Morphology-driven photocurrent enhancement in PTB7/PC71BM bulk heterojunction solar cells via the use of ternary solvent processing blends

PTB7/PC₇₁BM bulk heterojunction solar cell devices with high photocurrents were fabricated through processing their active layers from a ternary solvent system. The active layers were deposited from solutions where chlorobenzene (CB) was used as the main solvent, DIO (3%) as the solvent additive and cyclohexanone (CHN), a solvent in which both the polymer and fullerene are poorly soluble. The morphology of the different active layers was characterized by atomic force microscopy (AFM) and grazing incidence X-ray scattering (GIXRS) while charge extraction (CE) and transient photovoltage (TPV) techniques were used to measure charge carrier recombination kinetics. It was found that this ternary solvent system affects the J-V characteristics to a significant extent, mainly affecting the J_SC and FF, in a trend where the J_SC is seen to increase with increasing ratio of CHN/CB, while the FF decreases concomitantly. This increase in J_SC has been correlated with an increase in crystallinity of the active layer and more specifically with an increase of the crystalline volume of the fullerene domains and the increase in the polymer crystallites size, while the decrease in FF was linked to non-optimal vertical segregation. Despite the drop in FF, the addition of CHN in the blend still leads to an overall increase in power conversion efficiency (PCE) with respect to the devices processed from pristine CB (6.86% vs. 7.31%).     

Improved mobility and lifetime of carrier for highly efficient ternary polymer solar cells based on TIPS-pentacene in PTB7: PC71BM

Highly efficient ternary polymer solar cells (T-PSCs) realized by the improved mobility and lifetime of carrier in PTB7: PC₇₁BM: TIPS-pentacene blends were fabricated. By adjusting the weight ratios of third component TIPS-pentacene in the binary PTB7: PC₇₁BM blends, we found that the short circuit current and fill factor (FF) were simultaneously enhanced, resulting in a maximum power conversion efficiency (PCE) of 8.09% with 21.3% improvement. The improved photovoltaic performance of T-PSC was mainly due to the enhanced light absorption, energy level cascading, optimized blend morphology, and increased hole mobility. It was also found that the incorporation of TIPS-pentacene increased the average hole lifetime, ensuring efficient hole transport and collection with suppressed bimolecular recombination, contributing to the photocurrent. Additionally, the low thickness dependent row-off of FF indicates TIPS-pentacene is a promising third component for the realization of thick film T-PSC. The improved PCEs were obtained as well for other ternary donor: acceptor: TIPS-pentacene systems, demonstrating that the incorporation of TIPS-pentacene is a wide practicable methodology for the development of highly efficient T-PSCs.     

Impact of alkyl chain length of 1,n-diiodoalkanes on PC71BM distribution in both bulk and air surface of PTB7:PC71BM film

The impact of alkyl chain length of different additives, such as 1,4-diiodobutane (DIB), 1,6-diiodohexane (DIH), 1,8-diiodooctane (DIO) and 1,10-diiododecane (DID), on the PC₇₁BM distribution in PTB7:PC₇₁BM-based polymer solar cells, is systematically investigated, for the first time. Among these additives, DIO is found to have the optimum alkyl chain length that maximizes the performance of PTB7:PC₇₁BM based polymer solar cells, attaining a power conversion efficiency as high as 8.84%, which is almost four times higher than that without any additives. For DID additives (longer alkyl chain length than DIO), a drop in efficiency to 7.91% was observed. Experimental investigations show that the microstructure of the bulk and the surface layer as well as the surface morphology of the PTB7:PC₇₁BM polymer film can be controlled simultaneously by varying the alkyl chain length of additives. Results also show that the substantial improvement in performance is attributed to the improved 1) phase segregation, 2) PC₇₁BM distribution uniformity in the bulk of the PTB7:PC₇₁BM film, 3) surface smoothness and 4) high PTB7 content at the interface between the active layer and the top electrode.     

