Eur. Phys. J. Appl. Phys. (2013) 62: 30202 DOI: 10.1051/epjap/2013130107

Film forming properties of electrosprayed organic heterojunctions

M. Ali, M. Abbas, S.K. Shah, E. Bontempi, A. Di Cicco, and R. Gunnella

Eur. Phys. J. Appl. Phys. (2013) 62: 30202DOI: 10.1051/epjap/2013130107

THE EUROPEANPHYSICAL JOURNAL

APPLIED PHYSICS

Regular Article

Film forming properties of electrosprayed organic heterojunctions

M. Ali1,2, M. Abbas3,4, S.K. Shah1, E. Bontempi5, A. Di Cicco1, and R. Gunnella1,6,a

1 CNISM – School of Science and Technology, Camerino University, via Madonna delle Carceri, 62032 Camerino (MC), Italy2 Physics Department, COMSATS Institute of Information Technology, Park Road Chak Shahzad, 44000 Islamabad, Pakistan3 Linz Institute for Organic Solar Cells, No. 69, Altenberger Strase, 4040 Linz, Austria4 Laboratoire IMS, UMR 5218 CNRS, ENSCBP, 16 avenue Pey-Berland, 33607 Pessac Cedex, France5 Chemistry for Technologies Laboratory, University of Brescia, via Branze 38, 25123 Brescia, Italy6 ISM-CNR-Area di Ricerca di Tor Vergata, via del Fosso del Cavaliere 100, 00133 Roma, Italy

Received: 26 February 2013 / Received in final form: 10 May 2013 / Accepted: 17 May 2013Published online: 18 June 2013 – c© EDP Sciences 2013

Abstract. We used the electrospray deposition (ESD) method to fabricate organic photovoltaic devices withpoly (3-hexyl-thiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) blends of differentcomposition ratios and different organic solvent solution, namely chloroform (CLF) and dichlorobenzene(DCB). The morphology and crystallinity of the active layers were investigated by means of atomic forcemicroscopy (AFM), two-dimensional X-ray diffraction (XRD2) and optical absorption, spreading lighton the peculiarities of the present growth technique, involving much faster solvent evaporation and filmforming processes, and comparatively much more ordered strutures with less need of thermal annealingprocesses. The power conversion efficiency (PCE) under AM 1.5G solar simulation obtained for devicesdeposited from DCB as compared to those deposited from CLF, showed significant improvement, in fairagreement with what was found by the overall characterization of the physical properties.

1 Introduction

In the development of renewable energy devices, organicphotovoltaics (OPV) cells have attracted significant at-tention because of their flexibility, large area and low costof materials, processing and installation.

The power conversion efficiency (PCE) of bulkheterojunction (BHJ) solution-processed polymer solarcells [1–3] has reached reported certified values up to8.62% [4]. Despite the various advantages over conven-tional inorganic counterparts, OPV has been facing somechallenges and limitations such as low charge carrier mo-bility, lack of low band gap polymers and short excitonlife-time, limiting power conversion efficiencies [5,6].

The fabrication of OPV devices, in which the activelayer is deposited by a solution casting technique, suchas the most widely used spin coating process, is affectedby small scaling of device area, larger material consump-tion, low production rate and high production cost. Forthese reasons devices fabrication by various novel materi-als growth techniques such as screen printing [7], doctorblading [8], inkjet printing [9], spray deposition and air-brush spray deposition was proposed [10]. For instance,spray deposition methods were applied successfully to thefabrication of OPV devices [11–15] and recently reached alevel of maturity required for the large-scale production.

a e-mail: roberto.gunnella@unicam.it

In some cases, such methods mostly adopted post-growththermal/solvent treatment processes and were based onlarge concentration values of the solutions. The present ap-proach points to a technology able to reduce post-growthtreatments, and by the use of low concentration solutionsto achieve a more refined route to the fabrication of com-plex heterostructures. Electro-spray, among other scalabletechniques, seems to accomplish with this task convinc-ingly well.

