Enhanced efficiency of perovskite solar cells by PbS quantum dot modification

Applied Surface Science 487 (2019) 32–40

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Enhanced efficiency of perovskite solar cells by PbS quantum dot modification Xinyi Zhu, Bei Cheng, Xiaohe Li, Jianjun Zhang, Liuyang Zhang

T

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State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Perovskite solar cell PbS quantum dots Interface engineering Carrier extraction

Organic-inorganic perovskite solar cells (PSCs) have achieved high power conversion efficiency (PCE) over the last few years. However, interfacial recombination of PSCs and the defects of perovskite film still seriously restrain the performance of PSCs. Hence, we introduced PbS quantum dots (QDs) between perovskite layer and hole transport layer (HTL) to modulate delicately both the interfacial contact and the perovskite film. With the incorporation of PbS QDs, the PSCs presented enhancement in hole extraction and retardation in interfacial recombination. Besides, proper concentration of PbS QDs contributed to improved perovskite film morphology by enlarging the grain size and healing the pinholes. Benefiting from all of these advantages, devices with PbS QDs modification achieved an impressive PCE of 19.24%, superior to that fabricated without the modification.

1. Introduction

tunable optoelectronic properties derived from quantum size effect [29–31]. For instance, CuInS2/ZnS QDs were spin-coated at the interface between inorganic PSK layer (CsPbBr3) and carbon to improve the charge extraction from PSK layer. The incorporation of QDs not only notably enhanced the PCE of PSCs from 6.01% to 8.42%, but also improved the devices stability under high humidity. However, the PCE of inorganic PSCs still cannot rival with that of organic-inorganic hybrid PSCs, and the two-step synthesis of CuInS2/ZnS core/shell QDs was complicated compared to those synthetic methods applied to the ordinary QDs [32]. Considering the proper intermediate energy level and hole transfer ability, PbS QDs are suitable substitute for CuInS2/ZnS QDs [33]. Meanwhile, as one of the most attractive narrow bandgap semiconductor nanocrystals, PbS QDs have exhibited wide-ranged light absorption, huge bandgap tunability owing to the large Bohr exciton radius (18 nm) and the multiple-exciton generation (MEG) effect [34–36]. Furthermore, PbS QDs and MAPbI3 have good compatibility owing to their interconvertible crystal phase (tetragonal for MAPbI3, cubic for PbS), the six-coordinated lead atom and similar PbePb distance (5.97 Å for PbS, 6.26 Å for MAPbI3), which favors the formation of ‘dots-in-a-matrix’ crystals [37,38]. Yi et al. have reported a PSC, where the PbS QDs served as the inorganic HTL. Thanks to the retardation of carrier recombination and the increase of Voc, an improved PCE of 2.36% was achieved compared to the PSCs without HTL, indicating the effective hole transfer of PbS QDs. However, the PCE of PSCs with PbS QDs cannot match the PCE of PSCs with Spiro-OMeTAD, and the high-concentration (10 mg mL−1) of PbS QDs may slow the

Organic-inorganic hybrid perovskite solar cells (PSCs) have received tremendous attention owing to their superb power conversion efficiency (PCE) and great potential for industrial applications [1–3]. As the absorber layer of PSCs, metal halide perovskite (PSK), which generally has a molecular formula of ABX3, where A is MA+ or FA+ (MA+ = CH3NH3+, FA+ = CH(NH)2+), B is Pb2+ or Sn2+ and X is halide anion (Cl−, Br− or I−), presents superior optical and electronic characteristics, including large light absorption coefficient, high carrier mobility and long charge diffusion length [4–7]. And the PCE of PSCs has surged swiftly from only 3.9% in 2009 to exceeding 23% nowadays [8–10]. In spite of the significant achievement for PSCs, there still exist some obstacles, such as surface defects of PSK film, interfacial recombination of carriers and long-time stability of devices, which impede the further increase of PCE [11–14]. Based on the existing materials, interfacial engineering is a facile way to retard the carrier recombination significantly and thus enhance the PCE of devices since the interfaces between adjacent layers are essential to charge collection and transport [15–19]. Numerous materials have been reported as interlayers between PSK layer and hole transport layer (HTL) or electron transport layer (ETL), such as 3-aminopropyltriethoxysilane (APTES) [20], pyridinesulfonic acid (PA) [21], CuS [22], AuCl3-doped graphene [23] and inorganic quantum dots (QDs) [24–28]. Among them, the inorganic QDs show unique strengths, encompassing their convenient solution-processing, low cost and

