Photovoltaic performance - In-situ Growing CdS Single-Crystal Nanorods via

Chapter 6 In-situ Growing CdS Single-Crystal Nanorods via

6.6 Photovoltaic performance

Figure 6.6 shows the photoluminescence emission spectra of P3HT and CdS /P3HT composites in toluene. In agreement with previous work [162] a reduction in the spectral intensity of the composites relative to the reference P3HT sample was observed and the reduction of PL intensity is increased with the aspect ratio of CdS nanorod. This reduction of PL intensity is due to photogenerated charge transfer between the CdS and P3HT [33]. According to previous studies, the decreased photoluminescence intensity of the composites is related to an improvement in photovoltaic performance [52]. The photoluminescence quenching can be also used as a powerful tool for evaluation of charge transfer efficiency in the donor-acceptor blend composites [30, 178]. Once the photogenerated excitons are dissociated, the probability for recombination should be significantly reduced. This is a well-known effect of the ultrafast electron transfer from the donor to acceptor and it is expected to increase the exciton dissociation efficiency in photovoltaic devices [30, 178, 179].

(a) (b)

450 500 550 600 650 700 750 800

In te ns ity (a .u .)

Wavelength (nm)

P3HT

CdS/P3HT(AR=4) CdS/P3HT(AR=8) CdS/P3HT(AR=16)

Thus the synthesis of the composites with high-aspect-ratio, single-crystal CdS nanorods provides a significant interest and encourages for further investigations onto the photoconductivity assessment for solar cell use.

Figure 6.6 Photoluminescence emission spectra for P3HT and CdS nanorods/P3HT composite solution (both solutions in toluene pumped at 450 nm).

Figure 6.7 gives a summary of operating characteristics for P3HT/CdS-based device using the synthesized CdS nanorods of various aspect ratios. Figure 6.7(a) shows the variation of both JSC and VOC with various aspect ratios of CdS nanorods. It was observed that JSC obviously increased with the aspect ratio of the CdS nanorods but VOC, which is limited by the difference between the HOMO of the donor and the LUMO of the acceptor [20, 180], remained identical at ~0.64 V. It suggests a percolation of the CdS forming connective network across the device, rendering the device behaves like a complete heterojunction. The influence of aspect ratio of the CdS nanorods on power conversion efficiency (PCE) and fill factor (FF) is illustrated in Figure 8(b). Comparing with Figures 8(a) and 8(b), it can be found that both JSC

and FF increased with the aspect ratio of the CdS nanorods, suggesting that the aspect ratio of CdS nanorods reduced charge recombination. The PCE of the well-assembled device using the nanorods with aspect ratio of 16 leads to a considerable improved power conversion efficiency as high as 2.9% due to an increase in both FF and JSC. It was inferred that for the photoactive layers consisting of equiaxial nanoparticles, electron transport is dominated by hopping, but the band conduction is prevalent in the photoactive layers consisting of nanorods [50] because the presence of nanorods with high aspect ratio can penetrate through a large portion of the device to develop percolation pathways for electron transport. In other words, longer nanorods are easier to form a percolation path between two electrodes and thereby can thus fully contribute to photovoltaic conversion. Therefore, the increase in PCE and JSC is primarily a result of efficient charge transport.

Figure 6.7 Hybrid solar cell characteristics. (a) VOC and JSC (b) PCE and FF.

6.7 Summary

Formation of CdS nanorods and the corresponding variation of the aspect ratio of the nanorods can be well manipulated via a soft templating technology using the

planar P3HT polymer, as a molecular template upon synthesis, and in the meantime, as a conducting matrix to form final P3HT-CdS composite structure, which, to the best of our knowledge, has been explored for the first time. The formation of the nanorods with various aspect ratios, ranging from ~8 to 16, is controlled by the DCB-to-DMSO ratio of the co-solvent. The nanorods with higher aspect ratio allow connective network to develop in the P3HT matrix, facilitating percolation pathways for electron transport. Therefore, the charge transport is considerably enhanced by using the CdS nanorods with higher aspect ratio. The enhancement in carrier mobility can be accomplished by improving the CdS-P3HT interface without the involvement of surfactants. All above merits give a PCE to a level as high as 2.9%, albeit there has rooms in technical development not yet being fully explored and optimized in terms of dimensional control of the nanorods and device assembly in this investigation. The results offer significant practical advantages in CdS nanorod-based solar cell device compared to those solar cells reported via the use of conventional hybrid composites.

