We herein report on the improved photovoltaic (PV) effects of using a polymer bulk-heterojunction (BHJ) layer that consists of a low-band gap electron donor polymer of poly(N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3′-benzothiadiazole)) (PCDTBT) and an acceptor of [6,6]-phenyl C71 butyric acid methyl ester (PCBM70), doped with an interface-engineering surfactant additive of poly(oxyethylene tridecyl ether) (PTE). The presence of an interface-engineering additive in the PV layer results in excellent performance; the addition of PTE to a PCDTBT:PCBM70 system produces a power conversion efficiency (PCE) of 6.0%, which is much higher than that of a reference device without the additive (4.9%). We attribute this improvement to an increased charge carrier lifetime, which is likely to be the result of the presence of PTE molecules oriented at the interfaces between the BHJ PV layer and the anode and cathode, as well as at the interfaces between the phase-separated BHJ domains. Our results suggest that the incorporation of the PTE interface-engineering additive in the PCDTBT:PCBM70 PV layer results in a functional composite system that shows considerable promise for use in efficient polymer BHJ PV cells.
©2012 Optical Society of America
There has been a great deal of recent interest in the study of polymer solar cells (PSCs), ever since the first demonstrations of the photo-induced transfer of electrons from electron donor-conjugated polymers to acceptor fullerenes [1–5]. Among the devices described, the bulk heterojunction (BHJ) PSC [2, 3] is of particular interest, due to the efficient photo-induced generation of charge in its blended photovoltaic (PV) layer, which is composed of interpenetrating, channel-like domains of separated polymer and fullerene phases. Following the annealing of the BHJ structure at elevated temperatures, PSCs with PV layers of poly(3-hexylthiophene) (P3HT) and phenyl C61-butyric acid methyl ester (PCBM60) have shown high power conversion efficiencies (PCEs) of 3-5%. The BHJ films fabricated using these materials with various structures and process advancements appear to have well defined bicontinuous interpenetrating networks [4, 5]. However, although the P3HT:PCBM60 system has previously been shown to produce efficiencies of about 5%, new systems of materials are needed to improve its efficiency further for practical applications. This is because of the limitations of conventional P3HT, whose bandgap lies at around 1.9 eV, which limits absorbance to wavelengths below 650 nm . By decreasing the bandgap of the active material, it is possible to harvest more solar photons, thereby increasing the PCE. One of the most efficient low-bandgap polymers to have been discovered is poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT), which has an optical bandgap of around 1.4 eV [7, 8]. PCEs of up to 3.0% have been reported for BHJ PSCs fabricated using PCPDTBT and [6,6]-phenyl C71 butyric acid methyl ester (PCBM70) . More recently, a PCE of 5-6% was reported for a BHJ PSC that used a blend of PCBM70 and poly[N-9”-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3′-benzothiadiazole)] (PCDTBT), which has a bandgap of around 1.88 eV [10, 11]. It has also recently been shown that by incorporating small amounts (a few volume per cent) of ‘processing additives’ of alkane dithiols (which act as bad solvents) in the solution used to process the blended films of PCPDTBT and PCBM70, the PCE can be increased to 5.5% through alterations of the morphology of the BHJ . Considerable efforts have been made to improve the device performance of PSCs through the use of ‘processing additives,’ and several groups of researchers have reported highly efficient PSCs fabricated using these additives [12–14].
In contrast, relatively little progress has been made to reduce recombination loss and/or increase carrier lifetimes in the BHJ PV layer, which may now be the main limiting factors in the fabrication of highly efficient PSC devices. In order to improve carrier lifetimes, further development of the BHJ interfaces is required. Hence, in order to modify the BHJ interfaces between the phase-separated domains of the donor-conjugated polymer and the acceptor fullerene, we added a non-ionic surfactant to the PV layer. To investigate the effect of this, we used the surfactant poly(oxyethylene tridecyl ether) (PTE) as an additive, in view of its low highest-occupied-molecular-orbital (HOMO; −8.1 eV) and high lowest-unoccupied-molecular-orbital (LUMO; −2.1 eV) [15–17]. By incorporating a PTE surfactant into the P3HT:PCBM60 system, we successfully increased the PCE from 3.9 to 4.5% via a reduction of the recombination loss .
In order to extend the scope of the practical applications of the PSC devices, we further investigated a low-bandgap PCDTBT:PCBM70 PV layer with a PTE additive. We found that by introducing the PTE interface-engineering additive (0.164 wt%) into a low-bandgap PCDTBT:PCBM70 (1:4 w/w) layer, we were able to increase both the fill factor and short-circuit current density. These increases resulted in a PCE of 6.0%, which was much higher than that (4.9%) of the reference PSCs without the PTE interface-engineering additive.
