# Solution-Processed Bulk Heterojunction Solar Cells with Silyl End-Capped Sexithiophene

Authors: Jung Hei Choi; Mohamed E. El-Khouly; Taehee Kim; Youn-Su Kim; Ung Chan Yoon; Shunichi Fukuzumi; Kyungkon Kim
Journal: International Journal of Photoenergy
 (2013)

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## Abstract

We fabricated solution-processed organic photovoltaic cells (OPVs) using substituted two sexithiophenes, a,w-bis(dimethyl-n-octylsilyl)sexithiophene (DSi-6T) and a,w-dihexylsexithiophene (DH-6T), as electron donors, and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor. Solution-processed OPVs using DH-6T and DSi-6T showed good photovoltaic properties in spite of their poor solubility. The best performance was observed on DSi-6T : PCBM 1 : 5 (w/w) blend cell with an open circuit voltage (Voc) of 0.63 V, short circuit current density (Jsc) of 1.34 mA/cm2, fill factor (FF) of 55%, and power conversion efficiency of 0.44% under AM 1.5 G illumination. Although DH-6T has higher hole mobility than DSi-6T, the DSi-6T : PCBM blend cell showed higher hole mobility than DH-6T : PCBM cell. Therefore, DSi-6T cell showed higher device performance than DH-6T cell due to its silyl substitutions, which lead to the increase of the solubility. The incorporation of solution-processed TiO2 interfacial layer in the DSi-6T : PCBM devices significantly enhances FF due to the reduced charge recombination near active layer/Al interface.


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## Body

## 1. Introduction

Organic solar cells have received strong attention due to the possibility to realize flexible and low-cost large photovoltaic cells [1–5]. Polymer based organic photovoltaic cells (OPVs) demonstrated power conversion efficiencies (PCEs) in excess of 7% by the results of synthesis of low bandgap polymers [6, 7]. However, the polymers still have high batch-to-batch variations and difficulty in purification, and there have been many efforts to discover solution-processed small molecules. Recently, oligothiophenes have received much attention as donor materials in OPVs because of their high hole mobility, easy multigram synthesis, high purity, and simple chemical modification [8–10]. Some studies on oligothiophene based solar cells have already been reported using bilayer heterojunction solar cells [11–15]. By controlling the film morphology using coevaporation of excess fullerene (C60), bulk heterojunction organic photovoltaic cells consisting of sexithiophene (6T) as a donor and C60 as an acceptor showed good photovoltaic properties [16]. However, the small molecules based on sexithiophene have not been investigated solution-processed OPVs. The low performance of sexithiophene-based solar cells was mainly attributed to the difficulties encountered in fabricating solution-processed bulk heterojunction cells.We chooseα,ω-dihexylsexithiophene (DH-6T) as the oligothiophene material because it is well known to exhibit high field-effect mobility as high as 1.0 cm2/Vs and a representative oligothiophene material [17]. Recently, we reported α,ω-bis(dimethyl-n-octylsilyl)sexithiophene (DSi-6T) for vacuum-deposited or solution processable organic thin film transistor application [18]. The silyl end-capped sexithiophene (DSi-6T, 2.2 g/L in CHCl3) was more soluble than hexyl substituted sexithiophene (DH-6T, 1 g/L in CHCl3) [19]. Thus, the silyl end-capped sexithiophene may be a good candidate for solution processable solar cell application.In regard to our continuing interest for sexithiophene-based organic solar cells, we report herein analysis of the performance of solution-processed organic solar cells based onα,ω-functionalized linear sexithiophene: PCBM systems. We fabricated the solution-processed OPVs using two linear sexithiophenes, α,ω-dihexylsexithiophene (DH-6T) and α,ω-bis(dimethyl-n-octylsilyl)sexithiophene (DSi-6T) as donors and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an acceptor (Figure 1). Solution-processed bulk heterojunction (BHJ) solar cells using DH-6T and DSi-6T with PCBM showed good photovoltaic properties in spite of their poor solubility. Although DH-6T has higher hole mobility than DSi-6T, the DSi-6T : PCBM blend cell showed higher hole mobility than DH-6T:PCBM cell. Therefore, DSi-6T cell showed higher device performance than DH-6T cell due to their silyl substitutions, which lead to the increase of the solubility. The higher photovoltaic performance of DSi-6T cell can be explained by the film morphology dependence on the solubility of donor. In addition, utilizing the nanosecond transient absorption spectroscopy in polar media in the visible and near-IR region proved electron-transfer reactions in DH-6T:PCBM and DSi-6T : PCBM mixtures.Figure 1
Chemical structures and energy band diagram ofDSi-6T, DH-6T, and PCBM.


## 2. Experimental Section

### 2.1. Instruments

The absorption spectra were measured using a Perkin Elmer Lambda 35 UV-vis spectrometer in spin-coated films at room temperature. The morphology of theDSi-6T : PCBM and DH6T : PCBM was determined using atomic force microscopy (AFM) (Park systems) operating in a noncontact mode, under ambient conditions.  Steady-state fluorescence measurements were carried out on a Shimadzu spectrofluorophotometer (RF-5300PC). Phosphorescence spectra were obtained by a SPEX Fluorolog τ3 spectrophotometer. Emission spectra in the visible region were detected by using a Hamamatsu Photonics R5509-72 photomultiplier. A deaerated 2-MeTHF solution containing DH-6T and DSi-6T at 77 K was excited at indicated wavelengths. Cyclic voltammograms (CV) and differential pulse voltammograms (DPV) techniques were carried on a BAS CV 50 W Voltammetric Analyzer. A platinum disk electrode was used as working electrode, while a platinum wire served as a counter electrode. SCE electrode was used as a reference electrode. All measurements were carried out in deaerated benzonitrile containing tetra-n-butylammonium hexafluorophosphate (TBAPF6; 0.10 M) as a supporting electrolyte. The scan rate = 50 mV/s.DSi-6T and DH-6T were excited by a Panther OPO pumped by Nd:YAG laser (Continuum, SLII-10, 4–6 ns fwhm) at λ  = 440 nm with the powers of 1.5 and 3.0 mJ per pulse. The transient absorption measurements were performed using a continuous xenon lamp (150 W) and an InGaAs-PIN photodiode (Hamamatsu 2949) as a probe light and a detector, respectively. The output from the photodiodes and a photomultiplier tube was recorded with a digitizing oscilloscope (Tektronix, TDS3032, 300 MHz). All measurements were conducted at 298 K. The transient spectra were recorded using fresh solutions in each laser excitation.


