Investigation Performance and Mechanisms of Inverted Polymer Solar Cells by Pentacene Doped P3HT : PCBM

The inverted polymer solar cells (PSCs)with pentacene-dopedP3HT : PCBMabsorption layerswere fabricated. It was demonstrated that the pentacene doping modulated the electron mobility and the hole mobility in the resulting absorption layer. Furthermore, by varying the doping content, the optimal carrier mobility balance could be obtained. In addition, the pentacene doping led to an improvement in the crystallinity of the resulting films and made an enhancement in the light absorption, which was partly responsible for the performance improvement of the solar cells. Using the space-charge-limited current (SCLC) method, it was determined that the balanced carrier mobility (μh/μe = 1.000) was nearly achieved when a pentacene doping ratio of 0.065 by weight was doped into the P3HT : PCBM : pentacene absorption layer. Compared with the inverted PSCs without the pentacene doping, the short circuit current density and the power conversion efficiency of the inverted PSCs with the pentacene doping ratio of 0.065 were increased from 9.73mA/cm to 11.26mA/cm and from 3.39% to 4.31%, respectively.


Introduction
Over the past decades, much effort has been devoted to improving energy utilization efficiency, to developing renewable energy, and to decreasing overall greenhouse gas emissions [1]. Recently, polymer solar cells (PSCs) have attracted much attention and are thought of as a potential candidate of the next generation solar cells, because they have many advantages, including low cost, flexibility, light weight, and easy fabrication [2,3]. However, compared with the inorganic solar cells [4][5][6], the PSCs suffer from two major drawbacks of a lower power conversion efficiency (PCE) and a worse stability [7]. Conventionally, the PSCs were constructed with an Al back cathode electrode, a poly(3,4-ethylene-dioxythiophene) : poly(styrene sulfonate) (PEDOT : PSS) hole transport layer inserted between the polymer absorption layer and the indium tin oxide (ITO) front anode electrode. Unfortunately, the oxygen could diffuse into the absorption layer through the pinholes and grain boundaries within the Al electrode. Consequently, the quality of the absorption layer of PSCs was degraded [8].
Furthermore, the ITO electrode was easily etched by the PEDOT : PSS [9]. These problems were responsible for the instability of the PSCs, which limited the application and commercialization of the devices. To improve the stability of the PSCs, an inverted cell structure for PSCs was previously proposed, where a high-work-function metal (Au or Ag) layer was used as the back contact anode electrode and the PEDOT : PSS hole transport layer was removed [10]. However, the conventional inverted PSCs still suffer from low PCE. To enhance the efficiency, many efforts have been carried out previously. For example, organic or inorganic materials were doped into the P3HT : PCBM absorption layers of the PSCs to enhance the light absorption or the carrier mobility. Various promising doping materials were previously reported, including cadmium selenide (CdSe) [11], zinc oxide (ZnO) [12], nanodiamonds [13], single wall carbon nanotubes (SWCNTs) [14], ferric oxide (Fe 3 O 4 ) [15], graphene [16], 3-hydroxyflavone (3-HF) [17], and perylene [18]. In these previous reports, the performances of PSCs were improved owing to an increase of light absorption. Consequently, the amount of the photoinduced charge carriers in the absorption  layer was increased. However, the PCE of these PSCs with the absorption layer doped with various materials was still not satisfactory, only 1.5%∼3.6%. It has been pointed out that the carrier mobility mismatching in the absorption layer was one of the main reasons for the low PCE [19]. The balanced carrier mobility could decrease the carrier recombination in the absorption layer and hence increase the photocurrent of the resulting solar cells [20].