2. Experimental methods
PCDTBT (Luminescence Technology Corp.), PCBM70 (Nanostructured Carbon Inc.), poly(oxyethylene tridecyl ether) (PTE, Aldrich), and poly(3,4-ethylenedioxythiophene): poly(4-styrenesulphonate) (PEDOT:PSS, Clevios PVP. Al 4083, H. C. Starck Inc.) were used as received from the manufacturers. The structures of the materials used in the photoactive layer are shown in Fig. 1 . The sample BHJ PSCs were fabricated in a sandwich structure with an indium tin oxide (ITO) anode and an Al:Li/Al cathode. Patterned 80-nm-thick ITO glass with a sheet resistance of 30 ohms/square was cleaned by sequential ultrasonic treatment in detergent, deionized water, acetone, and isopropanol, and then treated in an ultraviolet-ozone chamber for 15 min. Then, a ca. 40-nm-thick hole-collecting PEDOT:PSS buffer layer was spin-coated onto the ITO electrode. The blended solution of PCDTBT (0.456 wt%), PCBM70 (1.824 wt%), and PTE additive in dichlorobenzene was spin-coated on top of the PEDOT:PSS layer. The PV layer was about 85 nm thick. Finally, for the cathode, a ca. 1-nm-thick Al:Li alloy (Li: 0.1 wt%) layer and a pure Al (ca. 50-nm-thick) layer were formed on the PV layer via thermal deposition (0.5 nm/s), at a base pressure below Pa. The sample device structure studied was therefore [ITO/PEDOT:PSS/ PCDTBT:PCBM70:PTE /Al:Li/Al]. The active area of the fabricated device was mm2. For comparison, a reference PSC was fabricated with the structure [ITO/PEDOT:PSS/PCDTBT:PCBM70/ Al:Li/Al]. Another reference PSC was also fabricated with an interfacial PTE layer (ca. 1~2 nm) on the PV layer, i.e., [ITO/PEDOT:PSS/PCDTBT:PCBM70/PTE/Al:Li/Al], by spin-coating a PTE solution (0.075 wt% in distilled water) on the PCDTBT:PCBM70 layer. After coating, all PSCs were thermally annealed at 65 °C for 60 min. The boiling point of PTE is higher than 205 °C, so the PTE remained in the PV layer even after thermal annealing; this would not be the case for conventional processing additives.
A model PSC device was also fabricated with a structure of stacked PV layers between an ITO anode and an Al cathode, according to the following procedure: A pure PCDTBT solution was made by dissolving PCDTBT (0.456 wt%) in dichlorobenzene. After routine cleaning of the ITO glass, the PCDTBT solution was spin-coated on top of the ITO, precoated with a hole-collecting PEDOT:PSS buffer layer. The coated PCDTBT layer was about 60 nm thick. PTE surfactant molecules were then coated onto the PCDTBT layer, by spin-coating a PTE solution (0.075 wt% in distilled water, 3500 rpm) to form an ultrathin PTE additive interlayer (ca. 1~2 nm). Next, a PCBM70 layer was further deposited on top of the PTE-coated PCDTBT layer using thermal evaporation (0.5 nm/s), in order to form a well defined interface. The thickness of the PCBM70 layer was about 25 nm. For the cathode, Al:Li/Al layers were then formed on top of the active PV layers using thermal deposition (0.5 nm/s). Thus, the device structure consisted of the sequence [ITO/PEDOT:PSS/PCDTBT/ PTE/PCBM70/Al:Li/Al] (model device with PTE additives). For comparison, we also fabricated another model device that consisted of the sequence [ITO/PEDOT:PSS/PCDTBT/PCBM70/Al:Li/Al] without PTE additives. After the model devices had been fabricated, they were also annealed at 65 °C for 60 min.