### 2.2. Fabrication of BHJ Cells

BHJ solar cells were fabricated on patterned ITO (indium tin oxide) coated glass substrates as the anode. ITO on glass substrates was sequentially cleaned with isopropyl alcohol, acetone, and isopropyl alcohol in ultrasonic baths. Afterwards, it was dried in oven at 80°C for 10 min and then treated with UV-ozone for 20 min. The surface of the ITO substrate was modified by spin-coating conducting poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT : PSS, H. C. Starck Baytron P VP Al 4083) with a thickness of around 40 nm, followed by baking at 120°C for 10 min in oven. The molecular structures of the donor (DSi-6T and DH-6T) and acceptor materials (PCBM) and their energy band diagram are shown in Figure 1. DSi-6T was synthesized according to literature procedures [18]. DH-6T was purchased from Aldrich Chemical Co. and was used without further purification. DSi-6T : PCBM (1 : 4, 1 : 5, 1 : 6 w/w) and DH-6T: PCBM (1 : 4, 1 : 6 w/w) were blended together and dissolved in chloroform at a total concentration of 20 mg mL−1. And then DSi-6T : PCBM blended solution was no filtration, but DH-6T : PCBM solution was filtered out through a 0.45 μm filter. The active layer was spin-cast at 4000 rpm from the solution of DSi-6T or DH-6T and PCBM (Nano-C) in chloroform. The thickness of the active layers was around 80 nm, as measured with an Alpha-step IQ. A solution containing 0.5 wt% TiO2 nanoparticles in ethanol was spin-coat at a rate of 4000 rpm on top of the active layer. The synthesis of TiO2 nanoparticles is described elsewhere [20, 21]. The cathode consists of 100 nm of aluminum and was thermally evaporated on top of the film through the use of a shadow mask to define an active area of 0.12 cm2 under a base pressure of 1 × 10−6 Torr (1 Torr = 133.32 Pa). The devices were annealed at 60°C for 10 min in the thermal evaporator. The current density-voltage curves were obtained using a Keithley 2400 source-measure unit. The photocurrent was measured under illumination using a Newport class-A 100 mW cm−2 solar simulator (AM 1.5 G) and the light intensity was calibrated with an NREL-calibrated Si solar cell with KG-1 filter for approximating one sun light intensity. External quantum efficiency (EQE) was measured in range from 300 to 800 nm using a specially designed EQE system (PV Measurements, Inc.). A 75 W xenon lamp was used as a light source for the generating monochromatic beam. A calibration was performed using a silicon photodiode, which was calibrated using the NIST-calibrated photodiode G425 as a standard, and IPCE values were collected under bias light at a low chopping speed of 10 Hz.


## 2.1. Instruments

The absorption spectra were measured using a Perkin Elmer Lambda 35 UV-vis spectrometer in spin-coated films at room temperature. The morphology of theDSi-6T : PCBM and DH6T : PCBM was determined using atomic force microscopy (AFM) (Park systems) operating in a noncontact mode, under ambient conditions.  Steady-state fluorescence measurements were carried out on a Shimadzu spectrofluorophotometer (RF-5300PC). Phosphorescence spectra were obtained by a SPEX Fluorolog τ3 spectrophotometer. Emission spectra in the visible region were detected by using a Hamamatsu Photonics R5509-72 photomultiplier. A deaerated 2-MeTHF solution containing DH-6T and DSi-6T at 77 K was excited at indicated wavelengths. Cyclic voltammograms (CV) and differential pulse voltammograms (DPV) techniques were carried on a BAS CV 50 W Voltammetric Analyzer. A platinum disk electrode was used as working electrode, while a platinum wire served as a counter electrode. SCE electrode was used as a reference electrode. All measurements were carried out in deaerated benzonitrile containing tetra-n-butylammonium hexafluorophosphate (TBAPF6; 0.10 M) as a supporting electrolyte. The scan rate = 50 mV/s.DSi-6T and DH-6T were excited by a Panther OPO pumped by Nd:YAG laser (Continuum, SLII-10, 4–6 ns fwhm) at λ  = 440 nm with the powers of 1.5 and 3.0 mJ per pulse. The transient absorption measurements were performed using a continuous xenon lamp (150 W) and an InGaAs-PIN photodiode (Hamamatsu 2949) as a probe light and a detector, respectively. The output from the photodiodes and a photomultiplier tube was recorded with a digitizing oscilloscope (Tektronix, TDS3032, 300 MHz). All measurements were conducted at 298 K. The transient spectra were recorded using fresh solutions in each laser excitation.


## 2.2. Fabrication of BHJ Cells

BHJ solar cells were fabricated on patterned ITO (indium tin oxide) coated glass substrates as the anode. ITO on glass substrates was sequentially cleaned with isopropyl alcohol, acetone, and isopropyl alcohol in ultrasonic baths. Afterwards, it was dried in oven at 80°C for 10 min and then treated with UV-ozone for 20 min. The surface of the ITO substrate was modified by spin-coating conducting poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT : PSS, H. C. Starck Baytron P VP Al 4083) with a thickness of around 40 nm, followed by baking at 120°C for 10 min in oven. The molecular structures of the donor (DSi-6T and DH-6T) and acceptor materials (PCBM) and their energy band diagram are shown in Figure 1. DSi-6T was synthesized according to literature procedures [18]. DH-6T was purchased from Aldrich Chemical Co. and was used without further purification. DSi-6T : PCBM (1 : 4, 1 : 5, 1 : 6 w/w) and DH-6T: PCBM (1 : 4, 1 : 6 w/w) were blended together and dissolved in chloroform at a total concentration of 20 mg mL−1. And then DSi-6T : PCBM blended solution was no filtration, but DH-6T : PCBM solution was filtered out through a 0.45 μm filter. The active layer was spin-cast at 4000 rpm from the solution of DSi-6T or DH-6T and PCBM (Nano-C) in chloroform. The thickness of the active layers was around 80 nm, as measured with an Alpha-step IQ. A solution containing 0.5 wt% TiO2 nanoparticles in ethanol was spin-coat at a rate of 4000 rpm on top of the active layer. The synthesis of TiO2 nanoparticles is described elsewhere [20, 21]. The cathode consists of 100 nm of aluminum and was thermally evaporated on top of the film through the use of a shadow mask to define an active area of 0.12 cm2 under a base pressure of 1 × 10−6 Torr (1 Torr = 133.32 Pa). The devices were annealed at 60°C for 10 min in the thermal evaporator. The current density-voltage curves were obtained using a Keithley 2400 source-measure unit. The photocurrent was measured under illumination using a Newport class-A 100 mW cm−2 solar simulator (AM 1.5 G) and the light intensity was calibrated with an NREL-calibrated Si solar cell with KG-1 filter for approximating one sun light intensity. External quantum efficiency (EQE) was measured in range from 300 to 800 nm using a specially designed EQE system (PV Measurements, Inc.). A 75 W xenon lamp was used as a light source for the generating monochromatic beam. A calibration was performed using a silicon photodiode, which was calibrated using the NIST-calibrated photodiode G425 as a standard, and IPCE values were collected under bias light at a low chopping speed of 10 Hz.


