Ubiquitous carrier harvesting in organic solar cells with embedded indium-tin-oxide nano-electrodes

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Ubiquitous carrier harvesting in organic solar cells with embedded

indium

–tin-oxide nano-electrodes

Min-Hsiang Hsu

a

, Po-Han Chen

a

, Peichen Yu

a,n

, Jen-Hsien Huang

b

, Chia-Hua Chang

a

,

Yu-Chih Cheng

a

_{, Chih-Wei Chu}

b

a

Department of Photonics and Institute of Electro-Optical Engineering, National Chiao-Tung University, 1001 University Road, Hsinchu 300, Taiwan, ROC

b

Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan

a r t i c l e i n f o

Article history: Received 5 March 2013 Received in revised form 19 July 2013

Accepted 30 July 2013

Available online 3 September 2013 Keywords:

Indium tin oxide nanorods Nano-electrodes Electrochemical deposition Organic solar cells

a b s t r a c t

Bulk heterojunction in organic solar cells allows adequate interfaces for the dissociation of photo-generated excitons. However, the intermixed donor- and acceptor-like materials do not guarantee successful carrier transport and collections. In this work, we demonstrate ubiquitous carrier harvesting characteristics by using embedded indium–tin-oxide nano-electrodes (NEs) with a conformal electro-polymerized coating of a hole conducting layer. Compared to its planar electrode counterpart, this device exhibits internal quantum efficiency insensitive to the distribution of carrier generation, as determined by angle-resolved absorption spectroscopy and photocurrent characteristics. The ubiquitous carrier harvesting characteristics offer a viable solution to organic thin-film devices fabricated on flexible substrates or operated under diffused ambient light.

1. Introduction

Organic solar cells (OSCs) are promising candidates for inex-pensive renewable energy sources made possible by solution processes and reel-to-reel production capabilities[1–3]. Assisted by the advent of materials engineering, the performance of OSCs has become very competitive with other thin film materials. For instance, a recently certified device with a record efficiency of over 10.7% has been reported [4], further promoting the applications of OSCs in next-generation photovoltaics. It has been known that the power conversion efficiency (PCE) of an OSC depends on photon absorption, exciton dissociation, and carrier collection, where the first two factors are rarely an issue for polymer–fullerene based solar cells due to highly absorptive active materials and randomly distributed bulk heterojunction (BHJ) structures [5_–7]. However, charge transfer still face challenges including unbalanced carrier mobilities, isolated islands, and dead ends formed by donor- and acceptor-like materials [8,9]. To improve carrier collection, recent studies have focused on modifying material properties and processing conditions via surface morphol-ogy engineering, solvent annealing, and synthesis methods[10–13]. Novel designs based on nano-materials and nano-fabrication tech-niques have also been extensively explored [14–19]. Among all, embedded nano-electrodes (NEs) protruded into the active layer

have shown the potential to enhance the PCE and to elongate device lifetime, which are both largely attributed to the balanced charge transport [20,21]. Nevertheless, evidence of omni-present carrier collection capability enabled by embedded NEs had not yet been explored. In this work, we demonstrate ubiquitous carrier harvesting characteristics by using embedded indium tin oxide (ITO) NEs. The surface roughness and carrier conduction are retained by applying a conformal electro-polymerization technique of a hole conducting layer (HCL) onto individual nanorods. Compared to the reference cell with a planar electrode (PE), the solar cell with NEs exhibits internal quantum efﬁciency insensitive to the distribution of carrier generation, determined by angle-resolved absorption spec-troscopy and photocurrent characteristics. The omnipresent carrier collection attribute of embedded NEs is very appealing in organic photovoltaic devices with ﬂexible substrates and operated under diffused ambient light.

2. Experimental

The ITO NEs were deposited onto a 1.5 cm 1.5 cm ITO glass substrate with a thickness of 260 nm and a sheet resistance of 15Ω⧸square using oblique electron-beam evaporation at an incli-nation angle of 70_{1 with respect to the surface normal of the} substrate. The target was composed of 5 wt% SnO2 and 95 wt% In2O3. During the deposition, 1 sccm nitrogen gas was introduced under a vacuum pressure of₁₀4Torr with a deposition rate of Contents lists available atScienceDirect

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0.15 nm/s at 2601C for 270 s, growing the rods to around 100– 150 nm in height.

