermore, Fig. 1c and d present the cross-sectional EDS mapping of Ti and Pt elements that confirms the graded doping of the TiO2 layer with Pt QDs. To have further proof of graded doping and its effect on the device performance, we have done a series of experiments like XRD and XPS that is discussed in the coming sections.

(a) Energy band diagram, (b) Cross-sectional SEM image, (c & d) Cross-sectional EDS mapping images of the PSC with graded Pt QD doped ETL.

It holds significant importance to know how the energy levels evolve with the doping effect. To understand this, light absorption of the PSC is measured and ultraviolet photoelectron spectroscopy (UPS) is done. The first one gives the absorption spectra, in which the optical band gap can be derived from the absorption edge, and the second measures the conduction band energy. The light absorption by PSC might not get affected much upon adding extra layers or application of extra procedures. Figure 2a compares the absorption spectrum of the above-discussed device with a TiO2-only device, uniformly doped TiO2 device, and graded doped TiO2 device. We found out that the graded doped device absorbs the light slightly more than the control device. But uniformly doped one absorbs much higher amount of light. As a result, the transparency of the graded doped TiO2 device is higher compared to the uniformly doped one.

In addition, as shown in the Tauc plot in Fig. 2b, the band gaps of the undoped and graded doped TiO2 are 3.31 eV and 3.22 eV, respectively, which shows that Pt doping has changed the band gap of TiO2 film and caused a better energy band alignment with perovskite film. On the other hand, the ultraviolet photoelectron spectra (UPS) of undoped and graded Pt-doped TiO2 were employed to calculate the valence band (EVB) energies. By using the equation EVB = hν – (Ecut off – Eonset), where hν is the photon energy of the excited light source (here 21.22 eV), we will be able to calculate the valance band energy. Ecut off and Eonset are obtained from Fig. 2c, d. The calculated EVB for undoped and graded Pt-doped TiO2 films were 7.49 and 7.32 eV, respectively. To calculate the conduction band (ECB), the value of the bang gap obtained from the Tauc plot in Fig. 2b is subtracted from EVB. Based on these findings, we set the energy levels as Fig. 1a illustrates with the structure of FTO/TiO2/MAPbI3/HTL/Ag.

(a) absorption spectra and Tauc plot (inset), (b) O 1s core level binding energy peak, and (c-d) The UPS spectra of pristine and graded doped TiO2 film.

To be sure that TiO2 ETL is deposited with a proper crystalline structure and also if Pt QDs doping is successful, XRD analysis is done. Figure 3a shows the XRD spectrum from the TiO2 ETL which shows proper stable anatase phase crystallization that is suitable for use as an electron transfer layer. Figure 3b shows the XRD pattern of a layer composed of Pt nanoparticles. Figure 3c demonstrates the XRD pattern of the planar TiO2 film after application and sintering of Pt paste on TiO2 ETL. If we compare all three patterns, we distinguish that both, the diffraction pattern of TiO2 and Pt nanoparticles, is present at the same time in the third spectrum. Therefore, we can conclude that the planar TiO2 film is doped properly. Now the question is how we could judge that the doping with this method is graded, meaning that we have a higher concentration of Pt QDs at the side of perovskite but the side of FTO cathode TiO2 ETL is almost free of Pt nanoparticles. To be sure about this, we reversed the order of the TiO2 /Pt paste structure. In other words, we made first Pt QDs layer first, and after sintering, TiO2 deposited and consequently sintered at 500 °C. We took the low-angle XRD spectrum out of this structure and did not get any sign of Pt nanoparticles and only the TiO2 pattern was observed (Fig. 3d). This means that the application of Pt pastes on top of the TiO2 layer accompanied by sintering causes diffusion of Pt inside the TiO2 layer so that we observe the said both structures at the same time. But the reverse structure spectrum indicates that Pt may diffuse inside the layer but not the whole layer to reach the FTO cathode. It may diffuse only a few nanometers which is confirmed by EDS map (Fig. 1c & d).

