• Energy Research

Distinguishing among different electrical loss mechanisms − such as interface and bulk recombination − is a common problem in thin film solar cells. In this work, we report a J–V measurement technique using different illuminating spectra to distinguish between these two recombination losses. The basic idea is to change the relative contribution of bulk recombination to the total losses of photo-generated charge carriers by generating them in different depths within the absorber layer using different spectral regions of the illuminating light. The use of modern LED sun-simulators allows an almost free design of illumination spectra at intensities close to 1 sun. The comparison of two simple J–V measurements, one recorded with illumination near the absorber's band-gap energy and one with light of higher energy, in combination with supporting measurements of the absorber properties, as well as device modeling, enables the extraction of the diffusion length and the interface recombination velocity. Using this technique, we show that in CIGS solar cells, an RbF post-deposition treatment does not only reduce interface recombination losses, as often reported, but also reduces bulk recombination in the CIGS absorber. Furthermore, we find that both cells, with and without RbF treatment, are dominantly affected by interface recombination losses.

Abstract Hydrogenated nanocrystalline silicon oxide (nc-SiOx:H) films have demonstrated a unique combination of low parasitic absorption and high conductivity. Here, we report on the use of n-type nc-SiOx:H as front surface field (FSF) in rear-emitter silicon heterojunction (SHJ) solar cells exhibiting excellent electrical cell parameters at a thickness down to only 5 nm. Using a seed layer, we are able to maintain excellent electrical performance (high fill factor (FF) and open circuit voltage (VOC)), while enhancing layer transparency for maximizing short circuit current (JSC). These results, together with the short deposition time (

AbstractOptical and electrical properties of p-type doped hydrogenated amorphous silicon (p-a-Si:H), used as window emitter layer in silicon based hetero junction (a-Si:H/c-Si HJ) solar cells, are investigated. These properties were investigated by comparing diborane (B2H6) and trimethylboron (B(CH3)3, TMB) as doping gas analyzing the deposition temperature dependence. A wider optical band gap (E04 >1.9eV) and lower refractive index (n) is observed for TMB-doped layers, which we ascribe to carbon incorporation. a-Si:H/c-Si HJ solar cells with p-doped emitter layers using TMB and B2H6 were fabricated. For cells with TMB-doped layers an increased photocurrent of up to 1.5mA/cm2 is found, which is due to a reduction in parasitic absorption. However, cells with TMB-doped emitter also show a higher series resistance, which we attribute to increased electrical losses at the TCO/p-contact.

Laser-based patterning for serial interconnection of chalcopyrite (i.e., Cu(In ${}_{\rm{x}},\text{Ga}_{\rm{1-x}}$ )Se2 or CIGSe) solar cells was obtained by 1) laser ablation using picosecond (ps) pulses, 2) local phase transformation using nanosecond (ns) laser pulses, and 3) conventional needle-based patterning. All three patterning approaches cause a modification of the material properties in the vicinity of the actual P2 scribing lines, which affects and limits the electrical functionality of the interconnection, and thus has to be considered for positioning the P3 scribe. Thus, the extension and the properties of the affected zone aside the P2 scribe was investigated through spectral and spatial photoluminescence (PL). From the depletion of the PL intensity when approaching the scribing line and a peak shift analysis it is concluded that the laser-affected zone is distinctively larger than visual inspections suggest. Even putatively ultrashort, nonthermal ps pulses cause material modifications which might be facilitating recombination losses and thus limiting solar cell efficiencies. For ps laser patterning the affected area is even larger than for the ns laser patterning, due to a modification of the band structure and to thermal decomposition. Evolving subpeaks at the low energy tails are found to originate from Cu-related flat defect levels, i.e., Cu vacancies ( $V_{{\rm{Cu}}}$ ) and antisites (Cu In), created upon laser impact. These findings provide insights into laser-based material modification and provide beneficial information for minimizing the dead area resulting from laser-based monolithic interconnection.

Combining the emerging perovskite solar cell technology with existing silicon approaches in a tandem cell design offers the possibility for new low-cost high-performance devices. In this study, the potential of liquid phase crystallized silicon (LPC-Si) solar cells as a bottom cell in an all-thin-film tandem device is investigated. By optimizing the current output of a four terminal tandem using optical simulations and state-of-the-art electrical properties of the top and bottom cells, we show that an efficiency of 23.3 $\%$ can be reached, where 7.2 $\%$ are attributed to the LPC-Si bottom cell. Including the potential of future developments of both sub cells, efficiencies of over 28 $\%$ are estimated. Electrical and optical measurements of the bottom cell are performed by attaching a perovskite and a cutoff filter to the front side of the interdigitated back contacted LPC-Si cells. The measurements using a cutoff filter show a high impact of the filtered incident light spectrum on the open circuit voltage of the LPC-Si cell. A comparison of the simulated and measured absorptance shows that especially the optical properties of the transparent conductive oxides and recombination losses in the LPC-Si cause high current losses. Combining the measured data of the filtered LPC-Si cells and the semitransparent perovskite cells, yields a realistic estimation for the efficiency of a state-of-the-art four-terminal tandem device of 19.3 $\%$ .

In this contribution, the impact of thermal stress on Cu(In,Ga)Se $_{2}$ (CIGSe) thin film photovoltaic devices is investigated. The tolerance of such devices to high temperatures is of particular interest for processing transparent conductive oxides (TCOs) in order to further close the gap to the theoretical efficiency limit and for their potential use as bottom devices in tandem applications in order to overcome the theoretical efficiency limit of single junction solar cells. When CdS-buffered CIGSe high efficiency solar cells are subjected to thermal stress, elemental interdiffusion of Na and Cd between the absorber and the window layers as well as chemical reactions at the CIGSe/CdS interface result in a degraded power conversion efficiency (PCE). Here, we compare the degradation mechanisms of CdS and GaO $_{x}$ buffered CIGSe solar cells under thermal stress. A model explaining the observed degradation behaviors is proposed.

Abstract This paper addresses the achievements and challenges in a-Si:H/µc-Si:H tandem solar cell technology including production aspects. As an example we show figures of the module production at Masdar PV, with module conversion efficiencies reaching 10%. The process integration was supported by PVcomB, namely, by developing improved PECVD processes and transferring them to Masdar PV. The important role of the TCO substrate for producing high-efficiency modules at lowest possible costs is discussed. We show the results of tandem cells on thermally annealed ZnO:Al substrates, resulting in an improvement of 0.4–0.5% (abs) as a result of the TCO annealing, reaching a stable efficiency of 12.1% for a 1 cm² solar cell and 11.6% for a mini module. The challenges in developing thin film silicon solar modules with higher efficiency than presently reached are discussed.

This paper reports the in situ application, for the first time, of simultaneous multispecies, multipoint near-infrared diode laser absorption spectroscopy (NIRDLAS) to the deposition of thin films by atmospheric pressure chemical vapour deposition (APCVD). Hydrogen fluoride as a precursor and hydrogen chloride as a reaction product were detected in tin oxide thin film deposition in a roll-to-roll pilot reactor. The process is commercially important, and we demonstrate results from the application of the technology to the production scale manufacturing process for photovoltaic coatings.