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Understanding the Effects of a High Surface Area Nanostructured ITO Electrode on Organic Solar Cell Performance Bing Cao, Xiaoming He, Jason Sorge, Abeed Lalany, Kaveh Ahadi, Amir Afshar, Brian C. Olsen, Tate C Hauger, Md Hosnay Mobarok, Peng Li, Kenneth C Cadien, Michael J. Brett, Erik J. Luber, and Jillian M. Buriak ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10610 • Publication Date (Web): 12 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017

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ACS Applied Materials & Interfaces

Understanding the Effects of a High Surface Area Nanostructured ITO Electrode on Organic Solar Cell Performance Bing Cao,†,‡ Xiaoming He,† Jason Sorge, Abeed Lalany, Kaveh Ahadi,⊥ Amir Afshar,⊥ Brian C. Olsen,†,‡ Tate C. Hauger, †,‡ Md Hosnay Mobarok,† Peng Li,§ Kenneth C. Cadien,⊥ Michael J. Brett, Erik J. Luber,†,‡* Jillian M. Buriak†,‡,* †

Department of Chemistry, University of Alberta, 11227 Saskatchewan Drive, Edmonton,

AB T6G 2G2, Canada. ‡

National Institute for Nanotechnology, National Research Council Canada, 11421

Saskatchewan Drive, Edmonton, AB T6G 2M9, Canada. Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada. ⊥

Department of Chemical and Materials Engineering, University of Alberta, Edmonton,

Alberta T6G 1H9, Canada §

nanoFAB Centre, University of Alberta, Edmonton, Alberta T6G 2V4, Canada

KEYWORDS: organic solar cells, photovoltaics, high surface area electrode, ITO, nanotree, bulk heterojunction, BHJ

ACS Paragon Plus Environment


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ABSTRACT: Organic solar cells (OSCs) are a complex assembly of disparate materials, each with a precise function within the device. Typically, the electrodes are flat, and the device is fabricated through a layering approach of the interfacial layers and photoactive materials. This work explores the integration of high surface area transparent electrodes to investigate the possible role(s) a three-dimensional electrode could take within an OSC, with a BHJ composed of a donor-acceptor combination with a high degree of electron and hole mobility mismatch. Nanotree indium tin oxide (ITO) electrodes were prepared via glancing angle deposition (GLAD), structures that were previously demonstrated to be single-crystalline. A thin layer of zinc oxide was deposited on the ITO nanotrees via atomic layer deposition (ALD), followed by a self-assembled monolayer of C60-based molecules that was bound to the zinc oxide surface through a carboxylic acid group. Infiltration of these functionalized ITO nanotrees with the photoactive layer, the bulk heterojunction (BHJ) comprising PC71BM and a high hole mobility low band gap polymer (PDPPTT-T-TT), led to families of devices that were analyzed for the effect of nanotree height. When the height was varied from 0, to 50 nm, 75 nm, 100, and 120 nm, statistically significant differences of device performance were noted, with the maximum device efficiencies observed with a nanotree height of 75 nm. From analysis of these results, it was found that the intrinsic mobility mismatch between the donor and acceptor phases could be compensated for when the electron collection length was reduced relative to the hole collection length, resulting in more balanced charge extraction, reduced recombination, leading to improved efficiencies. However, as the ITO nanotrees increased in height and branching, the decrease in electron collection length was offset by an increase in hole collection length and potential deleterious electric field redistribution effects, resulting in decreased efficiency.


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1. INTRODUCTION Organic solar cells (OSCs) continue to attract intense research attention, due in part to the vast potential of organic synthesis to tailor the properties of the bulk heterojunctions and interfacial layers with exquisite precision.1–4 OSCs also have the advantages of being potentially low cost, light-weight, and manufacturable via ambient high-throughput methods such as spray-coating and roll-to-roll techniques.5–11 The photoactive layer is typically composed of a bulk heterojunction (BHJ), which consists of a nanoscale phasesegregated mixture of donor/acceptor polymers and/or small molecules12–15 sandwiched between two electrodes, one of which must be transparent.12,15,16 Charge transport within the BHJ strongly depends upon the percolating pathways of the donor/acceptor phases, and their respective charge carrier mobilities, which are typically substantially lower than inorganic materials.17–22 As such, the three-dimensional morphology of the BHJ, in addition to composition, plays a significant role in the resulting power conversion efficiency (PCE).23-28 Planar indium tin oxide (ITO) most commonly serves as the transparent electrode in OSCs. One concept being considered is to shorten the collection length meandering path of free charge carriers through the donor and acceptor materials by using high interfacial area nanostructured electrodes.29,30 The photocurrent and charge collection might be improved if the electrode were composed of vertical nanopillars or other threedimensional structures, enabling more direct pathways for charge transport.30–32 Nanostructuring may also be advantageous for optical trapping to maximize light absorption within the BHJ.33–35 A nanostructured electrode has also been proposed as a means of compensating for mismatched electron-hole mobilities.30 Examples of three dimensional ITO electrodes applied to BHJ-based OSCs include pillars,36,37 and helices,38 accessed via glancing angle deposition (GLAD), nanobranched structures prepared via ebeam evaporation,39 etched grooves of ITO,40 and ITO nanorod arrays.41,42 Surprisingly, only small increases in efficiencies were noted (a ~10% increase of the power conversion efficiency is typical), and many questions remain as to why this approach has not proven more effective. The device architecture is, however, considerably more complex than that



