Ye-Chan
Kim
a,
Ho-Jung
Jeong
b,
Sung-Tae
Kim
a,
Young Hyun
Song
b,
Bo Young
Kim
b,
Jae Pil
Kim
b,
Bong Kyun
Kang
c,
Ju-Hyung
Yun
d and
Jae-Hyung
Jang
*a
aSchool of Electrical Engineering and Computer Science, Gwangju Institute of Science and Technology (GIST), Cheomdangwagi-ro, Buk-gu, Gwangju, 61005, Republic of Korea. E-mail: yckim@gist.ac.kr
bLighting Materials and Components Research Center, Korea Photonics Technology Institute (KOPTI), Gwangju 61007, Republic of Korea. E-mail: hojung@kopti.re.kr
cNano Materials and Components Research Center, Korea Electronics Technology Institute, Seongnam 463-816, Republic of Korea
dDepartment of Electrical Engineering, Incheon National University, Yeonsu-gu, Incheon 406-772, Republic of Korea
First published on 28th November 2019
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.
Another approach to prevent UV light absorption in the TCO layer is to integrate luminescent down-shifting (LDS) materials on top of the solar cells.15,16 The schematic designs of the conventional CIGS solar cells and the ones integrated with the LDS layer are shown in Fig. 1(a) and (b), respectively. The LDS layer absorbs high energy photons and emits lower energy photons which pass through the TCO layer and reach the CIGS absorption layer. The low energy photons are absorbed in the absorber layer and contribute to photocurrent generation.15 Quantum dots based on Cd or Cu, organic dyes, and rare earth elements have been investigated for LDS materials.15,17 However, these LDS materials have drawbacks, such as the high material cost and complex fabrication process. Low cost LDS material synthesis processes and a simple integration process still require investigation.
The CsPbX3 (X = Cl, Br, I) perovskite nanocrystal is one of the candidates for low cost LDS materials because it is easy to synthesize using a solution process. It also provides a couple of advantages, such as tunability of the emission wavelength from the visible to the infrared region by controlling its composition,18,19 and narrow band emission characteristics that prevent the re-absorption loss of the emitted photons.20,21 Given these excellent optical properties, CsPbX3 perovskite nanocrystals deserve to be applied as the LDS layer. Recently, Donglei Zhou et al. achieved highly improved power conversion efficiency (PCE) by integrating a CsPbBr3 perovskite based LDS layer on a Si solar cell.22 However, it required rare earth dopants such as Ce and Yb. It is also difficult to use in CIGS solar cells because its emissions, centered at 450 nm, are absorbed in the CdS buffer layer.
In this work, pure CsPbBr3 perovskite nanocrystals were integrated as the LDS layer on flexible CIGS solar cells to overcome the parasitic absorption of UV light in the TCO layer. The structural and optical characteristics of the CsPbBr3 perovskite nanocrystals were analysed and the device performances of the CIGS solar cell with and without the CsPbBr3 perovskite LDS layer were compared, to study the effect of the CsPbBr3 perovskite LDS layer.
The flexible CIGS solar cells were fabricated on top of 100 μm thick stainless steel (STS) substrates. A 300 nm thick Na-doped Mo (Mo:Na) and 1 μm thick Mo were deposited by dc sputtering in sequence. The CIGS absorber layer was grown on Mo:Na/Mo coated STS by the three stage co-evaporation process.4 Details of the CIGS growth and device fabrication processes are reported elsewhere.15 As shown in Fig. 1(a), the completed CIGS device has a Al–Ni metal grid/ZnO:Al–ZnO window layer/CdS buffer layer/CIGS absorber layer/Mo based back contact/STS substrate (1.0/0.4/0.06/2.0/1.3/100 μm) structure.
To investigate the effects of the CsPbBr3 based LDS layer on the photovoltaic performance of the devices, a 730 nm thick CsPbBr3 perovskite LDS layer was deposited on top of the CIGS devices by spin coating at 2000 rpm for 30 seconds. Afterwards, to evaporate the remaining toluene solvent, the coated samples were annealed at 80 °C for 10 min on a hot plate.
