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1. Spectroscopic Ellipsometry Investigation of CuInSe2 as a Narrow Bandgap Component of Thin Film Tandem Solar Cells

   https://doi.org/10.1109/PVSC.2018.8548177  

   Sapkota, Dhurba R. ; Koirala, Prakash ; Pradhan, Puja ; Shrestha, Niraj ; Junda, Maxwell M. ; Phillips, Adam B. ; Ellingson, Randy J. ; Heben, Michael J. ; Marsillac, Sylvain ; Podraza, Nikolas J. ; et al ( June 2018 , 2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC))

   Spectroscopic ellipsometry (SE) was performed on CuIn Se 2 (CIS) thin films and solar cells with a goal toward optimizing this low bandgap absorber for tandem applications. The CIS thin films and the absorbers in devices were deposited by one-stage thermal co-evaporation on silicon and on Mo-coated soda-lime glass substrates in a deposition system that has yielded CuIn 1-x Ga x Se 2 (CIGS) cells with > 17% efficiency using standard thickness (2.0 μm)x = 0.3 absorbers and > 13% using 0.7 μm low-Ga absorbers. In this study, a mapping capability for CIS Cu stoichiometry y = [Cu]/[In] over the film area was established based on a y-dependent parametric dielectric function (ε 1 , ε 2 ) with bandgap critical point E g decreasing linearly from 1.030 eV for y = 0.7 to 1.016 eV for y = 1.1. In addition, a full set of (ε 1 , ε 2 ) spectra measured for the CIS cell components enables analysis of SE data in terms of an accurate structural model for the device. With this model, spectra in the external quantum efficiency can be predicted, and deviations from this prediction can be attributed to incomplete collection of photogenerated electrons and holes as simulated with a carrier collection profile. 

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2. High‐Performance Ternary Perovskite–Organic Solar Cells

   https://doi.org/10.1002/adma.202109348  

   Zhu, Tao ; Shen, Lening ; Xun, Sangni ; Sarmiento, Julio S. ; Yang, Yongrui ; Zheng, Luyao ; Li, Hong ; Wang, He ; Bredas, Jean‐Luc ; Gong, Xiong ( February 2022 , Advanced Materials)

   Perovskite solar cells in which 2D perovskites are incorporated within a 3D perovskite network exhibit improved stability with respect to purely 3D systems, but lower record power conversion efficiencies (PCEs). Here, a breakthrough is reported in achieving enhanced PCEs, increased stability, and suppressed photocurrent hysteresis by incorporating n‐type, low‐optical‐gap conjugated organic molecules into 2D:3D mixed perovskite composites. The resulting ternary perovskite–organic composites display extended absorption in the near‐infrared region, improved film morphology, enlarged crystallinity, balanced charge transport, efficient photoinduced charge transfer, and suppressed counter‐ion movement. As a result, the ternary perovskite–organic solar cells exhibit PCEs over 23%, which are among the best PCEs for perovskite solar cells with p–i–n device structure. Moreover, the ternary perovskite–organic solar cells possess dramatically enhanced stability and diminished photocurrent hysteresis. All these results demonstrate that the strategy of exploiting ternary perovskite–organic composite thin films provides a facile way to realize high‐performance perovskite solar cells.

    

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3. Environmental analysis of perovskites and other relevant solar cell technologies in a tandem configuration

   https://doi.org/10.1039/c7ee01650f  

   Celik, Ilke ; Phillips, Adam B. ; Song, Zhaoning ; Yan, Yanfa ; Ellingson, Randy J. ; Heben, Michael J. ; Apul, Defne ( January 2017 , Energy & Environmental Science)

   Future high performance PV devices are expected to be tandem cells consisting of a low bandgap bottom cell and a high bandgap top cell. In this study, we developed a cradle-to-end of use life cycle assessment model to evaluate the environmental impacts, primary energy demand (PED), and energy payback time (EPBT) of four integrated two-terminal tandem solar cells composed of either Si bottom and lead-based perovskite (PK Pb ) top cells (Si/PK Pb ), copper indium gallium selenide (CIGS) and PK Pb (CIGS/PK Pb ), copper zinc tin selenide (CZTS) and PK Pb (CZTS/PK Pb ), or tin-lead based perovskite (PK Sn,Pb ) and PK Pb (PK Sn,Pb /PK Pb ). Environmental impacts from single junction Si solar cells were used as a reference point to interpret the results. We found that the environmental impacts for a 1 m 2 area of a cell were largely determined by the bottom cell impacts and ranged from 50% (CZTS/PK Pb ) to 120% of those of a Si cell. The ITO layer used in Si/PK Pb , CZTS/PK Pb , and PK Sn,Pb /PK Pb is the most impactful after the Si and CIGS absorbers, and contributed up to 70% (in PK Sn,Pb /PK Pb ) of the total impacts for these tandem PVs. Manufacturing a single two-terminal device was found to be a more environmentally friendly option than manufacturing two constituent single-junction cells and can reduce the environmental impacts by 30% due to the exclusion of extra glass, encapsulation, front contact and back contact layers. PED analysis indicated that PK Sn,Pb /PK Pb manufacturing has the least energy-intensive processing, and the EPBTs of Si/PK Pb , CIGS/PK Pb , CZTS/PK Pb , and PK Sn,Pb /PK Pb tandems were found to be ∼13, ∼7, ∼2, and ∼1 months, respectively. On an impacts per kW h of Si basis the environmental impacts of all the devices were much higher (up to ∼10 times). These results can be attributed to the low photoconversion efficiency (PCE) and short lifetime that were assumed. While PK Sn,Pb /PK Pb has higher impacts than Si based on current low PCE (21%) and short lifetime (5 years) assumptions, it can outperform Si if its lifetime and PCE reach 16 years and 30%, respectively. Among the configurations considered, the PK Sn,Pb /PK Pb structure has the potential to be the most environmentally friendly technology. 

