- Email: [email protected]
Inverted polymer bulk heterojunction solar cells with ink-jet printed electron transport and active layers
Organic Electronics 35 (2016) 118e127
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Inverted polymer bulk heterojunction solar cells with ink-jet printed electron transport and active layers Arjun Singh a, b, *, Shailendra Kumar Gupta a, b, Ashish Garg a, b, ** a b
Department of Materials Science and Engineering, Indian Institute of Technology, Kanpur, Kanpur, 208016, India Samtel Centre for Display Technologies, Indian Institute of Technology, Kanpur, Kanpur, 208016, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 March 2016 Received in revised form 8 May 2016 Accepted 10 May 2016
Ink-jet printing is a potentially attractive technique for printing of components for organic electronic devices primarily due to its ability to print patterned layers and reduced ink wastage. However, the mechanism of film formation is quite complex and needs an understanding of various printing parameters on the film growth. In this manuscript, we successfully demonstrate ink-jet printing of smooth zinc oxide (ZnO) thin films with controlled thickness as electron transport layers for inverted organic solar cell devices fabricated on indium tin oxide coated glass substrates. The parameters that strongly affect the formation of a continuous ZnO thin film with controlled thickness are ink concentration and viscosity, substrate surface treatment, drop spacing, substrate temperature during printing and the annealing temperature, affected by a combination of surface energetics, surface tension of the ink and the rate of solvent evaporation. The results suggest that one can achieve a transmittance of >85% for a 45 nm thin ZnO film possessing uniform structure and morphology, fabricated using a drop spacing of 40 e50 mm at an ink viscosity of 4.70 cP with substrate held at room temperature. The P3HT:PC61BM inverted organic solar cell devices fabricated using printed ZnO films as electron transporting layers exhibit an efficiency of ~3.4e3.5%, comparable to that shown by the devices fabricated on spin coated ZnO films. Finally, the device with printed P3HT:PC61BM active layer on printed ZnO layer showed a device efficiency of ca. 3.2% suggesting that nearly completely printed devices can deliver a comparable performance to the spin coated devices. © 2016 Elsevier B.V. All rights reserved.
Keywords: Organic solar cells Ink-jet printing Zinc oxide Electron transport layer Active layer P3HT:PC61BM
1. Introduction One of the attractive features of organic electronic devices is the feasibility of their fabrication by solution processing methods, in particular printing based methods offering enormous potential for large area fabrication of devices with reasonably high throughput [1e3] and hence possible reduction in costs. Whilst the blend of Poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61butyric acid methyl ester (PC61BM) has been a workhorse system for bulk heterojunction (BHJ) organic solar cells, development of new low band-gap polymers and blend systems such as those based on Poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b0 ]
* Corresponding author. Department of Materials Science and Engineering, Indian Institute of Technology, Kanpur, Kanpur, 208016, India. ** Corresponding author. Department of Materials Science and Engineering, Indian Institute of Technology, Kanpur, Kanpur, 208016, India. E-mail addresses: [email protected] (A. Singh), [email protected] (A. Garg). http://dx.doi.org/10.1016/j.orgel.2016.05.015 1566-1199/© 2016 Elsevier B.V. All rights reserved.
dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno [3,4-b]thiophenediyl}) or PTB7 and Phenyl-C71-butyric acid methyl ester (PC71BM) has seen laboratory scale device efficiencies exceed 10%. These OSC devices are typically fabricated in normal and inverted architectures on indium tin oxide (ITO) coated glass substrates with ITO acting as anode in normal devices and as cathode in inverted devices. Poor life times of normal organic solar cell devices primarily using hole transporting poly(3,4ethylenedioxythiophene) polystyrene sulfonate) (PEDOT:PSS) layer on ITO electrode [4e6] and problems with the use of Al cathode [7] led to the development of inverted OSC devices. In inverted OSC devices, the hole transporting PEDOT:PSS layer between the active layer and ITO electrode is replaced by a thin electron transport layer (ETL), typically made of a n-type material such as zinc oxide (ZnO) or titanium dioxide (TiO2) [8,9] with ZnO being a typical choice as ETL whilst another oxide such as molybdenum trioxide (MoO3) [10,11] is used as a hole transport layer (HTL) between the active layer and the top electrode. Presence of stable oxides [12,13] at the electrode - active layer interfaces makes
A. Singh et al. / Organic Electronics 35 (2016) 118e127
119
Fig. 1. Variation of drop shape vs elapsed time at a jetting voltage of 12 V for inks with different concentration (a) 0.25 M, (b) 0.35 M, (c) 0.45 M, (d) 0.55 M and (e) 0.70 M with Fromm number mentioned in each figure.
this inverted structure exhibit longer life times as compared to the normal OSC devices [14,15]. For mass production of OSCs, various printing methods are being considered. Among these, ink-jet printing, although not widely considered as a roll-to-roll process, is one of the attractive printing methods due to its ability to form patterned films eliminating or minimizing the need for lithography. Also, the technique offers superior control on the delivery of ink and its reduced wastage. Moreover, one can manipulate the speed of printing by employing multiple print heads and nozzles [16]. Ink-jet printing has been successfully explored for the deposition of common semiconducting polymeric materials such as PEDOT:PSS [17,18] and P3HT:PC61BM [19e24] for OSCs with devices yielding acceptable performances. However, the process of thin film formation in inkjet printing is quite complex and is strongly affected by the parameters such as ink viscosity, drop spacing, substrate temperature
and surface treatment, determined by the type of material. From the perspective of inverted OSC device development by printing on ITO coated glass or plastic substrates, the first step has to be the printing of ZnO films followed by printing of active layer blend and other layers. However, in comparison to the printing of active layer blend and PEDOT:PSS, printing of oxides is a rather poorly addressed topic highlighting the need for detailed explorations. Specifically, in case of ZnO, whilst researchers have reported fabrication of ZnO in various forms such ZnO seed layer for nanorods [25], ZnO nanoparticles for gas sensors [26], amorphous oxides [27], zinc tin oxide [28e31], indium zinc tin oxide [28] and a few other oxides capped with nanoparticles using ink-jet printing method [32], none of these reports conclusively demonstrate the fabrication of ZnO films with controlled thickness which were further integrated into devices such as a working OSC device. Although there are recent reports on the ink-jet printing of ZnO for
120
A. Singh et al. / Organic Electronics 35 (2016) 118e127
Fig. 2. Optical micrographs of ZnO films printed at drop spacing of 25 and 30 mm on ITO/glass substrates (a) without surface treatment (b) with UVO surface treatment. Images on the right show the corresponding contact angle measurements.
OSC devices [24,33], the details are sketchy. The challenges one can encounter in the printing of an oxide such as ZnO can be formation of ink with appropriate viscosity and surface tension, wetting onto the substrate and formation of a thin oxide film with dense microstructure and uniform thickness. Printing of active layer for OSCs, in particular P3HT:PC61BM, on the other hand is a relatively well studied topic using a variety of printing techniques such as gravure printing [34], screen printing [35,36], flexographic printing [37] or ink-jet printing [22,24,33,38] with most efforts relating to normal architecture devices. For example Hoth et al. [22] reported the ink-jet printing of P3HT:PC61BM with different solvents such as tetraline, O-Dichlorobenzene and mesitylene and reported the device efficiencies around 2.9%. Use of additives such as 1,8-octanedithiol (ODT), odichlorobenze and chloronephthalene (Cl-naph) leads to further improvement in device efficiencies [39]. However, one would still need to develop the process for printing the active layer on a new buffer layer such as ZnO, albeit with a reduced level of optimization. In this report, we successfully demonstrate ink-jet printing of ZnO thin films via a detailed study with a focus on understanding the formation of ZnO thin films. We have studied the ink-jetting characteristics and stable drop formation by varying the concentration of the ink and have investigated the effect of process parameters such as drop spacing, substrate temperature, substrate surface modification and annealing temperature on the film formation, critical to the formation of a good quality film for OSC devices. This was followed by printing of P3HT:PC61BM active layer on the printed ZnO film with printed device efficiencies remarkably comparable to those obtained on spin coated devices.
ambient and were transferred to the glove box for device fabrication. To fabricate inverted organic solar cell devices, a blend solution of P3HT and PC61BM was prepared in chlorobenzene in 1:0.8 wt ratio followed by stirring for 12 h in a nitrogen filled glove box. After spin coating the P3HT:PC61BM solution on the printed ZnO layer, the films were annealed for 10 min at 150 C on a hot plate. For ink-jet printing of P3HT:PC61BM, the ink was formulated by using low and high boiling point solvents to avoid fast evaporation of chlorobenzene using tetralene (20% v/v) as an additive to chlorobenzene. The P3HT and PC61BM concentration in the ink was 5 mg/ml each. After mixing all the components, the solution were stirred for 12 h at 45 C in N2 filled glove box to yield a stable ink solution. For comparison, a control sample was fabricated on spin coated ZnO layer. Further, 10 nm thin hole transport layer of MoO3 and a 100 nm Ag layer as top electrode were deposited by thermal evaporation through a shadow mask. The final device structure was Glass/ITO/ZnO/P3HT:PC61BM/MoO3/Ag. Photovoltaic device characteristics were measured in dark as well as light (1 Sun/1.5G solar spectrum) using a Keithley 2400 source meter. Surface topography and morphological studies were carried out using Zeiss optical microscope and Asylum research MFP-3D atomic force microscope (AFM), latter also being used for Kelvin probe force microscopy (KPFM) measurements. KLA Tencor surface profilometer was used for thickness measurement of the layers. Optical absorbance and transmittance of ZnO layers were measured using a Perkin Elmer Lambda 750 UVevis spectrophotometer. 3. Results and discussion
2. Experimental details 3.1. Role of ZnO ink viscosity on ink jetting behavior Zinc oxide inks of concentrations 0.25 M, 0.35 M, 0.45 M, 0.55 M and 0.70 M were formulated using zinc acetate (Zn(CH3COO)2) as a precursor material, dissolved in 2-methoxyethanol as a solvent and with further addition of controlled amounts of monoethanolamine (C2H7NO) as a stabilizing agent. The clear ink solution was filtered through 0.45 mm PVDF filter before transferring into the printer cartridge followed by holding it for 20 min to remove any trapped air bubbles. The ZnO layer was printed on cleaned ITO coated glass substrates using DiMatix2831 ink-jet printer with 16-nozzles (nozzle diameter ~ 21 mm). Prior to the printing of ZnO films, ITO substrates were subjected to UVeOzone treatment for 15 min. The printed films were annealed at 175 C and 250 C for 10 min in the
First, we investigated the jetting behavior of ZnO ink through the nozzle of ink-jet printer by changing the ink viscosity at a fixed jetting voltage (12 V) applied to the print head of the printer. The experiments were performed on the inks of five different viscosities: 2.07 cP, 2.46 cP, 2.90 cP, 3.62 cP and 4.70 cP, corresponding to the inks concentrations 0.25 M, 0.35 M, 0.45 M, 0.55 M and 0.70 M respectively and CCD camera images of jetting behavior of the formulated ZnO ink are shown in Fig. 1. Jetting behavior of the drops is highly dependent on the viscosity which is manifested in the dimensionless parameter, called as Fromm number [40], Z and is given as:
A. Singh et al. / Organic Electronics 35 (2016) 118e127
121
Fig. 3. Optical micrographs of ZnO films, ink-jet printed using inks of different concentration and viscosities (a) 2.07 cP (0.25 M) (b) 2.43 cP (0.35 M) (c) 2.90 cP (0.45 M) (d) 3.62 cP (0.55 M) and (e) 4.70 cP (0.70 M). The drop spacing was kept at 35, 45 and 50 mm.
