Band offset at TiO₂/MDMO PPV and TiO₂/PEDOT PSS interfaces studied using photoelectron spectroscopy

Abstract

We report band alignment and band offset studies across the interfaces of hetero-structures of TiO₂ with poly[2-methoxy-5-(3′,7′-dimethyl-octyloxy)-p-phenylene vinylene] (MDMO PPV) and poly(styrenesulfonate) doped poly(3,4-ethylenedioxythiophene) (PEDOT PSS) using photoelectron spectroscopy. In both the cases the band alignment was found to be type II with significant band bending in the range 0.2 to 0.3 eV. In the case of the TiO₂/MDMO PPV hetero-structure, the valance band offset (VBO)/conduction band offset (CBO) were found to be 2.68 ± 0.1 eV/1.68 ± 0.1 eV, while the TiO₂/PEDOT PSS hetero-structure exhibited VBO/CBO as 2.3 ± 0.1 eV/0.7 ± 0.1 respectively. Based on these results, schematic band diagrams for these organic/inorganic hetero-structures were proposed. Our studies are important for the optimum design of various photovoltaic devices based on these organic/inorganic hetero-junctions.

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Introduction

Organic–inorganic hybrid solar cells have become an exciting alternative to next generation dye sensitized solar cells since they combine the advantages of both the systems.^1–3 Despite its special features like low fabrication cost, flexibility and light weight, its relatively low power conversion efficiency (PCE) hinders its practical application possibilities. PCE could be significantly improved by the use of bulk hetero junctions (BHJ) where efficient charge separation and charge injection occurs due to a large junction surface area.^4–6 Organic–inorganic bulk hetero junctions, where nano-structured metal oxides are embedded in a conjugated polymer matrix, provide a large junction surface area. Among metal oxides, TiO₂ has been identified as a suitable choice of inorganic material in organic–inorganic BHJ solar cells due to its robust nature, low cost, strong photo catalytic activity and high photoelectric conversion efficiency.⁷ This has motivated various research groups to study the performance of TiO₂/conjugated polymer based BHJ for photovoltaic applications.^8–11 Among these studies, tremendous attention has been drawn towards PEDOT:PSS/MDMO PPV:TiO₂ based BHJ solar cells due to the stable optical and electrical properties of PEDOT PSS and MDMO PPV.^12–14 Similarly, Paul A. van Hal et al. reported a hybrid BHJ solar cell (ITO/PEDOT:PSS/MDMO PPV:TiO₂/LiF/Al) with a PCE of ∼0.2%.¹⁵ In this study, the active medium of the BHJ solar cell consisted of important organic/inorganic hetero-interfaces like TiO₂/MDMO PPV and TiO₂/PEDOT PSS which ensured charge separation and carrier transport respectively. In all these reports, it has been noticed that band offset and band alignment of these organic–inorganic hybrid systems plays a crucial role in determining the overall performance of the aforesaid photovoltaic systems. Clear insight into the band offset and band alignment of these hetero-interfaces is highly necessary to further improve the photovoltaic performance of these photovoltaic systems. Despite this, detailed studies on the direct measurement of band offset and band alignment of these hybrid organic–inorganic hetero-structures are not reported so far. Therefore, it is imperative to study the band offset and their alignment across the interfaces of such hybrid organic–inorganic hetero-structures to further improve the performance of these photovoltaic systems. X-ray photoelectron spectroscopy (XPS) and ultra-violet photoelectron spectroscopy (UPS) have been identified as the appropriate tools for determining the band offset and band alignment of hetero-structures.^16–23

In this work, we report band offset and alignment studies of TiO₂/MDMO PPV and TiO₂/PEDOT PSS hetero-structures using photoelectron spectroscopy. Our studies show that the TiO₂/MDMO PPV interface exhibits type II band alignment and wide conduction and valence band offsets which is desirable to initiate fast exciton breaking and charge separation in a high interface potential arising due to wide band offsets. On the other hand, the observed low value of the conduction band offset in the TiO₂/PEDOT PSS interface indicates a requirement for other organic material for efficient electron blocking in the aforesaid photovoltaic systems.

For these studies, high quality, smooth, uniform and pin hole free TiO₂ thin films are required. In recent years, atomic layer deposition has emerged as a suitable technique to satisfy these requirements.^24–26 Hence, in the present work, we used the atomic layer deposition technique to deposit TiO₂ thin films followed by growth of the respective conjugated polymers using the spin coating method. Atomic layer deposition is a versatile technique to grow high quality TiO₂ thin films.^24–26 Atomic layer deposition is based on two self limiting and complimentary half reactions from the gas phase to produce thin films and over layers in the nanometer range. Atomic layer deposition techniques maintain high precision, perfect conformality and fine process control as compared to various other deposition methods like the sol–gel method, pulsed laser deposition, the hydrothermal method, sputtering etc.^27–30 Detailed discussions about the growth techniques and parameters are given in the next section.

