Optimization and Development of ITO-Free Plasmonic Gold Nanoparticles Assisted Inverted Organic Solar Cells

This paper focuses on the fabrication of an ITO-free plasmonic assisted inverted organic solar cell (OSC) constituting aluminium doped zinc oxide (AZO) as front cathode and ultraviolet (UV) ltering layer. The gold nanoowers are introduced in the device to increase the eciency using localized surface plasmon resonance (LSPR) shown by plasmonic nanoparticles. We used GPVDM software to rst optimize the cell, based on the geometry AZO/ZnO/PTB7:PC71BM/MoO3/Ag where AZO acts as the transparent conducting oxide (TCO) cathode and UV lter, zinc oxide (ZnO) behaves as the electron transport layer (ETL), Thieno[3,4 b]thiophene-alt-benzodithiophene: [6,6]-phenyl C71 butyric acid methyl ester (PTB7: PC71BM) mixture as the active layer, molybdenum trioxide (MoO3) as the hole transport layer (HTL) and silver (Ag) serves as the anode layer. By modelling, we nd that the optimized device with maximum power conversion eciency (PCE) includes 10 nm thick HTL, 200 nm thick photoactive layer and ETL thickness of 30 nm. Using the optimized thicknesses, we have fabricated three structurally identical inverted OSCs: rst having AZO as the front cathode (AZO based device); second with ITO as the front cathode (ITO based control device); third includes AZO as cathode and plasmonic gold nanoowers embedded inside the active layer (plasmonic assisted AZO based device). The AZO based device exhibited the PCE value of 6.19%, slightly less than the eciency of 6.83% for ITO based control device. However, a remarkable increase in the lifetime was achieved for AZO based device under UV assisted acceleration ageing test. The stability enhancement of AZO based device is because of the UV ltering properties of AZO which prevent degradation in the device due to UV exposure. Also, the PCE of AZO based device was further enhanced to 7.01% when plasmonic gold nanoparticles were included in the active layer. This work provides a feasible way to develop an ITO free plasmonic assisted inverted organic solar cell to achieve cost-effectiveness, high eciency and stability.


