Fig. 1(a) shows the extinction spectra of Au-NPs in adequate solution determined by UV-vis spectroscopy, exhibiting a plasmonic resonance peak located at 530 nm. The average particle size are ca. 50±7 nm, estimated from the image of scanning electron microscopy (SEM) (see inset in Fig. 1(a)). Those Au-NPs are doped into the PEDOT:PSS hole transport layer before contacting with the top Ag electrode. As an example, the finished device structure with (x, y) = (2-NT, Au) is shown in Fig. 1(b).
Fig. 2(a) depicts the current density(J)–voltage(V) characteristics for those four kinds of cells and the corresponding performance parameters are summarized in Table 1. The standard cell denoted as (x, y)
= (none, none) shows the short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF), and power conversion efficiency (PCE) of 9.9 mA/cm2, 0.5 V, 41.3%, and 2.02%, respectively, which agrees well with that reported in the literature [4]. Doping the rear interlayer with Au-NPs results in a moderate improvement in Jsc (~8%) and FF (~14%) at a constant V oc, and an approximately 21% enhancement in PCE. By simply modifying the ZnO-nanorod array with 2-NT molecules, there are
~ 21%, ~24%, ~22%, and ~86% improvement in Jsc, Voc, FF, and PCE, respectively. Apparently, in addition to the much higher Jsc and FF values than the Au-NP doped case, modifying the front interlayer with 2-NT molecules also enhances Voc. By manipulating both interlayers, the PCE of the device is further improved to 4.20% with a Jsc, Voc, and FF of 12.8 mA/cm2, 0.61 V, and 53.8%, respectively. For the best cell, the PCE can be up to 4.36%, an improvement factor of ~120%. The largely
enhancement in IPCE values over the wavelength range from 400 to 600 nm for all modified cells. Fig. 2(c) exhibits the corresponding IPCE enhancement factor with respect to the standard cell. In the wavelengths from 400 to 600 nm, the IPCE enhancement factor is about 1.2 for 2-NT modified device, while it is ~1.1 for Au-NPs doped cells. By combining both approaches, there is approximately 30% increment in the IPCE value, indicating that the collected extra charge is almost the sum of the individual case.
We then measured the UV-vis absorption spectra for the BHJ film under different modification conditions. According to Fig. 3(a), the structure with neat interlayers shows typical absorption characteristics for P3HT in the wavelengths of 400 – 650 nm with three vibronic state transition peaks at 515, 550, and 600 nm [5,6]. Modifying the front interlayer with 2-NT molecules neither alters the spectra feature nor the photon absorption efficiency. However, incorporating Au NPs into the rear interlayer enhances light absorption of the photoactive film in 450 – 600 nm region. The average enhancement factor is ~1.05 (see inset in Fig.
3(a)), suggesting a harvest of additional 5% of incident photons by the photoactive layer as Au-NPs is added.
Because the photoactive layer absorbs only additional 5% photons, it is unlikely to produce ~30% extra charge carriers for the collection as shown by the IPCE measurements. The largely enhanced Jsc should arise from some other factors. Based on Mihailetchi and co-workers’ analytical approach [7,8], we compared the photocurrent behavior for these cells.
The photocurrent density (Jph) is defined as the current density difference between the cell under illumination and in the dark, and the effective voltage (Veff) is determined by V0 – Va, where V0 is the voltage when Jph = 0 and Va is the applied voltage. As shown in Fig. 3(b), the Jph increased linearly with Veff at low voltages and then saturated at a certain high value of Veff. The saturated photocurrent density (Jsat) is independent of the bias and temperature and can be correlated with the maximum exciton generation rate (Gmax) through Jph = qGmaxL, where q is the elementary charge and L is the thickness of the photoactive layer, by assuming that
all of the photogenerated excitons are separated into free carriers and contributed to the current [7,8]. The obtained Jsat for (x, y) = (none, none), (none, Au), (2-NT, none), and (2-NT, Au) were 11.8, 12.8, 13.9, and 14.1 mA/cm2, respectively, which correspond to Gmax of 3.7 ×1027, 4.0 ×1027, 3.9 ×1027, and 4.4 ×1027 m – 3s – 1, respectively. It is known that Gmax is governed by the maximum number of photon absorbed [6,7]. The incorporation of Au-NPs in PEDOT:PSS increases Gmax, suggests that more photons are absorbed in the photoactive layer, which is consistent with UV-vis absorbance measurement in Fig. 3(a). Sample treated with only 2-NT molecules also exhibits a subtle increase in Gmax. Since the photon absorption efficiency remains the same (see Fig. 3(a)), the enhancement can be attributed to the passivation effect of 2-NT molecules on the ZnO-nanorod surface to minimize the carrier loss at the surface defect states, such that more free charges can contribute to Jph. This surface passivation effect has been resolved by the fluorescence measurement previously [3]. By taking the advantages of both treatments;
i.e., SPR and surface passivation effects, Gmax displays the highest value among the cells.
