Abstract
Hybrid solar cells (HSCs) with water soluble polythiophene sodium poly[2-(3-thienyl)-ethyloxy-4-butylsulfonate] (PTEBS) thin films produced using electrospray deposition (ESD) were fabricated, tested, and modeled and compared to devices produced using conventional spin coating. A single device structure of FTO/TiO2/PTEBS/Au was used to study the effects of ESD of the PTEBS layer on device performance. ESD was found to increase the short circuit current density () by a factor of 2 while decreasing the open circuit voltage () by half compared to spin coated PTEBS films. Comparable efficiencies of 0.009% were achieved from both device construction types. Current-voltage curves were modeled using the characteristic solar cell equation and showed a similar increase in generated photocurrent with an increase by two orders of magnitude in the saturation current in devices from ESD films. Increases in are attributed to an increase in the interfacial contact area between the TiO2 and PTEBS layers, while decreases in are attributed to incomplete film formation from ESD.
1. Introduction
Hybrid solar cells (HSCs) consist of an inorganic semiconducting electron acceptor and an organic polymer electron donor sandwiched between a transparent conducting oxide anode and a metal cathode. Polymer-based solar cells have attracted considerable attention due to low production cost, solution processing, and an array of different materials suitable for device fabrication [1–3]. HSCs take advantage of the high electron mobilities ( cm2 V−1 s−1) [4, 5] in inorganic semiconductors to compensate for the poor hole mobilities in organic polymers (polythiophene polymers ≈ 0.00001–0.1 cm2 V−1 s−1) [6–9] which severely limit the electron diffusion length (~10 nm) [10, 11]. The interface between the inorganic semiconductor and the organic polymer in HSCs must be within one diffusion length of exciton generation to effectively separate excitons into holes and electrons. This presents a challenge to bilayer devices due to a limited interfacial contact area between the electron donor and acceptor layers. HSC devices have been designed with nanostructures (nanorods, nanoribbons, and interpenetrating layers) to increase the interfacial surface area [12–14].
An electrospray is a fine aerosol produced when electrical forces overcome surface tension forces in a liquid resulting in nebulization. While electrospray aerosols can be formed in a number of ways, the most common method is to apply a high electrical potential to a liquid inside of a metallic capillary tube. The liquid at the tip of the capillary forms a Taylor cone, formed by the competition between the electric force and surface tension. A fine jet of liquid is emitted from the tip of the Taylor cone and this jet breaks up into a fine aerosol of charged droplets [15]. The droplets may be on the order of 10’s of nanometers and ideally can be controlled by controlling the electric potential and solution properties [16]. Electrospray deposition (ESD) has become a viable technology for preparing polymer thin films of less than 100 nm from a precursor solution for organic photovoltaic devices [17–20] and for light-emitting diodes [21–23]. This technique offers several advantages towards large scale commercialization of organic photovoltaic devices over traditional spin coating for thin film deposition. ESD is inexpensive, efficient (minimal waste), and has a relatively high throughput [16] making it practical for large scale commercialization.
The polymers poly(3-hexylthiophene) [P3HT], poly(2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene) [MEH-PPV], and poly(3-octylthiophene) [P3OT] have widely been reported as electron donor materials in TiO2 HSCs [24–27]. These polymers are soluble in organic solvents such as toluene, chloroform, chlorobenzene, tetrahydrofuran, and xylene which are toxic in nature [28]. Commercialization of polymer based solar cells will require generation of several gigawatts of power [29], which corresponds to a solar cell (~5% efficient) area of thousands of square meters [30]. The toxicity of organic solvents makes them a poor option on this order of magnitude due to increased costs and difficulty of fabrication associated with environmental hazards. Water-soluble polymers are an obvious alternative due to water’s prevalence, low cost, and safety. The water soluble-polymer sodium poly[2-(3-thienyl)-ethyloxy-4-butylsulfonate] (PTEBS) (Figure 1) has been reported as an electron donor material in TiO2 HSCs by Qiao et al. [31–33] and in polymer solar cells using fullerene derivatives [34, 35].