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of an OSC with a planar ITO electrode - the embedded high surface area metal oxide electrode within the intricate organic BHJ could affect the nanoscale phase segregation, in addition to the interplay of internal electric fields, charge carrier pathways, and light management. Challenging experimental aspects such as complete filling of the electrode with the BHJ also need to be carefully analyzed and verified, prior to any device testing.36,37 In this work, we examined the use of high surface area single-crystalline ITO nanotree electrodes, with a BHJ composed of a high mobility mismatched donor and acceptor combination. Previous work looking at applying a high surface area electrode to compensate for mobility mismatch used the now classic combination of P3HT and PC61BM as the BHJ,30 a combination that has only a small electron-hole mobility mismatch (an order of magnitude).22 Here we examine a very high mobility donor polymer, PDPPTT-T-TT, and the molecular acceptor, PC71BM; in this case, the donor phase has a hole mobility reported to be 4 orders of magnitude greater than the electron mobility of the acceptor fullerene phase.43 The high surface area electrode would collect the electrons from the lower mobility fullerene phase of the BHJ. The single-crystalline ITO nanostructures used here are made up of well-defined vertical nanocolumnar structures, with branches that grow epitaxially from the nanopillar ‘trunk’.44 ITO nanotrees with average heights of 50, 75, 100, 120, and 150 nm were prepared, coated with charge-selective interfacial layers, and filled with the PDPPTT-T-TT:PC71BM BHJ, to produce inverted OSCs. In this architecture, shown schematically in Figure 1, electrons are collected at the nanotree ITO cathode, and holes at the Ag anode. By varying the heights of the ITO nanotree electrode, it was possible to systematically investigate how the structure of these three-dimensional electrodes impacts the performance of these bulk heterojunction solar cells. A high surface area electrode for electron extraction could provide a platform for investigating the general utility of high surface area electrodes to compensate for high mobility mismatch.


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Figure 1. Cross-sectional schematic of the three-dimensional solar cells investigated in this work. Zoomed-in schematic of functional ITO nanobranches shown (bottom) as well as the molecular structures of the donor (PDPPTT-T-TT), acceptor (PC71BM) and selfassembled interfacial layer (C60-SAM). Interfacial layers not drawn to scale.

2. EXPERIMENTAL SECTION 2.1. Materials. ITO coated glass substrates were purchased from Delta Technologies (8-12 Ω/sq). The high-hole-mobility polymer PDPPTT-T-TT was synthesized according to prior literature,43 and the molecular weight was measured to be Mw = 178 kDa, Mn = 13 kDa using a Viscotek TDA305 Triple Detection GPC system by Malvern Instruments Ltd., where chloroform was used as the mobile phase. PC71BM was purchased from American Dye Sources; C60-SAM was purchased from One-Material Inc. All solvents were purchased from Sigma Aldrich. All materials and solvent were used as received without further treatment unless otherwise stated.