Introduction
Sun is the most powerful and sustainable energy source available on the planet. Even though the earth receives only a small part of the sun's enormous energy, an hour of solar energy is equivalent to the energy consumed by humans in an entire year [1]. Photovoltaic (PV) devices exhibit direct and e cient solar energy to electrical energy conversion. Organic solar cells (OSCs) have enormous potential in solar energy conversion technology because of interesting associated properties like economical large scale production, greater mechanical exibility, dynamic structural variation etc [2,3]. Also, OSC modules are thin, lightweight and exible that can be installed easily on any regular structure like buildings or irregularly shaped structures like tents or even fabrics [4,5]. Though the eld of OSCs is in substantial progress, there are hurdles to boost its commercial realization, like low power conversion e ciency, less lifetime and inability for large scale production [6]. One of the prominent problems is the exploitation of indium tin oxide (ITO) as a cathode material. Due to the scarcity of indium, a multifold price rise is there for indium which has implicitly led to the increased cost of ITO based OSCs [7,8]. AZO is a potential substitute for ITO because of its concomitant properties like high electrical conductivity, good optical transmission and economical cost [9,10]. In addition, AZO has a bandgap around 3.26 eV due to which it can e ciently block UV rays and restrict the device from degradation. There are reports on developing OSCs with inverted structures, having e ciencies comparable to regular structure OSCs, with a lot of scope for stability improvement and the ability to result in roll-to-roll mass production [11,12].
Nowadays, bulk heterojunction (BHJ) OSCs utilizing polymer-fullerene blend show great potential in photovoltaic devices by virtue of their solution-processable and e cient exciton dissociation properties.
There have been tremendous efforts by researchers all over the world to enhance the PCE of the OSCs by employing various strategies like using novel active and interfacial materials, varying device architecture, introducing plasmonic nanoparticles, using more e cient device fabrication techniques etc. [13]. The PCE mainly depends upon the absorption of photons inside the photoactive layer. To increase the photon absorption, the active layer thickness should be high. However, a highly thick active layer results in increased device resistance and more exciton recombination because of the low mobility of carriers and low diffusion length of exciton inside the active layer. Thus, it is imperative to use some other lighttrapping techniques to enhance photon absorption inside the active layer while keeping the optimum thickness. There are reports on the introduction of noble metallic nanoparticles into organic photovoltaic devices to enhance light absorption through LSPR shown by plasmonic nanoparticles [14]. Here we report a 13% enhancement in PCE by adding gold nano owers inside the photoactive layer of the AZO based inverted OSC. The incorporation of gold nano owers exhibit the advantages of light absorption enhancement over a wide range, increased generation rate of exciton, higher exciton dissociation e ciency and charge carrier density enhancement which result in higher performance of the device [15][16][17].
We rst theoretically investigated the effect of variation in ETL, active layer and HTL thickness on PCE and other critical parameters of OSC and optimized the same [18]. Then subsequently, three devices with the optimized thickness (AZO based device, ITO based control device, plasmonic assisted AZO based device) were fabricated and parameters like V OC , J SC , P max, FF and PCE were tabulated from the current density-voltage (J-V) measurements. The goal of the present study is to fabricate cost-effective, highly e cient and stable ITO free plasmonic assisted inverted OSC by replacing expensive ITO with economical aluminium doped zinc oxide (AZO) and introducing gold nano owers inside the active layer.
The bilayer geometry consisting of ZnO and AZO is utilized for better extraction of electrons [19][20][21].
Fabrication of AZO based plasmonic OSC is important for economical viability, better performance and stability enhancement, as it will drastically trim the cost of production, increase e ciency and enhance the lifetime of the device [22].

Device Modelling
The ITO free inverted OSC geometry is depicted in Fig. 1(a). The OSC consists of ve layers viz:-AZO (transparent bottom cathode), ZnO (ETL), PTB7:PC 71 BM blend (active layer), MoO 3 (HTL), Ag (top anode) [23,24]. The energy levels of the different layers of the OSC are depicted in Fig. 1(b). All the simulations Page 4/16 are done using General Purpose Photovoltaic Device Model (GPVDM) solar simulator software [25]. We have investigated the electrical properties of the device through the GPVDM simulations. GPVDM solves poisson's, drift-diffusion and continuity equations for both electrons and holes. The drift-diffusion equations are solved in coordinate space using the nite difference method to explain the ow of charge inside the device [26]. This simulation software solves the carrier trapping and escape equations in energy space corresponding to each mesh point in coordinate space. Therefore, GPVDM resolves the carrier density in both position and energy space. Shockley-Read-Hall (SRH) recombination is utilized to explain the recombination of the carrier in the 1D and time domain [27].
The drift-diffusion equations (electrons and holes) are solved in position space to explain charge carrier transport.
The continuity equations (electrons and holes) are used to preserve the conservation of charge carriers.
where R = net recombination rate per unit volume and G = net generation rate per unit volume.
The internal potential distribution inside the cell is calculated using poisson's equation.
The incident photon gets absorbed in the active material where it generates an exciton in the donor (PTB7). The diffusion of exciton takes place within the donor to the interface of Donor/Acceptor and gets dissociated into free electron and hole which are nally collected at their respective electrodes. The photocurrent is produced by connecting a load to the external circuit. Through simulations, variation in thickness of HTL, ETL and active layer is investigated and optimized for maximum PCE from the device.

