Efficiency enhancement of perovskite solar cells via electrospun CuO

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Surfaces, Interfaces, and Applications

Efficiency enhancement of perovskite solar cells via electrospun CuO nanowires as buffer layers Qinjun Sun, Shaolong Zhou, Xiaolei Shi, Xiaochun Wang, Liyan Gao, Zhanfeng Li, and Yuying Hao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19335 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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ACS Applied Materials & Interfaces

Efficiency enhancement of perovskite solar cells via electrospun CuO nanowires as buffer layers Qinjun Sun*,Shaolong Zhou, Xiaolei Shi, Xiaochun Wang, Liyan Gao, Zhanfeng Li, Yuying Hao*

Key Laboratory of Advanced Transducers and Intelligent Control System, Ministry of Education, Shanxi Province, College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, China. E-mail: [email protected]; [email protected] KEYWORDS: perovskite solar cell, CuO nanowires, electrospin, interface, buffer layers

ABSTRACT: CuO nanowires (NWs) with the diameters ranging from 130 to 275 nm have been successfully prepared by electrospinning and calcination technique, followed by a calcination process. Inverted planar heterojunction perovskite solar cells

(PSCs)

with

the

structure

of

ITO/CuO

NWs/PEDOT:PSS/CH3NH3PbI3/PCBM/Bphen/Ag were designed, achieving a best power conversion efficiency (PCE) of 16.87% , which is 21% improvement compared with that of the control PSCs without CuO NWs. By the characterizations of optical microscope, X-ray diffraction (XRD) and scanning electron microscopy (SEM), it is found that CuO NWs have uniform morphology and orderly arrangement. The electrochemical impedance spectrometry (EIS) and external quantum efficiency (EQE)

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were used to reveal the effect of CuO NWs on the performance of PSCs. By compared with ZnO NWs with the same diameters and quantitative analysis based on a simple model, we conclude that the improvement of PCE by about 13% can be ascribed to the increase of the PEDOT:PSS/CH3NH3PbI3 interface area, and the remaining increase of 8% are attributed to the higher hole mobility of CuO NWs/PEDOT:PSS composite film. The results indicate that the efficiency of PSCs will have a significant enhancement when the optimal CuO NWs are introduced into the charge transport layer.

INTRODUCTION

Since it was first reported in 2009, perovskite solar cells (PSCs) have been developed in a terrific speed in recent years due to their high charge mobility, broadband absorption, low cost and ease of fabrication1-4. Especially, the power conversion efficiency (PCE) of lead halide perovskite solar cells has skyrocketed from 3.8% to over 22%5,6. Because of the simple processing procedure and little hysteresis, inverted planar heterojunction PSCs have been widely studied with the structure of ITO/ PEDOT:PSS/CH3NH3PbI3/PCBM/cathode. However, the highest occupied molecular orbital (HOMO) of PEDOT:PSS is only 5.2 eV, which leads to low open-circuit voltage (Voc)7-9. In addition, PEDOT:PSS usually causes the degradation of PSCs due to its acidity and hygroscopicity10. High cost and low stability of the commonly used organic hole transport materials (HTMs) limit the commercialization of PSCs. Compared with organic HTMs, transition metal oxides, such as MoO311,12, V2O313,


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and NiO14, are more promising because of their high optical transmittance, high stability, high hole mobility and low cost. Solution-processed CuOx have been used as HTMs in PSCs, achieving the average PCEs of 13.35%15 and 14.61%16. Copper oxide (CuO) is well-known as p-type semiconductor and has the valence band of 5.4 eV, which matches well with that of CH3NH3PbI3. Therefore, CuO could be an ideal HTM for PSCs. Nano-materials such as nano-metals or oxides17-20, quantum dots21 and nanowires22 have been employed to modify the interfaces of PSCs due to their unique properties. In comparison with the nanoparticles, one-dimensional NWs have a large specific surface area, more conductivity and lower charge carrier recombination 23

. However, there are few works on CuOx NWs used as HTMs in PSCs. Meanwhile,

the effect of nanowires on solar cells is still not clear.