Chapter 7 Annealing Effect in Photovoltaic Performance for Hybrid P3HT/Elongated CdS nanocrystal solar cells

7.1 Introduction

Recently, solar cells based on soluble conjugated polymers have attracted attention because conjugated polymers are promising materials for the development of low-cost optoelectronic devices [221, 181]. Organic photovoltaic cells using conjugated polymer/fullerene blends in a bulk heterojunction architecture have been reported with efficiencies up to 6.5% [182]. A bulk heterojunction is usually composed of two phases in the photoactive layer: fullerenes or nanocrystals for the acceptor phase and polymers for the donor phase. In a bulk heterojunction, the large interfacial area between the donor and acceptor phases provides sufficient charge separation, which is a prerequisite for solar-cell operation [183]. Upon irradiation, the conjugated polymer absorbs the photons and produces excitons. Exciton dissociation is known to occur at the interface between two materials with different excited-state energy levels; the electron will move to the lower excited-state energy level of the acceptor because it is energetically favorable. Upon irradiation the electrons from the excitons in the conjugated polymer transfer to the nanocrystals, and the holes from the excitons in the nanocrystals transfer to the conjugated polymer. The optimum overall performance of a bulk heterojunction cell can only be achieved by balancing the various requirements of photogeneration, transport, and extraction of charge carriers.

The bulk heterojunction system still has some difficulty achieving the 10%

power conversion efficiency (PCE) required for commercialization. The most serious problem is the difficulty of controlling the solid-state phase morphology of the two phases in the photoactive layer because the exciton diffusion length in conjugated polymers is typically between 5 and 10 nm [184, 185]. A variety of ideal photovoltaic structures have been proposed with interdigitated, pure phases spaced by a distance equal to or less than the exciton diffusion length. Some approaches to control donor/acceptor morphology have recently been reported, including the choice of spin-coating solvent [186, 187], slow drying of spin-coated films [28, 188], and controlled thermal annealing [189, 190]. Among them, thermal annealing is indispensable for complete solvent evaporation and phase separation for the bulk heterojunction formation, and this annealing greatly influences the evolution of the film morphology and device efficiency of hybrid cells in polymer/fullerene systems [104-106, 191]. For instance, Reyes-Reyes et al. reported that a significant improvement in the power conversion efficiency from 1.1% to 5% for P3HT-PCBM blends can be obtained by subjecting the sample to thermal annealing, even at low PCBM loading fractions [191], because annealing changes not only the film crystallinity but also the aggregation within the PCBM nanophase. However, only a few reports have focused on polymer/inorganic semiconductor composites. Olson et al.

reported that phase separation is heavily influenced by nanocrystal ligand choice [192]. The ligands often dominate the electrical performance of the nanocrystals because they are generally insulators (alkyl chains) that impede charge transport between individual nanocrystals. The ligands can also play an important role in controlling the degree of nanocrystal aggregation, the limitation of which can improve charge transport [54]. Though surface ligands are insulators that prevent charge transport between nanocrystals, surfactants or ligands have been widely used in most literature reports in polymer or polymer/semiconductor hybrid systems. In other

words, there are very few reports in the literature in which no ligands were used to control or fabricate composites. An in-situ-growth of the polymer/semiconductor hybrid system could be developed without ligands in order to enhance the performance. In our previous report [193], we discussed the development of an in-situ growth of CdS/P3HT with different aspect ratios of CdS and demonstrated that the photovoltaic efficiency can be much enhanced by using a larger aspect ratio of CdS nanorods in the CdS/P3HT system.

