Kristopher Wieland

10% efficiency solar cells with 0.5 µm of CdTe

Fabricating high-efficiency CdS/CdTe solar cells with ultra-thin (below 1µm) absorber layers is a challenging yet highly desirable step in improving CdTe technology. Typically solar cell performance decreases due to shunting, incomplete absorption (deep penetration loss), fully depleted CdTe layers or interference between the main and the back contact junction when the CdTe layer thickness approaches a certain limit. While some of these losses are fundamental, others can be minimized by careful optimization of the fabrication steps. We present the results of such optimization.

The correlation of performance in CdTe photovoltaics with grain boundaries

It is assumed that the performance of polycrystalline CdTe photovoltaic thin films is correlated to microstructure, especially aspects of the microstructure related to grain boundaries. However the role of grain boundaries on the electrical performance of these films is not well understood. In an effort to gain understanding into the correlation between the performance and grain boundaries in CdTe thin films, electron backscatter diffraction (EBSD) has been used to characterize the the grain size, grain boundary structure and texture of sputtered CdTe at varying deposition pressures before and after CdCl2 treatment. Weak axisymmetric (or fiber) textures were observed in the as-deposited films. At lower deposition pressures (111) fiber textures were observed and (110) fiber textures were observed at higher lower deposition pressures. Samples treated with CdCl2 exhibited significant grain growth with a high fraction of twin boundaries. A much stronger correlation of performance with grain size was found when the grain size was corrected to exclude the twin boundaries. This observation confirms that the twin boundaries do indeed have different electrical properties than random high-angle boundaries.

Effects of Cu and CdCl 2 treatment on the stability of sputtered CdS/CdTe solar cells

We have studied the effects on cell stability of the amount of copper in the back contact and the dependence on CdCl2 activation treatment for sputtered CdS/CdTe cells. Unencapsulated cells were tested in the dark and at one sun at 85 °C and higher. We find that cells with > 3 nm of evaporated Cu (covered with 20 nm Au) had initially higher short-circuit current but were less stable than cells with limited Cu (1–3 nm) and that cells without intentional Cu in the back contact had neither better stability nor better initial performance. For cells with Cu in the back contact, a window of CdCl2 treatment time was found for optimal stability. The cells without back-contact Cu were not very sensitive to the treatment time. With optimized CdCl2 treatment and Cu thickness, cells have shown very good stability. For cells with unoptimized Cu thickness or activation treatment, the decreases in fill factor (FF) and open-circuit voltage (Voc) were significant. We believe these changes may arise from possible formation of an oxide or telluride layer involving copper at the back contact and the migration of Cu from the back contact.

Sputtered HRT layers for CdTe solar cells

This work focuses on demonstrating the suitability of various high resistivity transparent (HRT) layers prepared by magnetron sputtering for sputtered CdS/CdTe cells. HRT buffer layers added between the transparent conducting oxide (TCO), and the CdS layer are important for reducing the effects of non-uniformities and shunts in large-area thin-film devices. CdS/CdTe cells were fabricated on Pilkington TEC 7 glass coated with a sputter deposited HRT layer of ZnO:Al or SnO2. In some cases O2 was added to the Ar sputter gas to increase the resistivity of the HRT buffer layers. Film properties were optimized for HRT performance by adjustments in the substrate deposition temperature, sputter gas pressure, and RF power. Best results have been obtained with reactively sputtered ZnO:Al with 2 % O2 in Ar.

Electron backscatter diffraction and photoluminescence of sputtered CdTe thin films

Electron backscatter diffraction (EBSD) has been used to characterize the grain size, grain boundary structure, and texture of sputtered CdTe at varying deposition pressures before and after CdCl2 treatment in order to correlate performance with film microstructure. It is known that twin boundaries may have different electrical properties than high-angle grain boundaries and in this work we have included the effects of twin boundaries. We found better correlation of solar cell device performance to the twin-corrected grain size than to the standard grain size. In addition, we have correlated the photoluminescence (PL) spectra with device performance and with the EBSD results. We find that sputtering at 18 mTorr yields the highest efficiency, largest twin-corrected grain size and the strongest PL.

CdTe cell stability vs. CdS thickness

We have studied the stability in air of sputtered CdTe/CdS cells with a focus on the thickness of the CdS layer and the substrate used during cell fabrication. The stability of cell efficiencies under light soak is compared for CdTe cells on both HRT-coated Pilkington TEC 15 glass and standard TEC15 glass for a range of seven different CdS layer thicknesses from 0 to 230 nm. Little difference in stability can be found between cells with or without the HRT layer for all CdS thicknesses. The buffer layer helps to produce high initial performance and uniformity of cells by maintaining high VOC for very thin CdS layers. But there is no indication that the buffer layer improves the stability of unencapsulated, sputtered cells under one-sun light soak at open circuit. Devices with the thicker CdS layers show better stability under one-sun light soak in air.

