Rapid progress in silicon wire array solar cells has enabled cells with high open circuit voltage (>600 mV) and high (>90%) quantum efficiency, in wire arrays grown by a metal-catalyzed vapor-liquid-solid growth process. Following growth on a crystalline (111) Si wafer, Si wire arrays are embedded in a polymethyldisiloxane (PDMS) film and can be peeled off the growth template substrate, yielding an unusual photovoltaic material: a flexible, bendable, wafer-thickness Si absorber. Following wire array peel off, the original growth template substrate can be reused for subsequent array growth without further lithography. In this paper, I will report progress on large-area (> 10cm x 10cm) peel-off of Si wire arrays and directions for high efficiency cell and module fabrication that can enable the < $1/W module manufacturing goal.

Nanostructured silicon thin film solar cells are promising, due to the strongly enhanced light trapping, high carrier collection efficiency, and potential low cost. Ordered nanostructure arrays, with large-area controllable spacing, orientation, and size, are critical for reliable light-trapping and high-efficiency solar cells. Available top-down lithography approaches to fabricate large-area ordered nanostructure arrays are challenging due to the requirement of both high lithography resolution and high throughput. Here, a novel ordered silicon nano-conical-frustum array structure, exhibiting an impressive absorbance of ∼99% (upper bound) over wavelengths 400-1000 nm by a thickness of only 5 μm, is realized by our recently reported technique self-powered parallel electron lithography that has high-throughput and sub-35-nm high resolution. Moreover, high-efficiency (up to 10.8%) solar cells are demonstrated, using these ordered ultrathin silicon nano-conical-frustum arrays. These related fabrication techniques can also be transferred to low-cost substrate solar energy harvesting device applications. The fabrication of large-area ordered controllable Si nanostructure arrays needs top-down planar lithography with both high throughput and high resolution. Conventional optical lithography has high throughput, but its critical dimension (CD) is limited to a fraction of the wavelength. Traditional electron beam lithography (EBL) has the highest resolution <10 nm, but EBL suffers from high cost and low throughput due to the required electron beam raster scanning serial exposure. Nanoimprint lithography could be used to achieve nanostructured arrays, but the prospect of mask mechanical contact to substrate leads to a large number of defects and short mask life. Our recently reported technique self-powered parallel electron lithography (SPEL), using large-area planar radioactive beta electron thin film emitters to parallel expose e-beam resist through a stencil mask, demonstrated sub-35-nm resolution. Using naturally emitted high-energy beta particles, the SPEL system can be compact as the electron focusing column needed in EBL systems is no longer needed. Elimination of vacuum in SPEL will significantly simplify the overall lithography system and greatly reduce the cost, while enabling large area massively parallel high-throughput electron lithography with high resolution. Therefore, SPEL is a very promising way for large-area ordered nanostructure array fabrication, especially for solar cells applications.

Atomic layer deposition (ALD) is a gas-phase deposition process that can penetrate into pores less than 5nm in diameter [1], making it a promising tool for the fabrication of nanostructured heterojunctions in extremely thin absorber solar cells. ALD and chemical vapor deposition (CVD) of Cu-rich CuxS has been performed on planar ZnO and nano-porous TiO2 using a new precursor (KI5) and H2S. Copper sulfides occur in five stable crystal phases ranging from Cu-rich Cu2S to Cu-poor CuS. The semiconducting Cu2S phase is a promising solar cell material consisting entirely of non-toxic and earth abundant materials. Cross-sectional SEM images taken of nanoporous TiO2 films with and without ALD treatment show backfilling and uniform coverage at penetration depths of over 200nm. X-ray absorption fine structure (EXAFS) data indicates that film crystal structures are disordered and dominated by Cu-rich phases for films deposited in the temperature range 150-400C. X-ray photoelectron spectroscopy was used to isolate the composition at the surfaces of the ALD-deposited films. Results are consistent with the Cu2S crystal phase. Optical absorption was measured using photothermal deflection spectroscopy for a wide range of CVD-deposited film thicknesses. The initial film growth (<100nm) shows high absorption at low photon energies, a characteristic of metallic, Cu-poor CuxS. As thickness increases, distinguishable direct and indirect band gaps appear in the ranges 1.11-1.15eV (indirect) and 1.81-2.03eV (direct). These values are consistent with accepted Cu2S values [2]. The sheet resistances of ALD and CVD-deposited CuxS films on planar ZnO do not scale linearly with thickness, indicating the presence of a Cu-poor material preferentially forming at the substrate/CuxS boundary. This Cu-poor region at the ZnO and TiO2 interface effectively shorts out the semiconducting Cu2S phase. [1] L. Reijnen, B. Feddes, A. M. Vredenberg, J. Schoonman, and A. Goossens, The Journal of Physical Chemistry B 108, 9133-9137 (2004). [2] O. Madelung, Semiconductors - Basic Data, 2nd ed. (Springer, 1996).

