Hybrid heterojunction solar cells based on single-walled carbon nanotubes and amorphous silicon thin films

Hybrid heterojunction solar cells based on silicon and single-walled carbon nanotube (SWCNT) thin films have a simple structure and their manufacture employ simple low-temperature processes. Moreover, their progress has been rapid during the last decade, wherein the efficiency of heterojunction solar cells combining hydrogenated amorphous silicon (a-Si:H) and SWCNTs thin film has increased from 0.03% to 8.80%. Here, we present a comprehensive overview of the state-of-the-art on SWCNTs/a-Si:H heterojunction solar cells. In addition to a comprehensive technology review, important special features such as adhesion of SWCNT film to a-Si:H, the interface between SWCNT and a-Si:H, and their influence on the performance of the heterojunctions are included. Future paths for improving the performance of such solar cells are also suggested. Finally, key challenges and trends for further research and development of SWCNTs/ amorphous silicon heterojunction solar cells are discussed. This article is categorized under: Photovoltaics > Science and Materials


| INTRODUCTION
The interest in renewable energy sources has significantly increased along the needs to reduce greenhouse gas emissions as part of the climate change mitigation.Photovoltaics (PV) is one of the future key clean energy technologies and its markets are growing fast (IEA, 2019).Among various solar cell types, crystalline silicon (c-Si) dominates the market showing high power conversion efficiency (PCE) exceeding 26% with heterojunction with intrinsic thin technology based on thin a-Si:H passivating layers and on interdigitated back contacts on n-type silicon wafers (Green et al., 2018;Yoshikawa et al., 2017).But as recent technologies are evolving into flexible, versatile, and portable electronics, there is also demand for manufacturing solar cells in roll-to-roll processes while maintaining an adequate PCE.Such solar cells would be thin, flexible and could be directly incorporated into applications such as buildings, for example, as roofing shingles.The structural complexity and ease of device fabrication are the key factors that determine the cost and the SWCNTs/a-Si:H thin film heterojunction solar cells.Therefore, in this review, we focus to present the application of SWCNT thin films in a-Si:H solar cells.We briefly discuss the working principles of SWCNTs/a-Si:H heterojunctions and summarize the development of SWCNT thin films in a-Si:H.The adhesion of SWCNT film to a-Si:H, the interface between SWCNT and a-Si:H, and their influence on the performance of the heterojunctions are discussed.This aims to highlight the progress and the advantages of SWCNT thin film as heterojunctions with a-Si:H in PV applications, and to provide possible hints that might help in the further development of SWCNTs-based PV devices.

| Working mechanism
Solar cells composed of a SWCNT film and Si form a heterojunction device that has been well studied (Jung et al., 2013;Kozawa et al., 2012;Z. Li et al., 2009).The SWCNT film acts not only as charge carrier transport, but also as a photoactive layer (Jia et al., 2008;Tune et al., 2012).The SWCNTs/Si heterojunction follows classical p-n or Schottky theory, where in the photo-generated carriers travel distance is determined by the carrier diffusion length, which is of the same order of magnitude as the cell layer thickness.The electric field responsible for the separation of photogenerated carriers (electron-hole pairs) is concentrated within a very thin zone at the p/n junction or in other words at the closest proximity of the SWCNTs and Si interface, thereby enabling the separation of photo-generated carriers.However, in amorphous silicon materials, the carriers can travel only short distances before recombination.Hence, a uniform electric field needs to be present from the origin of photo-generation throughout the entire cell thickness.This electric field assists carrier travel and immediately separates electron and holes, thereby avoiding recombination.The field assisted travel distance is given by the drift length.Therefore, an intrinsic (i)-layer is inserted between p-and n-layers, to form a pin-diode.For more details on the pin-diode theory, the reader is suggested