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We present herein the results of a study of the reflective polarizing photovoltaic (PV) effects in an aligned polymer bulk-heterojunction PV layer. The PV layer consisted of a composite of regioregular poly(3-hexylthiophene) and methanofullerene (P3HT:PCBM) and the fairly uniform in-plane alignment of the P3HT:PCBM PV layer was achieved by means of a simple rubbing technique. The macroscopic axial orientation of the P3HT polymer in the aligned PV layer was observed to be significantly increased in the direction of rubbing with an axial orientational order parameter of 0.40. Moreover, it was also found that the reflective polarizing polymer solar cells (PSCs) that contained the aligned P3HT:PCBM layers exhibited a greater degree of anisotropy of 1.60 for the PV efficiencies under polarized illumination along the two principal axes. These reflective polarizing PSCs were applied to new reflective type solar cell-liquid crystal displays (Solar-LCDs), which exhibited a contrast ratio of 1.7. These results form a promising foundation for various energy-harvesting polarization-dependent opto-electrical Solar-LCD device applications.
©2012 Optical Society of America
1. Introduction
Following reports of the photo-induced transfer of electrons from π-conjugated polymers (donors) to fullerenes (acceptors), a number of studies have been carried out on the development of polymer solar cells (PSCs), in order to achieve higher efficiencies in the generation of electrical power through the absorption of light [1–6]. Among them, one of particular importance to the efficient generation of photo-induced charges in a polymer photovoltaic (PV) layer is the bulk-heterojunction (BHJ) structure [1–3], which is composed of interpenetrating channel-like domains of phase-separated polymer and fullerene aggregates within a composite PV layer. By using the BHJ structure together with the pre- and/or post-thermal annealing of the PSCs, the power conversion efficiencies (PCEs) of the PSCs can be significantly increased to 2-7% [4–8]. In addition to these improvements, another important development is the fabrication of polymer PV films with an anisotropic orientation to generate anisotropic optoelectronic properties, which are not readily achievable for inorganic semiconductor PV materials [9–12]. The optoelectronic properties for the transport, decay, and generation of charges and excitons in orientationally ordered polymer PV films with a preferred direction are expected to be anisotropic and, thus, different in the directions parallel and perpendicular to the alignment direction of the polymer chains [9–11]. Indeed, several experiments have been conducted which suggest that films of aligned conjugated polymers absorb and emit parallel and perpendicular polarized photons with different probabilities. Quite recently, we also demonstrated in-plane anisotropic PV effects that are sensitive to the polarization state of the incident light, based on the alignments in the polymer BHJ PV layers fabricated by a simple rubbing technique, viz. transparent polarizing BHJ PSCs that showed different photocurrent behaviors for incident light polarized parallel and perpendicular to the aligned direction [12]. It was also shown that these transparent polarizing BHJ PSCs preferentially absorb light polarized parallel to the dipole moment of the aligned BHJ PV polymers, causing their optical transmission to be linearly polarized. This is the same as thatobserved in an ordinary dichroic polarizer [13, 14], except that the transparent polarizing BHJ PSC converts the absorbed photons into electrical energy, instead of dissipating it as heat in the dichroic polarizer. Such in-plane anisotropic PV devices can be applied to various energy-harvesting polarization-dependent applications. However, in contrast to the several studies that have been conducted on the anisotropic PV effects of inorganic [15], organic single-crystal [16], and polymeric bi-layer [17] semiconducting devices, very few related studies on the effects of the in-plane anisotropic polymer BHJ PVs and their related opto-electrical devices have been conducted till now.
In this study, we introduced reflective polarizing BHJ PSCs as replacements for conventional, purely absorptive, linear reflective polarizers in various optical systems, such as reflective type liquid crystal displays (LCDs), for use in energy harvesting applications. This concept may allow the reflecting polarizing PV device to be located under the frame of the LCD device, which would maximize its useful area, leaving the entire front surface available for the display.
