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Diketopyrrolopyrrole latent pigment-based bilayer solar cells

Diketopyrrolopyrrole latent pigment-based bilayer solar cells Org. Photonics Photovolt. 2018; 6:8–16 Research Article Open Access Myles Rooney, Francesco Carulli, Silvia Luzzati, Roland Resel, Benedikt Schrode, Riccardo Ruffo, Mauro Sassi, and Luca Beverina* Diketopyrrolopyrrole latent pigment-based bilayer solar cells DOI: https://doi.org/10.1515/oph-2018-0002 tween donor and acceptor. Even when completely orthogo- Received November 16, 2018; accepted December 21, 2018 nal systems can be established, underlying layers are sus- ceptible to swelling with the orthogonal solvent allowing Abstract: Two Diketopyrrolopyrrole based latent pigment for sequentially deposited materials to permeate into the donor materials were fabricated into thin film bilayer pho- bulk of the underlying layer [1]. This diffusion of organic tovoltaic devices featuring PCBM as the acceptor. Ther- materials is advanced by common processes such as ther- mal deprotection of the thin film, carried out at 200 C, mal annealing. Although this intermixing at the interface returns the dye-like small molecule to the corresponding can often be useful for exciton splitting in photovoltaics, pristine pigment quantitatively. The connected evolution the resulting reproducibility and stability remain a con- of electrical and morphological features of pure thin films cern [2]. So far the only reliable way to produce stacked and blends are examined. A significant decrease in extinc- devices with sharp interfaces is vacuum deposition. This tion coefficient was noted and correlated both to intrinsic technology also holds the record for highest efficiency in changes of the electronic structure upon cleavage and to organic, molecular based single junction solar cells [3]. an increase in internal scattering due to extensive crys- The latent pigment approach may offer advantages in tallization. Power conversion efficiencies of 0.33% were this respect. Latent pigments are organic pigments bear- achieved for bilayer devices, nearly doubling previous re- ing a chemically or thermally removable functionality, sults with latent pigment DPP devices, under comparable which induces a non-permanent, dye-like solubility char- experimental conditions. acter [4]. The labile group silences inter- and intramolecu- lar hydrogen bond networks (Figure 1). They have already 1 Introduction demonstrated use in numerous fields such as OPV [5, 6], thermochromic devices for food industries [7], lumines- In the field of organic optoelectronics, solution processed cent solar concentrators [8] and more recently Organic devices are usually made by a standard bottom up build, Thin Film transistors [9]. The nature of the material crys- with sequential layering of active materials. To achieve tallisation through this deprotection process has been ex- highly resolved and reproducible interlayers orthogonal amined [4]. and the possibility of creating thin heterojunc- solvent processing is used: the bottom layers of material tion and BHJ films from materials has been exemplified are ideally completely insoluble in the solvent used to through pigments such as indigo [10, 11]. process the subsequent layers. Unfortunately, the nature Di-tert-butyl dicarbonate (t-BOC) is the most common of the materials commonly used in Organic Photovoltaics latent pigment protection group. It allows for facile func- (OPV) are such that the solubility characteristics of two lay- tionalisation of pigments at near quantitative yields [12]. ers (donor and acceptor) are often very similar. Completely This functionality is sterically bulky and is success- orthogonal solvent approaches are thus quite rare. Gener- ful in disrupting π-π stacking networks and H-bonding ally, what occurs is a fine intermixing of the interface be- sites to afford soluble semiconductor materials. The quantitative deprotection by a thermal treatment hap- pens in a temperature range far below the degradation *Corresponding Author: Luca Beverina: Department of Materials Science, University of Milano, Via R. Cozzi, 55, Milano, Italy, I-20125 point of common pigments such as diketopyrrolopyrrole Myles Rooney, Riccardo Ruffo, Mauro Sassi: Department of Mate- (DPP), quinacridone (QC), perylenediimide (PDI) and iso- rials Science, University of Milano, Via R. Cozzi, 55, Milano, Italy, indigo [13–15]. The removal of the t-BOC is observed by the I-20125 evolution of two gases CO and isobutylene. Francesco Carulli, Silvia Luzzati: ISMAC-CNR, Via Corti 12, 20133 Amongst the various industrial pigment classes al- Milano, Italy ready exploited in organic electronics, Diketopyrrolopy- Roland Resel, Benedikt Schrode: Institute of Solid State Physics, Graz University of Technology, Austria Open Access. © 2018 Myles Rooney et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution- Non-Commercial-NoDerivs 4.0 License. Myles Rooney et al. , Diketopyrrolopyrrole latent pigment-based bilayer solar cells Ë 9 tovoltaic devices [5]. The thermal deprotection of DPP BOC at 200 C was used to trigger a phase segregation effect with respect to PCBM, thereby accessing a favourable BHJ morphology by a post deposition technique. Photovoltaic characterisation demonstrated a remarkable increase in Jsc upon formation of cleaved 3,6-di(biphenyl- 4-yl)- 1,4-dioxopyrrolo[3,4-c]pyrrole-2,5(1H,4H)-dicarboxylate (DPP ) within the film. This resulted in a 20-fold increase power conversion efficiency with respect to the as pre- pared cells based on protected DPP . The increase BOC in current was attributed both to the improved charge carrier mobilities, arising from the hydrogen bonding net- work which improves material packing and to improved charge generation due to the formation of an extended Donor-Acceptor interface. Conversion efficiencies peaked at a modest 0.15 % due to the relatively wide optical gap of DPP along with unfavourable level alignment with respect to PCBM. Here we report DPP materials with thiophene rings substituted in place of Phenyl rings in order to reduce the optical gap and raise the HOMO level Fig. 1. Top: The latent pigment approach demonstrated on derivative of this molecule for increasing the spectral overlap to the DPP-Th . Bottom: The structure of latent pigment derivative DPP-Th 2 3 sunlight spectrum and for a more suitable alignment with PCBM to favour charge separation at the hetero-interface. rroles (DPP) are recognised to be particularly successful. Probably the best known DPP pigment at an industrial level is Pigment 254, a.k.a. Ferrari red [16]. While this well 2 Experimental known derivative has inappropriate HOMO/LUMO levels to be of practical value as an organic photovoltaic mate- rial, the chemistry of the DPP core has been extensively 2.1 Materials and methods developed giving access to highly conjugated derivatives with good performances both in OPV and OFET technolo- All reagents were used as delivered from Sigma Aldrich gies [17, 18]. It has been shown that in its parent pigment and TCI chemicals without further purification. ITO pat- form DPP crystals have improved charge transport and terned glass substrates were supplied by TFD Inc. with a mobilities over N-alkylated “dye” counterparts. This cor- work function of 4.8 eV. The thermal window for deprotec- responds to the activation of a H-bonding network result- tion was measured by specular X-ray diffraction and ther- ing in more efficient π-π packing [13]. These characteristics mogravimetric analysis. make DPP cores a perfect example for use with the latent For X-ray scattering experiments, thin films were pre- pigment technique. pared on silicon oxide wafers with 150 nm thermal silicon Recently, through our work to develop industrially as surface coverage. compatible methods for creating cheap, green chemistry X-ray reflectivity measurements were carried out on an compliant organic semiconductors, we have demonstrated PANalytical Empyrean system using a sealed copper tube, that the conjugation of simple DPP derivatives, bearing the the radiation was parallized and monochromatized with protecting t-BOC functionality, can be extended via a mod- an X-ray mirror so that a primary x-ray beam was formed ified Suzuki-Miyaura protocol in mild conditions in a wa- with a vertical height of 0.1 mm and CuKa radiation. Fit- ter/micelle media. This procedure lead to the straightfor- ting of the reflectivity data was performed with the soft- ward synthesis of derivatives DPP-Th and DPP-Th in high ware X’Pert Reflectivity. 