Polyethylenimine-Ethoxylated Interfacial Layer for Efficient Electron Collection in SnO2-Based Inverted Organic Solar Cells

In this work, we studied inverted organic solar cells based on bulk heterojunction using poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C71-butyric acid methyl ester (P3HT:PCBM) as an active layer and a novel cathode buffer bilayer consisting of tin dioxide (SnO2) combined with polyethylenimine-ethoxylated (PEIE) to overcome the limitations of the single cathode buffer layer. The combination of SnO2 with PEIE is a promising approach that improves the charge carrier collection and reduces the recombination. The efficient device, which is prepared with a cathode buffer bilayer of 20 nm SnO2 combined with 10 nm PEIE, achieved Jsc = 7.86 mA/cm2, Voc = 574 mV and PCE = 2.84%. The obtained results exceed the performances of reference solar cell using only a single cathode layer of either SnO2 or PEIE.


Introduction
Third-generation photovoltaic technology based on conjugated polymers has drawn much attention over the past decade due to advantages such as simple preparation, light weight, low cost, solution-based fabrication on large-area and low-temperature solution process ability that allows for fabrication on flexible substrates via roll-to-roll manufacturing for solar energy conversion [1][2][3][4]. Recently, efforts have been made to enhance power conversion efficiency (PCE) of organic solar cells (OSCs), reaching a power conversion efficiency above 15-17% [5,6]. To do so, the research was focused on the development of device architecture [7][8][9], design semiconductor polymers and development of non-fullerene acceptors (e.g.,2,2 -[[6,6,12,12-Tetrakis(4-hexylphenyl)-6,12dihydrodithieno [2,3- [10][11][12], controlmorphology [13,14] and interfacial engineering between layer materials and the active layer that provides a powerful strategy to enhance both stability and efficiency of organic photovoltaic devices. Generally, the structure of OSCs Smartlab®diffractometer (9 kW) equipped with a CuKα 1 incident source (λ = 1.54056 Å) and a Ge (220) × 2-bounce front monochromator. Morphological analysis was carried out by scanning electron microscopy (SEM) on a Zeiss GEMINI 500 scanning electron microscope. A UV-Visible-NIR spectrophotometer (Perkin-Elmer Lambda 950) with an integrating sphere of 150 mm was used to obtain the transmission/absorption spectra of the films in the wavelength range of 200-1000 nm. Atomic force microscopy (AFM) images were obtained in tapping mode with a Nanoscope Dimension 3100 from Veeco Instruments. Scanning electron microscopy (SEM) images were recorded by JEOL 6700F microscope. The resistivity, carrier concentration and Hall mobility of the SnO 2 films were determined by a Hall effect measurement system at room temperature using ECOPIA equipment (HMS-5500 Van der Pauw). The electrical characterizations of the inverted devices were done inside a glovebox under a nitrogen atmosphere; the current density-voltage (J-V) characteristics were measured with an HP Agilent source measurement unit, in dark and under illumination using a solar simulator with AM1.5G conditions (Oriel Xenon 150 W, model 96000), after calibration using a standard silicon solar cell with 100 mW/cm 2 . The external quantum efficiency (EQE) was also measured inside the glovebox for the elaborated devices.

SnO 2 Thin Film Deposition
The cleaning process of the pre-patterned ITO (sheet resistance 5-15 Ω, Lumtec) glass substrates was done in an ultrasonic bath successively in detergent, deionized water, acetone, 2-isopropanol and de-ionized water, then dried with nitrogen gas flow followed by UV-ozone treatment for 15 min. SnO 2 layers were deposited onto clean ITO at 200 • C by radio frequency (RF) reactive magnetron sputtering in a commercial sputtering system (Orion 3 device from AJA International Co.) using pure target of Sn (99.99%). The SnO 2 sputtering was done in an argon-oxygen gas mixture, with 8 sccm argon and 4 sccm oxygen. The sputtering parameters used in this work were optimized in a previous work [37,38], and the deposition time was varied to deposit different thicknesses of the SnO 2 layer: 10, 20, 30, 40 and 60 nm.

PEIE Interlayer Fabrication
PEIE (Mw = 70,000 g/mol, Sigma-Aldrich), an 80% ethoxylated-based solution, was prepared by diluting 0.2% by mass in 2-methoxyethanol (99.8%, Sigma-Aldrich), which was deposited onto ITO/SnO 2 substrates by a spin-coater at 1500 rpm for 60s. Afterward, a part of the PEIE was wiped away from the borders with ethanol. Then, the prepared structures were placed inside the nitrogen-filled glove box for further treatment and depositions to complete the device structure; the samples were annealed at 100 • C for 15 min before depositing the active layer.