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2.2. ITO Nanotree Electrode Preparation. All nanostructured electrodes were fabricated on commercially available flat ITO-coated substrates. Flat ITO slides were cleaned successively by 10 min ultrasonication in dichloromethane, Millipore deionized water (18.2 MΩcm), and 2-propanol. Before ITO nanotree deposition, the flat substrates were dried under a nitrogen flow and further cleaned for 10 minutes with air plasma (100 mTorr, Harrick PDC 32G, 18W). ITO nanotrees were then deposited onto the flat substrate using electron beam evaporation of ITO pieces (Cerac, Inc., 91:9 In2O3:SnO2, 99.99% pure) in a custom glancing angle deposition (GLAD) chamber (AXXIS, Kurt J. Lesker, base pressure 1 x 10-7 Torr) with a throw distance of 42 cm and nominal deposition rate of ~0.5 nm/s. Substrate tilt (the angle (α) between the substrate normal and material flux) and rotation (about the substrate normal, φ) was controlled in real time with custom software (α = 85°, pitch = 20 nm). Substrate temperature (TS) was adjusted using halogen bulbs to illuminate the substrate surface and enable a hybrid VLS-GLAD growth mode (room temperature – 240 °C). The temperature was monitored using a Type T thermocouple elevated approximately 1 cm above the center of the deposition chuck, with the deposition chuck being thermally isolated from the substrate motion drive shaft by a Macor headpiece (see Beaudry et al,44,45 for calibration details). The height of the ITO nanotrees was controlled by changing deposition times. A thin layer of ZnO (~ 8 nm) was coated onto ITO nanotree electrodes by atomic layer deposition (ALD) using a low temperature ALD reactor (Kurt J. Lesker 150LX). The substrate temperature was maintained at 53 °C and the chamber pressure kept at 1.07 Torr during thin film growth. Diethylzinc (Sigma-Aldrich >99.99%) was utilized as a precursor for zinc ,and water vapor was used as an oxidant. The alternating precursor pulses width/purge times were, respectively, diethylzinc (0.03/5 s) and water vapor (0.5/10 s). Accordingly, 8-nm thick zinc oxide was grown at a rate of 0.075 nm/cycle. The growth per cycle was resolved using in-situ spectroscopic ellipsometry (J.A. Woollam M2000DI) during growth on the planar structure. 2.3. Photovoltaic Devices Fabrication. A solution of PDPPTT-T-TT:PC71BM 1:2 (wt%), 20 mg/ml total in CHCl3:o-dichlorobenzene (ODCB) was prepared in a glovebox and stirred at ~ 70 °C overnight. C60-SAM solutions of 1 mM concentrations were


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prepared in a 1:1 (v/v %) cosolvent mixture of tetrahydrofuran:chlorobenzene (THF:CB). Self-assembled monolayers were formed on the surface of ITO nanotree electrode/ZnO surface by immersing into the solution for 10 min, followed by thorough rinsing with THF:CB and acetone. PDPPTT-T-TT: PC71BM films were spin-cast onto the electrode at 1500 rpm for 60 s before being placed in a petri dish, sealed with parafilm and left to dry. Samples were then transported in a sealed glass vial to a nitrogen glovebox for thermal evaporation of metal the top contacts. MoOx/Ag/Mg contact swere evaporated at base pressures less than 5 × 10−6 Torr, at rates of 0.1 Å/s, ∼2 Å/s and 2.5 Å/s, respectively. Device areas were 0.155 ± 0.01 cm2. The PV characteristics of the OSC devices were characterized at 25–30 °C in air under simulated AM 1.5 G conditions (Xenon source from Oriel 911911000 W) equipped with a custom made water filter and calibrated to a certified Si reference cell with a KG-5 filter (PV Measurements, PVM624). The light intensity was then subsequently measured immediately preceding any J-V curves using a thermopile (XLP12-3S-H2). J–V characteristics were recorded using a computercontrolled Keithley 2400 source meter. 2.4 Characterization. ITO nanotree electrodes were characterized with a Hitachi S4800 scanning electron microscope (SEM) and a Sigma Zeiss FESEM. Scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDX) analysis was performed on a JEOL JEM-ARM200cF S/TEM, equipped with a cold Field-Emission Gun (cFEG) and a probe Cs corrector. EDX maps were acquired with a Silicon Drift (SDD) EDX detector at an acceleration voltage of 200 kV. TEM samples were prepared on a Zeiss Orion NanoFab Helium Ion Microscope, which is equipped with a Ga-FIB column. ITO nanotree films were filled with HPR 504 photoresist and plan-view samples were sectioned and polished with a 30 keV Ga-FIB before imaging. Atomic force microscopy (AFM) was performed in tapping mode on a Digital Instruments/Veeco multimode AFM, and the collected data were analyzed using the open source software Gwyddion.46 Contact angle measurements were carried out using a Ramé-Hart contact angle goniometer and Millipore deionized water (18.2 MΩcm) was used as a probe liquid. Reflectivity measurements were carried out using a Perkin Elmer Lambda 1050 Spectrophotometer equipped with an integrating sphere. An aperture was used to decrease the spot size until the beam could fit entirely onto the



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sample. 0% and 100% transmission correction scans were then performed to calibrate beam intensity. Samples were measured with the reflective electrode deposited on the backside to prohibit any optical transmission, placed at the rear side of the integrating sphere with the glass sider close to the integrating sphere. A black diffuse reflectance standard (Labsphere SRS – 02 – 010) was placed behind the sample to absorb any transmitted light.

