Enhancement of the performance of organic solar cells by electrospray deposition with optimal solvent system

Solar Energy Materials & Solar Cells 121 (2014) 119–125

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Enhancement of the performance of organic solar cells by electrospray deposition with optimal solvent system Xin-Yan Zhao a,c, Xizu Wang c, Siew Lay Lim c, Dongchen Qi c, Rui Wang c, ZhiQiang Gao d, BaoXiu Mi a,d,n, Zhi-Kuan Chen b,c,nn, Wei Huang a,b,nnn, Weiwei Deng e,nnnn a

Institute of Advanced Materials, Nanjing University of Posts & Telecommunications, Nanjing 210046, China Institute of Advanced Materials, Nanjing University of Technology, Nanjing 210009, China c Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore d School of Materials Science & Engineering, Nanjing University of Posts & Telecommunications, Nanjing 210046, China e Department of Mechanical and Aerospace Engineering, University of Central Florida, 4000 Central Florida Blvd, Orlando, FL 32816, USA b

art ic l e i nf o

a b s t r a c t

Article history: Received 19 June 2013 Received in revised form 5 October 2013 Accepted 21 October 2013

Electrospray (ES) as a thin film deposition method that is uniquely suited for manufacturing organic photovoltaic cells (OPVs) with desired characteristics of atmospheric pressure fabrication, roll-to-roll compatibility, less material loss, and possible self-organized nanostructures. The additional solvent with high electrical conductivity plays an important role in ES deposition process to fabricate OPVs with active layer composed of polymer mixture poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PC61BM). Here we introduced acetic acid, which possesses high electrical conductivity, as additive solvent in ES process. The dependence of device performance on the concentration of acetic acid was investigated, and optimal ratio was obtained. To further demonstrate the influence of additive solvents with different electrical conductivity, OPV devices with active layer deposited by ES method using solutions containing acetic acid, acetone or acetonitrile were fabricated. The characteristics of active layers were revealed by optical microscope, atomic force microscopy, UV–vis spectroscopy and X-ray diffraction. Compared with additive solvents of acetone and acetonitrile, the active layer formed by electrospraying solvent containing acetic acid demonstrated enhanced vertical segregation distribution and improved P3HT crystallinity, which resulted in better device performance. OPV device using acetic acid as additive achieved power convention efficiency (PCE) of 2.99 70.08% under AM 1.5 solar simulation, which is on par with that of the spin coated device (PCE 3.12 7 0.07%). & 2013 Published by Elsevier B.V.

Keywords: Electrospray Organic photovoltaics Additive Electrical conductivity Morphology

1. Introduction Organic photovoltaics (OPVs) have gained broad interest in the past two decades due to its potential to fabricate flexible, lightweight, low-cost and large-area photovoltaic devices. Most recently, power conversion efficiency (PCE) of OPVs has achieved over 8% [1,2]. To date, the reported OPVs with high efficiencies

n Corresponding author at: Institute of Advanced Materials, Nanjing University of Posts & Telecommunications, Nanjing 210046, China. nn Corresponding author at: Institute of Advanced Materials, Nanjing University of Technology, Nanjing 210009, China. nnn Corresponding author at: Institute of Advanced Materials, Nanjing University of Posts & Telecommunications, Nanjing 210046, China. nnnn Corresponding author at: Department of Mechanical and Aerospace Engineering, University of Central Florida, 4000 Central Florida Blvd, Orlando, FL 32816, USA. E-mail addresses: [email protected] (B. Mi), [email protected] (Z.-K. Chen), [email protected] (W. Huang), [email protected] (W. Deng).

0927-0248/$ - see front matter & 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.solmat.2013.10.020

have been fabricated by spin coating, which is a process only suited for laboratory scale and rigid substrate with significant material waste [3–5]. Therefore, there is an urgent need to develop fabrication methods that are scalable and roll-to-roll compatible. Recently, electrospray (ES) has emerged as an attractive approach for fabricating OPVs. ES is an electrohydrodynamic liquid atomizing technique that can generate monodisperse droplets with diameter of a few nm to 100 mm. Studies on using ES as deposition method in the field of organic thin-film electronic devices have been reported in fabrication of organic lightemitting diodes (OLEDs) [6,7] and organic photoconductive devices [8]. These studies showed that ES was competitive to other solution-process techniques for organic thin-film electronic device fabrication. ES has several unique advantages for OPV production, such as compatibility with roll-to-roll process, operation in atmospherical pressure, less material loss, and possible self-organized nano-structures [9,10]. Most recently, ES has been used in OPV fabrication to deposit P3HT:PCBM active layers with