Amorphous silicon is widely used in thin film solar cells, however, a large portion of incident light is reflected back from flat surface of a-Si due to its high refractive index. Certain nanostructures have been demonstrated for broadband reflection suppression. Here we report on the design and fabrication of a-Si hollow nanospheres using a wafer-scale Langmuir-Blodgett assembly technique and chemical vapor deposition. We investigated light absorption and reflection properties of these a-Si nanostructures for solar cell applications. These Si hollow nanospheres display greatly enhanced absorption to the flat control film, particularly in the spectral range from 550 nm to 800 nm. Full-wave electromagnetic simulation of the absorption in the active a-Si:H layer agrees well with experimental results. We show strong absorption peak due to resonance mode in the shell of Si nanosphere.

We fabricate ultra-thin Cadmium Telluride (CdTe) Schottky diode photovoltaics in a substrate geometry. Devices were comprised of a 350nm thick CdTe microcrystalline layer deposited by spin-casting and sintering colloidal nanorods onto RF sputtered Molybdenum films on glass. A transparent conducting Aluminum doped Zinc Oxide (AZO) top contact was then RF sputtered on the CdTe. A typical device has a 0.2 mA/cm2 short-circut current density (Jsc), a 100mV open-circuit voltage (Voc) and a 25% fill factor under AM1.5G, 100mW/cm2, illumination. We compare these results to our best superstrate devices with 5% power conversion efficiency on pre-patterned ITO with a structure of ITO/CdTe/Al and a 22mA/cm2 Jsc, a 520mV Voc and a 43% fill factor. Our substrate device performance is limited by the high series resistance of our RF sputtered AZO films and heat damage to the CdTe during the RF sputtering process. To alleviate this problem moving to a low-temperature solution-deposited transparent electrode is required. This work highlights the possibility of successfully fabricating a CdTe Schottky diode solar cell on Molybdenum metal foils.

As PrimeStar Solar transitions from CdTe PV module pilot production to high volume manufacturing we are focused on minimizing production variability and costs while maximizing yield, performance and reliability. Steps being taken include a factory design with in-line metrology and characterization to enhance real time monitoring and process control feedback; distributed flow to optimize equipment capacity and facilitate statistical process control; and careful automation to minimize product handling defects. Our equipment design is a balance between minimizing capital cost, maximizing availability and throughput as well as preserving flexibility for anticipated future process enhancements. We are also tightening source material specifications to minimize product variability. A number of the opportunities and challenges with this transition are discussed.

Successful deployment of photovoltaic power generation at the terawatt level will require synthesis techniques that can produce low-cost, large-area devices with performance that rivals single crystal materials. We report here on two physical vapor deposition approaches that have large-scale potential. The first approach utilizes ion beam assisted deposition (IBAD) to produce biaxial texture in thin films. The ion beam is incident on the substrate concurrent with deposition and at an inclined angle corresponding to a channeling direction in the growing film. This produces in-plane and out-of-plane alignment of the crystals in the film. We have developed a seeded epitaxy technique that uses a seed layer that is easily aligned with IBAD, upon which the semiconductor of interest can be grown. Here we report on Si films grown on IBAD deposited CaF2. We have used a variety of approaches for Si film growth including evaporation, sputtering and hot wire CVD. We will report on structural characterization as well as electronic properties. The second approach utilizes reactive sputter deposition to produce sulfide absorber layers. The material Cu2ZnSnS4 (CZTS) has many advantages for application in photovoltaic devices, including self-doping, favorable band-gap and no expensive or rare constituents. However, conventional growth approaches require an anneal to react with form the desired sulfide phase. Since the metal species are the fast diffusing species, formation of the sulfide is accompanied by incorporation of Kirkendal voids and defects. Our approach is to incorporate the sulfur in the growth process and directly form the desired phase. Here we report on material quality and device performance.

