Characterization of Organic Thin Film Solar Cells of PCDTBT : PC 71 BM Prepared by Different Mixing Ratio and Effect of Hole Transport Layer

The organic thin film solar cells (OTFSCs) have been successfully fabricated using PCDTBT : PC 71 BM with different mixing ratios (1 : 1 to 1 : 8) and the influence of hole transport layer thickness (PEDOT : PSS). The active layers with different mixing ratios of PCDTBT : PC 71 BM have been fabricated using o-dichlorobenzene (o-DCB). The surface morphology of the active layers and PEDOT : PSS layer with different thicknesses were characterized by AFM analysis. Here, we report that the OTFSCs with high performance have been optimized with 1 : 4 ratios of PCDTBT : PC 71 BM. The power conversion efficiency (PCE = 5.17%) of the solar cells was significantly improved by changing thickness of PEDOT : PSS layer. The thickness of the PEDOT : PSS layer was found to be of significant importance; the thickness of the PEDOT : PSS layer at 45 nm (higher spin speed 5000 rpm) shows higher short circuit current density (Jsc) and lower series resistance (Rs) and higher PCE.


Introduction
The present world population exceeds seven billion and is expected to increase beyond nine billion per year by 2050.As the population growth, energy consumption was double in the year 2010 and it would be increased in the future.The energy demand is big issue, which would be widening the gap between rich and poor countries [1,2].In order to reduce energy problem, solar energy is the best alternative of fossil fuel.Total amount of solar energy might be approximately estimated about 1000 TW.Considering the current amount of total energy consumption (10 TW) for mankind, solar energy could be regarded as an infinite source of energy.Besides, solar energy generation does not cause CO 2 emission; this would be to reduce environment problem.However, the silicon based solar cells fabrication has high cost, because of that supply of resource materials could be limited to the material processing.
In this present investigation, we demonstrated that the PCDTBT : PC 71 BM with different mixing ratios (1 : 1 to 1 : 8) and changing thickness of the hole transporting layers (PEDOT : PSS) for high performance OTFSCs have been studied.The PCDTBT : PC 71 BM active layers with four different ratios (1 : 1, 1 : 2, 1 : 7, and 1 : 8) have been prepared using o-dichlorobenzene as a solvent, which was compared with mixing ratio of 1 : 3, 1 : 4, 1 : 5, and 1 : 6 to our previous reports [21].The surface morphologies of different blending ratios of active layers and different thickness PEDOT : PSS films were observed by AFM analysis.The electrical properties of different mixing ratio of active layers and PEDOT : PSS layer thickness on the performance of OTFSC were examined by measuring the open circuit current voltage ( oc ),  sc ,   , and PCE using current density-voltage (-) analysis.The open circuit current voltage ( oc ), short circuit current density ( sc ), series resistance (  ), and power conversion efficiency (PCE) have been carried out for the different thickness of PEDOT : PSS layers.was purchased from Heraeus, Germany, and used as a hole transport layer between ITO and active layer (PCDTBT : PC 71 BM).Chlorobenzene, o-dichlorobenzene, ethanol, and acetone were purchased from Aldrich and used without further purification.Figure 1 shows the molecular structures of the PCDTBT, PC 71 BM, and PEDOT : PSS.

Fabrication of Organic
Thin Film Solar Cells.Indium doped tin oxide (ITO) conducting glass substrate (Aldrich; 8-12 Ω/sq) was used to fabricate the OTFSC device with a structure of ITO/PEDOT : PSS/PCDTBT : PC 71 BM/Al.Figure 2 shows schematic diagram of the OTFSC device.ITO conducting glass substrate was ultrasonicated thoroughly in acetone, ethanol, and distilled water for 10 min for each step to remove the organic contaminations.The cleaned ITO conducting glass substrate was then exposed to UV Ozone for 10 min.PEDOT : PSS was coated at spin speed of 3000 rpm for 30 sec followed by heat treated at 150 ∘ C in atmospheric air for 10 min.Four different active layers were prepared by changing the mixing ratio of PCDTBT and PC 71 BM (1 : 1, 1 : 2, 1 : 7, and 1 : 8) in o-DCB solvent.Al was deposited by vacuum evaporation technique on the active layer and used as cathode electrode in OTFSCs.The effect of the hole transporting layer thickness (PEDOT : PSS) on photovoltaic performance was studied by different thickness of PEDOT : PSS layers.The different thickness of PEDOT : PSS layers was achieved by changing the spin speed from 2000 to 5000 rpm and the Al thin films were deposited with 5 mm (effective area of the device: 25 mm 2 ).

