H. Jessica
Pereira
,
Oliver S.
Hutter‡
,
G. Dinesha M. R.
Dabera
,
Luke A.
Rochford§
and
Ross. A.
Hatton
*
Department of Chemistry, University of Warwick, CV4 7AL, UK. E-mail: Ross.Hatton@warwick.ac.uk
First published on 30th March 2017
Optically thin copper films with a random array of sub-optical wavelength apertures couple strongly with light in the wavelength range 600–800 nm due to excitation of surface plasmonic resonances. Herein we show that this trapped light can be used to excite electronic transitions in a nearby strongly absorbing organic semiconductor before the plasmonic excitations dissipate their energy as heat into the metal. This energy transfer process is demonstrated using model small molecule and polymer photovoltaic devices (based on chloro-aluminium phthalocyanine:C60 and PCE-10:PC70BM heterojunctions respectively) in conjunction with a nano-hole copper electrode formed by thermal annealing an optically thin Cu film supported on polyethylene terephthalate. The efficiency of this process is shown to be highest for wavelengths in the range 650–750 nm, which is part of the solar spectrum that is weakly absorbed by today's high performance organic photovoltaic devices, and so these findings demonstrate that this type of electrode could prove useful as a low cost light catching element in high performance organic photovoltaics.
An alternative scalable approach that has received relatively little attention to date, but is well matched to OPV device architectures, is to trap the incident light6 as surface plasmonic excitations at the surface of a noble metal electrode having a random array of sub-optical wavelength apertures.7 Similar to metal nanoparticles, nano-holes in a noble metal film have a large absorption cross-section enabling strong coupling with plane wave incident light,15 whilst having the advantage of being confined to the plane of the electrode. Consequently, unlike metal nanoparticles, nano-apertures in a metal film electrode do not risk compromising the shunt resistance of a device with a very thin light harvesting layer, such as in an OPV. Also, unlike metal nanoparticles, metal nano-hole electrodes can support both propagating and localised plasmonic-excitations, which are strongly coupled.6,15,16 This approach to light trapping in OPVs was first proposed by Reilly et al. who used a 30 nm thick Ag electrode with a random distribution of circular apertures of the same radius formed by micro-sphere lithography.17 Stec et al.18 advanced that work in three important ways: (i) by developing a scalable method for the formation of a dense array of random apertures over large areas based on annealing optically thin (∼10 nm) Ag films; (ii) by demonstrating that Au could be used in place of Ag; and (iii) by showing that OPV devices using random aperture nano-hole electrodes could actually outperform conventional ITO glass window electrodes in OPVs. Importantly the apertures formed by thermal annealing have a broad distribution of shapes and sizes which ensures a broad-band plasmonic response, which is well-matched to OPV applications.15 However, whilst Ag and Au are the preferred metals for this application, because of their high conductivity and relatively low optical losses,14 the use of such high cost metals in OPVs inevitably erodes the cost advantage of OPVs over other types of thin film photovoltaics, and so there is a need to identify lower cost alternatives to these metals. Cu is attractive as an alternative to Ag owing to its abundance, high electrical conductivity and because it is ∼100th of the cost of Ag.19 The optical losses in Cu due to intra-band transitions and free electron (Drude) losses are comparable to that of Au so it can support intense surface plasmon resonances in the visible region,20,21 although Cu has not received much attention as a plasmon-active material for photovoltaics due to its susceptibility to oxidation in air, which results in the formation of a surface oxide that damps surface plasmonic excitations.20,21 However, in recent years there has been considerable progress in the development of ultra-thin surface passivation layers for Cu which offer a means of addressing this problem,22,23 and so nano-structured Cu may yet prove useful for light trapping and concentration in OPVs. Herein we demonstrate for the first time a proof-of-principle that Cu electrodes with a random array of sub-optical wavelength apertures can be used to catch incident light and efficiently transfer that energy to an adjacent organic semiconductor in a model OPV device. Based on these findings we propose the optimal location for such an electrode to maximise its potential benefit in an OPV.
All electrode and device fabrication was carried out in a nitrogen filled glove box with a O2 level <1 ppm unless otherwise stated. Evaporation of metals and metal oxides was carried out with a CreaPhys Organic Molecular evaporator co-located in the same glove box. The thickness of deposited layers was monitored using a carefully calibrated quartz-crystal microbalance mounted adjacent to the substrate. All metals were thermally evaporated from tungsten boats. MoO3 was thermally evaporated using boron nitride crucibles. Chloro-aluminium phthalocyanine (ClAlPc) and C60 were thermally evaporated from boron nitride crucibles. The working pressure of the evaporator was <1 × 10−5 mbar.
