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DOI: 10.1039/C4NR05707D

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Lu Han , Jie Liu , Ningning Yu , Zeke Liu , Jinan Gu , Jialing Lu and Wanli Ma * 5

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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Nanocrystal array solar cells based on lead chalcogenide quantum dots (QDs) have recently achieved high power conversion efficiency over 8%. The device performance is expected to further increase by using 1dimensional nanorods (NRs), due to their improved carrier transport over zero-dimensional quantum dots. However, previously reported PbSe NRs haven’t been used in solar cells mainly because of their large diameters, resulting in small bandgap unsuitable for photovoltaic application. In this work, we have demonstrated a new method for synthesizing monodisperse ultra-small PbSe NRs with diameter approaching 2 nm (Eg> 1.2 eV), which can be attributed to the use of dipenylphosphine (DPP) and trans2-octenoic acid (t-2-OA). The introduction of trace DPP can greatly lower the reaction temperature, leading to reduced diameters for obtained PbSe NRs as well as largely increased yield. The use of shortchain t-2-OA together with oleic acid as capping ligands results in high monomer reactivity, fast nuclei diffusion and high growth rate, which realize the anisotropic growth of ultra-small PbSe NRs at such low reaction temperature. The PbSe NRs show n-type property and high electron mobility as measured by field-effect transistors. The PbSe NRs with narrow diameters also demonstrate proper bandgap for photovoltaic application. They were used for the first time in solar cells and improved efficiency was demonstrated when used together with QDs.

1. Introduction

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In recent years, lead chalcogenide nanocrystals (NCs) such as PbS and PbSe have attracted more attentions for their potential applications in telecomunications 1, 2 3-5 photodetectors , photovoltaics , and light emitters6. Especially, their remarkably large exciton Bohr radius 7, size and shape-tunable physical attributes 8-10 and multiple exciton generation(MEG) 11,12 effect make PbS and PbSe ideal candidates for solar energy conversion. Nanocrystal array solar cells based on PbS/PbSe have been extensively studied in recent years3,13-16 and high power conversion efficiency (PCE) over 8% has been achieved by Sargent and Bawendi groups17,18. The first efficient NC solar cell based on PbSe has been reported by Luther et al. in 2008 19 and we then improved the efficiency to 4.57% 4,20. However, currently all the PbS/PbSe nanocrystal solar cells use quantum dots (QDs) as the active material, which may hinder the carrier mobility due to frequent inter-dots hopping. In hybrid nanocrystalpolymer photovoltaics, it has been reported that 1dimensional (1D) cadmium selenide nanorods (NRs) led to more efficient electron transport than zero-dimensional (0D) QDs and hence the performance was improved 21-23.

This journal is © The Royal Society of Chemistry [year]

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Additionally, latest breakthrough showed increased MEG efficiency in PbSe NRs compared with QDs6,24,25. The MEG efficiency can also be enhanced by adjusting the aspect ratio of PbSe NRs and exhibits a maximum at an aspect ratio of 67 with the multi-exciton yields two-fold higher than QDs 26. Moreover, PbSe NRs also exhibit other interesting properties such as anisotropic absorption27, high absorption coefficients28, and long biexciton lifetime 26,29. However, the anisotropic growth of PbSe colloidal NCs has been less well developed compared to II-VI semiconducting NCs30-32, mainly due to its highly symmetrical (rock salt) crystal structure. So far the synthesis of colloidal PbSe NRs has only been reported by a few groups. Prasad et al. have prepared PbSe NCs with anisotropic shapes by using noble metal 33 or hybrid nanoparticles34 as seeds, which, however, may change the intrinsic physical properties of synthesized NCs 35. Jawaid et al. showed that PbSe rods and multipods with nearly 4.5 nm diameter can be synthesized in the inadvertent oxidation polyunsaturated solvents36. Recently, Murray’s group has reported PbSe NRs with ~4 nm diameter by using a new phosphine selenide precursor 37. However, none of the PbSe NRs reported previously have been employed in solar cells likely due to their large diameters (> 4 nm), resulting in [journal], [year], [vol], 00–00 | 1