Z¼
ðargÞ1=2
h
where, a is the radius of the printing orifice (~21 mm), r is the density (~1009 kg/m3), g is the surface tension and h is the viscosity of the ink. Corresponding Z values are (a) 14.02 (0.25 M) (b) 11.12 (0.35 M), (c) 9.15 (0.45 M), (d) 7.90 (0.55 M) and (e) 5.62 (0.70 M). To form a drop without a satellite, while Fromm [40] et al. suggested a value of Z > 2, recent works [41] suggest a range of 4 Z 14 as more appropriate. Subsequent work by Reiss and
Derby [42] suggested that printable ink should have Z values between 1 and 10. They observed that the upper values of Z (i.e. lower viscosity) are determined by the satellite formation in the ink whilst the lower limit of Z is governed by dissipation of pressure wave by fluid viscosity. For a spherical drop to form, the head and the tail drops need to merge after a short time i.e. when head and tail drop velocities are the same. As Fig. 1 shows, we get spherical drops only for the case of Z ¼ 5.62 when the tail droplets merges with the head droplet after an elapsed time of 60 ms. All other inks of higher Z values did not yield spherical drops even after an
122
A. Singh et al. / Organic Electronics 35 (2016) 118e127
elapsed time of 60 ms as visible from the jetting behavior shown in Fig. 1(aee) with tail drop velocity being lower than the head velocity. 3.2. Effect of surface treatment on the printing of ZnO ink Subsequently, we investigated the printing of zinc oxide precursor ink with and without surface UV Ozone (UVO) treatment of the substrate surface at different drop spacing of 25 mm and 30 mm and results are shown below in Fig. 2. As the figure shows, for both the drop spacing values, the ink did not spread uniformly over the untreated substrates leading to the formation of droplets due to coalescence of different drops, indicating towards poor wetting characteristics of the substrate surface by the ink. However, after a 15 min UVO surface treatment of the surface, the jetted drops spread to give rise to a uniform film over the ITO substrates for both the drop spacing values suggesting an improved wettability of the substrate by ZnO ink. Further contact angle measurements (Fig. 2) showed that this is due to a decrease in the contact angle on the surface after ozone treatment: the contact angle value was 45.9 without any surface treatment which decreased to 14.3 after UVO treatment for 15 min. 3.3. Influence of ZnO ink viscosity and drop spacing on ink-jet printing The viscosity of the ink affects not only the jetting behavior of the ink as shown earlier, it also affects the morphology of the printed film in a significant manner. This is because the spreading of the ink is dependent on the ink viscosity as well as the drop spacing. The optical micrographs of the printed ZnO films with 15 min UVeOzone treatment at different ink viscosities as well as drop spacing are shown in Fig. 3. In this experiment, the lines were printed on the substrates, which gradually merge to form the film on their own. The images seem to suggest a mixed effect of drop spacing and the ink viscosity on film’s uniformity. The results suggest that at lower viscosity of 2.07 cP, the films are not uniform with the formation of pin-holes in the film as shown in Fig. 3 (a). At the viscosity of 2.43 cP, film consists of lines at all the drop spacings which tend to be sparsely spaced at higher drop spacing as shown in Fig. 3(b) and never disappear completely. In contrast, the printed ZnO films appear to be more uniform for higher viscosity of the ink as shown in Fig. 3(cee)) with lines being less prominent. Particularly, for the viscosity of 4.70 cP, the film is more uniform and dense with no lines present at a drop spacing of 45 mm. Our results suggest that the ink with higher viscosity spreads better especially at the drop spacing of 45e50 mm whilst ink with low viscosity leads to undulations. These observations, although counterintuitive, suggest a competition between the evaporation of the solvent from the printed lines and lateral spreading on the substrate. Formation of smooth films at higher viscosity appears to suggest an optimum balance between these parameters preventing the build-up on the edges of the printed lines. This factor is examined again in the next -vis substrate temperature. Based on the above results, section vis-a all further experiments were carried on the ZnO films printed using an ink of viscosity 4.70 cP with a concentration of 0.70 M. 3.4. Effect of substrate temperature on the morphology of printed ZnO films
Fig. 4. (a) Optical micrographs of printed films (b) schematic diagram showing flow of ink during solvent evaporation and (c) line profiles of printed ZnO lines at different substrate temperatures.
After investigating the effect of surface treatment and viscosity, we investigated the effect of substrate temperature during ink-jet printing on the morphology of printed ZnO films. Fig. 4 (a) shows the optical microscope images of ink-jet printed ZnO films at different substrate temperatures with corresponding 2-D profiles
A. Singh et al. / Organic Electronics 35 (2016) 118e127
123
Fig. 5. (a) Optical transmittance of printed ZnO film annealed at 175 C and 250 C (inset shows the absorbance of P3HT:PC61BM blend film deposited on printed ZnO films); (bec) AFM images (5 5 mm2) showing topography of corresponding ZnO films.
Fig. 6. (a) Schematic of inverted OSC device structure; plots of current density (J) vs voltage (V) of OSC devices in (b) dark (c) under illumination (AM 1.5 G, 1 Sun); (d) external quantum efficiency of the devices on printed ZnO films.
Table 1 Performance of inverted OSC devices on printed ZnO films (Ci: ink concentration, Ta: annealing temperature of ZnO film). Reference sample was fabricated on spin coated ZnO films. Ci
Ta ( C)
Jsc (mA/cm2)
Voc (V)
FF (%)
PCE (%)
Best PCE (%)
Rs (U-cm2)
Rsh (U-cm2)
0.70 M
175 250 175
9.8 ± 0.3 9.7 ± 0.1 8.8 ± 0.1
0.60 0.58 0.64
58.9 ± 3.8 51.9 ± 3.9 59 ± 3.6
3.4 ± 0.1 2.9 ± 0.2 3.4 ± 0.1
3.5 3.1 3.5
11.0 13.3 11.5
717.4 456.4 700.5
Ref.
shown in Fig. 4 (c). The results show that while the films printed at 25 C (room temperature or RT) and 30 C are quite uniform, those printed at 40 C and 50 C tend to possess the wavy edges. This can be explained on the basis of flow of ink and solvent evaporation
during film formation due to competition between the precursor ink movement due to surface tension and the evaporation of the solvent, as shown schematically in Fig. 4(b). The increase in the substrate temperature leads to faster evaporation of the solvent and
124
A. Singh et al. / Organic Electronics 35 (2016) 118e127
Fig. 7. AFM images of 115 nm thin printed P3HT:PC61M film annealed at (a) 120 C (15 min), (b) 120 C (10 min) followed by 150 C (5 min) and (c) 150 C (15 min).
dependence of transmittance on the annealing temperature, the films were examined using AFM whose results are shown in Fig. 5(b and c). The images show that although topography of both the films shows a nanocrystalline structure, ZnO films annealed at 175 C show lower roughness (0.8 ± 0.1 nm) as compared to those annealed at 250 C (1.2 ± 0.1) which could result in reduced surface scattering leading to slightly enhanced transmission of the films annealed at 175 C. 3.6. Fabricated of organic solar cells on printed ZnO films
Fig. 8. UVevisible absorption spectra of 115 nm thin printed P3HT:PC61BM films at different annealing temperatures.
hence causing non-uniformity on film surface as the edges become thicker with increase in the substrate temperature. The printed line width also decreases with increase in the substrate temperature, as one can observe from Fig. 4(c). The line width is 0.25 mm at RT and decreases to 0.16 mm at a higher substrates temperature of 50 C implying that the drop spreading is lesser at higher temperatures, resulting in non-uniformity in the printed film surface at higher substrate temperature.