3. RESULTS 3.1. ITO Nanotree Electrode Characterization. The ITO nanotree electrodes were prepared as previously described,43 and characterized using scanning electron microscopy (SEM). Shown in Figure 2 are cross-sectional and corresponding tilted SEM micrographs of as-deposited ITO nanotree electrodes with average heights of 50, 75, 100 and 120 nm. From these micrographs several important observations regarding the nanotree morphology are noted. First, the 45° tilted view (Figure 2) images shows a significant increase in nanotree areal density as the average nanotree height changes from 50 to 75 nm. Second, the degree of nanotree ‘branching’ increases substantially when the height exceeds 75 nm - the number and size of branches perpendicular to the nanotree ‘trunks’ is greater for the 100 nm and 120 nm thick films, compared to the 75 and 50 nm films.


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Figure 2. Tilted (top pane) and cross-sectional (bottom pane) SEM images of (a) 50 nm, (b) 75 nm, (c) 100 nm, and (d)120 nm tall as-deposited ITO nanotree electrodes. Tilted images were acquired at 45°. Scale bar: 100 nm. To fulfill the requirement of electron extraction at the ITO electrode, a hole blocking interfacial layer was used. Defective ZnO is a n-type semiconducting metal oxide that has a deep valence band level, (~7.7 eV), which effectively blocks hole transport, while the conduction band level (4.4 eV) is close to the LUMO of PC71BM, allowing electron extraction.47 Common methods for ZnO thin film deposition on planar substrates such as spin casting, either from a precursor solution or nanoparticle solution, cannot be used with these nanostructured substrates because they will not produce a conformal coating. With this consideration, atomic layer deposition (ALD) was selected to deposit interfacial ZnO, resulting in coatings of uniform thickness and composition on the surface of the ITO nanotrees. Planar BHJs were fabricated and tested as a function of ZnO deposition temperature and thickness (Table S1), where it was found that 8 nm-thick thermal ALD



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ZnO deposited at 50 °C had the highest average power conversion efficiencies of the devices tested. As such, these deposition conditions were used for all other ITO nanotree BHJ devices reported in this work. Figure S1 shows cross-sectional SEM micrographs of 8 nm ALD ZnO-coated ITO nanotrees, which clearly reveal that both the trunk and branches of the nanotrees become thicker as a result of the ZnO coating. To further investigate the nature of the ZnO coating, TEM and EDX elemental analysis were performed. As shown in Figure 3, the EDX elemental mapping of an individual ITO nanotree shows that the Zn is found primarily on the exterior of the nanotree, while the core is indium rich, confirming the conformal nature of the ZnO coating. This observation is further substantiated by EDX line scans of a ZnO-coated ITO nanotree that has been cut in half (perpendicular to the trunk) using a focused ion beam (FIB), where the EDX elemental line scan indicates that there is a strong indium signal from the inner trunk, while the zinc signal is mainly from the outer skin (Figure S2).


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Figure 3. (a) Dark field plan-view TEM image and (b) combined EDX mapping of an ITO nanotree sample coated with 10 nm of ZnO by ALD. Individual EDX compositional maps of (c) tin (d) zinc (e) indium and (f) oxygen. The plan-view sample was sectioned and polished with 30 keV Ga-FIB (helium ion microscope) prior to imaging with TEM. Accompanying low magnification TEM images can be seen in Figure S3 (supplementary materials). Scale bar: 50 nm. 3.2. Solar Cell Device Fabrication. Bulk heterojunctions of PDPPTT-T-TT and PC71BM were chosen due to their previous use in OSCs. Earlier studies showed that these two materials are significantly different with respect to their electron and hole mobilities,48 which provide an opportunity to evaluate the ability of the high surface area nanotree electrodes to compensate for mobility mismatch.30 PDPPTT-T-TT has a reported hole mobility of 1.42 cm2/V·s, and PC71BM has an electron mobility of 10-4 cm2/V·s.22,43,48 As per the architecture chosen for these devices, electrons from the low mobility electron acceptor phase, PC71BM, are collected by the high surface area ITO nanotree electrode. To fabricate OPV devices, the ITO nanotree electrodes needed to be infiltrated with the



