Dimension- and Surface-Tailored ZnO Nanowires Enhance Charge

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Dimension- and Surface-tailored ZnO Nanowires Enhance Charge Collection in Quantum Dot Photovoltaic Devices Jayce J. Cheng, Chia-Hao Marcus Chuang, Olivia Hentz, Paul Rekemeyer, Moungi G. Bawendi, and Silvija Gradecak ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00204 • Publication Date (Web): 06 Apr 2018 Downloaded from http://pubs.acs.org on April 12, 2018

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ACS Applied Energy Materials

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Dimension- and Surface-Tailored ZnO Nanowires

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Enhance Charge Collection in Quantum Dot

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Photovoltaic Devices

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Jayce J. Cheng1, Chia-Hao Marcus Chuang2, Olivia Hentz1, Paul H. Rekemeyer1, Moungi G.

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Bawendi2, Silvija Gradečak1*

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1

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Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02141, USA

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Abstract. The use of zinc oxide (ZnO) nanowires improves charge collection, and consequently

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power conversion efficiency, in quantum dot (QD) based photovoltaic devices. However, the

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role of the nanowire geometry (e.g. density, length, morphology etc.) relative to the QD

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properties remains unexplored, in part due to challenges with controlled nanowire synthesis.

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Here, we independently tailor nanowire length and the active device layer thickness to study

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charge collection in lead sulfide (PbS) QD photovoltaic devices. We then demonstrate

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consistently high internal quantum efficiency in these devices by applying quantum efficiency

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and total reflectance measurements. Our results show that significant losses originate from ZnO

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nanowire-QD interfacial recombination, which we then successfully overcome by using

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nanowire surface passivation. This geometry-tailored approach is generally applicable to other

Department of Materials Science and Engineering, 2Department of Chemistry, Massachusetts

1 ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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nanowire-QD systems and the surface passivation schemes will play significant role in future

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development of nanostructured photovoltaics.

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Keywords. Nanostructured photovoltaics, lead sulfide, quantum dots, zinc oxide, nanowires,

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charge extraction, interface recombination, surface passivation

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ZnO nanowires have been a critical component in increasing power conversion efficiency

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(PCE) of quantum dot photovoltaic devices (QD PV)1–4, mostly by improving charge collection.

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QD PV utilize semiconductor nanocrystals with sub-Bohr radius dimensionality as their active

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layer5–7, providing bandgap tunability. In particular, PbS QDs can be synthesized with excellent

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size control8,9 in a facile one-pot process10,11 and PbS QD PV devices have demonstrated

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certified PCE up to 13.4%13. Their scalability14,15 and long-term stability in ambient

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atmosphere16 makes them an attractive photovoltaic technology. However, the carrier diffusion

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length in QD films falls short of the optimum thickness for absorbing all of the incident light17–

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19

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combined depletion width and carrier diffusion length (estimated at 300 and 100 nm,

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respectively, for PbS QDs passivated with tetrabutylammonium iodide (TBAI)18,20,21), thus

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sacrificing carrier collection22,23. Incorporating ZnO nanowires into PbS QD PV enabled

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formation of thicker active layers by providing high electron mobility pathways within the

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device that greatly increase the efficiency of charge extraction3,24 by increasing short-circuit

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current (Jsc)25,26. ZnO nanowires with excellent crystal quality can be hydrothermally synthesized

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under benign temperature and pH conditions27–30 and on a range of substrates2, making them

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excellent candidates for PV applications. Moreover, we have recently demonstrated31,32 synthesis

: a 700 nm thick PbS QD film absorbs 85% of incident light, but this thickness exceeds the


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of high quality nanowire arrays with controlled density, length and morphology, which makes

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them an ideal platform for investigating the role of nanostructures in optical absorption and

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charge extraction in QD PV devices.

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Despite the success of ZnO nanowire PbS QD PV devices, there are several outstanding

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questions concerning the role of ZnO nanowires in the active layer. First, it remains unclear how

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nanowire dimensions affect optical and electronic processes in QD PV devices, and therefore the

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ideal device geometry for maximizing Jsc. Second, ZnO nanowire QD PV devices typically show

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lower open circuit voltage (Voc) compared to planar ZnO devices. Reduced Voc can result from

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multiple

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recombination22,36; identifying the dominating factors in ZnO nanowire devices would further

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boost PCE. Finally, increased scattering at the nanowire-QD interface can increase the optical

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pathlength3, resulting in higher net absorption, but the full influence of different nanowire

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geometries on the optical scattering profile remains unclear.

sources

such

as

shunting,

interfacial

recombination33–35,

or

trap-mediated

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Here, we show that morphological tailoring and surface engineering of ZnO nanowire arrays

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is critical for enhancing performance of PbS QD PV devices by increasing light trapping and

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reducing interfacial recombination. We first show that Jsc increases monotonically with device

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thickness for devices with constant nanowire length, which demonstrates that nanowires enable

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thicker active layers without sacrificing charge collection. Subsequently, the parameter space

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was explored by fabricating devices with varying nanowire length and active layer thickness. As

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nanowire length increases, there is an increasing mismatch between the measured Jsc and the

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corresponding value calculated from external quantum efficiency (EQE) measurements. In



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seeking to explain the deficit, we exclude light trapping and light concentration effects and show

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that interfacial recombination is a significant loss mechanism in nanowire devices.

