This manuscript describes how to design and fabricate efficient inverted SMPV1:PC71BM solar cells with ZnO nanorods (NRs) grown on a high quality Al-doped ZnO (AZO) seed layer. The well-aligned vertically oriented ZnO NRs exhibit high crystalline properties. The power conversion efficiency of solar cells can reach 6.01%.
This manuscript describes how to design and fabricate efficient inverted solar cells, which are based on a two-dimensional conjugated small molecule (SMPV1) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), by utilizing ZnO nanorods (NRs) grown on a high quality Al-doped ZnO (AZO) seed layer. The inverted SMPV1:PC71BM solar cells with ZnO NRs that grew on both a sputtered and sol-gel processed AZO seed layer are fabricated. Compared with the AZO thin film prepared by the sol-gel method, the sputtered AZO thin film exhibits better crystallization and lower surface roughness, according to X-ray diffraction (XRD) and atomic force microscope (AFM) measurements. The orientation of the ZnO NRs grown on a sputtered AZO seed layer shows better vertical alignment, which is beneficial for the deposition of the subsequent active layer, forming better surface morphologies. Generally, the surface morphology of the active layer mainly dominates the fill factor (FF) of the devices. Consequently, the well-aligned ZnO NRs can be used to improve the carrier collection of the active layer and to increase the FF of the solar cells. Moreover, as an anti-reflection structure, it can also be utilized to enhance the light harvesting of the absorption layer, with the power conversion efficiency (PCE) of solar cells reaching 6.01%, higher than the sol-gel based solar cells with an efficiency of 4.74%.
Organic photovoltaic (OPV) devices have recently undergone remarkable developments in the application of renewable energy sources. Such organic devices have many advantages, including solution-process compatibility, low cost, light weight, flexibility, etc.1,2,3,4,5 Up until now, polymer solar cells (PSCs) with a PCE of more than 10% have been developed by utilizing the conjugated polymers blended with PC71BM6. Compared to polymer-based PSCs, small molecule-based OPVs (SM-OPVs) have attracted more attention when it comes to fabricating OPVs due to their several distinct advantages, including well-defined chemical structures, facile synthesis and purification, and generally higher open-circuit voltages (Voc)7,8,9. At present, a 2-D structure conjugated small molecule SMPV1 (2,6-Bis[2,5-bis(3-octylrhodanine)-(3,3-dioctyl-2,2':5,2''-terthiophene)]-4,8-bis((5-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene) with BDT-T (benzo[1,2-b:4,5-b']dithiophene) as the core unit and 3-octylrodanine as the electron-withdrawing end-group10 has been designed and used to blend with PC71BM for promising sustainable OPVs application. The PCE of conventional small molecule solar cells (SM-OPVs) based on SMPV1 blended with PC71BM has reached more than 8.0%10,11.
In the past, PSCs could be enhanced and optimized simply by adjusting the thickness of the active layer. However, unlike PSCs, SM-OPVs in general have a shorter diffusion length, which greatly limits the thickness of the active layer. Hence, to further increase the short current density (Jsc) of SM-OPVs, utilizing the nano-structure12 or NRs9 to improve optical absorption of SM-OPVs became necessary.
Among these methods, the anti-reflection NRs structure is generally effective for light harvesting of the active layer over a broad range of wavelengths; therefore, knowing how to grow well-aligned vertically oriented zinc oxide (ZnO) NRs is very critical. The surface roughness of the seed layer below the ZnO NRs layer has a great influence on the orientation of the NR arrays; therefore, in order to deposit well-oriented NRs, the crystallization of the seed layer needs to be precisely controlled9.
In this work, the AZO films are prepared by theRadio-Frequency (RF) sputtering technique. Compared with other techniques, RF sputtering is known to be an efficient technology that is transferable to industry for it is a reliable deposition technique, which allows the synthesis of high purity, uniform, smooth, and self-sustainable AZO thin films to grow over large area substrates. Utilizing the RF sputtering deposition enables the forming of high quality AZO films that exhibit high crystallization with reduced roughness of surface. Therefore, in the subsequent growth layer, the orientations of the NRs are highly aligned, even more so when compared to ZnO films prepared by the sol-gel method. Using this technique, the PCE of the inverted small molecule solar cells based on well-aligned vertically oriented ZnO NR arrays can reach 6.01%.
