Enhanced Power Conversion Efficiency of P 3 HT : PC 71 BM Bulk Heterojunction Polymer Solar Cells by Doping a High-Mobility Small Organic Molecule

The effect of molecular doping with TIPS-pentacene on the photovoltaic performance of polymer solar cells (PSCs) with a structure of ITO/ZnO/poly(3-hexylthiophene-2,5-diyl) (P3HT) : [6,6]-phenyl C71-butyric acid methyl ester (PC 71 BM) : TIPSpentacene/MoOx/Ag was systematically investigated by adjusting TIPS-pentacene doping ratios ranged from 0.3 to 1.2 wt%. The device with 0.6 wt% TIPS-pentacene exhibited the enhanced short-circuit current and fill factor by 1.23mA/cm and 7.8%, respectively, resulting in a maximum power conversion efficiency of 4.13%, which is one-third higher than that of the undoped one. The photovoltaic performance improvement was mainly due to the balanced charge carrier mobility, enhanced crystallinity, and matched cascade energy level alignment in TIPS-pentacene doped active layer, resulting in the efficient charge separation, transport, and collection.


Introduction
Polymer solar cells (PSCs), as one of the most promising energy conversion technologies, have attracted much attention in last decades due to their unique properties of low cost, being easily manufactured, large scale, and being flexible [1][2][3][4][5].The PSCs have many excellent applications, such as incorporation with wearable products, decoration of buildings, and space application for optimizing the design of space solar power.Recently, the power conversion efficiencies (PCEs) have reached 10% and 11% for the PSCs using single-junction and multijunction structures, respectively [6,7].However, the PCEs are still not high enough for commercialization.Therefore, great effort is devoted to further improve the photovoltaic performance of PSCs.The major drawbacks of PSCs are often attributed to low light absorption, limited exciton migration, and low hole transport ability [8].Particularly, the low hole transport ability could increase the carrier recombination in the active layer and suppress the charge carrier collection [9].Hence, a relatively low short-circuit current ( SC ) and fill factor (FF) are often observed in such PSCs.
In order to increase the hole transport ability of organic active layers, numerous approaches have been applied, such as modifying metal/semiconductor interface [10], introducing multisolvents [11], processing solvent additives [12], and doping a small amount of high-mobility materials [13].Among them, molecular doping is an effective method to enhance hole transport ability of PSCs.For example, Liu et al. enhanced the PCE of PSCs by adding a high-mobility conjugated polymer with suitable energy band structure [13].P-type molecular doping of F4-TCNQ improved the hole density and hole mobility in the polymer: fullerene derivative blends [14].Pentacene, a high hole mobility small molecule used in organic thin-film transistors (OFETs), was successfully added in the poly(3-hexylthiophene-2,5diyl) (P3HT) : [6,6]-phenyl C61-butyric acid methyl ester (PC 61 BM) blends to balance hole and electron mobility and improve the photovoltaic performance of relative PSCs [15][16][17].In this work, a high hole-mobility pentacene derivative of TIPS-pentacene (0.8 cm 2 V −1 s −1 , which is nearly 4 folders as high as that of P3HT) [18,19], with high solubility in organic solvents and deeper highest occupied molecular orbital (HOMO) of 5.20 eV compared with 5.00 eV of P3HT [20], was introduced in the P3HT : [6,6]-phenyl C71-butyric acid methyl ester (PC 71 BM) blends.By adjusting the doping ratios of TIPS-pentacene from 0.3 to 1.2 wt%, the optimized PSC with 33% PCE improvement was obtained.The mechanism of TIPS-pentacene doping was elucidated through characterizing the morphology of active layers by X-ray diffraction (XRD) and atomic force microscopy (AFM).Furthermore, the variation of charge carrier mobility was investigated in the active layers from the hole-only and electron-only devices by using the space-charge-limited current (SCLC) method.

