Leakage Current Induced by Energetic Disorder in Organic Bulk Heterojunction Solar Cells: Comprehending the Ultrahigh Loss of Open-Circuit Voltage at Low Temperatures

Abstract

In organic bulk heterojunction solar cells, the open circuit voltage (V oc ) suffers from an ultrahigh loss at low temperatures. In this work we investigate the origin of the loss through calculating the V oc − T plots with the device model method systematically and comparing it with experimentally observed ones. When the energetic disorder is incorporated into the model by considering the disorder-suppressed and temperature-dependent charge carrier mobilities, it is found that for nonselective contacts the V oc reduces drastically under the low temperature regime, while for selective contacts the V oc keeps increasing with the decreasing temperature. The main reason is revealed that as the temperature decreases, the reduced mobilities give rise to low charge extraction efficiency and small bimolecular recombination rate for the photogenerated charge carriers, so that in the former case they can be extracted from the wrong electrode to form a leakage current which counteracts the photocurrent and increases quickly with voltage, leading to the anomalous reduction of V oc . In addition, it is revealed that the charge generation rate is slow-varying with temperature and does not induce significant V oc loss. This work also provides a comprehensive picture for the V oc behavior under varying device working conditions.

Origin of Open‐Circuit Voltage Turnover in Organic Solar Cells at Low Temperature

“…[34,35] The disorder-suppressed mobility at low temperatures then leads to a greatly reduced bimolecular recombination rate where a large amount of charge carriers can thus accumulate in the device. [9] In devices with electrodes that are not perfectly selective, wrong charge carriers can be extracted at the electrodes because they are not effectively blocked, contributing to the leakage current and the V OC turnover at low temperatures. The leakage current caused by this effect can be reduced by using Ohmic contacts.…”

“…Moreover, the different temperature dependences of leakage current observed in the measured systems, as shown in Figure 4 and in Figure S1 and S3, Supporting Information, possibly originate from the temperature dependence of charge transport and extraction properties. This might be due to their differences in the degree of order, [9] the charge transport mechanism, [36] or the effect of charge transporting layers. [37] The temperature-dependent leakage current of the PFBDB-T:C8-ITIC system in particular is intriguing, and suggests a very different origin of leakage current in this system compared to the other systems.…”

“…Different quantitative descriptions of V OC have been proposed from different experimental perspectives, including dark current density-voltage ( J-V) characteristics, [5] spectroscopic studies, [6] analysis of energetic disorder, [7] and measurement of the temperature dependence of V OC . [8,9] Among these studies, investigations of the temperature dependence of V OC are less common, likely due to the need for cryogenic equipment. Recently, however, low-temperature studies have gained more attention and are being applied more broadly to investigate OSCs.…”

“…[8] On the other hand, Yang et al explained V OC turnover in terms of energetic disorder-suppressed mobilities together with nonselective contacts that can lead to large parasitic leakage current. [9] Although Yang et al successfully simulated a V OC reduction at low temperatures, they did not provide experimental results to support their theory.…”

Temperature and Light Modulated Open‐Circuit Voltage in Nonfullerene Organic Solar Cells with Different Effective Bandgaps

“…[16][17][18][19][20][21] Even less attention has been paid to the investigation of temperature-dependent generationrecombination processes in BHJ layers and, specifically, at their interfaces with electrical contacts. [22][23][24] In this contribution we analyzed temperature and light modulated V oc in a series of nonfullerene BHJ solar cells with conventional or inverted device architecture and a systematically tuned effective bandgap E g,eff in a wide range from 1.0 to 1.62 eV. The temperature-dependent V oc versus light intensity slopes The relationship of the temperature-light intensity dependence of opencircuit voltage V oc in nonfullerene-based organic solar cells with their material characteristics and multimechanism recombination parameters is described.…”