Effect of incorporation of CdS NPs on performance of PTB7: PCBM organic solar cells

It has been well known that incorporation of nano-heterostructures of various metals, semiconductors and dielectric materials in the active layer of organic solar cells (OSCs) helps in improving power conversion efficiency (PCE). In the present study, we demonstrated microwave synthesis of CdS nanoparticles (NPs) for their application in one of most efficient OSCs consisting of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl]] (PTB7): [6,6]-phenyl C₇₁-butyric acid methyl ester (PCBM) photoactive blend. This is crucial to fully explore the promising features of low cost and scalability in organic-inorganic hybrid solar cells. Synthesized CdS NPs are slightly elongated and highly crystalline with their absorption lies in the visible region as confirmed by High resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), UV–Vis absorption spectroscopy studies. Our experimental results for the devices in an inverted geometry having a structure ITO/ZnO/PTB7: CdS: PCBM/MoO₃/Ag has shown increase in Jsc and PCE by nearly 10%. However, it was observed that this increase is only when NPs were added in the low concentration in active layer. UV–Vis absorption spectroscopy, Photoluminescence (PL) and atomic force microscopy (AFM) studies were carried out in order understand the device performance.     

Photo-physics of PTB7, PCBM and ICBA based ternary solar cells

Indene-C₆₀bisadduct (ICBA) is one of the rare acceptors which can supersede commonly used phenyl-C71-butyric acid methyl ester (PCBM70) in enhancing the performance of bulk heterojunction (BHJ) solar cells owing to its shallower lowest unoccupied molecular orbital (LUMO) level. However, ICBA tends to decrease the photocurrent for most of the low band-gap polymers synthesized to date. Here we examine the interaction of ICBA with the one of the popular low band-gap polymers poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl) carbonyl] thieno[3,4-b]thiophenediyl]] (PTB7), at femtosecond level, to explore key dynamics governing the operation of BHJ cells involving ICBA. The photo-physics of binary and ternary systems based on PTB7 coupled with PCBM70 and/or ICBA are studied by means of transient absorption spectroscopy (TAS) and electrochemical impedance spectroscopy (EIS) and supported by morphology analysis. Our study suggests that both inefficient charge-separation and poor charge transport of ICBA is responsible for relatively low photocurrent generation.     

Two effects of 1,8-diiodooctane on PTB7-Th:PC71BM polymer solar cells

To obtain higher device performance, the ideal bulk heterojunction (BHJ) morphology should feature both nanophase separation to increase charge generation and bi-continuous percolating networks to increase charge transport. In this paper, solvent additive, 1,8-diiodooctane (DIO), was used in PTB7-Th:PC₇₁BM blend to improve BHJ morphology. The effect of DIO on charge generation and charge transport were studied carefully. Experimental study indicated that the effect of DIO on charge generation and charge transport are conflicted. Positive effects of DIO, which were induced by nanophase separation for charge generation in BHJ, are proved by the results of internal quantum efficiency (IQE) and photocurrent density (J_ph), and negative effects of DIO on charge transport has been investigated according to the time-of-flight secondary ion mass spectrometer (TOF-SIMS).     

Highly efficient organic tandem solar cell based on SubPc:C70 bulk heterojunction

Highly efficient organic tandem solar cell with peak power conversion efficiency (PCE) of 7.66% has been demonstrated by simply stacking two same boron subphthalocyanine (SubPc):C₇₀ bulk heterojunction devices, with a high active inter-connecting layer composed of bathophenanthroline (Bphen)/Silver/hexadecafluoro-copper-phthalocyanine (F₁₆CuPc)/MoO₃. We find that F₁₆CuPc plays an important role which extends the recombination zone, facilitates the extraction of hole and the carrier recombination. The measured PCE of the tandem solar cell corresponds to a 38% increase compared to that the optimal single cell.     