In this work HSC devices were fabricated with a bilayer structure of FTO/TiO2/PTEBS/Au. ESD was compared to spin coating thin films of the water-soluble polymer PTEBS onto an interpenetrating TiO2 layer. Figure 2 shows an energy band diagram of the HSC devices fabricated. Light is absorbed by the PTEBS and an exciton is excited from the HOMO (−5.2 eV) to the LUMO (−3.2 eV) energy level [31]. The exciton is then separated into an electron and hole at the LUMO (−4.2 eV) of the TiO2 is [12]. The electron travels to the fluorine-doped tin oxide (FTO) (−4.4 eV) electrode and the hole goes to the gold (−5 eV) electrode [37, 38]. Average current densities were then modeled using the characteristic equation of an equivalent circuit of a solar cell.
2. Experimental
Transparent conductive FTO coated glass substrates of 12.5–14.5 Ω sheet resistance were purchased from Hartford Glass Company. FTO substrates were then cleaned in ultrasonic baths of detergent, acetone, isopropanol, and water for 15 minutes each. Degussa P25 TiO2 powder was suspended in 0.1 M acetic acid by sonication for 12 hours. The TiO2 suspension was then left alone for 2 days to allow large particles to settle to the bottom before spin coating onto the FTO at 1700 rpm for 40 seconds to obtain a 2.1 μm thick layer. The TiO2 was annealed for 1 hour at 500°C.
PTEBS was purchased from QCR solutions and dissolved in water at a 2% (by weight) concentration. The PTEBS layer was deposited by spin coating at 800 rpm to create a ~100 nm thick layer. Devices with a PTEBS layer from ESD were from a 0.5% solution in a water and ethanol blend with a 1 : 1 ratio before being loaded into a 1 mL syringe with a 30 G hypodermic needle. The ethanol cosolvent was used to lower the surface tension of the solution to maintain a steady Taylor cone for adequate film formation. Figure 3 illustrates a schematic of the electrospray configuration used to deposit PTEBS layers from ESD. The TiO2 coated FTO substrate was adhered to an aluminum plate with copper tape. The distance between the needle tip and the grounded aluminum plate holding the FTO substrate was 7 cm. The flow rate through the needle, controlled by a syringe pump, was 2 μL/min and the applied voltage was 7.5–9 kV. The ITO substrate was grounded to the aluminum plate and sprayed for 1 hour to create a ~100 nm thick layer. The 80 nm thick gold electrodes were then deposited via sputter coating through a mask to create 0.06 cm2 devices.
Device characterization was performed under 100 mW/cm2 illumination using 1.5 AM light from a Spectra-Physics 96000 150 W Solar Simulator. Current and voltage source/measurements were obtained using a Keithley 236. A VEECO Icon Dimension AFMandan Ambios XP-1 profilometer was used to characterize films. A Hitachi Scanning Electron Microscope (SEM) was used to image TiO2 samples and measure pore sizes.
3. Modeling
A single diode electrical equivalent circuit (Figure 4) of a solar cell was used as a model to compute output current densities (). The equation describing the current-voltage characteristics of a solar cell is represented by where is the output current density, is the saturation current, is the output voltage, is the series resistance, is the diode ideality factor, is the thermal voltage ( is the Boltzmann constant, is the temperature, and is the elementary charge), is the shunt resistance, and is the photocurrent. Equation (1) is a transcendental equation that expresses the current density output as a function of itself and . An explicit exact solution for (1) can be computed using the Lambert function [39, 40]. Equation (1) must be expressed in the form of to utilize the Lambert function, where where . The solution to is the function Lambert where the branch () of the solution corresponds to which satisfies Lambert . For simplification Lambert , the explicit solution of (1) becomes [39, 40]:
4. Results and Discussion
Table 1 displays the minimum, maximum, and average values of the short circuit current (), open circuit voltage (), and efficiency () of 15 different HSC with PTEBS layers from ESD and spin coating. Figure 5 is a current density ()-voltage () plot of the average current density values of all devices for each construction type. The average for ESD devices was 0.0443 mA/cm2 which was double the average of 0.0220 mA/cm2 for spin coated devices. The average for ESD devices was 0.3406 V which was nearly half the average of 0.6493 V for spin coated devices. The average efficiency for ESD devices was 0.006%, slightly lower than the spin coated device efficiency of 0.0076%. The maximum efficiencies achieved from both device constructions were comparable at 0.009% but were significantly less than the highest reported identically structured spin coated devices of 0.17% [32]. The average standard deviation of the current densities was 0.0040 mA/cm2 for the spin coated device construction and 0.0085 mA/cm2 for the ESD device construction. ESD constructed devices have greater variations in morphology of the PTEBS layer which led to higher standard deviations and greater performance variation.
