Symposium ES20 : Thin-Film Chalcogenide Semiconductor Photovoltaics

Thin-film Ag-CIGS (or ACIGS) solar cells and modules produced by MiaSolé Hi-Tech yield high efficiencies rivaling polycrystalline silicon-based modules but are based on a high-throughput, PVD-based deposition process on flexible stainless steel substrates. The process is unique among thin-film solar manufacturers in that a complete device stack is produced without a vacuum break in less than one hour. Using this manufacturing system, a high level of process control and stability is possible via real-time control of the deposition process.

To continue performance improvements that close the gap between manufacturing material and champion small devices, a better understanding of the fundamental device properties is required. This requires development of a device model that accounts for electrical performance characteristics observed under light and voltage stress over time for comparison to real-world solar cell performance.

Our recent effort to generate a fundamental device model began with employing a wide variety of characterization results from our own measurements, those of our research partners, and features of many CIGS materials that have previously been excluded in prior device models. Additionally, device parameters must be based on actual device measurements and constituents, as heterojunction ACIGS solar cell devices are complex and errors compound quickly. The overall objective was to keep the model as simple as possible and look for generally good agreement among a variety of simulated measurements.

Most of the effort has been devoted to understanding the defect structure of the ACIGS absorber layer. Conventional capacitance-voltage profiling is supplanted in our measurements by high-speed C-V profiling (HSCV) that avoids many complications of transient charge effects in highly defective materials such as ACIGS. An important consequence of these measurements is the discovery that MiaSolé absorber layers (as well as other CIGS devices) can be defined by an acceptor-rich layer (ARL) adjacent to the CdS buffer layer. This ARL has been shown to have profound consequences for device electrical performance, as it can degrade fill factor and reduce efficiency.

The physical basis for the ARL is a matter of some debate. A variation in static shallow acceptor concentration is one possible source, but it predicts neither device metastability nor measured high concentrations of DLTS-visible defects. Charge accumulation at high concentrations of deep acceptors would be possible, but accounts for neither metastability nor carrier capture kinetics observed in DLTS. Instead we hypothesize that V(Se)-V(Cu) divacancy defects are responsible for charge accumulation in the ARL, and their simulated response to light- and voltage-stressing is consistent with our measurements. This hypothesis is qualitatively consistent with positron annihilation spectroscopy (PAS) measurements of our absorber. Predicted shallow acceptor and donor levels of the divacancy also provide an explanation for the long-observed high dopant compensation in CIGS, and at the concentrations predicted by our model, it is possible that the divacancies alone can be used to explain most of the charge and doping profile of the absorber.

Emphasis of the divacancy defect in our device model can guide MiaSolé Hi-Tech to new directions for device improvement that leverage our ability to directly manipulate absorber stoichiometry in our unique thin-film solar manufacturing system.

In this work, we present a suite of characterization tools developed to accurately and systematically analyze losses in non-Si photovoltaic solar cell technologies, in particular thin-film chalcogenide devices. This characterization platform will enable easy access to the implied open-circuit voltage (iV_oc) and fill factor (iFF) of devices, to their pseudo fill factor (pFF) and thus series resistance (R_s) at maximum power point, and to their complete-stack contact resistance (R_c). Together with calculated detailed-balance performance limits and traditional J-V measurements, these metrics will enable a precise accounting of recombination and resistive losses, from fundamental efficiency limits to experimental device performance. The characterization platform includes systematic determination of the absorber optical parameters (n & k), optical modeling of the device, calculation of the iV_oc from measurement of the external radiative efficiency (ERE) of the absorber, J_sc-V_oc measurements, and measurement on the contact stack resistance using through-the-absorber transfer line method (TLM).
Cd(Se)Te solar cells, our test system, provide a prime example of the potential impact of the techniques we propose to develop and implement: although they exhibit strong photoluminescence response, record poly-Cd(Se)Te cells have bandgap-voltage deficits (W_oc) of approximately 550 mV, as compared with below 400 mV for all other mature PV technologies. Similarly, these record Cd(Se)Te devices have FFs below 80%, when other mature cells are near or above 85%. Frustratingly, a systematic identification of the origin of these sub-par performances – for example recombination or resistive losses – has been lacking, thus slowing down the development of these technologies. Similarly, it is often asserted that Cd(Se)Te cells need a better back (hole) contact. Although we believe this is true, it is currently unknown how high the V_oc and FF could be for a given cell if it had a perfect back contact.
Using the proposed set of characterization techniques, users will be able to identify the sources of inefficiencies in thin-film chalcogenide solar cells, from bulk material quality and surface passivation to contact selectivity and resistivity. Rapid absorber and contact improvement will thus be possible.