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The development of hybrid inorganic/organic thin-film solar cells on flexible, lightweight, space-qualified, durable substrates provides an attractive solution for space power generation with high mass specific power (W/kg). The high-volume, low-cost fabrication potential of organic cells will allow for square miles of solar cell production at one-tenth the cost of conventional inorganic materials. Plastic solar cells take a minimum of storage space and can be inflated or unrolled for deployment. We explore a cross-section of NASA in-house and sponsored research efforts that aim to provide new hybrid technologies that include both inorganic and polymer materials as active and substrate materials. For NASA applications, any solar cell or array technology must not only meet weight and AMO efficiency goals, but also must be durable enough to survive launch and space environments. Also, balance of system technologies must be developed to take advantage of ultra-lightweight solar arrays in power generation systems.

A framework for modeling the power generation of laterally-contacted n-type/intrinsic/p-type/intrinsic (nipi) diodes coupled with an alpha-particle radioisotope source is developed. The framework consists of two main parts, the alpha-particle energy deposition profile (ADEP) and a lumped parameter equivalent circuit model describing the nipi device operation. Experimental measurements are used to verify the ADEP modeling approach which determines the spatially varying energy deposited within the device. Using these results, nipi-diode radioisotope batteries are simulated and the effect of the number of junctions, the thickness of the junction, and the alpha-particle flux on output voltage and power are investigated. The modeling results indicate that a 1 cm2 bi-layer device (consisting of one source and two adjacent nipi-diodes) with a source activity of 300 mCi can reach a power output of 2 mW, excluding radiation damage effects.

Presented here is the recent progress of the NASA Glenn Research Center OMVPE group's efforts in the development of high efficiency thin-film polycrystalline III-V photovoltaics on optimum substrates. By using bulk polycrystalline germanium (Ge) films, devices of high efficiency and low mass will be developed and incorporated onto low-cost flexible substrates. Our progress towards the integration of high efficiency polycrystalline III-V devices and recrystallized Ge films on thin metal foils is discussed.

An alpha voltaic battery utilizes a radioactive substance, which emits energetic alpha particles, that is coupled to a semiconductor p/n junction diode. Alpha voltaics have not been technologically successful to date primarily because the alpha particles damage the semiconductor material, thus degrading the electrical output of the solar cell in just a matter of hours. The key to future development resides in the ability to limit this degradation. Several approaches to solving this problem have been investigated. One approach uses photovoltaic devices which have good radiation tolerance such as InGaP. Another involves the use of non-conventional cell designs, such as a lateral junction n-type/intrinsic/p-type/intrinsic cell, which minimizes the effect of radiation damage on the overall cell performance. A third approach uses an intermediate absorber which converts the alpha energy into light which can be converted by the photovoltaic junction. The intermediate absorbers used in this approach are inherently radiation-hard semiconducting quantum dots. The synthesis of both the quantum dots and the InGaP devices are presented. A summary of the various approaches and resulting performance of the alpha voltaic devices is given.

The majority of high-efficiency space solar cells being produced today are based on multi-junction devices of lattice-matched III-V materials. An alternative which has been receiving an increasing amount of attention is the lattice mis-matched or metamorphic approach to multi-junction cell development. In the metamorphic triple junction cell under development by ERI and its partners, the InGaAs junction (bottom cell) of the three-cell stack is the current limiting entity, due to the current matching which must be maintained through the device. This limitation may be addressed through the incorporation of InAs quantum dot array into the depletion region of an InGaAs cell. The InAs quantum dots in the InGaAs cell will provide sub-gap absorption and thus improve its short circuit current. This cell could then be integrated into the three-cell stack to achieve a space solar cell whose efficiency exceeds current state-of-the-art standards. A theoretical estimate predicts that a InGaAlP(1.95eV)/InGaAsP(1.35 eV)/InGaAs(1.2 eV) triple junction cell incorporating quantum dots to improve the bottom cell current would have an efficiency exceeding 40%. In addition, theoretical estimates have demonstrated that the use of quantum dot structures may also hold other cell benefits such as improved temperature coefficients and better radiation tolerance, which are especially important for utilization in space. As a first step towards achieving that goal, we have initiated the development of InAs quantum dots on lattice-mismatched InGaAs (1.2 eV bandgap) grown epitaxially on GaAs by metallorganic vapor phase epitaxy (MOVPE). These quantum dots have been characterized via photoluminescence (PL) and atomic force microscopy (AFM). A correlation exists between the quantum dot size and resulting optical band structure and can be controlled via the synthesis parameters. Quantum dots were incorporated into prototype InGaAs devices. A comparison of the resulting photovoltaic efficiency under simulated 1 sun intensity and air mass zero (AM0) illumination and spectral response demonstrated that an improvement in the long-wavelength photoconversion efficiency was achieved through the incorporation of the InAs quantum dots.