Experiment
The chemical structures of organic materials are shown in Figure 1(a), and the structure of PSCs is indium tin oxide (ITO)/ZnO (30 nm)/P3HT : PC 71 BM : TIPS-pentacene (180 nm)/MoO  (15 nm)/Ag (100 nm) as depicted in Figure 1(b).The ITO-coated glass substrates with a sheet resistance of 10 Ω/sq were consecutively in an ultrasonic bath containing detergent, acetone, deionized water, and isopropyl alcohol for 10 min each step and finally dried in an oven for 30 mins [21].The ZnO precursor solution was spincoated on the ITO-glass substrates.After baking at 200 ∘ C for 60 min in atmosphere, the substrates were transferred to a glove box (1 ppm O 2 and H 2 O).P3HT (99.9%,Rieke Metals) and PC 71 BM (99.9%,Solarmer) were dissolved in 1,2-dichlorobenzene (DCB) and mixed in the glove box to obtain blend solutions (30 mg/mL) with a weight ratio of 1 : 1. TIPS-pentacene (99.9%,Rieke Metals) solution was separately prepared in DCB at a concentration of 2 mg/mL and then mixed with the blend solutions of P3HT : PC 71 BM.TIPS-pentacene doping ratios in P3HT : PC 71 BM blends were adjusted from 0.3, 0.6, and 0.9 to 1.2 wt%.Then, P3HT : PC 71 BM : TIPS-pentacene blend solutions were spin-coated on ZnO thin layer.After that, the substrates were solvent-annealed in a covered Petri dish for 20 mins and then were thermal-annealed at 120 ∘ C for 10 mins [22].MoO  (99.98%,Aldrich) layer was deposited onto the active layers at a rate of 1 to 3 Å/s at a pressure of 3.0 × 10 −3 Pa in vacuum, followed by the deposition of Ag anode at a rate of 10 Å/s under a pressure of 3.0 × 10 −3 Pa.The typical area of PSCs was 0.02 cm 2 .All measurements were performed under ambient condition without encapsulation.
A light source integrated with a xenon lamp (CHF-XM35, Beijing Trust Tech) with an illumination power of 100 mW/cm 2 was used as a solar simulator.The curves under illumination and in the dark were measured with  a Keithley 4200 programmable current-voltage source, and the external quantum efficiency (EQE) spectra were measured under the lump light passing through a monochromator.The ultraviolet-visible (UV-Vis) absorption spectra of the active layer on quartz substrates were measured using a Shimadzu UV1700 system.The film preparation condition for XRD (D1-HR XRD, Bede, Inc.) and AFM (MFP-3D-BIO, Asylum Research) measurement was kept the same as the device fabrication for comparison.

