Stability enhancement in organic solar cells by incorporating V₂O₅ nanoparticles in the hole transport layer

Abstract

The synthesis of vanadium pentaoxide (V₂O₅) nanoparticles by a hydrothermal method and their utilization in a PEDOT:PSS buffer layer in a PCDTBT:PC₇₁BM device structure is demonstrated. V₂O₅ nanoparticles were dispersed in the PEDOT:PSS hole transport layer (HTL) in normal architecture bulk heterojunction (BHJ) solar cells. The device performance for both pure PEDOT:PSS and hybrid HTLs were studied and demonstrated to effectively work in bulk heterojunction organic solar cells (BHJ OSCs). From the stability test initially for one week and subsequently for another three weeks, it was confirmed that the OSC device with the incorporation of V₂O₅ nanoparticles in the standard HTL leads to a decrease in device degradation and significantly improves the lifetime as compared to the standard HTL based device. Moreover, the hybrid HTL exhibits better optical properties and a relatively stable band gap as compared to its pristine PEDOT:PSS counterpart. Our results indicate that V₂O₅ could be a simple addition into the PEDOT:PSS layer to overcome its stability and degradation issues leading to an effective HTL in BHJ OSCs.

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1. Introduction

Organic solar cells have gained considerable attraction due to their friendliness to the environment, cheaper generation cost, compatibility with green energy systems and large scale roll to roll production.^1,2 Bulk heterojunction organic solar cells (BHJ OSCs) are based on an intimate blend of a donor and an acceptor material within the active layer matrix which are then phase separated while coating to create a donor acceptor interface.^3–5 Photons are absorbed by these materials to generate excitons which diffuse to the donor–acceptor interface and dissociate into the charge carriers. These charge carriers are then extracted by the respective electrodes.^6,7 These solar cells have layered geometric structure and optimization of each layer is essential to achieve overall performance of the device. It is very important in BHJ OSCs to achieve good ohmic contacts and charge transportation between the electrodes and the active layer. This is limited by the deep energy levels of the donor polymers which create a barrier at the interface between the active layer and either of the electrodes.⁸ In order to overcome these issues, hole transport layer, HTL, (also called as hole extraction layer) and electron transport layer ETL (also called as electron extraction layer) are introduced at the interface between active layer and the electrodes.^4,9,10

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is considered as one of the most widely used HTL materials for the efficient charge transportation. However, there are several stability issues associated with this material due to its shallower energy levels and acidic nature which turns the device to degrade rapidly.^11–13 In recent years, a lot of research has been carried out to overcome the shortcomings of PEDOT:PSS. Metal oxides have been proved to be a good alternative to it.¹⁴ These metal oxides serve many functions in an OSC such as their use as an active material, transparent electrodes, charge blocking and extraction layers, optical spacers and at the same time they are helpful in enhancing the stability and efficiency.¹⁵ There are several transition metal oxides such as V₂O₅, NiO, Cs₂CO₃, WO₃ and MoO₃ which are being used in the BHJ solar cells and exhibit a wide range of energy levels. Many of them are proved to be more stable than PEDOT:PSS due to their intrinsic optical, structural and electrical properties.^16–22 In general, metal-oxide charge extraction layers demonstrated to be a good addition in OSCs with overall improvement in efficiency and stability.^16,17

V₂O₅ is one of the most widely explored materials as an n-type semiconductor due to its very good transparency, wide optical ban dap, and good stability.^18,19 In addition, this metal oxide also provide good interfacial adhesion and enhances the device stability significantly when exposed to ambient environment which are limiting factors in case of PEDOT:PSS HTL.^20,21 There are several methods which have been employed in the past to deposit this material as an HTL in BHJ OSCs both in normal as well as in inverted structures. Vacuum deposited processes had been widely used to deposit V₂O₅ in the past²² but these processes are not compatible with the large scale production on account of their high manufacturing costs which lead to a substantial addition in the overall fabrication cost of the device.²³ In recent years, efforts have been made to solution deposit the V₂O₅ by using different techniques such as brush painting,²⁴ solution processed spin coating,¹¹ sol–gel derived vanadium oxide,^20,25 and several other methods discussed in ref. 15–21. However, still a lot of work is needed to be done to find the best and cheaper ways to deposit these oxides, for optimizing the material properties to get the highly efficient devices compatible with PEDOT:PSS and exhibiting high stability.