However, as mentioned before, the efficiency and morphology of hybrid cells eare strongly influenced by thermal annealing because it is able to change the interaction or bonding between the P3HT polymer and the nanoparticles, which will affect the device performance. To effectively employ this ligand-free poly(3-hexylthiophene-2,5-dyl) (P3HT)/CdS nanocrystals with various nanocrystal ARs material system, it is imperative to understand the interaction of P3HT with the CdS nanocrystals and how it affects the blend morphology, phase separation and ultimately the photovoltaic device performance. The effect of annealing on the physical interaction between P3HT polymer and CdS nanocrystals was investigated on in-situ-grown P3HT/CdS nanocrystals systems with various nanocrystal ARs. The UV-vis results revealed the presence of a strong interaction between the P3HT and the CdS nanocrystals, which would further influence the extent of CdS aggregation in these blend films during the annealing process depending on the AR of CdS nanocrystals. This study on the annealing-condition-dependent PCE reveals that the polymer-nanocrystal interaction has a dramatic effect on the photovoltaic performance of hybrid solar cell devices.

7.2 Annealing effect on optical properties

Figure 7.1(a) shows the UV-Vis absorption spectra for the thin films of P3HT and the

P3HT/CdS composites with various ARs as spun cast on glass substrates. For the pure P3HT film, the solid-state absorption spectra showed two peaks at 521 nm and 556 nm and one shoulder at 605 nm. The shoulder at 605 nm is generally attributed to a higher crystallization or ordering of intra-chain interactions in semiconducting polymers and the peak intensity depends on the order degree in the intermolecular chains of the microcrystalline domains [103, 190]. These bands can be attributed to the π–π* transition [103]. As the AR of CdS nanocrystal increases, the absorption bands are blue-shifted. The shift of π–π* transition absorption peaks to shorter wavelength indicates an increasing density of conformational defects, which causes the loss of conjugation [194, 195]. This indicates that the blue shifts are due to the in-situ growth of CdS nanocrystals in the P3HT matrix, which induced the consequent destruction of the P3HT chain ordering during solvent evaporation. Figure 7.1(b) and 1(c) show the UV–vis spectra measured for P3HT/CdS (AR=4 and 16) films before and after annealing at different temperatures for 60 min. After thermal annealing, the spectra shifted towards longer wavelengths and the shoulder at 610 nm became more distinguishable when the annealing temperature increased because the interchain interaction between P3HT chains is stronger [103]. It is implied that the annealing treatments can improve ordering of P3HT chains.

Figure 7.1 (a) UV-Vis absorption spectra obtained for P3HT/CdS thin films with various ARs. UV-Vis absorption spectra of P3HT/CdS thin films with AR of (b) 4 and (c) 16 before and after annealing treatments.

The PL spectra in Figure 7.2 display the same trend, which supports this understanding of the structural variation of the P3HT/CdS composite film after annealing treatment. The PL intensity of the annealed sample was higher than that of the as-grown (or unannealed) samples. This indicates that the photo-induced electron transfer from the P3HT to CdS becomes less efficient upon annealing. The efficiency of electron transfer obviously depends on the mean distance between the conjugated polymer and CdS because the film thickness was much larger than the P3HT exciton diffusion length. The increase of the PL intensity on thermal annealing is consistent with an assumed increase in the size or number of P3HT crystallites with dimensions

(a)

