Uniaxial anisotropy in PEDOT:PSS electrodes enhances the

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Uniaxial anisotropy in PEDOT:PSS electrodes enhances the photocurrent at oblique incidence in organic solar cells Jonas Bergqvist, Hans Arwin, and Olle Inganas ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00221 • Publication Date (Web): 14 Jun 2018 Downloaded from http://pubs.acs.org on June 15, 2018

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Uniaxial anisotropy in PEDOT:PSS electrodes enhances the photocurrent at oblique incidence in organic solar cells Jonas Bergqvist1*, Hans Arwin2, Olle Inganäs1 1

Biomolecular and Organic Electronics, IFM, Linköping University, SE-581 83 Linköping, Sweden 2 Laboratory of Applied Optics, IFM, Linköping University, SE-581 83 Linköping, Sweden * [email protected], [email protected]

PEDOT:PSS is a well studied organic conductor, commonly used as a transparent electrode material in printed organic devices such as organic solar cells. PEDOT:PSS films are known to be uniaxially anisotropic and exhibit a lower extinction coefficient and lower refractive index in the out of plane direction. To determine the maximum attainable photocurrent in thin film solar cells the optical power dissipation can be calculated by the transfer matrix method. However, until now the anisotropic properties of PEDOT:PSS films has not been included in the model. In this work we have included an uniaxial anisotropic treatment of PEDOT:PSS films. We investigate reversed and semitransparent solar cells, with aluminum and PEDOT:PSS respectively as the second electrode and PEDOT:PSS as the top electrode, as compared to devices with isotropic PEDOT:PSS electrodes. For p-polarized light at large oblique incidence the inclusion of anisotropy shows a gain of over 7% for the maximum photocurrent in reversed solar cells. In semitransparent solar cells the photocurrent enhancement reaches 4-5% for p-polarized light. However, an enhancement of optical power dissipation and thus photocurrent generation of close to 40% is calculated for wavelengths close to the absorber bandgap. This work shows that for correct calculations of optical power dissipation in devices with PEDOT:PSS electrodes anisotropy should be included in the optical model. This will be especially important to determine the daily energy output of organic solar cells as their expected first markets are on building facades and indoor applications with larger fractions of diffuse light at large oblique incidence. Keywords PEDOT:PSS, anisotropy, transfer matrix method, optical model, organic solar cells.

During the coming decades there will be calls for a huge supply of renewable energy sources and a strong demand for devices with a low ratio of energy input relative to the energy output over their lifetime. Organic photovoltaics (OPV) is potentially a perfect candidate for high energy return of investment (EROI) due to features like large area production via roll to roll (R2R) printing methods1 and very low materials use as the devices usually are comprised of a thin film stack less than 500 nm thick. Power conversion efficiencies (PCE) exceeding 13% have been reported2-4 as well as life times up to 7 year5. However, commercial modules are unlike to compete with silicon based modules within the near future and most likely surfaces optimal for solar energy conversion will be covered with silicon solar cells. Hence, market introduction of OPV will likely be focused on less optimal surfaces such as facades or indoor energy harvesting with large fractions of diffuse light. For such applications the relative high performance of OPV at oblique incidence will be of utmost importance. 1 ACS Paragon Plus Environment

