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Cite This: ACS Appl. Energy Mater. 2019, 2, 4873−4881
Vanadium Oxide as Transparent Carrier-Selective Layer in Silicon Hybrid Solar Cells Promoting Photovoltaic Performances Chia-Yun Chen,*,†,‡ Ta-Cheng Wei,† Po-Hsuan Hsiao,† and Chia-Hsiang Hung† †
Department of Materials Science and Engineering and ‡Hierarchical Green-Energy Materials (Hi-GEM) Research Center, National Cheng Kung University, Tainan 70101, Taiwan
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S Supporting Information *
ABSTRACT: Vanadium pentoxide (V2O5) has been proposed as a promising selective contact for holes in organic electronic devices. In this study, the strategy was undertaken for minimizing the possible charge recombination at electrode surfaces in the siliconbased hybrid solar cells and further allowed the exceptional capability for hole extraction. This was accomplished by inserting the mixed vanadium oxide (VOx) phases based on a simplified low-temperature fabrication process. Detailed examinations of crystallinity, chemical states and compositions, topography, and optical transmittance of such VOx layer were performed, revealing the coexistence of V2O3 and V2O5 phases that facilitated the hole-selective contact due to establishing the ohmic-like interfaces as well as blocking the transport of electrons, and the optimal anneal treatment at 200 °C was further validated. On the basis of this, the insertion of a VOx electron-blocking layer in the integrated organic/inorganic hybrid solar cells was fully fabricated with solution processing methods, presenting the leading conversion efficiency of 14.4% being approximately 1.6 times superior than the conventional VOx-free hybrid solar cells. These results provided a viable rule toward realizing the high-performance and low-cost solar cells and might further offer the high potential for other functional applications based on hybrid material designs. KEYWORDS: hybrid solar cells, efficiency, silicon nanowires, vanadium oxide, solution processing method
1. INTRODUCTION In recent years, organic photovoltaics (OPVs) have attracted worldwide attention from the scientific community due to their promising characteristics including low-cost manufacturing procedures, attainable flexibility, and toward large-scale industrial applications, Nevertheless, the conversion efficiency of state-of-the-art OPVs was still less than 15%, which remained insufficient for the practical operation needs in current mobile electronics.1−6 Also, the drawbacks in manufacturing aspects laid in poor stability under the longterm operation and were subjected to efficiency degradation due to environmental deterioration. By contrast, crystalline silicon (Si) solar cells, the dominant commercialized photovoltaics, enabled one to push the practical limit efficiency approaching the ultimate theoretical value of 29.4%.7,8 Moreover, the comparably high operation stability of crystalline Si solar cells could practically meet the needs of sustainable electronic power and therefore dominated the renewable-energy market in the past decade. However, the existing indelible disadvantages such as exorbitant cell design and complicated and long-period manufacturing processes turned out to be the remaining challenges of existing Si-based photovoltaic technique that inevitably hindered the extensive replacement of the current petroleum-related energy. Recently, great efforts have been made for the development of next generation solar cells shooting to fulfill the criteria of both cost-effective approach and high-efficient energy conversion. Among them, several schemes have been applied to organic/inorganic hybrid solar cells that could potentially impact the photovoltaic designs.9−14 Basically, the hybrid solar © 2019 American Chemical Society