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photoactive BHJ, where the compatibility and degree of infiltration of the BHJ into the three-dimensionally structured electrode are critical characteristics for determining the performance of these solar cells.36 Intimately mixed PDPPTT-T-TT:PC71BM solutions were spin-cast onto freshly deposited ITO electrodes, and cross-sectional SEM images were taken to observe the degree of infiltration. Cross-sections of BHJ solar cells with ITO nanotree electrodes were characterized by imaging with backscattered electrons in SEM, as shown in Figure 4. In a backscattered SEM image, the signal scales with the atomic mass of the nuclei from which the electrons are scattered. As a result, regions made of atoms with higher Z-numbers appear brighter than those with lower Z-numbers. Consequently, the bright regions in Figure 4b correspond to the inorganic ITO, and the dark regions to the organic BHJ. From these data it can be concluded that the ITO nanotrees survived the spin-casting of the PDPPTT-T-TT:PC71BM, and appeared to be intimately infiltrated by the active layer components. To further check if the ITO nanotrees were damaged by spin-coating, the BHJ was washed off, and the remaining ITO nanotree film was imaged by SEM (Figure S4) No structural changes were noted. For inverted devices, it is generally considered ideal if the BHJ is PCBM-rich at the cathode, which facilitates electron transfer at this interface.49–52 To promote the accumulation of PC71BM on the ITO surface, the surface energy of the cathode can be modified.53-55 The surface of the ALD-deposited ZnO was functionalized with a selfassembled monolayer of a fullerene derivative, C60-substituted benzoic acid, (molecular structure shown in Figure 1) to induce local accumulation of the PC71BM derivative at this electrode. It has previously been shown that the application of a C60-SAM interfacial layer to the ZnO surface of planar ITO/ZnO/C60-SAM/P3HT:PCBM/PEDOT:PSS/Ag devices resulted in a 26% increase in the average PCE of the solar cells.53 The carboxylic acid group of the C60-SAM binds to the hydrophilic ZnO, and modifies both the work function and surface energy of the interface. The change in surface energy is evidenced by the change of the water contact angle of an ALD-deposited ZnO layer on a flat ITO surface, which increases from 16° (bare ZnO) to 59° after applying the C60-SAM monolayer (Figure S5). Following application of the C60-SAM monolayer, the nanotree electrode was infiltrated with the BHJ. The infiltration step of the BHJ consisted of spincasting (in air) a 1:2 (wt.%) blend of PDPPTT-T-TT and PC71BM in CHCl3:ODCB (4:1


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v/v %) solution (20 mg/ml) onto the functionalized, nanostructured ITO electrode, and slowly evaporating the residual casting solvent over ∼15–20 min in a petri-dish sealed with parafilm, and then transferred to an evaporator for MoOx (8 nm) and Ag (20 nm) deposition.

Figure 4. Backscattered SEM micrographs of PDPPTT-T-TT:PC71BM/ITO nanotree BHJ with (a) flat ITO and (b) nanotree ITO electrode. Scale bar: 100 nm. 3.3. Solar Cell Performance. The OSCs comprising ZnO/C60-SAM modified nanotree ITO electrodes of different heights, with a PDPPTT-T-TT:PC71BM BHJ photoactive layer, were assembled and tested. The solar cell device structure is depicted schematically in Figure 1. The general device architecture is defined as: commercial ITO/nanotree ITO (nanotree height = t)/ZnO/C60-SAM/PDPPTT-T-TT:PC71BM (~130 nm thickness)/MoOx(8 nm)/Ag (20 nm)/Mg(60 nm), where t equals 0, 50, 75, 100 or 120 nm. Due to the low sublimation temperature of magnesium, it was deposited on top of the 20 nm Ag layer to reduce the series resistance of cells, while also achieving a deposition temperature of less than 100 °C (as measured by a thermocouple near the substrate).56



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Figure 5. (a) J-V curves of champion solar cell devices for ITO nanotree electrodes of different heights. (b) Normalized average J-V parameters (Jsc, Voc, FF and PCE) as a function of average tree height. All parameters are normalized to those of the planar electrode. The performance of these solar cells is summarized in Table 1, and the J-V curves of the champion devices, for each nanotree height, are shown in Figure 5. From these data, it can be seen that the solar cell performance changes with the height of the ITO nanotrees. Specifically, optimum performance was observed at a height of 75 nm, beyond which increasing the height reduced the average power conversion efficiency of the solar cells. The control devices fabricated on commercially available ITO coated glass had an average PCE of 3.8%, Jsc of 16.9 mA/cm2, Voc of 0.50 V, and FF of 0.44. By using 50 nm tall ITO nanotree electrodes, an increase in FF to 0.48 was obtained, which was the major contributor to the higher average PCE of 4.2%. A further increase of the ITO nanotree


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height to 75 nm enhanced the PCE to 4.7%, primarily due to an increased Jsc of 17.9 mA/cm2. However, further increasing the height of ITO nanotrees degraded device performance. Major decreases in Jsc, Voc and FF were observed in solar cell devices fabricated with 100 nm and 120 nm ITO nanotree electrodes, which both had an average PCE of 2.9%. The devices with 150 nm ITO nanotree electrodes had PCEs of ~0%, which is believed to be the result of the taller ITO nanotrees shorting the devices. In summary, the performance of the solar cell devices did not have a linear relationship with the height of the ITO nanotree electrodes - optimum performance was obtained with 75 nm tall ITO nanotree electrodes. The J-V curves of the champion devices for each different ITO nanotree height are depicted in Figure 5a, while the photovoltaic parameters (normalized to the planar devices), Jsc, Voc, PCE, and FF are plotted against the height of ITO nanotrees in Figure 5b. Table 1. Summary of photovoltaic parameters of BHJ solar cells as a function of ITO nanotree height. Uncertainties were taken as the standard deviation from the mean. Table entries with a (*) denote that there is a statistically significant change of that parameter compared to devices of the previous height of ITO nanotrees, e.g. The Voc of devices with 100 nm tall nanotrees has a (*), which indicates that it is statistically different from the Voc of devices with 75 nm tall nanotrees. Statistical significance was measured using a 2-sided Kolmogorov-Smirnov test with a significance level of 0.5%. Outliers were rejected using Pierce’s criterion.57 (N = number of devices tested) NT height