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Consequently, interfacial recombination and the Jsc deficit were minimized by titania surface

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passivation via atomic-layer deposited (ALD). These findings show that precise control of

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nanostructured morphology and surface properties to match the optical and electronic properties

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of the active layer is essential for achieving maximum performance in PV devices.

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To investigate the effect of QD overlayer (i.e. the thickness of QDs above the nanowire

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array) on device properties, we first fabricated ZnO nanowire/PbS QD PV devices with varying

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active layer thickness between 335 – 575 nm and constant nanowire length of 200 ± 10 nm

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(Figure 1a). As this comparative study focuses on varying nanowire geometry between devices,

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readily available QDs with a first exciton peak at 880 nm were used. We achieved excellent

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nanowire length uniformity from substrate-to-substrate, while the active layer thickness

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increased linearly with number of spincoated layers (Supporting Information Figure S1). Jsc

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increases continuously with device thickness even above 575 nm (Figure 1b) – significantly

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beyond the carrier collection depth – confirming that nanowire arrays enhance charge extraction

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by extending the depths from which carriers can be efficiently collected. Furthermore, net optical

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absorption increases with thickness (the total volume of QDs is increasing); this effect coupled

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with enhanced carrier extraction results in the increase of Jsc. As also shown in Figure 1b, the

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measured Jsc values are in agreement with the current extracted from EQE measurements on the

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same devices (Jsc, EQE). The increase in Jsc involves a tradeoff in Voc and fill factor (Supporting

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Information Figure S2), both of which decrease with device thickness. These relationships can be

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attributed to bulk recombination and increasing series resistance, respectively. These competing


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processes culminate in a maximum PCE for devices with an active layer of 470 nm for the fixed

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nanowire length of 200 nm (Figure 1b, inset).

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Figure 1. Effect of varying device thickness with constant nanowire length. (a) Cross-section

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scanning electron microscope (SEM) images of nanowire QD PV devices where the nanowire

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length is fixed at 200 nm and the number of QD layers is indicated above the respective cross-

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section. Active layer thickness refers to the total thickness of nanowire array and QD overlayer.

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(b) Corresponding device performance metrics Jsc, Jsc, EQE and PCE (inset) as a function of the

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device thickness. Each data point represents six individual devices.

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Having ascertained the effect of QD thickness while holding nanowire length constant, we

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next fabricated devices with varied nanowire length (Figure 2a) to investigate the role of 5 ACS Paragon Plus Environment


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nanowires in charge collection. Sets of devices with 9, 12, and 15 QD layers were fabricated,

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with each set featuring nanowire lengths increasing in 50 nm intervals, beginning from planar

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ZnO until device failure (short-circuited device due to excessive nanowire length). Again, we

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were able to precisely control nanowire length across each set; nanowire length increased

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linearly with growth time and device thickness increased linearly with number of QD layers

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(Supporting Information Figure S3). These device architectures span a significant parameter

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space and therefore allow us to probe the dual effects of nanowire length and device thickness on

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PV metrics.

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Device performance parameters as a function of the nanowire length are shown in Figures 2b

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– d. It is immediately obvious that both device thickness and nanowire length strongly affect

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device performance. For devices with thick (15-layers) or thin (9-layers) QD overlayers,

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incorporation of nanowires only slightly enhances PCE before it starts to decrease as nanowires

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extend deeper into the active layer (Figure 2b). In contrast, nanowire devices significantly

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outperform planar devices in the 12-layer device set, and we obtain maximum PCE of 8.5% with

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200 nm ZnO nanowires and 470 nm active layer thickness, corresponding to the dimensions at

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which current collection is maximized (Figure 2d).

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To get deeper understanding of the factors that influence – and eventually limit – PCE, we

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next consider Voc and Jsc for different nanowire lengths and device thicknesses. Voc displays

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distinct variation with nanowire length within each set (Figure 2c). In the 15-layer set (thickest

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active layer), Voc is a weak function of nanowire length. However, decreasing the number of

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spincoated QD layers from 12 to 9 causes the effect of nanowire length to become significant:

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Voc decreases substantially as nanowire length increases. Voc has been shown to be sensitive to

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nanowire-QD interfacial traps28, which act as recombination sites and may also pin the Fermi-


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level. Since increasing nanowire length from 200 – 475 nm causes at least a doubling in

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nanowire surface area (details in Supplementary Figure S4), the observed Voc behavior suggests

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that nanowire surfaces strongly impact device performance.

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Figure 2. Effect of varying device thickness and nanowire length. (a) Cross-section SEM

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images of devices with varying nanowire length (nominal length indicated) and number of QD

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layers, thus the overall device thickness. All micrographs are at same scale. Corresponding

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device performance metrics (b) PCE, (c) Voc and (d) Jsc for sets of nanowire length-varied

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devices with 9, 12 and 15 spincoated QD layers. For each set, nanowire length was increased in

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50 nm intervals until device failure, i.e. for each set, the subsequent device failed due to short-

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circuiting. Darker background shading indicates relatively higher magnitude within each set.

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Overall Jsc increases with the number of QD layers (Figure 2d), confirming the results

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obtained from active layer thickness-varied devices. Whereas the thinnest 9-layer device series 7 ACS Paragon Plus Environment


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displays minimal variation in Jsc with nanowire length, both the 12- and 15-layer devices display

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a distinct maximum in Jsc: compared to planar devices, Jsc increases until nanowires are about

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200 nm long, subsequently decreasing rapidly as nanowires become longer. The initial increase

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in Jsc shows that nanowires improve charge collection over planar devices for thicker active

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layers. Furthermore, even small differences in nanowire length (200 nm vs. 150 nm) are

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sufficient to increase Jsc by ~ 2 mA cm-2 (8% increase), showing that precise control of nanowire

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length is critical for maximizing overall device performance.

