Karsten Bothe - Profile on Academia.edu (original) (raw)

Papers by Karsten Bothe

The IEC 61853 standard series aims to provide a standardized measure for photovoltaic (PV) module... more The IEC 61853 standard series aims to provide a standardized measure for photovoltaic (PV) module energy rating, namely the Climate Specific Energy Rating (CSER). For this purpose, it defines procedures for the experimental determination of input data and algorithms for calculating the CSER. However, some steps leave room for interpretation regarding the specific implementation. To analyze the impact of these ambiguities, the comparability of results, and the clarity of the algorithm for calculating the CSER in Part 3 of the standard, an intercomparison is performed among research organizations with ten different implementations of the algorithm. We share the same input data, obtained by measurement of a commercial crystalline silicon PV module, among the participating organizations. Each participant then uses their individual implementations of the algorithm to calculate the resulting CSER values. The initial blind comparison reveals differences of 0.133 (14.7%) in CSER. After several comparison phases, a best practice approach is defined, which reduces the difference by a factor of 210 to below 0.001 (0.1%) in CSER for two independent PV modules. The best practice presented in this article establishes clear guidelines for the numerical treatment of the spectral correction and power matrix extrapolation, where the methods in the standard are not clearly defined. Additionally, we provide input data and results for the PV community to test their implementations of the standard's algorithm. To identify the source of the deviations, we introduce a climate data diagnostic set. Based on our experiences, we give recommendations for the future development of the standard.

37th European Photovoltaic Solar Energy Conference and Exhibition, Oct 27, 2020

In recent years, solar cell development has undergone a major change in metallization layout. In ... more In recent years, solar cell development has undergone a major change in metallization layout. In their most radical form, busbarless solar cells completely omit the busbar and leave the fingers as solely contacting area. Consequently, characterization and calibration laboratories were forced to develop new contacting units. At the same time, the question of the correct arrangement of current and sense contacts arises. To perform accurate and precise measurements of the current-voltage characteristic of busbarless solar cells, we transfer the well-established concept of busbar-resistance neglecting contacting to the measurement of busbarless cells. The result is a universally valid gridresistance neglecting contacting scheme, which provides the same fill factor as one would get if one had contacted the entire metallized area of the solar cell. We demonstrate that a variety of contacting schemes are able to determine this fill factor if the sensing contact is correctly placed. We provide experimental evidence of consistent results for a contacting with 12 contact bars at ISFH CalTeC and 30 wires at Fraunhofer ISE CalLab, respectively. For the fill factor of 15 silicon solar cells with finger line resistances ranging from 0.6 to 12 /cm we show that the En-values between both calibration laboratories are well below 1, demonstrating a very good agreement within the accompanied measurement uncertainty.

Progress in Photovoltaics, Jun 6, 2022

Consolidated tables showing an extensive listing of the highest independently confirmed efficienc... more Consolidated tables showing an extensive listing of the highest independently confirmed efficiencies for solar cells and modules are presented. Guidelines for inclusion of results into these tables are outlined, and new entries since January 2022 are reviewed. An appendix describing temporary electrical contacting of large‐area solar cells approaches and terminology is also included.

A conflict between previous and recently published data on the two-stage lightinduced degradation... more A conflict between previous and recently published data on the two-stage lightinduced degradation (LID) of carrier lifetime in boron-doped oxygen-containing crystalline silicon is addressed. The previous experiments showed the activation of two boron-oxygen recombination centers with strongly differing recombination properties for the fast and slow stages of LID, whereas more recent studies found only a single center for both stages. To resolve this controversy, the historic silicon samples of these previous examinations are reexamined in this study after more than one decade. It is found that, in the historic samples, the fast stage can be either described by two different centers or a mixture of the two, depending on the duration of previous dark annealing. A possible solution is suggested based on the involvement of different activating impurities in the boron-oxygen defect. In dark-annealed samples, the defect consisting of boron, oxygen, and the activation impurity is present in two latent configurations, which reconfigure during LID at a fast and a slow stage. In the examined historic silicon samples, which did not undergo a gettering pretreatment, a significant concentration of an additional boron-oxygen defect with a different kind of activating impurity attached exists. The historic and modern results are thus reconciled.

World Conference on Photovoltaic Energy Conversion, Nov 18, 2009

We present a calibration-free dynamic carrier lifetime imaging approach, based on the infrared li... more We present a calibration-free dynamic carrier lifetime imaging approach, based on the infrared lifetime mapping (ILM) technique. Introducing the dynamic evaluation procedure we show that the lifetime is determined analytically from the signal ratio of infrared camera images recorded directly after turning on an excitation source and after steady-state conditions are established within the sample. A comparison with microwave-detected photo conductance decay measurements shows an excellent quantitative agreement. We show that signal blurring is a fundamental issue of the dynamic approach and introduce a method to overcome this limitation. This method combines dynamic and static-ILM measurements and allows for high resolution lifetime measurements in measurement times as fast as 1 sec per wafer.