2. Experimental methods
Regioregular poly (3-hexylthiophene) (P3HT, Aldrich) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM, Nano-C.) were used as received. The reflective sample cells were fabricated as follows: cleaned indium tin oxide (ITO, 80 nm, 30 ohm/square) on glass was used as the anode. After the routine cleaning of the substrate by UV-ozone treatment, a blended solution of P3HT and PCBM (1.2:0.88 by weight, 85 nm) [18] was spin-coated onto the ITO precoated with a hole-collecting poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS, CLEVIOSTM 4083, H. C. Starck Inc., 40 nm) layer. The P3HT:PCBM PV layer was then rubbed three times unidirectionally (x-direction) by a roller covered with a cotton velvet material under a pressure of ~120 g/cm2 with a translation speed of the substrate of 10 mm/s by using a modified coating machine (CT-AF300, Core Tech.) at a temperature of 110°C (Fig. 1(a) ). It is noted that a highly reproducible layer structure of the rubbed layer was achieved. After rubbing the PV layer, 1 nm Al:Li alloy (Li: 0.1 wt%) / Al cathode (50 nm) layers were then formed on the aligned PV layer by thermal deposition (0.5 nm/s) at a base pressure of less than 2.7 × 10−4 Pa. For comparison, a reflective reference PSC was fabricated with an unrubbed P3HT:PCBM layer. All of the fabricated cells were annealed for 10 min at 150 °C under an N2 atmosphere (post-thermal annealing) and then tested under ambient conditions without encapsulation.
Fig. 1 (a) Schematic of rubbing process for in-plane aligned PV layers. (b) AFM images of the reference layer (Left panel) and the rubbed sample layer (Right panel). The x-axis represents the direction of rubbing.
The PV layers were studied using polarized microscopy with a UV-vis spectrometer (Varian, Cary 1E). Topographic images were obtained by atomic force microscopy (AFM, Nanosurf AG Switzerland Inc., Nanosurf easyscan2). The PV performance was measured using a source meter (Keithley 2400) and calibrated using a reference cell (Bunkoh-keiki, BS-520) under an illumination of 100 mW/cm2 generated by an AM 1.5G light source (Newport, 96000 Solar Simulator). The in-plane anisotropy was evaluated under polarized illumination (26.1 mW/cm2) produced by passing light from the solar simulator through a linear polarizer. The average power loss due to the polarizer was corrected. The reported values from several (~10) individual cells were averaged. The incident photon-to-current collection efficiency (IPCE) spectra were obtained using a Titan Electro-optics Co., QE-IPCE 3000 measurement system.
To fabricate the reflective Solar-LCD cells, we used the nematic LC mixture, ZLI-2293 (from Merck), having an optical anisotropy, ∆n, of 0.132. The LC cell gaps were 5 μm and the cell has transparent ITO electrodes on both glass substrates. Standard rubbed polyimide alignment layers on the ITO were used to align the LC molecule to form the 90° twist nematic (TN) LC cell. The TN LC cell was placed between a sheet polarizer at the input end and a reflective polarizing PSC at the reflecting end to form the Solar-LCD device. The LC director of the TN LC cell at the input end was parallel to the passing axis of the sheet polarizer and the illuminating light was normally incident on the sheet polarizer. The alignment (rubbing) axis (x-direction) of the reflective polarizing PSC was set to be perpendicular to the passing axis of the sheet polarizer to achieve normal black mode (NB) [19] operation.
3. Results and discussion
The studied PV layer was composed of a blend of PCBM and P3HT, which exhibited supermolecular self-ordering [20, 21], and became aligned linearly in the plane of the substrate after simple rubbing (Fig. 1(a)). The surface texture of the PV layer was investigated by means of AFM (Fig. 1(b)). The isotropic topographic image of the reference layer (unrubbed P3HT:PCBM layer) shows a smooth surface with a root mean square roughness of2~3 nm. Compared with the smooth and isotropic surface of the reference layer, a grooved morphology with a roughness of 3~4 nm can be seen along the direction of rubbing in the image of the sample layer (rubbed P3HT:PCBM layer).