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Facile synthesis of ultra-small PbSe nanorods for photovoltaic application

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unsuitably small bandgap for photovoltaics. We have pointed out that in PbSe NCs array solar cells, the optimal diameter for PbSe NCs should be as small as 2 ~ 3 nm to achieve decent open circuit voltage and good performance 4. Thus to take advantage of the enhanced carrier transport of PbSe NRs in solar cells, it still remains a challenge to synthesize PbSe NRs with ultra-small diameters. In this work, we reported a solution-process method to synthesize high-yield, monodisperse PbSe NRs with narrow diameters approaching 2 nm (Eg> 1.2 eV) by firstly introducing trans-2-octenoic acid (t-2-OA) as the synergetic ligand together with conventional oleic acid (OA). The introduction of short-chain t-2-OA led to high monomer reactivity, fast nuclei diffusion and high growth rate for NCs, resulting in anisotropic growth of PbSe NCs. The aspect ratio of the PbSe NRs can be adjusted by changing the length of the short-chain carboxylic acids. Furthermore, we intentionally added a small amount of dipenylphosphine (DPP) to induce low-temperature nucleation and high synthetic conversion yields, which enables the growth of PbSe NRs at low temperatures (80 oC-120 oC) to achieve NRs with extremely small diameters. The photovoltaic and carrier transport properties of these PbSe NRs were investigated for the first time in solar cells and field-effect transistors (FETs) respectively, and improved performance than conventional QDs has been demonstrated.

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2.1 Chemicals and Materials. Lead oxide (99.9%), Tris(diethylamino)phosphine (TDP) (97%), oleic acid (tech., 90%), 1,3-benzenedithiol (BDT) (97%) were purchased from Alfa-Aesar; trans-2-hexenoic acid (t-2-HA) (>98%), trans2-octenoic acid (t-2-OA) (>95%), trans-2-decenoic acid (t-2DA) (>95%) were purchased from TCI; selenium powder (99.5%), diphenylphosphine (DPP) (98%), 1-octadecene (ODE) (90%) were purchased from J&K; trioctylphosphine (TOP) (97%) was purchased from Strem; bis(trimethylsilyl)selenide ((TMS) 2Se) (97%) was acquired from Gelest. 1-octadecene was dried by heating to 100°C under vacuum for 24 h and then placed in glovebox. The phosphine selenide solution was obtained by stirring 10 ml phosphine and 10 mmol Se powder overnight and then stored in glovebox.

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2.2.1 Control of the t-2-XA:OA molar ratio. All operations were under nitrogen atmosphere using standard air-free Schlenk line techniques. A solution of 0.4 mmol PbO (89 mg), 1 mmol carboxylic acid ligands (t-2-XA:OA) and 8 g dried ODE was heated at 130 oC in a 50 mL threeneck flask under nitrogen. The molar ratio of t-2-XA:OA was adjusted to specific values, with the total amount kept at 1 mmol. The solution was then degassed for additional 1 h at 100 oC under vacuum before setting the solution at 120 oC under nitrogen. Then a mixture of 1.2 ml 1M TDPSe and 10 ul DPP in 1.2 ml ODE was injected. The reaction was allowed to continue for 2 min, and then it was rapidly

