Improvement in durability of flexible plastic dye-sensitized solar cell modules

Large-area integrated modules of flexible plastic type dye-sensitized solar cell (DSC) have been fabricated based on polyethylene naphthalate (PEN) film for practical applications such as ubiquitous power sources. From the view point of improving durability, composition of organic solvent-based electrolytes has been investigated. As a result, a plastic DSC module using LiI-free electrolyte maintained its energy conversion efficiency of 2% over 220 h under the accelerated condition of 55 °C and 95% relative humidity.     

High-efficiency flexible CdTe solar cells on polymer substrates

Development of flexible and lightweight solar cells is interesting for terrestrial and space applications that require a very high specific power (kW/kg) and flexibility for curved shaping or rolling. Flexible CdTe/CdS solar cells of 11% efficiency in superstrate and 7.3% efficiency in substrate configurations have been developed with a “lift-off” approach. However, roll-to-roll manufacturing is desired in future.Therefore, flexible superstrate solar cells were directly grown on commercially available 10 μm thin polyimide (Upilex™) foils. A process for the deposition of ITO (front contact) has been developed to have a stable front contact on the Upilex™ foil. Post-deposition annealing treatments of the ITO/polyimide stacks bring a significant stability to the front contact, having almost the same sheet resistance at the beginning and at the end of the cell fabrication process. Solar cells with AM1.5 efficiency of 11.4% on Upilex™ foils (highest efficiency recorded for flexible CdTe cell) have been developed. A comparison of the cells prepared on different polyimides is presented.     

CdTe/CdS solar cells on flexible substrates

The development of CdTe/CdS solar cells on flexible substrates is reviewed in this article. Photovoltaic structures on lightweight and flexible substrates have several advantages over the heavy glass based structures in both terrestrial and space applications. The cells mounted on flexible foil are not fragile, the requirements of the supporting structures are minimum and they can be wrapped onto any suitably oriented or curved structures. The specific power of the solar cells is an important factor in space applications and hence development of photovoltaic devices on light weight substrates is interesting. CdTe is one of the leading candidates for photovoltaic applications due to its optimum band gap for the efficient photo-conversion and robustness for industrial production with a variety of film preparation methods. Flexible solar cells with conversion efficiencies exceeding 11% have been developed on polyimide foils. The development of CdTe devices on metallic substrates is impeded due to the lack of a proper ohmic contact between CdTe and the substrate. The polymer substrate has the advantage that the devices can be prepared in both “superstrate” and “substrate” configurations.     

A round robin study of flexible large-area roll-to-roll processed polymer solar cell modules

A round robin for the performance of roll-to-roll coated flexible large-area polymer solar-cell modules involving 18 different laboratories in Northern America, Europe and Middle East is presented. The study involved the performance measurement of the devices at one location (Risø DTU) followed by transportation to a participating laboratory for performance measurement and return to the starting location (Risø DTU) for re-measurement of the performance. It was found possible to package polymer solar-cell modules using a flexible plastic barrier material in such a manner that degradation of the devices played a relatively small role in the experiment that has taken place over 4 months. The method of transportation followed both air-mail and surface-mail paths.     

Comparative study of conventional vs. one-step-interconnected (OSI) monolithic CdTe modules

We report on the fabrication of 5 × 7.5 cm² CdTe photovoltaic module devices using two alternative routes. One which uses the conventional approach where laser scribing and active layer deposition steps are inter-mixed, and the other via the one-step-interconnection (OSI) process. OSI combines three laser processes with two inkjet processes, depositing insulating and conductive materials. This allows the series interconnection to occur after the deposition of all active layers reducing fabrication time, capital equipment cost and interconnect dead zone. Its suitability for the manufacture of CdTe mini-modules has previously been demonstrated but no direct comparison was made against the conventional process. The structural properties and performance of conventional vs. OSI processed CdTe modules are presented and show comparable performance using both approaches with the OSI showing considerable process simplification.     

Ultra-thin bifacial CdTe solar cell

Developing a high-quality transparent back contact, while maintaining efficient light transmission through the top absorber layer, are key components for achieving high-efficiency II–VI polycrystalline thin-film tandem solar cells. Combining these two elements, we fabricated ultra-thin bifacial CdTe solar cells (0.68 μm) with ZnTe:N/ITO transparent back contact and achieved efficiencies of 5.7% and 5.0% with illumination from the glass and the contact side, respectively. Device analysis, using (J–V) and QE measurements, show that the loss in efficiency is due to higher R_S and J₀ as well as lower, side-dependent, photons absorption.     