(b) (c)

larger than the exciton diffusion length, allowing for radiative relaxation rather than electron transfer at surrounding heterojunctions. Because the concentration of the CdS nanocrystals in the film does not change upon annealing, the change of the photoluminescence intensity should primarily originate from the CdS aggregation and the morphology evolution of the active layer. A larger increase in PL intensity was found for the P3HT/CdS composites with an AR of 4 (from line 1 to 3) than those with an AR of 16 (from line 4 to 6). This indicates that upon annealing, the photo-induced electron transfer from the P3HT to CdS for the CdS nanocrystals of AR=4 becomes less efficient than that with CdS nanocrystals of AR=16. Furthermore, the PL spectra of P3HT showed resolved vibronic structures at around 647 and 708 nm and found that Gaussian curves fit the PL curve well. The data of the Gaussian-fitting of the two peaks are shown in Table 1. The PL emission peak located at ~ 647 nm (1.92 eV) was assigned to the pure electronic transition and the peak at ~ 708 nm (1.75 eV) was assigned to the first vibronic band [196]. The red shifts in PL emission after annealing treatment suggest that the polymer chains are π-stacked on each other [94, 197]. The increase of the conformational order in the composites was attributed to the alignment of the polymer chains in the vicinity of nanocrystals during annealing treatments.

Figure 7.2 The fluorescence spectra of P3HT/CdS nanocrystals with an AR of 4 and 16 before and after heat treatment. Line (1): AR=4, unannealed. Line (2): AR=4, 140

°C. Line (3): AR=4, 160 °C. Line (4): AR=16, unannealed. Line (5): AR=16, 140 °C.

Line (6): AR=16, 160 °C.

Table 3 PL measurement of the P3HT/CdS nanocrystals with different ARs.

7.3 AR effect on thermal properties

Figure 7.3 shows the DSC thermograms of pure P3HT and the P3HT/CdS nanocrystal composites. An exothermic peak showing the typical features of a melting transition

Annealing

550 600 650 700 750 800 850

Wavelength (nm)

was detected for all the samples. The melting temperature of the P3HT/CdS nanocrystal composites depends on the crystallization conditions. The DSC data show that the exothermic peak was shifted to a lower temperature for the P3HT/CdS composites as compared to the pure P3HT, which is clear evidence of the influence of the CdS nanocrystals on the thermal properties. The shifting of the P3HT melting peak to a lower temperature indicates that the semi-crystalline nature of P3HT is partially hindered by CdS nanocrystals. We believe that the decrease in the crystallinity of the P3HT/CdS nanocrystal composites is related to the fact that the in-situ-grown CdS nanocrystals embedded between the P3HT chains inhibit the main-chain crystallization due to the strong interaction between the CdS nanocrystals and the P3HT [198]. Furthermore, it is noted that the P3HT/CdS nanocrystal composite with an AR=16 shows a larger shift than that with an AR of 4. This indicates that the degree of order in the P3HT is further reduced when the CdS of AR=16 is used. This observation, along with UV-vis absorption and PL measurements, indicates that on annealing, the growth of ordered polymer domains is more hindered by P3HT/CdS composites with an AR of 16. These differences from thermal annealing effects are important in the determination of the morphology of the device active layers and the improvement of the photovoltaic performance of P3HT/CdS composite-based solar cells.

0 50 100 150 200

Heat Flow

Temperature (^OC)

P3HT

CdS/P3HT(AR=4) CdS/P3HT(AR=16)

Figure 7.3 DSC thermograms of pure P3HT and P3HT/CdS nanocrystals with an AR of 4 and 16 at a heating rate of 20 °C /min.

7.4 Annealing effect on photovoltaic performance

It is well known that the performances of photoactive layers based on P3HT/nanocrystal blends are usually enhanced when a thermal-annealing step is applied. Therefore, an optimization concerning the annealing temperature and duration was carried out for P3HT/CdS nanocrystals with an AR of 4 and 16. As shown in Figure 7.4, the device using the nanocrystals with an AR=16 shows a higher power conversion efficiency (PCE) as compared to that with an AR of 4 at different annealing conditions. Generally, for the photoactive layers consisting of equiaxial nanocrystals, electron transport is dominated by hopping. In contrast, for the photoactive layers consisting of elongated nanocrystals, the band conduction is prevalent in the P3HT/CdS composites [50] because nanorods with a high AR can penetrate through a large portion of the device to develop percolation pathways for electron transport. In other words, longer nanocrystals more easily form a percolation

path between two electrodes and can thus more fully contribute to photovoltaic conversion. Therefore, the PCE of the AR=16 composite is higher than that of the AR=4 composite because of more efficient charge transport.

