Perfluoroalkyl-substituted conjugated polymers as electron acceptors for all-polymer solar cells

A series of six diketopyrrolopyrrole (DPP) based conjugated polymers with a varying content of solubilizing per ﬂ uoroalkyl chains were synthesized. Based on a systematic investigation of the in ﬂ uence of the solvent on the photovoltaic performance, it is found that 1,6-diiodoper ﬂ uorohexane (IC 6 F 12 I) is an e ﬀ ective solvent additive to enhance the power conversion e ﬃ ciency (PCE) of DPP polymers with per ﬂ uoroalkyl side chains. The polymers consist of thiazole-ﬂ anked DPP units that alternate along the main chain with varying ratios of thiophene (T) and per ﬂ uoroalkyl benzodithiophene (FBDT) units. The polymers possess high molecular weights, narrow band gaps and good crystalline properties. The DPP polymers were used as electron acceptors in bulk heterojunction solar cells with another DPP polymer as the electron donor. A solvent mixture of CHCl 3 : 1-chloronaphthalene (1-CN) is found to provide the best PCE of 2.9% in non-ﬂ uorine based DPP polymer solar cells, but yields a low PCE of 0.52% for per ﬂ uoroalkyl-containing polymer solar cells. Per ﬂ uoroalkyl-containing polymer solar cells fabricated from CHCl 3 with IC 6 F 12 I as the processing additive show a signi ﬁ cantly improved PCE of 2.1%. The morphology analysis of the blend ﬁ lms reveals that IC 6 F 12 I as an additive improves the micro-phase separation between the polymer donor and acceptor, which results in enhanced charge generation.


Introduction
2][3] A signicant number of conjugated small molecules [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] and polymers [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] with excellent electron transport properties and aligned energy levels have been designed and synthesized, which are well-suited as electron acceptors in organic photovoltaic devices.Currently, perylenediimide and naphthalenediimide based materials are considered as the most promising non-fullerene acceptors, 36,37 since these materials have shown high electron mobilities of about 1 cm 2 V À1 s À1 (ref.38) and deep lowest unoccupied molecular orbital (LUMO) levels which are similar to those of fullerene derivatives.1 In terms of the large possible variation in materials, conjugated polymers possess an advantage compared to fullerene derivatives, such as [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM), for which variations in the optical band gap and LUMO energies are more restricted.In recent years, conjugated polymers have been widely exploited as electron donors in polymer solar cells (PSCs), 42,43 and design principles from these studies can also be utilized for the development of new acceptor polymers.In particular, when introducing electron-rich and electron-decient moieties into a conjugated backbone, the socalled donor-acceptor polymers exhibit distinct variations in frontier orbital energy levels, absorption spectra, and chargecarrier mobilities, which allow the conjugated polymers to be used as electron donors or acceptors in solar cell devices.For instance, the diketopyrrolopyrrole (DPP) unit is widely used as an electron-decient building block to construct narrow band gap polymers with near-infrared absorption.][51] Meanwhile, organic eld-effect transistors (FETs) based on DPP polymers also present high electron mobilities above 5 cm 2 V À1 s À1 , 46,52 which are higher than those of fullerene derivatives, 53 indicating their potential for use as electron acceptors in PSCs.One recent example is the design of a polymer in which the DPP is anked by two thiazole rings to effectively lower the LUMO level.34 The resulting DPP polymer can act as an electron acceptor in combination with another DPP polymer as the electron donor in PSCs to reach a PCE of 2.9%.At present this represents the highest PCE when using DPP polymers as electron acceptors.It will be of importance to further explore new DPP polymer acceptors for efficient photovoltaic devices.
Recently, we designed and synthesized a DPP polymer acceptor PDPP2TzFBDT (Scheme 1) that has similar frontier energy levels to those of PCBM and can be potentially used as a universal acceptor for PSCs. 54However, our initial attempt to use PDPP2TzFBDT as an electron acceptor with PDPP5T 34 as an electron donor was not successful and resulted in very poor performance with PCEs of 0.19%.PDPP2TzFBDT has long per-uoroalkyl chains that are partially responsible for the deep LUMO levels, but the lipophobic peruoroalkyl chains also cause poor miscibility with PDPP5T, resulting in large phase separation in blends of these two polymers, poor charge generation, and consequently low PCEs.Similar behaviour was also reported in donor polymer : fullerene systems, in which the donor polymers that bear long peruoroalkyl chains showed large phase-separated domains and hence provide poor PCEs in PSCs. 55,56Therefore, it is important to nd a way to improve the micro-phase separation in these blend lms in order to apply peruoroalkyl-based conjugated polymers in organic photovoltaic devices.
In this work, we explore the use of diiodoperuoroalkanes as additives in solution-processed PSCs based on peruoroalkyl DPP acceptor polymers.Based on the similarity principle, we assume that polymers with peruoroalkyl units have better solubility in peruoroalkanes as additives, which would prevent the fast precipitation and therefore reduce the domain size of the polymers.A similar function of o-dichlorobenzene (o-DCB) as an additive in DPP polymer solar cells was also observed. 57 synthesized several DPP acceptor polymers, in which the thiazole-anked DPP segment was linked with thiophene (T) and peruoroalkyl benzodithiophene (FBDT) units in different ratios (Scheme 1).The polymers were found to have small optical band gaps (E g ) and good crystalline properties, depending on the ratio of T and FBDT units.The resulting polymers were used as electron acceptors in polymer-polymer solar cells with PDPP5T as the electron donor, which were solution-processed from chloroform (CHCl 3 ) with 1-chloronaphthalene (1-CN) or -diiodoperuoroalkanes as processing additives.The solar cells based on PDPP5T : PDPP2TzFBDT fabricated from CHCl 3 : IC 6 F 12 I exhibited a much improved PCE of 2.1% compared to cells from CHCl 3 or CHCl 3 : 1-CN due to their better micro-phase separation.The results reveal that diiodoperuoroalkanes can be used to effectively tune the morphology of peruoroalkyl-based polymer solar cells.This encourages the design of peruoroalkyl-based conjugated polymers for efficient solar devices.