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As the nanowire length (and consequently device thickness) increases, electron collection

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improves because ZnO nanowires provide high electron mobility pathways. Electrons can thus

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be efficiently collected from deeper regions in the device, but the distance travelled by holes to

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the Au anode also increases with nanowire length (the optical generation rate decreases

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exponentially with the path length so the bulk of carrier generation takes place at the ZnO-QD

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interface). Therefore, Jsc increases with the nanowire length, but eventually drops due to poor

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collection of holes26. Furthermore, optical effects, such as internal light scattering and light

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concentration due to nanowires, may result in changes to carrier recombination dynamics.

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Finally, incorporating increasingly longer nanowires into the active layer introduces defects into

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the QD film or ZnO-QD interface, which cause poorer film quality and hence reduction in light

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absorption and charge collection efficiency. We therefore next provide more in-depth

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investigation of individual effects via quantum efficiency measurements.

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Different types of quantum efficiency measurements isolate specific optoelectronic

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processes, thereby letting us identify and address any limiting factors. EQE convolves several


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optical and electronic processes into a single measurement, as elucidated in Figure 3a. The ratio

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of incident photon flux to total charge generation rate is known as absorption efficiency.

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Following collection of generated carriers to their respective electrodes, the ratio of total charge

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generation rate to collection rate is known as collection efficiency; the ratio of collection

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efficiency to absorption efficiency is termed internal quantum efficiency (IQE), that is, the

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ability of a material to generate charge from light. EQE spectra for the 15-layer device set are

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shown in Figure 3b. The planar device spectrum displays pronounced interference peaks whereas

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spectra for nanowire devices are smoother, indicating that enhanced optical scattering in

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nanowire devices influences the optical modes that form interference peaks in the active layer.

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Internal scattering by nanowires improves light harvesting efficiency by increasing the optical

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pathlength resulting in overall higher EQE for nanowire vs. thin film devices. It is also important

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to note the opposed quantum efficiency behavior in the short (~400 nm) and long (~900 nm, at

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the excitonic peak) wavelength regions as a function of nanowire length. At short wavelengths

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the quantum efficiency decreases with increasing nanowire length. Because these wavelengths

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are absorbed near the substrate, i.e. at the ZnO-QD interface, increasing the nanowire length

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causes poorer carrier collection as holes have to travel further to the Au contact. However, at

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longer wavelengths quantum efficiency increases as nanowires become longer. These

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wavelengths are absorbed deeper in the device, near the QD-Au contact interface, and therefore

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enhanced EQE in this spectral region reflects improved carrier collection by longer nanowires in

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this region of the device.

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Figure 3. External quantum efficiency measurements. (a) Individual optical and electronic

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efficiencies measured during EQE. Following illumination of AM 1.5G, light is transmitted,

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reflected specularly, or reflected diffusely at each material interface, completing a double pass

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after full reflection at the Au contact. (b) EQE spectra for the 15-layer device series including

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spectra for planar, 150, 210, 370 and 570 nm long nanowires. All spectra display the PbS QD

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exciton peak at ~ 930 nm. Interference effects in the spectrum for planar device (black dashed

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line) can be seen in the peaks at 380 and 725 nm. Black arrows at 400 and 930 nm indicate

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change in EQE arising from increasing nanowire length.

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The theoretical short circuit current Jsc, EQE is shown in Figure 4a, b, and c for the 15-, 12-,

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and 9-layer device sets, respectively. In the absence of white light bias, Jsc,EQE typically

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underestimates Jsc because our EQE setup utilizes low intensity monochromatic perturbation, 10 ACS Paragon Plus Environment

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whereas J-V testing is performed under broadband 1-Sun intensity, where the higher generation

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rate causes trap-filling from increased carrier concentration thereby prolonging the carrier

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lifetime. However, in this case Jsc, EQE exceeds Jsc substantially (up to 5 mA cm-2, Figure 4d) for

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devices with longer nanowires. This behavior is consistent across each device series – as

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nanowire length increases, Jsc, EQE initially lies below Jsc, but while Jsc reaches a maximum and

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subsequently decreases, no similar precipitous drop in Jsc,

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recall (Figure 1b) that Jsc,

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increased with constant nanowire length, and therefore the current discrepancy is not a function

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of QD overlayer thickness. The consistently high Jsc,

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potentially be obtained if the specific carrier loss mechanism could be identified and alleviated,

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thereby greatly boosting PCE. The subsequent analysis therefore focuses on using three tools,

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namely UV/Vis integrating sphere measurements, finite element analysis, and ALD surface

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passivation with titania to isolate and consider the effects of light scattering, light concentration,

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and interfacial recombination, respectively.

EQE

EQE

was observed. Furthermore, we

remains a good indicator for Jsc even as device thickness is

EQE

suggests that much higher Jsc could

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The internal quantum efficiency (IQE) of the ZnO nanowire/PbS QD active layer was

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calculated by performing integrating sphere measurements to obtain the light harvesting

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efficiency (LHE) (Supporting Figure S5) and using Equation 1:

‫= ܧܳܫ‬

‫ܧܳܧ‬ ‫ܧܪܮ‬

(1)

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IQE and EQE differ in that EQE reflects collected carriers as a function of total incident photons,

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whereas IQE only considers photons that are actually absorbed in the material.