Optical Modeling of Photovoltaic Modules with Ray Tracing Simulations

John Wiley & Sons, Inc. eBooks, Aug 3, 2018

The IEC 61853 standard series "Photovoltaic (PV) module performance testing and energy ratin... more The IEC 61853 standard series "Photovoltaic (PV) module performance testing and energy rating" aims to provide a standardized measure for PV module performance, namely the Climate Specific Energy Rating (CSER). An algorithm to calculate CSER is specified in part 3 based on laboratory measurements defined in parts 1 and 2 as well as the climate data set given in part 4. To test the comparability and clarity of the algorithm in part 3, we share the same input data, obtained by measuring a standard photovoltaic module, among different research organizations. Each participant then uses their individual implementations of the algorithm to calculate the resulting CSER values. The initial blind comparison reveals differences of 0.133 (14.7%) in CSER between the ten different implementations of the algorithm. Despite the differences in CSER, an analysis of intermediate results revealed differences of less than 1% at each step of the calculation chain among at least three participants. Thereby, we identify the extrapolation of the power table, the handling of the differences in the wavelength bands between measurement and climate data set, and several coding errors as the three biggest sources for the differences. After discussing the results and comparing different approaches, all participants rework their implementations individually and compare the results two more times. In the third intercomparison, the differences are less than 0.029 (3.2%) in CSER. When excluding the remaining three outliers, the largest absolute difference between the other seven participants is 0.0037 (0.38%). Based on our findings we identified four recommendations for improvement of the standard series.

IEEE Journal of Photovoltaics, Mar 1, 2018

The determination of the spectral responsivity is an essential part of solar cell characterizatio... more The determination of the spectral responsivity is an essential part of solar cell characterization. Since solar simulators only approximate the reference spectrum, a spectral mismatch correction has to be performed. This correction procedure requires spectral responsivity data. Apart from the complete differential spectral responsivity procedure, the IEC 60904-8 standard defines four simplifications. In this paper, we provide information on the variations in the spectral responsivity curves for these simplifications. We show that for nonlinear front junction cells, deviations predominantly occur at wavelengths above 700 nm and become largest around 1000 nm. While we found a maximum deviation of 30% for the simplification with lowest requirements in bias irradiance, all other simplifications yield deviations below 10%. For a nonlinear cell measured relative to a world photovoltaic scale reference cell using a class A solar simulator, this transfers to a deviation below 0.01% in the spectral mismatch factor. If one depends on the use of a simplification, we recommend using the multicolor approach. Even though the singlecolor approach might yield lower deviations, this approach requires knowledge about the maximum in the spectral responsivity, which is not generally known in advance of the measurement. Accepting a slightly higher deviation, the white bias approach is a recommendable alternative.

Physica Status Solidi (rrl), Oct 28, 2010

Journal of Applied Physics, Dec 7, 2015

Supersaturated levels of interstitial oxygen in Czochralski silicon can lead to the formation of ... more Supersaturated levels of interstitial oxygen in Czochralski silicon can lead to the formation of oxide precipitates. Although beneficial from an internal gettering perspective, oxygen-related extended defects give rise to recombination which reduces minority carrier lifetime. The highest efficiency silicon solar cells are made from n-type substrates in which oxide precipitates can have a detrimental impact on cell efficiency. In order to quantify and to understand the mechanism of recombination in such materials, we correlate injection level-dependent minority carrier lifetime data measured with silicon nitride surface passivation with interstitial oxygen loss and precipitate concentration measurements in samples processed under substantially different conditions. We account for surface recombination, doping level, and precipitate morphology to present a generalised parameterisation of lifetime. The lifetime data are analysed in terms of recombination activity which is dependent on precipitate density or on the surface area of different morphologies of precipitates. Correlation of the lifetime data with interstitial oxygen loss data shows that the recombination activity is likely to be dependent on the precipitate surface area. We generalise our findings to estimate the impact of oxide precipitates with a given surface area on lifetime in both n-type and p-type silicon. V

Progress in Photovoltaics, Nov 21, 2022

Consolidated tables showing an extensive listing of the highest independently confirmed efficienc... more Consolidated tables showing an extensive listing of the highest independently confirmed efficiencies for solar cells and modules are presented. Guidelines for inclusion of results into these tables are outlined, and new entries since July 2022 are reviewed. Graphs showing progress with each cell technology over the 30‐year history of the tables are also included plus an updated list of designated test centres.

