Copolymers based on fluorene-thiophene units have presented promising efficiencies in photovoltaic devices applications. They present good transport properties mainly after thermal treatment of the polymer films. Here, we investigate the properties of bilayer devices formed by the heterojunction of the polymer F8T2 with variable thickness and the fullerene. The series resistance of the equivalent circuit associated with the device increases as the polymer film gets thicker. The current-voltage characteristics of the bilayer devices follow the Mott-Gurney law of SCLC. For the best performing device we measured 2.1% of power conversion efficiency.

1. Introduction

Fluorene-thiophene-conjugated copolymers have been extensively investigated and used in the field of organic electronics [1, 2]. The main applications are organic photovoltaic cells (PVs) [310], organic light-emitting diodes (OLEDs) [11], and field effect transistors (FETs) [12]. These copolymers have high absorption coefficient in the visible wavelength range; high quantum yields values and high charge mobility. Additionally, the incorporation of carbon-based materials, such as carbon nanotubes [9], fullerenes or its derivatives [1315] brought interesting effects in organic PV cells built in bulk heterojunction (BHJ) or layered geometry. These carbon compounds have high electron affinity relatively to the conjugated polymers; in this way, they act as electron acceptor materials favoring the exciton dissociation prior the radiative/nonradiative recombinations, thus the charge collection through the electrodes.

This combination has resulted in high power conversion efficiencies (η). For instance, BHJ cell based on poly(9,9′-dihexylfluorene-alt-thiophene) polymer (PFT1) and PCBM displayed open circuit voltage (𝑉oc) of 0.77 V and η of 0.62% [3], while using poly(9,9′-dihexylfluorene-alt-bithiophene) (F6T2) or poly[9,9′-dioctyl-fluorene-co-bithiophene] (F8T2) displayed 𝑉oc around 1 V, η of 2.7% [4] or 2.13% [5, 6], respectively. These results were both attributed to the high hole mobility and absorption coefficient in these polymers with efficient photocurrent generation. A bilayer solar cell prepared with F8T2 but different fullerene C70 displayed the highest η of 3.4%, after thermal annealing at 200°C [7]. The increase in 𝜂 in this device was attributed to a better chain organization and higher roughness of the film, achieving higher donor-acceptor heterojunction area for exciton dissociation. Recently, we reported a bilayer PV cell prepared with poly [9,9′-hexyl-fluorene-alt-bithiophene] and fullerene (C60) displaying an increase of η from 0.4 to 2.8% after annealing at 200°C. Comparing these results and the behavior of the current versus voltage characteristics, we concluded that the π-π stacking in solid state is enhanced after thermal annealing with a reduction of traps and thus reflecting higher hole mobility in this polymer after annealing [8].

Herein, we report the studies of bilayer devices formed by the heterojunction of the polymer F8T2 with variable thickness (18 up to 60 nm) and the fullerene C60 sandwiched between two electrodes: FTO/PEDOT : PSS as the anode and aluminium as the cathode. The transport properties of the devices follow the Mott-Gurney law of space charge law current (SCLC).

2. Experimental

Poly(9,9-dioctylfluorene-alt-bithiophene) (F8T2) polymer with number average molecular weight (Mn) >20,000 gmol−1 was purchased from Aldrich. The devices were fabricated as follows: poly(3,4-ethylenedioxythiophene)-polystyrene sulfonic acid (PEDOT : PSS) was filtered in a 0.45 μm filter, then a film with thickness of 40 nm was spin coated at 6000 rpm onto the cleaned fluorine-tin-oxide (FTO-, 5–10 Ω) [16] patterned glass substrate and annealed at 100°C by 15 min in air. F8T2 polymer film was spin coated from chlorobenzene solutions with concentrations of 3–7.5 mgmL−1 and treated at 100°C by 15 min (in vacuum). These parameters were set in order to obtain active layer thickness ranging from 18 to 60 nm. Then, 30 nm of C60 and 100 nm of Al were thermally evaporated through a shadow mask at vacuum pressure of 6 × 10−6 mbar, respectively. Subsequently, the FTO/PEDOT : PSS/F8T2/C60/Al were encapsulated upon nitrogen atmosphere. The F8T2 and C60 molecular structures as well as the device structure are depicted in Figure 1.

The polymer films thickness was determined in a Dektak 3 profilometer. Topography images were acquired by using an atomic force microscope (AFM, Shimadzu SPM 9500J3) in dynamic mode. The photovoltaic characterization was performed with a Keithley picoammeter with power supply, model 6487 and a monochromator/spectrometer (1/4 m Oriel). The solar simulation was made using air mass (AM1.5) filter with a power illumination of 100 mWcm−2 from a 150 W Oriel Xenon lamp.

3. Results and Discussion

F8T2 films were analyzed by AFM as illustrated in Figures 2(a)2(d). Similar morphology is observed by varying the thickness from 18 up to 60 nm, however the film with 18 nm has root-mean-square (rms) roughness equal to 1.2 nm, and this value is slightly higher in comparison with thicker films that display roughness below 1 nm. This may arise due to the roughness related with the FTO substrate; PEDOT : PSS and thicker polymer films cover more efficiently the valleys and hills observed in polycrystalline FTO films.