The optical properties of the PV layers were investigated using a Cary 1E (Varian) UV-vis spectrometer. Topographic images of the surfaces of the PV layers were obtained using scanning atomic force microscopy (AFM, Nanosurf easyscan2 FlexAFM, Nanosurf AG Switzerland Inc.). The nanocrystalline structures of the PV layers were investigated using X-ray diffraction (XRD-Rigaku D/max 2200, Cu Kα, ). We observed the surface wettability of the PV material layers studied by measuring the water contact angle (Phoenix 300 Touch, Surface Electro Optics). The performance of the PSCs was measured under an illumination intensity of 100 mW/cm2, generated by an AM 1.5G light source (Newport, 96000 Solar Simulator). The photocurrent-versus-voltage (J-V) characteristics were measured using a source meter (Keithley 2400), and calibrated using a reference cell (Bunkoh-keiki, BS-520). The incident photon-to-current collection efficiency (IPCE) spectra were measured for the PSCs studied using an IPCE measurement system. The charge carrier mobility of the PV layers was measured using the time-of-flight (TOF) technique [18, 19], with irradiation by pulsed laser light (532 nm, Nd-YAG laser, pulse width; 4 ns). The sample was subjected to an applied bias, and the resulting photocurrent was measured using a digital oscilloscope (TDS420, Tektronix). Transient photovoltage (TPV) experiments were performed following a previously reported procedure, by connecting the devices to an oscilloscope with high input impedance (1 MΩ), which allows for the measurement of open-circuit voltage (VOC) under varying white light illumination . An Nd-YAG pulsed laser with a wavelength of 532 nm was used to generate a small perturbation in open-circuit voltage.
3. Results and discussion
The schematic structure of the BHJ PSC device may be seen in Fig. 1(a); when the PTE molecules are doped into the PCDTBT:PCBM70 BHJ PV layer to form a blended PCDTBT:PCBM70:PTE layer, the PTE surfactant molecules can spread through the domains in the BHJ PV layer, but some of the PTE molecules may become lodged at the BHJ interfaces between the phase-separated domains, as well as at the interfaces between the PV layer and either electrode. In this case, it is to be expected that the PTE molecules that lie between the phase-separated domains can block holes from the polymer domains of the donor PCDTBT, due to its low HOMO level, and transport electrons to the PCBM70 acceptor domains via the interfacial dipole effect [15–17, 21]. Thus, it may be possible to suppress the recombination of the separated charge carriers at the BHJ interfaces (e.g. polarons), thereby affording an improvement in PV performance.
Figure 2(a) shows the J-V characteristics of the BHJ PSCs with the PCDTBT:PCBM70 (reference 1), the PCDTBT:PCBM70/PTE (reference 2), and the PCDTBT:PCBM70:PTE (sample) PV layers. In the dark, all three devices showed excellent diodic behaviors with high rectification ratios, indicating good coverage of the BHJ PV layers. However, there were slight but distinct differences among the three PSCs, which implied that there were some differences in the mechanisms of the charge carrier flows through the BHJ PV layers. Under an illumination of AM 1.5G and 100 mW/cm2, fairly good PV performance was observed for the BHJ PSCs. Table 1 summarizes the PV performances of the BHJ PSC devices. With the reference active PCDTBT:PCBM70 layers, we recorded an open-circuit voltage (VOC) of ca. 0.886 V, a short-circuit current density (JSC) of 11.7 mW/cm2, and a fill factor (FF) of 47.3%. This corresponded to a PCE of 4.9%, a value comparable with those reported by others . In reference device 2 with the PCDTBT:PCBM70/PTE structure, an improved PCE of 5.4% was obtained. We attribute this improvement largely to the improved collection of electrons at the cathode that resulted from a reduction in the potential barrier between the PV layer and the cathode, which was produced by the interfacial dipole effects of the PTE molecules [15–17, 21]. Further improvements were observed in the sample PSCs with the PCDTBT:PCBM70:PTE active layer, which had a VOC of 0.904 V, and higher values of JSC and FF of 13.8 mA/cm2 and 48.2%, respectively, for a PTE concentration of ca. 0.164 wt%. These increased values resulted in an improved efficiency of 6.0%, which led to a PCE that was up to 22% higher than that of reference device 1 with the PCDTBT:PCBM70 layer. These results clearly demonstrate the superior performance of the PCDTBT:PCBM70:PTE PV layer compared to that of either of the other BHJ PV layers considered.
We further evaluated the PV performance of the PSCs that incorporated the PTE additive by measuring the IPCE spectra. The observed IPCE spectra of the PSC devices are shown in Fig. 2(b). It can be seen that the IPCE values are consistent with the variations in JSC for the PSCs with and without the PTE additive. The maximum IPCE was 73.0% at 470 nm for the sample device with the PTE additive, which corresponded to the highest JSC, while the IPCE value was about 60.9% for the reference device without the additive, which had the lowest JSC.