1. Growth of AZO Sputtered Seed Layer on ITO Substrate
2. Growth of the Sol-gel Processed ZnO Seed Layer on ITO Substrate
3. Growth of ZnO NR Array on a Seed Layer
4. Fabrication and Measurement of Inverted Small Molecule Solar Cells
5. Characterization Techniques
The layered structure of the devices consisted of an ITO substrate/AZO (40 nm)/ZnO NRs layer, SMPV1:PC71BM (80 nm)/MoO3 (5 nm)/Ag (150 nm) as shown in Figure 1. In general, the AZO or ZnO seed layer is widely used to function as the electron transport layer (ETL) in PSCs devices. Apart from PSCs, SM-OPVs usually have a shorter active layer, limited by the shorter diffusion length8. Hence, to further improve the light-harvesting capability of devices, the ZnO NRs layer is introduced to be grown on the seed layer, to work as an anti-reflection layer to enhance collection of the incident light, and to increase the interface area for carrier collection at the same time12,14.
The surface morphology and roughness of the seed layer have a significant influence on the orientation of the NR arrays. Figure 2a and Figure 2b areAFM images of the seed layer based on the sputtering method and the sol-gel method, respectively. The surface morphology of the sol-gel processed seed layer can be seen to not only exhibit higher roughness, but also to form a natural ridge pattern. As a result, the orientation of the NR arrays grown on the sol-gel processed layer will be much rougher than layers grown using the sputtering technique. Figure 2c and Figure 2d show the scanning electron microscope (SEM) images of the NR arrays grown on the sputtered seed layer and the sol-gel processed seed layer respectively. Clearly, the orientation of the NR arrays grown on the sputtered AZO layer can be observed to be better than those grown on the sol-gel processed ZnO layer.
In addition to the SEM images, to further estimate the orientation of the NR arrays, XRD analysis (Figure 3) is used to identify the orientation and crystallization of the NR arrays. Compared with the XRD spectra of the NRs grown on a sol-gel processed seed layer, the spectra of NR arrays based on a sputtered seed layer show a relatively stronger peak at 34.5 °, indicating that not only the orientation but also the crystallization of the ZnO NR arrays is better on the sputtered layer than on the sol-gel process layer.
As well as the XRD measurement of the seed layer, the µ-PL spectra of NRs are also measured. Figure 4 shows the PL spectra of the NR arrays with different deposition methods. The emission peak at 385 nm originates from the excitonic recombination19. On the other hand, the green emission of the spectra comes from oxygen vacancies (intrinsic defects), again implying that the film quality of the sputtered layer is better than the quality of film formed by the sol-gel method. It can be noticed that the PL spectra of the ZnO NRs on sputtered AZO shows a considerably weaker peak at 385 nm compared to that of the ZnO NRs on sol-gel ZnO. This significant PL quenching occurs in the ZnO NR array on the sputtered AZO seed layer, implying that the AZO seed layer contains better exciton dissociation and charge separation capability than that of the ZnO sol-gel seed layer. The results reveal that the AZO/ZnO NRs layer based on the sputtering process appears to be a better electron transport layer than that based on the solution process.
Figure 5 shows the J-V characteristics of the devices with a sputtered AZO seed layer and a sol-gel processed ZnO seed layer. The short circuit current Jsc, open circuit voltage Voc, FF, and the PCE can be derived from the J-V curves. The devices with a sputtered seed layer exhibit Jsc of 11.96 mA/cm2, Voc of 0.87 V, FF of 57.8%, and PCE of 6.01%, which is better than the sol-gel processed solar cell with Jsc of 10.01 mA/cm2, Voc of 0.88 V, FF of 53.8%, and PCE of 4.74%.