Experimental section

TiO₂ thin films of thickness ∼250 nm were deposited on n-type Si (100) wafers using a thermal atomic layer deposition (ALD) system (make: Beneq, Finland; Model TFS-200). Titanium tetrachloride (TiCl₄) and deionized water (H₂O) were used as the Ti and oxidant precursors respectively. High purity (99.999%) nitrogen was used for purging after every precursor pulse. Each cycle of TiO₂ growth consisted of the following sequences: (1) 500 ms of TiCl₄ pulsing, (2) 1 s nitrogen purging, (3) 500 ms of H₂O pulsing, and (4) 1 s nitrogen purging. The total number of ALD cycles was fixed at 3000 and the substrate temperature was kept at 250 °C for the growth of all TiO₂ thin films. MDMO PPV and PEDOT PSS polymers (make: Sigma Aldrich) with concentrations of 15 mg ml⁻¹ in chlorobenzene and water respectively were spin coated on the respective substrates. The as grown MDMO PPV and PEDOT PSS thin films were kept in a vacuum oven at 50 °C for 12 hours for drying. For photoelectron spectroscopy studies, we used a set of three samples for each hetero-structure: a TiO₂ thin film of thickness ∼250 nm (A1), a MDMO PPV thin film of thickness ∼250 nm (A2) and a TiO₂ (∼250 nm)/MDMO PPV (∼5 nm) hetero-junction (A3). Similarly for the TiO₂/PEDOT PSS system we prepared three sets of samples: TiO₂ thin film of thickness ∼250 nm (A1), PEDOT PSS thin film of thickness ∼250 nm (B2) and TiO₂ (∼250 nm)/PEDOT PSS (∼5 nm) hetero-junction (B3). The valance bands of these hetero-structures and their individual layers were recorded using ultra-violet photoelectron spectroscopy (UPS) measurements. Core level spectra of these samples were recorded using X-ray photoelectron spectroscopy (XPS). UPS measurements were performed with the angle integrated photoelectron spectroscopy beam line at the INDUS-I synchrotron radiation source with a photon energy of 65 eV. An ESCA instrument with Al Kα (hν = 1486.6 eV) as the X-ray excitation source was used for core level spectroscopy. Exposure to atmosphere can contaminate the sample surface and this may affect the precision of the measurements. All the sample surfaces, prior to measurement, were therefore cleaned using Ar sputtering. After Ar sputtering, peaks related to contaminants were significantly reduced. The C 1s peak (284.6 eV) was used to calibrate all XPS spectra. All the core-level peak positions were obtained by fitting the respective core-level spectra using Shirley background and Voigt (mixed Gaussian–Lorentzian curve) line shape functions.

Results and discussion

The X-ray diffraction pattern (XRD) of the TiO₂ thin film showed only an anatase phase with a (101) peak at 2θ = 25.4° and its detailed studies were reported in our earlier work.³¹ The optical band gaps of TiO₂, MDMOPPV and PEDOT PSS were found to be 3.2 eV, 2.2 eV and 1.6 eV respectively (not shown here). Firstly, we discuss the analysis of the photoelectron spectroscopy data of the TiO₂/MDMOPPV hetero-structure. Fig. 1(a) and (b) show the Ti 2p peak in TiO₂ (A1) and the TiO₂/MDMO PPV (A3) hetero-structure respectively. The Ti 2p peak was de-convoluted mainly to two peaks, 2p_1/2 at ∼464.5 eV and 2p_3/2 (Ti⁴⁺) at ∼458.9 eV with a separation of ∼5.6 eV. These values are consistent with the previously reported XPS data of TiO₂.³² An additional shoulder peak was observed at ∼457.3 eV which could be assigned to the Ti³⁺ state.²¹ This shoulder peak shows the formation of reduced oxidation states during Ar sputtering.²¹ We used binding energy values of the 2p_3/2 (Ti⁴⁺) peaks of samples A1 and A3 for the calculation of the valance band offset. In the case of sample A3, de-convoluted peak positions were at ∼459.2 eV (2p_3/2 (Ti⁴⁺)), ∼457.5 eV (2p_3/2 (Ti³⁺)) and ∼464.5 eV (2p_1/2) as shown in Fig. 1(b). Fig. 1(c) and (d) represent the C 1s peaks in samples A2 and A3 respectively. The C 1s peak in Fig. 1(c) was de-convoluted to two peaks, 284.6 eV and 286.1 eV, which correspond to carbon atoms in the aromatic ring and C–O–C bonds in the polymer chain respectively.³³ We used the binding energy of the C 1s peak of the carbon atoms in the aromatic ring for the band offset calculation. De-convoluted C 1s peaks of hetero-structure (sample A3) were positioned at ∼284.3 eV and ∼285.9 eV as shown in Fig. 1(d). Fig. 1(e) and (f) show the UPS (valance band) spectra of TiO₂ (A1) and MDMO PPV (A2) thin films. The VBM positions of the samples were obtained by extrapolating linearly the leading edge to the extended base line of the valance band (VB) spectra as depicted in Fig. 1(e). The VBM positions of TiO₂ (A1) and MDMO PPV (A2) were estimated as 3.2 eV and 1.1 eV below the Fermi level (E_f) respectively. All these values are listed in the Table 1. Using these core level and VB data, we calculated the valance band offset (VBO) of TiO₂/MDMO PPV hetero-structures using the Kraut’s equation³⁴ given below.