Solar RRL, May 18, 2023

Herein, a method for the determination of the spectral responsivity (SR) and the short‐circuit cu... more Herein, a method for the determination of the spectral responsivity (SR) and the short‐circuit current under standard test conditions of photovoltaic devices (e.g., solar cells) is presented. This multi‐spectrum SR method requires a spectrally tunable broadband light source irradiating the photovoltaic device with a large number of different spectra. For each spectrum, the light response of the device and the spectral irradiance in the measuring plane are measured. The spectral irradiances are integrated within predefined wavelength intervals and are incorporated together with the measured light response into an equation system which relates them to the (unknown) SR of the photovoltaic device. By solving the equation system, mathematically using regression algorithms, the SR is determined. Due to the usage of a broadband light source, the device operates at realistic injection conditions during measurements. The mathematical background of the multi‐spectrum SR method is described and its applicability is demonstrated on three world‐photovoltaic‐scale‐type solar cells and one large‐area reference cell. Short‐circuit currents from all SR curves are calculated using the tabulated AM1.5 G spectrum. In comparison to the SR reference data, the short‐circuit currents from the multi‐spectrum SR method deviate by less than 0.68%.

Solar RRL, Sep 23, 2022

Interdigitated back contact (IBC) silicon solar cells with a passivating n‐type poly‐Si on oxide ... more Interdigitated back contact (IBC) silicon solar cells with a passivating n‐type poly‐Si on oxide emitter and an aluminum‐doped p+ base contact on M2‐sized Ga‐doped p‐type Cz wafers are reported. The Al‐doped base contact forms during the firing of the printed contacts and allows for a lean process flow. The device optimization balances recombination at the base contacts against resistive losses and respects constraints set by the need of interconnecting cells in a module and contacting the cells temporally by a measurement chuck. A special sample holder is designed for measuring the Isc–Voc curve of the IBC cell with a busbar‐less metal grid. The pseudo‐efficiency is 24.7%. All fingers of each polarity are connected with wires and an efficiency of 22.3% is measured. The comparison of simulations and measurements reveals that the cell has 23.4% efficiency without the series resistance losses due to the wires. A huge part of the resistive losses in the cell are the transport losses of the majorities in the base dissipating a power that corresponds to 0.76%abs efficiency and the resistive losses at the Al‐doped base contact (0.29%abs).

Nucleation and Atmospheric Aerosols, 2022

In this contribution we present a novel method to determine the base doping concentration of sola... more In this contribution we present a novel method to determine the base doping concentration of solar cells from current-voltage (IV) curves measured under illumination. Our method is based on counting the charge carriers which are stored during an IV-sweep inside the Si bulk of the solar cell. We derive this charge from a comparison of a single sweep IV-curve and a steady-state IV-curve. Besides a detailed explanation of our new method, we analyze its applicability and repeatability and present an independent confirmation based on state-of-the-art capacitance-voltage (CV) characterization. We perform this confirmation of our results on a number of cells with different cell architectures ranging from p-type PERC solar cells over n-PERT cells to TOPCon and HJT structures. The base doping concentration determined by our method match the results of the reference CV measurements adequately with an approximate measurement uncertainty of 20%. In contrast to time consuming laboratory methods that require offline sample characterization, our evaluation is performed without the need of additional measurement equipment or measurement time and is ready for inline usage in solar cell mass production.

Energy Procedia, Sep 1, 2017

District heating networks are commonly addressed in the literature as one of the most effective s... more District heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat sales. Due to the changed climate conditions and building renovation policies, heat demand in the future could decrease, prolonging the investment return period. The main scope of this paper is to assess the feasibility of using the heat demand-outdoor temperature function for heat demand forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were compared with results from a dynamic heat demand model, previously developed and validated by the authors. The results showed that when only weather change is considered, the margin of error could be acceptable for some applications (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations.

EPJ Photovoltaics, 2023

I-V measurements are sensitive to the number and positioning of current and voltage sensing conta... more I-V measurements are sensitive to the number and positioning of current and voltage sensing contacts. For busbarless solar cells, measurement setups have been developed using current collection wires and separate voltage sense contacts. Placing the latter at a defined position enables a grid resistance neglecting measurement and thus I-V characteristics independent from the contacting system. This technique has been developed for solar cells having a finger grid and good conductivity in the direction of the fingers. The optimal position of the sense contact in case of finger-free silicon heterojunction solar cells has not yet been studied. Here, the lateral charge carrier transport occurs in a transparent conductive oxide layer resulting in a higher lateral resistance. We perform finite difference method simulations of HJT solar cells without front metallization to investigate the impact of high lateral resistances on the I-V measurement of solar cells. We show the high sensitivity on the number of used wires for contacting as well as the position of the sense contact for the voltage measurement. Using the simulations, we are able to explain the high difference of up to 7.5% in fill factor measurements of metal free solar cells with varying TCO sheet resistances between two measurement systems using different contacting setups. We propose a method to compensate for the contacting system to achieve a grid-resistance neglecting measurement with both systems allowing a reduction of the FF difference to below 1.5%.

