Bio-based, cellulosic-CuInS 2 nanocomposites for optoelectronic applications

A generic approach to design optoelectronic devices using renewable biopolymers is demonstrated. As a proof of principle, a biopolymer/CuInS 2 nanocomponent-based solar cell has been assembled by using a cellulose derivative with a reasonable life cycle performance, namely trimethylsilyl cellulose (TMSC). The solar cells are manufactured using a mixture of copper and indium xanthates as precursors, which decompose and form CIS nanoparticles within the biopolymer matrix during a thermal treatment, which was investigated by in situ combined grazing incidence small and wide angle X-ray scattering experiments. The growth of the nanoparticles is thereby controlled by the TMSC matrix. The nanocrystals exhibit an average diameter of approx. 4 nm. Using this composite, it was possible to fabricate solar cells, generating current in a wide range of the solar spectrum and exhibiting power conversion efficiencies of ca. 1%.


INTRODUCTION
To meet the challenges of climate change and sustainable use of global resources, tremendous efforts have been made to replace fossil derived energy resources and synthetic petrochemicalbased materials by those derived from renewable resources. This development is mainly driven by the rather large CO 2 emissions associated with both, production of energy and of synthetic materials. 1 Among renewable energy resources, photovoltaics are one of the main pillars. One currently followed research area is the use of nanocrystals and nanocomposites in solar cells using at present mainly toxic compounds such as PbS or Cd-based chalcogenides. [2][3][4][5] In order to overcome environmental concerns, alternatives have been thoroughly investigated, among which CuInS 2 represents a non-toxic alternative. [6][7][8][9] However, also in this case, CuInS 2 is usually synthesized via wet-chemical/colloidal synthesis routes employing long-chained capping ligands, which have to be removed or exchanged using potentially toxic reagents (e.g. 1,3-benzenedithiol, hexanethiol, etc.) before the nanocrystalline absorber films for the solar cells can be prepared. 10,11 A convenient way to avoid capping agents and ligand exchange reagents is to generate the metal sulfide particles directly in the photovoltaic active layer. In this approach, nanocrystals are formed from precursor compounds which are converted to metal sulfides usually by a mild thermal treatment. Usually, a conjugated polymer matrix is used for controlling the nanocrystal growth. 12 Among the precursors, metal xanthates have proven to be most efficient since they decompose accompanied by the formation of only volatile by-products leaving the layer leading to pure conjugated polymer/nanocrystal nanocomposite thin films which can be directly applied as absorber layers in solar cells. [12][13][14] In this work, we investigate if it is possible to fully circumvent the use of synthetic capping ligands, ligand exchange reagents and polymers in the fabrication of CuInS 2 -based nanocrystal solar cells by replacing them in the in-situ route by the bio-based trimethylsilyl cellulose. before the image was captured was 2 seconds. Each sample was measured at least three times.
The contact angle (CA) calculations were performed with the Young-Laplace equation and the surface free energy calculation with the Owens-Wendt-Rabel & Kaelble method.
UV-Vis spectroscopy. The UV-Vis absorption spectra of the samples were measured with a Shimadzu UV-1800 UV spectrophotometer. The absorbance was determined from 270 -1100 nm at 25°C and in ambient atmosphere.
Grazing incidence small and wide angle X-ray scattering. 2D grazing incidence small and wide angle X-ray scattering (GISAXS, GIWAXS) measurements were conducted at the Austrian SAXS Beamline 5.2L of the electron storage ring ELETTRA (Trieste, Italy), 24 using a similar setup as described before. 25 The beamline has been adjusted to a q-resolution (q=4π/λ*sin(2θ/2), 2θ represents the scattering angle) between 0.1 and 3.5 nm -1 (GISAXS). The X-ray energy was 8 keV. For the time-resolved measurements, the nanocomposite samples were placed in a heating cell (DHS 1100 from Anton Paar GmbH, Graz, Austria) equipped with a custom-made dome with Kapton polyimide film windows and were heated from 30 °C up to 230 °C at a heating rate of approx. 10 °C min -1 under nitrogen atmosphere. During the temperature scan, data were recorded with framing rate of 6 s using a Pilatus 1M detector (Dectris). For detection of the GIWAXS signal, a Pilatus 100K detector from Dectris was used. The angular calibration of the detectors was carried out using silver behenate powder (d-spacing of 58.38 Å) and p-bromo benzoic acid, respectively.

