MULTILAYER HETEROSTRUCTURES FOR APPLICATION IN OLEDS AND PHOTOVOLTAIC DEVICES

Abstract

This invention relates to a supported polymer heterostructure and methods of manufacture. The heterostructure is suitable for use in a range of applications which require semiconductor devices, including photovoltaic devices and light-emitting diodes.

Claims

1. A method of making a supported heterostructure comprising a diffuse interface, the method comprising or including the steps of: i) depositing on a substrate a first polymer; ii) crosslinking the polymer; and iii) depositing on the first polymer a further material with lower molecular weight than the molecular weight of the first polymer, wherein the further material diffuses into the first polymer.

2. The method of claim 1, wherein the lower molecular weight material comprises an oligomer.

3. The method of claim 2, wherein the oligomer comprises an oligomer of thiophene, phenylene, fluorene or oxadiazole.

4. The method of claim 1, wherein the lower molecular weight material comprises a small molecule.

5. The method of claim 4, wherein the small molecule comprises fullerene or a derivative thereof.

6. The method of claim 1, wherein the lower molecular weight material comprises one or more nanocrystals or nanowires.

7. The method of claim 1, wherein the substrate is pre-patterned.

8. The method of claim 1, comprising the further steps of incorporating the supported heterostructure in a semiconductor device.

9. The method according to claim 1, wherein the device is a photovoltaic device.

10. The method according to claim 1, wherein the device is a light-emitting diode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0114] Embodiments of the invention will now be described, by way of example only and without limitation, with reference to the accompanying drawings and the following Examples, in which:

[0115] FIGS. 1a and 1b describe general schemes for the formation of nanoscale heterostructures in accordance with the present invention;

[0116] FIG. 2 shows the photovoltaic performance of devices according to the present invention compared to known blended composites; and

[0117] FIG. 3 shows the dielectric function of a composition formed in accordance with the fourth aspect of the present invention.

[0118] With reference to FIG. 1, a solution (1) comprising a first polymer and a phase control material is deposited onto a substrate (2). Phase separation into distinct phases of polymer (3) and phase control material (4) then occurs. In a third step, the phase control material (4) can be selectively removed through use of a solvent which dissolves the phase control material (4) but does not dissolve the first polymer (3) or in the case of the phase control material being a solvent, by evaporation, leaving behind a series of voids and nanostructured elevations of polymer (3), which may then be immobilized by crosslinking. As a fourth step, deposition of a second material (5) onto the first polymer (3) completes fabrication of the heterostructure.

[0119] In order to increase the height of the nanostructure, crosslinked polymer (3), immobilized on substrate (2), may undergo a subsequent deposition of solution (1) using the first layer as a template, as shown in FIG. 1b. This intermediate structure would then be processed as before: selective removal of phase control material (4) followed by crosslinking, and deposition of a second material (5) to back-fill into first polymer (3).

[0120] As shown in FIG. 1, the deposition of the second material (5) may be such that it completely or substantially encapsulates the first polymer (3) upon the substrate (2). The steps of (i) deposition of the first polymer (3) and phase control material (4) and (ii) removal of the phase control material (4) and crosslinking of the first polymer (3) may be repeated as many times as is necessary, depending on the required morphology of the heterostructure.

[0121] Dimension (a) in FIG. 1 refers to the lateral length scale of the elevations of deposited polymer. For convenience, this is indicated as being the sum of the width of one elevation and the width of the neighbouring void. Dimension (b) refers to the height or thickness of the elevations. Dimension (c) refers to the total height or thickness of the supported heterostructure.

[0122] As can be seen in FIG. 2, in comparison with the bulk distributed heterostructure devices given by the bi-blend of PFB and F8BT (maximum photon to electron conversion efficiency of 1.5% at 475 nm), it is clear that the columnar heterostructures of the present invention give much better performance (up to 7% at the same wavelength) by a factor of greater than 4.5.

[0123] In summary, a natural phase separation between polymers or solvent and polymer may be achieved according to the present invention. Tall polymer nanostructures can be built using the layer by layer upon polymer nanotemplate approach. Also a diffuse interface may be achieved according to the present invention. The methods according to the present invention are more easily processable than processes that require lithography. The methods according to the present invention are also solution processable which enables large area production by using techniques such as ink jet printing.

EXAMPLES

Materials

[0124] All chemicals and reagents were obtained from Sigma-Aldrich unless otherwise stated. (6,6)-phenyl C.sub.61-butyric acid methyl ester (PCBM) was obtained from Nano-C.