X-ray diffraction pattern from (a) TiO2 layer, (b) Pt, (c) Pt-doped TiO2 film, and (d) back side of Pt-doped TiO2 film.

For a more reliable and detailed investigation of doping and graded doping of Pt in the TiO2 film x-ray photoelectron spectroscopy (XPS) was employed. Figure 4a indicates that Pt QDs insertion does not impact the Ti 2p core level. The O 1s core levels of the TiO2 film are illustrated in Fig. 4b. As we see, an explicit change of the XPS spectrum happened after doping Pt QDs into the TiO2 film, and the shoulder at higher binding energy gets a little wider and bigger which is related to oxygen interaction with Pt34. Examination of the ratio of two peaks of O 1s indicates that Pt was doped into the TiO2 film. The peaks, at 532 eV and 529 eV in the spectra of O 1s, are related to the Pt-O and Ti-O bond in the TiO2 lattice, respectively. With regards to the constitution of Pt-O in the Pt-doped TiO2 film, it can be concluded that the new composite was produced. Another conclusion that can be drawn from XPS spectra is that Pt QDs doping has a significant impact on the TiO2 film’s surface oxygen vacancy deactivation. It is well-established that oxygen atoms can easily escape from the TiO2 structure, particularly on the layer surface. This leaves behind Ti atoms with an incomplete electron configuration (Ti4+) that can readily capture free electrons. Building upon previous research, we hypothesize that treating TiO2 with Pt partially converts Ti4+ to Ti3+ within the crystal lattice. This transformation is believed to neutralize electronic defects caused by missing oxygen atoms, similar to the effect of lithium doping in TiO235.

Likewise, to investigate that doping is graded, we etched 15 nm of TiO2-Pt film by the electron beam etching method and carried out the XPS analysis again. As we see in Fig. 4b, according to the intensity decrement of the peak at 530 eV, we conclude the bonds between Pt-O still exist so we see still the peak, but its intensity is lowered compared to the surface of the TiO2 film. Now the direct conclusion is that the density of Pt QDs in the depth of 20 nm of the TiO2 film is much lower than its surface and this implies that the doping of Pt QDs into the TiO2 film is graded. This further confirms the XRD and EDS mapping results that we discussed above.

XPS spectrum of undoped, uniformly doped, and graded doped TiO2, (a) Ti 2p, and (b) O 1s binding energy peak,

Figure 5a shows the J-V curves of PSCs with TiO2 and two differently Pt-doped TiO2 ETLs under light intensity of 1000 Wm− 2. These results clearly show that all the solar cell parameters including open circuit voltage (VOC), short circuit current density (JSC), and the power conversion efficiency (PCE) decrease for uniformly doped ETL but increase for graded doping at the ETL/perovskite interface. Uniformly doping of the perovskite deteriorates the performance of the cell (PCE = 9.21%) compared to the control undoped device (PCE = 12.92%). This could be attributed to the permeation of holes to the cathode and thus enhancement of recombination of the electron-hole pairs at the interface and nearby cathode which lowers the efficiency of charge extraction and consequently the cell efficiency. Therefore, from now on we exclude this device from our consideration and discuss the control device performance together with the graded doped one. As shown in Fig. 5a, the measured average PCEs of the PSCs with TiO2 and graded Pt-doped TiO2 ETLs obtained 12.92% and 14.36%, respectively. Measured PCE statistics of 15 fabricated devices are shown in Fig. 5b. It shows that graded Pt-doped devices are superior compared to the others. Furthermore, the PCE distribution of the PSC with graded Pt-doped TiO2 ETL compared to PSC with undoped TiO2 ETL is more concentrated. This means that the graded doping with Pt QDs in the ETL/perovskite interface yields better repeatability with a favorable impact on device performance. The reason could be because of the passivation of the electronic defects and trap states that exist at the interfaces as we rationalized from XPS spectra, and these states are common recombination centers which is considered bad for charge extraction and subsequently the cell’s efficiency. We talk about graded doping in which the density of quantum dots is higher at the interface and in none at the other end of TiO2. So, a higher amount of QDs at the interface can effectively passivate surface defects and also the TiO2 bulk traps. This deactivation of traps and passivation of surface defects will reduce the thermally-non-radiative recombination. Therefore, reduced recombination at the interfaces by optimizing the layers’ interface, results in minimizing recombination losses that leads to higher VOC. Moreover, the presence of the traps and defects lowers the effective electron mobility. To reach the trap-free current, traps might be eliminated completely but it is not possible completely to do so. But, a few times dilution or elimination of traps can lead to current enhancement considerably36. In the coming paragraphs by measuring electron transport, we show that traps are reduced by implementing QDs and this reduction leads to better conduction of electrons and consequently JSC enhancement. Table 1 summarizes the average photovoltaic parameters of all PSCs fabricated in the normal ambient.