The majority of satellites and near-earth probes developed to date have used photovoltaic (PV) arrays for power generation. For future missions probing closer to the sun, solar cells need to be developed that can function at higher temperatures, light intensity, and radiation conditions. The theoretical and experimental performance of wide-bandgap materials for use as high-temperature solar cells has been studied. The dependence of solar cell parameters as a function of temperature and cell bandgap was investigated. Several wide-bandgap materials (i.e., GaP, SiC, and GaN) are compared to the conventional materials used for space solar cells.

We report on radiation-induced changes in the electrical resistivity of carbon nanotube (CNT) matrices, or “Buckey” paper intended for use in space photovoltaic applications in combination with quantum dots. Irradiation with 2 MeV protons causes the roomtemperature resistivity of the CNT paper to increase nearly linearly, from 0.06 Ω cm to 0.32 Ω cm at a fluence of 7x10 H/cm. Comparing the radiation responses of QDs and CNT paper to that of conventional bulk solar cells shows that QD/CNT-based photovoltaic devices have the potential to be as many as five orders of magnitude more resistant to radiation damage. The reason for the high radiation tolerance of CNT paper is currently unknown, but several possible explanations are discussed.

Solar cells based on organic photoconductive thin films have shown to possess attributes that enable them to compete effectively with modules based on crystalline silicon. It is exp ected that these attributes will allow realization of a lower $/W p cost figure for organic solar cells compared to crystalline silicon. These attributes are large optical absorption coefficient, high reliability, high efficiency and lower materials costs. The cost reductions and reliability of thin film technologies result from monolithic integration which requires fewer processing steps, avoids expensive crystal growing in large areas, the handling of fragile wafers and their discrete interconnection. Nano structure -polymer blend solar cells generally have the following structure : indium tin oxide ( ITO )/active polymer /metal electrode , where the active polymer layer is composed of a weak donor polymer doped with nanostructures , such as carbon nanotubes, that act as electron acceptors. We have investigated the material properties of the various layers of nanostructure -polymer blend solar cells . Single wall carbon nanotubes (SWNT) were fabricated by a laser vaporization method and characterized by transmission electron microscopy . The interface between metal electrode and the polymer layer was investigated by sputtering two types of metals, Au and Ag, on poly(3 -octylthiophene) (P3OT). The metal/polymer interface was s tudied by focus ion beam (FIB) and seconda ry ion mass spectroscopy (SIMS). A design of experiment (DOE) study was implemented to determine the processing parameters that had the most significant effect on the film properties of poly(3,4 -ethylenedioxythiophene) doped with polystyrenesulfonate (PED OT:PSS) spin coated on ITO by looking at the relationship between controllable factors such as spin coating speed, solution concentration, anneal time, anneal temperature and the resulting % roughness of the films. It was found that a broad interfacial la yer is present between the metal and the polymer layer and the DOE showed that solution concentration had the most effect on the % roughness.

Publisher Summary This chapter discusses the operation of solar cells in a space environment. The first 30 years of space solar cell development focused on silicon solar cells. The concept of a tandem cell was proposed in the early days to enhance the overall efficiency. An optimized three-cell stack followed with a theoretical optimum efficiency of 37%. However, it was 40 years later before a multijunction solar cell flew in space. Today, silicon cells still fly in space, but the cell of choice is a multijunction solar cell. While commercial satellites use silicon, dual, or triple junction GaInP/GaAs/Ge, there is a marked interest in military applications of thin-film cells. NASA also has planned missions in which a large specific power (kW/kg) and lower cost would be beneficial. The advantages of thin film solar cells are their large specific power when deposited on a flexible, lightweight substrate with a suitably lightweight support structure. This chapter discusses space missions and their environments. Concepts of air mass zero spectrum, trapped radiation environment, solar flares, neutral environment, the particulate environment, and thermal environment are explained in detail.

Minority-carrier diffusion lengths in n-type 6H-SiC solar cells were measured using the planar electron-beam induced current (EBIC) technique. Experimental values of electron beam current, EBIC and beam voltage were obtained for n-type SIC with a carrier concentration of 1.7E17 cm/sup -3/. This data was fit to theoretically calculated diode efficiency curves, and the diffusion length and metal layer thickness extracted. The extracted hole diffusion length ranged from 0.68 /spl mu/m to 1.46 /spl mu/m. The error for these values was /spl plusmn/15%. Additionally, we introduce a novel variation of the planar technique. This "planar mapping" technique measures diffusion length along a linescan creating a map of diffusion length versus position. This map is overlaid onto the EBIC image of the linescan, allowing direct visualization of the effect of crystal defects on minority carrier diffusion length. Diffusion length maps of both n and p-type 6H SiC show that large micropipe defects severely limit the minority carrier diffusion length, reducing it well below 0.1 /spl mu/m inside large defects.

Solar cell thin films of CuInS/sub 2/ were deposited on fused silica, stainless steel, Kapton/sup TM/ and polybenzobisoxazole using the single source organometallic precursor (PPh/sub 3/)/sub 2/CuIn(SEt)/sub 4/, in conjunction with spray chemical vapor deposition. Films were deposited at temperatures ranging from 325-360/spl deg/C. As deposited, the films had a thickness on the order of 200 /spl Aring/. The grain structure of the films was found to vary with carrier gas flow rate and substrate temperature.

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