cells were made with the incorporation of conductive polymers such as PEDOT:PSS, P3HT, fullerene derivative PCBM, and others as charge-carrier transport layers with inorganic semiconductor materials in order to realize the p−n junction for the separation of photogenerated carriers.15−18 Specifically, PEDOT:PSS polymers were extensively utilized for the construction of high-performance hybrid solar cells owing to its highly visible transparency, sound ambient stability, and superior hole-transport conductivity.19−21 Previous studies have reported that the hole-transport conductivity could be further improved through modification of PEDOT:PSS polymers with either emerging polar organic solvents, acid treatments, or post-treatment.22−24 For instance, Pettersson et al. reported the significant enhancement of PEDOT:PSS conductivity based on adding the sorbitol into the PEDOT:PSS solutions prior to the film formation. 25 Furthermore, Kim et al. systematically demonstrated the influences of adding various solvents for polymer preparation, and the conductivity of the hybrid polymeric layers were increased from ∼0.8 to ∼80 S/cm.26 In addition, they further clarified the screening effect owing to the polar solvents between the involved dopants and the main polymeric chains that could greatly impact the carrier transport within cell designs. Although the transport characteristics of PEDOT:PSS conductive layers could be manipulated, the intrinsic low Received: March 15, 2019 Accepted: May 29, 2019 Published: May 29, 2019 4873
DOI: 10.1021/acsaem.9b00565 ACS Appl. Energy Mater. 2019, 2, 4873−4881
Article
ACS Applied Energy Materials
Figure 1. (a) Schematic illustration of VOx-inserted organic/inorganic hybrid solar cells. (b) The involving band diagram of cell design. were subjected to the electron-beam evaporator for the deposition of 200 nm thick Al layer on the rear side of Si substrates and then preserved in the vacuum chamber. 2.3. Device Fabrication. In device construction, the ITO glass with sheet resistance of 7 Ω/square that served as the upper electrode was subject to oxygen plasma (18W, PDC-32G, Harrick Scientific Inc.) for 1 min to improve the surface wettability and remove the residual hydrocarbon contaminations on ITO surfaces. Next, the saturated V2O5 powders as a precursor dissolved in DI water with the concentration of 44 mM was prepared with ultrasonication for 10 min and followed by a magnetic stir with 300 rpm for 20 h. The suspensions were further centrifuged (5000 rpm) and filtered with a PTFE membrane (0.45 μm) and directly deposited onto the ITO glass using a spin coater with a spin rate of 2000 rpm for 30 s. After that, the samples were dried at 140 °C for 5 min and followed by the annealing treatment under various operating temperatures including 140, 200, and 300 °C for 30 min, respectively. In addition, the binary p-type polymers containing 6 wt % of PEDOT:PSS molecules were uniformly dispersed in ethylene glycol. The dispersants were spun onto the ITO glass with a spin rate of 1000 rpm for 1 min and subsequently dried at 140 °C for 2 min. On the other hand, the prepared NW samples with the area of 2 × 2 cm2 were spin-coated with 6 wt % of PEDOT:PSS solutions under a spin rate of 2750 rpm for 45 s. The as-prepared ITO glass was carefully placed onto the polymer-coated NWs and then the loadings of 200 g in weight were applied to assist the device integration and finalize the fabrication of hybrid solar cells. The corresponding morphology and photograph of a photovoltaic device is shown in the Supporting Information. 2.4. Characterizations. The structural crystallinity was investigated with a Raman system (Renishaw inVia) and the crystallographic patterns of VOx layer were characterized with an X-ray diffractor (XRD, Bruker AXS Gmbh, D2 Phaser). The XRD patterns were indexed with the orthorhombic phase of V2O5 (JCPDS # 01089-0612) and the rhombohedral phase of V2O3 (JCPDS # 34-0187). In addition, the surface structures and chemical states of the VOx layer were analyzed with a Fourier transform infrared spectrometer (FTIR, Thermo Scientific, Nicolet 6700) and X-ray photoelectron spectrometer (XPS, PHI 5000 Versa Probe), respectively, and the surface topography was obtained from atomic force microscopy (AFM; Bruker Dimension ICON). Energy dispersive spectroscopy (EDS, Oxford EDS INCA 350) was used to characterize the chemical compositions of the formed VOx layer, and transmission/reflection spectra of the samples were measured with a UV−vis spectrophotometer (Hitachi, U3900H). The J−V characteristics along with cell performances were measured under a standard solar simulator (AM 1.5 G, 100 mW/cm2) equipped with a current−voltage measurement system (Keithley 2400).
