L.
Yu
,
T.
Grace
,
M.
Batmunkh
,
M.
Dadkhah
,
C.
Shearer
and
J.
Shapter
*
Centre for Nanoscale Science and Technology, School of Chemical and Physical Sciences, Flinders University, Bedford Park, South Australia 5042, Australia. E-mail: joe.shapter@flinders.edu.au
First published on 25th October 2017
Graphene oxide/single-wall carbon nanotube (GOCNT) hybrid films have been used to fabricate heterojunction solar cells with silicon (Si) due to their compatibility with both aqueous and organic processing. In these cells GOCNT films are required to be both highly transparent and conducting. Different approaches are used to improve these optoelectronic properties of the GOCNT films, including hybridization with silver nanowires (AgNWs) and p-type doping with CuCl2, AuCl3, SOCl2, HCl, H2SO4, HNO3 and HClO4. UV-vis-NIR absorbance, Raman spectroscopy, and the sheet resistance of the films were used to evaluate the properties of the treated films and quantify doping. The most effective way to improve the optoelectronic properties of the GOCNT films was the incorporation of AgNWs which improved the figure of merit (FOM, the ratio of transparency and conductivity) by over 600%. However, GOCNT/Si heterojunction photovoltaic devices with HNO3 doped GOCNT films showed the highest solar photocurrent conversion efficiency (11.38 ± 0.26%). In terms of stability, CuCl2 and HCl doped films have the best electrode FOM stability, and devices made with such films have the most stable efficiency as well. This report suggests that the electronegativity of the active elements in the dopants has a strong influence on the optoelectronic properties of the films as well as the solar cell performance.
Recently, graphene oxide (GO) has been shown to be able to both help to disperse CNTs in water and enhance the aqueous compatibility of the as-prepared film due to its amphiphilic nature.22–25 Such hybrid films made with GO and CNTs (GOCNTs) have been used as transparent window electrodes and applied on silicon to fabricate graphene oxide carbon nanotube/silicon heterojunction solar cells (GOCNT/Si).26 Briefly, in a typical device formed with p-type GOCNT films as the window electrode and an n-type silicon base, the silicon is the light absorbing layer which produces excitons. The resulting excitons are then separated into charge carriers under the influence of the built-in potential at the interface of the heterojunction, as shown in Fig. 1(a).27–31 Holes are transported through the GOCNT network acting as a window electrode and collected by the front metal electrode while electrons are collected at the back metal contact via the silicon layer.21 The exact nature of the heterojunction is still not clear although it has been discussed thoroughly in the recent literature.32–37 It can be regarded as a Schottky barrier, a metal–insulator–semiconductor or a p–n junction. In some cases, the thin silicon oxide layer between Si and GOCNTs improves the device performance via a reduction in the reverse saturation current.38
Fig. 1 (a) Schematic energy diagram of GOCNT/Si heterojunction solar cells; (b) schematic of the bath doping process. MCE is the mixed cellulose ester filter and the substrate can be a glass slide or Si substrate coated with metal electrode. The filter paper dissolves in acetone and the GOCNT electrodes are then transferred to a dopant bath to conduct chemical doping. The GOCNT film is picked up with the substrate after being transferred to the next acetone bath. The band gap of GOCNT is estimated from the S11 peak position of the UV-vis-NIR spectra of the GOCNT films.32 |
In these solar cells, devices made with the as-deposited CNT films typically show mediocre performance due to high sheet resistance and low native p-doping. Chemical doping of the CNT film is normally performed to satisfy the practical requirements of the optoelectronic properties of CNT based window electrodes.39 The general p-type dopants include HCl,40 HNO3,41 H2SO4,42 SOCl2,43 AuCl3,44 and CuCl2.37 The sheet resistance (Rsheet) of CNT based networks is dependent on the conductivity of individual CNTs as well as the contact resistance between CNTs.1 Chemical doping results in the shifts of the Fermi level, which can increase the density of charge carriers and reduce the Schottky barrier height, ϕB, between metallic and semiconducting species, and as a result, the optical absorption peak caused by the first and second interband transitions in SWCNTs is suppressed or completely bleached (as shown in Fig. S1†).45 Recently, a bilayer structure of AgNWs/SWCNTs has been used to improve the efficiency of SWCNT/Si devices by nearly two-fold (from 4.31 to 7.89%) and the main improvement was attributed to the dramatic improvement of the optoelectronic properties with the addition of the AgNW top layer which provides highly conducting paths between CNTs.33,46
In this report, different p-type dopants as well as AgNWs are used to improve the optoelectronic properties of the CNT-based (GOCNT) transparent conducting films. Bath doping, as shown in Fig. 1(b), was used due to the benefit of the previously developed organic–aqueous transfer process making the films available for bath processing.21 This approach increases the doping period compared to simply dropping the chemicals on the films and eliminates the effect of the chemicals on the metal coated Si substrates (for example, the acid might dissolve the Cr layer) at the same time. AgNWs, due to their limited dispersity in water, could not be processed this way and they were filtered with GOCNTs on filter papers. The treated transparent films were then deposited on n-type Si to create GOCNT/Si heterojunction solar cells. Both the efficiency and the stability of the devices were studied and compared.
