EDOT – diketopyrrolopyrrole copolymers for polymer solar cells †

a The photovoltaic properties of a series of diketopyrrolo[3,4-c ]pyrrole (DPP) copolymers containing 3,4-ethylenedioxythiophene (EDOT) as a comonomer are reported. With use of di ﬀ erent aryl ﬂ anking units on the DPP core, namely thiophene, pyridine or phenyl, optical gaps ranging from 1.91 eV to 1.13 eV are achieved. When blended with the fullerene derivative [6,6]-phenyl C 71 -butyric acid methyl ester (PC 71 BM), the thiophene-ﬂ anked copolymer PDPP[T] 2 -EDOT with an optical gap of 1.13 eV was found to have the best photovoltaic performance, with an e ﬃ ciency of 2.5% in an inverted device architecture. Despite having the lowest open circuit voltage of the three polymers studied, PDPP[T] 2 -EDOT -based devices were able to achieve superior e ﬃ ciencies due to the high short circuit current of up to (cid:1) 15 mA cm (cid:3) 2 . PDPP[T] 2 -EDOT - based devices also exhibit higher external quantum e ﬃ ciencies which are associated with a superior microstructure – as revealed by transmission electron microscopy (TEM) and grazing incidence wide-angle X-ray scattering (GIWAXS) – which is associated with the enhanced aggregation tendency of PDPP[T] 2 - EDOT chains. In particular PDPP[T] 2 -EDOT : PC 71 BM blends were found to have a ﬁ ner phase separated morphology with superior thin-ﬁ lm crystallinity. Surface morphology was also investigated with atomic force microscopy and near-edge X-ray absorption ﬁ ne-structure spectroscopy.


(A) Introduction
Harnessing solar energy is a promising way to meet the world's rapidly increasing energy demand.Intensive research has been conducted on organic photovoltaic technologies (OPV) over the past two decades since the rst demonstration of a donor/ acceptor bilayer planar heterojunction in 1979. 1,2OPVs offer several advantages compared to conventional solar technologies including solution processability, low cost, exible substrates, semitransparency and ease of fabrication.The bilayer heterojunction architecture has certain limitations including a limited donor-acceptor interfacial area and the requirement of long exciton diffusion lengths in order to ensure sufficient exciton dissociation.Excitons in most organic semiconductors have very short lifetimes with associated diffusion lengths of about 4-20 nm, 3,4 and thus only excitons within an exciton diffusion length of the donor-acceptor interface are able to dissociate into free charges before recombining.[10] The development of OPV technology has gone hand-in-hand with the innovation of new materials.The power conversion efficiency of organic solar cells is a product of open-circuit voltage (V OC ), short-circuit current (J SC ) and ll factor (FF).The highest occupied molecular orbital (HOMO) level of the electron donor material and the lowest unoccupied molecular orbital (LUMO) level of the electron acceptor material roughly determine the maximum achievable V OC , while the lowest optical gap (E g ) of the two materials largely determines maximum possible J SC . 11With regard to polymer donors, the key issues of polymer design include engineering the band gap and energy levels to achieve high J SC and V OC .In an ideal case, a low band-gap polymer, whose absorption extends into the inferred region, can greatly improve the J SC as more photons can be converted to electrons.Thus ne tuning of HOMO and LUMO levels is required to achieve a high V OC with a small band gap, while maintaining a LUMO level high enough for efficient charge separation. 12In addition, high performance polymeric donor materials should possess high hole mobilities and the ability to be processed with a suitable acceptor to form an optimal phase-separated morphology.
