Organic solar cells based on acceptor-functionalized diketopyrrolopyrrole derivatives

Abstract. The synthesis and characterization of three solution processable diketopyrrolopyrrole (DPP) derivatives featuring acceptor units attached to the core by alkyne linker units is reported. Cyclic voltammetry and density functional theory calculations indicate that the DPP derivatives possess similar HOMO and LUMO energies. Solar cells were fabricated by blending the synthesized DPP derivatives with [6,6]-phenyl-C71-butyrate methyl ester. The influence of donor:acceptor blend ratio, film thickness, annealing temperature, and annealing time on device performance was studied. Differences in device performance were related to atomic force microscopy measurements of the films. The highest power conversion efficiency of 1.76% was achieved for the DPP derivative functionalized with an aldehyde electron-withdrawing group with a 1∶0.7 donor:acceptor ratio when the active layer was annealed for 10 min at 110°C.


Introduction
Solution-processed organic solar cells based upon bulk heterojunction (BHJ) architectures continue to attract significant interest due to their potential for low-cost fabrication by simple printing techniques and the ability to produce flexible and lightweight devices. 1 Significant progress was achieved in the development of BHJs featuring reasonable power conversion efficiencies (PCEs) from conjugated polymers as the donor component and methyl [6,6]-phenyl-C 71 -butyrate methyl ester (PC 71 BM) as the acceptor. 2 However, conjugated polymers have some important drawbacks, including limited batch-to-batch reproducibility and laborious purification, which limit their large-scale synthesis. BHJs featuring small molecule donors have emerged as important alternatives to conjugated polymer-based solar cells due to their better synthetic reproducibility, well-defined structure, and ability to be produced with high purity. [3][4][5][6][7][8] Significant progress has recently been made in the reported power conversion efficiencies of small molecule-based devices, which has resulted in systems of this type becoming viable alternatives for fabricating active layers within BHJs.
In this article, we describe the synthesis and characterization of three small molecule diketopyrrolopyrrole (DPP) derivatives (DPP-BZ, DPP-ES, and DPP-AM) and their subsequent fabrication as BHJ devices with PC 71 BM. DPP was selected as the core moiety for this *Address all correspondence to: Graeme Cooke, E-mail: Graeme.Cooke@glasgow.ac.uk and Ifor D. W. Samuel, E-mail: idws@ standrews.ac.uk † Authors contributed equally. study in view of its convenient synthesis, excellent light absorption properties, good photochemical/thermal stability, and excellent charge carrier mobility. [9][10][11][12] We have further functionalized the DPP core with electron-withdrawing aldehyde (DPP-BZ), cyanoacrylic ester (DPP-ES), and cyanoacrylic amide (DPP-AM) units ( Fig. 1) in an effort to tune the optical and redox properties of the DPP core. Furthermore, for DPP-AM and DPP-ES, we have included 2-ethylhexyl side chains to promote good solubility of these derivatives and, in the case of DPP-AM, an amide unit that could have the propensity to participate in intermolecular hydrogen bonding.

Synthesis
All melting points are uncorrected. The NMR spectra were recorded on a Bruker AVIII 500 MHz spectrometer as CDCl 3 solutions. The 1 H and 13 C spectra were recorded at 500 and 125 MHz, respectively, with either residual nondeuterated chloroform [δ 7.26 ( 1 H) and δ 77.0 ( 13 C)] or tetramethylsilane as the internal standard. Mass spectrometry measurements electrospray ionisation (ESI) were undertaken using a Bruker microTOFq high-resolution mass spectrometer. Reactions were performed under an atmosphere of nitrogen (oxygen-free) using oven-dried glassware (dried for 24 h at 100°C). Dry solvents were obtained using the Pure-Solv™ Solvent Purification System (Innovative Technology). 2-ethylhexyl-2-cyanoacetate, 13 N-(2-ethylhexyl)-2-cyanoacetamide, 14 and DPP-Br 2 15 were synthesized according to literature procedures.

Electrochemistry
Cyclic voltammetry (CV) experiments were performed using a CH Instruments 440A electrochemical workstation. All measurements were undertaken under an oxygen-free nitrogen atmosphere. A solution of electrochemical grade tetrabutylammonium hexafluorophosphate dissolved in dry DCM (0.1 M) was employed as the supporting electrolyte. A platinum disc working electrode, a platinum wire counter electrode, and a silver wire pseudo-reference electrode were used for all measurements. The half-wave potentials were referenced to ferrocene (Fc) (internal reference) with the Fc/Fc+ redox couple adjusted to 0 V. The HOMO and LUMO energies were estimated from the CV derived half-wave potentials of the first oxidation and reduction waves, respectively, using the following equations: where −4.8 eV is the E ðHOMOÞ for Fc against the vacuum. 16

Density functional theory calculations
Calculations were performed using the Spartan '08 software suite. 17 Molecular geometries were initially optimized semi-empirically (AM1) and then reoptimized using density functional theory (DFT) (B3LYP/6-31G*). The optimized structures were shown to be local minima as the vibrational frequencies in the optimized geometries did not generate imaginary frequencies.
To facilitate the convergence of the geometry optimizations, the 2-ethylhexyl side chains of the DPP derivatives were replaced by methyl units.

