Development of a solar cell spectral response mapping system using multi-LBIC excitation

This work presents a new multi-laser LBIC measurement system that is currently under development at CREST. The final set-up uses 11 lasers, 6 of which are currently operational, to form a spatially resolved spectral response map of the device under test. The design aspects of the measurement system are detailed and first measurements of a crystalline and amorphous silicon solar cell are demonstrated. Measurements show how a crack in a crystalline silicon solar cells affects the local quantum efficiency and the effects of discoloration in amorphous silicon. Thus, highlighting the advantages in multi-wavelength and absorption depth profiling of device performance and defects

Compressive current response mapping of photovoltaic devices using MEMS mirror arrays

Understanding the performance and aging mechanisms in photovoltaic devices requires a spatial assessment of the device properties. The current dominant technique, electroluminescence, has the disadvantage that it assesses radiative recombination only. A complementary method, laser beam-induced current (LBIC), is too slow for high-throughput measurements. This paper presents the description, design, and proof of concept of a new measurement method to significantly accelerate LBIC measurements. The method allows mapping of the current response map of solar cells and modules at drastically reduced acquisition times. This acceleration is achieved by projecting a number of mathematically derived patterns on the sample by using a digital micromirror device (DMD). The spatially resolved signal is then recovered using compressed sensing techniques. The system has fewer moving parts and is demonstrated to require fewer overall measurements. Compared with conventional LBIC imaging using galvanic mirror arrangements or xy scanners, the use of a DMD allows a significantly faster and more repeatable illumination of the device under test. In this proof-of-concept instrument, sampling patterns are drawn from Walsh–Hadamard matrices, which are one of the many operators that can be used to realize this technique. This has the advantage of the signal-to-noise ratio of the measurement being significantly increased and thus allows elimination of the standard lock-in techniques for signal detection, reducing measurement costs, and increasing measurement speed further. This new method has the potential to substantially decrease the time taken for measurement, which demonstrates a dramatic improvement in the utility of LBIC instrumentation

Compressed sensing current mapping of PV devices using a DLP projector

A commercial Digital Light Processing (DLP) projector has been utilised for compressed sensing current mapping of photovoltaic (PV) devices. Through the projector, the necessary patterns are projected to apply compressive sampling for measurement acquisition. The reconstruction of the current map is achieved by an optimisation algorithm. The main advantage of this method is that measurement time is significantly reduced, compared to conventional LBIC measurement systems. This is achieved mainly by acquiring fewer measurements than a raster scan would need. Initial current maps of cells and modules have been acquired, showing the feasibility of the method. The issues of such a system have been investigated and its potential for fast and simple current mapping of PV modules is demonstrated

Evaluation of effective carrier lifetime of CdTe solar cells using transient photovoltage decay measurements

A transient photovoltage decay (TPVD) measurement system is currently being developed at CREST and measurements were conducted on several CdTe solar cells. The extracted effective carrier lifetimes were around 100ns. The effect of external illumination biasing was investigated and was found to reduce the effect of junction capacitance and saturate trap states in the devices. This resulted in shorter extracted effective carrier lifetimes. Increasing the illumination of the pulsed-laser intensity also increased the effective carrier lifetime

Towards current mapping of photovoltaic devices by compressed imaging

A new photovoltaic (PV) device current mapping method has been developed, utilizing the recently introduced compressed sensing sampling theory. The aim is to significantly reduce measurement time of Light Beam Induced Current measurements. A prototype setup has been built at National Physics Laboratory (NPL) to implement the method. Initial results are presented and illustrate the feasibility of the method

Compressed sensing current mapping methods for PV characterisation

The Compressed Sensing (CS) sampling theory has been combined with the Light Beam Induced Current (LBIC) method, to produce an alternative current mapping technique for photovoltaic (PV) devices. Compressive sampling of photocurrent is experimentally implemented using a Digital Micro-mirror Device (DMD). The main advantage of this new method for current mapping is that measurement time can be significantly reduced compared to conventional LBIC measurement systems. This is achieved mainly by acquiring fewer measurements than a raster scan would need and by utilizing the fast response of the micro-mirror array. Two different experimental layouts are considered in this work. The first is a small area optical set-up based on a single wavelength laser source. The second layout utilizes a commercial Digital Light Processing (DLP) projector through which compressive sampling is applied. Experimental results with both experimental schemes demonstrate that current maps can be produced with less than 50% of the measurements a standard LBIC system would need. The ability to acquire current maps of individual cells in encapsulated modules is also highlighted. The advantages and drawbacks of the method are presented and its potential to significantly reduce measurement time of current mapping of PV cells and modules is indicated