The recent success of polymer/fullerene solar cells that utilise a polymeric donor and fullerene acceptor has resulted largely from the development of new low band-gap donor materials.The donor-acceptor design, where electron rich and electron decient units alternate along the copolymer backbone is commonly used to tune the HOMO and LUMO energy levels and the optical gap of these polymers. 13While many different building blocks have been utilised for the construction of donor-acceptor copolymers, the electron decient diketopyrrolopyrrole (DPP) unit has proved to be a versatile acceptor unit with DPP-based copolymers possessing low band gaps for polymer solar cell applications 14,15 and high carrier mobilities in eld-effect transistors. 16In particular power conversion efficiencies of up to 9.4% have been achieved, [17][18][19] with bandgaps as low as 1.13 eV also realised for DPP-based polymers. 20It is the strong electron decient character of the DPP moiety, combined with high planarity and aggregation, that allows these novel materials to absorb in the near infrared region and exhibit good electron and hole mobilities. 16ere we report the photovoltaic properties of novel DPPbased copolymers that incorporate the electron rich 3,4-ethylenedioxythiophene (EDOT) as comonomer with different aryl anking units on either side of the DPP core, see Fig. 1. 21The use of EDOT as a comonomer is motivated by its strong electron rich character which can enhance the p-type character of PDPP materials and provide non-covalent, diffusive H/O interactions to the adjacent aryl units in the backbone. 19Through the use of different anking units, tuning of the bandgap is achieved with an optical bandgap as low as 1.13 eV achieved with the use of thiophene anking units which promote a more coplanar backbone.Interestingly, the highest efficiencies are achieved with the lowest bandgap material that exhibits a broad spectral response from 350 nm to over 1000 nm.As well as characterising the performance of these novel polymers in inverted and standard device congurations, the microstructure of blends with the fullerene derivative [6,6]-phenyl C 71butyric acid methyl ester (PC 71 BM) are also reported.

Materials
Fig. 1 presents the chemical structures of the three DPP-EDOT polymers investigated.These three polymers employ the DPP moiety as the electron decient unit along with the EDOT unit as an electron rich comonomer.For the polymer PDPP[T] 2 -EDOT, thiophene units are used as anking units either side of the DPP moiety.For the polymer PDPP[Py] 2 -EDOT pyridine units are used as anking units while for PDPP[Ph] 2 -EDOT phenyl units are used as anking units.The synthesis of these materials has been reported separately. 21The electrochemical and optoelectronic properties are strongly inuenced by the choice of anking unit, with the HOMO, LUMO and bandgap values of these polymers summarised in Table 1.The low band gap of the thiophene containing polymer is attributed not only to the planar structure of the polymer backbone but also to a very strong push-pull effect of the electron rich EDOT and the electron decient DPP core. 21 Both polymer and fullerene components contribute to the absorption spectrum of the blend lm, with PC 71 BM contributing at wavelengths lower than 600 nm and a distinct peak in the visible at around 465 nm and further peaks in the UV range.The absorption onsets of the polymers vary from 650 nm for PDPP[Ph] 2 -EDOT to 780 nm for PDPP[Py] 2 -EDOT and extend out to 1100 nm for PDPP[T] 2 -EDOT, matching the absorption features previously seen in neat polymer lms. 21The high band gap PDPP[Ph] 2 -EDOT has a relatively narrow absorption range with only one distinct peak at 550 nm, with the lack of vibronic features attributed to its low planarisation due to large dihedral angles and hence decreased delocalization. 21The intermediate optical gap PDPP[Py] 2 -EDOT : PC 71 BM has two distinct peaks at around 640 and 710 nm consistent with vibronic features while the low band gap PDPP[T] 2 -EDOT has a broad near infrared absorption band ranging from 640 nm to 1100 nm with two distinct vibronic peaks at 950 and 850 nm.

Device performance
In this study, the performance of both standard and inverted devices has been evaluated.Devices with a standard structure consisted of ITO/MoO 3 /active layer/Ca/Al, while devices based on an inverted structure consisted of ITO/ZnO/PEIE/active layer/ MoO 3 /Ag.The active layers are all spin-coated from chloroform solutions with 2 vol.%DIO with the active layer thickness separately optimised for each polymer.Details of device fabrication and testing are described in the Experimental section.Fig. 3a-d presents the current density versus voltage characteristics (J-V curves) of devices under AM1.5G simulated sunlight and in the dark for both standard and inverted structures.Device performance parameters such as J SC , V OC , FF and power conversion efficiency (PCE) are summarized in Table 2.In general, EDOT-polymer devices with a high band gap show a higher V OC and a lower J SC , while low band gap EDOT-polymer devices exhibit a higher J SC and a lower V OC .The standard architecture PDPP[T] 2 -EDOT : PC 71 BM device shows an efficiency of only 0.69% with a short circuit current of 6.8 mA cm À2 and an open-circuit voltage of 0.25 V.In comparison, we found signicant improvement in performance for the inverted device structure for PDPP[T] 2 -EDOT : PC 71 BM blends with an almost 4fold increase in efficiency to 2.5%, mainly resulting from an increase in J SC to 15.5 mA cm À2 along with improvements in V OC (an increase from 0.25 V to 0.32 V) and FF (from 0.40 to 0.50).