In this work, CuO NWs with an optimal diameter of about 190 nm were prepared via electrospinning technique, followed by calcination. Electrospun technique have been developed rapidly in recent years and applied in optoelectronics devices widely, due to its simple manufacturing equipment, low cost, diverse of spinable materials and controllability 24, 25. The CuO NWs with different coverage were used in PSCs as anode buffer layer and an average PCE of 16.35% over 30 cells with optimal coverage of CuO NWs was obtained, which is an obvious increase compared with that of the reference cells (13.50%). This enhancement can be ascribed to the increase of hole transport and extract ability. By quantificational calculation based on a simple model, we found that the coverage and the diameter of NWs are the main factors for the increase of hole extract. Further studies still need to be carried out for investigating



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the effect mechanism of nano-wires on the performance of PSCs.

2. EXPERIMENTAL SETION

2.1. Materials

Most of the reagents and solvents were purchased from chemical companies (Shanghai Aladdin biological technology co., LTD), including poly(vinylpyrrolidone) (PVP, average Mw=58,000, Aladdin), alcohol (99%, Aladdin), dimethyl formamide (DMF, ≥99.9%, Aladdin), dimethyl sulfoxide (DMSO, ≥99%, Aladdin), and sec-butanol (99.5%, Aladdin). The other reagents were purchased from other companies, such as Cupric nitrate (99%, Tianjin Kay Tong Chemical Reagen), and Acetic

acid

(99%,

Kermel).

Poly

(3,4-ethylenedioxythiophene)

poly

(styrenesulphonate) (PEDOT:PSS, Clevios PVP AI 4083) was purchased from Heraeus

(Germany).

The

CH3NH3I

(MAI,

99%),

PbI2

(99.99%)

and

phenyl-C61-butyric acid methyl ester (PC61BM, 99.5%) were supplied by Xi’an Polymer Light Technology Corp. All materials were used as received, without further purification.

2.2 Synthesis of CuO NWs

The CuO NWs were prepared using the electrostatic spinning equipment (HD-2335, Ucalery). Polymer concentration, distance between the spinneret and the substrate, applied voltage, and flow rate have been studied in detail to obtain optimal nano-fibers (Figure S1-S4). Typically, 0.4 g of Cu(CH3COO)2•H2O powders was


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dissolved in 10 mL ethanol and then 0.69 g of PVP was added. To form a homogeneous precursor solution, the mixture was stirred at about 20

for 10 hours.

Subsequently, the resulting solution was electrospun via the electrospinning apparatus. 5 mL of the precursor solution was taken into a syringe (fixed on a spring pump) and then

a

hemispherical

droplet

presented

on

the

tip

of

syringe.

The

spinneret-to-substrate distance, flow rate and applied voltage were set as 12 cm, 0.1 mm/min and 10 kV, respectively. The hemispherical droplet would be polarized under such a high voltage and thus turned a Taylor cone. Charged jets on the bottom of Taylor cone would produce when the repulsive electrostatic force is larger than the surface tension. The jets often pass in a nearly straight line before bending into a complex path in the electric field. The volatile solution underwent volatilization during the jets in the air26. Finally, the fiber films with the diameters at the nanosized level were obtained on the collector. In order to obtain the pure CuO NWs, the as-prepared PVP/Cu(CH3COO)2 composite fibrous membranes were heated on a hot plate at 150

for 15min, and then calcined at 500

for 2 h in air.

2.3 Fabrication of perovskite solar cells

The detailed preparation procedures were shown in Figure 1a. The ITO glass substrates were cleaned sequentially in detergent, clean water, alcohol, and isopropanol. Afterwards, the substrates were attached to the collector of electrostatic spinning. The uniform CuO NWs with the desired diameter (e.g. about 190 nm) were obtained on the ITO glass by carefully adjusting the spinning distance, flow rate and