3.5. Effect of annealing temperature on the optical behavior of printed ZnO films To investigate the effect of annealing temperature on optical transmission, 45 nm thin ZnO films were printed using a drop spacing of 45 mm at RT from an ink of viscosity 4.70 cP on UVOzone treated ITO coated glass substrates followed by annealing at 175 C and 250 C, for 10 min in air and the results are shown in Fig. 5(a). Fig. 5(a) suggests that the optical transmittance of both the films above 450 nm is quite similar and comparable to that of ITO/ glass. However, between 350 and 450 nm, the transmission of film annealed at 175 C is superior to that annealed at 250 C which is also manifested in the absorbance of P3HT:PC61BM blend film as shown in the inset. To examine the role of surface topography in
To demonstrate the viability of printed ZnO films for photovoltaic applications, we fabricated the inverted OSC devices with structure Glass/ITO/ZnO/P3HT:PC61BM/MoO3/Ag having printed ZnO films as ETL (Fig. 6(a)) and the photovoltaic measurement results are shown in Fig. 6 and Table 1. The devices fabricated on printed ZnO films annealed at 175 C (p-ZnO175) show power conversion efficiency (PCE) of 3.4 ± 0.1%, comparable to those obtained on spin coated ZnO (s-ZnO) films (3.4 ± 0.1%) and with superior reproducibility. Interestingly, the short circuit current density (Jsc) of the device on p-ZnO175 film is a bit higher than that on spin coated ZnO despite slightly higher absorbance of the latter which could be due to slightly reduced series resistance (Rs) and improved shunt resistance (Rsh) attributed to higher roughness of s-ZnO films (1.7 nm after annealing at 175 C) leading to higher interfacial roughness. On the other hand, Voc of the device on s-ZnO is slightly better and as a result both devices have nearly comparable performance. Comparison of the devices fabricated on ZnO films annealed at different temperature shows that the device on p-ZnO175 exhibits much improved device fill factor (FF) and hence higher PCE compared to the device on p-ZnO250 (printed ZnO film annealed at 250 C). This is a desirable finding because lower annealing temperature of the device components is a positive attribute for large area processing of organic solar cells reducing the thermal budget as well as the process applicability to the substrates which cannot withstand high temperature annealing operation. The improved device performance at low annealing temperature of ZnO films could be related lower roughness of ZnO and hence low interfacial roughness with the active layer leading to a decrease in the shunt resistance of the devices and higher series resistance, as shown in Table 1. The semi-log dark J-V characteristics, as shown in Fig. 6(b), also show that the rectification ratio decreases with an increase in the annealing temperature: the rectification ratio for devices made on p-ZnO175 films is ~102 at ± 0.5 V with minimum lowest leakage current indicating better diode performance. The decrease in the
A. Singh et al. / Organic Electronics 35 (2016) 118e127
125
Fig. 9. J-V characteristics of the inverted OSC devices with printed P3HT:PC61BM active layer and ETL of printed ZnO of different thicknesses in (a) dark and (b) light conditions.
Table 2 Parameters of inverted OSC devices with 115 nm thin printed active layer (P3HT:PC61BM) and printed ETL (ZnO) with varying ETL thickness. Printed ZnO thickness (nm)
Voc (V)
45 38 35 25
0.62 0.62 0.63 0.60
nm nm nm nm
± ± ± ±
Jsc (mA/cm2) 0.01 0.02 0.01 0.02
7.5 8.3 8.1 9.2
± ± ± ±
0.5 0.4 0.6 0.1
FF (%) 51.8 51.4 54.4 57.0
rectification ratio (ION/IOFF ratio) is indicative of increased interface trap density with increasing annealing temperature of printed ZnO films. Higher roughness of p-ZnO250 films in comparison to pZnO175 films results in an effectively larger ZnO/active layer interfacial area leading to higher interfacial trap density which causes undesired trap assisted recombination of the carriers and hence lowers the FF [10]. This is also reflected in the external quantum efficiency (EQE) of all the devices as shown in Fig. 6(d). The maximum EQE is achieved for the devices on p-ZnO175 films with a value of ca. 57% at 500 nm which is in agreement with higher optical transmittance of the films (~85% in visible range). In contrast, the EQE is lowered to ca. 47% at 500 nm for devices made on pZnO250 films. Further, using the dark J-V data of the devices from Fig. 6 (b), we calculated the built-in voltage (Vbi) of the devices fabricated on pZnO175 and p-ZnO250 films following Mantri et al. [43]. The results show that the device on p-ZnO175 films is 0.83 V, higher than 0.75 V shown by the device on p-ZnO250 films. The Vbi is equal to [(EFS-Ec)D] where EC is Fermi level of cathode, EFS is the work function of semiconductor at equilibrium and D is the voltage drop due to interfacial layer [44]. The increase in Vbi of the devices on p-ZnO175 films is also concomitant with the lower surface roughness of pZnO175 films as compared to p-ZnO250 films. This emphasizes that p-ZnO175 films provide an improved interface leading to reduced voltage drop at the interface (D) resulting in enhanced charge extraction in the device. 3.7. Device fabrication with printed active layer and electron transport layer Next, we printed P3HT:PC61BM active layers on printed ZnO films with device structure shown in Fig. 6(a). The jetting of P3HT:PC61BM ink was acceptable at 12 V without any nozzle blocking. Subsequently, 115 nm thin printed films were annealed at various temperatures: 120 C (15 min), 150 C (15 min) and a twostep annealing (120 C (10 min) followed by 150 C (5 min)). The films were cooled in the glove box by taking them off the hot plate and keeping them on a surface at RT. Although the printed films are
± ± ± ±
PCE (%) 6.9 4.9 3.7 2.3
2.4 2.7 2.8 3.1
± ± ± ±
0.3 0.3 0.3 0.1
Best PCE (%)
Rs (U-cm2)
Rsh (U-cm2)
2.7 3.0 3.1 3.2
14.2 15.5 13.5 10.8
550 684 613 862
quite uniform at all temperatures as suggested by the AFM images of the samples (Fig. 7), with increase in the annealing temperature, phase separation of PC61BM appears to occur in the films annealed at 150 C (Fig. 7(a)) as suggested by bright spots on the surface. The images show that the film annealed at 120 C (15 min) possesses highest roughness 6.7 ± 0.6 nm while the film annealed at 150 C (15 min) has lowest roughness 4.4 ± 0.5 nm and the film 120 C (10 min) þ 150 C (5 min) yields an intermediate roughness of 5.3 ± 0.2 nm. Annealing at higher temperature make render P3HT polymer chain becoming soft and flexible and hence the interdiffusion of PC61BM may occur [45] with the growth of crystallites on the surface. Further, UVevis absorption spectra of printed P3HT:PC60BM films with variation in the heat treatment temperature, as shown in Fig. 8, show that the highest absorbance is achieved when the film is annealed in two steps i.e. 120 C (10 min) followed by 150 C (5 min). In contrast, intermediate absorbance is achieved when the film was annealed at 120 C and is minimum when film was directly annealed at 150 C, latter attributed to the phase separation of PC61BM crystallites. Finally we fabricated the inverted OSC devices by printing both ZnO layer to act as ETL and P3HT:PC61BM blend as active layer with device structure Glass/ITO/ZnO(p)/P3HT:PC61BM(p)/MoO3/Ag (p: printed). The devices consisted of a 115 nm thin P3HT:PC61BM blend layer annealed in two stages (120 C (10 min) þ 150 C (5 min)). The thickness of ZnO layer was varied as per the details in the preceding sections as its a crucial factor in determining OSC device performance due to optical field effects [46]. The device performance at various thicknesses of printed ZnO i.e. 25, 35, 38 and 45 nm is shown in Fig. 9 and summarized in Table 2 suggesting that the device performance is superior at lower thickness of ZnO. The dark device characteristics exhibit a diode like behavior with highest rectification ratio at a ZnO thickness of 25 nm. Under illumination, the device shows a maximum PCE of 3.2% with a FF of 60%. This efficiency is comparable to the control samples which were prepared using spin coating (see Table 1) proving that the printed device can be as good as spin coated devices. The issue that may next be taken up is that of scalability as ultimately the efficacy
126
A. Singh et al. / Organic Electronics 35 (2016) 118e127
of the process is to be judged by its performance on large areas. The device showed a series resistance of about 10.8 Ohm-cm2 and reasonably high shunt resistance implying that the charge extraction is superior at lower ZnO thickness. It has been shown previously that in inverted OSC devices, varying ZnO thickness affects the series resistance of the device by influencing the film roughness as well as in tuning the optical electric field in the active layer [46]. With increase in ZnO thickness, the drop in PCE is accompanied by a reduction in the fill factor and decrease in the shunt resistance which are related to the loss of carriers at the interface between the ITO electrode and active layer mediated by ZnO. 4. Conclusions In conclusion, we have successfully fabricated inverted OSC devices with device structure Glass/ITO/ZnO/P3HT:PC61BM/MoO3/ Ag using ink-jet printed ZnO as ETL and printed P3HT:PC61BM blend as active layer on ITO coated glass substrates. To achieve device quality ZnO films with uniform thicknesses and morphology, we investigated the effect of the various printing parameters on its printing characteristics. Our results suggest that viscosity of the ink strongly affects the drop formation as well as printing quality of film with a viscosity of 4.70 cP leading to the formation of a good quality film at RT. We also found that the film characteristics are strongly affected by drop spacing, substrate temperature during printing and surface modification, determined by the ink spreading on the substrate surface as well as the annealing temperatures. The device fabricated on ZnO films printed at RT at a drop spacing of 45 mm followed by annealing at 175 C showed a maximum power conversion efficiency of ~3.5%, comparable to those obtained on spin coated ZnO films. Finally, printing of P3HT:PC61BM active layer on printed ZnO layer led to a device efficiency of 3.2% showing that inverted devices can be nearly completely printed and can deliver performance similar to the spin coated devices. Acknowledgements Authors thank Department of Science and Technology, India for the financial support through Indo-UK APEX Project (Grant no. SR/ RC-UK/Solar(F)/2010 and SR/RC-UK/APEX Phase II/2015(G)), and DST Nanoscience and Nanotechnology Centres at IIT Kanpur for use of their facilities. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.orgel.2016.05.015 References [1] F.C. Krebs, Fabrication and processing of polymer solar cells: a review of printing and coating techniques, Sol. Energy Mater. Sol. Cells 93 (2009) 394e412. € sel, D. Angmo, T.T. Larsen-Olsen, F.C. Krebs, Roll-to-roll [2] R. Søndergaard, M. Ho fabrication of polymer solar cells, Mater. Today 15 (2012) 36e49. €sel, F.C. Krebs, Roll-to-Roll fabrication of large area [3] R.R. Søndergaard, M. Ho functional organic materials, J. Polym. Sci. Part B Polym. Phys. 51 (2013) 16e34. [4] K. Kawano, R. Pacios, D. Poplavskyy, J. Nelson, D.D.C. Bradley, J.R. Durrant, Degradation of organic solar cells due to air exposure, Sol. Energy Mater. Sol. Cells 90 (2006) 3520e3530. [5] M. Jørgensen, K. Norrman, F.C. Krebs, Stability/degradation of polymer solar cells, Sol. Energy Mater. Sol. Cells 92 (2008) 686e714. [6] W. Yang, Y. Yao, C.-Q. Wu, Mechanisms of device degradation in organic solar cells: influence of charge injection at the metal/organic contacts, Org. Electron. 14 (2013) 1992e2000. [7] K. Norrman, N.B. Larsen, F.C. Krebs, Lifetimes of organic photovoltaics: combining chemical and physical characterisation techniques to study
degradation mechanisms, Sol. Energy Mater. Sol. Cells 90 (2006) 2793e2814. [8] T. Salim, Z. Yin, S. Sun, X. Huang, H. Zhang, Y.M. Lam, Solution-processed nanocrystalline TiO2 buffer layer used for improving the performance of organic photovoltaics, ACS Appl. Mater. Interfaces 3 (2011) 1063e1067. [9] J. Xiong, B. Yang, C. Zhou, J. Yang, H. Duan, W. Huang, X. Zhang, X. Xia, L. Zhang, H. Huang, Y. Gao, Enhanced efficiency and stability of polymer solar cells with TiO2 nanoparticles buffer layer, Org. Electron. 15 (2014) 835e843. [10] K. Zilberberg, H. Gharbi, A. Behrendt, S. Trost, T. Riedl, Low-temperature, solution-processed MoOx for efficient and stable organic solar cells, ACS Appl. Mater. Interfaces 4 (2012) 1164e1168. [11] Z. Su, L. Wang, Y. Li, G. Zhang, H. Zhao, H. Yang, Y. Ma, B. Chu, W. Li, Surface plasmon enhanced organic solar cells with a MoO3 buffer layer, ACS Appl. Mater. Interfaces 5 (2013) 12847e12853. [12] W.L. Leong, Y. Ren, H.L. Seng, Z. Huang, S.Y. Chiam, A. Dodabalapur, Efficient polymer solar cells enabled by low temperature processed ternary metal oxide as electron transport interlayer with large stoichiometry window, ACS Appl. Mater. Interfaces 7 (2015) 11099e11106. [13] S. Venkatesan, E. Ngo, D. Khatiwada, C. Zhang, Q. Qiao, Enhanced lifetime of polymer solar cells by surface passivation of metal oxide buffer layers, ACS Appl. Mater. Interfaces 7 (2015) 16093e16100. [14] M.J. Tan, S. Zhong, J. Li, Z. Chen, W. Chen, Air-stable efficient inverted polymer solar cells using solution-processed nanocrystalline ZnO interfacial layer, ACS Appl. Mater. Interfaces 5 (2013) 4696e4701. [15] F. Yang, E.-K. Park, J.-H. Kim, Y.-S. Kim, Improved performance of inverted polymer solar cells using pentacene, Int. J. Energy Res. 40 (5) (April 2016) 677e684. [16] A. Teichler, J. Perelaer, U.S. Schubert, Inkjet printing of organic electronics comparison of deposition techniques and state-of-the-art developments, J. Mater. Chem. C 1 (2013) 1910e1925. [17] A. Singh, M. Katiyar, A. Garg, Understanding the formation of PEDOT: PSS films by ink-jet printing for organic solar cell applications, RSC Adv. 5 (2015) 78677e78685. [18] S.H. Eom, S. Senthilarasu, P. Uthirakumar, S.C. Yoon, J. Lim, C. Lee, H.S. Lim, J. Lee, S.-H. Lee, Polymer solar cells based on inkjet-printed PEDOT: PSS layer, Org. Electron. 10 (2009) 536e542. [19] A. Lange, M. Wegener, C. Boeffel, B. Fischer, A. Wedel, D. Neher, A new approach to the solvent system for inkjet-printed P3HT: PCBM solar cells and its use in devices with printed passive and active layers, Sol. Energy Mater. Sol. Cells 94 (2010) 1816e1821. [20] C.N. Hoth, S.A. Choulis, P. Schilinsky, C.J. Brabec, On the effect of poly(3hexylthiophene) regioregularity on inkjet printed organic solar cells, J. Mater. Chem. 19 (2009) 5398e5404. [21] C.N. Hoth, P. Schilinsky, S.A. Choulis, C.J. Brabec, Printing highly efficient organic solar cells, Nano Lett. 8 (2008) 2806e2813. [22] C.N. Hoth, S.A. Choulis, P. Schilinsky, C.J. Brabec, High photovoltaic performance of inkjet printed polymer:fullerene blends, Adv. Mater. 19 (2007) 3973e3978. [23] M. Neophytou, W. Cambarau, F. Hermerschmidt, C. Waldauf, C. Christodoulou, R. Pacios, S.A. Choulis, Inkjet-printed polymerefullerene blends for organic electronic applications, Microelectron. Eng. 95 (2012) 102e106. [24] T.M. Eggenhuisen, Y. Galagan, E.W.C. Coenen, W.P. Voorthuijzen, M.W.L. Slaats, S.A. Kommeren, S. Shanmuganam, M.J.J. Coenen, R. Andriessen, W.A. Groen, Digital fabrication of organic solar cells by Inkjet printing using non-halogenated solvents, Sol. Energy Mater. Sol. Cells 134 (2015) 364e372. [25] R. Kitsomboonloha, S. Baruah, M.T.Z. Myint, V. Subramanian, J. Dutta, Selective growth of zinc oxide nanorods on inkjet printed seed patterns, J. Cryst. Growth 311 (2009) 2352e2358. [26] W. Shen, Y. Zhao, C. Zhang, The preparation of ZnO based gas-sensing thin films by ink-jet printing method, Thin Solid Films 483 (2005) 382e387. [27] Y. Wang, X.W. Sun, S.W. Liu, A.K.K. Kyaw, J.L. Zhao, Oxide thin film transistors with ink-jet printed In-Ga-Zn oxide channel layer and ITO/IZO source/drain contacts, in: Nanoelectronics Conference (INEC), 2013 IEEE 5th International, 2013, pp. 168e171. [28] D.-H. Lee, S.-Y. Han, G.S. Herman, C.-h. Chang, Inkjet printed high-mobility indium zinc tin oxide thin film transistors, J. Mater. Chem. 19 (2009) 3135e3137. [29] D. Kim, Y. Jeong, K. Song, S.-K. Park, G. Cao, J. Moon, Inkjet-printed zinc tin oxide thin-film transistor, Langmuir 25 (2009) 11149e11154. [30] B. Kim, S. Jang, M.L. Geier, P.L. Prabhumirashi, M.C. Hersam, A. Dodabalapur, High-speed, inkjet-printed carbon nanotube/zinc tin oxide hybrid complementary ring oscillators, Nano Lett. 14 (2014) 3683e3687. [31] B. Kim, J. Park, M.L. Geier, M.C. Hersam, A. Dodabalapur, Voltage-controlled ring oscillators based on inkjet printed carbon nanotubes and zinc tin oxide, ACS Appl. Mater. Interfaces 7 (2015) 12009e12014. [32] Y. Wu, T. Tamaki, T. Volotinen, L. Belova, K.V. Rao, Enhanced photoresponse of inkjet-printed ZnO thin films capped with CdS nanoparticles, J. Phys. Chem. Lett. 1 (2010) 89e92. € dlmeier, K. Sudau, G. Hernandez-Sosa, [33] S. Sankaran, K. Glaser, S. G€ artner, T. Ro A. Colsmann, Fabrication of polymer solar cells from organic nanoparticle dispersions by doctor blading or ink-jet printing, Org. Electron. 28 (2016) 118e122. [34] P. Kopola, T. Aernouts, S. Guillerez, H. Jin, M. Tuomikoski, A. Maaninen, J. Hast, High efficient plastic solar cells fabricated with a high-throughput gravure printing method, Sol. Energy Mater. Sol. Cells 94 (2010) 1673e1680. [35] F.C. Krebs, M. Jørgensen, K. Norrman, O. Hagemann, J. Alstrup, T.D. Nielsen,
A. Singh et al. / Organic Electronics 35 (2016) 118e127
[36] [37]
[38] [39]
[40] [41]
J. Fyenbo, K. Larsen, J. Kristensen, A complete process for production of flexible large area polymer solar cells entirely using screen printingdfirst public demonstration, Sol. Energy Mater. Sol. Cells 93 (2009) 422e441. Z. Bing, C. Heeyeop, C. Sung Min, Screen-printed polymer:fullerene bulkheterojunction solar cells, Jpn. J. Appl. Phys. 48 (2009) 020208. A. Hübler, B. Trnovec, T. Zillger, M. Ali, N. Wetzold, M. Mingebach, A. Wagenpfahl, C. Deibel, V. Dyakonov, Printed paper photovoltaic cells, Adv. Energy Mater. 1 (2011) 1018e1022. P. Schilinsky, C. Waldauf, C.J. Brabec, Performance analysis of printed bulk heterojunction solar cells, Adv. Funct. Mater. 16 (2006) 1669e1672. S.H. Eom, H. Park, S.H. Mujawar, S.C. Yoon, S.-S. Kim, S.-I. Na, S.-J. Kang, D. Khim, D.-Y. Kim, S.-H. Lee, High efficiency polymer solar cells via sequential inkjet-printing of PEDOT: PSS and P3HT: PCBM inks with additives, Org. Electron. 11 (2010) 1516e1522. J.E. Fromm, Numerical calculation of the fluid dynamics of drop-on-demand jets, IBM J. Res. Dev. 28 (1984) 322e333. D. Jang, D. Kim, J. Moon, Influence of fluid physical properties on ink-jet printability, Langmuir 25 (2009) 2629e2635.
127
[42] N. Reis, B. Derby, Ink jet deposition of ceramic suspensions: modeling and experiments of droplet formation, MRS Online Proc. Libr. 625 (2000) (nullnull). [43] P. Mantri, S.M.H. Rizvi, B. Mazhari, Estimation of built-in voltage from steadystate currentevoltage characteristics of organic diodes, Org. Electron. 14 (2013) 2034e2038. [44] A. Guerrero, L.F. Marchesi, P.P. Boix, S. Ruiz-Raga, T. Ripolles-Sanchis, G. Garcia-Belmonte, J. Bisquert, How the charge-neutrality level of interface states controls energy level alignment in cathode contacts of organic bulkheterojunction solar cells, ACS Nano 6 (2012) 3453e3460. [45] D.E. Motaung, G.F. Malgas, S.S. Nkosi, G.H. Mhlongo, B.W. Mwakikunga, T. Malwela, C.J. Arendse, T.F.G. Muller, F.R. Cummings, Comparative study: the effect of annealing conditions on the properties of P3HT: PCBM blends, J. Mater. Sci. 48 (2012) 1763e1778. [46] S.K. Gupta, A. Sharma, S. Banerjee, R. Gahlot, N. Aggarwal, Deepak, A. Garg, Understanding the role of thickness and morphology of the constituent layers on the performance of inverted organic solar cells, Sol. Energy Mater. Sol. Cells 116 (2013) 135e143.