One of the main constraints in OSC performance is its stability. OSCs are very sensitive towards various environmentally induced degradations such as decay in chemical, physical, mechanical, structural and optical performance. Consequently the life time of these devices is very short which put limitations for their commercial use. Our goal to establish an easy solution processable fabrication procedure to develop a highly stable device by overcoming the reliability issues associated with PEDOT:PSS layer stimulates us to investigate the effect of incorporating the V₂O₅ nanoparticles in PEDOT:PSS layer. Our investigations show that HTL with the dispersion of V₂O₅ nanoparticles exhibit much stable device as compared to its pristine counterpart.

2. Materials and methods

2.1. Active materials and the synthesis of V₂O₅ nanoparticles

Both poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′ benzothiadiazole)] (PCDTBT), and (6,6)-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) were purchased from Lumtec. While PEDOT:PSS solution (PH1000) has been purchased from H.C. Starck and used as received. Fig. 1 shows the molecular structure of PCDTBT, PC₇₁BM and PEDOT:PSS.

Fig. 1 Molecular structures of (a) poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), (b) (6,6)-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM), and (c) poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) or PEDOT:PSS.

V₂O₅ nanoparticles were synthesis by hydrothermal method. 1 mM of vanadium acetylacetonate was dissolved into 40 mL of deionised water. 10 mg of Hexa-decyl-trimethyl ammonium bromide (CTAB) and 5 mg of trimethylamine N-oxide were dissolved into 20 mL of deionised water. This solution was added drop by drop into the vanadium solution under stirring at 80 °C and left at this temperature for 30 minutes. Whole mixture was transferred to Teflon-lined stainless steel autoclave with capacity of 80 mL and put into an electric oven at 150 °C for 2 hours. The product was collected by centrifuge at 5000 rpm for 1 min, then washed several times with deionised water and left to dry at 50 °C in the oven under vacuum overnight. The powder was followed by calcinations at 450 °C for 2 hours to finally obtain yellowish-brown V₂O₅ nanoparticles. The flow chart for the synthesis procedure has been presented below in Fig. 2.

Fig. 2 Flow chart of V₂O₅ nanoparticles synthesis via hydrothermal method.

2.2. Solar cell fabrication procedure

Pre-patterned indium tin oxide (ITO) coated glass substrates, for device fabrication, were provided by Ossila Ltd UK. The dimensions of the rectangular substrates were 1.5 × 2.0 cm, with six active pixels on each substrate and an active area of 30 × 1.5 mm (4.5 mm²) for every pixel. Those were cleaned first by using normal soap and then sequential ultrasonication in deionised water, acetone and isopropanol for 15 min each. At the end, substrates were dried by N₂ stream and treated by O₂ plasma for 5 minutes. PEDOT:PSS was spun coated at 4000 rpm for 1 min and then dried at 120 °C for 30 minutes to achieve the final layer thickness of around 40 nm. The active layer is consisting of PCDTBT as donor material and (6,6)-phenyl C₇₁ butyric acid methyl ester or PC₇₁BM as an acceptor material. The active layer ingredients were dissolved in chloroform with a concentration of 10 mg mL⁻¹ and then mixed with the volume ratio of 1 : 4 (PCDTBT : PC₇₁BM). Mixing was performed by stirring them for 24 hours. After transferring samples into nitrogen-filled glove-box, the final blended active layer was spun coated at 2000 rpm for 20 seconds. Aluminium electrode with thickness of 100 nm was deposited on top of the active layer in vacuum (<10⁻⁶ torr) by means of thermal evaporation. Finally, all the OSCs devices were encapsulated by sticking a thin glass slide to cover active area of the device using a drop of epoxy glue. In the second set of experiments, PEDOT:PSS was mixed with the V₂O₅ nanoparticles at the concentration of 1 mg mL⁻¹ and coating parameters were optimized for the desired thickness of 40 nm. Same fabrication procedure was adopted which is described previously. Fig. 3 shows two types of fabricated OSC's varied by their HTLs including the energy band diagram for both of them.