applied voltage. In the premise of these optimizations, the receiving time was adjusted to tune the coverage of NWs on the ITO glass. Subsequently, the PEDOT:PSS layer was spun onto the CuO NWs at 5000 rps for 30 s in air, followed by the perovskite precursor solution spin-coating at 6000 rps for 30s in a nitrogen glove box. Here the perovskite precursor solution was prepared by dissolving PbI2 and CH3NH3I with molar ratio of 1:1.1 in the mixture of DMF and DMSO with the volume ratio of 9:1. After the perovskite spin-coating, the sec-butyl alcohol was added rapidly on the rotating perovskite film with a delay time of 7–9 s to induce the fast crystallization of perovskite. The specific delay time depends on the concentration of the perovskite precursor solution. After that, the resulting films were transferred on a hot plate, first annealed at 100 then annealed at 100

for 10-20 min in ambient air (real-time humidity of ~60%) and for 5-15 min in DMSO atmosphere27. Next, the PCBM was

spun at 2500 rps for 30s and then Bphen as interfacial layer was spun at 6000 rps for 30s to collect electron efficiently8. Finally, a 100 nm silver electrode was deposited by thermal evaporation. The finished PSCs based on CuO NWs/PEDOT:PSS as a composite hole transport layer was shown in Figure 1b.

Figure 1. (a) Schematic illustration of the main fabrication procedure. (b) Schematic device structure of the proposed PSC.


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2.4 Characterization

The film thickness was measured with the stylus profiler (Dektak XT, Bruker). The surface morphology was analyzed using optical microscope (ECLIPSE LV150, Nikon), scanning electron microscopy (SEM, Jeol JSM-7100F) and atomic force microscopy (AFM, SPA-3000HV). The transmission spectra were recorded by R1 Macroscopic Angle Distinguish spectrophotometer (Shanghai Ideaoptics Corporation). The steady-state and transient-state photoluminescence (PL) spectra were measured using a PG2000-Pro-EX spectrophotometer (Shanghai Ideaoptics Corporation) and transient

fluorescence

spectrometer

(FLS980,

Edinburgh

Instruments,

EI),

respectively. The X-ray diffraction (XRD) was recorded using a Rigaku D/Max-B X-ray diffractometer. Current density-voltage (J-V) characteristics were measured using a Keithley 2400 source meter under a simulated AM 1.5G solar irradiation (ABET Sun 3000). The active area of device is defined as 0.04 cm2 by a black mask in J-V measurement. The external quantum efficiency (EQE) was measured using a power source (Zolix Sirius-SS) with a monochromator (Zolix Omni-λ). All measurements were performed in air environment without encapsulation.

3. RESULTS AND DISCUSSION

In order to avoid the adverse effect on the crystallization of perovskite layer, the thin CuO NWs layer was introduced into PSCs. We chose the receipt time of 10 s, 25 s, 45 s and 60 s to control the coverage of CuO NWs on the ITO glass substrate, as shown in Figure 2. The coverage of corresponding CuO NWs was estimated as 2.1%, 4.2%,



6.3% and 9.5%, respectively. Here, the coverage, i.e. to the ratio of the occupied area of nanowires to ITO area on the substrate, was calculated by the pixel count of different colors in optical microscopy images. It can be observed that the NWs are arranged orderly on the substrate in a straight manner, and the number of CuO NWs increases linearly with the increase of receipt time. The statistical distribution of the diameters was calculated by a simple software of Nano Measurer, as shown in Figure 2(e). More specifically, the line width of every CuO NW displayed in the optical microscopy image were measured automatically using this software by setting a ruler, and then the statistical distribution of diameters were offered automatically. The diameters of CuO NWs are distributed from 130 nm to 275 nm, and the majority (about 90%) are concentrated at 190±20nm, which indicates that the prepared CuO NWs are uniform, having an average diameter of about 190 nm. The X-ray diffraction (XRD) of the NWs was displayed in Figure 2(f). The characteristic diffraction peaks located at 2θ =35.47°, 38.77° and 48.82° are ascribed to (002), (110) and (-202) lattice planes of monoclinic CuO, respectively. The characteristic diffraction peaks can be indexed to the standard monoclinic CuO28, and there are no additional peaks of impurities, indicating that it is pure CuO. No CuO2 characteristic diffraction peaks are found in the XRD patterns, which confirms that the PVP/Cu(CH3COO)2 composite films are oxidized completely and pure CuO NWs are obtained.