The incorporation of apertures into the Cu film results in a large reduction in the far-field transparency across a broad wavelength range between 450–850 nm, with the largest absolute reduction of 10% for wavelengths in the range 590–700 nm. Light scattering measurements performed using an integrating sphere (Fig. 1(c)) confirmed that this reduction in transparency is not the result of far-field light scattering, which is consistent with the small size of the apertures compared to the wavelength of the incident light. Since the largest reduction in transparency occurs for wavelengths in the range 590–700 nm and peaks at ∼625 nm, the reduction in transparency for wavelength >600 nm can be attributed (primarily) to light absorption arising from excitation of surface plasmon resonances in the Cu film, since this wavelength range coincides with that over which Cu can support surface plasmon resonance excitations and the optical losses due to inter-band transitions and free electron losses are at their lowest.14 Finite-element modelling of the near-field intensity in Cu films with and without apertures of the size and shapes formed by annealing Cu films corroborate the conclusion that light is trapped at the surface as a near-field evanescent wave (Fig. 2). The simulations also show that the intensity distribution across the electrode surface is highly heterogeneous, consistent with the complex interaction between plasmonic excitations associated with different features on the electrode surface. In principle these regions of intense near-field radiation can be used to excite electronic transitions from the HOMO to the LUMO in the organic semiconductor before the surface plasmons dissipate their energy as heat due to ohmic losses in the metal.25 To explore this possibility, model OPVs based on a bilayer heterojunction of the small molecule donor ClAlPc and acceptor C60 (Fig. 1(c) – inset) were fabricated using Cu electrodes with and without a random array of apertures shown in Fig. 1(a) and (b). This simple device architecture and the thickness of the organic semiconductor layers were specifically selected as a model structure to test the aforementioned hypothesis: ClAlPc was used because its intense Q-band26,27 is located between 600–850 nm (Fig. 1(c) – inset) which coincides with the range over which surface plasmon resonant excitations are most intense in the Cu nano-hole electrode. MoO3 was used as the hole-extraction layer because it is well-known to facilitate efficient extraction of holes from most donor type organic semiconductors for thicknesses in the range 3–20 nm and its thickness can be accurately controlled by thermal evaporation.28–30 The results of the finite-element modelling given (Fig. 2(c) and (e)) show that a 5 nm thick MoO3 layer is sufficiently thin for there to be significant spatial overlap between the ClAlPc layer and the near-field evanescent wave extending from the electrode surface, since the range of the trapped light extends to several tens of nanometres from the electrode surface.
The device current–voltage characteristics in the dark and under 1 sun illumination are shown in Fig. 3(a) and Table 1. It is evident that the short-circuit current density (Jsc) is comparable for devices using Cu electrodes with and without nano-holes, despite the lower far-field transparency of the Cu nano-hole electrode, particularly over the wavelength range between 600–800 nm which coincides with that over which ClAlPc most strongly absorbs (Fig. 1(c) inset). This finding is consistent with the hypothesis that the light trapped as surface plasmonic excitations in the Cu nano-hole electrode (which results in the reduced transparency for λ > 575 nm in the nano-hole electrode) is to a large extent coupled into the adjacent ClAlPc layer, since in the bilayer device architecture the ClAlPc is the only layer within range of the evanescent wave extending from the surface of the nano-hole electrode (Fig. 2(c) and (e)). This conclusion is corroborated by the difference in the shape of the external quantum efficiency (EQE) spectra (Fig. 3(b)) from which it is evident that photons are converted to electrons in the external circuit more efficiently for devices using the nano-hole electrode for λ = 600–800 nm, which is the same range over which the far-field optical transparency is observed to be reduced. Indeed the EQE is increased by ∼25% over the wavelength range of 655–715 nm, which coincides with the wavelength range over which the optical losses in the Cu nano-hole electrode due to inter-band transitions and free electron damping are at a minimum (Fig. 3(c)) and so surface plasmon resonances will be most intense. Conversely for λ < 575 nm the EQE is reduced in line with the reduction in far-field transparency for devices using the Cu nano-hole electrode. Normalizing the difference in EQE to the transparency (% T) (Fig. 3(c)) show that the onset of enhanced absorption actually occurs very close to the minimum in the optical losses for Cu.