The standard PDPP[Py] 2 -EDOT : PC 71 BM device on the other hand shows a higher PCE (2.2%), compared with inverted structure device (1.4%).The J SC of both types of PDPP[Py] 2 -EDOT : PC 71 BM devices are very similar (5.1 mA cm À2 versus 4.9 mA cm À2 ), while the V OC and FF of the standard structure device are both slightly higher (0.80 V versus 0.70 V and 0.54 versus 0.41 respectively).Despite a relatively high V OC (0.97 V for standard and 0.91 V for inverted structure), PDPP[Ph] 2 -EDOT : PC 71 BM devices gave a very moderate PCE of 1.7% for both architecture devices largely due to the low J SC (4.1 mA cm À2 for standard and 4.4 mA cm À2 for inverted structures).
Fig. 3c and d present the dark J-V characteristics for both standard and inverted architectures.For standard structure devices, the current leakage values of PDPP[Py] 2 -EDOT : PC 71 BM and PDPP[Ph] 2 -EDOT : PC 71 BM devices are about 2 orders of magnitude lower than that of the PDPP[T] 2 -EDOT : PC 71 BM device, with the high leakage of standard PDPP[T] 2 -EDOT : PC 71 -BM device likely affecting the FF of this device.In comparison, for the inverted structure, the dark current value of the PDPP[T] 2 -EDOT device is strongly suppressed, and is one order of magnitude lower than that of the PDPP[Py] 2 -EDOT device.
The external quantum efficiency (EQE) curves of devices are presented in Fig. 3e and f, which agree well with the reported J SC values.The low band gap PDPP[T] 2 -EDOT : PC 71 BM device based on an inverted structure has a broad photo-response from 350 nm to over 1000 nm with the highest EQE value of 51.6% at 415 nm.Despite the lower band gap of PDPP[T] 2 -EDOT, the inverted PDPP[T] 2 -EDOT : PC 71 BM still has higher EQE values than the inverted devices based on the other two polymers.In the standard conguration, the peak EQE is highest for the highest band gap material, with the PDPP[T] 2 -EDOT : PC 71 BM device having the lowest peak EQE.The fact that the peak EQE values are generally less than 50% (and below 30% for the case of (PDPP[Py] 2 -EDOT and PDPP[Ph] 2 -EDOT)) indicates that the relatively low efficiencies of these cells compared to current high performance systems are due to relatively low quantum efficiency of charge collection.

Morphology
In order to understand the relationship between the device performance, morphology, and chemical structure, the microstructure of these PDPP-EDOT copolymer : fullerene blends have been probed with a series of characterization techniques.Atomic force microscopy (AFM) was used to image the surface features while transmission electron microscopy (TEM) was employed to image the bulk morphology.Synchrotron-based grazing incidence wide-angle X-ray scattering (GIWAXS) was employed to study thin-lm crystallinity with near-edge X-ray   4a) shows a smooth surface and brous structures with a surface roughness (R q , root mean squared) value of 2.0 nm.In comparison, the surface features become larger and rougher in the PDPP[Py] 2 -EDOT : PC 71 BM lm (Fig. 4b) with a surface roughness of R q ¼ 4.0 nm.A dramatic change is observed for the PDPP[Ph] 2 -EDOT : PC 71 BM blend, Fig. 4e, which is characterised by relatively large surface undulations with a surface roughness of R q ¼ 6.3 nm.The AFM images suggest variations in the underlying morphology which are conrmed by the TEM images.For the PDPP[T] 2 -EDOT : PC 71 -BM lm (Fig. 4b) a ne continuous brous structure is seen indicating a relatively ne phase separated structure and consistent with the smooth surface observed with AFM.Measurement of bril width yielded an average bril width of 9 AE 3 nm.Some larger features are observed which are attributed to polymer aggregates and are likely associated with the higher regions seen in the AFM image.A coarser phase-separated structure is observed for the PDPP[Py] 2 -EDOT : PC 71 BM lm (Fig. 4d) with average bril width of 23 AE 4 nm.A different morphology is observed in TEM image of the PDPP[Ph] 2 -EDOT : PC 71 BM lm as shown in Fig. 4f, with dark, enclosed domains of average diameter of 70 AE 10 nm surrounded by an interconnected phase with average with of 30 AE 10 nm.Due to the higher density of the fullerene, the enclosed domain with darker contrast is likely to be fullerene-rich with the continuous phase polymer rich.The variations in thin lm morphology observed by TEM go some way to explain the observed trends in device performance.Indeed, Li et al. have previously established a relationship between bril width and solar cell performance in polymer : fullerene blends based on DPP polymers. 