The polarized transmission and absorption (inset) spectra of the PV layer (Fig. 2(a) ) exhibited a strong absorption at 510 nm, which was mainly attributable to the π - π* transition of the polymer chains of P3HT. These spectra showed pronounced vibronic peaks, thereby indicating a considerable degree of interaction between the P3HT chains [22, 23]. Moreover, the rubbed sample layer shows low transmittance (T||) (or strong absorption) for incident light polarized parallel to the x-direction, while it shows high transmittance (T┴) (or weak absorption) for light polarized perpendicular to the x-direction, causing its optical transmission to be linearly polarized, as in the case of a dichroic polarizer. Thus, a high extinction ratio (T||/T┴) and dichroic ratio (DR) of 1.60 and 3.04, respectively, at 590 nm were observed for the rubbed sample layers, which show that the P3HT was preferentially aligned in the x-direction, because it is most absorptive along the axis of the chains [24, 25]. Here, DR = A||/A┴, and A|| and A┴ are the polarized absorbances of the layer for incident light polarized parallel and perpendicular to the x-direction, respectively [26, 27]. Thus, the dichroism observed in the polarized optical spectra provides a direct estimate of the average orientation of the chains with respect to the x-direction. The macroscopic orientational order parameter, S, which is defined as the statistical average (< >) of the second Legendre polynomial (P2 (cos θ)), i.e., S = <P2(cos θ)> = 1/2 (3 <cos2 θ > - 1), can be determined using the equation: S = (DR - 1)/(DR + 2) [26]. Here, θ is the angle between the P3HT molecular axis and the preferred (x-) direction. The estimated S value for the sample (rubbed) film was ca. 0.4, ascompared to 0 for the reference (unrubbed) film (at 590 nm, Fig. 2(a)). The aligned P3HT:PCBM layers (sample layers) also demonstrated clear optical birefringence (Fig. 2(b)), thus confirming the two in-plane principal axes of the rubbed sample layer, with x being the direction of rubbing and y being orthogonal to it. It is noted that the chromatic dispersion due to the absorption of the PV layer with a strong emission peak near 530~560 nm by an LED light source for the polarized-light microscope is mainly responsible for the greenish color of the microscopic textures in Fig. 2(b).
Fig. 2 (a) Polarized transmission and absorption (inset) spectra of the reference and sample layers for incident light polarized parallel (||) and perpendicular (┴) to the x-direction of the layers. (b) Polarizing microscopic textures of the sample layer at four angles of rotation of the layers under an optical polarizing microscope. The white arrows indicate the rubbing (x-) direction and the orientation of the crossed polarizers is shown by the crossed arrows (A, P).
The designed structure of the reflective polarizing PSC, as shown in Fig. 3(a) , comprises a transparent ITO anode with a hole collecting PEDOT:PSS buffer layer, an in-plane aligned P3HT:PCBM layer and a reflective Al cathode. The aligned P3HT:PCBM layer selectively absorbs propagating light whose polarization is parallel to the aligned direction (x-direction), causing the optical reflection to be linearly polarized perpendicular to the direction of alignment. Simultaneously, the aligned P3HT:PCBM layer can generate electron-hole pairs from the selectively absorbed polarized light, thus forming a reflective polarizing PV cell. We then investigated the PV performance of the produced PSCs having a structure of [ITO/PEDOT:PSS/aligned P3HT:PCBM layer (85 nm)/Al:Li (1 nm)/Al] (sample cell). A reference cell with an isotropic P3HT:PCBM layer (reference cell) was used for comparison: [ITO/PEDOT:PSS/isotropic P3HT:PCBM layer (85 nm)/Al:Li (1 nm)/Al] (reference cell). Figure 3(b) shows the current density-voltage (J–V) characteristics of the reference and sample cells in the dark and under illumination. In the dark, both cells clearly behaved as diodes with high rectification ratios (Fig. 3(b)). However, their current flows were slightly different, which may be due to the differences in the internal resistance and variations in the interfacial potential barrier between the rubbed PV layer and the cathode. The PV characteristics of the PSCs were also investigated (Fig. 3(b)) by means of 100 mW/cm2, AM 1.5G simulated solar illumination. Table 1 summarizes the device performance. When tested under non-polarized light illumination, the sample cell with the reflective Al electrode reached a power conversion efficiency of 1.73%, with a short-circuit current density (JSC) of 5.69 mA/cm2, an open-circuit voltage (VOC) of 0.65 V, and a fill factor (FF) of 0.47. Compared with the reference cell using the unrubbed P3HT:PCBM layer (Table 1), the low JSC in the sample device may be because only light polarized parallel to the x-direction of the incident light can be efficiently absorbed and transformed into a photocurrent. The low FF for the sample may be primarily caused by the high series resistance of the interface between the rubbed