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The photon to charge carrier converting layer in OPV devices is a binary composite material composed of different blends of polymers, fullerenes and/or small molecules. Numerous materials have been evaluated as electrodes, where indium tin oxide (ITO) has been the primary transparent electrode used, whereas silver or aluminum are used as counter electrode in opaque devices. However, to fully utilize the benefits of R2R printing also the electrodes (including possible interlayers) should be printable. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is processed from solution and has successfully been used as transparent electrode in both opaque and semitransparent devices with aluminum6 and ITO7 as counter electrodes. PEDOT:PSS has also been used both as anode and cathode, the latter upon a surface modification8. By connecting narrow subcells in series, the resistive loss/voltage drop in the PEDOT:PSS electrodes can be minimized, and an acceptable conductivity versus transmission balance can be achieved. Furthermore, thin current collectors of silver can be printed on the PEDOT:PSS electrode to reduce the series resistance of the electrodes9-11, albeit to the cost of a reduced aperture area, and also a reduced EROI. The use of two thin PEDOT:PSS electrodes allows for semitransparent solar cells for specific applications, but with lower efficiency due to transmitted photons which are lost. However, the transmitted photons can be recycled via a back scatterer so that the device can reach a performance similar to a device with a reflecting electrode12. Thus, controlling unwanted absorption losses in the PEDOT:PSS is of utmost importance, as the light will make at least three passes through PEDOT:PSS before the second pass through the active layer. Furthermore, predicting the transmittance will be important for devices which combine solar cells and solar screening. PEDOT:PSS thin films is known to be uniaxially anisotropic and exhibit a lower extinction coefficient and lower refractive index in the out of plane direction13-15. However, the effect of the uniaxial anisotropy present in thin PEDOT:PSS films on the light coupling into the solar cell stack has to the best of our knowledge not been investigated so far. In this work we extend the transfer (scattering) matrix method (TMM) to simulate organic solar cell stacks with anisotropic top PEDOT:PSS electrodes. The results using an anisotropic PEDOT:PSS model are compared to the case with an isotropic PEDOT:PSS model, where the in plane refractive index is used. Most TMM simulations of organic solar cells have been performed under normal incidence with the in plane refractive indices of all materials. When the model is expanded to oblique incidence, we here highlight the importance of also expanding the refractive index of PEDOT:PSS to an anisotropic model, in contrast to keeping an isotropic model. We investigate the effects of anisotropy of PEDOT:PSS electrodes in devices with reflective bottom electrodes, and in semitransparent devices with PEDOT:PSS also as bottom electrode. For both stacks the power dissipation in the active layer and in the electrodes is simulated as a function of incidence angle and active layer thickness.We show that the transmittance for longer wavelengths through the PEDOT:PSS film is increased when including anisotropy, and that this also reduce the parasitic absorption at longer wavelengths. For reversed solar cells an increased active layer absorption increases the photo current generation by 8% at large incident angles when including anisotropy, but here a reduced parasitic absorption in the PEDOT:PSS is affected also by the thin film interference. For semitransparent solar cells our simulations show relative power dissipation gains of 40% in the active layer, for p-polarized light incident at 900 nm upon inclusion of anisotropy. These results will be important for the modelling and prediction of energy output for OPV in non-optimal locations and inclinations.

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The transfer (scattering) matrix method was introduced by Pettersson et. al to model the power dissipation in OPV devices and account for the interference occurring in thin films in organic solar cells under normal incidence16 and have been extensively used since then to determine the maximum current and internal quantum efficiency (IQE) for numerous active layer material systems. The model has also been extended to treat oblique incidence without17 and with an incoherent glass lid18 and also to include anisotropy in the active layer, but then via the transmission line method19. Other studies have used TMM to calculate the current response over a full solar day20. The TMM is based on Maxwells equations where the reflection and transmission at each interface in a thin film stack is calculated and the phase shift and attenuation in each layer considered. From a superposition of waves propagating downwards and upwards in the stack the electric field is calculated as a function of stack depth. From the divergence of the time-average Poynting vector the optical power dissipation for each wavelength in layer j is expressed as a function of the electric field squared according to equation 1,

, = ,

(1)

where is the speed of light, vacuum permittivity, the real value of the refractive index in layer j and the absorption coefficient of layer j. From the absorptance for each in layer j can be calculated from equation 2, =

,

(2)

where is the irradiance incident to the thin film stack. Assuming 100% internal quantum efficiency the maximal photo current can be calculated from the absorptance in the active layer j from equation (3)

!"

'

= #$%& = &(

(3)

where q is the elementary charge, $%& the photon flux and h Planck constant.

PEDOT:PSS exhibit uniaxial anisotropy with a considerably lower absorption coefficient, , in the plane perpendicular to the film surface as compared to the plane of the surface. From here on we will refer to the direction perpendicular to the surface as the extra ordinary direction with absorption coefficient ) and in directions parallel to the surface we use the ordinary absorption coefficient ) . The difference between ) and ) is expected to enhance the coupling of light into the active layer under oblique incidence, with a reduced parasitic absorption when PEDOT:PSS is used as the transparent electrode. However, also the real parts of the refractive index differ for the extra ordinary and the ordinary directions. This will modify the reflection and transmission coefficients. Furthermore, the interference within the stack will modify the power dissipation. A full TMM calculation is therefore necessary to determine the influence of electrodes with optical anisotropy on the absorption in the active layer, and thereby on the maximum photo current accessible. The explicit formulas for reflection and transmission for general absorbing uniaxial media can be found in ref21 and the modifications necessary for the standard TMM model in the case of oblique incidence are clarified in the supporting information. Nevertheless, we wish to emphasize the effect of anisotropy on light impinging from an isotropic medium. The refracted wave exhibit birefringence and splits up into two waves, one propagating in the ordinary and one extra 3 ACS Paragon Plus Environment

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ordinary direction. Further the electric field E and the electric displacement D vectors are not parallel in the extra ordinary direction as

) * = + 0 0

0

) 0

0 0 -.