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bulk heterojunction [11–15] and multilayer structure [16], as well as to deposit poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) as hole transport layer [15] or electrode [17]. Notably, the structural and electronic properties study of electrosprayed multilayer active film [16] showed ES is capable of creating organic thin films with complex heterostructures by using different materials and/or solvents, or differently aggregated phase of the same active material, which is in sharp contrast to spin coating. Cells fabricated by electrospray have reached PCEs that are comparable to spin coated devices [10,15]. In addition, due to the Coulombic attraction force between the droplet and the conducting substrate, nearly 100% materials utilization rate is achievable, while spin coating suffers material loss as high as 95% in our spin coating tests. Among numerous physical properties of the solution used in ES, the electrical conductivity plays the most critical role because it directly affects the droplet diameter and subsequently the film nanostructure and morphology. The most commonly used solvents to dissolve the OPV active materials are dichlorobenzene (DCB) and chlorobenzene (CB). However, both DCB and CB are non-polar and their electrical conductivities are too low for ES process. Therefore, small amounts of additives are usually used to boost the conductivity of the solutions. Typical polar solvents such as 1,1,2,2-tetrachloroethane (TCE) [10], acetone [11], dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), and acetonitrile [12] have been used as additives. The polarity of the above mentioned additives is relatively weak and the electrical conductivity enhancements are still modest. This prompts us to consider organic solvents with strong polarity such as acetic acid, which has not been reported as conductivity booster for electrospray fabrication of OPV devices. In this study, we used DCB as primary solvent for active materials, and acetic acid as solvent additive to fabricate the bulkheterojunction active layers based on P3HT:PCBM blend. We optimized the concentration of the acetic acid, and also compared the effect of acetic acid with two other polar solvents (acetone and acetonitrile). We characterized the morphology, the surface roughness, the light absorption, and the nanoscale domain organization of the electrospray fabricated films using various techniques including optical microscopy (OM), UV–vis spectroscopy, and high-resolution X-ray diffraction (XRD). We correlated the solution properties, morphology, and the current density–voltage (J–V) characteristics of the ES devices. We found the PCE of the ES OPV device fabricated in air was comparable to that of the devices fabricated in pure nitrogen environment.

2. Experimental Patterned indium tin oxide (ITO) coated glass substrates were cleaned by sonication in a sequence of diluted Hellmanex solution, deionized (DI) water, acetone, and isopropyl alcohol (IPA) baths for 15 min each and then dried at 80 1C for 1 h prior to use. After drying, the substrates were UV/ozone treated for 10 min. Then the PEDOT:PSS (Clevios P VP AI 4083) solution was spin-coated at 4000 rpm for 60 s onto the ITO/glass substrates and dried at 120 1C for 10 min. The additives of acetone, acetonitrile or acetic acid were mixed with the P3HT:PCBM solution (with the ratio of 1 mg:0.8 mg in 1 mL of DCB) separately. Fig. 1(a) shows the ES setup, which consisted of a syringe pump, a DC high voltage power supply, a metal nozzle and a substrate stage. The electrospray nozzle was a metal needle with the tip tapered to 100 μm outer diameter (New Objective Company). The distance between the needle tip and the substrate was kept at 5 cm. A high voltage of 4–6 kV was applied between the nozzle and the substrate to generate stable electrospray in the

Emitter HV Norm Surface tension electric stress

Camera

Electric shear stress

FTO glass

x

Hot plate on motorized stage

Fig. 1. (a) The electrospray setup; (b) cone-jet mode in electrospray; and (c) the unstable mode because of low electrical conductivity.

