A grating-structured interface of a poly(3-hexylthiophene) (P3HT) and n-type [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)-based bulk-heterojunction (BHJ) photovoltaic (PV) cell was designed and fabricated to obtain a desirable thickness distribution of the deposited bathocuproine (BCP) buffer layer to efficiently utilize its potentials. As a master mold of the grating-structure, a commercially available recordable digital versatile disc (DVD-R) substrate was employed. The grating-structured surface of the P3HT:PCBM layer was successfully produced by duplication from a poly(dimethylsiloxane) secondary mold using the spin cast molding technique. From morphological observations of the grating-structured surface covered with vapor-deposited BCP, we roughly estimated the ratio of the BCP thickness at “walls” to that at “top” and “bottom” regions to be ∼0.5. The grating-type BHJ PV cell with a 5-nm-thick BCP layer exhibited the maximum power-conversion efficiency (ηp) of 3.51%. Compared with the conventional flat-type BHJ PV cell with a 20-nm-thick BCP layer, the performance of the grating-type BHJ PV cell with a 20-nm-thick BCP layer was remarkably improved, owing to the contribution of the wall side contact, which provides a lower-barrier path of the electrons toward the cathode through the thinner BCP layer.

Organic photovoltaic (PV) cells have been attracting increased attention owing to their potentials to meet the demands of inexpensive, renewable, and clean energy resources. In particular, over the past decade, the bulk-heterojunction (BHJ) organic PV cells1,2 composed of p-type poly(3-hexylthiophene) (P3HT) and n-type [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) have been intensively investigated as one of the most prominent candidates for a polymer PV cell.3–9 In contrast to planar-heterojunction (PHJ) architectures,10–14 the interpenetrating networks in the BHJ’s blended active layer of P3HT:PCBM enable a large expansion of the total area of the p-n interface. The enhanced p-n junction significantly facilitates the exciton dissociation into charge carriers. In order to achieve BHJ PV cells with high performances, the resulting negative and positive carriers should be efficiently collected at the electron-collecting cathode and hole-collecting anode electrodes, respectively. As the carrier collections are significantly affected by the electrical properties at the electrode interfaces, an appropriate choice of the buffer layers, introduced at the blended-polymer-active-layer–cathode and –anode interfaces, is required for highly efficient BHJ PV cells.15–20 

In this study, bathocuproine (BCP) was selected as the cathode buffer layer’s material in BHJ PV cells with a structure of indium-tin-oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/P3HT:PCBM/BCP/Al. BCP is a commonly used material for an electron collection (hole-blocking) layer in organic light-emitting devices (OLEDs) and PHJ PV cells made of low-molecular-weight organic materials.21,22 Significant performance improvements of PHJ PV cells have been observed upon the insertion of a BCP cathode buffer layer, which can be attributed to several factors, as follows. The BCP layer acts (i) as an exciton-blocking layer, which prevents cathode quenching by limiting the exciton diffusion toward the cathode metal electrode;23 (ii) as a hole-blocking layer, which prevents the holes in the p-type organic donor layer to recombine at the cathode;24 (iii) as an optical spacer, which increases the light absorption in the active layer by changing the spatial distribution of the optical electric field inside the device;25,26 (iv) as a protective layer, which prevents damage of the underlying organic layer during the cathode metal deposition;27,28 and (v) as an interfacial electronic-structure modifier, which suppresses the generation of an unfavorable interface dipole at the direct contact between the n-type organic acceptor layer and the cathode electrode.21 

In addition, BCP has been employed in BHJ PV cells15,29–31 as one of the most effective materials of the cathode buffer layer, owing to the above functions.17,20Figure 1 illustrates the energy diagram of the BHJ PV cell with the BCP layer.8,30,32,33 The energy difference between the work function of the PEDOT:PSS modified cathode electrode (∼5.2 eV)8 and the highest occupied molecular orbital (HOMO) energy level of the P3HT donor (∼5.1 eV)34,35 seems to be small enough for ohmic charge injection. On the contrary, the electrons at the lowest unoccupied molecular orbital (LUMO) level of the PCBM acceptor (∼4.3 eV)32 must overcome the energy barrier of ∼1.2 eV to reach the Al cathode in case of electron transport via the BCP LUMO level of ∼3.5 eV36 (path i in Fig. 1). The more probable conduction pathway through the BCP layer is electron transport via gap-states lying below the BCP LUMO level (path ii in Fig. 1). Gap-states in the BCP layer are thought to be caused by the incorporation of Al atoms during the deposition of the Al cathode electrode on BCP12,37,38 and by the diffusion of P3HT molecules into the BCP layer during the deposition of BCP on the P3HT:PCBM active layer.39 

FIG. 1.

