• Energy Research

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AbstractOrganic bulk heterojunction photodiodes (OPDs) attract attention for sensing and imaging. Their detectivity is typically limited by a substantial reverse bias dark current density (Jd). Recently, using thermal admittance or spectral photocurrent measurements, Jd has been attributed to thermal charge generation mediated by mid‐gap states. Here, the temperature dependence of Jd in state‐of‐the‐art OPDs is reported with Jd down to 10−9 mA cm−2 at −0.5 V bias. For a variety of donor‐acceptor bulk‐heterojunction blends it is found that the thermal activation energy of Jd is lower than the effective bandgap of the blends, by ca. 0.3 to 0.5 eV, but higher than expected for mid‐gap states. Ultra‐sensitive sub‐bandgap photocurrent spectroscopy reveals that the minimum photon energy for optical charge generation in OPDs correlates with the dark current thermal activation energy. The dark current in OPDs is attributed to thermal charge generation at the donor‐acceptor interface mediated by intra‐gap states near the band edges.

Perovskites with bandgaps between 1.7 and 1.8 eV are optimal for tandem configurations with crystalline silicon (c-Si) because they facilitate efficient harvest of solar energy. In that respect, achieving a high open-circuit voltage (V-OC) in such wide-bandgap perovskite solar cells is crucial for a high overall power conversion efficiency (PCE). Here, we provide key insights into the factors affecting the V-OC in wide-bandgap perovskite solar cells. We show that the influence of the hole transport layer (HTL) on V-OC is not simply through its ionization potential but mainly through the quality of the perovskite-HTL interface. With effective interface passivation, we demonstrate perovskite solar cells with a bandgap of 1.72 eV that exhibit a V-OC of 1.22 V. Furthermore, by combining the high-V-OC perovskite solar cell with a c-Si solar cell, we demonstrate a perovskite-Si four-terminal tandem solar cell with a PCE of 27.1%, exceeding the record PCE of single-junction Si solar cells.

AbstractMinimizing the reverse bias dark current while retaining external quantum efficiency is crucial if the light detection sensitivity of organic photodiodes (OPDs) is to compete with inorganic photodetectors. However, a quantitative relationship between the magnitude of the dark current density under reverse bias ( Jd) and the properties of the bulk heterojunction (BHJ) active layer has so far not been established. Here, a systematic analysis of Jd in state‐of‐the‐art BHJ OPDs using five polymers with a range of energy levels and charge transport characteristics is presented. The magnitude and activation energy of Jd are explained using a model that assumes charge injection from the metal contacts into an energetically disordered semiconductor. By relating Jd to material parameters, insights into the origin of Jd are obtained that enable the future selection of successful OPD materials.

AbstractFor 19 diketopyrrolopyrrole polymers, the highest occupied molecular orbital (HOMO) energies are determined from i) the oxidation potential with square‐wave voltammetry (SWV), ii) the ionization potential using ultraviolet photoelectron spectroscopy (UPS), and iii) density functional theory (DFT) calculations. The SWV HOMO energies show an excellent linear correlation with the open‐circuit voltage (Voc) of optimized solar cells in which the polymers form blends with a fullerene acceptor ([6,6]‐phenyl‐C61‐butyl acid methyl ester or [6,6]‐phenyl‐C71‐butyl acid methyl ester). Remarkably, the slope of the best linear fit is 0.75 ± 0.04, i.e., significantly less than unity. A weaker correlation with Voc is found for the HOMO energies obtained from UPS and DFT. Within the experimental error, the SWV and UPS data are correlated with a slope close to unity. The results show that electrochemically determined oxidation potentials provide an excellent method for predicting the Voc of bulk heterojunction solar cells, with absolute deviations less than 0.1 V.

The electrical properties of MOS capacitors with an indium tin oxide (ITO) gate are studied in terms of the number density of the fixed oxide charge and of the interface traps N/sub f/ and N/sub it/, respectively. Both depend on the deposition conditions of ITO and the subsequent annealing procedures. The fixed oxide charge and the interface-trap density are minimized by depositing at a substrate temperature of 240 degrees C at low power conditions and in an oxygen-rich ambient. Under these conditions, as-deposited ITO films are electrically conductive. The most effective annealing procedure consists of a two-step anneal: a 45-s rapid thermal anneal at 950 degrees C in N/sub 2/, followed by a 30 min anneal in N/sub 2//20% H/sub 2/ at 450 degrees C. Typical values obtained for N/sub it/ and N/sub f/ are 4.2*10/sup 10/ cm/sup -2/ and 2.8*10/sup 10/ cm/sup -2/, respectively. These values are further reduced to 1.9*10/sup 10/ cm/sup -2/ and >

Monolithic perovskite/silicon tandem photovoltaics have fueled major research efforts as well as gaining rapid industrial interest. So far, most of the literature has focused on the use of currently more expensive silicon heterojunction bottom cell technology. This work demonstrates a perovskite/silicon tandem solar cell based on the industrially dominant passivated emitter and rear cell (PERC) technology. In detail, we investigate a tunnel recombination junction (TRJ) consisting of ITO/NiO/2-(9H-carbazol-9-yl)ethyl] phosphonic acid (2PACz) and compare it with an ITO/2PACz TRJ. Specifically, the NiO layer is deposited by atomic layer deposition (ALD). Although ITO/2PACz-based tandem devices can reach more than 24% conversion efficiency, we observe that they suffer from a large spread in photovoltaic parameters due to electrical shunts in the perovskite top cell, caused by the inhomogeneity of the 2PACz layer on ITO. Instead, when ALD NiO is sandwiched between 2PACz and ITO, the surface coverage of 2PACz improves and the yield of the devices, in terms of all device parameters, also improves, i.e., the standard deviation decreases from 4.6% with ITO/2PACz to 2.0% with ITO/NiO/2PACz. In conclusion, thanks to the presence of NiO, the TRJ consisting of ITO/NiO/2PACz leads to a 23.7% efficient tandem device with narrow device efficiency distribution.

Thermal annealing in air of p–i–n metal halide perovskite solar cells processed on PEDOT:PSS restores the work function of this hole transport layer, resulting in power conversion efficiency.