• Energy Research

The authors investigated the effect of an applied high voltage (1 kV) across the thickness of a soda-lime glass substrate of Cu(In,Ga)Se2 (CIGS) thin-film solar cells. Two types of CIGS cells were tested, differing only in the deposition process of the molybdenum (Mo) back contact. Whilst one cell type was susceptible to potential induced degradation (PID), the other exhibited highly increased stability against PID. PID occurs for PID-susceptible cells after the transfer of a certain amount of charge through the soda-lime glass substrate when the Mo back contact of the cell operates as a cathode (negatively biased versus backside of the substrate). Capacitance–voltage and electron-beam-induced current measurements showed an enlarged space charge region expanding to the Mo back contact and a lowered doping density by a negative potential for PID-susceptible cells. Glow discharge optical emission spectroscopy (GDOES) revealed an accumulation of sodium (Na) in the solution-grown CdS buffer layer and a segregation on the surface of the ZnO:Al window layer for higher charges for PID-susceptible cells. Cells with increased PID immunity did not show an increase of Na for charges up to around 9 mC/cm². We demonstrate that it is possible to improve the PID stability of CIGS solar cells by modification of the molybdenum back contact.

Abstract An effective encapsulation solution for flexible CIGS is urgently needed to ensure a competitive market entry of the technology. In this work, we demonstrate the feasibility to effectively encapsulate module-level (10 × 10 cm2) CIGS/glass solar cells by employing a thin Al2O3 barrier layer grown by atomic layer deposition (ALD). As determined by a direct methodology, 10 nm ALD-Al2O3 is proved to be sufficient in preventing electrical degradation of the Al:ZnO (AZO) window layer upon exposure to damp heat test (DHT) and equally effective to encapsulate 10 × 10 cm2 CIGS/glass mini-modules by efficient blockage of moisture ingress. CIGS mini-modules encapsulated by ALD-Al2O3 barrier layer retain an average of 80% and 72% of initial efficiency after 1000 and 2000 h of DHT, respectively. Whereas unencapsulated modules drop to an average of 67% (1000 h DHT) and 22% (2000 h DHT) of initial efficiency. Thanks to the presence of ALD-Al2O3 barrier layer, less electrical degradation occurred in AZO window layer and P3 interconnection; also less shunting paths appeared – both led to a lower FF drop in encapsulated CIGS mini-modules. However, an issue of Na migration out of the CIGS layer is observed, which negatively impacts the module stability during DHT.

Partial substitution of Cu by Ag in Cu(In,Ga)Se2 (CIGS) solar cells is advantageous as it allows lower temperature growth while maintaining high performance. To understand the role of Ag on device performance, we present a comprehensive analysis of (Ag,Cu)(In,Ga)Se2 (ACIGS) samples with an [Ag]/([Ag]+[Cu]) (AAC) ratio varying from 7% to 22%. The analysis involves a set of material and device characterization techniques as well as numerical simulations. Multiple electrical and material properties show a systematic dependence on the increased Ag content. These include a carrier-density decrease, a grain-size increase, and a flattened [Ga]/([Ga] + [In]) (GGI) profile leading to a higher minimum band gap energy and a reduced back grading. Although the best performing device (PCE = 18.0%) in this set has an AAC = 7%, cells with higher Ag contents have an advantage of a smoother absorber surface which is attractive for tandem applications, despite their slightly inferior conversion efficiencies (PCE = 16.4% for 22% Ag).

Thin-film solar cells based on Cu(In,Ga)(Se,S) 2 (CIGS) have demonstrated both high efficiencies and a high cost-reduction potential in industrial production. This way, future CIGS module production lines can be profitable even for scales below the GW range. Among the different technologies, only the coevaporation method has demonstrated efficiencies above 20%, approaching the record values of polycrystalline Si cells. The main focus of this contribution is on the new results of the ZSW cell line with efficiencies above 20%, as well as on the mini-module line on glass substrates. Mini modules (10 cm × 10 cm) with efficiencies in the range of 17% give a proof of concept for industrial-sized modules. ZSW is also developing flexible cells and modules, transferring the processes from the glass-based technology. We achieved 18.6% cell efficiency on metal substrates and a 15.4% efficient mini module could be demonstrated with adapted methods of module patterning. In order to develop industrially relevant processes for foils, we are running a roll-to-roll deposition plant. Additionally, we have improved CIGS cell efficiencies with alternative buffers to certified 19.0% for solution-grown Zn(O,S), to 16.4% for sputtered Zn(O,S), and 17.1% for evaporated In 2S 3. Our cells deposited by vacuum-free methods exhibit an efficiency of 8.5% with a nanoparticle-based process.

All-inorganic perovskites, such as CsPbI2Br, have emerged as promising compositions due to their enhanced thermal stability. However, they are very prone to degradation due to moisture.

Schematic depiction of spiro-OMeTAD degradation causing perovskite instability under pseudo-operating conditions and its prevention using an ultra-thin ALD layer of alumina.

AbstractDespite the efficiencies above 20% achieved with (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells, further efficiency improvements are necessary. One possibility is the implementation of a postdeposition treatment (PDT) process. The aim of this study is therefore to investigate the effect of rubidium‐fluoride (RbF)‐PDT on the performance and physical properties of the ACIGS absorber. For this purpose, the RbF source temperature of the PDT process of ACIGS films with Ag/(Ag+Cu) (AAC) ratios of 5% from a multistage co‐evaporation inline process was systematically varied. It was shown that the efficiency of the devices is reduced by the PDT process, unlike observed for Cu(In,Ga)Se2 (CIGS) absorber, and is strongly influenced by the amount of rubidium. The behavior can be attributed to a strong reduction of the doping, which results from a changed doping mechanism. Furthermore, evidence for the formation of an additional layer was found. In addition, deep level transient spectroscopy (DLTS) measurements were performed on the samples showing a strong signal at low temperatures. This minority trap signal is strongly influenced by the amount of Rb and shows a systematically changing energetic position towards the middle of the band gap and an increasing density. Based on pulse variation measurements, the associated defect could be identified as an extended defect, indicating a location of the defect at grain boundaries.

We present a method for high-rate solution growth of the Zn(O,S) buffer layer to achieve deposition rates and material consumptions far below the standard Zn(O,S) and CdS deposition method. We replace the organosulfide thiourea by the more quickly decomposable thioacetamide and control the reaction kinetics by the use of chelating ligands and ammonia. We characterize the produced layers by secondary neutral mass spectrometry, X-ray diffraction, and optical transmission. For cell preparation, we use high-efficiency Cu(In,Ga)Se2 with an alkali-modified surface, as well as industrially relevant inline absorber material. We realize a certified 21% cell efficiency with the standard thiourea-based Zn(O,S) and first cells with over 19 % with the high-rate Zn(O,S) buffer.