The photovoltaic devices were characterized by their spectral response and their current density versus voltage curves under AM 1.5 illumination of 100 mW cm−2. The spectral response is expressed by the external quantum efficiency (IPCE) which is the ratio of the photocurrent to incoming photons flux: IPCE = 1240 𝐽ph/𝜆𝐼0, where 𝐽ph is the photocurrent density (Acm−2), 𝐼0 is the light intensity (Wm−2), and 𝜆 is the wavelength (nm) [17]. Figure 3 shows the spectral response of FTO/PEDOT : PSS/F8T2/C60/Al devices for different active layer thickness: 18, 30, 40, and 60 nm. The external quantum efficiency spectra of the devices follow the absorption coefficient (𝛼=4𝜋𝑘/𝜆) dependence on wavelength of F8T2 [5, 18] and C60, reflecting the contribution of the polymer and the molecule to the photocurrent [19]. The light harvesting contribution of F8T2 can be observed in the wavelength range from 400 to 500 nm. The comparison of the action spectra for each thickness shows that the photoconversion efficiency is higher for the device with active layer of 30 nm: the maximum IPCE value is around 80% under monochromatic illumination of 𝜆=450 nm. Devices with thicker F8T2 layer presented maximum IPCE around 50%. It is well known that although thicker layers can absorb more light, the light intensity at the interface is smaller, thus the exciton dissociation in the F8T2/C60 device [20, 21].

The electrical characteristics under illumination of AM1.5 radiation of 100 mWcm−2 are shown in Figure 4, and the photovoltaic parameters obtained from these curves are summarized in Table 1. The device performance dependence on F8T2 thickness corroborates with the external quantum efficiency results. The 30 nm F8T2 layer exhibits a short-circuit current (𝐽sc) of 4.79 mAcm−2, 𝑉oc of 0.91 V, a fill factor of 50% and of 2.1%. Bilayer devices based on F8T2/C70 which were prepared by using trichlorobenzene as solvent and annealed at 100°C, displayed η = 1.2% as it was previously reported by Kekuda et al. [7]. Devices with F8T2 thickness of 60 nm presented poor photovoltaic performance. It is well known that devices based on thick active layers have the efficiency compromised by the transport due to higher resistance of the layer, as it can be clearly seen in Figure 4. In Figure 4 is also presented an equivalent circuit for photovoltaic device under illumination considered as a current source with a diode in parallel. We calculate the series resistance 𝑅s of equivalent circuit associated with the solar cells by using the modified method for IEC 60891 following Kunz and Wagner [22]. Generally, the loss mechanisms in photovoltaic devices are divided in series resistance (𝑅𝑠), which is related with the loss by charge carrier trapping or recombination of free charges, and shunt resistance (in parallel with the current source) (𝑅sh), which is related with leakage phenomena. 𝑅𝑠 increases with the active layer thickness due to the increasing of the probability of a charge carrier to be trapped. In our devices, the series resistance increased with the F8T2 thickness for 30 nm the 𝑅𝑠 was 77 Ω, for 40 nm the 𝑅𝑠 was 198 Ω, and for 60 nm the 𝑅𝑠 was 445 Ω. In the case of 18 nm the 𝑅𝑠 was 150 Ω. This value did not follow the general behavior of resistance versus polymer thickness suggesting that the film formation indeed modifies the charge transport.

The electrical properties of these bilayer devices were also investigated by measuring the current versus voltage characteristics in dark. The study of forward current of bi-layer diodes is not so straight forward to do. F8T2 and C60 have different electron affinities values in order to promote the exciton dissociation. The interface barrier of F8T2/C60 is high enough to prevent the passage of charges through it, forming high charge density in each layer with a maximal recombination zone near the interface, resulting in a monopolar current density of hole inside the polymer layer and electrons inside the molecule layer [23]. Also, as the devices were fabricated with two different work functions electrodes (ΦPEDOT 5.2 eV and ΦAl 4.2 eV), a built-in potential (𝑉bi) is established in the organic layers at zero bias. Before attempting to model current versus field data [24], this inherent potential 𝑉bi must be subtracted from the applied voltage. Based on earlier reports in literature, it was found that the built-in potential is equal to the open-circuit photovoltage (𝑉oc) of the device at low temperature [25]. In this work, the 𝑉oc for each F8T2 thickness from Figure 2(b) is considered as the built-in potential value. Considering that, plotting the data from the current versus voltage curve in dark as a log-log plot of the 𝐽×𝐿(C60) versus ((V𝑉bi)/L(C60))2 (Figure 5), it is possible to notice that all the linear fittings have angular coefficient equal to one, which characterize the Mott-Gurney law of space charge limited current (SCLC) described by the expression [26]: 9𝐽=8𝜀0𝜀𝑟𝜇𝑉𝑉bi2𝐿3,(1) where 𝜀0𝜀𝑟 is the permittivity of the polymer, μ the electron mobility, and L the thickness of the C60 layer. The space charge limited current flowing in the device is induced by effective thickness-dependent mobilities of the charges in the respective layer of the device, spatial charge of holes in the polymer, and electrons in the molecule layers. The linear fittings of SCLC scaling plots for the fixed C60 thickness show that in each material layer we have a simple Mott-Gurney law. Thus, the charge transport in these devices is a property of the interface polymer/C60. From the different curves for different polymer thickness, it follows that the effective mobility of holes and electrons are dependent on the ratio between the polymer and C60 layers’ thickness. Despite the fact that a power conversion efficiency of 2.1% was achieved for one combination of thickness in these bi-layer devices, the analysis of the electrical properties in dark indicates that the mobility of charges in these devices still limits the efficiency.

4. Conclusion

We have investigated the effect of altering the F8T2 film thickness on the photovoltaic performance of devices based on the bilayer heterojunction with C60 molecules. It has been demonstrated that a power conversion efficiency of 2.1% can be achieved for F8T2 films with thickness equal to 30 nm and submitted to annealing at 100°C. From experimental and theoretical analyses, we have results that for such kind of systems the forward current is limited by spatial charge of holes in the polymer layer and electrons in the molecule layer. The series resistance calculated for an equivalent circuit of solar cells applied to our diodes plays an important part, depending not only on the layer thickness but also film morphology.