2 3 yield in an ambient atmosphere. (Figure 1) [19, 20]. Grazing incidence X-ray diffraction (GIXD) measure- Previously we have employed the simple di-tert- ments were performed at the Diamond Light Source (Did- butyl3,6-di(biphenyl- 4-yl)-1,4-dioxopyrrolo[3,4-c]pyrrole- cot, UK), beamline I07 21. X-rays with a wavelength of 2,5(1H,4H)-dicarboxylate (DPP ) small molecule 1.00 Å were used at an incident angle of ~0.12 , slightly BOC derivative in the preparation of bulk heterojunction pho- below the critical angle of the substrate to reduce the back- 10 Ë Myles Rooney et al. , Diketopyrrolopyrrole latent pigment-based bilayer solar cells ground signal. Diffracted intensities were measured using at 5000 rpm for 45 seconds. LiF was evaporated as a 1 nm a Pilatus 2M area detector. Reciprocal space maps were cal- interlayer. Barium was thermally evaporated as an 8 nm culated from the measured data using the xrayutilities li- interlayer. The cells were closed with 90-100 nm of ther- brary for Python [22]. Extraction of peak positions and in- mally evaporated aluminium. Each substrate contains 4 dexation was performed using the custom-made software cells and 3 substrates were used per fabrication with a to- PyGID [23]. tal of 12 cells made in tandem per fabrication. The active area of each cell is 6.2 mm . BHJ cells were also created with DPP:PCBM (weight ratio 1:1) being spincast from chlo- Synthesis roform simultaneously. Thermal deprotection was carried DPP-Th and DPP-Th derivatives were prepared according out on half of the cells while the other half were tested ther- 2 3 to the literature procedure [19]. mal treatment/annealing free. The current density-voltage measurements were performed directly in glovebox where the solar cells were assembled, with a Keithley 2602 source Preparation of the PMMA slabs meter, under AM 1.5G solar simulation (ABET 2000). 100 ml of freshly distilled methyl methacrylate is added to 100 mg of the free radical initiator azobisisobutyroni- trile (AIBN). This mixture is stirred vigorously and heated 3 Results and Discussion slowly until just under the boiling point. The viscosity slowly changes. Heat until the solution becomes a viscous Derivatives DPP-Th and DPP-Th were selected by taking 2 3 syrup. Evolving bubbles should be slow to rise through the into account the two weaknesses of the previously em- solution. Quench this reaction in an ice bath to 20 C. In a ployed derivative DPP : HOMO energy levels lower than separate beaker dissolve 150 mg of lauryl peroxide and the that of PCBM (leading to faulty level alignment) and an −5 latent pigment of interest (1x10 M) in 60 ml of freshly dis- absorption spectrum covering only the high energy region tilled methyl methacrylate. This solution is added directly of the UV-Vis spectrum (cut off at 560 nm). Indeed, both to the partially polymerised syrup. Stir until homogeneous new materials feature the use of thiophene over benzene before casting into a square mould. Heat the mould in a thus ensuring higher conjugation through improved pla- water bath for 24 hours at 58 C. Remove the slab from the narity and increased π-electron density according to the mould and heat in an oven for a further 12 hours at 100 C π-excessive nature of thiophene. Derivative DPP-Th in to ensure complete polymerisation. The optical absorption particular was already employed, in its pristine protected of a series of PMMA slabs was measured using a Jasco v- form, to prepare BHJ devices with moderate to good con- 570 Uv-spectrometer before and after thermal cleavage in version efficiencies [24]. a heated oil bath. The HOMO levels of DPP-Th and DPP-Th were esti- 2 3 mated from the corresponding Cyclic voltammetry plots re- ported in g fi ure 2, which also shows the energy level di- 2.2 BHJ and bilayer device fabrication and agram of both DPP derivatives, alongside with the previ- characterisation ously employed DPP . The addition of Th and Th moi- H 2 3 eties to DPP backbone induces a reduction of pigments Bilayer devices were fabricated with the following proto- band gap and allows also a better energy level alignment col. ITO substrates underwent a standard sonication for with respect to PCBM. It should be noted that such energy 10 minutes subsequently in 2% mucasol solution, dis- levels refer to the protected, latent pigment form of the DPP tilled water, acetone and isopropanol. Substrates were derivatives. CV characterization of deprotected pigments then dried under a nitrogen flow before 10 minutes ozone is not possible due to their extremely low solubility in or- cleaning. PEDOT:PSS was then spincoated at 2000 rpm ganic solvents. The effect of protecting a strong electron for 60 seconds before being annealed at 110 C for 10 min- withdrawing group with the t-BOC functionality is the low- utes in a nitrogen glove box (45-50 nm). Active layer de- ering of both the HOMO. As such, the thermally activated position was then carried out with DPP-Th and DPP-Th 2 3 cleavage process should raise the HOMO levels, with ben- with 8.5 mg/mL solutions in chloroform spincoated at eficial impact on level offsets with respect to the PCBM ac- 4000 rpm for 45 seconds. The substrates were then placed ceptor. on a hot plate for deprotection at 200 C for 7 minutes. The impact on optical properties is much more diffi- The insolubility of the deprotected films allows PCBM to cult to predict. Pigments are known to be affected by ag- be also spincoated from a chloroform solution of 8 mg/ml Myles Rooney et al. , Diketopyrrolopyrrole latent pigment-based bilayer solar cells Ë 11 gregation both in solution and in the solid state. The most aggregation), according to procedures elucidated from Lu- striking example of such an effect is quinacridone whose minescent Solar Concentrator fabrication processes [8]. pigment form is deep purple while the corresponding la- Latent pigments are dissolved in a solution of PMMA tent pigment one is pale yellow [25]. in stabilizer free MMA. This viscous solution can be poured in a mould of suitable thickness (2 mm in our case). Poly- merisation at a temperature well below the cleavage of the t-BOC unit affords a solid solution of latent pigment in PMMA. These slabs are then heated to 100 C to com- plete the polymerisation of all unreacted monomers and can undergo a further annealing in oil baths to depro- tect the dispersed latent pigment within. The deprotection is performed on the unaggregated latent pigment within the high viscosity polymeric medium. Thus, we avoid the formation of aggregates as the pigments cannot appre- ciably diffuse. This strategy allows us to study the opti- cal properties of the unprotected pigments behaving as highly dispersed molecules. Returning to the example of quinacridone, the deprotection of its latent pigment af- fords a yellow slab while the deprotection of its thin film equivalent gives very hazy purple layers. PMMA slabs containing DPP-Th and DPP-Th were 2 3 compared via UV-Vis spectroscopy before and after the cleavage with thin films measured using the same thermal cycle. Fig. 2. Top: Cyclic voltammetry and structure of DPP-Th2 and Th3. Reversible redox steps evaluate HOMO levels to be −5.9 eV and 5.6 eV, LUMO levels of −4.01 eV and 3.9 eV respectively. Bottom: level diagrams for PCBM, DPPH, DPP-Th , DPP-Th . 