Tuning the viscosity of halogen free bulk heterojunction inks for inkjet printed organic solar cells

For the solution processing of organic photovoltaics on an industrial scale, the exclusion of halogenated solvents is a necessity. However, the limited solubility of most semiconducting polymer/fullerene blends in non-halogenated solvents results in ink formulations with low viscosities which poses limitations to the use of roll-to-roll compatible deposition processes, such as inkjet printing. We propose to add polystyrene as a rheological modifier to increase the viscosity of bulk heterojunction (BHJ) non-halogenated inks. The printing and performance of P3HT/PCBM photoactive layer inks are characterized as a function of polystyrene concentration and three different molecular weights. Addition of 1 wt% polystyrene provided a near two-fold gain in viscosity, with the largest viscosity gains coming from the polymer with the highest molecular weight. However, this coincided with greater viscoelastic behavior, which reduced the jetting performance of the inks. Differences in solvent compatibility of the polystyrene/P3HT/PCBM ternary blend resulted in phase separation upon layer drying, whereby polystyrene segregated to the layer-air interface to form an isolated domain or network like topology. Nevertheless, a 1.7-fold increase in dynamic viscosity was obtained for devices with printed BHJ layers containing polystyrene at the expense of a 20% reduction in OPV performance. The improved viscosity and good printing behavior achieved with small additions of polystyrene demonstrates its potential to overcome the limited viscosity resulting from typical non-halogenated ink formulations for semiconducting polymers. These results offer a step forward to the industrialization of inkjet printing as an effective deposition technique for functional layers of organic electronics.     

Efficient PTB7-Th:Y6:PC71BM ternary organic solar cell with superior stability processed by chloroform

Blend morphology is crucial for the efficiency and stability of organic solar cells. Exploring and understanding the correlations between is meaningful and greatly desired. In this work, based on polymer donor (PTB7-Th), fullerene and non-fullerene acceptors (PC₇₁BM and Y6), we systematically study the influence of ternary strategy and solvent system on device performance and stability. It is found that insufficient and excessive phase separation of blend could result in the depressed performance of corresponding devices. Appropriate phase separation/blend morphology can be achieved by utilizing a ternary strategy or suitable solvent. Chloroform-processed ternary blend PTB7-Th:Y6:PC₇₁BM delivers efficiency of 9.55%, with dramatically enhanced J_SC of 24.68 mA cm⁻² due to optimized absorption, blend morphology and optoelectronic properties. More importantly, superior device stability is demonstrated for the optimal ternary device under both thermal stress and maximum power point operation, by maintaining 80% of initial efficiency at 85 °C for 880 h and presenting almost zero efficiency decay in 200 h under MPP operation.     

Influence of molar mass ratio,annealing temperature and cathode buffer layer on power conversion efficiency of P3HT:PC71BM based organic bulk heterojunction solar cell

In bulk heterojunction (BHJ) solar cells, the molar mass ratio of donor-acceptor polymers, the annealing temperature (T_an) and the cathode buffer layer plays very consequential role in improving the power conversion efficiency (PCE) by tuning the film morphology and enhancing the charge carrier dynamics. A comprehensive understanding of each of these factors is essential in order to optimize the performance of organic solar cells (OSCs). Albeit there are several fundamental reports regarding these factors, an altogether meticulous correlation of these physical processes with experimental evidence of the photo active layer are required. In this work, we systematically analyzed the influence of different molar mass ratio, the annealing temperature (T_an) and the cathode buffer layer of rrP3HT:PC₇₁BM based BHJ solar cells and their corresponding photovoltaic performances were correlated carefully with their thin film growth structure and energy level diagram. The device having 1:0.8 molar mass ratio of rrP3HT:PC₇₁BM and T_an = 150 °C annealing temperature with Bathocuproine (BCP) as the cathode buffer layer having ITO/PEDOT:PSS/rrP3HT:PC₇₁BM (molar mass ratio = 1:0.8; (T_an = 150 °C)/BCP/Al) configuration showed the best device performance with PCE, ɳ = 4.79%, Jsc = 14.21 mA/cm², V_oc = 0.58 V and FF = 57.8%. This drastic variation in PCE of the device having BCP/Al as the cathode contact compared to the other device configurations is due to the coalesced effects of better hole-blocking capacity of BCP along with Al and better phase separation of the active blend layer at 150 °C annealing temperature. These results explicate the cumulate role of all these physical parameters and their combined contribution to the PCE amendment and overall device performance with rrP3HT:PC₇₁BM based organic BHJ solar cell.     