The chalcopyrite solid solution Cu(In_xGa_1-x)Se₂ (CIGS) is a commercially emerging thin film absorber material based on the promise of low manufacturing cost and high conversion efficiency (champion cell now exceeds 20%). The primary challenge in achieving low processing cost is increasing the synthesis rate of CIGS at lower temperature. Recognizing this challenge the national solar technology roadmap calls for decreasing the absorber synthesis time to 2 min by 2015 while retaining high efficiency. To assist in indentifying improved synthesis routes, we have been using in-situ high temperature X-ray diffraction to better understand reaction pathways and determine rate constants¹. These studies have suggested that synthesis of Cu(In_xGa_1-x)Se₂ is diffusion limited for most precursor structures. This suggests that pathways that include a liquid phase or involve an interstitial or high vacancy concentration diffusion mechanism would be good candidates.Time-resolved selenization is used to study the formation of CuInSe₂ (CIS) from novel compound bilayer precursors. Specifically the bilayer structures glass/Mo/γ-In₂Se₃/CuSe_y (sample A) and glass/Mo/γ-In₂Se₃/β-Cu₂Se (sample B) were investigated. The structures were deposited by thermal evaporation on sputtered Mo/thin sodium-free glass substrates. ICP analysis indicated both samples were copper-rich with a Cu/In ratio = 0.97 for sample A and 1.18 for Sample B. Intermediate liquid phases are expected for Cu-rich and high Se activity conditions. Initial temperature ramp experiments using the glass/Mo/γ-In₂Se₃/CuSe_y sample revealed the reaction sequence of formation of β-CuSe, selenization to CuSe₂, decomposition of this compound to γ-CuSe and γ-In₂Se₃ to InSe, and final synthesis of CIS. The sequence for the glass/Mo/γ-In₂Se₃/β-Cu₂Se precursor showed β-Cu₂Se reacts with Se to form CuSe₂, then melts peritectically giving L + γ-CuSe, and γ-In₂Se₃ disproportionates yielding InSe and Se, followed by final synthesis of CIS. Isothermal experiments were performed to quantitatively extract kinetic parameters using the Avrami and parabolic growth models. SEM images revealed significant grain growth for both temperature ramp annealed samples. Based on these results, these precursor structures were annealed for 2 min using rapid thermal annealing in a Se atmosphere to test the feasibility of this precursor. Interestingly the reaction was complete in 2 minutes at a very low temperature (390 ^οC for samples A and 370 ^οC for sample B) while showing large grain growth. Additionally, TEM was performed to provide compositional and structural support for the indentified pathways.[1]W. Kim, S. Kim, E. Payzant, S. Speakman, S. Yoon, R. Kaczynski, R. Acher, T. Anderson, O. Crisalle, and S. Li, Journal of Physics and Chemistry of Solids 66/11 (2005) 1915.

The development of Raman scattering based strategies for process monitoring in chalcopyrite based photovoltaic thin film technologies is reported. Raman spectra measured at different process steps during the fabrication of the absorbers are very sensitive to features related to their crystalline quality, presence of secondary phases and polytypes and alloy composition. All these are features that have a significant impact on the characteristics of the final solar cells. New strategies based in the use of quasi-resonant Raman measurements are described for the non destructive assessment of the composition of quaternary alloys. The methodology developed can be used to monitor the fabrication of CIGS absorbers by using several techniques, such as sputtering, solution based processes, electrodeposition, .... In particular, in this work the implementation of Raman scattering for monitoring of electrodeposition processes used in the fabrication of low cost electrochemical based CuIn(S,Se)2 solar cells is reported at both on-line and in-situ levels due to the strong interest for the development of technologies with low fabrication costs of the single step electrodeposition of CuInSe2 precursors followed by a Rapid Thermal Process (RTP) sulphurisation process. The potential of using RS as a quality assessment and monitoring tool in the production of chalcopyrite absorbers is described, with a particular emphasis on the most relevant structures: CuInSe2 (CISe), CuInS2 (CIS), CuGaS2 (CGS) and CuGaSe2 (CGSe). We will discuss the most important information that can be inferred from the analysis of the Raman spectra to process monitoring, including crystalline quality, crystallographic structure, chemical composition in the case of quaternary alloys, and the presence of secondary phases. One remarkable advantage of the developed methology is the identification of Ordered Vacancy Compounds (OVCs) in CISe based absorbers. These phases arise as a result of a deficiency of Cu during the film formation, leading to the introduction of randomly distributed In[Cu] antisite defects in the chalcopyrite lattice, which are electrically compensated by Se vacancies, and which are difficult to be identified in line with other monitoring techniques. Several OVCs with different stoichiometries have been studied in this work, including CuIn2Se3.5, CuIn3Se5, and CuIn5Se8.The advantages of quasi-resonant measurements can be achieved by selecting an excitation wavelength close enough to the band-gap of the alloy is also discussed. This determines a strong increase of the intensity of the Raman modes, which allows for a significant decrease of the measuring time, improving the potentiality the implementation of this technique as an in-line in-site quality control technique.

