energies Article Efficient Planar Hybrid n-Si/PEDOT:PSS Solar Cells with Power Conversion Efficiency up to 13.31% Achieved by Controlling the SiOx Interlayer Chenxu Zhang 1, Yuming Zhang 1,*, Hui Guo 1, Qubo Jiang 2, Peng Dong 1 and Chunfu Zhang 1,* ID 1 Wide Bandgap Semiconductor Technology Disciplines State Key Laboratory, School of Microelectronics, Xidian University, Xi’an 710071, China; [email protected] (C.Z.); [email protected] (H.G.); [email protected] (P.D.) 2 School of Electronic Engineering and Automation, Guilin University of Electronic Technology, No. 1 Jinji Road, Guilin 541004, China; [email protected] * Correspondence: [email protected] (Y.Z.); [email protected] (C.Z.) Received: 25 April 2018; Accepted: 21 May 2018; Published: 30 May 2018 Abstract: In this work, the effects of the SiOx interface layer grown by exposure in air on the performance of planar hybrid n-Si/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solar cells are investigated. Compared to the cell with a hydrogen-terminated Si surface, the cell with an oxygen-terminated Si surface reveals improved characteristics in power conversion efficiency, increased from 10.44% to 13.31%. By introducing the SiOx, the wettability of the Si surface can be improved, allowing an effective spread of the PEDOT:PSS solution and thus a good contact between the PEDOT:PSS film and Si. More importantly, it can change the polarity of the Si surface from a negative dipole to a positive dipole, owing to the introduction of the SiOx interface. The Si energy band will bend up and give rise to a favorable band alignment between Si and PEDOT:PSS to promote carrier separation. These results could be potentially employed to further development of this simple, low-cost heterojunction solar cell. Keywords: SiOx interface; n-Si/PEDOT:PSS solar cells; surface polarity 1. Introduction Si/organic hybrid solar cells utilizing poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) combined with an n-type silicon (n-Si) have attracted significant research interest in recent years due to their obvious advantages of a low-temperature process, remarkably low fabrication cost, and potential high efficiency [1]. In this type of hybrid solar cells, as shown in Figure1, the PEDOT:PSS layer acts as a hole transporting path [2] when forming a heterojunction with the n-type Si substrate. Si has a strong absorption ability in a very wide spectrum range and excellent carrier transport ability. PEDOT:PSS is a water-soluble polymer which has a high conductivity, a transmission window in the visible spectral range, and excellent chemical and thermal stability. This type of Si/PEDOT:PSS hybrid solar cell combines the superior absorption property of Si in a wide spectrum range and the advantage of aqueous solution-based processes of PEDOT:PSS. Over the past few years, considerable progress towards creating highly efficient hybrid n-Si/PEDOT:PSS solar cells has been achieved by adjusting major factors such as electrical conductivity, chemical affinity, and passivation on the device surface [3–9]. Sun et al. introduced perovskite nanoparticles to the PEDOT:PSS layer to generate a positive electrical surface field [10]. Pham et al. presented adding functionalized graphene to the PEDOT:PSS solution based on Silicon nanowire (SiNW), and demonstrated an enhancement of photoelectric conversion efficiency (PCE). Wen et al. [11] fabricated an n+ amorphous silicon layer on Energies 2018, 11, 1397; doi:10.3390/en11061397 www.mdpi.com/journal/energies Energies 2018, 11, 1397 2 of 10 Energies 2018, 11, x FOR PEER REVIEW 2 of 10 the back of the c-Si, which could significantly improve the extraction of the photon-generated carrier andextraction suppress of the the recombination photon-generated of hole carrier electrons and suppress at the rear the cathode recombination [12]. Despite of hole significant electrons effortsat the to improverear cathode solar cell[12]. performance, Despite significant the highest efforts power to improve conversion solar efficiencycell performance, (PCE) reported the highest to date power is still lowconversion when compared efficiency to (PCE) the value reported theoretically to date is estimated still low when by Shockley–Queisser compared to the value for a single-junctiontheoretically deviceestimated [13]. by Shockley–Queisser for a single-junction device [13]. Figure 1. Schematic structure of planar hybrid n-Si/PEDOT:PSS solar cells. Figure 1. Schematic structure of planar hybrid n-Si/PEDOT:PSS solar cells. To further improve the performance of the hybrid n-Si/PEDOT:PSS solar cells, the interface propertiesTo further between improve n-Si theand performance PEDOT:PSS are of theof great hybrid importance. n-Si/PEDOT:PSS This is because solar cells,photogenerated the interface propertiesholes in n-Si between are required n-Si and to PEDOT:PSSbe effectively are transmitted of great importance.through the heterojunction This is because interface photogenerated [1] into holesthe inPEDOT:PSS n-Si are required layer and to then be effectively collected by transmitted the anode.through