## 3. Results and Discussion

### 3.1. Electron-Transfer Reaction of DSi-6T/PCBM and DH-6T/PCBM

The UV-vis absorption spectra of spin-coated films ofDSi-6T and DH-6T blended with PCBM are shown in Figure 2. Both DH-6T and DSi-6T exhibited similar absorption spectra in a dilute chloroform solution, with absorption maxima observed at 440 nm and 442 nm, respectively. The absorption spectra of sexithiophene films exhibited a noteworthy blue shift with respect to that in solution for both compounds, but this shift is more significant for DH-6T than for DSi-6T. While the DH-6T film showed the strongly blue-shifted absorption spectrum with maxima at 362 nm, the DSi-6T film showed an absorption peak at 415 nm. Such feature is attributed to molecular excitons formed as a consequence of the intermolecular interactions in the solid state [22] and is found to be blue-shifted by about 0.6 eV with respect to the characteristic absorption peaks of the isolated molecules. The PCBM film has an absorption band of about 340 nm and significantly greater absorption in the visible region (350–750 nm) compared to the PCBM solution, consistent with the literature [23]. The UV-vis absorption spectra of DSi-6T : PCBM(1 : 5 w/w) andDH-6T : PCBM(1 : 6 w/w) blend films showed similar shape due to their same backbone of donor and strong absorption of PCBM.Figure 2
UV-vis absorption spectra of theDSi-6T : PCBM (1 : 5 w/w) and DH-6T : PCBM (1 : 6 w/w) blend films.Fluorescence spectra of the singlet-excited state ofDH-6T and DSi-6T in benzonitrile exhibited emission bands at 514 and 520 nm, respectively, from which the energy of the singlet state of DH-6T and DSi-6Twere estimated as 2.41 and 2.39 eV, respectively (See Supporting Information, Figure S1 of Supplementary Material available online at http://dx.doi.org/10.1155/2013/843615). Fluorescence quantum yields of DH-6T and DSi-6T was determined as 0.13 and 0.36, respectively. Fluorescence lifetimes of the singlet states of DH-6T and DSi-6Twere determined as 1.0 and 1.8 ns, respectively. In the case of the DH-6T/PCBM and DSi-6T/PCBM mixtures, it was found that the fluorescence intensities and the lifetimes are quite close to those of the DH-6T and DSi-6T, respectively. This observation suggests the intermolecular electron transfer between the DH-6T and DSi-6Twith PCBM, but not the intramolecular electron transfer.The redox potentials of the examinedDH-6T,DSi-6Tand PCBM have been examined by using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques to evaluate the driving forces for the electron transfer (-ΔGetT). The first reduction potential (Ered) of the PCBM was located at –856 mV versus Ag/AgNO3, while the oxidation potentials (Eox) of DH-6T and DSi-6T were located at 732 and 600 mV versus Ag/AgNO3 (see Supporting Information, Figure S2–S5). The redox potential measurements of the DH-6T/PCBM and DSi-6T/PCBM mixtures did not show significant interaction suggesting no interaction in the ground state. The redox potentials of the examined DH-6T, DSi-6T, and PCBM suggest their potentials as promising materials in the photovoltaic cells. The feasibility of the electron-transfer process via the triplet-excited states is controlled by the free energy change (ΔGetT), which can be expressed by the Rehm-Weller relation [24] (1):
(1)ΔGetT=Eox-Ered-ET+Ec,
where Eox is the first oxidation potential of the DH-6T and DSi-6T, Ered is the first reduction potential of PCBM, ET is the triplet energy of DH-6T (1.78 eV) and DSi-6T (1.79 eV) (we found from phosphorescence measurements in deaerated benzonitrile that energy level of the triplet DH-6T and DSi-6T at 1.78 and 1.79 eV, resp.), and Ec is the Coulomb energy term (approximately 0.06 eV in the polar benzonitrile) [25]. The free energy change (ΔGetT) values viathe triplet DH-6T and DSi-6T were estimated as −0.05 and −0.17 eV, respectively. The negative ΔGetT values suggest that the quenching process should be close to the diffusion-controlled limit (kdiff) [26].By photoexcitation ofDSi-6T in deaerated benzonitrile using 440 nm laser photolysis, the transient absorption spectrum immediately after the laser pulse exhibited only an absorption band at 700 nm, which assigned to the triplet-excited state of DH-6T (3DH-6T*) (See Supporting Information; Figure S6). By fitting the decay profile of 3DSi-6T*, the decay rate constant was found to be 6.60 × 104 s−1, from which the lifetime of 3DSi-6T*was estimated as 15.2 μs. By photoexcitation of DSi-6T in the presence of PCBM [0.01–0.10 mM] in Ar-saturated benzonitrile using 440 nm laser photolysis, the transient spectra exhibit the characteristic band of 3DSi-6T* at 700 nm. With its decay, the concomitant rises of the DSi-6T radical cation (DSi-6T∙+) at 800 and 1500 nm and the PCBM radical anion (PCBM∙-) at 1000 nm were observed (Figure 3(a) and Figure S7). These observations show clear evidence of occurrence of intermolecular electron transfer from the triplet-excited state of DSi-6T to PCBM. In oxygen-saturated solutions, an intermolecular energy transfer from 3DSi-6T* to oxygen emerges, suppressing the electron-transfer process. Similar electron-transfer features were recorded in the case of DH-6T/PCBM mixture in deaerated benzonitrile (See supporting information, Figure S8).(a) Nanosecond transient spectra ofDSi-6T (0.05 mM) in the presence of PCBM (0.05 mM) in deaerated benzonitrile; λex=440nm. Inset: Decay time profile of 3DSi-6T* (700 nm) and rise and decay profiles of DSi-6T∙+(800 nm). (b) Dependence of rate constant of the formation of DSi-6T∙+ at 800 nm on concentration of PCBM in deaerated benzonitrile. Inset: first-order plot.
(a)
(b)A more detailed picture of the kinetic is shown in Figure3(b), where the rate constant of the electron-transfer process (ket) was evaluated by monitoring the formation of the DSi-6T∙+ as function of the concentrations of PCBM. The formation of DSi-6T∙+ was fitted with clean first-order kinetics; each rate constant is referred to (k1st). The linear concentration dependence of the observed k1st values gives the ket, which is calculated as 2.24 × 109 M−1 s−1, which is near the diffusion-controlled limit (kdiff = 5.6 × 109 M−1 s−1) in benzonitrile [27]. Similarly, the ket value of 3DH-6T*/PCBM mixture was found to be 2.65 × 109 M−1 s−1, which is slightly larger than that of 3DSi-6T*/PCBM mixture (See supporting information, Figure S9).