In order to understand the influence of the PTE additive on the PCDTBT:PCBM70 blended film, we measured the UV–visible absorption spectra, XRD patterns, and AFM topographic images of the PCDTBT:PCBM70 layers prepared both with and without the PTE additive. Figure 3(a) shows the optical absorption spectra of the PCDTBT:PCBM70 and PCDTBT:PCBM70:PTE PV films after thermal annealing at 65 °C for 60 min. The PCDTBT:PCBM70 PV film showed strong absorption bands in the visible range from 350 to 650 nm, extending to an absorption onset at 720 nm, and containing two distinct but broad absorption bands centered at ca. 380-420 nm and ca. 475-570 nm. The two broad absorption bands with peaks at 398 and 576 nm were caused by the PCDTBT, and the absorption near 450 nm was caused by the PCBM70. As shown in Fig. 3(a), the absorption spectra of the PCDTBT:PCBM70 layers were almost identical to those of the PCDTBT:PCBM70:PTE layers, indicating that the addition of small amounts of PTE additive barely altered the characteristics of the optical absorption, which are directly related to the crystalline ordering of the PV layer.
Figure 3(b) shows the XRD patterns obtained for the BHJ PV layers after thermal annealing both with and without the PTE additive. For comparison, the figure also shows the XRD pattern for a BHJ PV layer of P3HT:PCBM60 (black), in which a clear single peak may be seen at , implying a corresponding lattice constant d of 1.65 nm . In contrast, the XRD peaks for the PCDTBT:PCBM70 PV layers both with and without the PTE additive were negligibly small, within the limits of the sensitivity of detection. (All layers showed only broad XRD patterns between 12° and 40°, which were attributable to the amorphous silicon dioxide in the crystalline silicon substrate.) These XRD results show that no crystalline domains were present in the PCDTBT:PCBM70 PV layers, indicating that the BHJ PV layers may have been amorphous . It is therefore clear that neither the thermal annealing of the PCDTBT:PCBM70 PV layer nor the addition of the PTE additive into the PV layer resulted in any improvement in the nano-crystallisation of the PV layer.
Figure 3(c) shows the AFM topographic images of the PCDTBT:PCBM70 and PCDTBT:PCBM70:PTE PV films, obtained using a scanning area of μm2, before and after thermal annealing. The surface of the as-prepared PCDTBT:PCBM70 film was fairly smooth, with a root mean square (RMS) roughness of 0.6 nm. After annealing at 65 °C for 60 min, the surface roughness of the PCDTBT:PCBM70 remained unchanged (ca. 0.6 nm). These figures clearly show that the annealed samples had a texture similar to that of the as-prepared samples. These surface morphologies contrasted with those of the annealed P3HT:PCBM60 PV layer , in that we observed no ‘coarse’ texture in the annealed PCDTBT:PCBM70 layers. In comparison with the surfaces of the PCDTBT:PCBM70 layer, it is clear that surfaces with almost identical surface roughnesses were formed on the PCDTBT:PCBM70:PTE layers (RMS roughness of 0.5 nm before annealing, and 0.7 nm afterwards). It is therefore clear that the surface morphology of the PCDTBT:PCBM70:PTE layer was almost identical to that of the PCDTBT:PCBM70 layer, which indicates that the addition of the PTE additive into the PCDTBT:PCBM70 PV layer had hardly any effect on the surface structure of the PV layer.
In order to understand the effects of the PTE additive on the electrical properties of the BHJ PV layers, we also investigated the mobilities  of the charge carriers in the PCDTBT:PCBM70:PTE PV layers. Figure 4 shows representative electron and hole transients observed using the TOF technique [18, 19, 24], for the PCDTBT:PCBM70 and PCDTBT:PCBM70:PTE layers (320 nm thick). By examining the carrier transit times (τTs) in the graphs, the PCDTBT:PCBM70:PTE was determined to have an electron mobility of ca. cm2/Vs and a hole mobility of ca. cm2/Vs at . These values are almost the same as those of the PCDTBT:PCBM70 reference layer without the PTE additive (electron mobility of ca. cm2/Vs and hole mobility of ca. cm2/Vs). It should also be noted that they are almost the same as those of a typical PCDTBT:PCBM70 layer . We therefore found that the addition of the PTE additive did not alter the mobilities of the electrons and holes in the PCDTBT:PCBM70-based PV layer.