Vak et al. [15] fabricated an OPV device based on a2:1 (by weight) blend of poly (3-hexyl-thiophene) (P3HT)and [6,6]-phenyl C61 butyric acid methyl ester (PCBM)from a dilute solution (≤2 mg/mL) using a handheld air-brush spray deposition. Although the optimized device ob-tained a power conversion efficiency of 2.83%, the solutionconcentration used was still four times larger than thatused in the present electrospray method. In another work,Kim et al. [16] recently proposed electrostatically sprayedmethod in air and obtained power conversion efficiency upto 0.38%, with solution concentration of about 1 mg/mLof P3HT:PCBM (1.2:1.0) using cosolvents due to unstableatomization with a single solvent [16,17]. Solution concen-tration was even higher (1.8–2.2 mg/mL) in the work ofFukuda et al. [17], with the reported PCE of 1.9%. Inthese works the solvent effect is perhaps one reason forinefficient separation from solute and for low efficiency ofas-prepared devices. This is confirmed by the significant

30202-p1

The European Physical Journal Applied Physics

increase of efficiency obtained after further treatments [16].In such a scenario, the lack of a detailed characterizationof morphology and crystallinity of the active layers andtheir effects on the performance of the novel devices is de-cisive and cannot simply be retrieved by previous studiesperformed for more conventional film forming techniques.

In this respect, we previously demonstrated the opti-mization of deposition parameters for organic thin filmsusing an electrospray method [18,19]. The P3HT thinfilms prepared with 2 kV applied voltage and 0.5 mg/mLsolution concentration exhibited predominant flat-on ori-entation of the molecular lamella plane and good dioderectification [19], which is essential for the fabrication ofOPV cells. By such a technique we recently prepared OPVdevices with state-of-the-art efficiency if post-growthannealing temperature did not exceed 130 ◦C, a manda-tory issue in case of fabrication on flexible plasticsubstrates [20]. The results are even more surprising, con-sidering that the comparison was done with samples grownby spin coating in glove box, without transfer in air. Thereason for this remarkable property is not completely clear,deserving further studies and characterization of the filmsproduced by ESD.

In the same work [20], we also successfully demon-strated the advantage in terms of efficiency obtainable byintroducing specially modeled multilayer heterojunctions,made possible by the specific technique adopted.

Here we present, by different investigation tools, moredetails on the overall picture of the OPV devices obtainedby the electrospray technique. Optical absorption wasmeasured to infer structural conformation while films sur-face morphology was investigated using atomic forcemicroscopy (AFM). Two-dimensional X-ray diffraction(XRD2) was performed to study the order of the filmalong the growth direction. Electrical characterization ofOPV devices was presented for the comparison of efficien-cies obtained using different solvents and donor/acceptorratios.

2 Experimental details

The electrospray technique is a promising method forthe preparation of many kinds of polymers thinfilms [21,22] from liquid solutions. A schematic diagramof the electrospray deposition apparatus was reported inreferences [18,19]. A teflon tubing (3 mm OD, 1.5 mm ID)and a flange brought the solution to a stainless steel tip,specifically modified through an electrochemical etchingin the H2SO4:H2O, 1:1 solution providing a conical shape(0.31 mm OD, 0.16 mm. ID). A high-voltage DC powersupply was connected to the spray tip. The tip was cen-tered to the pinhole of the grounded skimmer by accu-rate positioning. The solution container was pressurizedwith nitrogen gas, and a controlled flow rate of solutionmoved consequently toward the tip through the teflon. Ahigh voltage, typically 1–3 kV, was required at the nee-dle tip with respect to the grounded sample for the effi-cient electro-ionization, depending mainly on the boiling

Fig. 1. Schematic sketch of the electrospray process for filmdeposition.

