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GCEP award #40654: High-Efficiency, Low-Cost Thin Film Solar Cells

Alberto Salleo, Dept. of Materials Science and Engineering, Stanford University Yi Cui, Dept. of Materials Science and Engineering, Stanford University

Peter Peumans, Dept. of Electrical Engineering, Stanford University 1. Motivation The goal of the project is to develop materials and processing techniques that will allow the fabrication of low-cost multi-junction solar cells entirely by solution-processing or other low-cost techniques amenable to roll-to-roll (R2R) fabrication such as lamination. Device modeling will help identify the best device architecture. Integration of different types of materials (metals, oxides, semiconductors, polymers) is one of the goals of this project. 2. High-Efficiency, Low-Cost Multi-Junction Solar Cells We propose to take advantage of nanostructured materials to fabricate high-efficiency multi-junction cells using solution-based processing techniques. As a result, our cells will be amenable to large-area fabrication, such as R2R, thereby dramatically reducing their cost. A crucial aspect of this technology is the introduction of transparent conducting materials that can be deposited in mild conditions in order to serve as intermediate electrodes in an unconstrained multi-junction architecture. This report is divided in two sections: materials development, and integration in photovoltaic devices. 2-1 Electrode development: ZnO-electrical properties Summary of previous results: In years I and II we showed that were able to synthesize highly doped ZnO nanowires and we showed how to measure carrier density and mobility in the wires. We obtained carrier densities of the order of 1020 cm-3 and mobilities of the order of 20-40 cm2/V.s. Dopant incorporation however was up to one order of magnitude higher than the carrier density indicating that a large fraction of the dopants (Al or Ga) was not in substitution sites. In an effort to increase the dopant activation we thermally annealed the nanowires, which caused the free carriers to disappear. New results: In order to understand what limits the carrier density in our solution-synthesized ZnO nanostructures and why annealing causes the free carriers to disappear, we performed anomalous X-ray diffraction (AXRD) experiments at SSRL. The goal of the experiments was twofold: determine whether certain synthesis conditions could lead to increased incorporation of dopants in the substitutional sites and determine whether the loss of carriers upon annealing was due to a displacement of the substitutional dopant (by precipitation into a new phase for example). The limitations of the energy range of the beamlines allowed us to study only Ga as a dopant. This limitation was not a major one as Ga-doped ZnO nanostructures offered the best performance anyway. In AXRD, the intensity of a reflection of the host crystal is monitored while the energy of the beam scans the absorption edge of the guest atom. If the guest atom is present as substitutional atom in the host lattice the intensity of the reflection changes as the beam probes the absorption of the guest atom (Fig. 1a).

We studied the effect of surfactant concentration (oleic acid) and solvent coordinating power. Increasing the solvent coordinating power led to unfavorable morphologies, we therefore studied in more detail the effect of the surfactant using 1-hexadecanol as the solvent. Increasing the surfactant concentration leads to a reduced incorporation of Ga in substitutional sites (Figs. 1b and c). Furthemore, as expected, the nanostructures became larger when the surfactant concentration was decreased. Finally, annealing did not cause any change in the substitutional Ga signal (Fig. 1b), hence we deduced that

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annealing did not cause Ga atoms to leave their substitutional sites. As a result, we concluded that the decrease in carrier concentration upon annealing at 400°C is caused by creation of a defect intrinsic to ZnO with acceptor-like character.

Figure 1: Structure factor of a ZnO reflection as a function of energy as Ga is incorporated into the lattice (a). Diffraction peak area (002) of ZnO synthesized with 5 at.% Ga in solution for two different oleic acid concentrations (b and c). The oleic acid affects the concentration of substitutional Ga as well as the size of the nanostructures.

The formation of defects is confirmed by the photothermal deflection spectroscopy, which reveals

the emergence of a broad sub-gap absorption upon annealing (Fig. 2). Further investigation is needed to identify the most likely culprit.

Figure 2: Sub-gap absorption of ZnO nanostructures before and after annealing. A broad defect band appears between 1.5 and 3 eV.

2-2 Electrode development: ZnO-optical properties Summary of previous results: In year II we demonstrated that different synthesis conditions can lead to different ZnO morphologies (nanowires, nanopyramids, fir-cones, nano-berries, smooth fir-cones) that have attractive optical properties in terms of light scattering.

