Efficiency enhancement of organic photovoltaic devices by embedding uncapped Al nanoparticles in the hole transport layer

Abstract

The effects of incorporating uncapped aluminum (Al) nanoparticles (NPs), fabricated by laser ablation in liquid, in the hole transport layer (HTL) of organic photovoltaic devices were systematically investigated. The integration of Al NPs in the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole transport layer resulted in 8.7% enhancement in the power conversion efficiency. This improvement can be attributed to a combination of optical and electrical effects. In particular, light trapping inside the photoactive layer takes place due to scattering of the incident light at wide angles by the embedded Al NPs. At the same time, the electrical conductivity of the HTL becomes enhanced, which in effect improves the hole collection and establishes a mobility balance. These findings were supported by spectroscopic analysis and photon-to-electron conversion efficiency measurements of the respective devices.

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Organic photovoltaic (OPV) devices are promising replacements for their inorganic counterparts due to their low cost production, easy manufacture and their flexibility targeting small scale power generation and portable consumer electronics applications,^1,2 which can benefit from room temperature large area printing.^3,4 OPVs are based on the bulk heterojunction (BHJ) concept, where a conjugated polymer (donor) and a fullerene (acceptor) material are blended together in a nanoscale morphology.⁵ In polymer BHJ OPVs, an important factor in determining the device performance is buffer layers,^6–9 wherein one of them is the hole transport layer (HTL) and the other is the electron transport layer (ETL). HTL provides good interfacial properties between the active layer and the transparent conductive electrode. PEDOT:PSS is most commonly used as the HTL¹⁰ in OPVs to increase the crystallinity of the subsequent active layer of donor:acceptor, as well as to enhance hole extraction.¹¹ It has many advantages such as high transparency, high work function (WF), smooth morphology and acceptable conductivity. So, the holes can be collected easily via the PEDOT:PSS layer from the highest occupied molecular orbital (HOMO) of the polymer donor.¹²

Conjugated polymers that are responsible for exciton generation in OPV devices, have a low charge carrier mobility and small exciton diffusion length,¹³ which limits the optimum thickness of the active layer¹⁴ and leads to poor absorption of incident photons. The typical thickness of the active layer is around 100 nm, whereas the exciton diffusion length is ∼10 nm.¹⁵ A thicker active layer offers higher light absorption, but, it comes at the expense of lowered charge collection and increased exciton recombination. In order to overcome this limitation, different light trapping strategies have been proposed, such as photonic crystals,¹⁶ 3D configurations^17,18 and metallic NPs.¹⁹

Metallic NPs have been recently identified as a breakthrough route for enhancing the OPVs' efficiency.^20,21 Thus, various types and shapes of NPs have already been placed in all the different components of an OPV device including the active layer,^22–25 the buffer layers^26–28 and between interfaces.^29–31 By adding metal NPs in OPVs, two effects can be excited; on the one hand the localized surface plasmon resonance (LSPR) effect^20,21 and on the other the light trapping effect by scattering the incident light at wide angles.^32–35 In the case of HTL, different metallic NPs have been implanted into the PEDOT:PSS layer showing that the performance enhancement was due to hole collection improvements and reduced exciton quenching.^27,36–40

The incorporation of surfactant free Al NPs, produced by laser ablation method⁴¹ is another promising approach, owing to their relative large diameter reaching tens of nms,⁴² combined with the high plasma frequency of Al; the frequency, in which the electrons oscillate back at forth. The Al NPs can potentially lead to significantly greater performance enhancement than Ag or Au NPs, due to the much higher plasma frequency of Al, which ensures a better overlap between plasmon resonance and absorption band of organic semiconductors. Al has a much higher plasma frequency (∼15.3 eV) than Ag (9.6 eV) and Au (8.55 eV), which causes the Al NP enhancement peak to overlap well with the absorption bands of the photoactive layer PCPDTBT:C70-PCBM.⁴³ Therefore, the combination of high plasma frequency and large NPs diameter could give rise to enhanced light-trapping ability by efficient light scattering. In addition, the wide availability and low cost of Al provides a strong motivation for its utilization in plasmonic, rather than other noble NPs.

Furthermore, Al NPs have already been introduced into the active layer, achieving an efficiency improvement, as well as a lifetime increase of the OPV device.^44,45 It was found that the embedded Al NPs act as performance stabilizers, giving rise to enhanced structural stability of the active blend.⁴⁶

Here, we report a facile method for the performance enhancement of OPV devices by blending Al NPs in the PEDOT:PSS HTL prior to spin coating. It was found that the incorporation of Al NPs into the PEDOT:PSS applied in poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]:phenyl-C₇₁-butyric acid methyl ester (PCDTBT:PC₇₁BM) based devices leads to a power conversion efficiency (PCE) improvement by 8.7%. This enhancement can be attributed to light trapping by efficient scattering of incident photons (optical effect), as well as to enhanced conductivity (electrical effect) of the PEDOT:PSS HTL, resulting in enhanced hole mobility of the device, and therefore more efficient hole transport and collection. Since the Al NPs were added and not doped in the HTL in low concentrations the energy level of the PEDOT:PSS remained the same.