point of the solvents used. Operatively, when the solu-tion reaches the tip, an accumulation of charges makesthe surface unstable and leads to the formation of theTaylor cone of droplets and to the separation of positiveand negative charges (Fig. 1). The solvent is removed fromthe droplets through a differential pumping stage. As aconsequence, the charge density on droplet increases withrepulsive forces between the droplet and the solute ionscausing the formation of a wide plume. The estimatedcharge on a droplet is, according to the Rayleigh theory,

QR = 8π(σlε0a

3) 1

2 , (1)

where σl is the liquid surface tension, ε0 is the electricpermittivity of the free space and a is the radius of thedroplet. The charge and size of the droplets can be con-trolled to some extent by adjusting the liquid flow rateand the voltage applied to the nozzle. Since the materialextracted through a grounded skimmer using the differen-tial pumping system reaches the substrate, a thin film isformed. This system allows the direct transfer of macro-molecules from solution environment into vacuum, wherethey can be deposited in the absence of contaminants.The first stage pumping system was maintained at 1 torr.The substrate which was mounted on the sample holderin the second stage pumping was maintained at a pressureof 1 mtorr while the charged molecules were driven awayfrom the solution toward the substrate by the differentialpumping system.

By ESD deposition technique, we prepared BHJdevices as shown in Figure 2 starting from differentcomposition ratios of blended materials using eitherdichlorobenzene (DCB) or chloroform (CLF). The deviceswere fabricated on indium tin oxide (ITO) coated glasssubstrates which were sequentially cleaned by sonicationin 2% Hellmanex solution, deionized water, acetone,isopropanol and deionized water for 20 min.

The conducting hole transport buffer layer, a 50 nmlayer of poly (3,4-ethylenedioxythiophene) doped withpolystyrene sulfonic acid (PEDOT:PSS), was subsequentlyspin coated from a water solution on ITO/glass sub-strates at 4000 rpm for 40 s, and subsequently driedin air for 30 min at 150 ◦C. Commercial electronic grade

30202-p2

M. Ali et al.: Film forming properties of electrosprayed organic heterojunctions

Fig. 2. (a) Chemical structure of P3HT, (b) PCBM and(c) model of bulk heterojunction organic solar cell.

regioregular P3HT and PCBM powders (sigma), as de-picted in Figure 2, were dissolved without further pu-rification in CLF and DCB solution (0.5 mg/mL, whichis approximately 40 times more diluted than the solu-tions used for spin coated devices). The active blend layerswith an optimized thickness of 120 nm corresponding to2 mL solution were deposited in various weight ratios of(P3HT:PCBM) in the range from 1:1.2 to 1:0.4.

Other parameters for voltage and flow rate were 2 kVand 1.3–2 μL/s respectively.

The optical absorption spectra were measured bymeans of UV-vis Varian Cary 3G spectrophotometer.AFM studies were carried out using digital instruments di-mension 3100 in tapping mode with quantitative analysisperformed using the Gwyddion software. Two-dimensionalX-ray diffraction (XRD2) images were collected by aDymax-RAPID X-ray microdiffractometer, with a cylin-drical imaging plate detector, allowing Cu kα diffractiondata from 0 to 160◦ (2θ) horizontally and from −45 to45◦ (2θ) vertically. The samples have been found to keeptheir characteristics for few weeks in air, before the metal-lic contacts (150 nm Al) are thermally evaporated. Themeasurements were led in a Steuernagel solar simulatorfor AM1.5 conditions and input power of 100 mW/cm2

white-light illumination calibrated using a standard silicondiode. I-V characteristics were measured using a KeithleySMU 236 in dry nitrogen glove box.

