Design and Fabrication of Photonic Structured Organic Solar Cells by

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Design and Fabrication of Photonic Structured Organic Solar Cells by Electrospraying Khadija Kanwal Khanum, Jagdish Anakkavoor Krishnaswamy, and Praveen Chandrashekarapura Ramamurthy J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01698 • Publication Date (Web): 03 Apr 2017 Downloaded from http://pubs.acs.org on April 4, 2017

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The Journal of Physical Chemistry

Design and Fabrication of Photonic Structured Organic Solar Cells by Electrospraying

Khadija Kanwal Khanum1 Jagdish A. K.2 and Praveen C. Ramamurthy1,2* Department of Materials Engineering, Indian Institute of Science, Bangalore, India 2 Center for Nanoscience and Engineering, Indian Institute of Science, Bangalore, India * email [email protected] 1

Abstract Photonic architectures in optoelectronic devices comprise of uniform periodic structural arrangement which aids in improved interaction with light. The processing of such photonic arrays generally, involves several steps of fabrication. This work proposes single step fabrication of equal submicron size porous photonic structure array through electrospraying. This distinctive approach involves a continuous supply of material with simultaneous removal of solvent for fabricating large area samples, compared to other processing techniques. The process optimization is carried out to regulate the pore diameter and spacing. Morphological studies showed that the continuity of porous array extending to large areas of the order of millimeters. Optical investigations demonstrated that this uniform periodic topography-assisted in improved light scattering and a consequent enhancement of light absorption. Further, the incorporation of this array structure in the active layer of organic photovoltaic devices is evaluated through both experiments and modeling. Modeling and simulations suggest that the optimized range of 200500 nm pore size aids in higher current density. Experiments reveal an improvement of 18 % in the photocurrent density at short circuit in the periodic array architecture compared to the planar reference. Therefore, in this study, a facile single-step novel method of obtaining photonic array structures with improved device performances is demonstrated.

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Introduction Organic solar cells (OSCs) are in the limelight due to their lower temperature processing methods1, lightweight and good performances even in the diffuse light.2 However, other factors such as power conversion efficiency and atmospheric stability still necessitate active research before commercialization.3,4 The power conversion efficiencies are usually dependent on an optimum thickness of active polymer layer to absorb a maximum number of photons and its heterojunction morphology for efficient charge transportation.5,6 Although on the one hand, thin active layers are required to suppress charge recombination,7,8 on the other hand, thick active layers are required for increased light absorption. However, thick active layers will limit the charge extraction as polymer semiconductors have low charge carrier mobilities and possess short carrier diffusion lengths.9,10 These contrasting requirements of maintaining a thin active layer with improved light absorption poses interesting problems in light harvesting research. Improved light harvesting is achieved either by increasing the excitons diffusion lengths by tailoring the polymer donor material in the active layer or by patterning the active layer.11,12 In the case of active layer patterning, the enhancement in optical absorption is through an increase in optical path lengths by extending oblique-angle scattering across the active layer.13,14 Although patterning of other layers, such as top metal electrode and electron transport layer have also been studied,15,16 this study is focused on the improvement in light absorption due to nanostructuring of the active layer. Modeling and simulations have been carried out on bulk heterojunction solar cells,17 especially for P3HT: PCBM (poly(3-hexylthiophene-2,5diyl):(phenyl-C61-butyric acid methyl ester)blend.18 It has been observed that the effect of nanostructuring19 has definite and distinct levels of enhancements in the device performance.