The morphology and crystallinity of the CsPbBr3 perovskite nanocrystals were characterized using scanning electron microscopy (SEM, S-4700, Hitachi) and transmission electron microscopy (TEM, JEM-2100F, JEOL) with an accelerating voltage of 200 kV. The absorbance and photoluminescence (PL) were investigated by using a UV-VIS-IR spectrophotometer (Cary-5000, Varian) and PL spectroscope (RPM 2000, ACCENT) equipped with a He–Cd laser (325 nm), respectively. The external quantum efficiency (EQE) of the solar cells was measured with an incident photon to electron conversion efficiency measurement system (QEX7, PV measurement) calibrated by certified reference Si and Ge photodiodes. The current density–voltage (J–V) characteristics of the CIGS solar cells were measured with an Air Mass 1.5G solar simulator (Oriel Sol3A Class AAA, Newport) with an irradiation intensity of 100 W m−2.
The absorbance and PL spectra are shown in Fig. 3. The CsPbBr3 perovskite nanocrystals absorb UV light ranging from 300 to 520 nm and emit photons with a wavelength of 532 nm. The emitted photons are transmitted to the CIGS absorber layer and generate photocurrent. The measured full width at half maximum (FWHM) of 25.3 nm in the emission spectrum is much narrower than those of the other LDS materials.23 Furthermore, the transmittance is greater than 90% in the wavelength range between 520 and 1200 nm (Fig. S2†). However, it decreases abruptly at wavelengths shorter than 520 nm due to light absorption by the CsPbBr3 perovskite. Photographs taken under UV light with a wavelength of 325 nm are shown in the inset of Fig. 3. Under UV illumination, a green light emission is observed in the CIGS solar cell integrated with the CsPbBr3 perovskite, whereas there is no PL on the conventional CIGS solar cell without the perovskite.
To analyse the spectral response, the EQEs of the CIGS devices with and without CsPbBr3 perovskites were characterized, as shown in Fig. 4(a). While the EQE of the conventional CIGS solar cell is nearly zero in the wavelength range between 300 and 340 nm, due to parasitic UV light absorption in the TCO layer, the EQE of the CIGS solar cell integrated with the CsPbBr3 perovskite is highly improved in the wavelength range between 300 and 390 nm (Region 1). It is because the CsPbBr3 perovskite based LDS layer absorbs short wavelength light between 300 and 520 nm and emits visible light with a wavelength of 532 nm. The energy down-converted photons reach the CIGS absorber layer without parasitic absorption in the TCO layer.
Fig. 4 (a) EQE graphs of the flexible CIGS solar cell with and without CsPbBr3 perovskite nanocrystals. (b) ΔEQE spectrum. The inset shows the ΔJsc calculated from eqn (1) in regions 1 and 2. |
In addition, the improved EQE is observed for the CIGS solar cell with the CsPbBr3 perovskite in the wavelength range between 500 and 1100 nm (Region 2), indicating that the LDS layer acts as an anti-reflection coating (ARC) as well. The large refractive index mismatch between the air (nair = 1.00) and TCO layer (nZnO = 1.96 at 800 nm wavelength) was reduced by the CsPbBr3 perovskite layer which has an effective refractive index of 1.82 at a wavelength of 800 nm (Fig. S3†). The surface reflectance of the CIGS device with a CsPbBr3 perovskite LDS layer is much lower than that of the CIGS solar cell without a LDS layer (Fig. S4†)
As shown in Fig. 4(b), the improvement in EQE (ΔEQE) distinctively observed in regions 1 and 2 results from the effects of LDS and ARC, respectively. The current density gain (ΔJsc) calculated from the EQE between λ1 and λ2 can be expressed as
(1) |
The J–V characteristics were measured by using a solar simulator under an irradiation intensity of 100 W m−2. The J–V characteristics of the CIGS solar cells with and without the CsPbBr3 perovskite are shown in Fig. 5, and the photovoltaic performance parameters of the two types of solar cells are shown in the inset of Fig. 5. The open circuit voltage (Voc) and the fill factor are invariant, whereas both short circuit current density (Jsc) and PCE of the CIGS solar cell with the CsPbBr3 perovskite are improved by 4.5% compared with those of the conventional CIGS solar cell without the CsPbBr3 perovskite LDS layer. The flexible CIGS solar cell integrated with the CsPbBr3 perovskite LDS layer exhibits a Jsc of 37.2 mA cm−2, Voc of 0.56 V, fill factor of 56.2% and PCE of 11.6%.
Fig. 5 J–V characteristics of the CIGS solar cells with and without the CsPbBr3 perovskite. The inset table compares photovoltaic performance parameters of the two types of solar cells. |
This work was done at the KOPTI supported by the Korea Institute for Advancement of Technology (KIAT) through the International Cooperative R&D program (P0006844_ development of color conversion nanocrystal luminescence materials for next generation displays).
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nr06041c |
This journal is © The Royal Society of Chemistry 2020 |