AgNWs were synthesized by following a typical polyol approach with slight modifications with the UV-vis spectrum and SEM images shown in Fig. S2.†48 The final concentration of AgNWs in water is 0.2 mg mL−1.
The redox colloidal solution (CuCl2 (99.0%, Sigma-Aldrich)/Cu(OH)2), (denoted as CuCl2 in the following section since Cu2+ in CuCl2 has been reported to be the active part for rapid p-type doping of SWCNTs by extracting electrons and being reduced to Cu1+ while Cu2+ hydroxide provides long-term doping stability)37 was prepared by adding a 1 M NaOH (98.0%, Chem-Supply) aqueous solution into a 0.1 M CuCl2 ethanol solution gradually. The volume ratio of these two solutions was kept at 1:2000.
118 mg of HAuCl4·3H2O (99.0%, Sigma-Aldrich) was dissolved in 30 mL of acetone at room temperature with gentle vortex mixing to prepare 10 mM AuCl3 solution. 10% v/v SOCl2 (99.0%, Sigma-Aldrich) was prepared by the dilution of pure SOCl2 with benzene. 10 wt% HCl (32.0%, RCI Labscan), 10 wt% HNO3 (70.0%, RCI Labscan), 10 wt% H2SO4 (98.0%, RCI Labscan) and 10 wt% HClO4 (70.0%, Merck) were prepared by dilution with deionized water.
In terms of the devices based on AgNWs, AgNWs–GOCNT films were fabricated by filtering a mixture of 150 μL GOCNT initial suspension and 50 μL AgNW stock solution diluted to 100 mL with deionized water. The reduced amount of GOCNT solution was needed in order to keep the transmittance at 550 nm (T) the same as that of the film without AgNWs. After dissolving the MCE in the first acetone bath, the AgNWs–GOCNT films were then picked up as described before.
The external quantum efficiency (EQE) of the devices was measured as a function of wavelength from 300 to 1100 nm by passing the chopped light from a xenon source through a monochromator.
