Ultrawide Spectral Response of CIGS Solar Cells Integrated with Luminescent Down-Shifting Quantum Dots

Effects of Alkali Elements on Copper Indium Gallium Aluminum Selenide Flexible Solar Cells Fabricated on Polyimide Substrates

Polycrystalline Cu(InGaAl)Se2 (CIGAS) thin films were prepared on polyimide (PI) foils by depositing aluminum (Al) and CIGS precursor layers. Three ceramic CIGS quaternary targets with different sodium (Na) contents were used for investigating the influences of alkali doping at an annealing temperature of 500 °C. The Al concentration was enriched at the front interfaces of absorber films with different Na doping amounts after annealing. Na in the precursor layer diffused to both interfaces during the annealing process, most Na diffused into the Mo layer, and Na existed in the annealed film as compounds Na2Sex and Na2SeO3. An appropriate amount Na element could be beneficial for grain growth in the region beneath the surface. Low Na doping had no significant effect on the crystallization property. High Na doping effected the diffusion of the Cu2-xSe liquid phase and reduced the grain size. On the basis of good crystallization, the passivation effect of Na can effectively improve the performance of cells. A certified power conversion efficiency of 16.19% of a CIGAS flexible solar cell with a 0.54 cm2 active area has been achieved.

Colored Silicon Heterojunction Solar Cells Exceeding 23.5% Efficiency Enabled by Luminescent Down-Shift Quantum Dots

Colored solar panels, realized by depositing various reflection layers or structures, are emerging as power sources for building with visual aesthetics. However, these panels suffer from reduced photocurrent generation due to the less efficient light harvesting from visible light reflection and degraded power conversion efficiency (PCE). Here, color-patterned silicon heterojunction solar cells are achieved by incorporating luminescent quantum dots (QDs) with high quantum yields as light converters to realize an asthenic appearance with high PCE. It is found that large bandgap (blue) QD layers can convert UV light into visible light, which can notably alleviate the parasitic absorption by the front indium tin oxide and doped amorphous silicon. Additionally, a universal optical path model is proposed to understand the light transmission process, which is suitable for luminescent down-shift devices. In this study, solar cells with a PCE exceeding 23.5% are achieved using the combination of a blue QD layer and a top low refractive index anti-reflection layer. Based on our best knoledge,the obtained PCE is the highest for a color-patterned solar cell. The results suggest an enhanced strategy involving incorporation of luminescent QDs with an optical path design for high-performance photovoltaic panels with visual aesthetics.

Recent Advances in Colloidal Quantum Dots or Perovskite Quantum Dots as a Luminescent Downshifting Layer Embedded on Solar Cells

The solar cell has a poor spectral response in the UV region, which affects its power conversion efficiency (PCE). The utilization of a luminescent downshifting (LDS) layer has been suggested to improve the spectral response of the photovoltaics in the short wavelength region through photoluminescence (PL) conversion and antireflection effects, which then enhance the PCE of the solar cell. Recently, colloidal quantum dots (CQDs) or perovskite quantum dots (PQDs) have been gaining prime importance as an LDS material due to their eminent optical characteristics, such as their wide absorption band, adjustable visible emission, short PL lifetime, and near-unity quantum yields. However, the instability of QDs that occurs under certain air, heat, and moisture conditions limits its commercialization. Thus, in this review, we will focus on the physical and optical characteristics of QDs. Further, we will discuss different synthesis approaches and the stability issues of QDs. Different approaches to improve the stability of QDs will be discussed in detail alongside the recent breakthroughs in QD-based solar cells for various applications and their current challenges. We expect that this review will provide an effective gateway for researchers to fabricate LDS-layer-based solar cells.

Improving Ultraviolet Responses in Cu2ZnSn(S,Se)4 Thin-Film Solar Cells Using Quantum Dot-Based Luminescent Down-Shifting Layer

Quantum dot (QD)-based luminescent down-shifting (LDS) layers were deposited on Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells via the drop-casting method. The LDS layers can easily widen the narrow absorption wavelength regions of single-junction solar cells and enable improvement of the short-circuit current. The optical properties of LDS layers deposited on glass and containing different QD contents were analyzed based on their transmittance, reflectance, and absorbance. The absorber films to be used in the CZTSSe solar cells were determined by X-ray diffraction measurements and Raman spectroscopy to determine their crystal structures and secondary phases, respectively. The completed CZTSSe solar cells with LDS layers showed increased ultraviolet responses of up to 25% because of wavelength conversion by the QDs. In addition, the impact of the capping layer, which was formed to protect the QDs from oxygen and moisture, on the solar cell performance was analyzed. Thus, a maximal conversion efficiency of 7.3% was achieved with the 1.0 mL QD condition; furthermore, to the best of our knowledge, this is the first time that LDS layers have been experimentally demonstrated for CZTSSe solar cells.

