Article pubs.acs.org/JPCC
Investigation of Phase Separation in Bulk Heterojunction Solar Cells via Supramolecular Chemistry Raja Bhaskar Kanth Siram, Meera Stephen, Farman Ali, and Satish Patil* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India S Supporting Information *
ABSTRACT: In this work, we have prepared two donor−acceptor−donor (D-A-D) πconjugated oligomers to investigate the effect of phase separation on the performance of bulk heterojunction (BHJ) solar cells. These charge transfer low band gap π-conjugated oligomers (TTB and NMeTTB) were synthesized by Knoevenagel condensation of terthiophenecarbaldehyde and barbiturate appended pyran derivative. The thin film morphology of both the oligomers and along with electron acceptor [6,6]-phenyl-C60butyric acid methyl ester (PC61BM) was investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The blend of NMeTTB and PC61BM thin film yield highly ordered thin film, whereas there was clear phase separation between TTB and PC61BM in thin film.The BHJ solar cell was fabricated using a blend of NMeTTB and TTB with PC61BM acceptor in 1:1 ratio as active layer, and a power conversion efficiency of 1.8% was obtained. This device characteristic was compared with device having TTB:PC61BM as active layer, and large difference is observed in photocurrents. This poor performance of TTB in BHJ devices was attributed to the difference in the nanoscale morphology of the corresponding derivatives. We rationalize our findings based on the low charge carrier mobility in organic field-effect transistors and miscibility/phase separation parameter of binary components (oligomers and PC61BM) in the active layer of bulk heterojunction solar cells.
1. INTRODUCTION In recent years, organic photovoltaics (OPV) is proving to be a promising technology to harvest solar energy.1 OPV has number of advantages over conventional silicon-based solar cells such as low cost, ease of processability, and flexibility.2−4 The high power conversion efficiency of OPVs is established based on the concept of bulk heterojunction (BHJ) in which the active layer is composed of physical mixture of donor and acceptor.5 Both small molecules6−11 and conjugated polymers12−20 have been used as p-type donor materials, and fullerene derivatives used as an acceptor in BHJ cells. The efficiency of an organic solar cell (OSC) critically depends on the dissociation of exciton into free charge carriers at the interface and efficient collection of free charge carriers at respective electrodes. After exciton formation, it is desirable to have a donor−acceptor (D/A) interface within the exciton diffusion length to facilitate exciton dissociation and then charge separation. These free charges require percolating pathways to reach the respective electrodes. In case of semiconducting polymers, the exciton diffusion length is in the order of 10−20 nm,21 and hence, the efficiency of charge separation in BHJ solar cells is significantly influenced by length scales of phase separation. However, phase separation in the active layer of BHJ is not limited to 20 nm due to inherent thermodynamic instability of the blend phase. The differences in crystallization tendency of the fullerene derivative and conjugated polymer drive them to form macroscopic phase separated structures larger than 20 nm. The unoptimized morphology accounts for low power conversion efficiency © 2013 American Chemical Society
(PCE) in many novel semiconducting polymers with near ideal molecular orbital energies. The film morphology can be finetuned by controlling phase separation during the film formation, and various strategies such as introduction of additives, post annealing, and so forth have been employed in this regard.22 The other strategy which can be employed to tune the microstructure in BHJ is via supramolecular chemistry.23 This approach requires complementary functional groups which can undergo hydrogen bonding and form microstructures. In this work, we intend to study phase separation in a donor/acceptor blend via supramolecular chemistry approach. We prepared two oligomers (TTB and NMeTTB) by coupling donor−acceptor−donor (D-A-D) by vinylene spacer. TTB has a functional group which undergoes self-assembly by hydrogen bonding, whereas we disrupted the self-assembly by introducing a methyl group in NMeTTB. We chose barbiturate as a functional group for intermolecular hydrogen bonding. Barbiturate with its acidic proton (N−H) and electronegative oxygen (CO) atoms presents itself as a suitable candidate to study the H-bonding effect on the microstructure and PCE in BHJ. Moreover, barbiturate has high electron affinity and can be coupled to electron rich donor moieties to prepare low band gap materials. Targeting on this aspect and exploiting the donor− acceptor−donor (D-A-D) concept to manipulate the electronic Received: February 12, 2013 Revised: April 10, 2013 Published: April 29, 2013 9129
dx.doi.org/10.1021/jp401523u | J. Phys. Chem. C 2013, 117, 9129−9136
The Journal of Physical Chemistry C
Article
Figure 1. Molecular structure of the D-A-D derivatives TTB and NMeTTB.
Powder X-ray diffraction studies were carried out by using a Philips X’pert X-ray diffractometer. The sample was prepared by spin coating the solution of TTB and NMeTTB in chloroform and dried at 50 °C for about 15 min. Atomic force microscopy (AFM) was carried out in tapping mode using a Bruker5500 atomic force microscope. Films were prepared on the silicon substrate by spin coating from 10 mg/ mL solutions of the derivatives in chloroform and dried at 50 °C for 10 min. For transmission electron microscopy (TEM) studies, the thin film was deposited on PEDOT:PSS coated ITO substrates in the same manner as solar cells are fabricated. After drying the film at 50 °C for about 10 min, the substrate was dipped into DI water and the films were floated onto the DI water/air interface. The films were transferred to carbon coated copper TEM grids with 300 mesh. TEM characterization was performed on a JEOL 2100F instrument operating at 200 kV. Organic Field-Effect Transistor (OFET) Fabrication. OFETs were fabricated on cleaned glass substrates. Thermally evaporated aluminum (40 nm) and spin-casted PVDF (800 rpm, 60 s + 150 °C for 2 h) acted as gate electrode and dielectric layer (Cox= 15 nF/cm2), respectively. Dielectric layer was treated with HMDS and heated at 110 °C for 2 h in N2 atmosphere. NMeTTB was spin coated (1000 rpm for 60 s) on to HMDS treated PVDF layer from 20 mg/mL solution in chlorobenzene. Gold source and drain contacts were evaporated on to the organic semiconductor at ∼10−6 mbar pressure. Measurements were performed using Keithley 2400 source meters and a 6514 electrometer. Solar Cell Device Fabrication. ITO/glass substrates were ultrasonically cleaned sequentially in detergent, water, acetone, and isopropyl alcohol. Then, the substrates were covered by 30 nm PEDOT:PSS (Clevios 4083 provided by H. C. Stark) and annealed in air at 150 °C for about 15 min. The active layer was deposited by spin coating of a blend of D-A-D derivative and PC61BM in 1:1 ratio for about 150 nm. The active layer was dried at 50 °C for 10 min, and the cathode made of calcium (20 nm) and aluminium (100 nm) was evaporated through a shadow mask under high vacuum (