VBO = (E^MDMOPPV_C1s − E^MDMOPPV_VB) − (E_Ti2p3/2^TiO₂ − E^TiO_VB²) − (E^MDMOPPV_C1s − E_Ti2p3/2^TiO₂) (1)

where (E^MDMOPPV_C1s − E_Ti2p3/2^TiO₂) is the binding energy difference between the Ti 2p_3/2 and C 1s core levels (CL) in the TiO₂/MDMO PPV hetero-structures (A3). (E_Ti2p3/2^TiO₂ − E^TiO_VB²) is the energy difference between the Ti 2p_3/2 core level and valance band maximum (VBM) in the TiO₂ thin film (A1) and (E^MDMOPPV_C1s − E^MDMOPPV_VB) is the energy difference between the C 1s core level and the valance band maximum (VBM) in MDMO PPV (A2). Substituting the respective XPS and UPS data in eqn (1), we obtained the VBO of the TiO₂/MDMO PPV hetero-structure as 2.7 eV.

Fig. 1 XPS core level spectra of the Ti 2p peaks in A1 (a) and A3 (b), C 1s peaks in A2 (c) and in A3 (d). VB spectra of TiO₂ (e), MDMO PPV (f) and the TiO₂/MDMO PPV hetero-structure (g).

Table 1 Values of the core level peaks and VBM positions of various samples. There is an error bar of ±0.1 in the values of the core level peaks and VBM

Samples	Ti 2p3/2 (eV)	C 1s (eV)	S 2p3/2 (eV)	VBM (eV)
A1	458.9	—	—	3.2
A2	—	284.6	—	1.1
A3	459.2	284.3	—	—
B2	—	—	163.4	0.5
B3	458.7	—	163.6	—

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Fig. 1(g) shows the UPS valance band (VB) spectra of the TiO₂/MDMO PPV hetero-structure and depicts two distinct valance band onsets. The valance band onset appearing at higher binding energy was assigned to that of the TiO₂ bottom and that in the lower binding energy was related to the MDMO PPV surface layer. The difference between these two valance band onsets was found to be 2.8 eV, which corroborated the VBO calculated using the Kraut equation. A difference of 0.1 eV was observed in the VBO calculations of the TiO₂/MDMO PPV hetero-structure using these two methods. This was related to the measurement error in our experiment. Considering the error bar of ±0.1 eV in the values of the core level and VBM, we concluded that the VBO of the TiO₂/MDMO PPV hetero-structure was 2.7 ± 0.1 eV. We calculated the conduction band offset (CBO) using the following equation

CBO = VBO + E_g (MDMOPPV) − E_g (TiO₂) (2)