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Patterning could be of spherical, pyramid, buckled, corrugated and well-type structures.20,21 Several fabrication methods have been reported for obtaining periodic patterns such as nanoimprinting lithography, and template synthesis.22,23 These are sophisticated lengthy processes and require multiple steps before the final pattern could be achieved.24,25 While patterning the front electrode interface is relatively simpler, it does not always lead to optical enhancement in an organic photovoltaic architecture.26 Thus patterning the back electrode interface is of interest. This poses a challenge because conventional structuring processes such as photolithography or electron beam lithography cannot be applied on polymer active layers due to the degradation of the active material on exposure to process chemicals. Thus a simple process to pattern the active layer/back electrode interface is of particular interest. Electrospraying is a simple process which can be controlled through its process parameters27 such as applied voltage, the flow rate of the solution, and the solvent,28,29 leading to various hierarchical morphologies30,31 such as photonic balls,32 network structure,33 and porous structures.34,35 Electrospraying has also been carried out for P3HT,36,37 a widely known conducting polymer in organic photovoltaic devices for its optical properties,38,39 and better air stability40 in creating various morphologies41 photonic crystal geometry in OSCs.42

In this study, initially P3HT: PCBM blend is electrosprayed at a wide range of voltages and tip to collector distances obtaining irregular porous structures. The process parameters dictating the evolution of regular structures are applied voltage, tip to collector distance and solvent vapor pressure. Subsequently, the process parameters are optimized resulting in regular porous structures of required dimensions. Morphological, topographical and optical characterizations are carried out along with the modeling and simulation to understand the effect of pore size on the



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device performance. Devices are fabricated using these photonic inspired structures, and photoelectrical responses are assessed. Thus, the aim of this study is to understand enhancement in light absorption by light scattering in organic photovoltaic architectures by incorporating an active layer with a porous structure and also to provide a generic fabrication technique which can be employed for a majority of solution processed active layers.

Experimental Materials Regioregular poly(3-hexylthiophene-2,5-diyl), P3HT is purchased from Rieke Metals, Inc. with an average molecular weight of 50,000 – 70,000 kDa and (phenyl-C61-butyric acid methyl ester), PCBM is purchased from Nano-C. Poly (3,4-ethylenedioxythiophene): poly(styrene sulfonate) PEDOT: PSS is purchased from Baytron Clevios. Chloroform solvent of an analytical grade is procured from S. D. Fine Chem. Ltd. Indium tin oxide (ITO) coated glass substrates with a sheet resistance of 70-100 Ω are purchased from Delta Tech. All chemicals are used as received, and the prepared solutions are filtered using 0.2 µm syringe filters.

Electrospraying P3HT: PCBM in the constant ratio of 1:0.8 (by wt) is dissolved in chloroform and stirred for 8 hours. All the electrospraying experiments are carried out in ambient conditions. The schematic configuration of the experimental setup is as depicted in Figure S1 (in the supplementary information). Electrospraying parameters such as applied voltage using high voltage source Gamma RR50 is varied from 5-8 kV, flow rate and tip to collector distance are varied from 0.8-3 ml hr-1 and 6-8 cm respectively, with 24 G (0.31 mm) needle diameter. It is


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important to observe that, the photonic-inspired arrays are formed instantaneously on a range of substrates such as aluminum foil, glass or ITO substrates without further post-processing.

Device fabrication Indium tin oxide coated glasses are patterned by selectively etching with a mixture of HCl:HNO3:DI water in the ratio of 1:1:5. Etched ITO glasses and glass substrates of 1 inch2 are thoroughly rinsed using soap solution and sonicated in water, acetone, and isopropanol for 10 min each. This is followed by UV-Ozone treatment for 20 min to remove organic residues. A layer of PEDOT: PSS with a drop of Triton X-100 ®, as a surfactant is spin coated (SC) on substrates at 4000 rpm and annealed at ~130°C for 15 min. Electrospraying is performed on grounded aluminum collector affixed with glass and ITO substrates at optimized conditions of 7 kV, 6 cm and 3ml hr-1 for 30 sec. P3HT:PCBM solution concentration is fixed at 27 mg ml-1 for both electrospraying and spin-coating. There are three batches of samples (i) spin-coated (SC) standard reference batch (ii), electro-sprayed (ES) and (iii) sequentially electrosprayed and spin coated (ES+SC) as represented in Figure1(a). Spin coating is carried out at 1000 rpm for ES+SC and SC (standard) batches. In ES+SC batch, to fill the empty areas between the electrosprayed patches containing the photonic structures, a layer of P3HT: PCBM is spin coated. The active layer thickness is 300±30 nm in both cases. All the batches are annealed at 110 °C for 15 min and allowed to cool down to room temperature. Aluminum electrodes approximately 100 nm thick are deposited on the substrates at the rate of 2-5 Å s-1 for an active area of 2 mm2 using thermal evaporation.