The substantial differences in and between the two different device constructions are likely due to the differences in the interfacial contact area between the TiO2 and the PTEBS and the morphology of the PTEBS layer. Figure 6 is an SEM image of the TiO2 interpenetrating network of nanoparticles. The average pore size is ~50 nm with a range of 10–100 nm.
When using the spin coating technique, the surface tension of the water inhibits the PTEBS solution from penetrating the nanopores of the TiO2 layer. Incomplete or inadequate filling of porous TiO2 nanostructures has been extensively reported as a deficiency in HSC [5, 6, 41–44]. By contrast, the ESD technique can lead to improved pore filling for two reasons. First, ESD generates particles small enough to penetrate the pores in the TiO2 film. A histogram of particle diameters from ESD (Figure 7) reveals that the majority of PTEBS particles upon deposition are in the first bin, which ranges in diameter between 20 and 51.9 nm and has a median of 39.5 nm. The aerosolized polymer nanoparticles are able to penetrate the pores of the TiO2 film and increase the interfacial contact area between the TiO2 and PTEBS layers. In addition, the nanoparticles generated by ESD are electrically charged and are driven by the applied electric field. The electrostatic force also helps push the particles into the nanopores leading to improved interfacial contact area.
The ESD film includes interfacial boundaries between individual PTEBS particles that do not fully coalesce upon deposition. Kim et al. reported that these boundaries can diminish the performance of polymer solar cells [17]. Figure 8 is an AFM image of a PTEBS film from ESD showing that the film surface has some interfacial boundaries between particles even though it is a contiguous film. Film irregularities are believed to be the reason for a decrease in . The ESD film had a root mean squared surface roughness of 20.2 nm. The uniformity of a spin coated film as compared to an ESD film is a likely source for an increased standard deviation of output current density in devices with ESD construction.
The average curves from both coating techniques were then modeled using the characteristic equation of solar cells (see (1)). Figure 9 shows the model curves along with the average illuminated curves. The following parameter values used in the model were kept constant and agree with the previously reported values [45–47]: Ω, Ω, , J/K, C, and K. The values of the series and shunt resistance were chosen because they gave the curves the proper shape. The reverse saturation current density () and the photocurrent density () were used to fit the experimental curves. The spin coated curve agreed with the model at values of mA/cm2 and mA/cm2, and the ESD curve agreed to the model with values of mA/cm2 and mA/cm2. Increases in due to an increase in the interfacial contact area between the polymer and TiO2 were also reported by Tanaka [48]. Our model shows that devices with PTEBS films from ESD generate double the amount of , and an increase of two orders of magnitude in the compared to devices constructed from spin coating. The increase in is attributed to poor film quality of ESD PTEBS films. The interfacial boundaries between PTEBS particles increase leakage current through the device sidewalls similar to decreasing , which is known to decrease [17, 39, 40, 49, 50]. Our model responds better by varying rather than to demonstrate the effect of poor film quality on .
Figure 10 shows the dark curves of typical devices of each construction type along with the model curves. The values in the model curves were the best fits ( mA/cm2 and mA/cm2) from the illuminated curves and mA/cm2. The ESD model agrees very well with the experimental data while the spin coated model deviates slightly at higher voltages. However, the general trend of the model is largely in agreement with the experimental data.
5. Conclusions
ESD of the water-soluble polymer PTEBS thin films results in comparable efficiencies to traditional spin coating of thin films in TiO2 solar cells. Devices produced from ESD generate nearly double the but approximately half the in comparison to spin coated films. Devices from ESD have a larger standard deviation in current density due to greater variations in film morphology. The increase in is attributed to a greater interfacial contact area between the PTEBS and TiO2 layers due to the superior penetration of electrosprayed PTEBS droplets into the pores of the TiO2. Surface tension of the water prevents adequate penetration of the TiO2’s pores during spin coating. Poor film quality and surface roughness from interfacial boundaries between particles in the PTEBS layers from ESD are responsible for decreased . An exact analytic solution using the Lambert function was used to model the experimental data from both devices construction types. The model showed an increase by a factor of 2 in in the ESD construction and an increase in of two orders of magnitude in the ESD construction. The model also agreed with dark curves when mA/cm2.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.