### 3.2. Photovoltaic Properties of DSi-6T/PCBM and DH-6T/PCBM

For investigating the photovoltaic properties of sexithiophenes, OPVs were fabricated with the configuration of ITO/PEDOT : PSS/Donor (DSi-6T or DH-6T) : PCBM/Al. DSi-6T : PCBM and DH-6T:PCBM were blended together and dissolved in chloroform at a total concentration of 20 mg/mL. After Al deposition, the devices were thermally annealed at 60°C for 10 min. Figures 4 and 5 show the current density-voltage (J-V) curves of the OPV devices with DSi-6T : PCBM and DH-6T:PCBM using different blended ratios under AM 1.5 illumination, 100 mW cm−2. Table 1 summarizes the photovoltaic performance of the functionalized sexithiophene based solar cells. The sexithiophene DSi-6T was mixed with PCBM to form bulk heterojunction active layers, for use as a donor with PCBM as an acceptor. The weight ratios of the donor to acceptor were 1 : 4, 1 : 5, and 1 : 6. For comparison, the OPV devices based on DH-6T : PCBM were also fabricated with commercial DH-6T as a donor and the weight ratio of 1 : 4 and 1 : 6. The short circuit current density (Jsc) values of the DSi-6T : PCBM BHJ solar cells increase as the weight ratio of donor to acceptor increases. However, the weight ratio of donor to accepter could not be larger than 1 : 4, due to low solubility of sexithiophenes. The photovoltaic performance was observed on DSi-6T : PCBM 1 : 5 (w/w) blend cell with an open circuit voltage (Voc) of 0.60 V, a Jsc of 1.29 mA/cm2, a fill factor (FF) of 42%, and power conversion efficiency of 0.32%. Under the same conditions, DH-6T : PCBM 1 : 6 (w/w) blend cell showed a Voc of 0.63 V, a Jsc of 0.60 mA/cm2, a FF of 44%, and PCE of 0.17%. The Jsc values of the DH-6T:PCBM cells were constant with 0.60 mA/cm2 regardless of the weight ratio of DH-6T to acceptor. This result could be ascribed to the saturated solution of DH-6T : PCBM (1 : 6 w/w). In fact, a factor strongly affecting the Jsc of DSi-6T : PCBM cells is the filtration of the active solution. The Jsc of solar cell with the filtration of DSi-6T : PCBM solutions is 20% lower than that of solar cell without filtration. The most DSi-6T donors remained in the filter due to their strong π-π interaction. However, the DH-6T cells without filtration of active solutions showed the poor photovoltaic performance.Table 1
Photovoltaic characteristics ofDSi-6T : PCBM and DH-6T : PCBM blended cells.
Device name
V
oc
(V)
J
sc
(mA/cm2)
FF
PCE (%)
DSi-6T : PCBM
1 : 4
0.61
1.26
0.39
0.30
1 : 5
0.60
1.29
0.42
0.32
1 : 6
0.57
0.88
0.37
0.18
DSi-6T : PCBM (with TiO2)
1 : 4
0.64
1.34
0.50
0.42
1 : 5
0.60
1.34
0.55
0.44
1 : 6
0.62
1.07
0.49
0.32
DH-6T : PCBM
1 : 4
0.52
0.62
0.33
0.11
1 : 6
0.63
0.60
0.44
0.17
DH-6T : PCBM (with TiO2)
1 : 4
0.51
0.61
0.42
0.13
1 : 6
0.65
0.57
0.56
0.21
aOpen circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and PCE at DSi-6T and DH-6T as electron donors: PCBM at weight ratio. The device performance was consistent and reproducible. The active area is 0.12 cm2.Figure 4
J-V characteristics of ITO/PEDOT : PSS/DSi-6T : PCBM/Al (active layer DSi-6T : PCBM 1 : 4, w/w (○), 1 : 5 w/w (△), 1 : 6 w/w (□)) and ITO/PEDOT : PSS/DSi-6T : PCBM/TiO2/Al (active layer DSi-6T : PCBM 1 : 4, w/w (●), 1 : 5 w/w (▲), 1 : 6 w/w (■)) under illumination of AM 1.5, 100 mW cm−2.Figure 5
J-V characteristics of ITO/PEDOT : PSS/DH-6T : PCBM/Al (active layer DH-6T:PCBM 1 : 4, w/w (◆), 1 : 6 w/w (▲)) and ITO/PEDOT : PSS/DH-6T:PCBM/TiO2/Al (active layer DH-6T:PCBM 1 : 4, w/w (●), 1 : 6 w/w (■)) under illumination of AM 1.5, 100 mW cm−2.For the development of OPVs with increased PCE and lifetime, OPVs with the TiO2 interfacial layer between the active layer and the Al electrode were fabricated together with typical OPVs without TiO2. According to the literature, the TiO2 interfacial layer improved the PCEs and reduced the sensitivity of such devices to oxygen and water vapour [21, 28]. As shown in Figure 4, the FF of DSi-6T : PCBM cells with TiO2 layer is enhanced up to 0.55 which is 30% higher than that of the solar cell. The best performance was observed on DSi-6T : PCBM 1 : 5 (w/w) blend cell with Voc of 0.63 V, Jsc of 1.34 mA/cm2, FF of 55%, and power conversion efficiency of 0.44%. Also the FF of DH-6T cells is exactly 27% higher than that of solar cells. The DH-6T : PCBM 1 : 6 (w/w) blend cell with TiO2 layer exhibited a Voc of 0.65 V, Jsc of 0.57 mA/cm2, FF of 56%, and PCE of 0.21% (Figure 5). The incorporation of solution processed TiO2 interfacial layer in the sexithiophene : PCBM BHJ devices significantly enhances FF, mainly due to the reduced charge recombination near active layer/Al interface.Figure6 shows the external quantum efficiency (EQE) plot for the DSi-6T : PCBM (1 : 5 w/w) and DH-6T:PCBM (1 : 6 w/w) cells. The sexithiophene-based cells absorb the solar light in narrow visible range and show low EQEs due to a small amount of donor by limited solubility. For the DH-6T : PCBM solar cell, the maximum EQE is 11.3% at 340 nm that is caused by strong absorption of PCBM. When the DSi-6T was used, the DSi-6T : PCBM solar cell shows a higher EQE in the all wavelength, with a maximum of 16.1% at 410 nm. The absorption of the DSi-6T donor was increased in the EQE spectrum to be different from the UV-vis spectrum. It is expected that horizontal intermolecular packing due to the bulky silyl side chains of DSi-6T may increase the charge transfer at the interfaces, so the EQE increases.Figure 6
