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Textured ZnO films from evaporation- triggered aggregation of nanocrystal dispersions and their use in solar cells Junfeng Yan a , Qing Lian a , Muhamad Z. Mokhtar a , Amir H. Milani a , Eric Whittaker b , Bruce Hamilton b , Paul O’Brien a,c , Nam T. Nguyen a and Brian R. Saunders a, * a School of Materials, MSS Tower, The University of Manchester, Manchester, M13 9PL, U.K. b Photon Science Institute, University of Manchester, Alan Turing Building, Oxford Road, Manchester, M13 9PL, U.K. c School of Chemistry, The University of Manchester, Manchester, M13 9PL, U.K. Due to its high electron mobility, good stability and potential for low-temperature synthesis ZnO has received considerable attention for use in solar cells, photodetectors and sensors. Whilst there have been reports involving the formation ZnO films with porous morphologies the majority of those have involved elaborate or time-consuming preparation methods. In this study we investigate a simple new method for preparing textured porous ZnO (tp-ZnO) films. We used colloidal instability triggered by the evaporation of a volatile stabilising ligand during spin-coating of a ZnO nanocrystal (NC) dispersion to deposit crack-free tp-ZnO films. The porosity of the tp-ZnO films was 56 % and they could be prepared using amine-based ligands with boiling points less than or equal to 78 o C. To demonstrate the ability to use the tp-ZnO films as electron acceptors they were infiltrated with poly(3-hexylthiophene) (P3HT) and solar cells prepared. The power conversion efficiencies of the tp-ZnO/P3HT 1

University of Manchester · Web viewZnO is a wide bandgap semiconductor with a high electron mobility1 that is of considerable interest in a number of areas including electronics2,

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Textured ZnO films from evaporation-triggered aggregation of nanocrystal dispersions and their use in solar cells

Junfeng Yana, Qing Liana, Muhamad Z. Mokhtara, Amir H. Milania, Eric Whittakerb, Bruce Hamiltonb,

Paul O’Briena,c, Nam T. Nguyena and Brian R. Saundersa,*

a School of Materials, MSS Tower, The University of Manchester, Manchester, M13 9PL, U.K.

b Photon Science Institute, University of Manchester, Alan Turing Building, Oxford Road, Manchester, M13 9PL, U.K.

c School of Chemistry, The University of Manchester, Manchester, M13 9PL, U.K.

Due to its high electron mobility, good stability and potential for low-temperature synthesis ZnO has received considerable attention for use in solar cells, photodetectors and sensors. Whilst there have been reports involving the formation ZnO films with porous morphologies the majority of those have involved elaborate or time-consuming preparation methods. In this study we investigate a simple new method for preparing textured porous ZnO (tp-ZnO) films. We used colloidal instability triggered by the evaporation of a volatile stabilising ligand during spin-coating of a ZnO nanocrystal (NC) dispersion to deposit crack-free tp-ZnO films. The porosity of the tp-ZnO films was 56 % and they could be prepared using amine-based ligands with boiling points less than or equal to 78 oC. To demonstrate the ability to use the tp-ZnO films as electron acceptors they were infiltrated with poly(3-hexylthiophene) (P3HT) and solar cells prepared. The power conversion efficiencies of the tp-ZnO/P3HT devices reached values that were three times higher than a control bilayer ZnO/P3HT device prepared using a sol-gel derived ZnO film. Because our method used a low temperature treatment and ZnO films are used in a wide variety of third-generation solar cells, the new tp-ZnO films introduced here may offer a low cost method for improving the efficiency of other solar cells.

INTRODUCTION

ZnO is a wide bandgap semiconductor with a high electron mobility1 that is of considerable interest in a number of areas including electronics2, 3, optics4, 5, water-purification6, solar cells7-10 and photocatalysis11, 12. ZnO can be prepared using solvothermal synthesis in a range of different morphologies, which include nanorods13, 14 and 3D superstructures15. Textured or porous ZnO films are of particular interest as they may provide an increased surface area and enhanced light scattering16. ZnO films have received arguably the greatest attention within optoelectronics2, 17, 18. The synthetic methods for preparing textured ZnO films include chemical bath deposition11, 19, electrochemical deposition14, 16, 20 or the sol-gel method21, 22. However, these approaches are either time-consuming, require high-temperature sintering or use elaborate equipment. We report here our unexpected discovery that textured and porous, crack-free, ZnO (tp-ZnO) films can be simply prepared using layer-by-layer spin-coating of ZnO nanocrystals (NC) dispersions containing a volatile stabilising ligand.

