Joris Libal

We present our own Interdigitated Back Contact (IBC) technology, which was developed at ISC Konstanz and implemented in mass production with and at SPIC Solar in Xining, China, with production efficiencies of over 24%. To our knowledge, this is the highest efficiency achieved in the mass production of crystalline silicon solar cells without the use of charge-carrier-selective contacts. With an adapted screen-printing sequence, it is possible to achieve open-circuit voltages of over 700 mV. Advanced module technology has been developed for the IBC interconnection, which is ultimately simpler than for conventional double-sided contacted solar cells. In the next step, we will realize low-cost charge-carrier-selective contacts for both polarities in a simple sequence using processes developed and patented at ISC Konstanz. With the industrialisation of this process, it will be possible to achieve efficiencies well above 25% at low cost. We will show that with the replacement of silver screen-printed contacts by copper or aluminium metallisation, future IBC technology will be the end product for the PV market, as it is the best performing c-Si technology, leading to the lowest cost of electricity, even in utility-scale applications.

In this paper we summarize the status of bifacial photovoltaics (PV) and explain why the move to bifaciality is unavoidable when it comes to e.g., lowest electricity generation costs or agricultural PV (AgriPV). Bifacial modules—those that are sensitive to light incident from both sides—are finally available at the same price per watt peak as their standard monofacial equivalents. The reason for this is that bifacial solar cells are the result of an evolution of crystalline Si PV cell technology and, at the same time, module producers are increasingly switching to double glass modules anyway due to the improved module lifetimes, which allows them to offer longer product warrantees. We describe the general properties of the state-of-the-art bifacial module, review the different bifacial solar cells and module technologies available on the market, and summarize their average costs. Adding complexity to a module comes with the increase of possible degradation mechanisms, requiring more thorough testing, e.g., for rear side PID (Potential Induced Degradation). We show that with the use of bifacial modules in fixed tilt systems, gains in annual energy yield of up to 30% can be expected compared to the monofacial equivalent. With the combination of bifacial modules in simple single axis tracking systems, energy yield increases of more than 40% can be expected compared to fixed tilt monofacial installations. Rudimentary simulations of bifacial systems can be performed with commercially available programs. However, when more detailed and precise simulations are required, it is necessary to use more advanced programs such as those developed at several institutes. All in all, as bifacial PV—being the most cost-effective PV solution—is now becoming also bankable, it is becoming the overall best technology for electricity generation.

The size and number of utility-scale bifacial photovoltaic (PV) installations has proliferated in recent years but concerns over modeling accuracy remain. The aim of this work is to provide the PV community with a validation study of eight tools used to simulate bifacial PV performance. We simulate real 26 kilowatt-peak (kW_p) bifacial arrays within a 420-kW_p site located in northern Europe (55.6° N, 12.1° E). The substructures investigated include horizontal single-axis trackers (HSATs) and fixed tilt racks that have dimensions analogous to those found in utility-scale PV installations. Each bifacial system has a monofacial reference system with similar front side power. We use on-site solar radiation (global, diffuse, and beam) and albedo measurements from spectrally flat class A sensors as inputs to the simulation tools, and compare the modeled values to field measurements of string level power, rear and front plane of array irradiance, and module temperature. Our results show that state-of-the-art bifacial performance models add ~0.5% uncertainty to the PV modeling chain. For the site investigated, 2-D view factor fixed tilt simulations are within ±1% of the measured monthly bifacial gain. However, simulations of single-axis tracker systems are less accurate, wherein 2-D view factor and 3-D ray tracing are within approximately 2% and 1% of the measured bifacial gain, respectively.

One of the prerequisites for a reliable energy yield prediction of bifacial photovoltaic (PV) systems is the capability of modeling the backside irradiance of those systems with high accuracy. Currently, the most important optical models used to quantify the reflected irradiance on the backside of a bifacial solar panel are view factor and ray tracing. The MoBiDiG simulation tool has been developed at ISC Konstanz uses the view factor (VF) concept to model the rear irradiance. In addition to the VF concept, ray tracing (RT) has been adopted to determine the backside irradiance of bifacial modules by using the open‐source tool bifacial_radiance that has been developed by the National Renewable Energy Laboratory (NREL). A customized monocrystalline silicon solar panel has been built in order to evaluate the accuracy of the existing optical models by locally resolved rear irradiance measurement. The performance of rear irradiance has been investigated along the rows of the customized PV module during sunny and cloudy days with typical back irradiance values of 50 and . The comparison of measured and modeled data has been carried out on hourly, daily, and monthly basis, and the results show lower deviations for solar cells located in the center of the PV module than on the edge. Moreover, the concept of decisive solar cells has been introduced and applied to both measured and modeled data, solar cells located in the center rows were found to act as the most decisive solar cells. Finally, considering the installation configuration studied here, ie, bifacial mounting with low clearance height (below 0.2 m), both hourly RT and VF approaches are able to model long‐term cumulative irradiance received by decisive solar cells with a very high accuracy ranging from 0.5% to 2%.

We present the characterisation of directionally solidified n-type Si ingots. Three ingots with a range of bulk resistivities and different n-type doping elements (Sb, P and As) were studied. We show from Hall measurements that the mc-Si material has excellent electrical transport properties. The mobilities are close to the theoretical limit, which is given mainly by scattering at acoustical phonons. Mobilities so close to the theoretical value have, to our knowledge, not been demonstrated for comparable p-type mc-Si wafers. Additional measurements on high quality p-type mc-Si material support this statement. This means that other scattering mechanisms reduce the mobility in p-type mc-Si material, but are not present in n-type silicon. Lifetime measurements were conducted by /spl mu/W-PCD using an iodine-ethanol surface passivation. This passivation was used preferably to SiN/sub x/, as in some experiments the hydrogen from the PECVD SiN seemed to passivate the bulk at deposition temperatures. Average values in excess of 120 /spl mu/s over large areas were measured. In order to exploit the good material properties of n-type mc-Si, solar cell concepts must be developed and the processes optimised. B-diffusion is the most problematic step as it is considered to be both destructive to material quality and energy consuming. In this paper, we show that a BBr/sub 3/-emitter diffusion is possible at moderate temperatures without degrading the carrier lifetime of the mc-Si material. An additional contribution from Libal et al. on solar cell processing is included in this conference.