band gap (∼1.5 eV) as well as involving HOMO state (5.0− 5.2 eV) might substantially limit the potential enhancement of cell efficiency. In fact, these might cause the inefficient characteristics of blocking the photogenerated electrons which led to carrier recombination particularly at interfaces of hole-transport layer and contact electrode. In order to overcome this demanding feature, several transition metal oxides (TMOs) such as V2O5, WO3, NiO, and MoO3 have been demonstrated and employed as a hole-transport layer in OPVs that yielded the air-stable photovoltaic performances.27−34 For instance, Lee et al. have demonstrated that the solar devices based on PEDOT:PSS doped with V2O5 could retain its conversion efficiency more than 98% after three weeks stored at ambient conditions.35 In light of these findings, in this study we performed the all-solution-processed method to prepare the highly transparent vanadium oxide (VOx) layer for boosting the photovoltaic performances in Si-nanowirebased hybrid solar cells. We found that utilizing the VOxinserted design sandwiched in between the ITO electrode and the underlying PEDOT:PSS layer did not merely provide the transparent and ohmic-like contact but further offered the hole-selectivity for blocking electrons that could reduce the carrier recombination at the interfaces.
2. EXPERIMENTAL SECTION 2.1. Chemicals. Acetone (99.8%, Honeywell), isopropyl alcohol (99.9%, Honeywell), nitric acid (65%, AppliChem), hydrofluoric acid (48%, Fisher Chemical), silver nitrate (99.85%, Acros Organics), ethylene glycol (extra pure 99+%, Acros Organics), PEDOT:PSS (PH-1000, UniRegion Bio-Tech), vanadium(V) oxide (99.6+%, Acros Organics) were purchased and used as received. 2.2. Fabrication of Si Nanostructures. The single crystalline Si substrates (resistivity = 1−10 Ω-cm and thickness = 525 μm) with the fixed size of 2 cm by 2 cm were used as the starting materials. Prior to the etching process, Si substrates were cleaned in the ultrasonication with acetone, isopropyl alcohol, and deionized water (DI) water several times. Subsequently, the as-cleaned substrates were immersed with the mixed solutions containing HF (4.6 M) and AgNO3 (0.01 M) for 3.5 min at 25 °C for the formation of regular Si nanowires (SiNWs) with a controlled length of 400 nm.36−41 After the etching process, the samples with formed NWs were dipped in the concentrated HNO3 solutions (65%) to completely remove the residual silver dendrite structures and followed by rinsing with DI water. The NW-based samples were then dipped in the dilute HF solutions (1%) to etch away the grown oxide on NW surfaces and followed by drying in the stream of N2 gas. After that, the samples 4874
DOI: 10.1021/acsaem.9b00565 ACS Appl. Energy Mater. 2019, 2, 4873−4881
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ACS Applied Energy Materials
Figure 2. (a) Raman spectra, (b) FTIR analysis, and (c) XRD patterns of the prepared VOx layer obtained from various annealing temperatures.
