Enhancing Efficiency of Organic Bulkheterojunction Solar Cells by Using 1,8-Diiodooctane as Processing Additive
Organic solar cells (OSCs) are attractive as an al- ternative to inorganic devices for their easy fabrication and solution-processability. A major and unsolved problem with bulk heterojunction devices remains the optimization of the network morphology. Here, we discuss the influence of the 1,8-diiodooctane (DIO) solvent additive on the efficiency of OSCs and show that by selectively controlling the crystallization of the organic ma- terial, the power conversion efficiency (PCE) can be increased by about 30%. For P3HT:PCBM-based devices, the power con- version efficiency (PCE) was increased from 3.7% to 4.9% for PCPDTBT:P3HT:PCBM-based devices from 3.2% to 4.1%. This improvement is due to the higher I/subSCsub/ , which is in agreement with the higher external quantum efficiency (EQE) observed on the de- vices fabricated with DIO. We correlate this to an increase of the surface roughness observed with atomic force microscopy (AFM) analysis. We demonstrate that the effect of the DIO additive is equivalent to a high-temperature thermal annealing.
I. INTRODUCTION
OVER the past few years, interest in cheap photovoltaic solutions has increased because of growing demand for renewable energy sources. Polymer-based organic solar cells (OSCs) offer a cost-effective option for solar energy conver- sion and are attractive as a solution-processable alternative to classical inorganic photovoltaic solutions.
The most commonly used materials for polymeric solar cell fabrication are Poly(3-hexylthiophen-2,5-diyl) (P3HT) as electron donor and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as electron acceptor [1]. Recent reports have shown that this material combination can reach power conversion efficiencies (PCE) over 5% [2] with reported ISC values up to 16 mA/cm2 [3]–[6]. By introducing new fullerene derivatives, the PCE of P3HT-based solar cells can be increased up to 6.5% [7].
OSCs are based on a generic stack, which includes an an- ode, an electron blocking interlayer, a semiconductor layer, and a cathode. The organic semiconductor layer is usually a Bulk Heterojunction (BHJ) [8] with an interpenetrating network of polymer/fullerene composites sandwiched between electrodes with different work functions for efficient charge extraction. Photon absorption results in the creation of an exciton that diffuses to a heterojunction and dissociates at the polymer/fullerene interface with the electron going to the fullerene and the hole to the polymer side. Because of the different work function of the electrodes, electrons move to the cathode and holes to the anode, where they can be extracted. While the good charge separation properties of this BHJ design can provide internal quantum efficiencies (IQE) up to almost 100% [9], reduced external quantum efficiencies (EQE) are usually observed because of different loss mechanisms. In order to efficiently extract all photogenerated charges, the complex mechanisms involved in the generation, transport, and extraction of the charge carriers generated in the semiconductor layer have to be further investigated. In particular, a critical point still to be addressed is the role of the complex morphology of the organic semiconductor blend and, in particular, how it affects the power conversion efficiency of the final device.
In general, increasing the structural order of the BHJ is desirable as it improves charge carrier mobility and extraction. However, since the exciton diffusion length in typical organic semiconductors is estimated to range on a length scale of 10-nm [10], [11], the formation of big crystals can lead to exciton relaxation to the ground state prior to reaching a polymer/fullerene interface and is, therefore, undesirable. Nonetheless, a certain level of ordering is required to ensure efficient charge transport in the material.
Over the past few years, many different and elegant techniques have been reported to control the BHJ morphology and, thus, improve solar cell efficiency. Postprocessing annealing treatments are commonly used to control the polymer/fullerene morphology. For a blend made from P3HT:PCBM, thermal annealing is known to significantly increase the efficiency of BHJ-based OSCs. This increase has been associated with P3HT reordering and improvement of the nanoscale morphology, the precise mechanism however remains unclear [12]. In particular, heating the P3HT:PCBM blend to temperatures above the P3HT glass transition temperature facilitates the phase separation that allows, in the case of BHJ-based organic photodiodes, further improvement of the EQE [13]. Other techniques commonly used to manipulate the blend morphology include varying the mixing ratio of the polymers in the blend or the solvent in which they are mixed or the rate of drying as well as changing the thermal and vapor annealing conditions. These techniques all operate on both p- and n-materials at the same time and usually all lead to a common arrangement of the components, which consists of a vertically and laterally phase-separated blend of crystalline P3HT and PCBM with the PCBM close to the PEDOT:PSS interlayer and the P3HT close to the cathode. This vertical composition profile is opposite to that of the ideal donor-rich structure close to the anode and acceptor-rich structure next to the cathode [14].
