Inverted organic solar cells with non-clustering bathocuproine (BCP) cathode interlayers obtained by fullerene doping

Bathocuproine (BCP) is a well-studied cathode interlayer in organic photovoltaic (OPV) devices, where it for standard device configurations has demonstrated improved electron extraction as well as exciton blocking properties, leading to high device efficiencies. For inverted devices, however, BCP interlayers has shown to lead to device failure, mainly due to the clustering of BCP molecules on indium tin oxide (ITO) surfaces, which is a significant problem during scale-up of the OPV devices. In this work, we introduce C70 doped BCP thin films as cathode interlayers in inverted OPV devices. We demonstrate that the interlayer forms smooth films on ITO surfaces, resulting from the introduction of C70 molecules into the BCP film, and that these films possess both improved electron extraction as well exciton blocking properties, as evidenced by electron-only devices and photoluminescence studies, respectively. Importantly, the improved cathode interlayers leads to well-functioning large area (100 mm2) devices, showing a device yield of 100%. This is in strong contrast to inverted devices based on pure BCP layers. These results are founded by the effective suppression of BCP clustering from C70, along with the electron transport and exciton blocking properties of the two materials, which thus presents a route for its integration as an interlayer material towards up-scaled inverted OPV devices.

the acceptor, through an ideally zero energy barrier at that interface 19 . Potentially, such layers also provide a transparent spacer to optimize the optical field distribution within the active layer, and thus enhance the OPV performance even further [20][21][22] . Peumans et al. 23 introduced for the first time a BCP (2,9-dimethyl-4,7-diphenyl-1,10-ph enanthroline) layer as a combined EBL and ETL in between the electron acceptor and cathode in bilayer, standard configuration OPV cells based on CuPc and fullerene, C 60.
Bathocuproine (BCP) is widely used as ETL and EBL material in OPVs. BCP has a highest occupied molecular orbital (HOMO) level at 7.0 eV and lowest unoccupied molecular orbital (LUMO) at 3.5 eV 24,25 . Despite the relatively high lying LUMO level, BCP efficiently transports electrons from the acceptor to the cathode due to the formation of a BCP-metal complex, formed when the metal cathode is evaporated on top of the BCP layer 26 . Additionally, BCP works as an EBL and electron selective contact due to its relatively low lying HOMO level of 7.0 eV compared to the HOMO level of e.g. C 70 at 6.1 eV 27 . The exciton blocking properties of BCP results in a higher charge generation yield at the D-A interface, which again leads to enhanced device performance through enhanced short-circuit current densities [28][29][30] . Work reported by Gommans et al. has documented that BCP also can act as an optical spacer layer, to best exploit optical interference effects in OPV cells 31 .
Several studies have focused on the function of BCP as ETL and EBL in OPV devices with standard device architecture 24,31,32 , highlighting the aforementioned properies. In our previous study, an area-dependent behavior of BCP used as ETL and EBL in inverted OPV devices was reported 33 . It was observed that while scaling up the OPV device area, the performance and device yield of the inverted OPV devices decrease significantly compared to standard configuration cells, which was demonstrated to be due to the clustering of BCP on ITO surfaces 33,34 . While BCP on small device areas works well as both EBL and ETL, the probability of BCP clusters penetrating the active layer (approx. 50 nm thick in that study) increases for increased device area. This potentially results in electrical shunting of the inverted OPV devices, which dramatically decrease the device yield for up-scaled cells. In recent work, this has also been demonstrated to lead to faster degradation of inverted OPV devices based on pure BCP ETL and EBL layers 35 . The integration of Ag doped BCP layers in inverted OPVs as buffer layers has previously been reported on 36,37 . However, although these layers provide improved electrical properties, Ag doped BCP may lead to unwanted exciton quenching processes in the fullerene acceptor layer and thus deteriorate the device performance 37 . Such quenching processes between metals and adsorbed molecules are well-know 38 .
Incorporation of interlayers or buffer layers fabricated from a blend of two or more organic materials is a common practice in OPV devices. The blended layers potentially improve the device performance by enhancing the electrical properties at the respective interface (interlayers), and/or the optical properties of the devices (interlayers or buffer layers) 17,39,40 . Bartynski et al. used a blend of C 60 and BCP layer as ETL and EBL in standard OPV devices that improved the electron conductivity, while efficiently blocking excitons and reducing exciton-polaron recombination 27 . Furthermore, Xiao et al. reported that a blend of BPhen:C 60 increases the electron conductivity and, as well, decreases exciton recombination effects in the devices 41 . Liu et al. used a BCP:C 60 layer as EBL and ETL in standard configuration OPV devices to optimize the optical properties of the devices, and also the device lifetime 42 . However, compared to C 60, C 70 offers higher stability upon air exposure 43 , and also a higher conductivity 43 , which may be beneficial when used as an interlayer material in organic photovoltaic devices.
