Substrate Temperature Effect on Charge Transport Performance of ZnO Electron Transport Layer Prepared by a Facile Ultrasonic Spray Pyrolysis in Polymer Solar Cells

A novel ultrasonic spray pyrolysis for high-quality ZnO films based on zinc-ammonia solution was achieved in air. To investigate the structural and optical properties as well as the performance of polymer solar cells (PSCs), ZnO films at different substrate temperatures and thicknesses were prepared. The performance of poly(3-hexylthiophene):[6,6]-phenyl C 61 -butyric acid methyl ester (P3HT:PCBM) based PSC was found to be improved due to the ZnO films. The crystal structure and roughness of the ZnO films fabricated at different temperatures were found to affect the performance of PSCs.The optimized power conversion efficiency was found to be maximum for PSCs with ZnO films prepared at 200C. The growth process of these ZnO films is very simple, cost-effective, and compatible for larger-scale PSC preparation. The precursor used for spray pyrolysis is environmentally friendly and helps to achieve ZnO film preparation at a relative low temperature.


Introduction
Bulk heterojunction (BHJ) PSCs have received a great deal of attention due to their light weight, flexibility, and lowcost fabrication by using solution processing methods [1][2][3].Typical BHJ PSCs are fabricated with a transparent conductive anode, an electron-transporting layer (ETL), a holetransporting layer (HTL), and an active layer sandwiched between the ETL and HTL [4,5].N-type metal oxides such as titanium oxide (TiO 2 ), zinc oxide (ZnO), and cesium carbonate (Cs 2 CO 3 ) have previously been reported as an efficient ETL in PSCs [6,7].Among the n-type metal oxides used in PSCs, ZnO is a promising candidate due to its good transparency across the entire visible spectral range, high electron mobility, environmental stability, and low cost.Furthermore, the band edge cut-off of ZnO is nearly 370 nm and this protects the photoactive layer from UV damage whilst also improving the life of PSCs [8].Recently, triple-junction PSCs using ZnO as ETL with high power conversion efficiency (PCE) of over 11.5% have been fabricated by Yang's group [9].
Solution processing techniques such as spin-coating and spray pyrolysis are particularly appealing as they offer a cheap and efficient way of preparing ZnO thin films.Spincoated ZnO films using a precursor composed of zinc acetate (Zn(CH 3 COO) 2 ), ethanolamine (NH 2 CH 2 CH 2 OH), and 2methoxyethanol (CH 3 OCH 2 CH 2 OH) have been reported as a means to fabricate efficient inverted PSC devices [1,[10][11][12].In these studies, the ZnO films required long annealing times of 30 minutes to 1 hour.Even so, the conventional spin-coating approach is only suitable for lab-scale ZnO preparation, limiting potential roll-to-roll variations.To acquire large-scale ZnO films, other techniques such as blade coating, inkjet printing, and spray pyrolysis are required [13][14][15].With the advantages of continuous production, high efficiency, and excellent control of chemical uniformity, spray pyrolysis is able to fabricate ZnO based devices in a rollto-roll processing way.However, traditional spray pyrolysis using zinc acetate or zinc nitrate solution as precursor needs a substrate temperature higher than 320 ∘ C in order to promote crystallization and realize high carrier mobility and transmittance [16,17].ZnO films prepared by spray pyrolysis with various precursors have been investigated previously.With ZnO ETLs prepared by spray pyrolysis from 2-methoxyethanol diluted sol-gel precursor at 250-300 ∘ C, P3HT:PCBM based PSCs had relative high performance [18], achieving PCEs of roughly 2.99-3.22%.Unfortunately, PSC devices with ZnO ETL prepared at 150 ∘ C exhibited a poor PCE of only 0.94%.Although ZnO films have been prepared on flexible substrates at 100 ∘ C using spray pyrolysis with diethylzinc (DEZ) solution [19,20], DEZ reacts violently with water and easily ignites upon contact with air.The toxicity and instability of DEZ leads to serious security problems to both operators and the environment, limiting its use in potential applications.
Here we have developed an ultrasonic spray pyrolysis (USP) technique in order to prepare highly dense ZnO thin films using zinc-ammonia [(Zn(NH 3 ) 4 ](OH) 2 solution as a precursor, which can be decomposed to the three components, ZnO, NH 3 , and H 2 O.The preparation of the ZnO films described here is extremely simple and cost-effective with a low temperature process under ambient conditions.Compared with the sol-gel approach, the whole process does not require an anneal and does not use any toxic organic solvents.In this study we show the thermal effect of the substrate temperature upon surface morphology, structure, optical transparency, and performance of ZnO based inverted PSCs.