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Figure 4. Difference between current predicted by EQE and J-V testing. Jsc, EQE and Jsc vs.

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nanowire length for (a) 15-layer, (b) 12-layer, and (c) 9-layer device sets. The shaded region

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represents the current shortfall between Jsc, EQE and Jsc. (d) The difference between Jsc, EQE and Jsc

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as a function of nanowire length for each device series showing deficit and excess predicted

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current.

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Generally higher LHE over the visible and near infrared region for nanowire devices

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confirms that part of the Jsc,

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increasing nanowire length, the LHE spectra between nanowire devices are remarkably similar,

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suggesting that it is the presence of nanowires and not the specific morphology that improves

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light harvesting. Therefore, light harvesting from external surface scattering is unlikely to be the

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reason for the discrepancy between Jsc, EQE and Jsc. Apart from optical absorption, processes like

EQE

enhancement is due to nanowire optical scattering. Despite


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transmission, refraction, and reflection are not intensity-dependent (at the intensities involved in

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PV operation). Therefore, we do not expect the ability of a PV device to trap incident light

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through scattering to vary with the intensity of light during measurement, whether it is low-

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intensity (EQE) or 1-Sun (J-V testing). It therefore can be concluded that the current extraction

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becomes a function of light intensity when nanowire length increases.

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Sublinear variation of photocurrent with light intensity is likely due to changes in the type

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and relative rates of recombination. Between low-intensity EQE measurements and 1-Sun

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illumination, the generation rate changes by several orders of magnitude and the relative rates

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from different recombination mechanisms are intensity dependent. Therefore, the divergence

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between Jsc, EQE and Jsc may be due to:

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1. Increased internal scattering with longer nanowires causes optical generation to take place

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deeper into the device. Unintentional light concentration causes increased recombination rates

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and these rates change drastically with light intensity, thus reducing collection efficiency.

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2. Increasing nanowire length involves a commensurate increase of total interfacial area.

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Although the dominant recombination mechanism could be either interfacial recombination or

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Shockley-Read-Hall (SRH) recombination via sub-bandgap states, the latter dominates in planar

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devices,22 but the large interface area in ZnO nanowire devices could make interfacial

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recombination more significant. Notably, it has been shown that recombination at the ZnO-QD

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interface can cause sub-linear scaling in photocurrent vs. intensity1,34 due to space-charge

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limitation.

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Identifying the cause of this deficit is important because EQE represents an upper bound on

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collected current. The consistent IQE obtained for our active layer shows that the material is of



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generally good quality, and therefore further improvements in efficiency should be possible by

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addressing the cause of the deficit. Subsequently, we use optical simulation to yield information

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about scattering at the nanowire-QD interface and use atomic layer deposition (ALD) to

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passivate titania on the nanowire surface to investigate interfacial recombination.

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Figure 5. Finite-element analysis and ALD titania surface passivated devices. (a) FDTD

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simulation of carrier generation rate Gop distribution in 15-layer device set cross sections using

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measured nanowire lengths and device thicknesses. The corresponding predicted Jsc, obtained by

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integrating Gop across the cross section, is shown below; this value assumes 100% collection

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efficiency. Jsc and Jsc, EQE vs. nanowire length for (b) unpassivated and (c) ALD titania surface

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passivated planar and nanowire devices. All devices have 12 QD layers spincoated. The shaded

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region represents the current shortfall between Jsc, EQE and Jsc.

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We used finite element analysis to visualize the spatial variation of generation rate in each

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device cross-section (Figure 5a). Frequency-domain time difference (FDTD) simulation is

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crucial for accurately modeling nanowire devices because the features have sub-wavelength

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dimensions. Dielectric constant and extinction (n, k) values measured using spectroscopic

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ellipsometry were used as the material parameters for FDTD simulation (Supporting Figure S6).

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FDTD yields both the spatial distribution of generation rate and the integrated generation rate,

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which corresponds to the expected Jsc assuming 100% collection efficiency.

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The Jsc, FDTD predicted by FDTD increases with nanowire length (Figure 5a) and compared to

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thin film devices, which is in agreement with the EQE results. Nanowire devices have non-

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uniform spatial generation rate profile along the x-axis, with carrier generation concentrated in

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nanowire interstices, in contrast to planar devices. Furthermore, optical generation extends

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deeper into the device (along the y-axis) in nanowire devices as compared to planar devices

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because of the optical scattering provided by nanowires in the active layer. Finally, the absolute

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value of the maximum generation rate rises, from 1.5 × 1022 to 1.8 × 1022 cm-3 indicating that

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light concentration in the nanowire interstices can raise the carrier concentration rate by about

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20%; such regions would thus experience a somewhat higher recombination rate and lower

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collection efficiency. This is, however, unlikely to be the cause of the large current discrepancy

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between Jsc, EQE and Jsc. The predicted Jsc, FDTD tracks Jsc, EQE closely (Supporting Figure S7): Jsc,

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FDTD

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Therefore, it is changes in collection efficiency that are causing the current deficit, moving on to

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probe interfacial recombination as a possible carrier loss mechanism in nanowire devices.

increases with increasing number of QD layers, and also increases with nanowire thickness.