Results and Discussion
The absorption spectra of thin films of P3HT, PC 71 BM, and TIPS-pentacene are shown in Figure 2(a).It can be seen that P3HT shows strong light absorption from 450 to 650 nm, while PC 71 BM has compensatory absorption from 350 to 550 nm.TIPS-pentacene exhibits a wide absorption in the visible region from 400 to 750 nm [20].Figure 2(b) shows the absorption spectra of P3HT : PC 71 BM blend films with various TIPS-pentacene doping ratios.It is found that the absorption in the wavelength from 400 nm to 700 nm does not change significantly with the increase of TIPS-pentacene doping ratio.For the small amount of TIPS-pentacene, the absorption contribution of TIPS-pentacene is negligible.This phenomenon also indicates that the active layer thicknesses are unchanged with the doping of TIPS-pentacene.The current density-voltage (-) characteristics of P3HT : PC 71 BM PSCs with various TIPS-pentacene doping ratios are displayed in Figure 3(a).The detailed parameters with error statistics including open circuit voltage ( OC ),  SC , FF, and PCE are listed in Table 1.For the precise comparison of photovoltaic performance of PSCs with different TIPSpentacene doping ratios, the relative change of the device parameters with increasing TIPS-pentacene doping ratios is summarized in Figure 3(b).The performance of pristine P3HT : PC 71 BM devices was chosen as the reference and set to 100%.All other devices data were normalized to that reference.
OC is 0.59 V for undoped devices and 0.3 wt% TIPSpentacene doped devices and slightly increased to 0.61 V for the relative high TIPS-pentacene doping ratios devices.The increased  OC is due to the multicharge separation phenomenon [23].As shown in Figure 1(c), the HOMO of TIPSpentacene is deeper than that of P3HT.After doping TIPSpentacene, donor : donor : accepter blends were formed, and the additional TIPS-pentacene : PC 71 BM interface could facilitate the exciton separation in the bulk heterojunction.Even though little exciton can be formed on TIPS-pentacene for its negligible light absorption, the exciton can be formed on PC 71 BM for the obvious light absorption of PC 71 BM.The exciton on PC 71 BM can be directly dissociated into free charge carrier by P3HT or TIPS-pentacene. OC of TIPSpentacene doped PSCs is dependent on the composition of P3HT : PC 71 BM and TIPS-pentacene : PC 71 BM interfaces as reported by the previous works [24].Thus, the additional TIPS-pentacene : PC 71 BM interface can affect the overall heterojunction energetics, resulting in the increased  OC .With doping of TIPS-pentacene, a remarkable enhancement of  SC and FF is observed.In the device doped with 0.3 wt% TIPS-pentacene,  SC is increased to 10.05 mA/cm 2 , and FF is increased to 58.4%.When doping 0.6% TIPS-pentacene, the device reaches its optimized performance with  SC of 10.86 mA/cm 2 , a FF of 62.4%, and a PCE of 4.13%.However, when the doping ratios are increased to 0.9% and 1.2%, the devices show decreased photovoltaic performance.
To further investigate the diode characteristics of PSCs, we modeled - characteristics with the Shockley diode equation as [25] where  ph is the photocurrent,   is the reverse saturation current density, and  is the ideal factor. is Boltzmann's constant, and  is the temperature. is the electron charge, and  is the device area.The relevant diode parameters, including series resistance (  ) and shunt resistance ( SH ), are summarized in Table 1.The decreased   and increased  SH of PSC with 0.6 wt% TIPS-pentacene made a contribution to the enhanced  SC and FF for efficient charge carrier transport and collection.
To illustrate the effect of molecular doping of TIPSpentacene on  SC of P3HT : PC 71 BM PSCs, EQE was employed.Figure 4 shows EQE spectra of P3HT : PC 71 BM PSCs with various TIPS-pentacene doping ratios in the wavelength ranging from 400 to 700 nm.The shape of EQE spectra resembles that of UV-Vis absorption spectra of the blend films.EQE values for devices exceed 50% in the region of 400-570 nm.It is found that the largest EQE of 68.9% is obtained for PSCs with 0.6 wt% TIPS-pentacene doped.However, when the doping ratios of TIPS-pentacene in P3HT : PC 71 BM blends were further increased, EQE of PSCs was decreased.
To further investigate EQE variation, the theoretical  SC values ( SC(EQE) ) are obtained via integrating EQE.The formula of integration is presented as where () is AM1.5 solar spectral density.The inset of Figure 4 shows  SC(EQE) of PSCs with various TIPS-pentacene doping ratios.All  SC(EQE) are about 1.31 mA/cm 2 less than the measured values in - curves, as the offered EQE spectra lack the part less than 400 nm. SC(EQE) is increased from 7.76 mA/cm 2 for undoped devices to 9.23 mA/cm 2 for 0.6 wt% TIPS-pentacene doped devices and then decreased to 7.61 mA/cm 2 for 1.2 wt% TIPSpentacene doped devices.This situation is similar to the characteristics of  SC as listed in Table 1.
The effect of different TIPS-pentacene doping ratios in P3HT : PC 71 BM blends on the charge carrier transport properties was further investigated by using SCLC model.The hole-only devices with a configuration of ITO/MoO  (15 nm)/P3HT : PC 71 BM : TIPS-pentacene (180 nm)/MoO  (15 nm)/Ag (100 nm) and electron-only devices with   a configuration of ITO/ZnO (30 nm)/P3HT : PC 71 BM : TIPSpentacene (180 nm)/Bphen (5 nm)/Ag (100 nm) were fabricated, respectively.The hole mobility in the hole-only devices and the electron mobility in the electron-only devices can be calculated using Mott-Gurney law as where  is the charge carrier mobility. is the relative permittivity of polymer assumed to be 3, and  0 is the vacuum dielectric constant of 8.85 × 10 −12 F/m. is the voltage, and  is the thickness of the layer.- characteristics of the hole-only and electron-only devices with various TIPS-pentacene doping ratios in P3HT : PC 71 BM blends are presented in Figures 5(a) and 5(b), respectively.The hole mobility and the electron mobility of corresponding devices are listed in Table 2.In the hole-only devices, the hole mobility of undoped P3HT : PC 71 BM device was 1.3 × 10 −4 cm 2 V −1 s −1 .After doping TIPS-pentacene, hole mobilities of the devices were calculated to be 1.7 × 10 −4 , 2.6 × 10 −4 , 2.0 × 10 −4 , and 9.5 × 10 −5 cm 2 V −1 s −1 for 0.3 wt%, 0.6 wt%, 0.9 wt%, and 1.2 wt% doping ratios.On the other hand, in the electron-only devices, the electron mobility is slightly decreased with the increase of TIPSpentacene doping ratio in P3HT : PC 71 BM : TIPS-pentacene blends.In particular, balanced charge carrier mobility of 1.00 was obtained in the active layer with 0.6 wt% TIPSpentacene, which ensures efficient charge transport and collection in P3HT : PC 71 BM : TIPS-pentacene blends, resulting in the enhancement of both  SC and FF.However, when TIPS-pentacene doping ratios are high, PCE is decreased.The degraded performances are due to the decreased hole mobilities, leading to an unbalance charge transport in P3HT : PC 71 BM : TIPS-pentacene blends.In this condition, hole with lower mobility induced cumulative charge carriers in the active layer due to the SCLC effect [26].Therefore, PCE of PSCs with high TIPS-pentacene doping ratios is decreased by the reduction of  SC and FF.
To demonstrate TIPS-pentacene doping effect on the crystallinity of P3HT, P3HT : PC 71 BM blends without and with 0.6 wt% TIPS-pentacene were characterized by XRD as shown in Figure 6.XRD spectra show obvious -axis orientation of P3HT crystallite at a peak of 2 near 5.4 ∘ , corresponding to the (100) crystal plane [21].Compared with P3HT : PC 71 BM blends, the 0.6 wt% TIPS-pentacene doped P3HT : PC 71 BM exhibits higher (100) peak.This result indicated that the enhancement of hole mobility for TIPSpentacene doped blends could result from the crystallinity enhancement of P3HT.
To further investigate the effect of molecular doping of TIPS-pentacene on the morphology of P3HT : PC 71 BM blends, the surface morphology of P3HT : PC 71 BM blends and 0.6 wt% TIPS-pentacene doped P3HT : PC 71 BM blends was examined by using AFM as shown in Figure 7.For P3HT : PC 71 BM blends, the root-mean-square (RMS) roughness is 6.14 nm.The rough texture suggests that the enhancement of ordered structure is due to the self-organization of thick P3HT : PC 71 BM blends during slow growing process.After doping 0.6 wt% TIPS-pentacene, RMS increases slightly to 6.28 nm.It has been reported that the increased RMS of P3HT : PC 71 BM blends demonstrated that P3HT had enhanced crystallinity [22].Based on this phenomenon, it could also be deducted that the crystallinity of P3HT in the active layer was improved with the molecular doping of TIPS-pentacene, which is consistent with the results of XRD measurement mentioned above.In addition, the rough surface could shorten the charge-transport distance from P3HT : PC 71 BM blends to Ag anode, resulting in an efficient hole collection.AFM phase images of P3HT : PC 71 BM blends and 0.6 wt% TIPS-pentacene doped P3HT : PC 71 BM blends are presented in Figures 7(c) and 7(d).It can be seen from the two phase images that no obvious ternary TIPS-pentacene domain was observed, indicating that small doping ratio would not change P3HT : PC 71 BM interpenetrating network.