Experimental
Zinc acetate (Zn(CH 3 COO) 2 ⋅2H 2 O) and ammonia water (NH 3 ⋅H 2 O, 25∼28%) were purchased from Shanghai Chemical Reagent Corp., China (analytical grade reagents), and used as received.P3HT and PCBM were purchased from Lumtec, Taiwan.The precursor solution was prepared using the following recipe.Zinc acetate (5 g) was dissolved in 150 mL deionized H 2 O. Next, 3.5 mL NH 3 ⋅H 2 O was dropped into the solution whilst magnetically stirring.After the solution had completely precipitated, the mixture was vacuum-filtrated and washed by more than 1 L of deionized water in order to remove CH 3 COO − .Finally, the precipitate was dissolved using 50 mL of ammonia-water and then diluted with 300 mL deionized water.
ZnO films were fabricated by using an ultrasonic spray pyrolysis apparatus developed in-house.This consists of a cylindrical ultrasonic transducer, an automatic - table, a 400 × 300 mm 2 temperature controlled heating block, a 15 mm diameter dropping-proof quartz nozzle, and pipe fittings, as shown in Figure 1.The nozzle was mounted above the heating block.In this study, the deposition of ZnO films upon ITO glass (6.7 Ω/sq), soda lime glass, and PET was investigated.All substrates were consecutively cleaned in an ultrasonic bath containing detergent, acetone, deionized water, and ethanol for each 10 min step and then dried with nitrogen [21,22].During the deposition process, compressed air was used as a carrier gas.The spray rate was held constant at 10 L/min.The distance between the nozzle and substrate was kept at 10 mm, which was chosen to ensure that the mist reached the substrate for a given flow rate.ZnO thin films were deposited at substrate temperatures of 125 ∘ C, 150 ∘ C, 200 ∘ C, and 250 ∘ C, respectively.
ZnO films were characterized before device fabrication.The composition and electron structure were tested by using X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB).The crystal structure was determined by using X-ray diffraction (XRD, PANalytical X'Pert PRO) in grazing incidence mode with a Cu K radiation (0.154 nm).The surface morphology of the films was characterized by using an atomic force microscope (AFM, Dimension Edge).The optical properties of ZnO films deposited on glass substrates were studied by measuring the optical transmittance spectra in the wavelength range 250-850 nm, with a UV-visible spectrophotometer (Hitachi U-3900).The thickness of all films was measured using a stylus profile meter (Alpha-Step D-100).To investigate the performance of ZnO ETLs in PSC devices, inverted PSCs with stack structure of ITO/ZnO/ P3HT:PCBM/MoO x /Ag were fabricated.For the active layer, a blend of P3HT and PCBM film in a 1 : 1 weight ratio up to about 200 nm was spin-coated on the ZnO film at 1000 rpm for 16 s.MoO x films (15 nm) and Ag films (100 nm) were then successively thermally evaporated onto the P3HT:PCBM blend at 10 −4 Pa.Current density-voltage (J-V) characteristics were measured with a Keithley 2400 under a xenon lamp with an illumination power of 100 mW/cm 2 .All measurements were performed in air at 25 ∘ C.

