Ultralow Dark Current in Near-Infrared Perovskite Photodiodes by Reducing Charge Injection and Interfacial Charge Generation

Solution-processed Pb and Pb-Sn metal-halide perovskite photodiodes (PPDs) are attracting attention for application in large-area light-sensing as they promise lower cost, high sensitivity, and spectral tunability from the visible to the near infrared. One of the key issues for reaching high specific detectivity (<i>D</i>^*) in photodiodes is minimizing the dark current density (<i>J</i>_D) and the noise (<i>i</i>_n), which are typically rather high under the reverse bias where PPDs are operated. Among the common strategies to reduce dark current density, the inclusion of charge-blocking layers between the electrodes and the perovskite layer has become popular. Whilst these blocking layers are successful in increasing the energy barrier for charge injection, the lower limits of dark current density reached experimentally (~10⁻⁶ mA cm⁻²) remain, however, orders of magnitude higher than the expected intrinsic bulk thermal-generated dark current density, which we calculated to be 1 × 10⁻¹² mA cm⁻² for bandgap of 1.23 eV. Hence, another process than bulk thermal generation and charge injection is responsible for the remaining dark current. To find its origin, we carefully analyzed the activation energy (<i>E</i>_a) of the dark current by studying its temperature dependence in optimized Pb-Sn perovskite photodiodes with different bandgaps, employing a series of electron-blocking layers (EBL). The results show that the obtained <i>E</i>_a, which ranges from 0.6 to 1 eV depending on the EBL-perovskite material, correlates with the energy offset (<i>Φ</i>) between the conduction band minimum (CBM) of the perovskite and the highest occupied molecular orbital (HOMO) of the EBL. This suggests that a thermal charge generation process at that interface is the main cause for <i>J</i>_D. Through this mechanism, at non-zero temperature an electron from the EBL HOMO is thermally excited to the conduction band (CB) of the perovskite and generates an electron-hole pair, which is then separated into free charge carriers by the applied reverse bias and collected, thereby producing a current. Challenging the long-held view that charge-blocking layers only reduce dark current, we reveal that <i>J</i>_D scales exponentially with <i>Φ</i> and thereby with the HOMO energy of the electron-blocking layer, which is in fact responsible for generating the dark current density of the PPD. By lowering the HOMO energy by using appropriate blocking layers, such as PTAA:poly-TPD and poly-TPD, <i>Φ</i> (and <i>E</i>_a) increases and <i>J</i>_D decreases by almost 4 orders of magnitude, going from 10⁻⁴ to values &lt; 10⁻⁷ mA cm⁻² for <i>Φ</i> &gt; 1 eV. To the best of our knowledge, no perovskite photodiodes have been reported with similarly low dark current density. These findings are supported by drift-diffusion simulations that reproduce the magnitude and the trend of <i>J</i>_D when interfacial charge generation is included. From the analysis of current noise spectral density, the noise current (<i>i</i>_n) is also found to scale with <i>J</i>_D, and thus with <i>Φ</i>, further highlighting the relevance of EBL HOMO energy on the specific detectivity of PPDs. These measurements also reveal a frequency independent spectrum of <i>i</i>_n for PPDs with maximized <i>Φ</i> (in the interval<i> f</i> = 1-100 Hz) and an arising 1/<i>f</i> behavior at low frequency for those exhibiting higher dark current density.<br/>Finally, based on this new understanding of the origin of the dark current we developed a near infrared PPD that features a wavelength sensitivity up to 1050 nm, a sub-microsecond response time, an ultralow dark current density of 5 × 10⁻⁸ mA cm⁻²and noise current of 2 × 10⁻¹⁴ A Hz^−1/2, resulting in a real specific detectivity of 2.5 × 10¹² Jones in the near infrared. The new insight on the origin of dark-current generation described in this work will fuel the development of novel, even more refined device architectures that further decrease dark current and increase specific detectivity, as well as integration of these exciting materials in a variety of different electronic and medical applications.