[0125] PFB and F8BT may be obtained, for example, from American Dye Sources.

Test Methods

Quantum Efficiency

[0126] External quantum efficiencies were measured in an inert atmosphere or in a vacuum (10.sup.−6 Torr) using a calibrated system comprising a solar simulator (Oriel), monochromator (Cornerstone) and sourcemeter (Keithley 2400). Short circuit current and open circuit voltage were measured using a semiconductor parametric analyzer (Keithley 4200). Incident light power was measured with a calibrated silicon diode (OPT301M). Quantum efficiencies were obtained as a function of wavelength by comparing the photocurrent output of the device under test with that of a calibrated photodiode. Power conversion efficiencies (PCE) were calculated according to Equation 1:

[00001]$\begin{array}{cc}P.Math..Math.C.Math..Math.E=\frac{\mathrm{Fill}.Math..Math.\mathrm{Factor}\times {\mathrm{Voltage}}_{\left(\mathrm{open}.Math..Math.\mathrm{circuit}\right)}\times {\mathrm{Current}}_{\left(\mathrm{short}.Math..Math.\mathrm{circuit}\right)}\times 100}{\mathrm{Incident}.Math..Math.\mathrm{Power}}& \left(1\right)\end{array}$

wherein the fill factor is the ratio of the actual maximum obtainable power to the theoretical power, given as a percentage.

Measurement of Nanostructures

[0127] The morphology of the nanostructures were measured by atomic force microscopy (Digital Instruments Dimension 3000 atomic force microscope in tapping mode).

[0128] All film thicknesses are measured on a Tencor P2 profilometer.

Ellipsometry

[0129] Ellipsometry measurements were carried out on a JA Woollam M2000V ellipsometer. Variable angle spectroscopic ellipsometry was measured at both the top and bottom interfaces, wherein the top interface refers to the air-polymer interface and the bottom interface refers to the polymer-glass interface. Del and Psi ellipsometric values were collected as a function of wavelength for angles of 50, 55 and 65 degrees.

[0130] Optical modelling was carried out using WVASE32 modelling software. Optical constants n and k were fitted by using the measured thickness.

Example 1a

[0131] The polymer poly(9,9′-dioctylfluorene-co-bis-N,N′(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylene-diamine) PFB with molecular weight ˜100,000 is dissolved in toluene at a concentration of 9 mg/ml. Polystyrene (PS) with molecular weight ˜10,000 is dissolved in toluene at a concentration of 9 mg/ml. The two solutions are added in equal amounts such that the weight ratio of the two polymers are equal. A photo-crosslinker is added at 6 wt % 12 mm by 12 mm silicon oxide wafers are pre-cleaned with RCA standard clean 1 solution for 15 minutes. The solution is then spin coated onto the wafer at 3000 revolutions per minute (rpm) for 40 seconds. The thickness of the resulting film is ˜40 nm. Methyl ethyl ketone (MEK) is dropped onto the film for 10 seconds to selectively remove the polystyrene. The wafer is then spun dry of methyl ethyl ketone with dissolved polystyrene at 3000 rpm for 40 seconds. The remaining polymer PFB reveals a nanostructure with feature sizes in the range of 100 nm to 500 nm, more specifically 300 nm. The PFB layer is then cross-linked by exposing the sample to ultra-violet light (254 nm) for 2 mins under nitrogen at ambient temperature (10 ppm O.sub.2/H.sub.2O).

Example 1b

[0132] Poly (9,9′-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylene-diamine) PFB with molecular weight ˜300,000 gmol.sup.−1 is dissolved in toluene at a concentration of 9 mg/mL. Polystyrene (PS) with a molecular weight ˜10,000 gmol.sup.−1 is dissolved in toluene at a concentration of 9 mg/mL. PFB is the desired first polymer to be nanostructured and is also the hole acceptor, e.g. for use in a photovoltaic cell. PS is the phase control material. The two solutions are mixed in equal amounts such that the weight ratio of the two polymers is 1:1. Bisazide photo-crosslinker ethylene bis(4-azido-2,3,5-trifluoro-6-isopropylbenzoate) is dissolved in toluene at a concentration of 9 mg/mL and added to the polymer solution mixture at 1 wt % with respect to the total polymer weight.