Deposited films of inorganic semiconducting NCs have major fundamental and practical interest23. From the fundamental viewpoint such films have optoelectronic properties that differ from the dispersed NCs and depend on the nature of the inter-NC contacts24. Deposited ZnO NCs are used as key components in at least three types of third-generation solar cells (quantum dot, hybrid-polymer and perovskite solar cells)7-9, 25-32 which is primarily due to the electron accepting properties of ZnO. Usually, semiconducting inorganic NCs are insoluble in the organic solvents used for deposition and require stabilisation with ligands33. Unfortunately, crack formation within films deposited from such NC dispersions often occurs34-36, which can adversely affect film electronic properties. Cracks may result from ligand removal when this causes a major volume decrease with a simultaneous local stress build-up35. Methods to achieve crack-free film formation within deposited NC films are urgently needed. There is evidence that films containing disordered NCs have a lower propensity to crack35. Here, we study a simple spin-coating approach that uses evaporation-triggered destabilisation of ZnO NC dispersions under conditions favouring disordered aggregate formation to provide crack-free tp-ZnO films.

Whilst ZnO NCs have been studied for many years37 there have been few systematic studies of their colloidal stability in non-aqueous solvents. In our previous work38, 39 we reported that concentrated non-aqueous ZnO NC dispersions could be stabilised with ligands such as 1-propylamine (PA) or destabilised by adding bifunctional ligands38. We found that amine-based ligands were moderately good dispersants for ZnO. However, they could also be readily displaced by bifunctional thiol-ligands to trigger aggregation. We recently observed in our laboratory that ZnO NC dispersion aggregation could also be triggered by evaporation of a volatile amine ligand, such as PA, and that this resulted in textured ZnO NC films during spin-coating when films were prepared using a layer-by-layer approach. Because such films have potential for optoelectronic applications we conducted a study of this new approach to preparing tp-ZnO films and the results are presented herein.

As part of this work bilayer tp-ZnO/poly(3-hexyl thiophene) (P3HT) films and solar cells were studied. ZnO/P3HT and closely related poly(3-hexyl selenophene)-based solar cells are well established and have been prepared using spin-coating of mixed dispersions21, 40, 41, in-situ ZnO NC formation42 or using pre-formed textured ZnO films8. The best efficiencies for ZnO/P3HT solar cells have been achieved to date using in situ ZnO formation42. However, a difficulty for the latter method is control of the NC size and distribution. The use of vertical ZnO nanorods represents an alternative approach where well aligned pre-formed nanorods were first prepared8. However, infiltration can be problematic if the acceptor phase has very small pores43. Here, we study tp-ZnO films with pores that favoured P3HT infiltration.

The approach used to prepare the textured porous ZnO films (tp-ZnO) is depicted in Scheme 1. A ZnO NC dispersion was dispersed in CHCl3 containing ligand and then spin-coated at room temperature. The ligands used were PA, 1-butylamine (BA) and 1-dodecylamine (DA). The latter was used as a high-boiling point, control. Evaporation of the volatile ligands (BA and DA) resulted in ZnO aggregation and deposition of a tp-ZnO film which was heated. The process was repeated a further five times to form a film with a thickness of ~ 120 nm. The ability of the textured tp-ZnO to act as an electron acceptor was tested by infiltrating the film with P3HT (Scheme 1) and constructing tp-ZnO/P3HT bilayer solar cells.

Scheme 1. Method used to prepare textured porous ZnO (tp-ZnO) films and their infiltration with P3HT. A concentrated ZnO NC dispersion in CHCl3 and stabilised with a volatile ligand was spin-coated to give a porous ZnO film. The film was dried at 80 oC and the process repeated a further five times. The tp-ZnO film was immersed in a dilute P3HT solution for 24 h and then P3HT spin-coated onto the tp-ZnO film using a concentrated P3HT solution. The ligands used were 1-propylamine (PA), n-butylamine (BA) and n-dodecylamine (DA).