Cu(In,Ga)Se2 Solar Cells Integrated with Subwavelength Structured Cover Glass Fabricated by One-Step Self-Masked Etching

We report an anti-reflective cover glass for Cu(In,Ga)Se₂ (CIGS) thin film solar cells. Subwavelength structures (SWSs) were fabricated on top of a cover glass using one-step self-masked etching. The etching method resulted in dense whiskers with high aspect ratio. The produced structure exhibited excellent anti-reflective properties over a broad wavelength range, from the ultraviolet to the near infrared. Compared to a flat-surface glass, the average transmittance of the glass integrated with the SWSs improved from 92.4% to 95.2%. When the cover glass integrated with the SWSs was mounted onto the top of a CIGS device, the short-circuit current and the efficiency of the solar cell were enhanced by 4.38 and 6%, respectively, compared with a CIGS solar cell without cover glass.

Luminescent down-shifting CsPbBr3 perovskite nanocrystals for flexible Cu(In,Ga)Se2 solar cells

To overcome the parasitic absorption of ultraviolet (UV) light in the transparent conductive oxide (TCO) layer of flexible Cu(In,Ga)Se2 (CIGS) thin film solar cells, a CsPbBr3 perovskite nanocrystal based luminescent down-shifting (LDS) layer was integrated on CIGS solar cells fabricated on a stainless steel foil. The CsPbBr3 perovskite nanocrystal absorbs solar irradiation at wavelengths shorter than 520 nm and emits photons at a wavelength of 532 nm. These down-shifted photons pass the TCO layer without parasitic absorption and are absorbed in the CIGS absorber layer where they generate photocurrent. By minimizing the parasitic absorption in the TCO layer, the external quantum efficiency (EQE) of the CIGS solar cell with the CsPbBr3 perovskite nanocrystal layer is highly improved in the UV wavelength range between 300 and 390 nm. Additionally, in the wavelength range between 500 and 1100 nm, the EQE is improved since the surface reflectance of the CIGS device with the CsPbBr3 perovskite LDS layer was reduced. This is because the CsPbBr3 perovskite nanocrystal layer, which has an effective refractive index of 1.82 at a wavelength of 800 nm, reduces the large refractive index mismatch between air (nair = 1.00) and the TCO layer (nZnO = 1.96 at a wavelength of 800 nm). Both the short circuit current density and power conversion efficiency of the flexible CIGS solar cell integrated with the CsPbBr3 perovskite are improved by 4.5% compared with the conventional CIGS solar cell without the CsPbBr3 perovskite LDS layer.

Enhanced efficiency of Cu2ZnSn(S,Se)4 solar cells via anti-reflectance properties and surface passivation by atomic layer deposited aluminum oxide

Reducing interface recombination losses is one of the major challenges in developing Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells. Here, we propose a CZTSSe solar cell with an atomic layer deposited Al₂O₃ thin film for surface passivation. The influence of passivation layer thickness on the power conversion efficiency (PCE), short-circuit current density (J_sc), open-circuit voltage (V_oc) and fill factor (FF) of the solar cell is systematically investigated. It is found that the Al₂O₃ film presents notable antireflection (AR) properties over a broad range of wavelengths (350–1000 nm) for CZTSSe solar cells. With increasing Al₂O₃ thickness (1–10 nm), the average reflectance of the CZTSSe film decreases from 12.9% to 9.6%, compared with the average reflectance of 13.6% for the CZTSSe film without Al₂O₃. The Al₂O₃ passivation layer also contributes to suppressed surface recombination and enhanced carrier separation. Passivation performance is related to chemical and field effect passivation, which is due to released H atoms from the Al–OH bonds and the formation of Al vacancies and O interstitials within Al₂O₃ films. Therefore, the J_sc and V_oc of the CZTSSe solar cell with 2 nm-Al₂O₃ were increased by 37.8% and 57.8%, respectively, in comparison with those of the unpassivated sample. An optimal CZTSSe solar cell was obtained with a V_oc, J_sc and η of 0.361 V, 33.78 mA and 5.66%. Our results indicate that Al₂O₃ films show the dual functions of AR and surface passivation for photovoltaic applications.

ALD-Al₂O₃ is used as a passivation layer in a CZTSSe device and optimal device parameters are obtained by precisely controlling Al₂O₃ thickness.