cone-jet mode. The P3HT:PCBM solution was injected through the nozzle at a rate of 5 μL min
1. The ES process was performed in air at room temperature. For the spin coated reference devices, 20 mg of P3HT and 16 mg of PCBM were dissolved in 1 mL of DCB and then stirred for more than 12 h prior to use. The solution was spin coated on the PEDOT:PSS coated ITO/glass at 500 rpm for 120 s and dried for more than 2 h. Then one device is spincoated in a glove box filled with nitrogen (N2) and another device is spin coated in air. All the samples were transferred in a thermal evaporator for metal electrode (100 nm of Al) deposition at the pressure of 4  10
6 mbar. The active area of each device was 9 mm2. Then the devices were post-annealed at 120 1C for 10 min in the glove box. The current density–voltage (J–V) characteristics were examined using a Keithley 2400 source measuring unit under simulated AM 1.5 illumination (100 mW/cm2) with a solar simulator. The conductivity of the solutions was tested by a megohmmeter. The optical microscopy and atomic force microscopy were used for the morphological investigation. The absorption spectra of the active films were obtained using a UV–vis–NIR spectrometer. The XRD results were obtained using a high-resolution X-ray diffractometer.

3. Results and discussions 3.1. Optimization of acetic acid concentration To produce quasi-monodisperse droplet with electrospray process, the liquid at the nozzle tip should take a conical shape termed cone-jet mode (Fig. 1(b)), which results from a balance between surface tension and electric stress normal at the liquid– gas interface [18,19]. The liquid electrical conductivity plays a key role in the cone-jet phenomenology. The cone-jet mode may not be obtained if the liquid electrical conductivity is too low with certain nozzle inner diameter and liquid flow rate. The electrical conductivity of the solution composed of 1 mg P3HT and 0.8 mg PCBM dissolved in 1 mL DCB in our experiment is only 1  10
7 S/ cm and could not support the cone-jet mode (Fig. 1(c)). One way to increase the electrical conductivity is to mix another solvent with high electrical conductivity with the solution. As a non-toxic solvent with high electrical conductivity (3.18  10
4 S/cm at 20 1C), acetic acid is a popular additive solvent to increase the solution electrical conductivity in electrospray. When 5, 10 or 15 vol% acetic acid are added into the P3HT:PCBM: DCB (1 mg:0.8 mg:1 mL) solutions, the electrical conductivity of the solutions are increased to 1.55  10–6, 2.46  10
6 and 3.41  10
6 S/cm, respectively, and a stable cone-jet mode can be obtained. Concentration of 20 vol% or more resulted in active

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Fig. 2. J–V curves of OPV devices with different ratios of acetic acid (a) under AM 1.5 G illumination at 100 mW/cm2 and (b) semilogarithmic plots under dark.

Table 1 Summary of average performance parameters of OPV devices fabricated by spin coating and ES with different ratios of acetic acid. All the parameters were averaged over 8 devices and the values in brackets indicate the standard deviation of the PCEs. Device

JSC (mA/cm2)

VOC (V)

FF (%)

PCE (%)

5 vol% Acetic acid 10 vol% Acetic acid 15 vol% Acetic acid Spin-coating in N2 Spin-coating in air

6.431 6.863 7.666 8.868 7.760

0.602 0.596 0.597 0.580 0.545

63.01 62.85 65.25 60.58 57.62

2.43 2.57 2.99 3.12 2.44

material aggregation in the solution. The typical J–V curves of OPV devices fabricated by ES with different acetic acid concentration are presented in Fig. 2. The statistical averages of the corresponding performance parameters and the standard deviations of PCE are summarized in Table 1. The OPV devices fabricated from 15 vol % acetic acid showed the highest PCE (2.99 70.06%) with average JSC of 7.666 mA/cm2, VOC of 0.597 V, and FF of 65.25%. As can be seen in Fig. 2 and Table 1, JSC and FF increases significantly with the increase of the additive ratio of acetic acid from 5 vol% to 15 vol%. Fukuda et al. reported the similar tendency in the ES OPV devices with P3HT:PCBM active layer using acetone as additive in DCB [11]. They attributed the improved performance to the smooth surface formed from the solution with higher concentration of acetone. The droplet diameter may be another fundamental parameter to explain the enhanced device performance with the increased ratio of added acetic acid in our work. Small droplet size is essential to form a high quality film because the active layer thickness is only 100– 200 nm and large droplet size may result in gaps among the pancakes and thus cause local shorts in the film. The droplet diameter of the electrospray is usually inferred from the scaling laws. At high conductivity regime the scaling law for droplet size yields [18] D ¼ GðεÞðQ εε0 =kÞ1=3 ¼ GðεÞðQ τe Þ1=3