Schematic energy diagram showing HOMO and LUMO levels of organic layers (P3HT, PCBM, and BCP), Fermi levels of electrodes (ITO and Al), and a modified work function of a PEDOT:PSS coated anode.

FIG. 1.

Schematic energy diagram showing HOMO and LUMO levels of organic layers (P3HT, PCBM, and BCP), Fermi levels of electrodes (ITO and Al), and a modified work function of a PEDOT:PSS coated anode.

Close modal

In the case of the blended active layer, excitons are generated close to the donor-acceptor interface and rapidly dissociate into charge carriers. Accordingly, the exciton-blocking function is not important.40 The HOMO energy level of BCP (7.0 eV) is deeper than those of P3HT and PCBM, which indicates that the BCP layer inserted between the P3HT:PCBM active layer and the Al cathode can provide the hole-blocking function. Furthermore, BCP has the wide band gap of 3.5 eV, which is suitable for use as a transparent spacer. Therefore, the BCP layer can be expected to have roles (ii) and (iii) but cannot be expected to have role (i). Roles (iv) and (v) are also thought to be effective as described below. Due to the expected functions of roles (ii)–(v), we employed the BCP layer as the promising cathode buffer used in the P3HT:PCBM-based BHJ PV cells.

Chang et al.15 investigated the influence of the BCP cathode buffer layer on the performance of a P3HT:PCBM-based BHJ PV cell by varying the buffer layer’s thickness in the range of 1–20 nm. They observed the highest performance of the BHJ PV cell for a thickness of the BCP buffer layer of 2 nm, whereas several studies on small-molecule PHJ PV cells reported that the highest PV performances were usually achieved at a thickness of the BCP buffer layer in the range of 5–10 nm.21,41 The optimized BCP thickness is determined by the above functions [(i)–(v)]. In general, functions (i)–(iv) require a thickness of the BCP layer of approximately 5–10 nm, or even larger;25,42–44 however, a thick buffer layer increases the series resistance (Rs) of the PV cell,15,30 leading to a reduced efficiency. As an ultrathin BCP layer (thickness <1 nm) can act as an interfacial electronic-structure modifier,45 the smaller optimized thickness of 2 nm suggested that the function (v) might be mainly responsible for the improved performance of the P3HT:PCBM-based BHJ PV cell with the BCP cathode buffer layer.15 Cathode buffer layers employed in P3HT:PCBM-based BHJ PV cells are effective as optical spacers and protective layers, as reported in several studies.20,46–50 For copper-phthalocyanine-(CuPc)/fullerene-(C60)-based small-molecule solar cells, Toumi et al.27 reported that the optimal thickness of the BCP protective layer to prevent metal atom diffusion into the organic sub-layer was 9 nm ± 1 nm. As the optical spacers, 10-nm-thick zinc oxide47 and 30-nm-thick titanium oxide46 were effectively introduced between the P3HT:PCBM active layer and the top Al cathode to reduce the “optical dead-zone” near the cathode electrode. These studies suggest that a thicker (with a thickness smaller than approximately 10 nm) BCP buffer layer used in a P3HT:PCBM-based BHJ PV cell would be beneficial for functions (iii) and (iv). However, an increased thickness of the BCP layer would lead to a high Rs of the PV cell and consequently to a low device efficiency.