[0133] The solution is spin-coated onto clean ITO-glass substrates pre-coated with a 50 nm thick poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDT:PSSH) at 3000 revolutions per minute (rpm) for 40 seconds. The thickness of the resultant film is 40 nm. Methyl ethyl ketone (MEK) is dropped onto the film for 10 seconds and spun-off to selectively remove the polystyrene.

[0134] The film remaining on the substrate is PFB (with dispersed crosslinker) in-plane nanostructured at a lateral length scale of ˜100 nm and thickness of 20 nm.

[0135] The nanostructured PFB layer is then cross-linked by exposing the sample to UV light (254 nm) for 2 minutes under a nitrogen atmosphere (<1 ppm O.sub.2/H.sub.2O).

[0136] Poly((9,9′-dioctylfluorene)-alt-(benzothiadiazole)) (F8BT), the electron accepting material for the photovoltaic cell, is then spun-cast from a 15 mg/mL solution in toluene to give a film thickness of about 50 nm.

[0137] After the crosslinking step in the two polymers plus one solvent approach, the same solution of (PFB/PS) is spin coated onto the polymer nanotemplate. The PFB will align to the PFB nanotemplate below. The polystyrene is then selectively removed using MEK. The resulting self-organized polymer nanostructure is rendered insoluble by exposure of The PFB layer can be cross-linked by exposing the sample to ultra-violet light (254 nm) for 2 mins under nitrogen ambient (10 ppm O.sub.2/H.sub.2O). Third and subsequent layers can be added by repeating the steps.

Example 1c

[0138] As described in the preceding paragraph, in a variation of the method of Example 1b, the steps of deposition of the PFB/PS containing solution, washing with MEK and crosslinking were carried out a second time prior to deposition of the F8BT solution, resulting in a nanostructured PFB layer with a thickness of 35 nm.

Example 1d

[0139] In a variation of the method of Example 1b, the steps of deposition of the PFB containing solution, washing with MEK and crosslinking were repeated twice prior to deposition of the F8BT solution, resulting in a nanostructured PFB layer with a thickness of 50 nm.

[0140] The external quantum efficiencies of Examples 1b, 1c and 1d are illustrated in FIG. 2.

Example 1e

[0141] The polymer poly(9,9′-dioctylfluoreneco-bis-N,N′(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylene-diamine) PFB with molecular weight ˜100,000 is dissolved in toluene at a concentration of 9 mg/ml. Polystyrene (PS) with molecular weight ˜10,000 is dissolved in toluene at a concentration of 9 mg/ml. The two solutions are added in equal amounts such that the weight ratio of the two polymers are equal. A photo-crosslinker is added at 6 wt %. 12 mm by 12 mm ITO substrate after 10 min oxygen plasma was span with PEDT:PSSH at 5000 revolutions per minute (rpm) for 40 seconds to give 40 nm thick PEDT:PSSH. The solution is then spin coated onto this PEDT:PSSH film at 3000 rpm for 40 seconds. The thickness of the resulting film is ˜40 nm. Methyl ethyl ketone (MEK) is dropped onto the film for 10 seconds to selectively remove the polystyrene. The wafer is then spun dry of methyl ethyl ketone with dissolved polystyrene at 3000 rpm for 40 seconds. The remaining polymer PFB reveals a nanostructure with feature sizes ˜100 nm. The PFB layer is then cross-linked by exposing the sample to ultra-violet light (254 nm) for 2 mins under nitrogen ambient (10 ppm O.sub.2/H.sub.2O).

Example 1f

[0142] The polymer poly [2-methoxy-5-(3′,7′-dimethyloctyloxy)]-p-phenylene (OC1C10-PPV) with molecular weight ˜400,000 is dissolved in toluene at concentration of 9 mg/ml. Polystyrene (PS) with molecular weight ˜40,000 is dissolved in toluene at a concentration of 9 mg/ml. The two solutions are added in equal amounts such that the weight ratio of the two polymers are equal. A photo-crosslinker is added at 6 wt %. 12 mm by 12 mm silicon oxide wafers are pre-cleaned with RCA standard clean 1 solution for 15 minutes. The solution is then spin coated onto the wafer at 3000 revolutions per minute (rpm) for 40 seconds. The thickness of the resulting film is ˜40 nm. Methyl ethyl ketone (MEK) is dropped onto the film for 10 seconds to selectively remove the polystyrene. The wafer is then spun dry of methyl ethyl ketone with dissolved polystyrene at 3000 rpm for 40 seconds. The remaining polymer PFB reveals a nanostructure with feature sizes ˜100 nm. The PFB layer is then cross-linked by exposing the sample to ultra-violet light (254 nm) for 2 mins under nitrogen ambient (10 ppm O.sub.2/H.sub.2O).