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Figure 2. (a-d) Optical microscopy images of CuO NWs with different coverage of 2.1%, 4.2%, 6.3% and 9.5%. (e) Histograms of CuO NWs diameters distribution calculated by Nano Measurer. (f) XRD patterns of the CuO NWs.

Figure 3. (a) J–V characteristics of the best PSCs for each type with different coverage of CuO NWs, i.e. different coverage of CuO NWs under 100 mW/cm2 illumination (AM 1.5G), (b) statistical distribution of (JSC) and (c) statistical distribution of (PCE) versus the coverage of CuO NWs. The results come from 150 cells (30 cells for each type).

The current density-voltage (J-V) characteristics of the devices with the structure of ITO/CuO NWs(t)/PEDOT:PSS/CH3NH3PbI3/PCBM/Bphen/Ag are shown in Figure 3(a), where t represents the coverage of CuO NWs on ITO, e.g., t=2.1%, 4.2%, 6.3%



and 9.5%. The J-V curve of PSCs with only PEDOT:PSS as hole transport layer is also presented as reference. The statistical distribution of Jsc and PCE for each type of devices are displayed in Figure 3b and Figure 3c, respectively. The statistical photovoltaic parameters for each type of device with different amount of CuO NWs are summarized in Table 1. Under one sun simulated illumination, the reference device yields an average open circuit voltage (VOC) of 0.97 V, an average short circuit current density (JSC) of 18.33 mA/cm2 and an average fill factor (FF) of 0.76, and then an average PCE of 13.50 %, being similar to those of Ref.16. Generally, when CuO is used as hole transport layer instead of PEDOT:PSS, VOC is obviously changed as the valance band of CuO is lower than the HOMO of PEDOT:PSS15-16. However, when CuO NWs were used in our experiment, VOC keeps nearly a constant of 0.97 V. The differences imply that our prepared CuO NWs do not change the anode work function, because CuO NWs did not form continuous film (see Figure 2). With the increase of CuO NWs coverage, the average JSC first increases from 18.33 mA/cm2 to 21.72 mA/cm2, and then declines to 17.98 mA/cm2. The PCE obtains an optimal average value of 16.35%, when t = 4.2%, with an enhancement of over 21% compared with that of reference devices. Moreover, it can be found from Table S1 that the corresponding device has the lowest series resistance (Rs). The above results demonstrate that CuO NWs as anode buffer layer is feasible to improve the efficiency of PSCs.


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Table 1. The statistical parameters from 30 solar cells for each type NWs-coverage 9.5% 6.3% 4.2% 2.1% 0

VocÄVÅ 0.97±0.01 0.97 ±0.01 0.98 ±0.01 0.97±0.01 0.97±0.01

JscÄmA/cm2Å 17.98±1.2 20.97±1.5 21.72±1.1 19.80±1.3 18.33 ±1.3

FF 0.75±0.01 0.77±0.01 0.77±0.01 0.76±0.01 0.76±0.01

PCE (%) 13.01 ±0.61 15.70 ±0.48 16.35±0.52 14.69±0.47 13.50±0.50

It is well known that, the crystallization of perovskite layer is a major factor, determining the efficiency of PSCs29,30. When adding CuO NWs with diameter of about 190 nm under the perovskite layer, could it improve the crystallization of perovskite film for higher efficiency of PSCs? The morphologies of perovskite films with and without CuO NWs were studied by scanning electron microscopy (SEM) and XRD, as shown in Figure 4. The top-view SEM images of the perovskite film without NWs are shown in Figure 4a. The surface of the reference perovskite film is flat and free of pinholes, exhibiting smooth morphology and large crystal grain with an average grain size of 765 nm. When CuO NWs are added (Figure 4b), the surface of perovskite keeps smooth and dense morphology. Moreover, some large crystal grain with an average grain size of 770 nm can be found. This indicates that the introduction of NWs with diameter of about 190 nm does not alter the crystallinity of the perovskite layer, which is in line with what we expected. This conclusion is further confirmed by XRD patterns in Figure 4d. It can be observed that the diffraction peaks of all CH3NH3PbI3 films with or without CuO NWs are located at 2θ= 14.14°, 28.36° and 31.9°, which can be indexed to (110), (220), and (310) planes in the tetragonal phase with 14 cm space group31. Compare with the reference films, the location of the



diffraction peaks of the perovskite film on CuO NWs are the same, and the diffraction intensity keeps almost the same as well. It is very interesting that some CuO NWs reach out the multilayer sample when the sample is torn apart along cross-section, as seen clearly in cross-sectional SEM (Figure 4c). This indicates that CuO NWs with good mechanical propertiy have been successfully embedded under the perovskite layer. Also, CuO NWs can also be observed obviously in the optical microscopy and AFM images of ITO/CuO NWs/PEDOT:PSS composite films, as shown in Figure S5 and Figure S6.