In recent research, the use of processing additives has become popular [15], [16], because of its effectiveness in enhancing the device efficiency and its ease of implementation: adding only a small amount of additive into the blend before film deposition is sufficient to increase the device efficiency significantly [17]. These additives have mainly been adopted with polymeric low-bandgap materials in combination with PCBM [16], [18]– [20]. Recently, it has been shown that the use of additives also improves the performance of small molecule [21] and perovskite-based solar cells [22]. Although the underlying working mechanism has yet to be completely understood, they seem to operate only one material. It has been reported that with the addition of a small amount of additive to the solution, the interpenetrating network of the p- and n-materials in the BHJ can be altered, by increasing phase separation between polymer and fullerene [18]. For PCPDTBT:PCBM devices, this usually leads to an increased PCE. Two criteria for such kind of processing additives were identified: a higher boiling point than the host solvent, usually chlorobenzene (CB) or dichlorobenzene (DCB), and selective solubility of the fullerene component of the blend. When the solvents evaporate after film deposition, the PCPDTBT will solidify first, while the PCBM remains in a liquid phase longer due to the high boiling point of the additive, thereby enabling control of the phase separation and the resulting morphology of the BHJ network [17], [18]. The resulting layer has a phase-separated morphology comprising domains of pure, crystalline PCPDTBT fibrils, and other do- mains of PCBM-rich mixture with amorphous PCPDTBT [17]. The most commonly used additives for low-band gap materials are 1,8-diiodooctane (DIO), 1,8-octanedithiol (ODT) and 1,8-dichlorooctane (DCO).
In this paper, we present a study of the effect of the processing additives in organic solar cells fabricated with P3HT:PCBM and show that up to 30% efficiency improvements can be achieved with the addition of a small amount of DIO. This improvement is caused by an increase of open circuit voltage, fill factor, and short circuit current. The same effect can be observed on a device fabricated with a ternary P3HT:PCBDTBT:PCBM blend. We show that in both cases, the observed increase in PCE is related to a morphology change of the semiconductor layer. Furthermore, we demonstrate that the addition of DIO has an effect similar to a thermal annealing, but without the need of a high temperature step. In brief, the DIO allows the fabrication of high-quality semiconductor films at low processing temperatures. The label “with DIO” is used in this paper to abbreviate P3HT:PCBM or P3HT:PCPDTBT:PCBM films fabricated in a dichlorobenzene solution where 2.5 vol.% DIO was added to the solution prior to spin deposition of the blend onto the PEDOT:PSS interlayer.
II. P3HT:PCBM SOLAR CELLS
At first, we tested the effect of DIO on a P3HT:PCBM (1:0.75 wt.) blend. Solar cells were fabricated with ITO as anode, 25-nm PEDOT:PSS as interlayer, different thicknesses of P3HT:PCBM with or without DIO as semiconductor and 6-nm Ca and 100-nm Ag as cathode (see methods for details). In Fig. 1(a), the energy band diagram of the device is shown. HOMO and LUMO levels for the P3HT and PCBM were taken from [23]. EQE measurements without bias of solar cells fabricated from P3HT:PCBM with DIO and varying the thickness of the active layer from 80 to 480-nm are presented in Fig. 1(b). Three different shapes can be observed. First, thin solar cells show an EQE with a maximum at about 520-nm wavelength and two additional peaks around 330 and 620-nm. The peaks at 520 and 620-nm are characteristic P3HT peaks which correspond to the π–π∗ transitions of different vibrational states of P3HT [24]; the peak at 330-nm is attributed to PCBM absorption. Second, with increasing layer thickness, the peak for green wavelengths remains almost constant at about 70%, while the others two peaks rise, which leads to a flatter shape with only one main peak. The highest EQE is found for a layer thickness of about 200-nm with 77% for visible wavelengths. Eventually with fur- ther increasing the active layer thickness, the EQE for green wavelength declines and the maximum shifts to about 620-nm wavelengths.