2 3 Indeed, unexpected absorption shifts and significant lowering of extinction coefficient after thermal cleavage of the t-BOC moiety was recorded. It is expected that the sig- nificant change in film crystallinity through deprotection of the latent pigment results in aggregation, leading to a Fig. 3. Left) thin lm fi UV absorption of DPP materials (Black) Pro- shift in absorption wavelength, and an increase in light tected (Red) After deprotection. Right) PMMA Slabs with DPP mate- rials dispersed throughout. (Black) Protected (Red) After deprotec- scattering throughout the film. However, this effect could tion. also be intrinsic and due to the removal of the electron withdrawing t-BOC from the accepting residue of the DPP dyes. In order to be able to decouple the effects of aggre- The behaviour of the two compounds is remarkably gation from the single molecule effect of the cleavage, we different. In the case of DPP-Th the PMMA absorption resorted to the preparation of PMMA slabs incorporating spectra before and after the cleavage are similar both in DPP-Th and DPP-Th at very low concertation (to avoid 2 3 terms of optical gap and oscillator strength. The main dif- 12 Ë Myles Rooney et al. , Diketopyrrolopyrrole latent pigment-based bilayer solar cells ference is the structuring of vibronic replicas going from als DPP-Th and DPP-Th , respectively. No change in the 2 3 the protected dye to the pigment form. This effect is co- film roughness was observed for DPP-Th , a value of 2 nm herent with the expected planarization due to the removal was obtained by the X-ray reflectivity fit. For DPP-Th the of the bulky t-BOC group. The comparison with the corre- roughness increases from 1 nm to 3 nm due to the decou- sponding thin films clearly shows a remarkable tendency pling process. for aggregation, affecting both pristine and cleaved films. GIXD patterns of spin coated films before and after the In both cases a band peaking at 700 nm attributed to a J decoupling process are shown in Figure 4. The diffraction type aggregate is clearly visible and survives the cleavage patterns of the as-prepared films (Figure 4a and 4b) are process. The optical density of the film is significantly re- highly crystalline with strong preferred orientation of the duced upon cleavage. The latter effect could be due to the crystallites. In case of the molecule DPP-Th the diffraction formation of smaller crystallites, sizeably increasing the peaks could be indexed. Based on the solved crystal struc- scattering of incident light. Such an interpretation is also ture (Figure 5) the positions of Bragg peaks are calculated supported by the GIXD data discussed later which shows and plotted together with the diffraction pattern (Figure quite clearly the formation of very small crystals. 4a). A preferred orientation with the 001 planes parallel As for DPP-Th derivative, the behaviour is remarkably to the substrate surface is found. The agreement of calcu- different. The PMMA slab shows the familiar low energy lated and experimentally observed peak positions is very broad absorption band common to all DPP materials, in good (e.g. compare for 0-10 and 011 / 0-11). Based on these this case red-shifted due to the increased conjugation with results we can conclude that in case of DPP-Th , the single respect to DPP-Th . Upon cleavage, a substantial widen- crystal structure is present within the spin coated films. ing of the optical gap is observed with shifting of the ab- A drastic change in the diffraction pattern is observed sorption maximum from 600 nm to 510 nm. Such a shift after the decoupling process. The peaks are smeared more is connected with a sizeable reduction in the optical den- along Debye-Scherrer rings which means that the pre- sity. In this case, the planarization occurring as a conse- ferred orientation is less pronounced in comparison to the quence of the formation of intramolecular hydrogen bonds as-prepared films. The number of peaks is drastically re- does not counterbalance the loss of electron withdraw- duced, and the peak width is considerably broader. This ing capability of the DPP core upon removal of the t-BOC means that the crystalline order in the system is consid- residue. The thin film spectra show the same feature, along erably decreased which is either related to a considerably with a remarkable increase in the scattering background smaller crystallite size or by increasing disorder within the for cleaved samples, likely due to the same crystallization individual crystalline domains. In case of DPP-Th we ob- −1 −1 effect affecting derivative DPP-Th . serve Bragg peaks at q = 0.20 Å (d = 31 Å), q = 1.15 Å −1 Such an effect can also be studied directly on thin films (d = 5.46 Å) and q = 1.83 Å (d = 3.43 Å) (Fig. 4d). In case of −1 of DPP-Th and DPP-Th by X-ray reflectivity and GIXD be- DPP-Th we observe Bragg peaks at q = 0.33 Å (d = 19 Å) 2 3 2 −1 fore and after the thermal treatment. A strong change in and q = 1.83 Å (d = 3.43 Å) (Fig. 4c). the thin film morphology as well as in the crystalline prop- The interplanar distance of 3.43 Å is dominantly erties is observed by X-ray reflectivity and GIXD, respec- present in both samples. This distance is the character- tively. In detail, X-ray reflectivity investigations (see Sup- istic for π-π stacking distance of aromatic units. Interest- porting Information) were performed on spin coated films ingly, already the single crystal solution of DPP-Th reveals before and after the decoupling process. Fitting of the X- stacks of the aromatic units with the comparable stack- ray reflectivity curves reveal basic information on the thin ing distance of 3.47 Å. Comparing both diffraction peaks film morphology. The films have a film thickness of 40 nm of the two different types of decoupled films there is a fun- and 46 nm and show due to the decoupling process a re- damental difference in the stacking of the aromatic units. duction of the film thickness of 26% and 19% for the mate- In case of DPP-Th the π-π stacks are oriented parallel to rials DPP-Th and DPP-Th , respectively. The total electron the substrate surface, since the diffraction peak is located 2 3 densities of the film were determined by using the criti- around q = 0. While in case of DPP-Th the aromatic units xy cal angle of total external reflection and converted to mass are oriented perpendicular to the substrate surface, con- densities by the knowledge of the chemical composition. cluded from the presence of the diffraction peak located −3 A value of 0.405 Å was obtained for DPP-Th2 which is dominantly at q = 0. −3 3 smaller than 0.450 Å (mass density 1.430 g/cm ), the cal- culated electron density for the molecule DPP-Th2 based on its single crystal solution. The decoupling process en- hances the mass density of 4% and 10% for the materi- Myles Rooney et al. , Diketopyrrolopyrrole latent pigment-based bilayer solar cells Ë 13 when compared with benchmark g fi ures for organic de- vices it does represent an almost doubling of the PCE% of previous Latent Pigment materials. This improvement is attributed to frontier orbital alignment between donor and acceptor within this cell. Fig. 4. a) Indexed GIXD of thin lm fi of protected DPP-Th . b) GIXD of thin lm fi of deprotected DPP-Th . c) GIXD of thin lm fi of protected DPP-Th . d) GIXD of thin lm fi of deprotected DPP-Th . Thick weak 3 3 Bragg peaks seen as light blue bands indicate small crystallites with a random orientation in respect to the substrate. Diffraction plots have been converted into the scattering vector q space so that they are independent of wavelength of X-ray source. Fig. 6. Initial JV curve for ITO/PEDOT:PSS/ DPP-Th /PCBM/LiF/Al bilayer device. A short post deposition annealing of 140 C for 5 min- utes was carried out on closed ITO/PEDOT:PSS/DPP- Th /PCBM/LiF/Al cells. Results are shown in Table 1. This is a standard post deposition annealing proce- dure which improves the interface mixing of the layers and should not be confused with the thermal cleavage of the latent pigment, happening at 200 C prior to the deposi- tion of the acceptor layer and contacts closing the cell. This