Given that the bulk film properties of the BHJ PV layers (the nano-texture, the optical absorption, the nano-crystallization, and the carrier mobility) of the PCDTBT:PCBM70 and PCDTBT:PCBM70:PTE layers were not significantly different, it may be supposed that the improved efficiency of the BHJ PSCs with the PCDTBT:PCBM70:PTE layer may have originated in the interfacial properties of the BHJ PSC structure. Therefore, in order to understand the interfacial properties of the PCDTBT:PCBM70 blend layer, we investigated the changes in contact angles due to the PTE additives for the BHJ PV layers to check the polarity of the BHJ interfaces between the donor/acceptor domains. Figure 5 shows examples of drops of water on the surfaces of pure PCDTBT and pure PCBM70 layers. The measured contact angles were ca. 98.1° for the pure PCDTBT surface, and ca. 71.5° for the pure PCBM70 surface, indicating that the water wettability of the PCDTBT was low, as a result of the hydrophobicity of its alkyl side-chains, while that of the PCBM70 surface was moderate, because of the hydrophilic nature of its acid groups. The contact angle decreased to ca. 83.3° for the mixed PCDTBT:PTE surface, while increasing to ca. 83.1° for the mixed PCBM70:PTE surface. This finding indicates that the alkyl chains of the PTE were anchored to the surface of the PCDTBT, and its hydroxyl polar head groups (–OH) protruded outwards, causing a decrease in contact angle, while the hydroxyl groups of the PTE were anchored to the surface of the PCBM70 and its alkyl chains protruded outwards, causing an increase in contact angle. In other words, the orientations of the PTE surfactant molecules at the surface were normal to both surfaces, with the molecules at one surface having the opposite orientation to those at the other. It is therefore possible that when some of the PTE additive became lodged at the interfaces of the BHJ, the PTE molecules may have become oriented in a direction normal to the interface, with the hydroxyl groups being directed towards the PCBM70 domain. It is also noteworthy that the contact angle of the PCDTBT:PCBM70:PTE layer was about 93.8°, almost the same as that of the PCDTBT:PCBM70 layer (95.7°), and it is therefore also possible that the orientation of the alkyl chains of the PTE molecules at the air-side surface of the PV layer could have been similar to that of the alkyl side-chain of the PCDTBT at the air-side surface.
When the PTE molecules were positioned and oriented at the BHJ interfaces in the PCDTBT:PCBM70:PTE PV layer, the hole-blocking and electron-transporting properties of the PTE molecules could induce changes in the charge carrier flow characteristics. In order to provide evidence that the PTE acted as an interface-engineering additive in the PV layer, we studied the characteristics of a model PSC device that contained the PTE additive. For the model device, we considered and tested a simple but well defined interface between the PCDTBT and the PCBM70 by using a stacked layer structure of a PCDTBT bottom layer and a top layer of PCBM70, which was similar to the bilayer PSC structure (see Fig. 1(b)). In the fabrication of the PCDTBT and PCBM70 layers, PTE additive molecules were placed only at the interface between the two layers using a simple solution-coating process, i.e., an ultrathin dipolar PTE layer was formed between the PCDTBT donor and the PCBM70 acceptor layers. The PV performance was then observed in order to investigate the effects of the PTE additives at the interface between the PCDTBT and the PCBM70 layers by measuring the J-V characteristics of the stacked model devices. Figure 6(a) shows the dark J-V characteristics of the stacked PSCs with and without the PTE interlayer. Good diodic behaviors with high rectification ratios were apparent for both devices in darkness. However, there were slight but distinct differences between the two stacked PSCs in terms of their current flows. The J-V curves were also measured under illumination (AM 1.5G, 100 mW/cm2), as shown in Fig. 6(b). For the PCDTBT/PCBM70 bilayer PSC without the PTE interlayer, the JSC, VOC, FF, and PCE values were 2.0 mA/cm2, 0.787 V, 56.4%, and 0.9%, respectively. In contrast, when the PTE interlayer was placed between the PCDTBT layer and the PCBM70 layer, the VOC of the model PSC (PCDTBT/PTE/PCBM70) increased from 0.787 to 0.806 V. Increases in JSC of 2.4 mA/cm2 and FF of 59.7% were also observed for the model PSC with the PTE interlayer, showing an increase in PCE from 0.9% to 1.2%. This result clearly demonstrates the improved performance of the stacked model PSC due to the effect of the PTE additives at the interface between the PCDTBT and the PCBM70 layers, compared with that of the stacked (bilayer) PSC without the additives. We attribute this improvement to the shift in energy leveloffset of the donor and acceptor, and the reduced recombination loss of charge transfer excitons due to the PTE dipolar layer at the PCDTBT/PCBM70 interface . This result clearly implies that the use of PTE interface-engineering additives at the BHJ interfaces can bring about improvements in PV performance.