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Corresponding author. E-mail address: [email protected] (L. Zhang).

https://doi.org/10.1016/j.apsusc.2019.05.067 Received 13 March 2019; Received in revised form 20 April 2019; Accepted 7 May 2019 Available online 08 May 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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UV–vis absorption spectra of the PSK films were measured by a UV–visible spectrometer (UV2600, Shimadzu, Japan). Steady-state photoluminescence (PL) spectra were acquired by a fluorescence spectrophotometer (F-7000, Hitachi, Japan) with 460 nm excitation wavelength. Photocurrent-voltage (J-V) characteristics and electrochemical impedance spectroscopy (EIS) were recorded by an electrochemical work station (CHI660C, Chenhua Instrument Crop., China). The J-V characteristics were measured under an illumination of 1 sun (100 mW cm−2, AM 1.5G) generated by a solar simulator (91160, Newport Corp., USA) and the active cell area was 0.09 cm2. The EIS was measured at dark under the bias of 0 V in the frequency range of 0.01–105 Hz. The incident monochromatic photoelectric conversion efficiency (IPCE) was tested using Newport's QE/IPCE Measurement Kit with monochromatic light from a 300 W Xe lamp (Newport, model no. 6258).

hole transfer due to their long-chain ligands [33]. Herein, we design a convenient method to reduce interfacial carrier recombination and enhance the device performance by incorporating the PbS QDs with proper bandgap at the interface between PSK layer and HTL. Resulting from the remarkable hole transport ability of PbS QDs and the suitable band alignment of PSCs, faster hole extraction and lower carrier recombination were realized. Therefore, we achieved a remarkable improvement of PCE by 10% for the PSCs with PbS QDs compared to that for the pristine PSCs. In addition, with the modification of the optimal concentration of PbS QDs, homogenous PSK film with large grains and a smooth surface was obtained, which is attributed to the great coordination between MAPbI3 and PbS. 2. Experimental section 2.1. PbS QDs preparation

3. Results and discussion PbS QDs were synthesized on the basis of a published method [36]. Briefly, 106 mg of sulfur powders was added in 4 mL of oleylamine (OLA) with stirring at 40 °C for 0.5 h. Meanwhile, 0.36 g of lead oxide (PbO), 1 mL of oleic acid (OA) and 15 mL of 1-octadecene (1-ODE) were mixed, stirred, and heated to 145 °C under nitrogen atmosphere for 1 h. Afterwards, 2 mL of solution of sulfur in OLA was injected into the PbO solution. The temperature was reset to 100 °C, and heated for 10 min. Subsequently, the QDs were purified and extracted. Finally, the PbS QDs were dispersed in hexane with different concentrations (0.5, 1.0 and 1.5 mg mL−1).

To investigate the influence of PbS QDs on the PSC performance, we adopted PbS QDs as an interlayer between PSK layer and HTL by spincoating. Distinguished by the concentrations of PbS QDs dispersion (0.5, 1.0 and 1.5 mg mL−1), the samples are denoted as pristine, P0.5, P1.0 and P1.5, respectively. Fig. 1a presents the XRD pattern of PbS QDs. There are four strong peaks at 26.1°, 30.1°, 43.2° and 51.1°, which correspond to the (111), (200), (220) and (311) plane of cubic PbS (JCPDS Card No. 65-0692), respectively [40]. Absorption spectrum in Fig. 1b shows that the first excitonic peak of PbS QDs resides at 830 nm, demonstrating an optical bandgap Eg = 1.49 eV [41]. The narrow bandgap of PbS QDs is also beneficial to the absorption of visible and infrared light. TEM image in Fig. 1c shows that PbS QDs exhibit a uniform distribution of particle size, and the inset presents that the average diameter of PbS QDs is 3.4 nm. Moreover, HRTEM in Fig. 1d shows clear lattice fringes with measured spacing of 0.34 and 0.30 nm, which correspond to the crystalline lattice (111) and (200) of PbS, respectively. And these results match well with the XRD pattern (Fig. 1a) of PbS, where PbS QDs show the cubic structure. The device structure of our solar cells in Fig. 2a displays the typical n-i-p planar structure, which is composed of FTO/bl-TiO2/MAPbI3/ PbS/Spiro-OMeTAD/Au. The cross-sectional SEM image of PSC is demonstrated in Fig. 2b. The thickness of bl-TiO2, MAPbI3 and PbS/SpiroOMeTAD layer were measured to be 40 nm, 310 nm and 140 nm, respectively. It has been reported that the highest occupied molecular orbital (HOMO) of PbS QDs is −5.2 eV (relative to vacuum energy level), which is close to the valence band maximum (VBM) of MAPbI3 (−5.4 eV) [37]. Because of the bandgap of PbS QDs is gauged to be 1.49 eV, the lowest unoccupied molecular orbital (LUMO) of PbS QDs is located at −3.7 eV. Considering the band alignment of each layer, the energy level diagram of our devices is displayed in Fig. 2c, where the hole transport path is from PSK to PbS/Spiro and then to the gold electrode, and the electrons move from PSK to bl-TiO2 and then to FTO substrate. Previous research has demonstrated that the accumulation of holes at the interface of PSK/HTL will lead to serious interfacial recombination, and thus restrain the device performance [42]. Owing to the better matching of energy level with the incorporation of PbS QDs, the higher hole mobility can be generated, and therefore the carrier recombination is retarded, which ultimately enhances the PCE of PSCs [32]. To further analyze how the morphologies of PSK films evolved in different concentrations of PbS QDs, the top-view FESEM images and AFM images are compared in Figs. 3 and 4, respectively. As reflected in Fig. 3, the pristine PSK film is composed of many small grains, with the average size of 190 nm, and the numerous grain boundaries (GBs) shown in Fig. 3a can induce crystal disorders and form unfavorable bonds (PbePb and IeI), meanwhile, the defect state at GBs of PSK can function as a trap state and slow down the hole transfer, which will