ITO-free flexible inverted organic solar cell modules with high fill factor prepared by slot die coating

We report on the fabrication of inverted ITO-free P3HT:PCBM solar cell modules on glass and PET foil as substrate where the organic functional layers are deposited with slot die coating, a reel to reel compatible coating technique. The active layers have been processed in ambient atmosphere, which will be of advantage in a future production and is especially remarkable as the metallic cathode is already deposited on the substrate at this stage of fabrication. The modules comprise two busses of 11 cell elements connected in series each. The series connection leads to an open circuit voltage of up to 6.88 V on glass substrate, which translates to 625 mV per cell element, a very competitive value for P3HT:PCBM based solar cells on glass. Although the designated area is as large as 41 cm² and the active area 26.4 cm², we obtain fill factors of up to 65% for these modules, which again is a typical value for small area laboratory cells. Remarkably the values for PET foil as substrate with an open circuit voltage of 6.5 V and a fill factor of 64% are very close to the results on glass and to our knowledge the highest fill factors for flexible organic solar cells, even if compared to small area devices. The short circuit current densities and therefore efficiencies are also comparable to small area devices, if only the photoactive area is accounted for. Therefore we have demonstrated that the scale up of organic solar cells can be achieved with a suited circuitry scheme.     

Technological aspects of flexible CIGS solar cells and modules

This paper describes the technological status of and some challenges in the manufacturing of Cu(In,Ga)Se₂ (CIGS)-based solar cells on flexible polymer and metal substrates. Substrate characteristics such as thermal expansion properties and stability, surface roughness, or substrate composition, strongly influence growth and properties of the following layers. For example, adhesion failure, cracking, or contamination by diffusion of impurities from the substrate may occur with some substrates. Aspects of (external) sodium incorporation into CIGS are discussed for high and low CIGS deposition temperature. Low-temperature deposition processes are particularly important when polyimide substrates are used. The electrical insulation of metal foils by dielectric barriers (e.g. SiO_x or Al₂O₃) allows the fabrication of monolithically integrated modules. A soft and selective patterning technique based on laser scribing and mask-free photolithography is described. Working modules as large as 20 cm × 30 cm were achieved with these methods.     

Fabrication of CdTe solar cells by laser-driven physical vapor deposition

Polycrystalline cadmium sulfide-cadmium telluride heterojunction solar cells were fabricated for the first time using a laser-driven physical vapor deposition method. An XeCl excimer laser was used to deposit both of the II–VI semiconductor layers in a single vacuum chamber from pressed powder targets. Results are presented from optical absorption, Raman scattering, X-ray diffraction, and electrical characterization of the films. Solar cells were fabricated by deposition onto SnO₂-coated glass with top contacts produced by gold evaporation. Device performance was evaluated from the spectral quantum efficiency and current-voltage measurements in the dark and with air mass 1.5 solar illumination.     

Manufacturing method for monolithic dye-sensitised solar cells permitting long-term stable low-power modules

A manufacturing technique for monolithic dye-sensitised solar cells is presented. Encapsulated modules designed for indoor low-power applications have been prepared using industrial methods and equipment. Under certain conditions (light intensity <5000 lx, temperature between –10°C and 50°C, and relative humidity of appr. 50%), the modules have performed well and shown excellent long-term stability. Moreover, modules withstand illumination in combination with storage at 100% relative humidity. However, a certain degradation of the module performance takes place at illuminations exceeding 5000 lx and temperatures exceeding 50°C.     

An inter-laboratory stability study of roll-to-roll coated flexible polymer solar modules

A large number of flexible polymer solar modules comprising 16 serially connected individual cells was prepared at the experimental workshop at Risø DTU. The photoactive layer was prepared from several varieties of P3HT (Merck, Plextronics, BASF and Risø DTU) and two varieties of ZnO (nanoparticulate, thin film) were employed as electron transport layers. The devices were all tested at Risø DTU and the functional devices were subjected to an inter-laboratory study involving the performance and the stability of modules over time in the dark, under light soaking and outdoor conditions. 24 laboratories from 10 countries and across four different continents were involved in the studies. The reported results allowed for analysis of the variability between different groups in performing lifetime studies as well as performing a comparison of different testing procedures. These studies constitute the first steps toward establishing standard procedures for an OPV lifetime characterization.     

How to design dye-sensitized solar cell modules

A precise numerical model based on Kirchhoff's laws is used in investigating dye-sensitized solar cell (DSC) modules. As the I-V curve of a small DSC is obtained, the performances of DSC modules with any geometrical character can be exactly simulated by this model. By simulation, we find that the poor synchronization of work status in each cell is the main cause of low efficiency of large area module besides the Joule losses in the resistances. The optimized geometry parameters of the DSC module are mainly influenced by the manufacturing technique rather than the performance of the small cell. Simulation shows that the large area DSC module with aperture area efficiency of 10.57% can be produced based on the small cell with efficiency of 11.1%.     