Jsc (mA/cm2)

Voc (V)

PCE (%)

FF

N

120 nm

13.9 ± 0.6

0.47 ± 0.01

2.9 ± 0.4

0.43 ± 0.03

17

100 nm

14.1 ± 1.1 *

0.48 ± 0.02 *

2.9 ± 0.4 *

0.43 ± 0.04 *

19

75 nm

17.9 ± 0.5 *

0.53 ± 0.01

4.7 ± 0.5 *

0.49 ± 0.03

27

50 nm

16.5 ± 0.3

0.52 ± 0.02 *

4.2 ± 0.5 *

0.48 ± 0.04 *

19

0 nm

16.9 ± 0.6

0.50 ± 0.02

3.8 ± 0.4

0.44 ± 0.03

26

4. DISCUSSION As seen from the backscattered SEM micrographs (Figure 4) the three-dimensional high surface area nanotree electrodes are well infiltrated by the organic BHJ. In principle this architecture should provide shorter pathways for electron collection - instead of travelling all the way through the BHJ to reach the flat electrode, electrons need only



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migrate to the surface of these nanotrees to be collected. As such, with a reduced average collection length of the electrons in the PCBM, the probability of non-geminate (or secondary geminate58) recombination events occurring will decrease. Conversely, the collection length for holes generated below the tops of the nanotrees is expected to increase as branches from the ITO nanotrees may block previously available transport pathways to the Ag electrode. However, the decrease/increase of electron/hole collection lengths is expected to have an asymmetric effect on the total number of recombination events, for multiple reasons. Firstly, the decrease in electron collection length will generally be greater than the increase in hole collection length. For example, holes that are photogenerated above the ITO nanotree will have no change in collection length (relative to a planar electrode), but photogenerated electrons will have a significant decrease in collection length, as they only need to be transported to a region near the tops of the nanotrees. Secondly, the hole mobility of the polymer (PDPPTT-T-TT) is reported to be several orders of magnitude greater than the PC71BM.43 As such, the limiting factor in planar devices is expected to be electron collection, where an initial decrease in electron collection length will have a net reduction in recombination, even if there was an equivalent increase in hole collection length. In order to the quantitatively analyze how these nanotree electrodes impacted the recombination properties of these devices, the charge extraction probability curves (𝑃! ) were calculated and shown in Figure 6. Briefly, the charge extraction probability curves were determined by first subtracting the solar cell current under illumination from the current in the dark. Next, the reverse saturation current, 𝐽!"# , was determined by fitting the dark curve subtracted data to the Hecht expression for carrier trapping,59 𝑃! (𝑉) =

𝐽(𝑉) 𝜇𝜏(𝑉! − 𝑉) 𝑑𝑑′ = 1 − exp 𝐽!"# 𝑑𝑑′ 𝜇𝜏(𝑉! − 𝑉)

where 𝑉! is the compensation voltage, 𝜇 is the effective mobility, 𝜏 is the effective recombination lifetime, 𝑑 is the thickness of the BHJ and 𝑑′ is the effective charge collection length. Finally, these curves were averaged, (rejecting poor fits to the model, defined as fits with a root-mean-square error greater than 0.25 mA/cm2) plotted in Figure 6, and the resulting fitting parameters given in Table 2.


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Figure 6. Average charge collection probability curves of solar cell devices for ITO nanotree electrodes of different heights. Table 2. Summary of charge extraction probability curve parameters of BHJ solar cells as a function of ITO nanotree height. Uncertainties are taken as the standard deviation from the mean. N = number of devices with a fitting root-mean-square error less than 0.25 mA/cm2. NT height

Jsat (mA/cm2) 𝞵𝛕/dd' (V-1)

120 nm

19.7 ± 0.6

2.7 ± 0.5

0.58 ± 0.01 4

100 nm

19 ± 3

3±1

0.55 ± 0.03 13

75 nm

21.6 ± 0.4

4.7 ± 0.5

0.56 ± 0.01 9

50 nm

19.3 ± 0.5

5.3 ± 0.4

0.57 ± 0.01 12

0 nm

23 ± 1

2.9 ± 0.2

0.58 ± 0.01 6

Vs (V)