(a) Current density–voltage (J–V) curves, (b) PCE statistics, (c) and (d) Reverse and forward J-V scans of the control and Pt-doped ETL device.

As we observed, the devices with uneven-graded doping give better efficiency than those devices with undoped and evenly doped ETL. We attribute this improvement in efficiency to the reduction of the recombination rate at the ETL/perovskite interface which is because of deactivation of electron traps at the oxide surface. This leads better to extraction of charges to the electrode. In the following, we focus on confirming this hypothesis that our method of doping eliminates traps and consequently reduces the non-radiative trapping in the interface, and also improves the charge extraction to the electrode. Moreover, we observe that hysteresis is reduced for the graded doped devices as compared to Fig. 5c and d. Since the hysteresis is considered bad for the PVs, we find out that graded doping of the perovskite layer improves the cell’s performance. The reason could be attributed to the elimination of trapped charges at the interface which changes the internal field of the device.

As shown schematically in Fig. 1a, XRD and XPS results, Pt nanoparticle concentration is higher at the interface of the TiO2 layer and the perovskite. But on the other side, the interface of FTO/ TiO2 is almost free of nanoparticles. We believe that sintering Pt QDs paste at 500 degrees burns paste extra ingredients and just leaves Pt nanoparticles at the surface of which some can diffuse inside the TiO2 ETL. This method of doping leaves a higher concentration of Pt-QDs at the TiO2 /Perovskite interface that by donating electrons to the trapping sites, trap states can be filled and therefore deactivated. On the other hand, the mobility of the ETL layer due to the introduction of doping states is enhanced and can improve the charge extraction to the cathode.

Measuring the external quantum efficiency (EQE) enables us to confirm the performance of PSCs relative to each other. As shown in Fig. 6a, the EQE of the graded Pt-QDs doped PSCs shows an increment in the long wavelength region. This declares that the extraction rate of charge carriers improved. In other words, a higher number of electrons and holes in the ETL-doped PSC compared to the undoped PSC is produced. This is clear from the short-circuit current in which the amount of the short-circuit current for the doped device is 18.80 mA/cm2 but for the undoped device is 16.82 mA/cm2. Similar to the JSC, VOC also increased for doped devices. This can be attributed to the deactivation of trap states at the TiO2 /perovskite interface. The presence of trap states and subsequently trapping of produced free electrons can perturb the internal field of the cell. This consequently decreases the open circuit voltage VOC thus the efficiency. The increment of VOC is desirable for all solar cells and empowers charge extraction in which we observed in our doped devices.

Figure 6b demonstrates the PL spectra of perovskite films deposited on the TiO2 ETL with and without Pt nanoparticles. Here, we see the PL intensity of the PSC device with Pt-doped ETL shows a significant quenching, which indicates that the traps are deactivated and free carriers do not recombine with deactivated traps anymore and electrons get extracted through ETL and the resulting current density will increase.