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4. Hybrid Perovskite‐Organic Flexible Tandem Solar Cell Enabling Highly Efficient Electrocatalysis Overall Water Splitting

   https://doi.org/10.1002/aenm.202000361  

   Li, Zhen ; Wu, Shengfan ; Zhang, Jie ; Lee, Ka Chun ; Lei, Hang ; Lin, Francis ; Wang, Zilong ; Zhu, Zonglong ; Jen, Alex K. Y. ( March 2020 , Advanced Energy Materials)

   Perovskite‐organic tandem solar cells are attracting more attention due to their potential for highly efficient and flexible photovoltaic device. In this work, efficient perovskite‐organic monolithic tandem solar cells integrating the wide bandgap perovskite (1.74 eV) and low bandgap organic active PBDB‐T:SN6IC‐4F (1.30 eV) layer, which serve as the top and bottom subcell, respectively, are developed. The resulting perovskite‐organic tandem solar cells with passivated wide‐bandgap perovskite show a remarkable power conversion efficiency (PCE) of 15.13%, with an open‐circuit voltage (V_oc) of 1.85 V, a short‐circuit photocurrent (J_sc) of 11.52 mA cm⁻², and a fill factor (FF) of 70.98%. Thanks to the advantages of low temperature fabrication processes and the flexibility properties of the device, a flexible tandem solar cell which obtain a PCE of 13.61%, withV_ocof 1.80 V,J_scof 11.07 mA cm⁻², and FF of 68.31% is fabricated. Moreover, to demonstrate the achieved highV_ocin the tandem solar cells for potential applications, a photovoltaic (PV)‐driven electrolysis system combing the tandem solar cell and water splitting electrocatalysis is assembled. The integrated device demonstrates a solar‐to‐hydrogen efficiency of 12.30% and 11.21% for rigid, and flexible perovskite‐organic tandem solar cell based PV‐driven electrolysis systems, respectively.

    

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5. Efficiency enhancement of ultrathin CIGS solar cells by optimal bandgap grading. Part II: finite-difference algorithm and double-layer antireflection coatings

   https://doi.org/10.1364/AO.474920  

   Ahmad, Faiz ; Civiletti, Benjamin J. ; Monk, Peter B. ; Lakhtakia, Akhlesh ( November 2022 , Applied Optics)

   In Part I [Appl. Opt.58,6067(2019)APOPAI003-693510.1364/AO.58.006067], we used a coupled optoelectronic model to optimize a thin-film${\mathrm{C}\mathrm{u}\mathrm{I}\mathrm{n}}_{1-<\#comment/>\mathrm{\xi <\#comment/>}}{\mathrm{G}\mathrm{a}}_{\mathrm{\xi <\#comment/>}}{\mathrm{S}\mathrm{e}}_2$(CIGS) solar cell with a graded-bandgap photon-absorbing layer and a periodically corrugated backreflector. The increase in efficiency due to the periodic corrugation was found to be tiny and that, too, only for very thin CIGS layers. Also, it was predicted that linear bandgap-grading enhances the efficiency of the CIGS solar cells. However, a significant improvement in solar cell efficiency was found using a nonlinearly (sinusoidally) graded-bandgap CIGS photon-absorbing layer. The optoelectronic model comprised two submodels: optical and electrical. The electrical submodel applied the hybridizable discontinuous Galerkin (HDG) scheme directly to equations for the drift and diffusion of charge carriers. As our HDG scheme sometimes fails due to negative carrier densities arising during the solution process, we devised a new, to the best of our knowledge, computational scheme using the finite-difference method, which also reduces the overall computational cost of optimization. An unfortunate normalization error in the electrical submodel in Part I came to light. This normalization error did not change the overall conclusions reported in Part I; however, some specifics did change. The new algorithm for the electrical submodel is reported here along with updated numerical results. We re-optimized the solar cells containing a CIGS photon-absorbing layer with either (i) a homogeneous bandgap, (ii) a linearly graded bandgap, or (iii) a nonlinearly graded bandgap. Considering the meager increase in efficiency with the periodic corrugation and additional complexity in the fabrication process, we opted for a flat backreflector. The new algorithm is significantly faster than the previous algorithm. Our new results confirm efficiency enhancement of 84% (resp. 63%) when the thickness of the CIGS layer is 600 nm (resp. 2200 nm), similarly to Part I. A hundredfold concentration of sunlight can increase the efficiency by an additional 27%. Finally, the currently used 110-nm-thick layer of${\mathrm{M}\mathrm{g}\mathrm{F}}_2$performs almost as well as optimal single- and double-layer antireflection coatings.

    

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