In addition, the improved device performance depends heavily on the annealing conditions, as clearly seen in Figure 5. The devices annealed at 160 °C show the best performance, with efficiency as high as 1.5% (t=40 min) and 2.9% (t=60 min) for AR=4 and 16, respectively. This improvement is primarily attributed to the higher photon absorption and better crystallinity of the P3HT chains, as evidenced by the UV–vis spectra (Figure 2). However, as increasing the annealing temperature over 160 °C, it results in performance degradation, which can be explained by morphological changes and low structural stability at high temperatures.

Figure 7.4 Power conversion efficiency plotted as a function of annealing temperature and time.

7.5 Interaction between P3HT and CdS surface

Figure 7.5 shows the surface morphology change as monitored by AFM. The morphology can be further investigated for the origin of the long-term stability of P3HT/CdS devices with different annealing temperatures. For the as-deposited

(a) (b)

(unannealed) film with AR=4 and 16, the surface is very smooth: the root mean square (rms) roughnesses are 1.067 nm and 1.184 nm, respectively. However, after thermal treatment at 160 °C, the rms roughnesses obtained from Figure 6(c) and 6(d) for AR=4 and 16 become 7.259 nm and 4.821 nm, respectively, because the CdS nanocrystals aggregate during thermal annealing. Moreover, it can be found that the scale of the aggregations for AR=4 is larger than that for AR=16, indicating that the CdS nanocrystals of AR=4 aggregate more easily than those of AR=16, which is in good agreement with the results of Figure 3. Compared with the film with AR=4, the film with AR=16 reveals rod-like texture due to a higher AR and smaller aggregations which consist of fewer CdS nanocrystals. As mentioned above, the best device performance and the highest PCE are obtained when the devices are annealed at 160 °C. Therefore, CdS nanocrystals aggregate excessively comparable to the exciting diffusion length, which leads to the reduced charge segregation and device efficiencies [199, 200].

Figure AFM height images of P3HT/CdS films with (a) AR=4 and (b) AR=16. (c) AR=4 and (d) AR=16 annealed at 160 °C for 60 min.

To understand the interaction and molecular structure between P3HT and CdS nanocrystals during in-situ growth of P3HT/CdS, ¹H NMR measurements were performed on the pristine P3HT and P3HT/CdS nanocrystals composites with an AR of 4 and 16. As shown in Figure 7(b) and 7(c), as compared to pure P3HT in Figure 7(a), it was found that the broader proton peaks at a chemical shift of ~6.98 (thiophene ring (a)) and ~2.79 ppm (hexyl chain (b)) were clearly observed for P3HT/CdS in Figure 7(b) and 6(c), which confirms that some interaction occurs between the polymer and the CdS. Additionally, the proton peaks from the hexyl chain (e) (~0.91 ppm) was only slightly broadened compared with the proton peaks

(a) (b)

(c) (d)

from the thiophene ring (a) and hexyl chain (b). A similar phenomenon also has been reported for polymer-CNT composites; the interactions of polymers with CNTs cause broadening and reduced intensity of ¹H NMR peaks [201, 202]. The closer the protons are to the surface of CNTs, the broader and weaker the peak will be. In addition, the degree of broadening in the peak corresponding to the thiophene ring is due to the corresponding protons coming very close to the nanoparticles [203]. This indicates that the thiophene ring is much closer to the CdS surface than the hexyl chain. This conclusion was further supported by the change of the relative intensity of proton peaks (a), (b), and (e). In the pristine P3HT solution, the ratio of protons (a), (b), and

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