22For the polymers investigated here, the PDPP[T] 2 -EDOT : PC 71 BM blend has the nest morphology with narrowest bril width, consistent with the high EQEs achieved in inverted PDPP[T] 2 -EDOT : PC 71 BM devices.With increased bril width, the PDPP[Py] 2 -EDOT : PC 71 BM blend has a lower maximum EQE than PDPP[T] 2 -EDOT-based devices.Interestingly the PDPP[Ph] 2 -EDOT : PC 71 BM lm does not have a brillar morphology, rather having a morphology suggestive of liquid-liquid phase separation seen in other systems such as the MDMO-PPV : PC 61 BM and PTB7-Th : PC 71 BM where spherical, fullerene-rich domains are surrounded by a continuous polymer-rich phase. 23,24In PDPP : PCBM blends the use of a solvent additive such as DIO is usually effective in suppressing liquid-liquid phase separation 22,25 however for the case of the PDPP[Ph] 2 -EDOT : PC 71 BM blend this does not seem to be the case.van Franeker 25 have recently found that for a PDPP : PCBM blend that the presence of the solvent additive DIO induces aggregation of polymer chains suppressing liquid-liquid separation.This observation may suggest that PDPP[Ph] 2 -EDOT chains with phenyl anking units have a reduced tendency to aggregate, which is supported by UV-vis measurements of neat PDPP[Ph] 2 -EDOT lms 21 and of PDPP[Ph] 2 -EDOT : PC 71 BM blends that exhibit a lack of vibrational structure.In contrast for the more strongly aggregating PDPP[Py] 2 -EDOT and PDPP[T] 2 -EDOT chains brillar morphologies are observed resulting from the increased tendency of these chains to aggregate.Thus the choice of anking unit not only strongly affects the optical properties of these polymers but it also strongly inuences the thin-lm morphologies achieved in blends with PC 71 BM.Certainly for the case of the PDPP[Ph] 2 -EDOT : PC 71 BM the coarse morphology observed is limiting cell efficiency, with morphological feature much greater than the exciton diffusion length.For the PDPP[Py] 2 -EDOT : PC 71 BM blend the large bril widths observed there (23 AE 4 nm) are also not optimal for cell performance.Therefore it appears that the morphologies achieved so far are limiting the overall efficiency achieved with these polymers, particularly for the PDPP[Py] 2 -EDOT and PDPP [Ph] 2 -EDOT-based cells.Interestingly Li et al. 22 also correlated  bril width with solubility, with less soluble DPP-based polymers giving narrower brils.Higher molecular weight polymers also lead to a reduced solubility which in turn results in a smaller bre width. 26Out of the three polymers, the DPP derivatives with a six-membered ring as the aryl anking units, i.e.PDPP[Py] 2 -EDOT and PDPP[Ph] 2 -EDOT, show a signicantly lower tendency to aggregate (PDPP[T] 2 -EDOT is also the most crystalline, see below) leading to a worse bre network formation.Even for the highest performing PDPP[T] 2 -EDOT : PC 71 BM blend efficiency could be signicantly increased by reducing the average bril width from $10 nm to $5 nm (ref.22) such as through the use of higher molecular weight samples.Thus these results highlight the importance of intermolecular interactions in determining nanoscale morphology and hence overall cell performance.GIWAXS.Fig. 5 presents GIWAXS measurements on the PDDP-EDOT : PC 71 BM blends assessing the thin-lm crystallinity.Despite the apparent ordered brillar network seen in the TEM images (and similar to previous results 22 ) the GIWAXS patterns indicate a low degree of crystalline ordering in the polymer phase compared to other systems such as regioregular P3HT : fullerene lms.The scattering patterns are largely dominated by the scattering of PC 71 BM aggregates (halo at $1.3 ÅÀ1 ) though polymer scattering features can also be discerned.PDPP [T] 2 -EDOT appears to be the most highly ordered on the 3 polymers, with a clear alkyl lamellar stacking peak observed at 0.34 ÅÀ1 consistent with previous bulk WAXS measurements on the neat polymer. 21For the other two polymers, an alkyl lamellar stacking peak can only barely be discerned, being much broader and consistent with a signicantly shorter crystalline coherence length.The peak positions of the alkyl lamellar stacking peaks for PDPP[Py] 2 -EDOT and PDPP[Ph] 2 -EDOT are consistent with the bulk SAXS measurements of $0.40 ÅÀ1 and 0.37 ÅÀ1 with the different lamellar stacking distances for the different polymers attributed to the different backbone conformations.In particular the thiophene-anked PDPP with the highest lamellar stacking distance ($18.4Å) has a more coplanar conformation compared to the phenyl-anked PDPP which has a more twisted backbone conformation and a smaller inter-lamellar distance ($15.9Å).
