John Kessler

ZnO/CdS/CuInSe2 thin‐film solar cells with improved performance

An important milestone in the development of photovoltaic thin‐film solar cells is the achievement of 15% conversion efficiency. This letter describes the highest efficiency single junction thin‐film cell reported to date. An active area efficiency of 14.8% is obtained with the cell structure n‐ZnO/n‐CdS/p‐CuInSe2 deposited on a soda‐lime glass substrate. The current achievements are due to improved properties of the CuInSe2 layer and the heterojunctions compared to previously reported results. The rate and substrate temperature profiles used during the coevaporation process yield a relatively large‐grained material with very strong 〈112〉 orientation and low porosity. This results in reduced recombination rates, hence higher open circuit voltage and fill factor.

Baseline Cu(In, Ga)Se2 device production : Control and statistical significance

Small- and-large area Cu(In,Ga)Se 2 - based solar cells, as well as 20 cm 2 mini-modules are produced using a baseline approach that privileges process simplicity and statistical significance. High-quality devices are controllably obtained, as well as a 14.7% world record mini-module. Both grided and conventional mini-modules are produced and compared. A few processes from our research areas are presented as candidates for baseline integration. Among these, the examples of fast CIGS and thin CIGS are shown. For the latter, Ga-grading is involved and fill factors above 81% have been measured.

Cu(In,Ga)Se2-based thin-film photovoltaic modules optimized for long-term performance

Abstract In this contribution we give an overview of the mechanisms behind degradation of Cu(In,Ga)Se 2 -based modules. Based on the results from a detailed analysis of power losses in modules, prior to and after extended damp heat exposure, we discuss to what extent modules can be designed to achieve enhanced long-term performance. For conventional modules, we show that the stability can be improved by optimizing the interconnect and the front contact. Furthermore, we argue that gridded modules are better from a long-term performance point of view. A novel interconnect structure, specifically designed for long-term durability, is briefly discussed.

Interface study of CuInSe2/ZnO and Cu(In, Ga)Se2/ZnO devices using ALD ZnO buffer layers

Abstract Solar cells based on CuInSe2 (CIS) and Cu(In,Ga)Se2 (CIGS), with a ZnO buffer layer deposited by atomic layer deposition (ALD), are compared to their reference cells; (CIS or CIGS)/CdS/ZnO. While the CIS/ZnO devices show only slightly lower efficiencies compared to their reference cells, the difference between the CIGS/ZnO devices and their reference cells is larger. In the latter case, the main difference is the lower open circuit voltage of approximately 200 mV of the direct ZnO devices. The valence band offset between CIS and ZnO is determined by ultraviolet photoelectron spectroscopy to −2.2 eV which gives a conduction band offset, ΔEc, of +0.1±0.2 eV. This slightly positive offset is in contrast to our previous result for the CIGS/ZnO interface of ΔEc=−0.2±0.2 eV, and is a possible explanation for the much lower voltage loss observed for the CIS/ZnO devices. Zn diffusion into the different absorbers is investigated by energy dispersive X-ray spectroscopy on transmission electron microscope cross-sections prepared from direct ZnO devices. These cross-sections also show very good coverage of the absorber surface by the (ALD)ZnO layer.

Highly efficient Cu(In, Ga)Se2 mini-modules

In this contribution, we present results and the philosophy of our mini-module efforts. These efforts have achieved world record levels as well as a reproducible process. Various mini-module designs are tested using two different baseline Cu(In,Ga)Se2 deposition recipes. Gridded mini-modules achieve highest efficiencies and are much less demanding on the ZnO:Al top contact than their conventional counterpart. For all of the designs tested, our experimental results are in the order of the expectations from our modeling. Gridded modules can achieve efficiency levels very close to those of the cells.

Growth of Cu(In,Ga)Se2 Films Using a Cu-poor/rich/poor Sequence: Substrate Temperature Effects

Growth of Cu(In,Ga)Se2 Films Using a Cu-poor/rich/poor Sequence: Substrate Temperature Effects

Thin film PV modules for low-concentrating systems

The high cost of photovoltaic (PV) energy has imposed extensive research efforts in order to provide alternatives to the conventional crystalline silicon (c-Si) PV technology. Thin film PV modules based on Cu(In,Ga)Se2 (CIGS) is considered one of the most promising alternatives for mass production of low-cost PV. In parallel to the development of new module technologies, there is an increasing interest for using concentrating optics in PV systems in order to increase radiation onto the modules. By replacing the relatively expensive PV absorbers with low-cost concentrators there is a potential reduction of overall system costs. The reflector types considered in this study are based on the compound parabolic concentrator (CPC) and the planar reflector. These are low-concentrating devices with concentration ratios of 1–4. With the CPC as well as the planar reflector, the illumination on the PV module will be non-uniform, with local light intensities that are considerably larger than the average 4 suns. For conventional c-Si modules, this is detrimental to module performance. It is demonstrated in the present work that modules based on thin film technology are better candidates for reflector applications. The principles of design and fabrication of CIGS thin film PV modules for low-concentrating systems are discussed, and experimental results from measurements of CIGS modules under concentrated illumination are evaluated.

Design of Grided Cu(In,Ga)Se2 Thin Film PV Modules

Abstract Results from modeling designs of Cu(In,Ga)Se2 thin-film PV modules show that grided modules, at standard test conditions as well as at low-concentrated light, exhibit significantly improved performance when compared with conventional designs. It is further discussed that a grided design is advantageous from a synthesis and manufacturing point of view, since it provides higher front contact process tolerance and throughput as well as improved degrees of freedom of the module geometry.

Rapid Cu(In,Ga)Se/sub 2/ growth using "end point detection"

Thin film Cu(InGa)Se/sub 2/ (CIGS) is grown using a two stage co-evaporation process where all of the Cu is evaporated in the first stage. Near the end of the second stage, a Cu-rich to Cu-poor transition occurs, where the power delivered to the substrate heater in order to sustain a constant substrate temperature, changes as a result of a change in the radiative behavior of the CIGS film. The output power signal is shown to respond quickly to, and be characteristic of the film composition near the transition. Using this signal to monitor the deposition process results in excellent control of the final Cu content, even when the evaporation rates are poorly known and poorly controlled. High quality devices result, even at high evaporation rates. Solar cells with efficiencies close to 15 % have been produced from CIGS deposition times below 15 minutes and are only marginally better for deposition times of up to 45 minutes, and this at constant substrate temperatures of 500/spl deg/C.

Effect of sulfurization on the microstructure of chalcopyrite thin-film absorbers

Polycrystalline CuInSe 2 (CIS) and Cu(In,Ga)Se 2 (CIGS) thin-films were grown by co-evaporation on a soda lime glass substrate. Rapid thermal processing (RTP) in H 2 S atmosphere with processing temperatures ranging from 350 to 550°C was used to sulfurize the absorber. The change in microstructure after sulfurization and annealing was characterized mainly by transmission electron microscopy. A non-uniform and porous surface reaction layer is evident in the CIS and CIGS structures after the RTP. The CIGS structure have a tendency towards a phase separation, whereas, the CIS films exhibit mixed sulfoselenides, CuIn(Se 1-x S x ) 2 , where x varies. In order to improve the device performance, the formation of two distinct phases should be avoided during the sulfurization processing.