Transmission electron microscopy. Electron diffraction patterns and STEM (Scanning
Transmission Electron Microscopy images) were acquired using a Tecnai F20 (operated at 200 kV) and a FEI Titan 3 G2 60-300 (operated at 300 kV). Both microscopes are equipped with a post-column electron energy filter from Gatan Inc. (GIF). The Titan microscope has a Cs probe corrector for the STEM mode and therefore the high resolution STEM images were observed on the Titan 3 G2 microscope. Selected area electron diffraction patterns (SAED) from the nanoparticles were recorded energy-filtered within a range of about 1400 nm in diameter.
Solar cell assembly. The nanocrystal solar cells were fabricated in the device architecture glass/ITO/PEDOT:PSS/biopolymer-CuInS 2 /Al. As substrates, glass/ITO slides with a sheet resistance of 10 Ω/sq were used, which were cleaned in deionized water and isopropanol in an ultrasonic bath followed by O 2 plasma cleaning (FEMTO, Diener Electronic, Germany).
Afterwards, a PEDOT:PSS layer (Clevios P VP.Al 4083, Heraeus) was doctor blading on the glass/ITO layer in ambient atmosphere and subsequently annealed at 150 °C for 10 min in a glove box. Next, the nanocomposite films were prepared by doctor blading of a chlorobenzene solution containing copper xanthates, indium xanthates and TMSC (concentration of TMSC in the precursor solution: 5 mg/mL; the weight ratio TMSC:CuInS 2 was 1:9, the molar ratio CuXa/InXa 1:1.7) and subsequent thermal treatment (temperature program: 15 min heating from room temperature to 195 °C followed by 15 min at 195 °C) on a programmable heating plate (MCS 66, CAT Ingenieurbüro M. Zipperer GmbH). In the last preparation step, approximately 100 nm thick aluminum electrodes were deposited via thermal evaporation at a base pressure of 5×10 -6 to 1×10 -5 mbar. The characteristic values of the prepared solar cells were determined from IV curves recorded using a Keithley 2400 SourceMeter, a custom made Lab-View software and a Dedolight DLH400D lamp providing a spectrum very similar to AM1.5G. The light intensity was set to 100 mW/cm 2 (determined using a KippZonen-CMP-11 pyranometer, no spectral mismatch was considered). The EQE spectrum was measured using a Multimode4 monochromator (AMKO) equipped with a Xenon lamp and a Keithley 2400 source meter.

9
The aim of this paper is to design a nanocrystal solar cell based on CuInS 2 and TMSC. In order to realize this goal (Scheme 1), different requirements must be met, namely solvent compatibility between the xanthate precursors and TMSC (i), the formation of homogeneous precursor films by spin-coating these solutions onto PEDOT:PSS/ITO (ii), the conversion of the metal xanthates to the metal sulfide nanoparticles in the TMSC matrix (iii), the final assembly of the device (iv) and finally the determination of the photoelectric activity of the assembled solar cells (v). In all these steps, comprehensive characterization techniques are employed to investigate the underlying processes.
Scheme1. Schematic representation of the manufacturing steps of CuInS 2 /TMSC nanocomposite films and the multi-layer optoelectronic device.
As mentioned above, solvent compatibility between the xanthates and TMSC needs to be ensured. Based on previous findings in literature to prepare CuInS 2 nanoparticles, the ratio between the two xanthates (CuXa/InXa) was fixed at 1.0:1.7. 12 Then, a solubility screening was performed. For this purpose, different amounts of CuXa/InXa (2-12 wt% in chloroform) were added to TMSC solutions (0.5 wt%). All concentrations resulted in clear yellow-brownish solutions. These solutions were further subjected to spin-coating experiments and all the ratios led to films with a rather smooth optical appearance. Afterwards, the nanoparticle growth was induced by exposing the films to elevated temperature (195 °C, up to for 230 °C for GISWAXS).
Decomposition of the xanthates via the Chugaev rearrangement 26 is initiated at a temperature of around 155 °C as shown earlier. 12 A benefit provided by the TMSC derivative is the rather high thermal stability (degradation starts at 280 °C, see Fig. S1, ESI †).