In Fig. 1(c), optical absorbance measurements on P3HT:PCBM with DIO layers with different thicknesses around the 200-nm PCE optimum are presented showing that the light absorption rises with increasing film thickness. This observation fits with the Lambert Beer law. Therefore, we can conclude the observed drop in EQE for very thick devices is not related to optical absorption but to electrical recombination phenomena.
In agreement with the EQE measurements, the highest PCE of 4.9% is observed on the device with 200-nm BHJ thickness [see Fig. 1(d)]. The cell has an Isc of 14.9 mA/cm2, a VOC of 0.60 V, and an FF of 55%. A solar cell fabricated without DIO is also shown as reference with an ISC of 11.65 mA/cm2, a VOC of 0.58 V, and a FF of 54%. The observed high FF > 50% suggests that the photocurrent depends only weakly on the internal field of the device. With the use of DIO, an improvement of about 30% in the PCE is observed.
In order to better understand the improvement caused by the DIO, we studied the semiconductor layers by optical absorption measurements. Two solutions were fabricated with P3HT:PCBM (1:0.75 wt.) in DCB (2 wt.% solution), one with and one without 2.5 vol.% DIO. The blends were stirred on a hotplate at 80 °C for 1 h prior to spin-coating onto a transparent glass slide. Fig. 2(a) and (b) shows the optical absorption spectra of the films measured after fabrication (Ref. in the figure) and then after several successive annealing steps up to 200 °C (each step taking 10 min).
After processing, the P3HT:PCBM layer shows low absorption in the visible range (see Fig. 2(a)). After successive annealing steps up to 140 °C, the absorption in the visible rises while the absorption at 320-nm from the PCBM stays constant.
For annealing temperatures over 140 °C (P3HT glass transition temperature), the absorption in the visible further increases, but the PCBM absorption at 320-nm decreases. Contrary, the layer fabricated with DIO [see Fig. 2(b)] already shows high absorption for visible wavelengths after fabrication and only a weak improvement with annealing. Again, for temperatures > 140 °C, we observe a small increase of the absorption in the visible range and a reduction of the PCBM absorption at 320-nm. In Fig. 2(c) to (f), microscope images of P3HT:PCBM films annealed at 140 and 160 °C are shown. When layers are annealed at 160 °C, domain structures appear which are due to the formation of PCBM aggregates [25]. We suppose that the reduction of absorbance for low wavelength is due to PCBM crystallization.
These results indicate that the use of DIO has a similar effect as a thermal annealing, improving the absorption of the P3HT:PCBM layer. By heating the layer with DIO at only 50 °C, we already observe the same absorbance as with the layer without DIO at 140 °C. This makes the use of processing additives an interesting alternative to thermal annealing for devices that cannot stand higher temperatures, e.g., devices fabricated from blends with three or more materials, in which some components degrade under high temperatures.
III. P3HT:PCPDTBT:PCBM SOLAR CELLS
In a next step, we further investigated the effect of DIO on a ternary blend. The possibility to mix several polymers with fullerenes to create ternary blends has been previously reported [26]. Of special interest is the incorporation of a low-bandgap material to extend the P3HT absorption spectrum into the near- infrared (NIR) and to increase the cell’s open circuit voltage. PCPDTBT is a promising low-bandgap material and a good candidate to fabricate ternary blends together with P3HT and PCBM because of its ideal energy levels and charge carrier properties [27].
For the first approach, a material ratio of 1:1 for P3HT:PCPDTBT was chosen. In Fig. 3(a), optical absorbance measurements of films made from P3HT:PCPDTBT:PCBM (1:1:2 wt.) with and without DIO (2.5 vol.%) are presented. Films were not annealed and the spectra are normalized to the isotropic PCBM peak at around 330-nm. Interestingly, the film fabricated with DIO shows a dark-red color, while the one without DIO has a green color. The layer without DIO shows an NIR peak at about 720-nm typical for the PCPDTBT [18] and a peak at about 330-nm from the PCBM but low absorption for green wavelength attributed to the P3HT. On the contrary, the layer fabricated with DIO shows the same peaks for PCPDTBT and PCBM, but also the characteristic P3HT peaks at 520 and 550-nm and the shoulder at 595-nm [24]. The additional PCPDTBT peak at about 800-nm is due to the aggregation of the π-conjugated polymer of PCPDTBT [24], [28]. Although the same amount of P3HT was present in both films, the layer fabricated without DIO has a much lower absorption of green wavelengths compared to the one with DIO. This result indi- cates that with the use of DIO the P3HT absorption in the blend is enhanced.