Enhanced intrinsic stability of the bulk heterojunction active layer blend of polymer solar cells by varying the polymer side chain pattern

Organic photovoltaics (OPVs) have acquired huge attention over the past years as potential renewable energy sources, adding attractive features such as aesthetics, semi-transparency, flexibility, large area printability, improved low-light performance, and cost-effectiveness to the well-known Si-based photovoltaics. Steady improvements in OPV power conversion efficiencies are continuously reported, notably for bulk heterojunction solar cells based on conjugated polymer:fullerene blends. However, apart from efficiency and cost, the stability of organic solar cell devices is of particular concern. Among the different factors contributing to OPV instability, gradual loss of the optimum phase-separated nanomorphology of the photoactive layer blend is a critical parameter. In this paper, we present the results of ‘shelf-life’ accelerated lifetime tests performed for devices containing a range of functionalized poly(3-alkylthiophene) (P3AT) donor polymers upon prolonged thermal stress. By the incorporation of functional moieties on the side chains of P3HT-based copolymers, a remarkable improvement of the intrinsic stability of the active layer blend morphology is accomplished, even for fairly low built-in ratios (5–15%) and without crosslinking to covalently anchor the polymer and/or fullerene molecules. Moreover, these alterations do not influence the initial power conversion efficiencies to a large extent. As such, the presented approach can be regarded as an attractive paradigm for OPV active layer stability.     

Incorporation of diketopyrrolopyrrole dye to improve photovoltaic performance of P3HT:PC71BM based bulk heterojunction polymer solar cells

Ternary blend solar cells have been intensively studied in recent years to harvest more photons over the near-IR region. In this work, the effects of adding a diketopyrrolopyrrole dye (py-DPP) into a conventional P3HT:PC₇₁BM based bulk heterojunction photovoltaic cell are investigated. The near infrared absorption of the blend is enhanced by the doped py-DPP dye, leading to more than 20% increased power conversion efficiency compared to the P3HT:PC₇₁BM binary system. The highest efficiency of 4.05% is achieved for a P3HT:PC₇₁BM blend with 2.4 wt % of py-DPP.     

Improved performance of organic photovoltaic cells with PTB7-Th:PC71 BM by optimized solvent evaporation time in electrospray deposition

As poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b; 4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] has good potential as a low-band gap donor polymer for organic photovoltaic cells (OPVs), we investigated the optimized electrospray deposition condition for realizing suitable polymer ordering and/or crystallite size by controlling the solvent evaporation time. Previous studies on the electrospray process have mainly focused on novel device structure owing to its unique characteristic of small droplet size, which is less than 1 μm. However, in this research, we investigated the spontaneous formation of interpenetrating continuous networks of the donor- and acceptor-rich domains of solvent evaporation during the electrospray process. By evaluating the ultraviolet–visible absorption spectrum, Raman spectroscopy, and direction of polymer ordering, it was shown that the polymer-stacking condition was not influenced by solvent evaporation time, even though poly(3-hexylthiophene-2,5-diyl) along the face-on direction was well stacked under the slow solvent evaporation condition. In contrast, the crystallite size, which was estimated from the full width at half maximum X-ray diffraction pattern, increased as the solvent evaporation time increased. This means that the crystalline grain spontaneously grew in the droplet and that the large crystalline grain was formed during the slow evaporation condition. Furthermore, the photovoltaic performance trend was the same as the performance trend of the crystallite size and were increased with increasing solvent evaporation time for both polymers. Therefore, the crystalline grain size was a dominant factor in determining the photovoltaic performance. Additionally, the crystalline grain size could be controlled by the solvent evaporation time. Finally, by optimizing the active-layer thickness, the highest photoconversion efficiency of 8.6% was achieved. This is the highest value of an electrospray-based device. These results indicate that the solvent evaporation time is an important factor in determining the crystallite size of an organic thin film, which directly affects the photoconversion efficiency of OPVs.     