Electrode | Champion Jsc/mA cm−2 | J sc/mA cm−2 ± 1 SD | V oc/V ± 1 SD | FF ± 1 SD | % PCE ± 1 SD |
---|---|---|---|---|---|
Non-annealed (without nano-holes) | 2.71 | 2.53 ± 0.09 | 0.48 ± 0.03 | 0.40 ± 0.02 | 0.49 ± 0.04 |
Annealed (with nano-holes) | 2.65 | 2.44 ± 0.11 | 0.60 ± 0.02 | 0.49 ± 0.01 | 0.72 ± 0.05 |
Interestingly, devices using a nano-hole electrode also consistently exhibit a higher Voc and FF (Fig. 3(a) and Table 1). It is evident from Fig. 3(a) that this difference stems from the earlier onset of dark current injection and increased current leakage at revere bias in devices using Cu electrodes without nano-holes. Since Cu is known to diffuse into evaporated MoO3−x films at room temperature30,31 it is possible that the former stems from a difference in the energy level alignment at the electrode-ClAlPc interface due to a variation in the extent of n-type doping of the MoO3−x layer by Cu30 since the increase in crystallinity upon annealing would be expected to impart increased lattice stability, thereby reducing the thermodynamic drive for Cu diffusion into the neighbouring MoO3−x layer. To test this hypothesis the crystallinity of the Cu electrodes before and after thermal annealing was probed using grazing angle X-ray diffraction films, and the work function of Cu/MoO3−x (5 nm) bilayer films were measured using the contact potential difference method. It is evident from the X-ray diffraction pattern (ESI Fig. S2†) that the crystallinity is increased upon annealing because the intensity of the peak corresponding to reflection from the Cu (111) plane is more intense after annealing. The work function of the electrode/MoO3−x bilayer is also significantly lower for the case in which the Cu electrode was not annealed; typical values were ∼5.7 eV and ∼5.1 eV for Cu electrodes with and without nano-holes respectively. Importantly, whilst this difference complicates the interpretation of device Jsc, it cannot explain the change in the shape of the EQE spectrum.
To test the generality of the findings, OPVs were also fabricated using a bulk-heterojunction based on the narrow band gap polymer PCE10 in conjunction with [6,6]-phenyl-C71-butyric acid methyl ester (PCE10:PC70BM) (100 nm) in place of ClAlPc:C60. Importantly in this device structure a smaller proportion of the total photoactive layer thickness is in the region of intense near-field near to the Cu electrode because the PCE10:PC70BM is 30% thicker than the ClAlPc:C60 bilayer, and unlike the ClAlPc:C60 the PCE10:PC70BM absorbs strongly over a broad range of wavelengths, including in the range 450–600 nm where the Cu nano-hole electrode is not plasmon-active. Consequently, the Jsc in devices using nano-hole electrodes is lower than that without (Table 2). However, the EQE in devices using the nano-hole electrode is still highest in that part of the spectrum where the electrode is least transparent, which corresponds to region over which Cu supports surface plasmonic excitations. It is also evident from Fig. 4(b) that the onset of plasmon enhanced absorption occurs when the optical losses are a minimum and at the same wavelength as seen in the ClAlPc/C60 OPVs; 590–600 nm (Fig. 3(c)).
Electrode | Thickness of MoO3/nm | J sc/mA cm−2 ± 1 SD | V oc/V ± 1 SD | FF ± 1 SD | % PCE ± 1 SD |
---|---|---|---|---|---|
Non-annealed | 5 | 7.08 ± 0.16 | 0.65 ± 0.04 | 0.56 ± 0.02 | 2.78 ± 0.52 |
Annealed | 5 | 6.16 ± 0.60 | 0.72 ± 0.02 | 0.56 ± 0.03 | 2.54 ± 0.35 |
Non-annealed | 15 | 6.21 ± 0.43 | 0.71 ± 0.01 | 0.57 ± 0.03 | 2.82 ± 0.34 |
Annealed | 15 | 4.75 ± 0.36 | 0.74 ± 0.01 | 0.53 ± 0.03 | 1.91 ± 0.22 |
To confirm the near-field nature of the light trapping mechanism, the thickness of the MoO3−x layer was increased from 5 to 15 nm. The key device performance parameters are summarised in Table 2. It is evident from Table 2 that the effect of increasing the MoO3−x layer thickness from 5 to 15 nm is much more pronounced for the device with the nano-hole electrode, which is consistent with the exponential decay of the near-field intensity into the photoactive layer (Fig. 2(e)).
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7se00077d |
‡ Current Address: Stephenson Institute for Renewable Energy, University of Liverpool, L69 7ZF, UK. |
§ Current Address: Department of Chemistry, University of Birmingham, B15 2TT, UK. |
This journal is © The Royal Society of Chemistry 2017 |