Inverted Device Elaboration
iOSCs were fabricated using P3HT (98.5% regioregular, Sigma-Aldrich) as an electron donor and PC71BM (99.5%, Solenne B.V.) as an electron acceptor. The photo-active layer was prepared based on P3HT:PC71BM solution with a ratio of 1:0.7 dispersed in 1 mL o-dichlorobenzene (99%, anhydrous, Sigma-Aldrich). The absorber layer was deposited onto the previous prepared films using a spin-coater with 1200 rpm for 60s and then annealed at 150 • C for 15 min. Next, MoO 3 (7 nm) and Ag (120 nm) were thermally evaporated in a vacuum of 1.10-6 Torr, acting as inverted device electrodes. We fabricated a series of inverted PSC devices with various SnO 2 thicknesses of PEIE with the following structure: glass/ITO/SnO 2 /P3HT:PCBM/MoO 3 /Ag; another kind of device was also fabricated by incorporating an ultrathin PEIE layer with structure glass/ITO/SnO 2 /PEIE/P3HT:PCBM/MoO 3 /Ag. The final architecture The device configuration, chemical structure and energy level diagram of each component are given in Figure 1a,b. Tin dioxide has conduction band and valence band energies around −7.95 eV and −4.01 eV [39], respectively, which suggests that electrons from the active layer can be transported into SnO2, while holes from the active layer can be blocked. At the same time PEIE contains simple aliphatic amine groups that can produce surface dipoles between the cathode and the active layer. Consequently, the work-function of the ETL is significantly reduced beyond the lowest unoccupied molecular orbital (LUMO) value of PCBM. The PEIE layer acts as a stepping stone, which decreases the energy barrier at the interface of the ITO/SnO2 layer and P3HT:PCBM. The electrical contact between the active layer and the ETL shift from a Schottky contact to an ohmic contact. The alignments of the energy levels facilitate electron transport and reduce the energy loss in the process of charge separation, contributing to the enhancement of the short-circuit density (Jsc) and the opencircuit voltage (Voc). Therefore, SnO2 has a relatively wider band gap and lower valence band position, which blocks holes and thus avoids undesirable carrier recombination during the electron transport from the photoactive layers to the ITO cathode.

X-ray Diffraction Study
The structural properties of SnO2 and P3HT thin films are depicted in Figure 2. The XRD pattern of SnO2 with 100 nm revealed peaks at 26.66°, 33.89°, 37.94°, 51.77° and 54.75° assigned respectively to planes (110), (101), (200), (211) and (220) of crystalline tin-dioxide tetragonal rutile structure with space group P42/mnm, which matched well with JCPDS card (Joint Committee on Powder Diffraction Standards) no 00-041-1445 and exhibited (101) plane as the preferred orientation. The XRD pattern of P3HT demonstrated a high crystalline structure, with peaks located at low angles 5.3°, 10.7° and 15.90°, which were attributed respectively to planes (100), (200) and (300), with a preferential orientation toward the (100) plane; these peaks were related to lamellar ordering of the phase separated structure by alkyl side chains [40,41]. The device configuration, chemical structure and energy level diagram of each component are given in Figure 1a,b. Tin dioxide has conduction band and valence band energies around −7.95 eV and −4.01 eV [39], respectively, which suggests that electrons from the active layer can be transported into SnO 2 , while holes from the active layer can be blocked. At the same time PEIE contains simple aliphatic amine groups that can produce surface dipoles between the cathode and the active layer. Consequently, the work-function of the ETL is significantly reduced beyond the lowest unoccupied molecular orbital (LUMO) value of PCBM. The PEIE layer acts as a stepping stone, which decreases the energy barrier at the interface of the ITO/SnO 2 layer and P3HT:PCBM. The electrical contact between the active layer and the ETL shift from a Schottky contact to an ohmic contact. The alignments of the energy levels facilitate electron transport and reduce the energy loss in the process of charge separation, contributing to the enhancement of the short-circuit density (J sc ) and the open-circuit voltage (V oc ). Therefore, SnO 2 has a relatively wider band gap and lower valence band position, which blocks holes and thus avoids undesirable carrier recombination during the electron transport from the photoactive layers to the ITO cathode.