In order to efficiently combine the above functions corresponding to various BCP thicknesses without increasing Rs, in this study, we introduced a BCP buffer layer at the interface between a grating-structured P3HT:PCBM active layer and the top Al cathode. As illustrated in Fig. 2(a), when a vacuum vapor deposition of BCP is performed on the surface of a grating-structured P3HT:PCBM layer, the thickness of the obtained BCP layer is large at the “top” and “bottom” parts, while at the “walls” it is small. The thicker BCP parts at the top and bottom planes, where the thermally evaporated Al atoms are incident almost vertically during the cathode deposition, can preferably act as a protective layer [Fig. 2(b)]. Similarly, they can act as an optical spacer under the irradiation of incident light from the back-side of the ITO-coated face of the device during its operation as a solar cell [Fig. 2(c)]. In contrast, the vapor flux and light are incident almost horizontally to the thinner BCP parts on the wall planes. Owing to the glancing angle of the incident Al atoms, the velocity components of the Al atoms normal to the wall planes are smaller than those normal to the top and bottom planes. Therefore, there should be no need to protect the wall planes by covering with a thicker BCP layer. Similarly, the components of the incident light rays normal to the wall planes are significantly smaller. Consequently, the thinner BCP parts on the wall planes are not required to act as a protective layer and optical spacer. The thinner BCP parts can suppress the generation of an unfavorable interface dipole at the contact between C60 and Al,21 which reduces the contact component of Rs in the BHJ PV cell. Therefore, the grating-structured interface is expected to enable a buffer layer deposition that leads to suitable BCP thicknesses for achieving the desired functions at each part.

FIG. 2.

Illustrations of the (a) vapor deposition of BCP on the grating-structured active layer, (b) succeeding deposition of an Al top cathode on the grating-structured surface covered with BCP, and (c) light irradiation from the ITO side of the resulting PV device. In (b) and (c), PEDOT:PSS is abbreviated as “PP.”

FIG. 2.

Illustrations of the (a) vapor deposition of BCP on the grating-structured active layer, (b) succeeding deposition of an Al top cathode on the grating-structured surface covered with BCP, and (c) light irradiation from the ITO side of the resulting PV device. In (b) and (c), PEDOT:PSS is abbreviated as “PP.”

Close modal

In this study, the effects of the insertion of a BCP buffer layer with various deposition thicknesses at the interface between the grating-structured active layer and the Al cathode are investigated. As an inexpensive master mold of the grating-structure, a commercially available recordable digital versatile disc (DVD-R) substrate was employed.51,52 The grating-structured surface of the active layer was successfully produced by duplication from a poly(dimethylsiloxane) (PDMS) secondary mold using the spin cast molding technique. Compared with the conventional BHJ PV cell, where the BCP buffer layer is inserted at the flat interface, the grating-structured BHJ PV cell exhibited a higher performance even for an excessively thick deposition of BCP (thickness ≥10 nm). Our results suggest that the grating-structured active-layer–cathode interface is an effective strategy to prevent the increase in Rs caused by the insertion of a BCP buffer layer as thick as the spacer layers employed in the BHJ PV cells.46,47

The grating-structured surface of the P3HT:PCBM active layer was fabricated by spin cast molding using a PDMS mold stamp. Vinyl poly(dimethylsiloxane) prepolymer (VDT-731), hydrosilane prepolymer (HMS-301), and Pt catalyst (SIP6831.1) were purchased from Gelest, Inc., and used to prepare the PDMS mold stamp. P3HT, PCBM, and BCP were purchased from Luminescence Technology Corp., Frontier Carbon Corp., and Tokyo Chemical Industry, respectively. The PEDOT:PSS solution (Clevios AI4083) was purchased from Heraeus. These materials were used as received. All other employed solvents and reagents were of analytical grade and purchased commercially.

The employed substrate was a 100-nm-thick-ITO-coated glass plate with a resistance of approximately 20 Ω/sq supplied by Sanyo Vacuum Industries. The substrates were cut into 14 × 18 mm2 sample slides, and the ITO layers were etched using an aqua regia solution to form 2-mm-wide stripes, which are employed as anodes. The patterned ITO substrates were cleaned and rinsed using two detergent solutions (Extran MA 03, pH 6.8, MERCK, and Kontaminon O, pH 10, WAKO) and de-ionized water. The ITO substrates were further cleaned, successively ultrasonicated in acetone and isopropanol, and transferred to boiled isopropanol.