The ITO glasses with nano-columns were then treated to an ultrasonic clean process in deionized water, acetone and isopropyl alcohol. The deposition of PEODT was controlled by Autolab potentiostat/galvanostat (Eco Chemie, model PGSTAT30). The 0.01 M monomers EDOT from Aldrich (97%) were dissolved in a 0.1 M LiClO4/acetonitrile solution for a triple-terminal electroche-mical process with a platinum foil as the counter electrode, an Ag/Ag+reference electrode (0.01 M AgNO3and 0.1 M TBAClO4in acetonitrile), and the ITO as the working electrode. The PEDOT ﬁlms were electro-polymerized onto the ITO nano-columns at 1.1 V versus the Ag/Ag+reference electrode with a pre-determined charge density, which controls the thickness of PEDOTﬁlm. Then the ITO rods with PEDOT were transferred to a glove box for the subsequent device fabrication process.

The active material was consisted of 2 wt% regioregular P3HT (Mw 3.7 104_{g mol}1_{) and 2 wt% PCBM in a dichlorobenzene} solution, which were obtained from Rieke Metals and Nano-C Inc., respectively. The solution blend was then spun-cast on to the PEDOT layer at a speed of 600 rpm for 60 s and dried in covered Petri glass dishes for 30 min. Finally, a 30-nm-thick layer of calcium followed by a 60-nm-thick layer of aluminum were thermally coated onto the active material at a pressure of 6 106_{Torr through a 0.1 cm}2 _{shadow mask. The reference} devices with a conventional ITO_{ﬁlm electrode were also} fabri-cated with the same processes and conditions.

Efficiency measurements were performed under the simulated AM1.5 G irradiation (100 mW/cm2) using a Xenon lamp-based Thermal Oriel 1000 W solar simulator and the angular character-istic analyses were carried out via the rotating stage to acquire the data under different incident angles. The spectrum of the solar simulator was calibrated with a PV-measurement (PVM-154) mono-Si solar cell (NREL calibrated), and a Si photo diode with KG-5 colorfilter (Hamamatsu, Inc.) was used to check the irradia-tion of the exposed area (100 mW/cm2_{). The PVM-154 combined} with a KG-5 filter (350–700 nm passed, Newport) was used to simulate a reference solar cell with a spectral response from 350 nm to 700 nm, where the calibration was based on the IEC-69094-1 spectrum[22]. The current density–voltage (J–V) curves were measured with a semiconductor source measure unit (Keith-ley 2400) using a four-wire connection and the measurements were corrected via the external series resistance. Stability in time was continuously monitored by routinely repeating the measure-ments on c-Si solar cells. The temperature of the cells was actively controlled during the measurement at 2571 1C.

The external quantum efficiency (EQE) system is equipped with a 450 W Xenon lamp (Oriel Instrument, model 6266) light source, a water-based IR filter (Oriel Instrument, model 6123 NS) and a monochromator (Oriel Instrument, model 74,100). The beam spot on the sample is rectangular, measuring roughly 1 mm 3 mm. A calibrated silicon photodetector (Newport 818-UV) was used to calibrate the EQE system before measurements. A lock-in amplifier (Standard Research System, SR830) and an optical chopper controller (SR540), set in voltage mode, were also equipped to lock the output signal and the photocurrent was converted to voltage using a 1-Ω resistor, which was parallel to the sample. The temperature of cells was again actively controlled at 2571 1C during the measurement.

The angle-resolved absorption measurement system is custom built, which utilizes a 15 cm-radius integrating sphere with a motor-controlled rotational sample stage in the center and a broadband Xenon lamp. The collected reﬂected photons were then analyzed by a spectrometer (QE65000, Ocean Optics) to obtain the angular reﬂective spectrum (R) up to an incident angle of 701. For the sample preparation, because the calcium (Ca) in the

devices could be easily oxidized, the layer was removed for the angular-response absorption measurement. The cell absorption (A) can be estimated by subtracting the reﬂective spectrum by the unity, i.e. A¼1R. The system was calibrated by the reﬂective spectrum of a NIST-standard, intrinsic Si at normal incidence.