to review relevant literatures (Schropp & Zeman, 1998;Shah, 2010;Street, 1991).During the last decade, the use of carbon nanotubes in thin film amorphous silicon has been of great interest for researchers as light trapping structures, antireflective coatings, transparent electrodes, and p-type window layer contacting a-Si:H to form heterojunction (Alekseeva et al., 2018;Del Gobbo et al., 2011;Funde et al., 2016;J. Kim et al., 2012;P. M. Rajanna et al., 2018P. M. Rajanna et al., , 2019;;Schriver et al., 2010;Tu et al., 2012;Zhou et al., 2014Zhou et al., , 2008Zhou et al., , 2009)).Similar to SWCNTs/c-Si heterojunction (Hu et al., 2019;Jeon et al., 2018;Tune et al., 2012), SWCNTs/a-Si:H also form semiconductor/semiconductor or metal/semiconductor junction as shown in Figure 1a or AZO or metals like In/Ga (Xu et al., 2016), Ti/Au (Jia et al., 2012;Jia, Li, et al., 2011), or Al (Xiaokai Li et al., 2013) that forms an ohmic contact with the amorphous silicon, and a-Si:H (p) or SWCNTs due to the built-in-voltage created by the applied electric field.Electrons are collected across n-type a-Si:H, ITO or AZO and holes are collected at p-type a-Si:H or SWCNTs itself that can be used as a p-type transparent electrode as shown in Figure 1.Furthermore, the front electrode made of Ag (Xu et al., 2015), Au/Cr (De Nicola et al., 2017;Yu, Batmunkh, et al., 2017), or Pt (Cui et al., 2014) contacts the SWCNT film for better transfer of holes to an external circuit.Similar process happens when the metallic SWCNT-film comes in contact with i-layer thus forming Schottky junction as shown in Figure 1b.The differences will be the magnitude of the reverse saturation current and its switching characteristics.
The complexity of the SWCNT film, in which semiconducting and metallic nanotubes coexist leads to the uncertainty at the interface between SWCNTs and a-Si:H.Several individual nanotubes are present in a device, and each forms a heterojunction with the i-layer of a-Si:H.As SWCNTs exhibit semiconducting and/or metallic behavior, a p-i-n junction can be respectively expected for the former (Figure 1a) and a Schottky junction for the latter (Figure 1b).

| SWCNTs as transparent electrodes
The first attempt to create a a-Si/CNT heterojunction was made by Schriver et al. using the carbon nanotube films as buckypaper and graphene in junctions with undoped a-Si thin films as shown in Figure 2a.The measured J-V characteristics of the buckypaper/a-Si heterojunction solar cell is as shown in Figure 2b (Schriver et al., 2010).The produced solar cells were air-stable without additional processing steps like doping, multilayer film deposition in high vacuum, or transparent conducting oxide deposition.In the subsequent year SWCNTs were sprayed on a-Si:H by Gobbo et al. to form Schottky barrier solar cells (Figure 2c) (Del Gobbo et al., 2011).They measured the external quantum efficiency up to 35% at a wavelength of about 460 nm (Figure 2d) and indicated that for lower density SWCNT/a-Si:H heterojunction, nanotubes dominate the photocurrent generation, separation, and transport mechanism thereby splitting the F I G U R E 2 (a) A schematic of device structure.a-Si:H is PECVD deposited on patterned ITO substrates.Either SiO 2 or Al 2 O 3 is deposited on top, and a window is patterned and etched.A carbon film is deposited over the window; (b) illuminated J-V curves for buckypaper on a-Si:H cells (Schriver et al., 2010).Reproduced (adapted) with permission from (Solid State Communications 2010, 150, 561-563)  electron-hole pair generation at the CNT-CNT or CNT-Si heterojunction.However, with increased density of SWCNT/ a-Si:H heterojunction more electron-hole pairs are generated in a-Si:H.In the next year, Kim et al. used SWCNT films obtained by vacuum filtration through a mixed cellulose ester membrane as transparent electrodes on a a-Si:H n-i-p solar cell (Figure 3a) (J.Kim et al., 2012).Despite their similar work functions, a Schottky barrier was formed at the SWCNTs/a-Si:H interface that resulted in an inoperable solar cell with a fill factor of 22% as shown in Figure 3b.In order to address this issue, gold nanodots were deposited at the p + a-Si:H/SWCNTs interface (Figure 3a).The nanodots were found to be effective in eliminating the interfacial Schottky barrier thus allowing ohmic