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quenched by placing the flask in a room-temperature water bath and injecting 5 ml anhydrous hexane. The NCs were purified by precipitation twice in hexane/isopropyl alcohol and stored in powder in a nitrogen-filled glovebox. 2.2.2 Control of the reaction temperature. The t-2OA:OA molar ratio was fixed at 4:1, while the PbSe NRs were grown at specific temperatures for optimal time (50 min, 15 min, 2 min, 1 min and 0.5 min for 80 oC, 100 oC, 120 oC, 140 oC and 160 oC respectively). 2.3 Characterization. Transmission electron microscopy (TEM) images were obtained using a Tecnai G2 F20 S-Twin transmission electron microscope. UV-vis-NIR spectra were recorded on a Perkin Elmer model Lambda 750. The NIR fluorescence spectra of PbSe NRs were collected on an Applied NanoFluorescence spectrometer at room temperature with an excitation laser source of 785 nm. The crystal structure of PbSe NRs was confirmed by X-ray powder diffraction (XRD) by using an X'Pert-ProMPD (Holand) D/max-gAX-ray diffractometer with Cu Ka radiation (λ= 0.15406 nm). For XRD measurement, samples were prepared by depositing NCs or NRs solutions in chloroform onto a Si substrate. The IR spectra of PbSe NRs with different t-2-OA:OA molar ratios were recorded in the range 3200-400 cm-1 on a HYPERION spectrometer with a pressed KBr pellet. The FET were characterized at room temperature in a high vacuum probe station (Lakeshore) connected to a semiconductor parameter analyzer (Keithley 4200). 2.4 Computational details. All the calculations were performed by DMol3 in MS developed by Accelrys Inc. Allelectron method that includes a generalized gradient approximation (GGA) for the exchange-correlation term was adopted. The DND basis set and PBE functional were chosen in this calculation. We perform the Self-consistent field (SCF) computations with a convergence criterion of 10−5a.u. on the total energy and electron density. We speed up the convergence by smearing 0.001Ha which is set in the orbital occupancy. All the structures have been optimized so that we can get the stable structure and the total energy. We calculate the binding energy in the atom level. 2.5 Device fabrication. The substrates were cleaned by sonication in detergent, acetone, and isopropyl alcohol for 10 min in sequence, and then ozone treatment for 30 min to remove any organic residue. A thin layer of PEDOT:PSS was first deposited at 4500 rpm onto the pre-cleaned indium tin oxide (ITO) substrate and then thermally annealed at 150 o C for 10 minutes. Then the substrates were transferred to a nitrogen glovebox. A solution of PbSe dots (20 mg/ml) dissolved in hexane was spinning-coated onto the PEDOT:PSS layer. The post ligand exchanged was carried out by soaking the as-prepared film in a solution of 0.001M 1,3-BDT in acetonitrile for 30s, then rinsed with pure acetonitrile to remove the excess of 1,3-BDT and ligands. Then another QD layer was deposited and ligand exchanged. Subsequently, two NRs layers were deposited on top of the dots film by spin-coating a 20 mg/ml PbSe NRs solution in hexane and then ligand-exchanged. Finally, a 0.6 nm LiF (0.1 Å S-1) and 100 nm Al (1 Å S -1 for the first 10 nm and 3 Å S-1 for the remaining 90 nm) were thermally evaporated at a pressure of 1×10-6 mbar through a shadow mask (active

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area 7.25 mm2). Devices for FETs measurements were fabricated on ndoped silicon wafers with 250 nm of thermally grown SiO 2, which serve as the back gate and part of the gate dielectric stack, respectively. The substrate was washed with DI water, acetone and then processed by hexamethyldisilazane (HMDS). After that, the substrates were transferred to a nitrogen glovebox. A solution of 20 mg/mL PbSe QDs in hexane was spin-coated at 1500 rpm on the Si substrate and then ligand-exchanged as described above. Similarly, a solution of 20 mg/ml PbSe NRs in hexane was deposited on the substrate and ligand-exchanged. 80 nm copper was evaporated on the active layer under high vacuum (Kurt J. Lesker, < 10-6 Torr) using a shadow mask with 1200 µm channel width and 50 µm channel length for the drain and source electrodes, respectively.

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oriented attachment along the (100) axis, as shown in Fig. 1b.