The effect of processing solvent dependent film aggregation on the photovoltaic performance of squaraine:PC71BM bulk heterojunction solar cells

In this work, we systemically investigated the processing solvent-dependent aggregation behavior of a squaraine dye, 2,4-bis[4-(N,N-dibutylamino)-2,6- dihydroxyphenyl] squaraine (DBSQ), in a DBSQ: [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM) blend film, as well as the aggregation effect on the photovoltaic performance of DBSQ:PC₇₁BM bulk heterojunctions (BHJs). Our finding shows that the aggregation behavior of DBSQ dye in the blend film can be controlled via the proper selection of the processing solvents. For a J-aggregate (head-to-tail molecule alignment) DBSQ:PC₇₁BM active layer based BHJ cells, a power conversion efficiency (PCE) of over 5% can be obtained, which is 75% higher than that of the H-aggregate (parallel molecule packing) active layer based BHJ cells. Our results indicate that the processing solvent controlled J-aggregation formation shall be considered as effective approach to tune the optical and electrical properties of thin films for high-performance BHJ solar cells.     

Effect of spatial distribution of generation rate on bulk heterojunction organic solar cell performance: A novel semi-analytical approach

We present a physics-based semi-analytical model of bulk heterojunction (BHJ) organic solar cell (OSC) for predicting the electrical characteristics of the device, taking into account the space dependency of generation rate profiles. The model enables us to derive the J-V characteristics of BHJ OSC without the need of a closed form expression of arbitrary carrier generation rate (which may not exist), hence avoiding the cumbersome numerical fitting method employed in literature previously. Using the proposed model, we perform an extensive analysis to study the effect of spatial distribution of generation rate profiles on the device performance. For this purpose, we use Gaussian shaped profiles that have a common average value thus retaining the total number of generated carriers same. We vary the position of the generation peak and its sharpness (width of the Gaussian peak) as well as number of peaks to analyze their effects on device efficiency. For the considered profiles, results show that the optimized profile has a peak carrier generation rate exactly halfway through the active layer and falls off sharply on either side. In the end, we propose methods of controlling the generation profiles by modifying the device structure and perform optical simulation to show the corresponding generation profiles. Thus, we show the usefulness of our derived model in finding the spatial distribution of a given number of carriers along the active layer that yields the best device performance.     

D-A-D-A-D push pull organic small molecules based on 5,10-dihydroindolo[3,2-b]indole (DINI) central core donor for solution processed bulk heterojunction solar cells

Two D-A-D-A-D small molecules based on same 5,10-dihydroindolo [3,2-b]indole central donor core and different benzothiadiazole (BT) and fluorine substituted BT (FBT) acceptor units, denoted as p-DINI-(BTTh₃)₂ (1) and p-DINI-(FBTTTh₃)₂ (2), respectively were synthesized and their optical and electrochemical properties were investigated. These molecules were applied as donor along with PC₇₁BM as electron acceptor for the fabrication of solution processed bulk heterojunction organic solar cells. The solar cells prepared from the optimized active blended layer (1:2) cast from dichlorobenzene (DCB) showed overall power conversion efficiency (PCE) of 2.02% and 2.70% for 1 and 2, respectively as donor. The higher PCE of 2 as compared to 1 is attributed to the higher hole mobility and broader IPCE spectra. In order to improve the PCE we have employed a two step treatment of active layer i.e. solvent vapor annealing after thermal annealing (SVA-TA) and the PCE has been enhanced up to 4.14% and 5.27% for optimized 1:PC₇₁BM and 2:PC₇₁BM active layers, respectively. The improvement in the PCE has been resulted from the improvement in the balanced charge transport and better crystallinity of the donor in the blended active layer.     

