Spectrally resolved photoluminescence is used to measure the band-to-band absorption coefficient α(BB)(ℏω) of organic-inorganic hybrid perovskite methylammonium lead iodide (CH₃NH₃PbI₃) films from 675 to 1400 nm. Unlike other methods used to extract the absorption coefficient, photoluminescence is only affected by band-to-band absorption and is capable of detecting absorption events at very low energy levels. Absorption coefficients as low as 10⁻¹⁴ cm⁻¹ are detected at room temperature for long wavelengths, which is 14 orders of magnitude lower than reported values at shorter wavelengths. The temperature dependence of α(BB)(ℏω) is calculated from the photoluminescence spectra of CH₃NH₃PbI₃ in the temperature range 80-360 K. Based on the temperature-dependent α(BB)(ℏω), the product of the radiative recombination coefficient and square of the intrinsic carrier density, B(T) × n(i)², is also obtained.

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