PHYSICAL REVIEW MATERIALS 6, 085402 (2022) Solution-processed CaMnO 3-δ -based all oxide solar cells with high open-circuit voltage Parul Garg , 1 Sanchari Bhattacharya, 2 Sakal Singla, 1 Priya Kaith , 1 Sanjoy Datta, 2 Biswanath Chakraborty, 1 and Ashok Bera 1 , * 1 Department of Physics, Indian Institute of Technology Jammu, Jammu and Kashmir 181221, India 2 Department of Physics and Astronomy, National Institute of Technology Rourkela, Odisha 769008, India (Received 27 May 2022; revised 8 July 2022; accepted 19 July 2022; published 9 August 2022) Finding a low-cost, stable solar absorber with suitable optical properties is one of the key aspects in promoting solar cell research. Perovskite-structured CaMnO 3 , composed of earth-abundant elements, is an environmentally friendly multifunctional material especially featuring thermoelectricity and G-type antiferromagnetism. In this paper, we explore the potential of CaMnO 3-δ (CMO) as an absorber layer in all oxide solar cells. Solution- processed CMO shows strong light absorption capacity in the wavelength range of 400–750 nm with the estimated optical bandgap of 1.77 eV. First-principles calculations show d -d transitions within the upper Mott- Hubbard bands, and lower Mott-Hubbard bands constitute the electronic bandgap, whereas the p-d transitions between the O 2 p and Mn 3d bands contribute to the optical absorption in the visible region. Furthermore, a gradual increase in the current value was observed under illumination in the photoconductivity measurement. We have designed a CMO absorber-layer-based all oxide solar cell using a conventional mesostructured TiO 2 photo-anode and NiO hole transport layer (HTL). The suitable band alignment of the CMO absorber with both TiO 2 and NiO enables us to achieve an average open-circuit voltage ( V OC ) of 0.92 V with the maximum value of 1.03 V, which is among the highest reported values so far for oxide-based solar cells. Solar cell parameters were also validated using simulations. Our results add a direction in the search for low-cost, stable solar absorber materials. DOI: 10.1103/PhysRevMaterials.6.085402 I. INTRODUCTION Perovskite-structured manganites with the chemical for- mula RMnO 3 , where R is a trivalent rare-earth cation or diva- lent alkaline-earth cation (R = La, Ba, Nd, Ca, Sr, Y, Pr, etc.), exhibit unique electric and magnetic properties [1–3] and have been extensively used in many practical applications like mag- netoresistance switching [4], fuel cells [5], and gas sensing [6], to name a few. Ca and Mn are present at the 5th and 12th positions [7,8] in the most earth-abundant metal list, making CaMnO 3 one of the cheapest materials among the existing sustainable manganites. The high negative Seebeck coefficient (S ∼−350 μVK −1 ) makes it a promising thermoelectric ma- terial [9]. Recently, CaMnO 3 has been used in energy storage [10,11], as an electrocatalyst [12], and as a buffer layer in solar cells [13], confirming its multifunctionality. In general, oxides are environmentally stable and exten- sively used as electron transport layer (ETL) or hole transport layer (HTL) in solar cells [14–19]. Some oxides have also been used as active absorber layers [20,21]. Complex ox- ides possess bandgap tunability with the change in chemical compositions providing an auxiliary advantage in designing multijunction solar cells or panchromatic solar cells [22–24]. Hence, some perovskite oxides like BaTiO 3 [25], BiFeO 3 [26,27], LaVO 3 [28], KNbO 3 [29,30], and Pb(Zr, Ti)O 3 [31] have been used as active absorber layers in solar cells. Among * ashok.bera@iitjammu.ac.in them, BaTiO 3 , BiFeO 3 and Pb(Zr, Ti)O 3 have large bandgaps, restricting their light absorption capacity to a shorter wave- length range in the solar spectrum, while La and V being expensive, do not provide cost-effective solutions. Environ- mentally friendly n-type semiconducting CaMnO 3 has an experimental direct bandgap of 1.55 [32] to 1.75 eV [33] with high absorption coefficient of 10 5 cm −1 and a fast charge carrier mobility of 0.02–0.05 cm 2 V −1 s −1 [34], making it a promising absorber material in solar cells. In this paper, we have synthesized CaMnO 3-δ (CMO) thin films using the sol-gel spin coating technique and studied their optical and optoelectronic properties. The solution-grown CMO thin film shows a direct optical bandgap of 1.77 eV with an absorption coefficient > 10 4 cm −1 . Density functional theory (DFT) has also been employed to explain the electronic band structure and the origin of light absorption in the vis- ible spectrum. Furthermore, on broadband light illumination, CMO film shows about a 2.5-fold increase in the current value over the dark current. Based on the suitable optical and pho- toconducting nature of CMO, we have fabricated an all oxide solar cell using CMO as an absorber. In a conventional sensi- tized solar cell architecture, with mesoporous TiO 2 (m-TiO 2 ) as an electron transporting layer and NiO as a solid-state HTL, we could achieve a maximum open-circuit voltage ( V OC ) of 1.03 V with an average of 0.92 V, which is among the highest reported V OC for all oxide-based solar cells. To validate our results, we have also simulated our device performance using SCAPS software. The increase in the V OC after adding a NiO HTL mainly originated due to the suitable band alignment of 2475-9953/2022/6(8)/085402(8) 085402-1 ©2022 American Physical Society