© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2439 wileyonlinelibrary.com COMMUNICATION Identifying Fundamental Limitations in Halide Perovskite Solar Cells Wei Lin Leong,* Zi-En Ooi, Dharani Sabba, Chenyi Yi, Shaik M. Zakeeruddin, Michael Graetzel, Jeffrey M. Gordon, Eugene A. Katz, and Nripan Mathews DOI: 10.1002/adma.201505480 by defect levels, [12] interfacial states, [13] energetic disorder, [14] as well as first-order [15] and bimolecular recombination. [16] qV oc for perovskite solar cells is typically in the range of 0.9–1.1 eV at room temperature ( q is the elementary charge). The small difference E g - qV oc ≈ 0.5 eV (at standard test conditions of one-sun irradiance, AM 1.5G spectrum and cell temperature T = 298 K) compares favorably to other photovoltaic technolo- gies, being lower than organic or dye-sensitized cells (≈0.7 eV), quantum-dot cells (≈0.8 eV), polycrystalline CdTe (≈0.6 eV), and only slightly higher than crystalline silicon (≈0.4 eV). [17] The small energy loss implicitly indicates a modest degree of both electronic disorder and recombination losses in halide perovskites, remarkable for a low-temperature solution- processed semiconductor. Our focus on the temperature dependence of the cell’s prin- cipal performance parameters is motivated by the need to: i) elucidate the dominant photogeneration and recombination processes, ii) estimate basic upper bounds for the key perfor- mance parameters, [18–20] and iii) be able to evaluate cell perfor- mance at the PV temperatures reached under actual field condi- tions (>330 K). The temperature range studied here (80–360 K) was chosen to be broad enough both to illuminate basic car- rier dynamics and to allow a more complete PV evaluation. Experiments and analyses of this nature are sparse, [21–23] in part due to cell degradation in air that compromises measurement reproducibility, including pronounced hysteresis in curves of current density J as a function of voltage V. Here, we report our measurements of nominally nondegraded mesoporous perovskite cells, performed in an evacuated optical cryostat, with little hysteresis that would not otherwise allow for the equilibrium behavior of the device to be evaluated. [24] We first show the temperature-dependent external quantum effi- ciency (EQE) measurements and extract the effective bandgap energy E g * of the absorber from the sub-bandgap region of the spectral EQE to study the effect of perovskite bandgap changes due to phase transitions on the photovoltaic performance. Subsequently, we analyzed the temperature-dependent device characteristics. Two distinct regimes of the temperature and irradiance dependence of V oc were observed: a linear regime at higher temperature with a logarithmic irradiance ( I) depend- ence; and a saturation regime at lower temperature where V oc is invariant to both irradiance and temperature. We will show that these limits can be understood in terms of recombination in the bulk of the perovskite absorber and that interface recombina- tion plays a minor role. Trap-assisted recombination due to the presence of shallow states in the perovskite absorber is found to be the dominant recombination mechanism. Our results also Organic–inorganic halide perovskite solar cells have quickly become the most efficient of solution-processed photovoltaic (PV) technologies. [1] These cells are able to combine the appealing aspects of both inorganic thin-film and organic pho- tovoltaics. The organometal trihalide perovskite absorbers, such as methylammonium lead halide, CH 3 NH 3 PbX 3 (X = Cl, Br, or I), are highly crystalline even when processed at tempera- tures as low as 70–100 °C. [2] Their favorable PV characteristics include tunability of optoelectronic properties, [3] high optical absorption (≈10 4 cm -1 ), [4] low exciton binding energy, [5,6] long- range charge transport, [7] and efficient charge collection [8] at the contacts. Power conversion efficiencies (PCEs) of 15–21% have already been demonstrated. [9,10] Despite rapid advances in device performance, estab- lishing the fundamental efficiency limits of perovskite cells has remained elusive, which prompts delving into the fun- damental physics governing device operation. Of particular interest is understanding their high open-circuit voltage ( V oc ) relative to their bandgap energy E g ≈ 1.55 eV. [11] V oc reflects basic thermodynamic loss mechanisms and is strongly affected Dr. W. L. Leong, Dr. Z.-E. Ooi Institute of Materials Research and Engineering (IMRE) Agency for Science, Technology and Research (A*STAR) 2 Fusionopolis Way, Innovis #08-03, Singapore 138634, Singapore E-mail: leongwl@imre.a-star.edu.sg Dr. D. Sabba, Prof. N. Mathews Energy Research Institute @NTU (ERI@N) Research Techno Plaza X-Frontier Block, Level 5 50 Nanyang Drive, Singapore 637553, Singapore Dr. C. Yi, Dr. S. M. Zakeeruddin, Prof. M. Graetzel Laboratory of Photonics and Interfaces Ecole Polytechnique Fédérale de Lausanne (EPFL) Lausanne CH 1015, Switzerland Prof. J. M. Gordon, Prof. E. A. Katz Department of Solar Energy and Environmental Physics Jacob Blaustein Institutes for Desert Research Ben-Gurion University of the Negev Sede Boqer Campus 84990, Israel Prof. N. Mathews School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue, Singapore 639798, Singapore Adv. Mater. 2016, 28, 2439–2445 www.advmat.de www.MaterialsViews.com