ð1Þ

where D is the droplet size, ε the dielectric constant, k the electrical conductivity, and Q the liquid flow rate. There appears to be no consensus on the functional form of G(ε), which, at any rate, is not critical in the present context. τe is the charge relaxation time, which indicates how quickly the charge on the liquid surface responds to a transient electric field. Eq. (1) reveals that both the liquid flow rate Q and the electrical conductivity k will affect the droplet size. To ensure sufficient throughput, the flow rate should be kept at a meaningful level, and this leaves us with the only option of increasing liquid conductivity to obtain smaller droplets. Higher acetic acid concentration boosts the

( 70.10) ( 70.08) ( 70.08) ( 70.07) ( 70.12)

RS (Ω cm2)

RSH (Ω cm2)

Conductivity (10
6  S/cm)

104 96 93 94 157

11258 11325 12804 10000 9110

1.51 2.66 3.41 NA NA

conductivity, thus reduces the droplet diameter. The values of RS and RSH in Table 1 show that RS decreases and meanwhile RSH increases with increasing acetic acid concentration, which proves that solution with higher conductivity can indeed reduce defects during the ES process and achieve better contact between the active layer and electrode. Consequently, device fabricated by solution with 15 vol% acetic acid demonstrated the highest JSC, FF and PCE. OPV devices with spin coated active layers are fabricated for comparison. It is well known that oxygen and moisture may deteriorate the OPV device performance. To account for the effect of oxygen and humidity, we fabricated spin-coating devices both in N2 glovebox and in air. Since the ES process lasted for 60 min, the spin coated active layer film deposited in air was also kept in air for 60 min before being transferred into the N2-filled glovebox. Compared with the spin coated device fabricated in the glovebox, the ES-coated devices demonstrate higher VOC and FF, but lower JSC. The devices spin coated in air showed significantly worse performance compared with the devices spin coated in N2. The lower JSC of the ES deposited device could partially be attributed to P3HT/PCBM degradation caused by oxygen and moisture, which quench the excitons in the film, thus resulted in decreased charge (hole and electron) transport and JSC [20]. However, the ES deposited devices have higher VOC and FF than devices spin coated both in air and N2. From the J–V curves in dark (Fig. 2b), OPV devices deposited by electrospraying solution with 15 vol% acetic acid demonstrated a significant reduction of leakage current at reverse bias, leading to a diode rectification ratio (current density at 1 V divided by current density at
1 V) over 10-fold higher than the devices spin coated in N2 (2.95  105 vs. 2.57  104). Consequently, the fill factor increased from 60.58% for the spin coated device (in N2) to 65.25% for device fabricated by electrospraying solution with 15% acetic acid. This result suggests that the ES deposition method can passivate the current leakage, which might be attributed to the vertical segregation distribution in the active

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Fig. 3. The optical microscope images of active layers fabricated by solutions with different solvent additives: (a) acetone, (b) acetonitrile, and (c) acetic acid.