contact to form between the SWCNTs and p + layer without any measurable impact on the J sc .This approach led to achieving a respectable FF of 58% which is comparable to that of a a-Si:Hp-i-n solar cell (FF of 62%) with conventional TCO (Figure 3c).In the same year, Khanal et al. used SWCNT films as electrodes to replace the p-layer and back contact in a-Si:H solar cell (Figure 3d) (Khanal et al., 2012).They varied the SWCNTs film thickness and inferred that the optical properties of the nanotubes affect the device performance than does the conductivity (Figure 3e).The cells were illuminated from each side (glass and SWCNTs), and a 25 nm thick SWCNTs film resulted in a PCE of 1.46% (Figure 3f).Funde et al. reported The schematic of a-Si:H single junction solar cells with SWCNTs; (b) J-V curves of a-Si:H solar cells with SWCNTs without any interface treatment at the p +/SWCNT interface and a-Si:H solar cells with ZnO:Al without SWCNTs as a control sample; (c) J-V curves of a-Si:H solar cells with SWCNTs with gold nanodots at the p +/SWCNT interface and a-Si:H solar cells with ZnO:Al and without SWCNTs as a control sample (J.Kim et al., 2012).Reproduced (adapted) with permission from (Advanced Materials 2012, 24, 1899-1902)  the first use of aerosol CVD synthesized SWCNT thin films as p-layer and transparent electrodes by a unique technique of dry-transfer in a-Si:H solar cells as in Figure 3g (Funde et al., 2016).Furthermore, the SWCNTs were doped with thionyl chloride (SOCl 2 ) that resulted in an improved PCE of 1.5% from 0.3% for pristine nanotubes (Figure 3h).They highlighted that there exists a substantial potential for further improvement as the parameters of the fabricated device were not optimized in terms of opto-electrical properties of SWCNTs, thickness of nanotube films, and top metal contact.The studies on acid doping and SOCl 2 treatment of SWCNTs originated in SWCNTs/Si solar cells by Z. Li et al. (2008).The doping shifts the Fermi level of SWCNTs below ν 1 , thus increasing the mobility and carrier density as in Figure 4a (Blackburn et al., 2008;Jeon et al., 2018).As a result, the S 11 transition is suppressed in the semiconducting SWCNTs and further doping would suppress the S 22 transition as well (Figure 4b), as observed by near-infrared absorption spectroscopy (Figure 4c).

| SWCNTs-PEDOT:PSS composite
Recently, it has been shown that both the environment and the substrate material influences the efficient usage of SWCNTs film (W.Li et al., 2019; P. P. Rajanna et al., 2020).The perpetual contact between the nanotubes film and the substrate material or the atmospheric medium impacts the interface properties that consequently effects net efficiency of the solar cell (Xiaokai Li et al., 2013).As an example, at the interface of SWCNTs/Si heterojunction solar cells there exists great profuse of nanotube-silicon junctions where SWCNTs are in physical contact with Si surface.However, within the SWCNT network many nanotubes overlap and suspend on each other without contacting the Si.As a result, the solar cell performance suffers due to the increased recombination at the interface where bare Si surfaces are exposed to air (Jia et al., 2008;Jia, Cao, et al., 2011).To solve this problem, researchers have tried to mix conducting polymers with the SWCNT films to improve their junction density, uniformity, and conductivity (Lin et al., 2013;Sarker et al., 2010;Singh et al., 2008;Xie et al., 2020).Poly(3,-4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a well-researched and commonly used hole transport material, which has high conductivity, work-function, and transmittance (X.Fan et al., 2019).Moreover, injecting PEDOT:PSS on SWCNTs film, the micropores are completely filled and surface roughness is decreased (Q.Fan et al., 2017; P. M. Rajanna et al., 2018).Researchers have therefore combined SWCNTs with PEDOT:PSS to further enhance the performance and form hybrid heterojunction PEDOT:PSS-SWCNTs/Si solar cells (B.Fan et al., 2008;He et al., 2016He et al., , 2017;;Kymakis et al., 2006;Liu et al., 2017;Thomas et al., 2018).For example, Fan et al. developed an effective way of fabricating hybrid PEDOT:PSS/SWCNT/Si solar cells by placing a SWCNT network on the surface of a Si wafer and then spinning a PEDOT:PSS solution onto it (Q.Fan et al., 2017).