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Inverted polymer bulk heterojunction solar cells with ink-jet printed electron transport and active layers Arjun Singh a, b, *, Shailendra Kumar Gupta a, b, Ashish Garg a, b, ** a b
Department of Materials Science and Engineering, Indian Institute of Technology, Kanpur, Kanpur, 208016, India Samtel Centre for Display Technologies, Indian Institute of Technology, Kanpur, Kanpur, 208016, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 March 2016 Received in revised form 8 May 2016 Accepted 10 May 2016
Ink-jet printing is a potentially attractive technique for printing of components for organic electronic devices primarily due to its ability to print patterned layers and reduced ink wastage. However, the mechanism of film formation is quite complex and needs an understanding of various printing parameters on the film growth. In this manuscript, we successfully demonstrate ink-jet printing of smooth zinc oxide (ZnO) thin films with controlled thickness as electron transport layers for inverted organic solar cell devices fabricated on indium tin oxide coated glass substrates. The parameters that strongly affect the formation of a continuous ZnO thin film with controlled thickness are ink concentration and viscosity, substrate surface treatment, drop spacing, substrate temperature during printing and the annealing temperature, affected by a combination of surface energetics, surface tension of the ink and the rate of solvent evaporation. The results suggest that one can achieve a transmittance of >85% for a 45 nm thin ZnO film possessing uniform structure and morphology, fabricated using a drop spacing of 40 e50 mm at an ink viscosity of 4.70 cP with substrate held at room temperature. The P3HT:PC61BM inverted organic solar cell devices fabricated using printed ZnO films as electron transporting layers exhibit an efficiency of ~3.4e3.5%, comparable to that shown by the devices fabricated on spin coated ZnO films. Finally, the device with printed P3HT:PC61BM active layer on printed ZnO layer showed a device efficiency of ca. 3.2% suggesting that nearly completely printed devices can deliver a comparable performance to the spin coated devices. © 2016 Elsevier B.V. All rights reserved.
Keywords: Organic solar cells Ink-jet printing Zinc oxide Electron transport layer Active layer P3HT:PC61BM
1. Introduction One of the attractive features of organic electronic devices is the feasibility of their fabrication by solution processing methods, in particular printing based methods offering enormous potential for large area fabrication of devices with reasonably high throughput [1e3] and hence possible reduction in costs. Whilst the blend of Poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61butyric acid methyl ester (PC61BM) has been a workhorse system for bulk heterojunction (BHJ) organic solar cells, development of new low band-gap polymers and blend systems such as those based on Poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b0 ]
* Corresponding author. Department of Materials Science and Engineering, Indian Institute of Technology, Kanpur, Kanpur, 208016, India. ** Corresponding author. Department of Materials Science and Engineering, Indian Institute of Technology, Kanpur, Kanpur, 208016, India. E-mail addresses: [email protected] (A. Singh), [email protected] (A. Garg). http://dx.doi.org/10.1016/j.orgel.2016.05.015 1566-1199/© 2016 Elsevier B.V. All rights reserved.
dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno [3,4-b]thiophenediyl}) or PTB7 and Phenyl-C71-butyric acid methyl ester (PC71BM) has seen laboratory scale device efficiencies exceed 10%. These OSC devices are typically fabricated in normal and inverted architectures on indium tin oxide (ITO) coated glass substrates with ITO acting as anode in normal devices and as cathode in inverted devices. Poor life times of normal organic solar cell devices primarily using hole transporting poly(3,4ethylenedioxythiophene) polystyrene sulfonate) (PEDOT:PSS) layer on ITO electrode [4e6] and problems with the use of Al cathode [7] led to the development of inverted OSC devices. In inverted OSC devices, the hole transporting PEDOT:PSS layer between the active layer and ITO electrode is replaced by a thin electron transport layer (ETL), typically made of a n-type material such as zinc oxide (ZnO) or titanium dioxide (TiO2) [8,9] with ZnO being a typical choice as ETL whilst another oxide such as molybdenum trioxide (MoO3) [10,11] is used as a hole transport layer (HTL) between the active layer and the top electrode. Presence of stable oxides [12,13] at the electrode - active layer interfaces makes
A. Singh et al. / Organic Electronics 35 (2016) 118e127
119
Fig. 1. Variation of drop shape vs elapsed time at a jetting voltage of 12 V for inks with different concentration (a) 0.25 M, (b) 0.35 M, (c) 0.45 M, (d) 0.55 M and (e) 0.70 M with Fromm number mentioned in each figure.
this inverted structure exhibit longer life times as compared to the normal OSC devices [14,15]. For mass production of OSCs, various printing methods are being considered. Among these, ink-jet printing, although not widely considered as a roll-to-roll process, is one of the attractive printing methods due to its ability to form patterned films eliminating or minimizing the need for lithography. Also, the technique offers superior control on the delivery of ink and its reduced wastage. Moreover, one can manipulate the speed of printing by employing multiple print heads and nozzles [16]. Ink-jet printing has been successfully explored for the deposition of common semiconducting polymeric materials such as PEDOT:PSS [17,18] and P3HT:PC61BM [19e24] for OSCs with devices yielding acceptable performances. However, the process of thin film formation in inkjet printing is quite complex and is strongly affected by the parameters such as ink viscosity, drop spacing, substrate temperature
and surface treatment, determined by the type of material. From the perspective of inverted OSC device development by printing on ITO coated glass or plastic substrates, the first step has to be the printing of ZnO films followed by printing of active layer blend and other layers. However, in comparison to the printing of active layer blend and PEDOT:PSS, printing of oxides is a rather poorly addressed topic highlighting the need for detailed explorations. Specifically, in case of ZnO, whilst researchers have reported fabrication of ZnO in various forms such ZnO seed layer for nanorods [25], ZnO nanoparticles for gas sensors [26], amorphous oxides [27], zinc tin oxide [28e31], indium zinc tin oxide [28] and a few other oxides capped with nanoparticles using ink-jet printing method [32], none of these reports conclusively demonstrate the fabrication of ZnO films with controlled thickness which were further integrated into devices such as a working OSC device. Although there are recent reports on the ink-jet printing of ZnO for
120
A. Singh et al. / Organic Electronics 35 (2016) 118e127
Fig. 2. Optical micrographs of ZnO films printed at drop spacing of 25 and 30 mm on ITO/glass substrates (a) without surface treatment (b) with UVO surface treatment. Images on the right show the corresponding contact angle measurements.
OSC devices [24,33], the details are sketchy. The challenges one can encounter in the printing of an oxide such as ZnO can be formation of ink with appropriate viscosity and surface tension, wetting onto the substrate and formation of a thin oxide film with dense microstructure and uniform thickness. Printing of active layer for OSCs, in particular P3HT:PC61BM, on the other hand is a relatively well studied topic using a variety of printing techniques such as gravure printing [34], screen printing [35,36], flexographic printing [37] or ink-jet printing [22,24,33,38] with most efforts relating to normal architecture devices. For example Hoth et al. [22] reported the ink-jet printing of P3HT:PC61BM with different solvents such as tetraline, O-Dichlorobenzene and mesitylene and reported the device efficiencies around 2.9%. Use of additives such as 1,8-octanedithiol (ODT), odichlorobenze and chloronephthalene (Cl-naph) leads to further improvement in device efficiencies [39]. However, one would still need to develop the process for printing the active layer on a new buffer layer such as ZnO, albeit with a reduced level of optimization. In this report, we successfully demonstrate ink-jet printing of ZnO thin films via a detailed study with a focus on understanding the formation of ZnO thin films. We have studied the ink-jetting characteristics and stable drop formation by varying the concentration of the ink and have investigated the effect of process parameters such as drop spacing, substrate temperature, substrate surface modification and annealing temperature on the film formation, critical to the formation of a good quality film for OSC devices. This was followed by printing of P3HT:PC61BM active layer on the printed ZnO film with printed device efficiencies remarkably comparable to those obtained on spin coated devices.
ambient and were transferred to the glove box for device fabrication. To fabricate inverted organic solar cell devices, a blend solution of P3HT and PC61BM was prepared in chlorobenzene in 1:0.8 wt ratio followed by stirring for 12 h in a nitrogen filled glove box. After spin coating the P3HT:PC61BM solution on the printed ZnO layer, the films were annealed for 10 min at 150 C on a hot plate. For ink-jet printing of P3HT:PC61BM, the ink was formulated by using low and high boiling point solvents to avoid fast evaporation of chlorobenzene using tetralene (20% v/v) as an additive to chlorobenzene. The P3HT and PC61BM concentration in the ink was 5 mg/ml each. After mixing all the components, the solution were stirred for 12 h at 45 C in N2 filled glove box to yield a stable ink solution. For comparison, a control sample was fabricated on spin coated ZnO layer. Further, 10 nm thin hole transport layer of MoO3 and a 100 nm Ag layer as top electrode were deposited by thermal evaporation through a shadow mask. The final device structure was Glass/ITO/ZnO/P3HT:PC61BM/MoO3/Ag. Photovoltaic device characteristics were measured in dark as well as light (1 Sun/1.5G solar spectrum) using a Keithley 2400 source meter. Surface topography and morphological studies were carried out using Zeiss optical microscope and Asylum research MFP-3D atomic force microscope (AFM), latter also being used for Kelvin probe force microscopy (KPFM) measurements. KLA Tencor surface profilometer was used for thickness measurement of the layers. Optical absorbance and transmittance of ZnO layers were measured using a Perkin Elmer Lambda 750 UVevis spectrophotometer. 3. Results and discussion
2. Experimental details 3.1. Role of ZnO ink viscosity on ink jetting behavior Zinc oxide inks of concentrations 0.25 M, 0.35 M, 0.45 M, 0.55 M and 0.70 M were formulated using zinc acetate (Zn(CH3COO)2) as a precursor material, dissolved in 2-methoxyethanol as a solvent and with further addition of controlled amounts of monoethanolamine (C2H7NO) as a stabilizing agent. The clear ink solution was filtered through 0.45 mm PVDF filter before transferring into the printer cartridge followed by holding it for 20 min to remove any trapped air bubbles. The ZnO layer was printed on cleaned ITO coated glass substrates using DiMatix2831 ink-jet printer with 16-nozzles (nozzle diameter ~ 21 mm). Prior to the printing of ZnO films, ITO substrates were subjected to UVeOzone treatment for 15 min. The printed films were annealed at 175 C and 250 C for 10 min in the
First, we investigated the jetting behavior of ZnO ink through the nozzle of ink-jet printer by changing the ink viscosity at a fixed jetting voltage (12 V) applied to the print head of the printer. The experiments were performed on the inks of five different viscosities: 2.07 cP, 2.46 cP, 2.90 cP, 3.62 cP and 4.70 cP, corresponding to the inks concentrations 0.25 M, 0.35 M, 0.45 M, 0.55 M and 0.70 M respectively and CCD camera images of jetting behavior of the formulated ZnO ink are shown in Fig. 1. Jetting behavior of the drops is highly dependent on the viscosity which is manifested in the dimensionless parameter, called as Fromm number [40], Z and is given as:
A. Singh et al. / Organic Electronics 35 (2016) 118e127
121
Fig. 3. Optical micrographs of ZnO films, ink-jet printed using inks of different concentration and viscosities (a) 2.07 cP (0.25 M) (b) 2.43 cP (0.35 M) (c) 2.90 cP (0.45 M) (d) 3.62 cP (0.55 M) and (e) 4.70 cP (0.70 M). The drop spacing was kept at 35, 45 and 50 mm.
