Light-emission organic solar cells with MoO₃:Al interfacial layer–preparation and characterizations

Abstract

A light-emitting organic solar cell (LE-OSC) with electroluminescence (EL) and photovoltaic (PV) properties is successfully fabricated by connecting the EL and PV units using a MoO₃:Al co-evaporation interfacial layer, which has suitable work function and good transmittance. PV and EL units are fabricated based on poly(3-hexylthiophene) (P3HT)-indene-C₆₀ bisadduct (IC₆₀BA) blends, and 4,4′-bis (N-carbazolyl) biphenyl-fac-tris (2-phenylpyridine) iridium (Ir(ppy)₃), respectively. The work function and the transmittance of the MoO₃:Al co-evaporation are measured and adjusted by the ultraviolet photoelectron spectroscopy and the optical spectrophotometer to obtain the better bi-functional device performance. The forward- and reverse-biased current density-voltage characteristics in dark and under illumination are evaluated to better understand the operational mechanism of the LE-OSCs. A maximum luminance of 1550 cd/m² under forward bias and a power conversion efficiency of 0.24% under illumination (100 mW/cm²) are achieved in optimized LE-OSCs. The proposed device structure is expected to provide valuable information in the film conditions for understanding the polymer blends internal conditions and meliorating the film qualities.

Graphical abstract

Keywords

organic solar cell (OSC) / polymer-fullerene / light emission / MoO ₃:Al interfacial layer

1 Introduction

Conjugated polymers blended with soluble fullerene derivatives have demonstrated a remarkable potential for low-cost, large-area and flexible organic solar cells (OSCs) with exceeding 10% in power conversion efficiency [1]. For commercialization, further improvement in power conversion efficiency and the cell reliability are desired [2,3]. To the power conversion efficiency, there still exists the room of improvement by synthesizing more efficient materials [4], optimizing more rational device structure [5] and even electrical doping for improving the light-harvesting efficiency in polymer solar cells [6,7]. Importantly, the issue of cell stability has to be paid more attentions after the power conversion efficiency approaching 10%. There are many degradation channels during cell operation and each contributes to the overall deterioration of photovoltaic (PV) performance that consequently shortens the device lifetime. Great efforts have been taken in the cell stability especially the materials stability in OSCs. According to each layer that usually constitutes an OSC, the materials stability can be subdivided: active layer [8,9], electrodes [10,11], electron transport layer [12,13], and hole transport layer [14,15] which contributes their own degradation mechanisms. In addition, the physical stabilities such as the cell structure [16], the interfacial condition [17,18], and even the film processing means also play important roles in the whole cell degradation [19]. The most significant contributor is believed to be a combined effect by above mentioned mechanisms due to the diffusion of atmosphere species into the cells. Electrical trap states, which are mainly induced by the impurities and the structural defects in the organic bulk films [20,21], can be easily evaluated by basis electrical and/or optical measurements [22–25].

Electroluminescence (EL) imaging has been widely used as a photographic evaluation technique to indicate the cell physical properties in silicon solar cells [26–29]. The same function of EL imaging is also desired in OSCs, which is expected to provide valuable information on the film defects for meliorating the film quality and improving the device performance. Recently, Cravino et al. reported a light-emitting OSC (LE-OSC) based on a conjugated system with internal charge transfer [30]. Hoyer et al. demonstrated a weak EL imaging in polymer-fullerene based OSCs [31]. Tvingstedt et al. observed the EL phenomenon from several types of organic polymer-fullerene bulk heterojunction solar cells [32]. The general understanding treats light and current generations as competing process due to their reverse generation mechanism, which leads to very weak EL intensity in past reports. The EL intensity needs to be improved as high enough for clearly observing the microstructure of organic films. Other than inorganic based EL devices, noticeably, organic light-emitting device (OLED) is an innate surface-emitting light source, which makes it very appropriate to evaluate the film quality especially the film uniformity as a back light illumination. We try to observe the film quality of polymer-fullerene blends by using OLED as the back light illumination. It can be realized by connecting the PV unit and EL unit via an appropriate intermediate layer, which generally functions as an internal electrode to produce charge carrier and to facilitate the carriers into the adjacent units.