Fast current mapping of photovoltaic devices using compressive sampling

Light Beam Induced Current (LBIC) measurements are a useful tool in photovoltaic (PV) device characterisation for accessing the local electrical properties of PV devices. The main disadvantage of a typical LBIC system is measurement time, as a raster scan of a typical silicon solar cell can last several hours. The focus of this paper is the reduction of LBIC measurement time by means of compressed sensing (CS). The CS-LBIC system described in this paper can theoretically reduce measurement time to less than 25% of that required for a standard LBIC raster scan. Measurement simulations of a CS-LBIC system are presented as well as a practical demonstration using a digital micro-mirror array, which further reduces the measurement time by an order of magnitude. Instead of a raster scan, the PV device under measurement is sampled by a series of patterns and the current map is reconstructed using an optimization algorithm. Simulations of CS-LBIC measurements using the 2D spatially-resolved PV-Oriented Nodal Analysis (PVONA) model developed at CREST are used as a tool to explore the capabilities and verify the accuracy of this measurement technique as well as its ability to detect specific defects, such as cracks and shunts. Simulation results confirm that the CS sampling theory can be applied as an effective method for significantly reducing measurement time of current mapping of PV devices. An initial CS-LBIC system prototype has been built at the National Physical Laboratory (NPL) and measurements of small area devices (1cm x 0.8cm) using this system are given. The current maps are created using a Digital Micromirror Device (DMD) kit as a pattern generator. The response time of the micro mirror array is less than 20μs. This is another factor in the reduction of measurement time, as the movement time of an x-y translation stage is considerably slower. Initial measurement results show that current maps of PV cells can be acquired with 75% fewer measurements which, combined with the fast response of the pattern generator, can reduce LBIC measurement time by an order of magnitude

Optical technique for photovoltaic spatial current response measurements using compressive sensing and random binary projections

Compressive sensing has been widely used in image compression and signal recovery techniques in recent years; however, it has received limited attention in the field of optical measurement. This paper describes the use of compressive sensing for measurements of photovoltaic (PV) solar cells, using fully random sensing matrices, rather than mapping an orthogonal basis set directly. Existing compressive sensing systems optically image the surface of the object under test, this contrasts with the method described, where illumination patterns defined by precalculated sensing matrices, probe PV devices. We discuss the use of spatially modulated light fields to probe a PV sample to produce a photocurrent map of the optical response. This allows for faster measurements than would be possible using traditional translational laser beam induced current techniques. Results produced to a 90% correlation to raster scanned measurements, which can be achieved with under 25% of the conventionally required number of data points. In addition, both crack and spot type defects are detected at resolutions comparable to electroluminescence techniques, with 50% of the number of measurements required for a conventional scan

Compressed sensing current mapping spatial characterization of photovoltaic devices

A new photovoltaic (PV) device current mapping method has been developed, combining the recently introduced Compressed Sensing (CS) sampling theory with Light Beam Induced Current (LBIC) measurements. Instead of a raster scan, compressive sampling is applied using a Digital Micro-mirror Device (DMD). The aim is to significantly reduce the time required to produce a current map, compared to conventional LBIC measurements. This is achieved by acquiring fewer measurements than a full raster scan and by utilizing the fast response of the micro-mirror device to modulate measurement conditions. The method has been implemented on an optical current mapping setup built at the National Physical Laboratory (NPL) in the UK. Measurements with two different PV cells are presented in this work and an analytical description for realisation of an optimised CS current mapping system is provided. The experimental results illustrate the feasibility of the method and its potential to significantly reduce measurement time of current mapping of PV devices

Compressed sensing current mapping methods for PV characterisation

The Compressed Sensing (CS) sampling theory has been combined with the Light Beam Induced Current (LBIC) method, to produce an alternative current mapping technique for photovoltaic (PV) devices. Compressive sampling of photocurrent is experimentally implemented using a Digital Micro-mirror Device (DMD). The main advantage of this new method for current mapping is that measurement time can be significantly reduced compared to conventional LBIC measurement systems. This is achieved mainly by acquiring fewer measurements than a raster scan would need and by utilizing the fast response of the micro-mirror array. Two different experimental layouts are considered in this work. The first is a small area optical set-up based on a single wavelength laser source. The second layout utilizes a commercial Digital Light Processing (DLP) projector through which compressive sampling is applied. Experimental results with both experimental schemes demonstrate that current maps can be produced with less than 50% of the measurements a standard LBIC system would need. The ability to acquire current maps of individual cells in encapsulated modules is also highlighted. The advantages and drawbacks of the method are presented and its potential to significantly reduce measurement time of current mapping of PV cells and modules is indicated