3. RESULTS AND DISCUSSION Figure 1a illustrated the basic features of organic/inorganic hybrid photovoltaic devices illuminated with solar lights on the ITO side. Basically, the involving photoelectric effect was activated by photoexcitation of SiNWs that generated the electron and hole pairs. Subsequently, the p−n heterojunction created with p-type PEDOT:PSS layer and n-type SiNWs readily separated the positive and negative charges; namely, the positive charges (holes) preferentially transported toward the polymer side, whereas the generated electrons with negative charges turned to move toward the SiNW counterparts. Nevertheless, the low lowest unoccupied molecular orbital (LUMO) level (3.5 eV) of p-type polymeric layer might allow the transport of photoexcited electrons reaching the ITO/ polymer interfaces according to the band diagram of integrated photovoltaic devices, as shown in Figure 1b. These resulted in a significant charge recombination at ITO surfaces and in turn degraded the photovoltaic performances. On the other hand, with the addition of V2O5 layers exhibiting a relatively high LUMO level (2.4 eV) and high bandgap energy (>3 eV) in between the ITO electrode and the p-type organic layer this feature favored to selectively block the transport of electrons toward the ITO electrode due to the created large potential barrier, thus reducing the possible recombination of charges occurred at ITO/polymer interfaces. Meanwhile, the offered comparable highest occupied molecular orbital (HOMO) energy of the V2O5 layer (4.7 eV) with PEDOT:PSS counterpart (5.1 eV) energetically stimulated the hole extraction through interfaces due to the formation of ohmiclike contact. Such induced V2O5 layer acted as a hole-selective contact that could efficiently benefit the current gain by both favoring the collection of holes and on the other hand minimizing the surface recombination at ITO electrodes, as schematically presented in Figure 1b. The structural crystallinity of the deposited VOx prepared with three different annealing temperatures was displayed in Figure 2a. The consistent Raman peaks representing vibration
modes that were assigned to be VO bending vibration (281 and 405 cm−1), VOV bending vibration (477 cm−1), V3O stretching (523 cm−1), V2O stretching (690 cm−1), and VO stretching modes (990 cm−1) could be found from these three samples, whereas the crystallinity quantified by the peak intensity increased with the increase of annealing temperature, owing to the supplement of thermal energy for the rearrangement of lattice atoms. To further characterize the bonding states of as-formed VOx layers, FTIR investigations were performed, as shown in Figure 2b. The characteristic absorption peaks spectrally located at 831 and 650−450 cm−1 were assigned to the VOV and VO coupled vibration and VOV stretching vibration, respectively. In addition, the pronounced peaks around 1021 cm−1 was contributed from the symmetric stretching vibration of the V5+O bonding states which was identified to be the characteristic of orthorhombic V2O5 structures. Moreover, one could observe the red-shifting of such V5+O features with respective to the decrease of annealing temperature, whereas no obvious shift of this characteristic absorption peak annealed at 300 °C compared with pure V2O5 powders. Botto et al. revealed that the shift of the V5+O vibration mode toward the lower wavenumber responded to the change of radius of the vanadium element in ordered lattice due to the variation of the oxidation state.42 This showed that the contained VOx no longer constituted the sole V2O5 phase; instead the low oxidation state of vanadium was implemented owing to the insufficiency of thermally activated atomic rearrangement at 200 and 140 °C. These findings could be further supported by the XRD characterizations where the VOx films were deposited with repeated cycles of 15 times for clearly revealing their crystallographic patterns. The diffracted analysis was indexed in Figure 2c, indicating the 21.6°, 32.3°, and 41.2° diffracted angles appearing in the samples annealed at 300 °C represented (101), (011), and (002) crystalline planes of orthorhombic V2O5 lattices, respectively. With lower annealing temperature of 200 and 140 °C, however, the corresponding 4875
DOI: 10.1021/acsaem.9b00565 ACS Appl. Energy Mater. 2019, 2, 4873−4881
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ACS Applied Energy Materials
Figure 3. XPS investigations of VOx layer synthesized under various annealing temperatures: (a) 140 °C, (b) 200 °C and (c) 300 °C, respectively. (d) Quantitative comparisons of V2O5 and V2O3 contents in the VOx layer prepared with three different annealing temperatures.