)

(4)

where ) and ) is the corresponding ordinary and extra ordinary relative permittivity. Consequently the Poynting vector will not be parallel to energy and wave front propagation are non-parallel. For the general absorbing anisotropic media this generates cross terms between the s- and p-polarizations, where s and p refers to perpendicular and parallel to the plane of incidence respectively. Thus, for higher asymmetries the 4x4 matrix formalism introduced by Berreman22 and Yeh23 must be used. However, for uniaxial anisotropy with the optical axis perpendicular to the reflecting plane, the ordinary wave is purely s-polarized and the extra ordinary purely p-polarized, which also is the case for the Poynting vector24. Results and discussion Device geometry Rigid encapsulation with glass or other transparent barrier materials is necessary for long time stability of organic solar cells. Glass and polyethylene terephthalate (PET) used for encapsulation both have a refractive index around 1.5 over the visible/NIR region. This lid puts a first limit to the coupling of light into the organic solar cell stack, which is illustrated in Figure 1 by the calculated transmittance for s- and p-polarized light at an air ambient (n = 1) / glass interface. Refractive indices The refractive index n and extinction coefficient k for all layers in the solar cell stacks, as determined by spectroscopic ellipsometry, are shown in Figure 2 a and b respectively. The difference in the complex-valued refractive index N = n + ik over an interface determines reflection and transmission efficiencies. An increasing difference enhances reflection and thus reduces transmission. For weak absorbers (all layers but aluminum in our devices) n is most significant. The birefringence Δ = ) − ) of PEDOT is 0.162 at 700 nm and 12345,) coincides with 67! over the full wavelength range resulting in low reflection for the field component in the eo-direction at the GLASS/PEDOT interface. Also, the transmission through the PEDOT/active layer will be more efficient in the extraordinary direction as the optical contrast is larger in the ordinary direction. The dichroism of PEDOT (see Figure 2b), i.e.Δ8 = 8) − 8) , is 0.067 at 700 nm, a wavelength at which the active layer polymer:fullerene blend poly[indacenodithieno [3,2-b] thiophene-alt-6- (2-ethylhexyl) -4,8[1,2,5] thiadiazolo-[3,4-f] isoindole-5,7-dione] : phenyl -C71- butyric acid methyl ester P21:PCBM has its maximum red peak absorption. Transmittance through isotropic and anisotropic PEDOT:PSS films As a first comparison we investigate the transmittance through a glass/PEDOT:PSS/glass stack with light incident from ambient air for isotropic and for anisotropic PEDOT:PSS films as a function of incident angle. Figure 3a shows that the transmittance is ~90% for blue light and ~80% for red. If the angle of incidence is increased up to 60°, the transmittance increases in the weakly absorbing blue region for both isotropic and anisotropic PEDOT:PSS films due to a reduced reflectance in the air/glass interface. However, at longer wavelengths the effect of the reduced ) becomes apparent, where the anisotropic film shows a higher transmittance 4 ACS Paragon Plus Environment

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for angles also over 70°, while the isotropic PEDOT:PSS film has a reduced transmittance below 60° at 700 nm, compared to normal incidence. At longer wavelengths the transmittance is reduced for all incidence angles. A thicker PEDOT:PSS layer results in a higher conductivity at the expense of transmittance. For a 250 nm thick top electrode the normal incidence transmittance at 500 nm is 79% (Figure 3b), which is on the limit to be acceptable. At larger incidence angles the reflectance at the air/glass interface is reduced and more light is transmitted through the stack. However, the path length through the PEDOT:PSS is also longer at higher angles and more light will be absorbed. For wavelengths longer than 550 nm the transmittance is increased relative to normal incidence for all incident angles up to 70° when including anisotropy, whereas in the isotropic case the transmittance is reduced. Thus we confirm the expected differences in transmittance through PEDOT:PSS films at longer wavelengths for anisotropic inclusion. However, implications for solar cell performance remains to be investigated. Photocurrent and absorption profiles for reversed solar cell We start by investigating a reversed solar cell with the stack Glass/PEDOT:PSS/P21:PCBM/ aluminium, illuminated with the AM1.5G spectrum (shown in supporting information Figure SI1). The power dissipation in the active layer is calculated for both s- and p-polarized light as a function of the active layer thickness for a 120 nm thick PEDOT:PSS electrode, both for isotropic and anisotropic PEDOT:PSS models. For the isotropic layer model no is used both for s- and p-polarized incident light. Thus, there will be no difference for the anisotropic and isotropic calculations for s-polarization. For s-polarized light a larger angle of incidence leads to smaller power dissipation in the active layer and thus smaller photo-current JMAX, as seen in Figure 4a. Note that the plotted photocurrent is the current assuming 1:1 s- and p-polarization of the incident light, thus at normal incidence JS = JP = J/2. This reduction with increasing angle of incidence is primarily due to larger reflection in the air/glass interface. However, noteworthy is that the interference maximum is shifted from an active layer thickness of 90 nm at normal incidence to 100 nm at 60°. Thus, a thicker layer may compensate some of the losses due to oblique incidence. For p-polarized light the interference maxima is further shifted to 110 nm for anisotropic treatment of the PEDOT:PSS layer (Figure 4b). Furthermore, the interference minima at normal incidence is lost and replaced with a weak increase in maximal Jp with layer thickness. The reduced reflectance for p-polarized light at the air/glass interface allows for a higher transmission and subsequently a higher photon flux to the active layer, increasing JMAX. At 60° incidence JMAX increase by ~0.5 mA/cm2 for 100 – 110 nm thick active layers whereas at 70° a slightly larger increase is seen for 80 - 120 nm thick active layers. At 70° incidence the maximum increase in JMAX with an anisotropic layer model compared to an isotropic layer model is more than 8% for 85-110 nm thick active layers (inset Figure 4b). However, solar light is best approximated with 1:1 ratio between s- and p-polarization. Figure 4c shows JMAX for light with equal parts of s- and p-polarization which displays a shift of the first interference maximum from 90 nm at normal incidence to 100 and 110 nm for 60 and 70° incidence. In absolute numbers JMAX increase ~0.5 mA/cm2, but as also s-polarized light is included the relative increase is now halved to 4%. Figure 4d shows JMAX as a function of incidence angle for 70, 90 and 110 nm thick active layers. JMAX for 70 nm thick active layers decreases for angles above 20 and 40° for isotropic and anisotropic layers, respectively. On the contrary, JMAX increases up to 60° and 70° for 110 5 ACS Paragon Plus Environment