288



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ZnO nanowires likely contain significant concentrations of midgap interfacial surface states

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as evidenced by surface roughness following hydrothermal synthesis31 and propensity for

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interacting with adsorbed molecules37 via high dangling bond density. Indeed, changes in band

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offset38 and Fermi level pinning22,39 have been observed in QD PV devices due to interface

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chemistry. We recall that the increasingly severe decline in Voc with nanowire length as device

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thickness decreases (Figure 2c) suggests that substantial interfacial recombination exists. To

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probe this effect, two sets of 12-layer devices were fabricated with one set surface passivated

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with 1 nm of ALD titania and the other set fabricated without surface passivation (this thin layer

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does not change the nanowire surface area significantly). We selected titania for its excellent

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surface properties and robustness towards adsorption of contaminating species, while ALD

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provides us the capability of precisely depositing conformal layers with highly controllable sub-

300

monolayer thickness.

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Jsc for unpassivated and passivated devices, shown in Figures 5b and 5c respectively, follows

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the familiar relationship with nanowire length, peaking at 200 nm long nanowires. However, all

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surface passivated nanowire devices have higher Jsc than unpassivated devices, demonstrating

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that ALD titania is indeed successful in improving current collection. Notably, surface

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passivation effectively halts the Voc decline (Supporting Figure S8b) whereas Voc for

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unpassivated devices decreases rapidly with increasing nanowire length, showing that surface

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states are significant sources of carrier loss. The enhancement in carrier collection leads to higher

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PCE in surface passivated nanowire devices (Supporting Figure S8d), especially for devices with

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longer nanowire arrays (~ 400 nm). With this result we show that engineering of surface states,

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in conjunction with nanowire array morphology tailoring, adds to our toolbox for boosting PCE


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of nanowire devices. Our work indicates a clear pathway toward development of effective

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surface passivation schemes that will be investigated in future.

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In conclusion, morphological tailoring and surface engineering of ZnO nanowire arrays were

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used to investigate convoluted optical and electronic processes in nanostructured PbS QD PV

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devices toward optimized device performance. Nanowires enhance current collection compared

318

to planar devices and thus enable thicker QD active layers, but achieving maximum PCE

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involves a trade-off between improving current collection (by using longer nanowires) and Voc

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(by decreasing surface area). By exploring parameter space using devices with varying nanowire

321

length and active layer thickness, we showed that the current extracted from EQE measurements

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Jsc,

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mechanisms in nanowire devices. The effect of external and internal light scattering were

324

excluded as nanowires contribute to enhanced light scattering and thus improved optical

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absorption. Instead, interfacial recombination is the major contributor to the current losses. We

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used this knowledge to significantly minimize the losses by passivating ZnO nanowire surface

327

using ALD titania. Our work demonstrates that precise control of all aspects of nanostructured

328

architecture – morphology and surface structure – can be used to probe multiple optical and

329

electronic phenomena and subsequently utilize their interaction to further enhance device

330

performance.

EQE

significantly exceeded that observed during J-V testing pointing out potential loss

331

332

Methods



333

ZnO nanowire arrays were fabricated by hydrothermal synthesis of ZnO nanowire arrays on

334

150 nm thick patterned ITO electrodes (Thin Film Devices Inc). ITO substrates were sonicated

335

in acetone for 10 min, boiled in IPA for 10 min, and finally subjected to O2 plasma cleaning for 2

336

min. Dense seed layers were then spincoated from sol-gel solution (0.3 M zinc acetate, 0.3 M

337

monoethanolamine in 2-methoxyethanol, all from Sigma-Aldrich) at 4000 rpm for 60 s, and

338

annealed at 200 oC for 10 min. Two layers were spincoated to produce ~ 30 nm thick seed layers.

339

ZnO nanowire arrays were then grown from the seed layers in 25 mM zinc nitrate and 25 mM

340

HMTA aqueous solutions at 90 oC for varying growth times between 40 and 100 min to yield

341

nanowire arrays with controlled lengths.

342 343

For studying interfacial recombination, a subset of planar and nanowire arrays was surface

344

passivated using atomic layer deposition (ALD) of TiO2, carried out on a Cambridge Nanotech

345

Savannah ALD system. Each layer deposition cycle consisted of a 200 ms pulse of tetrakis-

346

dimethylamido-titanium, a 20 ms pulse of water, and a 7 s nitrogen purge. Deposition

347

temperature was 150 oC and 20 cycles were carried out to conformally surface passivate the ZnO

348

surface with 1 nm of TiO2.

349 350

The nanowire arrays, along with planar control n-type layers consisting of only the dense

351

seed layer (30 nm thickness), were then used as substrates for QD PV device fabrication. PbS

352

QDs suspended in octane were prepared using our previously established procedure3,16,22.

353

Deposition of a single QD layer was conducted as follows: 50 mg/ml PbS QDs in octane was

354

spincoated at 1500 rpm for 35 s. Following deposition, ligand exchange was carried out. Two

355

types of ligands were used: 10 mg/ml TBAI in methanol, and 0.02 wt% EDT in acetonitrile


Page 18 of 31

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356

(ACN). The QD film was soaked in either TBAI/methanol or EDT/ACN for 30 s, and spun off at

357

2500 rpm for 30 s. The substrate was then rinsed with methanol or ACN respectively, depending

358

on the ligand exchange solution, (2500 rpm, 30s) three times. The thickness of each layer was ~

359

30 nm; layers were repeated until the desired thickness was obtained. The final two layers of

360

every device were always EDT layers, i.e. a device with 8 layers would consist of the first 6

361

layers done with TBAI, finished off with 2 layers of EDT. Finally, gold top electrodes (100 nm)

362

were deposited using thermal evaporation to complete the device. The nominal active device area

363

is 5.44 mm2 as defined by the overlap of the ITO and gold electrodes. For measurements, masks

364

with 3 mm2 apertures were used to define the device area in order to obtain accurate current

365

density measurements.