2.2. Device fabrication Fluorine-doped tin oxide (FTO) glass substrate was cleaned in succession with deionized water, ethanol and acetone by ultrasonication for 15 min each. After dried in oven at 80 °C, the substrate was further cleaned in ultraviolet-ozone for 15 min. Then, the TiO2 blocking layer (bl-TiO2) was deposited on the FTO glass by spin-coating according to the reported method [39]. The PSK precursor solution was prepared by dissolving 0.484 g of PbI2 and 0.159 g of CH3NH3I in DMF/DMSO mixed solution (4:1 v/v) and stirring at 60 °C for 1 h. The solution was spin-coated on the FTO/TiO2 substrate at 4000 rpm for 30 s, and 200 μL of toluene was dropped on the substrate at 5 s. Then, the substrate was annealed at 100 °C for 10 min. Different concentrations of PbS QDs were dynamically spin-coated on PSK film at 2000 rpm for 30 s. After heating for 2 min in 100 °C, Spiro-OMeTAD solution (74 mg of SpiroOMeTAD dissolved in 1 mL of chlorobenzene with the addition of 28.8 μL of TBP and 17 μL of Li-TFSI/acetonitrile (520 mg mL−1)) was deposited on PSK/PbS film by spin-coating at 3000 rpm for 30 s. By contrast, the pristine sample was spin-coated the Spiro-OMeTAD without the PbS QDs layer. Finally, a 50 nm-thick counter electrode (Au) was deposited on Spiro-OMeTAD layer by thermal evaporation. 2.3. Characterization UV–vis absorption analysis of PbS QDs in hexane dispersion was performed by UV–vis spectrophotometer (Lambda 750 S, PerkinElmer, USA). Electronic images of PbS QDs, PSK films and PSCs were collected by high-resolution transmission electron microscope (HRTEM, Titan G2, FEI, USA) and JSM-7500F field emission scanning electron microscope (FESEM, JEOL, Japan). Contact potential differences (CPDs) were measured via a gold probe (3 mm diameter, Instytut Fotonowy, Poland) as the reference and a Kelvin control (Instytut Fotonowy, Poland) with a sensitivity of 1 mV and 460 nm excitation light on the indium-doped tin oxide (ITO) glass substrates. The surface of PSK films was evaluated by a multimode 8 atomic force microscope (AFM, Bruker, USA) in Scan Analyst mode. The contact angles were obtained from contact angle meters (Theta Lite, Blolin, Finland). X-ray diffraction (XRD) patterns were obtained by a D/MAX-RB X-ray diffractometer (Rigaku, Japan). 33

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Fig. 1. (a) XRD pattern of PbS QDs. (b) Absorption spectrum of PbS QDs dispersed in hexane. (c) TEM image of PbS QDs. Inset: size distribution of PbS QDs. (d) HRTEM image of PbS QDs.