The study begins by characterising the morphologies of the tp-ZnO films prepared using PA. The role of evaporation in the triggering of aggregate formation is then probed. The importance of ligand boiling point on textured morphology is investigated using ligands with different boiling points and a mechanism describing film formation is proposed. P3HT films are then coated on the tp-ZnO films and the morphologies assessed. The tp-ZnO/P3HT films are used to construct solar cells and the device performance studied. The power conversion efficiency is shown to reach at least three times that of a control bilayer ZnO/P3HT device which confirms the potential for the tp-ZnO films to act as an improved electron accepting layer for solar cells. Because our new method for preparing textured ZnO films used low temperatures and inexpensive equipment it should be widely applicable.

EXPERIMENTAL DETAILS

Materials

Zn(CH3COO-)2.2H2O (98%), CHCl3 (99.8%), BA (99.5%), DA (> 99%), ethanolamine (99.5%), 2-methoxyethanol (99.8%) and methanol (MeOH, 99.8%) were purchased from Aldrich. PA (> 99%) and chlorobenzene (CBZ) were purchased from Alfa Aesar. P3HT (average molecular weight of 15,000 - 45,000 g/mol and 99.995% trace metals basis) was purchased from Aldrich and had a regioregularity > 95%. All materials were used as received.

ZnO nanocrystal synthesis

ZnO NCs were synthesised according to the method reported previously38. Briefly, Zn(CH3COO-)2.2H2O (2.95 g, 13.4 mmol.) was dissolved in MeOH (135 mL), then reacted with a KOH (1.48 g, 23 mmol) solution in MeOH (65 mL) at 60 oC for 135 min44. The ZnO NC precipitate was collected and re-dispersed in CHCl3 and PA was gradually added until the PA volume fraction was 13 vol.% The dispersion became transparent after agitation and the ZnO concentration (CZnO) was 18.1 mg/mL. The stock dispersion was stored at -18 oC. The dispersion stability was assessed before use by means of dynamic light scattering (below).

tp-ZnO film preparation

A portion of stock ZnO NC dispersion (300 L) was added onto a pre-cleaned glass substrate (~ 2.5 x 2.5 cm2). Spin-coating was conducted using a Laurell, Model WS-650Mz-23NPP spin processor. The latter was programmed to first spin at 300 rpm for 3.0 s (phase 1) and then at 2,000 rpm for another 10.0 s (phase 2). The ZnO film was heated in an oven 80 oC for 5 min. [CARE: fumes from 1-PA should not be inhaled.] The process was conducted a total of six times to give a total film thickness of 116 nm.

tp-ZnO/P3HT film preparation

P3HT solutions (2 – 20 mg/mL) were prepared by dissolving the proper mass of P3HT in CBZ and heating the solution at 60 oC together with ultrasonication. Following the method of Ravirajan et al.13, the tp-ZnO films were immersed overnight in a solution of P3HT in CBZ (2 mg/mL) and then briefly dried with a flow of dry nitrogen gas. Finally, a more concentrated P3HT solution in CBZ (of various concentrations) was spin-coated at 3,000 rpm. The films were then heated at 50 °C in air for 5 min.

tp-ZnO/P3HT device fabrication

ITO coated glass was sonicated in water for 30 min, then in NaOH solution (2.0 M) for 30 min which was followed by exhaustive washing with high purity water. The substrate was then dried under N2. A compact hole blocking layer (bl-ZnO) of ZnO (thickness 35 nm) was prepared via sol–gel method21 which involved spin-coating a Zn(CH3COO-)2.2H2O solution (0.75 M) with ethanolamine (45 l) in 2-methoxyethanol (100 L) onto ITO at 1000 rpm and annealing in air on a hotplate at 300 oC for 10 min. The latter process was then repeated. Subsequently, tp-ZnO/P3HT films were prepared as described above. The P3HT concentrations (CP3HT) used to prepare devices were 7, 15 and 20 mg/mL and the total thickness of the tp-ZnO/P3HT films were, respectively, 130, 133 and 149 nm. The films used for devices were subjected to an additional annealing step in order to improve the mobility of the P3HT phase45. For all devices, the films were annealed for a further 140 oC for 5 min on a hotplate. An Ag (or Au) layer (80 nm) was deposited onto the P3HT.