In the subsequent year, our group reported the synergistic effect of SWCNTs and PEDOT:PSS as a composite ptype transparent electrode to be more effective in forming coupled continuous hybrid heterojunction with a-Si:H as shown in Figure 5a (Alekseeva et al., 2018).The performance of the SWCNTs/a-Si:H, PEDOT:PSS/a-Si:H and SWCNTs-PEDOT:PSS/a-Si:H was compared (Figure 5b).It was found that SWCNTs-PEDOT:PSS composite film has better solar cell output performance with a PCE of 1.6%, FF of 54% and V oc of 0.803 V when compared SWCNTs (PCE of 1.1%) and PEDOT:PSS (1.0%) alone.It was explained that by the introduction of PEDOT:PSS, the F I G U R E 4 Density of states of pristine and doped (a) semiconducting; (b) metallic SWCNTs, respectively; and (c) absorbance spectra of pristine, ethanol densified, and HNO 3 functionalized SWCNTs film micropores of randomly oriented SWCNTs film is filled thereby forming continuous contact with underneath a-Si: H (Figure 5d).Although the FF and V oc was improved when compared to all the previous reported SWCNT/a-Si:H devices, the J sc was lower, owing to the increased absorption of incident photons in the p-type SWCNTs-PEDOT: PSS layer.The improvement in FF and V oc of SWCNT/a-Si:H in Alekseeva et al. could be primarily attributed to SWCNT film synthesis using Aerosol CVD, conformal deposition by dry transfer method, significantly higher electrical properties of SWCNT films, and better interface between SWCNT and a-Si:H (Del Gobbo et al., 2011;Funde et al., 2016;Khanal et al., 2012;J. Kim et al., 2012;Schriver et al., 2010).These results suggest that the SWCNT films should be carefully chosen to further optimize the SWCNTs-PEDOT:PSS composite.Following this, Rajanna et al. reported a SWCNTs/a-Si:H heterojunction solar cell with a much improved PCE of 2.7% by optimizing the thickness of SWCNTs film in the SWCNTs-PEDOT:PSS composite used as p-type layer and transparent electrode (Figure 5c) (P.M. Rajanna et al., 2018).The fabricated devices showed increased PCEs of 3.4% and V oc of 0.9 V with incorporation of PMMA as an anti-reflection layer on top of the SWCNTs-PEDOT:PSS composite (Figure 5c).Moreover, a decrease in the sheet resistance of the SWCNT-PEDOT:PSS (composite) film and an increase in its work function was measured compared to the pristine SWCNT film.This can be attributed, for the fact that every single carbon atom is on the surface exposed to the environment.Therefore, any atom/molecule put on a SWCNT cause changes in their electronic structure and charge transfer between the atom/molecule and a nanotube.Therefore, when PEDOT:PSS is injected it filled micropores in the SWCNT film and, that the holes in the PEDOT:PSS patches can transfer to the interconnected SWCNT network consequently, doping the SWCNTs (Q.Fan et al., 2017; Ki  Kim et al., 2010).Although, a significant progress had been made in SWCNTs/a-Si:H heterojunction solar cell from initial PCE of less than 1.0% to 3.4%, yet this was very low and beyond the scope for practical applications.The challenge was to overcome the problems at the SWCNTs and a-Si:H interface, improve the ptype transparent electrode, and thereby improve the overall solar cell performance.The significant problems as indicated in the previous works was the large interface resistance, high Schottky barrier leading to band-offsets between a-Si:H and SWCNTs, and high series resistance of SWCNTs resulting in high carrier recombination.