366 367

Current-voltage (J-V) testing was carried out using a Keithley 2636A source-meter and a

368

Newport 96000 solar simulator with an AM1.5G filter. Before measurements, the light intensity

369

was calibrated to 1-Sun, 100 mW cm-2 using a Newport 91150V reference cell. EQE

370

measurements were conducted using monochromatic light obtained by passing white light from a

371

xenon lamp (Thermo Oriel 66921) successively through a chopper, monochromator, and finally

372

through an optical fiber butt-coupled to the device. The device was illuminated through the glass

373

substrate in an underfilled geometry. The output photocurrent was measured with a lock-in

374

amplifier (Stanford Research System SR 830). Jsc,

375

spectrum with the AM 1.5G photon flux and integrating under the resulting curve from 380 –

376

1100 nm. Total reflectance measurements were taken on a Perkin-Elmer LAMBDA 1050

377

UV/Vis/NIR spectrophotometer with 100 mm Integrating Sphere accessory. Finally, devices

EQE

was calculated by convolving the EQE



378

were cross-sectioned using focused ion beam (FIB) on a FEI Helios Nanolab scanning electron

379

microscope. Micrographs were taken at 5 kV accelerating voltage and 21 pA beam current.

380 381 382

383

Supporting Information

384

Supporting information contains details regarding device dimension measurements, active layer

385

thickness varied device performance, nanowire surface area, total reflectance measurements,

386

frequency-domain time difference simulation, and ALD titania surface passivation. This material

387

is available free of charge via the Internet at http://pubs.acs.org.

388

Corresponding Author

389

Address correspondence to: [email protected]

390

Acknowledgements

391

This work was supported by Eni S.p.A. under the Eni-MIT Alliance Solar Frontiers Center.

392

J.J.C. acknowledges support from the Agency for Science, Technology and Research, Singapore.

393

This work made use of the MRSEC Shared Experimental Facilities at MIT, supported by the

394

National Science Foundation under award number DMR-1419807. Any opinion, findings, and

395

conclusions or recommendations expressed in this material are those of the authors and do not

396

necessarily reflect the views of the National Science Foundation.

397


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398 399 400

401

References

402 403 404

(1)

Jean, J.; Chang, S.; Brown, P. R.; Cheng, J. J.; Rekemeyer, P. H.; Bawendi, M. G.; Gradečak, S.; Bulović, V. ZnO Nanowire Arrays for Enhanced Photocurrent in PbS Quantum Dot Solar Cells. Adv. Mater. 2013, 25, 2790–2796.

405 406 407

(2)

Park, H.; Chang, S.; Jean, J.; Cheng, J. J.; Araujo, P. T.; Wang, M.; Bawendi, M. G.; Dresselhaus, M. S.; Bulović, V.; Kong, J.; Gradečak, S. Graphene Cathode-Based ZnO Nanowire Hybrid Solar Cells. Nano Lett. 2013, 13, 233–239.

408 409 410

(3)

Rekemeyer, P. H.; Chang, S.; Chuang, C. H. M.; Hwang, G. W.; Bawendi, M. G.; Gradečak, S. Enhanced Photocurrent in PbS Quantum Dot Photovoltaics via ZnO Nanowires and Band Alignment Engineering. Adv. Energy Mater. 2016.

411 412 413 414

(4)

Lan, X.; Bai, J.; Masala, S.; Thon, S. M.; Ren, Y.; Kramer, I. J.; Hoogland, S.; Simchi, A.; Koleilat, G. I.; Paz-Soldan, D.; Ning, Z.; Labelle, A. J.; Kim, J. Y.; Jabbour, G.; Sargent, E. H. Self-Assembled, Nanowire Network Electrodes for Depleted Bulk Heterojunction Solar Cells. Adv. Mater. 2013, 25, 1769–1773.

415 416

(5)

Yuan, M.; Liu, M.; Sargent, E. H. Colloidal Quantum Dot Solids for Solution-Processed Solar Cells. Nat. Energy 2016, 1, 16016.

417 418

(6)

Kramer, I.; Sargent, E. Colloidal Quantum Dot Photovoltaics: A Path Forward. ACS Nano 2011, 5, 8506–8514.

419 420

(7)

Sargent, E. H. Infrared Photovoltaics Made by Solution Processing. Nat. Photonics 2009, 3, 325–331.

421 422 423

(8)

Murray, C. B.; Norris, D. J.; Bawendi, M. G. Synthesis and Characterization of Nearly Monodisperse CdE (E = Sulfur, Selenium, Tellurium) Semiconductor Nanocrystallites. J. Am. Chem. Soc. 1993, 115, 8706–8715.

424 425 426

(9)

Weidman, M. C.; Beck, M. E.; Hoffman, R. S.; Prins, F.; Tisdale, W. A. Monodisperse, Air-Stable PbS Nanocrystals via Precursor Stoichiometry Control. ACS Nano 2014, 6, 6363–6371.