The transparency of the GOCNT hybrid and AgNWs–GOCNT films at 550 nm was determined by UV-vis-NIR spectroscopy (Lambda 950 PerkinElmer) with the background subtraction of a clean glass slide. The Rsheet of the GOCNT films was measured using a four-point probe in linear configuration (Keithlink). The optoelectronic properties of the hybrid films could then be evaluated with a figure of merit (FOM) which is calculated by using eqn (1) and rearranged to eqn (2):
(1) |
(2) |
Raman spectra of the films were obtained by using an AFM-Raman system (Nanonics-Horiba) using an excitation energy of 1.96 eV. X-ray photoelectron spectroscopy (XPS) spectra of pure CNT and GOCNT films were collected at the soft X-ray beamline of the Australian synchrotron with more details shown in the ESI.†53
Chemical names (sum of electronegativities) | Working mechanisms |
---|---|
AgNWs (1.93) | The hybridized AgNWs act as the additional charge transport channels to bridge the less conductive GOCNT network which is regarded as a mechanical support33 |
CuCl2 (8.22) | Instant electron transfer from CNTs to Cu2+ in CuCl2 which is reduced to Cu1+ while the less active Cu2+ in Cu(OH)2 provides a persistent charge transfer effect on the CNTs.37 In addition, it is also possible that CNT–Cl is formed by the doping in which Cl attracts electrons from CNTs44 |
AuCl3 (12.02) | The doping mechanism is not clear. Some reports suggest the reduction of cationic Au3+ to Au nanoparticles causes p-type doping while some attribute the doping effect to the formation of CNT–Cl where electron transfer from CNT to Cl takes place44 |
SOCl2 (6.32) | The doping effect is caused by the decomposition of the molecules, 2SOCl2 + 4e− → S + SO2 + 4 Cl−, which naturally withdraw electrons from CNTs43 due to adsorption of SO2 and possibly production of CNT–Cl |
H2SO4 (16.54) | The intercalated SO3−/SO42− withdraws electrons from CNTs and shifts the Fermi level with the formation of more holes in the valence band. In addition, it could also remove some polymeric and amphipathic dispersant and therefore results in the reduction in the contact resistance between individual CNTs56 |
HNO3 (13.46) | Electrons are transferred from the surface of CNTs to the physisorbed molecules with NO3− groups, which causes the shift of the Fermi level to the valence band. A better contact is also created by the removal of impurities, such as metal catalysts and amorphous carbon56,57 |
HCl (3.44) | The mechanism is similar to those of HNO3 and H2SO4 by intercalation. The doping can shift the Fermi level down to the valence band56 |
HClO4 (16.92) | ClO4− groups might withdraw electrons from the surface of CNTs and result in the downshift of the Fermi level of SWCNTs into their valence band |
The optoelectronic properties of the prepared and modified GOCNT films (on microscope slides) were studied by UV-vis-NIR absorbance and sheet resistance. As shown in Fig. 2(a), all of the GOCNT films have a very similar absorbance at 550 nm and thus the visible transmittance of these films can be considered to be essentially the same (80%). In addition, the untreated GOCNT film shows two broad optical absorption peaks at about 1945 nm (S11) (shown in the inset of Fig. 2(a)) and 1010 nm (S22), which are consistent with those of large diameter arc discharge SWCNTs.58 The S11 peak remains and a wide absorption feature above 1000 nm is observed after the hybridization of GOCNTs with AgNWs, which indicates the existence of AgNWs in the electrode and suggests they may not have a p-type doping effect on CNTs.59 In contrast, both S11 and S22 peaks are suppressed to various extents after GOCNT films are treated with p-type dopants (the degree of the S11 and S22 suppression: HNO3 ≈ H2SO4 ≈ SOCl2 ≈ AuCl3 ≈ CuCl2 > HCl, as shown in Fig. S6†),60 which is a result of the shift of the Fermi level of CNTs into the valence band with electrons transferred from CNTs to dopants.61 As shown in Fig. S1,† a lower population of electrons in the valence band after p-doping is responsible for the suppressed S11 peak. Further evidence of doping is obtained from the Raman spectra, as shown in Fig. 2(b), the G/D ratios of all the films are very close, which indicates that both hybridization and p-type doping do not introduce structural defects into the CNTs.62 The untreated film has an evident metallic contribution for the G band on the low wavenumber side of the G peak (Breit–Wigner–Fano (BWF) peak), which is due to the strong interactions between electrons and phonons in metallic CNTs (Fig. 2(b) and S7†).63,64 GOCNT films treated with p-type dopants show a reduction in the metallic contribution in the G band (the reduction of the BWF intensity is more evident in films treated with dopants of higher electronegativity, as shown in Fig. 2(c and d), see Fig. S7† for the peak fitting of the Raman spectra) with a blue shift of the G peak position to different extents. The G band position of films treated with dopants with active elements of higher electronegativity seems to shift further, as shown in (Fig. 2(c and e)), which infers CNT stiffening leading to higher phonon energies after electron transfer from CNTs to dopants.65,66 However, films hybridized with AgNWs behave in a different way compared to the p-typed dopants. They show a broader feature in the BWF peak range and there is a red shift in the G band position with a similar BWF intensity to that of the untreated film, which suggests that AgNWs have a different working mechanism in GOCNT films.