secondary annealing resulted in a noticeable reduc- tion in short circuit current. LiF provides an ohmic con- tact between semiconductor and metal contact and low- ers the work function of aluminium. Lithium has also been observed migrating into active semiconductor layers, usu- Fig. 5. Single crystal structure of DPP-Th was grown from slow evap- ally this lithium doping of the organic results in improved oration in chloroform. device performances [27]. Improvements to FF and Voc due to an optimised charge transfer across the interface is 3.1 Bilayer Heterojuction devices also commonly seen with the use of this interlayer [28–30]. However, LiF can also crystalize into heterogeneous plate Bilayer heterojuction devices based on derivatives DPP- islands on the surface of semiconductors resulting in non- Th and DPP-Th were prepared and thermally annealed 2 3 optimum interlayer properties. according to the protocol described in the materials and To investigate the possible effects of LiF dissipation method section. during annealing, a similar close cell thermal treatment Preliminary device results for DPP-Th show a moder- was also carried out using Ba rather than LiF. ate PCE of 0.31%. Although this result is not outstanding 14 Ë Myles Rooney et al. , Diketopyrrolopyrrole latent pigment-based bilayer solar cells Table 1. Results of the annealing process carried out on a ITO/PEDOT:PSS/ DPP-Th /PCBM/LiF/Al bilayer device at 140 C for 5 minutes on closed cell, i.e. after the LiF/Al electrode evaporation. The remeasured cell showed a reduction in current and lfi l factor. Sample Voc [V] FF [%] Jsc [mA/cm ] PCE [%] Not annealed (closed cell) 0.43 0.50667 1.44 0.31% 140 C annealed (closed cell) 0.45 0.38032 1.33 0.23% Table 2. ITO/ PEDOT:PSS/ DPP-Th2/ PCBM/ interlayer/Al bilayer device results. Average device data taken over 12 cells for each interlayer and annealing treatment. Voc [V] FF [%] PCE %] Jsc [mA/cm ] Barium 0.455 0.96491 0.431655 0.19 Barium annealed 0.441 1.254683 0.46415 0.2571 LiF 0.3975 1.4558 0.4987875 0.2882 LiF annealed 0.436 1.295613 0.3739425 0.2117 Interestingly the closed cell annealing in the case of isation shows evidence of poor charge generation and a devices fabricated with barium improves the device perfor- high level of thermal instability in terms of bulk morphol- mance contrasting with the behaviour of LiF. This may sug- ogy. This is most likely attributed to permeability of the gest that LiF is indeed forming non-uniform islands upon donor layers due to the formation of very small crystals annealing. upon thermal cleavage of the latent pigment form. Upon The further symmetrical extension of the DPP-Th core heating permeable channels are available for PCBM to mi- with a further hexyl thiophene to form the DPP-Th struc- grate through causing unbalanced, weakly performing de- ture was thought as a facile means to enhance the Jsc. vices which have a structure more similar to an intermixed Unfortunately, the desired Voc was not achieved when bilayer or bulk heterojunction device. The highest per- DPP-Th was fabricated into bilayer devices. The average forming devices for these materials was 0.33% PCE aver- short circuit current for devices fabricated with DPP-Th aged over 12 devices. This is double the power conversion increases but the Voc decreases in comparison to DPP-Th , efficiency previously produced by a bulk heterojunction this is consistent with the electronic modifications of the of DPP /PCBM. The improvements in performance is ac- materials. Film thicknesses was optimised by adapting the counted to alignment of HOMO and LUMO levels between spincoating speed for the deposition of the donor layer. By donor and acceptor. PMMA slabs containing highly dis- increasing the thickness of this active layer, we see a very persed latent pigments were created in order to examine small improvement in the fill factor of the devices (see ta- the optical properties of the latent pigments and the effects ble 3 below). Unlike DPP-Th further thermal annealing af- of agglomeration and crystalline properties on the absorp- ter deposition of PCBM at 140 C for 5 minutes does not ap- tive properties of these materials in thin films. A shift in the pear to have a substantial effect on device performance. absorption spectra is accounted for by the electron with- drawing nature of t-BOC on each of the materials exam- ined. It is established that the decrease in extinction coef- ficient seen during deprotection is greatly exaggerated for 4 Conclusion a thin film on a substrate compared to the highly dispersed materials trapped within the bulk PMMA. It is evident that A latent pigment strategy has been developed to fabricate less crystalline order and agglomerative tendencies of the planar bilayer organic photovoltaics. Two lead diketopy- deprotected films has an adverse effect on the absorption rrolopyrrole (DPP-Th and DPP-Th ) based semiconduc- 2 3 properties of these materials. High level of crystalline or- tors were chosen for a detailed study in both device char- der with low levels of material interconnectivity seems to acteristics and film quality. The small molecules in ques- work against the device performances. tion undergo a significant crystallographic rearrangement upon the thermal cleavage of solubilising groups. A sig- Acknowledgment: We thank Università degli Studi nificant reduction in crystal size is noted with a result- Milano-Bicocca (grant n 2016-ATESP-0047) and MIUR ing powder-like film. No crystal orientation is observed af- (Progetto Dipartimenti di Eccellenza 2017 “Materials for ter deprotection. The result is a disordered and more than energy”) for financial support. L.B. and M.R. thank the likely permeable layer of material. The device character- Myles Rooney et al. , Diketopyrrolopyrrole latent pigment-based bilayer solar cells Ë 15 Table 3. Device characteristics for ITO/PEDOT:PSS/ DPP-Th /PCBM/LiF/Al cells. All data is averaged over 3 substrates containing 4 cells each. DPP-Th spin speed and post-deposition annealing 140 C 3 Voc [V] Jsc [mA/cm ] FF% PCE% 8 mg/ml 2000 rpm thermally annealed 0.369 2.186 39.21 0.317 8 mg/ml 1000 rpm thermally annealed 0.371 2.099 42.33 0.329 8 mg/ml 800 rpm thermally annealed 0.375 2.105 42.58 0.336 8 mg/ml 1000 rpm as cast 0.368 2.277 39.40 0.330 8 mg/ml 800 rpm as cast 0.353 2.242 42.23 0.334 European Community’s Seventh Framework Programme [12] Wuts, P. G. M. Protective Groups in Organic Synthesis. (1999). [13] Głowacki, E. D. et al. Hydrogen-bonded diketopyrrolopyrrole (FP7/2007-2013) under grant agreement N. 607232 for (DPP) pigments as organic semiconductors. Org. Electron. 15, financial support (THINFACE). 3521–3528 (2014). The authors also thank Diamond Light Source for ac- [14] Jones, F. & Okui, N. The Thermal Stability of Linear Trans- cess to beamline I07 (proposal SI13569-1). quinacridone Pigments. Soc. Dye. Colour. 91, 361–365 (1975). [15] Moreno-lo, J. C., Grizzi, O. & Sa, E. A. 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Fully solution Synchrotron Radiat. 23, 1245–1253 (2016). processed p-i-n organic solar cells with an industrial pigment [22] Kriegner, D., Wintersberger, E. & Stangl, J. xrayutilities: a ver- - Quinacridone. Org. Electron. physics, Mater. Appl. 12, 1126– satile tool for reciprocal space conversion of scattering data 1131 (2011). recorded with linear and area detectors. Appl. Crystallogr. 46, [7] Galliani, D. et al. Thermochromic Latent-Pigment-Based Time 1162–1170 (2013). – Temperature Indicators for Perishable Goods. 1164–1168 [23] Moser, A. Thesis. (Graz University of Technology, Austria, (2015). doi:10.1002/adom.201500073 2012., 2012). [8] Mattiello, S., Sanzone, A., Brazzo, P., Sassi, M. & Beverina, L. [24] Tamayo, A. B., Walker, B. & Nguyen, T.-Q. A Low Band Gap, First Demonstration of the Applicability of the Latent Pigment Solution Processable Oligothiophene with a Diketopyrrolopy- Approach to Plastic Luminescent Solar Concentrators. 