Control bilayer sol-gel ZnO devices were also prepared. In this case the sol-gel bl-ZnO layer was prepared as described above. The P3HT film was then deposited onto the bl-ZnO layer using a P3HT solution concentration of 20 mg/mL and annealed as described above. The thickness of the sol-gel ZnO layer and P3HT layer were 35 and 30 nm, respectively.

Physical measurements

All SEM measurements were conducted using a Philips FEGSEM XL30 instrument. Atomic force microscopy (AFM) images were taken from an Asylum Research MFP-3D operating in AC (“tapping”) mode. Imaging was performed using Olympus high aspect ratio etched silicon probes (OTESPA) with nominal spring constant of 42 N/m (Bruker AXS S.A.S, France). The cantilever oscillation frequency varied between 300 and 350 kHz and was determined by the auto-tune facility of the Asylum software, as was the drive amplitude. The set-point was adjusted to just below the point at which tip-sample interaction was lost to minimise sample damage. The thickness of the films was measured using a Dektak 8 (Bruker) via scratch a line on the film surface (Stylus Profilometer). UV-visible spectra were obtained using a Hitachi U-1800 spectrophotometer. Dynamic light scattering (DLS) data were obtained using a Malvern Zetasizer Nano-2S instrument.

Porosity determination for tp-ZnO film

The porosity of tp-ZnO film was determined via Inductively Coupled Plasma Mass Spectrometer following the method of Goux et al.46 The ZnO film (area of 1.2 cm2) was fully dissolved using aqueous HCl solution (10 %) to give a Zn2+ solution with total volume of 2.0 mL. The digested solution was diluted with high purity water to give a final solution volume of 10 mL. This solution was analysed using ICP analysis. From the calculated mass of ZnO and the dimensions of the films, the average porosity (six samples) was determined.

Device characterization

The current density-voltage (J–V) characteristics were measured using a Keithley 2420 Sourcemeter and 100 mW/cm2 illumination (AM 1.5G) and a calibrated NREL certified Oriel Si-reference cell. An Oriel SOL3A solar simulator was used for these experiments. The active area of the devices was defined using a square aperture within a mask and fixed at 0.025 cm2. Forward and reverse direction sweeps were measured with a sweep rate of 50 mV/s.

RESULTS AND DISCUSSION

Porous ZnO film morphology

The ZnO NCs studied here were characterised by TEM (Fig. S1a) and had a number-average TEM diameter (dTEM) of 4.1 ± 0.4 nm. The NC diameter was also determined from UV-visible spectra (Fig. S1b) for NCs dispersed in CHCl3/PA using the value of the wavelength at half peak height (1/2 = 346 nm). From Meulenkamp’s relationship37 the diameter (dUV-vis) was 3.5 nm. DLS data for the NCs dispersed in CHCl3/PA (Fig. S1c) gave a z-average diameter (dz) of 7.9 nm. This result is indicative of good dispersion of the ZnO NCs, which was due to the PA. The higher dz value compared to dUV-vis and dTEM was due to the presence of ligand and the stronger scattering of light by the largest particles.

ZnO films were deposited by repeated layer-by-layer spin-coating of the ZnO NC dispersions which was conducted a total of six times (Scheme 1). While control ZnO films deposited from CHCl3 did not form uniform films and contained very large aggregates (Fig. S2a), the films deposited from CHCl3/PA had a uniform morphology at the scale of 10s of m (Fig. S2b). This is because 1-PA acted as a ligand. The ZnO film thickness increased linearly with cycle number (See Fig. S3). Unless otherwise stated a total of six spin-coating cycles were used and the final thickness was 116 ± 12 nm as measured by surface profilometry. Higher magnification SEM images of the films showed that they had a textured porous morphology (Fig. 1a - c). The morphology consisted of ridges with valleys in between. Whilst many different types of textured morphologies have been reported in the literature14-16, 47, the present approach was very simple and only required a spin-coater and an oven. The textured crack-free morphology extended over the entire film (100s of m, See Fig. S2b). This is an important observation because cracking of NC films is an important problem that hinders their use35, 48.

Figure 1. SEM images of ZnO film deposited from CHCl3/PA (a – c). (d) and (e) shows a representative tapping mode AFM image and line profile of the films. (f) shows a higher magnification AFM image and the yellow arrows highlight the particulate nature of the ZnO NC clusters. Scale bars: (a) 10 m; (b) - (d): 2 m; (f); 200 nm.