In BHJ PSCs, it has previously been reported that not only the mobility (μ) but also the lifetime (τ) of the photo-generated charge carriers strongly influence their collection, because their mean path distance is proportional to both μ and τ (μ ·τ) for a given electric field . Therefore, in order to assess the lifetime and recombination loss of charge carriers due to the PTE at the PCDTBT/PCBM70 BHJ interface, we further investigated changes in the current flow characteristics that are related to the process of recombination at the BHJ interfaces [15–17] by monitoring the TPV decays of the BHJ PSC devices. Due to the open-circuit conditions used in TPV measurements, TPV decay is proportional to the excess carrier relaxation, which allows the direct measurement of the carrier lifetime . Figure 7(a) shows representative examples of TPV decays (ΔVOC) in the studied BHJ PSCs for a steady-state light-bias (VOC) of 0.85 V, following the subtraction of the constant light-bias from the decay kinetics. It is clear that the TPV decays for all the layers appear as exponential functions with lifetimes (τ) of e−1. This lifetime corresponds to a charge carrier decay due to a material-specific recombination of the photo-generated charges of the PSCs . For a given light-bias (), lifetime τ values of ca. 55.2, 76.6, and 232.2 μs were extracted from the numerical least-squares fit to the exponential function for the PCDTBT:PCBM70, PCDTBT:PCBM70/PTE, and PCDTBT:PCBM70:PTE films, respectively. It is clear that the carrier lifetime was much greater in the PCDTBT:PCBM70:PTE film than in the PCDTBT:PCBM70 and PCDTBT:PCBM70/PTE films. Thus, charge recombination occurred more slowly in the sample BHJ PV layer that contained the PTE additive, while the carrier lifetime in the PCDTBT:PCBM70/PTE layer was rather closer to that seen in the PCDTBT:PCBM70 reference layer. We therefore suggest that through the use of PTE additives at the BHJ interfaces, PCBM70 fullerene-rich domains can capture more dissociated electrons with hole-blocking from the PCDTBT polymer domains, and as a consequence, the carrier lifetime can be improved through the suppression of the recombination rate. We also observed the carrier lifetimes (τ's) of the BHJ PSCs as a function of VOC by varying the illumination intensity. Inset in Fig. 7(a) shows a semi-log plot of the variations in τ for various values of VOC for the devices studied. The observed values of ln(τ) decrease linearly with increasing VOC, implying a decay behavior of the bimolecular recombination process . A least-squares fit of the data yielded slopes of 0.67, 0.70, and 0.61 for the PCDTBT:PCBM70, PCDTBT:PCBM70/PTE, and PCDTBT:PCBM70:PTE PV layers, respectively. The slopes obtained for the studied PSCs are in good agreement with the results of similar previous analyses of PSC devices . We also used our results to extract recombination times under an approximate intensity of 1 sun (), yielding lifetimes of about 17, 22, and 89 μs for the PCDTBT:PCBM70, PCDTBT:PCBM70/PTE, and PCDTBT:PCBM70:PTE PV layers, respectively. It is thus clear that the addition of the PTE interface-engineering additive to the PCDTBT:PCBM70 PV layer causes the carrier lifetime of the film to increase significantly, by a factor of up to 5.
Finally, in order to investigate the carrier recombination loss , we also measured the J-V characteristics under a reverse bias and an illumination of 100 mW/cm2, using simulated AM 1.5G conditions. Figure 7(b) shows the photocurrent (JPH) as a function of the effective applied bias , where V is the applied bias. JPH was obtained by subtracting the dark current from the current under illumination, i.e., . V0 is the bias for which . At a large reverse bias (), JPH values of ca. 123, 135, and 138 A/m2 were obtained for the PCDTBT:PCBM70, PCDTBT:PCBM70/PTE, and PCDTBT:PCBM70:PTE films, respectively. This clearly indicates that the photocurrents in the PCDTBT:PCBM70/PTE and PCDTBT:PCBM70:PTE films were nearly identical, and were higher than the photocurrent in the PCDTBT:PCBM70 film without the PTE additive. It is therefore clear that the PTE molecules at the cathode interface improved the collection of photogenerated charge carriers. As shown in Fig. 7(b), under a low reverse bias (), the photocurrent in the PCDTBT:PCBM70/PTE layer approached that of the PCDTBT:PCBM70 film, and was lower than the photocurrent in the PCDTBT:PCBM70:PTE film, indicating that the PTE molecules at the BHJ interfaces in the PV layer contribute to the inhibited recombination of dissociated charge carriers at the BHJ interfaces.
By considering the foregoing effects of the PTE additive on PV performance, it is clear that the PTE interface engineering additives largely suppress the recombination of the dissociated charges, and the charges in the BHJ PV layer preferentially leave the device rather than being annihilated within the PCDTBT:PCBM70:PTE layer of the sample device. This clear improvement in PV performance in the interface-engineered BHJ PSC device confirms the advantages of using PTE additives in the low-band gap BHJ PSC structure.