5723– rrole Core for Use in Organic Solar Cells. J. Phys. Chem. C 112, 5729 (2015). doi:10.1002/ejoc.201500554 11545–11551 (2008). [9] Maqueira-Albo, I. et al. A latent pigment strategy for robust [25] Głowacki, E. D. et al. Hydrogen-Bonded Semiconducting Pig- active layers in solution-processed, complementary organic ments for Air-Stable Field-Effect Transistors. Adv. Funct. Mater. field-effect transistors. J. Mater. Chem. C 5, 11522–11531 25, 1563–1569 (2013). (2017). [26] Scharber, B. M. C. et al. Design Rules for Donors in Bulk- [10] Liu, T. H., Cheng, W. T. & Huang, K. T. Crystallization and mor- Heterojunction Solar Cells — Towards 10 % Energy-Conversion phology of indanthrone converted from latent pigment in the Eflciency. Adv. Mater. 5090, 789–794 (2006). solution with photo acid generator. Dye. Pigment. 105, 137– [27] Bruder, F. & Brenn, R. Photoelectron Spectroscopy of the Con- 144 (2014). tact between the Cathode and the Active Layers in Plastic [11] Głowacki, E. D. et al. A facile protection–deprotection route for Solar Cells: The Role of LiF. Jpn. J. Appl. Phys. 44, 3695 (2005). obtaining indigo pigments as thin lms fi and their applications [28] Brabec, C. J. et al. Effect of LiF / metal electrodes on the perfor- in organic bulk heterojunctions. Chem. Commun. 49, 6063– mance of plastic solar cells. Appl. Phys. Lett. 80, 1288 (2002). 6065 (2013). 16 Ë Myles Rooney et al. , Diketopyrrolopyrrole latent pigment-based bilayer solar cells [29] Lee, S., Jeong, S., Kim, D., Kim, C. & Han, Y. Inverted Polymer Solar Cells with an Ultrathin Lithium Fluoride Buffer Layer. J. Nanosci. Nanotechnol. 12, 3205–3209 (2012). [30] Ahlswede, E. et al. Comparative study of the influence of LiF , NaF , and KF on the performance of polymer bulk heterojunc- tion solar cells bulk. Appl. Phys. Lett. 90, (2007). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Organic Photonics and Photovoltaics de Gruyter

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© by Luca Beverina, et al., published by De Gruyter
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Abstract

Org. Photonics Photovolt. 2018; 6:8–16 Research Article Open Access Myles Rooney, Francesco Carulli, Silvia Luzzati, Roland Resel, Benedikt Schrode, Riccardo Ruffo, Mauro Sassi, and Luca Beverina* Diketopyrrolopyrrole latent pigment-based bilayer solar cells DOI: https://doi.org/10.1515/oph-2018-0002 tween donor and acceptor. Even when completely orthogo- Received November 16, 2018; accepted December 21, 2018 nal systems can be established, underlying layers are sus- ceptible to swelling with the orthogonal solvent allowing Abstract: Two Diketopyrrolopyrrole based latent pigment for sequentially deposited materials to permeate into the donor materials were fabricated into thin film bilayer pho- bulk of the underlying layer [1]. This diffusion of organic tovoltaic devices featuring PCBM as the acceptor. Ther- materials is advanced by common processes such as ther- mal deprotection of the thin film, carried out at 200 C, mal annealing. Although this intermixing at the interface returns the dye-like small molecule to the corresponding can often be useful for exciton splitting in photovoltaics, pristine pigment quantitatively. The connected evolution the resulting reproducibility and stability remain a con- of electrical and morphological features of pure thin films cern [2]. So far the only reliable way to produce stacked and blends are examined. A significant decrease in extinc- devices with sharp interfaces is vacuum deposition. This tion coefficient was noted and correlated both to intrinsic technology also holds the record for highest efficiency in changes of the electronic structure upon cleavage and to organic, molecular based single junction solar cells [3]. an increase in internal scattering due to extensive crys- The latent pigment approach may offer advantages in tallization. Power conversion efficiencies of 0.33% were this respect. Latent pigments are organic pigments bear- achieved for bilayer devices, nearly doubling previous re- ing a chemically or thermally removable functionality, sults with latent pigment DPP devices, under comparable which induces a non-permanent, dye-like solubility char- experimental conditions. acter [4]. The labile group silences inter- and intramolecu- lar hydrogen bond networks (Figure 1). They have already 1 Introduction demonstrated use in numerous fields such as OPV [5, 6], thermochromic devices for food industries [7], lumines- In the field of organic optoelectronics, solution processed cent solar concentrators [8] and more recently Organic devices are usually made by a standard bottom up build, Thin Film transistors [9]. The nature of the material crys- with sequential layering of active materials. To achieve tallisation through this deprotection process has been ex- highly resolved and reproducible interlayers orthogonal amined [4]. and the possibility of creating thin heterojunc- solvent processing is used: the bottom layers of material tion and BHJ films from materials has been exemplified are ideally completely insoluble in the solvent used to through pigments such as indigo [10, 11]. process the subsequent layers. Unfortunately, the nature Di-tert-butyl dicarbonate (t-BOC) is the most common of the materials commonly used in Organic Photovoltaics latent pigment protection group. It allows for facile func- (OPV) are such that the solubility characteristics of two lay- tionalisation of pigments at near quantitative yields [12]. ers (donor and acceptor) are often very similar. Completely This functionality is sterically bulky and is success- orthogonal solvent approaches are thus quite rare. Gener- ful in disrupting π-π stacking networks and H-bonding ally, what occurs is a fine intermixing of the interface be- sites to afford soluble semiconductor materials. The quantitative deprotection by a thermal treatment hap- pens in a temperature range far below the degradation *Corresponding Author: Luca Beverina: Department of Materials Science, University of Milano, Via R. Cozzi, 55, Milano, Italy, I-20125 point of common pigments such as diketopyrrolopyrrole Myles Rooney, Riccardo Ruffo, Mauro Sassi: Department of Mate- (DPP), quinacridone (QC), perylenediimide (PDI) and iso- rials Science, University of Milano, Via R. Cozzi, 55, Milano, Italy, indigo [13–15]. The removal of the t-BOC is observed by the I-20125 evolution of two gases CO and isobutylene. Francesco Carulli, Silvia Luzzati: ISMAC-CNR, Via Corti 12, 20133 Amongst the various industrial pigment classes al- Milano, Italy ready exploited in organic electronics, Diketopyrrolopy- Roland Resel, Benedikt Schrode: Institute of Solid State Physics, Graz University of Technology, Austria Open Access. © 2018 Myles Rooney et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution- Non-Commercial-NoDerivs 4.0 License. Myles Rooney et al. , Diketopyrrolopyrrole latent pigment-based bilayer solar cells Ë 9 tovoltaic devices [5]. The thermal deprotection of DPP BOC at 200 C was used to trigger a phase segregation effect with respect to PCBM, thereby accessing a favourable BHJ morphology by a post deposition technique. Photovoltaic characterisation demonstrated a remarkable increase in Jsc upon formation of cleaved 3,6-di(biphenyl- 4-yl)- 1,4-dioxopyrrolo[3,4-c]pyrrole-2,5(1H,4H)-dicarboxylate (DPP ) within the film. This resulted in a 20-fold increase power conversion efficiency with respect to the as pre- pared cells based on protected DPP . The increase BOC in current was attributed both to the improved charge carrier mobilities, arising from the hydrogen bonding net- work which improves material packing and to improved charge generation due to the formation of an extended Donor-Acceptor interface. Conversion efficiencies peaked at a modest 0.15 % due to the relatively wide optical gap of DPP along with unfavourable level alignment with respect to PCBM. Here we report DPP materials with thiophene rings substituted in place of Phenyl rings in order to reduce the optical gap and raise the HOMO level Fig. 