The PEDOT:PSS solution was filtered using a polyvinylidene difluoride (PVDF) filter (pore size: 0.45 µm) and ultra-sonicated for more than 1 h before use. A PEDOT:PSS layer (∼40 nm) was fabricated on the cleaned surface of the ITO substrate using a spin-coater (MS-A100, Mikasa) at a spin rate of 3300 rpm for 30 s in a nitrogen-filled glove box and annealed at 85 °C for 5 min.

As a cheap and convenient source of grating pattern, a commercially available DVD-R was used.52,53 The polycarbonate support substrate of the DVD-R was carefully peeled off to obtain the underlying grating pattern. The remaining dye and metal layers on the grating-patterned surface were removed using aqua regia, followed by rinsing with de-ionized water. After drying, it was used as a master mold.

A PDMS solution made by mixing 6.0 g of the vinyl PDMS prepolymer with 2.0 g of the hydrosilane prepolymer and 0.1% w/w platinum catalyst was poured onto the master mold. The casted PDMS layer, as thick as approximately 2 mm, was degassed under vacuum for 20 min to remove air bubbles and cured at 90 °C for 1 h. The master mold was removed, and the PDMS stamp was obtained.

Figure 3 illustrates the fabrication of the grating-structure at the polymer-blend layer’s surface. A photoactive solution with a P3HT:PCBM weight ratio of 1:0.8 was prepared in 1,2-dichlorobenzene (17 g/l). The solution was ultra-sonicated for more than 2.5 h and filtered using a polytetrafluoroethylene (PTFE) filter (pore size: 0.45 µm) before use. The filtrate P3HT:PCBM solution was spin-casted (1800 rpm for 60 s) on the PDMS mold stamp under a nitrogen atmosphere. Simultaneously, an adhesive layer was fabricated on the target surface of the ITO substrate and coated with the PEDOT:PSS layer by spin-coating with the same P3HT:PCBM solution (900 rpm for 60 s). Immediately after this procedure, the PDMS mold stamp was placed on top of the target surface. The contact was continued for 20 min in a closed petri dish at room temperature to promote adhesion between the two layers by suppressing the evaporation of the residual solvent. It was then annealed on a hotplate at 110 °C for 10 min. After cooling to a moderate temperature, the stamp was carefully removed from the substrate, and the grating-structured polymer active layer was transferred onto the target surface.

FIG. 3.

Illustration of the fabrication procedures for the grating-structured blend polymer film.

FIG. 3.

Illustration of the fabrication procedures for the grating-structured blend polymer film.

Close modal

The surface morphologies of the grating-structured surfaces were observed using atomic force microscopy (AFM) (SPA400 with an SPI4000 controller, Seiko Instruments).

The BCP layers (with thicknesses of 5 nm, 10 nm, and 20 nm) and Al cathode layer (∼200 nm) were deposited using thermal evaporation in a vacuum chamber (∼2 to 5 × 10−4 Pa) with deposition rates of ∼1.0 to 1.5 and 2–5 Å s−1, respectively, on top of the grating-structured surface of the ITO/PEDOT:PSS/P3HT:PCBM multilayered structure. In addition, a grating-structured device without BCP was fabricated as a reference device, using a similar procedure (without BCP deposition). The grating-type polymer-blend solar device configuration is illustrated in the left side of Fig. 4. The film thickness defined as the distance from the back plane to the raised region of the grating-structured front of the blend polymer film was estimated to be ∼260 nm by AFM height measurements of an indentation made on the film by scratching with a syringe needle.54 

FIG. 4.

Device configurations of the BHJ PV cells with grating-structured and flat cathode–organic-layer interfaces.

FIG. 4.

Device configurations of the BHJ PV cells with grating-structured and flat cathode–organic-layer interfaces.

Close modal

For comparison, conventional flat-type devices, illustrated in the right side of Fig. 4, without and with a BCP layer (5 nm, 10 nm, and 20 nm) were also fabricated, by the following procedure. After spin-coating (900 rpm), the photo-active layer (∼150 nm) was fabricated on the PEDOT:PSS-coated ITO from the same solution as that used for the grating-type device; the blend film was annealed at 110 °C for 10 min. The BCP layer for the flat-type devices was prepared using the same methods, as described above. The BCP layers (5 nm, 10 nm, and 20 nm) and Al cathode layer (∼100 nm) were deposited at the same conditions as described above on the flat surface of the P3HT:PCBM/PEDOT:PSS/ITO multilayered structure.