annealing temperatures, and these features might affect the optical and electrical aspect of photovoltaic cells. The surface topography of VOx thin films deposited on ITO glass was investigated by AFM measurements, as shown in Figure 4a. In comparison, the surface roughness of bare ITO glass was also measured. The results indicated that the surface roughness of samples with or without VOx coating was correspondingly comparable. Also, after experiencing PEDOT:PSS deposition on them, the deviation of resulting surface roughness was still limited to less than 12%, as compared in Figure 4a,d, respectively. These revealed the introduction of a VOx thin film did not substantially roughen the contact area of ITO with underlying organic layer and thus allowed them to maintain the well processing reliability of the cell integration. Moreover, the composition uniformity was further characterized with EDS mapping, as shown in Figure 5a. The representative sample annealed with 200 °C presented the rather uniform elemental composition of both vanadium and oxygen elements, revealing the successful deposition of VOx layer that acted as intermediate layer in between ITO electrode and p-type polymer layer for achieving the intended modification of carrier transport at interfaces. To further discriminate the capability of carrier separation, a sweeping bias from −4 to 4 V was applied on the VOxincorporated photovoltaic devices under the light illuminations with wavelength of 580 nm while the excited photocurrents were monitored, as shown in Figure 5b. It was found that the photocurrent enhancement deduced from the evaluation of Iphotocurrent − Idark current was highest in the case of 200 °C anneal-based devices, which was approximately 1.4 times and
characteristic diffraction intensities were reduced as well, whereas the additional diffraction pattern indexing (012) plane of V2O3 metastable crystal coexisted. The presence of rhombohedral V2O3 phase confirmed that the complete transformation of stable V2O5 phase was accompanied by the heat treatment at 300 °C; otherwise the coexistence of V2O3 and V2O5 mixed phases emerged at either 200 or 140 °C of the annealing treatment. The surface compositions as well as oxidation states of VOx thin films were further characterized with XPS analysis, as shown in Figure 3a−c. In addition to the consistent XPS peak located at 525 eV that originated from a V2p1/2 signal, the oxidation transition of VOx due to oxygen loss could be observed from low annealing temperature, shown in Figure 3a,b, where the dual-peak deconvolution of the XPS spectra revealed the appearance of V2O5 (517.3 eV) and V2O3 (515.9 eV) that reflected the resultant mixed phases of VOx. On the other hand, such structural inhomogeneity was restored and configured into the single V2O5 phase by raising the annealing temperature toward 300 °C, as evidenced in Figure 3c. Furthermore, the relative content of each oxidation state of VOx was quantified, as presented in Figure 3d. These results brought into view the phase transformation of VOx with respect to the varied thermal treatment, which indicated that the mixed phases mainly appeared at surfaces of the VOx layer at both 200 and 140 °C with different contents of the V2O3 phase of 10% and 15%, respectively, and were followed by the formation of a single stoichiometric configuration of the V2O5 phase at 300 °C. This accounted for the transformation of a VOx thin film particularly at surface structures with varied 4876
DOI: 10.1021/acsaem.9b00565 ACS Appl. Energy Mater. 2019, 2, 4873−4881
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Figure 4. AFM results of various samples without (figure left) and with (figure right) deposition of PEDOT:PSS coating: (a) bare ITO glass, ITO/ VOx prepared with (b) 140 °C, (c) 200 °C, and (d) 300 °C of annealing process, respectively.
Figure 5. (a) EDS characterizations of VOx layer (200 °C anneal). The inserted figures presented the 2D EDS mapping of involved vanadium and oxygen elements, respectively. (b) Photocurrent investigations of various samples. The dark currents were measured with sweeping bias from −4 to 4 V from the VOx-inserted sample without light illuminations. The inserted panel shows the comparisons of photocurrent enhancement from four different samples.
1.9 times higher than 300 °C anneal-based and reference cells, respectively, as compared in the inserted panel of Figure 5b.
This speculated that the hole extraction from light excitation of devices was accomplished because of the accessible ohmic-like 4877
DOI: 10.1021/acsaem.9b00565 ACS Appl. Energy Mater. 2019, 2, 4873−4881
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Figure 6. Transmission spectra of samples (a) without and (b) with PEDOT:PSS deposition. (c) The corresponding images for the validation of sample transparency.
Figure 7. Evolution of (a) J−V characteristics and (b) EQE spectra of various hybrid solar cells.
respectively. The results indicated that light transmittivity of samples was not significantly altered by inserting the VOx as an electron-blocking layer, where the high transmission (>87%) was maintained regardless of preparing PEDOT:PSS coating. This could be further discovered in Figure 6c, showing the comparably transparent VOx/ITO electrode prepared with various annealing temperatures. Furthermore, it reflected the ignorable absorption of solar light with such introduced VOx layers, which benefited the effective carrier generation from the underlying SiNW counterparts under solar illuminations. In addition, the light-absorption properties of photovoltaic devices were presented in the Supporting Information.