3 Microscopy

Though strong redistribution of composition and readjust-ment of morphology along the vertical direction are wellknown [23] and must be taken into consideration in a real

device, especially after contacts fabrication [24], we re-ported in Figures 3 and 4 the AFM images of active lay-ers surfaces with P3HT:PCBM weight ratios of 1:0.6, 1:0.8and 1:1, prepared with CLF and DCB, respectively. Weobserved that the films prepared with DCB show rathersmooth surface compared to the ones with CLF. We as-sumed that solvent and solute dissociation in electrosprayprocess were dependent on the boiling point of the sol-vents used. While lower boiling point made CLF easierto evaporate, the polymer blend was much drier whenit reached the substrate compared to the DCB solution.Dewetting of the trace solvent was playing an importantrole in smoothening the film. Much drier polymer blendstended to aggregate to form much rougher surface. More-over, surface phase separation of P3HT and PCBM wasmore evident in the case of the blend in CLF.

Nevertheless, such observations are very indicative ofstill unclear mechanisms in the film composition. In factthe whole method was intended to strenghten the indepen-dence from the boiling point of the solvent, with solventfree film formation being its real peculiarity.

Higher film roughness and poorer percolation of thedonor and acceptor materials could contribute to the lowerefficiency (shown later on) of the devices prepared fromCLF if the effects were seriously involving a large portionof the sample volume. Such an hypothesis will be con-firmed by the XRD diffraction reported below.

Figures 3 and 4 also show in the lower right cornercharacteristic line profiles scanned through the images.Here it is clearly shown that a fullerene content around50% contributes to smoothen the roughness value of thefilm in a sizeable way for both classes of samples, indi-cating an optimal morphology for a quite balanced blendcomposition with respect to the predominance of onespecies with respect to the other.

4 X-ray diffraction

The XRD2 images are shown in Figure 5 for the samplesgrown with different blend ratio in CLF fromFigures 5a to 5c and in DCB from 5d to 5e. 2D detectorshave been chosen for analysis in order to allow the collec-tion of large portions of the diffraction rings. The 2θ andβ integration directions are shown in Figure 5. Integrat-ing the intensities of 2D images along θ allows to obtain1D XRD patterns, that can be analyzed by conventionalpowder diffraction analysis software. Integrating the in-tensities of 2D images along β direction allows to evaluatepreferred orientation. Indeed, in the case of an ideally mis-oriented sample, the Debye rings would appear continuousand uniform in the intensity; on the contrary, when themicrostructure is far from ideal conditions, Debye ringsare modified in the intensity distribution (along β) orshape [25]. Spectra of samples deposited from DCB showan intense peak, at about 5◦ (2θ), that indicates a de-gree of crystallinity of P3HT:PCBM system. The imagesshow that Debye ring, at about 5◦, has concentrated in-tensity, indicating the presence of preferred orientation.

30202-p3

The European Physical Journal Applied Physics

(a) (b)

(d)(C)

Fig. 3. AFM images (surface scan area: 10 × 10 μm2) of active layers deposited from CLF with weight ratios (P3HT:PCBM)of 1:0.6, 1:0.8 and 1:1 respectively. (d) Representative line scans of the samples surfaces.

(a) (b)

(d)(C)

Fig. 4. AFM images (surface scan area: 10 × 10 μm2) of active layers deposited from DCB with weight ratios (P3HT:PCBM)of 1:0.6, 1:0.8 and 1:1, respectively. (d) Representative line scans of the samples surfaces.

The concentrated intensity of the diffraction peak is in-dicative of a higher order of the system in the verticaldirection. It is possible to conclude that these spectra area proof of the miscibility of PCBM in P3HT domains oc-curring without disrupting the crystalline P3HT domainsin almost all sample volume, giving origin to a balancedmixing of contiguous P3HT and PCBM nanoaggregates.On the contrary, XRD2 images of samples deposited fromCLF show only a very weak but isotropic XRD peak, due

to a poorly ordered but homogeneous system. Such inten-sities result not so concentrated as those found for samplesdeposited from DCB. These spectra are due to the diffu-sion of the fullerene component in the polymer matrixprobably forming a dispersion of PCBM with disorderedP3HT molecules [26]. To semiquantitatively evaluate theorientation, the change in the XRD intensity along onesingle Debye ring can be studied. In Figure 6 the plot ofthe XRD intensity of the peaks at about 5◦ (2θ) versus

30202-p4

M. Ali et al.: Film forming properties of electrosprayed organic heterojunctions

(a)

(d) (e) (f)

(c)(b)

2qb

Fig. 5. XRD2 images of P3HT:PCBM blend films deposited from CLF are presented from (a) to (c), while films deposited fromDCB from (d) to (f); in both cases the P3HT to PCBM ratio was increased from 0.6 (left) to 1.0 (right).