Gold Nano owers Synthesis
To synthesise gold nano owers, 0.01 M gold salt (100 µl) was added into 20 mM ice-cold L-AA (35 ml) placed over a magnetic stirrer. The reaction was maintained at a constant temperature of 5 ˚C throughout. When gold salt was added, the colourless solution rst turned purple and then changed to faint blue which con rmed the formation of gold nano owers. In this procedure, Au nano owers were synthesized by the self-assembly process in which nanoparticles with identical surface arrangements approach one another and combine together by oriented attachment. The gold ions present in the aqueous solution of hydrogen tetrachloroaurate (III) trihydrate salt are reduced by the ascorbate ions and stimulate nucleation for the synthesis of Au nano owers. Finally, these nanoparticles turned into nano owers through ostwald ripening process.

Device Fabrication
AZO cathode was sputtered on the pre-cleaned glass substrate utilizing rf sputtering. It was then patterned through the laser scriber. The cleaning of the patterned AZO glass (sheet resistance around 20 Ω/sq) was done rst in detergent, then in water, acetone and 2-propanol under sonication and nally dried using an oven [28]. Spin coating of a ZnO layer (~30 nm) was done over AZO sputtered glass substrate at 3500 rpm and subsequently annealed at 200 ˚C for 30 min [29]. The PTB7:PC 71 BM photoactive layer (~200 nm) having a blend ratio 1:1.5 was deposited over ZnO through spin-coating the solution containing chlorobenzene solvent (25 mg/ml) [30]. A 10 nm thick MoO 3 (Molybdenum trioxide) lm was thermally deposited over the photoactive layer [31]. Finally, the Ag anode of around 100 nm thickness was thermally coated in a high vacuum (~8×10 -6 mbar) using a shadow mask. This completes our rst device (AZO based device). Similarly, a second device (ITO based control device) with an identical structure based on ITO window cathode was also developed for comparison purposes. Finally, a third device (plasmonic assisted AZO based device) was developed using AZO as front cathode and introducing gold nano owers in the active layer blend. All these fabricated devices had an active area ~ 4 mm 2 .

Characterization
The absorption spectrum of the gold nano owers was recorded using EVOLUTION 300 UV-VIS (Thermo Scienti c). Transmission electron microscopy (TEM) (JEOL-2100F, Japan) was performed to examine the shape and size of the gold nano owers. The X-ray diffraction data have been collected with a PANalytical diffractometer (EMPYREAN). The current-voltage (I-V) characteristic curve for the AZO lm was recorded using the Keithley I-V setup. The optical spectra in transmission mode for ITO and AZO cathode while in absorbance mode for active layer (PTB7:PC 71 BM) were carried out using EVOLUTION 300 UV-VIS (Thermo Scienti c). Raman mapping is done for AZO lms, to con rm the uniformity, using a 514.5 nm argon ion laser excitation. The J-V curves of the OSCs were recorded through the source meter (Keithley 2450) with devices illuminated under AM1.5G (100 mW/cm 2 ) solar simulator. The external quantum e ciency (EQE) measurements were recorded with the help of a Tunable Light Source instrument (Newport) equipped with a standard silicon diode. The UV assisted acceleration ageing test was performed using a mercury vapour lamp having an intensity of 90 mW/cm 2 .