Materials and measurements
All synthetic procedures were performed under an argon atmosphere.Commercial chemicals were used as received.
1 H-NMR spectra of the polymers were recorded on a Bruker AVIII 500WB NMR Spectrometer at 100 C with 1,1,2,2-tetrachloroethane-d 2 as the solvent and tetramethylsilane (TMS) as the internal standard.The molecular weight was determined by gel permeation chromatography (GPC) at 140 C on a PL-GPC 220 system (Agilent Technologies with a Knauer PDA detector) using a PLgel 10 mm MIXED-B LS column and o-DCB as the eluent against polystyrene standards.A low concentration of 0.1 mg mL À1 polymer in o-DCB was applied to reduce aggregation.Optical absorption spectra were recorded on a JASCO V-570 spectrometer with a slit width of 2.0 nm and a scan speed of 1000 nm min À1 .Cyclic voltammetry (CV) was performed under an inert atmosphere at a scan rate of 0.1 V s À1 and 1 M tetrabutylammonium hexauorophosphate in acetonitrile as the electrolyte.An ITO glass slide covered with a thin layer polymer (approx.20 nm) was used as the working electrode.The counter and reference electrodes were a Pt wire and Ag/AgCl, respectively.Atomic force microscopy (AFM) images were recorded using a Digital Instruments Nanoscope IIIa multimode atomic force microscope in the tapping mode under ambient conditions.The tips were purchased from Bruker (Model: SCANA-SYST-AIR with one cantilever, T ¼ 650 nm, L ¼ 115 mm, W ¼ 25 mm and spring constant of 0.4 N m À1 ).2D grazing-incidence wide angle X-ray scattering (2D-GIWAXS) measurements were performed by using a Xenocs WAXS/SAXS system, with an X-ray wavelength of 1.5418 Å.The incident angle was 0.2 .The sample-to-detector distance was 127.5 mm.The scattered X-rays were detected by using a Dectris Pilatus 100k counting detector.The counting time was 4 h.All lm samples were prepared by spin-coating solutions on Si/SiO 2 substrates.Steady state uorescence spectra were recorded at room temperature using an Edinburgh Instruments FLS980 double-monochromator luminescence spectrometer equipped with a nitrogen-cooled near-IR sensitive photomultiplier (Hamamatsu).Photovoltaic devices with an inverted conguration were made by spin coating a ZnO sol-gel at 4000 rpm for 60 s onto pre-cleaned, patterned ITO substrates.The photoactive layer was deposited by spin coating a chloroform solution containing PDPP5T and thiazole-based DPP polymers and an appropriate amount of processing additives such as 1-CN, 1,4-diiodoper-uorobutane (IC 4 F 8 I), 1,6-diiodoperuorohexane (IC 6 F 12 I) or 1,8-diiodoperuorooctane (IC 8 F 16 I) in air.MoO 3 (10 nm) and Ag (100 nm) were deposited by vacuum evaporation at ca. 4 Â 10 À5 Pa as the back electrode.
The active area of the cells was 0.04 cm 2 .The J-V characteristics were measured by using a Keithley 2400 source meter unit under AM1.5G spectrum from a solar simulator (Enlitech model SS-F5-3A).The illumination intensity was determined at 100 mW cm À2 using a monocrystalline silicon reference cell with a KG5 lter.The short-circuit current density (J sc ) under AM1.5G conditions was estimated from the spectral response and convolution with the solar spectrum.The external quantum efficiency was measured by using a Solar Cell Spectral Response Measurement System QE-R3011 (Enli Technology Co., Ltd.).The thickness of the active layers in the photovoltaic devices was measured on a Veeco Dektak XT prolometer.