Z¼
ðargÞ1=2
h
where, a is the radius of the printing orifice (~21 mm), r is the density (~1009 kg/m3), g is the surface tension and h is the viscosity of the ink. Corresponding Z values are (a) 14.02 (0.25 M) (b) 11.12 (0.35 M), (c) 9.15 (0.45 M), (d) 7.90 (0.55 M) and (e) 5.62 (0.70 M). To form a drop without a satellite, while Fromm [40] et al. suggested a value of Z > 2, recent works [41] suggest a range of 4 Z 14 as more appropriate. Subsequent work by Reiss and
Derby [42] suggested that printable ink should have Z values between 1 and 10. They observed that the upper values of Z (i.e. lower viscosity) are determined by the satellite formation in the ink whilst the lower limit of Z is governed by dissipation of pressure wave by fluid viscosity. For a spherical drop to form, the head and the tail drops need to merge after a short time i.e. when head and tail drop velocities are the same. As Fig. 1 shows, we get spherical drops only for the case of Z ¼ 5.62 when the tail droplets merges with the head droplet after an elapsed time of 60 ms. All other inks of higher Z values did not yield spherical drops even after an
122
A. Singh et al. / Organic Electronics 35 (2016) 118e127
elapsed time of 60 ms as visible from the jetting behavior shown in Fig. 1(aee) with tail drop velocity being lower than the head velocity. 3.2. Effect of surface treatment on the printing of ZnO ink Subsequently, we investigated the printing of zinc oxide precursor ink with and without surface UV Ozone (UVO) treatment of the substrate surface at different drop spacing of 25 mm and 30 mm and results are shown below in Fig. 2. As the figure shows, for both the drop spacing values, the ink did not spread uniformly over the untreated substrates leading to the formation of droplets due to coalescence of different drops, indicating towards poor wetting characteristics of the substrate surface by the ink. However, after a 15 min UVO surface treatment of the surface, the jetted drops spread to give rise to a uniform film over the ITO substrates for both the drop spacing values suggesting an improved wettability of the substrate by ZnO ink. Further contact angle measurements (Fig. 2) showed that this is due to a decrease in the contact angle on the surface after ozone treatment: the contact angle value was 45.9 without any surface treatment which decreased to 14.3 after UVO treatment for 15 min. 3.3. Influence of ZnO ink viscosity and drop spacing on ink-jet printing The viscosity of the ink affects not only the jetting behavior of the ink as shown earlier, it also affects the morphology of the printed film in a significant manner. This is because the spreading of the ink is dependent on the ink viscosity as well as the drop spacing. The optical micrographs of the printed ZnO films with 15 min UVeOzone treatment at different ink viscosities as well as drop spacing are shown in Fig. 3. In this experiment, the lines were printed on the substrates, which gradually merge to form the film on their own. The images seem to suggest a mixed effect of drop spacing and the ink viscosity on film’s uniformity. The results suggest that at lower viscosity of 2.07 cP, the films are not uniform with the formation of pin-holes in the film as shown in Fig. 3 (a). At the viscosity of 2.43 cP, film consists of lines at all the drop spacings which tend to be sparsely spaced at higher drop spacing as shown in Fig. 3(b) and never disappear completely. In contrast, the printed ZnO films appear to be more uniform for higher viscosity of the ink as shown in Fig. 3(cee)) with lines being less prominent. Particularly, for the viscosity of 4.70 cP, the film is more uniform and dense with no lines present at a drop spacing of 45 mm. Our results suggest that the ink with higher viscosity spreads better especially at the drop spacing of 45e50 mm whilst ink with low viscosity leads to undulations. These observations, although counterintuitive, suggest a competition between the evaporation of the solvent from the printed lines and lateral spreading on the substrate. Formation of smooth films at higher viscosity appears to suggest an optimum balance between these parameters preventing the build-up on the edges of the printed lines. This factor is examined again in the next -vis substrate temperature. Based on the above results, section vis-a all further experiments were carried on the ZnO films printed using an ink of viscosity 4.70 cP with a concentration of 0.70 M. 3.4. Effect of substrate temperature on the morphology of printed ZnO films
Fig. 4. (a) Optical micrographs of printed films (b) schematic diagram showing flow of ink during solvent evaporation and (c) line profiles of printed ZnO lines at different substrate temperatures.
After investigating the effect of surface treatment and viscosity, we investigated the effect of substrate temperature during ink-jet printing on the morphology of printed ZnO films. Fig. 4 (a) shows the optical microscope images of ink-jet printed ZnO films at different substrate temperatures with corresponding 2-D profiles
A. Singh et al. / Organic Electronics 35 (2016) 118e127
123
Fig. 5. (a) Optical transmittance of printed ZnO film annealed at 175 C and 250 C (inset shows the absorbance of P3HT:PC61BM blend film deposited on printed ZnO films); (bec) AFM images (5 5 mm2) showing topography of corresponding ZnO films.
Fig. 6. (a) Schematic of inverted OSC device structure; plots of current density (J) vs voltage (V) of OSC devices in (b) dark (c) under illumination (AM 1.5 G, 1 Sun); (d) external quantum efficiency of the devices on printed ZnO films.
Table 1 Performance of inverted OSC devices on printed ZnO films (Ci: ink concentration, Ta: annealing temperature of ZnO film). Reference sample was fabricated on spin coated ZnO films. Ci
Ta ( C)
Jsc (mA/cm2)
Voc (V)
FF (%)
PCE (%)
Best PCE (%)
Rs (U-cm2)
Rsh (U-cm2)
0.70 M
175 250 175
9.8 ± 0.3 9.7 ± 0.1 8.8 ± 0.1
0.60 0.58 0.64
58.9 ± 3.8 51.9 ± 3.9 59 ± 3.6
3.4 ± 0.1 2.9 ± 0.2 3.4 ± 0.1
3.5 3.1 3.5
11.0 13.3 11.5
717.4 456.4 700.5
Ref.
shown in Fig. 4 (c). The results show that while the films printed at 25 C (room temperature or RT) and 30 C are quite uniform, those printed at 40 C and 50 C tend to possess the wavy edges. This can be explained on the basis of flow of ink and solvent evaporation
during film formation due to competition between the precursor ink movement due to surface tension and the evaporation of the solvent, as shown schematically in Fig. 4(b). The increase in the substrate temperature leads to faster evaporation of the solvent and
124
A. Singh et al. / Organic Electronics 35 (2016) 118e127
Fig. 7. AFM images of 115 nm thin printed P3HT:PC61M film annealed at (a) 120 C (15 min), (b) 120 C (10 min) followed by 150 C (5 min) and (c) 150 C (15 min).
dependence of transmittance on the annealing temperature, the films were examined using AFM whose results are shown in Fig. 5(b and c). The images show that although topography of both the films shows a nanocrystalline structure, ZnO films annealed at 175 C show lower roughness (0.8 ± 0.1 nm) as compared to those annealed at 250 C (1.2 ± 0.1) which could result in reduced surface scattering leading to slightly enhanced transmission of the films annealed at 175 C. 3.6. Fabricated of organic solar cells on printed ZnO films
Fig. 8. UVevisible absorption spectra of 115 nm thin printed P3HT:PC61BM films at different annealing temperatures.
hence causing non-uniformity on film surface as the edges become thicker with increase in the substrate temperature. The printed line width also decreases with increase in the substrate temperature, as one can observe from Fig. 4(c). The line width is 0.25 mm at RT and decreases to 0.16 mm at a higher substrates temperature of 50 C implying that the drop spreading is lesser at higher temperatures, resulting in non-uniformity in the printed film surface at higher substrate temperature.
3.5. Effect of annealing temperature on the optical behavior of printed ZnO films To investigate the effect of annealing temperature on optical transmission, 45 nm thin ZnO films were printed using a drop spacing of 45 mm at RT from an ink of viscosity 4.70 cP on UVOzone treated ITO coated glass substrates followed by annealing at 175 C and 250 C, for 10 min in air and the results are shown in Fig. 5(a). Fig. 5(a) suggests that the optical transmittance of both the films above 450 nm is quite similar and comparable to that of ITO/ glass. However, between 350 and 450 nm, the transmission of film annealed at 175 C is superior to that annealed at 250 C which is also manifested in the absorbance of P3HT:PC61BM blend film as shown in the inset. To examine the role of surface topography in
To demonstrate the viability of printed ZnO films for photovoltaic applications, we fabricated the inverted OSC devices with structure Glass/ITO/ZnO/P3HT:PC61BM/MoO3/Ag having printed ZnO films as ETL (Fig. 6(a)) and the photovoltaic measurement results are shown in Fig. 6 and Table 1. The devices fabricated on printed ZnO films annealed at 175 C (p-ZnO175) show power conversion efficiency (PCE) of 3.4 ± 0.1%, comparable to those obtained on spin coated ZnO (s-ZnO) films (3.4 ± 0.1%) and with superior reproducibility. Interestingly, the short circuit current density (Jsc) of the device on p-ZnO175 film is a bit higher than that on spin coated ZnO despite slightly higher absorbance of the latter which could be due to slightly reduced series resistance (Rs) and improved shunt resistance (Rsh) attributed to higher roughness of s-ZnO films (1.7 nm after annealing at 175 C) leading to higher interfacial roughness. On the other hand, Voc of the device on s-ZnO is slightly better and as a result both devices have nearly comparable performance. Comparison of the devices fabricated on ZnO films annealed at different temperature shows that the device on p-ZnO175 exhibits much improved device fill factor (FF) and hence higher PCE compared to the device on p-ZnO250 (printed ZnO film annealed at 250 C). This is a desirable finding because lower annealing temperature of the device components is a positive attribute for large area processing of organic solar cells reducing the thermal budget as well as the process applicability to the substrates which cannot withstand high temperature annealing operation. The improved device performance at low annealing temperature of ZnO films could be related lower roughness of ZnO and hence low interfacial roughness with the active layer leading to a decrease in the shunt resistance of the devices and higher series resistance, as shown in Table 1. The semi-log dark J-V characteristics, as shown in Fig. 6(b), also show that the rectification ratio decreases with an increase in the annealing temperature: the rectification ratio for devices made on p-ZnO175 films is ~102 at ± 0.5 V with minimum lowest leakage current indicating better diode performance. The decrease in the
A. Singh et al. / Organic Electronics 35 (2016) 118e127
125
Fig. 9. J-V characteristics of the inverted OSC devices with printed P3HT:PC61BM active layer and ETL of printed ZnO of different thicknesses in (a) dark and (b) light conditions.