β angle [25] is shown. If there were no preferred orienta-tion, the Debye ring intensity should be constant alongβ (infinite spread). Thus, the behavior of the diffractionintensities shown in Figure 6 demonstrates that samplesdeposited from CLF, showing lower diffraction signal, areslightly oriented. On the contrary samples deposited fromDCB are highly oriented [27]. The XRD intensity alongthe Debye ring at 5◦ was fitted by a pseudo-Voigt functionand the results of FWHM values are shown in Figure 6.It results that sample P3HT:PCBM in 1:1 ratio in DCBhas the lowest FWHM value of the diffraction intensityalong β direction, indicating the highest orientation, con-sistently with the proof of a regular surface morphologyobtained for the same sample in AFM analysis.

5 Optical absorption

The normalized absorption spectra of P3HT:PCBM blendfilms with different compositions are reported in Figure 7.They showed three main features mainly due to the P3HTabsorption: a first shoulder structure at about 596 nm(2.08 eV); a second one at about 549 nm (2.26 eV);a broad feature centered around 515 nm (2.41 eV). Thespectra are normalized at the minimum of the absorp-tion around 400 nm. Following Clark and coworkers [28],the scheme of a broad peak, due to disordered P3HT do-mains, and of low energy vibronic features, due to ordereddomains, is well established [19,29].

An overall decrease of vibronic peaks was noticed atthe increase of the PCBM concentration, especially above67% (not shown here) due to strong disorderingeffects.

Fig. 6. Plot of the signal of diffraction peak around 5◦ (2θ)along the Debye ring direction (β direction). DCB solu-tion deposited blend films are presented in the upper partwhile lower curves are samples deposited from CLF. Themeasured FWHM is reported for DCB samples diffractionintensity.

30202-p5

The European Physical Journal Applied Physics

(a)

(b)

Fig. 7. Normalized absorption spectra of ESD blendfilms with weight ratio 1:1, 1:0.8, and 1:0.6 respectively(a) samples prepared with CLF, (b) samples prepared withDCB.

Nevertheless, at the P3HT:PCBM ratio variation, themain vibronic features remain well visible, indicatingstrong interchain interactions among the regioregularP3HT chains and high ordering of the polymer chains [29].This was especially true in the case of DCB films whichdo not seem to be affected by the PCBM amount, keep-ing a constant ratio of ordered and disordered domainsvolumes, for a wide range of compositions.

Quite different is the behavior of the sample grownby CLF solution, less efficient in dissolving the strong ag-gregations, leading to an increase of amorphous domainswith respect to ordered domains as dramatically describedin Figure 7a. Here, even if more intense vibronic features(Fig. 7a) are initially present due to larger volume of crys-talline P3HT regions, the strong interaction among thetwo components led to a relevant amorphization.

