COMMUNICATION © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (1 of 6) 1500799 wileyonlinelibrary.com Monolithic Perovskite-CIGS Tandem Solar Cells via In Situ Band Gap Engineering Teodor Todorov,* Talia Gershon, Oki Gunawan, Yun Seog Lee, Charles Sturdevant, Liang-Yi Chang, and Supratik Guha* Dr. T. Todorov, Dr. T. Gershon, Dr. O. Gunawan, Dr. Y. S. Lee, C. Sturdevant, Dr. L.-Y. Chang, Dr. S. Guha IBM T. J. Watson Research Center Yorktown Heights, NY 10598, USA E-mail: tktodoro@us.ibm.com; guha@us.ibm.com DOI: 10.1002/aenm.201500799 lead iodide perovskite top cell on Cu 2 ZnSn(S,Se) 4 kesterite and Si-based bottom cells. [10,12] The remarkable efficiencies of perovskite devices above 15%, the possibility to process at tem- peratures below 150 °C, and the highly tunable band gap range from 1.6 to 2.25 eV make these materials especially attractive for monolithic tandem integration with CIGS. [12–20] Here, we report perovskite-CIGS tandem solar cells in which each cell was customized for monolithic integration in the following sequence: transparent conducting electrode (TCE)/phenyl-C61-butyric acid methyl ester (PCBM)/perovs- kite/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/ITO/CdS/CIGS/Mo/Si 3 N 4 /glass. By modifying our solution-based process, [21] the CIGS absorber band gap was reduced to 1.04 eV for improved photon management. A 30 nm thick ITO was selected as a transparent recombination layer that was deposited directly onto the CdS without the intrinsic ZnO layer commonly used in CIGS devices. The elimination of ZnO from the device structure was critical for achieving func- tional perovskite tandems, as we discovered that the presence of ZnO in proximity to the perovskite layer degrades device per- formance. Perovskite devices with electron-selective ZnO layers have been reported previously, [22–24] however we found that pro- cessing at temperatures above 60 °C resulted in deterioration of the perovskite layer. In contrast, perovskite layers processed on our ZnO-free CIGS structure could withstand annealing treat- ments for several hours at 120 °C without any damage. Considering the importance of precise band gap control in monolithic tandem devices, we designed a reactor for contin- uous in situ monitoring and precise control of the optical prop- erties of the perovskite layer via vapor-based halide exchange reactions, as illustrated in Figure 1a. The system provided temperature and pressure control as well as spectroscopic measurement capability. In a typical process, a sample of spin- coated lead halide PbX 2 (X = I and/or Br) is placed inside the temperature-controlled vacuum chamber and annealed in the presence of methylammonium halide (CH 3 NH 3 X) vapor. A transparent port at the top of the chamber provides an optical path for a broad-spectrum light source to illuminate the sample in the reactor, and a port beneath the sample leads to a spec- trophotometer, which collects the transmission spectra. The real-time feedback is used for precise engineering of the per- ovskite absorber properties by adjusting the temperature, pres- sure, and precursor type. The process was designed to have better compatibility with the CIGS device than previously reported vapor-assisted approaches that employ temperatures in excess of 120 °C and focus on pure iodide perovskite with fixed band gap. [25,26] In our reactor the conversion temperature was Enhanced light harvesting by using two or more absorbers with different band gaps has so far been the most effective approach to improve the performance of photovoltaic (PV) devices beyond the practical limits of single p–n junctions. [1,2] The tandem solar cell concept has been implemented commercially for several PV technologies ranging from the relatively low- performance amorphous silicon to the highest performance III–V group materials. A suitable partner absorber could also benefit the rapidly growing Cu(In,Ga)(S,Se) 2 (CIGS) technology with already highly efficient commercial PV products. [3] If the second absorber could be directly integrated into existing mod- ules, any resultant increase in efficiency would then lead to pro- portional cost savings per Watt. There are two common types of tandem solar cells. [2] The first configuration is the mechanically stacked, consisting of two independent solar cells one on top of the other that due to simplicity is commonly used for preliminary studies. [4,8] The second is the monolithic two-terminal device built in series on a single substrate. This, more practical structure, is the one most commonly used in industry, and preferred from a techno- logical and performance perspective. It contains the minimum number of functional layers and auxiliary elements such as transparent conductive coatings, substrates, and interconnects and therefore has the potential for lowest costs as well as optical and series resistance losses. However, monolithic device inte- gration requires sophisticated simultaneous optimization of a series of elements, including current matching between the devices, development of appropriate recombination layers, and a careful design of the full fabrication sequence to ensure pro- cess compatibility with all layers and interfaces formed during preceding steps. [9,10] Despite the obvious advantages of monolithic tandem devices, achieving high-efficiency monolithic tandems using CIGS absorbers has been challenging due to degradation of the p–n junction at temperatures above 200 °C. The highest reported efficiency of 3.8% was reached through a sophisticated layer transfer of an organic top device. [11] Improved performances were achieved using a monolithically grown methylammonium Adv. Energy Mater. 2015, 5, 1500799 www.MaterialsViews.com www.advenergymat.de