Solar cell fabrication and characterization
Solar cells were fabricated on patterned indium tin oxide coated glass substrates purchased from Xinyan Technology Ltd. (10 to 13 ohm∕sq). The substrates were cleaned by ultra-sonication in acetone and isopropyl alcohol successively for 10 min each. The dried substrates were then treated with oxygen plasma for 3 min. The hole conducting poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) layer (CLEVIOS™ P VP AI 4083) was then spin-coated at 4000 rpm to obtain films of 30 nm thickness and was annealed on a hot plate at 140°C for 10 min to remove the residual water. The dried substrates were then transferred into a nitrogen glove box to deposit the active layer made from a blend of the DPP material with PC 71 BM (99% purity from Solenne) spin-coated at 1000 rpm from the chloroform solution. These were then annealed at 110°C for 10 min. The solution concentration was varied to provide films of different thicknesses. Thereafter, a 15 nm calcium layer followed by a 100 nm aluminum layer were evaporated at a pressure of 2 × 10 −6 mbar. Active areas of devices (2 × 4 mm 2 ) were defined by a shadow mask. The devices were then encapsulated by using UV-curable epoxy to glue a glass slide to the top of the device. The current-voltage (J-V) characteristics were measured in ambient conditions under illumination at 100 mW∕cm 2 intensity by using a Keithley 2400 source measure unit and a Science-Tech, SS150 solar simulator. The light intensity at the test surface was measured using a Newport Oriel reference cell. For J-V measurement of the solar cells, a shadow mask equal to the size of the test pixel was used to avoid the generation of charge carriers by photons absorbed outside the cell area. Absorption spectra were recorded by using a Cary 300 spectrophotometer. Atomic force microscopy (AFM) images were recorded by using a Veeco diCaliber instrument in the tapping mode. All the film thicknesses were measured using a Dektak 150 surface profiler. External quantum efficiency (EQE) measurement was performed with an incident photon to charge carrier efficiency setup, which consists of a xenon lamp, a photodiode calibrated by the National Physical Laboratory (UK), Keithley 6517A picoammeter, and a TMc300 monochromator.