Table 1 shows the performance of the devices with different seed layers. By utilizing the sputtered seed layer, well-aligned vertically oriented ZnO NR ETL can be formed, and thereby not only the absorption but also the carrier collection efficiency can be enhanced. As a result, compared with the sol-gel processed devices, devices with a sputtered seed layer exhibit higher Jsc value (11.96 mA/cm2) and better FF value (57.8%), as shown in Table 1.
Figure 1: Schematic diagram of the inverted small molecule solar cell structure. Layered structure of the devices consisted of ITO substrate/AZO (40 nm)/ZnO NRs layer, SMPV1:PC71BM (80 nm)/MoO3 (5 nm)/Ag (150 nm). Please click here to view a larger version of this figure.
Figure 2: AFM and SEM images of ZnO NR array. AFM images of ZnO NR array grown on (a) a sputtered AZO seed layer and (b) a sol-gel processed ZnO seed layer; SEM top-view images of ZnO NR array grown on (c) a sputtered AZO seed layer and (d) a sol-gel processed ZnO seed layer. The surface morphology and roughness of the ZnO NRs layer can be observed via the AFM and SEM images. Please click here to view a larger version of this figure.
Figure 3: XRD spectra of ZnO NR array. The XRD pattern of ZnO NR array grown on a sputtered AZO seed layer and a sol-gel processed ZnO seed layer. The orientation and crystallization of the NRs can be identified by the XRD spectra. The ZnO NR array grown on different seed layers exhibits almost the same orientation (002). The strength of the (002) peak for the NRs on sputtered AZO seed layer is stronger than that on sol-gel processed ZnO seed layer, revealing that the ZnO NRs on sputtered AZO seed layer exhibits better vertical orientation along the (002) axis. Please click here to view a larger version of this figure.
Figure 4: PL spectra of AZO and ZnO seed layer. The PL spectra of a sputtered AZO seed layer and a sol-gel processed ZnO seed layer. The defects and the exciton dissociation capability of the NRs can be evaluated by the PL spectra. The emission peak at 385 nm originates from the excitonic recombination and the green emission of the spectra comes from oxygen vacancies of the ZnO NR array. Please click here to view a larger version of this figure.
Figure 5: J-V curve of the devices with different seed layers. The J-V characteristics of devices under illumination with a sputtered AZO seed layer and a sol-gel processed ZnO seed layer. The performance of the solar cells can be derived from the J-V curves14. Please click here to view a larger version of this figure.
Devices | Voc (V) | Jsc (mA/cm2) | FF (%) | PCE(%) |
Sputtering seed layer | 0.87 | 11.96 | 57.8 | 6.01 |
Sol-gel processed seed layer | 0.88 | 10.01 | 53.8 | 4.74 |
Table 1: The performance of the devices with different seed layers. A summary of the performance of the devices derived from J-V curves including short circuit current, open circuit voltage, fill factor, and the power conversion efficiency
By utilizing the NRs interlayer, both the Jsc and the FF of the devices can be improved. However, the surface roughness of NRs will also influence the subsequent processes. Thus, the orientation and the surface morphology of the NRs should be carefully manipulated. For a long time, the sol-gel processed ETL such as TiO2 and ZnO were commonly used in PSCs due to their simple procedures. However, the crystallization of sol-gel processed layers is generally of the amorphous type, and the surface morphology of the layers is rough in the majority of the cases. Hence, in this study, to precisely control the film quality of the seed layer, the sputtered seed layer has been selected to replace the sol-gel processed seed layer. The ZnO NRs grown on the sputtered AZO seed layer also show better vertical alignment, which is beneficial for subsequent processes. It is noted that at the end of the NRs growth process, the residual precursor solvent on the NRs needs to be removed, and thus the sample needs to be baked on the hot plate to ensure the residual solvent dries out completely. Furthermore, to avoid the annealing effect changing the surface morphology, the drying temperature is set at 250 °C, which is below the recrystallization temperature of the ZnO.