In a next step, devices were fabricated with the same solutions, as described in the method section. Ternary blends need to be annealed to optimize device performance [26]. Accordingly, before cathode deposition, samples were annealed at 80 °C for 10 min.
In Fig. 3(c), I–V measurements under illumination (1 sun) of P3HT:PCPDTBT:PCBM (1:1:2 wt.) devices with and without 2.5 vol.% DIO are presented. The cell fabricated with DIO shows a Voc of 0.59 V, an Isc of 11.2 mA/cm2, and FF of 46.4%, which result in a measured PCE of 3.08%. The reference device without DIO has an Isc of 1.73 mA/cm2 and a Voc of 0.72 V, resulting in a poor PCE.
This extreme performance difference is also represented in the EQE measurements (without bias) in Fig. 3(d). While the device with DIO shows a maximal EQE of 50% for green wavelengths, the one without only goes up to 4% for the same wavelengths. In agreement with the absorbance measurements [see Fig. 3(a)], the device with DIO features peaks for all three components of the BHJ, while the device without DIO lacks a response from the P3HT. This behavior is also reflected by the different observed Voc; the device with DIO shows a Voc close to 0.6 V, which is typical for P3HT:PCBM OSCs, while the device with- out DIO features a Voc close to 0.7 V, which is typical for PCPDTBT:PCBM-based devices.
In order to try to increase the P3HT absorption, an additional annealing step of 140 °C for 10 min was performed on the samples. For the OSC without DIO, we observe that the en- capsulated area of the device change color from green (typical PCPDTBT color) to brown-red (typical P3HT color); the nonencapsulated area remains green [see Fig. 3(b)]. As can be seen in Fig. 3(e), after annealing, the Vo c of the cell shifts to 0.59 V and the Isc rises to 2.09 mA/cm2. The shift of the Voc to a value close to 0.6 V indicates that now the main light absorbing material is the P3HT. Accordingly, an extra peak at about 540-nm after annealing can be observed in the EQE [see Fig. 3(f)].
This phenomenon cannot be observed on the device fabricated with DIO. EQE measurements show that the device has sensitivity in the visible and in the NIR before as well as after a second annealing step of 140 °C. However, after the second annealing, the Isc decreases, probably due to a degradation of the PCPDTBT (not shown). These results confirm that it is possible to use DIO to fabricate high efficiency OSCs at low temperatures.
A higher PCE for ternary blend devices is observed, when a lower amount of PCPDTBT relative to P3HT is used [26]. In Fig. 4(a), the energy band diagram of the device with ternary blend is shown. PCPDTBT HOMO (4.9 eV) and LUMO (3.5 eV) values were taken from [16], [23], [29]. Absorbance measurements of P3HT:PCPDTBT:PCBM (1:0.2:1 wt.) layers with and without DIO (2.5 vol.%) are presented in Fig. 4(b). The spectra have been normalized to the isotropic PCBM peak at around 330-nm. Compared with the previous measurements in Fig. 3(a), P3HT absorption is now much higher than the PCPDTDT absorption due to the difference in concentration of the polymers in the blend. With this material ratio, the addition of DIO causes no extra peaks in the absorption measurements, but slightly increases polymer absorption.
The maximal PCE of about 4.1% is observed on a solar cell fabricated with ITO/PEDOT:PSS/P3HT:PCPDTBT:PCBM (1:0.2:1 wt.) + DIO (2.5 vol.%)/Ca/Ag [see Fig. 4(c)]. The cell was fabricated with 25-nm PEDOT Al4083, about 230-nm semiconductor and 6-nm Ca and 100-nm Ag.