Results and Discussion
The synthesis of DPP-BZ, DPP-ES, and DPP-AM from DPP-Br 2 is outlined in Fig. 2. To help promote planarity of the derivatives, the electron-withdrawing moieties [18][19][20][21][22] were introduced using Sonogashira-coupling reactions of DPP-Br 2 and 4-ethynylbenzaldehyde. 23 Further, Knoevenagel condensation of DPP-BZ with 2-ethylhexyl-2-cyanoacetate and N-(2-ethylhexyl)-2-cyanoacetamide afforded DPP-ES and DPP-AM, respectively. CV was used to estimate the HOMO and LUMO energies of the DPP derivatives from the half-wave potentials of their redox waves ( Table 1). The CV of DPP-BZ is provided in Fig. 3. The electrochemical data reveal similar oxidation and reduction potentials for all three materials, indicating that the differing groups at the termini of the molecules play a minimal role in modulating the ionization potential and electron affinity of these derivatives.
DFT calculations performed on the DPP derivatives indicated a planar structure for all three derivatives, and the HOMO/LUMO energy levels are shown in Table 1. The HOMO/LUMO distributions are shown in the contour maps provided in Fig. 4. For all derivatives, the HOMO is mainly localized on the DPP core and the directly attached alkyne residue. The LUMO maps are also similar for each derivative, and partially extend onto the electron-withdrawing aldehyde moieties of DPP-BZ and the cyano residues of DPP-ES and DPP-AM. The calculated LUMO/HOMO energy values are in good agreement with the CV-estimated values, further indicating that the differing electron-withdrawing units had surprisingly little effect in perturbing the electronic properties of these systems ( Table 1). The optical band gaps (E opt ) obtained from solution UV-Vis spectra of the DPP derivatives are close to the HOMO-LUMO gaps provided by DFT and electrochemistry (∼1.8 eV).
The absorption spectra of the materials in thin films are shown in Fig. 5. They show an onset around 750 nm and peaks in the range of 600 to 700 nm. The absorption peaks of the films are   redshifted by ∼40 nm compared to that in solution (data not shown). This is consistent with a more rigid and ordered structure in the solid state. 24,25 In addition to this, significant broadening of the absorption in the region of 300 to 450 nm was also observed. The absorption of DPP-ES and DPP-AM is slightly redshifted compared to DPP-BZ largely due to the presence of the CN group in DPP-ES and DPP-AM. Solar cells containing each of the DPP materials as the electron donor blended with PC 71 BM as the electron acceptor were fabricated to evaluate the photovoltaic properties of the materials. The J-V characteristics of the devices using DPP-ES:PC 71 BM and DPP-AM:PC 71 BM blends as the photoactive layer [1∶0.7 donor:acceptor (D:A) ratio and 83 nm thick] are shown in Fig. 6. Both the devices show poor performance with low short circuit current density (J sc ) and, hence, very low overall PCE of 0.08 to 0.10%.
The poor device performance was investigated by recording AFM images. Figure 7 shows that both active layers have similar morphologies with domains around 150 to 250 nm in diameter. Smaller aggregates are desirable allowing increased interfacial area and better charge separation. 26 This, in general, results in an increase in J sc and hence an enhanced overall device performance. 27 The domain sizes in the present study are much larger than the optimal required for efficient exciton dissociation. This limits the overall performance of these devices. In addition, there was substantial surface roughness of the blend films: the RMS roughness values were  18.6 nm for DPP-ES:PC 71 BM and 19.6 nm for DPP-AM:PC 71 BM in as coated films. Upon annealing, the roughness does not significantly change. This roughness is undesirably high as it is known that smooth active layers are essential for optimum device performance. 28 A possible reason for the larger than desirable phase separation could be the intramolecular dipoles due to the cyano units. Dipole-dipole interactions could facilitate aggregation and discourage mixing with the acceptor. 29 Figure 8 shows the J-V characteristics of the devices using a DPP-BZ:PC 71 BM blend as the photoactive layer under 1 sun illumination. The effect of D:A blend ratios on device performance was investigated for films of thickness 83 nm and the results are shown in Table 2. As the acceptor proportion decreases, the highest PCE was obtained for a 1∶0.7 D:A ratio annealed at 110°C for 10 min. With a 1∶2 D:A ratio, an overall PCE of 0.73% was obtained with a short-circuit current density of 2.45 mA∕cm 2 , an open circuit voltage (V oc ) of 0.79 V, and a fill factor (FF) of 37.4%. Further reductions in PC 71 BM content reduces the performance. With a 1:0.6 blend, a PCE of 1.44% was obtained.
AFM measurements were performed to investigate the effect of the D:A ratio on the blend film morphology (Fig. 9). Blend films with a 1∶2 D:A ratio show aggregates of size 150 to 250 nm in diameter. Upon reducing the content of PC 71 BM in the blend films, the domain size decreases considerably. It is, therefore, clear that with reduced PC 71 BM content, the donor and acceptor are more homogeneously mixed. This is crucial for efficient charge separation in BHJ devices. 30 Upon thermal annealing, the pore size and pore density at the film surface decreased. Thermal annealing also helps in the removal of air voids formed due to inherent instability involved in the spin-coating process. For a 1∶0.7 D:A ratio, the homogeneity of     the film is considerably enhanced and the RMS roughness reduces considerably upon thermal annealing. For the as coated film, the RMS roughness is 7.4 nm compared to 4.9 nm after thermal annealing. A significant change in the height profile upon thermal annealing [ Fig. 9(g)] shows enhanced homogeneity and improved surface smoothness of the blend films due to thermal annealing. The reverse saturation current is a measure of the number of carriers that can overcome the energetic barrier created by the p-n junction in the reverse bias condition. 31 In the device with a 1∶0.7 D:A, the rectification ratio improved by a factor of 29 compared to that of a 1∶2 D:A ratio device at AE1 V. This is an indication of the large reduction in reverse saturation current in the devices with lower PC 71 BM content. For further reduced ratios of PC 71 BM, the films were smoother, but the solar cell performance decreased. The effect of annealing conditions was also explored. Devices annealed for 5 and 15 min at 110°C (1:0.7 D:A ratio) showed reduced performance mainly due to a decrease in J sc . Upon annealing for 5 and 15 min, the J sc was 4.0 mA∕cm 2 and 3.9 mA∕cm 2 , respectively, which is ∼10% lower than when annealed for 10 min. The PCEs for devices where the active layer was annealed for 5 and 10 min were 1.55 and 1.44%, respectively. Thermal annealing mainly affects the donor and acceptor crystallinity and their phase separation. Annealing at a particular temperature and for a specific time gives optimum phase separation and crystallinity and, hence, improved device performance. Devices annealed at higher temperatures also showed reduced J sc and, hence, lower PCE. Figure 10 shows the absorption spectra of the blend films of various D:A ratios before and after thermal annealing at 110°C for 10 min. For the 1:0.7 ratio of DPP-BZ and PC 71 BM, the J sc and PCE obtained was 0.99 mA∕cm 2 and 0.28% for the as coated photoactive layers. Upon thermal annealing at 110°C for 10 min, these improved to 4.42 mA∕cm 2 and 1.76%, respectively. The shoulders/peaks corresponding to DPP-BZ absorption become prominent/sharp upon thermal annealing, which indicates improvement in the crystalline order in DPP. This, in turn, improves the charge carrier mobility in the donor phase. The absorption of the films in the visible region also increases. The AFM topography images indicate that the blend films become more compact and homogeneous upon thermal annealing. The above changes in the film properties upon thermal annealing result in large improvements in J sc and FF and, hence, also in the PCE. 32,33 The inset in Fig. 8 shows the EQE of the best performing device. The quantum efficiency reaches a maximum of 28.2% at 615 nm. The DPP absorption in blend films is slightly blueshifted compared to that of pristine DPP films. The blueshift is more pronounced in films with a 1:2 D:A ratio compared to 1:0.7 D:A ratio blend films. It has been suggested that fullerene acceptors in the blend films distort the molecular packing of the donor polymer/small molecule and, thereby, decrease the effective conjugation length. 24,34 This effect would be larger for higher fullerene concentrations. Another important parameter is the device thickness because it affects light absorption and charge extraction. The device performance parameters with different thicknesses of active layer for 1:0.7 D:A ratio are given in Table 3. Thermal annealing was performed at 110°C for 10 min. The best device efficiency was obtained for a 83-nm-thick active layer. For higher thicknesses, the J sc and overall PCE decrease. The series resistance of the solar cell is contributed by three factors viz. the contact resistances between the different layers in the device, resistance of the top and rear metal contacts, and the resistance of the layers in the device including the BHJ active layer. 35 The series resistance increases with increasing active layer thickness. For an active layer thickness of 83 nm, the series resistance was 8 Ωcm 2 , which increases to 18 Ωcm 2 for a 101 nm thickness and 40 Ωcm 2 for a 118-nm-thick active layer device. High series resistance reduces the current flow and also decreases the FF. Once the charges are separated at the D-A interface, the electric field created between the electrodes of different work functions sweeps these charges to the respective electrodes. 36 For larger thicknesses of the active layer, this field decreases, which, in turn, limits the charge transport toward the respective electrodes.