layer caused by PCBM aggregates after thermal annealing and this will be discussed further in Section 3.2. 3.2. Morphology of active layers fabricated using acetic acid and other additives To compare the effects of different additives, we also fabricated OPV devices with acetone and acetonitrile as additional solvents. The concentration of acetone and acetonitrile was 15 vol%, same as the optimized concentration of acetic acid. Although 5 and 10 vol% of both acetone and acetonitrile were attempted, none of those solutions can sustain stable cone-jet mode, possibly due to the relatively lower electrical conductivity of acetone (2  10
7 S/cm) and acetonitrile (7  10
6 S/cm) at 20 1C. Further increase of the concentration to 20% for both acetone and acetonitrile caused aggregation in the solution. Fig. 3 shows the optical microscope (OM) images of the P3HT/ PCBM blend films that were prepared using the ES method on the PEDOT:PSS coated ITO/glass. The OM images show the film consists primarily of circular residue of the dried deposited droplets. The film is sparsely filled with fiber-like crystals. Kim et al. reported that the boundary between circular residues can resist charge flow, and reducing the boundary density by solvent vapor soaking drastically improved the device performance [10]. The overlap of the boundaries is mainly dependent on the evaporation of solvents. The vapor pressure of solvent is an important factor in the droplet evaporation. At 25 1C, the saturated vapor pressures of acetone, acetonitrile and acetic acid are 230, 80, and 17 mmHg respectively. Lower vapor pressure results in slower drying speed of droplets, which benefits to form a continuous film with fewer circular boundary overlap. The fiber-like crystals were PCBM aggregates induced after post-annealing at 120 1C for 10 min [21–23]. The PCBM aggregates have been found in both thermally annealed and solvent annealed bulk-heterojunction active layers [21–24]. Since the devices fabricated by ES method in our experiment are all under the same conditions except for the additives, thus the variation in density and size of the fiber-like crystals should be attributed to the different additional solvents. It has been suggested that lower vapor pressure solvent may produce more and larger PCBM agglomerates [23], which is in accordance with our experimental results as revealed in Fig. 3(a)–(c). Contradicting claims and explanations abound in the literatures for PCBM aggregation. Arguments have been proposed that the segregate PCBM towards the surface evolved vertical composition distribution with PCBMrich near the cathode and P3HT-rich adjacent to the anode, which enhances the selectivity of the contacts towards one type of charge carrier and so reduce charge leakage [21,25]. Other works postulated that the formation of large PCBM crystallites should be inhibited to achieve higher device performance and morphological stability [23,24]. In our experiment, the JSC of the ES devices are much lower than device spin coated in N2 and slightly lower than

Fig. 4. The XRD of the active layers fabricated by solutions with different solvent additives.

that of the spin coated device in air. Besides the degradation caused by oxygen and moisture in the ES process, the large PCBM domains may inhibit the exciton diffusion toward donor–acceptor interfaces and the boundaries of pancakes may retard the charge transport. The FF of the ES devices is higher than that of the spin coated device in our experiments, which implies that the vertical segregation distribution is beneficial to the reduction of leakage current (see Fig. 2b). 3.3. Spectral analysis of active layers fabricated using acetic acid and other additives Increasing the mesoscopic order and crystallinity scale of P3HT can enhance interchain interaction and hole mobility within the active layer and thus improve PCE of the OPV devices. The crystallinity of P3HT domains can be inferred from relative (100) plane peak intensities of P3HT at a specific position (2θ¼5.41) of the XRD spectra. From Fig. 4, the peak intensities of the ES devices increase in the following order: acetoneoacetonitrileoacetic acid. The relatively high peak intensity obtained using the acetic acid additive reveals the formation of higher order P3HT structures during the solvent evaporation. Generally, the hole mobility in the donor P3HT polymer is lower than the electron mobility in the acceptor PCBM [26,27]. The enhanced crystallinity of P3HT can facilitate the hole transport and result in a better-balanced electron and hole mobility. Fig. 5 shows the UV–vis absorption spectra of the P3HT:PCBM active layers fabricated from solutions with different additives. The absorption spectra were normalized to the maximum P3HT absorption for comparison. In the spectra, the absorption peak at about 330 nm is attributed to PCBM, and the absorption peak locates at  515 nm with two shoulders at  550 nm and  600 nm is attributed to P3HT [28–30]. The positions of the PCBM absorption peak 330 nm and the shoulders at 550 and 605 nm of P3HT absorption do not shift with different solvents. However, the absorption peak of 515 nm was slightly blue shifted

X.-Y. Zhao et al. / Solar Energy Materials & Solar Cells 121 (2014) 119–125

Fig. 5. The normalized light absorption of the films fabricated from solutions containing different solvent additives.

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Fig. 7. The J–V curves of OPV devices with different solvent mixture (15 vol%) under AM 1.5 G illumination at 100 mW/cm2 and the corresponding average parameters (inset).

have been discussed above, the exposure of the ES films to oxygen and moisture, the large PCBM aggregates and the circular boundaries formed in the ES active layers will also lead to lower photocurrent of the ES devices compared to that of the spin coating devices.