PV layer and the PEDOT:PSS layer with Al cathode. The performance of the PSCs was consistent with the IPCE spectra (Fig. 3(c)). Next, the polarization-dependent J-V characteristics of the PSCs were investigated under polarized illumination (Fig. 3(d)), in which 26.1 mW/cm2 polarized light was incident on the ITO of the PSCs. When illuminated with light polarized in the preferred (x-) direction, the reference cell showed a PCE of 3.19% (PCE||), which is similar to that (PCE┴) observedunder illumination polarized along the y-direction, implying an isotropic PV effect. Compared with the reference cell, the sample cell showed a greater degree of anisotropy of the PV effects, i.e., the PV effect in the sample cell was maximized when the polarization of the incident light was parallel to the x-direction, while it was minimized when the polarization was perpendicular to the x-direction, as shown in Table 1. The estimated anisotropy of the PCE (PCE|| / PCE┴) had values of up to 1.59, which is slightly higher than that (1.42) of the transparent polarizing PSCs [12]. This may be because the incident light in the reflective polarizing PSC is reflected by the electrode, causing it to re-propagate through the PV layer and be absorbed twice, while the incident light is not reflected and only absorbed once in the transparent polarizing PSCs. These results also confirm that the rubbed P3HT:PCBM layer with reflective Al cathode provides effective anisotropic PV performance. It is therefore clear from the foregoing results that the sample cell acted as a reflective polarizing PSC. Polarized illumination gave average values of the anisotropic PCEs along the principal axes that were close to those measured under unpolarized light (PCEunpol, Fig. 3(b)) and so followed the relationship 1/2 (PCE|| + PCE┴) = PCEunpol. These anisotropic PV effects provide clear evidence of the polarizing photovoltaic activity in the oriented P3HT:PCBM layers and it can thus be inferred that the improved anisotropy of the PV effect mainly originates from the anisotropic generation of excitons by the polarized absorption of the aligned PV layer.
Fig. 3 (a) Schematic structure of the reflective polarizing PSC containing an in-plane aligned PV layer. The red arrows indicate the polarization of the propagating light. (b) J-V characteristics of the reference and sample cells in the dark and under illumination. The inset shows a semilogarithmic plot of the performance of the sample cell in the dark and under illumination. (c) IPCE spectra of the reference and sample cells. (d) Polarization-dependent J-V characteristics of the PSCs under polarized illumination.
Table 1. Device performance of reference and sample BHJ PSCs studied in this work.
The polarizing PSC was assessed as a potential replacement for a typical dichroic polarizer, which is an important component of most LCDs. To investigate the functionality of the reflecting polarizing PSC as a power-generating polarizer, energy-harvesting reflective LCD cells were made and tested. Figure 4 depicts the structure of the combined TN LC cell and eflective polarizing PSC, i.e., a reflective Solar–LCD cell consisting of a linear sheet polarizer, a TN LC cell, and a reflective polarizing PSC. Two operation modes of the Solar-LCD device may be possible, depending on the orientation of the alignment (rubbing) axis of the reflective polarizing PSC. If the alignment axis of the PSC is perpendicular to the passing axis of the polarizer, the operation mode is similar to the NB mode [19]: when no voltage is applied (left pixel in the figure), the polarized light after the polarizer enters the TN cell, where the twisted orientation of the LC molecules causes it to change its polarization by 90°, making it parallel to the alignment axis of the polarizing PSC; therefore, the light is absorbed and blocked by the polarizing PSC. In contrast, when a voltage is applied to the LC cell (right pixel in the figure), the LC molecules align along the electric field and the polarization of the light passingthrough the TN LC cell does not change. Thus, light passes through the polarizing PSC and is reflected, because its polarization is perpendicular to the alignment axis of the polarizing PSC. Thus, the reflected output of the system ranges from the dark extinction state to the bright reflection state, depending on the voltage level applied to the LC cell. It is noted that if the rubbing axis of the polarizing PSC is parallel to the passing axis of the polarizer, the reflective Solar-LCD may operate in normal white mode. In this study, the normal black mode was used for the experiments. In addition to its polarizing ability, as mentioned above, the reflective polarizing PSC can generate electricity from the selectively absorbed light, rather than dissipating it as waste heat. This allows the reflective polarizing PSC to be used as a power generating polarizing element in reflective LC displays, which, unlike conventional isotropic PSCs, has its entire surface available for use in the display.