N

From the average charge extraction probability curves in Figure 6, several observations are noted. First, the recombination rates of the devices with 50 and 75 nm tall ITO nanotree electrodes are less than the planar devices (e.g. higher charge extraction probability). The observed reduction in recombination rate is in accordance with the previously proposed mechanism of decreasing the effective collection length as a result of having a 3-dimensional ITO nanotree electrode.30,37 Furthermore, this reduction in effective collection length can be roughly quantified from the fit parameter 𝜇𝜏/𝑑𝑑′ (see Table 2). Specifically, as the height of the nanotree electrodes vary, it is expected that the values of 𝜇 and 𝜏 remain mostly unchanged, as they are bulk properties of the BHJ



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materials, while 𝑑 is kept constant at ~130 nm. As such, the change of 𝜇𝜏/𝑑𝑑′ from 2.9 V-1 for planar devices to 5.4 V-1 for planar to 50 nm ITO electrode represents a ~45% reduction in the effective charge collection length. As the height of the nanotrees is further increased from 50 nm the reduction in effective charge collection length is ~38% and 9% from 75 and 100 nm tall ITO electrodes. These data imply that beyond a certain height of ITO nanotrees, the electron collection length is sufficiently reduced such that it is no longer the rate-limiting step to recombination and/or the hole collection becomes significantly impeded by the ITO nanotrees. As previously discussed, as the ITO nanotree height and degree of branching increases, the hole collection length will subsequently increase. Despite the expected asymmetry between changes in hole and electron collection length, with the introduction of an ITO nanotree electrode, if the nanotrees are sufficiently dense, tall and branched, hole extraction could become significantly impeded. Secondly, as the fraction of ITO embedded in the polymer phase increases, the chance of holes diffusing towards the ITO electrode (resulting in recombination) also increases. However, another very important effect of the 3-dimensional ITO electrode to consider is the redistribution of the electric field compared to a planar electrode.60 Specifically, the presence of ITO branches, which are parallel to the Ag electrode, will generate built-in electric fields that run anti-parallel and perpendicular to the desired direction for charge extraction. This effect will have the net result of increasing the residence time of holes that are in the polymer phase before they can be extracted, subsequently increasing the recombination rate. Previous studies investigating three dimensional ITO nanoscale electrodes and OSCs found moderate efficiency enhancements (~10% increase in PCE), which were primarily a result of increases in Jsc.30,38,39 These increases in Jsc were attributed to the anti-reflective properties of these nanostructured electrodes, where a reduction in reflection at the glass/ITO interface was observed as a result of the nanotree branches. In order to quantify the anti-reflective properties of these ITO nanotree electrodes as a function of ITO nanotree height, the reflectivity of these devices were measured using an integrating sphere (Figure 7a). From these data it appears that is indeed an anti-reflective effect, as the reflectivity of the ITO nanotree substrates is generally less than that of the planar ITO


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substrate. Since these reflectivity spectra are acquired on actual devices with a metallic top electrode, the measured reflectivity can be directly converted into absorption within the solar cell (A = 1– R). From the absorption spectra the absorption factor for each solar cell is calculated,61 𝜂! =

! !(!)! ! ! ! ! !

! !" !"

where 𝐼(𝜆) is the measured intensity of the solar simulator spectrum (AM 1.5 G) and 𝐴(𝜆) is the measured absorption spectrum of the solar cells. The absorption factor gives a direct measure of the fraction of incident photons absorbed by the solar cell and is shown in Figure 7b. Calculation of the absorption factor shows that there is very little change in the anti-reflective properties of these cells as the height of the ITO nanotrees change, where the 50 and 75 nm tall electrodes have less than a 2% increase in the absorption fraction relative to planar ITO. (It is also noted that the increase in absorption fraction of the BHJ will actually be less than the values shown in Figure 7b, since additional parasitic absorption from the ITO has not been accounted for and will increase slightly with the height of the ITO nanotrees).