Fig. 2. (a) Schematic diagram of the device architecture. (b) Cross-sectional SEM image and (c) energy level diagram of PSC incorporated with PbS QDs. 34

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Fig. 3. Top-view FESEM images of (a) pristine PSK film and (b–d) PSK films incorporated with different concentrations of PbS QDs.

Fig. 4. AFM images of (a) pristine PSK film and (b-d) PSK films spin-coated with different concentrations of PbS QDs.

aggravate the trap-assistant recombination and decrease the PSC performance [43–45]. After covered by PbS QDs interlayer, the mean grain size increases to 290 nm for P1.0 film. With even higher concentration of dispersion, indiscernible change of the grain size is attended. Apart from the rise in grain size, the drop in GB is apparent. It can be concluded that PbS QDs play a significant role in the improvement of PSK films. As mentioned in the previous study, PbS has a good compatibility with MAPbI3, such as the similar crystal structure, the same coordination of Pb atoms, and the similar PbePb distance [37]. Meanwhile,

Table 1 Roughness valuesa of PSK films without and with PbS QDs. Roughness Ra (nm) Rq (nm)

Pristine

P0.5

P1.0

P1.5

10.3 13.1

9.5 11.9

8.1 10.3

9.0 11.4

a Ra and Rq stand for arithmetic mean deviation and root mean square deviation, respectively.

35

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Fig. 5. Static contact angles of water droplet on (a) pristine PSK film and (b–d) PSK films covered with different concentrations of PbS QDs.

Fig. 6. (a) CPD image of pristine PSK film and P1.0 sample. (b) Schematic of the energy band position for PSK and PbS.

gradually with the corporation of PbS QDs [13]. In other words, PbS QDs promote the movement and annihilation of GBs, and consequently increase the grain size and reduce the GBs, which will be advantageous to retard the trap-assistant recombination and enhance the open circuit voltage (Voc) of devices [28]. AFM image in Fig. 4a shows that the pristine film contains many pinholes, which indicates the inhomogeneous film coverage, and the pinholes can become trapping centers during the carrier movement, which will be unfavorable to obtain the high PCE of PSCs. Encouragingly, when the concentration of PbS QDs increases, the amount of pinholes is reduced, which can be attributed to the good coordination of PbS QDs and MAPbI3. Furthermore, roughness indicators of each sample, attained from the AFM images, suggest that the Ra and Rq of PSK films dwindle initially and rise later with increasing the concentration of PbS QDs. As shown in Table 1, the P1.0 film has the minimum Ra and Rq, indicating its smoothest surface among all the samples, and the drop of Ra and Rq also demonstrates that PSK film with fewer defects can be achieved by the incorporation of PbS QDs, which is in consistency with the SEM images [13]. However, too high concentration of PbS QDs is harmful, and the reason is that excessive PbS QDs can agglomerate on the films, which leads to the rise of roughness [28]. Since moisture corrosion is the key culprit which leads to the

Fig. 7. XRD patterns of PSK films on FTO glass without and with different concentrations of PbS QDs.

since GB passivation can be achieved by replacing the I− in IeI bond with Cl− and O2−, and S exhibits small atomic radius similar to Cl and valence electrons same with O, S2− can also contribute to the GB passivation [45]. By taking all of these advantages, when the films are annealed twice, the adjacent grains tend to merge into each other 36

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Fig. 8. (a) UV–vis absorption spectra of PSK films on FTO glass without and with different concentrations of PbS QDs. (b) Steady-state PL quenching spectra of samples without and with PbS QDs interlayer on the FTO/MAPbI3/Spiro-OMeTAD substrates.

Fig. 9. (a) Current-voltage characteristics of the optimized solar cells with and without PbS QDs. (b) IPCE spectra and Jcal of pristine and P1.0 solar cells. J-V curves of (c) pristine and (d) P1.0 device under reverse scan and forward scan.

sample, the CPDs of them are measured via a gold probe as the reference and a Kelvin control on the ITO substrates. And the work function (W) of samples is calculated according to the following equation.