It is instructive to consider the implications of the ligands on the morphology of the tp-ZnO films because the ligands were removed after deposition. The ratio of the ligand volume (VL) to NC particle volume (VNC) for the dispersed ZnO NCs can be estimated from

(1)

where L, ML, dNC and L are the surface area per ligand adsorbed, ligand molecular weight , NC diameter and ligand density, respectively. NA is Avogadro’s number. (The derivation of equation (1) is given in the ESI.) If we assume a L value of49 22 Å2 (for a monovalent ligand adsorbed to NCs with a similar diameter to those studied here) and use ML, dNC and L values of 59 g/mol, 4.0 nm and 0.72 g/cm3 then VL/VNC = 0.92. The latter corresponds to a ligand volume fraction (L) of ~ 48 vol.% using equation S4 (See ESI and Table S1). The latter value is slightly lower than the measured ZnO NC film porosity of 56 ± 4 %

Agreement between the calculated L value and the film porosity may be expected if all the ligand were removed without significant overall volume change. The absence of film cracking is also attributed to a disordered NC arrangement caused by evaporation triggered aggregation as well as the layer-by-layer (LBL) approach used for film formation which is known to reduce internal stresses35. It is proposed that evaporation resulted in diffusion controlled aggregation which gave disordered aggregates that were unable to rearrange in to more ordered packing.

AFM data (Fig. 1d) confirmed that the textured morphologies apparent from SEM (e.g., Fig. 1c) were not an artefact of the high vacuum using during SEM. Rather, the texture was formed as a consequence of spin-coating and oven drying. A representative line-profile (Fig. 1e) shows that the pores were deep and almost spanned the whole depth of the film. The RMS roughness was 67 nm. The highest magnification AFM image (Fig. 1f) shows that the film peaks were aggregates of smaller ZnO NC aggregates with diameters of ~ 20 to 50 nm (indicated by yellow arrows). 3D AFM images (Fig. S5) highlight the relatively rough morphology present within the tp-ZnO films. SEM images of tp-ZnO films prepared using 1, 3 and 6 layers show that porosity was present from the first layer and became more pronounced with subsequent layers (Fig. S6).

To further probe the morphology the surface was scratched and the film regions next to the scratch were imaged (Fig. S7). Whilst the film showed evidence of deformation and flow under shear, the higher magnification images revealed smaller pores in the lower layers. These pores support the view that the films were fully porous in the z-direction as indicated by line profile in Fig. 1e. We fortuitously found a region of film where the uppermost layer adjacent to a scratch had been removed to expose the under-layer (See Fig. S8). Examination of the latter region revealed relatively small pores with diameters of 60 - 100 nm. These data provided additional evidence that the pores were connected. Taken together, these data show that the tp-ZnO films had porosity that extended through the film depth.

Mechanism for porous ZnO film formation

The most important physical chemistry question regarding the present system concerns what the mechanism is for the formation of the textured and porous ZnO NC films. During spin-coating the CHCl3 and ligand (PA) evaporated rapidly from the deposited (wet) film. PA has a significantly lower boiling point (48 oC) compared to CHCl3 (61 oC) and would have been relatively rapidly evaporated during spin-coating. It is proposed that as ligand was preferentially lost, ZnO colloidal stability was compromised. To qualitatively probe the effect of solvent evaporation on ZnO colloidal stability we prepared a stock ZnO NC dispersion (18.1 mg/ml) dispersed in CHCl3/PA and then used UV-visible spectroscopy and DLS to study the dispersion stability changes that occurred as the solvent evaporated (See Fig. 2). The dispersion became visibly turbid as the evaporation proceeded (Fig. 2a) and light scattering was evident from the UV-visible spectra (Fig. 2b). The numerical value of the absorbance at 500 nm (A500) is a measure of light scattering because ZnO does not absorb light at this wavelength (compare to Fig. S1b). Fig. 2c shows that the value for A500 increased strongly with CZnO. (The latter was estimated from the dispersion volumes.) This trend is due to aggregates which are apparent from the DLS data (Fig. 2d) and were triggered by ligand (and solvent) evaporation. It is likely that as ligand (and solvent) evaporated during spin-coating aggregation also occurred. Because the timescale for aggregation was much shorter for spin-coating the aggregates would have been smaller than those apparent from Fig. 2d.