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Figure 7. (a) Reflectivity of ITO nanotree BHJ solar cells measured with an integrating sphere. (b) Corresponding absorption factor measured as a function of ITO nanotree height. Unlike previous studies using ITO nanotree electrodes, 30,38,39 light trapping is not found to be a significant effect in these solar cells and cannot account for the 7% increase in Jsc that is observed for the 75 nm tall ITO nanotree electrodes (relative to planar ITO). The reason for these differences may be the variation of the areal density of nanotrees, where other works used much denser arrays of nanotrees, 38,39 which would more effectively modulate the refractive index within the devices, resulting in anti-reflective properties. Reasons why the Jsc of the 75 nm tall ITO nanotree electrodes increases by 7% are not well understood at this point. Specifically, as shown in the charge extraction probability curves (Figure 6), the recombination rates of unbound charges are slightly lower for the


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50 nm devices compared to the 75 nm devices, while both devices absorb roughly the same number of photons. Given these two facts, it is difficult to rationalize the increase in Jsc if neither light absorption nor charge extraction probability increase. During film formation, it is possible that the morphology of the BHJ changes due to the increased height and interfacial area of the ITO nanotree electrode, which could result in an increased exciton generation rate,62 or bound pair dissociation rate.63 Further work will be required to understand the origin of the Jsc enhancement of the 75 nm tall ITO nanotree electrodes relative to the 50 nm tall ITO nanotree electrodes. Lastly, it is also worth noting that there is a statistically significant change in Voc, which improves by up to 6% (75 nm tall ITO electrodes) and worsens up to 6% (120 nm tall ITO electrodes). This is somewhat surprising, as the most rudimentary theory of opencircuit voltage in OSCs posits that Voc is determined by the quasi-Fermi levels of the acceptor and donor materials,64 which presumably remain constant with ITO nanotree height. However, there is a vast body of literature on the many factors that can influence the open-circuit voltage of OSCs, ranging from carrier density, charge transfer states, density of states, donor/acceptor interfacial area, morphology and recombination.65 Although any number of the these factors could theoretically be changing with the height of the ITO nanotree electrodes, we believe that the most likely candidate is the change in recombination rates. The interaction between Voc and recombination can be most easily understood by recognizing that Voc occurs when the recombination rate is equal to the photo-generation rate.65 As such, given that the charge extraction probability curves (Figure 6) show that the recombination rate is suppressed in the devices with 50 and 75 nm tall ITO nanotree electrodes (relative to planar), we would expect an increase in the open-circuit voltage if the generation rate of free charge carriers stays constant. Given that the rate of photon absorption stays essentially constant with ITO nanotree height (Figure 7b), it is reasonable to expect an increase in open-circuit voltage for the 50 and 75 nm tall ITO nanotree electrodes. However, as previously noted, there may be an increase in exciton generation rate or bound-pair dissociation rate due to morphological changes. Nonetheless, the observed increase in open-circuit voltage suggests that the reduction in recombination rate for the 50 and 75 nm tall ITO nanotree electrodes is greater than any possible increases in free charge generation rate. Moreover, inspection of



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Figure 5b reveals a very strong correlation between fill factor and open-circuit voltage, which further suggests that the changes in Voc are a result of recombination.

5. CONCLUSIONS This work attempted to determine and reconcile the many possible effects of transparent ITO nanotree electrodes within bulk heterojunction solar cells that have a mobility mismatch, with the donor polymer phase having a much higher reported mobility than the acceptor fullerene phase. The branched electrodes impact device performance in several ways, either improving or worsening device performance. These effects are highly dependent on the height and morphology of the nanotrees, relative to the BHJ. For shorter and less branched nanotrees (less than half the BHJ film thickness) the device efficiency is found to improve, which is a result of an asymmetric reduction in charge extraction length for electrons compared to an increase for holes, since the nanotrees are only present in the bottom half of the BHJ. This geometric asymmetry in electron/hole collection length compensates for the asymmetry in electron/hole mobilities, leading to more balanced charge extraction. On the other hand, when the nanotree electrodes are sufficiently tall (greater than half the BHJ thickness), there is a decrease in hole extraction rates and undesirable internal electric field redistribution, which begin to offset improvements in electron extraction, leading to a reduction in efficiency. Thus, integration of three-dimensional complex electrodes into OSCs will require very careful consideration to properly balance the undesirable asymmetries of planar devices without causing unwanted detrimental effects.

ASSOCIATED CONTENT Supporting Information SEM images of ALD ZnO coated ITO nanotrees; average photovoltaic parameters of BHJ solar cells made with ZnO coating deposited at different thicknesses and


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temperatures; EDX line scan of an individual ITO nanotree; SEM images of recovered ITO nanotree electrodes; AFM height maps of ZnO coated and flat ITO surfaces (PDF) AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Future Energy Systems (FES) Research Institute of the University of Alberta, Alberta Innovates-Technology Futures (AITF iCORE IC50-T1 G2013000198), the Canadian Foundation for Innovation (CFI), Saudi Basic Industries (SABIC), and NSERC (the NSERC Photovoltaics Innovation Network, NSERC Discovery Grant RGPIN-201405195) are thanked for support. We would also like to thank Anqiang He and Shihong Xu from University of Alberta nanoFAB for assistance with SEM imaging.

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Understanding the Effects of a High Surface Area ... - ACS Publications

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