Example 1g

[0143] Polymer poly [2-methoxy-5-(3′,7′-dimethyloctyloxy)]-p-phenylene (OC1C10-PPV) with molecular weight ˜1,000,000 gmol.sup.−1 is dissolved in toluene at a concentration of 5 mg/mL. Polystyrene (PS) with molecular weight ˜40,000 is dissolved in toluene at a concentration of 9 mg/mL. The two solutions are added in equal amounts such that the weight ratio of the two polymers are equal. Bisazide photo-crosslinker ethylene bis(4-azido-2,3,5-trifluoro-6-isopropylbenzoate) is dissolved in chlorobenzene at a concentration of 9 mg/mL and added to the polymer solution mixture to give 1 wt % with respect to the total polymer weight. The remaining steps are identical to those in Example 1b.

Example 1h

[0144] Polymer poly [2-methoxy-5-(3′,7′-dimethyloctyloxy)]-p-phenylene (OC1C10-PPV) with molecular weight ˜100,000 is dissolved in toluene at concentration of 9 mg/ml. Polystyrene (PS) with molecular weight ˜10,000 is dissolved in toluene at concentration of 9 mg/ml. The two solutions are added in equal amounts such that the weight ratio of the two polymers are equal. A photo-crosslinker is added at 6 wt %. 12 mm by 12 mm ITO substrate after 10 min oxygen plasma was spun with PEDT:PSSH at 5000 revolutions per minute (rpm) for 40 seconds to give 40 nm thick PEDT:PSSH. The solution is then spin coated onto this PEDT:PSSH film at 3000 rpm for 40 seconds. The thickness of the resulting film is ˜40 nm. Methyl ethyl ketone (MEK) is dropped onto the film for 10 seconds to selectively remove the polystyrene. The wafer is then spun dry of methyl ethyl ketone with dissolved polystyrene at 3000 rpm for 40 seconds. The remaining polymer PFB reveals a nanostructure with feature sizes ˜100 nm-500 nm. The PFB layer is then cross-linked by exposing the sample to ultra-violet light (254 nm) for 2 mins under nitrogen ambient (10 ppm O.sub.2/H.sub.2O).

Example 2a

[0145] The polymer poly(9,9′-dioctylfluoreneco-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylene-diamine) PFB with molecular weight ˜100,000 is dissolved in toluene at a concentration of 9 mg/ml. 6 wt % of a photo-crosslinker is added next. A dissimilar solvent (dimethylformamide) which has low entropy to PFB is added at 5 vol %. The solution is spin coated onto pre-cleaned silicon oxide wafers at 3000 rpm at 40 seconds. PFB is naturally phase separated with length scales in the order of a few micrometers. The PFB layer can be cross-linked by exposing the sample to ultra-violet light (254 nm) for 2 mins under nitrogen ambient (10 ppm O.sub.2/H.sub.2O).

[0146] Tall polymer nanostructures can also be built using the one polymer and two solvent approach as the nanotemplate layer and subsequent layers can be built using either the two polymer one solvent or one polymer two solvent approach.

Example 3a

[0147] Polymer poly [2-methoxy-5-(3′,7′-dimethyloctyloxy)]-p-phenylene (OC1C10-PPV) with molecular weight 100,000 is dissolved in toluene at a concentration of 9 mg/ml. 6 wt % of a photo-crosslinker is added next. The solution is spin coated onto pre-cleaned silicon oxide wafers at 3000 rpm at 40 seconds to give a 55 nm thick film. The OC1C10-PPV layer can be cross-linked by exposing the sample to ultra-violet light (254 nm) for 2 mins under nitrogen ambient (10 ppm O.sub.2/H.sub.2O). (6,6)-phenyl C.sub.61-butyric acid methyl ester (PCBM) with molecular weight ˜910.9 is dissolved in chlorobenzene. PCBM is spin-coated twice onto OC1C10-PPV layer at 3000 rpm at 40 seconds to give a 35 nm thick OC1C10-PPV/14 nm PCBM.