3.4. J–V characteristics of OPV devices using acetic acid and other additives

Fig. 6. IPCE spectra of OPV devices.

from 51 nm of spin coated film to 514 nm of electrosprayed films. The exact position of the main P3HT absorption peak is associated with the effective conjugation length of the chain segments in P3HT. When the mean conjugation length increases, the peak position is red shifted [31,32]. Therefore the slight blue shift of the main P3HT absorption peak suggests that ES process leads to shorter efficient conjugation length in polymer backbones. Meanwhile, the absorption shoulders at 550 nm and 600 nm of spin coated film are significantly higher than those of the ES films, which indicates stronger π–π interchain interactions of the polymer backbones in the spin coated film. It has been suggested that slowing growth rate of the bulk-heterojunction film, or in other words, increasing the time it takes for the wet films to solidify, leads to a higher order in the π–π stacking of P3HT chains, and thus enhanced optical absorption [27,33]. In our experiments, we observed that the spin coated film experienced a color change from yellow to red, corresponding to the drying of the film. However, the color of ES film did not change during the deposition, which indicates that the droplets were almost dried when they reached the substrate. The stronger absorption of spin coated film may account for the much higher JSC of the spin coated devices than the electrosprayed devices. Among the ES films, the optical absorption of film from acetic acid additive is higher than those deposited with acetone or acetonitrile additive. This indicates that acetic acid is beneficial for better PCBM crystallinity and efficient π–π stacking of P3HT, which is in accordance with the characteristics revealed by XRD intensity in Fig. 4. The incident-photon-to-current efficiency (IPCE) spectra in Fig. 6 show that the spin coated device has two higher peaks at 350 nm and  550 nm and more vibronic than the electrosprayed devices. This can be partially attributed to the lower optical absorption of the ES active layers (Fig. 5). In addition, as

The J–V curves of OPV devices with different additives are shown in Fig. 7 and detailed device operational parameters are summarized in the inset Table. The highest PCE was achieved by using acetic acid as additive (2.9970.08%), followed by acetonitrile (2.8270.10%) and then acetone (2.6570.09%). The highest VOC and FF were obtained by using acetic acid. The highest JSC was achieved by device fabricated from solution containing acetonitrile, which can be explained by the optical microscope images in Fig. 3. Firstly, the film deposited using acetonitrile as additive has the least circular boundaries that may retard the charge transport. Secondly, PCBM aggregates, which may inhibit the exciton diffusion toward donor–acceptor interfaces, were less and smaller in the film from the solution containing acetonitrile additive compared to the films from other solutions. The JSC values are in accordance with the photocurrent values illustrated by the IPCE spectra. The device deposited by solution with acetic acid additive, which exhibits the highest FF and PCE, has most PCBM aggregates. This result is consistent with the standpoint of Campoy-Quiles et al. that this vertical segregation distribution is expected to enhance the selectivity of the contacts towards one type of charge carrier and therefore reduce charge leakage [21]. The peak intensities of XRD of the ES devices increase in the order of acetone, acetonitrile, and acetic acid, which is in accordance with the FF tendency of OPV devices. The adsorption of oxygen molecules on the polymer chain may lead to VOC loss [20]. Interestingly, the device fabricated from electrospraying solution containing acetic acid, in air for about 1 h, has significantly higher VOC than that of the spin coated device fabricated in N2 in our experiments. The VOC is primarily determined by the energy difference between the highest occupied molecular orbital (HOMO) of the polymer and the lowest unoccupied molecular orbital (LUMO) of the PCBM. As observed in Fig. 5, there is a slight blue shift in the main absorption peak of P3HT in the ES films, which means that the band gap of P3HT in the ES film is broadened. The increase in the optical band gap indicates a lower HOMO level of P3HT upon less crystallization, thus increases the VOC [34]. In addition, the VOC is related to the saturated dark current of the device and the reduction of the leakage current leads to higher VOC [35,36].

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Fig. 8. The normalized photovoltaic characteristics as a function of storage time for OPV devices fabricated by electrospray (solid symbols) and spin coating (open symbols) methods, which are stored (a) in glovebox, and (b) in ambient.