Results and Discussion
To investigate the effect of thermal treatment upon the crystal structure of ZnO, films were fabricated at different temperatures with thickness of ∼200 nm on ITO glass.XRD patterns of the ZnO films prepared at 125 ∘ C, 150 ∘ C, 200 ∘ C, and 250 ∘ C are shown in Figure 2. All of the samples exhibit a wurtzite ZnO structure compared with standard PDF card (number 361451).Three typical peaks occur at 31.8 ∘ , 34.3 ∘ , and 56.6 ∘ which correspond to (100), (002), and (101) directions, reflecting nonoriented polycrystalline ZnO thin films, which were different with single-crystalline ZnO films grown at the 800 ∘ C high temperature and a lower growth rate by the similar mist chemical vapor deposition technique [23].Figure 2 shows that the crystal structure of the ZnO films was modified by thermal effects.The sample prepared at 125 ∘ C exhibited weak diffraction peaks which indicates poor crystal structure.For samples with substrate temperature of 150 ∘ C, 200 ∘ C, and 250 ∘ C, the intensity of diffraction peaks was almost the same.It indicates that ZnO films tend to form a stable crystal structure at the substrate temperature above 150 ∘ C.Although ZnO films deposited at 180 ∘ C using zinc acetate solution have been previously reported [24], however, X-ray diffraction results showed that all these samples seemed to be amorphous ZnO.From the discussion above, it is feasible to deposit ZnO films with good crystal structure upon ITO glass or flexible substrate such as an ETL of PSCs.
The thermal effect upon electron structure was characterized by XPS.Before testing, ZnO films were etched using an argon plasma for approximately 10 nm. Figure 3 shows core level XPS spectra of Zn 2p and O 1s for the ZnO films that were not annealed.From Figure 3(a), it can be seen that the binding energy of the Zn 2p 3/2 peak was at 1021.8 eV in the ZnO film prepared at 125 ∘ C. The maximum of the Zn 2p 3/2 peak shifts toward higher binding energy by 0.8 eV as the substrate temperature increases to 150 ∘ C.This shift implies that less Zn atoms are bound to O atoms [12].A further increase of the substrate temperature leads to the Zn 2p 3/2 peak shifting towards higher binding energies.This indicates that the oxygen-deficient component had decreased and the number of Zn-O bonds in the films had increased by higher substrate temperature.The O 1s XPS spectra exhibit asymmetric line shapes (Figure 3(b)).The peak with lower binding (529.8 to 530.4) energy corresponds to O atoms in a ZnO matrix.The second peak at 531.7 to 531.9 eV is probably due to Zn(OH) 2 [25,26].The relative magnitude of the low binding energy O atoms was 64% when the substrate temperature is 125 ∘ C and this increased to 71% after the substrate temperature was increased to 250 ∘ C. It can therefore be seen that a higher substrate temperature increases the number of Zn-O bonds in ZnO films.
The thermal effect upon morphology in ZnO films was investigated by using AFM.All samples were scanned over an area of of 1 × 1 m 2 .AFM 3D morphology images of samples prepared at 125 ∘ C, 150 ∘ C, 200 ∘ C, and 250 ∘ C are shown in Figure 4.The film prepared at 125 ∘ C exhibited a suedelike textured surface.The trichomes on the surface become thinner when the substrate temperature was increased to 150 ∘ C. A further increase of the substrate temperature to 200 ∘ C and 250 ∘ C leads to the observation of spherical grains upon the film surface.This is typically produced by fast nucleation and growth rate [27].The thermal effect induced morphology also leads to varying roughness.The root mean square (RMS) roughness values were 1.95 nm, 2.25 nm, 3.71, and 4.85 nm for ZnO films prepared at 125 ∘ C, 150 ∘ C, 200 ∘ C, and 250 ∘ C, respectively.Notice that grains on the surface of the ZnO films prepared at lower substrate temperature were smaller and more uniform than those prepared at higher substrate temperature.Thus, the ideal substrate temperature in this study for high performance ZnO ETL fabrication should be lower than 250 ∘ C.
To investigate the thermal effect upon optical performance of ZnO films, several samples with the thickness of ∼100 nm were prepared at different substrate temperatures and tested.Figure 5 Shows the optical transmittance spectra of ZnO films prepared at 125 ∘ C, 150 ∘ C, 200 ∘ C, and 250 ∘ C. The film prepared at the low temperature of 125 ∘ C had a lower optical transmittance due to the poor crystal structure.The rest of the films prepared at higher temperature have a greater transmittance in the range of 400-850 nm with a value of up to 83% when compared to air.ZnO films prepared at higher temperature exhibit an obvious absorption onset red-shift at approximately 350 nm.The shift is due to  the narrower energy gap of ZnO films that were prepared at higher substrate temperature.The energy gap of the ZnO film prepared at 125 ∘ C was 3.27 eV, while it shifted to 3.22 eV when the substrate temperature increased up to 250 ∘ C. The effect of temperature upon ZnO films of different thicknesses in inverted PSC devices was investigated in this study.J-V characteristics of the inverted PSC devices with ZnO ETLs prepared at different temperature are shown in Figure 6.The performances of the inverted PSCs are summarized in Table 1.