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nm thick active layers. The JMAX ratio between anisotropic and isotropic layer models increase with incidence angle by almost 5% for 70 nm thick films whereas the ratio for 90 and 110 nm thick active layers increase by 6% at 80°. Thus we conclude that calculations using an anisotropic PEDOT:PSS layer model predict an enhanced absorption in the active layer and thereby also enhanced photocurrent generation, as expected.

However, from the modelled photocurrent we cannot conclude the cause of the enhancement. Hence, we plot the relative power dissipation for p-polarized light for a 90 nm thick active layer with isotropic and anisotropic treatment of the PEDOT:PSS in Figure 5a and b, respectively. The relative losses due to reflectance is comparable for both cases whereas for anisotropic PEDOT:PSS the dissipation in the active layer increases between 600 and 800 nm and the PEDOT:PSS layer shows a reduced power dissipation in the same interval. The parasitic absorption in the aluminium also increases around 800 nm. To further investigate where the dissipation occurs in the stack we plot the spectral power dissipation for the full depth of the stack for isotropic and anisotropic PEDOT:PSS in Figure 5c and d, respectively. The main difference observed here is a slight decrease in the PEDOT:PSS dissipation in the 600-800 nm range. However, to highlight the differences we also plot the ratio in Figure 5e. The PEDOT:PSS absorption decreases between ~500 and ~800 nm, whereas the dissipated power in the active layer and aluminium increases 10-20% between 600 and 850 nm. However, there is no clear gradient with less absorption in the PEDOT:PSS layer for longer wavelengths, as one would expect from the transmission data, due to the phase contributions to the interference. As the results were not fully intuitive we also plot the spectral power dissipation as a function of stack depth for the stronger red absorbing active layer poly[N,N'- bis(2hexyldecyl) isoindigo- 6,6'- diyl- alt- thiophene- 2,5- diyl]:PCBM (P3TI:PCBM) in the supporting information Figure SI2, where a similar behaviour for the active layer thickness showing the largest enhancement from an anisotropic PEDOT:PSS description is noticed. Hence, optical modelling is of utmost importance to understand the power dissipation in thin film solar cells with reflective bottom electrodes. Photocurrent and absorption profiles for semitransparent PEDOT:PEDOT solar cells We also investigate the effect on transmittance and JMAX for semitransparent solar cells with PEDOT:PSS as both anode and cathode. For real devices the cathode PEDOT:PSS is modified by a monolayer to a few nm thick layer of polyethyleneimine ethoxylate25, not included in the optical modelling. We start by calculating the transmittance for isotropic and anisotropic PEDOT:PSS films for 60° incidence angle. In Figure 6a a larger transmittance for p-polarized light and an anisotropic layer model compared to an isotropic layer model is observed for wavelengths above 400 nm. The difference increases towards the red as also the dichroism increases. For semitransparent devices with a back reflector attached to the bottom transparent substrate the interesting spectral region is the one where the active layer absorbs. Figure 6b shows the transmission ratio for p-polarized light between anisotropic and isotropic PEDOT:PSS layer models together with the absorption coefficient of P21:PCBMb. At the bandgap, around 800 nm, the transmission increases by 30% whereas at the red absorption peak at 680 nm the transmission increase is 10%. This provides more photons possible to recycle by a backside scatterer. A stronger red absorber would lead to even more gain. The