427 428 429

(10)

Caruge, J. M.; Halpert, J. E.; Wood, V.; Bulovic, V.; Bawendi, M. G. Colloidal QuantumDot Light-Emitting Diodes with Metal-Oxide Charge Transport Layers. Nat Phot. 2008, 2, 247–250.

430 431

(11)

Yin, Y.; Alivisatos, A. P. Colloidal Nanocrystal Synthesis and the Organic-Inorganic Interface. Nature 2005, 437, 664–670.

432 433

(12)

Liu, M.; Voznyy, O.; Sabatini, R.; García de Arquer, F. P.; Munir, R.; Balawi, A. H.; Lan, X.; Fan, F.; Walters, G.; Kirmani, A. R.; Hoogland, S.; Laquai, F.; Amassian, A.; Sargent, 21 ACS Paragon Plus Environment


E. H. Hybrid Organic–inorganic Inks Flatten the Energy Landscape in Colloidal Quantum Dot Solids. Nat. Mater. 2016, 16, 258–263.

434 435 436 437

(13)

NREL. Best Research-Cell Efficiencies http://www.nrel.gov/pv/assets/images/efficiency_chart.jpg (accessed Oct 30, 2017).

438 439 440

(14)

Coe-Sullivan, S.; Steckel, J. S.; Woo, W.-K.; Bawendi, M. G.; Bulović, V. Large-Area Ordered Quantum-Dot Monolayers via Phase Separation During Spin-Casting. Adv. Funct. Mater. 2005, 15, 1117–1124.

441 442 443 444

(15)

Kramer, I. J.; Minor, J. C.; Moreno-Bautista, G.; Rollny, L.; Kanjanaboos, P.; Kopilovic, D.; Thon, S. M.; Carey, G. H.; Chou, K. W.; Zhitomirsky, D.; Amassian, A.; Sargent, E. H. Efficient Spray-Coated Colloidal Quantum Dot Solar Cells. Adv. Mater. 2015, 27, 116– 121.

445 446 447

(16)

Chuang, C.-H. M.; Brown, P. R.; Bulović, V.; Bawendi, M. G. Improved Performance and Stability in Quantum Dot Solar Cells through Band Alignment Engineering. Nat. Mater. 2014, 13, 1–6.

448 449 450

(17)

Barkhouse, D. A. R.; Kramer, I. J.; Wang, X.; Sargent, E. H. Dead Zones in Colloidal Quantum Dot Photovoltaics: Evidence and Implications. Opt. Express 2010, 18, A451– A457.

451 452 453 454 455

(18)

Lan, X.; Voznyy, O.; Kiani, A.; García De Arquer, F. P.; Abbas, A. S.; Kim, G. H.; Liu, M.; Yang, Z.; Walters, G.; Xu, J.; Yuan, M.; Ning, Z.; Fan, F.; Kanjanaboos, P.; Kramer, I.; Zhitomirsky, D.; Lee, P.; Perelgut, A.; Hoogland, S.; Sargent, E. H. Passivation Using Molecular Halides Increases Quantum Dot Solar Cell Performance. Adv. Mater. 2016, 28, 299–304.

456 457

(19)

Lan, X.; Masala, S.; Sargent, E. H. Charge-Extraction Strategies for Colloidal Quantum Dot Photovoltaics. Nat Mater 2014, 13, 233–240.

458 459 460 461

(20)

Ning, Z.; Voznyy, O.; Pan, J.; Hoogland, S.; Adinolfi, V.; Xu, J.; Li, M.; Kirmani, A. R.; Sun, J.; Minor, J.; Kemp, K. W.; Dong, H.; Rollny, L.; Labelle, A.; Carey, G.; Sutherland, B.; Hill, I.; Amassian, A.; Liu, H.; Tang, J.; Bakr, O. M.; Sargent, E. H. Air-Stable NType Colloidal Quantum Dot Solids. Nat. Mater. 2014, 13, 4–10.

462 463 464

(21)

Speirs, M. J.; Dirin, D. N.; Abdu-Aguye, M.; Balazs, D. M.; Kovalenko, M. V.; Loi, M. A. Temperature Dependent Behaviour of Lead Sulfide Quantum Dot Solar Cells and Films. Energy Environ. Sci. 2016, 9, 2916–2924.

465 466 467

(22)

Chuang, C. H. M.; Maurano, A.; Brandt, R. E.; Hwang, G. W.; Jean, J.; Buonassisi, T.; Bulovic, V.; Bawendi, M. G. Open-Circuit Voltage Deficit, Radiative Sub-Bandgap States, and Prospects in Quantum Dot Solar Cells. Nano Lett. 2015, 15, 3286–3294.

468 469 470 471

(23)

Johnston, K. W.; Pattantyus-Abraham, A. G.; Clifford, J. P.; Myrskog, S. H.; Hoogland, S.; Shukla, H.; Klem, E. J. D.; Levina, L.; Sargent, E. H. Efficient Schottky-Quantum-Dot Photovoltaics: The Roles of Depletion, Drift, and Diffusion. Appl. Phys. Lett. 2008, 92, 122111.