The influence of the heating step on the chemical composition of the films was monitored by ATR-IR spectroscopy. Fig. 1 compares the different materials before and after the heating procedure. The obtained results revealed that the TMSC was neither altered by exposure to elevated temperature (e.g. no hydrolysis indicated by the appearance of OH bands associated to  OH at 3200-3600 cm -1 ) nor by the addition of copper or indium xanthate. Furthermore, it is demonstrated that the heating step led to decomposition of both xanthates while the TMSC remains unaffected. In order to get insights into the formation of the CuInS 2 nanocrystals in the TMSC matrix, we conducted combined time resolved GIWAXS and GISAXS experiments using synchrotron radiation on a temperature controlled sample stage. The temperature-dependent evolution of the GIWAXS patterns of a TMSC/metal xanthate sample is shown in Fig. 2. Between 120 and 150 °C, an intense broad peak between 26 and 32° and a minor one around 47° 2 theta evolved.
These peaks can be assigned to the (112) and (204) reflections, which are characteristic for chalcopyrite CuInS 2 . Therefore, it can be concluded that the conversion of the precursors to the  These short intervals are beneficial as the formation of nanocrystalline metal sulfides from metal xanthates proceeds rather fast. 29,30 The GISAXS patterns at selected temperatures during the heating run are presented in Fig. 3A demonstrating a strong increase of scattering due to the formation of CuInS 2 particles. The areas used for horizontal integration are indicated with a red box in the GISAXS images and the resulting horizontal cuts at q z = 0.45 nm -1 are presented in The evolution of the integrated intensities of the GISAXS patterns (Fig. 3C), calculated between q y =0.1 and 2.5 nm -1 ) revealed that minor structural changes were taking place in the TMSC/metal xanthate film already starting at a temperature of around 120 °C. At approx. 145-150 °C, a significant increase in the integrated intensity was observed. This originated from the 25  Intensity / a.u. 13 decomposition of the metal xanthates, the evaporation of volatile organic decomposition products and the formation of CuInS 2 nanocrystals in this temperature range as already revealed by the GIWAXS investigations. By these processes, the overall electron density in the nanocomposite film was increased, leading to an enhanced scattering intensity in the GISAXS patterns. After the formation of the CuInS 2 nanocrystals, the changes in the integrated intensities were minor. Any evidence for further compaction of the nanocomposite film and decomposition of the organic TMSC matrix was not observed until a temperature of 230 °C. A further important point for the usage of the nanocomposite layers in a solar cell or other optoelectronic devices is the connection of the separate nanoparticles inside the film to ensure a continuous pathway allowing for electronic conduction throughout the material. The bright field TEM images at different magnifications (Fig. 4A, B) revealed a dense network of nanoparticles with diameters from 2 nm to 5 nm in the TMSC matrix. Also here crystallographic data on the nanoparticles' structure can be derived by selected area electron diffraction patterns (SAED) (Fig. 4C)). The diffraction patterns feature three main diffraction rings at r = 3.18 nm -1 (112), r = 5.16 nm -1 (204/220) and r = 6.01 nm -1 (116/312) which are in excellent agreement with reference data for chalcopyrite CuInS 2 (PDF 032-0339) and a proof for the high crystallinity of the nanoparticles inside the film. More data is available in the ESI † (Fig. S4).
An important aspect for the design of optoelectronic devices is the morphology of the hybrid layer. Especially in the assembly of multi-layer devices, the surface roughness of the different layers is crucial for their performance. For that reason the surface was analyzed by atomic force microscopy (AFM) before and after the heat treatment. The UV-VIS absorption of the film was determined to prove the suitability of the hybrid material for the application as an absorber material in a nanocrystal solar cell. The untreated film showed a distinctive absorption band for CuXa/InXa starting at ca. 420 nm, which vanished after the heat treatment (see Fig. 6). At the same time, the characteristic spectrum of CuInS 2 nanoparticles with an onset between 800-900 nm appeared. 12 Furthermore, the TMSC matrix did not show any absorption in the measured range and did not affect the absorption of the CuInS 2 nanoparticles.   The formation of CuInS 2 -investigated again by combined GIWAXS and GISAXS experimentstakes place at slightly higher temperatures of approx. 170 °C (see Figures S11 and S12, ESI †) compared to the conversion to the CuInS 2 nanocrystals in the TMSC matrix. Fig. 8B shows the final GIWAXS trace (measured at 210 °C) in comparison to the CuInS 2 reference pattern proving also the formation of CuInS 2 in this system. However, the dispersability and the stability upon heat treatment have been shown insufficient so far to prepare a working solar cell with satisfying performance. Especially the obtained high roughness (see AFM images in Figure S9 in the ESI †) is detrimental for thin film solar cells. Nevertheless, this approach might be interesting for templated porous metal sulfide films analogous to porous silicates obtained by ChNCstemplating. 31 In summary, we successfully demonstrated the replacement of synthetic ligands by the use of a