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absorption coefficient for the blend P3TI:PCBM which has a larger absorption in the red part of the spectra is also shown in Figure 6b. Figure 6c shows the calculated JMAX for P21:PCBM for which the photocurrent versus active layer thickness for unpolarised light is very similar at normal and 60° incidence, but a slight decrease is observed at 70°. The difference between isotropic and anisotropic PEDOT:PSS layer models for p-polarization is only 0.2 mA cm-2 for a 100 nm thick active layer. For the same stack P3TI:PCBM active layer the enhancement is larger, but still only 0.25 mA cm-2. The JMAX ratio for inclusion of anisotropy in PEDOT:PSS is shown in Figure 6d for both active layers. For very thin active layers an inclusion of anisotropy results in a potential photocurrent increase of 10%, whereas the enhancement stays at a modest 4-5% for active layers thicker than 80 nm. We analyse the spectral power dissipation versus depth in each layer. The isotropic and anisotropic spectral profiles in Figure 7a and Figure 7b, respectively, clearly show a higher absorption in the isotropic PEDOT:PSS films. The ratio image in Figure 7c shows the gradient, expected from transmittance, with less power dissipated in the PEDOT:PSS layer for longer wavelengths when including anisotropy. Furthermore, the power dissipation in the active layer increases by 15% for wavelengths at 740 nm, whereas it increases by over 40% deep in to the bandgap at 900 nm. This implies that active layers with a larger absorption in the red and near infrared part of the spectrum require an anisotropic treatment of the PEDOT:PSS electrodes in optical simulations. Conclusions In this work we have studied the optical power dissipation in organic solar cells with PEDOT:PSS electrodes, under oblique incidence. We have included an uniaxial anisotropic treatment of the PEDOT:PSS layers, and compared to an isotropic treatment. The transmittance is enhanced for the anisotropic PEDOT:PSS layer models for oblique incidence for wavelengths with a pronounced dichroism. As a result, the optical power dissipation in the active layer in organic solar cells can be enhanced. In solar cells with a metal bottom electrode, thin film interference results in a dependence on enhancement factor on active layer thickness and angle of incidence. For 90-110 nm thick active layers the photocurrent can be enhanced by close to 7%. For semi-transparent solar cells the photocurrent enhancement is limited to 4-5% for p-polarized light. However, at longer wavelengths of ~900 nm deep down in to the bandgap of the active layer, the optical power dissipation enhancement reaches 40%. Thus, for future materials where a redshifted absorption spectrum is attained with a high open circuit voltage, the uniaxial anisotropy in PEDOT:PSS electrodes may prove highly valuable. This work shows that for correct future calculations of optical power dissipation in devices with PEDOT:PSS electrodes, optical anisotropy should be included in the model. This will be especially important to determine the daily energy output of organic solar cells as their expected first markets are on building facades and indoor applications with larger fractions of diffuse light at large oblique incidence.

Materials and methods A detailed derivation of the alterations needed to expand TMM calculations to include uniaxial anisotropy at oblique incidence in a thin film stack with an incoherent lid can be found in the supplementary information. The method described in ref26 was used to calculate reflectance and transmittance of the full stack. Also the Generalized Transfer Matrix Method 7 ACS Paragon Plus Environment

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(GTM) could be used for optical systems with incoherent and coherent stacks27, but here we have chosen Ref 22 as the GTM requires calculation of the Poynting vector. GTM however enables an efficient further development including both anisotropy and a combination of coherent and incoherent stacks. Validation of the Matlab code used for calculations is found in supporting information Figure SI3-4. Spectroscopic ellipsometry measurements were performed in the spectral range 245 – 1690 nm at angles of incidence 45,55,65 and 75° with an RC2 ellipsometer from J.A. Woollam Co., Inc. (USA). The software (CompleteEASE, J.A. Woollam Co., Inc.) was used to model the optical constants of the active layers and PEDOT:PSS thin films spin coated on silicon substrates, using Kramers-Kronig consistent B-spline models28 for the active layers and a Drude-Lorentz dispersion model for the PEDOT:PSS film. The PEDOT:PSS film was spun at 1000 rpm from a PH1000 solution (Heraeus) with 5 vol% DMSO and 0.5 vol% Zonyl FS-62 to a dry film thickness of 110 nm. The PEDOT:PSS film was annealed for 5 minutes at 50°C prior to measurement. The film was not possible to fit using an isotropic model. The fit gave a high confidence for ) and 8) and the modelled optical constants are comparable to published data16 where sorbitol was used as the processing additive. P21:PCBM was prepared from a 25 gl-1 oDCB solution spun at 1000 rpm to 100 nm dry film thickness while optical constants for P3TI:PC71BM were taken from reference29. Glass and aluminium optical constants was attained from the CompleteEASE library. To approximate the insolation the AM1.5 solar spectrum with 1:1 p- and s-polarization was used, unless otherwise mentioned. The solar spectrum does of course change over the course of the day, but rather weighted towards more red photons, why we consider our results a lower boundary of the possible benefits with an anisotropic treatment of PEDOT:PSS.

Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interests Author contributions JB performed the optical simulations and wrote the manuscript. HA contributed to the data analysis and discussions and revised the manuscript. OI initiated the project and contributed to analysis and discussions and revising the manuscript. Acknowledements Funding was provided from the Swedish Energy Agency and the Knut and Alice Wallenberg foundation through a Wallenberg Scholar grant to O.I. is gratefully acknowledged.


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References 1. Søndergaard, R.; Hösel, M.; Angmo, D.; Larsen-Olsen T.T.; Krebs, F.C.; Roll-to-roll fabrication of polymer solar cells, Mater. Today. 2012, 15, 36–49. 2. Zhao, W.; Li, S.; Yao, H.; Zhang, S.; Zhang, Y.; Yang, B.; and Hou, J.; Molecular Optimization Enables over 13% Efficiency in Organic Solar Cells, Am. Chem. Soc., 2017, 139 (21), 7148–7151 3. Xu, X.; Yu , T.; Bi , Z.; Ma, W.; Li, Y.; Peng, O.; Realizing Over 13% Efficiency in Green‐Solvent‐Processed Nonfullerene Organic Solar Cells Enabled by 1,3,4‐ Thiadiazole‐Based Wide‐Bandgap Copolymers, Adv. Mater. 2018, 30, 1703973 4. An Alkylated Indacenodithieno[3,2-b]thiophene-Based Nonfullerene Acceptor with High Crystallinity Exhibiting Single Junction Solar Cell Efficiencies Greater than 13% with Low Voltage Losses; Fei, Z.; Eisner, F.D.; Jiao, X.; Azzouzi, M.; Röhr, J.A.; Han, Y.; Shahid, M.; Chesman, A.S.R.; Easton, C.D.; McNeill, C.R.; Anthopoulos, T.D.; Nelson, J.; Heeney, M. Adv. Mater. 2018, 30, 1705209 5. Peters, C.H.; Sachs-Quintana, I.T.; Kastrop, J.P.; Beaupré, S.; Leclerc, M.; McGehee, M.D.; High efficiency polymer solar cells with long operating lifetimes, Adv. Energy Mater. 2011, 1, 491–494. 6. Tang, Z.; Andersson, L.M.; George, Z.; Vandewal, K.; Tvingstedt, K.; Heriksson, P.; Kroon, R.; Andersson, M.R.; Inganäs, O.; Interlayer for modified cathode in highly efficient inverted ITO-free organic solar cells, Adv Mater. 2012, 24, 554–558. 7. Tang, Z.; George, Z.; Ma, Z.; Bergqvist, J.; Tvingstedt, K.; Vandewal, K.; Wang, E.; Andersson, L.M.; Andersson, M.R.; Zhang, F.; Inganäs, O.; Semi-transparent tandem organic solar cells with 90% internal quantum efficiency, Adv. Energy Mater. 2012, 2, 1467–1476. 8. Zhou, Y.; Cheun, H.; Choi, S.; Potscavage Jr. W.J.; Fuentes-Hernandez, C.; Kippelen, B.; Indium tin oxide-free and metal-free semitransparent organic solar cells, Appl Phys Lett. 2010, 97, 153304 9. Tvingstedt, K.; Inganäs, O.; Electrode grids for ITO-free organic photovoltaic devices, Adv Mater. 2007, 19, 2893–2897. 10. Krebs, F.C.; Søndergaard, R.; Jørgensen, M.; Printed metal back electrodes for R2R fabricated polymer solar cells studied using the LBIC technique, Solar Energy Materials and Solar Cells. 2011, 95, 1348–1353. 11. Dos Reis Benatto, G.A.; Roth, B.; Madsen, M.V.; Hösel, M.; Søndergaard, R.R.; Jørgensen, M.; Krebs, F.C.; Carbon: The Ultimate Electrode Choice for Widely Distributed Polymer Solar Cells, Adv. Energy Mater. 2014, 4, 1400732 12. Tang, Z.; Elfwing, A.; Bergqvist, J.; Tress, W.; Inganäs, O.; Light trapping with dielectric scatterers in single- and tandem-junction organic solar cells, Adv. Energy Mater. 2013, 3, 1606–1613. 13. Pettersson, L.A.; Ghosh, S.; Inganäs, O.; Optical anisotropy in thin films of poly(3,4ethylenedioxythiophene)–poly(4-styrenesulfonate), Organic Electronics. 2002, 3, 143– 148. 14. Optical anisotropy in solvent-modified poly(3,4-ethylenedioxythiophene): poly (styrenesulfonic acid) and its effect on the photovoltaic performance of crystalline silicon/organic heterojunction solar cells, Liu, Q.; Imamura, T.; Hiate, T.; Khatri, I.; Tang, Z.; Ishikawa, R.; Ueno, K.; Shirai, H.; Appl. Phys. Lett. 2013, 102, 243902 15. Measuring optical anisotropy in poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) films with added graphene, Isoniemi, T.; Cameron, D.C.; Simonen, J.; Toppari. J.J.; Organic Electronics., 2015, 25, 317-323 16. Pettersson, L.A.A.; Roman, L.S.; Inganäs, O.; Modeling photocurrent action spectra of photovoltaic devices based on organic thin films, J Appl Phys. 1999, 86, 487–496. 9 ACS Paragon Plus Environment