Fig. 3 Device structure of PCDTBT:PC₇₁BM solar cells with (a) PEDOT:PSS, (b) PEDOT:PSS + V₂O₅ as hole transport layer, and the energy diagram for solar cells with (c) PEDOT:PSS, and (d) PEDOT:PSS + V₂O₅ as hole transport layer.

2.3. Characterizations

2.3.1. Characterization of V₂O₅ nanoparticles. V₂O₅ nanoparticles were first characterized for their compositional and structural properties. XRD was performed in 2θ scanning range of 10° to 60° by using Ultima-IV (Rigaku, Japan) multipurpose X-ray diffraction system. Powder diffractometer, equipped with Cu K-α source (λ = 0.154060 nm) was used to obtain the X-ray diffraction pattern. Energy dispersive X-ray analysis along with FESEM was performed to obtain the elemental composition by using EDS OXFORD INCA ENERGY 250 (Oxford Instruments, UK) attachment. Raman spectroscopy was performed to measure Raman shifts by a DXR Raman Microscope (Thermo Scientific, USA), equipped with green light excitation (532 nm) laser source at 6 mW power.

2.3.2. Characterization of OSC device. Thickness of the HTL consisting of a mix blend of PEDOT:PSS and V₂O₅ was measured by surface Profiler (DEKTAK 150 Veeco, UK). Morphological characterizations were carried out by field emission scanning electron microscope (FESEM) model JEOL JSM-7600F, Japan and atomic force microscopy (AFM) model SPM PROBE VT AFM XA 50/500 Omicron, Germany. Optical measurements of the samples were carried out with the UV-vis spectrophotometer (Perkin Elmer Lambda 750, USA). The current–voltage (I–V) characteristics of the solar cells were measured by Keithley 236 Source Measure Unit. Solar cell performance was tested by using an air mass 1.5 Global (AM 1.5 G) solar simulator with an irradiation intensity of 100 mW cm⁻². The light intensity calibration was performed with a Newport power meter 1918-R with calibrated Si-detector 818-UV.

3. Results and discussion

3.1. Structural and morphological analysis

XRD pattern of the prepared V₂O₅ nanoparticles is presented in Fig. 4(a). All the peaks are well indexed to the ICDD PDF-2 (Release 2011) DB card number 01-077-2418 and represent the intense reflections at 15.4126, 20.3651, 21.7817, 26.1387, 31.0528, 32.4078, 33.3502, 34.3418, and 41.3024°. 2θ diffracted peaks show sharp and noise free spectra which confirm the high crystallinity and well-arranged orthorhombic symmetry of V₂O₅ crystal structure (space group: Pmmn (59)) with cell dimensions as a = 11.5120, b = 3.5640, c = 4.3680. Both XRD and EDS (Fig. 4(c)) results confirm the high purity of the prepared material.

Fig. 4 (a) XRD pattern of the V₂O₅ nanoparticles synthesis by hydrothermal method, (b) Raman spectra of pristine PEDOT:PSS HTL and its hybrid variant. Inset shows Raman fingerprints for V₂O₅ nanoparticles and (c) EDS spectra of synthesized V₂O₅ nanoparticles including the elemental composition.