In conclusion, we have reported on the use of a low-bandgap PCDTBT:PCBM70-based PV layer that incorporates a PTE surfactant, which was used to engineer the BHJ interfaces in PSCs. We have shown that BHJ PSCs that contain the interface-engineering PTE additive are more efficient than conventional PSCs. A high PCE (6.0%) was obtained for our PCDTBT:PCBM70 (1:4 w/w) PSC device using 0.164 wt% of the PTE additive, which yielded improvements in PCE of up to 22%. This improvement may be attributed to the increased selective flow of dissociated charge carriers, not only at the interfaces between the PV layer and the electrodes, but also at the BHJ interfaces between the PCDTBT and PCBM70 domains. Our findings show that a combination of PTE interface-engineering additives and high-performance low-band gap PV materials holds great potential for the development of a new generation of highly efficient PSCs.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (2012R1A2A2A01015654), and by the Converging Research Center Program through the Ministry of Education, Science and Technology (2012K001303), and by the Basic Science Research Program through the National Research Foundation of Korea (NRF) Grant funded by the Korea Government (MEST) (20100029416).
References and links
1. N. S. Sariciftci, L. Smilowitz, A. J. Heeger, and F. Wudl, “Photoinduced electron transfer from a conducting polymer to buckminsterfullerene,” Science 258(5087), 1474–1476 (1992). [CrossRef] [PubMed]
2. G. Yu, J. Gao, J. C. Hummelen, F. Wudl, and A. J. Heeger, “Polymer photovoltaic cells: enhanced efficiencies via a network of internal donor-acceptor heterojunctions,” Science 270(5243), 1789–1791 (1995). [CrossRef]
3. C. J. Brabec, N. S. Sariciftci, and J. C. Hummelen, “Plastic solar cells,” Adv. Funct. Mater. 11(1), 15–26 (2001). [CrossRef]
4. G. Li, V. Shrotriya, Y. Yao, and Y. Yang, “Investigation of annealing effects and film thickness dependence of polymer solar cells based on poly(3-hexylthiophene),” J. Appl. Phys. 98(4), 043704 (2005). [CrossRef]
5. W. Ma, C. Yang, X. Gong, K. Lee, and A. J. Heeger, “Thermal stable, efficient polymer solar cells with nanoscale control of the interpenetrating network morphology,” Adv. Funct. Mater. 15(10), 1617–1622 (2005). [CrossRef]
6. K. M. Coakley and M. D. McGehee, “Conjugated polymer photovoltaic cells,” Chem. Mater. 16(23), 4533–4542 (2004). [CrossRef]
7. G. Dennler, M. C. Scharber, and C. J. Brabec, “Polymer-fullerene bulk-heterojunction solar cells,” Adv. Mater. (Deerfield Beach Fla.) 21(13), 1323–1338 (2009). [CrossRef]
8. M. Morana, M. Wegscheider, A. Bonanni, N. Kopidakis, S. Shaheen, M. Scharber, Z. Zhu, D. Waller, R. Gaudiana, and C. Brabec, “Bipolar charge transport in PCPDTBT-PCBM bulk-heterojuction for photovoltaic application,” Adv. Funct. Mater. 18(12), 1757–1766 (2008). [CrossRef]
9. J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T.-Q. Nguyen, M. Dante, and A. J. Heeger, “Efficient tandem polymer solar cells fabricated by all-solution processing,” Science 317(5835), 222–225 (2007). [CrossRef] [PubMed]
10. S. H. Park, A. Roy, S. Beaupré, S. Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee, and A. J. Heeger, “Bulk heterojunction solar cells with internal quantum efficiency approaching 100%,” Nat. Photonics 3(5), 297–302 (2009). [CrossRef]
11. J. Zhou, X. Wan, Y. Liu, F. Wang, G. Long, C. Li, and Y. Chen, “Synthesis and photovoltaic properties of a poly(2,7-carbazole) derivative based on dithienosilole and benzothiadiazole,” Macromol. Chem. Phys. 212(11), 1109–1114 (2011). [CrossRef]
12. J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger, and G. C. Bazan, “Efficiency enhancement in low-bandgap polymer solar cells by processing with alkane dithiols,” Nat. Mater. 6(7), 497–500 (2007). [CrossRef] [PubMed]