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Uniaxial anisotropy in PEDOT:PSS electrodes enhances the photocurrent at oblique incidence in organic solar cells Jonas Bergqvist1*, Hans Arwin2, Olle Inganäs1 1

Biomolecular and Organic Electronics, IFM, Linköping University, SE-581 83 Linköping, Sweden 2 Laboratory of Applied Optics, IFM, Linköping University, SE-581 83 Linköping, Sweden

A semitransparent organic solar cell stack with an active layer sandwiched between two PEDOT:PSS electrodes. The optical power dissipation in the stack is modeled for p-polarized light incident at 70◦ with isotropic and uniaxially anisotropic models decribing the optical properties of the PEDOT:PSS layer. The right figure shows the spectrally resolved optical power dissipation after taking the ratio between an anisotropic PEDOT:PSS model to an isotropic model. This work investigates different optical properties in electrodes for organic solar cells, which potentially could reduce the future climate impact of electrical energy generation.

70° Air

Optical power dissipation ratio for OPV stack with anisotriopic vs isotropic PEDOT:PSS

Glass incoherent

50

PEDOT:PSS 120 nm P3TI:PCBM 100 nm PEDOT:PSS 120 nm Glass

Stack depth (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1,400

PEDOT:PSS 1,200

100 150

1,000

P3TI:PCBM

200

0,8000

250

0,6000

PEDOT:PSS 300

0,4000

300

400

500

600

700

800

Wavelength (nm)


900

1000

(a)

Transmittance Air / Glass

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 (b) 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

y

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x

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z θi

Air

θr

Glass PEDOT:PSS Active layer Aluminum or PEDOT:PSS θt

Glass

1,0 0,8

TS

0,6

TP T

0,4 0,2 0

0

20 40 Environment 60 ACS Paragon Plus Angle of incidence ()

80

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ACS Photonics PEDOT:PSS o

Air

Extinction coefficient, k

Refractive index, n

1 PEDOT:PSS eo Glass P21:PCBM Aluminum 2 3 3 4 5 2 6 7 1 8 9 10 11 0 400 500 600 700 800 900 1000 12 Wavelength (nm) 13 14 15 16 9 (b) 17 6 Air PEDOT:PSS o 18 3 Glass PEDOT:PSS eo 19 Aluminum P21:PCBM 20 0,3 21 22 0,2 23 24 25 0,1 26 27 28 0 400 500 600 700 800 900 1000 29 ACS Paragon Plus Environment Wavelength (nm) 30 31

(a)

Transmittance, TP

Transmittance, TP

1 2 3 4 5 6 7 8 9 10 11 12 (b) 13 14 15 16 17 18 19 20 21 22 23 24 25

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1,0

Angle of incidence

0 20 40 60 70 80

anisotropic isotropic

0,9 0,8 0,7

Air

Glass incoherent

Glass incoh.

0,5 400

500 600 700 800 Wavelength (nm)


0,9 0,8 0 0,7

TP

900 1000

1,0

Angle of incidence

0 20 40 60 70 80

θ

Air

Glass incoherent PEDOT:PSS 250 nm

0,6

Glass incoh.

0,5 0,4 300

θ

PEDOT:PSS 120 nm

0,6

0,4 300

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400

ACS Paragon Environment 500 600 700 800Plus900 1000 Wavelength (nm)

TP

ACSofPhotonics Angle incidence

J = (JS + JP)/2 Air/Glass/PEDOT/P21:PCBM/Al -/-/120/d/100 nm 50

300

0 60 60 70 70


100 150 200 Thickness (nm)

6

(d)

ratio

4 2 0

Angle of incidence

0 60 60 70 70

JP

8 1,10 1,08 1,06 1,04 1,02 1,00 0

60 70

100

200

300

Thickness (nm)

0

50


250

15

300 1,08

13 -2

250


-2

-2


-2

JMAX (mA cm )

50

10

JMAX (mA cm )