Recently, many efforts have been devoted to balancing the carrier mobility in the absorption layer. It was reported that the carrier mobility could be modulated by doping pentacene into the absorption layer of the conventional PSCs and the performances of the resulting devices were improved [21]. In order to further improve the performance of the PSCs, in this work, the inverted PSCs with pentacene-doped absorption layer were fabricated and investigated. To clearly identify the electron mobility and the hole mobility in the absorption layers, the electron-only devices and the holeonly devices with the corresponding absorption layers were analyzed, respectively, using the space-charge-limited current (SCLC) method. By varying pentacene doping content in the absorption layer, the optimal mobility balance condition was obtained. It is clarified that the PCE of the inverted PSCs was enhanced by properly balancing the carrier mobility in the absorption layer. Figure 1 shows the schematic configuration of the inverted polymer solar cells (PSCs). The 25 nm thick Al-doped ZnO (AZO) film was deposited on the ITO-coated glass substrate using a magnetron radio-frequency (RF) sputtering system. The AZO film worked as the electron transportation and hole blocking layer. The mixed solution of poly(3-hexylthiophene) (P3HT), (6, 6)-phenyl-C 61 -butyric acid methyl ester (PCBM), and pentacene with given mixing ratio in the 1,2-dichlorobenzene (DCB) was then spread on the AZO film using a spin-coating technique to form a P3HT : PCBM : pentacene absorption layer of the inverted PSCs. The thickness of the absorption layer was 200 nm. Subsequently, the deposited absorption layer was annealed in a nitrogen glove box at 110 ∘ C for 20 minutes. Finally, the 10 nm thick MoO 3 layer and the 100 nm thick Ag layer were subsequently deposited on the absorption layer as the anode electrode of the inverted PSCs using a thermal evaporator. The absorption area of the inverted PSCs was about 4 mm 2 . Thus fabricated inverted PSCs with various pentacene doping ratios (0, 0.05, 0.06, 0.065, and 0.07 by weight) in the P3HT : PCBM (1 : 0.8) absorption layers were, respectively, named solar cells A, B, C, D, and E, hereafter. For estimating the hole mobility and the electron mobility in the absorption layer, the hole-only devices of Au/P3HT : PCBM : pentacene/MoO 3 /Ag (100/ 200/10/100 nm) and the electron-only devices of ITO/AZO/ P3HT : PCBM : pentacene/Al (300/25/200/100 nm) were fabricated. In this work, ten batches, each batch had six devices, of the electron-only devices, the hole-only devices, and the inverted PSCs were fabricated and measured.

Experiments
The space-charge-limited current (SCLC) method was used to estimate the hole mobility and the electron mobility in the absorption layer for the hole-only devices and the electron-only devices, respectively. The crystallinity and surface morphology of the absorption layers with various pentacene doping contents were measured using Xray diffraction (XRD) and atomic force microscopy (AFM), respectively. The current density versus voltage ( -) characteristics of the inverted PSCs were measured at room temperature using a -curve tracer (Keithley 2400) with an AM 1.5 G solar simulator (100 mW/cm 2 ). The external quantum efficiency (EQE) was measured using a chopped calibrated light beam from a xenon lamp combined with a lock-in amplifier. The absorption and the diffuse reflection spectra of the absorption layer with various pentacene doping contents and the resulting cells were measured using an UV-Vis spectrometer (Hitachi, U4100).

Experimental Results and Discussion
The SCLC method was used to estimate the electron mobility ( ) and the hole mobility ( ℎ ) in various absorption layers by using the corresponding electron-only and hole-only devices, respectively. The dark current density-voltage characteristics of the electron-only devices and the hole-only devices with the absorption layers of various pentacene doping contents were shown in Figure 2. The electron mobility of the electrononly devices and the hole mobility of the hole-only devices were estimated by Mott-Gurney law equation shown as follows [22]: where is the dark current density, 0 is the permittivity of the P3HT : PCBM : pentacene absorption layer, which was estimated to be (average value ± standard deviation) (4.50 ± 0.02) × 10 −11 F/m from the capacitance-voltage measurement results, is the carrier mobility, is the applied voltage, and International Journal of Photoenergy  is the thickness of the absorption layer of the devices. This equation could be also rewritten and shown as follows: To conform the Mott-Gurney law, the slope of log( )-log( ) curve for the electron-only devices and the hole-only devices should be 2. In this work, the applied voltage of 1.7 V matched in Mott-Gurney law was chosen to estimate the hole mobility of the hole-only devices and the electron mobility of the electron-only devices. Thus, the electric field ( ) of all devices estimated by the formula of = / , where of 1.7 V is the applied voltage and of 200 nm is the thickness of the absorption layer, was 8.5 × 10 4 V/cm. The resulting hole mobility and the electron mobility in the absorption layers with various pentacene doping contents are listed in Table 1. It can be seen that the hole mobility of the absorption layer increased and the electron mobility of the absorption layer decreased with increasing the pentacene doping content. In particular, for the P3HT : PCBM absorption layer with pentacene doping ratio of 0.065, the hole mobility, compared with the P3HT : PCBM absorption layer, was increased from (0.94 ± 0.01) × 10 −3 cm 2 /Vs to (1.16 ± 0.01) × 10 −3 cm 2 /Vs. Contrarily, the electron mobility in the absorption layer was decreased from (1.37 ± 0.01) × 10 −3 cm 2 /Vs to (1.16 ± 0.01) × 10 −3 cm 2 /Vs. The opposite variation of the electron mobility and the hole mobility with pentacene doping content indicated that the ratio of the hole mobility and the electron mobility was accordingly modulated. As seen from the results listed in Table 1, the carrier mobility ratio varied with the pentacene content and, in particular, balanced carrier mobility of 1.000 ± 0.001 was obtained in the absorption layer with a pentacene doping ratio of 0.065 by weight.