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Figure 1. (a) Isometric representation of device architectures for spin coated, electrosprayed and electrosprayed & spin coated batches and two-dimensional cross-sectional schematics of the device architecture studied; (b) structured and (c) planar

Characterization The morphological studies are performed on gold sputtered samples using Scanning electron microscope, SEM (ESEM Quanta 200 FEI) at 10 kV while high magnification studies are carried out with Field emission scanning electron microscope, FESEM (Carl Zeiss Ultra 55) at 5 kV. The topographical analyses are carried out using Atomic Force Microscope (AFM) (Bruker Dimension Icon) The optical studies are done based on diffuse reflectance spectroscopy on aluminum coated glass substrates samples, using Perkin Elmer Lambda 35 UV/Vis spectrophotometer attached with integrating sphere. The optoelectronic studies are operated under AM 1.5 solar radiations with a power density of 100 mWcm-2 using Newport Oriel full spectrum while measurements are analyzed using Keithley 4200 semiconductor parametric 6 ACS Paragon Plus Environment

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analyzer. The external quantum efficiency (EQE) measurements are evaluated using Enlitech, QE-R in the wavelength range of 300 – 800 nm with a step size of 5 nm.

Simulation details The schematic representation of two-dimensional device architecture studied is as in Figure 1(b) for structured and Figure 1(c) for planar where the z-axis points out of the plane. The optical transport simulations are carried out by finite element method using COMSOL 5.0 package. The maximum mesh size used is 5 nm, whereas the minimum wavelength simulated in 330 nm. A spectral input corresponding to standard AM 1.5G power spectral density is employed. Total absorbed photon flux is obtained by evaluating the net power density Ptr(f) transferred to the active layer (Eq 1). This is the difference between the flux entering the PEDOT:PSS/P3HT: PCBM interface and the flux exiting the P3HT: PCBM/Al interface.

φ( f ) =

Ptr ( f ) (1) hf

Here Φ is photon flux density in m-2s-1 Hz-1, f is the frequency, and h is the Planck’s constant (6.62 x 10-34 m2 kg s-1). The maximum current density at short circuit is obtained by integrating

φ ( f ) across the spectral range of interest (Eq 2) f2

J

max sc

= q ∫ φ ( f )df (2) f1

Here f1 = 4 x1014 Hz and f2 = 9 x1014 Hz, q is the electronic charge and J scmax represents the maximum photocurrent density obtainable for a given photon flux distribution. The actual current density would be reduced by other factors including series resistance, bulk, and interface recombination. The values of various device layers used for simulation are as follows; thickness of ITO (tITO) is 30 nm, thickness of PEDOT: PSS (tPP) is 50 nm, thickness of active layer (d) is 7 ACS Paragon Plus Environment


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150 nm, depth of extension (dpr) of back electrode is 100 nm width of structured electrode (wpr) and spacing (s or s/2).

Further, the charge transport behavior in the steady state is simulated using a drift-diffusioncontinuity formalism (Eq. 3-6) with a steady state potential distribution governed by the charge density distribution, given by the Poisson’s equation (Eq. 7)

J n = qDn∇.n − qµ n∇.ψ (3) J p = qD p ∇. p − qµ p ∇.ψ (4)

−

1 ∇.J n = G ( x, y ) − R( x, y ) (5) q

1 ∇.J p = G ( x, y ) − R( x, y ) (6) q ∇ 2ψ = −

q

ε 0ε r

( p − n +N eff ) (7)

Where Jn and Jp are the current densities due to electrons and holes, n and p are the electron and hole densities, µn and µp are electron and hole mobilities, Dn and Dp are electron and hole diffusivities, given by the Einstein relationship (Dn/p = (kT/q)µn/p), ψ is the electrostatic potential, q is the electronic charge, T is the temperature, k is the Boltzmann constant, ε0 is the permittivity of free space, and εr is the relative permittivity of the active material. G is the exciton generation rate obtained from frequency domain optical simulations. R is the recombination rate of electrons and holes governed by a Langevin second order recombination process (Eq. 8).19