13. G. Garcia-Belmonte and J. Bisquert, “Open-circuit voltage limit caused by recombination through tail states in bulk heterojuction polymer-fullerene solar cells,” Appl. Phys. Lett. 96(11), 113301 (2010). [CrossRef]
14. Y. Liang, Z. Xu, J. Xia, S.-T. Tsai, Y. Wu, G. Li, C. Ray, and L. Yu, “For the bright future-bulk heterojunction polymer solar cells with power conversion efficiency of 7.4%,” Adv. Mater. (Deerfield Beach Fla.) 22(20), E135–E138 (2010). [CrossRef] [PubMed]
15. Y. I. Lee, M. Kim, Y. Ho Huh, J. S. Lim, S. Cheol Yoon, and B. Park, “Improved photovoltaic effect of polymer solar cells with nanoscale interfacial layers,” Sol. Energy Mater. Sol. Cells 94(6), 1152–1156 (2010). [CrossRef]
16. B. Park, Y. H. Huh, and M. Kim, “Surfactant additives for improved photovoltaic effect of polymer solar cells,” J. Mater. Chem. 20(48), 10862–10868 (2010). [CrossRef]
17. J. H. Park, S. S. Oh, S. W. Kim, E. H. Choi, B. H. Hong, Y. H. Seo, G. S. Cho, B. Park, J. Lim, S. C. Yoon, and C. Lee, “Double interfacial layers for highly efficient organic light-emitting devices,” Appl. Phys. Lett. 90(15), 153508 (2007). [CrossRef]
18. G. Dennler, A. J. Mozer, G. Juška, A. Pivrikas, R. Österbacka, A. Fuchsbauer, and N. S. Sariciftci, “Charge carrier mobility and lifetime versus composition of conjugated polymer/fullerene bulk-heterojuction solar cells,” Org. Electron. 7(4), 229–234 (2006). [CrossRef]
19. J. Huang, G. Li, and Y. Yang, “Influence of composition and heat-treatment on the charge transport properties of poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester blends,” Appl. Phys. Lett. 87(11), 112105 (2005). [CrossRef]
20. H. C. Hesse, J. Weickert, M. Al-Hussein, L. Dössel, X. Feng, K. Müllen, and L. Schmidt-Mende, “Discotic materials for organic solar cells: effect of chemical structure on assembly and performance,” Sol. Energy Mater. Sol. Cells 94(3), 560–567 (2010). [CrossRef]
21. H.-L. Yip, S. K. Hau, N. S. Baek, H. Ma, and A. K.-Y. Jen, “Polymer solar cells that use self-assembled-monolayer-modified ZnO/Metals as cathodes,” Adv. Mater. (Deerfield Beach Fla.) 20(12), 2376–2382 (2008). [CrossRef]
22. B. D. Cullity, Elements of X-Ray Diffraction (Addison-Wesley, 1956).
23. S. Cho, J. H. Seo, S. H. Park, S. Beaupré, M. Leclerc, and A. J. Heeger, “A thermally stable semiconducting polymer,” Adv. Mater. (Deerfield Beach Fla.) 22(11), 1253–1257 (2010). [CrossRef] [PubMed]
24. A. J. Mozer, G. Dennler, N. S. Sariciftci, M. Westerling, A. Pivrikas, R. Österbacka, and G. Juška, “Time-dependent mobility and recombination of the photoinduced charge carriers in conjugated polyer/fullerene bulk heterojuction solar cells,” Phys. Rev. B 72(3), 035217 (2005). [CrossRef]
25. S. R. Cowan, R. A. Street, S. Cho, and A. J. Heeger, “Transient photoconductivity in polymer bulk heterojuction solar cells: Competition between sweep-out and recombination,” Phys. Rev. B 83(3), 035205 (2011). [CrossRef]
26. B. Yang, Y. Yuan, P. Sharma, S. Poddar, R. Korlacki, S. Ducharme, A. Gruverman, R. Saraf, and J. Huang, “Tuning the energy level offset between donor and acceptor with ferroelectric dipole layers for increased efficiency in bilayer organic photovoltaic cells,” Adv. Mater. (Deerfield Beach Fla.) 24(11), 1455–1460 (2012). [CrossRef] [PubMed]
27. P. P. Boix, J. Ajuria, R. Pacios, and G. Garcia-Belmonte, “Carrier recombination losses in inverted polymer: Fullerene solar cells with ZnO hole-blocking layer from transient photovoltage and impedance spectroscopy techniques,” J. Appl. Phys. 109(7), 074514 (2011). [CrossRef]
28. V. Shrotriya, Y. Yao, G. Li, and Y. Yang, “Effect of self-organization in polymer/fullerene bulk heterojuctions on solar cell performance,” Appl. Phys. Lett. 89(6), 063505 (2006). [CrossRef]