Device Optimization Through Simulations
Optimization of Active Layer:-The thickness of the photoactive layer affects the incident light absorption and hence exciton generation in OSC, therefore its optimization is very crucial. To improve the PCE of the inverted OSC, the active layer was varied between 100-300 nm thickness [32]. and electron a nity similar to AZO is the most suitable material for ETL. Zinc oxide (ZnO) is chosen as ETL and its thickness is varied between 10-50 nm. Fig. 3 depicts the observed J-V characteristics and the electrical parameters extracted. The variations of J sc , P max and PCE show a similar trend with the increase in ETL thickness. Also, the increase in ETL thickness has only minimal effect on all the parameters in our simulated device. The V oc and FF remain almost constant with variations in ETL thickness [33]. Overall, it can be concluded that ETL thickness slightly affects the performance parameters of the OSC and the optimized thickness for ETL observed to be 30 nm.
Optimization of HTL:-From the simulations results of the preceding sections, the thickness of ETL and active layer is optimized to 30 nm and 200 nm respectively. In this section, the variation of HTL thickness is investigated through simulations. MoO 3 is chosen as HTL due to its concomitant properties like better transparency, high conductivity and better hole transportation [34]. The MoO 3 thickness is varied between 5-15 nm. The J-V characteristic curves and other parameters obtained through the simulations are drawn in Fig. 4 (a) and Fig. 4 (b) respectively. As seen in Fig. 4(b), the variation in HTL thickness has no prominent effect on FF and V OC , which is due to the low hole mobility. The optimized inverted organic solar cell is obtained at 10 nm thickness of HTL. J sc , P max and PCE slightly vary with variation in the thickness of HTL and exhibit the optimum values at 10 nm. FF and Voc remain almost insensitive to variation in thickness of HTL. Consequently, the thickness of HTL has a minor effect on electrical parameters of simulated inverted OSC due to the low hole mobility [35].

Cathode Investigation
As the aim of this work is to deposit AZO as TCO having comparable characteristics to ITO, we started with deposition and characterization of AZO rst. The AZO lm over glass substrate was deposited using rf sputtering and the AZO lm thickness was determined to be 800 nm with the help of Dektak pro lometer. The XRD pattern for the AZO cathode lm is shown in Fig. 5 (a). The AZO crystallites exhibit a hexagonal wurtzite structure (ICDD-00-051-0037). The spectra show a major peak at 62.85˚ re ecting the lateral growth along the crystallographic axis (103). Other peaks at 34.28˚, 36.18˚, 47.39˚ and 56.41ẘ ith small intensity are detected at axes (002), (101), (102) and (110), respectively. The (002) plane is related to vertical growth, however, the presence of a relatively intense peak along (103) is indicative of non-vertical growth. Another critical parameter of TCO is the low sheet resistance, which corresponds to better conductivity. The sheet resistance of AZO lm was calculated using a four-point probe. Fig. 5 (b) shows the room temperature I-V curve, with sheet resistance around 20 Ω/sq which approaches the sheet resistance of ITO electrode 15 Ω/sq.
The UV-Vis spectra in transmission mode for AZO and ITO lms were measured and the wavelength dependent transmittance for both the lms are plotted Fig. 5(c). It is evident from the plot that AZO lm has an average transmittance of around 80% in the visible region (400 nm to 800 nm), comparable to ITO/glass. Also, the transmittance curve of AZO depicts a cut off at the wavelength ~ 380 nm which is due to the UV absorption properties of AZO (bandgap 3.26 eV). Although AZO blocks the UV part of the solar spectrum, most of the sunlight passes through it as the major portion of the solar radiation lies in the spectrum having λ > 380 nm, hence enabling e cient absorption in the device. This UV ltering capacity of the AZO lm enhances the stability of the device by preventing device degradation due to UV exposure. Uniformity is a prerequisite for the cathode of a solar cell. To ensure the uniformity in AZO cathode, raman mapping was done at various locations over the lm. doped zinc oxide has demonstrated electrical and optical properties that are comparable to ITO, it can effectively replace ITO to achieve cost-effective and stable photovoltaic devices.
Gold Nano owers Characterization E cient light absorption in inverted OSCs is important for their better performance and the inclusion of gold nano owers in the active layer serves the purpose without compromising their architecture. The incorporated plasmonic nanoparticles (5-50 nm) act like antennas and the incoming light energy is stored in LSPR modes resulting in multiple times enhancement of the local electromagnetic eld leading to enhanced light absorption. The increase in exciton dissociation is also possible through interactions between the plasmons and the excitons thereby lowering the recombination loss of excitons. The introduction of the gold nano owers inside the active layer improves the PCE of the OSCs using the optical property of LSPR without affecting their electrical characteristics. Gold is preferably used as it is highly resistive to oxidation and hence ensures a longer lifetime of the cell. The size and shape of nano owers were investigated using TEM apparatus. The TEM picture shown in Fig. 6(a) suggests the formation of gold nano owers. The average size for gold nano owers analyzed by ImageJ software was 33 nm. The core of the nano owers was around 25 nm with the length of each petal around 4 nm. The absorption spectrum for the gold nano owers is the convolution of two absorption peaks as seen from g. 6(b). There exist two absorption peaks within the visible regime, attributed to the surface plasmons in the gold nano owers. These nanoparticles show surface plasmon peaks at two different frequencies, the rst peak at 588 nm arises from the core of nano owers and the other peak at 748 nm is due to free electrons present in petals.