Figure 5(a) shows an AFM image of the grating-structured P3HT:PCBM surface. The periodic line pattern with a pitch of ∼0.82 µm, approximately equal to that of the DVD-R master mold grating (0.80 µm), indicates the successful replication of the grating-structure. However, the cross-sectional profile shown in the lower panel of Fig. 5(a) exhibits sinusoidal shape, which is not consistent with the rectangular profile of the master mold grating pattern. This deviation in shape is mainly caused by the tip convolution effect.55 

FIG. 5.

AFM images of the grating-structured P3HT:PCBM surface (a) without and (b) with a 50-nm-thick BCP deposition. The upper panels show the topographic images, while the lower panels show the cross-sectional profiles along the lines drawn in each upper panel.

FIG. 5.

AFM images of the grating-structured P3HT:PCBM surface (a) without and (b) with a 50-nm-thick BCP deposition. The upper panels show the topographic images, while the lower panels show the cross-sectional profiles along the lines drawn in each upper panel.

Close modal

In order to determine the ratio of the BCP film formed on the wall side of the grating-structure, we investigated the morphology of the grating-structured surface with a deposition of a 50-nm-thick BCP layer, as shown in Fig. 5(b). Although it is a rough estimation, we estimated the BCP layer’s thickness at the wall side using the increase in the width at half of the peak height of the cross-sectional profile after the deposition of the 50-nm-thick BCP layer. The comparison between the two cross-sectional profiles in Figs. 5(a) and 5(b) showed that the increase was approximately 50 nm. Therefore, when the thickness of the deposited BCP layer was 50 nm, we estimated that the thickness of the BCP film on the walls of the grating-structure is approximately 25 nm. According to the estimated ratio of the wall-side thickness with respect to the nominal thickness of the film, the thicknesses of the BCP layers at the wall side are expected to be approximately 2.5 nm, 5 nm, and 10 nm when the nominal thicknesses of the BCP layers are 5 nm, 10 nm, and 20 nm, respectively.

Figure 6 shows the current-density–voltage (JV) characteristics of the flat-type PV cells without and with the BCP buffer layer under a simulated AM 1.5 illumination. In order to investigate the influence of the buffer layer on the device performances, buffer layers with three different thicknesses of 5 nm, 10 nm, and 20 nm were considered. For all cells, an almost constant open-circuit voltage (Voc) of approximately 0.6 V was observed. The Voc value is consistent with those obtained from JV characteristics of devices with similar compositions.15,31,56 On the contrary, the introduction of the BCP layer with a thickness of 5 nm increased Jsc to its maximum value, which then decreased with the increase in the BCP layer’s thickness.

FIG. 6.

JV characteristics of the flat-type BHJ PV cells without (0 nm) and with (5 nm, 10 nm, and 20 nm) a BCP buffer layer under a simulated AM 1.5 illumination (100 mW cm−2).

FIG. 6.

JV characteristics of the flat-type BHJ PV cells without (0 nm) and with (5 nm, 10 nm, and 20 nm) a BCP buffer layer under a simulated AM 1.5 illumination (100 mW cm−2).

Close modal

The observed photovoltaic parameters, Jsc, Voc, fill factor (FF), and ηp, of the flat-type PV cells are summarized in Table I. Compared with the cell without a BCP layer, significant enhancements of Jsc from 8.24 mA cm−2 to 9.43 mA cm−2 and FF from 0.57 to 0.61 were achieved using the 5-nm-thick BCP layer; consequently, the calculated ηp was improved from 2.93% to 3.62%. In contrast to the positive effect of the 5-nm-thick BCP layer, the BCP layer with a larger thickness of 10 nm caused rapid decreases in Jsc to 5.16 mA cm−2 and FF to 0.51. Moreover, the further increase in the BCP layer’s thickness to 20 nm induced an s-shaped J–V curve (Fig. 6) with an extremely low FF of 0.21; consequently, ηp was significantly reduced to 0.59%. The observed kink in the curve could be attributed to the energy barrier for carrier extraction at the interfaces of the PV cell.57 Therefore, the s-shaped feature suggests the presence of a charge accumulation near the cathode interface in the structure with the 20-nm-thick BCP layer. The increased Rs accompanying the charge accumulation near the cathode interface would be mainly responsible for the lowering of the performances of the flat-type PV cells with the thicker BCP layers.