X-ray Diffraction Study
The structural properties of SnO 2 and P3HT thin films are depicted in Figure 2 which were attributed respectively to planes (100), (200) and (300), with a preferential orientation toward the (100) plane; these peaks were related to lamellar ordering of the phase separated structure by alkyl side chains [40,41].

UV-Visible Study
To investigate the transmittance of SnO2 layers as well as photoactive layer absorption, a UVvisible study was conducted on our samples. Figure 3a presents the absorption spectra of P3HT, PCBM and the P3HT:PCBM mixture. The conjugated polymer P3HT had a wide and strong absorption band in the range of 400-650 nm and revealed a maximum at ~550 nm. Nevertheless, PCBM presented a broad absorption in ultraviolet in the range of 300-575 nm with a maximum absorption at ~375 nm. One can see that the optical absorption spectra of P3HT:PCBM (1:0.7)-based thin film exhibited a first absorption shoulder at 300-350 nm attributed to PCBM and a second dominant peak located at higher wavelengths corresponding to P3HT absorption. The peak at 450 nm (2.75 eV) corresponded to band-to-band transitions inferred from π-π* transitions from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) [42], and this peak was located at a lower photon energy of 2.25 eV for pure pristine P3HT films. The absorption shoulders located at ~600 nm (2.07 eV), ~550 nm (2.25 eV) and ~515 nm (2.41 eV) revealed the three absorption features (S0, S1, S2) of P3HT excitonic absorption with Frenkel excitons absorption; these bands were the so-called vibronic absorption shoulders [43,44]. The excitonic absorption S0 at 600 nm indicated singlet exciton generation; the absorption S1 at 558 nm implied one exciton and one phonon generation. The peak S2 at 509 nm revealed the beginning of one exciton and two phonons.
The optical transmittance spectra of SnO2 thin films with different thicknesses were measured in the wavelength range of 200-1000 nm and are depicted in Figure 3b. One can noticed that, in the visible range 380-780 nm, the transmittance of the SnO2 buffer layer was found to decrease gradually from 87 to 75.1% with increasing SnO2 thickness from 10 to 60 nm. The average transmittance value of ~82% in the visible range was recorded for the prepared SnO2 samples.

UV-Visible Study
To investigate the transmittance of SnO 2 layers as well as photoactive layer absorption, a UV-visible study was conducted on our samples. Figure 3a presents the absorption spectra of P3HT, PCBM and the P3HT:PCBM mixture. The conjugated polymer P3HT had a wide and strong absorption band in the range of 400-650 nm and revealed a maximum at~550 nm. Nevertheless, PCBM presented a broad absorption in ultraviolet in the range of 300-575 nm with a maximum absorption at~375 nm. One can see that the optical absorption spectra of P3HT:PCBM (1:0.7)-based thin film exhibited a first absorption shoulder at 300-350 nm attributed to PCBM and a second dominant peak located at higher wavelengths corresponding to P3HT absorption. The peak at 450 nm (2.75 eV) corresponded to band-to-band transitions inferred from π-π* transitions from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) [42], and this peak was located at a lower photon energy of 2.25 eV for pure pristine P3HT films. The absorption shoulders located at~600 nm (2.07 eV),~550 nm (2.25 eV) and 515 nm (2.41 eV) revealed the three absorption features (S0, S1, S2) of P3HT excitonic absorption with Frenkel excitons absorption; these bands were the so-called vibronic absorption shoulders [43,44]. The excitonic absorption S0 at 600 nm indicated singlet exciton generation; the absorption S1 at 558 nm implied one exciton and one phonon generation. The peak S2 at 509 nm revealed the beginning of one exciton and two phonons.