Therefore, the a heterojunctionhigh-quality interface interface between [1] into n- the PEDOT:PSSSi and PEDOT:PSS layer and is then critical collected for high-performance by the anode. Therefore, hybrid solar a high-quality cells. However, interface usually, between there n-Si are and PEDOT:PSSnumerous ismicrovoids critical for at high-performance the Si/PEDOT:PSS hybridinterface solar because cells. of However, the insufficient usually, wettability there are of numerous the Si microvoidssurface, which at the leads Si/PEDOT:PSS to a bad surface interface coverage because and severe of the recombination. insufficient wettability A proper ofinterface the Si surface,layer whichwith leadsa hydrophilic to a bad nature surface between coverage the andPEDOT:PSS severe recombination. and n-Si can improve A proper the interfacewettability layer of the with Si a hydrophilicsurface, which nature is helpful between in the the formation PEDOT:PSS of ideal and interface n-Si can contacts improve between the wettability the PEDOT:PSS of the film Si surface, and whichthe Si is substrate, helpful in and the thus formation a more ofideal ideal p–n interface junction, contacts resulting between in less charge the PEDOT:PSS recombination film and and a the Silarger substrate, photocurrent and thus density a more [14]. ideal At p–n the junction,same time, resulting the inserted in less interface charge layer recombination may help to andpassivate a larger photocurrentthe surface of density n-Si, which [14]. Atcould the further same time, enhance the insertedthe device interface performance. layer mayAs an help effective to passivate interface the surfacelayer between of n-Si, which PEDOT:PSS could and further n-Si, enhance its thickness the device is a critical performance. parameter. As When an effective the interface interface layer layer is betweentoo thin, PEDOT:PSS it is insufficient and n-Si, to provide its thickness good issurface a critical wettability parameter. and When passivation. the interface However, layer once is too the thin, it isinterface insufficient layer to is providetoo thick, good the photogenerated surface wettability carriers and have passivation. difficulty However, when they once tunnel the interfacethrough the layer insulating layer. is too thick, the photogenerated carriers have difficulty when they tunnel through the insulating layer. Previous work has shown improved solar cell performance when using a thin atomic layer Previous work has shown improved solar cell performance when using a thin atomic layer deposition (ALD) Al2O3 layer [15,16], which was achieved based on increased build-in potential and deposition (ALD) Al2O3 layer [15,16], which was achieved based on increased build-in potential improved charge collection by the electron blocking of Al2O3. However, the ALD instrument is and improved charge collection by the electron blocking of Al O . However, the ALD instrument expensive, and the process is complex. Other oxide layers [17,18]2 3are also reported to provide the is expensive,crucial interface and thelayer process for the is hybrid complex. solar Other cells. oxideHowever, layers all [ 17the,18 reported] are also deposition reported methods to provide are the crucial interface layer for the hybrid solar cells. However, all the reported deposition methods are complicated. As is well known, when Si is exposed to the air, a native oxide (SiOx) will grow easily. complicated.As an efficient As interface is well known, layer, its when thickness Si is exposedis also important. to the air, We a native could oxideremove (SiO thex )unintentionally will grow easily. Asgrown an efficient oxide interfacelayer by dipping layer, its the thickness Si substrate is also in a important. dilute HF solution We could and remove intentionally the unintentionally regrow the grownSiOx layer oxide by layer exposing by dipping the Si substrate the Si substrate to air in a in simple a dilute and HF controllable solution andway intentionallyso that the wanted regrow SiOx the SiOthicknessx layer bycould exposing be obtained. the Si However, substrate whether to air in this a simple oxide layer and can controllable effectively way improve so that the theinterface wanted SiOqualityx thickness and enhance could bethe obtained.device performance However, or whether not is still this not oxide well investigated. layer can effectively improve the interfaceIn qualitythis work, and the enhance effects the of devicethe SiO performancex layer intentionally or not isgrown still not though well investigated.exposure to air on the performanceIn this work, of planar the effects hybrid of n-Si/PEDOT:PSS the SiOx layer solar intentionally cells are investigated. grown though Compared exposure to the to cell air with on the performancethe bare Si surface, of planar the hybrid cell with n-Si/PEDOT:PSS an oxygen-terminated solar cells Si surface are investigated. reveals improved Compared characteristics to the cell in with thepower bare Siconversion surface, theefficiency cell with (PCE), an oxygen-terminated increased from 10.44% Si surfaceto 13.31%.