From the c-dependence (polar angle-dependence) of the lamellar stacking peak it is possible to calculate Herman's orientational parameter, S, to quantify the orientation of crystallites relative to the substrate. 27The calculated values for S range from S ¼ À0.5 to S ¼ 1 corresponding to fully face-on or fully edge-on, respectively, with S ¼ 0 indicating no preferential orientation.S is plotted as a function of X-ray angle of incidence a based on the analysis of 2D GIWAXS patterns taken at different grazing angles ranging from a ¼ 0.05 to a ¼ 0.40 .For angles below the critical angle, identied as a C ¼ 0.17 in this case based on the angle which gives the highest scattering intensity, scattering is primarily from the top 10 nm of the lm, while for angles above the critical angle X-rays scatter from throughout the lm.For the PDPP[T] 2 -EDOT : PC 71 BM blend, a value of S ¼ 0.2 is determined independent of angle of incidence indicating that the PDPP[T] 2 -EDOT crystallites have a slight edge-on preference that is uniform throughout the depth of the lm.For the PDPP[Py] 2 -EDOT : PC 71 BM blend a consistent value of S $ 0 is calculated for different values of a indicating that the PDPP[Py] 2 -EDOT crystallites are randomly oriented throughout the lm.Finally, for the PDPP[Ph] 2 -EDOT : PC 71 BM blend, a strong a-dependence of S is observed, with S ranging from 0.8 at the surface (indicating a strong edgeon orientation preference) to S ¼ 0 within the bulk, indicating no strong orientational preference lower in the lm.
NEXAFS spectroscopy.To complement the above microstructural analyses, NEXAFS spectra were acquired of the top surface with a view to determining surface composition.Total electron yield (TEY) spectra were collected from blend lms and are compared to that from neat polymer and PC 71 BM lms, see Fig. 6.In the TEY modality, X-ray absorption is determined by measuring the number of electrons that leave the sample via measurement of the drain current that ows into the sample to compensate for the lost electrons.Due to the limited mean-freepath of electrons in solids, TEY has a surface sensitivity of $3 nm, 28 providing information about the chemical composition of the top surface layer.Spectra were collected at an X-ray angle of incidence of 55 to mitigate orientational effects. 29y comparing the blend spectra with that of the reference neat spectra, it is clear that the blend spectra better resemble the neat polymer spectra indicating that the top surface is polymer-rich.By tting the blend spectra to a linear combination of neat spectra it is possible to determine a %-surface composition.Interestingly, when tting over the full spectral range the best-t linear combination deviates from the measured blend spectrum in each case, see ESI. † This deviation is attributed to the polymer chains having a different surface conformation in the neat lms compared to blends, with more side-chains exposed to the surface in the blends compared to the neat lms, which changes the weighting of the spectra in the p* and s* regions.To account for this discrepancy, in order to obtain reliable chemical compositions, ts have been made over (i) the entire spectral window (280 eV to 320 eV), (ii) over the p* region (282 eV to 287 eV) and (iii) over the s* region (287 to 320 eV).Details of these ts can be found in the electronic ESI.† Based on these ts, the surface composition of the PDPP[T] 2 -EDOT : PC 71 BM blend was determined to be 78 AE 9 wt% polymer, the surface composition of the PDPP[Py] 2 -EDOT : PC 71 BM blend to be 92 AE 8 wt% polymer and the surface composition of the PDPP[Ph] 2 -EDOT : PC 71 BM blend to be 70 AE 3 wt% polymer.