Substituting the band gaps of TiO₂ and MDMO PPV as E_g (TiO₂) ∼ 3.2 eV and E_g (MDMO PPV) ∼ 2.2 eV respectively, the CBO was estimated to be 1.7 ± 0.1 eV. We observed binding energy shifts between a particular core level in the thin film sample (A1, A2) and the same core level in hetero-structure (A3) sample. This showed the formation of an appreciable space charge like layer (band bending) at the interface of the TiO₂/MDMO PPV hetero-structure.³⁵ We used the binding energy values of C 1s in MDMO PPV and Ti 2p_3/2 in TiO₂ for the estimation of band bending. Binding energy shifts were found to be 284.6–284.3 = 0.3 eV for the C 1s peak and 458.9–459.2 = −0.3 eV for the Ti 2p_3/2 peak which were considered as the band bending in the MDMO PPV and TiO₂ sides respectively. Using these band offset values, band bending and band gaps, a schematic band diagram was constructed which showed type II band alignment for the VBO of the TiO₂/MDMO PPV hetero-structure as shown in Fig. 3(a). Previous reports show that type II band alignment has been observed in similar widely investigated photovoltaic systems like TiO₂/MEH PPV, TiO₂/P3HT, 6H–SiC/P3HT etc.^2,36,37 There are reports where the interfaces of TiO₂/polymer have been modified by inserting multiple donor absorbers to enhance photo-generation of excitons and their subsequent breaking/charge separation.^36,38,39 Interface modification generally demands a wider band offset across the organic/inorganic interface to incorporate donor absorbers in the maximum range of the solar spectrum. The value of the band offset estimated here in the TiO₂/MDMOPPV system is higher than/comparable to that of the similar aforesaid organic/inorganic photovoltaic systems. This relatively higher band offset can initiate two things: (1) enhanced photo-generation by incorporation of efficient multiple donor absorbers, and (2) fast exciton breaking in a high interface potential. This wide band offset and type II band alignment in the TiO₂/MDMOPPV hetero-structure offers a favorable condition to improve the PCE of related photovoltaic systems using suitable interface modification.

Fig. 2 XPS core level spectra of the S 2p peaks in B2 (a) and B3 (b), the Ti 2p peak in B3 (c). VB spectra of PEDOT PSS (d) and the TiO₂/PEDOT PSS hetero-structure hetero-structure (e).

Fig. 3 Schematic diagram of type II band alignment of (a) TiO₂/MDMO PPV and (b) TiO₂/PEDOT PSS hetero-structures.

Similar analyses were performed on the photoelectron spectroscopy data of the TiO₂/PEDOT PSS hetero-structure. Fig. 2(a)–(c) show the XPS core level spectra of S 2p in B2, S 2p in B3 and Ti 2p in B3. The S 2p peak in PEDOT PSS generally consists of spin-split doublets (S 2p_1/2,3/2); lower binding energy peaks correspond to spin-split components of sulfur in PEDOT while the higher binding energy peaks correspond to that in the PSS.³³ The S 2p peaks of samples B2 and B3 were de-convoluted to four peaks as shown in Fig. 2(a) and (b) respectively. Fig. 2(a) shows the S 2p spin-split doublets (PEDOT) of sample B2 at ∼163.4 eV (2p_3/2) and ∼164.8 eV (2p_1/2) where as that of PSS is depicted in the same figure at ∼167.5 eV (2p_3/2) and ∼168.9 eV (2p_1/2). Fig. 2(b) depicts the S 2p doublets (PEDOT) of sample B3 at ∼163.6 eV (2p_3/2) and ∼164.7 eV (2p_1/2) whereas the peaks positioned at ∼167.9 eV (2p_3/2) and ∼169.0 eV (2p_1/2) correspond to the S 2p doublets of PSS. We used the sulfur line (S 2p_3/2) of PEDOT of the samples B2 and B3 for the band offset calculation of the TiO₂/PEDOT PSS hetero-structure. The Ti 2p peak of sample B3 was de-convoluted mainly to two peaks with a shoulder peak as described earlier. Fig. 2(c) shows the de-convoluted peaks positioned at ∼458.7 eV (2p_3/2 (Ti⁴⁺)), ∼457.3 eV (2p_3/2 (Ti³⁺)) and ∼464.2 eV (2p_1/2). All these data are in line with previously reported data.^21,32 We used the binding energy values of the 2p_3/2 (Ti⁴⁺) peaks of samples A1 and B3 for the calculation of the valance band offset of the TiO₂/PEDOT PSS hetero-structure in the present work. Fig. 2(d) shows the UPS valance band (VB) spectra of the PEDOT PSS thin film (B2). The VBM positions of sample B2 were estimated as 0.5 eV below the Fermi level (E_f). All these values are tabulated in Table 1. We calculated the valance band offset (VBO) of TiO₂/PEDOT PSS hetero-structure using the Kraut’s equation³⁴ given below.