The variation of the carrier mobility upon pentacene doping can be understood based on the photovoltaic process in the polymer solar cells (PSCs) described below. Figure 3 shows the carrier transport process as well as the energy level diagram of the component materials in the inverted PSCs with P3HT : PCBM : pentacene absorption layer. In the process, the generation and transport of the carriers in the absorption layer played the most important role and were illustrated in detail in Figure 4. As seen from Figure 4, the domains of the donor material were separated in the acceptor materials. When the incident light was absorbed by the donor material, electron-hole excitons were generated in P3HT, as shown in Figure 4(a). The excitons diffused to the interface between the donor and acceptor materials, as shown in both Figures 3 and 4(b). At the interface, the excitons were dissociated and the resulting electrons are transited into the electronegative acceptor materials, whereas the resulting holes remained in the P3HT, as shown in Figure 4(c). Afterwards, the resultant electrons and holes are transported towards the corresponding electrodes [23] as shown in Figure 3. However, due to the energy difference between LUMO of pentacene (electron affinity, 2.9 eV) [24] and the LUMO of PCBM (electron affinity, 3.7 eV) [25], as shown in Figure 3, the addition of pentacene doped into the absorption layer obstructed the electron transportation and decreased the electron mobility in the absorption layer. Consequently, the electron mobility in the absorption layer decreased with an increase of the pentacene doping content. On the other hand, the hole mobility enhancement could be attributed to the improvement in the crystallinity of the P3HT, as reported previously [26]. To demonstrate this phenomenon, the crystallinity analyses of the absorption layers with various pentacene doping contents were carried out using XRD and the results are shown in Figure 5. As shown in Figure 5, the XRD spectra of all the deposited absorption layers exhibited a (100) diffraction peak of the a-axis orientation of P3HT [27]. Moreover, the intensity of the diffraction peak increased with an increase of the pentacene doping content. These results indicated that the crystallinity of the P3HT in the absorption layer was improved by doping the pentacene. This phenomenon indicated that the hole mobility enhancement could be attributed to the enhancement in the crystallinity, induced by pentacene doping, of the P3HT in the absorption layer.
Except the mobility balance in the absorption layer, changes in the other properties of the absorption layer upon pentacene doping might affect the performances of the resulting solar cells. Figure 6 shows the absorption spectra, in which the wavelength ranged from 300 nm to 800 nm, of the absorption layers with various pentacene doping contents. As shown in Figure 6, the absorption of the absorption layers increased with an increase of the pentacene doping weight ratio. The absorption enhancement is obviously favorable to the solar cell performance. In general, the absorptivity of the polymer is larger as the electric field of the incident light is aligned parallel to the orientation of the polymer main chains [28]. In other words, the improvement of the P3HT crystallinity enhances its absorption for the light incident perpendicularly to the main chains of the crystalline P3HT. In our case, as observed above by XRD analysis, the intensity of the (100) diffraction peak for the P3HT, which corresponded to an alignment of P3HT main chain parallelly to the substrate [27], increased with an increase of the pentacene doping content. It implied an improvement of P3HT crystallization with its main chain parallel to the substrate. Therefore, according to the previous observation [28], the absorptivity enhancement of the absorption layer was attributed to the crystallinity improvement of the P3HT in the pentacene-doped absorption layer. Furthermore, Figure 7 shows the surface morphologies of the absorption layers with various pentacene doping contents.  [13,29]. Based on this observation, it could also be deducted that the crystallinity of the P3HT : PCBM : pentacene absorption layer was improved with an increase of the pentacene content, which was consistent with the above-mentioned XRD measurement results. Moreover, the increased surface roughness could enhance the light utilization via internal reflection and scattering at the roughened surface, which was also beneficial to the exciton production. To clearly demonstrate this feature, the reflectivity spectra of the P3HT : PCBM : pentacene inverted PSCs with various pentacene doped absorption layers were measured and the results are shown in Figure 8. As shown in Figure 8, the reflectivity of the inverted PSCs was slightly decreased with an increase of the pentacene doping content, which implied that the diffused reflection light from the roughened surface was more effectively absorbed by absorption layer. Besides, the