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We describe here a new synthesis scheme producing monodisperse PbSe NRs at low temperatures by using shortchain carboxylic acid and OA as hybrid capping agents and tris(diethylamino)phosphine selenium (TDPSe) with trace DPP as Se precursor in the noncoordinating solvent 1octadecene (ODE). Previously, researchers observed that short ligands can lead to elongated NRs in Ⅱ-Ⅵ system38,39. Here, short-chain t-2-OA was used together with conventional OA in our system to achieve PbSe NRs. We successfully obtained high-quality PbSe NRs with a suitable bandgap at an optimal molar ratio of 4:1 for t-2-OA and OA, as shown in Fig. 1a. The synthesized PbSe NRs are monodisperse with a remarkably small diameter of 2.6±0.4 nm and an average length of 14±4 nm, yielding an aspect ratio of 6±2. The single crystallinity and structural integrity of the PbSe NRs were demonstrated by the high-resolution transmission electron microscopy (HRTEM) images (Fig. 1b). The PbSe NRs shown in Fig. 1b have fringe spacing of 0.31 nm, which corresponds to the (200) lattice planes for the cubic rock salt structure of PbSe, as also reflected in the inset Fourier transform. The HRTEM image also indicates that the [100] crystallographic axis is parallel to the long axis of the rods as previously observed in anisotropic PbS NCs 40. Meanwhile, very few multipods are also observed in the HRTEM images (Fig. S1), which show two perpendicular sets of (200) facets, in accordance with the growth direction of PbSe NRs. Oriented attachment is one of the most reported mechanisms for the anisotropic growth of PbS and PbSe NCs41,42. The attachment of PbSe NCs can be along the axis of nonpolar (100) or polar (111) facets, favoring facets with higher surface energy. Without capping ligands, the surface energy of (111) in PbSe NCs is larger than (100). With capping ligands, the surface energy of (111) can be dramatically reduced and become smaller than that of (100)43. In addition, Jeong et al. demonstrated that (111) capped with short-chain carboxylic acids led to much lower surface energy than that capped with long-chain ones44. Thus in our work, the (100) facets of PbSe NCs should have higher surface energy than (111) capped with short-chain t2-XA (X=H, O or D) and hence more active, leading to

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Fig. 1 (a) The TEM image of ultra-small PbSe NRs with a molar ratio of 4:1 for t-2-OA and OA. (b) HRTEM image of PbSe NRs with lattice fringes of 0.31 nm. The inset shows the corresponding Fourier transform.

3.1 The effect of t-2-OA:OA ratio on PbSe NCs morphology.

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Previous studies have shown that the organic capping ligands used in the colloidal NCs synthesis can play a profound role in the growth mechanism and will affect the physical properties of colloidal NCs38,45-47. In this study, t-2-OA and OA were mixed together as the capping agents. Fig. 2 shows that the molar ratio of t-2-OA/OA can largely affect the morphology of synthesized PbSe NRs at the optimal growth temperature of 120 oC. When t2-OA is used as the only capping agent, the NRs have an aspect ratio of 9±4 with a high yield of branched NCs (Fig. 2a). At an optimal molar ratio of 4:1 for t-2-OA and OA, more monodisperse NRs can be obtained and the aspect ratio decreases to 6±2 (Fig. 2b). With further increased OA ratio, the aspect ratio of PbSe NRs gradually decreases to nearly 1 (spherical NCs), as shown in Fig. 2b-f. The detailed sizes of the NRs shown in Fig. 2 are listed in Table S1. In addition, the yield of PbSe NCs and the percent of branched nanostructures are also listed in Table S1. It is worth noting that the typical reaction conditions for PbSe nanorods37 require high reaction temperature of 170 oC and almost five times higher lead precursor concentration compared to our optimal conditions. After the addition of short-chain carboxylic acid t-2-OA, more lead precursor can be released at elevated temperature. Thus a high percentage of multiple-

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branched PbSe NCs can be observed (in Fig. S2), which can be attributed to the increased growth rate and extremely large monomer concentration at high temperature48. Based on the data, we conclude that the addition of short-chain t-2-OA can promote the growth of PbSe NRs, leading to elongated and branched NCs. In contrast, the use of long chain OA can reduce the growth speed and improve the monodispersity of PbSe NRs (Fig. 3). It is noteworthy that nearly spherical QDs are observed in Fig. 2f when OA is used as the sole capping agent, while Murray et al. has obtained PbSe NRs instead37. We attributed the discrepancy to our significantly lower synthesis temperature of 120 oC compared to 150-170 oC in their experiment. In addition, we also investigated the role of both ligands on the capping of synthesized NCs. FTIR spectra of free carboxylic acids and PbSe NRs coated with mixed t-2-OA : OA capping ligands are shown in Fig. S4 and S3. In Fig. S3, the characteristic peaks of free carboxylic acids at ~1700 cm-1 shown in Fig. S4 are not observed, indicating that t-2-OA and OA on the surface of PbSe NR form carboxylates with Pb atoms. Meanwhile, with t-2-OA as the sole capping ligand, the characteristic peaks of–COO- group are at 1464 cm-1 and 1380 cm-1, while those of –COO- are at 1574 cm-1 and 1406 cm-1 if only OA is used instead. For mixed t-2-OA : OA, PbSe NCs will be capped by both ligands. Nonetheless, the peak