(a) EQE spectra and the integrated JSC of devices with TiO2 and TiO2 -Pt ETLs, (b) PL spectra of perovskite film (PF) with TiO2 and TiO2 -Pt ETLs.

To test how the conductivity of the TiO2 layer changes upon doping, we fabricated devices with FTO/ TiO2 /Ag structures. The slope of the graph in Fig. 7a is proportional to the conductivity of the sandwiched layer and as can be seen, the conductivity of ETL has increased with doping Pt, which can be concluded to increase the effective mobility of charge carriers. Obviously, this feature directly impacts the increment of the charge carriers’ extraction rate. Graded doping, unlike the uniform doping that allows holes to penetrate into the FTO cathode or to be recombined with trapped electrons, will block holes in the perovskite layer to be extracted to the anode. Therefore, drawbacks of uniform/even doping are eliminated.

Elimination of the traps can lead to a change in the slope and enhancement of the space charge-limited current (SCLC)37. Therefore, measuring the SCLC will show if the electron trap states are deactivated/eliminated. I-V measurements of the undoped and graded-doped devices (FTO/TiO2/ MAPbI3/PCBM/Ag and FTO/graded Pt QD: TiO2/ MAPbI3/PCBM/Ag) are shown in the Fig. 7b. The slope of the undoped device in the region of SCLC amounts to 3.62 and the doped one to 3.23. This decrement in the I-V slope can happen due to the decrement in trap state density and results in the current enhancement. So, this reduction of traps and passivation of the defects in the interface will lead to the enhancement of JSC.

The device with undoped TiO2 films exhibited a trap state density of 2.48 × 1015 cm− 3 and effective electron mobility, µe, of 1.24 cm2 (V− 1 s− 1) and for graded Pt-doped TiO2 films a trap state density of 1.67 × 1015 cm− 3 and effective electron mobility, µe, 2.05 cm2 (V− 1 s− 1) is calculated. The improvement of effective electron mobility, µe, in the Pt-doped TiO2 films mainly stems from the reduction of internal traps and the better-matched band structure, leading to efficient carrier transfer.

The ideality factor is a powerful tool to identify if the non-radiative recombination occurs in the device or not38,39,40. This can be measured by monitoring the VOC change under different light intensities. Figure 7c shows the VOC vs. light intensity for the PSCs with doped and undoped ETLs. The ideality factor of a semiconductor is a measure of how closely the diode follows the ideal diode equation (the ideality factor is 1kbT/q). If the ideality factor is higher than the ideal value, the non-radiative recombination happens in the diode. The ideality factor that we measured for the undoped devices is of the order of 1.80kbT/q, which clearly demonstrates non-radiative recombination rate is higher than the doped devices whose ideality factor is 1.5kbT/q. This can be attributed to the higher quality interfacial contact between ETL and perovskite achieved by graded doping of ETL. The reduction of non-radiative recombination, elimination, or deactivation of traps, is reflected in the ideality factor value change that we observe apparently for our device with doped ETL41,42. In the following, we took further techniques to confirm that non-radiative recombination at the interface of ETL/Perovskite is responsible for efficiency lowering and our doping method eliminates recombination at the interface.

(a) I-V characteristics of pristine and graded QD doped TiO2 ETL, (b) the electron-only devices space charge-limited current (SCLC) measurements, and (c) VOC versus light intensity curves, of control and Pt-doped ETL device.

Many researches have shown that structure uniformity greatly affects the performance of PSCs43,44,45. Normally, uniformity is the outcome of uniform crystallinity. To investigate the surface morphology of the TiO2 ETL upon different routes of doping, AFM (atomic force microscopy) images have been taken. Uniformity as mentioned has a great impact on the quality of the ETL/perovskite contact and consequently on the crystallization of the perovskite layer. As Fig. 8 demonstrates, the root mean square (RMS) of the pristine and graded Pt-doped TiO2 layer was 6.54 nm and 2.75 nm, respectively. The roughness of the doped TiO2 film is much lower than the pristine TiO2 layer, and this provides a much uniform interface and subsequently, the quality and crystallinity of the perovskite are further enhanced.