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R ( x, y ) =

q

ε 0ε r

min( µ n , µ p )( np − ni2 ) (8)

Results and Discussion Control of pore diameter and spacing The effects of process parameters the pore sizes as well as the spacing between the pores are as explained below, Figure 2. In Figure 2(a) the pore size of 1100 ± 64 nm has a pore-spacing of around 600 ± 20 nm at operating conditions of (applied voltage, tip to collector distance and flow rate, respectively) 7 kV, 6 cm and 3 ml hr-1. Whereas in Figure 2(b) the pore size of 520 ± 40 nm has a pore-spacing of 310 ± 30 nm at operating conditions of 7 kV, 8 cm and 3 ml hr-1 and Figure 2(c) has the pore size range between 240-520 nm at operating conditions of 5 kV, 6 cm and 3 ml hr-1. Therefore it could be observed that the spacing between the pores generally follows a scaling rule, that is, it is almost 60 % of the size of the pores. With an increase in the distance between the tip to the collector, solvent evaporation increases. This is evident by changing the distance from 6 cm (Figure 2(a)) to 8 cm (Figure 2(b)) which results in decreased pore size. Whereas, a simultaneous reduction in the traveled distance and the applied voltage leads to a decreased the pore size (Figure 2(c)). However, other interacting parameters also have to be optimized to get uniform sized pores. Therefore by controlling the process parameters and their interactions the tuning of the structure is obtained.



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Figure 2. FESEM images demonstrating the approximately equal submicron photonic inspired structures for (a) wpr=1100 nm (b) wpr=500 nm and (c) wpr>240 nm Surface profiling using AFM facilitated in obtaining information such as, the topography and the pores size across the surface, Figure 3. Pore size and pore size distribution are measured by the topographical analysis of the fabricated film. Figure 3(a) suggests a uniform pore size of 1100 ± 64 nm with a spacing of 600 ± 20 nm. Similarly, Figure 3(b) and 3(c) represent corresponding topography and the pore sizes of 500 nm and more than 240 nm, respectively. These results are in agreement with the morphological studies carried out through FESEM (Figure 2).


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Figure 3. AFM images of varying pore size with depth profile (of the respective highlighted position) for (a) wpr=1100 nm (b) wpr=500 nm and (c) wpr>240 nm

Morphological analysis To illustrate the concept the morphological analysis is carried out only on sub-micron scale. The morphological analyses are performed at various magnifications for the electrosprayed sample to demonstrate the regular pores formation of equal size spread across an area of more than 100 µm, as shown in Figure 4(a). Figure S2 (in the supplementary information) illustrates more FESEM images of the photonic structured layer in the large area. Figure 4(b) and 4(c), respectively gives the information that these pores are arranged at regular lines and equal spaced 11 ACS Paragon Plus Environment


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intervals of 0.6 ± 0.02 µm. Further higher magnifications suggest that these pores have a diameter of approximately 1.1 ± 0.064 µm as seen in Figure 4(d). The dependence of morphology on the solvent, applied voltage, and tip-to-collector distance could be observed in Figure S3-S5 (in the supplementary information), wherein the irregular size and spaced pores before the optimization of electrospraying process parameters are demonstrated. Whereas, in optimized electrospraying (Figure 4) as the polymer blend solution deposits on the collector there is simultaneous evaporation of chloroform due to its high vapor pressure (26.2 kPa) leaving behind the regular size porous array.32 It is therefore observed that the solvent dictates the characteristics of evolving morphology along with, applied voltage and tip to collector distance being equally important factors. Since the pores are periodically extended across the area, the term ‘photonic structured’ is preferred in further discussions.