Phase purity of the synthesised V₂O₅ nanoparticles was confirmed by the Raman spectroscopy due to its high sensitivity towards crystalline V₂O₅ nanoparticles.²⁶ Determined spectrum matched well with the reported spectra and shown in the inset of Fig. 4(b).^27,28 The peak at 995 cm⁻¹ is assigned to the terminal oxygen (V–O) stretching mode occurring due to unshared oxygen. Bands at 696 and 526 cm⁻¹ are attributed to V–O–V stretching modes. Peaks at 406 and 283 cm⁻¹ are exhibiting the bending vibrations of V=O bonds whereas peaks at 487 and 303 cm⁻¹ also represent the bending vibrations of different oxygen bonds. Raman peaks at 194 and 142 cm⁻¹ represent the lattice vibrations.^27,28

Two variants of HTL were also characterized for Raman shifts since it is a powerful tool to study the conducting polymers. Resulting spectra for both HTLs are represented in the Fig. 4(b). Attributions of peak positions are consistent with several reported results,^29,30 showing dominantly the characteristics for PEDOT in either case and contribution of PSS is almost negligible in both cases. Peaks observed at 1563, 1500, 1440, 1360, 1252 and 986 cm⁻¹ are assigned to PEDOT. Moreover, the most obvious difference brought by the addition of V₂O₅ nanoparticles was observed between 1252 cm⁻¹ and 1563 cm⁻¹. These nanoparticles reduced the intensity of the Raman fingerprints for the peaks attributed to 1252, 1360, 1440, 1500 and 1563 cm⁻¹, furthermore they exhibited narrower band as compared to pristine HTL.^31,32

It is crucial to study surface morphology to ensure the device performance by improving different surface parameters. To compare the two HTL morphologies, we used FESEM and AFM characterizations. High resolution FESEM images and AFM morphology of both HTLs are presented in Fig. 5.

Fig. 5 FE-SEM and AFM images for (a and c) pristine PEDOT:PSS HTL and (b and d) hybrid HTL.

SEM image of the pristine PEDOT:PSS (Fig. 5(a)) layer shows the smooth and featureless surface morphology as compared to hybrid HTL. Whereas, the hybrid HTL exhibits a loosely packed surface morphology (Fig. 5(b)); the globules aggregation of discrete nanoparticles on the entire surface are likely to be V₂O₅ particles uniformly distributed within the polymer matrix which confirms the method efficacy of the solution processed fabrication technique that was used to prepare HTLs. Hybrid HTL shows formation of channels like morphology likely caused by nucleation effect of V₂O₅ particles. Fig. 5(c) and (d) shows the non-contact tapping mode AFM images of pristine and hybrid HTLs deposited on ITO coated glass substrates. The morphology of the PEDOT:PSS HTL is much smoother than hybrid HTL and surface roughness increased to double due to addition of V₂O₅ nanoparticles. The increase in the roughness was expected and agrees with the reported results in ref. 33. Although the roughness of the HTL had increased with the addition of V₂O₅ nanoparticles but it is believed that the particles dispersed on the surface of the HTL can effectively extract the charged carrier due to their high conductivity and at the same time they assist in reducing the leakage current by blocking the negative charge carriers which in turns guarantees a better and stable device. It is reported that the RMS roughness of modified HTL increases slightly with the increase of V₂O₅ concentration in PEDOT : PSS except for high ratio of doping concentration (1 : 1).³⁴ In our work, it is observed that the addition of V₂O₅ in small amount (1 mg) into PEDOT:PSS has also altered the roughness of modified HTL as shown in Fig. 5(d). However, the change in device performance is believed to be due to the change in electronic and optical properties of the HTL³⁴ compared to the morphological change such as the roughness. Further details are presented in Section 3.2.