Figure 2. (a) Image of a ZnO NC dispersion with CZnO of 18 mg/mL (far left). The other images show the dispersion as evaporation proceeded. The calculated CZnO values (mg/mL) are shown. (b) UV-visible spectra for the dispersions are shown in (a). The variation of the absorbance at 500 nm with CZnO is shown in (c). (d) DLS data from selected samples from (a).

It is proposed that the key reason for the formation of porous ZnO morphologies was the low boiling point of the ligand (PA) which favoured aggregation during spin-coating. To test this hypothesis we investigated the importance of ligand boiling point by preparing ZnO dispersions using BA (boiling point = 78 oC). As a control, we also prepared ZnO dispersions using a high boiling point ligand (DA, boiling point = 248 oC). (The structures of the ligands are shown in Scheme 1.) The dz values obtained for the ZnO NCs stabilised with BA and DA were 8.5 and 10.6 nm, respectively. (The DLS data are shown in Fig. S9.) These data confirm that BA and DA act as stabilising ligands for ZnO NCs. ZnO films prepared using the CHCl3/BA had a porous morphology (Fig. 3a and b). There was evidence of smaller pores present for this film compared to tp-ZnO prepared using 1-PA (Fig. 1c) which is suggestive of morphology tuneability. In contrast a non-porous film was produced using DA/CHCl3 (Fig. 3c and d). These data support our hypothesis that the porous ZnO morphology was due to the low boiling point of the ligand.

Figure 3. (a and b) SEM images of as-prepared ZnO film deposited from CHCl3/BA dispersion. (c and d) SEM images of ZnO film deposited from CHCl3 /DA dispersion. All scale bars are 2 m.

Fig. 4 depicts the mechanism proposed to explain the formation of the tp-ZnO morphology. This mechanism has some similarities to that proposed by Snaith et al. for polymer infiltration of mesoporous scaffolds50. Accordingly, a wet film was initially present (Fig. 4a) which lost a significant proportion of dispersion during initial spin-coating (Fig. 4b). Solvent and ligand evaporation preferentially occurred from the top of the film (Fig. 4c) which concentrated the dispersion. Above a critical CZnO value the dispersion formed NC aggregates. The aggregates deposited onto the substrate and residual solvent/ligand was removed by heating. The process was repeated a further five times. The relatively small pores formed at the bottom of the films during the early cycles (Fig. S6a) were not able to be effectively filled by successive ZnO layers because of aggregate formation. The latter tended to deposit onto pre-existing ZnO aggregates, which gave rise to the textured morphology. In the case of high boiling point ligand (e.g., DA) the ligand was not removed which prevented NC destabilisation and pore formation. In that case the solvent was removed first. It is well known that for binary co-solvent blends the less volatile solvent resides in a spin-coated film for the longest time51. It follows from this discussion that to prepare textured tp-ZnO films by spin-coating the ligand boiling point should be less than or equal to that for BA (78 oC).

Figure 4. Depiction of the proposed evaporation-triggered aggregation mechanism for tp-ZnO formation. The initial wet film (a) contained dispersed ZnO NCs. Volatile solvent and ligand were lost during spin-coating (b – d) and evaporation-triggered aggregation occurred (c) resulting in deposition of aggregates (e). The process was repeated to give a textured tp-ZnO film (f). The components are not drawn to scale.

Morphology and optoelectronic properties of tp-ZnO/P3HT hybrid films

We next investigated hybrid films consisting of P3HT spin-coated onto tp-ZnO film. Following Ravirajan et al.8 the ZnO substrate was first pre-soaked in dilute P3HT solution (2 mg/mL) overnight as a conditioning step before spin-coating P3HT solutions with a range of P3HT concentrations (CP3HT). Compared to the unfilled tp-ZnO film (Fig. 1c) there was a progressive loss of morphological detail apparent from SEM images as CP3HT increased from 7 (Fig. 5a), through 15 (Fig. 5b) to 20 mg/mL (Fig. 5c). AFM data for the film prepared using CP3HT = 15 mg/mL are shown in Fig. 5d and a 3D image for this film appears in Fig. S10a. The images show the morphology was less undulating. This suggestion is supported by line profile (Fig. 5e) and the RMS roughness of 41 nm, which was significantly less than that for the parent tp-ZnO film (67 nm, above). P3HT likely filled the pores and coated the ZnO aggregates. The higher magnification AFM image (Fig. 5f) for the film prepared using 15 mg/mL P3HT is relatively featureless. The 3D image (Fig. S10b) confirms that much of the texture had been mostly lost and that the P3HT had covered the ZnO and filled in the pores to a large extent (especially when compared to Fig.S5b).