Al NPs generation was performed by femtosecond (∼170 fs@1 kHz) laser ablation of Al metallic target (99.99%) placed into a Pyrex cell and covered by a layer of absolute ethanol using an Yb:KGW pulsed laser. This technique produces a large variety of NPs that are free of both surface-active substances and counter-ions. Additionally, the advanced pulsed laser ablation method used is capable of highly uniform target irradiation, and consequently NPs generation over a controlled size range with a high degree of reproducibility. More details can be found elsewhere.⁴¹

PCDTBT and PC₇₁BM were purchased from Solaris Chem and Solenne B.V. respectively. PCDTBT:PC₇₁BM were dissolved in 1,2-dichlorobenzene : chlorobenzene (3 : 1) (o-DCB : CB) in a 1 : 4 (4 mg : 16 mg) ratio and stirred for at least 72 h at 80 °C before used. The photovoltaic devices reported were fabricated on 20 mm by 15 mm indium-tin-oxide (ITO) glass substrates with a sheet resistance of 20 Ω sq⁻¹. The impurities are removed from the ITO glass through a 3-step cleaning process. As HTL, an aqueous solution of poly(ethylene-dioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT:PSS), purchased from Heraeus, in which 0.6, 0.8 and 1% (v/v) Al NPs were added, was then spin-casted onto ITO substrate at 6000 rpm for 60 s, with average layer thickness of 30 nm, followed by baking for 15 min at 120 °C in air. The photoactive layer was subsequently deposited by spin-coating the blended solution at 1200 rpm on top of PEDOT:PSS layer forming a thick layer of approximately 80 nm, as determined from cross-sectional SEM images. Lastly, using a thermal evaporator inside a nitrogen-filled glove box, Ca and Al were thermally deposited, through a shadow mask, reaching thicknesses of 5 nm and 100 nm, respectively, with an overall active area of 4 mm².

Fig. 1a and b show the transmission electron microscopy (TEM) images of the fabricated Al NPs. While, Fig. 1c shows the size distribution obtained by statistical analysis of the TEM images. It can be seen that the size distribution is relative wide (20–45 nm), displaying an average diameter of ∼30 nm. The HRTEM images and analysis of the fabricated Al NPs can be found in previous work.⁴⁷

Fig. 1 (a and b) TEM images and (c) size distribution of the synthesized Al NPs.

Fig. 2a presents the ultraviolet-visible (UV-vis) absorption spectra of Al NPs solution, exhibiting strong absorption in the UV region, while the absorption edge protrudes to the visible region. UV region, while the absorption edge protrudes to the visible region. Moreover, in Fig. 2b UV-vis absorption spectra of PCDTBT:PC₇₁BM films, into the PEDOT:PSS with and without Al NPs, are presented.

Fig. 2 UV-vis absorption spectrum of (a) Al NPs solution and (b) the active layer (PCDTBT:PC₇₁BM) with and without Al NPs.

The photoactive layer presents a uniform enhancement in its absorbance spectrum within the entire wavelength range after the incorporation of NPs, due to efficient light scattering at the large diameter NPs, which increase the effective length of the optical path and subsequently the probability of photons to be absorbed by the polymer. The current voltage (J–V) curves of the pristine device and the devices with different Al NPs concentrations (under illumination with 100 mW cm⁻² power intensity) and the external quantum efficiency (EQE) of pristine and the device with 0.8% concentration of Al NPs, as well as the BHJ OPV device structure are displayed in Fig. 3.

Fig. 3 (a) Current density–voltage (J–V) characteristics, (b) quantum efficiency of the pristine and the device with 0.8% Al NPs embedded into the HTL. (c) Device structure of the OPV with Al NPs.

The respective averaged photovoltaic characteristics are summarized in Table 1. To account for experimental errors, the reported averages and deviations for each device are taken for 10 identical devices, consisting of six cells each. It is shown that the incorporation of Al NPs with 0.8% concentration induces the best improvement of device characteristics. An enhancement of ∼5% to the short-circuit current (J_SC), whereas the fill factor (FF) and open-circuit voltage (V_OC) remains almost constant. As a result, the maximum improvement of 8.7% in the device PCE is obtained from the device with the Al NPs concentration of 0.8%. In order to have a clear evidence for the increased J_SC to the Al NPs incorporated devices, EQE spectra where collected. An almost broad and uniform increase in the entire wavelength rage is observed as shown in Fig. 3b indicating that the responsible mechanism for the enhanced J_SC is the light trapping caused by the efficient scattering of the reflected photons through the ITO side. It should also be noted that the integrated J_SC values from the IPCE spectrum as presented in Table 1 for the pristine and the Al NPs based devices are 10.92 and 11.47 mA cm⁻² respectively for PCDTBT:PC₇₁BM. The IPCE calculated values are less than 4% different than the actual measured J_SC values, indicating good accuracy of the OPV measurement.