3.2. Optical transmittance and band gap calculations

In Fig. 6(a), we have reported the transmittance spectra of pristine (PEDOT:PSS) HTL and hybrid HTL (PEDOT:PSS + V₂O₅) to distinguish the ageing effect on the optical properties of the both variants. Data are shown for the freshly prepared and one month aged samples. The transmittance curves for hybrid HTL show high transmittance of more than 94% in the visible region and did not show any significant reduction in transmittance over one month ageing period. Whereas, the transparency of the pristine PEDOT:PSS of freshly prepared sample is exhibiting around 89% transmittance which diminished to 84% after one month of preparation. It is notable that transmittance of the pristine PEDOT:PSS thin film reduced with time which is undesirable.³⁵ Relatively higher degree of transparency in the hybrid HTL ensures more light absorption by the active layer and therefore an increase in the photo-generation of charges.³⁶

Fig. 6 The transmittance spectra, for (a) pristine PEDOT:PSS and PEDOT:PSS enhanced with V₂O₅ nanoparticles, the Tauc plot of (αE)² against the photon energy (E) for (b) freshly prepared HTL variants and (c) aged HTL variants.

In Fig. 3(c) and (d), the energy band diagram has been presented based on their known energy levels which consist of LUMO and HOMO values. It can be seen that the presence of V₂O₅ has further facilitated the extraction of holes from the active component (PCDTBT) to PEDOT:PSS layer and then to the ITO via V₂O₅. In principle, a significant difference of V₂O₅ LUMO level as compared to LUMO levels of both PEDOT:PSS and the active component (PCDTBT) has potentially created a restriction for electrons flow to the ITO and consequently made V₂O₅ as good electron blocking component between the ITO/active layer interfaces. In this section, the optical energy gaps for the freshly prepared and aged samples of pristine and hybrid PEDOT:PSS HTLs have been determined via the optical absorption spectroscopy. The square of absorption coefficient and photons energy (αE)² is related to the band gap (E_g) and calculated by the following expression.^18,37

αhv = α_o(hv − E_g)ⁿ (1)

where E_g is the band gap corresponding to a particular transition occurring in the samples, v is the transition frequency, h is Planck's constant, n is the exponent which can be taken as 1/2, 3/2, 2 and 3 depending on the band transition classifications. Whereas, α = 2.303A/t, A and t are absorption and thickness of the films respectively.³⁸ Extrapolated values from the straight line to the abscissa determine the optical band-gap. Fig. 6(b) and (c) shows the Tauc plot, i.e., variation of (αE)² versus photon energy of pristine and hybrid HTLs. For the direct allowed transition, the optical band gap transition energy for pristine PEDOT:PSS varied from 1.46 to 1.55 eV when the data were collected after one month. Whereas, the optical energy gap for hybrid HTL was found to be around 1.51 eV slightly higher than the energy gap of fresh pristine PEDOT:PSS HTL which is believed to be due to the presence of V₂O₅ nanoparticles, a well-known high work function metal oxide. However, it remained unchanged during this period of time exhibiting high degree of stability in the transition energy which consequently helps in improving the device performance especially in cell stability. Thus, the improvement in the device performance is also attributed to the optical properties of V₂O₅.³⁹ The variation in transmittance and band gap energy of the both variants of HTL are summarised in Table 1.

Table 1 The variations in transmittance and band gap energy of pristine and hybrid HTLs due to ageing for one month

Sample	Transmittance (%)		Optical energy gap (eV)
	Fresh	Aged	Fresh	Aged
Pristine PEDOT:PSS	89	84	1.46	1.55
PEDOT:PSS + V2O5	94	94	1.51	1.51

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3.3. Organic solar cell device characteristics