Synthesis and characterization
Peruoroalkyl-DPP based polymers were prepared by Stille polymerization (Scheme 1).The synthesis of two derivatives, PDPP2TzT 35 and PDPP2TzFBDT, 54 has been described previously.The copolymers PDPP2Tz10FBDT, PDPP2Tz30FBDT, PDPP2Tz50FBDT, and PDPP2Tz70FBDT were synthesized by copolymerizing the dibromo-DPP monomer 1 with the bisstannyl monomers of T (2) and FBDT (3), in which the ratio of the co-monomers 2 and 3 was adjusted to prepare polymers with different contents of peruoroalkyl units.Stille polymerizations were performed under identical conditions using Pd 2 (dba) 3 /PPh 3 as the catalyst system and toluene/DMF as the solvent at 115 C.
The polymers show different 1 H-NMR spectra (ESI, Fig. S1 †), but it is difficult to determine the ratio of the polymer segments.Therefore, the m : n ratio is denoted according to the feed ratio of the co-monomers 2 and 3 (Scheme 1) and does not necessarily represent the actual ratio of T and FBDT units in the polymer backbone.The molecular weight of these polymers has been determined by GPC using o-DCB as the eluent.As shown in Table 1 and Fig. S2 (ESI †), most polymers possess a similarly high molecular weight between 60 and 80 kg mol À1 .The solubility of PDPP2TzFBDT is very poor in o-DCB such that only a soluble small molecular weight fraction can be measured by GPC.The similar molecular weight of these DPP copolymers is also benecial for investigation of solvent inuence on photovoltaic devices.

Optical and electrochemical properties
The optical absorption spectra of the DPP polymers in CHCl 3 solution and in solid state thin lms are shown in Fig. 1 and the parameters are summarized in Table 1.All polymers exhibit nearinfrared absorption with absorption onsets ranging from 772 to 805 nm in CHCl 3 solution and red-shied absorptions in thin lms with onsets between 787 and 861 nm.PDPP2TzT in which the thiazole-anked DPP unit is alternating with T as the aromatic units has the lowest E g of 1.44 eV, while PDPP2TzFBDT where the thiazole-anked DPP unit is alternating with FBDT units shows the largest E g of 1.58 eV among these polymers.When using a ratio of T and FBDT as 9 : 1 in PDPP2Tz10FBDT, the E g slightly increases to 1.46 eV.Interestingly, as the ratio of T and FBDT is further changed to 7 : 3, 5 : 5, and 3 : 7, the polymers PDPP2Tz30FBDT, PDPP2Tz50FBDT, and PDPP2Tz70FBDT have very similar E g of 1.49 to 1.51 eV in thin lms, which are red-shied compared to that of the copolymer PDPP2TzFBDT.The absorption spectra of the polymers in CHCl 3 solution with a concentration of 0.001 g L À1 are shown in Fig. S3 (ESI †).Interestingly, when increasing the content of FBDT units, the intensity of the absorption at 700 nm decreases, indicating a lower absorption coefficient for FBDT-based polymers.
The electrochemical properties of the DPP polymers were determined by CV measurements (ESI †, Fig. S4 and Table 1).The highest occupied molecular orbital (HOMO) level of PDPP2TzT is at À5.47 eV and the LUMO level is at À4.03 eV.When the T unit is replaced by the FBDT unit, the resulting PDPP2TzFBDT polymer exhibits deeper HOMO and LUMO levels at À6.14 and À4.56 eV, respectively.The copolymers with T and FBDT show HOMO levels around À5.75 and À5.85 eV and LUMO levels around À4.24 and À4.39 eV.The results illustrate that FBDT units can effectively lower the energy levels of electron acceptors.It is also noted that the LUMO levels were not linearly reduced with increasing FBDT content, which is possibly from the different aggregation of the polymers in o-DCB and the deviation when exacting reduction potential from CV curves in Fig. S4, ESI.†