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New results: We studied the light-scattering properties of these ZnO nanostructures in more detail. In collaboration with D. Knipp at Jacobs University in Bremen, films of ZnO nanocones (~250 nm on the side) were spin-coated using spin-on-glass as a binder. The effect of nanocone concentration on transmission and scattering was studied. Diffuse scattering was found to increase at increasing nancone concentration, with haze factors up to 0.8 in the visible and 0.3 in the near-IR (λ=1200 nm). Angle-resolved scattering showed that the enhanced scattering persisted in an angular range comprised between 5° and 55°. The ability to tune the reflectivity edge by controlling the onset of plasma absorption with doping makes these nanostructures particularly attractive as intermediate electrode materials in tandem cells. Finally, preliminary results indicated that the wavelength dependence of the haze depends on the lengthscale of the scatterer: large nanocones are more efficient at scattering at long wavelengths while small nanoparticles scatter better at shorter wavelengths. Hence, future studies will be aimed at understanding these effects quantitatively and designing and synthesizing nanostructures suitably optimized.

2-3 Electrode development: ZnO/Ag nanowire hybrids Summary of previous results: In years I and II, the ZnO and Ag nanowire activities were carried out in parallel. We realized that each materials system had its advantages and shortfalls. While low (<10 Ω/sq.) sheet resistances were produced with Ag , Ag nanowire electrodes exhibited little diffuse transmittance. ZnO on the other hand showed outstanding diffuse transmittance but insufficient conductivity (>1 kΩ/sq.). New results: This year we successfully developed a process to combine ZnO nanostructures and Ag nanowires to produce a transparent electrode with low sheet resistance and high haze. In order to deposit this new electrode we used a spray-coating set-up built and developed during years I and II. The spray-coater is capable of depositing materials over relatively large areas (10x10 cm2) at high speed. Pressure, flow rate, distance to substrate, substrate temperature and raster pattern can all be controlled. In order to disentangle these process parameters, a Design of Experiment matrix was produced and optimal deposition conditions were determined. Due to their interesting scattering properties, we spray-coated ZnO nanoberries first (Fig. 3). The resulting films showed haze factors up to 50% in the near-IR, however their conductivity was insufficient.

Figure 3: Total and diffuse transmission of spray-coated ZnO nanoberry films for different numbers of spraying passes. LPCVD-deposited ZnO films are included for comparison.

Films of Ag nanowires co-sprayed with ZnO nanoberries solved this problem. Interestingly, the

ZnO nanostructures were found to preferentially decorate the Ag nanowires (Fig. 4). The optical

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properties of the hybrid film depended on the Ag to ZnO ratio (Fig. 4). Total transmission was between 80 and 90% and the haze in the near-IR was around 0.3. Figure 4 shows a comparison of our electrode against a state-of-the-art LPCVD-deposited ZnO layer deposited under optimized conditions for scattering. Our electrode has the same total transmission as the LPCVD layer and a haze in the near-IR that is a factor of 4 to 6 higher than that of the LPCVD film. In addition to these outstanding optical properties, our films displayed sheet resistances of the order of 10 Ω/sq. or less. The variation in properties was less than 15% over ~60 cm2. A provisional patent application for this invention was filed under GCEP sponsorship. Future work will address the optimization of the scattering geometries and Ag/ZnO ratios.

Figure 4: Total transmission and haze of hybrid ZnO/Ag nanowire films at different ZnO concentrations (left). A film deposited by LPVD (literature data) is included for comparison. SEM micrographs of spray-coated hybrid films (right)

2-4 Active material development: CuInS2 and Cu2ZnSnS4

Summary of previous results: In addition to nanowire synthesis (year I)a new air-stable ink rolling (AIR) process was developed (year II) to form CuInS2 (CIS) absorber thin films. The process consist of the following steps (a) Ink deposition process using roller-bar. (b) Oxide bilayer formation after heating at 370oC in air. (c). Film after sulfurization in sulfur vapor at 525oC. New results: The cross section image and current-voltage measurement for a CuInS2 solar cell is shown in Figure 5. The short-circuit current, Jsc, is 18.49 mA/cm2, which is comparable to high efficiency CuInS2 solar cells. The power conversion efficiency (η) in this initial device is 2.15%. The fill factor (FF=0.37) and open-circuit voltage (Voc = 320 mV) are not yet high, which is most likely due to the low shunt resistance. We believe that this might be due to film cracking induced by mechanical stresses created during sulfurization, a chemical transformation involving structural and volumetric changes.

The conversion of the mixed oxide to the ternary compound CuInS2 occurs in three steps. First, the CuO transforms into copper sulfide and forms plates on the film surface (Figure 6b,d). No holes are observed in the film at this stage of the growth. Next, the film is converted to cubic CuInS2 at ~400°C. Lastly, the tetragonal phase used as an absorber layer for solar cells forms above 535°C. Since no reaction interface was observed during the growth process, we speculate that the formation of cubic CuInS2 from a mixture of CuxS/In2O3 occurs via a nucleation-limited growth mechanism that causes pothole formation due to the volume change. These holes continue to grow larger during the cubic to

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tetragonal transformation. Figure 6a shows the 20-50 µm potholes formed in a film sulfurized at 535°C. We found that a two stage heating profile that allows copper sulfide to completely diffuse to the film surface mitigates pothole formation. This allows the formation of continuous, large-grain CuInS2 with no potholes (Figure 6c). This study could be used to improve the efficiency of CuInS2 solar cells by increasing the shunt resistance of the film and minimizing deleterious void formation during film growth.