Table 2 Photovoltaic performances of PSCs with and without PbS QDs. Devices

Jsc (mA cm−2)

Voc (V)

FF (%)

PCE (%)

Pristine P0.5 P1.0 P1.5

22.71 22.77 23.17 22.97

1.08 1.11 1.14 1.12

71.20 72.00 72.83 72.43

17.46 18.20 19.24 18.63

Wsample = e × CPDsample + Wprobe

(1)

where e and CPDsample are the electron charge and CPD of the sample, respectively. Wsample and Wprobe stand for the work function of the sample and the gold probe (4.25 eV, vs. vacuum), respectively. Fig. 6a exhibits the CPD image of pristine and P1.0 samples. In dark condition, Wpristine and WP1.0 are 4.69 and 4.58 eV, respectively. While under illumination, Wpristine and WP1.0 increase to 4.78 and 4.67 eV, respectively [48]. The rise of work function under light illumination reveals that the electrons transfer from the valence band to the conduction band, leading to the drop of Fermi level (EF). Taken the above results together, the PSK is found to be a p-type semiconductor. Yet, the down-shift of work function of P1.0 sample is attributed to electron diffusion from PbS QDs to PSK to balance the Fermi level, illustrating

decomposition of PSK and thus the reduction of PSCs efficiency, we carry on the water contact angle measurements for PSK film with and without PbS QDs (Fig. 5) [13,46]. With increasing concentration of PbS QDs, the contact angles increase monotonously from 60.43° to 78.76°. The reasons are speculated as follows: one is that the surface contact for the water droplet turns difficult owing to the smoother PSK film; another is the eminent ability of long-chain hydrophobic ligands of PbS QDs [34,47]. To investigate the electronic properties of pristine PSK film and P1.0 37

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Fig. 10. Photovoltaic metrics of devices. Average performance parameters are obtained based on 40 tested cells.

carrier transfer [38,45]. To further analyze the optical property of PSK films, UV–visible spectra are measured as shown in Fig. 8a. The absorption onsets of the PSK films are located at 780–785 nm, showing that the bandgap of MAPbI3 is 1.58–1.59 eV [51]. Obviously, no shift of absorption onsets indicates that the impact PbS QDs interlayer on the bandgap of PSK is marginal. Nevertheless, it should be noted that, owing to the coverage of PbS QDs, which contributes to larger grains and fewer pinholes in PSK film, the significantly enhanced absorption in visible light is manifest for the P0.5, P1.0 and P1.5 samples. Moreover, the small upshift of absorption in the near-infrared is presumably because of the narrow bandgap of PbS QDs [38]. As mentioned, high concentration of PbS QDs dispersion has an adverse effect on the growth of grains. For this cause, the absorption of P1.5 film is weaker in comparison with that of P1.0. In addition, the steady-state PL is conducted to gain deep insight into the function of PbS QDs on carrier segregation of PSK film. As shown in Fig. 8b, the PL peaks of all samples are located at approximately 770 nm, which correspond to the absorption onsets displayed in Fig. 8a. Furthermore, the strong quenching of PL intensity is exhibited with increasing concentration of PbS QDs, indicating that the PbS QDs interlayer can facilitate the hole transfer from active layer to SpiroOMeTAD layer, which is related to the high hole mobility of PbS QDs and the improved band alignment originated from the suitable bandgap of PbS QDs [33,52]. However, the higher PL intensity for P1.5 film compared to that of P1.0 is mainly because of the excess of PbS QDs with long-chain ligands, which will retard the hole diffusion in the interface of PSK/HTL. Considering the abovementioned better properties in different aspects achieved by PSK films with PbS QDs, we assembled solar cells to investigate whether the embedment of PbS QDs elevates the optoelectronic quality of devices. The J-V curves of best-performing PSCs without and with PbS QDs are displayed in Fig. 9a, and the detailed photovoltaic metrics are summarized in Table 2. After spin-coating of PbS QDs, the Voc of device is boosted from 1.08 V (pristine) to 1.14 V (P1.0). However, the further increase of concentration (1.5 mg mL−1) leads to the decrease of Voc (1.12 V). On account of the obvious

Fig. 11. Nyquist plots of pristine and PbS QDs incorporating PSCs under dark condition. Inset: the equivalent circuit model.

that PbS QDs exhibit higher EF than that of MAPbI3 (Fig. 6b) [48]. Therefore, the interlayer of PbS QDs can facilitate the separation of photo-generated electron-hole pairs, and ultimately improve the device performance. For the sake of identifying the crystal structures of PSK films, XRD analysis is employed and the results are shown in Fig. 7. The main peaks centered at 14.0°, 28.4° and 31.9° can be well assigned to the (110), (220) and (310) planes of tetragonal MAPbI3 [25,39]. Moreover, in comparison with the pristine films, owing to the better surface morphology shown in Figs. 3–4, the intensities of the first three peaks is markedly enhanced for the films covered with PbS QDs, illustrating the superior crystallinity of MAPbI3, which would be beneficial for the charge generation [49,50]. The strongest intensities of the first three peaks are achieved when the dispersion concentration of PbS QDs increases to 1.0 mg mL−1 (P1.0). However, a further increase of concentration inflicts the drop of peak intensity for P1.5 film because the excessive substitution of I− by S2− distorts the crystal structure of MAPbI3, which will inhibit the merging of grains, and thus impede 38