Table 2 Parameters of inverted OSC devices with 115 nm thin printed active layer (P3HT:PC61BM) and printed ETL (ZnO) with varying ETL thickness. Printed ZnO thickness (nm)
Voc (V)
45 38 35 25
0.62 0.62 0.63 0.60
nm nm nm nm
± ± ± ±
Jsc (mA/cm2) 0.01 0.02 0.01 0.02
7.5 8.3 8.1 9.2
± ± ± ±
0.5 0.4 0.6 0.1
FF (%) 51.8 51.4 54.4 57.0
rectification ratio (ION/IOFF ratio) is indicative of increased interface trap density with increasing annealing temperature of printed ZnO films. Higher roughness of p-ZnO250 films in comparison to pZnO175 films results in an effectively larger ZnO/active layer interfacial area leading to higher interfacial trap density which causes undesired trap assisted recombination of the carriers and hence lowers the FF [10]. This is also reflected in the external quantum efficiency (EQE) of all the devices as shown in Fig. 6(d). The maximum EQE is achieved for the devices on p-ZnO175 films with a value of ca. 57% at 500 nm which is in agreement with higher optical transmittance of the films (~85% in visible range). In contrast, the EQE is lowered to ca. 47% at 500 nm for devices made on pZnO250 films. Further, using the dark J-V data of the devices from Fig. 6 (b), we calculated the built-in voltage (Vbi) of the devices fabricated on pZnO175 and p-ZnO250 films following Mantri et al. [43]. The results show that the device on p-ZnO175 films is 0.83 V, higher than 0.75 V shown by the device on p-ZnO250 films. The Vbi is equal to [(EFS-Ec)D] where EC is Fermi level of cathode, EFS is the work function of semiconductor at equilibrium and D is the voltage drop due to interfacial layer [44]. The increase in Vbi of the devices on p-ZnO175 films is also concomitant with the lower surface roughness of pZnO175 films as compared to p-ZnO250 films. This emphasizes that p-ZnO175 films provide an improved interface leading to reduced voltage drop at the interface (D) resulting in enhanced charge extraction in the device. 3.7. Device fabrication with printed active layer and electron transport layer Next, we printed P3HT:PC61BM active layers on printed ZnO films with device structure shown in Fig. 6(a). The jetting of P3HT:PC61BM ink was acceptable at 12 V without any nozzle blocking. Subsequently, 115 nm thin printed films were annealed at various temperatures: 120 C (15 min), 150 C (15 min) and a twostep annealing (120 C (10 min) followed by 150 C (5 min)). The films were cooled in the glove box by taking them off the hot plate and keeping them on a surface at RT. Although the printed films are
± ± ± ±
PCE (%) 6.9 4.9 3.7 2.3
2.4 2.7 2.8 3.1
± ± ± ±
0.3 0.3 0.3 0.1
Best PCE (%)
Rs (U-cm2)
Rsh (U-cm2)
2.7 3.0 3.1 3.2
14.2 15.5 13.5 10.8
550 684 613 862
quite uniform at all temperatures as suggested by the AFM images of the samples (Fig. 7), with increase in the annealing temperature, phase separation of PC61BM appears to occur in the films annealed at 150 C (Fig. 7(a)) as suggested by bright spots on the surface. The images show that the film annealed at 120 C (15 min) possesses highest roughness 6.7 ± 0.6 nm while the film annealed at 150 C (15 min) has lowest roughness 4.4 ± 0.5 nm and the film 120 C (10 min) þ 150 C (5 min) yields an intermediate roughness of 5.3 ± 0.2 nm. Annealing at higher temperature make render P3HT polymer chain becoming soft and flexible and hence the interdiffusion of PC61BM may occur [45] with the growth of crystallites on the surface. Further, UVevis absorption spectra of printed P3HT:PC60BM films with variation in the heat treatment temperature, as shown in Fig. 8, show that the highest absorbance is achieved when the film is annealed in two steps i.e. 120 C (10 min) followed by 150 C (5 min). In contrast, intermediate absorbance is achieved when the film was annealed at 120 C and is minimum when film was directly annealed at 150 C, latter attributed to the phase separation of PC61BM crystallites. Finally we fabricated the inverted OSC devices by printing both ZnO layer to act as ETL and P3HT:PC61BM blend as active layer with device structure Glass/ITO/ZnO(p)/P3HT:PC61BM(p)/MoO3/Ag (p: printed). The devices consisted of a 115 nm thin P3HT:PC61BM blend layer annealed in two stages (120 C (10 min) þ 150 C (5 min)). The thickness of ZnO layer was varied as per the details in the preceding sections as its a crucial factor in determining OSC device performance due to optical field effects [46]. The device performance at various thicknesses of printed ZnO i.e. 25, 35, 38 and 45 nm is shown in Fig. 9 and summarized in Table 2 suggesting that the device performance is superior at lower thickness of ZnO. The dark device characteristics exhibit a diode like behavior with highest rectification ratio at a ZnO thickness of 25 nm. Under illumination, the device shows a maximum PCE of 3.2% with a FF of 60%. This efficiency is comparable to the control samples which were prepared using spin coating (see Table 1) proving that the printed device can be as good as spin coated devices. The issue that may next be taken up is that of scalability as ultimately the efficacy
126
A. Singh et al. / Organic Electronics 35 (2016) 118e127
of the process is to be judged by its performance on large areas. The device showed a series resistance of about 10.8 Ohm-cm2 and reasonably high shunt resistance implying that the charge extraction is superior at lower ZnO thickness. It has been shown previously that in inverted OSC devices, varying ZnO thickness affects the series resistance of the device by influencing the film roughness as well as in tuning the optical electric field in the active layer [46]. With increase in ZnO thickness, the drop in PCE is accompanied by a reduction in the fill factor and decrease in the shunt resistance which are related to the loss of carriers at the interface between the ITO electrode and active layer mediated by ZnO. 4. Conclusions In conclusion, we have successfully fabricated inverted OSC devices with device structure Glass/ITO/ZnO/P3HT:PC61BM/MoO3/ Ag using ink-jet printed ZnO as ETL and printed P3HT:PC61BM blend as active layer on ITO coated glass substrates. To achieve device quality ZnO films with uniform thicknesses and morphology, we investigated the effect of the various printing parameters on its printing characteristics. Our results suggest that viscosity of the ink strongly affects the drop formation as well as printing quality of film with a viscosity of 4.70 cP leading to the formation of a good quality film at RT. We also found that the film characteristics are strongly affected by drop spacing, substrate temperature during printing and surface modification, determined by the ink spreading on the substrate surface as well as the annealing temperatures. The device fabricated on ZnO films printed at RT at a drop spacing of 45 mm followed by annealing at 175 C showed a maximum power conversion efficiency of ~3.5%, comparable to those obtained on spin coated ZnO films. Finally, printing of P3HT:PC61BM active layer on printed ZnO layer led to a device efficiency of 3.2% showing that inverted devices can be nearly completely printed and can deliver performance similar to the spin coated devices. Acknowledgements Authors thank Department of Science and Technology, India for the financial support through Indo-UK APEX Project (Grant no. SR/ RC-UK/Solar(F)/2010 and SR/RC-UK/APEX Phase II/2015(G)), and DST Nanoscience and Nanotechnology Centres at IIT Kanpur for use of their facilities. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.orgel.2016.05.015 References [1] F.C. Krebs, Fabrication and processing of polymer solar cells: a review of printing and coating techniques, Sol. Energy Mater. Sol. Cells 93 (2009) 394e412. € sel, D. Angmo, T.T. Larsen-Olsen, F.C. Krebs, Roll-to-roll [2] R. Søndergaard, M. Ho fabrication of polymer solar cells, Mater. Today 15 (2012) 36e49. €sel, F.C. Krebs, Roll-to-Roll fabrication of large area [3] R.R. Søndergaard, M. Ho functional organic materials, J. Polym. Sci. Part B Polym. Phys. 51 (2013) 16e34. [4] K. Kawano, R. Pacios, D. Poplavskyy, J. Nelson, D.D.C. Bradley, J.R. Durrant, Degradation of organic solar cells due to air exposure, Sol. Energy Mater. Sol. Cells 90 (2006) 3520e3530. [5] M. Jørgensen, K. Norrman, F.C. Krebs, Stability/degradation of polymer solar cells, Sol. Energy Mater. Sol. Cells 92 (2008) 686e714. [6] W. Yang, Y. Yao, C.-Q. Wu, Mechanisms of device degradation in organic solar cells: influence of charge injection at the metal/organic contacts, Org. Electron. 14 (2013) 1992e2000. [7] K. Norrman, N.B. Larsen, F.C. Krebs, Lifetimes of organic photovoltaics: combining chemical and physical characterisation techniques to study