1. Top: The latent pigment approach demonstrated on derivative of this molecule for increasing the spectral overlap to the DPP-Th . Bottom: The structure of latent pigment derivative DPP-Th 2 3 sunlight spectrum and for a more suitable alignment with PCBM to favour charge separation at the hetero-interface. rroles (DPP) are recognised to be particularly successful. Probably the best known DPP pigment at an industrial level is Pigment 254, a.k.a. Ferrari red [16]. While this well 2 Experimental known derivative has inappropriate HOMO/LUMO levels to be of practical value as an organic photovoltaic mate- rial, the chemistry of the DPP core has been extensively 2.1 Materials and methods developed giving access to highly conjugated derivatives with good performances both in OPV and OFET technolo- All reagents were used as delivered from Sigma Aldrich gies [17, 18]. It has been shown that in its parent pigment and TCI chemicals without further purification. ITO pat- form DPP crystals have improved charge transport and terned glass substrates were supplied by TFD Inc. with a mobilities over N-alkylated “dye” counterparts. This cor- work function of 4.8 eV. The thermal window for deprotec- responds to the activation of a H-bonding network result- tion was measured by specular X-ray diffraction and ther- ing in more efficient π-π packing [13]. These characteristics mogravimetric analysis. make DPP cores a perfect example for use with the latent For X-ray scattering experiments, thin films were pre- pigment technique. pared on silicon oxide wafers with 150 nm thermal silicon Recently, through our work to develop industrially as surface coverage. compatible methods for creating cheap, green chemistry X-ray reflectivity measurements were carried out on an compliant organic semiconductors, we have demonstrated PANalytical Empyrean system using a sealed copper tube, that the conjugation of simple DPP derivatives, bearing the the radiation was parallized and monochromatized with protecting t-BOC functionality, can be extended via a mod- an X-ray mirror so that a primary x-ray beam was formed ified Suzuki-Miyaura protocol in mild conditions in a wa- with a vertical height of 0.1 mm and CuKa radiation. Fit- ter/micelle media. This procedure lead to the straightfor- ting of the reflectivity data was performed with the soft- ward synthesis of derivatives DPP-Th and DPP-Th in high ware X’Pert Reflectivity. 2 3 yield in an ambient atmosphere. (Figure 1) [19, 20]. Grazing incidence X-ray diffraction (GIXD) measure- Previously we have employed the simple di-tert- ments were performed at the Diamond Light Source (Did- butyl3,6-di(biphenyl- 4-yl)-1,4-dioxopyrrolo[3,4-c]pyrrole- cot, UK), beamline I07 21. X-rays with a wavelength of 2,5(1H,4H)-dicarboxylate (DPP ) small molecule 1.00 Å were used at an incident angle of ~0.12 , slightly BOC derivative in the preparation of bulk heterojunction pho- below the critical angle of the substrate to reduce the back- 10 Ë Myles Rooney et al. , Diketopyrrolopyrrole latent pigment-based bilayer solar cells ground signal. Diffracted intensities were measured using at 5000 rpm for 45 seconds. LiF was evaporated as a 1 nm a Pilatus 2M area detector. Reciprocal space maps were cal- interlayer. Barium was thermally evaporated as an 8 nm culated from the measured data using the xrayutilities li- interlayer. The cells were closed with 90-100 nm of ther- brary for Python [22]. Extraction of peak positions and in- mally evaporated aluminium. Each substrate contains 4 dexation was performed using the custom-made software cells and 3 substrates were used per fabrication with a to- PyGID [23]. tal of 12 cells made in tandem per fabrication. The active area of each cell is 6.2 mm . BHJ cells were also created with DPP:PCBM (weight ratio 1:1) being spincast from chlo- Synthesis roform simultaneously. Thermal deprotection was carried DPP-Th and DPP-Th derivatives were prepared according out on half of the cells while the other half were tested ther- 2 3 to the literature procedure [19]. mal treatment/annealing free. The current density-voltage measurements were performed directly in glovebox where the solar cells were assembled, with a Keithley 2602 source Preparation of the PMMA slabs meter, under AM 1.5G solar simulation (ABET 2000). 100 ml of freshly distilled methyl methacrylate is added to 100 mg of the free radical initiator azobisisobutyroni- trile (AIBN). This mixture is stirred vigorously and heated 3 Results and Discussion slowly until just under the boiling point. The viscosity slowly changes. Heat until the solution becomes a viscous Derivatives DPP-Th and DPP-Th were selected by taking 2 3 syrup. Evolving bubbles should be slow to rise through the into account the two weaknesses of the previously em- solution. Quench this reaction in an ice bath to 20 C. In a ployed derivative DPP : HOMO energy levels lower than separate beaker dissolve 150 mg of lauryl peroxide and the that of PCBM (leading to faulty level alignment) and an −5 latent pigment of interest (1x10 M) in 60 ml of freshly dis- absorption spectrum covering only the high energy region tilled methyl methacrylate. This solution is added directly of the UV-Vis spectrum (cut off at 560 nm). Indeed, both to the partially polymerised syrup. Stir until homogeneous new materials feature the use of thiophene over benzene before casting into a square mould. Heat the mould in a thus ensuring higher conjugation through improved pla- water bath for 24 hours at 58 C. Remove the slab from the narity and increased π-electron density according to the mould and heat in an oven for a further 12 hours at 100 C π-excessive nature of thiophene. Derivative DPP-Th in to ensure complete polymerisation. The optical absorption particular was already employed, in its pristine protected of a series of PMMA slabs was measured using a Jasco v- form, to prepare BHJ devices with moderate to good con- 570 Uv-spectrometer before and after thermal cleavage in version efficiencies [24]. a heated oil bath. The HOMO levels of DPP-Th and DPP-Th were esti- 2 3 mated from the corresponding Cyclic voltammetry plots re- ported in g fi ure 2, which also shows the energy level di- 2.2 BHJ and bilayer device fabrication and agram of both DPP derivatives, alongside with the previ- characterisation ously employed DPP . The addition of Th and Th moi- H 2 3 eties to DPP backbone induces a reduction of pigments Bilayer devices were fabricated with the following proto- band gap and allows also a better energy level alignment col. ITO substrates underwent a standard sonication for with respect to PCBM. It should be noted that such energy 10 minutes subsequently in 2% mucasol solution, dis- levels refer to the protected, latent pigment form of the DPP tilled water, acetone and isopropanol. Substrates were derivatives. CV characterization of deprotected pigments then dried under a nitrogen flow before 10 minutes ozone is not possible due to their extremely low solubility in or- cleaning. PEDOT:PSS was then spincoated at 2000 rpm ganic solvents. The effect of protecting a strong electron for 60 seconds before being annealed at 110 C for 10 min- withdrawing group with the t-BOC functionality is the low- utes in a nitrogen glove box (45-50 nm). Active layer de- ering of both the HOMO. As such, the thermally activated position was then carried out with DPP-Th and DPP-Th 2 3 cleavage process should raise the HOMO levels, with ben- with 8.5 mg/mL solutions in chloroform spincoated at eficial impact on level offsets with respect to the PCBM ac- 4000 rpm for 45 seconds. The substrates were then placed ceptor. on a hot plate for deprotection at 200 C for 7 minutes. The impact on optical properties is much more diffi- The insolubility of the deprotected films allows PCBM to cult to predict. Pigments are known to be affected by ag- be also spincoated from a chloroform solution of 8 mg/ml Myles Rooney et al. , Diketopyrrolopyrrole latent pigment-based bilayer solar cells Ë 11 gregation both in