| SWCNTs-PEDOT:PSS-SWCNT fibers as novel transparent electrode
To solve the problems mentioned in the previous section, we proposed a rational design of a novel p-type transparent conductor developed using a multicomponent composite that combines the superior properties of SWCNTs with PEDOT:PSS, MoO 3 and SWCNT fibers into a single composite (Figure 6a) (P.M. Rajanna et al., 2019).Various configurations were developed and examined as a p-type window layer and electrode in a-Si:H solar cells.A configuration of SWCNTs-MoO 3 -PEDOT:PSS/SWCNT fibers composite (p-type) measured a record equivalent sheet resistance (R sh ) of 17 Ω/sq with a transmittance of 90% at 550 nm (Kaskela et al., 2010;P. M. Rajanna et al., 2019).Adsorption doping of SWCNT films and fibers by HAuCl 4 is one of the major factors for record R sh .As all carbon atoms are exposed to the environment, any atom/molecule put on a SWCNT causes charge transfer between the atom/molecule and nanotube (Ki et al., 2008) Rajanna et al., 2019) the observed p-type behavior due to its high electronegativity results in a strong HAuCl 4 doping which shifts the Fermi level deeper in the valence band (Ki et al., 2008;K. K. Kim et al., 2010;Tsapenko et al., 2018).Moreover, the developed p-type composite displayed a high degree of mechanical flexibility with ≤5% change in resistance (Figure 6b).The solar cells made from the developed p-type composite electrode on a-Si:H absorber yielded an outstanding J sc of 15.03 mA/cm 2 and record PCE up to 8.8% for SWCNTs/a-Si:H heterojunction solar cells (Figure 6c) which was an effective 18% improvement over a standard n-i-p configured solar cell (Figure 6d).Moreover, SWCNT fibers by itself can be used as replacement for traditional metal contacts due to its high conductivity and simple deposition process (P.M. Rajanna et al., 2019).
Solar cells based on SWCNTs as p-type transparent electrodes in a-Si:H have been studied to substitute standard ptype a-Si:H, transparent conductive oxide such as ITO and FTO, and front metal contacts.The PCEs on rigid structures have progressed steadily from 0.03% up to 8.80% as tabulated in Table 1.Moreover, the mechanical properties of SWCNTs and well-established a-Si:H are promising for future low-cost flexible and wearable solar cells.

| SUMMARY AND FUTURE OUTLOOK
Here the recent success in the application of SWCNTs in a-Si:H heterojunction solar cells has been reviewed.The unique structure and extraordinary opto-electrical properties of SWCNTs give them notable advantages for PV applications, and therefore the future direction is undoubtedly SWCNTs based p-type transparent electrodes.Notable progress has been made on the SWCNTs/a-Si:H heterojunction-based devices, however challenges remain in their practical use.For example, the working mechanism of SWCNTs in solar cells has not been fully clarified due to the statistical presence of both semiconducting and metallic nanotubes in a thin film.Also, the performance of SWCNTs based solar cells is determined by the structure and properties of SWCNTs, such as chirality, sheet resistance, and work function (Ren & Wang, 2010).For this, either pure semiconducting or metallic SWCNTs with high purity and quality are needed to fully understand the mechanism.