Characterization Techniques.
The thickness of hole transport layer (PEDOT : PSS) was measured using a Dektak II profilometer.Surface morphologies of the different blend of active layer and PEDOT : PSS layer are observed by atomic force microscopy (AFM: Seiko Instruments SPA400-SPI4000).All AFM images were taken in dynamic force mode at optimal force.Absorption characteristics of eight different active layers of PCDTBT : PC 71 BM, pristine PCDTBT, and PC 71 BM are measured using UV-visible spectroscopy (Shimazu, UV2450).The photovoltaic current density-voltage (-) characteristic of PCDTBT is as follows: PC 71 BM based OTFSCs were performed in standard Newport 66902 150 W solar simulator with an Oriel Cornerstone 130 monochromator and a Xe lamp equipped with an air mass of 1.5 G (AM 1.5 G) filter as the white-light source (100 mW/cm 2 ).An Advantest-R-6441 A.C. source meter was used for measuring the - parameter such as  oc ,  sc , fill factor (FF),   , and PCE.All measurements were carried out in air without any encapsulation.shows the surface roughness parameters with smaller P-V of PCDTBT : PC71BM surface which suggests smoother surface with nanomorphology (Figure S1) [21].Slow evaporation prevented the growth of the rough structure during the drying processes, resulting in smoother surface with nanomorphology, which could improve the device performance [21,24,25].