UV-Visible Study
To investigate the transmittance of SnO2 layers as well as photoactive layer absorption, a UVvisible study was conducted on our samples. Figure 3a presents the absorption spectra of P3HT, PCBM and the P3HT:PCBM mixture. The conjugated polymer P3HT had a wide and strong absorption band in the range of 400-650 nm and revealed a maximum at ~550 nm. Nevertheless, PCBM presented a broad absorption in ultraviolet in the range of 300-575 nm with a maximum absorption at ~375 nm. One can see that the optical absorption spectra of P3HT:PCBM (1:0.7)-based thin film exhibited a first absorption shoulder at 300-350 nm attributed to PCBM and a second dominant peak located at higher wavelengths corresponding to P3HT absorption. The peak at 450 nm (2.75 eV) corresponded to band-to-band transitions inferred from π-π* transitions from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) [42], and this peak was located at a lower photon energy of 2.25 eV for pure pristine P3HT films. The absorption shoulders located at ~600 nm (2.07 eV), ~550 nm (2.25 eV) and ~515 nm (2.41 eV) revealed the three absorption features (S0, S1, S2) of P3HT excitonic absorption with Frenkel excitons absorption; these bands were the so-called vibronic absorption shoulders [43,44]. The excitonic absorption S0 at 600 nm indicated singlet exciton generation; the absorption S1 at 558 nm implied one exciton and one phonon generation. The peak S2 at 509 nm revealed the beginning of one exciton and two phonons.
The optical transmittance spectra of SnO2 thin films with different thicknesses were measured in the wavelength range of 200-1000 nm and are depicted in Figure 3b. One can noticed that, in the visible range 380-780 nm, the transmittance of the SnO2 buffer layer was found to decrease gradually from 87 to 75.1% with increasing SnO2 thickness from 10 to 60 nm. The average transmittance value of ~82% in the visible range was recorded for the prepared SnO2 samples.  The optical transmittance spectra of SnO 2 thin films with different thicknesses were measured in the wavelength range of 200-1000 nm and are depicted in Figure 3b. One can noticed that, in the visible range 380-780 nm, the transmittance of the SnO 2 buffer layer was found to decrease gradually from 87 to 75.1% with increasing SnO 2 thickness from 10 to 60 nm. The average transmittance value of 82% in the visible range was recorded for the prepared SnO 2 samples.

SEM and AFM Micrographs
In order to study the thickness effect on SnO 2 surface morphology and topography as well as the film's roughness, AFM and SEM images were recorded for different film thicknesses with and without PEIE on the ITO substrate, as illustrated in Figure 4. In order to study the thickness effect on SnO2 surface morphology and topography as well as the film's roughness, AFM and SEM images were recorded for different film thicknesses with and without PEIE on the ITO substrate, as illustrated in Figure 4. One can see that the SnO2 film surface changed with thickness, and the surface morphology was rather homogeneous with surface RMS roughness (root mean square roughness) of 1.92, 2.05, 2.12, 2.67 and 4.45 nm, with an approximate error of 1%, for SnO2 films with thicknesses of 10, 20, 30, 40 and 60 nm, respectively without an ultrathin layer of PEIE. As can be seen from the AFM and SEM micrographs images, continuous films were formed on ITO substrates, and the nucleation process was completed and nucleus growth had already preceded. After PEIE deposition onto SnO2, AFM and SEM micrographs showed no agglomeration of PEIE on the surface and no significant changes compared to samples without PEIE.

Hall Effect Study
Hall effect measurement was performed to investigate electrical properties of SnO2 films with One can see that the SnO 2 film surface changed with thickness, and the surface morphology was rather homogeneous with surface RMS roughness (root mean square roughness) of 1.92, 2.05, 2.12, 2.67 and 4.45 nm, with an approximate error of 1%, for SnO 2 films with thicknesses of 10, 20, 30, 40 and 60 nm, respectively without an ultrathin layer of PEIE. As can be seen from the AFM and SEM micrographs images, continuous films were formed on ITO substrates, and the nucleation process was completed and nucleus growth had already preceded. After PEIE deposition onto SnO 2 , AFM and SEM micrographs showed no agglomeration of PEIE on the surface and no significant changes compared to samples without PEIE.

Hall Effect Study
Hall effect measurement was performed to investigate electrical properties of SnO 2 films with different thicknesses. The obtained parameters of bulk concentration (n), resistivity (ρ) and mobility (µ) are listed in Table 1. The electrical properties confirmed the n-type character of SnO 2 ; moreover, the resistivity increased with SnO 2 thickness, and the charge carrier concentration varied with the thickness and presented a maximum value at 20 nm.