EQE curves ofDSi-6T : PCBM (1 : 5) and DH-6T:PCBM (1 : 6) blend cells.To investigate the charge transporting property of the OPV devices, we measured hole mobility of the donor: PCBM blend layers with the space-charge limited-current (SCLC) model. Hole-only devices were fabricated with a device configuration of ITO/PEDOT : PSS/Donor : PCBM/MoO3/Al, because high work function of molybdenum oxide (MoO3) blocks the injection of electrons from the Al cathode. The blend ratios of the Donor: PCBM layers were 1 : 5 (w/w) and 1 : 6 (w/w) for DSi-6T : PCBM and DH-6T : PCBM, respectively.The hole transport through the active layers is limited by the space charge and the SCLC is described by(2)J=98εrε0μhV2L3,
where J is the current density, ε0 is the permittivity of vacuum, εr is the dielectric constant of the material (assumed to be 3), μh is the mobility, V is the applied voltage (Vapplied) corrected from built-in voltage (Vbi) arising from difference in the work function of the contacts and voltage drop (Vr) due to the series resistance of the electrodes, and L is the thickness of the blend layer [29]. Equation (1) is valid when the mobility is field independent. It has been found that the behaviour of electric field dependence of the mobility can be described by an empirical rule; μh=μh0exp(γE1/2), where μh0 is the hole mobility at zero field, γ is field dependence prefactor, and E is the electric field. Therefore, the electric field dependent SCLC model can be expressed by
(3)J=98εrε0μh0exp(γE)V2L3.The zero-field hole mobilityμh0 and the prefatory γ of DSi-6T : PCBM (1 : 5 w/w) and DH-6T:PCBM (1 : 6 w/w) blend films were obtained from (2) as shown in Figure 7(a) (μh0 = 7.7 × 10−6 cm2/Vs, γ = 3.3 × 10−4 m0.5 V−0.5 for DSi-6T : PCBM, and μh0 = 1.6 × 10−6 cm2/Vs, γ = 4.5 × 10−4 m0.5 V−0.5 for DH-6T:PCBM). The low γ of DSi-6T : PCBM indicates low energetic disorder related to the interaction of each hopping charge in randomly oriented dipoles [30, 31]. Figure 7(b) shows the SCLC fitting of DSi-6T : PCBM (1 : 5 w/w) and DH-6T:PCBM (1 : 6 w/w) blend films according to (1) and (2). It is observed that the SCLC calculated from the field independent model deviates from the experimental data at the high voltage region. In actual operating devices, the active materials are in the effective bias of ~1 V (corresponding to the energy level difference of HOMO and LUMO of an electron donor and acceptor) at the short circuit conditions. Taking this into consideration, we estimated the field dependent hole mobility of the active blend layers at 1 V; μh = 2.7 × 10−5 cm2/Vs for DSi-6T : PCBM, and μh = 8.8 × 10−6 cm2/Vs for DH-6T : PCBM. The higher hole mobility of the DSi-6T : PCBM blend film than that of the DH-6T : PCBM film is a key property of the enhanced photovoltaic performance. It seems that the higher hole mobility of DSi-6T : PCBM blend layer is inconsistent with the pure donor’s field-effect mobility. However, it should be noted that the mobility in organic thin-film transistors strongly depends on the molecular orientation due to the highly anisotropic charge carrier mobility. In the OPV device configuration, the DSi-6T : PCBM BHJ layer yielded the better vertical charge transport than the DH-6T: PCBM due to the improved solubility of DSi-6T.(a) ln (JL3/V2) versus E1/2 curves according to the SCLC equation (2). (b) J1/2 versus V curves showing the electric-field-dependence of hole mobilities at highly biased region. Dotted and solid lines are calculated curves with (1) and (2), respectively.
(a)
(b)To study the thermal annealing effect of this system, the morphologies of sexithiophene: PCBM films before and after annealing at 60°C were studied by atomic force microscopy (AFM) (Figure8). The morphological studies indicate the more amorphous nature of DSi-6T : PCBM compared to DH-6T: PCBM films. The root-mean-square (rms) roughness is 47, 38, and 23 nm for DSi-6T : PCBM 1 : 4, 1 : 5, and 1 : 6 (w/w), respectively, and 7.1 nm for DH-6T:PCBM 1 : 6 (w/w). In DSi-6T : PCBM films, the weight ratio donor to acceptor increased as the roughness of films decreased. After thermal annealing, the roughness was 37, 29, and 14 nm for DSi-6T : PCBM 1 : 4, 1 : 5, 1 : 6 (w/w), respectively, and 7.6 nm for DH-6T:PCBM 1 : 6 (w/w). For the solar cell based on the DSi-6T : PCBM cell, the rms roughness of films after thermal annealing at 60°C was significantly reduced as compared to that of films. In the case of DH-6T:PCBM cell, the morphological change after annealing was slightly observed. Unfortunately, all films were quite rough compared to the polymer solar cells. The inferior morphologies of sexithiophene based BHJ solar cells may be related to the solubility of donors, which is expected to influence the photovoltaic performance. For a comparison, the DSi-6T : PCBM cell was thermally annealed at 135°C which was the temperature employed for the P3HT : PCBM cell. When a film was annealed at a high temperature, the decrease in PCE (0.034%) became more significant. The future work may concentrate on optimization of the annealing temperature of the DSi-6T : PCBM devices to allow better efficiency.AFM images of the blended films under various ratios (a)DSi-6T : PCBM 1 : 4 w/w nonannealed (e) annealed and (b) DSi-6T : PCBM 1 : 5 w/w non-annealed (f) annealed and (c) DSi-6T : PCBM 1 : 6 w/w non-annealed (g) annealed and (d) DH-6T: PCBM 1 : 6 w/w non-annealed (h) annealed. The image scan sizes is 5 μm × 5 μm.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)


