Nanorod, Nanowire, and Nanotube Arrays

Compared to the bilayer devices, bulk heterojunction HSCs have much higher conversion efficiencies. However, they are still limited by the inefficient charge transport caused by the discontinuous percolation pathways as shown in Fig. 9.10a, b. Therefore, an ordered heterojunction which has direct charge transport pathways is generally regarded as an ideal structure for HSCs, as shown in Fig. 9.10c [77].

The ordered heterojunction can be formed by infiltrating conjugated polymers into the vertically aligned nanostructures of the inorganic nanocrystals, such as nanorod, nanowire, and nanotube arrays that can be prepared by a variety of physical or chemical methods including nanoimprint [41, 78], low temperature liquid phase deposition [79], template method [80], anodization [81], etching [82], and so on.

Cadmium Chalcogenide nanorod and nanowire arrays have been extensively studied for preparing high performance ordered heterojunction HSCs in the past years. Kang et al. [83] first reported such a device with an admirable PCE of 1 %

Fig. 9.9 a Schematic of the TiO2 nanofiber/P3HT device. b low magnification, c high magnification, SEM images of the electrospun TiO2 network. d SEM image of electrospun ZnO network. e SEM cross-section view of the ZnO nanofiber/P3HT device. a, b, c reproduced with permission from Ref. [22], d, e reproduced with permission from Ref. [31]

based on CdTe nanorod array and poly(3-octylthiophene) (P3OT), then the MEH – PPV/CdS nanorod array device was fabricated with an efficiency of about 0.6 % [13]. Schierhorn et al. [84] successfully synthesized vertically aligned CdSe nanorods on ITO glass and systematically investigated the influence of nanorod length on the device performance (see Fig. 9.11), As shown in Fig. 9.12, the Jsc increased linearly with nanorod length and the device based on 612 ± 46 nm long nanorod gave the highest efficiency of 1.38 % [85].

Shankar et al. [86] demonstrated a single heterojunction solid-state solar cell by sensitizing the anodic TiO2 nanotube array with a P3HT derivative. However, the device exhibited poor performance due to the bad contact between the active layer and electrode. When they infiltrated both P3HT and PCBM into TiO2 nanotube arrays to form double heterojunction solar cells, a 1 % PCE was achieved. Vertically aligned TiO2 nanorods were used to fabricate HSC with P3HT by Kuo et al. [87], but only a 0.12 % PCE was obtained. Higher efficiency could be realized by the control of the dimensions of the nanorods and/or sizes of the D/A domains. Tepavcevic et al. [88] found that, when the polymer was in situ poly­merized in the TiO2 nanotube arrays, the device showed a much stronger (>103) photocurrent density than that of device fabricated with ex-situ synthesized polymer. However, the white light efficiency was not reported by the authors. Mor et al. [89] successfully demonstrated an efficient TiO2 nanotube arrays/dye/P3HT device structure (see Fig. 9.13), where the dye accounted for the absorption of the red and near-infrared portion of the solar spectrum while P3HT absorbed the higher energy photons and served as a hole transport material. Such a device that combined the advantages of both solid-state DSSC and BHJ solar cell showed an average PCE as high as 3.2 %. Recently, highly oriented TiO2 nanotubes were synthesized with ZnO nanorod template and a 3.32 % efficiency was reported for the HSC fabricated by infiltrating P3HT/PCBM into the TiO2 matrix [90].

Vertically aligned ZnO nanorods and nanowires that could be easily prepared were also extensively studied for preparing high performance HSCs. Figure 9.14 presents the typical morphology of ZnO nanorods array. The charge recombination

(b)

rate in ZnO nanorods array-based device was found to be over two orders of magnitude slower than that of device based on ZnO nanoparticles [91]. Although many efforts have been done to optimize the morphologies of the active layers for the ZnO nanorod and nanowire arrays based HSCs [92-97], the PCEs of such devices were still very low, typically lower than 1 %. Similar to TiO2 nanotube

arrays [88], higher efficiencies could be achieved by infiltrating the blend of P3HT and PCBM into the ZnO nanorods array [98, 99], but the reported efficiencies were still lower than that of state-of-the-art BHJ organic solar cells.

Besides the above mentioned materials, Si [100, 101], InP [102] and CuO [103] nanowire arrays were also used for fabricating hybrid solar cells with conjugated polymers with the efficiencies normally lower than 2 %.