3.5. Stability comparison between OPV devices fabricated by ES and spin coating We measured device stabilities under different storage conditions. Here, the ES devices were fabricated with the optimal solvent system (15 vol% acetic acid additive) and the spin coated devices were fabricated in N2 glovebox. Eight OPV cells by ES and spin coating were fabricated respectively to ensure data reliability. The devices for this study were not encapsulated and tested either in N2 glovebox or under ambient conditions according to the ISOS-D-1 protocol [37]. Eight devices in the same group showed similar stability behavior. The tested results of devices stored in glovebox are illustrated in Fig. 8(a). The electrosprayed devices showed a little bit faster degradation than the spin coating devices. The spin coating devices demonstrated 4.2% of PCE drop after being stored in glovebox for 96 h; while the ES devices dropped by 8.8%. As the content of H2O and O2 in the N2 glovebox is quite low (o2 ppm), the degradation of the tested devices may be mainly attributed to the residual of air/H2O in the PEDOT:PSS layer and the active layer and the diffusion of cathode materials into the active layer [38,39]. Since the ES process was performed in air, there might be more residual air/H2O in the PEDOT:PSS and active layers in ES devices than in spin coating devices. The residual air/H2O in the devices could result in increased series resistance and reduced fill factor of the device [38]. The diffusion of Al atoms into the P3HT/ PCBM active layer may act as a recombination site and hence accelerate device degradation [40]. From this point of view, devices fabricated by ES may have more recombination sites because of the rougher surface of the active layers prepared by ES method. For devices stored and tested under ambient conditions (results shown in Fig. 8(b)), the performance of both groups of devices decayed much faster than the devices stored in glovebox. As the sample devices were unsealed and without cathode buffer layer, oxygen and moisture are expected to diffuse into the device from the microscopic pinholes of the aluminum cathode and the edges of the films. Subsequently, O2 and moisture will cause oxidation of the aluminum electrode and chemical degradation of the active layer [20,40]. It is worth noting that the ES devices decayed slower than spin coating devices tested in ambient conditions. The ES devices showed 31% performance loss after being exposed in air for 24 h; while the spin coating devices lost performance by 56%. After exposure in air for 72 h, we found the PCE of the ES devices dropped to 40% of the original value; while the spin coating devices only retained 12% of the original PCE. Although the reason of slower degradation of the ES devices than spin coating devices in ambient conditions is not clear at this moment, the results reported here may indicate that electrospray process has one more advantage over spin coating in terms of better device storage lifetime in ambient conditions. Further investigation is planned to study the mechanism of this interesting behavior.

4. Conclusions Acetic acid was mixed with DCB to enhance the electrical conductivity of P3HT:PCBM solution to obtain stable deposition in the ES process. Higher concentrations of acetic acid in DCB led to better device performance, which was related to the higher electrical conductivity of the mixed solution. Morphology of P3HT: PCBM active layers deposited using ES with additives of acetic acid, acetone, and acetonitrile were investigated and compared. Acetic acid additive led to significant vertical phase separation, stronger optical absorption, and the highest XRD intensity. These properties of the active layer led to the best PCE (2.99 70.08%) of device deposited using acetic acid as additive in ES process, which is comparable to that of the OPV device fabricated by spin coating in N2 (3.12 70.07%) and higher than device spin coated in air (2.44 70.12%). This result indicated that ES is very promising for OPV fabrication. However, the photocurrent of OPV devices fabricated by ES was lower than that of the device spin coated in N2. This can be partially attributed to the properties of ES films, such as lower ordering of the π–π conjugated structure in P3HT that reduces the optical absorption, and overlap of circular boundaries that retards the carrier transport. Lifetimes of devices stored in glovebox and in ambient conditions were measured. Results indicate that ES devices have better device storage lifetime in ambient conditions. Further study is on going to improve the morphology of active layers of ES devices and to investigate the mechanism of device degradation fabricated through different methods. Acknowledgments This work is partially supported by National Basic Research Program of China (973 Program, No. 2009CB930600), National Natural Science Foundation of China (Nos. 61077021, 61076016), funding from Nanjing University of Posts and Telecommunications (Nos. NY212076, NY212050). WD thanks the partial financial support from National Science Foundation (CMMI 1335295). References [1] R.F. Service, Outlook brightens for plastic solar cells, Science 332 (2011) 293. [2] M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Solar cell efficiency tables (version 39), Prog. Photovolt. Res. Appl. 20 (2012) 12–20. [3] L.-M. Chen, Z. Hong, W.L. Kwan, C.-H. Lu, Y.-F. Lai, B. Lei, C.-P. Liu, Y. Yang, Multi-source/component spray coating for polymer solar cells, ACS Nano 4 (2010) 4744–4752. [4] F.C. Krebs, Fabrication and processing of polymer solar cells: a review of printing and coating techniques, Sol. Energy Mater. Sol. Cells 93 (2009) 394–412.

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