## 3.1. Electron-Transfer Reaction of DSi-6T/PCBM and DH-6T/PCBM

The UV-vis absorption spectra of spin-coated films ofDSi-6T and DH-6T blended with PCBM are shown in Figure 2. Both DH-6T and DSi-6T exhibited similar absorption spectra in a dilute chloroform solution, with absorption maxima observed at 440 nm and 442 nm, respectively. The absorption spectra of sexithiophene films exhibited a noteworthy blue shift with respect to that in solution for both compounds, but this shift is more significant for DH-6T than for DSi-6T. While the DH-6T film showed the strongly blue-shifted absorption spectrum with maxima at 362 nm, the DSi-6T film showed an absorption peak at 415 nm. Such feature is attributed to molecular excitons formed as a consequence of the intermolecular interactions in the solid state [22] and is found to be blue-shifted by about 0.6 eV with respect to the characteristic absorption peaks of the isolated molecules. The PCBM film has an absorption band of about 340 nm and significantly greater absorption in the visible region (350–750 nm) compared to the PCBM solution, consistent with the literature [23]. The UV-vis absorption spectra of DSi-6T : PCBM(1 : 5 w/w) andDH-6T : PCBM(1 : 6 w/w) blend films showed similar shape due to their same backbone of donor and strong absorption of PCBM.Figure 2
UV-vis absorption spectra of theDSi-6T : PCBM (1 : 5 w/w) and DH-6T : PCBM (1 : 6 w/w) blend films.Fluorescence spectra of the singlet-excited state ofDH-6T and DSi-6T in benzonitrile exhibited emission bands at 514 and 520 nm, respectively, from which the energy of the singlet state of DH-6T and DSi-6Twere estimated as 2.41 and 2.39 eV, respectively (See Supporting Information, Figure S1 of Supplementary Material available online at http://dx.doi.org/10.1155/2013/843615). Fluorescence quantum yields of DH-6T and DSi-6T was determined as 0.13 and 0.36, respectively. Fluorescence lifetimes of the singlet states of DH-6T and DSi-6Twere determined as 1.0 and 1.8 ns, respectively. In the case of the DH-6T/PCBM and DSi-6T/PCBM mixtures, it was found that the fluorescence intensities and the lifetimes are quite close to those of the DH-6T and DSi-6T, respectively. This observation suggests the intermolecular electron transfer between the DH-6T and DSi-6Twith PCBM, but not the intramolecular electron transfer.The redox potentials of the examinedDH-6T,DSi-6Tand PCBM have been examined by using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques to evaluate the driving forces for the electron transfer (-ΔGetT). The first reduction potential (Ered) of the PCBM was located at –856 mV versus Ag/AgNO3, while the oxidation potentials (Eox) of DH-6T and DSi-6T were located at 732 and 600 mV versus Ag/AgNO3 (see Supporting Information, Figure S2–S5). The redox potential measurements of the DH-6T/PCBM and DSi-6T/PCBM mixtures did not show significant interaction suggesting no interaction in the ground state. The redox potentials of the examined DH-6T, DSi-6T, and PCBM suggest their potentials as promising materials in the photovoltaic cells. The feasibility of the electron-transfer process via the triplet-excited states is controlled by the free energy change (ΔGetT), which can be expressed by the Rehm-Weller relation [24] (1):
(1)ΔGetT=Eox-Ered-ET+Ec,
where Eox is the first oxidation potential of the DH-6T and DSi-6T, Ered is the first reduction potential of PCBM, ET is the triplet energy of DH-6T (1.78 eV) and DSi-6T (1.79 eV) (we found from phosphorescence measurements in deaerated benzonitrile that energy level of the triplet DH-6T and DSi-6T at 1.78 and 1.79 eV, resp.), and Ec is the Coulomb energy term (approximately 0.06 eV in the polar benzonitrile) [25]. The free energy change (ΔGetT) values viathe triplet DH-6T and DSi-6T were estimated as −0.05 and −0.17 eV, respectively. The negative ΔGetT values suggest that the quenching process should be close to the diffusion-controlled limit (kdiff) [26].By photoexcitation ofDSi-6T in deaerated benzonitrile using 440 nm laser photolysis, the transient absorption spectrum immediately after the laser pulse exhibited only an absorption band at 700 nm, which assigned to the triplet-excited state of DH-6T (3DH-6T*) (See Supporting Information; Figure S6). By fitting the decay profile of 3DSi-6T*, the decay rate constant was found to be 6.60 × 104 s−1, from which the lifetime of 3DSi-6T*was estimated as 15.2 μs. By photoexcitation of DSi-6T in the presence of PCBM [0.01–0.10 mM] in Ar-saturated benzonitrile using 440 nm laser photolysis, the transient spectra exhibit the characteristic band of 3DSi-6T* at 700 nm. With its decay, the concomitant rises of the DSi-6T radical cation (DSi-6T∙+) at 800 and 1500 nm and the PCBM radical anion (PCBM∙-) at 1000 nm were observed (Figure 3(a) and Figure S7). These observations show clear evidence of occurrence of intermolecular electron transfer from the triplet-excited state of DSi-6T to PCBM. In oxygen-saturated solutions, an intermolecular energy transfer from 3DSi-6T* to oxygen emerges, suppressing the electron-transfer process. Similar electron-transfer features were recorded in the case of DH-6T/PCBM mixture in deaerated benzonitrile (See supporting information, Figure S8).(a) Nanosecond transient spectra ofDSi-6T (0.05 mM) in the presence of PCBM (0.05 mM) in deaerated benzonitrile; λex=440nm. Inset: Decay time profile of 3DSi-6T* (700 nm) and rise and decay profiles of DSi-6T∙+(800 nm). (b) Dependence of rate constant of the formation of DSi-6T∙+ at 800 nm on concentration of PCBM in deaerated benzonitrile. Inset: first-order plot.
(a)
(b)A more detailed picture of the kinetic is shown in Figure3(b), where the rate constant of the electron-transfer process (ket) was evaluated by monitoring the formation of the DSi-6T∙+ as function of the concentrations of PCBM. The formation of DSi-6T∙+ was fitted with clean first-order kinetics; each rate constant is referred to (k1st). The linear concentration dependence of the observed k1st values gives the ket, which is calculated as 2.24 × 109 M−1 s−1, which is near the diffusion-controlled limit (kdiff = 5.6 × 109 M−1 s−1) in benzonitrile [27]. Similarly, the ket value of 3DH-6T*/PCBM mixture was found to be 2.65 × 109 M−1 s−1, which is slightly larger than that of 3DSi-6T*/PCBM mixture (See supporting information, Figure S9).


