Cell-to-Module Losses Key to Improving PV Efficiencies

December 15, 2016 by Jeff Shepard

Researchers in Europe and the U.S. are focused on understanding ways to lower cell-to-module (CTM) interconnection losses as one of the keys to improving the efficiency of next-generation PV modules. In the U.S. Natcore Technology will be the industrial partner working in a $800K SunShot grant awarded to Arizona State University (ASU) by the U.S. Department of Energy (DoE) for a project titled "Monolithic silicon module manufacturing at < 0.40 $/W."

In Europe, Fraunhofer ISE has developed SmartCalc.CTM a software tool that helps reduce the cell-to-module power losses in PV module manufacturing. Research and industry invest lots of know-how in improving solar cell efficiency. In order that PV modules benefit from the advances in cell efficiency, the cell-to-module integration process must be performed reliably with low losses.

With this in mind, the photovoltaic module group at Fraunhofer ISE developed the software SmartCalc.CTM, which enables manufacturers of PV modules and materials to optimize the assembly and material combination in a PV module, before fabricating a prototype.

In the U.S., ASU received the DoE grant to develop and demonstrate technology that enables a mass-manufactured solar module that costs less than $0.40/W, is 22% efficient, has an annual degradation rate of less than 0.2%, and is warrantable for 50 years. The National Renewable Energy Laboratory (NREL) is also a partner in this grant.

Natcore was selected for this project because it has all the tools needed for complete n/p or p/n homojunction silicon solar cell fabrication, either with standard top/bottom contacts or all-back-contact configurations. The latter capability also applies to any silicon heterojunction structures that may be investigated in the proposed project. The project gives Natcore access to measurement and diagnostic tools for cell and module characterization, as well as to manufacturing quality control expertise and monitoring systems.

Natcore, through its Science Advisory Board and its own two-party agreements, has direct connections with two leading international manufacturers of integrated flex circuit materials that will enable all-back-contact cells to be integrated into full 60-cell panels with a significant reduction in CTM losses.

Dr. Dennis Flood, Natcore's co-founder and Chief Technology Officer, and one of the authors of the grant proposal, will be responsible for evaluating the manufacturability of the new flex-circuit-based interconnection schemes implemented by ASU, including determining cell-to-flex-circuit alignment tolerances, evaluating suitable handling and alignment technologies, and tracking the throughput of all module assembly steps. Natcore will commit $25,000 in cost share to this project for this effort at the rate of one month per year (between two scientists).

"This grant will provide Natcore with considerable help with testing, evaluation, cost analysis, etc., at no cost other than in-kind contributions," says Dr. Flood. "For example, ASU will measure CTM energy loss on cells that are made in our Rochester lab."

"The CTM figure is a critical component of a solar cell's bottom-line value in the marketplace," says Chuck Provini, Natcore president and CEO. "Currently CTM losses average from 8% to 10%. We believe that our all-back-contact structure will lower that loss almost to zero. So any panel manufacturer who uses cells made with our technology will in effect be getting that much more energy at no extra cost."

"We are in good company with this grant," adds Provini. "We have a significant opportunity with this that can help us with other interactions as well. We have an integral role very early and can have an even bigger one later if we want. The ASU team wants to come to Rochester soon after the first of the year, and wants to work through us with our substrate resource."

Franunofer’s SmartCalc.CTM determines the CTM effects which occur when solar cells are integrated into a module. Starting with the cell power, the software calculates the optical losses and gains (e. g. reflection), electrical losses (e. g. due to resistances) and the geometrical losses (inactive areas) in solar modules. The software tool assists in analyzing potential yields, thus enabling PV manufacturers to determine how new materials or concepts would affect module efficiency. In the model, the interaction between components and process steps are optimized in such a way that the best possible module – with the highest power or the maximum efficiency under the given boundary conditions – is achieved.

“SmartCalc.CTM was conceived for module manufacturers and material suppliers,” explains Matthieu Ebert, team leader of “Module Efficiency and New Concepts” at Fraunhofer ISE. “Our software offers added value in that it can quickly analyze the interplay between the influencing factors for a module design and present these in a clear manner.” The underlying models in SmartCalc.CTM provide detailed yet flexible control at the same time. Properties of solar cells, encapsulation material or module glass can be easily adapted.

“Thus, the use of new materials and components and their effect on the module efficiency can be easily determined, enabling new technologies and module designs to be evaluated rapidly. The software can also be used to optimize costs. For example, different less-expensive materials can be compared and evaluated with respect to the module’s performance efficiency,” Ebert continued.

SmartCalc.CTM is based on a simulation model which has been published and is under development at Fraunhofer ISE since 2008. The model considers the many factors that influence module performance such as optical and electrical effects as well as singular components, for example, solar cells, glazings or the module frame. As a result, potential improvements can be indentified easily and applied in practice. The CTM factors can change drastically depending on the choice of solar cell, module materials and the module assembly.

“With a well-selected combination of materials and module design, an optimization of all factors can even lead to a CTM > 100%, indicating a higher output as compared to the sum of all solar cells before integration,” says Ebert.

SmartCalc.CTM has a user-friendly interface with an accompanying operating manual. As well as the software license, the team at Fraunhofer ISE offers consulting and R&D support for analyzing and optimizing the simulation results. The software shall be continually developed in order to keep pace with future module assemblies.