Fig. 2 (a) UV-vis-NIR and (b) Raman spectra of untreated, AgNW hybridized, CuCl2, AuCl3, SOCl2, HCl, H2SO4, HNO3 and HClO4 doped GOCNT films; (c) an expanded view of the G band in Raman spectra from 1500 to 1700 cm−1; (d) BWF peak intensity (estimated from Fig. S7†)67,68 of GOCNT films treated with different materials and (e) G band position versus electronegativity. The blue dashed lines (in d and e) do not indicate any prediction but are added to highlight a trend. |
The influence of the hybridization and p-type doping on the optoelectronic properties is further studied with measured Rsheet and FOM (calculated by eqn (1) and (2)) versus the sum of electronegativity of the molecules or species acting on the CNTs, as shown in Fig. 3(a and b). Among all of the films, the untreated film has the highest Rsheet with the lowest FOM and the AgNW hybridized GOCNT film has the lowest Rsheet (120 Ω sq−1 (AgNWs–GOCNT hybrids) versus above 350 Ω sq−1 (p-doped GOCNT films)) with the highest FOM (12 (AgNWs–GOCNT hybrids) versus less than 4 (p-doped GOCNT films)), which indicates that the optoelectronic properties of the GOCNT film are significantly improved after the hybridization of AgNWs. Compared to the FOM value (above 90) in the previous literature, our value for AgNW hybridization is much lower, which might be due to the fact that the film in the literature had a much higher content of AgNWs (only 6 wt% SWCNT and the absorption peaks of SWCNTs were not observed in the UV-vis-NIR spectrum for the hybrid at all)69 than the films in this study. In terms of the p-type doped GOCNT films, it seems that the Rsheet/FOM decreases/increases exponentially with the sum of electronegativity of active elements in these dopants. Since the p-type doping is realized by electron transfer from the CNTs to adsorbed dopant molecules, it is not surprising that molecules containing more and/or higher electronegativity atoms have better ability to withdraw electrons. In order to test the validity of this relationship, we decided to use a dopant which had not previously been used in the literature to determine if it followed a similar trend. Doping with HClO4 (electronegativity sum = 16.92) was performed and the Rsheet and FOM seem to fit well in the correlation found before, as shown in Fig. 3(a and b) (the star). In addition, both the bleaching of S11 in the UV-vis-NIR and the upshifting of G band in the Raman spectrum revealed the p-doping nature of HClO4 treatment (Fig. 2).
Overall, hybridization of GOCNTs with AgNWs is the most effective way to enhance the optoelectronic properties of the films, and materials containing active atoms with higher electronegativity values are better dopants than those with lower values. Since the working mechanism of AgNWs (creating a more conductive metallic path between less conductive GOCNT networks, as shown in Fig. S5†) is different from that of p-type dopants, it does not fit the trend found in this study. We used a measure of electronegativity as opposed to (for example) redox potentials because in many cases there is more than one mechanism of action suggested and we used the sum of electronegativity to try to take into account all means of interaction.
Fig. 4 Performance of the solar cells based on GOCNT films with different treatments (a) J–V light curves; (b) J–V dark curves; (c) efficiency; (d) JSC; (e) EQE (the integrated JSC values based on EQE are listed in Table S3†); (f) VOC; (g) FF; (h) Rshunt; (i) Rseries; (j) ideality; (k) Jsat and (l) ϕB. The blue dashed lines in (c), (d) and (f–l) do not indicate any prediction but show the trends in these plots. |
The properties of the junction were further studied by analysis of dark J–V curves. The shunt resistance (Rshunt) of the interfaces increases with the electronegativity while the series resistance (Rseries) has a decreasing trend, as shown in Fig. 4(h and i). In terms of the diode properties, there is no clear trend in ideality, Jsat and ϕB due to the relatively large error bars related to the values (Fig. 4(j–l)). However, the general observation is that devices with untreated GOCNT films have poorer diode performance (the highest ideality and Jsat with the lowest ϕB) while solar cells whose films were treated with dopants with a high total electronegativity, such as HNO3 and H2SO4, have excellent diode properties (with ideality approaching 1 and increasing ϕB).