Photovoltaic properties of the organic solar cell based on PCDTBT:PC₇₁BM active layer with pristine HTL, PEDOT:PSS, and its modified variant with the incorporation of V₂O₅ nanoparticles were investigated. The efficiency and life time stability of both types of devices were investigated by performing I–V measurements and reduction in the PCEs was recorded over a period of 4 weeks. The solar simulations were performed at the light intensity of 100 mW cm⁻². Fig. 7 shows the I–V characteristics for both OSCs. The two variants of OSCs showed a decline in both short circuit current (J_sc) and open circuit voltage (V_oc) as shown in Fig. 7(a) and (b). However, OSC devices with pristine PEDOT:PSS HTL exhibited more decay in both parameters than that of with hybrid HTL. After the first week, the pristine device demonstrated a minor degradation, mainly the decrease of the device V_oc from 0.8 to 0.75 V. After four weeks the device continued to degrade and V_oc value reduced to 0.70 V. Whereas, in the hybrid device the V_oc for freshly prepared device is about 0.85 V which showed minor degradation in first week with V_oc of 0.80 V. After four weeks the hybrid device showed no further reduction in the V_oc.

Fig. 7 The current–voltage (I–V) characteristic in fresh condition, after one week, and after 4 weeks for OSCs with (a) PEDOT:PSS, and (b) PEDOT:PSS + V₂O₅ HTL layer.

From the normalized efficiency calculated over the period of four weeks time as shown in Fig. 8, it is evident that that device with the hybrid HTL was consistently much stable as compared to the device with pristine PEDOT:PSS layer. It retained about 90% of its efficiency, whereas the standard device reduced to around 65% of its total efficiency over this entire period of time.

Fig. 8 Normalized efficiency (η/η_o) of both OSCs with pristine PEDOT:PSS layer and PEDOT:PSS + V₂O₅ nanoparticles layer measured for 28 days (four weeks).

The improvement in the device stability and overall performance was due to the intrinsic features of the metal oxide nanoparticles which were incorporated in the HTL. Our results were also consistent with the previous studies showing that V₂O₅ effectively prevent the leakage current in both normal as well as inverted geometries. It proved to be more stable as compared to conventional PEDOT:PSS layer.^16,40–42 The addition of these nanoparticles forms an organic–inorganic hybrid buffer layer which protects the active layer and at the same time maintains the device performance.⁴³ Moreover, in recent years⁴⁴ it is studied that conventional PEDOT:PSS layer does not provide good adhesion at the interfaces. Whereas V₂O₅ proved to be better in adhesion⁴¹ at the interface between active layer and HTL and at the same time it also helps in transport of charged carriers to the electrode. Relatively better performance of the hybrid device is also attributed to the better transmittance and stable band gap of the hybrid HTL as compared to its pristine counterpart. Furthermore, it mitigated the adverse effects of PEDOT:PSS acidic nature on ITO over the time due to its favourable optical, structural and electrical properties which in turns had given us a more stable device as compared to the standard cell.

4. Conclusions

This work has successfully demonstrated the fabrication of air stable PCDTBT:PC₇₁BM based polymer solar cells with V₂O₅ modified HTL to address the reliability issues associated with conventional PEDOT:PSS layer. Our results show that the mixing of V₂O₅ PEDOT:PSS has a major impact on the device life time stability. Based on this study, results were evident that V₂O₅ based HTL could significantly reduce the degradation in OSCs device performance than pristine PEDOT:PSS HTL. Our device with hybrid HTL retained 90% of its PCE even after four weeks of ageing compared to the pure PEDOT:PSS HTL where overall efficiency reduced to around 65% of its initial value. In addition to remarkably better stability, both HTLs and active layers were fabricated by the simple and scalable solution processed method, which greatly simplifies the fabrication process and reduced the overall cost. This study thus showed the importance of buffered layers in OSC with an emphasis on improving the HTL if long operational devices are to be achieved. A further in depth study of metal oxide based HTLs and their interfaces with the BHJ and ITO electrodes are needed to be investigated in future.

Acknowledgements

The work was supported by High Impact Research (HIR) grant UM.S/625/3/HIR/MoE/SC/26 with account number UM.0000080/HIR.C3 from the Ministry of Higher Education, Malaysia. The authors also acknowledge the financial support from University of Malaya Research Grant with grant number RP007/13AFR.

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