## 3.2. Photovoltaic Properties of DSi-6T/PCBM and DH-6T/PCBM

For investigating the photovoltaic properties of sexithiophenes, OPVs were fabricated with the configuration of ITO/PEDOT : PSS/Donor (DSi-6T or DH-6T) : PCBM/Al. DSi-6T : PCBM and DH-6T:PCBM were blended together and dissolved in chloroform at a total concentration of 20 mg/mL. After Al deposition, the devices were thermally annealed at 60°C for 10 min. Figures 4 and 5 show the current density-voltage (J-V) curves of the OPV devices with DSi-6T : PCBM and DH-6T:PCBM using different blended ratios under AM 1.5 illumination, 100 mW cm−2. Table 1 summarizes the photovoltaic performance of the functionalized sexithiophene based solar cells. The sexithiophene DSi-6T was mixed with PCBM to form bulk heterojunction active layers, for use as a donor with PCBM as an acceptor. The weight ratios of the donor to acceptor were 1 : 4, 1 : 5, and 1 : 6. For comparison, the OPV devices based on DH-6T : PCBM were also fabricated with commercial DH-6T as a donor and the weight ratio of 1 : 4 and 1 : 6. The short circuit current density (Jsc) values of the DSi-6T : PCBM BHJ solar cells increase as the weight ratio of donor to acceptor increases. However, the weight ratio of donor to accepter could not be larger than 1 : 4, due to low solubility of sexithiophenes. The photovoltaic performance was observed on DSi-6T : PCBM 1 : 5 (w/w) blend cell with an open circuit voltage (Voc) of 0.60 V, a Jsc of 1.29 mA/cm2, a fill factor (FF) of 42%, and power conversion efficiency of 0.32%. Under the same conditions, DH-6T : PCBM 1 : 6 (w/w) blend cell showed a Voc of 0.63 V, a Jsc of 0.60 mA/cm2, a FF of 44%, and PCE of 0.17%. The Jsc values of the DH-6T:PCBM cells were constant with 0.60 mA/cm2 regardless of the weight ratio of DH-6T to acceptor. This result could be ascribed to the saturated solution of DH-6T : PCBM (1 : 6 w/w). In fact, a factor strongly affecting the Jsc of DSi-6T : PCBM cells is the filtration of the active solution. The Jsc of solar cell with the filtration of DSi-6T : PCBM solutions is 20% lower than that of solar cell without filtration. The most DSi-6T donors remained in the filter due to their strong π-π interaction. However, the DH-6T cells without filtration of active solutions showed the poor photovoltaic performance.Table 1
Photovoltaic characteristics ofDSi-6T : PCBM and DH-6T : PCBM blended cells.
Device name
V
oc
(V)
J
sc
(mA/cm2)
FF
PCE (%)
DSi-6T : PCBM
1 : 4
0.61
1.26
0.39
0.30
1 : 5
0.60
1.29
0.42
0.32
1 : 6
0.57
0.88
0.37
0.18
DSi-6T : PCBM (with TiO2)
1 : 4
0.64
1.34
0.50
0.42
1 : 5
0.60
1.34
0.55
0.44
1 : 6
0.62
1.07
0.49
0.32
DH-6T : PCBM
1 : 4
0.52
0.62
0.33
0.11
1 : 6
0.63
0.60
0.44
0.17
DH-6T : PCBM (with TiO2)
1 : 4
0.51
0.61
0.42
0.13
1 : 6
0.65
0.57
0.56
0.21
aOpen circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and PCE at DSi-6T and DH-6T as electron donors: PCBM at weight ratio. The device performance was consistent and reproducible. The active area is 0.12 cm2.Figure 4
J-V characteristics of ITO/PEDOT : PSS/DSi-6T : PCBM/Al (active layer DSi-6T : PCBM 1 : 4, w/w (○), 1 : 5 w/w (△), 1 : 6 w/w (□)) and ITO/PEDOT : PSS/DSi-6T : PCBM/TiO2/Al (active layer DSi-6T : PCBM 1 : 4, w/w (●), 1 : 5 w/w (▲), 1 : 6 w/w (■)) under illumination of AM 1.5, 100 mW cm−2.Figure 5
J-V characteristics of ITO/PEDOT : PSS/DH-6T : PCBM/Al (active layer DH-6T:PCBM 1 : 4, w/w (◆), 1 : 6 w/w (▲)) and ITO/PEDOT : PSS/DH-6T:PCBM/TiO2/Al (active layer DH-6T:PCBM 1 : 4, w/w (●), 1 : 6 w/w (■)) under illumination of AM 1.5, 100 mW cm−2.For the development of OPVs with increased PCE and lifetime, OPVs with the TiO2 interfacial layer between the active layer and the Al electrode were fabricated together with typical OPVs without TiO2. According to the literature, the TiO2 interfacial layer improved the PCEs and reduced the sensitivity of such devices to oxygen and water vapour [21, 28]. As shown in Figure 4, the FF of DSi-6T : PCBM cells with TiO2 layer is enhanced up to 0.55 which is 30% higher than that of the solar cell. The best performance was observed on DSi-6T : PCBM 1 : 5 (w/w) blend cell with Voc of 0.63 V, Jsc of 1.34 mA/cm2, FF of 55%, and power conversion efficiency of 0.44%. Also the FF of DH-6T cells is exactly 27% higher than that of solar cells. The DH-6T : PCBM 1 : 6 (w/w) blend cell with TiO2 layer exhibited a Voc of 0.65 V, Jsc of 0.57 mA/cm2, FF of 56%, and PCE of 0.21% (Figure 5). The incorporation of solution processed TiO2 interfacial layer in the sexithiophene : PCBM BHJ devices significantly enhances FF, mainly due to the reduced charge recombination near active layer/Al interface.Figure6 shows the external quantum efficiency (EQE) plot for the DSi-6T : PCBM (1 : 5 w/w) and DH-6T:PCBM (1 : 6 w/w) cells. The sexithiophene-based cells absorb the solar light in narrow visible range and show low EQEs due to a small amount of donor by limited solubility. For the DH-6T : PCBM solar cell, the maximum EQE is 11.3% at 340 nm that is caused by strong absorption of PCBM. When the DSi-6T was used, the DSi-6T : PCBM solar cell shows a higher EQE in the all wavelength, with a maximum of 16.1% at 410 nm. The absorption of the DSi-6T donor was increased in the EQE spectrum to be different from the UV-vis spectrum. It is expected that horizontal intermolecular packing due to the bulky silyl side chains of DSi-6T may increase the charge transfer at the interfaces, so the EQE increases.Figure 6
EQE curves ofDSi-6T : PCBM (1 : 5) and DH-6T:PCBM (1 : 6) blend cells.To investigate the charge transporting property of the OPV devices, we measured hole mobility of the donor: PCBM blend layers with the space-charge limited-current (SCLC) model. Hole-only devices were fabricated with a device configuration of ITO/PEDOT : PSS/Donor : PCBM/MoO3/Al, because high work function of molybdenum oxide (MoO3) blocks the injection of electrons from the Al cathode. The blend ratios of the Donor: PCBM layers were 1 : 5 (w/w) and 1 : 6 (w/w) for DSi-6T : PCBM and DH-6T : PCBM, respectively.The hole transport through the active layers is limited by the space charge and the SCLC is described by(2)J=98εrε0μhV2L3,