Similarly, when investigating optoelectronic properties, in order to test the validity of the correlation between the efficiency and the sum of the active element electronegativity in the dopants, devices with HClO4 treated top electrodes were fabricated and the results fit well as shown in Fig. 4(c). In addition, most of the other parameters of the HClO4 related devices, including JSC, VOC, FF, Rshunt, Rseries, ideality, Jsat, and ϕB, fit the general trend as other dopants, as shown in Fig. 4. The trend in VOC, FF, ideality and ϕB infers that a better junction has been created between Si and the GOCNT film after different treatments with the improvement in the optoelectronic properties as well as the formation of a better contact.
Overall, the devices treated with dopants with higher electronegativity values for the active elements have better efficiency (HNO3 treated devices have the highest efficiency among them, 11.38 ± 0.26%) as well as the best diode properties while the solar cells fabricated with the films of the best optoelectronic properties (AgNWs–GOCNT) have efficiency just higher than that of control devices (8.14 ± 0.27% versus 7.11 ± 0.35%). This suggests that the performance of the GOCNT/Si heterojunction solar cells is not solely dependent on the optoelectronic properties of the transparent conducting window electrodes. Since AgNWs can significantly enhance the charge transport through the GOCNT network while the improvement in the FOM for p-type doped GOCNT films is mainly due to the increased density of free charge carriers,65 it is inferred that the density of free charge carriers is the most important parameter in improving the solar cell performance. Furthermore, the role of the GOCNT network is more than that of a window electrode to transport the separated holes. More importantly, it contributes to the separation of the excitons by forming a p–n or Schottky junction between Si.
The degradation of the solar cell performance is shown in Fig. 5(b) and the normalized efficiency divided by FOM is plotted in Fig. 5(c) to study the influence of electrode properties on the solar cell performance. The efficiency decay is mainly caused by a combination of the degradation of the optoelectronic properties of GOCNT films and the growth of the insulating oxide layers (SiOx) between Si and the GOCNT electrode. After 10 days, the untreated devices have an efficiency of about 75% of their original state (this is similar to previous reports),21 which is solely due to the growth of the oxide layer. Solar cells with GOCNT films doped with CuCl2, SOCl2 and HCl (the three most stable dopants) have very similar stability to the control devices while the efficiencies of AgNW, HNO3, AuCl3 and HClO4 (the least stable four dopants) treated devices degrade to about 65% of their starting state. This suggests that the main reason for the degradation might be the growth of SiOx rather than the degradation of the GOCNT optoelectronic properties, which is further evidenced by the fact that the value of the normalized efficiency divided by FOM for most of the treated devices is very similar (about 0.8) and slightly higher than that of the untreated devices (about 0.75) after 10 days (with the exception of H2SO4 treated devices), as shown in Fig. 5(c). Interestingly, H2SO4 doped devices degrade to about 35% efficiency of its starting point within 5 days but the properties of GOCNT films are stable for the first 5 days. In addition, the value of the normalized efficiency divided by FOM for such devices is much lower than that of untreated samples. Both facts suggest that the oxidation rate of Si might be faster for the devices with the H2SO4 treated electrodes due to the presence of oxidizing species (HSO4− and H2SO4).
Different doping strategies have been widely explored and applied to CNT based transparent conducting films in order to improve the optoelectronic properties. As shown in this report, the FOM of GOCNT electrodes is improved using various dopants and the resulting solar cells show improved performance compared to the control devices. The bath doping approach shows excellent compatibility in the processing in terms of dopants, doping periods and the limited adverse effect on the final devices. Based on the results in this report, an ideal dopant would contain atoms of high electronegativity. However, the instability of the doping as well as the devices is still a concern which must be addressed in any commercial development.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ta08445e |
This journal is © The Royal Society of Chemistry 2017 |