Conclusions
In conclusion, three DPP derivatives featuring electron-withdrawing units have been synthesized in respectable yields. Solution electrochemistry and DFT calculations have shown that these materials possess similar HOMO/LUMO energies and molecular orbital distributions. Solar cells have been fabricated with PC 71 BM, which revealed that DPP-BZ gave the highest power conversion efficiency, while DPP-ES and DPP-AM devices had poor active layer morphology and hence low efficiency. The differences in device performance of the different materials can be explained by differences in the morphology seen in AFM measurements. postdoctoral research fellow. Currently, he is working as an assistant professor in the Department of Physics, North Maharashtra University, India.
Luis A. Serrano received his BSc honors degree in chemistry from the University of Extremadura and the University of the West of Scotland in 2011. He is currently completing his PhD on the subject of synthesis of redox-active materials with photoelectronic properties in the group of Professor Graeme Cooke.
Bernd Ebenhoch is currently completing his PhD on the subject of organic solar cells-novel materials, charge transport, and plasmonic studies, in the group of Professor Ifor Samuel.
Vincent M. Rotello is the Goessmann professor of chemistry and university distinguished professor at the University of Massachusetts-Amherst. He joined UMass in 1993, and received the NSF CAREER, Cottrell Scholar, and Camille Dreyfus Teacher-Scholar awards, the Sloan Fellowship, and the Langmuir Lectureship, and is a fellow of the AAAS and the Royal Society of Chemistry (United Kingdom). His research program focuses on engineering the interface between hard and soft materials, with >430 papers published to date.
Graeme Cooke is a professor of physical organic chemistry and head of nanoscience and materials chemistry within the School of Chemistry at the University of Glasgow. He is currently director of the Scottish Institute for Solar Energy Research (SISER). His research focuses on the design, synthesis, characterization, and self-assembly of functional molecules, polymers, and nanoparticles. He has published >120 papers to date.