CdS/CdTe solar cell using sputtering technique

Cadmium telluride films were deposited from a CdTe target using rf planar magnetron system on CdS/ITO/Glass substrates. The as grown films were polycrystalline and consisted of closepacked array of preferentially oriented single crystal grains. Chemical heat treatment using cadmium chloride was used to form the junction between cadmium sulphide and cadmium telluride layers. It also improves the structure of the sputtered CdTe film and the lifetime of carriers.     

Diamond/CdTe: a new inverted heterojunction CdTe thin film solar cell

Recently the concept of the inverted photovoltaic cell has become more attainable as a practical cell. This thin film cell consists of a p–n heterojunction in which the window layer is p-type and the absorber layer is n-type. The feasibility of a new inverted p–n heterojunction p-diamond/n-CdTe solar cell has been demonstrated. The non-optimized solar cell structure grown on semi-transparent p-diamond yielded an open circuit voltage of 0.23 V and a short circuit current of 1.54 mA/cm² when illuminated.     

Low-cost encapsulation materials for terrestrial solar cell modules

Solar cell modules must undergo dramatic reductions in cost in order to become economically attractive as practical devices for the production of electricity. A federal goal seeks to have, by 1986, an industrial capability of producing solar cell modules at a cost of 50¢ per W (in 1975 dollars) and a service lifetime of 20 yr. Today's modules cost more than $11.00 per W, and they have an undefined lifetime. Part of the cost reductions must be realized by the encapsulation materials which are used to package, protect, and support the solar cells, electrical interconnects, and other ancillary components. It is estimated that to meet a cost goal of 50¢ per W, encapsulation materials, including the structural substrate or superstrate, should cost between $2.70 and $5.00 per m² of module area (in 1975 dollars). This article presents the findings of material surveys intended to identify low cost materials which could be functional as encapsulants. This article further assesses the prognosis for achieving an encapsulation system at the lower cost goal of $2.70 per m², and identifies the technologies which must be advanced or developed to achieve 20-yr life with the lowest costing materials.     

Thermal performance of integral bypass diode solar cell modules

Without appropriate protection, solar cell arrays can be vulnerable to thermal instability because of mismatched cell characteristics caused by effects such as cell cracking or partial shading of the array. The normal method of protecting against such thermal instability in both small and large arrays is to include bypass diodes across groups of series-connected cells. Experimental results are described for the thermal performance of a novel approach whereby bypass diodes are integrated into the solar cell structure during cell fabrication. The main conclusion is that the present integral bypass diode approach automatically protects against thermal instability in both small and large arrays without the need for any other protective measures. This is in addition to the improved tolerance of the array output power to partial shading.     

Amorphous silicon solar cell modules fabricated with a single-chamber load-lock deposition system

Solar cell modules were fabricated in a single-chamber load-lock deposition system. This system routinely produces high efficiency solar cell modules on both small- and large-area substrates. The best solar cell efficiencies were 10.4% on 0.265 cm², 8.7% on 420 cm² and 8.1% on 1000 cm². The high throughput at one or two runs per hour makes the single-chamber load-lock deposition system a valuable research tool which permits rapid progress with great process control. Because of the large-area deposition capability, it can support pilot and manufacturing deposition systems.     

硅太阳电池组件工艺缺陷的红外检测技术

介绍了一种利用半导体电致发光原理检测硅太阳电池组件工艺缺陷的设备,并简要分析了工艺缺陷的产生及检测原理。利用电致发光原理,能够检测出虚焊、断栅、碎片和隐裂四种较明显的工艺缺陷,具有广阔的应用前景。     

Investigation into ultrathin CdTe solar cell Voc using SCAPS modelling

Abstract

Ultrathin CdTe photovoltaic solar cells were produced by metal organic chemical vapour deposition in a single horizontally configured growth chamber. Solar cell activation was investigated by varying the duration of the CdCl₂ layer deposition and 420°C thermal anneal to promote Cl diffusion into the CdTe. Thicker CdCl₂ layers used in activation treatment resulted in a greater degree of sulphur interdiffusion, up to 2 at.-%, into the CdTe layer. The thicker CdCl₂ activation layer was necessary to lower the reverse saturation current density for obtaining optimum experimental photovoltaic (PV) device performances. Modelling of the PV performances with equivalent solar cell structure for optimised devices using solar cell capacitance simulation software resulted in an overestimated open circuit voltage (V_oc). The simulations showed that reduced acceptor states at the CdTe interface with the intermixed region resulted in the largest decrease in V_oc when considering large back surface recombination velocities.