where J is the current density, ε0 is the permittivity of vacuum, εr is the dielectric constant of the material (assumed to be 3), μh is the mobility, V is the applied voltage (Vapplied) corrected from built-in voltage (Vbi) arising from difference in the work function of the contacts and voltage drop (Vr) due to the series resistance of the electrodes, and L is the thickness of the blend layer [29]. Equation (1) is valid when the mobility is field independent. It has been found that the behaviour of electric field dependence of the mobility can be described by an empirical rule; μh=μh0exp(γE1/2), where μh0 is the hole mobility at zero field, γ is field dependence prefactor, and E is the electric field. Therefore, the electric field dependent SCLC model can be expressed by
(3)J=98εrε0μh0exp(γE)V2L3.The zero-field hole mobilityμh0 and the prefatory γ of DSi-6T : PCBM (1 : 5 w/w) and DH-6T:PCBM (1 : 6 w/w) blend films were obtained from (2) as shown in Figure 7(a) (μh0 = 7.7 × 10−6 cm2/Vs, γ = 3.3 × 10−4 m0.5 V−0.5 for DSi-6T : PCBM, and μh0 = 1.6 × 10−6 cm2/Vs, γ = 4.5 × 10−4 m0.5 V−0.5 for DH-6T:PCBM). The low γ of DSi-6T : PCBM indicates low energetic disorder related to the interaction of each hopping charge in randomly oriented dipoles [30, 31]. Figure 7(b) shows the SCLC fitting of DSi-6T : PCBM (1 : 5 w/w) and DH-6T:PCBM (1 : 6 w/w) blend films according to (1) and (2). It is observed that the SCLC calculated from the field independent model deviates from the experimental data at the high voltage region. In actual operating devices, the active materials are in the effective bias of ~1 V (corresponding to the energy level difference of HOMO and LUMO of an electron donor and acceptor) at the short circuit conditions. Taking this into consideration, we estimated the field dependent hole mobility of the active blend layers at 1 V; μh = 2.7 × 10−5 cm2/Vs for DSi-6T : PCBM, and μh = 8.8 × 10−6 cm2/Vs for DH-6T : PCBM. The higher hole mobility of the DSi-6T : PCBM blend film than that of the DH-6T : PCBM film is a key property of the enhanced photovoltaic performance. It seems that the higher hole mobility of DSi-6T : PCBM blend layer is inconsistent with the pure donor’s field-effect mobility. However, it should be noted that the mobility in organic thin-film transistors strongly depends on the molecular orientation due to the highly anisotropic charge carrier mobility. In the OPV device configuration, the DSi-6T : PCBM BHJ layer yielded the better vertical charge transport than the DH-6T: PCBM due to the improved solubility of DSi-6T.(a) ln (JL3/V2) versus E1/2 curves according to the SCLC equation (2). (b) J1/2 versus V curves showing the electric-field-dependence of hole mobilities at highly biased region. Dotted and solid lines are calculated curves with (1) and (2), respectively.
(a)
(b)To study the thermal annealing effect of this system, the morphologies of sexithiophene: PCBM films before and after annealing at 60°C were studied by atomic force microscopy (AFM) (Figure8). The morphological studies indicate the more amorphous nature of DSi-6T : PCBM compared to DH-6T: PCBM films. The root-mean-square (rms) roughness is 47, 38, and 23 nm for DSi-6T : PCBM 1 : 4, 1 : 5, and 1 : 6 (w/w), respectively, and 7.1 nm for DH-6T:PCBM 1 : 6 (w/w). In DSi-6T : PCBM films, the weight ratio donor to acceptor increased as the roughness of films decreased. After thermal annealing, the roughness was 37, 29, and 14 nm for DSi-6T : PCBM 1 : 4, 1 : 5, 1 : 6 (w/w), respectively, and 7.6 nm for DH-6T:PCBM 1 : 6 (w/w). For the solar cell based on the DSi-6T : PCBM cell, the rms roughness of films after thermal annealing at 60°C was significantly reduced as compared to that of films. In the case of DH-6T:PCBM cell, the morphological change after annealing was slightly observed. Unfortunately, all films were quite rough compared to the polymer solar cells. The inferior morphologies of sexithiophene based BHJ solar cells may be related to the solubility of donors, which is expected to influence the photovoltaic performance. For a comparison, the DSi-6T : PCBM cell was thermally annealed at 135°C which was the temperature employed for the P3HT : PCBM cell. When a film was annealed at a high temperature, the decrease in PCE (0.034%) became more significant. The future work may concentrate on optimization of the annealing temperature of the DSi-6T : PCBM devices to allow better efficiency.AFM images of the blended films under various ratios (a)DSi-6T : PCBM 1 : 4 w/w nonannealed (e) annealed and (b) DSi-6T : PCBM 1 : 5 w/w non-annealed (f) annealed and (c) DSi-6T : PCBM 1 : 6 w/w non-annealed (g) annealed and (d) DH-6T: PCBM 1 : 6 w/w non-annealed (h) annealed. The image scan sizes is 5 μm × 5 μm.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)


## 4. Conclusion

In conclusion, we have demonstrated solution processed organic photovoltaic cells that incorporate sexithiophene derivatives having silyl side chain or hexyl chain in the end of thiophene ring as donor materials and PCBM as an acceptor. Owing to the particular electronic properties ofDSi-6T, DH-6T, and PCBM, such combinations seem to be perfectly suited for the study of electron-transfer processes in the polar media via the triplet states. The electron-transfer process from the triplet states ofDSi-6T, DH-6T to PCBM was confirmed in this study by utilizing the nanosecond laser photolysis technique in the visible and NIR regions. DSi-6T showed higher photovoltaic performance than DH-6T, despite its poor quality film morphology. The optimal DSi-6T : PCBM blend ratio was found to be 1 : 5, and its power conversion efficiency was 0.44% under AM 1.5 G illumination. Although the power conversion efficiency remains to be improved, the present study provides valuable insight into the further development of the solution-processed efficient OPV devices.



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*Source: 843615-2013-12-16.xml*