Figure 4. Top-view (a-b) and cross-sectional (c) SEM image of perovskite films prepared with and without CuO NWs. (d) XRD patterns of perovskite films prepared with and without CuO NWs.


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Figure

5.

J-V

characteristics

of

the

hole-only

devices

of

ITO/CuO

NWs/PEDOT:PSS/MoO3/Ag with different coverage of CuO NWs of 0 , 2.1%, 4.2%, 6.3% and 9.5%.

To figure out the origin of CuO NWs enhancing the performance of PSCs, the light transmission property of ITO/CuO NWs/PEDOT:PSS composite film was investigated and the measurement result are presented in Figure S7. We can see that the introduction of CuO NWs leads to a slight decrease in light transmission relative to ITO/PEDOT:PSS reference film. Therefore, the light absorption of CuO NWs has a negligible influence on the performance of PSCs.

Next, the hole-only devices of ITO/CuO NWs/PEDOT:PSS/MoO3/Ag with different coverage of CuO NWs on ITO substrate were prepared and the corresponding J-V characteristics were investigated, as shown in Figure 5. When CuO NWs are used in the hole-only devices, the conductivity of the devices has an obvious change. The devices with CuO NWs coverage of 4.2% exhibit the best conductivity, matching well with the change of Jsc and Rs of PSCs with increase of CuO NWs coverage. The conductivity divided by layer thickness versus the coverage



of CuO NWs is displayed in the inset figure of Figure 5, indicating the optimal devices receive 2.5 times improvement in conductivity compared with the reference devices. The results represent that the efficiency enhancement of PSCs partly originates from the conductivity improvement of CuO NWs/PEDOT:PSS composite film when the appropriate amount CuO NWs are introduced.

To further study the effect of the nanowires, we measured the EQE and J–V characteristic in the dark for the optimal CuO NWs PSCs (with CuO NWs coverage of 4.2%) and reference PSCs.

Figure 6. (a) EQE spectra, and (b) J–V characteristic in the dark of PSCs with and without optimal CuO NWs, (c) Steady-state PL spectra and (d) transient-state PL spectra of the ITO/PEDOT:PSS/CH3NH3PbI3 films with and without optimal CuO NWs

Figure 6a displays the EQE spectra of the PSCs with or without CuO NWs. Both of the devices exhibit a broad response in the range from 300 nm to 800 nm, and the device with CuO NWs has a higher photon-to-electron conversion efficiency in the whole response region compared with the reference device. The EQE values are more than 80% over the visible light region, reaching a maximum value of 90.74% at 680


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nm. The Jsc calculated from the EQE spectra is 21.05 mA/cm2 for the CuO embedded device, and 17.83 mA/cm2 for the reference device, which is consistent with J-V measurement. The J–V characteristics of the PSCs were also measured in the dark, see Figure 6b. We can see that the dark current of the CuO NWs device is lower than that of the reference device in the low voltage/current regime. The lower dark current indicates a higher interface-dependant shunt resistance (Rsh) and hence an improvement charge extraction efficiency, as explained earlier7. The steady-state PL spectra of ITO/PEDOT:PSS/CH3NH3PbI3 film with and without CuO NWs are shown in Figure 6c, all samples exhibit the similar spectral profile with a PL peak at 776 nm. However, comparing with the reference film, the PL peak intensity of the perovskite film with CuO NWs is reduced greatly. Such a dramatic PL quenching offers a useful indicator for a significantly enhancement of charge carrier extraction from the perovskite layer to the electrode32. To further investigate the hole-extraction ability, time-resolved PL decay was conducted for the perovskite films on substrates with and without CuO NW. As shown in Figure 6d, introducing CuO NWs into ITO/PEDOT:PSS/CH3NH3PbI3 film results in a shorter charge carrier lifetime, suggesting that the photogenerated charges can transfer more efficiently from the perovskite layer to the hole transport layer. All these may be due to the fact that the incorporation of NWs increases the contact area between the hole transport layer and the perovskite layer, and higher hole mobility of CuO NWs/PEDOT:PSS composite film.