TABLE I.

Device parameters for the flat-type BHJ PV cells without (0 nm) and with (5 nm, 10 nm, and 20 nm) a BCP buffer layer.

BCP layer’s thicknessJsc (mA cm−2)Voc (V)FFηp (%)
Without BCP (0 nm) 8.24 0.62 0.57 2.93 
5 nm 9.43 0.63 0.61 3.62 
10 nm 5.16 0.63 0.51 1.66 
20 nm 4.55 0.63 0.21 0.59 
BCP layer’s thicknessJsc (mA cm−2)Voc (V)FFηp (%)
Without BCP (0 nm) 8.24 0.62 0.57 2.93 
5 nm 9.43 0.63 0.61 3.62 
10 nm 5.16 0.63 0.51 1.66 
20 nm 4.55 0.63 0.21 0.59 

Figure 7 shows the J–V characteristics of the grating-type PV cells without and with BCP layers with different thicknesses (5 nm, 10 nm, and 20 nm). An almost constant Voc of approximately 0.6 V was observed, as in the flat-type PV cells. However, the grating-type reference cell without a BCP layer had a low ηp of 1.20% owing to the significantly lower Jsc of 4.75 mA cm−2 than that observed for the flat-type reference. The low Jsc value is most likely caused by the exciton quenching and charge recombination at the enhanced area of the grating-structured metal cathode. The insertion of the 5-nm-thick BCP layer at the grating-structured cathode–active-layer interface significantly improved Jsc to 9.75 mA cm−2 and ηp to 3.51%, comparable with those of the flat-type PV cell with a 5-nm-thick BCP layer. Taking into account the above rough estimation, for the 5-nm BCP deposition, the thickness of the BCP layer on the wall side is estimated to be ∼2.5 nm. This thickness value is consistent with the optimized BCP layer’s thickness of 2 nm for a flat-type device studied by Chang et al.15 The significant improvement of the device performance achieved by the 5-nm BCP deposition suggests that an almost optimized device configuration is achieved, whose BCP layer’s thickness is ∼2.5 nm at the walls and 5 nm at the top and bottom regions of the grating-structure.

FIG. 7.

J–V characteristics of the grating-type BHJ PV cells without (0 nm) and with (5 nm, 10 nm, and 20 nm) a BCP buffer layer under a simulated AM 1.5 illumination (100 mW cm−2).

FIG. 7.

J–V characteristics of the grating-type BHJ PV cells without (0 nm) and with (5 nm, 10 nm, and 20 nm) a BCP buffer layer under a simulated AM 1.5 illumination (100 mW cm−2).

Close modal

Table II shows the photovoltaic parameters for the grating-type PV cells without and with a BCP layer. Similarly to the flat-type PV cells, the photovoltaic parameters, Jsc, FF, and ηp, of the grating-type PV cell increase to their maximum values with the insertion of the 5-nm-thick BCP layer and then decrease with the increase in the BCP layer’s thickness. However, compared with the results of the flat-type PV cells, the ranges of the decreases in the photovoltaic parameters of the grating-type PV cells are remarkably suppressed. It is worth noting that the s-shape feature observed in the J–V curve of the flat-type PV cell with a 20-nm-thick BCP layer is not present in the J–V curve of the grating-type PV cell with a 20-nm-thick BCP layer, as shown in Fig. 7. The JV characteristic for the cell with a 20-nm-thick BCP layer exhibits a moderate FF of 0.40, which is significantly larger than that of the flat-type PV cell with the 20-nm-thick BCP layer (FF = 0.59), and ηp of 1.45%, which is larger than that of the grating-type PV cell without a BCP layer (ηp = 1.20%). The BCP thickness of ∼10 nm on the wall side estimated for the grating-structured PV cell with a 20-nm-thick BCP layer is comparable to a moderate BCP thickness of 10 nm, which corresponds to a moderate cell performance with photovoltaic parameters of Jsc = 5.16 mA cm−2, FF = 0.51, and ηp = 1.66% of the flat-type PV cells. These results suggest that the wall side contact at the grating-structured interface contributes to preferably establish electron pathways toward the cathode even when a deposition of a thicker BCP layer (20 nm) is performed.