The fabrication and characterization of SWCNTs based solar cells has been limited to small active area.Large area solar cells are a big challenge.For large area fabrication and performance of SWCNTs based solar cells, a SWCNT film with high transparency and low sheet resistance is desired.Although, significant progress has been made in lowering the sheet resistance at a transmittance of 90% of pristine SWCNTs film, much needs to be done to replace ITO.One of the possible ways is to reduce the inter-tube junction resistance in the most commonly used randomly oriented SWCNTs film that is much higher than the intrinsic tube resistance and the electrical conductivity of SWCNTs film (Fuhrer et al., 2000;Nirmalraj et al., 2009).Recently, carbon-welding on tube-tube junction was proposed to convert inter-tube Schottky contacts into near-ohmic ones (Jiang et al., 2018).To further reduce the inter-tube junction resistance, SWCNT arrays or aligned SWCNTs is another alternative as previously shown in Si solar cells, but mainly with multi-walled nanotubes (Di et al., 2013;R. Li et al., 2014).As SWCNTs have superior properties, an ideal case would be to make high quality aligned SWCNTs (Ma et al., 2011;Seah et al., 2011), whose opto-electrical properties are optimized.On the other hand, front contact fingers should be introduced in SWCNTs based solar cells, which can significantly enhance the FF.SWCNT strips/fibers have been used as front contacts, when the PCE increased to 6.52% from initial 3.97% (Xu et al., 2016).More recently, our group used SWCNT fibers to increase the overall conductivity of SWCNTs as p-type transparent electrode, thereby improving the PCE of SWCNT/a-Si:H heterojunction solar cells (P.M. Rajanna et al., 2019).Moreover, SWCNT fibers itself can be used to replace traditional metal contacts due to their high conductivity and simple deposition process.Therefore, this is good way to increase the efficiency of solar cells, and also, reduce the sheet resistance of SWCNTs film.
The work-function of SWCNT films need to be improved for better separation and extraction of photo-generated carriers, that is, the work-function of carbon nanotubes have to be fine-tuned in such a way that the energy levels match with Si.Moreover, the increased work function can increase the barrier height than can result in improved built-in voltage at the interface, thereby resulting in increased V oc .Therefore, effective p-type doping needs to be found that are stable other than the existing acid-based treatments with SOCl 2 , HNO 3 , chlorosulfonic acid, AuCl 3 , and HAuCl 4 , which are unstable (Ki et al., 2008;Tsapenko et al., 2018).This result in poor stability of the fabricated SWCNTs based solar cells.
The device architecture of SWCNTs/a-Si:H also needs to be further optimized.Only a few reports exist on the interface between SWCNTs as front electrode and Si (Jia et al., 2012;Tune et al., 2013;Yu, Grace, et al., 2017), but none exists on the interface between Si and back electrode.Therefore, this needs further investigation for SWCNTs/Si and SWCNT/a-Si:H new type heterojunction solar cells.
Finally, flexible solar cells are becoming of high relevance to the development of flexible and wearable devices (Fu et al., 2018).SWCNTs have excellent flexibility and can be combined with Si thin films like a-Si:H to fabricate flexible SWCNTs/a-Si:H solar cells, respectively.It is envisaged that the SWCNT films are novel p-type transparent conductors in combination with the use of pure high quality semiconducting or metallic SWCNTs that are aligned with better passivation, improved doping stability of SWCNTs, light trapping schemes and nanostructuring can potentially improve future PV devices.Nevertheless, the applications of the novel p-type composite material are not limited to solar cells (P.M. Rajanna et al., 2019).Rational design and room-temperature processing broaden the horizon for transparent and flexible electrode implementation in diverse applications in other fields of science and technology.ORCID Pramod M. Rajanna https://orcid.org/0000-0002-6175-4442Peter D. Lund https://orcid.org/0000-0002-4819-3847

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. As detailed by Tsapenko et al., the adsorption of formed AuCl − 4 anions, in which Cl − is the origin for F I G U R E 6 (a) Schematic of the device architecture with optimized TCF (SWCNTs-MoO 3 -PEDOT:PSS/SWCNT fibers composite) as ptype window layer and front contact; (b) TCF resistance change during 50,000 bending cycles at angles of 20 , 45 , 90 , and 180 with the radii of curvature from 10 to 1.6 in mm −1 ; (c) J-V characteristics of TCF solar cell (inset shows (bottom-left) a photograph of TCF on a polyimide substrate, and (bottom-right) photograph of the fabricated solar cell on a-Si using developed TCF showing its transparency); and (d) comparison of J-V parameters of standard n-i-p configured a-Si:H solar cell with fabricated solar cells using different TCFs (P.M. Rajanna et al., 2019).Reproduced (adapted) with permission from (Nano Energy 2020, 67, 104183) with License Number 4791271297717.Copyright (2020) Elsevier Ltd T A B L E 1 Reported SWCNTs/a-Si:H heterojunction solar cells