roughened surface increased the contact area between the polymer film and the metal anode. Therefore, the photocurrent of the PSCs could be increased [30]. As demonstrated in the above discussion, pentacene doping improved the crystallinity of the P3HT in  the absorption layer, which in turn caused changes in the carrier mobility, absorption, and surface roughness of the absorption layer. All of these changes affected the performances of the resulting devices. Figure 9 shows the -characteristics of the inverted PSCs with absorption layer of various pentacene doping contents. The photovoltaic characteristics of solar cells A, B, C, D, and E, including short circuit current density ( sc ), open circuit voltage ( oc ), fill factor (FF), and power conversion efficiency (PCE), were derived from the measured -characteristics and the results are listed in Table 2 be evidenced from the relationship of oc with the reverse saturation current density 0 [31]: where is the ideality factor, is the electron charge, is Boltzmann's constant, is the absolute temperature, ph is the photocurrent density, and 0 is deduced by extrapolating the linear regions of the dark current density-voltage curve ( Figure 10) to = 0. These devices with various pentacene doping contents exhibited similar dark current density and correspondingly had the similar oc . It could be seen that the performances, other than oc , of the inverted PSCs were improved by doping pentacene of a low ratio into the absorption layer and were optimized when the weight ratio of pentacene was 0.065 (solar cell D). For this optimized cell, the sc and PCE were 11.26 ± 0.04 mA/cm 2 and 4.31 ± 0.03%, respectively, which were obviously better than those of 9.73 ± 0.03 mA/cm 2 and 3.39 ± 0.02% for solar cell A. To further investigate the variation of the above-mentioned solar cell performances, the external quantum efficiency (EQE) of the inverted PSCs with various pentacene doping contents was measured in the wavelength ranged from 300 nm to 800 nm. The results, as shown in Figure 11, exhibited a similar variation as the pentacene content varied. For example, at the wavelength of 515 nm, the EQE of solar cells A, B, C, D, and E was 51.4 ± 0.1%, 55.0 ± 0.1%, 57.4 ± 0.1%, 59.5 ± 0.1%, and 58.3 ± 0.1%, respectively, in which solar cell D was the best one. The results discussed above indicated that the solar cell efficiency (EQE or PCE) was increased with an increase of the pentacene doping content in the absorption layer when the doping ratio was low and reached the maximum at the doping ratio of 0.065. When the pentacene doping ratio was further increased to 0.07 (solar cell E), the EQE was 8 International Journal of Photoenergy   degraded. The efficiency improvement at lower pentacene doping ratio could be attributed to both the enhancement in the absorptivity of the absorption layer and the improvement in the carrier mobility balance. Obviously, the efficiency degradation at higher doping ratio could not be ascribed to the change in the absorptivity of the absorption layer. As mentioned above, the absorptivity increased monotonically with the pentacene doping ratio, which tended to enhance the solar cell efficiency. However, it was noticed that the efficiency of the solar cell varied in a similar way as the carrier mobility ratio did (see Table 1). The optimal efficiency was achieved at the same pentacene doping ratio when the mobility in the absorption layer was properly balanced. At a higher doping ratio, say 0.07, the EQE degraded, while the carrier mobility ratio was increased to 1.044 ± 0.001, departing obviously from the balance. This unbalanced mobility resulted in an accumulation of low mobility carriers, causing an increase in carrier recombination and a decrease in PCE and EQE. This kind of correlation between EQE and the mobility balance implied that the carrier mobility balance played an important role in the efficiency variation.

Conclusion
In summary, the inverted PSCs with various pentacenedoped absorption layers were fabricated. Using the SCLC method to measure and estimate the hole mobility and the electron mobility in the resulting absorption layer with various pentacene doping contents, it was revealed that the carrier mobility in the absorption layer could be modulated by doping various pentacene contents. In particular, the required carrier mobility balance ( ℎ / = 1.000) was obtained in the P3HT : PCBM absorption layer with the pentacene doping ratio of 0.065. Using the absorption layer with the balanced carrier mobility could reduce the carrier recombination in the absorption layers and hence enhance the photocurrent of the resulting inverted PSCs. Moreover, more electron-hole excitons were generated in the pentacenedoped absorption layer due to larger absorptivity and larger surface roughness, which provided additional contribution to the performance improvement. The maximum PCE of 4.31 ± 0.03% was obtained for the inverted PSCs with the pentacene doping ratio of 0.065 in the absorption layer.