Figure 7. Electrochemical impedance spectrometry (EIS) of PSCs without and with the optimal coverage of CuO-NWs in dark

Electrochemical impedance spectroscopy (EIS) is commonly used to explore charge transport properties of solar cells by the parameters such as transport resistance, recombination resistance, and chemical capacitance etc33. Here EIS measurements were performed under short-circuit in the dark to investigate the charge transport recombination within the devices. Nyquist spectra in low-frequency region were Figure 7, we can see that the Nyquist plot of the device with the optimal CuO NWs (4.2% coverage) is ifferent greatly from that of the references devices. We all know that the semicircle in low-frequency region represents the charge recombination resistance (Rct). After fitting, the Rct of the device with CuO NWs is 2315 Ω, while the reference device is 1459 Ω, which is in well agreement with the change tendency of Rsh deduced from J-V curves in the dark. It is known that the main ways of charge loss include the interface- and bulk-dependent carrier charge recombination in the PSCs. Considering other layers are identical, the larger Rct in the propoesd PSCs should indicate a less interface charge recombination loss by the introduction of the


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CuO NWs. Therefore, CuO NWs can be confirmed to promote charge extracion from perovskite to ITO anode and then improve the short-circuit current of PSCs.

Figure 8.

Energy-level diagram of the CuO NW PSCs (a) and the ZnO NWs

device (b)

Figure 9. (a) The model diagram of substrate and NW (the average radius of NW is 95 nm, the thickness of PEDOT:PSS layer is 30 nm, which were measured by stylus profiler). (b)and (c) Schematic illustration of hole transport with CuO NWs and ZnO NWs, respectively.



Dose the improvement of conductivity of devices originate from the increase of interface area or the better hole transport of CuO, or both? To answer this question, we prepared ZnO NWs with the same diameter as CuO NWs to incorporate into in the PSCs as control experiment. The J-V characteristics of the devices with different coverage of ZnO NWs are shown in Figure S9. The relevant photovoltaic parameters are summarized in Table S2. The same trend was observed as CuO NWs devices. For the optimal ZnO NWs device with about 4.2% coverage (calculated from Figure S8), the and PCE of 15.37 % is obtained with an enhancement of 13.01%, compared with PCE(13.6%) of reference device. The energy levels of CuO and ZnO are shown in Figure 8. ZnO as a typical electron transport material lead to a large energy barrier for hole transport from perovskite layer to ITO. However, an obvious increase in Jsc and PCE can be also obtained in the control experiment. This inspires us to construct a model of the NWs in hole transport layer to solve this doubt, as shown in Figure 9a. We suggest that the PEDOT:PSS is coated on the NWs, otherwise, the device efficiency would decline. However, PEDOT:PSS film should be very thin as its thickness can not be measured by the stylus profiler. From the sketch of cross-section of single NW shown in Figure 9a, one can deduce that the boundary line between perovskite/PEDOT:PSS is lengthened by 3.2 times, which can be easily calculated by arc length formula. This means that the interfacial area at the perovskite/PEDOT:PSS interface is increased by 3.2 times if single NW is considered. For the optimal ZnO NWs device with about 4.2% coverage, the total interfacial area between perovskite and PEDOT:PSS is increased by 4.2%*3.2=13.4%, which is very consistent with


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13.01% enhancement in PCE. So we ascribe the efficiency enhancement of ZnO NWs device to the increase of interfacial area, see Figure 9c. The hole mobility of CuO is one order of magnitude higher than that of PEDDOT:PSS as reported in Ref.34,35. Therefore, the holes would be inclined to transport through CuO NWs, as shown in Figure 9b. This is the reason for another 8% enhancement of PCE for CuO NWs devices. However, when the coverage of CuO NWs increases, the PCE declines rapidly, as shown in Table 1, which could be attributed to overlapping of NWs increasing the transport distance of holes and breaking the crystalline of perovskite layer.