VBO = (E_S2p3/2^{PEDOT PSS} − E^{PEDOT PSS}_VB) − (E_Ti2p3/2^TiO₂ − E^TiO_VB²) − (E_S2p3/2^{PEDOT PSS} − E_Ti2p3/2^TiO₂) (3)

where (E_S2p3/2^{PEDOT PSS} − E_Ti2p3/2^TiO₂) is the energy difference between the core levels (CL); S 2p_3/2 and Ti 2p_3/2 in TiO₂/PEDOT PSS hetero-structures (A3) and (E_S2p3/2^{PEDOT PSS} − E^{PEDOT PSS}_VB) is the energy difference between the S 2p_3/2 core level and the valance band maximum (VBM) in the PEDOT PSS thin film (B2). Substituting the CL and VBM values in eqn (3), we obtained the VBO of TiO₂/PEDOT PSS hetero-structures as 2.3 eV.

The UPS valance band spectra of the TiO₂/PEDOT PSS hetero-structure consisted of two distinct valance band onsets as shown in Fig. 2(e). The valance band onset (at higher binding energy) was assigned to the TiO₂ bottom layer and that in the lower binding energy was related to the PEDOT PSS surface layer. The difference between these two valance band onsets was found to be 2.4 eV which was in close agreement with the VBO calculated using the Kraut equation. An acceptable difference of 0.1 eV (with an error bar of ±0.1 eV) was observed between the VBO calculations for the TiO₂/PEDOT PSS hetero-structure using these two approaches. Thus, we concluded that the VBO of the TiO₂/PEDOT PSS hetero-structure was 2.3 ± 0.1 eV. We calculated the conduction band offset (CBO) using the following equation,

CBO = VBO + E_g (PEDOT PSS) − E_g (TiO₂) (4)

Substituting the band gaps of TiO₂ and PEDOT PSS as E_g (TiO₂) ∼ 3.2 eV and E_g (PEDOT PSS) ∼ 1.6 eV respectively, the CBO was estimated as 0.7 ± 0.1 eV. Here also we observed binding energy shifts between the core levels in the thin film sample (A1, B2) and the same core levels in the hetero-structure (B3) sample. Hence we calculated band bending of the hetero-structure from the binding energy values of S 2p_3/2 in PEDOT PSS and Ti 2p_3/2 in TiO₂. The calculated binding energy shifts; 163.4–163.6 = −0.2 eV (for S 2p_3/2 peak) and 458.9–458.7 = 0.2 eV (for Ti 2p_3/2 peak) were considered as the band bending in the PEDOT PSS and TiO₂ sides respectively. We constructed a schematic band diagram of the TiO₂/PEDOT PSS hetero-structure using these values of band offset, band bending and band gaps, which showed a type II band alignment as shown in Fig. 3(b). This kind of type II band alignment has been observed in similar widely investigated photovoltaic systems like ZnO/PEDOT PSS, ZnO/P3HT, TiO₂/PTAA etc.^40–42 PEDOT PSS is generally considered as a hole transporting/electron blocking (charge selective transport) material which can initiate the hole conduction between the active medium and anode.⁴³ But, there are also reports which show that PEDOT PSS does not work as an efficient charge selective transport layer in similar kinds of other photovoltaic systems whereas materials like PTAA, PCPDTBT and TPA-PFCB work as excellent charge selective transport layers.^42,44,45 The low value of the CBO (∼0.7 ± 0.1 eV) obtained in the present case reveals that PEDOT PSS is not an effective electron blocking material in the present systems. Modification of the TiO₂/MDMOPPV interface by the introduction of suitable donor absorbers and replacement of PEDOT PSS with an efficient electron blocking layer is a plausible solution to improve the efficiency of aforesaid photovoltaic systems.

Conclusions

In conclusion, we studied the band alignment and band offset of TiO₂/MDMO PPV and TiO₂/PEDOT PSS hetero-structures using XPS and UPS. TiO₂/MDMO PPV and TiO₂/PEDOT PSS hetero-structures exhibited type II band alignment with significant band bending. TiO₂/MDMO PPV exhibited wide conduction and valance band offsets which facilitate efficient exciton breaking and charge separation. The wide band offsets observed in this case also open up the opportunity of modifying the organic/inorganic interface by the incorporation of suitable donor absorber materials which enhance the absorption range of pertinent photovoltaic system. The low value of the conduction band offset observed in the TiO₂/PEDOT PSS hetero-structure suggests a need for an alternate electron blocking layer in such organic/inorganic photovoltaic systems. These findings may be highly important for further optimization and improvement of the aforesaid organic/inorganic bulk hetero-junction photovoltaic systems.

Acknowledgements

We acknowledge Shri A. D. Wadikar for providing experimental support in PES measurements.

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