solution and in the solid state. The most aggregation), according to procedures elucidated from Lu- striking example of such an effect is quinacridone whose minescent Solar Concentrator fabrication processes [8]. pigment form is deep purple while the corresponding la- Latent pigments are dissolved in a solution of PMMA tent pigment one is pale yellow [25]. in stabilizer free MMA. This viscous solution can be poured in a mould of suitable thickness (2 mm in our case). Poly- merisation at a temperature well below the cleavage of the t-BOC unit affords a solid solution of latent pigment in PMMA. These slabs are then heated to 100 C to com- plete the polymerisation of all unreacted monomers and can undergo a further annealing in oil baths to depro- tect the dispersed latent pigment within. The deprotection is performed on the unaggregated latent pigment within the high viscosity polymeric medium. Thus, we avoid the formation of aggregates as the pigments cannot appre- ciably diffuse. This strategy allows us to study the opti- cal properties of the unprotected pigments behaving as highly dispersed molecules. Returning to the example of quinacridone, the deprotection of its latent pigment af- fords a yellow slab while the deprotection of its thin film equivalent gives very hazy purple layers. PMMA slabs containing DPP-Th and DPP-Th were 2 3 compared via UV-Vis spectroscopy before and after the cleavage with thin films measured using the same thermal cycle. Fig. 2. Top: Cyclic voltammetry and structure of DPP-Th2 and Th3. Reversible redox steps evaluate HOMO levels to be −5.9 eV and 5.6 eV, LUMO levels of −4.01 eV and 3.9 eV respectively. Bottom: level diagrams for PCBM, DPPH, DPP-Th , DPP-Th . 2 3 Indeed, unexpected absorption shifts and significant lowering of extinction coefficient after thermal cleavage of the t-BOC moiety was recorded. It is expected that the sig- nificant change in film crystallinity through deprotection of the latent pigment results in aggregation, leading to a Fig. 3. Left) thin lm fi UV absorption of DPP materials (Black) Pro- shift in absorption wavelength, and an increase in light tected (Red) After deprotection. Right) PMMA Slabs with DPP mate- rials dispersed throughout. (Black) Protected (Red) After deprotec- scattering throughout the film. However, this effect could tion. also be intrinsic and due to the removal of the electron withdrawing t-BOC from the accepting residue of the DPP dyes. In order to be able to decouple the effects of aggre- The behaviour of the two compounds is remarkably gation from the single molecule effect of the cleavage, we different. In the case of DPP-Th the PMMA absorption resorted to the preparation of PMMA slabs incorporating spectra before and after the cleavage are similar both in DPP-Th and DPP-Th at very low concertation (to avoid 2 3 terms of optical gap and oscillator strength. The main dif- 12 Ë Myles Rooney et al. , Diketopyrrolopyrrole latent pigment-based bilayer solar cells ference is the structuring of vibronic replicas going from als DPP-Th and DPP-Th , respectively. No change in the 2 3 the protected dye to the pigment form. This effect is co- film roughness was observed for DPP-Th , a value of 2 nm herent with the expected planarization due to the removal was obtained by the X-ray reflectivity fit. For DPP-Th the of the bulky t-BOC group. The comparison with the corre- roughness increases from 1 nm to 3 nm due to the decou- sponding thin films clearly shows a remarkable tendency pling process. for aggregation, affecting both pristine and cleaved films. GIXD patterns of spin coated films before and after the In both cases a band peaking at 700 nm attributed to a J decoupling process are shown in Figure 4. The diffraction type aggregate is clearly visible and survives the cleavage patterns of the as-prepared films (Figure 4a and 4b) are process. The optical density of the film is significantly re- highly crystalline with strong preferred orientation of the duced upon cleavage. The latter effect could be due to the crystallites. In case of the molecule DPP-Th the diffraction formation of smaller crystallites, sizeably increasing the peaks could be indexed. Based on the solved crystal struc- scattering of incident light. Such an interpretation is also ture (Figure 5) the positions of Bragg peaks are calculated supported by the GIXD data discussed later which shows and plotted together with the diffraction pattern (Figure quite clearly the formation of very small crystals. 4a). A preferred orientation with the 001 planes parallel As for DPP-Th derivative, the behaviour is remarkably to the substrate surface is found. The agreement of calcu- different. The PMMA slab shows the familiar low energy lated and experimentally observed peak positions is very broad absorption band common to all DPP materials, in good (e.g. compare for 0-10 and 011 / 0-11). Based on these this case red-shifted due to the increased conjugation with results we can conclude that in case of DPP-Th , the single respect to DPP-Th . Upon cleavage, a substantial widen- crystal structure is present within the spin coated films. ing of the optical gap is observed with shifting of the ab- A drastic change in the diffraction pattern is observed sorption maximum from 600 nm to 510 nm. Such a shift after the decoupling process. The peaks are smeared more is connected with a sizeable reduction in the optical den- along Debye-Scherrer rings which means that the pre- sity. In this case, the planarization occurring as a conse- ferred orientation is less pronounced in comparison to the quence of the formation of intramolecular hydrogen bonds as-prepared films. The number of peaks is drastically re- does not counterbalance the loss of electron withdraw- duced, and the peak width is considerably broader. This ing capability of the DPP core upon removal of the t-BOC means that the crystalline order in the system is consid- residue. The thin film spectra show the same feature, along erably decreased which is either related to a considerably with a remarkable increase in the scattering background smaller crystallite size or by increasing disorder within the for cleaved samples, likely due to the same crystallization individual crystalline domains. In case of DPP-Th we ob- −1 −1 effect affecting derivative DPP-Th . serve Bragg peaks at q = 0.20 Å (d = 31 Å), q = 1.15 Å −1 Such an effect can also be studied directly on thin films (d = 5.46 Å) and q = 1.83 Å (d = 3.43 Å) (Fig. 4d). In case of −1 of DPP-Th and DPP-Th by X-ray reflectivity and GIXD be- DPP-Th we observe Bragg peaks at q = 0.33 Å (d = 19 Å) 2 3 2 −1 fore and after the thermal treatment. A strong change in and q = 1.83 Å (d = 3.43 Å) (Fig. 4c). the thin film morphology as well as in the crystalline prop- The interplanar distance of 3.43 Å is dominantly erties is observed by X-ray reflectivity and GIXD, respec- present in both samples. This distance is the character- tively. In detail, X-ray reflectivity investigations (see Sup- istic for π-π stacking distance of aromatic units. Interest- porting Information) were performed on spin coated films ingly, already the single crystal solution of DPP-Th reveals before and after the decoupling process. Fitting of the X- stacks of the aromatic units with the comparable stack- ray reflectivity curves reveal basic information on the thin ing distance of 3.47 Å. Comparing both diffraction peaks film morphology. The films have a film thickness of 40 nm of the two different types of decoupled films there is a fun- and 46 nm and show due to the decoupling process a re- damental difference in the stacking of the aromatic units. duction of the film thickness of 26% and 19% for the mate- In case of DPP-Th the π-π stacks are oriented parallel to rials DPP-Th and DPP-Th , respectively. The total electron the substrate surface, since the diffraction peak is located 2 3 densities of the film were determined by using the criti- around q = 0. While in case of DPP-Th the aromatic units xy cal angle