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work puts forward a simple yet efficient method for fabricating highefficiency PSCs with the aid of quantum dots.

improvement of Voc, PCE of devices is significantly enhanced. What is more, the champion device with P1.0 PSK film delivers a PCE of 19.24%, with Voc of 1.14 V, short-current density (Jsc) of 23.17 mA cm−2 and fill factor (FF) of 72.83%, which exhibits a remarkable increase of PCE by 10% in contrast with that of the pristine one (17.46%). In fact, the notably improved Voc is mainly attributed to the following two reasons: one is that the PbS QDs interlayer can effectively promote the hole diffusion and retard the interfacial recombination between PSK layer and HTL; the other is that the homogeneous active layer with larger grains and less pinholes obtained by the incorporation of PbS QDs can obviously reduce the trapping states in PSK film and generate more electron-hole pairs under illumination. In accordance with the slightly improved Jsc in J-V measurement, IPCE for P1.0 sample shows a small increment compared to the pristine one, and the integrated current densities (Jcal) for P1.0 (22.56 mA cm−2) and pristine sample (21.61 mA cm−2) based on the IPCE curves match well with the J-V curves (Fig. 9b) [44]. In addition, for pristine and P1.0, the hysteresis of the PSC with best performance are investigated by the reverse-forward J-V curves (Fig. 9c–d). Hysteresis index (HI) of the device is defined by HI = (PCERev − PCEFw) / PCERev, where PCERev and PCEFw represent PCE of reverse scan and forward scan, respectively [53]. Similarly, with the covering of PbS QDs, the HI decreases from 25% (PCEFw = 13.10% and PCERev = 17.46%) for pristine device to 16% (PCEFw = 16.19% and PCERev = 19.24%) for P1.0, which is owing to the strong GB passivation in PSK film and valid interfacial recombination retardation between PSK layer and HTL [54–56]. To examine the reproducibility of our PSCs, the photovoltaic metrics of 40 individual devices of each type are tested and summarized in Fig. 10. The relatively narrow distribution of photovoltaic parameters illustrates the good reproducibility of P1.0 devices [57]. Nyquist plots of the four devices without bias voltage are presented in Fig. 11, all of which show a clear semicircle in the low frequency region. And the inset shows the equivalent circuit, where Rs and Rrec stand for series resistance and recombination resistance, respectively [58]. In view of the same structure of our devices, the difference of Rrec comes from the change of interfacial recombination of photo-injected electrons with holes between PSK layer and HTL owing to the introduction of PbS QDs interlayer. On the basis of equivalent circuit, the Rrec of each device is obtained from the fitted impedance spectra, where the PSCs with PbS QDs (132, 188 and 149 kΩ for P0.5, P1.0 and P1.5) have larger Rrec values in contrast with the pristine PSCs (94 kΩ). The remarkable enhancement of Rrec corroborates the retardation of interfacial recombination, and the main factor is the improved interfacial contact and facile carrier transport between PSK layer and HTL. However, it should be noted that the higher concentration of PbS QDs (1.5 mg mL−1) leads to the decrease of Rrec, indicating the hindrance of hole transfer, which is originated from the aggregation of inordinate PbS QDs and their long-chain ligands. Overall, proper concentration of PbS QDs plays a significant role in smoothing the surface of PSK film and accelerating the carrier transfer from PSK layer to HTL.

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4. Conclusion In summary, by introducing the PbS QDs interlayer between the PSK and Spiro-OMeTAD layer via spin-coating, we propose and verify a practical and feasible way to fabricate PSCs with high efficiency. The addition of PbS QDs significantly enhances the light absorption of PSK films. Moreover, PbS QDs interlayer also contributes to the larger recombination resistance and faster carrier transport. Integrating all these merits, the PSC with the 1.0 mg mL−1 PbS QDs dispersion shows impressive PCE of 19.24% in comparison with the PCE of 17.46% for the pristine one. Such enhancement of photovoltaic parameters is mainly accredited to two reasons: one is the better band alignment of PSC caused by PbS QDs with proper bandgap; the other is the homogenous PSK film with smooth surface and great crystallinity of MAPbI3. This 39

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