In this work, we studied bathocuproine:fullerene (BCP:C 70 ) acting as ELB and ETL functional blends in inverted architecture OPV devices, as sketched in Fig. 1a. The optimization of the BCP:C 70 ratio as well as the thickness of the blend layer was investigated. The optimized BCP:C 70 layers were employed in inverted OPV devices having active areas of up to 100 mm 2 , and the results were compared against inverted OPVs based on pure BCP layers. The investigation shows that the BCP:C 70 blends suppress the clustering of BCP on top of ITO surfaces, leading to a significantly improved device performance and especially device yield for up-scaled inverted OPV devices.  Figure 1 shows the inverted bilayer OPV device architecture studied in this work, having BCP:C 70 as ETL and EBL, as well as a schematic energy diagram of the device stack made from literature reported energy level values. DBP possesses a high optical absorption strength in the visible wavelength regime and a HOMO level at 5.5 eV 44 , making it a good match to fullerene acceptors such as C 60 and C 70 11 . Figure 2a shows an atomic force microscopy (AFM) image of 3 nm pure BCP deposited on top of an ITO coated glass substrate. The clustering of BCP occurs due to a large interface energy between ITO and BCP 33 , and may take place immediately after BCP deposition even at room temperature 34 . Such clustering can be explained by Ostwald ripening, in which some aggregates grow at the expense of others by adsorbing molecules from the surrounding surface area 45 . At larger surface area, the probability of forming clusters that cause device shunts are larger 33 , making device upscaling more challenging in inverted OPV architectures. One possible solution to overcome BCP aggregation is to conduct co-evaporation with another organic small molecule in order to obtain smoother films 34 . As shown in Fig. 2b, doping C 70 molecules into the BCP film via co-evaporation is effective in preventing the aggregation of BCP molecules, resulting in a nano-grained surface on ITO.

Results and Discussion
As reported in our previous work, the optimized BCP ETL thickness for small area inverted DBP/C 70 based bilayer OPV devices is 1.5 nm 33 , which was therefore chosen as initial ETL thickness in this work. Optimization results from 2 mm 2 inverted OPV cells with various ratios of the BCP:C 70 ETL and EBL are listed in Table S1, showing that devices with 2:1 ratio show slightly higher Fill Factor (FF) with an average value of 55%, V oc with an average value of 0.82 V, as well as PCE reaching an average value of 2.28%. Even though the 2:1 blend films lead to reasonable device performance, an improvement in the short-circuit current density is not seen, compared to reference cells, which is otherwise expected from the exciton blocking properties of the interlayer. This could be due to the relatively low thickness (1.5 nm) of the blend layer.
As a next step, we turned our attention to optimizing the thickness of the ETL and EBL blend layer in 2 mm 2 OPV devices. The JV characteristics and performance parameters of the OPV devices are shown in Fig. 3 and in Table 1, respectively. A summary of the performance parameters from Table 1 is plotted in Fig. 4. As shown in Fig. 4b, as the thickness of the BCP:C 70 layer increases from 1.5 nm to 3 nm, the J SC also increases, which can be well explained by the exciton blocking properties of BCP 46 . When increasing the thickness of the BCP:C 70 layer above 5 nm, the device performance parameters decrease, and the JV curves show clear s-shape characteristics (Fig. 3). The S-shape could be attributed to charge accumulation close to the active layer and ETL interface 42,47,48 . Charge accumulation close to the thicker ETL and EBL films could take place due to the non-ideal energy level alignment between C 70 and BCP, although further studies are required to understand that interface in detail. Electron-polaron accumulation at the electron acceptor and blocking interface may lead to exciton-polaron recombination effects 20,31 , a well-known cause for performance drops in OPV devices 49 .