Poly(3-hexylthiophene) (P3HT) and indene-C₆₀ bisadduct (IC₆₀BA) is the most intensely investigated conjugated polymer donor material and acceptor material, respectively. 4,4′-bis (N-carbazolyl) biphenyl (CBP)/fac-tris (2-phenylpyridine) iridium (Ir(ppy)₃) is a typical host/dopant system in OLEDs. In this paper, we demonstrate a bi-functional LE-OSC by connecting separated PV unit (P3HT:IC₆₀BA as the active layer) and EL unit (CBP:Ir(ppy)₃ as the emitting layer) using a MoO₃:Al co-deposited interfacial layer. The work function and the transmittance of the MoO₃:Al co-evaporation are measured and adjusted by the ultraviolet photoelectron spectroscopy and the optical spectrophotometer to obtain the better bi-functional device performance. With efforts, a maximum EL of 1550 cd/m² is achieved by optimizing the composition in the connecting layer. The forward- and reverse-biased current density-voltage characteristics in dark and under illumination are evaluated to better understand the operational mechanism of the bi-functional LE-OSCs. The finding in this work gives deep understanding of carrier transporting in tandem devices with combined PV and EL units. And the developed LE-OSCs with PV and EL units also provide a direct route to evaluate the microstructures (i.e., pinholes and defects) in organic semiconductor films.

2 Experimental

2.1 Device fabrication

Blend solutions of P3HT:IC₆₀BA (1:1, in wt%) were prepared using chloroform as the solvent at a solid content of 12 mg/mL. The solutions were prepared in a nitrogen circumstance and rigorously stirred for more than 12 h at room temperature. A thin layer of poly (3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) and the active layer of P3HT:IC₆₀BA (1:1, in wt%) were first deposited onto the patterned indium-tin-oxide (ITO) substrate sequentially by spin coating. After that, a co-evaporated interlayer of MoO₃:Al (or Ag) and a light-emitting layer CBP:Ir(ppy)₃ was formed, subsequently. 2,9-dimethyl-4,7-diphenylphenanthroline (BCP) was evaporated as a buffer layer. Finally, Al electrode was deposited via thermal evaporation through a shadow mask, giving an active device area of 0.04 cm². The whole device has a structure of ITO/PEDOT:PSS (50 nm)/P3HT:IC₆₀BA (150 nm)/MoO₃:Al (5 nm)/CBP:Ir(ppy)₃ (15 nm)/BCP (10 nm)/Al (70 nm). Figure 1 shows the schematic diagram of the device structure and the molecule structure of the materials upon investigation. The interlayer MoO₃:Al was co-deposited in the volume ratio of 1:1.

2.2 Measurements and characterization

The current-voltage characteristics were measured using a semiconductor parameter analyzer (HP 4155B) under a simulated air mass (AM) 1.5G spectra illumination. EL characteristics were measured using both the semiconductor parameter analyzer (HP 4155B) and a luminance meter (Topcon BM-3). The transmittance spectra were collected by a spectrophotometer (Hitachi 330). The surface morphology of the blend films was evaluated using atomic force microscopy (AFM). Ultraviolet photoelectron spectroscopy (UPS) analyses were carried out with the excitation photo energy of 21.2 eV (HeI) from an unfiltered He gas discharge lamp and a hemispherical analyzer. The graphic EL imaging was taken using a digital camera (CASIO EX-F1) combined with a microfocus lens system.

3 Results and discussion

Interfacial layers are commonly used in tandem organic electronic devices by connecting two and more separated PV and/or EL units. Typical intermediate interfacial layers can be formed by conductive films [33], organic heterojunction [34], and n-doped/metal oxide junction [35] with good optical and electrical characteristic [36,37]. Among them, MoO₃ is widely used as an anode interfacial material in OLEDs and OSCs owing to its relative low evaporation temperature, non-toxicity and deep electronic states [38–40]. The work function of MoO₃ can be tuned by doping using some metal materials with lower work function. The UPS spectra of pure MoO₃, and co-evaporated MoO₃:Al are displayed in Fig. 2. The inset is the enlarged regions of the photoemission secondary-electron cutoffs. The work function of the pure MoO₃ was estimated to be 5.1 eV, which is lower than the ultra-high vacuum in situ value due to the effect of oxygen and moisture by air exposure. When doping by Al, a downward shift of the vacuum level was observed, corresponding to a decrease in work function with 1.0 eV for MoO₃:Al.

