John F. Roller

Versatile light-emitting-diode-based spectral response measurement system for photovoltaic device characterization.

An absolute differential spectral response measurement system for solar cells is presented. The system couples an array of light emitting diodes with an optical waveguide to provide large area illumination. Two unique yet complementary measurement methods were developed and tested with the same measurement apparatus. Good agreement was observed between the two methods based on testing of a variety of solar cells. The first method is a lock-in technique that can be performed over a broad pulse frequency range. The second method is based on synchronous multifrequency optical excitation and electrical detection. An innovative scheme for providing light bias during each measurement method is discussed.

Fast and reliable spectral response measurements of PV cells using light emitting diodes

We present a measurement system for absolute differential spectral responsivity of solar cells based on high-powered LED arrays coupled to an optical light guide capable of large area illumination. Two different measurement techniques were developed and tested with the same measurement apparatus on a variety of solar cells. The first method is an individual LED lock-in technique that can be performed over a broad frequency range. The second method is based on synchronous multi-frequency optical excitation, called the Fourier transform (FT) technique, using the LEDs and detection with a spectrum analyzer. A scheme for providing light bias using the LEDs during either measurement scheme is discussed.

Absolute spectral responsivity measurements of solar cells by a hybrid optical technique

An irradiance mode, absolute differential spectral response measurement system for solar cells is presented. The system is based on combining the monochromator-based approach of determining the power mode spectral responsivity of cells with an LED-based measurement to construct a curve representing the light-overfilled absolute spectral response of the entire cell. This curve can be used to predict the short-circuit current (I(sc)) of the cell under the AM 1.5 standard reference spectrum. The measurement system is SI-traceable via detectors with primary calibrations linked to the NIST absolute cryogenic radiometer. An uncertainty analysis of the methodology places the relative uncertainty of the calculated I(sc) at better than ±0.8%.

Large-area irradiance-mode spectral response measurements of solar cells by a light-emitting, diode-based integrating sphere source

An irradiance-mode absolute differential spectral response (SR) measurement system based on a light emitting diode (LED) array is described. The LEDs are coupled to an integrating sphere whose output irradiance is uniform to better than 2% over an area of 160 mm by 160 mm. SR measurements of solar cells when subject to diffuse irradiation, as provided by the integrating sphere, are compared with collimated irradiance SR measurements. Issues originating from the differences in angular response of the reference versus the test cells are also investigated. The SR curves of large-area cells with dimensions of up to 155 mm are measured and then used to calculate the cells short circuit current (I(sc)), if illuminated by a defined solar spectrum. The resulting values of I(sc) agree well with the values obtained from secondary measurements.

Non-linearity measurements of solar cells with an LED-based combinatorial flux addition method

We present a light emitting diode (LED)-based system utilizing a combinatorial flux addition method to investigate the nonlinear relationship in solar cells between the output current of the cell and the incident irradiance level. The magnitude of the light flux is controlled by the supplied currents to two LEDs (or two sets of them) in a combinatorial fashion. The signals measured from the cell are arranged within a related overdetermined linear system of equations derived from an appropriately chosen Nth degree polynomial representing the relationship between the measured signals and the incident fluxes. The flux values and the polynomial coefficients are then solved for by linear least squares to obtain the best fit. The technique can be applied to any solar cell, under either monochromatic or broadband spectrum. For the unscaled solution, no reference detectors or prior calibrations of the light flux are required. However, if at least one calibrated irradiance value is known, then the entire curve can be scaled to an appropriate spectral responsivity value. Using this technique, a large number of data points can be obtained in a relatively short time scale over a large signal range.

Spectral dependence of carrier lifetimes in silicon for photovoltaic applications

Charge carrier lifetimes in photovoltaic-grade silicon wafers were measured by a spectral-dependent, quasi-steady-state photoconductance technique. Narrow bandwidth light emitting diodes (LEDs) were used to excite excess charge carriers within the material, and the effective lifetimes of these carriers were measured as a function of wavelength and intensity. The dependence of the effective lifetime on the excitation wavelength was then analyzed within the context of an analytical model relating effective lifetime to the bulk lifetime and surface recombination velocity of the material. The agreement between the model and the experimental data provides validation for this technique to be used at various stages of the solar cell production line to investigate the quality of the passivation layers and the bulk properties of the material.

Challenges of irradiance-mode spectral response measurements

Irradiance-mode spectral response measurements of solar cells are important because they provide a direct, reliable and accurate route for determination of the short circuit current (Isc) of solar cells under air mass 1.5 standard reference spectrum. In this work, a monochromator-based approach was combined with a light emitting diode (LED) array-operated system based on an integrating sphere design to obtain the irradiance-mode SR curves for a variety of cells, including large-area cells with dimensions up to 155 mm. Challenges associated with these measurements, including light uniformity, collimation issues and light bias effects are discussed. Isc calculations using this approach and intercomparison with other laboratories confirm the accuracy of the outlined methodology.