contact through the introduction of a VOx layer, which resulted in the photocurrent enhancement compared with the reference cells. More importantly, the V2O5 counterparts within a VOx film essentially emerged as electron-blocking components that effectively reduced the charge recombination encountered at ITO/polymer interfaces, which accounted for the superior enhancement of photogenerated currents through the introduction of a VOx layer. In addition to the reduced charge recombination, the light reflectivity representing the number of incoming photons that could enter the solar cells was examined, as shown in Figure 6. Figure 6a,b compares the spectral transmission of VOxdeposited ITO glass without and with polymer coating, 4878
DOI: 10.1021/acsaem.9b00565 ACS Appl. Energy Mater. 2019, 2, 4873−4881
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in VOx/hybrid solar cells (200 °C anneal), followed by 140 °C anneal and 300 °C anneal of VOx/hybrid solar cells and then the reference cells while examining the broad wavelength range from 400 to 1000 nm. It was believed that the mixed VOx phases as an insertion layer acted a decisive role on cell performances, which offered dual functionalities on both electron blocking and hole transport phenomena. The dominant V2O5 components essentially provided the carrier selectivity by displaying the potential barrier that blocked the possible gathering of electrons at ITO sides, as shown in Figure 1b. Meanwhile, the minor V2O3 counterpart with rather low contact resistance enabled one to guide the effective transport of holes reaching the electrode surfaces,43−45 thus providing the framework of carrier selective contact for high-performance hybrid solar cells. It could be also intuitively understood that the optimal relative composition of mixed phases directly corresponded for raising both Jsc and FF values, where the latter was positively correlative with the series resistance (Rs), as shown in Table 1. These features explained that the lowest Rs was achieved to be 4.86 Ω from the mixed VOx layer annealed at 200 °C, whereas the Rs from the sole V2O5 phase obtained at 300 °C anneal (6.03 Ω) and reference cells (6.31 Ω) were both comparably high. These results corresponded to the lowest contact resistance of VOx film annealed at 200 °C among these three cases. By considering the photovoltaic characteristics, foremost the highest conversion efficiency with a value of 14.4% was realized by inserting a VOx layer in hybrid solar cells prepared with 200 °C anneal, demonstrating an above 36% enhancement in conversion efficiency compared with the reference cells, evidencing its activation for the improvement of cell performances by well managing the carrier selectivity and hole extraction properties. Finally, characterizations of spatial photocurrent distributions were performed to examine the conversion uniformity of photons to currents under light illuminations. This was particularly crucial in nanostructure-based solar cells where the existence of structural defects, contact voids or cracks
Figure 7a presented the current density versus voltage (J−V) characteristics of the three fabricated VOx-based hybrid solar cells measured with standard AM 1.5 illumination systems. In addition, the performance of hybrid solar cells without the presence of the VOx electron-blocking layer, termed as reference cells, was compared, and the cell parameters extracted from corresponding J−V data are shown in Table 1. The noticeable improvement of open-circuit voltage (Voc) Table 1. Photovoltaic Characteristics of Reference Cells and VOx-Inserted Hybrid Solar Cells Prepared with Various Annealing Conditions type
efficiency (%)
Jsc (mA/cm2)
refereence cell 140 °C anneal 200 °C anneal 300 °C anneal
9.1 12.5 14.4 10.4
35.6 41.5 45.4 38.2
Voc (V) FF (%) Rs (Ω) 0.499 0.540 0.541 0.530
50.91 55.77 58.31 51.28
6.31 5.12 4.86 6.03
was found from all VOx-introduced hybrid solar cells (Voc > 0.530 V) in comparison with the reference cells (Voc = 0.499 V). In view of the band diagram at the created interfaces of inserted VOx and the underlying p-type polymeric layer, it could be elucidated that the upbending of LUMO level due to the insertion of VOx layer could create a potential barrier for electrons, which accounted for the promotion of carrier separations and in turn improved the collection efficiency of excited currents in ITO electrode. Aside from Voc, the substantial increase of short-circuit current density (Jsc) was also encountered, as shown in Table 1. These results readily matched the responses from external quantum efficiency (EQE) measurement through considering the combined effects of optical management and carrier recombination, as presented in Figure 7b. A comparison of EQE analysis explicitly indicated the greater efficiency gain of photoexcitation mechanism appearing