3. Results and discussion

The solar cells presented in this work incorporate distributed ITO nanorods which are vertically protruded into the active layers as three-dimensional (3D) electrodes. The highly conductive nanorods allow multiple pathways for low-mobility holes by rendering shortened routes to reach the anode. In other words, the time it took for holes to arrive at the anode is reduced, prompting an increase in the effective hole mobility with respect to the ﬁlm electrode. Previously, we have observed balanced charge transport achieved with this highly scaffolded cell archi-tecture [21]. The growth of ITO nanorods has been intensively studied over the past few years using various techniques [23]. Herein, we employ an oblique electron-beam deposition technique to place ITO nanorods on a 260-nm-thick ITO-coated glass sub-strate in an oxygen de_{ﬁcient ambient environment}[20,21]. The rod formation involves a self-catalytic vapor-liquid-solid (VLS) process, assisted by introduced nitrogen and high temperature in a vacuum

[24]. The incident vapor is set at an angle of 70_{1 with respect to the} substrate to control theflux density of molecules on the surface, which facilitates the separation and formation of nuclei. Moreover, the oxygen deficiency allows the segregation of tin-doped indium to form a liquid surface that promotes the absorption of incident vapors, resulting in vertical column growth. Since the nanorods may have different stoichiometric compositions compared to the conventional ITO glass substrate, we also perform an ultraviolet photoelectron spectroscopy (UPS) measurement to determine the work function of ITO nanorods from three samples and use the ITO glass substrate as reference. Preliminary results have shown that the average work function of ITO nanorods is 4.72 eV, which is slightly lower than that of the reference counterpart at 4.95 eV. The modi_{fication of work function is favored to hole transport,} which may contribute to charge collection. Next, to function properly as embedded NEs for organic solar cells, a conformal coating of a HCL around nanostructures is essential in order to maintain the morphology that provides sufficient contact area while effectively blocking electrons. Therefore, an electrochemical deposition of poly-(3,4-thylenedioxythiophene) (PEDOT) was employed via a triple-terminal electrochemical cell. Since the nanorods are highly conductive, EDOT monomers can be gradually polymerized and deposited in the conductive area and therefore, wrap around individual nanorods. Subsequently, the cell fabrica-tion follows the convenfabrica-tional spin-cast of active materials and is capped with calcium and aluminum as the cathode by thermal evaporation. Fig. 1a provides a clear cross-sectional image of a fabricated device, where the active layer is partially released from the substrate. In the dashed red frame, we observe that randomly-oriented nanorod electrodes freely stood on the ITO-coated glass substrate. The orientation and surface morphology was not affected by the device fabrication process or by peeling-off, indicating robust mechanical properties. Fig. 1b shows the distribution of deposited nanorods without the ECD process, where the heights range from 100 to 150 nm with a density of 5 109_{–1 10}10_cm2_{. Next, the deposition of a PEDOT HCL is} performed at a charge quantity setting of 50μC under an applied voltage of 1.1 V based on the optimized conditions described in the previous work[21]. With a lower setting, the coverage of HCL onto the rods is not complete, resulting in shunt losses that lower the fill factor and open circuit voltage. Conversely, at higher settings

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the large deposition rate at the tips of nanorods, where the electric field is strongest, leads to a cross-linked PEDOT network which planarizes the surface and lowers the output photocurrent.Fig. 1c shows the morphology of nanorods after the optimized ECD process, where the rods become slightly thicker with the PEDOT coating than the bare rods shownFig. 1b. It can be seen that the spacing between the NEs is still on the order of a few tens to over a hundred nanometers, which is sufficient for the penetration of active materials without altering the organization of BHJ[25]. The X-ray diffraction analysis also shows no significant differences on the P3HT crystallinity for samples with nano-electrodes and with a reference planar electrode (See SupportingFig. S1). The reference cell is chosen to be a conventional ITO film electrode with an optimized, 30 nm-thick PEDOT layer which is electro-chemically deposited onto the substrate at 500μC, as shown inFig. 1d. The PEDOT layer reveals a relatively dense porous structure, which has in the past been shown to enhance the cell performance [26]. Furthermore, an electron dispersive spectroscopic (EDS) analysis is

conducted to verify the coating of PEDOT onto the ITO nanorods. As the indicated inFig. 1e, the spectrum reveals the existence of sulfur, which is one of the characteristic elements of PEDOT, indicating the successful coating of the HCL via the ECD process. However, we note that due to the large beam probe size of an EDS in the micrometer range, it is difﬁcult to resolve the coverage of PEDOT onto the side walls of individual nanorods using this method. In such cases, solar cell characteristics could be employed to reﬂect the quality of the HCL coating as discussed previously in

[21].