One requirement for the connecting layer in tandem devices is the good optical transparency which can guarantee sufficient light transmittance between the PV and/or EL units. Figure 3 shows the transmission spectra of P3HT:IC₆₀BA blend (150 nm), P3HT:IC₆₀BA (150 nm)/MoO₃:Ag (5 nm) and P3HT:IC₆₀BA (150 nm)/MoO₃:Al (5 nm). Compared to the as-coated P3HT:IC₆₀BA blending film, additional deposition of a thin MoO₃:Ag (Al) results in an upward shift of transmission spectra, which is assumed to be associated with the localized surface plasmon resonance effect of Ag and Al nanoparticles formed in MoO₃:Ag (Al) film during co-evaporation [41,42]. In addition, the transmittance of P3HT:IC₆₀BA (150 nm)/MoO₃:Al (5 nm) is slightly higher than that of P3HT:IC₆₀BA (150 nm)/MoO₃:Ag (5 nm) at 512 nm (EL peak of Ir(ppy)₃). It means that more light generated in the EL unit would pass through the interfacial layer if using MoO₃:Al as a connecting layer.

Figure 4 shows the typical current density−voltage (J−V) characteristics in tandem devices with and without interfacial layer (IL) under illumination (100 mW/cm²). The J−V characteristics of a P3HT:IC₆₀BA based OSC device (OSC only) with structure of ITO/PEDOT (50 nm)/P3HT:IC₆₀BA (150 nm)/LiF (1 nm)/Al (70 nm) is also shown for comparison. The referenced device (OSC only) shows a power conversion efficiency (PCE) of 4.45% with short-circuit current density (J_sc) = 10.85 mA/cm², open-circuit voltage (V_oc) = 0.76 V and fill factor (FF) = 54%. For tandem devices without any interfacial layer (W/O IL), poor PV property with J_sc = 0.08 mA/cm², V_oc = 0.45 V and FF= 22% is achieved. The competing EL and PV mechanisms lead to the poor device performance because the charge carrier behavior is not tuned by a suitable interfacial layer. After using MoO₃:Al as an interfacial layer, the carrier injection and transport characteristics in both PV and EL units are improved, resulting in enhanced J_sc (0.96 mA/cm²), V_oc (0.70 V), and FF (36%), corresponding to an improved PCE of 0.24%. The detailed PV and EL characteristics in all tandem devices are summarized in Table 1.

Figure 5 shows the EL properties for (a) J−V and (b) luminance vs current density (L−J) characteristics, respectively, in LE-OSCs with and without interfacial layer in dark condition. The EL property of a CBP:Ir(ppy)₃ based OLED device (OLED only) with structure of ITO/PEDOT (50 nm)/CBP:Ir(ppy)₃ (80 nm)/BCP (10 nm)/Al (70 nm) is also shown for comparison. The reference device (OLED only) exhibits a turn-on voltage (V_t) of 1.6 V and a maximum luminance (L_max) of 40800 cd/m². For LE-OSC without any interfacial layer, the EL property largely deteriorates with a V_t of 4.4 V and an L_max of 140 cd/m² due to no tuneable carrier behavior by a suitable interfacial layer. The decreased device performance of EL unit was originated from the reduced electron extraction due to its direct contact with the PV unit. Since MoO₃:Al is a co-evaporation interfacial layer with bipolar carrier transporting properties, the hole can be injected into the EL unit from the PV unit. By inserting an interfacial layer of MoO₃:Al between the PV and EL units, the EL properties of LE-OSCs are remarkably improved. For example, in the case of using MoO₃:Al as an interfacial layer, V_t is lowered from 4.4 to 3.2 V, and a maximum luminance of 1550 cd/m² is achieved.