of total external reflection and converted to mass are oriented perpendicular to the substrate surface, con- densities by the knowledge of the chemical composition. cluded from the presence of the diffraction peak located −3 A value of 0.405 Å was obtained for DPP-Th2 which is dominantly at q = 0. −3 3 smaller than 0.450 Å (mass density 1.430 g/cm ), the cal- culated electron density for the molecule DPP-Th2 based on its single crystal solution. The decoupling process en- hances the mass density of 4% and 10% for the materi- Myles Rooney et al. , Diketopyrrolopyrrole latent pigment-based bilayer solar cells Ë 13 when compared with benchmark g fi ures for organic de- vices it does represent an almost doubling of the PCE% of previous Latent Pigment materials. This improvement is attributed to frontier orbital alignment between donor and acceptor within this cell. Fig. 4. a) Indexed GIXD of thin lm fi of protected DPP-Th . b) GIXD of thin lm fi of deprotected DPP-Th . c) GIXD of thin lm fi of protected DPP-Th . d) GIXD of thin lm fi of deprotected DPP-Th . Thick weak 3 3 Bragg peaks seen as light blue bands indicate small crystallites with a random orientation in respect to the substrate. Diffraction plots have been converted into the scattering vector q space so that they are independent of wavelength of X-ray source. Fig. 6. Initial JV curve for ITO/PEDOT:PSS/ DPP-Th /PCBM/LiF/Al bilayer device. A short post deposition annealing of 140 C for 5 min- utes was carried out on closed ITO/PEDOT:PSS/DPP- Th /PCBM/LiF/Al cells. Results are shown in Table 1. This is a standard post deposition annealing proce- dure which improves the interface mixing of the layers and should not be confused with the thermal cleavage of the latent pigment, happening at 200 C prior to the deposi- tion of the acceptor layer and contacts closing the cell. This secondary annealing resulted in a noticeable reduc- tion in short circuit current. LiF provides an ohmic con- tact between semiconductor and metal contact and low- ers the work function of aluminium. Lithium has also been observed migrating into active semiconductor layers, usu- Fig. 5. Single crystal structure of DPP-Th was grown from slow evap- ally this lithium doping of the organic results in improved oration in chloroform. device performances [27]. Improvements to FF and Voc due to an optimised charge transfer across the interface is 3.1 Bilayer Heterojuction devices also commonly seen with the use of this interlayer [28–30]. However, LiF can also crystalize into heterogeneous plate Bilayer heterojuction devices based on derivatives DPP- islands on the surface of semiconductors resulting in non- Th and DPP-Th were prepared and thermally annealed 2 3 optimum interlayer properties. according to the protocol described in the materials and To investigate the possible effects of LiF dissipation method section. during annealing, a similar close cell thermal treatment Preliminary device results for DPP-Th show a moder- was also carried out using Ba rather than LiF. ate PCE of 0.31%. Although this result is not outstanding 14 Ë Myles Rooney et al. , Diketopyrrolopyrrole latent pigment-based bilayer solar cells Table 1. Results of the annealing process carried out on a ITO/PEDOT:PSS/ DPP-Th /PCBM/LiF/Al bilayer device at 140 C for 5 minutes on closed cell, i.e. after the LiF/Al electrode evaporation. The remeasured cell showed a reduction in current and lfi l factor. Sample Voc [V] FF [%] Jsc [mA/cm ] PCE [%] Not annealed (closed cell) 0.43 0.50667 1.44 0.31% 140 C annealed (closed cell) 0.45 0.38032 1.33 0.23% Table 2. ITO/ PEDOT:PSS/ DPP-Th2/ PCBM/ interlayer/Al bilayer device results. Average device data taken over 12 cells for each interlayer and annealing treatment. Voc [V] FF [%] PCE %] Jsc [mA/cm ] Barium 0.455 0.96491 0.431655 0.19 Barium annealed 0.441 1.254683 0.46415 0.2571 LiF 0.3975 1.4558 0.4987875 0.2882 LiF annealed 0.436 1.295613 0.3739425 0.2117 Interestingly the closed cell annealing in the case of isation shows evidence of poor charge generation and a devices fabricated with barium improves the device perfor- high level of thermal instability in terms of bulk morphol- mance contrasting with the behaviour of LiF. This may sug- ogy. This is most likely attributed to permeability of the gest that LiF is indeed forming non-uniform islands upon donor layers due to the formation of very small crystals annealing. upon thermal cleavage of the latent pigment form. Upon The further symmetrical extension of the DPP-Th core heating permeable channels are available for PCBM to mi- with a further hexyl thiophene to form the DPP-Th struc- grate through causing unbalanced, weakly performing de- ture was thought as a facile means to enhance the Jsc. vices which have a structure more similar to an intermixed Unfortunately, the desired Voc was not achieved when bilayer or bulk heterojunction device. The highest per- DPP-Th was fabricated into bilayer devices. The average forming devices for these materials was 0.33% PCE aver- short circuit current for devices fabricated with DPP-Th aged over 12 devices. This is double the power conversion increases but the Voc decreases in comparison to DPP-Th , efficiency previously produced by a bulk heterojunction this is consistent with the electronic modifications of the of DPP /PCBM. The improvements in performance is ac- materials. Film thicknesses was optimised by adapting the counted to alignment of HOMO and LUMO levels between spincoating speed for the deposition of the donor layer. By donor and acceptor. PMMA slabs containing highly dis- increasing the thickness of this active layer, we see a very persed latent pigments were created in order to examine small improvement in the fill factor of the devices (see ta- the optical properties of the latent pigments and the effects ble 3 below). Unlike DPP-Th further thermal annealing af- of agglomeration and crystalline properties on the absorp- ter deposition of PCBM at 140 C for 5 minutes does not ap- tive properties of these materials in thin films. A shift in the pear to have a substantial effect on device performance. absorption spectra is accounted for by the electron with- drawing nature of t-BOC on each of the materials exam- ined. It is established that the decrease in extinction coef- ficient seen during deprotection is greatly exaggerated for 4 Conclusion a thin film on a substrate compared to the highly dispersed materials trapped within the bulk PMMA. It is evident that A latent pigment strategy has been developed to fabricate less crystalline order and agglomerative tendencies of the planar bilayer organic photovoltaics. Two lead diketopy- deprotected films has an adverse effect on the absorption rrolopyrrole (DPP-Th and DPP-Th ) based semiconduc- 2 3 properties of these materials. High level of crystalline or- tors were chosen for a detailed study in both device char- der with low levels of material interconnectivity seems to acteristics and film quality. The small molecules in ques- work against the device performances. tion undergo a significant crystallographic rearrangement upon the thermal cleavage of solubilising groups. A sig- Acknowledgment: We thank Università degli Studi nificant reduction in crystal size is noted with a result- Milano-Bicocca (grant n 2016-ATESP-0047) and MIUR ing powder-like film. No crystal orientation is observed af- (Progetto Dipartimenti di Eccellenza 2017 “Materials for ter deprotection. The result is a disordered and more than energy”) for financial support. L.B. and M.R. thank the likely permeable layer of material. The device character- Myles Rooney et al. , Diketopyrrolopyrrole latent pigment-based bilayer solar cells Ë 15 Table 3. Device characteristics for ITO/PEDOT:PSS/ DPP-Th /PCBM/LiF/Al cells. All data is averaged over 3 substrates containing 4 cells each. 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Organic Photonics and Photovoltaicsde Gruyter

Published: Dec 1, 2018

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