The fabricated solar cells with a PE and with NEs were characterized with an angle-resolved reﬂectance spectroscopy to determine the absorption characteristics (A¼1–R) modiﬁed by the nanorods, as plotted inFig. 2a and b, respectively. As there have been several technical reports regarding the enhanced optical absorption using surface texturing techniques on either the front or back side of the cell, we note that the height and density of the present nanorods are still not adequate for an evident

Fig. 1. (a) The cross-sectional image of a fabricated solar cell with embedded NEs, where the active layer is partially released from the substrate. The labeled region indicates that the free-standing NEs are protruded into the active materials, offering mutiple pathways for hole conduction. (b) Indium tin oxide nanorods deposited by oblique electron-beam evaporation. (c) The nanorods coated with a PEDOT hole conducting layer via electro-chemical deposition at an optimized charge quantity of 50μC under 1.1 V. (d) The reference ITO planar electrode covered with electrochemically deposited PEDOT at an optimized charge setting of 500μC. The scale bars are 200 nm. (e) Electron dispersive spectroscopic analysis of a single nanorod indicates the exisitence of sulfur, conﬁrming the successful coating of PEDOT on to the side wall.

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antireflection effect, as the AM1.5 G solar-spectrum-weighted absorption is nearly identical across the angles of irradiance incidence from 01 to 701 (see supporting Fig. S2). Nevertheless, we note that the proposed ITO nanostructures for enhanced carrier collection are also compatible with other advanced light harvesting schemes. Although not particularly significant, the modifications of absorption in terms of spectrum and angles of irradiance incidence still need to be taken into account for later device characteristic analyses.

Fig. 3a plots the current density–voltage curves under dark and illuminated conditions for solar cells with a PE and with NEs. The corresponding photovoltaic characteristics are summarized in

Table 1, including Jsc, open-circuit voltage (Voc), fill factor (FF), and PCE. In general, the cell with NEs exhibits a nearly 10% improvement in Jsc and 15% in PCE, which may be ascribed to the increased probability of hole collection resulting from both the modified work function and shortened transport routes via embedded NEs. Previously, we demonstrated that the ratio of carrier mobility with NEs is nearly unity at this optimized ECD setting of 50μC, where the Jscand PCE is also highest, by making electron- and hole-only devices. However, there had been little evidence as to omnipresent carrier collection capability in a functional device with embedded NEs. The enhancement in Jsc or PCE could be attributed to either or both improved optical absorption and carrier collection. Therefore, wefirst follow the standard procedures to analyze the measured external quantum

efﬁciency (EQE) and absorption (A) to obtain the extracted internal quantum efﬁciency (IQE), using IQE¼EQE/A, as plotted inFig. 3b, c, and d, respectively. It can be seen that the EQE spectrum indicates a broadband enhancement with the incorporation of NEs, while the absorption merely shows differences of 2–3% in the spectral ranges below 450 nm and between 480 and 560 nm. Conse-quently, the enhancement in Jscis mostly due to improved carrier collection across the broad absorption range of the photoactive materials, as shown in the IQE spectrum in Fig. 3d. Moreover, previous studies have indicated that parasitic absorptions in the ITO substrate and metals may lead to incorrect IQE analyses which need to be addressed for these devices. We have adopted the methodology proposed by Burkhard et al. to recalibrate the IQE and present the analysis in the supporting information [27]. Because the parasitic absorptions that occur in the PE and NE devices are nearly identical; the broadband IQE enhancement still holds after the calibration. A more detailed description can be founded in the supporting information.

To further investigate carrier collection characteristics, an angular J–V measurement was conducted by mounting the device on a rotatable chuck, and the results are shown in Fig. 4. The calibrated angular photocurrent, Jsc(θ) of both devices decays with the incident angle because the normal component of the irradi-ance is determined by the cosine of the oblique incidence. In fact, if the measured Jsc(θ) is normalized with the corresponding value at the normal incidence, i.e. Jsc(0) for both devices, the angular photocurrent of the cell with NEs exactly follows the cosine curve, while that of the planar counterpart decays faster than the cosine. Still, the difference in decaying trend could arise from either or both optical and electrical transport effects. We then make a further analysis using Eq. (1) by incorporating the measured angle-resolved absorption and IQE spectra.

JscðθÞ ¼

e hc

Z 650 nm 400 nm

IQ Eðλ; θÞ Aðλ; θÞ IAM1:5ðλÞ cos ðθÞdλ ð1Þ

where IQE(_{λ,θ) denotes the internal quantum efﬁciency and A(λ,θ)} the absorption, both as functions of the wavelength, λ and the angle of irradience incidence,θ; e is the electric charge, h is Plank's constant, and c is the speed of light. Since A(_{λ,θ) is experimentally} determined as shown in Fig. 2 and only IQE(λ,0) is available as depicted inFig. 3d, we plot the calculated Jsc(θ) by taking IQE(λ,θ)¼ IQE(λ,0) as shown inFig. 4. It can be seen that the measured Jsc(θ) from the cell with NEs matches the calculated values exactly, which conﬁrms the assumption that IQE(λ,θ)¼IQE(λ,0). On the other hand, the cell with a PE does not show this characteristic, indicating that the carrier collection capability is dependent on the incident angle of irradience. Since the distribution of carrier generation varies with different angles of irradiance, the angular insensitive IQE characterisitcs indicate that carrier collection cap-ability is spatially invariant to the generation distribution.

