Jonathan Mann

Kesterite Successes, Ongoing Work, and Challenges: A Perspective From Vacuum Deposition

Recent years have seen dramatic improvements in the performance of kesterite devices. The existence of devices of comparable performance, made by a number of different techniques, provides some new perspective on what characteristics are likely fundamental to the material. Here, we review progress in kesterite device fabrication, aspects of the film characteristics that have yet to be understood, and challenges in device development that remain for kesterites to contribute significantly to photovoltaic manufacturing. Performance goals, as well as characteristics of midgap defect density, free carrier density, surfaces, grain boundaries, grain-to-grain uniformity, and bandgap alloying are discussed.

A comparative study of Zn(O,S) buffer layers and CIGS solar cells fabricated by CBD, ALD, and sputtering

Zn(O,S) thin films were deposited by chemical bath deposition (CBD), atomic layer deposition, and sputtering. Composition of the films and band gap were measured and found to follow the trends described in the literature. CBD Zn(O,S) parameters were optimized and resulted in an 18.5% efficiency cell that did not require post annealing, light soaking, or an undoped ZnO layer. Promising results were obtained with sputtering. A 13% efficiency cell was obtained for a Zn(O,S) emitter layer deposited with 0.5%O2. With further optimization of process parameters and an analysis of the loss mechanisms, it should be possible to increase the efficiency.

Reflection Optimization for Alternative Thin-Film Photovoltaics

The recent improvements in efficiencies for kesterite (copper zinc tin selenide, CZTS) devices warrant an investigation into how the kesterite device stack can best be capped to minimize losses due to reflection. Additionally, ongoing efforts to replace the cadmium sulfide (CdS) layer in copper indium gallium selenide (CIGS)-based devices, most notably with zinc sulfide (ZnS), need to be accompanied by a similar investigation into how to best finish a CIGS/ZnS stack to minimize reflection losses. An optical analysis of how CZTS/CdS and CIGS/ZnS devices reflect light has been performed for the purpose of optimizing the transparent conducting oxide and antireflection layers for each stack. This research addresses what is similar and what is different between the alternative stacks and the routine CIGS/CdS stack and how to best reduce the reflection losses for each situation.

Outdoor performance prediction of photovoltaic modules based on indoor measurements

The performance of various CIGS and c-Si PV modules determined indoors as function of temperature and irradiance is used to develop an algorithm to predict actual outdoor performance of representative modules for a given set of temperature and irradiance. Deviations between predicted and measured values can be partially explained based on a combination of light-soaking effects and spectral composition of the light.

Effects of pre-conditioning and testing protocols on performance of different PV technologies

In order to compare panels produced using different technologies in a meaningful manner, it is important to use testing protocols that are relevant to each technology, and that provide results representative of the actual performance of the module under working conditions. Preconditioning process and testing protocols affect the module performance and must be controlled in order to obtain accurate comparisons. The effects of preconditioning processes and testing protocols on various PV technologies are studied and analyzed.

Evaluation of protocols for temperature coefficient determination

The temperature coefficients of CIGS and c-Si modules were determined using both a Class C sun simulator and a Class A+ flash tester. The temperature coefficients were calculated following various light soaking procedures, and varying the heating and cooling ramps. The temperature coefficient determination was also carried out for bare cells (without lamination), and the results compared to measurements carried out on related modules. It was observed that in order to obtain a reliable value for the temperature coefficient, hysteresis effects in the performance as a function of temperature must be avoided.

A Dry Heat-Induced Effect of Using Silver in CIGS Gridlines

The ease with which screen-printed silver is deposited and its high conductivity make it an appealing choice for gridline material on CIGS-based photovoltaic devices. However, present results suggest silver diffusion into the device can cause severe reductions in efficiency after as little as 200 h at 85 °C. Dramatic reductions in fill factor, characterized by unusual inflections in the power quadrant of current-voltage curves, are observed for devices with silver gridlines but not for those with nickel or aluminum gridlines. The shape of the current-voltage curves demonstrate that the degradation mode is not simply due to changes in resistance but is consistent with the creation of a secondary barrier near the device junction.