472 473 474 475

(24)

Kramer, I. J.; Zhitomirsky, D.; Bass, J. D.; Rice, P. M.; Topuria, T.; Krupp, L.; Thon, S. M.; Ip, A. H.; Debnath, R.; Kim, H.-C.; Sargent, E. H. Ordered Nanopillar Structured Electrodes for Depleted Bulk Heterojunction Colloidal Quantum Dot Solar Cells. Adv. Mater. 2012, 24, 2315–2319. 22 ACS Paragon Plus Environment

Page 22 of 31

Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


476 477 478

(25)

Jean, J.; Chang, S.; Brown, P. R.; Cheng, J. J.; Rekemeyer, P. H.; Bawendi, M. G.; Gradečak, S.; Bulović, V. ZnO Nanowire Arrays for Enhanced Photocurrent in PbS Quantum Dot Solar Cells. Adv. Mater. 2013, 25, 2790–2796.

479 480 481 482

(26)

Wang, H.; Gonzalez-Pedro, V.; Kubo, T.; Fabregat-Santiago, F.; Bisquert, J.; Sanehira, Y.; Nakazaki, J.; Segawa, H. Enhanced Carrier Transport Distance in Colloidal PbS Quantum-Dot-Based Solar Cells Using ZnO Nanowires. J. Phys. Chem. C 2015, 119, 27265–27274.

483 484

(27)

Vayssieres, L. Growth of Arrayed Nanorods and Nanowires of ZnO from Aqueous Solutions. Adv. Mater. 2003, 15, 464–466.

485 486

(28)

Tak, Y.; Yong, K. Controlled Growth of Well-Aligned ZnO Nanorod Array Using a Novel Solution Method. J. Phys. Chem. B 2005, 109, 19263–19269.

487 488 489

(29)

Greene, L. E.; Law, M.; Goldberger, J.; Kim, F.; Johnson, J. C.; Zhang, Y.; Saykally, R. J.; Yang, P. Low-Temperature Wafer-Scale Production of ZnO Nanowire Arrays. Angew. Chemie (International Ed. 2003, 42, 3031–3034.

490 491 492

(30)

Greene, L. E.; Law, M.; Tan, D. H.; Montano, M.; Goldberger, J.; Somorjai, G.; Yang, P. General Route to Vertical ZnO Nanowire Arrays Using Textured ZnO Seeds. Nano Lett. 2005, 5, 1231–1236.

493 494

(31)

Cheng, J. J.; Nicaise, S. M.; Berggren, K. K.; Gradečak, S. Dimensional Tailoring of Hydrothermally Grown Zinc Oxide Nanowire Arrays. Nano Lett. 2016, 16, 753–759.

495 496 497

(32)

Nicaise, S.; Cheng, J.; Kiani, A.; Gradecak, S.; Berggren, K. Control of Zinc Oxide Nanowire Array Properties with Electron Beam Lithography Templating for PV Applications. Nanotechnology 2015, 26, 075303–075311.

498 499 500

(33)

Brown, P. R.; Kim, D.; Lunt, R. R.; Zhao, N.; Bawendi, M. G.; Grossman, J. C.; Bulović, V. Energy Level Modification in Lead Sulfide Quantum Dot Thin Films through Ligand Exchange. ACS Nano 2014, 8, 5863–5872.

501 502 503 504

(34)

Ehrler, B.; Musselman, K. P.; Böhm, M. L.; Morgenstern, F. S. F.; Vaynzof, Y.; Walker, B. J.; MacManus-Driscoll, J. L.; Greenham, N. C. Preventing Interfacial Recombination in Colloidal Quantum Dot Solar Cells by Doping the Metal Oxide. ACS Nano 2013, 7, 4210– 4220.

505 506 507

(35)

Kemp, K. W.; Labelle, A. J.; Thon, S. M.; Ip, A. H.; Kramer, I. J.; Hoogland, S.; Sargent, E. H. Interface Recombination in Depleted Heterojunction Photovoltaics Based on Colloidal Quantum Dots. Adv. Energy Mater. 2013, n/a-n/a.

508 509 510

(36)

Bozyigit, D.; Lin, W. M. M.; Yazdani, N.; Yarema, O.; Wood, V. A Quantitative Model for Charge Carrier Transport, Trapping and Recombination in Nanocrystal-Based Solar Cells. Nat. Commun. 2015, 6, 6180.

511 512

(37)

Li, Q. H.; Wan, Q.; Liang, Y. X.; Wang, T. H. Electronic Transport through Individual ZnO Nanowires. Appl. Phys. Lett. 2004, 84, 4556–4558.

513 514 515 516

(38)

Kim, G. H.; García De Arquer, F. P.; Yoon, Y. J.; Lan, X.; Liu, M.; Voznyy, O.; Yang, Z.; Fan, F.; Ip, A. H.; Kanjanaboos, P.; Hoogland, S.; Kim, J. Y.; Sargent, E. H. HighEfficiency Colloidal Quantum Dot Photovoltaics via Robust Self-Assembled Monolayers. Nano Lett. 2015, 15, 7691–7696. 23 ACS Paragon Plus Environment


517 518

(39)

Kramer, I. J.; Sargent, E. H. The Architecture of Colloidal Quantum Dot Solar Cells: Materials to Devices. Chem. Rev. 2014, 114, 863–882.

519 520


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521


TOC Figure

522



Figure 1 82x119mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Figure 2 165x123mm (300 x 300 DPI)



Figure 3 83x135mm (300 x 300 DPI)


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Figure 4 165x130mm (300 x 300 DPI)



Figure 5 132x123mm (300 x 300 DPI)


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Table of content figure 77x36mm (300 x 300 DPI)


Dimension- and Surface-Tailored ZnO Nanowires Enhance Charge

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