We describe the design, development and manufacture of monolithically integrated photovoltaic modules based on high-quality high-uniformity copper indium gallium selenide (CIGS) thin films produced with the unique combination of ink based and physical vapor deposition (PVD) based nanoengineered precursor thin films, and a reactive transfer printing method. Reactive transfer is a two-stage process relying on chemical reaction between two separate precursor films to form CIGS, one deposited on the substrate and the other on a printing plate in the first stage. In the second stage, these precursors are brought in proximity and rapidly reacted under pressure while heat is applied. The use of two independent thin films provides the benefits of independent composition and flexible deposition technique optimization, and eliminates pre-reaction prior to the synthesis of CIGS. When atmospheric deposition of inks is utilized, the approach provides lower energy consumption, higher throughput, and reduced capital equipment cost with higher uptime. High quality CIGS with large grains on the order of several microns, and of preferred crystallographic orientation, are formed in under a minute based on compositional and structural analysis by XRF, SIMS, SEM and XRD. Cell efficiencies of 14% and module efficiencies of 12% have been achieved using this method. HelioVolt commercialized the reactive transfer process on a 20 MW pilot line, and is in the process of scaling the process on multiple 125 MW lines in a mass production GW-scale factory.

The development of colloidal inks that can be used to yield high quality semiconductor layers are a key step in the development of low-cost solar cells since they enable the use of fast and inexpensive coating processes such as spray coating and roll coating to form a thin film photoabsorbing layer. Chalcopyrite structure copper indium gallium diselenide (CIGSe) and stannite or kesterite copper zinc tin sulfides (CZTS) are key photoabsorbing materials for thin film solar cells due to their near ideal band gap and their serendipitous defect chemistry (CIGSe) and Earth abundance (CZTS). Due to their unique defect chemistry, high quality layers of these materials can be formed from solution phases processing techniques. The presentation will focus on our recent advances in the development of molecular precursor and nanocrystal ink routes to thin film photovoltaic devices [1]. In particular, we have recently reported the solution-phase synthesis of stoichiometric chalcopyrite structured CuInSe2 nanocrystals [2], Cu(In,Ga)S2 [3], and the very first synthesis of Cu2ZnSnS4 nanocrystals [4]. The syntheses proceeds rapidly from elemental and halide reagents via a simple batch reaction without “hot injection” in a single component coordinating solvent. We have demonstrated the use of these nanocrystals for low-cost solar cells by fabricating devices without using any oxygen-free techniques (after NC synthesis) and employing a scalable roll coating method. The nanocrystal inks are first coated on a back contact (Mo coated sodalime glass in this case). The nanocrystal layer is then easily consolidated into large crystalline domains by a brief thermal treatment in a selenium rich atmosphere to prevent selenium loss or to replace sulfur with selenium. The fabricated cells are robust and increase in efficiency with time, exhibiting similar serendipitous defect chemistry as layers formed by vacuum co-evaporation. We have fabricated solar cells by roll coating CIGS or CZTS nanocrystal-inks over large areas. CIGS devices fabricated by roll coating over large areas with a device architecture of Mo/CIGSSe/CdS/i-ZnO/ITO/Ni/Al are (at the time of the abstract submission) 12.0% efficient under standard AM1.5G illumination while CZTS devices are now at 7.2%. The presentation will focus on the key aspects of the nanocrystal synthesis, ink coating, nanocrystal consolidation, and device fabrication and characterization for both CIGS and CZTS solar cells.[1] Hillhouse H.W. & Beard M.C., Current Opinion in Colloid & Interface Science, 14, 245 (2009).[2] Guo, Q.J., Kim, S.J., Kar, M., Shafarman, W.N., Birkmire, R.W., Stach, E.A., Agrawal, R., Hillhouse, H.W.,Nano Letters 8, 9, 2982 (2008).[3] Guo, Q.J., Ford, G.M., Hillhouse, H.W., Agrawal, R., Nano Lett. 9, 8 3060 (2009).[4] Guo, Q.J., Hillhouse, H.W., Agrawal, R., J. Am. Chem. Soc. 131, 11672 (2009).