Crystalline properties
In order to investigate the crystalline properties and molecular packing in solid state lms, 2D-GIWAXS 59 was applied on thin lms spin coated from CHCl 3 on Si/SiO 2 substrates and the results are shown in Fig. 2 and Table 2.The q values are directly extracted from the peaks of GIWAXS line cuts.PDPP2TzT shows a distinct (100) diffraction peak in the in-plane direction, which correlates with a lamellar packing distance of 22.4 Å of the 2octyldodecyl side chains.The (010) diffraction peak in the outof-plane direction for PDPP2TzT is related to the p-p stacking distance of 3.67 Å of the conjugated backbone.These results reveal that PDPP2TzT is a semi-crystalline polymer with a distinct "face on" orientation of the polymer chains, which is benecial for charge transport in the vertical direction in bulk heterojunction solar cells. 40The polymers incorporating both T and FBDT into the main chain also exhibit the (100) and ( 010)   This journal is © The Royal Society of Chemistry 2016 diffraction peaks indicating a "face on" orientation for the 9 : 1 and 7 : 3 ratios, but the intensity of the diffractions is strongly reduced for the 5 : 5 and 3 : 7 co-monomer ratios.This is not unexpected because the introduction of the differently sized T and FBDT units in a random fashion will destroy the translational symmetry along the chain, which precludes obtaining highly ordered polymer domains.For PDPP2TzFBDT the diffraction peaks in the 2D-GIWAXS are restored, demonstrating that it is not the FBDT unit itself that causes the reduced crystallinity of the DPP polymers that were made with both co-monomers.PDPP2TzFBDT exhibits both (100) and (010) diffractions peaks in the out-of-plane direction, while in the in-plane direction the diffraction peaks are much less pronounced.We tentatively attribute this behaviour to the absence of a clear preference for the "face-on" or "edge-on" orientation of the PDPP2TzFBDT polymer chains on the surface.Apparently both types of domains are present, but the orientation of the crystallites is random.All thiazole-based DPP polymers presented here exhibit a similar d-spacing of about 23.0 Å for the (100) diffraction peak, originating from the lamellar packing distance induced by the 2-octyldodecyl side chains.The p-p stacking distances gradually increased from 3.67 Å to 3.90 Å for the polymers PDPP2TzT to PDPP2Tz70FBDT due to the increasing number of sterically demanding 5-peruorohexylthiophene-2-yl substituents, but then decreased to 3.83 Å for PDPP2TzFBDT possibly because of a more regular polymer structure (Table 2).A closer p-p stacking distance provides an improved wave function overlap between neighbouring chains which is benecial for charge transport.It is also interesting to note that PDPP2TzFBDT shows a new diffraction peak at q z ¼ 0.41 ÅÀ1 with a distance of d ¼ 15.3 Å, which is possibly induced by the lamellar stacking of peruoroalkyl units (ESI, Fig. S5 †).
Finally, as a comparison and because it is used as an electron donor, PDPP5T with ve anking thiophene units shows a (100) diffraction peak in the in-plane direction with d ¼ 19.04 Å and an (010) peak in the out-of-plane direction with d ¼ 3.83 Å, indicating a "face on" orientation (ESI, Fig. S6 †).
In summary, the 2D-GIWAXS data reveal that PDPP2TzT and PDPP2TzFBDT are more crystalline than their T/FBDT mixed copolymers and that the absence of the 5-peruorohexylthiophene-2-yl substituents in PDPP2TzT enables a closer p-p stacking (3.67 Å) than in PDPP2TzFBDT (3.83 Å), which can enhance charge transport for the former.