Figure 5: (a) SEM cross-section of solar cell and inset top-view of a CIS film. (b) CIS solar cell characteristics.

Figure 6: Steps of CIS thin film formation. Plan view SEM showing holes in the film after sulfurization at 535°C (a). Plan view of the surface after initial formation of CuxS (b). Plan view SEM after a 2-stage heating process that eliminates surface holes (c). XRD analysis indicating the formation of CuS and its transoformation from cubic to tetragonal (d). SEM cross-section of the CIS film (e).

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The AIR process has been adapted to analogous material systems, Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe), which are composed of more environmentally friendly and abundant elements. The highest efficiency of any CZT(S,Se) solar cell, 9.66% is currently obtained by Mitzi et al, by means of solution deposition. While this result is groundbreaking, it involves the use of highly toxic and explosive hydrazine, so a safer solution phase deposition route is desirable. Conveniently, our AIR process involves only common organic solvents, elemental sulfur, and organometallic complexes. A recent focus in the CZT(S,Se) project has been optimizing the ink synthesis so as to produce carbon-free oxide films which could lead to high quality CZT(S,Se) films.

In the AIR process, an ink is synthesized by mixing zinc salts with sulfur in pyridine to form the coating agent. This solution is mixed with copper and tin salts to complete the CZTS ink. This ink is cast onto Mo coated substrates and heated in air at 300°C, to remove carbonaceous residues. The dry oxide film is then sulfurized or selenized by elemental chalcogenide vapor at 525oC. Samples are analyzed with Scanning Electron Microscopy (SEM), and Auger Electron Spectroscopy (AES).

Early ink formulations attempted to use the acetylacetonate (acac) salts of Cu, Zn, and Sn, due to their facile decomposition observed in the CuInS2 system. However, Sn(acac)4 is a liquid at room temperature, which prevented even drying of the film. Sn(acac)Br2 and SnCl2 were also promising candidates due to their solubility in pyridine. Yet films were found to be Sn poor by varying amounts due to the unpredictable vaporization of SnCl2 during heating steps, and films from Sn(acac)Br2 had significant carbon residues. Sn(acetate)4 was settled upon due to its moderate stability in pyridine, and rapid decomposition upon oxidation. With this final ink formulation, clean oxide films of 2-3um could be created.

After selenizing or sulfurizing the oxide thin films created from these inks, interesting features were seen in AES and XRD. XRD and AES revealed the presence of many phases besides tetragonal CZTSe or CZTS). Reaction of the oxide with Se or S produced similar results, and the results for the selenide are shown in Figure 7. In addition to the expected CZTSe, residual CuxSe, SnO2 and ZnSe were commonly observed, or the the sulfide analogues in the case of sulfurization. AES confirmed the presence of SnO2 precipitates, which were found under grains of a mixture of CZTSe, CuxSe, and ZnSe. It is hypothesized that this residual SnO2 is present due to a large discrepancy in the kinetics of CuO, ZnO, and SnO2 sulfurization/selenization. This could be due to greater crystallinity of the SnO2, which could decrease its reactivity with sulfur vapor. This issue is likely solvable by more careful control of oxide formation and possibly more aggressive S/Se reaction conditions.

Figure 7: Elemental map showing a mixture of CZTSe, ZnSe, CuxSe and SnOx with CZTSe outlined in red (a). SEM showing the SnOx underlayer (b).

3. Future goals A no-cost extension was asked on this grant. If allowed, the following work will be done in its last year Electrodes

• Characterize the intrinsic defect that compensates free carriers in ZnO • Develop a reproducible process for the hybrid Ag/ZnO electrodes

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• Optimize electrical and optical properties of the hybrid Ag/ZnO electrodes • Improve adhesion of hybrid electrodes

Semiconductor layers

• Further development of the AIR process for CZTS and CIS • Integrate with other materials

Device fabrication and characterization

• Integrate hybrid ZnO/Ag electrodes in solar cells • Fabricate multi-junction cell using lamination, solution processing or a combination • Further develop simulation codes for unconstrained cells • Optimize device design

4. Publications 1. B. D. Weil, S. T. Connor, and Y. Cui, J. Am. Chem. Soc., 2010, 132 (19), 6642–6643 2. L. Hu, H. S. Kim, J.-Y. Lee, P. Peumans and Y. Cui, ACS Nano 2010, 4, 2955-2963 3. W. Gaynor, G. F. Burkhard, M. D. McGehee, P. Peumans, Advanced Materials (in press)