TABLE II.

Device parameters for the grating-type BHJ PV cells without (0 nm) and with (5 nm, 10 nm, and 20 nm) a BCP buffer layer.

BCP layer’s thicknessJsc (mA cm−2)Voc (V)FFηp (%)
Without BCP (0 nm) 4.75 0.61 0.41 1.20 
5 nm 9.75 0.62 0.58 3.51 
10 nm 6.16 0.62 0.44 1.69 
20 nm 5.92 0.61 0.40 1.45 
BCP layer’s thicknessJsc (mA cm−2)Voc (V)FFηp (%)
Without BCP (0 nm) 4.75 0.61 0.41 1.20 
5 nm 9.75 0.62 0.58 3.51 
10 nm 6.16 0.62 0.44 1.69 
20 nm 5.92 0.61 0.40 1.45 

As shown in Table I, the significant decrease in ηp from 2.93% to 1.66% was caused by the insertion of a 10-nm-thick BCP layer in the flat-type PV cell. From the result of the negative effect of the insertion of a 10-nm-thick BCP layer on the flat-type PV cell, the negative effect of the insertion of a 20-nm-thick BCP layer on the grating-type PV cell had been expected because the wall side of the grating-type PV cell with a 20-nm-thick BCP layer is covered with BCP of the similar thickness (∼10 nm). However, the slight increase in ηp from 1.20% to 1.45% is achieved by the insertion of a 20-nm-thick BCP layer in the grating-type PV cell. The reason why the positive effect was observed on the grating-type PV cell with the thicker BCP layer is not clear. It should be noted again that the observed ηp of the grating-type reference cell without a BCP layer, 1.20%, was much lower than that of the flat-type reference cell, 2.93%. In addition, contrary to the expectations, Rs of the reference grating-type PV cell, which is estimated from the reciprocal J-V slope at Voc in Fig. 7, seems to be larger (or comparable) than those of the grating-type PV cells with 5-nm-, 10-nm-, and 20-nm-thick BCP layers. Such high Rs might be ascribed to void formation at the wall side interface between Al and P3HT:PCBM during a glancing angle Al deposition process.58,59 The mechanism for the void formation is known as “self-shadowing” in which initially formed nuclei of deposited atoms prevent vapor atoms toward the substrate at a glancing angle from reaching the regions situated behind them. If an increase in Al film porosity is caused by the self-shadowing effect,60 the effective electrical contact area at the Al/P3HT:PCBM interface on the wall side of the grating structure would be reduced, which results in an increase in Rs. Meanwhile, surface diffusion due to molecular migration at room temperature easily takes place in an amorphous BCP film.61 The positive effect observed on the grating-type PV cell with the thicker BCP may be presumably explained by the surface diffusion effect of the pre-coated BCP, which is expected to reduce the self-shadowing effect during an Al deposition.62,63 Efforts are underway to precisely elucidate the BCP thickness and the Al porosity on the wall side by detailed AFM and SEM studies to determine an optimized structure for an efficient grating-type PV cell.

We first investigated the dependence of the device performance of the grating-type BHJ PV cell on BCP buffer layer thickness. From the observation of the morphology of the grating-structured surface before and after the BCP deposition, we roughly estimated that the BCP layer’s thickness on the wall side was approximately half of that at the “top” and “bottom” regions. The grating-type BHJ PV cell with a 5-nm-thick BCP layer exhibited the maximum ηp of 3.51%, which was comparable with that of the flat-type BHJ PV cell. Compared with the conventional flat-type BHJ PV cell with a 20-nm-thick BCP layer, the performance of the grating-type BHJ PV cell with a 20-nm-thick BCP layer was remarkably improved, owing to the contribution of the wall side contact. As a result, we successfully demonstrate that the wall side contact of the grating-type PV cell can provide lower-barrier paths of the electrons toward the cathode through the thinner BCP layer.

This study was partially supported by a Grant-in-Aid for Scientific Research (C) (Grant No. 21550170) to M.S. from the Japan Society for the Promotion of Science.

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