Energy Procedia, Sep 1, 2017

Energy Procedia, 2014

We investigate the impact of an injection-dependent carrier lifetime in crystalline silicon on dy... more We investigate the impact of an injection-dependent carrier lifetime in crystalline silicon on dynamic photoluminescence lifetime imaging (dynamic PLI). Although the dynamic lifetime approach is a technique that evaluates the time-dependence of a quantity proportional to the excess carrier density, it is only weakly influenced by the injection-level dependence of the lifetime. The reason for the little impact is the fact that the evaluation of dynamic PLI measurements does not only involve the decay of the carrier density, as it is common for photoconductance decay measurements, but also the increase of the carrier density directly after switching on the excitation source. In this contribution, we present injection-dependent lifetime measurements that are acquired with the camera-based dynamic PLI technique. We find that the deviation of the actual steady-state carrier lifetime from the lifetime obtained with dynamic PLI is less than 20 % for a wide range of measurement conditions.

IEEE Journal of Photovoltaics, Oct 1, 2012

We present a camera-based technique for the local determination of reverse saturation current den... more We present a camera-based technique for the local determination of reverse saturation current densities J 0 of highly doped regions in silicon wafers utilizing photoconductance calibrated photoluminescence imaging (PC-PLI). We apply this approach to 12.5 Â 12.5 cm 2 float zone silicon samples with textured surfaces and a homogeneous phosphorous diffusion with sheet resistances between 24 and 230 O/&. We find enhanced photoluminescence emission at metallized regions of a sample due to reflection of long-wavelength light at the rear side of the sample. Our measurement setup comprises an optical short pass filter in front of the camera effectively blocking wavelengths above 970 nm and therefore ensuring a correct calibration of the PL signal in terms of excess charge carrier density Dn. We analyze two sets of samples comprising metal contacts to highly doped regions prepared by Laser Transfer Doping (LTD) as well as standard tube furnace phosphorus diffusion. We find a considerably smaller J 0 value of 370 fA/cm 2 for the LTD approach compared to a standard diffusion process resulting in J 0 ¼ 570 fA/cm 2. On the basis of these results we demonstrate that J 0 imaging is a powerful analysis technique for process optimization.

The determination of the spectral responsivity is an essential part of solar cell calibration. Ap... more The determination of the spectral responsivity is an essential part of solar cell calibration. Apart from the complete differential spectral responsivity procedure, which yields the most accurate results, the IEC 60904-8 defines four simplifications. We provide detailed information on the expected variations in the calculated spectral responsivity curves for the different simplifications compared to the complete procedure. For non-linear crystalline silicon front junction solar cells, we show that deviations mainly occur at wavelengths above 700 nm and become largest between 1000 to 1200 nm. Even though we found a maximum deviation in spectral responsivity of 7% for the simplification with lowest requirements in bias irradiance, all other simplifications yield deviations well below 3%. This transforms into a deviation of 0.01% in the spectral mismatch factor for an industrial PERC solar cell when using a typical world photovoltaic scale (WPVS) reference solar cell and a class A two-lamp solar simulator. If you are reliant on the use of a simplification, we recommend using the multicolour approach. Even though the singlecolour approach might yield lower deviations in specific cases, it requires knowledge about the maximum in the spectral responsivity, which is not generally known in advance of the measurement. Accepting a slightly higher deviation, the white bias approach is a recommendable alternative.

37th European Photovoltaic Solar Energy Conference and Exhibition, Oct 27, 2020

Progress in Photovoltaics, Jun 6, 2022

World Conference on Photovoltaic Energy Conversion, Nov 18, 2009

Optical Modeling of Photovoltaic Modules with Ray Tracing Simulations

John Wiley & Sons, Inc. eBooks, Aug 3, 2018

IEEE Journal of Photovoltaics, Mar 1, 2018

Physica Status Solidi (rrl), Oct 28, 2010

Journal of Applied Physics, Dec 7, 2015

Progress in Photovoltaics, Nov 21, 2022

Consolidated tables showing an extensive listing of the highest independently confirmed efficienc... more Consolidated tables showing an extensive listing of the highest independently confirmed efficiencies for solar cells and modules are presented. Guidelines for inclusion of results into these tables are outlined, and new entries since July 2022 are reviewed. Graphs showing progress with each cell technology over the 30‐year history of the tables are also included plus an updated list of designated test centres.

Solar RRL, May 18, 2023

Solar RRL, Sep 23, 2022

Nucleation and Atmospheric Aerosols, 2022

Energy Procedia, Sep 1, 2017

EPJ Photovoltaics, 2023

Energy Procedia, Sep 1, 2017

Energy Procedia, 2014

IEEE Journal of Photovoltaics, Oct 1, 2012