Results and Discussion
The effect of the PEDOT : PSS layer thickness on the surface morphology was studied using AFM analysis.Figure 3 shows the AFM image of PEDOT : PSS layer with different thickness.The thickness of PEDOT : PSS layer is controlled by different spin speed.Surface roughness parameters obtained from AFM analysis for different thickness of PEDOT : PSS layer are given in Table 2. AFM image of PEDOT : PSS layer at different thickness is shown in Figure 3.
Surface roughness parameters show that there is increase in the RMS roughness of 1.34, 1.45, and 1.57 nm and P-V of 15, 21, and 35 nm when increasing the thickness 45, 62, and 90 nm, respectively.The AFM result shows that the PEDOT : PSS layer thickness about 45 nm is smoother surface due to higher spinning speed (5000 rpm) and evaporation rate of the solvent is higher.The smoother surface of PEDOT : PSS layer enables more uniform interfacial contact between active layer and PEDOT : PSS layer, which might increase the charge transport and extraction results improvement in the device performance [16,20,22].The AFM result shows that the RMS value increases when the thickness of PEDOT : PSS layer was increased around 62 nm, which implies that the uniformity of the PEDOT : PSS layer is less.In particular, a ring of solution is formed at the edge of the substrate due to incomplete centrifugation of the applied solution and there is some variation in film thickness across the substrate [16][17][18][19][20], which would lead to changes in charge International Journal of Photoenergy carrier transport property [16,18,22].Further increasing the thickness, the drying of the film becomes significantly slower and possible for different drying dynamics are responsible for a change in orientation of the PEDOT grains within the film and surface properties, which would lead to changes in charge transport property [16,[18][19][20]22].Thus it might be suggested that the surface roughness increases at higher thickness due to PEDOT : PSS aggregation [14,16,17,20]; it might decrease the charge transport and extraction properties and also reduce the PCE [16,18,19,22].
The OTFSC device with 1 : 1 ratio of PCDTBT : PC 71 BM reveals distinctively lower PCE = 1.53% compared with higher composition of PC 71 BM of other devices.It can be assumed that a lower composition of PC 71 BM would be relatively smaller number of electron-path, causing a lot of electronhole recombination, which reduces the charge collection efficiency as well as increasing the series resistance (  = 80 Ω cm 2 ) [26,27].The higher   = 80Ωcm 2 decreases the  sc = 5.86 mA/cm 2 as well as FF = 30%, resulting in lowest PCE = 1.53% for 1 : 1 ratio of PCDTBT : PC 71 BM device.Whereas, 1 : 4 ratio of PCDTBT : PC 71 BM the OTFSC device shows that there is an increase of  sc = 8.41 mA/cm 2 and a decrease of   = 21 Ω cm 2 .As a result, a higher PCE = 4.15% with good FF = 55% was achieved [21,23,24].It implies that  when the relative composition of PC 71 BM ratio was increased which lead to increase in the number of electron-path and a decrease in the electron-hole recombination have been optimized in the case of 1 : 4 ratio of PCDTBT : PC 71 BM.This reveals an increase in the charge carrier transport properties as well as the increase in  sc and decrease in the   results the enhancement in the device performance.From this observation, when the relative composition of PC 71 BM ratio was increased from 1 : 1 to 1 : 4, which lead to increases in the  sc , FF as well as the PCE.This might be attributed to increasing electron-path; it prevents the recombination and enhances the charge collection results improving the device efficiency.Further, increasing the relative composition of PC 71 BM ratio from 1 : 5 to 1 : 8 reveals a gradual decrease in  sc as well as PCE for the resulting OTFSC devices [21,23,24,26].It seems that a decrease of relative composition of PCDTBT over PC 71 BM would cause unbalanced hole-electron-path (or excessive hole-path) according to increase of relative composition of PC 71 BM over PCDTBT [23,24,26,27].The effect of the hole transporting layer (PEDOT : PSS) thickness on photovoltaic performance was studied by - measurement.The thickness of PEDOT : PSS layers was varied by changing the spin speed from 3000 rpm to 5000 rpm and active layer thickness kept constant with 1 : 4 ratio of PCDTBT : PC 71 BM.The - parameter and device performance of PCDTBT : PC 71 BM based OTFSC device with respect to PEDOT : PSS layer thickness are given in Table 4. Figure 5 shows the - characteristics of PCDTBT : PC 71 BM based OTFSC device with different thicknesses of PEDOT : PSS layer.The photovoltaic performance of the OTFSC device of PEDOT : PSS layers with a thickness of 45 nm shows significantly improved PCE = 5.15% when compared with higher thickness of PEDOT : PSS layer at 62 nm (PCE = 4.15%) obtained for lower spin speed with 3000 rpm as per our earlier report [21].When the spin speed was increased about 5000 rpm, PEDOT : PSS layer thickness as well as surface roughness will decrease.The enhancement of the device performance might be due to decreasing the series resistance   = 16 Ω/cm 2 resulting in enhanced  sc of 11.01 mA/cm 2 , FF 55%.The   is closely related with active layer morphology, intrinsic resistance, and thickness of the active layer.The enhancement of the device performance (PCE = 5.15%) due to the change in the properties of the PEDOT : PSS layer thickness itself directly or indirectly influences of the PEDOT : PSS surface properties on the properties of the blend spin-coated on top [16,17,19,20,22].This suggests that the thickness of PEDOT : PSS layer was decreased at 45 nm which leads to the decrease in the series resistance and the results will enhance the charge carrier transport properties and suppress the charge carrier recombination at the interface with an improvement in the device efficiency.This implies that the decreases in the series resistance as well as increase in the shunt resistance and FF might be depending on PEDOT : PSS layer thickness and influence of the PEDOT : PSS surface properties.The AFM results reveal that the surface of the PEDOT : PSS is smoother with the thickness of 45 nm, which enables more uniform interfacial contact between the active and PEDOT : PSS layer, which might reduce the local fluctuation and enhance the charge collection efficiency [16,18,19,22,28].The PEDOT : PSS layer thickness has increased around 62 nm and the results slightly increase in  oc = 0.9 V.However, there are increases in the   = 21 Ω cm 2 and decreases in the  sc = 8.41 mA/cm 2 , as a result of moderate performance of PCE = 4.15%.The decrease in   can be attributed to recombination loss.Further, increase in the PEDOT : PSS layer thickness about 92 nm will increase the   = 29 Ω cm 2 and decrease the device performance of PCE = 3.52 %.The device performance depends on PEDOT : PSS layer thickness and its surface.These results suggested that device performance significantly improved by changing thickness of PEDOT : PSS layer 45 nm at higher spin speed with 5000 rpm as well as decreasing the surface roughness and hence improvement in the OTFSCs performance.However, at higher thickness of PEDOT : PSS layer, the surface roughness will increase which results in a poor device performance due to PEDOT : PSS aggregation which leads to decrease in the charge transport and extraction [16][17][18][19][20][21][22].