Conclusion
In summary, the solution-processed P3HT : PC 71 BM : TIPSpentacene PSCs with 0, 0.3, 0.6, 0.9, and 1.2 wt% TIPSpentacene doping ratios were fabricated.0.6 wt% TIPSpentacene doped PSC exhibited an enhanced  SC and FF, resulting in a maximum PCE of 4.13%, which is 33% higher compared with the undoped PSC.The improved photovoltaic performance was originated from the balanced charge carrier mobility, enhanced crystallinity, and matched cascade energy level alignment in P3HT : PC 71 BM : TIPS-pentacene blends, resulting in efficient charge separation, transport, and collection.This work can significantly enhance our understanding of the mechanism of molecular doping with high-mobility small organic molecules on the photovoltaic performance of PSCs.

2 InternationalFigure 1 :
Figure 1: (a) Chemical structures of P3HT, PC 71 BM, and TIPS-pentacene.(b) Schematic structure of PSCs in this work.(c) Energy band diagram of materials used in PSCs.

Figure 3 :
Figure 3: (a) - characteristics of P3HT : PC 71 BM PSCs with various TIPS-pentacene doping ratios.The inset shows the equivalent circuit model of PSC.(b) Relative change of the device parameters with increasing TIPS-pentacene doping ratios.The performance of pristine P3HT : PC 71 BM PSCs was chosen as reference and set to 100%.All other devices data were normalized to that reference.

Figure 4 :
Figure 4: EQE spectra of P3HT : PC 71 BM PSCs with various TIPSpentacene doping ratios.The inset shows the integral  SC of PSCs with various TIPS-pentacene doping ratios.

Figure 7 :
Figure 7: AFM morphology images of (a) P3HT : PC 71 BM blend film and (b) P3HT : PC 71 BM with a 0.6 wt% TIPS-pentacene doping ratio blend film.Corresponding AFM phase images ((c) and (d)) are shown below each respective morphology image.Image size is 5 m × 5 m.

Table 1 :
Comparison of device characteristics of P3HT : PC 71 BM PSCs with various TIPS-pentacene doping ratios.

Table 2 :
The hole mobility and electron mobility of the holeonly and electron-only devices with various TIPS-pentacene doping ratios.Doping ratio (wt%)  ℎ