Solar Cell Case Study

[Sanny Olafeso]

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Active layer morphology is of key importance in bulk heterojunction (BHJ) solar cells. Bi-continuous nanoscale domains of donor and acceptor materials are required for efficient charge separation and transport1,2,3,4,5,6,7. To achieve the optimum active layer morphology, various methods, such as thermal annealing, solvent annealing, solvent additives, have been used in the process of solar cells fabrication1,2,3,4,5,6,7. From a molecular point of view, chemists want to use self assembly or self organization to control the morphology by molecular design8,9,10,11,12,13. In this respect, hydrogen bonding is the first choice due to its strong intermolecular interaction, high selectivity and directionality14,15,16,17. In some cases, improved device performance had been demonstrated when hydrogen bonding was employed in BHJ solar cells18,19,20,21,22. Here, we describe a case study in which the performance of two similar small molecular donor materials, one with amide group to form intermolecular hydrogen bonding, the other with ester group, are compared in BHJ solar cells. In addition, an amide containing fullerene derivative was also synthesized and used in conjunction with the donor materials in devices.

The presence of hydrogen bonding moieties on the electron acceptor fullerene derivatives may provide further insights into the effect of hydrogen bonding on BHJ solar cell devices. As with the electron donor material, the acceptor should ideally phase separate on a suitable length scale to allow maximum ordering and thus charge can transport effectively in continuous pathways to the electrodes and the recombination of free charges can be minimized6. In this regard, an analogue of [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) with an amide group was also designed and synthesized (see SI for details), resulting in [6,6]-phenyl-C61-N-methyl butyramide (PC61MBA, Figure 1)19.

Two small molecular donor materials, one end capped with ester group and the other end capped with amide group, were synthesized and fully characterized. The amide group of the donor material M2 formed intermolecular hydrogen bonds which led to fibrous aggregates in solid state. Application of the two small molecular donor materials in BHJ solar cells revealed that the amide group containing donor material showed lower performance due to the hydrogen bonding induced aggregation and disadvantageous morphology. Replacement of PCBM with its amide analogue further increased aggregation and large scale phase separation was observed in the blend films. Well defined morphology34,35,36, enhanced stability22,35, improved performance18,19,20,21,22,34,37,38,39 have been discussed in the literature for materials with hydrogen bonding moieties. In this work, the results clear showed that a cautious approach must be taken when using strong intermolecular forces such as hydrogen bonding in organic solar cell materials.

This work was made possible by support of the Victorian Organic Solar Cell Consortium, with funding provided by the Victorian State Government Department of State Development, Business and Innovation (Energy Technology Innovation Strategy and Victorian Science Agenda) and the Australian Renewable Energy Agency (ARENA Project 2-A018). Dr W. H. H. Wong is supported by an Australian Research Council Future Fellowship (FT130100500). We thank Andrew B. Holmes for helpful discussion. We thank the Australian Synchrotron for the SAXS/WAXS beam time to perform the GIWAXS experiments.

Z.X. synthesized and characterized the materials. Z.X. and W.W.H.W. prepared the manuscript. K.S. performed morphological studies and device fabrication. J.S. and S.J. did device fabrication. D.J.J. and W.W.H.W. supervised the project. All authors analyzed the data, discussed the results and contributed to revisions.

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The $200 million renovation of the former Bell Labs facility in Holmdel, New Jersey, includes the largest PV glass skylight currently in the United States at a quarter-mile long. The skylight filters sun through to the atrium, the focal point of the building. The building, designed between 1957 and 1962, looks like a relic with its concrete and steel makeup, but the open-air concourse illuminated mostly by natural light looks like a scene from the future.

Onyx Solar worked with the building architects to create custom PV glass skylights that resembled the previous glass. Onyx used its amorphous silicon (a-Si) solar cells, manufactured in Spain, between two plates of glass to create the skylight. Every glass panel in the new skylight is a PV panel. Amorphous silicon cells were not included in the latest solar tariffs.

Other than the unique teamwork between glazer and electrician, PV glass installations are usually similar to conventional solar installations. The Bell Works project was complex because it called for a total of 24 different glazings to make up the different schemes of the large skylight.

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Grand Valley Electric Power, a Colorado rural electric cooperative, has developed two community solar projects with an on-bill financing option. The first, built in 2011, was self-developed by the co-op. The second, built in 2014, was done in partnership with the non-profit Grid Alternatives. Grand Valley Power constructed these solar farms as a way to better provide service to its members and anticipate to changing business models, with a key objective to provide solar access to low-income households.

There are no fees or penalties if the participating co-op member chooses to stop making on-bill payments and leave the program. If a participant moves out of the co-op serving territory, the solar panel is bought back by the co-op. No cancellations or buy-backs have occurred to date.

Grand Valley Power evaluated lengthening the payment schedule for the on-bill payment option from five to 20 years. This would have made the monthly payment equal to the solar bill credit, creating a bill-neutral program.. The co-op decided not to do so mainly because longer-term loans would create more uncertainty and higher risk for the co-op.

For its next project, Grand Valley Power partnered with the non-profit Grid Alternatives to develop a community solar farm exclusively for low-income families. Energized in June 2015, the 24 kW (122 panel) system provides low-cost solar energy to select co-op members that need it the most.

With both solar farms sold out, Grand Valley Power is looking to expand. The original 17 kW array may grow by another 50-60 kW. The co-ops is also in talks with Grid Alternatives for the second and third phases of the low-income solar project.

Simple Solar subscribers receive a monthly $1.30 solar bill credit per unit for the electricity generated at the solar garden. The subscription leases provide the monthly bill credits for 20 years, and it is estimated that the $270 investment will be paid back within 15 years. Subscriptions can be sold back to Cedar Falls Utilities or transferred to someone else, provided they are a Cedar Falls Utilities customer.

The Simple Solar program sold out quickly and currently has a waiting list. Cedar Falls Utilities made 8,882 units (6,516 panels) available to the public. The rest went to retail and wholesale customers. Cedar Falls-based University of Northern Iowa purchased approximately 20% of the total project. More than 1,250 residents and businesses have subscribed to the solar farm. Due to the great demand, the utility began selling several hundred additional units that it had initially reserved for institutional reasons.

Cedar Falls Utilities uses a specific software in its Oracle Customer Care & Billing (CC&B) system to track both the solar credits and on-bill subscription payments. The software is able to provide reports that track results for participating customers.

To market the Simple Solar program, Cedar Falls Utilities engaged customers early on in the community solar planning process as part of a multi-phase outreach push to attract subscribers to the solar project. Before construction took place, the utility launched a pre-enrollment multimedia campaign. In the pre-enrollment phase, Cedar Falls Utilities unveiled artist designs of the solar farm, created a web page for customers to reserve a solar panel, and even ran a show on the local-access TV station in conjunction with the City of Cedar Falls.

During the enrollment phase, marketing included promotional videos and TV ads run through the Cedar Falls Utilities cable/internet utility, online materials, open houses, and direct mail. Extensive community outreach included events at farmers markets and presentations at the University of Northern Iowa. Cedar Falls Utilities staff actively participated in all of these events to talk about the project and its benefits, and to answer any questions.

Energy generated by the community solar farm is bought by Cedar Falls Utilities through a 25-year power purchasing agreement (PPA) with RER Energy Group, the private developer that built the array. With this financing format, the developer captures the 30% federal Investment Tax Credit (ITC) available for solar projects. Cedar Falls Utilities, as a tax-free entity, is ineligible for the ITC. Cedar Falls Utilities retains the rights to purchase the solar array from the developer once the tax benefits are exhausted.

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