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Fig. 2 TEM images of PbSe NRs synthesized at 120 oC with decreased t-2OA : OA molar ratio: (a) 5:0 (b) 4:1 (c) 3:2 (d) 2:3 (e) 1:4 (f) 0:5. The aspect ratio, diameter and length of these NRs are listed in Table S1.

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Fig. 3 Illustration of the effect of t-2-OA/OA ratio on PbSe NCs morphology.

intensity of OA is evidently higher than that of t-2-OA. We conclude that the NRs are mainly capped by long-chain OA, which helps to improve the dispersity of NRs in organic solvent. The effect of ligands on the optical properties of synthesized PbSe NCs was investigated. The absorbance and photoluminescence spectra of NCs were measured and shown in Fig. 4a. The change of PbSe aspect ratio with increased OA ratio is shown in Fig. 4b. It is worth noting that when the molar ratio of t-2-OA/OA decreases from 5:0 to 3:2, the NR diameter keeps almost constant at around 2.6 nm. Then the nanorod diameter decreases rapidly to 2.2 nm as the molar ratio further decreases to 0:5, indicating the large effect of ligand ratio on the size of PbSe NRs. It is considered that the NCs absorbance peak is primarily determined by their diameters49,50 as the result of quantum confinement. Therefore in the absorbance spectra (Fig. 4a), the first exciton absorbance peaks of the PbSe NRs are also changed with different ligand ratios, following the same trend. The photoluminescence spectra of PbSe NRs have well-defined emission peaks with full-width at half-maxima similar to the first excitonic absorption peaks, indicating good monodispersity. Murray37 and Tischler10 et al have reported that the Stokes shift for PbSe is sensitive to the shape of NCs. We also found that the Stokes shift for our PbSe NRs varies with the NC morphology, while further investigation is needed to understand the mechanism. The wide-angle X-ray scattering (WAXS) spectra of synthesized PbSe NRs with different ligand ratios are displayed in Fig. 5, which are consistent with previously reported ones for PbSe NRs and NCs37. The spectra indicate that the PbSe NRs have typical rock-salt structure (JCPDS: 06-0354) and the diffraction peaks of NRs have a slightly narrower diffraction peak (200) than the spherical NCs. As the aspect ratio of PbSe NRs decreases from 9 to nearly 1, the corresponding (200) diffraction peak gradually broadens. The NRs with large aspect ratio can easily align parallel to the substrate, while spherical NCs do not

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exhibit preferred crystallographic orientation relative to the substrate. As a result, the NRs demonstrate sharper diffraction peaks than QDs, consistent with the previous report37.

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Fig. 4 (a) Absorbance and photoluminescence spectra of the PbSe NRs with the molar ratio of t-2-OA : OA ranging from 5:0 to 0:5. (b) Average aspect ratio (blue squares) and diameter (red circles) of corresponding PbSe NRs with different molar ratio of t-2-OA to OA.

PbSe NRs were synthesized at different reaction temperatures with a fixed ratio of 4:1 for t-2-OA/OA. Fig. 6a displays the absorbance spectra of PbSe NRs synthesized at different reaction temperatures from 80 oC to 160 oC, with the optimal growth time varying from 50 min to 0.5 min respectively. The first exciton absorbance peaks of the resultant NRs red-shift with increased reaction temperature. The size and shape of PbSe NRs were investigated by TEM and shown in Fig. 6c-g. We can observe that the monodispersity of the synthesized PbSe NRs is optimal with a growth temperature of 120 oC, The correlation between NCs dimensions and growth temperatures is shown in Fig. 6b, with the detailed dimensions listed in Table S2. We find that the diameters of the resultant NCs gradually increase with elevated growth temperatures. Nevertheless, the length of PbSe NRs follows a more complex trend, suggesting a tradeoff between the NRs nucleation and growth process. The shorter PbSe NRs observed at high temperature may be attributed to Oswald ripening as a result of exhaustion of the precursors. Without DPP, NCs with long arms and high branched ratio can be observed (Fig. S5), suggesting the existence of large amount of precursors after nucleation, which indicates that the main reason for the exhaustion of precursors is not high temperature but DPP. Since