Table 1 Averaged photovoltaic characteristics of the devices fabricated with and without Al NPs inside the HTL

	JSC (mA cm^−2)	Calculated JSC (mA cm^−2)	VOC (V)	FF (%)	PCE (%)
PEDOT:PSS	10.92 ± 0.12	10.59	0.883 ± 0.01	60.0 ± 0.5	5.78 ± 0.12
PEDOT:PSS with 0.6% Al NPs	11.33 ± 0.10		0.884 ± 0.02	60.0 ± 0.4	6.00 ± 0.12
PEDOT:PSS with 0.8% Al NPs	11.47 ± 0.15	11.22	0.883 ± 0.02	62.0 ± 0.6	6.28 ± 0.16
PEDOT:PSS with 1% Al NPs	11.13 ± 0.09		0.885 ± 0.01	59.6 ± 0.4	5.87 ± 0.09

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Another point to be noted, is that, the conductivity of the PEDOT:PSS was notably enhanced upon the addition of Al NPs, as displayed in Table 2, which results in an increase in the hole mobility of the PEDOT:PSS layer and thus the hole transport and collection ability of the device.⁴⁸ The conductivity data was obtained from the Hall-effect measurements (Ecopia HMS-3000) in air based on the van der Pauw method, utilizing Ag paste on four corners of the pristine and doped PEDOT:PSS films as the electrodes.

Table 2 Conductivity of the pristine and doped with Al NPs PEDOT:PSS films. Hole mobility of the OPV devices with and without Al NPs obtained by SCLC model

	Conductivity (S cm^−1)	Hole mobility (cm^2 V^−1 s^−1)
PEDOT:PSS	5.086 × 10^−4	(2.09 ± 0.06) × 10^−5
PEDOT:PSS with Al NPs	8.953 × 10^−3	(3.49 ± 0.02) × 10^−5

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Fig. 4a shows the reflectivity spectra of the pristine device and the device with the Al NPs from 350–750 nm. The lower reflectivity of the devices with NPs indicates stronger absorption than the pristine due to the scattering of large diameter Al NPs. By introducing Al NPs based reflectors inside the HTL, a significant fraction of transmitted photons from the active layer to ITO side are reflected back to the active layer and thus the possibility of the absorption inside the photoactive medium is increased, which in turn increases electron–hole pairs generation.⁴⁹

Fig. 4 (a) Reflectivity spectra and (b) hole mobility measurements of the pristine and the devices with Al NPs.

Hole mobility measurements of the devices with and without Al NPs are shown in Fig. 4b. In order to investigate the responsible mechanism for PCE enhancement, as well as to prove the observed enhancement conductivity of PEDOT:PSS, we studied the impact of Al NPs in hole mobility of the devices. We have fabricated hole-only diodes based on both pristine PEDOT:PSS and PEDOT:PSS doped with Al NPs using PCDTBT:PC₇₁BM, as photoactive layer. A thin layer of Au was thermally evaporated onto the active layer with thickness of ∼20 nm. Calculations were based on Mott–Gurney equation (Space Charge Limited Current, SCLC) [eqn (1)]:

J = 9/8μ_hε₀ε_r(V − V_bi)²/L³, (1)

in which ε_r is the relative dielectric constant, ε₀ is the permittivity of free-space, μ_h is the charge carrier mobility, V is the applied voltage, V_bi the built-in potential and L is the thickness of the active layer. In Table 2, hole mobility measurements of the devices with and without Al NPs are shown. The incorporation of Al NPs significantly improves hole mobility compared with the undoped device, leading to an improved hole transport and hole collection through the HTL.^27,50

Fig. 5 shows the AFM images of PEDOT:PSS with and without Al NPs. The root-mean-squared (RMS) roughness of PEDOT:PSS layer on ITO glass is measured to be 1.124 nm, while PEDOT:PSS mixed with Al NPs showing the RMS roughness slightly improved than the PEDOT:PSS layer (1.101 nm). This improvement helps the transfer of the holes from the active layer to the HTL, since there is less probability of deflecting the holes. This leads to an increase of the hole mobility, as confirmed by SCLC measurements.

Fig. 5 AFM images of (a) pristine PEDOT:PSS and (b) PEDOT:PSS with Al NPs at 0.8% concentration. The films were spin-coated on ITO glass sheets.

Conclusions

To summarize, the incorporation of Al NPs into the HTL of an OPV device improves both its optical and electrical properties, leading to an efficiency enhancement of 8.7% with an average PCE of 6.28%. This efficiency improvement is primarily attributed to the effective light scattering of the incident photons, which increases the light path into the active layer. At the same time, the introduction of Al NPs into the HTL gives rise to an increase of its conductivity and, in turn, to an improvement of the hole transport ability. Moreover, the utilization of the most efficient donor polymer poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl) carbonyl]thieno [3,4-b] thiophenediyl}) (PTB7),⁵¹ instead of PCDTBT could lead to a further performance enhancement, since in this case the OPV device exhibits a broader absorption spectrum, which is expected to give rise to a significantly higher efficiency.

Acknowledgements

This research has been financially supported by the PeNElOPe (3116) project which is implemented under the ARISTEIA II Action of the Operational Programme Education and Life-long Learning and is co-funded by the European Social Fund (ESF) and National Resources.

Notes and references

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