Figure 5. Morphologies of tp-ZnO/P3HT films. The films were coated with P3HT at a concentration of (a) 7 mg/mL (b) 15 mg/mL and (c) 20 mg/mL. (d) to (f) show AFM images and a line profile for the film prepared using 15 mg/mL P3HT. Scale bars: (a) – (d): 2 m; (f); 200 nm.

The tp-ZnO/P3HT films absorbed UV-visible light strongly in the 500 – 600 nm region due to vibronic bands52 (Fig. S11a). With an increase of CP3HT used for spin-coating step the maximum absorption (~ 520 nm) increased, which is a consequence of increased film thickness. The photoluminescence (PL) spectra were also measured for the films (Fig. S11b) and the spectra show similar features to that reported elsewhere53 with the vibronic transitions from P3HT evident54. The PL spectra for P3HT films is known to originate from aggregates55 and is dependent on the extent of chain order56. We attribute the change in the relative intensities of the main PL peak with increasing CP3HT to differences in packing of the chains. It is proposed that the P3HT structure was less influenced by the tp-ZnO film as CP3HT increased based on the relative similarity of the PL spectra for the films prepared using CP3HT = 15 and 20 mg/mL. Furthermore, the PL intensity increased with CP3HT which indicates a greater number of photo-excited charge carriers were available for charge transport. One might then expect improved device performance for hybrid tp-ZnO/P3HT devices and this was explored.

tp-ZnO/P3HT hybrid solar cells were constructed using tp-ZnO films with P3HT deposited as a light harvesting layer. The CP3HT values used were 7, 15 and 20 mg/mL. Control bilayer devices were also prepared using only the sol-gel bl-ZnO layer and a P3HT layer (CP3HT = 20 mg/mL). The device architectures used are depicted in Fig. 6a. The sol-gel ZnO layer was relatively featureless as shown by the 3D AFM images shown in Fig. S12. Representative J-V curves for all four devices are shown in Fig. 6b. The device parameters measured for the sol-gel device are similar to those reported by Monson et al for their bilayer ZnO/P3HT device57. The Jsc values for the tp-ZnO/P3HT devices (Fig. 6c) were not significantly affected by CP3HT, whilst the FF value (Fig. 6d) was highest for the tp-ZnO/P3HT prepared using CP3HT = 20 mg/mL. This result is presumably because of the greater absorption from P3HT (Fig. S11a) which increased charge generation (Fig. S11b).

Figure 6. (a) Depiction of device architectures used. The sol-gel device was a control. (b) J-V data for the tp-ZnO/P3HT and sol-gel devices. The effect of P3HT concentration used for deposition on (c) short-circuit current density, (d) Fill factor, (e) open circuity voltage and (f) power conversion efficiency are shown.

The Voc values increased with CP3HT (Fig. 6e) which may be due to less recombination. The latter suggestion is also consistent with the PL data(Fig. S11b). However, the Jsc values were not highest for the tp-ZnO/P3HT device prepared using CP3HT = 20 mg/mL and other explanations for the change in Voc should also be considered. The value for Voc for P3HT/ZnO solar cells originates from the offset between the conduction band of ZnO and the HOMO of P3HT58. For the tp-ZnO/P3HT devices the band gaps of ZnO NCs and P3HT would be independent of CP3HT. Consequently, Voc should not depend on CP3HT. Beek et al.44 showed that UV light (380 to 420 nm) caused Voc to decrease for ZnO based bulk heterojunction devices. One of the potential advantages of the inverted geometry used for our cells is improved protection of the ZnO from UV-light. Indeed, the UV-visible spectrum for tp-ZnO films (Fig. S11a) shows an increasing absorption due to P3HT in the 380 – 420 nm region as CP3HT increased. Consequently, an additional contribution to the increase in Voc with CP3HT (Fig. 6e) is increased UV-protection afforded by the P3HT films with increasing CP3HT.