Fig. 5 WAXS patterns of the PbSe NRs synthesized at 120 oC with the molar ratio of t-2-OA : OA ranging from 5:0 to 0:5.

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the bandgaps of the NCs mainly depend on their diameters, we can precisely controlling the diameters and hence bandgaps of PbSe NRs by adjusting the growth temperatures.

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Previous reports have studied the dramatic effect of DPP as impurities in the synthesis of PbSe QDs using Pb-oleat and TOP51,52. They found that DPP in TOPSe can largely affect the formation of PbSe QDs by enhancing NCs nucleation and accelerating the reaction rate. In this work, we show that trace DPP added in TDPSe can significantly lower the optimal reaction temperature and reduce growth time during the synthesis of PbSe NRs, which is beneficial to produce rods with narrow diameters. In order to reveal the effect of DPP on PbSe morphology, we synthesized PbSe NCs at 120 oC with and without DPP. The TEM images of NCs w/wo DPP at optimal growth time are shown in Fig. S7. With DPP, the transparent precursor solution quickly turned to dark after the injection of TDPSe:DPP and monodisperse PbSe NRs with small diameters were obtained after only 2 min at 120 oC as a result of enhanced nucleation (Fig. S7a). The detailed dimension information of NRs at different growth time is listed in Table S3. In contrast, we noticed that the color change of the reaction solution without DPP was much slower than that using DPP (Fig. S8), which suggests the apparent effect of DPP on nucleation. The corresponding TEM images of PbSe NRs without DPP at different growth time are shown in Fig. S7b and Fig. S9. It is evident that the length and branched ratio of PbSe NC without using DPP are significantly increased than those using DPP, which can be attributed to the large amount of precursor left after limited nucleation. We find that the average diameter of optimal PbSe NRs without DPP is around 2.6 nm, corresponding to a bandgap of 1.09 eV (close to that of silicon), which makes them good candidates for NCs solar cells. The PbSe NRs without using DPP have an average diameter of 3.3 nm (corresponding to a bandgap of 0.98 eV), which are less suitable for photovoltaic application due to the resultant small open circuit voltage. However, both PbSe NRs have smaller diameters than previously reported ones36,37 due to the use of t-2-OA. It is worth noting that the yield of PbSe NRs increased to nearly 50% with DPP, which is much higher than the former reported 10%53. The high yield of PbSe NRs is an indispensible premise for their

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large-scale commercial applications in photovoltaics. Fig. S10 shows the absorption spectra of PbSe NRs with DPP and without DPP. The PbSe NRs with DPP have significantly sharper absorbance peak than those without DPP, suggesting largely improved monodispersity. From the point view of NC aspect ratio, the use of trace DPP in TDPSe tends to produce shorter PbSe NRs compared to systems without DPP (Table S3). We speculate that DPP can function as a secondary phosphine impurity in our synthesis, which may help to dissociate Pb-oleate to form a more reactive Pb-phosphine complex and accelerate the releasing of Se in TDPSe51,52,54. In comparison, bis(diethylamino)phosphorous53, the previously reported impurity in PbSe nanorod synthesis, can only form Pb-phosphine complex by their P−H moiety. Thus DPP can increase the reactivity of both Pb and Se precursors, resulting in more efficient nucleation at lower temperature. Consequently, a large amount of monomers were consumed during nuclei stage and only small quantities of monomers were remained for NC growth, leading to nanorods with high yields and ultra-small diameters.