Polymer-polymer solar cell performance
The thiazole-anked DPP polymers were applied as electron acceptors using PDPP5T as the electron donor in polymer-polymer photovoltaic devices with an inverted polarity conguration, in which ITO/ZnO and MoO 3 /Ag were used as electron and hole extracting contacts, respectively.PDPP5T has a similar absorption spectrum to that of the DPP acceptor polymers (Fig. 1).
When the photoactive layers were spin coated from CHCl 3 without processing additives low PCEs of 0.11-0.39%(ESI, Table S1 †) were obtained.The low performance is attributed to the large micro-phase separation between the donor and acceptor polymers as inferred from the corresponding AFM images (ESI, Fig. S7 †).This behaviour is similar to that of PDPP5T : PCBM solar cells when solution processed from CHCl 3 solution. 57o improve the blend morphology, the photoactive layers of the PDPP5T : DPP-polymer (1 : 1 w/w) blends were spin coated from CHCl 3 solutions using 1-CN, IC 4 F 8 I or IC 6 F 12 I as high boiling point solvent additives.
When the active layer was spin coated from CHCl 3 with 3% 1-CN as the additive, PDPP5T : PDPP2TzT cells resulted in a PCE of 2.8% with a J sc of 7.1 mA cm À2 , an open-circuit voltage (V oc ) of 0.81 V, and a ll factor (FF) of 0.49.Using the same solvent mixture, the solar cells based on PDPP5T with PDPP2Tz10FBDT, PDPP2Tz30FBDT, PDPP2Tz50FBDT, and PDPP2Tz70FBDT as electron acceptors showed gradually decreasing PCEs from 2.3% down to 1.5%, 0.52%, and 0.26%.The lower PCEs were mainly caused by a reduction of J sc from 5.7 to 0.82 mA cm À2 .PDPP5T : PDPP2TzFBDT cells had a slightly increased PCE of 0.43% when spin coated from CHCl 3 : 1-CN solution, but is still much lower than that of PDPP5T : PDPP2TzT cells (Table 3).
We then tested diiodoperuoroalkanes as processing additives.For PDPP5T : PDPP2TzFBDT cells we used IC 4 F 8 I, IC 6 F 12 I and IC 8 F 12 I and found that both IC 4 F 8 I and IC 6 F 12 I signicantly increase the PCE.With IC 8 F 16 I as the additive, the solar cells showed very poor J-V characteristics with large leakage current.This may due to the high melting point ($75 C) of IC 8 F 16 I, producing holes in the active layers when it was removed via a high vacuum process.Above 5%, the volume ratio of IC 6 F 12 I in CHCl 3 has little inuence on the device performance of PDPP5T : PDPP2TzFBDT cells (ESI, Table S3 †).Aer optimization, the PCE of PDPP5T : PDPP2TzFBDT cells spin coated from CHCl 3 : IC 6 F 12 I was found to be 2.0%, with J sc ¼ 7.4 mA cm À2 , V oc ¼ 0.67 and FF ¼ 0.41 at a lm thickness of 75 nm (Fig. 3a).The external quantum efficiency (EQE) of the optimized PDPP5T : PDPP2TzFBDT cells shows a broad photoresponse from 300 to 850 nm with a maximum EQE over 0.3 in the nearinfrared spectral region where the polymer absorbs light (Fig. 3b).PDPP5T : PDPP2TzFBDT cells fabricated from CHCl 3 with 20% IC 4 F 8 I also provided PCEs up to 1.8% (ESI, Table S3 †).
We then optimized the other PDPP5T : DPP-polymer solar cells using CHCl 3 : IC 6 F 12 I (9 : 1) as the solvent mixture (Fig. 3 Table 2 Crystallographic parameters of the polymer thin films from 2D-GIWAXS measurements Polymer Lamellar spacing p-p spacing and Table 3).We failed to fabricate solar cells for PDPP2TzT and PDPP2Tz10FBDT with PDPP5T using CHCl 3 : IC 6 F 12 I as solvent due to the poor wettability of the solutions on the ITO/ZnO surface, but for the polymers with an increased number of FBDT units, working devices were obtained.The current densityvoltage (J-V) characteristics and EQE of the optimized solar cells are shown in Fig. 3 and Table 3. PDPP5T : PDPP2Tz30FBDT cells fabricated from CHCl 3 : IC 6 F 12 I provided a low PCE of 0.38% with J sc ¼ 1.3 mA cm À2 compared to the PCE of 1.5% for the same blend processed from CHCl 3 : 1-CN.For the PDPP2Tz50FBDT and PDPP2Tz70FBDT based cells with a higher content of FBDT units, the PCEs obtained with IC 6 F 12 I as the co-solvent were similar to the PCEs obtained with 1-CN.
The effect of peruoroalkyl-based additives on the device performance was further investigated by using different donor polymers, PTB7-Th 60 and PDPP2T-DTP 61 (ESI, Fig. S8 †).Solar cells based on PTB7-Th or PDPP2T-DTP as the donor and PDPP2TzFBDT as the acceptor show high performance with PCEs of 2.5% and 1.0% when fabricated from CHCl 3 with IC 6 F 12 I as additives.In comparison, the PCEs of the cells processed from CHCl 3 with 1-CN are 2.1% and 0.6% (ESI, Table S4 and Fig. S9 †).The results conrm that IC 6 F 12 I as the additive can enhance the photovoltaic performance based on acceptor polymers bearing peruoroalkyl side units such as PDPP2TzFBDT.However, all cells show relatively low absolute PCE compared to other high efficiency non-fullerene solar cells, indicating a suboptimal morphology.

