3.1 Experimental system
3.1.1 Working principle of blade coating
Fig. 3-1-a shows the schematic working principle of blade coating. The thickness of the wet polymer film is defined by the gap. The polymer dry film thickness is tuned by the polymer concentration in solution and the gap of the blade coater. The polymer wet film is deposited by dragging the blade coater at a certain speed about 15 cm/s.
Fig. 3-1-a Schematic working principle of blade coating
3.1.2 Device structures
The development of the organic solar cells is to develop multilayer device structure and the bulk hetero-junction devices. Considering all of the development processes, the main idea is to increase the separation region for excitons. In this study, we used the bulk hetero-junction structure with single layer. The bulk hetero-junction device is made by the P3HT donor and PCBM acceptor blending. The device structure is designed for ITO/PEDOT:PSS/P3HT:PCBM/Ca/Al shown in Fig. 3-1-b (a). ITO is the anode and PEDOT:PSS (Baytron PVP AI 4083) is the hole transport layer. The active layer is used the
Glass substrate Polymer wet film
Blade coater
Gap
Fast moving Polymer solution
P3HT and PCBM blending in organic solvent, and the cathode Ca/Al is used. The energy band for the structure is shown in Fig. 3-1-b (b).
Fig. 3-1-b Single layer structure of bulk hetero- junction cells (a) device structure (b) energy band
3.1.3 Material introduction
A. Hole transport layer
For Fig. 3-1-c, the hole transport layer is poly-(3,4-ethylenedioxythiophene):
poly-(styrenesulfonate) (PEDOT:PSS) with LUMO of 3.6 eV and HOMO of 5.2 eV. The PEDOT is an aqueous solution of polymer. It is not only for the hole transport but also for flattening the ITO substrate. In addition, the PEDOT:PSS layer conforms with the advantage of the light transmission. According to the different ratio between PEDOT and PSS has the different types and resistance. As the type of AI4083 is 1:6 ratio for PEDOT:PSS and the volume resistivity is about 500-5000 ohm-cm.
PEDOT:PSS
Fig. 3-1-c Molecular structures of PEDOT and PSS
B. Active layer
The active layer is used the P3HT and PCBM blending in organic solvent. The poly(3-hexylthiophene) (P3HT) with LUMO of 2.8 eV and LUMO of 4.8 eV is a common donor of polymer material that with the absorption wavelength among 500nm~600nm. In general, the electron mobility (μe= 10-4~10-9 cm2/V-s) is lower than the hole mobility (μh= 10-1~10-7 cm2/V-s) for polymers. Therefore the better donor material is chosen with the higher hole mobility. P3HT is a better hole mobility among the conjugate polymer. It had been measured the hole mobility about 10-1 cm2/V-s [33]. Besides, the (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) with LUMO of 3.8 eV and HOMO of 6.2 eV is a common acceptor of small molecular material. It is a derivative for C60that has the higher electron transport ability and the absorption wavelength is about 350nm. But the PCBM could not be easily dissolved in organic solvents by itself that needs some other polymer material as guest to increase solubility.
Fig. 3-1-d The monomer structure of P3HT
Fig. 3-1-e The molecule structure of PCBM
3.2 Experimental process and measurement
3.2.1 Deposition of the hole transport layer
The patterning ITO coated glass substrates were cleaned by ultrasonic treatment with organic solvent, detersive and deionizer water successively. After removing the smirch on the substrate, the cleaned substrates were baked on hot plate with 100 ℃ for several minute. Then ITO substrates are treated by UVO3 with 20 minutes for the formation a hydrophilic surface. PEDOT:PSS (Baytron PVP AI 4083) film with the thickness about 40 nm as hole transport layer was spin coated in air and then baked at 200 ℃ for 15 minutes in nitrogenous glove box.
3.2.2 Deposition of the active layer
The blend of P3HT:PCBM with 1:1 ratio formulated 17 mg/ml respectively dissolved in toluene, chlorobenzene (CB), and dichlorobenzene (DCB). The solutions were heated at 60 ℃ for 12 hours at least. We first focus on solar cells with P3HT:PCBM blend dissolved in toluene. Device performances are compared for different coating methods including spin coating, blade coating, blade coating on a hot plate, as well as blade and spin coating. For the blade and spin coating the polymer wet film is deposited by blade coating which is followed by spinning until the dry film is formed.
Seven series of devices with six devices in each are made to study the different coating processes with different solvents statistically. Among them, five series are made with toluene solution to compare the coating processes, including spin coating (series A), blade coating (series B), blade coating on a hot plate at 60℃ (series C), as well as blade and spin coating (series D). In addition, the devices with the PEDOT:PSS layer by blade coating at 100℃ and P3HT:PCBM by blade and spin coating from toluene solution is made to study
the feasibility of bladed PEDOT:PSS layer (series G). The other two series made from two conventional high boiling point solvents, chlorobenzene (series E) and dichlorobenzene (series F), are compared with those from toluene by blade and spin coating. After coating all the P3HT:PCBM layers are annealed at 140℃ for 20 minutes in nitrogen.
The Ca(35nm)/Al(100nm) cathode was deposited by thermal evaporation. The active area of the device was 0.04 cm2. All the devices were packaged in the glove box and measured in the ambient environment.
3.3 Measurement
3.3.1 Photovoltaic property
The PCE is measured by the solar simulator (PEC-L11, Peccell Technologies) under AM1.5G irradiation.
3.3.2 External quantum efficiency
The incident photon-to-electron conversion efficiency (IPCE) is measured by the spectral response measurement system (SR300, Optosolar GMBH).
3.3.3 Morphology
The morphology of P3HT:PCBM is monitored by atomic force microscope (AFM, Dimension 3100, Digital Instruments).