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Figure 4. FESEM images of photonic inspired structure obtained through electrospraying at various magnifications



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Assessment of other processing techniques A comparison is carried out with spin-coating and drop casting methods using the same concentration of P3HT: PCBM solution as employed for electrospraying. The pore formation as explained is predominantly due to the evaporation of chloroform solvent as seen in both electrospraying (Figure 2 and 4) and spin-coated (Figure 5(a)-(d)) samples. In the spin-coated sample, the effect of additional centrifugal phenomenon results in uneven solvent evaporation and hence non-periodic pores formation for 2000 rpm, Figure 5(a) and 5(b). Whereas at an even higher speed of 3000 rpm, Figure 5(c) and 5(d) show that there are comparatively a larger number of pores and better uniformity. In the drop cast samples as shown in Figure 5(e), because of uneven solution distribution during drop formation more solution accumulates at the center and less at the edges with the further evaporation of solvent leaves a circular mark around the circumference, indicating that more material is accumulated at the periphery. This imbalance in the distribution, perhaps, has led to the absence of pores in drop cast samples.

Figure 5. FESEM images (a) and (b) for spin-coated samples at 2000 rpm,(c) and (d) spin coated at 3000rpm, and (e) drop-casted sample


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Hence these comparative results suggest that for the formation of an optimized photonic structure, the two important factors are; (i) controlled thin layer deposition of the solution and (ii) subsequent uniform solvent evaporation. Controlled layer deposition is present in both the electro-spun and spin-coated samples, whereas it is missing in the drop-casted samples. Further, the uniform solvent evaporation is dictated by the type of driving force applied for solvent evaporation. In the case of electrospraying, it is through a continuous supply of material and subsequent continuous solvent evaporation facilitating photonic type structure formation. Whereas in spin coating the total solution is dispensed at once followed by rotation of solution allowing solvent evaporation, however, this disturbs the uniform solvent evaporation. Although it could be observed, at higher rpm in spin-coated samples, due to higher driving force (rotational motion), there is more pores formation. However, the solution wetting property on the substrate is negotiated as solvent evaporation is quicker, thus preventing the complete spreading of the solution on the substrate. Hence, the electrospraying process has an advantage over the spin coating process, and simple method such as drop casting of solution where there is no further driving force could not generate pores.

Optical analysis The cross-section of the active layer shows that the pores are blind holes, which are open on the top surface and a thin film layer of P3HT: PCBM lies at the bottom of these holes as seen in Figure 6(a). As photonic structures have the characteristic of interacting with light in manifolds, reflectance studies are more appropriate, and therefore, the optical reflectance is measured using UV-Vis in diffuse reflectance spectroscopy mode. P3HT has major light absorption between λ=400 nm and 15 ACS Paragon Plus Environment


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650 nm region, Figure 6(b) shows the percentage reflectance of ES and SC samples in the same region. A decrease in percentage reflectance of ~16.6 % is observed for ES samples compared to SC samples. It is to be noted that the spin coating is carried out at 1000 rpm as employed in the device fabrication process, wherein pore formation is negligible compared to samples spincoated at 2000 and 3000 rpm. As discussed earlier, the formation of a repetitive pattern of equal size pores on the surface in ES samples has aided in increasing the path way for the light to travel inside the active material matrix, leading to improved absorption which subsequently leads to a decreased reflectance. Figure 6(c) schematically represents the proposed mechanism wherein in the photonic structure, multiple scattering occurs due to repetitive pores aiding in light trapping and hence more light absorption, unlike for planar structure (Figure 6(d)).


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Figure 6. (a) Cross-sectional image of electrosprayed photonic structure,(b) percent reflectance spectra for ES and SC photonic structured films and schematic of light interaction with (c) structured films and (d) planar films