Discussion
For the series of PDPP-EDOT polymers investigated here, the polymer with thiophene anking units achieved the highest efficiency despite having the lowest V OC .The superior performance of the PDPP[T] 2 -EDOT-based cells are largely due to the higher J SC with the other polymers achieving higher V OC and FF values.The high J SC of the PDPP[T] 2 -EDOT-based devices is a product of the higher EQE values obtained and the broader spectral response, with PDPP[T] 2 -EDOT having an optical gap of 1.13 eV, one of the lowest band gaps reported for a DPP polymer. 20,30While the low bandgap accounts for the broad spectral coverage, the higher EQE values for the PDPP[T] 2 -EDOT : PC 71 BM cells can be attributed to the more optimal morphology of PDPP[T] 2 -EDOT : PC 71 BM blends, with a narrower bril width and superior crystalline structure.
Furthermore, space-charge limited current

Device characterization
The photovoltaic characteristics of the solar cells were measured with a Keithley 2635 source measurement unit under a 1 sun, AM1.5G spectrum from a Photo Emission Tech.SS50AA solar simulator.The intensity of simulated solar light was calibrated by using a calibrated silicon solar cell with KG3 lter to obtain an accurate light intensity.External quantum efficiency (EQE) was measured as a function of wavelength by using light from a tungsten halogen lamp dispersed through a monochromator and focused onto the cell.The power density calibration was performed by placing a calibrated photodiode under test position and referencing the intensity measured to that of another silicon photodiode that samples a portion of the beam via a beam-splitter and serves to account for any intensity uctuations.
UV-vis-NIR absorption spectroscopy was measured over the wavelength range from 300 to 1200 nm with a PerkinElmer Lambda 950 Spectrometer.Surface topography was measured with a Veeco Nanoscope V atomic force microscope (AFM) using ScanAsyst mode.TEM images were obtained using a JEOL JEM-2100F TEM operating at a voltage of 200 kV.Defocussed bright-eld images were collected using a Gatan UltraScan 1000 (2k Â 2k) CCD camera.A 20 mm objective aperture and a large value of defocus (À10 000 nm) were employed to increase contrast between the phases.
GIWAXS measurements.GIWAXS measurements were conducted at the SAXS/WAXS beamline of the Australian Synchrotron. 33Samples were prepared by spin-coating lms onto MoO xcoated silicon wafers.Highly collimated 9 keV X-rays were calibrated to be at a tilt angle of 0 AE 0.01 when parallel to the surface of each sample by use of a silicon crystal analyser.A Dectris Pilatus 1M detector collected 2D scattering patterns.Each scattering pattern was tiled together from three 1 second images with the detector slightly moved between exposures, such that the resulting image removes gaps between the detector modules.The sample to detector distance was measured using a silver behenate scattering standard.Data was analysed using a modied version of the NIKA small angle scattering analysis package. 34EXAFS spectroscopy.NEXAFS spectroscopy was performed at the so X-ray beamline of the Australian Synchrotron 35 using a nearly perfectly linearly polarized X-ray beam.Total electron yield data was acquired by measuring the drain current owing to the sample under X-ray illumination while partial electron yield data was acquired using a Channeltron detector with a retarding voltage of 210 V.An X-ray angle of incidence of 55 was used for all spectra.The recorded signal was normalized by the "stable monitor method", 36 with the spectra normalized by setting the pre-edge to 0 and the intensity at 320 eV to 1. NEX-AFS data were analysed with QANT. 37

(D) Conclusions
We have studied the photovoltaic performance and morphology of polymer solar cells based on a series of novel EDOT-containing DPP polymers.By varying the aryl anking units on the DPP core from thiophene, to pyridine to phenyl a range of optical bandgaps spanning 1.91 to 1.
