Ultrathin Cu(In,Ga)Se2 solar cells are a promising way to reduce costs and to increase the electrical performance of thin film solar cells. An optical lithography process that can produce sub‐micrometer contacts in a SiO2 passivation layer at the CIGS rear contact is developed in this work. Furthermore, an optimization of the patterning dimensions reveals constrains over the features sizes. High passivation areas of the rear contact are needed to passivate the CIGS interface so that high performing solar cells can be obtained. However, these dimensions should not be achieved by using long distances between the contacts as they lead to poor electrical performance due to poor carrier extraction. This study expands the choice of passivation materials already known for ultrathin solar cells and its fabrication techniques.

Herein, it is demonstrated, by using industrial techniques, that a passivation layer with nanocontacts based on silicon oxide (SiOx) leads to significant improvements in the optoelectronical performance of ultrathin Cu(In,Ga)Se2 (CIGS) solar cells. Two approaches are applied for contact patterning of the passivation layer: point contacts and line contacts. For two CIGS growth conditions, 550 and 500 °C, the SiOx passivation layer demonstrates positive passivation properties, which are supported by electrical simulations. Such positive effects lead to an increase in the light to power conversion efficiency value of 2.6% (absolute value) for passivated devices compared with a nonpassivated reference device. Strikingly, both passivation architectures present similar efficiency values. However, there is a trade‐off between passivation effect and charge extraction, as demonstrated by the trade‐off between open‐circuit voltage (Voc) and short‐circuit current density (Jsc) compared with fill factor (FF). For the first time, a fully industrial upscalable process combining SiOx as rear passivation layer deposited by chemical vapor deposition, with photolithography for line contacts, yields promising results toward high‐performance and low‐cost ultrathin CIGS solar cells with champion devices reaching efficiency values of 12%, demonstrating the potential of SiOx as a passivation material for energy conversion devices.

Currently, one of the main limitations in ultrathin Cu(In,Ga)Se2 (CIGS) solar cells are the optical losses, since the absorber layer is thinner than the light optical path. Hence, light management, including rear optical reflection and light trapping is needed. In this work we focus on increasing the rear optical reflection. For this, a novel structure based on having a metal interlayer in between the Mo rear contact and the rear passivation layer is presented. In total, eight different metallic interlayers are compared. For the whole series, the passivation layer is aluminum oxide (Al2O3). The interlayers are used to enhance the reflectivity of the rear contact and thereby increasing the amount of light reflected back into the absorber. In order to understand the effects of the interlayer in the solar cell performance both from optical and/or electrical point of view, optical simulations were performed together with fabrication and electrical measurements. Optical simulations results are compared with current density-voltage (J-V) behavior and external quantum efficiency (EQE) measurements. A detailed comparison between all the interlayers is done, in order to identify the material with the greatest potential to be used as rear reflective layer for ultrathin CIGS solar cells and to establish fabrication challenges. The Ti-W alloy is a promising rear reflective layer since it provides solar cells with light to power conversion efficiency values of 9.9 %, which is 2.2 % (abs) higher than the passivated ultrathin sample and 3.7 % (abs) higher than the unpassivated ultrathin reference sample.

In recent years, the strategies used to break the Cu(In,Ga)Se2 (CIGS) world record of light to power conversion efficiency, were based on improvements of the absorber optoelectronic and crystalline properties, mainly using complex post-deposition treatments. To reach even higher efficiency values, advances in the solar cell architecture are needed focusing in the CIGS interfaces. In this study, we evaluate the structural, morphological and optoelectronic impact on the CIGS properties of using an Al2O3 layer as a potential front passivation layer. The impact of Al2O3 tunnelling layer between CIGS and CdS is also addressed in this study. Morphological and structural analyses reveal that the use of Al2O3 alone is not detrimental to CIGS, although it does not resist to the CdS chemical bath deposition. When CdS is deposited on top of Al2O3, the CIGS optoelectronic properties are heavily degraded. Nonetheless, when Al2O3 is used alone, optoelectronic measurements reveal a positive impact of its inclusion such as a very low concentration of interface defects and the CIGS keeping the same recombination channels. With the findings of this study the best use of Al2O3 front passivation layer could be with alternative buffer layers. The Al2O3 layer will keep the CIGS surface with a low density of defects while keeping its structural and optoelectronic properties as good as the ones when CdS is deposited. It can also be reported that a comparison between the different analyses allowed us to strongly suggest for the first time that low-energy muon spin spectroscopy (LE-μSR) is sensitive to both charge carrier separation and bulk recombination in complex semiconductors.

The incorporation of nanostructures in optoelectronic devices for enhancing their optical performance is widely studied. However, several problems related to the processing complexity and the low performance of the nanostructures have hindered such actions in real‐life devices. Herein, a novel way of introducing gold nanoparticles in a solar cell structure is proposed in which the nanostructures are encapsulated with a dielectric layer, shielding them from high temperatures and harsh growth processing conditions of the remaining device. Through optical simulations, an enhancement of the effective optical path length of approximately four times the nominal thickness of the absorber layer is verified with the new architecture. Furthermore, the proposed concept in a Cu(In,Ga)Se2 solar cell device is demonstrated, where the short‐circuit current density is increased by 17.4%. The novel structure presented in this work is achieved by combining a bottom‐up chemical approach of depositing the nanostructures with a top‐down photolithographic process, which allows for an electrical contact.

A novel architecture that comprises rear interface passivation and increased rear optical reflection is presented with the following advantages: i) an enhanced optical reflection is achieved by depositing a metallic layer over the Mo rear contact; ii) the addition of a sputtered Al2O3 layer improves the interface quality with CIGS; and, iii) the rear-openings are refilled with Mo to maintain the optimal ohmic electrical contact as generally observed from the growth of CIGS on Mo. Hence, a decoupling between the electrical function and the optical function of the substrate is achieved. We present in detail the manufacturing procedure of such type of architectures together with its benefits and caveats. A preliminary electrical analysis of resulting solar cells showing a proof-of-concept of the architecture is presented and discussed.

On the importance of joint mitigation strategies for front, bulk and rear recombination in ultrathin Cu(In,Ga)Se2 solar cells. ACS Appl. Mater. Interfaces 2021, which has been published in final form at

One of the trends making its way through the Photovoltaics (PV) industry, is the search for new application possibilities. Cu(In,Ga)Se2 (CIGS) thin film solar cells stand out due to their class leading power conversion efficiency of 23.35 %, flexibility, and low cost. The use of sub-µm ultrathin CIGS solar cells has been gaining prevalence, due to the reduction in material consumption and the manufacturing time. Precise CIGS finite-difference time-domain (FDTD) and 3D-drift diffusion baseline models were developed for the Lumerical suite and a 1D electrical model for SCAPS, allowing for an accurate description of the optoelectronic behavior and response of thin and ultrathin CIGS solar cells. As a result, it was possible to obtain accurate descriptions of the optoelectronic behavior of thin and ultrathin solar cells, and to perform an optical study and optimization of novel light management approaches, such as, random texturization, photonic nanostructures, plasmonic nanoparticles, among others. The developed light management architectures enabled to push the optical performance of an ultrathin solar cell and even surpass the performance of a thin film solar cell, enabling a short-circuit current enhancement of 6.15 mA/cm 2 over an ultrathin reference device, without any light management integrated.