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We also investigated the effect of chain length of carboxylic acids on the anisotropic growth of PbSe NRs. Trans-2-decenoic acid (t-2-DA), trans-2-hexanoic acid (t-2-HA) and t-2-OA with different chain-lengths were used and compared. As shown in Fig. S11, the ratio of t-2-DA has significant effect on the aspect ratio of synthesized PbSe NRs, quite similar to t-2-OA. For t-2-HA with a very short chain-length, the synthesized NCs have poor dispersity in solvents if OA is not used. Thus we fixed the t-2-XA

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(X= H, O or D) : OA molar ratio at 3:2 and then compare the morphology of resultant NCs. As shown in Fig. 7, when the chain-length is getting shorter, the shape of PbSe NCs with the same reaction conditions changes from dots, short rods to multipods. A clear trend is that the shorter the carboxylic acid, the higher the aspect ratio for the resulting PbSe NRs and the more likely the branched NCs will appear. We speculate that the length of the ligands and the binding strength between the capping ligands and Pb will determine the shape of the particles, as reported in the earlier work38. To testify our hypothesis, we examined the binding strength between Pb and four kinds of carboxylic acids including OA, t-2HA, t-2-OA and t-2-DA by using computational modeling at atomic level. Table S4 lists the related energetic parameter ΔE, which represents the binding strength between the Pb cation and the acid radical ion. It was calculated according to the equation: ΔE= [Etotal(Pb(XA)2)-Etotal(Pb)-2Etotal(XA)]/2 A lower value of ΔE means stronger bond strength between Pb and carboxylic acids, indicating that the corresponding lead precursor is more stable. ΔE is calculated in the optimized geometries (as shown in Fig. S12) and listed in Table S4. The value of ΔE for Pb-OA is significantly lower than that for Pb(t-2XA) (X=H, O or D), suggesting a much stronger bond strength. Meanwhile, the value of ΔE increases with decreased chainlength of carboxylic acids. Precursors with weaker binding strength are more likely release monomers during nuclei stage, which can promote the growth of nanocrystals55. Thus Pb(t-2-XA) with short chains is supposed to benefit the anisotropic growth of PbSe NCs, which is consistent with our results in Fig. 7.

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To investigate the photovoltaic performance of narrow PbSe NRs, Schottky junction devices with a simple ITO/PEDOT:PPS/PbSe/LiF/aluminum structure was adopted, as shown in the inset of Fig. 8. PbSe NRs with a bandgap of 1.09 eV and PbSe QDs were both used for comparison. Three types of devices using PbSe QDs, NRs or QDs/NRs as the active materials were fabricated, with the results shown in Fig. 8. The detailed photovoltaic parameters are listed in Table 1. We can see that devices using QDs/NRs have the best performance, while the devices using pure PbSe NRs show very poor efficiency. To interpret the performance difference, the morphology of the three active films was characterized by TEM (Fig. S13). We can observe that in Fig. S13a the QDs are well packed and the whole film is very dense and uniform. In contrast, the film made of pure NRs has poor quality with many aggregates and pinholes, which is very likely the reason leading to poor device performance. For the best performing film shown in Fig. S13c, a dense QD film was firstly deposited beneath the top NRs film to prevent pinhole while the top NRs film can facilitate electron transport to the electrode. The slightly enhanced performance of solar cells based on QDs/NRs stems from improved open circuit voltage and fill factor (FF). Since NRs are considered to have improved carrier

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mobility than QDs, less recombination25, and improved Voc as well as FF will be expected. However, the short circuit current density of the QDs/NRs film is slightly less than the one using pure QDs, which may be due to the lower absorbance coefficient of the less dense NRs film. According to the best of our knowledge, this is the first report of solar cells using PbSe NRs. Note that the surface states of NRs are usually higher than well passivated QDs56. Thus the device photovoltaic performance can be further improved if we can better passivate the PbSe NRs in the future. To confirm the enhanced carrier transport in NRs film, we fabricate field-effect transistors (FETs) by depositing PbSe NRs film on a doped n-type Si wafer with a gate dielectric stack of SiO2 (Fig. S14a). For comparison, we also fabricate FETs based on PbSe QDs19 with nearly the same diameter. Asdeposited PbSe QDs arrays with the treatment of 1,3benzenedithiol (BDT) turn out to be p-type (Fig. S14b). The resultant QDs film shows a room-temperature hole mobility of 1*10-7 cm2 V-1 s-1. The PbSe NRs film demonstrates n-type property with a high electron mobility of 5*10-4 cm2 V-1 s-1 (Fig. S14c). Thus the insertion of PbSe NRs between QDs and Al cathode is more beneficial to electron transport and collection, as proved by our device results. We noticed that PbSe NRs FETs display large hysteresis likely due to the device interface57 and high density trap states on the surface of PbSe NRs. )