Absorption Studies.
The solar energy conversion efficiency mainly depends on its light absorption property of active layers.The steady state absorption spectral properties of the PCDTBT : PC 71 BM active layer were investigated by UV-visible spectroscopy.Figure 6 shows that the absorbance spectra for the PCDTBT : PC 71 BM layer prepared with different ratios of PCDTBT and PC 71 BM in o-DCB and active layer show similar shape of light absorption.The spectra reveal that difference in ratio of PCDTBT and PC 71 BM could hardly be contributed to formation of different morphology in the resulting bulk heterojunction of PCDTBT : PC 71 BM.The 1 : 1 ratio of PCDTBT : PC 71 BM layer was higher than that of other ratios.This result indicates that light absorption property of PCDTBT : PC 71 BM could be strongly dependent on relative quantity of PCDTBT [21,24,26].
To verify that supposition, absorption studies were carried out for a single layer of PCDTBT and single layer of PC 71 BM. 18 mg of PCDTBT was dissolved in 1 mL of o-DCB and 18 mg of PC 71 BM was dissolved in 1 mL of o-DCB.Figure 7 shows light-absorbance spectra of the resulting individual layer of PCDTBT and PC 71 BM, respectively.Pristine PCDTBT layer shows strong light absorption peaks at wavelength of about 390 nm and 557 nm, while pristine PC 71 BM shows weak absorption peaks at wavelength of about 377 nm and 552 nm.Pristine PCDTBT shows four times higher light absorption peak at 390 nm than that of PC 71 BM at 377 nm, and about five times higher light absorption peak at 557 nm than that of PC 71 BM at 552 nm.Even though the weight percentage of PCDTBT and PC 71 BM is same, the PCDTBT layer reveals drastically higher light-absorbance than that of PC 71 BM layer.Therefore, the difference in the absorption property according to ratio of PCDTBT and PC 71 BM in o-DCB (Figure 7) is originated from the difference of quantity of PCDTBT.Absorption spectra for the PCDTBT : PC 71 BM with mixing ratios of 1 : 1 show higher absorption than the other pristine PCDTBT and PC 71 BM layers.The absorption  spectra of PCDTBT : PC 71 BM with mixing ratios of 1 : 1 show two absorbance maxima at 384 nm, 554 nm and a notable one shoulder band was observed around 440 nm, which could not be seen for the single layer of PCDTBT and PC 71 BM [21,24,26,27].It was reported that bulk heterojunction layer of PCDTBT/PC 71 BM revealed a strong absorption at 440 nm [26], which could play an important role to improve power conversion efficiency of organic thin film solar cells.The increase in the light absorption property of active layer leads to enhance the power conversion efficiency of organic thin film solar cell [21,24,26,27].

Conclusions
The characteristics of OTFSCs device have been fabricated using PCDTBT : PC 71 BM with mixing ratios of 1 : 1∼1 : 8 and the influences of PEDOT : PSS film thicknesses on the photovoltaic performances have been studied and demonstrated.The mixing ratio of 1 : 4 device shows higher PCE of 4.15% with PEDOT : PSS layer at 3000 rpm; it confirms the optimized mixing ratio which results in a higher PCE compared with the other mixing ratios.by changing thickness of PEDOT : PSS layer about 45 nm than PCE = 4.15% with higher thickness of 62 nm.The improvement in the device performance is due to the change in the PEDOT : PSS layer thickness and influence of the PEDOT : PSS surface properties.The optimized thickness of PEDOT : PSS layer at 45 nm shows uniform interfacial surface which leads to enhance the interfacial properties of the PEDOT : PSS layer and an increase in the charge transport and extraction.As a result, an improvement in the device performance was achieved.

Figure 6 :
Figure 6: UV-visible absorption spectra for the active layer with different ratio of PCDTBT and PC 71 BM.

Figure 7 :
Figure 7: UV-vis absorption spectra of pristine PCDTBT and PC 71 BM and PCDTBT : PC 71 BM layer in o-DCB.

Table 2 :
Surface roughness parameters obtained from AFM analysis for different thickness of PEDOT : PSS layer.