In order to further elaborate on the electron transport properties of the 3 nm BCP:C 70 blend ETL and EBL layer, electron-only devices (EODs) were fabricated. The structure of the EODs is shown in Fig. 5b, where 0 and 3 nm of BCP:C 70 (2:1) blends were investigated. In the EODs, electrons were injected into the devices through the Ag electrode and extracted out at the ITO electrode. The JV characteristics of the EODs with 3 nm BCP:C 70 ETL show an improvement in the electron extraction properties at the ITO electrode, compared to EODs without the combined ETL and EBL layer (Fig. 5a). Such improvements has also recently been demonstrated for pure, ultrathin BCP layers in small area inverted OPV devices 33 . To this point, the exact energy level alignment scheme across the ITO/BCP:C 70 interface needs to be examined in detail to point on the origin of the improved electron extraction properties. This highlights the importance of future photoelectron measurements to elucidate the detailed interfacial electronic structure and energetic alignment across the interface.    www.nature.com/scientificreports www.nature.com/scientificreports/ Photoluminescence (PL) intensity measurement was performed in order to elucidate the exciton blocking properties of the BCP:C 70 blend layers. In Fig. 6a, PL spectra of the pristine C 70 layer on ITO shows two peaks, a distinct narrow peak at around 690 nm followed by a broader peak at higher wavelengths, corresponding to characteristic electronic and vibrational modes for polycrystalline C 70 , as previously reported 50 . The PL spectra show significant increase in PL intensity from C 70 when deposited on top of the 3 nm BCP:C 70 layer, compared to reference stacks based on pure C 70 layers. The increase in the PL intensity is attributed to the enhanced exciton blocking properties of the BCP:C 70 blend layers, and thus minimum quenching at the ITO/C 70 interface 44 . The change in the relative intensity between the two peaks in the PL spectra can be explained by reduced quenching of specific vibronic transitions, upon insertion of the new ETL.
The reduced symmetry of the C 70 results in more allowed optical transitions and therefore significantly stronger absorption in the visible region compared to symmetrical C 60 46 . However, for the investigated ETLs, we have used ultra-thin interlayers of BCP:C 70 (only 3 nm at 2:1 ratio i.e. ~1 nm of C 70 ). Hence, the impact of light absorption due to either C 60 or C 70 should be negligible in this case. This can be seen in Fig. 6b, where the transmittance spectra of the pure BCP as well as the C 70 doped BCP:C 70 (2:1) layer on ITO-coated glass are shown. Clearly 3 nm of BCP or BCP:C 70 (2:1) show almost no change in optical transmittance, and thus negligible absorption when inserted as an ETL in the inverted device configuration used here. The blended ETL consists of a mixture of C 70 , which efficiently conducts electrons, and the wide energy gap bathocuproine (BCP) that blocks excitons, as demonstrated by our electron only devices and the photoluminescence results. This ETL therefore appears to separate excitons and electrons at the blocking interface as an effective filter, blocking excitons from quenching at the cathode, while promoting electron extraction through the same interlayer in the devices.
As demonstrated from Fig. 2, the BCP:C 70 blend layer possesses a smooth surface without BCP aggregation, which otherwise is a main problem in employing BCP in large area inverted devices, due to device shunting 33 . As a final investigation, the optimized BCP:C 70 blend layers were thus employed in cells with up-scaled device areas of 100 mm 2 , see Fig. 7. As a general observation, a reduced of J SC , FF (Table 2) and hence PCE were observed when increasing the active area from 2mm 2 to 100mm 2 , which in part can be understood from the increased ITO  www.nature.com/scientificreports www.nature.com/scientificreports/ resistance for up-scaled areas 14,33,51 . Devices with 3 nm BCP:C 70 (2:1) show V OC and J SC of 0.85 V and 4.9 mA/cm 2 , respectively, but low FF values of 44% (Table 2). This reduction in FF may be attributed to surface defects of the BCP:C 70 layer, which may arise due to thickness variations in the very thin ETL and EBL layer. Increasing the BCP:C 70 thickness leads to an enhancement of the FF, and devices with 5 nm BCP:C 70 (2:1) ETL and EBL show the highest Fill Factor (FF) values of 51%, and power conversion efficiencies (PCE) of 2.04% ( Table 2). The performance of the OPV devices reduces significantly when the thickness of the BCP:C 70 is increased to 10 nm. This can be explained by the increased series resistance and exciton-polaron recombination 20,31 taking place at the acceptor and blocking layer interface. Initial aggregation could potentially also promote further recombination effects.
For the large area inverted OPV devices with the BCP:C 70 (2:1) layers, the device yield was at 100%, even for OPV devices with incorporated blends of up to 10 nm in thickness. This is notable when compared to inverted OPV devices based on pure BCP as ETL and EBL, where very low device yields for 100 mm 2 cells are observed, mainly due to BCP clustering 33 . Doping of BCP with C 70 thus suppresses the clustering of the BCP molecules, resulting in smoother BCP:C 70 ETL and EBL on ITO surfaces, giving rise to 100% device yields even for large area devices.