3D AFM images of (a) TiO2 film, (b) Pt doped TiO2 layer and perovskite layer deposited on (c) the TiO2 layer, and (d) the graded Pt doped TiO2 layer.

Also, the water contact angle (WCA) measurement in Fig. 9a indicates an increase in surface energy after doping and optimization of TiO2 surface morphology. The WCA measured before and after Pt-doping was 44° and 15°, respectively. This means that the wettability of the ETL is enhanced after doping and this results in a more uniform perovskite layer formation and finally a better PSC performance.

(a) Water contact angles of the TiO2 and TiO2 + Pt films. (b) Stability test of the control and Pt-doped ETL-based PSCs.

The final check was the stability check of the fabricated PSCs (Fig. 9b). We kept our devices after the first test for 2 weeks in the ambient air in dark conditions and 25 °C with 30% humidity. Results demonstrate that PSC with the graded Pt-doped ETL presents 91% of its primary PCE, whereas the control device PSC decayed to 78% of its primary PCE. Therefore, here we figure out that upon graded doping not only improvements like layer uniformity, trap deactivation, and higher electron mobility occur but also device stability enhances. The device stability improvement mainly can be due to the trap’s passivation inside the devices and the modified ETL/perovskite interface and increasing the quality of the contact surface of ETL and perovskite layers.

The conductivity of the ETL and HTL layers plays a crucial role in the performance of a solar cell since they have the duty of charge extraction in the device. A new approach has been taken to dope TiO2 ETL using Pt QDs. In this method, we pasted Pt QDs from paste compound on top of TiO2 ETL that followed by sintering the structure at 500 °C in which a graded Pt-doped ETL layer is obtained. A few techniques like XRD, XPS, and also cross-section SEM images with EDS mapping, all together show that QDs diffuse inside the TiO2 and result in graded doping. This route gives a uniform layer with high wettability that enables us to make a more uniform perovskite layer on its top which is critical to the performance of PSCs. The solar cell made of the mentioned doped ETL exhibits a PCE of 14.36%, a higher efficiency relative to the TiO2 ETL-based Device. The improved cell performance is attributed to the enhancements of electron mobility and trap deactivation on the oxide surface. Due to the improvement of mobility of electrons and deactivation of traps, reduction of the non-radiative recombination in the perovskite/ETL interface happens, which effects on the PSC performance shown on the efficiency and stability.

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

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The authors acknowledge the Ghazanfarian family through their financial support of the project and through their donation to the Amir-alam Ghazanfarian Electronic Materials Lab at the Institute for Advanced Studies in Basic Sciences (IASBS).

Department of Physics, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan, 45137-66731, Iran

Shahriar Mohammadi & Davood Abbaszadeh

Institute of Physics – CSE, Silesian University of Technology, Konarskiego 22B, 44-100, Gliwice, Poland

Sakineh Akbari Nia

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Shahriar Mohammadi: Investigation, Methodology, figures preparation, original draft, Validation, Writing & editing. Sakineh Akbarinia: Investigation, Original draft, figures preparation.Davood Abbaszadeh: Supervision, Writing - review & editing, Formal analysis, Methodology, Investigation, Conceptualization, Data curation.

Correspondence to Davood Abbaszadeh.

The authors declare no competing interests.

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Mohammadi, S., Akbari Nia, S. & Abbaszadeh, D. Reduction of recombination at the interface of perovskite and electron transport layer with graded pt quantum dot doping in ambient air-processed perovskite solar cell. Sci Rep 14, 24254 (2024). https://doi.org/10.1038/s41598-024-75495-x

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Received: 04 June 2024

Accepted: 07 October 2024

Published: 16 October 2024

DOI: https://doi.org/10.1038/s41598-024-75495-x

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