These observations are in fair agreement with AFManalysis, where much rougher surface of films depositedfrom CLF with respect to DCB solution is obtained. They

-2.0 -1.0 0.0 1.0 2.0

10-2

10-1

100

101

102

0.0 0.2 0.4 0.6-4

-2

0 P3HT:PCBM

(b)

1:1 1:0.8 1:0.6

Cur

rent

den

sity

(mA/

cm2 )

Voltage (V)

CLF

illumination

-2.0 -1.0 0.0 1.0 2.0

10-7

10-5

10-3

10-1

101

Voltage (V)

P3HT:PCBM

Cur

rent

den

sity

(mA/

cm2 )

CLF

dark

1:1 1:0.8 1:0.6

(a)

Fig. 8. J-V curves of BHJ devices prepared with CLF withdifferent P3HT:PCBM weight ratios of 1:1, 1:0.8 and 1:0.6,respectively: (a) dark current, (b) current under illumination.Inset in panel (b) showed an expansion of the low positive Vrange.

also confirm XRD of CLF prepared films, showing larger(from the width along 2θ of the XRD patterns) but disor-dered P3HT crystals, due to the faster CLF evaporationrate.

In addition, samples grown from CLF solution showedsizeable Mie scattering in the high wavelengths region dueto microsize agglomeration.

As a general consideration, when compared with sim-ilar studies reported in the literature, it is worth to stressthat the present UV-vis data present a remarkable amountof P3HT in crystalline phase even in the absence of post-growth thermal annealing in both cases of solvents. Thisis pointing to a peculiar film forming mechanism inducedby the electrospray capable of a suitable control of thenanoparticle size similar to the use of dipolar solvent tocontrol the amorphous and aggregated P3HT particles ra-tio [30]. Such an effect would be mainly due to the forcesinteracting among polymers along the route from dropletsto the substrate, and if finely tuned could represent anadded value of the present technique.

30202-p6

M. Ali et al.: Film forming properties of electrosprayed organic heterojunctions

-2.0 -1.0 0.0 1.0 2.0

10-1

100

101

102

0.0 0.2 0.4

-6

-3

0P3HT:PCBM

1:1 1:0.8 1:0.6

Cur

rent

den

sity

(mA/

cm2 )

DCB

Voltage (V)

illumination

(b)

-2 .0 -1 .0 0 .0 1 .0 2 .010 -6

10 -4

10 -2

10 0

10 2

Voltage (V)

1:1 1:0.8 1:0.6

P3HT:PCBM

DCB

Cur

rent

den

sity

(mA/

cm2 ) dark

(a)

Fig. 9. J-V curves of BHJ devices prepared with DCB withdifferent P3HT:PCBM weight ratios of 1:1, 1:0.8 and 1:0.6,respectively: (a) dark current and (b) current under illumi-nation. Inset (panel b) showed an expansion of the low posi-tive V range.

6 Current density-voltage measurements

The current density-voltage (J-V) characteristics underdark and illumination in AM1.5G (100 mW/cm2) con-ditions for ESD devices prepared with different P3HT:PCBM weight ratios are presented in Figures 8 and 9, us-ing CLF and DCB solutions respectively. The dark J-Vcurves (Fig. 8a) showed a poor rectification in the case ofdevices deposited from CLF solution, indicating that, dueto the inhomogeneities, the active layer thickness was notpreventing formation of pinholes between the electrodes;as a result these devices had low shunt resistance and size-able leakage current. On the contrary, the devices preparedfrom DCB showed better rectification (Fig. 9a), suggest-ing good diode behavior and a high shunt resistance, dueto smoother surface of the active layer able to efficientlycover the PEDOT:PSS layer. From the J-V characteris-tics under illumination, we could see that the compositionratio strongly influenced the device electrical properties.When the weight ratio of P3HT:PCBM composition wasequal to 1:0.8 in CLF, the device exhibited the highestPCE (0.53%). The values of short-circuit current density(Jsc) and fill factor (FF) were 3.63 mA/cm2 and 28%,

Table 1. Performance parameters obtained from J-V curves ofdifferent devices prepared in this study.

