Ingrid Repins

Photovoltaic manufacturing: Present status, future prospects, and research needs

In May 2010 the United States National Science Foundation sponsored a two-day workshop to review the state-of-the-art and research challenges in photovoltaic (PV) manufacturing. This article summarizes the major conclusions and outcomes from this workshop, which was focused on identifying the science that needs to be done to help accelerate PV manufacturing. A significant portion of the article focuses on assessing the current status of and future opportunities in the major PV manufacturing technologies. These are solar cells based on crystalline silicon (c-Si), thin films of cadmium telluride (CdTe), thin films of copper indium gallium diselenide, and thin films of hydrogenated amorphous and nanocrystalline silicon. Current trends indicate that the cost per watt of c-Si and CdTe solar cells are being reduced to levels beyond the constraints commonly associated with these technologies. With a focus on TW/yr production capacity, the issue of material availability is discussed along with the emerging technologies of dye-sensitized solar cells and organic photovoltaics that are potentially less constrained by elemental abundance. Lastly, recommendations are made for research investment, with an emphasis on those areas that are expected to have cross-cutting impact.

Long lifetimes in high-efficiency Cu(In,Ga)Se2 solar cells

Time-resolved photoluminescence measurements on polycrystalline Cu(In,Ga)Se2 (CIGS) thin films corresponding to high-efficiency solar cells indicate recombination lifetimes as long as 250ns, far exceeding previous measurements for this material. The lifetime decreases by two orders of magnitude when exposed to air. Charge separation effects can be observed on CIGS∕CdS∕ZnO devices in low-intensity conditions. The ZnO layer forms a robust junction critical for charge separation, whereas the CdS layer alone forms a much weaker junction. Recombination at the CIGS/CdS interface is negligible. The results significantly adjust the previous picture of recombination in CIGS solar cells.

The state and future prospects of kesterite photovoltaics

A recent meeting of experts in kesterite, chalcopyrite, and related thin-film solar cell devices; characterization; and modeling from industry, academia, and national labs identified high-impact pathways forward in kesterite photovoltaics research, towards the end-goal of achieving high-efficiency (>18%) devices in an accelerated timeframe. This paper summarizes the conclusions of this meeting while providing background on key areas of kesterite research. This paper does not aim to provide a comprehensive status-of-the-field review but rather to suggest specific and targeted areas where additional focus might yield the highest-impact results.

Effects of sodium incorporation in Co-evaporated Cu2ZnSnSe4 thin-film solar cells

Sodium incorporation into Cu2ZnSnSe4 (CZTSe) substantially improves the device efficiency by enhancing the open-circuit voltage (VOC) and fill factor. Sodium increases hole density, makes the acceptor shallower, shifts the Fermi level lower, and leads to higher built-in voltage and, consequently, higher VOC. Sodium reduces the concentration of certain deep recombination centers, which further benefits VOC. The increase of hole density and mobility enhances the CZTSe conductivity leading to higher fill factor. Sodium causes smaller depletion width, hence, lower short-circuit current. The minority-carrier lifetime decreases slightly after sodium is incorporated via the Mo-coated soda-lime glass, although adding NaF provides some amelioration.

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.

Phase Identification and Control of Thin Films Deposited by Co-Evaporation of Elemental Cu, Zn, Sn, and Se

Kesterite thin films [(i.e., Cu2ZnSn(S,Se)4 and related alloys] have been the subject of recent interest for use as an absorber layer for thin film photovoltaics due to their high absorption coefficient (>104 cm−1), their similarity to successful chalcopyrites (like CuInxGa1−xSe2) in structure, and their earth-abundance. The process window for growing a single-phase kesterite film is narrow. In this work, we have documented, for our 9.15%-efficient kesterite co-evaporation process, (1) how appearance of certain undesirable phases are controlled via choice of processing conditions, (2) several techniques for identification of phases in these films with resolution adequate to discern changes that are important to device performance, and (3) reference measurements for those performing such phase identification. Data from x-ray diffraction, x-ray fluorescence, Raman scattering, scanning electron microscopy, energy dispersive spectroscopy, and current-voltage characterization are presented.

Chemical and electronic surface structure of 20%-efficient Cu(In,Ga)Se2 thin film solar cell absorbers

The chemical and electronic surface structure of 20%-efficient Cu(In,Ga)Se2 thin film solar cell absorbers was investigated as a function of deposition process termination (i.e., ending the growth process in absence of either Ga or In). In addition to the expected In (Ga) enrichment, direct and inverse photoemission reveal a decreased Cu surface content and a larger surface band gap for the “In-terminated” absorber.

Required material properties for high-efficiency CIGS modules

Relatively high proven efficiencies of CIGS devices are often cited regarding its choice as a semiconductor for photovoltaic manufacturing. Module efficiency is an important parameter, as a number of factors in the cost per watt are driven downward by increasing efficiency. Some of these factors include materials costs, throughput for a given capital investment, and installation costs. Thus, realizing high-efficiency (e.g. 15%) large-area CIGS modules is key in both reducing cost per watt and differentiating the technology from other thin films. This paper discusses the material properties required of each layer of the CIGS device such that large-area CIGS modules can achieve efficiencies 15%, which is substantially higher than the current industrial state-of-the-art. The sensitivity of module performance to the important material parameters is quantified based on both experimental data and modeling. Necessary performance differences between small-area devices and large-area modules imposed by geometry are also quantified. Potential technical breakthroughs that may relax the requirements for each layer are discussed.

Indications of short minority-carrier lifetime in kesterite solar cells

Solar cells based on kesterite absorbers consistently show lower voltages than those based on chalcopyrites with the same bandgap. We use three different experimental methods and associated data analysis to determine minority-carrier lifetime in a 9.4%-efficient Cu2ZnSnSe4 device. The methods are cross-sectional electron-beam induced current, quantum efficiency, and time-resolved photoluminescence. These methods independently indicate minority-carrier lifetimes of a few nanoseconds. A comparison of current-voltage measurements and device modeling suggests that these short minority-carrier lifetimes cause a significant limitation on the voltage produced by the device. The comparison also implies that low minority-carrier lifetime alone does not account for all voltage loss in these devices.

Characterization of 19.9%-efficient CIGS absorbers

We recently reported a new record total-area efficiency, 19.9%, for CuInGaSe2 (CIGS)-based thin-film solar cells [1]. Current-voltage analysis indicates that improved performance in the record device is due to reduced recombination. The reduced recombination was achieved by terminating the processing with a Ga-poor (In-rich) layer, which has led to a number of devices exceeding the prior (19.5%) efficiency record. This paper documents the properties of the high-efficiency CIGS by a variety of characterization techniques, with an emphasis on identifying near-surface properties associated with the modified processing.