Characterization of Sputtered CdTe Thin Films with Electron Backscatter Diffraction and Correlation with Device Performance.

The performance of polycrystalline CdTe photovoltaic thin films is expected to depend on the grain boundary density and corresponding grain size of the film microstructure. However, the electrical performance of grain boundaries within these films is not well understood, and can be beneficial, harmful, or neutral in terms of film performance. Electron backscatter diffraction has been used to characterize the grain size, grain boundary structure, and crystallographic texture of sputtered CdTe at varying deposition pressures before and after CdCl2 treatment in order to correlate performance with microstructure. Weak fiber textures were observed in the as-deposited films, with (111) textures present at lower deposition pressures and (110) textures observed at higher deposition pressures. The CdCl2-treated samples exhibited significant grain recrystallization with a high fraction of twin boundaries. Good correlation of solar cell efficiency was observed with twin-corrected grain size while poor correlation was found if the twin boundaries were considered as grain boundaries in the grain size determination. This implies that the twin boundaries are neutral with respect to recombination and carrier transport.

Improvements in ultra-thin CdS/CdTe solar cells

Thinning the CdTe is advantageous, inter alia, to reduce the consumption of Te in solar modules; however the CdTe cell performance typically decreases with CdTe thickness. In comparison with other deposition techniques, sputtering appears to be well suited for ultra-thin device preparation partly due to control over the growth rate, grain size, and film stress. The films were magnetron sputtered on Pilkington TEC15, SnO2:F-coated, soda-lime glass substrates with a high resistivity transparent (HRT) interfacial layer. The thickness of CdS was constant at 50–55 nm while the CdTe thicknesses ranged from 0.25μm–2.1 μm. With optimum cell post-deposition processing, we obtained cells with efficiencies of 8.0%, 11.0%, 12.5% for CdTe thicknesses, respectively, of 0.25, 0.50, 0.75 μm. We believe that these represent the highest efficiencies yet obtained for CdS/CdTe cells with such submicron absorber-layer thicknesses.

Development of wide operational range fiber laser for processing thin film photovoltaic panels

Continued reduction of production costs for the manufacture of thin film photovoltaic solar panels is required to support the ongoing broad adoption of this important technology. Laser scribing is an integral manufacturing process for the production of monolithically-integrated thin film solar cells. An advanced generation 1.06 um pulsed fiber master oscillator power amplifier (MOPA) has recently been developed with performance attributes designed to meet current and next generation thin film laser scribing requirements for improving thin film solar cell device efficiencies, throughput, and yield. It features broad pulse width flexibility (200ps to 20 ns), excellent beam quality, and stable pulse output over a wide range of pulse repetition frequencies (50 kHz to 1 MHz). These performance attributes are expected to enable advanced laser process developments for amorphous (a)-Si, Cd-Te, CIS (CuInSe), and next-generation thin film PV cell. As a result of the experiment, we have found that 10 to 20-ns pulse width gives the good results of structuring TCO layers while the pulse width of shorter than 1ns seems suitable for Mo-layer ablation. The scanning speed of 2500mm/s and 6000mm/s has been achieved, respectively.Continued reduction of production costs for the manufacture of thin film photovoltaic solar panels is required to support the ongoing broad adoption of this important technology. Laser scribing is an integral manufacturing process for the production of monolithically-integrated thin film solar cells. An advanced generation 1.06 um pulsed fiber master oscillator power amplifier (MOPA) has recently been developed with performance attributes designed to meet current and next generation thin film laser scribing requirements for improving thin film solar cell device efficiencies, throughput, and yield. It features broad pulse width flexibility (200ps to 20 ns), excellent beam quality, and stable pulse output over a wide range of pulse repetition frequencies (50 kHz to 1 MHz). These performance attributes are expected to enable advanced laser process developments for amorphous (a)-Si, Cd-Te, CIS (CuInSe), and next-generation thin film PV cell. As a result of the experiment, we have found that 10 to 20-ns pulse ...

Real-time Optical Thickness Monitor for thin film growth

In order to achieve maximum efficiency and performance in a photovoltaic solar cell module, the precise thickness of the semiconductor layers must be controlled. An Optical Thickness Monitor (OTM) based on optical interference can be used to better understand and to monitor the deposition process in real time. The OTM utilizes laser light either in transmission mode or in reflection mode which is detected with a photodiode. By measuring the interference fringes produced, the OTM displays real-time film thickness and material growth rate. Predicted film thicknesses are in good agreement with measured values.