XRD and GIXRD Study
The effect of PEIE at the SnO 2 /P3HT:PCBM interface was investigated through various structural, optical and electrical analyses in order to understand the buffer bilayer effect on inverted solar cell performance. XRD and GIXRD patterns of the prepared solar cells were based on the following structure: glass/ITO/SnO 2 /PEIE/P3HT:PCBM, as depicted in Figure 5a,b. Figure 5a reports the XRD patterns of the prepared solar cells with different SnO 2 thicknesses (10,20,30,40, and 60 nm). One can notice that the patterns demonstrated diffraction peaks corresponding to ITO, SnO 2 and P3HT, suggesting a well-defined crystal structure of these materials. Diffraction peaks identifying polycrystalline structure of ITO were attributed to (211), (400) and (411) planes. Comparing the P3HT:PCBM blend diffractograms with that of pure P3HT (Figure 2), it was evidenced that the blend was characterized by (100), (200) and (300), which were attributed to the P3HT crystalline structure; the diffraction peak (100) at 2θ = 5.3 • corresponded to the interchain spacing of 16.32 Å associated with inter-digitated alkyl [44]. The SnO 2 diffraction pattern was dominated by the (110) peak positioned at 2θ~26.52 • regardless of the SnO 2 thickness. This peak, for all prepared devices with different SnO 2 thicknesses, was an indication of the well-textured growth of SnO 2 by sputtering [45].
The elaborated devices were characterized by XRD and GIRXD using an incident angle of 0.5 • (Figure 5b) to localize all the peaks attributed to SnO 2 ; the main peak of the latter was positioned at 2θ~26.52 • and was attributed to the (110) plane; these patterns exhibited another weak peak at 2θ~25 • that belonged to the SnOx orthorhombic phase (in accordance with JCPDS card no. 00-013-0111). Moreover, the GIXRD patterns revealed an increase in the peak intensity corresponding to the SnO 2 tetragonal phase by increasing SnO 2 thickness from 10 to 60 nm, which became sharper with a slight shift with respect to the JCPDS card; therefore, the crystallinity improved as well. This indicates that SnO 2 surface morphology depends on SnO 2 thickness, which was also proven by SEM and AFM images.
2θ~26.52° and was attributed to the (110) plane; these patterns exhibited another weak peak at 2θ~25° that belonged to the SnOx orthorhombic phase (in accordance with JCPDS card no. 00-013-0111). Moreover, the GIXRD patterns revealed an increase in the peak intensity corresponding to the SnO2 tetragonal phase by increasing SnO2 thickness from 10 to 60 nm, which became sharper with a slight shift with respect to the JCPDS card; therefore, the crystallinity improved as well. This indicates that SnO2 surface morphology depends on SnO2 thickness, which was also proven by SEM and AFM images.

J-V Characterization and EQE Measurements
The metrics parameters of the elaborated inverted organic solar cells with and without PEIE were extracted from the current density-voltage (J-V) characteristics under illumination using AM1.5G with an irradiance of 100 mW/cm 2 . The determined parameters are summarized in Table 2, displaying short-circuit current density (J SC ), open-circuit voltage (V OC ), fill factor (FF) and power conversion efficiency (PCE). The J-V characteristics of glass/ITO/SnO 2 /P3HT:PCBM/MoO 3 /Ag inverted solar cells based on SnO 2 exhibited lower efficiencies (see Figure 6a). These low efficiencies were due to the high work function (WF) of SnO 2 that prevented charge carrier mobility, which interrupted charge transfer from the active layer to SnO 2 and/or charge trapping by interfacial ITO/SnO 2 and SnO 2 /P3HT:PCBM. In order to enhance the performance of these devices, the surface modifier PEIE was introduced on top of SnO 2 to elaborate a cathode bilayer device with the configuration ITO/SnO 2 /PEIE/P3HT:PCBM/MoO 3 /Ag while maintaining a constant thickness of PEIE (≈ 10 nm) and changing SnO 2 thickness (10,20,30,40 and 60 nm).
The J-V characteristics for the elaborated devices with and without PEIE are depicted in Figure 6. The inverted organic solar cells with an SnO 2 /PEIE cathode bilayer demonstrated better performance compared to those without PEIE; the device with 20 nm thickness of SnO 2 showed the highest performance with a short-circuit current density J SC = 7.86 mA.cm −2 , an open-circuit voltage Voc = 570 mV, a fill factor FF = 63.5% and a power conversion efficiency PCE = 2.84%. The use of SnO 2 /PEIE as a cathode bilayer increased the device parameters compared to those obtained for devices with a single PEIE or SnO 2 cathode layer only; the extracted photovoltaic parameters of the elaborated cells are presented in Table 2. The incorporation of the PEIE layer improved the interface quality as well as enhanced charge collection ability, which revealed an improvement of the photovoltaic performance [46,47]. The difference in device performance, with different electron transport layers, was mostly due to the increase or decrease in the short circuit current densities, which were consistent with external quantum efficiency (EQE) measurements. The EQE is defined as number of charges (Ne) extracted at the electrodes divided by the number of photons (Nph) of a certain incident wavelength on the solar cell.
The external quantum efficiency and internal quantum efficiency spectra of the best 20 nm device of SnO2 with/without the PEIE layer are depicted in Figure 7a,b. These spectra exhibited broad and high spectral responses ranging from 400 to 600 nm. The best device with SnO2 (20 nm)/PEIE exhibited a maximum of 60% and 64% for EQE and IQE, respectively, around 550 nm. The device with only SnO2 showed maxima of 55% and 53% for EQE and IQE, respectively, around 550 nm. The best performing device with combined layers of SnO2 (20 nm) and PEIE was attributed to the wellaligned dipoles of PEIE on top of the SnO2 layer.