For the device with ZnO ETLs deposited at 125 ∘ C, the PCE was only 1.64% with an open circuit voltage ( OC ) of 0.56 V, a short circuit current density ( SC ) of 9.64 mA/cm 2 , and a fill factor (FF) of 43.47%.The device had a relatively high series resistance (  ) and a low shunt resistance ( SH ).We propose that the poor performance was caused by low light transmittance and conductivity that is attributed to poor surface morphology and amorphous structure as demonstrated by AFM and XRD data.When the substrate temperature was increased to 200 ∘ C, the best device performance was obtained, where the PCE was 3.05% with  OC of 0.61 V,  SC of 9.26 mA/cm 2 , and FF of 53.63%.The devices with ZnO ETL prepared at 250 ∘ C and 300 ∘ C showed gradual degradation of  SC , FF, and PCE.The degradation might be caused by the rough surface morphology of ZnO ETLs.
J-V characteristics of the inverted PSC performance with ZnO ETLs of different thickness are shown in Figure 7. Table 2 is the summary of the device performance.For all devices with ZnO ETLs, both  OC and  SC were improved when compared to devices without ZnO.This indicates that ZnO can effectively modify the work function of ITO for electron transport and collection.PSC devices with 50 nm ZnO ETL showed the highest PCE.For the devices with a 25 nm ZnO ETL, the PCE was 2.75% with  OC of 0.59 V,  SC of 9.64 mA/cm 2 , and FF of 48.11%.When the thickness of ZnO ETL was increased, the performance of the device decreased gradually.For the device with a 100 nm ZnO ETL, the PCE was 2.58% with low  SC of 8.64 mA/cm 2 .
The general function of ETLs is to select negative carriers and block positive carriers to the cathode in a charge collection process and thus to increase  SH of PSCs [7,8,28].The performance of ZnO films and PSC devices discussed above demonstrated that crystal structure and composition of ZnO ETLs induced by a thermal effect are essential in   that coarse grains of different sizes on the ZnO ETL surface induce serious interfacial problems between ETL and the spin-coated active layer.This leads to the charge transport channel between the active layer and the ZnO ETL becoming narrow.Thus,   is increased by the degradation of electron collection efficiency.Incidentally, the interfacial problem may not exist or become a tiny problem when the active layer was prepared by spray coating or printing. SC also decreased gradually as the thickness of ZnO ETLs increased, while  OC remained stable.This indicates that contact resistance of ZnO ETLs is sensitive to thickness.Thin ZnO ETL with low   is beneficial for efficient PSC, unless it is not thick enough to cover the entire range of the active layer.This investigation suggests that a suitable thickness of ZnO ETL prepared by this spray pyrolysis method is approximately 50 nm.
Compared to PSC devices with spin-coated ZnO ETL, PSCs with ZnO ETLs prepared by this technique have relatively low FF and PCE.This might be attributed to the microscopic rough surface with coffee rings that induces serious contact problems between the spin-coated active layer and ETLs [18].Practically, the spin-coating approach is generally suitable for lab-scale fabrication of ZnO and devices.However, the use of this low temperature spray pyrolysis technique means that ZnO films can be processed upon a flexible substrate without annealing and with good thickness control.Additionally, this is compatible with fabrication of commercial PSCs using low-cost production methods like the well-known roll-to-roll process.Figure 8 shows a ZnO/PET film sample prepared at 150 ∘ C. In our next work, flexible PSCs with ZnO ETLs prepared by this technique will be discussed in detail.

Conclusion
High-quality ZnO films were successfully processed by low temperature spray pyrolysis using a [(Zn(NH 3 ) 4 ](OH) 2 solution at a low temperature of 150 ∘ C. The substrate temperature and the thickness of ZnO films had some influences on the composition, crystal structure, and optical transmittance as well as the performance of inverted PSCs.The optimized PCE of the PSCs based on ZnO ETLs prepared at 200 ∘ C with the thickness of 50 nm reached a maximum of 3.05%.In addition, this technique is quite easy and low-cost, and it is compatible with the roll-to-roll production of large-scale flexible PSCs.

Figure 2 :
Figure 2: X-ray diffraction patterns of ZnO films grown on ITO glass at different temperatures.

Figure 3 :
Figure 3: (a) Zn 2p and (b) O 1s XPS spectra of ZnO films fabricated with different substrate temperatures.

Figure 7 :Figure 8 :
Figure 7: J-V characteristics of devices with ZnO ETLs prepared with different thicknesses.

Table 1 :
Device characteristics of PSCs prepared at different substrate temperatures.

Table 2 :
Device characteristics of PSCs prepared at different substrate temperatures.