Active layer thickness effect on the recombination process of PCDTBT:PC71BM organic solar cells

We investigated the effect of active layer thickness on recombination kinetics of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C71-butyric acid methyl ester (PC₇₁BM) based solar cells. Analysis of the fitted Lambert W-function of illuminated current density–voltage (J–V) characteristics revealed increased recombination processes with increased active layer thicknesses. The ideality factor extracted from PCDTBT:PCBM solar cells continuously increased from 1.89 to 3.88 when photoactive layer thickness was increased from 70 to 150 nm. We found that such increase in ideality factor is closely related to the defect density which is increased with increased photoactive layer thickness beyond 110 nm. Therefore, the different density of defect states in PCDTBT:PCBM solar cells causes the different recombination paths where solar cells with a thicker active layer (?110 nm) are considered to undergo coupled trap-assisted recombination processes while single-defect trap-assisted recombination is dominant for thinner (70–90 nm) PCDTBT:PCBM solar cells. As a result, we found that the optimal efficiencies of PCDTBT:PC₇₁BM solar cells were limited to the active layers between 70 and 90 nm. Particularly, when PCDTBT:PC₇₁BM solar cells were optimized with an active layer thickness of 70 nm, energy conversion efficiency reached 6.5% while an increase in thickness led to the reduction of efficiency to 4.7% at 133 nm but then an increase to 5.02% at 150 nm.     

Enhancement of power conversion efficiency of PTB7:PCBM-based solar cells by gate bias

We designed and fabricated poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl]] (PTB7): [6,6]-phenyl-C70-butyric-acid-methyl-ester (PC₇₀BM)–based solar cells with gate electrodes, which can introduce an additional electric field within the devices just as in organic thin film transistors (OTFTs). Our proposed realize the simple and convenient modulation of electric field within the device, and power conversion efficiency (PCE) of 8.1% is reached at 2.0 V gate bias, significantly higher than the PCE of 6.8% at the case of no gate structure. By calculating the carrier mobility and the rate of exciton dissociation efficiency in detail, the role of electric field to the exciton dissociation and carrier transport was investigated, respectively. Meanwhile, the feasibility of the proposed device structure in practical application was discussed. The results suggest that such a gate structure has a great of prospects in achieving high efficiency polymer solar cells.     

Effect of blend layer thickness on the dielectric response of PCDTBT:PC71BM-based bulk heterojunction solar cells

Bulk heterojunction solar cells based on a blend of poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM), were studied. The organic photoactive layers were spin coated onto a poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT-PSS) interfacial layer at speeds of 600 and 2000 rpm. The molecular structure of PCDTBT, PC₇₁BM, and the PCDTBT:PC₇₁BM blend was investigated using Fourier-transform IR (FTIR) spectroscopy, which confirmed the absence of interactions between the individual components of the composite. The dielectric properties of PCDTBT:PC₇₁BM-based solar cells were studied under illumination by means of impedance analysis. The dielectric constant, impedance, and ac conductance were analyzed as a function of frequency at different bias voltages close to the open circuit voltage (Voc). We found that the dielectric constant, dielectric loss, and conductance increased with increasing PCDTBT:PC₇₁BM thickness. Impedance spectroscopy analysis revealed decreases in charge recombination and the resistance of the whole device with increasing spin coating speed for the active layer. Moreover, an increase in recombination resistance for the solar cells was observed close to V_OC.     

Synthesis and characterization of the conjugated polymers tethered with dipolar side chains containing a benzothiadiazole entity for bulk heterojunction solar cells

New conjugated copolymers (P1?P3) containing dipolar side chains connected to the main chain via triphenylamine donors have been synthesized and characterized. The side chains of these polymers have an electron deficient benzothiadiazole moiety in the spacer, but with different acceptors at the end. By changing the acceptor moieties of the side chain, the absorption spectra and HOMO/LUMO gaps of the polymers can be fine-tuned, ranging from 1.86 to 1.59 eV. Solution processed bulk heterojunction (BHJ) solar cells using these polymers as the donor and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as the acceptor were fabricated and measured under 100 mW cm^?2 of AM 1.5 illumination. The cell based on the blend of P1/PCBM (1:1, w/w) exhibited the highest power conversion efficiency of 1.78%, with open circuit voltage (V_oc) = 0.79 V, short circuit current (J_sc) = 6.63 mA cm^?2 and fill factor (FF) = 0.34, respectively.