Conclusion
In this work, we demonstrated an efficiency enhancement of inverted organic solar cells based on heterojunction by combining aqueous solution processed PEIE and sputtered SnO2 as a cathode EQE(λ) = Ne Nph(λ) The difference in device performance, with different electron transport layers, was mostly due to the increase or decrease in the short circuit current densities, which were consistent with external quantum efficiency (EQE) measurements. The EQE is defined as number of charges (Ne) extracted at the electrodes divided by the number of photons (Nph) of a certain incident wavelength on the solar cell.
The external quantum efficiency and internal quantum efficiency spectra of the best 20 nm device of SnO 2 with/without the PEIE layer are depicted in Figure 7a,b. These spectra exhibited broad and high spectral responses ranging from 400 to 600 nm. The best device with SnO 2 (20 nm)/PEIE exhibited a maximum of 60% and 64% for EQE and IQE, respectively, around 550 nm. The device with only SnO 2 showed maxima of 55% and 53% for EQE and IQE, respectively, around 550 nm. The best performing device with combined layers of SnO 2 (20 nm) and PEIE was attributed to the well-aligned dipoles of PEIE on top of the SnO 2 layer. The difference in device performance, with different electron transport layers, was mostly due to the increase or decrease in the short circuit current densities, which were consistent with external quantum efficiency (EQE) measurements. The EQE is defined as number of charges (Ne) extracted at the electrodes divided by the number of photons (Nph) of a certain incident wavelength on the solar cell.
The external quantum efficiency and internal quantum efficiency spectra of the best 20 nm device of SnO2 with/without the PEIE layer are depicted in Figure 7a,b. These spectra exhibited broad and high spectral responses ranging from 400 to 600 nm. The best device with SnO2 (20 nm)/PEIE exhibited a maximum of 60% and 64% for EQE and IQE, respectively, around 550 nm. The device with only SnO2 showed maxima of 55% and 53% for EQE and IQE, respectively, around 550 nm. The best performing device with combined layers of SnO2 (20 nm) and PEIE was attributed to the wellaligned dipoles of PEIE on top of the SnO2 layer.

Conclusion
In this work, we demonstrated an efficiency enhancement of inverted organic solar cells based on heterojunction by combining aqueous solution processed PEIE and sputtered SnO2 as a cathode EQE(λ) = Ne Nph(λ)

Conclusions
In this work, we demonstrated an efficiency enhancement of inverted organic solar cells based on heterojunction by combining aqueous solution processed PEIE and sputtered SnO 2 as a cathode buffer bilayer by overcoming the limitations using a single cathode buffer layer and tuning the work function of ITO. The PEIE surface modifier within the sputtered SnO 2 layer is a promising alternative buffer bilayer to maximize the percolation of electron charge carriers by the formation of a thin interfacial dipole between the electron-transporting layer ITO/SnO 2 and the LUMO of PCBM. This facilitates electron transport to the electrode by lowering the energy barrier between the active layer and the ITO/SnO 2 interface, and by suppressing trap-assisted recombination, as well as improving the wettability between these layers, which leads to increases in the short-circuit current density and the fill factor. The optimization of SnO 2 deposition parameters led to a highly efficient device using the following structure: ITO/SnO 2 (20nm)/PEIE/P3HT:PCBM/MoO 3 /Ag. An increase from 0.08% for a single buffer layer up to 2.84% using the SnO 2 (20 nm)/PEIE cathode buffer bilayer was obtained.