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Additionally, steric hindrance of the ligands may play an important role in the NC growth. Monomers bound by shorter chain ligands are less sterically hindered than those by their longer chain counterparts, leading to faster diffusion of the nuclei and a higher growth rate. In contrast, the longer and bulkier ligands with larger steric hindrance would prefer to stay as far apart as possible, resulting in NCs with high curvature like spheres. Therefore, short-chain Pb(t-2-XA) lead to higher monomer reactivity, concentration and diffusion speed than those with long chains, favoring the anisotropic growth of PbSe NCs. Compared to the work in Ref 53, the concentration of our lead precursor is only one fifth of theirs and our synthesis temperature is also significantly lower. Under such low precursor concentration and temperature, if we used OA as the sole capping agent as in Ref 53, only spherical PbSe NCs could be obtained (Fig. 2f). Importantly, we used t-2-OA/OA together with DPP and achieved PbSe nanorods with high aspect ratio and small diameter. The weak binding strength between Pb and t-2-OA led to fast nuclei diffusion and high growth rate at temperature as low as 120 oC, promoting oriented attachment.

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Fig. 8 The J-V characteristic of device with QDs (blue line), QDs/NRs (green line) and NRs (red line) as the active materials respectively under AM1.5G (100 mW/cm2) illumination.

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Fig. 7 TEM images of PbSe NCs syntheiszed at 120 oC with fixed t-2-XA:OA molar ratio (3:2) for (a) t-2-DA; (b) t-2-OA; (c) t-2-HA.

Current density(mA/cm

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Table 1. The performance parameters of device with the only QD layers, the QD/NR layers, the only NR layers respectively. Layers

Voc (V)

QD (4L) QD(2L)/NR(2L) NR (4L)

0.46 0.48 0.20

Jsc (mA/cm-2) 13.910 12.947 0.062

FF(%)

PCE(%)

49.9 56.5 26.4

3.126 3.515 0.003

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We have demonstrated a new method for synthesizing monodisperse ultra-small PbSe NRs with diameter approaching 2 nm, which can be attributed to the use of DPP and t-2-OA. The introduction of trace DPP can greatly lower the reaction temperature, leading to reduced diameters for obtained PbSe NRs as well as largely increased yield. The use of short-chain trans-2octenoic acid together with OA as capping ligands results in high monomer reactivity, fast nuclei diffusion and high growth rate, which results in the anisotropic growth of ultra-small PbSe NCs at such low reaction temperature. The PbSe NRs show n-type property and high electron mobility as measured by field-effect transistors. Due to quantum confinement, these PbSe NRs with narrow diameters show proper bandgap for photovoltaic application. They were used for the first time in solar cells and improved efficiency were demonstrated when used together with QDs.

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This work was supported by the National High Technology Research and Development Program of China (863 Program) (Grant No. 2011AA050520), the National Natural Science Foundation of China (Grant No. 61176054), the Natural Science Foundation of Jiangsu Province, China (Grant No. BK2011279), the Doctoral Fund of Ministry of Education of China (Grant No. 20113201120019), the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, 199 Ren-Ai Road, Suzhou Industrial Park, Suzhou, Jiangsu 215123, P. R. China E-mail: [email protected] † Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/b000000x/ ‡ Footnotes should appear here. These might include comments relevant to but not central to the matter under discussion, limited experimental and spectral data, and crystallographic data. 1.

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Facile synthesis of ultra-small PbSe nanorods for photovoltaic application.

Nanocrystal array solar cells based on lead chalcogenide quantum dots (QDs) have recently achieved a high power conversion efficiency of over 8%. The ...
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