In general, the transport layer of the OPV devices dominates the carrier collection and transportation of the solar cells. As a result, improving the mobility of the transport layers is very critical9. Unlike the sol-gel processed film, by adjusting the RF power, deposition temperature, and doping concentration of the AZO target, the sputtered AZO seed layer film can maintain high crystallization and high electron mobility.
Even under various environments or conditions of this fabrication process, it is still easy to replicate the results of the experiment. As long as the film quality of the seed layer is well controlled, the well-aligned vertically oriented ZnO NR array can be easily obtained.
Although the ZnO NR array shows great potential to function as ETL in OPVs, the sheet resistance of the ZnO NR array is still high. Hence, the ZnO NR arrays cannot replace the ITO and need to be compatible with ITO or other transparent electrodes during the applications.
Other than functioning as the ETL in the SM-OPVs, the well-aligned vertically oriented ZnO NR arrays can also work as an anti-reflection layer in an organic light-emitting diode (OLED) to increase light emission20. Moreover, for illumination applications, it can function as a donor to recombine with holes to emit light of a specific wavelength21. Consequently, we believe that high quality sputtered AZO film and well-aligned vertically oriented ZnO NR arrays will play an important role in the optoelectronics industry in the future.
The authors have nothing to disclose.
The authors would like to thank the National Science Council of China for the financial support of this research under Contract No. MOST 106-2221-E-239-035, and MOST 106-2119-M-033-00.
AZO target | Ultimate Materials Technology Co., Ltd. | none | AZO (2 wt% Al2O3 in ZnO) , 3”ψx 3mmt + 3mmt Cu B/P + Bonding |
SMPV1 | Luminescence Technology Corp. | 1651168-29-4 | 2,6-Bis[2,5-bis(3-octylrhodanine)-(3,3-dioctyl-2,2':5,2''-terthiophene)]-4,8-bis((5-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene |
RF sputtering system | Kao Duen Technology Co., Ltd | none | http://www.kaoduen.com.tw/index.php?action=product |
Zinc Acetate Dihydrate | J. T. Baker | 5970456 | 4.39 g |
Monoethanolamine | J. T. Baker | 141435 | 1.22 g |
2-methoxyethanol | Sigma-Aldrich | 109864 | 40 mL |
Zinc Nitrate Hexahydrate | J. T. Baker | 10196186 | 1.49 g |
Hexamethylenetetramine | Sigma-Aldrich | 100-97-0 | 0.7 g |
Indium tin oxide (ITO) | RiTdisplay | none | coated glass substrates (<10 Ω sq–1) |
AFM | Veeco | Innova SPM | |
SEM | FEI | Nova 200 NanoSEM | operation voltage: 10 kV |
XRD | Bruker | D8 X-ray diffractometer | 2θ range: 10–90 °; step size: 0.008 ° |
PL | Horiba | Jobin-Yvon HR800 | excitation source: 325 nm UV Laser 20 mW |
solar simulator | Newport | 91192A | AM 1.5G |
Precision Semiconductor Parameter Analyzer | Keysight Technologies | Agilent 4156C | sweep from -1 to +1 V |
toluene | Sigma-Aldrich | 108-88-3 | 1 mL |
PC71BM | Sigma-Aldrich | 609771-63-3 | 11.25 mg |
Thermal evaporation system | Kao Duen Technology Co., Ltd | Kao Duen PVD System | http://www.kaoduen.com.tw/index.php?action=product |
HCl | Sigma-Aldrich | 7647-01-0 | |
MoO3 | Alfa Aesar | 1313-27-5 | 99.50% |
silver ingot | ADMAT Inc. | none | 100.00% |
Thin Film Deposition Controller | INFICON | XTC | |
anti-corrosion tape (Polyimide Film) | 3M Taiwan Corporation | none | http://solutions.3m.com.tw/wps/portal/3M/zh_TW/InsulatingTape/home/product/Polyimide/ |
spin-coater | Chemat Technology, Inc | KW-4A | http://www.chemat.com/chematscientific/KW-4A.aspx |