0 60 60 70 70

JS

JMAX (mA cm )


JMAX (mA cm )

10

1 8 2 3 6 4 5 4 6 7 2 8 0 9 0 10 11 (c) 12 20 13 14 15 15 16 17 10 18 19 5 20 21 22 00 23 24

Angle of incidence

(b)

1,06

Actice layer thickness 11 (nm) Isotropic 70 110 9 Anisotropic 70 110

1,04

1,02

7

90 nm Ratio

5 ACS Paragon Plus Environment 250 300 0 20

40

60

Angle of incidence ()

80

1,00

Ratio

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(a)

Isotropic

Anisotropic

Glass incoherent PEDOT:PSS 120 nm

0,6

P21:PCBM 90 nm

0,4 0,2

Aluminum 100 nm 400

500

600

700

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900 1000

Wavelength (nm)

(d)

(e) 500

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1.3

0.01555

Wavelength (nm)

0.01409

700

0.01233

0.01057

0.008808

600

0.007046

0.005285

0.003523

500

0.001762

0.000

0.01555

0.01409

0.01233

0.01057

0.008808

0.007046

0.005285

0.003523

0.001762

0.000

400

800

200 250

Q(z,λ) (mW m-2 nm-2) 6 0 ACS Paragon Plus Environment 1000

150

0.8

70

70

Stack depth (nm)

Page 16 of 18 70° Air

Reflectance PEDOT:PSS AP21:PCBM Aluminum

0,8

0,0 300

900 1000

500

1000

1500

2000

2500

3000

1 2 0,8 3 0,6 4 5 0,4 6 7 0,2 8 0,0 9 300 400 500 600 700 800 10 Wavelength (nm) 11 (c) 12 1350 14 100 15 150 16 200 17 18 250 Q(z,λ) (mW m-2 nm-2) 19 6 0 300 20 300 400 500 600 700 800 900 21 Wavelength (nm) 22

b

1,0

Relative power distribution

a

Relative power distribution

1,0

(b)ACS Photonics

Reflectance PEDOT:PSS AP21:PCBM Aluminum

900

0.3 300

400

500

600

700

800

Wavelength (nm)

900

300 1000

Stack depth (nm)

(a)

(b) 1,6 ACS Photonics

TP anisotropic TP isotropic

0,5

TS isotropic

0,4 0,3 0,2 0,1 300

16

400

500 600 700 800 0 Wavelength (nm)

(d)

 () P21:PCBM 0 P21:PCBM 60 P21:PCBM 70 P3TI:PCBM 70

14 12 10

JS+JP

JP

8 6 4

Anisotropic Isotropic

2

1,5

P21:PC71BM

1,4

0,8

1,3 1,2

0,4

1,1 1,0 400

900 1000

JMAX-ANISOTROPIC/JMAX-ISOTROPIC

-2

JMAX (mA cm )

Transmittance

1 2 3 4 5 6 7 8 9 10 11 (c) 12 13 14 15 16 17 18 19 20 21 22 23 24

TP-ANISOTROPIC / TP-ISOTROPIC

TS anisotropic

0,6

1,2 P3TI:PC71BM

 (x105 cm-1)

(a) 0,7 Page 17 of 18

500

0

50

100

150

200

Thickness (nm)

250

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0,0 1000

1,12 1,10 P3TI:PCBM P21:PCBM

1,08 1,06 1,04 1,02

p-polarization 70

ACS Paragon Plus Environment 1,00

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600 700 800 Wavelength (nm)

0

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ACS Photonics 500

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Glass 100 1 incoherent 2 150 PEDOT:PSS 3 120 nm 4 200 5 250 P3TI:PCBM 6 100 nm Q(z,λ) (mW m-2 nm-2) 7 300 4 PEDOT:PSS 0 8 120 nm 300 400 500 600 700 800 900 1000 9 10 Wavelength (nm) Glass (c) 11 (b) 12 13 50 50 14 100 15 100 16 150 150 17 200 18 200 19 250 20 250 21 300 Q(z,λ) (mW m-2 nm-2) 300 0 4 22 ACS Paragon Plus Environment 23 300 400 500 600 700 800 900 1000 300 400 500 600 700 800 24 Wavelength (nm) Wavelength (nm)

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30

40

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0.01555

0.01409

0.01233

500

1000

1500

2000

2500

3000

20

Stack depth (nm)

10

1.200

30 40 50 60 70

Stack depth (nm)

0.01057

0.008808

0.007046

0.005285

0.003523

0.001762

0.000

1.400

1.000

0.8000

0.6000

0.4000

0.01555

0.01409

0.01233

0.01057

0.008808

0.007046

0.005285

0.003523

0.001762

0.000

900

1000

Uniaxial anisotropy in PEDOT:PSS electrodes enhances the

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