degradation mechanisms, Sol. Energy Mater. Sol. Cells 90 (2006) 2793e2814. [8] T. Salim, Z. Yin, S. Sun, X. Huang, H. Zhang, Y.M. Lam, Solution-processed nanocrystalline TiO2 buffer layer used for improving the performance of organic photovoltaics, ACS Appl. Mater. Interfaces 3 (2011) 1063e1067. [9] J. Xiong, B. Yang, C. Zhou, J. Yang, H. Duan, W. Huang, X. Zhang, X. Xia, L. Zhang, H. Huang, Y. Gao, Enhanced efficiency and stability of polymer solar cells with TiO2 nanoparticles buffer layer, Org. Electron. 15 (2014) 835e843. [10] K. Zilberberg, H. Gharbi, A. Behrendt, S. Trost, T. Riedl, Low-temperature, solution-processed MoOx for efficient and stable organic solar cells, ACS Appl. Mater. Interfaces 4 (2012) 1164e1168. [11] Z. Su, L. Wang, Y. Li, G. Zhang, H. Zhao, H. Yang, Y. Ma, B. Chu, W. Li, Surface plasmon enhanced organic solar cells with a MoO3 buffer layer, ACS Appl. Mater. Interfaces 5 (2013) 12847e12853. [12] W.L. Leong, Y. Ren, H.L. Seng, Z. Huang, S.Y. Chiam, A. Dodabalapur, Efficient polymer solar cells enabled by low temperature processed ternary metal oxide as electron transport interlayer with large stoichiometry window, ACS Appl. Mater. Interfaces 7 (2015) 11099e11106. [13] S. Venkatesan, E. Ngo, D. Khatiwada, C. Zhang, Q. Qiao, Enhanced lifetime of polymer solar cells by surface passivation of metal oxide buffer layers, ACS Appl. Mater. Interfaces 7 (2015) 16093e16100. [14] M.J. Tan, S. Zhong, J. Li, Z. Chen, W. Chen, Air-stable efficient inverted polymer solar cells using solution-processed nanocrystalline ZnO interfacial layer, ACS Appl. Mater. Interfaces 5 (2013) 4696e4701. [15] F. Yang, E.-K. Park, J.-H. Kim, Y.-S. Kim, Improved performance of inverted polymer solar cells using pentacene, Int. J. Energy Res. 40 (5) (April 2016) 677e684. [16] A. Teichler, J. Perelaer, U.S. Schubert, Inkjet printing of organic electronics comparison of deposition techniques and state-of-the-art developments, J. Mater. Chem. C 1 (2013) 1910e1925. [17] A. Singh, M. Katiyar, A. Garg, Understanding the formation of PEDOT: PSS films by ink-jet printing for organic solar cell applications, RSC Adv. 5 (2015) 78677e78685. [18] S.H. Eom, S. Senthilarasu, P. Uthirakumar, S.C. Yoon, J. Lim, C. Lee, H.S. Lim, J. Lee, S.-H. Lee, Polymer solar cells based on inkjet-printed PEDOT: PSS layer, Org. Electron. 10 (2009) 536e542. [19] A. Lange, M. Wegener, C. Boeffel, B. Fischer, A. Wedel, D. Neher, A new approach to the solvent system for inkjet-printed P3HT: PCBM solar cells and its use in devices with printed passive and active layers, Sol. Energy Mater. Sol. Cells 94 (2010) 1816e1821. [20] C.N. Hoth, S.A. Choulis, P. Schilinsky, C.J. Brabec, On the effect of poly(3hexylthiophene) regioregularity on inkjet printed organic solar cells, J. Mater. Chem. 19 (2009) 5398e5404. [21] C.N. Hoth, P. Schilinsky, S.A. Choulis, C.J. Brabec, Printing highly efficient organic solar cells, Nano Lett. 8 (2008) 2806e2813. [22] C.N. Hoth, S.A. Choulis, P. Schilinsky, C.J. Brabec, High photovoltaic performance of inkjet printed polymer:fullerene blends, Adv. Mater. 19 (2007) 3973e3978. [23] M. Neophytou, W. Cambarau, F. Hermerschmidt, C. Waldauf, C. Christodoulou, R. Pacios, S.A. Choulis, Inkjet-printed polymerefullerene blends for organic electronic applications, Microelectron. Eng. 95 (2012) 102e106. [24] T.M. Eggenhuisen, Y. Galagan, E.W.C. Coenen, W.P. Voorthuijzen, M.W.L. Slaats, S.A. Kommeren, S. Shanmuganam, M.J.J. Coenen, R. Andriessen, W.A. Groen, Digital fabrication of organic solar cells by Inkjet printing using non-halogenated solvents, Sol. Energy Mater. Sol. Cells 134 (2015) 364e372. [25] R. Kitsomboonloha, S. Baruah, M.T.Z. Myint, V. Subramanian, J. Dutta, Selective growth of zinc oxide nanorods on inkjet printed seed patterns, J. Cryst. Growth 311 (2009) 2352e2358. [26] W. Shen, Y. Zhao, C. Zhang, The preparation of ZnO based gas-sensing thin films by ink-jet printing method, Thin Solid Films 483 (2005) 382e387. [27] Y. Wang, X.W. Sun, S.W. Liu, A.K.K. Kyaw, J.L. Zhao, Oxide thin film transistors with ink-jet printed In-Ga-Zn oxide channel layer and ITO/IZO source/drain contacts, in: Nanoelectronics Conference (INEC), 2013 IEEE 5th International, 2013, pp. 168e171. [28] D.-H. Lee, S.-Y. Han, G.S. Herman, C.-h. Chang, Inkjet printed high-mobility indium zinc tin oxide thin film transistors, J. Mater. Chem. 19 (2009) 3135e3137. [29] D. Kim, Y. Jeong, K. Song, S.-K. Park, G. Cao, J. Moon, Inkjet-printed zinc tin oxide thin-film transistor, Langmuir 25 (2009) 11149e11154. [30] B. Kim, S. Jang, M.L. Geier, P.L. Prabhumirashi, M.C. Hersam, A. Dodabalapur, High-speed, inkjet-printed carbon nanotube/zinc tin oxide hybrid complementary ring oscillators, Nano Lett. 14 (2014) 3683e3687. [31] B. Kim, J. Park, M.L. Geier, M.C. Hersam, A. Dodabalapur, Voltage-controlled ring oscillators based on inkjet printed carbon nanotubes and zinc tin oxide, ACS Appl. Mater. Interfaces 7 (2015) 12009e12014. [32] Y. Wu, T. Tamaki, T. Volotinen, L. Belova, K.V. Rao, Enhanced photoresponse of inkjet-printed ZnO thin films capped with CdS nanoparticles, J. Phys. Chem. Lett. 1 (2010) 89e92. € dlmeier, K. Sudau, G. Hernandez-Sosa, [33] S. Sankaran, K. Glaser, S. G€ artner, T. Ro A. Colsmann, Fabrication of polymer solar cells from organic nanoparticle dispersions by doctor blading or ink-jet printing, Org. Electron. 28 (2016) 118e122. [34] P. Kopola, T. Aernouts, S. Guillerez, H. Jin, M. Tuomikoski, A. Maaninen, J. Hast, High efficient plastic solar cells fabricated with a high-throughput gravure printing method, Sol. Energy Mater. Sol. Cells 94 (2010) 1673e1680. [35] F.C. Krebs, M. Jørgensen, K. Norrman, O. Hagemann, J. Alstrup, T.D. Nielsen,
A. Singh et al. / Organic Electronics 35 (2016) 118e127
[36] [37]
[38] [39]
[40] [41]
J. Fyenbo, K. Larsen, J. Kristensen, A complete process for production of flexible large area polymer solar cells entirely using screen printingdfirst public demonstration, Sol. Energy Mater. Sol. Cells 93 (2009) 422e441. Z. Bing, C. Heeyeop, C. Sung Min, Screen-printed polymer:fullerene bulkheterojunction solar cells, Jpn. J. Appl. Phys. 48 (2009) 020208. A. Hübler, B. Trnovec, T. Zillger, M. Ali, N. Wetzold, M. Mingebach, A. Wagenpfahl, C. Deibel, V. Dyakonov, Printed paper photovoltaic cells, Adv. Energy Mater. 1 (2011) 1018e1022. P. Schilinsky, C. Waldauf, C.J. Brabec, Performance analysis of printed bulk heterojunction solar cells, Adv. Funct. Mater. 16 (2006) 1669e1672. S.H. Eom, H. Park, S.H. Mujawar, S.C. Yoon, S.-S. Kim, S.-I. Na, S.-J. Kang, D. Khim, D.-Y. Kim, S.-H. Lee, High efficiency polymer solar cells via sequential inkjet-printing of PEDOT: PSS and P3HT: PCBM inks with additives, Org. Electron. 11 (2010) 1516e1522. J.E. Fromm, Numerical calculation of the fluid dynamics of drop-on-demand jets, IBM J. Res. Dev. 28 (1984) 322e333. D. Jang, D. Kim, J. Moon, Influence of fluid physical properties on ink-jet printability, Langmuir 25 (2009) 2629e2635.
127
[42] N. Reis, B. Derby, Ink jet deposition of ceramic suspensions: modeling and experiments of droplet formation, MRS Online Proc. Libr. 625 (2000) (nullnull). [43] P. Mantri, S.M.H. Rizvi, B. Mazhari, Estimation of built-in voltage from steadystate currentevoltage characteristics of organic diodes, Org. Electron. 14 (2013) 2034e2038. [44] A. Guerrero, L.F. Marchesi, P.P. Boix, S. Ruiz-Raga, T. Ripolles-Sanchis, G. Garcia-Belmonte, J. Bisquert, How the charge-neutrality level of interface states controls energy level alignment in cathode contacts of organic bulkheterojunction solar cells, ACS Nano 6 (2012) 3453e3460. [45] D.E. Motaung, G.F. Malgas, S.S. Nkosi, G.H. Mhlongo, B.W. Mwakikunga, T. Malwela, C.J. Arendse, T.F.G. Muller, F.R. Cummings, Comparative study: the effect of annealing conditions on the properties of P3HT: PCBM blends, J. Mater. Sci. 48 (2012) 1763e1778. [46] S.K. Gupta, A. Sharma, S. Banerjee, R. Gahlot, N. Aggarwal, Deepak, A. Garg, Understanding the role of thickness and morphology of the constituent layers on the performance of inverted organic solar cells, Sol. Energy Mater. Sol. Cells 116 (2013) 135e143.