Charge transport in the blends
To study the inuence of different additives on the charge transport in the blend lms, we determined hole and electron mobilities from space charge limited current (SCLC) measurements using a device conguration consisting of ITO/MoO 3 / active layer/Au for hole-only devices and ITO/ZnO/active layer/ LiF/Al for electron-only devices.PDPP5T : PDPP2TzT layers fabricated from CHCl 3 : 1-CN have well balanced hole and electron mobilities around 10 À4 cm 2 V À1 s À1 (Table 3).With the same solvent mixture, the hole mobilities of other layers were similar to those of the PDPP2TzT cell, but the electron mobilities gradually decreased to 10 À6 cm 2 V À1 s À1 with increasing content of FBDT in acceptors.The unbalanced hole and electron mobilities could explain the low PCEs in FBDT-based polymer solar cells from CHCl 3 : 1-CN.When using IC 6 F 12 I as the additive, the electron mobilities were slightly improved compared to those from CHCl 3 : 1-CN, but the hole mobilities were reduced (Table 3).As a consequence, more balanced hole and electron mobilities can be achieved.As an example the m h /m e ratio of the PDPP5T : PDPP2TzFBDT blend decreases from   265 to 25 when replacing 1-CN by IC 6 F 12 I.The more balanced charge transport is accompanied by an enhancement of the PCE from 0.52% to 2.1%.

Morphology investigation
Because the efficiency of solar cells is intimately related to the morphology, the photoactive layers were further investigated by AFM, 2D-GIWAXS and photoluminescence (PL).When the lms were fabricated from CHCl 3 without additives, large domains were observed in AFM images (ESI, Fig. S7 †).These disappeared in thin lms processed from CHCl 3 with additives (Fig. 4).Fig. 4 shows that for PDPP5T : PDPP2Tz and PDPP5T : PDPP2Tz10FBDT blends processed from CHCl 3 : 1-CN the lateral dimensions and the height of the surface corrugation are smaller (Fig. 4a and b) than for the blends with a higher FBDT content (Fig. 4c-f), suggesting that the latter blends have a coarser micro-morphology with larger domains.When the same blends were fabricated from CHCl 3 : IC 6 F 12 I the AFM images showed a strongly reduced surface roughness (Fig. 4g-j) compared to those from CHCl 3 : 1-CN (Fig. 4c-f) and smaller lateral dimensions, strongly suggesting an enhanced mixing of the donor and acceptor polymers in the bulk-heterojunction systems.For PDPP5T : PDPP2TzFBDT (Fig. 4j), the surface corrugation is enhanced compared to the other blends.
The blended thin lms were further analysed using 2D-GIWAXS measurements.A PDPP5T : PDPP2TzFBDT lm spin coated from CHCl 3 : IC 6 F 12 I showed a lower intensity for the inplane (100) and out-of-plane (010) diffraction peaks (Fig. 5b and   d) as compared to the diffraction of the same blend spin coated from CHCl 3 : 1-CN (Fig. 5a and c).Hence, the higher mixing induced by IC 6 F 12 I reduces the crystallization of the polymers.Steady state PL was applied to further study the morphology difference originating from the spin coating solvent (Fig. 6).The pure polymer and blend thin lms were excited at 760 nm and show uorescence between 800 and 1200 nm.The PL spectra were corrected for the fraction of absorbed photons at the excitation wavelength using the absorption spectra of the same lms (ESI, Fig. S10 †).Fig. 6 shows that the PL intensity of blend lms is signicantly reduced compared to that of pure lms, indicating the charge transfer from PDPP5T to thiazole-bridged DPP polymers.When processed from CHCl 3 : IC 6 F 12 I, the blend lms have generally a lower PL intensity compared to the PL of lms from CHCl 3 : 1-CN.In particular, the PL intensity of PDPP5T : PDPP2TzFBDT spin coated from CHCl 3 : IC 6 F 12 I was greatly quenched (Fig. 6f).In this case the luminescence of the PDPP5T : PDPP2TzFBDT blend is almost the same as that of pure PDPP5T when the lm is spin coated from CHCl 3 : 1-CN.A higher PL quenching indicates a better charge generation which is expected when the mixing of the two components is enhanced.