Figure 8. (a) LBIC 3D maps of hybrid solar cells (a) without and (b) with VOx insertion obtained from 200 °C anneal. Photoluminescence images of hybrid solar cells (c) without and (d) with VOx insertion prepared with 200 °C anneal. 4879
DOI: 10.1021/acsaem.9b00565 ACS Appl. Energy Mater. 2019, 2, 4873−4881
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ACS Applied Energy Materials ORCID
might reflect to the degradation of photoresponse. For this, an emerging nondestructive technique, laser beam induced current (LBIC), was performed using a 450 nm laser beam with the spot size of approximately 10 μm and scanning rate of 25 μm/sec, as presented in Figure 8a,b. The 3D mapping results clearly indicated the profiles of photogenerated currents covering the junction sites as a function of their spatial positions in both VOx-inserted hybrid solar cells and reference cells, respectively. The relatively uniform photoresponse appearing in the hybrid solar cells incorporated with a VOx thin layer corresponded to the less nonuniformity of hybrid nanostructures, which yielded the larger effective area for activating the photovoltaic operation. In addition, the photoluminescence imaging was conducted to acquire the structural defects in planar view, as compared in Figure 8c,d. The visible nonuniform spots could be frequently observed in the reference cells, which might be attributed to the incomplete coverage of p-type polymer film on either ITO electrode or SiNWs. These inevitable processing defects could be compromised through modifying the ITO surfaces with a VOx layer, where the coverage of p-type polymer could be greatly improved due to the moderate surface wettability, as indicated in the Supporting Information, thus leading to the less defective luminescence signature.
Chia-Yun Chen: 0000-0002-8357-3968 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Ministry of Science and Technology of Taiwan (MOST 107-2221-E-006-013-MY3), and Hierarchical Green-Energy Materials (Hi-GEM) Research Center, from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) and the Ministry of Science and Technology (MOST 107-3017-F-006-003) in Taiwan. The authors greatly thank the Center for Micro/Nano Science and Technology, National Cheng Kung University with the facilities provided for conducting material characterizations.
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CONCLUSIONS In conclusion, the efficiency improvement of organic/inorganic hybrid solar cells was achieved with the introduction of VOx layer as electron-blocking contact at ITO surfaces, obtaining the conversion efficiency of 14.4% with around 1.6 times beyond the conventional VOx-free hybrid solar cells. It was found that the introduced VOx layer contained mixed V2O5 and V2O3 phases, where the former essentially acted as electron-blocking components for minimizing the charge recombination at ITO surfaces and the later facilitated the transport of photogenerated holes collected by the ITO electrode. In addition, by imaging the laser-induced currents and photoluminescence behaviors, it was confirmed that the optimal anneal temperature of 200 °C for the preparation of the inserted VOx layer provided the uniform photoresponse in the hybrid solar cells. Given the combined effects, the prospective applications based on such hybrid design could be further extended to other functional optoelectronic devices by emerging the presented facile and inexpensive solutionprocessing techniques.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00565.
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REFERENCES
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Crystal structures of V2O3 and V2O5, cross-section SEM image of the constructed heterojunctions, morphological observation of the hybrid solar cells, photographs of the prepared devices and detailed photovoltaic measurements, light-absorption characteristics of the solar cells, Mott−Schottky plot of the VOx-coated ITO glass, and wettability of ITO/VOx surfaces (PDF)
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DOI: 10.1021/acsaem.9b00565 ACS Appl. Energy Mater. 2019, 2, 4873−4881
Article
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DOI: 10.1021/acsaem.9b00565 ACS Appl. Energy Mater. 2019, 2, 4873−4881