Figure 6 shows the forward- and reverse-biased J−V characteristics of tandem devices in dark and under illumination (100 mW/cm²). The insets are the corresponding double-logarithmic plots. In both cases, EL emission is observed only at forward bias. And there exists no large difference on the EL properties. In addition, the contribution of light harvesting in the PV unit to the total current is observed in the low forward bias region (<3 V) because the EL current is far larger than the photo-induced current. While, the photo-induced current is very obvious in whole reverse bias region. Particularly, a trap-assistant current appears in high reverse-bias (>10 V) from the double-logarithmic plot. As shown in Fig. 1(b), the highest occupied molecular orbital (HOMO) of IC₆₀BA in the active layer of the PV unit is similar to that of CBP in the light-emitting layer of the EL unit. It is reported that the hole injection can be realized either through direct injection from the anode or subsequent transfer of holes from polymer to fullerene [29]. At the same time, the high work function of MoO₃ in the MoO₃:Al interfacial layer produces a route to the hole injection from the PV unit to the EL unit. Considering the direct electron injection through BCP and further blocking of holes by BCP, EL unit can operate effectively. Under illumination, light harvesting, exciton generation and carrier separation in the PV unit can also be carried out effectively. However, the electron extraction is largely confined by the large difference of the lowest unoccupied molecular orbital (LUMO) between IC₆₀BA and CBP. The functions and charge transporting, such as charge transport, light-emission, and light harvesting in PV and EL devices, are very different. And the light production and current generation in them are a competing phenomenon due to their reverse generation mechanism. We developed MoO₃:Al connecting layer enables the PV and EL tandem device operate very well. The current density−voltage characteristics analysis gave a further understanding of carrier transporting in tandem devices with combined PV and EL units.

As an application, the EL imaging in the tandem device is used as a background light source for observing the film conditions of P3HT:IC₆₀BA blending films. Figure 7 shows the photograph of EL imaging (2 mm × 2 mm) of (a) pinholes, (b) film uniformity and (c) strip lines of P3HT:IC₆₀BA blending films in different samples. The dusts, the hot spots, the defects, and even the polymer aggregation in P3HT:IC₆₀BA bulk film can be seen clearly by an optical microscope with black light illumination. Whereas, as shown in Fig. 7(a), the dusts, the defects and the pinholes can be distinguished clearly by using the OLED as a back illumination. Interestingly, an uneven film region (1.5 mm × 1.0 mm) of P3HT:IC₆₀BA bulk film, which could not be reflected in the optical microscopic image, is observed obviously by using an OLED as the back light source as shown in Fig. 7(b). Generally, the polymer:fullerene film in practical application is very thin (~100 nm), uneven film region with large area is not effectively reflected by a point light source based technique. In present case, OLED is an innate surface-emitting light source. In addition, the EL layer (OLED back light source) directly contacted with the P3HT:IC₆₀BA bulk film. The film quality especially the film uniformity of P3HT:IC₆₀BA blends can be reflected easily. Actually, the cell degradation is seriously associated with the film uniformity of the active layer besides the film defects. The uneven region of the active layer will be gradually broken from the center during the degradation process. We attributed it to the poor uniformity of the P3HT:IC₆₀BA blending films. In addition, the strip lines caused during the spin-coating process can also be evaluated effectively as shown in Fig. 7(c). In addition, the dark spots formed during the device degradation due to the incomplete encapsulation, were also clearly reflected from the right image of Fig. 7(c) when using OLED as the back light. In a word, direct evaluation of active film quality in OSCs can be easily realized by using the OLED as a background light source.

Actually, for most organic semiconductor films, the photo-oxidation due to the molecular oxygen and water will disrupt the delicate electrochemical processes, which mainly determines the key parameters and the device stability. Although the trapped charge carriers do not directly participate in the charge transport in the organic films, they still strongly affect the charge carrier behavior by their columbic forces. Therefore, the evaluation of film quality such as the trap states and the defect conditions in the active layers are very important for deeply understanding the mechanism of the cell degradation with a final goal of good cell stability. The EL imaging in our developed LE-OSCs can evaluate the film quality, such as film defect, film uniformity and the film degradation directly.

4 Conclusions

In conclusion, we have fabricated a LE-OSCs by connecting a P3HT:IC₆₀BA based PV unit and a CBP:Ir(ppy)₃ baded EL unit using a MoO₃:Al interfacial layer. The forward- and reverse-biased J−V characteristics in dark and illumination conditions are investigated to better understand the operational mechanism of the LE-OSCs. A maximum luminance of 1550 cd/m² under forward bias and a power conversion efficiency of 0.24% under illumination (100 mW/cm²) are achieved by optimizing the work function and the optical property of the MoO₃:Al interfacial layer. In addition, direct observation of film condition in P3HT:IC₆₀BA blends by using the EL imaging as a background light source. Since the advantages of self-emitting, surface-emitting for OLEDs, the film quality especially the uniformity of P3HT:IC₆₀BA blending films was evaluated using the OLED as a background light source by operating the tandem devices under a bias mode. EL imaging in the LE-OSCs we proposed is expected to evaluate the OSC stability based on the analysis of film quality such as film defect, film uniformity and the film degradation.

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