Role of diiodoperuoroalkane additives
The use of peruoroalkyl based processing additives to CHCl 3 resulted in more intimately blended lms for peruoroalkylsubstituted DPP acceptor polymers with donor polymers such as PDPP5T than with 1-CN as the additive.The PCE, however, was only signicantly enhanced for PDPP2TzFBDT.For the other co-polymers, with varying ratios of T and FBDT units, the blends become better mixed using IC 6 F 12 I but the PCE is not really improved.As a result of their irregular structure the mixed T/FBDT co-polymers are less crystalline and possibly the resulting morphology is too well-mixed.
Although diiodoperuoroalkane additives effectively enhance the PCE of PDPP5T : PDPP2TzFBDT solar cells, the PCE is still lower than that of the PDPP5T : PDPP2Tz blends without the peruoro substituents on the acceptor polymer.The main reason is the lower V oc and lower FF.The latter is likely related to the lower electron mobility of PDPP5T : PDPP2TzFBDT lms compared to PDPP5T : PDPP2Tz lms (Table 3).Hence, designing peruoroalkyl-conjugated polymers with high electron mobilities and optimizing the morphology by looking for new solvents will be the routes to further improve the PCEs of these cells.

Conclusions
A series of conjugated polymers based on thiazole-anked DPP units were synthesized by incorporating different ratios of T and FBDT units into the main chain to tailor the peruoroalkyl content of the polymers.The peruoroalkyl based polymers were applied as electron acceptors in polymer solar cells with PDPP5T as the electron donor.While a CHCl 3 : 1-CN solvent mixture provides a PCE of 2.9% for PDPP5T : PDPP2TzT layers without peruoroalkyl units, this solvent combination only provides a PCE of 0.52% for PDPP5T : PDPP2TzFBDT with peruoroalkyl units.We showed that the PCE of PDPP5T : PDPP2TzFBDT can be dramatically enhanced to 2.1% by using a CHCl 3 : IC 6 F 12 I solvent mixture.Detailed analysis using AFM, 2D-GIWAXS and PL measurements reveals that IC 6 F 12 I as the additive is helpful to enhance the mixing of the donor and peruoroalkyl-based acceptor in the blended lms that enhance the charge generation.The results demonstrate that diiodoperuoroalkane solvents can be efficient processing additives to improve the performance of solution processed peruoroalkyl based polymer solar cells.

a
Determined with GPC at 140 C using o-DCB as the eluent.b PDI is the polydispersity index.c E LUMO ¼ À5.23 À E red .d E HOMO ¼ E LUMO À E g .e The molecular weight was determined by using a low molecular weight fraction that dissolved in o-DCB at 140 C.

Fig. 1
Fig. 1 Optical absorption spectra of the DPP polymers (a) in CHCl 3 solution and (b) in solid state thin films.

25 aJ
sc was calculated by integrating the EQE spectrum with the AM1.5G spectrum.b The poor wettability of the mixed solution on the surface of ITO/ ZnO prevents forming continuous thin lms for solar devices.The weight ratio of PDPP5T to the acceptor DPP-polymers is 1 : 1.The thickness of the active layers is around 75 nm.

Fig. 3
Fig. 3 (a) and (c) J-V characteristics in the dark (dashed lines) and under white light illumination (solid lines).(b) and (d) EQE of the optimized PDPP5T : DPP-polymer solar cells.(a) and (b) The active layers fabricated from CHCl 3 solution with 3% 1-CN.(c) and (d) The active layers fabricated from CHCl 3 solution with 10% IC 6 F 12 I.

Table 1
Molecular weight and optical and electrochemical properties of the thiazole-based DPP polymers Additional diffraction peak was present: q z ¼ 0.41 ÅÀ1 and d ¼ 15.3 Å. a