Thursday continued cloudy and a good day to be indoors talking with colleagues and listening to their research presentations.
The day began with two parallel sessions of extended oral presentations. In Area 1 three talks by Stephen Forrest, Alex Zunger, and Frank Dimroth. Forrest’s talk described the use of crystalline organic materials to increase carrier mobility as an alternative to the bulk heterojunction. He described work in his group using a vapor phase transport technique and a liquid-phase growth method that results in large-grained crystals. He noted that diffusion lengths scale with grain size but that the grain size has to be twenty to forty times the size of the diffusion length before the grain boundaries stops influencing the carrier properties. Zunger described the fundamental physics of intermediate band and nanostructured solar cells. He described various reasons why these device concepts are difficult to implement in operating devices. Dimroth described methods for producing metamorphic triple junction solar cells. He reported a potentially record performance of 41.1% in a triple junction IMM device. He indicated that the key concept in achieving this performance was to optimize the 
tunnel junctions that join the cells and minimize defects in the grown layers. Several methods were described to eliminate these defects.
In the Area 7 extended oral session three presentations were given by John Wohlgemuth, Jennifer Granata, and Steve Ransome. John Wohlgemuth gave an overview of the progress achieved with BP Solar's Technical Pathway Partnership contract with the U.S. Dept. of Energy, and announced that they are on target for a 25% overall module cost reduction and a 14-cent/kWh price target by 2010. Part of the cost reductions are being achieved with a new wafer process called Mono2 that has already resulted in an 8% performance increase over the conventional BP Solar cast poly-silicon process. Granata discussed results of the characterization of solar module arrays on site at Sandia National Laboratory. The tests were intended to study inverter behavior but the results revealed several key points. Proper commissioning is essential to detecting errors in the installation, which may not be immediately obvious in the operation of the array. Failures and incipient failures in individual modules can be detected by a variety of methods including the use of infrared imaging cameras. The third talk in the session by Steve Ransome detailed results of a study into issues with kWh/kWp module ratings. Using actual module data from several climates, he showed that differences in these ratings between module types are within the uncertainty of the measurements. Also, the ratings calculated by several widely used PV sizing programs have large errors that were attributed to the built-in efficiency versus temperature profiles. He concluded that these ratings have very limited utility in selecting modules for system designs.
Area 9 presented a workshop on recycling that ran concurrently with the other sessions. The workshop was organized by Vasilis Fthenakis and described how modules containing toxic materials can be safely recovered and recycled. Lisa Krueger described how First Solar has established a reliable recycling program for their modules. Later Alex Mezei discussed how the returned modules are recycled using hydrometallurgical methods. This is a necessary component of their product acceptance by local governments. Voluntary recycling methods were described and a cost analysis was presented. The workshop also included a general overview of recycling issues and models for recycling methods. Virgina Gomex, presented PVCYCLE, the European Industry Wide Voluntary Recycling Scheme and Jun-Ki Choi, presented a new mathematical model that can be used to optimize theory of recycling. The workshop encouraged audience participation and discussion was lively.
During the morning session in Area 2 R. Klenk of the Helmholtz Zentrum in Berlin presented real progress in wide bandgap chalcopyrites resulting in > 13% efficiency devices with Voc > 800 mV. Phillip Dale described the growth and characterization of kesterite materials and solar cells made there from including the production of a new record performance solar cell from this material. Gregory Hanket described results producing solar cells from (Ag,Cu)(In,Ga)Se2 absorber layers with a variety of x=Ag/(Cu+Ag) ratios at different y=Ga/(In+Ga) ratios. Two optimal devices were described with x=0.15 and y=0.45 achieving 17.6% efficiency and with x=0.75 and y=0.8 yielding a 13% efficient device at an energy gap of 1.6 eV and a Voc of 890 meV. Ag substitution for Cu had little effect over most y composition ranges but improved devices at higher y values. L. Gutay described the use of confocal PL to determine the lateral variations in CIGS devices. The spatial resolution in this technique is 0.8 um and showed large variations in luminescence across the materials.
The afternoon sessions included an interesting session in Area 1 on organic PV devices and modules. The session began with a presentation by Annick Anctil from the Rochester Institute of Technology describing their recent developments with the use of organic die solar cells and coupling them with traditional bulk heterojunction solar cells to form tandem organic solar cells with improved efficiencies. An interesting discussion was provided by Sushobhan Avasthi from Princeton University regarding the use of small molecules between the Si-Metal intereface in c-Si solar cells to reduce carrier recombination velocity and improve lifetimes. Stephen Fonash (Penn State) provided a summary of the use of AMPS solar cell modeling software for simulating bulk heterojunction solar cells. Good correlation was observed between the simulations and previously reported data. Jiangeng Xue (University of Florida) utilized biased dependent spectroscopic photocurrent measurements to investigate the carrier lifetimes in CuPC based materials. Oblique angle deposition of CuPC materials, leading to the formation of interdigitated molecular heterojunction structures was shown to improve the efficiency by a factor of 2. Finally, Woo-Jun Yoon from Ohio State University illustrated increased P3HT bulk heterojunction performance by incorporating Ag nanoparticles into the structure increasing the absorption through plasmonic and scattering effects.
In Area 5, multiple new approaches to thin film Si (1-5 microns) including APC, e-beam, and epitaxial thickening on a seed layer by hot wire CVD were described. Excellent solar cell efficiencies of A-Si/A-SiGe dual junction devices (>14%) were reported.
Later in the afternoon in Area 1 an overview was presented of various 3rd-generation concepts by Prof. Martin Green. The talk was well attended with an estimated audience of 700. Six more talks followed with several topics based on quantum effects for solar cells, including both quantum wells and quantum dots in a number of material systems. The session was capped with a talk by Prof. Antonio Marti on intermediate band solar cells. He reported many approaches to realize this 3rd-generation concept in a multi-nation European project.
In Area 2 there were seven well presented papers. Of particular note, Ingrid Repins from NREL presented TRPL data showing CIGS minority carrier lifetimes ranging from 7 to 63ns. The carrier lifetimes were shown to depend strongly on time of air exposure of the device and surface treatments. Changing the deposition conditions and the nature of the deposited material was shown to change the lifetime. When the lifetime was measured immediately after growth and the device was immediately finished the lifetime was shown to correlate well with the performance of the resulting device. Anthony Vasko from University of Toledo disclosed that a 10.5% CdTe cell on a flexible polyimide substrate had been produced. Dennis Coyle described methods for damp heat accelerated lifetime tests on standard CIGS solar cells. Activation energies for degradation of devices were described and a general model for simulating damp heat tests and generalizing the results to module testing were covered. The session also included Timothy Potts describing a new metrology tool for characterizing glass defects.