In fact, not all the photogenerated excitons are completely dissociated into free carriers. The excition dissociation properties P(E,T) can be related to Jph through Jph = Jsat P(E,T) [9]. The calculated P(E,T) values under Jsc condition are 83.6%, 85.3%, 86.3%, and 87.1% for (x, y)
= (none, none), (none, Au), (2-NT, none), and (2-NT, Au), respectively.
Therefore, modifying either the front or the rear interlayer shows an improved exciton dissociation rate at different extent, which suggests that either SPR and 2-NT modification can effectively assist exciton separation. This role for 2-NT molecule is also evidenced by the shortening of fluorescence decay lifetime [3]. According to Wu et al. [10], SPR enhanced exciton dissociation probability can be understood as the plasmon–exciton coupling participating in the charge transfer process and can also be further interpreted by the concept of “hot excitons,” which possess excess energy to overcome their initial Coulombic potential [11,12]. With the integration of both treatments, P(E,T) can be further
enhanced by a factor of 4.1%. The enhancement of exciton dissociation rate can have significant contribution to the largely enhanced Jsc.
Open-circuit voltage decay (OCVD) technique was employed to obtain the carrier lifetime [13]. The cell was initially illuminated at a constant light intensity followed by an interruption of the illumination and the evolution of Voc with time (t) was monitored simultaneously. The Voc (t) follows the exponential decay behavior of a specific time constant τ. By exciting the cell at different initial steady states, a set of Voc (t) curves can be obtained and the corresponding τ can be calculated. Fig.
3(c) displays the recorded decay transient curves of Vocs under the application of a light pulse at various incident intensities for the standard sample. Using the same measurement procedures, the resulting decay lifetimes of Vocs for the four types of cells along with the corresponding fits are summarized in Fig. 3(d). Based on the fitting results, one can extrapolate the carrier lifetime for each type of cell to one sun condition and the obtained values for (x, y) = (none, none), (none, Au), (2-NT, none), and (2-NT, Au) cells are 25, 67, 140, and 200 μs, respectively.
The carrier lifetime for the standard cell obtained in one sun using the OCVD method has similar order of magnitude as that measured by using the impedance spectroscopy [14]. The incorporation of either Au-NPs or 2-NT molecules can effectively prolong the carrier lifetime for approximately 3 or 6-fold. By combining both approaches, the carrier lifetime can be further extended to almost 8-fold. The extension of the carrier lifetime by the inclusion of Au-NPs is a result of improved carrier mobility as suggested by Lu et al. [9], which is also the same for the effect of 2-NT molecules as shown in Ref. 32. We have also measured the carrier mobility by employing charge extraction in a linearly increasing voltage (CELIV) method and the obtained mobility values for (none, none), (none, Au), (2-NT, none), and (2-NT, Au) are 4.0×10 –5, 4.6×10 –5, 7.5×10 –5, and 8.2×10 –5 cm2V – 1s – 1, respectively. There is an improvement of carrier mobility for the cells with treated interlayer and the higher mobility can reduce the probability of carrier recombination, and hence the longer carrier lifetime. Therefore, both the surface
modification and SPR effect can improve the charge transport through the active layer and interface to the electrode. The coupling effect of both approaches further maximizes the charge transport of the cell structure.