Next, we present an electromagnetic simulation to verify the spatial distribution of carrier generation rates, which are derived from optical absorption, at normal and oblique illumination conditions, namelyθ¼01, 451, and 701. The corresponding carrier generation distributions, normalized to total generation rates, are presented in color maps overlaid with the profiles of simulated unit cells in Fig. 5a and b for devices with a PE and with NEs, respectively. Here, the incident illumination source is composed of plane waves with an equal component of transverse-electric (TE) and transverse-magnetic (TM) polarizations and weighted with the solar spectrum to represent the broadband and un-polarized sun light. As shown inFig. 5a, the layered solar cell structure with a PE exhibits a wave-guiding effect to the incident light, where the electromagnetic field distribution resembles a standing wave pat-tern resulting from multiple reflections at each dielectric interface. At different incident angles, the coupling of solar radiation to the

Fig. 2. The measured angular absorption spectra of the (a) PE device and (b) NE device.

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solar cell device leads to a spatial shift of absorption peaks, which changes the distribution of carrier generation. The incorporation of nanorods at the interface between the ITO substrate and the active materials reduces the wave-guiding effect, where the contrast of

the standing wave pattern is not as strong as with the PE device. However, the spatial occurrences of absorption peaks and dips are still identical due to the sparse distribution of short nanorods which have minimal impact on the dielectric environment to the incident photons. We then average the distributions of the carrier generation rate presented inFig. 5a and b along the horizontal direction, in order to compare the normalized carrier generation as a function of height, as shown inFig. 5c–e, for incident angles of 01, 451, and 701. The distributions of carrier generation in the PE and NE devices are similar and indeed vary with the angles of irradiance incidence, which also agree with the observation of angular absorption characterization (see supporting Fig. S2). Since absorption and carrier generation for both devices are nearly identical, we deduce that the carrier collection capability associated with the IQE results in different output photocurrent. Based on our previous analysis, the IQE for the device with a PE shows strong angle dependence, indicating that the carrier collection is indeed affected by distribu-tion of the carrier generadistribu-tion determined by illuminadistribu-tion. On the other hand, the device with NEs exhibit angle-independent IQE characteristics, which mitigates the effect of oblique illumination and is an indicator for ubiquitous carrier collection. As solar radiation has more than 30% of diffused components, the omni-present carrier harvesting property uniquely omni-presented in pro-truded ITO NEs is particularly desirable for organic thin-ﬁlm devices onﬂexible substrates and operated under diffused ambient light.

4. Conclusions

In conclusion, we show evidence of a polymer–fullerene based solar cell with embedded indium–tin-oxide nano-electrodes that demonstrate an omni-present carrier collection capability. Compared to a planar-electrode reference, the internal quantum efﬁciency spectrum with nano-electrodes exhibits broadband enhancements and is insensitive to the angle of irradiance incidence, as well as the distribution of carrier generation. The unique carrier harvesting

Fig. 3. The measured (a) current density–voltage; (b) external quantum efﬁciency, EQE and (c) absorption, A. (d) The derived internal quantum efﬁciency, IQE characteristics of solar cells with a PE and NEs using IQE¼EQE/A.

Table 1

Photovoltaic characteristics of solar cells with a planar electrode and with nano-electrodes.

Device Jsc(mA/cm2) Voc(V) FF (%) PCE (%)

Planar electrode 7.36 0.55 65 2.61 Nano-electrode 8.08 0.56 67 3.01

Fig. 4. The measured and calculated photocurrent density (Jsc) plotted as a function

of the angle of irradiance incidence for cells with a PE and with NEs. The NE device exhibit well matched experimental and calculated values, indicating that the internal quantum efﬁciency of the cell is insensitive to the illumination incident angle.

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attribute is vital to devices operated under diffused ambient light and also provides an insight into the attainment of high-ef_ficiency organic thin-film photovoltaics with flexible substrates.

Acknowledgment

This work is founded by the National Science Council in Taiwan under Grant no. 100-2628-E-009-020-MY3.

Appendix A. Supplementary material

Supplementary data associated with this article can be found in the online version athttp://dx.doi.org/10.1016/j.solmat.2013.07.045.

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