Example 3b

[0148] Polymer poly [2-methoxy-5-(3′,7′-dimethyloctyloxy)]-p-phenylene (OC1C10-PPV) with molecular weight 100,000 is dissolved in toluene at a concentration of 9 mg/ml. 6 wt % of a photo-crosslinker is added next. The solution is spin coated onto 40 nm thick PEDT:PSSH at 3000 rpm at 40 seconds to give a 55 nm thick film. The OC1C10-PPV layer can be cross-linked by exposing the sample to ultra-violet light (254 nm) for 2 mins under nitrogen ambient (10 ppm O.sub.2/H.sub.2O). (6,6)-phenyl C.sub.61-butyric acid methyl ester (PCBM) with molecular weight ˜910.9 is dissolved in chlorobenzene. PCBM is spin-coated twice onto OC1C10-PPV layer at 3000 rpm at 40 seconds to give a 35 nm thick OC1C10-PPV/14 nm PCBM.

Example 3c

[0149] Polymer poly [2-methoxy-5-(3′,7′-dimethyloctyloxy)]-p-phenylene (OC1C10-PPV) with molecular weight 100,000 is dissolved in toluene at a concentration of 9 mg/ml. 6 wt % of a photo-crosslinker is added next. The solution is spin coated onto 40 nm thick PEDT:PSSH at 3000 rpm at 40 seconds to give a 55 nm thick film. The OC1C10-PPV layer can be cross-linked by exposing the sample to ultra-violet light (254 nm) for 2 mins under nitrogen ambient (10 ppm O.sub.2/H.sub.2O). (6,6)-phenyl C.sub.61-butyric acid methyl ester (PCBM) with molecular weight ˜910.9 is dissolved in chlorobenzene to give final concentrations between 0.1 mg/mL and 10 mg/mL. The PCBM solution is inkjet-printed over the crosslinked (OC1C10-PPV) over multiple passes to give a layer of (OC1C10-PPV) of approximately 35 nm in thickness on top of which is deposited a layer of PCBM of approximately 30 nm thickness.

Example 3d

[0150] Poly [2-methoxy-5-(3′,7′-dimethyloctyloxy)]-p-phenylene (OC1C10-PPV) with molecular weight 1,000,000 gmol.sup.−1 was dissolved in toluene at a concentration of 3 mg/mL. Bisazide photo-crosslinker ethylene bis(4-azido-2,3,5-trifluoro-6-isopropylbenzoate) was added to the mixture to give a final crosslinker concentration of 0.5 w/w % with respect to the OC.sub.1C.sub.10-PPV. This solution was then spin coated onto clean ITO-glass substrate pre-coated with a 50 nm thick PEDT:PSSH film at 3000 rpm for 40 seconds to give a 55 nm thick film.

[0151] The OC.sub.1C.sub.10-PPV layer was then crosslinked by exposing to UV light (254 nm) for 2 minutes under a nitrogen atmosphere (<1 ppm O.sub.2/H.sub.2O) and rinsed with toluene on the spinner. (6,6)-phenyl C.sub.61 butyric acid methyl ester (PCBM) (molecular weight 910.9 g mol.sup.−1) was dissolved in chlorobenzene to a concentration of 9 mg/mL. PCBM was then infiltrated into the polymer layer by spinning at 3000 rpm for 40 seconds.

[0152] In this example, the PCBM is the electron acceptor and electron transporter, while the OC.sub.1C.sub.10-PPV is the hole acceptor and hole transporter of the photovoltaic cell. The primary light absorber is OC.sub.1C.sub.10-PPV, which has an absorption coefficient five times greater than that of PCBM at wavelengths between 450 and 550 nm.

[0153] Infiltration of PCBM into the OC.sub.1C.sub.10-PPV film results in a diffuse interface and a graded composition, as confirmed by top-side and bottom-side spectroscopic ellipsometry, the results of which are shown in FIG. 3. The dielectric function determined from the top-side (air-polymer) shows a higher PCBM content compared to the bottom-side (polymer-substrate). This can be seen from the relatively higher k values at wavelengths below 450 nm, where the OC.sub.1C.sub.10-PPV absorption tails off and the strong absorption band of PCBM begins. The ellipsometry results also suggest the possible presence of a thin layer of PCBM (about 3 nm) on the surface of the graded composite film.

[0154] Optical modeling suggests that the average PCBM:OC.sub.1C.sub.10-PPV volume ratio in the film is between 0.5:1 to 1:1 with slightly higher PCBM content at the top (air-polymer) interface than at the bottom.

Example 3e

[0155] Following infiltration of PCBM in Example 3c, an aluminium metal cathode was deposited onto the heterostructure to give a photodiode pixel.

[0156] Semiconductor devices made from such heterostructures exhibit higher efficiencies than conventional blend devices.