Opto – electrical analysis The current density-voltage characteristics for the reference SC, ES and ES+SC devices are shown in Figure 7(a). The ES device characteristics suggest that there is electrical shorting consequently generating negligible photo-current, which is due to highly irregular and discontinuous electrosprayed active layer surface. The device characteristics of reference SC and ES+SC samples have nearly the same open circuit voltage (VOC). However, the current density (JSC) in the ES+SC sample is enhanced by 18 % compared to the SC reference sample, which results in enhanced power conversion efficiency of 3.3 % compared to 2.7 % in the case of the SC reference. The power versus voltage curve in Figure 7(b), illustrates the maximum output power for each batch in comparison with current density versus voltage. The local maximum in the P-V plot occurs at nearly the same voltage, which shows that the fill factor is not degraded, in which the maximum would have shifted to lower voltages in the structured device. The simulated J-V characteristics for the structured and planar device are as shown in Figure 7(c), wherein the current density for a periodic structured device is higher (by 16 %) than that of the planar reference device. The simulated and experimental J-V curves show close agreement in all the parameters as seen from Table-1. The simulations assume perfect Ohmic contacts without Schottky barriers, and thus this excellent agreement between the simulated and experimental optoelectronic results shows that the fabricated devices have excellent interface quality and are practically the best devices possible for the active layer thickness here (around



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300 nm) Figure S8 (in the supplementary information). Therefore electrospraying tuned photonic structure results in improved light absorption resulting in the improvement in efficiency by approximately 23 %. Figure 7(d) gives the mean values and standard deviations for the obtained photovoltaic parameters for various devices. Further, Figure 8(d) shows the top view FESEM images of the active layer wherein the nanostructures are seen to be extended across the active layer, hence confirming that the structure in the active layer has been transferred to the top electrode.


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Figure 7. Device characteristics of photonic structured organic solar cells for; ES, SC and ES+SC (a) Current density – voltage, experimental devices (b) Power – voltage, experimental devices (c) Current density – voltage, simulated devices (d) statistical distribution of the device parameters, and (e) are the top view FESEM images of the complete nanostructured device



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Table-1. J-V characteristic data both experimental and simulated for photonic structured organic solar cells (the best experimental data are given in brackets)

Experimental

Simulated

Batches

JSC (mA/cm2)

VOC (V)

FF (%)

η (%)

ES

0.6 ± 0.3 (0.4)

0.015 ± 0.007 (0.02 )

17 ± 13 (26 )

0.002 ± 0.001 (0.003 )

SC

9.9 ± 0.9 (10.9)

0.57 ± 0.002 (0.57 )

31± 1.5 (32 )

2.37 ± 0.31 (2.7 )

ES+SC

11.4 ± 1.5 (12.9 )

0.55 ± 0.024 (0.57 )

33 ± 1 (33)

2.8 ± 0.34 (3.3 )

Planar/ SC

10.5

0.54

36

2.0

Patterned/ ES+SC

12.2

0.54

40

2.7

Additionally, optical properties and its effect on device characteristics are further evaluated by the modeling and simulation. Modeling and simulation of the system are carried out, to understand the effect of periodic structure on the photocurrent density as well as the effect of various pore diameter wpr, ranging from 50 to 1000 nm on the light harvesting capability. In these simulations, the pore depth dpr is assumed to be 100 nm in accordance with topographical information obtained from the AFM images. The active layer thickness d is taken as 300 nm as seen in the experiments, Figure S8 (in the supplementary information). It was seen in the section on topographical characterization, that the pore diameter wpr, and pore spacing s, are related following a simple scaling rule. Here for simplicity, these are assumed to be equal.


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Figure 8. (a) Relative change in absorptance in the structured device with respect to planar device architecture, as a function of wavelength λ, and pore diameter wpr, (b) The absolute values 21 ACS Paragon Plus Environment


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of the x,y, and z components of the electric fields for λ=750 nm, (c) Relative change in experimental EQE and (d) Simulated maximum photocurrent density as a function of pore diameter wpr. The black dashed line represents the value of the planar reference architecture

The relative change in the absorptance of the structured device architecture over the planar reference is plotted as a function of the free space wavelength λ and the pore diameter wpr, Figure 8(a). It is seen that strong enhancement peaks appear at larger wavelengths (roughly λ> 650 nm). These represent an improvement in the absorptance in the sub-bandgap spectral region, where the absorption strength of the material is small. As seen in the magnified portion of the plot (Figure 8(a) inset) for λ

Design and Fabrication of Photonic Structured Organic Solar Cells by

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