Whilst the number of CP3HT values used was limited, it can be seen that the PCE appeared to increase linearly with CP3HT for our tp-ZnO/P3HT devices (Fig. 6f). Importantly, the PCE values for the tp-ZnO/P3HT devices prepared using CP3HT values of 15 and 20 mg/mL were greater than that measured for the sol-gel control by factors of 2 and 3, respectively. The superior device performance for these tp-ZnO/P3HT devices is attributed to the greater ZnO-P3HT interfacial area (caused by the textured tp-ZnO morphology) which increased the Jsc values (Fig. 6c) as well as an increase of the Voc values.

A referee suggested that Au should be used instead of Ag as the top contact and that a device should be prepared without the bl-ZnO layer. We therefore constructed bl-ZnO-free, ITO/tp-ZnO/P3HT/Au cells (Fig. S13 and Table S2). The average PCE was 0.146%. The latter value was the highest PCE of all the device types studied. These data further confirm that tp-ZnO was an effective electron transport layer.

Conclusions

In this study we have investigated a new, simple and potentially versatile method for preparing textured ZnO NC films. The process simply used a volatile ligand and a spin-coater. Evaporation was shown to trigger aggregation of the ZnO NC dispersion stabilised by volatile ligands, which resulted in deposition of aggregates that formed the textured porous tp-ZnO films. The tp-ZnO films were crack-free. It appears that textured morphologies can be prepared by our LBL film preparation method when the ligand boiling point is less than or equal to 78 oC using amine-based ligands. The textured films were infiltrated with P3HT and the hybrid bilayer films provided improved PCE values that were at least a factor of three higher than that for a sol-gel control film. These results demonstrate the good potential for the tp-ZnO films to act as an effective electron transport layer for hybrid bilayer solar cells. Because ZnO layers are used for other solar cells the new tp-ZnO approach may offer benefits for other devices where an increased surface area compared to bl-ZnO layers is required. Moreover, the deposition method used relatively low toxicity amine-based ligands as well as low temperatures which are potentially advantageous for future studies.

Acknowledgements

BRS and POB would like to thank the EPSRC funding for support this work (K010298/1). We thank one of the referees for their experimental suggestions concerning construction of new devices.

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17

012345678

0

50

100

Height / nm

X / m

(f)(a)(c)(d)(e)(b)

1 10 100 1000

18

22

Intensity / AU

26

Size / nm

33

15 20 25 30 35

0.0

0.2

0.4

0.6

0.8

A

500

C

ZnO

/ (mg/ml)

300400500600700800

0.0

0.5

1.0

1.5

2.0

Absorbance

Wavelength / nm

33

26

24

22

20

18

(c)(d)(b)(a)

182022242633

(d)(a)(c)(b)

ZnONCLigandInitial wet filmExcessdispersionremovedEvaporation of solvent & ligandTriggeredaggregation80

o

CGlasstp-ZnOtp-ZnO6 cycles1

st

layer(a)(b)(c)(d)(e)(f)

012345678

0

50

100

Height / nm

X / m

(a)(b)(c)(d)(e)(f)

0.0 0.2 0.4 0.6

0.0

0.2

0.4

0.6

20

15

7

Solgel

Voltage / V

J

sc

/

(

mA/cm

2

)

5 10 15 20

0.00

0.05

0.10

Porous

Sol-gel

PCE / %

C

P3HT

/ (mg/mL )

99.0

0054.0

2

3

R

CPCE

HTP

5 10 15 20

0.0

0.2

0.4

0.6

FF

C

P3HT

/ (mg/mL )

5 10 15 20

0.0

0.2

0.4

0.6

V

oc

/ V

C

P3HT

/ (mg/mL )

5 10 15 20

0.0

0.2

0.4

0.6

J

sc

/

(

mA/cm

2

)

C

P3HT

/ (mg/mL )

(a)(b)(d)(c)(f)(e)

tp-ZnO/P3HTSol-gel

tp-ZnO/P3HT

bl-ZnOITO

Ag

bl-ZnOITO

Ag

P3HT

NH

2

NH

2

NH

2

ZnONCdispersionSpin-coatZnONCLigandLigandPADABAtp-ZnO/P3HTGlassGlassGlassSpin-coatP3HT/CBZ6 cyclestp-ZnO