Device Characterization
The simulation results of section 4.1 provided the optimized thickness of ETL (ZnO), active layer (PTB7:PC 71 BM) and HTL (MoO 3 ) to be 30 nm, 200 nm and 10 nm respectively. Utilizing these optimized thicknesses, we fabricated the three devices:-(1) AZO based device. (2) ITO based control device. (3) plasmonic assisted AZO based device. The rst AZO based device is fabricated based on the device geometry AZO/ZnO/PTB7:PC 71 BM/MoO 3 /Ag. The second device contains ITO as a front cathode keeping the remaining structure identical and is used for comparison studies (control device). The third device consists of plasmonic gold nano owers (average size of 33 nm) embedded into the active layer and using AZO as the front transparent conducting cathode. The J-V measurements for all three fabricated inverted OSCs were recorded using 100 mW/cm 2 AM 1.5 G and it was observed that AZO based device showed a PCE of 6.19% which was slightly less than PCE 6.83% for the ITO based control device. Also, the PCE of the AZO based OSC was further enhanced to 7.01% when gold nano owers were embedded within the active layer of the OSC. The J-V characteristic curves for the three fabricated cells are shown in Fig. 7(a) and the extracted important parameters like PCE, FF, P max , V OC and J SC are included in Table 1. Also, the parameters extracted from the J-V curves of the experimentally fabricated AZO based inverted OSC matches well with the theoretically optimized device using GPVDM simulation which ensures the credibility of the simulation results. The EQE measurements for the experimentally fabricated cells are depicted in Fig. 7(b) and it is evident that the cell with AZO front transparent cathode shows a red-shift in the short wavelength region associated with cut off in transmittance (λ < 380 nm) for AZO cathode. The EQE curve for the AZO based device follows the absorbance in the visible portion of the solar spectrum indicating the non-contribution of the UV in the measured J SC . This can be attributed to the UV blocking property of AZO and is further augmented by the use of ZnO as ETL. It is interesting that AZO is acting as a transparent cathode and blocks the UV, which can increase the lifetime of the fabricated OSC.
Upon introduction of gold nano owers inside the active region, there is a considerable enhancement in EQE in the wavelength regime 400-780 nm for the AZO cathode based device. This enhancement in EQE is due to near eld enhancement shown by plasmonic gold nano owers which increases absorption inside the active layer. Fig. 7(c)  The OSC degradation due to UV exposure can be eliminated by restricting the UV portion from the solar spectrum to reach the photoactive layer and this can be effectively done by the AZO front cathode. It is clear that AZO lm has very low transmittance in the UV region showing its potential candidature as a UV ltering layer. The UV assisted acceleration ageing test was done to investigate the stability of the AZO based and ITO based inverted OSCs [37]. Both the devices with different front cathodes were illuminated under UV irradiation for an exposure time of 0 to 30 min from the cathode side. This test was performed inside the nitrogen lled glove box which neglects the possible degradation of OSC due to moisture and oxygen. Fig. 7 shows the change in device parameters (PCE, FF, V OC , J SC ) recorded for the AZO and ITO (control) based inverted OSC under UV exposure of 0 to 30 min. Though ITO based control device possesses slightly higher e ciency initially, a fast decrease in all important device parameters is observed from the graphs in Fig. 8