Conclusion
In this work, development of inverted organic solar cells using mixed bathocuproine:fullerene (BCP:C 70 ) electron transport and exciton blocking layers has been demonstrated. Incorporation of C 70 molecules into the BCP layer suppresses clustering of the BCP molecules, resulting in smooth layers on ITO surfaces, a prerequisite for using them as efficient ETL and EBL in inverted OPV device configurations. While electron-only devices demonstrate improved electron extraction in the cells, photoluminescence studies reveals strong exciton blocking properties of the interface layer. Combining these material properties leads to well performing bilayer C 70 /DBP based inverted devices, reaching power conversion efficiencies up to 3.28%. While BCP clustering is know to be a severe problem for large area OPV cells, leading to significant reduction in device efficiency and device yield, the novel interlayer leads to well-functioning large area cells (100 mm 2 ), reaching an impressive device yield of 100%. This work thus demonstrates a viable route for the use of the well-known interlayer material bathocuproine (BCP) in inverted OPV devices.

Materials and Device Fabrication. Pre-patterned ITO coated glass substrates (Kintec Company, Hong
Kong) were used for 2 and 100 mm 2 cell area OPV devices. The sheet resistance of ITO was approximately 15Ω/ sq. The substrates were cleaned sequentially in an ultrasonic water bath with detergent, deionized water, Acetone and IPA (10 min for each) then blow dried with a nitrogen gun.
In the first step, OPV devices were fabricated on the cleaned ITO substrates with 2mm 2 cell areas. The BCP:C 70 (Sigma-Aldrich, Germany) blend layers with 1.5 nm thickness and different ratio (1:1, 2:1 and 4:1) were grown by co-evaporation, simultaneously depositing from two sublimation sources at a base pressure of 3 × 10 −8 mbar. This was followed by 30 nm C 70 at a growth rate of 0.2 Å/s and 20 nm DBP (Luminescence Technology Corp.,   www.nature.com/scientificreports www.nature.com/scientificreports/ Taiwan) deposited at 0.3 Å/s without breaking vacuum in between the steps. Then, 10 nm of molybdenum oxide (MoO 3 ) (Sigma-Aldrich, Germany) and 100 nm of Silver (Ag) (AESpump ApS, Denmark) were deposited by thermal evaporation at a base pressure of 5 × 10 −7 mbar. The deposition rates for the MoO 3 and Ag were 0.3 Å/s and 0.5 Å/s, respectively.
In the second step, 2 mm 2 OPV devices were fabricated using optimized BCP:C 70 ratio (2:1) with different thickness (1.5, 3, 5 and 10 nm). The deposition rates for the BCP and C 70 were 0.2 Å/s and 0.1 Å/s, respectively. Finally, optimized BCP:C 70 blend layers were used for fabrication of the up-scale OPVs devices (100 mm 2 cell area). All deposition parameters of the other layers were kept the same as in the first step. Electron-only devices (EODs), having the structure shown in Fig. 5b, were fabricated by sandwiching the BCP:C 70 mixed layers between the respective contact bottom ITO and top C 70 (100 nm)/BCP(10 nm)/Ag(100 nm) layers, using the same deposition rates as for OPV device fabrication.
Device Characterization. All characterizations were performed in an ambient environment. The current density-voltage (J-V) characteristics of the OPV devices were measured using a 2400 source measure unit (Keithley Instruments Inc., USA) and a class AAA solar simulator (Sun 3000, Abet Technologies Inc., USA). The J-V characteristics were measured by applying a voltage sweep from +1 to −0.5 V under a calibrated lamp intensity of 100 mW/cm 2 . Atomic force microscopy (AFM) images were taken using a Veeco Dimension 3100 scanning probe microscope. JV characteristics of the EODs were measured by applying a sweeping voltage from +1 to −1 V using a Keithley 2400 source measure unit (Keithley Instruments Inc., USA). For Photoluminescence (PL) intensity measurements of the ITO/C 70 (100 nm) and ITO/BCP:C 70 (3 nm)/C 70 (100 nm) structures, a microscope objective (Nikon E Plan 50 × 0.75 EPL) with a fluorescence microscope (Nikon Eclipse ME600) connected to a Maya2000Pro Spectrometer (from Ocean optics) was used to record the spectra. A mercury short arc lamp having a filtered excitation wavelength centered between 330-380 nm was used as excitation light source. Transmittance spectra were obtained from a Shimadzu 2700 spectrophotometer.