Fig. 4 Structure of the reflective Solar-LCD pixels (dark and bright), consisting of a linear sheet polarizer, TN-LC pixels, and a reflective polarizing PSC with normal black mode.
Figure 5(a) shows the experimental ratios of the luminance of the bright color (white) to that of the dark color (black) (ION/IOFF) for the reflected output from the fabricated Solar-LCD cell (see Experimental) as a function of the voltage applied to the TN cell in normal black mode (parallel-polarizer condition) under R, G, and B light illuminations. From the Fig. 5(a), it can be seen that in the voltage-off state, the reflectance is as dark as that of a TN cell between parallel polarizers and there is a dark state for the applied voltage (Vapp) below 2 Vrms. Moreover, as expected, when the voltage applied to the TN LC cell exceeds 2 Vrms, the amount of light reflected starts to increase and, thus, the ratio increases and reaches a maximum contrast ratio (CR) value of ca. 1.7 at about Vapp = 7 Vrms for red light. Although the CR value at a given voltage is different for each color, R, G or B, owing to the phase retardation effect of the TN LC cell, ION/IOFF ratio clearly increases in the high voltage regime and the threshold voltage for the bright state is only about 2 Vrms, which is well suited for various applications. After the full optimization of the TN LC cell for the purpose of controlling the phase retardation of the LC cell by adjusting the cell gap, birefringence and/or twist angle of the used LCs, one may obtain high and equal CR values for the R, G, and B light. Future experiments will focus on the optimization of the phase retardation effect for the sake of achieving low color dispersion and decreasing the operating voltage. To demonstrate the functionality of the reflective polarizing PSCs, we also fabricated and operated a Solar-LCD watch, which was made by combining a reflective polarizing PSC and a TN LCD watch, consisted of two pre-patterned ITO glasses and a commercial TN LC with a driving circuit. The photograph in Fig. 5(b) shows the operational reflective Solar-LCD watch displaying “12:00” in the normal black mode using a reflective polarizing PSC (inside dotted square) under ambient room light illumination. In addition to its polarizing ability, the inset figureclearly shows that the reflective polarizing PSC also generated electricity (*) from ambient illumination, demonstrating its functionality as a power-generating polarizer. This is unique feature of the reflective Solar-LCD. The polarizing PSC can generate in excess of 3~11 mW/m2 under typical office room light conditions (ca. 250~500 lux) and ca. 18 W/m2 under direct sunlight.
Fig. 5 (a) Voltage-dependent ION/IOFF of reflective Solar-LCD in the normal black condition. (b) Photograph of reflective Solar-LCD in operation displaying “12:00” in the normal black mode using a reflective polarizing PSC (inside dotted square) under ambient room light illumination. The inset figure shows that the polarizing PSC generated electricity (*) from ambient illumination.
4. Conclusions
In summary, we described reflective polarizing PV effects in uniformly aligned P3HT:PCBM layers achieved by simple rubbing. The macroscopic orientation of the main chain of the P3HT in the aligned P3HT:PCBM PV layer was observed to be significantly greater in the direction of rubbing, being as high as S ~0.4. It was proven that the aligned BHJ PSCs exhibit a greater degree of anisotropy of 1.59 for the PV efficiencies along the two principal axes. It was also demonstrated that the reflective polarizing BHJ PSC can be used in new reflective type Solar-LCDs, exhibiting a maximum contrast ratio of 1.7. The use of these novel anisotropic PV effects for the fabrication of reflective polarizing BHJ PSCs with highly ordered PV layers can form the basis of new types of energy-harvesting polarization-dependent opto-electrical applications, such as reflective Solar-LCDs.
Acknowledgments
The present research was supported by the Converging Research Center Program through the Ministry of Education, Science and Technology (2011K000613), by the technology development project of new and renewable energies of the Ministry of Knowledge Economy of the Republic of Korea (2011), and by Basic Science Research Program through the National Research Foundation of Korea (NRF) Grant funded by the Korea Government (MEST) (20100029416).
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