Figure 10. Hysteresis measurement for the CuO NWs based on PSCs and reference devices with a forward and backward voltage scan rate of 0.08 V/s.

It had been reported that there is anomalous photocurrent hysteresis for both mesoscopic and planar heterojunction junction PSCs. To check the hysteresis behavior in the CuO NWs device, its J−V characteristics under illumination conditions were measured under forward and backward scans as seen in Figure 10. The results indicate that the CuO NWs devices exhibit a negligible hysteresis. Compared with the



reference devices, the lower hysteresis implies that CuO introduction of NWs indeed promote the charge transport and extract efficiently.

4. CONCLUSION

In summary, CuO NWs with the diameters ranging from 130 to 270 nm have been prepared by a simple electrospinning method. The CuO NWs with optimized diameter and coverage were introduced into the inverted planar PSCs as an anode buffer layer. An average PCE of 16.35% are obtained, with more than 21% enhancement compared with that of the reference device with only PEDOT:PSS (13.50%). The increase of hole transport and extraction ability is the main reason for the PCE enhancement, as the crystalline of pervoskite layer is almost the same. Compared with the effect of ZnO NWs on the PSCs, the PCE improvement is divided into two parts. 13% enhancement originates from the increase of interfacial areas between hole transport layer (HTL) and perovskite layer, another 8% is from the higher hole mobility of CuO NWs/PEDOT:PSS composite material. This work suggests that when NWs are introduced into PSCs as buffer layer, four factors should be considered: the diameter, the coverage and material of NWs, and the thickness of perovskite layer. The diameter of NWs plays a central role for the improvement of extraction ability. The longer diameter, and the larger interface area is. However, when the diameter becomes too long, it may break the crystalline of perovskite layer. That is, there is a tradeoff between the optimal diameter of NWs and thickness of perovskite layer. The thicker of perovskite layer, the optimal diameter of NWs would be longer. The large NWs


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coverage can also increase the interface area, too large coverage will introduce more overlapping of NWs, causing a decline in PCE. Therefore NWs coverage should be optimized carefully. At last, the material of NWs with a better hole transport ability is also needed. Further work should be done to quantify the correlation between the optimal diameter, the coverage of NWs and the thickness of perovskite layer for NWs large-scale application in perovskite solar cells. In conclusion, this work provides a simple strategy to improve the PCE of planar perovskite solar cells.

ASSOCIATED CONTENT

Supporting Information The electrospinning conditions, such as solution viscosity, spinning distance, impressed voltage and flow rate, have been carefully adjusted in order to synthesize CuO NWs with a suitable diameter and uniform morphology. CuO NWs with an optimal diameter of 190 nm were prepared, the table summarized the parameters of best PSCs without and with different CuO NWs coverage. The ITO/CuO NWs/PEDOT:PSS layers were observed under an optical microscope, and the transmission of the devices with different coverage of CuO NWs. The roughness of PEDOT:PSS film and CuO NWs with PEDOT:PSS film were measured with AFM, respectively. The ZnO NWs with different coverage of 0, 2.1%, 4.2%, 6.3% and 9.5% under an optical microscope, the J–V curve of the devices with different coverage of ZnO NWs. The table summarized the parameters of PSCs without and with different ZnO NWs coverage.



AUTHOR INFORMATION *E-mail: [email protected], [email protected]

ORCID

Qinjun Sun: 0000-0001-9929-0308

Yuying Hao: 0000-0002-9691-7109

Zhanfeng Li: 0000-0002-8237-3622

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This research work was supported by NSFC-Joint Fundation program of Shanxi Coal Based Low Carbon Nurturing Project (U1710115), Key Research and Development (International Cooperation) Program of Shanxi (201603D421042), Platform and Base Special Project of Shanxi (201605D131038), and National Natural Scientific Foundation program of China (61571317, 61475109 and61274056).

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Efficiency enhancement of perovskite solar cells via electrospun CuO

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