Based upon the above results, manipulating the dual interlayers results in a significant improvement in the performance of solar cells and the underlying mechanisms can be understood as follows. Doping the rear interlayer with Au-NPs produces an approximately 5% additional harvested photon numbers, which can contribute to photocurrent. Because there is no changes in Voc, it suggests that the interlayer property is not altered in the presence of Au-NPs and still remains ohmic contact with Ag [15,16]. Though the series resistance (Rs) slightly increases from 5.6 to 6.0 Ωcm2 and the shunt resistance (Rsh) subtly decreases from 274 to 263 Ωcm2, the cell performance is improved, revealing that the cell performance does not result from a reduction in cell resistance. Instead, there is a subtle improvement in exciton dissociation rate and carrier lifetime owing to the SPR effect from Au-NPs. In general, an enhanced exciton dissociation probability reduces the carrier recombination rate, which is also supported by the improved carrier lifetime and, therefore, the FF of cells. Thus, we attribute the increased FF to the enhancement of exciton dissociation probability [10] and extended carrier lifetime [9]
resulting from the locally enhanced electromagnetic field originating from the excitation of the SPR. Consequently, the application of the SPR concept can lead to an improved efficiency of the cell from 2.02% to 2.45% (~21% enhancement). Modifying the front interlayer with 2-NT molecules results in ~86% enhancement in PCE due to largely enhanced Jsc, Voc and FF. The multiple functions of 2-NT layer enhance Voc due to the surface passivation effect and Jsc as results of improved exciton dissociation rate and a longer carrier lifetime. The higher FF can be partially attributed to reduced Rs from 5.6 to 5.0 Ωcm2 and enhanced Rsh
from 274 to 294 Ωcm2 and partially due to the increased exciton dissociation rate and lifetime. By integrating both approaches in a single cell, we obtain a giant enhancement in the cell performance because of taking the advantages of both treatments. Doping the rear
interlayer with Au-NPs has the unique feature of benefiting the photon absorption quantity while manipulating the front interlayer is more crucial for diminishing the surface defect states of the metal-oxide layer to minimize surface recombination events. For instance, as shown in Fig. 3(a), the additional 5% harvested photons due to Au-NPs scattering produce ~30% increment in Jsc. The reason for this magnification effect is that, in addition to the extra absorbed photons, the carrier dynamics including both the exciton dissociation rate and the carrier lifetime are maximized through the coupling of the effects of surface modified conjugated small molecules and the SPR. In such case, more free charges can be obtained and collected at electrodes by travelling through a better charge transport pathways. Further, the surface passivation of the front metal-oxide layer can raise the Voc of the cell. Additionally, the high FF is attributed to the highly reduced recombination rate due to the much increased exciton dissociate probability from combining both approaches.
Thus, the cell efficiency can be further maximized and a PCE of 4.36%
for the best cell with ~ 120% increment is achieved. This sets the record of the inverted polymer solar cell using P3HT:PCBM as the photoactive layer.
Acknowledgement
This work is supported by the National Science Council, Taiwan (Project No. NSC 102 - 2112 - M - 239 - 001 - MY3).
Figure Fig.4.1
(a) (b)
Fig.4.1. (a) UV-vis absorbance for Au-NPs in adequate solution. The inset shows the SEM image of Au-NPs on a silicon wafer. (b) The schematic structure for the cell
(b)
(c)
Fig.4.2 Evaluation of cell performance. (a) Current density (J)- voltage (V) characteristics, (b) incident-photon-conversion-efficiency (IPCE) curves, and (c) enhancement factor of IPCE values of the standard and cells under different manipulation conditions.
Fig.4.3
(c)
(d)
Fig.4.3 (a) UV-vis spectra for cells under different manipulation conditions. (b) Photocurrent density (Jph) vs. effective voltage (Veff) characteristics of the standard and manipulated cells. (c) The response of open circuit voltage (Voc) as a function of time to a light pulse with various light intensities. (d) The carrier lifetime as a function of Voc along with a fitting curve for each type of the cell.
0.0 0.1 0.2 0.3
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Chapter5
Conclusion
In conclusion, a giant enhancement of inverted solar cell efficiency has been demonstrated with the integration of different approaches based on the fabrication feasibility and impact on the cell performance. The front metal-oxide interlayer adopts the surface modification strategy by self-assembled a layer of 2-NT molecules on the ZnO-nanorod surface while the rear interlayer employs the surface plasmon resonance effect through doping the PEDOT:PSS with Au-NPs. Particularly, the former one can effectively passivate the metal-oxide surface and the latter one can improve the photon absorption efficiency. In addition, both approaches can also effectively enhance the exciton dissociation rate and extend the carrier lifetime. With the integration of both approaches, the fabricated cell can not only take the advantages of the individual treatment, but also have the benefits of the coupling between these two approaches. Therefore, the cell efficiency can be enhanced from 2.02 % to 4.36%, which represents the highest record reported so far in inverted solar cells using ZnO-nanorod as electron transporting layer and P3HT:PCBM as the photoactive material. The proposed method can be generalized to other polymer blend systems as well and open up a new route for designing high efficiency polymer solar cells.