Fig. 2 presents the normalized optical absorption spectra of blended lms of PDPP[T] 2 -EDOT : PC 71 BM, PDPP[Py] 2 -EDOT : PC 71 BM and PDPP[Ph] 2 -EDOT : PC 71 BM.These blends were are all spin-coated from chloroform solutions with 2 vol% of DIO additive and a weight ratio of 1 : 2 polymer : fullerene.Both polymer and fullerene components contribute to the absorption spectrum of the blend lm, with PC 71 BM contributing at wavelengths lower than 600 nm and a distinct peak in the visible at around 465 nm and further peaks in the UV range.The absorption onsets of the polymers vary from 650 nm for PDPP[Ph] 2 -EDOT to 780 nm for PDPP[Py] 2 -EDOT and extend out to 1100 nm for PDPP[T] 2 -EDOT, matching the absorption features previously seen in neat polymer lms. 21The high band gap PDPP[Ph] 2 -EDOT has a relatively narrow absorption range with only one distinct peak at 550 nm, with the lack of vibronic features attributed to its low planarisation due to large dihedral angles and hence decreased delocalization. 21The intermediate optical gap PDPP[Py] 2 -EDOT : PC 71 BM has two distinct peaks at around 640 and 710 nm consistent with vibronic features while the low band gap PDPP[T] 2 -EDOT has a broad near infrared

Fig. 5
Fig. 5 GIWAXS results of the PDDP-EDOT : PC 71 BM blends: (a) 2D scattering images, (b) plot of Herman's orientation parameter of the (100) peak vs. X-ray angle of incidence a, (c) out-of-plane and in-plane line profiles taken from sector cuts of the 2D images in part (a).

Fig. 6
Fig. 6 Total electron yield NEXAFS spectra of the top surface of neat films and blends.
13 eV was achieved, attributed to variations in backbone planarity.Despite having the lowest open circuit voltage, the thiophene anked polymer exhibited the highest efficiency of 2.5% attributed to the more favourable morphology, crystallinity and mobility of PDPP[T] 2 -EDOT : PC 71 BM blends that enabled EQEs of over 50%.The performance of inverted PDPP[T] 2 -EDOT : PC 71 BM devices were superior to that of standard devices attributed in part to the superior optical properties of the top electrode used in the inverted structure.

Table 2
Device performance of both standard and inverted devices based on PDPP[T] 2 -EDOT, PDPP[Py] 2 -EDOT and PDPP[Ph] 2 -EDOT 21PC 71 BM was purchased from Nano-C, while polyethylenimine, 80% ethoxylated solution, 1,8-diiodooctane (98%) and zinc acetate dehydrate (99.999% trace metals basis) were all purchased from Sigma-Aldrich.The standard device structure employed was ITO/MoO x /active layer/Ca/Al while the inverted device structure was ITO/ZnO/ PEIE/active layer/MoO x /Ag.For standard devices MoO x was thermally evaporated in vacuo ($10 À6 mbar) onto cleaned ITO/ glass with a thickness of 15 nm.For inverted devices, a 0.073 M ZnO precursor solution was prepare by dissolving 160 mg of zinc acetate dehydrate in 61 mg of ethanolamine and 10 mL of for 15 min to form a thin hole-blocking layer.The active layers of the standard devices were all spin-coated from blend solutions at 4000 rpm for 1 min for standard devices, and 4000 rpm, 6000 rpm and 8000 rpm for 1 min for inverted PDPP[T] 2 -EDOT : PC 71 BM, PDPP[Py] 2 -EDOT : PC 71 BM and PDPP[Ph] 2 -EDOT : PC 71 BM devices respectively.The solvent employed was chloroform with 2 vol% DIO additive and total concentration of the blend solutions were all set at 12 mg mL À1 with a PDPP-EDOT : PC 71 BM ratio of 1 : 2. The average active layer thickness were 96 nm, 68 nm and 65 nm for standard devices and 96 nm, 61 nm and 40 nm for inverted structure PDPP[T] 2 -EDOT : PC 71 BM, PDPP[Py] 2 -EDOT : PC 71 BM and PDPP[Ph] 2 -EDOT : PC 71 BM devices respectively.A 15 nm Ca layer and a 100 nm Al layer for standard structure devices or a 15 nm MoO x layer and a 100 nm Ag layer for inverted structure devices were subsequently evaporated in vacuo ($10 À6 mbar) through a shadow mask to dene electrodes with an active area of 4.5 mm 2 .All processing steps subsequent to the weighing of solid organic semiconductor powders were conducted in a nitrogen glove box.The devices were encapsulated with epoxy resin and glass cover slides before being removed from the glove box for testing.
2-methoxyethanol with vigorous stirring for 12 hours for the hydrolysis reaction at 60 C. 0.073 M ZnO solution was then spin-coated onto cleaned ITO/glass at 3000 rpm for 30 s and annealed on hot plate at 200 C for 30 minutes to form a thin conducting layer.Subsequently a 0.4 wt% PEIE solution was spin-coated at 5000 rpm for 30 s and annealed on hot plate at 110 C