Layer Jsc (mA) Voc (V) FF% Efficiency (%)P3HT:PCBM1:0.6 (CLF) 2.51 0.56 32 0.451:0.8 (CLF) 3.63 0.52 28 0.531:1 (CLF) 2.15 0.5 31 0.39

1:0.6 (DCB) 5.10 0.40 22 0.451:0.8 (DCB) 5.34 0.44 25 0.601:1 (DCB) 7.00 0.44 41 1.30

respectively, as shown in Figure 8b, while, by increas-ing the PCBM concentration to values higher than 50%,the solar cell characteristics quality was drastically re-duced. But almost a threefold (from 0.53 to 1.3%) in-crease has been observed in the PCE for the devices de-posited from DCB with composition ratio of P3HT:PCBMof 1:1 (Fig. 9b); similarly the current density increased to7.0 mA/cm2 and the FF to 41%, which suggests a per-colating structure with interpenetrating networks. On theother hand, the open-circuit voltage (Voc) did not changerelevantly (0.40–0.55 V) for both sets of devices, becausethe Voc mainly depends on the electronic donor/acceptorenergy levels [10]. From these results it is evident thatthe PCE of electrosprayed devices obtained here is almostfour times larger than the corresponding electrostaticallysprayed devices without any further treatment obtainedby Kim and coworkers [16] because of the specific para-meters allowed by the present setup.

Table 1 summarizes the characteristics of the devicesobtained in this study.

7 Conclusions

In conclusion, we have shown the characteristics of BHJactive layers grown by electrospray and their dependenceon the composition ratio from the point of view of themorphological and electrical properties. We recentlydemonstrated that the electrospray technique, under cer-tain conditions [20], is competitive, with more conven-tional ones, and that additional peculiar functionalities,like multilayer BHJs, were easy to achieve. Furthermore,low solution concentrations used in these preparationswould allow a broader set of novel materials for possi-ble implementation in future OPV devices. Therefore, thepeculiar characteristics of ESD blend films to be used asactive layers in OPV devices cannot be retrieved from pre-vious studies of films prepared by a different procedure.In particular the ESD films show sizeable order also with-out post-growth annealing. We proposed a picture wherethe ESD deposition technique acts by promoting encap-sulation of crystalline nanoparticles within the amorphousmatrix and fine-tuning of the size of the components couldbe hopefully controlled by solvent choice and other para-meters of the deposition (composition, high voltage andflow rate).

We also correlated PCE values with the physical char-acteristics of the active layers. Here, we have looked in

30202-p7

The European Physical Journal Applied Physics

details at the role of the active layers deposited from DCBwith composition ratio of 1:1 (P3HT:PCBM). They exhib-ited the maximum PCE of 1.30% and current density of7.0 mA/cm2 under AM 1.5 solar simulation.

Specific technical solutions could furtherly improve theperformances of such devices, for instance by using mul-titip spray for a better films planarity and integrated de-position systems to reduce ambient contamination dur-ing transfer to facilities for device characterization, but astrong control on the material side is a matter of priorityfor future progresses and the advancements of knowledgein the field of hybrid devices.

References

1. N.S. Sariciftci, L. Smilowitz, A. Heeger, F. Wudl, Science258, 1474 (1992)

2. S.E. Shaheen, C.J. Brabec, S. Sariciftci, F. Padinger, T.Fromherz, J.C. Hummelen, Appl. Phys. Lett. 78, 841(2001)

3. W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Adv.Funct. Mater. 15, 1617 (2005)

4. L. Dou, J. You, J. Yang, C.-C. Chen, Y. He, S. Murase,T. Moriarty, K. Emery, G. Li, Y. Yang, Nat. Photon. 356,2011 (2012)

5. C. Winder, N.S. Sariciftci, J. Mater. Chem. 14, 1077(2004)

6. H. Hoppe, N.S. Sariciftci, J. Mat. Res. Soc. 19, 1924 (2004)7. S.E. Shaheen, R. Radspinner, N. Peyghambarian, G.E.

Jabbour, Appl. Phys. Lett. 79, 2996 (2001)8. P. Schilinsky, C. Waldauf, C.J. Brabec, Adv. Funct. Mater.

16, 1669 (2006)9. C.N. Hoth, S.A. Choulis, P. Schilinsky, C.J. Brabec, Adv.