The cell shows a Voc of 0.60 V, a Isc of 11.19 mA/cm2, and a FF of 61%. The EQE of the device measured without bias is presented in Fig. 4(d). The same solar cell fabricated without DIO is shown for comparison. The device shows a Voc of 0.62 V, a Isc of 9.05 mA/cm2, and a FF of about 56.5%, which result in a measured PCE of 3.17%. Again, the addition of DIO to the blend results in an OSC with an improved Isc. In addition, a slight reduction of the Vo c with the use of DIO in the ternary blend is observed [28]. This observation is consistent with the absorption measurements as well as with the EQE measurements [see Fig. 4(b) and (d)], where the increase of the P3HT absorption/EQE is higher than the increase of the PCPDTBT absorption/EQE upon DIO addition.
IV. ATOMIC FORCE MICROSCOPY ANALYSIS
Finally, the morphologies of the BHJ layers fabricated with and without DIO were investigated by atomic force microscopy (AFM). P3HT:PCBM and P3HT:PCPDTBT:PCBM blends show comparable low surface roughness (see Fig. 5); the addition of the PCPDTBT to the binary blend does not significantly change the layer topography. With the use of DIO, the active layer surface morphology changes and the surfaces rough- ness increases significantly [28]. The increase of the roughness suggests that an additional phase separation occurs in the BHJ due to the DIO.
V. CONCLUSION
In conclusion, using processing additives such as DIO plays an important role in enhancing the PCE of an organic solar cell. This method is attractive due to its ease of implementation; adding only a small amount of additive into the blend before film deposition will result in a significant improvement of the solar cell performance.
Here, we studied the effect of the DIO additive on P3HT:PCBM and PCPDTBT:P3HT:PCBM devices, and we demonstrated that the efficiency for both material systems can be increased by about 30%. For P3HT:PCBM devices, the PCE was increased from 3.7% to 4.9%, for PCPDTBT:P3HT:PCBM devices from 3.2% to 4.1%. This improvement is due to the higher Isc in agreement with the higher EQE observed on the devices fabricated with DIO. We correlate this to an increase of the surface roughness observed with AFM analysis.
Furthermore, for both studied stacks, we have shown that the DIO has an effect similar to thermal annealing, enhancing the polymer absorption at low temperatures. We propose that this propriety of the DIO can be of interest to fabricate a high-quality hybrid film with three or more materials in which one or more components degrade upon high-temperature treatments, and an annealing step to improve the blend morphology is not possible.
VI. METHODS
Solar cells were fabricated entirely inside a glove box with a controlled nitrogen atmosphere on prestructured, ITO-coated transparent glass substrates. After cleaning the substrates in acetone, isopropanol and deionized water, the ITO surface was activated by oxygen plasma prior to spin-coating about 25-nm of Clevios PEDOT:PSS (AL 4083) diluted in isopropanol with a ratio of 1:3 wt. Then, the substrate was annealed on a hot- plate at 140 °C for 10 min. For the P3HT:PCBM devices, a 2% blend of P3HT:PCBM (1:0.75 wt.) in DCB with or without DIO (2.5 vol.%) was spin-coated onto the PEDOT: PSS layer with varying spinning velocities resulting in differ- ent layer thicknesses. Samples were annealed at 140 °C for 10 min. For the P3HT:PCPDTBT:PCBM devices, a 2% blend of P3HT:PCPDTBT:PCBM (1:1:2 or 1:0.2:1 wt.) in DCB with or without DIO (2.5 vol.%) was spin-coated onto the PEDOT:PSS layer with varying spinning velocities. Samples were annealed at 80 °C for 10 min. Finally, 6-nm Ca and 100-nm Ag were thermally evaporated as cathode. The devices were encapsulated with epoxy glue and a transparent thin glass slide. The active area of the devices, determined by the anode and cathode overlap, was 9 mm2.
I–V characteristics of the diodes were recorded using a Keythley 2602A. For solar cell performance, an Air Mass 1.5 Global (AM 1.5 G) solar simulator (Oriel Sol1A) with an irradiation intensity of 100 mW/cm2 was used. EQE spectra were recorded using a lock-in setup and a Si reference diode for calibration. AFM pictures were taken with a JSPM-5000 Scanning Probe Microscope.
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