Mater. 19, 3973 (2007)10. C.J. Brabec, J.R. Durrant, MRS Bulletin 33, 670 (2008)11. G. Susanna, L. Salamandara, T.M. Brown, A. Di Carlo, F.

Brunetti, A. Reale, Sol. Energy Mater. Sol. Cells 95, 1775(2010)

12. C.N. Hoth, R. Steim, P. Schilinsky, S.A. Choulis, S.F.Tedde, O. Hayden, C.J. Brabec, Org. Electron. 10, 587(2009)

13. R. Green, A. Morfa, A.J. Ferguson, N. Kopidakis, G.Rumbles, S.E. Shaheen, Appl. Phys. Lett. 92, 033301(2008)

14. T. Ishikawa, M. Nakamura, K. Fujita, T. Tsutsui, Appl.Phys. Lett. 84, 2424 (2004)

15. D.J. Vak, S.S. Kim, J. Jo, S.H. Oh, S.I. Na, J.W. Kim,D.Y. Kim, Appl. Phys. Lett. 91, 081102 (2007)

16. J.S. Kim, W.S. Chung, K. Kim, D.Y. Kim, K.J. Paeng,S.M. Jo, S.Y. Jang, Adv. Funct. Mater. 20, 3538(2010)

17. T. Fukuda, K. Takagi, T. Asano, Z. Honda, N. Kamata,K. Ueno, H. Shirai, J. Ju, Y. Yamagata, Y. Tajima, Phys.Status Solidi RRL 5, 229 (2011)

18. M. Abbas, M. Ali, S.K. Shah, F. D’Amico, P. Postorino,S. Mangialardo, M.C. Guidi, A. Cricenti, R. Gunnella, J.Phys. Chem. B 115, 11199 (2011)

19. M. Ali, M. Abbas, E. Bontempi, P. Colombi, S.K. Shah, A.Di Cicco, R. Gunnella, J. Appl. Phys. 110, 054515 (2011)

20. M. Ali, M. Abbas, S.K. Shah, R. Tuerhong, A. Generosi,B. Paci, L. Hirsch, R. Gunnella, Org. Electron. 13, 2130(2012)

21. J.C. Swarbrick, J.B. Taylor, J.N. Oshea, Appl. Surf. Sci.252, 5622 (2006)

22. N. Dam, M.M. Beerbom, J.C. Braunagel, R. Schlaf, Appl.Phys. 97, 24909 (2005)

23. B. Xue, B. Vaughan, C.-H. Poh, K.B. Burke, L. Thomsen,A. Stapleton, X. Zhou, G.W. Bryant, W. Belcher, P.C.Dastoor, J. Phys. Chem. C 114, 15797 (2010)

24. A. Dupuis, A. Tournebize, P.-O. Bussire, A. Rivaton, J.-L.Gardette, Eur. Phys. J. Appl. Phys. 56, 34104 (2011)

25. E. Bontempi, L.E. Depero, Thin Solid Films 450, 183(2004)

26. A. Swinnen, I. Haeldermans, P. Vanlaeke, J. D’Haen,J. Poortmans, M. D’Olieslaeger, J.V. Manca, Eur. Phys.J. Appl. Phys. 36, 251 (2006)

27. Y.S. Kim, Y. Lee, J.K. Kim, E.-O. Seo, E.-W. Lee, W. Lee,S.-H. Han, S.-H. Lee, Current Appl. Phys. 10, 985 (2010)

28. J. Clark, C. Silva, R.H. Friend, F.C. Spano, Phys. Rev.Lett. 98, 206406 (2007)

29. M. Abbas, F. D’Amico, M. Ali, I. Mencarelli, L. Setti,E. Bontempi, R. Gunnella, J. Phys. D: Appl. Phys. 43,035103 (2010)

30. J. Moule, K. Meerholz, Adv. Mater. 20, 240 (2008)

30202-p8